**Deposition Technologies**

**Chapter 6**

Provisional chapter

**Epitaxial Nitride Thin Film and Heterostructures: From**

DOI: 10.5772/intechopen.79525

Epitaxial nitride thin films and heterostructures are one of the most celebrated class of materials not only due to their utility in fundamental materials science and device physics studies, but also for their numerous industrial applications from hard coating technology to solid-state lighting. Transition metal nitrides such as TiN and others have been utilized for decades in hard coating and tribology applications. The last two decades have also seen the emergence and dominance of GaN for solid-state lighting and power electronic applications. Though TiN, and other wurtzite III-nitride semiconductor such as GaN remain the most important nitride coating materials for a range of applications, several other rocksalt nitride thin film and superlattice heterostructures such as ScN, CrN, and TiN/(Al,Sc)N metal/semiconductor superlattices have attracted significant interests in recent years for applications in thermoelectricity, plasmonics, solar energy conversion, and in high temperature electronic, optoelectronic, and plasmonic devices. In this chapter, we present an up-to-date summary of rocksalt nitride thin film and heterostructure coating materials for their applications in energy transport and conversion research fields. The suitability and usefulness of such nitride coating materials in the most recent scientific and engineering advances related to the energy transport and conversion research fields are

Keywords: transition metal nitrides, epitaxy, superlattice, refractory electronics,

Epitaxial nitride thin film and superlattice heterostructures are one of the most celebrated materials class, not only for their wide ranging industrial applications such as in corrosion resistant hard coating, light emitting diode (LED) and power electronic devices, but also for

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Epitaxial Nitride Thin Film and Heterostructures: From

**Hard Coating to Solid State Energy Conversion**

Hard Coating to Solid State Energy Conversion

Shashidhara Acharya and Bivas Saha

Shashidhara Acharya and Bivas Saha

http://dx.doi.org/10.5772/intechopen.79525

Abstract

highlighted.

1. Introduction

thermoelectric, plasmonics

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion** Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

DOI: 10.5772/intechopen.79525

Shashidhara Acharya and Bivas Saha Shashidhara Acharya and Bivas Saha

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79525

### Abstract

Epitaxial nitride thin films and heterostructures are one of the most celebrated class of materials not only due to their utility in fundamental materials science and device physics studies, but also for their numerous industrial applications from hard coating technology to solid-state lighting. Transition metal nitrides such as TiN and others have been utilized for decades in hard coating and tribology applications. The last two decades have also seen the emergence and dominance of GaN for solid-state lighting and power electronic applications. Though TiN, and other wurtzite III-nitride semiconductor such as GaN remain the most important nitride coating materials for a range of applications, several other rocksalt nitride thin film and superlattice heterostructures such as ScN, CrN, and TiN/(Al,Sc)N metal/semiconductor superlattices have attracted significant interests in recent years for applications in thermoelectricity, plasmonics, solar energy conversion, and in high temperature electronic, optoelectronic, and plasmonic devices. In this chapter, we present an up-to-date summary of rocksalt nitride thin film and heterostructure coating materials for their applications in energy transport and conversion research fields. The suitability and usefulness of such nitride coating materials in the most recent scientific and engineering advances related to the energy transport and conversion research fields are highlighted.

Keywords: transition metal nitrides, epitaxy, superlattice, refractory electronics, thermoelectric, plasmonics

### 1. Introduction

Epitaxial nitride thin film and superlattice heterostructures are one of the most celebrated materials class, not only for their wide ranging industrial applications such as in corrosion resistant hard coating, light emitting diode (LED) and power electronic devices, but also for

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

their utility as model systems for fundamental materials science studies as well as device physics research. Transition metal nitrides (TMNs) such as TiN have long been used as a coating material in every day home appliances such as watches and others, while III-Nitride semiconductors such as AlN is used in many devices as a dielectric and piezoelectric material. The last 20–30 years have also seen the emergence of GaN as one of the most celebrated nitride semiconductors for its applications in LED and power-electronics/optoelectronics devices.

Given such excellent set of physical properties, it is not surprising that nitrides are researched and developed for decades, and many industrial products are available in commercial market

Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

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113

However, more recently, a large number of new properties and functionalities that were previously neglected and were relatively unexplored have emerged with nitrides. TMN semiconductor ScN, and its solid state alloys (Al1xScxN and others) [1] and superlattices ((Hf, Zr)N/ScN and TiN/(Al, Sc)N) [2] have emerged as attractive materials for solid state energy transport and conversion research fields with applications ranging from thermoelectricity, plasmonics, and solar thermo-photovoltaics . Though several books, books chapter and research articles have addressed the broader implications of nitrides in materials science and solid-state physics, a detailed discussion on the most recent advances of TMNs thin film and superlattice heterostructures on energy transport and conversion research fields are lacking. In this chapter, we will discuss the recent progress and development on the epitaxial nitride thin film and superlattice heterostructures for

Traditionally nitride thin film and superlattice heterostructures are deposited by a variety of deposition techniques such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), magnetron sputtering, arc discharge method and others. Each of these deposition methods have their own advantages, and both research and development in academia and industries employ deposition techniques suitable for specific applications. For example, corrosion resistant hard coating technology, which has traditionally been the most prevalent application space for nitrides usually employ magnetron sputtering and arc discharge methods. The use of sputtering and arc discharge for hard coating applications are justified as several of such materials such as TiN, Ti1xAlxN and others are TMNs having metallic characteristics, and controlling electronic defects are not of great concern. Instead the sputtering and arc discharge methods allow industries large volume scalability, which may not be possible with some of the

LED and other optoelectronic industries, however, prefer several types of CVD methods due to their high through-puts and industrial scalability. Use of CVD are also necessary due to the requirement of controlling defects in epitaxial nitrides, and selective doping of the materials to n-type or p-type. For example, GaN based LED and power electronic devices are generally deposited via. Metal organic chemical vapor deposition (MOCVD) technique in industries.

Research and development of nitrides, however, employ almost all available techniques including ultra-high vacuum MBE, magnetron sputtering and others. As unwanted impurities such as oxygen, carbon, halogens, and others could significantly alter the electronic properties of semiconducting nitrides, device research with nitride thin film and heterostructures usually require ultra-high vacuum-based methods. A detailed discussion on each of these deposition techniques are beyond the scope of this book chapter, however, readers could refer to [2] for

having nitride materials as components.

other methods.

further details.

applications in solid state energy conversion and transport.

2. Nitride thin film and heterostructure growth

Such wide-ranging applications of nitride thin film and superlattice heterostructures are inherently related to their excellent properties, which are scarce in most other class of materials. Some of these interesting properties are


Given such excellent set of physical properties, it is not surprising that nitrides are researched and developed for decades, and many industrial products are available in commercial market having nitride materials as components.

However, more recently, a large number of new properties and functionalities that were previously neglected and were relatively unexplored have emerged with nitrides. TMN semiconductor ScN, and its solid state alloys (Al1xScxN and others) [1] and superlattices ((Hf, Zr)N/ScN and TiN/(Al, Sc)N) [2] have emerged as attractive materials for solid state energy transport and conversion research fields with applications ranging from thermoelectricity, plasmonics, and solar thermo-photovoltaics . Though several books, books chapter and research articles have addressed the broader implications of nitrides in materials science and solid-state physics, a detailed discussion on the most recent advances of TMNs thin film and superlattice heterostructures on energy transport and conversion research fields are lacking. In this chapter, we will discuss the recent progress and development on the epitaxial nitride thin film and superlattice heterostructures for applications in solid state energy conversion and transport.

### 2. Nitride thin film and heterostructure growth

their utility as model systems for fundamental materials science studies as well as device physics research. Transition metal nitrides (TMNs) such as TiN have long been used as a coating material in every day home appliances such as watches and others, while III-Nitride semiconductors such as AlN is used in many devices as a dielectric and piezoelectric material. The last 20–30 years have also seen the emergence of GaN as one of the most celebrated nitride semiconductors for its applications in LED and power-electronics/optoelectronics

Such wide-ranging applications of nitride thin film and superlattice heterostructures are inherently related to their excellent properties, which are scarce in most other class of materials.

a. Diversity in electronic properties: nitrides comprise materials having a full spectrum of electronic properties (from highly conductive metals to excellent dielectric or insulators). For example, TMNs such as TiN, ZrN and others are highly conductive metals with their electrical conductivity as high as traditional noble metals such as Cu, Ag, Au and others in some cases. AlN, BN and others are insulators with large bandgap (>5 eV) and highly resistive, which makes them essential parts in many devices as dielectric layers. GaN, InN, ScN and others are excellent semiconductors with bandgap ranging from few 100 s of meV to several eV. Because of such great diversity in electrical properties nitride have attracted

b. High melting temperature: nitrides usually possess extremely high melting temperature in 2000–3000C temperature range, which make them suitable for several high-temperature electronic, optoelectronic and plasmonic applications. For example, TiN has a melting tem-

c. Corrosion resistant and high mechanical hardness: nitride thin film and multilayer heterostructures are well-known to be corrosion resistant and mechanically hard materials with hardness at room temperature ranging more than 25 GPa for thin film to 40 GPa or more for multilayers. Such high hardness and corrosion resistant properties of are extremely useful for many applications such as cutting tools, bearings and in tribology, especially for

d. Potential for large acoustic impedance mismatch: due to the mass difference of metal atoms forming mono-nitrides, the nitride family of materials offer tremendous opportunity to create large acoustic impedance mismatch. Acoustic impedance mismatch in a heterostructure material creates phonon bandgap, which help in reducing thermal conductivity necessary for several applications such as thermoelectricity. For example, the acoustic impedance mismatch between HfN and ScN resulting from the mass difference between Hf (178.49 u) and Sc (44.95 u) atoms, results in a significantly lower cross-plane thermal conductivity in

e. Low homologous growth temperature: deposition temperature of nitrides is typically much smaller in comparison to their melting temperatures, which assist in uniform thin film and superlattice heterostructure growth with standard deposition methods such as magnetron

sputtering, molecular beam epitaxy, chemical vapor deposition and others.

perature of about 2600C, while the same for GaN is 2500C.

HfN/ScN multilayers compared to the individual thin films.

devices.

112 Coatings and Thin-Film Technologies

Some of these interesting properties are

many industrial applications.

harsh conditions.

Traditionally nitride thin film and superlattice heterostructures are deposited by a variety of deposition techniques such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), magnetron sputtering, arc discharge method and others. Each of these deposition methods have their own advantages, and both research and development in academia and industries employ deposition techniques suitable for specific applications. For example, corrosion resistant hard coating technology, which has traditionally been the most prevalent application space for nitrides usually employ magnetron sputtering and arc discharge methods. The use of sputtering and arc discharge for hard coating applications are justified as several of such materials such as TiN, Ti1xAlxN and others are TMNs having metallic characteristics, and controlling electronic defects are not of great concern. Instead the sputtering and arc discharge methods allow industries large volume scalability, which may not be possible with some of the other methods.

LED and other optoelectronic industries, however, prefer several types of CVD methods due to their high through-puts and industrial scalability. Use of CVD are also necessary due to the requirement of controlling defects in epitaxial nitrides, and selective doping of the materials to n-type or p-type. For example, GaN based LED and power electronic devices are generally deposited via. Metal organic chemical vapor deposition (MOCVD) technique in industries.

Research and development of nitrides, however, employ almost all available techniques including ultra-high vacuum MBE, magnetron sputtering and others. As unwanted impurities such as oxygen, carbon, halogens, and others could significantly alter the electronic properties of semiconducting nitrides, device research with nitride thin film and heterostructures usually require ultra-high vacuum-based methods. A detailed discussion on each of these deposition techniques are beyond the scope of this book chapter, however, readers could refer to [2] for further details.

### 3. Historical perspective: TiN as hard coating and GaN as solid-state lighting materials

### 3.1. Hard coating materials

Hard coating industries are one of the early adopters of nitride thin film and heterostructures. Since tribology applications require materials that are mechanically hard, chemically stable at elevated temperatures with low wear rate and coefficient of frictions over wide working conditions, TMNs such as TiN found immediate attention and applications. Titanium nitride (TiN) is a leading coating material and is used for edge retention and corrosion resistance on machine tooling, such as drill bits and milling cutters, often improving tool lifetime by a factor of three or more (Figure 1). However, the hardness of TiN is relatively low (20–24 GPa) and the oxidation resistance of TiN in air is limited to temperatures below 700C, beyond which TiN forms TiO2 and nitrogen bubbles, which significantly limits its application range [3, 4]. To overcome these limitations, several ternary nitrides thin film coating material have been developed starting from 1980s by solid state alloying TiN with other metals such as aluminum (Al), vanadium (V), molybdenum (Mo), zirconium (Zr), etc. (see Table 1).

Titanium-aluminum-nitride (Ti1xAlxN) is the most celebrated among these ternary nitrides, as it overcomes several limitations of TiN as a coating material. Aluminum atoms exhibit higher mobility compared to titanium atoms, therefore, exposure of Ti1xAlxN in air or in oxygen environment results in the formation of thin aluminum oxide layer on Ti1xAlxN surfaces at elevated temperatures [3]. Aluminum oxide layer acts as a barrier for further oxygen diffusion inside the Ti1xAlxN thin film, thus preventing further oxidation of the nitride film. Such oxidation resistant properties of Ti1xAlxN makes it an effective tooling material at elevated temperatures, where TiN fail to operate. Apart from the oxidation resistant behavior, Ti1xAlxN also exhibits enhanced hardness (35 GPa) in comparison to TiN (24 GPa) necessary for tribology applications. Ti1xAlxN coated cutting tools have also shown excellent wear resistance in machining sticky metals such as aluminum alloys and austenite stainless steel, and widely used in other industries.

(Ti1xMoxN), etc. [4]. Ti1xVxN is a technologically important thin film coating and it is used in a diverse range of areas such as the packaging industry, transparent barrier coatings, microelectronics and others. Magnetron sputtered Ti1xVxN thin films have high hardness (28 GPa) and excellent thermal stability [4]. Similarly, Ti1xMoxN is also an effective alternative ternary thin film coating material and is used in industries improve the mechanical properties of TiN. Ti1xMoxN have a lower friction coefficient and wear rates compared to

Apart from these ternary nitride coating materials, recent study on quaternary titaniumaluminum-scandium-nitride (Ti1<sup>x</sup>yAlxScyN) has shown significantly higher hardness of 46 GPa at room temperature [5]. Experimental results showed that incorporation of a small amount of scandium nitride (ScN) inside Ti1xAlxN matrix improved the crystal quality and the hardness of the alloy thin film. The exact mechanism of the hardness enhancement,

Thin-film multilayers and superlattices are also a potential configuration that realize extraordinarily hard materials with long lifetime at high operating temperatures. Koehler had proposed in the 1970s that the interfaces in multilayers should act as high energy barriers for dislocation motion, thereby increasing hardness [6]. Based on that suggestion, several nitrides

TiN thin film. The hardness of Ti1xMoxN is significantly high at 30 GPa [4].

Table 1. Hardness of various TMN coating materials are presented along with the deposition methods.

however, still remains to be addressed in details.

Material Deposition technique Hardness

deposition

TiN/ Al1xScxN Multiple arc vapor deposition

Arc discharge method 25

Cathodic arc ion plating 26 Arc ion plating 24

TiN/VN Magnetron sputtering 54 –

(GPa)

TiN Magnetron sputtering 24 Oxidation above 700C

transformation Cathodic arc vapor

27

35

Ti1xVxN Magnetron sputtering 28 Lower hardness values

Ti1xMoxN Magnetron sputtering 30 Low solubility of Mo in TiN

TiN/AlN Pulsed laser deposition 30 Extremely low AlN layer thickness

Magnetron sputtering 46 –

Significant challenges of each of these coating materials are also highlighted.

Challenges

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Ti1ixAlxN Magnetron co-sputtering 24 High temperature stability and AlN cubic-to-wurtzite phase

TiN/NbN Magnetron sputtering 48 Porosity at column boundary weakens the multilayer

TiN/CrN Magnetron sputtering 37 Poor oxidation resistance and interdiffusion at interface above 700C

Apart from Ti1xAlxN, several other ternary nitrides have found applications in tribology applications such as titanium-vanadium-nitride (Ti1xVxN), titanium-molybdenum-nitride

Figure 1. Nitride coating materials used in hard coating applications (adapted from Advanced Coating Service, Rochester, NY, USA).


Significant challenges of each of these coating materials are also highlighted.

3. Historical perspective: TiN as hard coating and GaN as solid-state

(Al), vanadium (V), molybdenum (Mo), zirconium (Zr), etc. (see Table 1).

austenite stainless steel, and widely used in other industries.

Hard coating industries are one of the early adopters of nitride thin film and heterostructures. Since tribology applications require materials that are mechanically hard, chemically stable at elevated temperatures with low wear rate and coefficient of frictions over wide working conditions, TMNs such as TiN found immediate attention and applications. Titanium nitride (TiN) is a leading coating material and is used for edge retention and corrosion resistance on machine tooling, such as drill bits and milling cutters, often improving tool lifetime by a factor of three or more (Figure 1). However, the hardness of TiN is relatively low (20–24 GPa) and the oxidation resistance of TiN in air is limited to temperatures below 700C, beyond which TiN forms TiO2 and nitrogen bubbles, which significantly limits its application range [3, 4]. To overcome these limitations, several ternary nitrides thin film coating material have been developed starting from 1980s by solid state alloying TiN with other metals such as aluminum

Titanium-aluminum-nitride (Ti1xAlxN) is the most celebrated among these ternary nitrides, as it overcomes several limitations of TiN as a coating material. Aluminum atoms exhibit higher mobility compared to titanium atoms, therefore, exposure of Ti1xAlxN in air or in oxygen environment results in the formation of thin aluminum oxide layer on Ti1xAlxN surfaces at elevated temperatures [3]. Aluminum oxide layer acts as a barrier for further oxygen diffusion inside the Ti1xAlxN thin film, thus preventing further oxidation of the nitride film. Such oxidation resistant properties of Ti1xAlxN makes it an effective tooling material at elevated temperatures, where TiN fail to operate. Apart from the oxidation resistant behavior, Ti1xAlxN also exhibits enhanced hardness (35 GPa) in comparison to TiN (24 GPa) necessary for tribology applications. Ti1xAlxN coated cutting tools have also shown excellent wear resistance in machining sticky metals such as aluminum alloys and

Apart from Ti1xAlxN, several other ternary nitrides have found applications in tribology applications such as titanium-vanadium-nitride (Ti1xVxN), titanium-molybdenum-nitride

Figure 1. Nitride coating materials used in hard coating applications (adapted from Advanced Coating Service, Roches-

lighting materials

114 Coatings and Thin-Film Technologies

ter, NY, USA).

3.1. Hard coating materials

Table 1. Hardness of various TMN coating materials are presented along with the deposition methods.

(Ti1xMoxN), etc. [4]. Ti1xVxN is a technologically important thin film coating and it is used in a diverse range of areas such as the packaging industry, transparent barrier coatings, microelectronics and others. Magnetron sputtered Ti1xVxN thin films have high hardness (28 GPa) and excellent thermal stability [4]. Similarly, Ti1xMoxN is also an effective alternative ternary thin film coating material and is used in industries improve the mechanical properties of TiN. Ti1xMoxN have a lower friction coefficient and wear rates compared to TiN thin film. The hardness of Ti1xMoxN is significantly high at 30 GPa [4].

Apart from these ternary nitride coating materials, recent study on quaternary titaniumaluminum-scandium-nitride (Ti1<sup>x</sup>yAlxScyN) has shown significantly higher hardness of 46 GPa at room temperature [5]. Experimental results showed that incorporation of a small amount of scandium nitride (ScN) inside Ti1xAlxN matrix improved the crystal quality and the hardness of the alloy thin film. The exact mechanism of the hardness enhancement, however, still remains to be addressed in details.

Thin-film multilayers and superlattices are also a potential configuration that realize extraordinarily hard materials with long lifetime at high operating temperatures. Koehler had proposed in the 1970s that the interfaces in multilayers should act as high energy barriers for dislocation motion, thereby increasing hardness [6]. Based on that suggestion, several nitrides multilayer system (e.g., TiN/NbN, TiN/VN, and TiN/CrN) have been developed in 1990s and 2000s that showed improved hardness compared to TiN and Ti1xAlxN thin films (see Table 1). However, all of the nitride multilayers mentioned above are miscible at temperatures exceeding 800C, which significantly limits their usefulness in cutting tool applications, where the surface temperature can reach as high as 1000C during the cutting process. Cubic (rocksalt)- TiN/AlN superlattices were developed to overcome the miscibility problem since TiN/AlN superlattices are immiscible up to 1000C. TiN/AlN superlattices also exhibit excellent oxidation resistance, relatively high hardness compared to TiN, and they are already used commercially as a coating material.

researchers started to grow high quality GaN films. The most significant of these efforts could be attributed to the work of Amano et al. where they deposited high quality GaN thin films by using a polycrystalline AlN as a buffer layer [8]. Similarly, Nakamura et al. at Nichi Chemical Company deposited high quality GaN films by using a low temperature GaN buffer layer with low background of n-type doping [9]. Development of low temperature GaN as a buffer layer proved to be an attractive and industry wide approach for mass production of GaN based light

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Apart from the materials quality issue, another important challenge in early years was on how to develop p-type GaN thin films. Though the idea of p-type doping of GaN through the incorporation of Mg was known, most experimental efforts to achieve p-type GaN were unsuccessful. Amano et al. managed to develop p-type GaN by electron beam irradiation [10]. However, the electron beam irradiation technique was an inefficient method and required a lot of time for hole-doping. Efforts to develop p-type GaN with Mg incorporation via chemical vapor deposition methods were also unsuccessful initially. Important breakthrough was made when modeling work showed that Mg bonds with hydrogen (H) thus forming Mg-H complex during the CVD growth process, which prevent the hole-doping activity of Mg and render development of p-type GaN difficult. Nakamura et al. addressed this important challenge by annealing Mg-doped GaN in N2 environment, where Mg frees up from the Mg-H complex and

The development of high quality GaN thin film growth technique and of p-type GaN lead to the ultimate discovery of blue LEDs, which has significantly changed our society. Akasaki, Amano and Nakamura were awarded the 2014 Noble Prize in Physics for their pioneering work on GaN LED development. GaN today is the workhorse for producing blue emission and along with a suitable dye, most commercially available LED light sources employ GaN. Significant efforts are also currently underway to develop GaN based electronic and optoelec-

Thermoelectric materials convert waste heat energy directly into electrical power and are attractive for harvesting energy in automobiles, power plants, and for deep space exploration. Such materials could also be used as a Peltier cooler in microelectronic chips and devices, where unwarranted heat generation (hot spots in integrated circuits (IC)) limit device efficiencies. Devices made from thermoelectric elements are environmentally friendly and they do not have any movable parts except for a fan in most cases. The efficiency of a thermoelectric

the Seebeck coefficient, σ is the electrical conductivity, κ<sup>e</sup> and κ<sup>p</sup> are the electronic and lattice contributions to the thermal conductivity, respectively, and T is the absolute temperature. The higher the thermoelectric figure-of-merit (ZT) of a material, the more efficient the energy

σ T)/ (κ<sup>e</sup> + κp), where S is

emitting devices.

enable p-type GaN [11].

4.1. Thermoelectrics

tronic devices for power-electronic applications.

4. Recent advances in energy transport and conversion

material is represented by its dimensionless figure-of-merit, ZT = (S<sup>2</sup>

However, TiN/AlN superlattice coatings have a significant drawback. The hardness of TiN/ AlN superlattices is around 33–35 GPa when the thickness of the AlN layers is less than 2– 3 nm but decreases sharply to 23–24 GPa as the AlN layer thickness is increased [5]. This large reduction in hardness is attributed to the transition from the epitaxially stabilized metastable cubic-AlN phase to the stable wurtzite-AlN phase when the AlN layer thickness exceeds the critical thickness of 2–3 nm [7]. The formation of wurtzite-AlN breaks the epitaxial relationship with cubic-TiN leading to polycrystalline grain growth and a significant hardness reduction. The same cubic-AlN to wurtzite-AlN transition is also the cause for deteriorating hardness in industrial Ti1-xAlxN tool coatings at times.

Saha et al. have successfully addressed this challenge by developing nominally single crystalline cubic-TiN/(Al, Sc)N epitaxial superlattices on MgO substrates, where the (Al, Sc)N is in metastable cubic (rocksalt) phase for more than 120 nm thickness [5]. The lattice-matched superlattices showed increased hardness as a function of the decreasing period thickness proposed by Koehler [6] and for a period thickness of 3 nm, a maximum hardness of 42 GPa was achieved at room temperature. Further analysis related to the temperature dependent hardness evolution and other mechanical properties are needed for TiN/(Al, Sc)N superlattices before its full potential for industrial applications can be realized. Therefore, nitrides are a reliable coating material for more than four-to-five decades, and significant research and development is currently underway with nitrides for their applications as coating materials.

### 3.2. Solid state lighting materials

While the coating industry continues to develop and utilize nitrides, the last two-to-three decades have seen the emergence of semiconducting GaN for solid state lighting applications and dominated the research and industrial materials development space. GaN have been attractive for blue emission from 1950s owing to its direct gap of 3.4 eV. However, high quality GaN growth on common substrates like Si and Sapphire was a significant challenge. Large lattice-mismatch of GaN with several common substrates rendered poor quality thin films, therefore, sub-standard electronic and optical properties. Moreover, development of p-type GaN necessary for LED and other electronic and optoelectronic and optoelectronic devices were also significantly difficult.

Significant breakthroughs were achieved in 1970s when using newly developed molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) techniques, researchers started to grow high quality GaN films. The most significant of these efforts could be attributed to the work of Amano et al. where they deposited high quality GaN thin films by using a polycrystalline AlN as a buffer layer [8]. Similarly, Nakamura et al. at Nichi Chemical Company deposited high quality GaN films by using a low temperature GaN buffer layer with low background of n-type doping [9]. Development of low temperature GaN as a buffer layer proved to be an attractive and industry wide approach for mass production of GaN based light emitting devices.

Apart from the materials quality issue, another important challenge in early years was on how to develop p-type GaN thin films. Though the idea of p-type doping of GaN through the incorporation of Mg was known, most experimental efforts to achieve p-type GaN were unsuccessful. Amano et al. managed to develop p-type GaN by electron beam irradiation [10]. However, the electron beam irradiation technique was an inefficient method and required a lot of time for hole-doping. Efforts to develop p-type GaN with Mg incorporation via chemical vapor deposition methods were also unsuccessful initially. Important breakthrough was made when modeling work showed that Mg bonds with hydrogen (H) thus forming Mg-H complex during the CVD growth process, which prevent the hole-doping activity of Mg and render development of p-type GaN difficult. Nakamura et al. addressed this important challenge by annealing Mg-doped GaN in N2 environment, where Mg frees up from the Mg-H complex and enable p-type GaN [11].

The development of high quality GaN thin film growth technique and of p-type GaN lead to the ultimate discovery of blue LEDs, which has significantly changed our society. Akasaki, Amano and Nakamura were awarded the 2014 Noble Prize in Physics for their pioneering work on GaN LED development. GaN today is the workhorse for producing blue emission and along with a suitable dye, most commercially available LED light sources employ GaN. Significant efforts are also currently underway to develop GaN based electronic and optoelectronic devices for power-electronic applications.

### 4. Recent advances in energy transport and conversion

### 4.1. Thermoelectrics

multilayer system (e.g., TiN/NbN, TiN/VN, and TiN/CrN) have been developed in 1990s and 2000s that showed improved hardness compared to TiN and Ti1xAlxN thin films (see Table 1). However, all of the nitride multilayers mentioned above are miscible at temperatures exceeding 800C, which significantly limits their usefulness in cutting tool applications, where the surface temperature can reach as high as 1000C during the cutting process. Cubic (rocksalt)- TiN/AlN superlattices were developed to overcome the miscibility problem since TiN/AlN superlattices are immiscible up to 1000C. TiN/AlN superlattices also exhibit excellent oxidation resistance, relatively high hardness compared to TiN, and they are already used com-

However, TiN/AlN superlattice coatings have a significant drawback. The hardness of TiN/ AlN superlattices is around 33–35 GPa when the thickness of the AlN layers is less than 2– 3 nm but decreases sharply to 23–24 GPa as the AlN layer thickness is increased [5]. This large reduction in hardness is attributed to the transition from the epitaxially stabilized metastable cubic-AlN phase to the stable wurtzite-AlN phase when the AlN layer thickness exceeds the critical thickness of 2–3 nm [7]. The formation of wurtzite-AlN breaks the epitaxial relationship with cubic-TiN leading to polycrystalline grain growth and a significant hardness reduction. The same cubic-AlN to wurtzite-AlN transition is also the cause for deteriorating hardness in

Saha et al. have successfully addressed this challenge by developing nominally single crystalline cubic-TiN/(Al, Sc)N epitaxial superlattices on MgO substrates, where the (Al, Sc)N is in metastable cubic (rocksalt) phase for more than 120 nm thickness [5]. The lattice-matched superlattices showed increased hardness as a function of the decreasing period thickness proposed by Koehler [6] and for a period thickness of 3 nm, a maximum hardness of 42 GPa was achieved at room temperature. Further analysis related to the temperature dependent hardness evolution and other mechanical properties are needed for TiN/(Al, Sc)N superlattices before its full potential for industrial applications can be realized. Therefore, nitrides are a reliable coating material for more than four-to-five decades, and significant research and development is currently underway with nitrides for their applications as coating materials.

While the coating industry continues to develop and utilize nitrides, the last two-to-three decades have seen the emergence of semiconducting GaN for solid state lighting applications and dominated the research and industrial materials development space. GaN have been attractive for blue emission from 1950s owing to its direct gap of 3.4 eV. However, high quality GaN growth on common substrates like Si and Sapphire was a significant challenge. Large lattice-mismatch of GaN with several common substrates rendered poor quality thin films, therefore, sub-standard electronic and optical properties. Moreover, development of p-type GaN necessary for LED and other electronic and optoelectronic and optoelectronic devices

Significant breakthroughs were achieved in 1970s when using newly developed molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) techniques,

mercially as a coating material.

116 Coatings and Thin-Film Technologies

industrial Ti1-xAlxN tool coatings at times.

3.2. Solid state lighting materials

were also significantly difficult.

Thermoelectric materials convert waste heat energy directly into electrical power and are attractive for harvesting energy in automobiles, power plants, and for deep space exploration. Such materials could also be used as a Peltier cooler in microelectronic chips and devices, where unwarranted heat generation (hot spots in integrated circuits (IC)) limit device efficiencies. Devices made from thermoelectric elements are environmentally friendly and they do not have any movable parts except for a fan in most cases. The efficiency of a thermoelectric material is represented by its dimensionless figure-of-merit, ZT = (S<sup>2</sup> σ T)/ (κ<sup>e</sup> + κp), where S is the Seebeck coefficient, σ is the electrical conductivity, κ<sup>e</sup> and κ<sup>p</sup> are the electronic and lattice contributions to the thermal conductivity, respectively, and T is the absolute temperature. The higher the thermoelectric figure-of-merit (ZT) of a material, the more efficient the energy conversion is. To be competitive with conventional power-generator and refrigeration technology, thermoelectric materials need to exhibit a ZT of about 3–4 over a wide temperature range. However, extensive research in the last decade has only improved the ZT to 2 at high operating temperatures [12]. Designing high-efficiency thermoelectric materials having ZT > 2 is particularly challenging due to the mutually conflicting design parameters. While the individual thermoelectric materials must exhibit high ZT values at the required temperature ranges of interest, practical thermoelectric devices require both n-type (electron conducting) and p-type (hole-conducting) materials with high ZTs, as well as effective methods to integrate them with metals having low contact resistances. Such restrictions naturally impose additional challenges in terms of material selection and device fabrication techniques.

Ti on Sc sites on ScN's electronic structure). For sputter deposited ScN grown on MgO substrate, Burmistrova et al. have showed that as-deposited ScN thin films exhibit a large n-type carrier concentration of (1–6) 1020 cm<sup>3</sup> due to the presence of oxygen as unwanted dopant impurities incorporated during deposition [14]. Several other studies have also reported the presence of carbon (C) and fluorine (F) as impurities inside sputter-deposited ScN thin films, which could

Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

concentration creates a favorable condition for high thermoelectric power factors in ScN, evidenced by the measured Seebeck coefficient of 156 μV/K, and an electrical conductivity of 1300 S/cm at 840 K [13] (Figure 2). Modeling analysis have showed that unwanted impurities such as oxygen incorporation during growth process dope the material heavily n-type and shift the Fermi level from inside the bandgap to inside the conduction band. Though the thermal conductivity of ScN thin films (14 W/m-K) are not as high as many other traditional III-nitrides, it is still higher than the values suitable for achieving high ZT. The best obtained ZT values for

Significant efforts have been made in recent years to reduce the thermal conductivity of ScN with the development of ScCrN and ScNbN solid-solution alloys, which exhibit reduced thermal conductivities due to increased alloy scattering [16, 17]. However, like the challenges encountered in most other thermoelectric materials systems, the reduction in thermal conductivity must be attained without reducing the power factor, for achieving higher figure-of-merit (ZT). In this regard, incorporation of nanoparticles, phase separation, a small amount of heavy element inclusion, and other approaches may be explored. For example, incorporation of rareearth metallic nanoparticles such as ErAs inside GaAs and InGaAlAs matrix have already demonstrated enhanced thermoelectric performance in such semiconductors [12]. Similarly, heavy metallic nitrides such as ZrN, HfN or WN could be incorporated inside ScN matrix, and

Figure 2. Temperature dependence of (a) Seebeck coefficient, (b) resistivity and (c) power factor of n and p-type of ScN.

Reprinted with permission from Saha et al. [15]. Copyright 2018 American Physical Society.

/Vs at room temperature and <sup>3</sup> 1020 cm<sup>3</sup> carrier

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119

potentially be electronically active [15].

ScN is 0.18 at 800 K temperature.

the thermoelectric properties could be explored.

The high electrical mobility of 100 cm2

Traditionally nitrides are regarded as poor thermoelectric materials, primarily because of their higher thermal conductivity. For example, GaN has a room temperature thermal conductivity in excess of 100 W/m-K, which is about two orders of magnitude higher compared to the most celebrated thermoelectric materials like Bi2Te3, having a thermal conductivity of 1 W/m-K at room temperature [2]. Other traditional III-nitride semiconductors (AlN, etc.) also have high thermal conductivity, which significantly limits their suitability in thermoelectricity. Apart from the high thermal conductivity, most commonly known nitrides also do not possess electronic properties that are commensurate for high ZT, such as (a) asymmetric distributions of density of states near the Fermi energy, (b) suitable carrier density (electron or hole) of 1019/cm3 and others, which has limited the exploration of nitrides as thermoelectric materials for a long time.

This situation has changed significantly in recent years with the emergence of scandium nitride (ScN) as a rocksalt (cubic) semiconducting material. ScN is a promising group III (B)-nitride semiconductor with an indirect bandgap and octahedral coordination [2]. Like most other transition metal nitrides (TMNs), ScN is structurally and chemically stable, mechanically hard (23 GPa), corrosion resistant, and possesses high melting temperatures in excess of 2873 K. Due to its rocksalt (cubic) crystal structure, ScN also offers a materials platform for engineering the band structure of alloys with the III–V nitride semiconductors (AlN, GaN, and InN, which adopt the wurtzite crystal structure without ScN) for applications where integration of the semiconductor with cubic(rocksalt) metals is required.

Although controversies persisted about the nature of its electronic structure during the 1990s and early 2000s, recent experimental results and theoretical modeling have demonstrated conclusively that ScN has an indirect bandgap of 0.9–1.2 eV and a direct gap of 2.2 eV. Kerdsongpanya et al. have demonstrated an extremely large power factor of 2.5 <sup>10</sup><sup>3</sup> W/m-K2 in ScN thin films grown on Al2O3 substrates [13]. While later research by Burmistrova et al. have improved the power factor values to (3.3–3.5) <sup>10</sup><sup>3</sup> W/m-K2 at 600–850 K in sputter deposited <sup>n</sup>-type ScN thin films grown on MgO substrates [14]. These power factors at 600–850 K temperature ranges are higher than those of Bi2Te3 and its alloys at 400 K, as well as the best high-temperature thermoelectric materials such as La3Te4 at 600 K and compare well with undoped crystalline SiGe in the same temperature range. The origin of such large power factors can be explained by the changes in ScN's electronic structure with respect to the presence of point defects and impurities (such as Sc and N vacancies, and doping effects of O and C on N-sites, and Ca and Ti on Sc sites on ScN's electronic structure). For sputter deposited ScN grown on MgO substrate, Burmistrova et al. have showed that as-deposited ScN thin films exhibit a large n-type carrier concentration of (1–6) 1020 cm<sup>3</sup> due to the presence of oxygen as unwanted dopant impurities incorporated during deposition [14]. Several other studies have also reported the presence of carbon (C) and fluorine (F) as impurities inside sputter-deposited ScN thin films, which could potentially be electronically active [15].

conversion is. To be competitive with conventional power-generator and refrigeration technology, thermoelectric materials need to exhibit a ZT of about 3–4 over a wide temperature range. However, extensive research in the last decade has only improved the ZT to 2 at high operating temperatures [12]. Designing high-efficiency thermoelectric materials having ZT > 2 is particularly challenging due to the mutually conflicting design parameters. While the individual thermoelectric materials must exhibit high ZT values at the required temperature ranges of interest, practical thermoelectric devices require both n-type (electron conducting) and p-type (hole-conducting) materials with high ZTs, as well as effective methods to integrate them with metals having low contact resistances. Such restrictions naturally impose additional

Traditionally nitrides are regarded as poor thermoelectric materials, primarily because of their higher thermal conductivity. For example, GaN has a room temperature thermal conductivity in excess of 100 W/m-K, which is about two orders of magnitude higher compared to the most celebrated thermoelectric materials like Bi2Te3, having a thermal conductivity of 1 W/m-K at room temperature [2]. Other traditional III-nitride semiconductors (AlN, etc.) also have high thermal conductivity, which significantly limits their suitability in thermoelectricity. Apart from the high thermal conductivity, most commonly known nitrides also do not possess electronic properties that are commensurate for high ZT, such as (a) asymmetric distributions of density of states near the Fermi energy, (b) suitable carrier density (electron or hole) of 1019/cm3 and others, which has limited the exploration of nitrides as thermoelectric materials

This situation has changed significantly in recent years with the emergence of scandium nitride (ScN) as a rocksalt (cubic) semiconducting material. ScN is a promising group III (B)-nitride semiconductor with an indirect bandgap and octahedral coordination [2]. Like most other transition metal nitrides (TMNs), ScN is structurally and chemically stable, mechanically hard (23 GPa), corrosion resistant, and possesses high melting temperatures in excess of 2873 K. Due to its rocksalt (cubic) crystal structure, ScN also offers a materials platform for engineering the band structure of alloys with the III–V nitride semiconductors (AlN, GaN, and InN, which adopt the wurtzite crystal structure without ScN) for applications where integration of the semiconductor with cubic(rocksalt) metals is

Although controversies persisted about the nature of its electronic structure during the 1990s and early 2000s, recent experimental results and theoretical modeling have demonstrated conclusively that ScN has an indirect bandgap of 0.9–1.2 eV and a direct gap of 2.2 eV. Kerdsongpanya et al. have demonstrated an extremely large power factor of 2.5 <sup>10</sup><sup>3</sup> W/m-K2 in ScN thin films grown on Al2O3 substrates [13]. While later research by Burmistrova et al. have improved the power factor values to (3.3–3.5) <sup>10</sup><sup>3</sup> W/m-K2 at 600–850 K in sputter deposited <sup>n</sup>-type ScN thin films grown on MgO substrates [14]. These power factors at 600–850 K temperature ranges are higher than those of Bi2Te3 and its alloys at 400 K, as well as the best high-temperature thermoelectric materials such as La3Te4 at 600 K and compare well with undoped crystalline SiGe in the same temperature range. The origin of such large power factors can be explained by the changes in ScN's electronic structure with respect to the presence of point defects and impurities (such as Sc and N vacancies, and doping effects of O and C on N-sites, and Ca and

challenges in terms of material selection and device fabrication techniques.

for a long time.

118 Coatings and Thin-Film Technologies

required.

The high electrical mobility of 100 cm2 /Vs at room temperature and <sup>3</sup> 1020 cm<sup>3</sup> carrier concentration creates a favorable condition for high thermoelectric power factors in ScN, evidenced by the measured Seebeck coefficient of 156 μV/K, and an electrical conductivity of 1300 S/cm at 840 K [13] (Figure 2). Modeling analysis have showed that unwanted impurities such as oxygen incorporation during growth process dope the material heavily n-type and shift the Fermi level from inside the bandgap to inside the conduction band. Though the thermal conductivity of ScN thin films (14 W/m-K) are not as high as many other traditional III-nitrides, it is still higher than the values suitable for achieving high ZT. The best obtained ZT values for ScN is 0.18 at 800 K temperature.

Significant efforts have been made in recent years to reduce the thermal conductivity of ScN with the development of ScCrN and ScNbN solid-solution alloys, which exhibit reduced thermal conductivities due to increased alloy scattering [16, 17]. However, like the challenges encountered in most other thermoelectric materials systems, the reduction in thermal conductivity must be attained without reducing the power factor, for achieving higher figure-of-merit (ZT). In this regard, incorporation of nanoparticles, phase separation, a small amount of heavy element inclusion, and other approaches may be explored. For example, incorporation of rareearth metallic nanoparticles such as ErAs inside GaAs and InGaAlAs matrix have already demonstrated enhanced thermoelectric performance in such semiconductors [12]. Similarly, heavy metallic nitrides such as ZrN, HfN or WN could be incorporated inside ScN matrix, and the thermoelectric properties could be explored.

Figure 2. Temperature dependence of (a) Seebeck coefficient, (b) resistivity and (c) power factor of n and p-type of ScN. Reprinted with permission from Saha et al. [15]. Copyright 2018 American Physical Society.

While the large power factor in the as-deposited n-type ScN is attractive for thermoelectricity, practical devices also require a highly efficient p-type material. Reducing the carrier concentration in ScN, and eventually turning it into a p-type semiconductor was also important for a host of other applications. Saha et al. have demonstrated p-type Sc1�xMnxN and Sc1�xMgxN thin film alloys by solid-state alloying of ScN with MnxNy and MgxNy, respectively [2]. The ptype ScxMg1�xN thin film alloys were found to be (a) substitutional solid solutions without any detectable MgxNy precipitation, phase segregation, or secondary phase formation; (b) exhibited a maximum hole-concentration of 2.2 � 1020 cm�<sup>3</sup> and hole mobility of 21 cm2 /Vs; (c) did not show any defect states inside the direct gap of ScN, thus retaining its basic electronic structure; and (d) exhibited impurity scattering by Mg addition that dominated hole conduction at high temperatures. The p-type ScxMg1�xN thin film alloys also exhibit very high Seebeck coefficients, in excess of 200 μV/K. However, due to the reduced mobility, the electrical conductivities are an order of magnitude lower, which results in a lower power factor values.

Apart from ScN, chromium nitride (CrN) has shown great promise for thermoelectric applications. Thermal conductivity of CrN is much smaller compared to ScN thin film, and as a result it may be possible to engineer CrN for high thermoelectric figure-of-merits (ZT). Similarly YN and several other rare-earth nitrides such as GdN, ErN, etc. exhibit semiconducting properties. Due to their higher atomic mass, such materials should exhibit lower thermal conductivity, and could be explored for thermoelectric applications.

the nitrides (Figure 3) however, are larger compared to noble metals due to increased inter-

(AZO, GZO and ITO) are plotted along with those of gold and silver. The arrows show the wavelength ranges in which nitrides and TCOs are respectively metallic. Reproduced with permission from Naik et al. [19]. Copyright 2011 Optical

) and (b) imaginary (ε00) parts of the dielectric permittivity of TMNs such as TiN, ZrN and TCOs

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Detailed studies on the plasmatic properties of TMNs (TiN and ZrN) such as surface plasmon polariton (SPP) propagation length, SPP mode size, second harmonic generation, absorption and emission have already demonstrated superior plasmonic qualities of nitrides (e.g. TiN, ZrN and others) in comparison to Au and Ag. Vigorous research activity is currently under-

Solar selective coatings are spectrally selective layers which has high absorptance in visible region and high reflectance (or low thermal emittance) in IR region of solar spectrum. These spectrally selective coatings are used to efficiently capture sunlight as heat in solar thermal converters, which has variety of applications such as solar water heating, solar thermal electricity generation, solar thermoelectric generator, solar thermophotovoltaics, etc. Though solar thermal conversion has been used for water heating purposes for several years, currently this technology is gaining significant attention in electricity production as it has achieved overall efficiency >30% and more suitable technology for large scale electricity production. In addition, solar thermal technology has efficient ways to store heat energy so that it can be used to

Conversion of solar radiation into useful heat energy is dependent on the optical properties of the solar selective materials such as α, the solar absorptance (fraction of the solar energy absorbed by a surface) and ε, the thermal emittance (faction of radiant energy emitted from the surface with respect to energy radiated by a blackbody at same temperature). Ideally a

way to convert the wonderful optical properties of nitrides into device applications.

band transitions, which is a limitation.

Figure 3. (a) Real (ε<sup>0</sup>

Society of America.

4.3. Solar selective coating technology

generate power in overcast conditions and also at night.

### 4.2. Plasmonics

Traditionally noble metals such as Au and Ag are regarded as the best plasmonic materials in the visible spectral range both for research as well as for limited number of industrial applications. However, noble metal-based plasmonic components have materials properties that significantly limit realization of practical plasmonic devices [18]. Some of the severe materials challenges with noble metals are (a) incompatibility with standard complementary metal oxide semiconductor (CMOS) fabrication processes, (b) noble metals are morphologically not stable at high temperatures >500�C, (c) the real part of the dielectric permittivity (ε<sup>0</sup> ) for noble metals are too large for several applications, (d) because of their high surface energies noble metals are difficult to fabricate in thin film or ultrathin film form, and (e) it is difficult to engineer optical properties of noble metals though materials engineering. Due to such materials challenges real life applications of plasmonics as a research field with noble metals have been extremely limited to only a handful of applications.

Transition metal nitrides and their epitaxial metal/semiconductor superlattices have enormous promise and potential in the plasmonics research field as they overcome several of the shortcomings of noble metals [18]. Optical characterizations have showed, that TiN and ZrN are excellent plasmonic materials in the visible spectral range (500–900 nm) [19]. The real part of the dielectric permittivity (ε<sup>0</sup> ) of TiN and ZrN (Figure 3) are smaller compared to noble metals (such as Au and Ag films) due to their relatively lower carrier concentrations. For several practical applications such as in devices for transformation optics, or in hyperbolic metamaterials, lower values of ε<sup>0</sup> are a necessity and the nitrides have already attracted significant interest in that pursuit. The imaginary parts of the dielectric permittivity (ε00) of

Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion http://dx.doi.org/10.5772/intechopen.79525 121

Figure 3. (a) Real (ε<sup>0</sup> ) and (b) imaginary (ε00) parts of the dielectric permittivity of TMNs such as TiN, ZrN and TCOs (AZO, GZO and ITO) are plotted along with those of gold and silver. The arrows show the wavelength ranges in which nitrides and TCOs are respectively metallic. Reproduced with permission from Naik et al. [19]. Copyright 2011 Optical Society of America.

the nitrides (Figure 3) however, are larger compared to noble metals due to increased interband transitions, which is a limitation.

Detailed studies on the plasmatic properties of TMNs (TiN and ZrN) such as surface plasmon polariton (SPP) propagation length, SPP mode size, second harmonic generation, absorption and emission have already demonstrated superior plasmonic qualities of nitrides (e.g. TiN, ZrN and others) in comparison to Au and Ag. Vigorous research activity is currently underway to convert the wonderful optical properties of nitrides into device applications.

#### 4.3. Solar selective coating technology

While the large power factor in the as-deposited n-type ScN is attractive for thermoelectricity, practical devices also require a highly efficient p-type material. Reducing the carrier concentration in ScN, and eventually turning it into a p-type semiconductor was also important for a host of other applications. Saha et al. have demonstrated p-type Sc1�xMnxN and Sc1�xMgxN thin film alloys by solid-state alloying of ScN with MnxNy and MgxNy, respectively [2]. The ptype ScxMg1�xN thin film alloys were found to be (a) substitutional solid solutions without any detectable MgxNy precipitation, phase segregation, or secondary phase formation; (b) exhibited a maximum hole-concentration of 2.2 � 1020 cm�<sup>3</sup> and hole mobility of 21 cm2

(c) did not show any defect states inside the direct gap of ScN, thus retaining its basic electronic structure; and (d) exhibited impurity scattering by Mg addition that dominated hole conduction at high temperatures. The p-type ScxMg1�xN thin film alloys also exhibit very high Seebeck coefficients, in excess of 200 μV/K. However, due to the reduced mobility, the electrical conductivities are an order of magnitude lower, which results in a lower power factor values. Apart from ScN, chromium nitride (CrN) has shown great promise for thermoelectric applications. Thermal conductivity of CrN is much smaller compared to ScN thin film, and as a result it may be possible to engineer CrN for high thermoelectric figure-of-merits (ZT). Similarly YN and several other rare-earth nitrides such as GdN, ErN, etc. exhibit semiconducting properties. Due to their higher atomic mass, such materials should exhibit lower thermal conductivity,

Traditionally noble metals such as Au and Ag are regarded as the best plasmonic materials in the visible spectral range both for research as well as for limited number of industrial applications. However, noble metal-based plasmonic components have materials properties that significantly limit realization of practical plasmonic devices [18]. Some of the severe materials challenges with noble metals are (a) incompatibility with standard complementary metal oxide semiconductor (CMOS) fabrication processes, (b) noble metals are morphologically not stable

are too large for several applications, (d) because of their high surface energies noble metals are difficult to fabricate in thin film or ultrathin film form, and (e) it is difficult to engineer optical properties of noble metals though materials engineering. Due to such materials challenges real life applications of plasmonics as a research field with noble metals have been extremely

Transition metal nitrides and their epitaxial metal/semiconductor superlattices have enormous promise and potential in the plasmonics research field as they overcome several of the shortcomings of noble metals [18]. Optical characterizations have showed, that TiN and ZrN are excellent plasmonic materials in the visible spectral range (500–900 nm) [19]. The real

noble metals (such as Au and Ag films) due to their relatively lower carrier concentrations. For several practical applications such as in devices for transformation optics, or in hyperbolic metamaterials, lower values of ε<sup>0</sup> are a necessity and the nitrides have already attracted significant interest in that pursuit. The imaginary parts of the dielectric permittivity (ε00) of

) of TiN and ZrN (Figure 3) are smaller compared to

at high temperatures >500�C, (c) the real part of the dielectric permittivity (ε<sup>0</sup>

and could be explored for thermoelectric applications.

limited to only a handful of applications.

part of the dielectric permittivity (ε<sup>0</sup>

4.2. Plasmonics

120 Coatings and Thin-Film Technologies

/Vs;

) for noble metals

Solar selective coatings are spectrally selective layers which has high absorptance in visible region and high reflectance (or low thermal emittance) in IR region of solar spectrum. These spectrally selective coatings are used to efficiently capture sunlight as heat in solar thermal converters, which has variety of applications such as solar water heating, solar thermal electricity generation, solar thermoelectric generator, solar thermophotovoltaics, etc. Though solar thermal conversion has been used for water heating purposes for several years, currently this technology is gaining significant attention in electricity production as it has achieved overall efficiency >30% and more suitable technology for large scale electricity production. In addition, solar thermal technology has efficient ways to store heat energy so that it can be used to generate power in overcast conditions and also at night.

Conversion of solar radiation into useful heat energy is dependent on the optical properties of the solar selective materials such as α, the solar absorptance (fraction of the solar energy absorbed by a surface) and ε, the thermal emittance (faction of radiant energy emitted from the surface with respect to energy radiated by a blackbody at same temperature). Ideally a

showed that the refractive index, absorptance or reflectance, can be tuned from that of metallic to dielectric characteristic by varying the composition and/or by incorporating O, N, C and Si. This can be attributed to changes in the electronic structure and bonding nature (i.e., metallic to covalent) with the change in stoichiometry of TiN based compounds and incorporation of elements such as Al or O, respectively [20]. In addition, these TMNs can act as diffusion barrier for metals, which may diffuse into the absorbers or oxidize at high operating temperatures (above 400 C). Consequently, preventing the degradation of optical properties of the coatings [23]. These properties give TMNs an edge over cermets for high temperature (i.e., above 500C)

Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

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Taking advantage of these properties, several groups have developed TMN based tandem (metal/ dielectric) solar selective coatings. For instance, Barshilia et al. fabricated TiAlN/TiAlON/Si3N4 solar selective tandem absorbers on copper substrate demonstrating high solar selectivity 0.95/0.06 and thermal stability up to 525C in air (50 h) and 800C in vacuum [23]. In this case, TiAlN (metallic in nature) acts as a main absorber, TiAlON (with low metallic content) acts as a semiabsorber and Si3N4 as an antireflection coating. The tandem absorber was fabricated such that the refractive index (in visible region) gradually increases from the coating surface to substrate. Such graded refractive index ensures that reflectance is minimized in visible region and absorptance is increased. Following this many other transition metals-based nitride, oxynitride, carbonitrides coatings have been developed in order to improve solar selectivity and thermal stability further, which are listed in Table 2. Detailed review on various combination of TMNs based solar selective

Solar water splitting is a well-known photocatalytic process used to produce molecular hydrogen, which is an attractive source of energy as it can be stored, transported and consumed as a fuel on demand. The photocatalytic reaction is driven by the electrons and holes generated in a semiconductor in response to the absorbed photons. Efficiency of such a solar-to-fuel conversion

Air Vacuum

900 (2 h) 650 (100 h)

325 (400 h)

Absorber α ε Thermal stability (C)

TiAlN/TiAlON/Si3N4 0.93–0.94 0.15–0.17 550 600 NbAlN/NbAlON/Si3N4 0.93–0.95 0.07 500 (2 h) 600 (2 h) TiAlN/CrAlON/Si3N4 0.94–0.95 0.05–0.07 500 (2 h) 800 (2 h) HfMoN/HfON/Al2O3 0.94–0.95 0.13–0.14 475 (34 h) 600 (450 h)

NbTiON/SiON 0.95 0.07 – 500 (40 h) Mo/ZrSiN/ZrSiON/SiO2 0.94 0.12 – 500 (500 h)

Table 2. TMNs based metallic/dielectric tandem absorber along with their absorptance, emittance and thermal stability

TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO 0.96 0.07 500 (2 h)

coatings, their optical properties and growth methods can be found in Ref. [20].

4.4. Hot-electron collection for solar water splitting

solar selective applications.

at different ambiences.

Figure 4. (a) Spectral reflectance for an ideal solar selective surface (green line) compared with solar spectrum (red line) and black body radiation (blue line) and (b) schematic diagram of different type of solar selective coating.

selective surface must have α close to 1 in the wavelength range of 0.3–2 μm of the solar emission spectrum, which covers the 95% of the solar energy and ε close to 0 beyond 2 μm (Figure 4a).

However, in practice most material surfaces do not meet these requirements perfectly. Therefore, the research has been focused on materials in order to maximize α at the same time reducing ε as lower values. Since solar thermal electricity production requires high operating temperatures in excess of 500C, in addition to these optical properties the solar selective materials must be stable at such operating temperature. Several types of spectrally selective coatings have been developed based on the following concepts (Figure 4b): (a) intrinsic absorber, (b) metalsemiconductor tandem, (c) multilayer absorbers, (d) multi-dielectric composite, (e) textured surface, and (f) selectively solar-transmitting coating on a blackbody-like absorber, etc.

Intrinsic absorbers such as W, Mo doped-MoO3, ZrB2, etc. inherently have spectral selectivity induced by dielectric dispersion as a function of wavelength. Though such materials are easy to fabricate, their spectral selectivity is less than ideal and require some kind of structural and compositional modification to achieve near-ideal spectral selectivity. Therefore, combination of design concepts such as metal-semiconductor tandem, multilayer absorbers, metal-dielectric composites, etc. are most widely studied. In metal-dielectric composites, cermets (metal nanoparticles dispersed in dielectric matrix) such as Mo-Al2O3, W-AlN, Mo-AlN, Mo-SiO2, W-Al2O3, Cr-Cr2O3, etc. have been investigated as selective coatings for high temperature applications. These composite coatings exhibit excellent solar selectivity and are thermally stable in vacuum. Hence they are already commercialized by many manufactures such as Luz International Ltd., USA, Siemens (formerly Solel), Germany, Archimede Solar Energy, Italy and Schott, Germany [20]. Major concerns with these cermets based coatings is their thermal stability in air above 400C. Beyond this temperature the optical properties, emittance in particular, starts degrading due to oxidation and/or diffusion of metal particles.

TMNs possess a good combination of chemical, mechanical properties, and are extremely stable at high-temperatures. Until last decade the optical properties of the TMNs were rarely studies. Initial studies on optical properties of TiAlN and TiAlON have exhibited high absorption coefficient and low reflectance in visible region [21, 22]. Further studies on TiN based compounds showed that the refractive index, absorptance or reflectance, can be tuned from that of metallic to dielectric characteristic by varying the composition and/or by incorporating O, N, C and Si. This can be attributed to changes in the electronic structure and bonding nature (i.e., metallic to covalent) with the change in stoichiometry of TiN based compounds and incorporation of elements such as Al or O, respectively [20]. In addition, these TMNs can act as diffusion barrier for metals, which may diffuse into the absorbers or oxidize at high operating temperatures (above 400 C). Consequently, preventing the degradation of optical properties of the coatings [23]. These properties give TMNs an edge over cermets for high temperature (i.e., above 500C) solar selective applications.

Taking advantage of these properties, several groups have developed TMN based tandem (metal/ dielectric) solar selective coatings. For instance, Barshilia et al. fabricated TiAlN/TiAlON/Si3N4 solar selective tandem absorbers on copper substrate demonstrating high solar selectivity 0.95/0.06 and thermal stability up to 525C in air (50 h) and 800C in vacuum [23]. In this case, TiAlN (metallic in nature) acts as a main absorber, TiAlON (with low metallic content) acts as a semiabsorber and Si3N4 as an antireflection coating. The tandem absorber was fabricated such that the refractive index (in visible region) gradually increases from the coating surface to substrate. Such graded refractive index ensures that reflectance is minimized in visible region and absorptance is increased. Following this many other transition metals-based nitride, oxynitride, carbonitrides coatings have been developed in order to improve solar selectivity and thermal stability further, which are listed in Table 2. Detailed review on various combination of TMNs based solar selective coatings, their optical properties and growth methods can be found in Ref. [20].

### 4.4. Hot-electron collection for solar water splitting

selective surface must have α close to 1 in the wavelength range of 0.3–2 μm of the solar emission spectrum, which covers the 95% of the solar energy and ε close to 0 beyond 2 μm

Figure 4. (a) Spectral reflectance for an ideal solar selective surface (green line) compared with solar spectrum (red line)

and black body radiation (blue line) and (b) schematic diagram of different type of solar selective coating.

However, in practice most material surfaces do not meet these requirements perfectly. Therefore, the research has been focused on materials in order to maximize α at the same time reducing ε as lower values. Since solar thermal electricity production requires high operating temperatures in excess of 500C, in addition to these optical properties the solar selective materials must be stable at such operating temperature. Several types of spectrally selective coatings have been developed based on the following concepts (Figure 4b): (a) intrinsic absorber, (b) metalsemiconductor tandem, (c) multilayer absorbers, (d) multi-dielectric composite, (e) textured sur-

Intrinsic absorbers such as W, Mo doped-MoO3, ZrB2, etc. inherently have spectral selectivity induced by dielectric dispersion as a function of wavelength. Though such materials are easy to fabricate, their spectral selectivity is less than ideal and require some kind of structural and compositional modification to achieve near-ideal spectral selectivity. Therefore, combination of design concepts such as metal-semiconductor tandem, multilayer absorbers, metal-dielectric composites, etc. are most widely studied. In metal-dielectric composites, cermets (metal nanoparticles dispersed in dielectric matrix) such as Mo-Al2O3, W-AlN, Mo-AlN, Mo-SiO2, W-Al2O3, Cr-Cr2O3, etc. have been investigated as selective coatings for high temperature applications. These composite coatings exhibit excellent solar selectivity and are thermally stable in vacuum. Hence they are already commercialized by many manufactures such as Luz International Ltd., USA, Siemens (formerly Solel), Germany, Archimede Solar Energy, Italy and Schott, Germany [20]. Major concerns with these cermets based coatings is their thermal stability in air above 400C. Beyond this temperature the optical properties, emittance in

face, and (f) selectively solar-transmitting coating on a blackbody-like absorber, etc.

particular, starts degrading due to oxidation and/or diffusion of metal particles.

TMNs possess a good combination of chemical, mechanical properties, and are extremely stable at high-temperatures. Until last decade the optical properties of the TMNs were rarely studies. Initial studies on optical properties of TiAlN and TiAlON have exhibited high absorption coefficient and low reflectance in visible region [21, 22]. Further studies on TiN based compounds

(Figure 4a).

122 Coatings and Thin-Film Technologies

Solar water splitting is a well-known photocatalytic process used to produce molecular hydrogen, which is an attractive source of energy as it can be stored, transported and consumed as a fuel on demand. The photocatalytic reaction is driven by the electrons and holes generated in a semiconductor in response to the absorbed photons. Efficiency of such a solar-to-fuel conversion


Table 2. TMNs based metallic/dielectric tandem absorber along with their absorptance, emittance and thermal stability at different ambiences.

is limited by the (a) short-range of light response of the semiconductor catalysts (used as photoelectrode) dictated by their bandgap and (b) smaller diffusion length of the photogenerated carriers than the photon absorption depth resulting in carrier recombination, instead of contributing in solar-to-fuel conversion. In order to overcome these limitations several strategies have been developed such as (a) doping/alloying of the semiconductor to tune its band structure, (b) fabrication of heterostructures to efficiently separate charge carrier at the junctions, (c) decreasing the particle size of the catalysts to efficiently collect carriers before they could recombine and (d) synthesize faceted nanostructures that are catalytically more active.

Tian and Tatsuma developed a new strategy to use plasmonic metal nanoparticles to enhance the photocurrent, and thereby improve solar-to-fuel conversion efficiency [24]. While traditionally plasmonics are used to confine light into nanoscale volume to provide intense electromagnetic localization and improved light scattering, in this work the plasmon decay was used to generate energetic electrons (hot electrons) and metal-semiconductor interface for charge separation. Using such phenomena in Au-TiO2 photoelectrode, the authors demonstrated an improvement in incident photon to current conversion efficiency (IPCE) by more than 20 in the presence of suitable donors. Advantage of using plasmonic photoelectrode over that of semiconductor is that photons with energy lesser than bandgap can also be absorbed through plasmon resonance, hence, has a broadband photo-response. In Au-TiO2 case, photon absorption and carrier generation for TiO2 takes place at UV region (≈380 nm) and that for Au at its plasmon band (≈520 nm). This will result in enhanced photocatalytic efficiency.

semiconductors, metals, and their heterostructures offering novel electronic and optoelectronic properties that are not present in other material systems. For high-temperature and high-power applications materials must have high melting point, large breakdown voltage, ability to dope preferentially n- and p-type and ability to form stable metal/semiconductor or metal/dielectric contacts (interface) for high temperature operations. Diverse properties of TMNs makes it a suitable candidate for such applications. Previously, stable epitaxial and low resistive contacts for high temperature applications with traditional metals was difficult due to their high surface energy and lack of crystal structure compatibility leading to misfit dislocations and defect states. However, recent efforts to grow epitaxial TMNs have shown that wide range of binary and ternaries such as TixMe1xN and TaxMe1xN (Me = Ti, Zr, Hf, Nb, Ta, Mo, W) with stable cubic rocksalt structure [29, 30]. With ternary alloying of TMNs, tunable lattice parameter (0.416–0.469 nm) and electronic properties such as bandgap, conductivity, work function (3.7–5.1 eV), etc. can be achieved [31]. This flexibility of TMNs in lattice parameter (also, close to III–N) and work function has made them attractive for electronic device application such as diffusion barriers [32], metallization and lattice matched growth templet for wide bandgap semiconductors [33], which are used in high power devices. Apart from these passive components, TMNs are also finding application in nanoelectromechanical systems (NEMS) such as TiN cantilever-type nanoelectromechanical switch fabricated using CMOS process [34]. The fabricated switches exhibited excellent performance and TMNs gives robustness to such devices at harsh environment such as radiation

Figure 5. Schematic representation of plasmon induced hot electron collection in (a) Au-TiO2 photoelectrode and (b) TiN/

Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

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125

Until recent years, realization of electronic devices with TMNs as active a component was not possible due to the fact that most known TMNs are metallic and non-availability of suitable semiconducting TMNs. However, recent research works revealed that ScN (indirect bandgap 0.9 eV) and rocksalt-AlxSc1xN (direct bandgap 2.2–3.7 eV) are semiconductors and can be doped preferentially both n- and p-type [15]. This has opened up a new direction for the development of new refractory electronic devices fully based on TMNs. Future research on the epitaxial growth of rocksalt nitride semiconductors heterostructures will be useful for the exploring device properties such as intersubband absorption and emission, confinement of

electrons in metallic wells, photodiodes, photo-conductors, and terahertz devices.

and high temperature.

TiO2 photoelectrode.

Several plasmonic based photocatalysis systems have been developed in terms of metalsemiconductor combinations and their structural configurations such as Au (nanoroad)/TiO2-Coborate (oxygen evolution catalyst) grown electrochemically using anodized alumina templet [25], Au/CeO [26], Au decorated 3D structured ZnO [27], etc. However, in such plasmonic structures, the extraction of hot electron (generated due to plasmon decay) from metal nanoparticle is limited by the Schottky barrier. The electrons with energy higher than the potential barrier (ϕB) can only be collected at electrode. This limitation is imposed by the larger difference between work function and electron affinity of largely used noble metal (i.e., Au and Ag) and semiconductors (TiO2 and ZnO), respectively.

In order to overcome the limitation imposed by large Schottky barrier, Naldoni et al., developed a novel photoanode using TiN based plasmonic nanoparticle decorated on TiO2 nanoroads [28]. With TiN(nanoparticle)/TiO2, they have demonstrated twice as many hot electrons as Au nanoparticle injection into TiO2, i.e., 25% increase in photocurrent in comparison to Au/TiO2. The observed enhancement in performance has been attributed to the broadband absorption (500– 1200) of cubic TiN as well as lower work function of TiN (ϕ<sup>M</sup> ≈ 4 eV) in comparison to Au (ϕ<sup>M</sup> ≈ 5.2 eV) (see Figure 5). This work has opened up a new avenue for several other metallic TMNs similar to TiN and ZrN, which found to exhibit Au-competitive optical properties.

#### 4.5. Refractory electronics and plasmonics

TMNs are well known refractory materials that are chemically, structurally, and morphologically stable at high temperature. Further, the TMNs are electronically diverse with insulators, Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion http://dx.doi.org/10.5772/intechopen.79525 125

is limited by the (a) short-range of light response of the semiconductor catalysts (used as photoelectrode) dictated by their bandgap and (b) smaller diffusion length of the photogenerated carriers than the photon absorption depth resulting in carrier recombination, instead of contributing in solar-to-fuel conversion. In order to overcome these limitations several strategies have been developed such as (a) doping/alloying of the semiconductor to tune its band structure, (b) fabrication of heterostructures to efficiently separate charge carrier at the junctions, (c) decreasing the particle size of the catalysts to efficiently collect carriers before they could

Tian and Tatsuma developed a new strategy to use plasmonic metal nanoparticles to enhance the photocurrent, and thereby improve solar-to-fuel conversion efficiency [24]. While traditionally plasmonics are used to confine light into nanoscale volume to provide intense electromagnetic localization and improved light scattering, in this work the plasmon decay was used to generate energetic electrons (hot electrons) and metal-semiconductor interface for charge separation. Using such phenomena in Au-TiO2 photoelectrode, the authors demonstrated an improvement in incident photon to current conversion efficiency (IPCE) by more than 20 in the presence of suitable donors. Advantage of using plasmonic photoelectrode over that of semiconductor is that photons with energy lesser than bandgap can also be absorbed through plasmon resonance, hence, has a broadband photo-response. In Au-TiO2 case, photon absorption and carrier generation for TiO2 takes place at UV region (≈380 nm) and that for Au at its

Several plasmonic based photocatalysis systems have been developed in terms of metalsemiconductor combinations and their structural configurations such as Au (nanoroad)/TiO2-Coborate (oxygen evolution catalyst) grown electrochemically using anodized alumina templet [25], Au/CeO [26], Au decorated 3D structured ZnO [27], etc. However, in such plasmonic structures, the extraction of hot electron (generated due to plasmon decay) from metal nanoparticle is limited by the Schottky barrier. The electrons with energy higher than the potential barrier (ϕB) can only be collected at electrode. This limitation is imposed by the larger difference between work function and electron affinity of largely used noble metal (i.e., Au and Ag) and semiconductors

In order to overcome the limitation imposed by large Schottky barrier, Naldoni et al., developed a novel photoanode using TiN based plasmonic nanoparticle decorated on TiO2 nanoroads [28]. With TiN(nanoparticle)/TiO2, they have demonstrated twice as many hot electrons as Au nanoparticle injection into TiO2, i.e., 25% increase in photocurrent in comparison to Au/TiO2. The observed enhancement in performance has been attributed to the broadband absorption (500– 1200) of cubic TiN as well as lower work function of TiN (ϕ<sup>M</sup> ≈ 4 eV) in comparison to Au (ϕ<sup>M</sup> ≈ 5.2 eV) (see Figure 5). This work has opened up a new avenue for several other metallic TMNs

TMNs are well known refractory materials that are chemically, structurally, and morphologically stable at high temperature. Further, the TMNs are electronically diverse with insulators,

similar to TiN and ZrN, which found to exhibit Au-competitive optical properties.

recombine and (d) synthesize faceted nanostructures that are catalytically more active.

plasmon band (≈520 nm). This will result in enhanced photocatalytic efficiency.

(TiO2 and ZnO), respectively.

124 Coatings and Thin-Film Technologies

4.5. Refractory electronics and plasmonics

Figure 5. Schematic representation of plasmon induced hot electron collection in (a) Au-TiO2 photoelectrode and (b) TiN/ TiO2 photoelectrode.

semiconductors, metals, and their heterostructures offering novel electronic and optoelectronic properties that are not present in other material systems. For high-temperature and high-power applications materials must have high melting point, large breakdown voltage, ability to dope preferentially n- and p-type and ability to form stable metal/semiconductor or metal/dielectric contacts (interface) for high temperature operations. Diverse properties of TMNs makes it a suitable candidate for such applications. Previously, stable epitaxial and low resistive contacts for high temperature applications with traditional metals was difficult due to their high surface energy and lack of crystal structure compatibility leading to misfit dislocations and defect states. However, recent efforts to grow epitaxial TMNs have shown that wide range of binary and ternaries such as TixMe1xN and TaxMe1xN (Me = Ti, Zr, Hf, Nb, Ta, Mo, W) with stable cubic rocksalt structure [29, 30]. With ternary alloying of TMNs, tunable lattice parameter (0.416–0.469 nm) and electronic properties such as bandgap, conductivity, work function (3.7–5.1 eV), etc. can be achieved [31]. This flexibility of TMNs in lattice parameter (also, close to III–N) and work function has made them attractive for electronic device application such as diffusion barriers [32], metallization and lattice matched growth templet for wide bandgap semiconductors [33], which are used in high power devices. Apart from these passive components, TMNs are also finding application in nanoelectromechanical systems (NEMS) such as TiN cantilever-type nanoelectromechanical switch fabricated using CMOS process [34]. The fabricated switches exhibited excellent performance and TMNs gives robustness to such devices at harsh environment such as radiation and high temperature.

Until recent years, realization of electronic devices with TMNs as active a component was not possible due to the fact that most known TMNs are metallic and non-availability of suitable semiconducting TMNs. However, recent research works revealed that ScN (indirect bandgap 0.9 eV) and rocksalt-AlxSc1xN (direct bandgap 2.2–3.7 eV) are semiconductors and can be doped preferentially both n- and p-type [15]. This has opened up a new direction for the development of new refractory electronic devices fully based on TMNs. Future research on the epitaxial growth of rocksalt nitride semiconductors heterostructures will be useful for the exploring device properties such as intersubband absorption and emission, confinement of electrons in metallic wells, photodiodes, photo-conductors, and terahertz devices.

Refractory optics and plasmonics with TMNs is yet another emerging and immensely promising research direction. Traditional plasmonic materials i.e., noble metals have highly limited absorption band width because of its resonant nature of plasmon excitation and high reflection in non-resonant frequencies. In addition, noble metals have low melting point, hence, poor thermal stability at high temperature and easily diffuses into surroundings. On the other hand, refractory metals such as W, Mo and others exhibit plasmon resonance in IR region with relatively high losses. Whereas TMN nitrides with their high melting temperatures and Aucompetitive optical properties could replace current polycrystalline refractory metals for hightemperature applications. Making use of these superior properties, Li et al. developed a TiN-SiO2-TiN broad band solar absorber, which can absorb 95% of light over the range of 400– 800 nm with 240 nm thick device [35]. Similarly, Ishii et al. fabricated TiN-ZnO-TiN structure, which showed superior performance in comparison to gold in visible region [36]. Both these devices were chemically robust, stable at high temperature and can be used in several emerging technologies such as solar-thermophotovoltaics (STPV), heat-assisted magnetic recording (HAMR).

research in currently underway to unlock several aspects of their physical properties to

Epitaxial Nitride Thin Film and Heterostructures: From Hard Coating to Solid State Energy Conversion

http://dx.doi.org/10.5772/intechopen.79525

127

TMNs have been the most important materials for industrial applications such as hard coatings, corrosion and wear resistant coatings for many decades owing to their high mechanical strength, thermal stability and chemical inertness. There has been significant progress in the understanding of process-structure-properties relationship of TMNs over the years. Now it is possible to fabricate wide variety of alloys and composites of TMNs with diverse and engineerable properties which were not available earlier. This has resulted in new device applications of TMNs such as solar selective absorbers, refractory plasmonics, photocatalysis, etc. Further, the development of stable epitaxial TMNs, heterostructures and superlattices has opened up new directions for refractory electronic device applications. Particularly, recent development of semiconducting rocksalt ScN and AlxSc1xN and their high n- and p-type dopability has further widened the scope for new device applications such as active components in thermoelectrics, electronic and optoelectronic devices. Future research on the epitaxial growth of rocksalt nitride semiconductors heterostructures will be useful for many emerging and novel industrial applications. Nitride superlattice heterostructures offer an excellent test bed for the exploration of refractory electronic and optoelectronic device properties such as intersubband absorption and emission, confinement of electrons in metallic wells, photodiodes, photo-conductors and terahertz devices.

International Centre for Materials Science (ICMS) and Chemistry and Physics of Materials Unit (CPMU), Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR), Bangalore,

[1] Saha B, Saber S, Naik GV, Boltasseva A, Stach EA, Kvam EP, et al. Development of epitaxial AlxSc1xN for artificially structured metal/semiconductor superlattice metamaterials. Physica

[2] Saha B, Shakouri A, Sands TD. Rocksalt nitride metal/semiconductor superlattices: A new class of artificially structured materials. Applied Physics Reviews. 2018;5:21101. DOI: 10.10

Status Solidi. 2015;252:251-259. DOI: 10.1002/pssb.201451314

develop practical devices.

6. Conclusions

Author details

India

References

63/1.5011972

Shashidhara Acharya and Bivas Saha\*

\*Address all correspondence to: bsaha@jncasr.ac.in

### 5. Transition metal nitride metal/semiconductor heterostructures

Transition metal nitrides have been utilized in recent years to develop the first epitaxial, nominally single crystalline TiN/(Al,Sc)N metal/semiconductor superlattices on MgO substrates. Rocksalt Al1-xScxN thin alloy films were developed with high AlN mole-fractions and critical thickness on TiN/MgO substrates by solid-state alloying of ScN and AlN. The latticematched TiN/(Al,Sc)N metal/semiconductor superlattices exhibit atomically sharp interfaces, structurally stable at high temperatures (950C), and amenable to doping, alloying and quantum size effects.

These novel (Ti,W)N/(Al,Sc)N metamaterials (Figure 6) have already exhibited lower thermal conductivity (1.7 W/mK at room temperatures) suitable for their thermoelectric applications. Coherent phonon thermal transport phenomenon was also demonstrated recently in these materials. Saha et al. have demonstrated hyperbolic dispersion of photonic isofrequency surfaces in these materials and enhanced photonic densities of states. Significant

Figure 6. HRTEM and HAADF-STEM image of the superlattices (Ti,W)N/(Al,Sc)N. Reproduced with permission from Saha et al. [37]. Copyright 2016 American Physical Society.

research in currently underway to unlock several aspects of their physical properties to develop practical devices.

### 6. Conclusions

Refractory optics and plasmonics with TMNs is yet another emerging and immensely promising research direction. Traditional plasmonic materials i.e., noble metals have highly limited absorption band width because of its resonant nature of plasmon excitation and high reflection in non-resonant frequencies. In addition, noble metals have low melting point, hence, poor thermal stability at high temperature and easily diffuses into surroundings. On the other hand, refractory metals such as W, Mo and others exhibit plasmon resonance in IR region with relatively high losses. Whereas TMN nitrides with their high melting temperatures and Aucompetitive optical properties could replace current polycrystalline refractory metals for hightemperature applications. Making use of these superior properties, Li et al. developed a TiN-SiO2-TiN broad band solar absorber, which can absorb 95% of light over the range of 400– 800 nm with 240 nm thick device [35]. Similarly, Ishii et al. fabricated TiN-ZnO-TiN structure, which showed superior performance in comparison to gold in visible region [36]. Both these devices were chemically robust, stable at high temperature and can be used in several emerging technologies such as solar-thermophotovoltaics (STPV), heat-assisted magnetic recording

5. Transition metal nitride metal/semiconductor heterostructures

Transition metal nitrides have been utilized in recent years to develop the first epitaxial, nominally single crystalline TiN/(Al,Sc)N metal/semiconductor superlattices on MgO substrates. Rocksalt Al1-xScxN thin alloy films were developed with high AlN mole-fractions and critical thickness on TiN/MgO substrates by solid-state alloying of ScN and AlN. The latticematched TiN/(Al,Sc)N metal/semiconductor superlattices exhibit atomically sharp interfaces, structurally stable at high temperatures (950C), and amenable to doping, alloying and

These novel (Ti,W)N/(Al,Sc)N metamaterials (Figure 6) have already exhibited lower thermal conductivity (1.7 W/mK at room temperatures) suitable for their thermoelectric applications. Coherent phonon thermal transport phenomenon was also demonstrated recently in these materials. Saha et al. have demonstrated hyperbolic dispersion of photonic isofrequency surfaces in these materials and enhanced photonic densities of states. Significant

Figure 6. HRTEM and HAADF-STEM image of the superlattices (Ti,W)N/(Al,Sc)N. Reproduced with permission from

(HAMR).

126 Coatings and Thin-Film Technologies

quantum size effects.

Saha et al. [37]. Copyright 2016 American Physical Society.

TMNs have been the most important materials for industrial applications such as hard coatings, corrosion and wear resistant coatings for many decades owing to their high mechanical strength, thermal stability and chemical inertness. There has been significant progress in the understanding of process-structure-properties relationship of TMNs over the years. Now it is possible to fabricate wide variety of alloys and composites of TMNs with diverse and engineerable properties which were not available earlier. This has resulted in new device applications of TMNs such as solar selective absorbers, refractory plasmonics, photocatalysis, etc. Further, the development of stable epitaxial TMNs, heterostructures and superlattices has opened up new directions for refractory electronic device applications. Particularly, recent development of semiconducting rocksalt ScN and AlxSc1xN and their high n- and p-type dopability has further widened the scope for new device applications such as active components in thermoelectrics, electronic and optoelectronic devices. Future research on the epitaxial growth of rocksalt nitride semiconductors heterostructures will be useful for many emerging and novel industrial applications. Nitride superlattice heterostructures offer an excellent test bed for the exploration of refractory electronic and optoelectronic device properties such as intersubband absorption and emission, confinement of electrons in metallic wells, photodiodes, photo-conductors and terahertz devices.

### Author details

Shashidhara Acharya and Bivas Saha\*

\*Address all correspondence to: bsaha@jncasr.ac.in

International Centre for Materials Science (ICMS) and Chemistry and Physics of Materials Unit (CPMU), Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR), Bangalore, India

### References


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[5] Saha B, Lawrence SK, Schroeder JL, Birch J, Bahr DF, Sands TD. Enhanced hardness in epitaxial TiAlScN alloy thin films and rocksalt TiN/(Al,Sc)N superlattices. Applied Phys-

[7] Madan A, Kim IW, Cheng SC, Yashar P, Dravid VP, Barnett SA. Stabilization of cubic AlN in epitaxial AlN/TiN Superlattices. Physical Review Letters. 1997;78:1743-1746. DOI: 10.11

[8] Hiramatsu K, Itoh S, Amano H, Akasaki I, Kuwano N, Shiraishi T, et al. Growth mechanism of GaN grown on sapphire with AlN buffer layer by MOVPE. Journal of Crystal

[9] Nakamura S, Senoh M, Mukai T, Makimoto T, Kido T, Guangrui Y, et al. GaN growth using GaN buffer layer. Japanese Journal of Applied Physics. 1991;30:L1705-7. DOI:10.

[10] Amano H, Kito M, Hiramatsu K, Akasaki I. P-type conduction in mg-doped GaN treated with low-energy electron beam irradiation (LEEBI). Japanese Journal of Applied Physics.

[11] Nakamura S, Mukai T, Senoh M, Naruhito I. Thermal annealing effects on P-type mg-23 doped GaN films. Japanese Journal of Applied Physics. 1992;139:L139-L142. DOI: 10.1143/

[12] Zide JMO, Bahk J-H, Singh R, Zebarjadi M, Zeng G, Lu H, et al. High efficiency semimetal/ semiconductor nanocomposite thermoelectric materials. Journal of Applied Physics. 2010;

[13] Kerdsongpanya S, Van Nong N, Pryds N, Žukauskaitė A, Jensen J, Birch J, et al. Anomalously high thermoelectric power factor in epitaxial ScN thin films Applied Physics Letters.

[14] Burmistrova PV, Maassen J, Favaloro T, Saha B, Salamat S, Rui Koh Y, et al. Thermoelectric properties of epitaxial ScN films deposited by reactive magnetron sputtering onto MgO (001) substrates. Journal of Applied Physics. 2013;113:153704. DOI:10.1063/1.4801886

[15] Saha B, Perez-Taborda JA, Bahk JH, Koh YR, Shakouri A, Martin-Gonzalez M, et al. Temperature-dependent thermal and thermoelectric properties of n-type and p-type

Sc1xMgxN. Physical Review B. 2018;97:1-9. DOI: 10.1103/PhysRevB.97.085301

[6] Koehler JS. Attempt to design a strong solid. Physical Review B. 1970;2:547-551

Growth. 1991;115:628-633. DOI: 10.1016/0022-0248(91)90816-N

1989;28:L2112-L2114. DOI: 10.1143/JJAP.28.L2112

108:123702. DOI: 10.1063/1.3514145

2011;99:232113. DOI:10.1063/1.3665945

ics Letters. 2014;105:151904. DOI: 10.1063/1.4898067

10.1116/1.573713

128 Coatings and Thin-Film Technologies

03/PhysRevLett.78.1743

1143/JJAP.30.L1705

24 JJAP.31.L139

2014.08.055


[29] Koutsokeras LE, Abadias G, Lekka CE, Matenoglou GM, Anagnostopoulos DF, Evangelakis GA, et al. Conducting transition metal nitride thin films with tailored cell sizes: The case of δ-TixTa1xN. Applied Physics Letters. 2008;93:1-4. DOI: 10.1063/1.2955838

**Chapter 7**

Provisional chapter

**CdTe Thin Films: Deposition Techniques and**

DOI: 10.5772/intechopen.79578

CdTe Thin Films: Deposition Techniques and

Antonio Arce-Plaza, Fernando Sánchez-Rodriguez, Maykel Courel-Piedrahita, Osvaldo Vigil Galán,

Antonio Arce-Plaza, Fernando Sánchez-Rodriguez, Maykel Courel-Piedrahita, Osvaldo Vigil Galán,

Sergio Ramirez-Velasco and Mauricio Ortega López

The II-IV semiconductor compound, CdTe, has suitable electrical and optical properties as photovoltaic and high-energy radiation sensor material. As an absorber material for thinfilm-based solar cells, CdTe holds the potentiality to fabricate high-efficiency solar cells by means of low-cost technologies. This chapter presents a comprehensive review on the CdTe thin-film deposition techniques as well as on the several configurations for the solar cell structures that have led the best efficiency conversion. Current CdTe thin-film deposition techniques include sputtering, close spaced vapor transport (CSVT), chemical spray pyrolysis, and electrodeposition. These techniques have easily been adapted to deposit polycrystalline CdTe films on various flexible and rigid substrates. In regard to the device structure configuration, a variety of partner materials (transparent contact, optical window, buffer layer) were tested, and CdTe film thickness was varied to develop opaque and semitransparent devices by some techniques mentioned above. In this chapter, we will discuss about each technique used for CdTe thin-film deposition as well as its advantages

Keywords: CdTe, thin films, deposit techniques, solar cell, semiconductor

Nowadays, the electrical power generation by photovoltaic conversion of solar light continuously increases. This can be attributed to the development of new photovoltaic materials and

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Sergio Ramirez-Velasco and Mauricio Ortega López

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Applications**

Abstract

and disadvantages.

1. Introduction

Applications

Viviana Hernandez-Calderon,

Viviana Hernandez-Calderon,

http://dx.doi.org/10.5772/intechopen.79578


#### **CdTe Thin Films: Deposition Techniques and Applications** CdTe Thin Films: Deposition Techniques and Applications

DOI: 10.5772/intechopen.79578

Antonio Arce-Plaza, Fernando Sánchez-Rodriguez, Maykel Courel-Piedrahita, Osvaldo Vigil Galán, Viviana Hernandez-Calderon, Sergio Ramirez-Velasco and Mauricio Ortega López Antonio Arce-Plaza, Fernando Sánchez-Rodriguez, Maykel Courel-Piedrahita, Osvaldo Vigil Galán, Viviana Hernandez-Calderon, Sergio Ramirez-Velasco and Mauricio Ortega López

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79578

#### Abstract

[29] Koutsokeras LE, Abadias G, Lekka CE, Matenoglou GM, Anagnostopoulos DF, Evangelakis GA, et al. Conducting transition metal nitride thin films with tailored cell sizes: The case of

[30] Abadias G, Guerin P. In situ stress evolution during magnetron sputtering of transition metal nitride thin films. Applied Physics Letters. 2008;93:1-4. DOI: 10.1063/1.2985814 [31] Matenoglou GM, Koutsokeras LE, Patsalas P. Plasma energy and work function of conducting transition metal nitrides for electronic applications. Applied Physics Letters.

[32] Lee YK, Latt KM, Osipowicz T, Sher-Yi C. Study of diffusion barrier properties of ternary alloy (TixAlyNz) in Cu/TixAlyNz/SiO2/Si thin film structure. Materials Science in Semi-

[33] Oliver MH, Schroeder JL, Ewoldt DA, Wildeson IH, Rawat V, Colby R, et al. Organometallic vapor phase epitaxial growth of GaN on ZrN/AlN/Si substrates. Applied Physics

[34] Jang WW, Lee JO, Yoon JB, Kim MS, Lee JM, Kim SM, et al. Fabrication and characterization of a nanoelectromechanical switch with 15-nm-thick suspension air gap. Applied

[35] Li W, Guler U, Kinsey N, Naik GV, Boltasseva A, Guan J, et al. Refractory plasmonics with titanium nitride: Broadband. Advanced Materials. 2014;26:7959-7965. DOI: 10.1002/adma.201

[36] Ishii S, Uto K, Niiyama E, Ebara M, Nagao T. Hybridizing poly(ε-caprolactone) and plasmonic titanium nitride nanoparticles for broadband photoresponsive shape memory films. ACS Applied Materials & Interfaces. 2016;8:5634-5640. DOI: 10.1021/acsami.5b12658 [37] Saha B, Koh YR, Comparan J, Sadasivam S, Schroeder JL, Garbrecht M, et al. Cross-plane thermal conductivity of (Ti,W)N/(Al,Sc)N metal/semiconductor superlattices. Physical

conductor Processing. 2000;3:191-194. DOI: 10.1016/S1369-8001(00)00031-7

δ-TixTa1xN. Applied Physics Letters. 2008;93:1-4. DOI: 10.1063/1.2955838

2009;94:152108. DOI:10.1063/1.3119694

130 Coatings and Thin-Film Technologies

Letters. 2008;93:023109. DOI:10.1063/1.2953541

401874

Physics Letters. 2008;92:103110. DOI:10.1063/1.2892659

Review B. 2016;93:1-11. DOI: 10.1103/PhysRevB.93.045311

The II-IV semiconductor compound, CdTe, has suitable electrical and optical properties as photovoltaic and high-energy radiation sensor material. As an absorber material for thinfilm-based solar cells, CdTe holds the potentiality to fabricate high-efficiency solar cells by means of low-cost technologies. This chapter presents a comprehensive review on the CdTe thin-film deposition techniques as well as on the several configurations for the solar cell structures that have led the best efficiency conversion. Current CdTe thin-film deposition techniques include sputtering, close spaced vapor transport (CSVT), chemical spray pyrolysis, and electrodeposition. These techniques have easily been adapted to deposit polycrystalline CdTe films on various flexible and rigid substrates. In regard to the device structure configuration, a variety of partner materials (transparent contact, optical window, buffer layer) were tested, and CdTe film thickness was varied to develop opaque and semitransparent devices by some techniques mentioned above. In this chapter, we will discuss about each technique used for CdTe thin-film deposition as well as its advantages and disadvantages.

Keywords: CdTe, thin films, deposit techniques, solar cell, semiconductor

### 1. Introduction

Nowadays, the electrical power generation by photovoltaic conversion of solar light continuously increases. This can be attributed to the development of new photovoltaic materials and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

inexpensive production technologies, which have led to the price reduction of the watt-hour generated by photovoltaic means. An interesting approach to the solar cells cost reduction consists of using polycrystalline semiconductor thin films.

A thin-film solar cell comprises different semiconductor layers as will be described below, including the so-called absorber or active material. Among these, cadmium tellurium (CdTe), silicon (Si), gallium arsenide (GaAs), CZTS, CIGS, and perovskites are currently used to fabricate high-efficiency solar cells at industrial level [1].

The cadmium telluride (CdTe) semiconductor belonging to II-VI family has been studied for many years. The oldest studies on CdTe synthesis and applications date 1890–1920 decades. In the last 10 years, CdTe has been mainly studied as a polycrystalline thin film and as a quantum dot. As a thin film, it has been prepared by close space vapor transport (CSVT) [2], laser ablation [3], electrodeposition [4] and spray pyrolysis [5], and sputtering [6], and it has been mostly used as the absorber material of thin-film solar cells. More recent deposition techniques of CdTe are based on taking CdTe nanocrystals dispersed in water or organic solvents [5] and transform them into CdTe thin films by using very simple and cheap deposition techniques such as dip-coating or spin-coating and an annealing process [7].

### 1.1. Solar cell operation: a brief

As stated above, thin-film solar cells consist of various semiconductor materials, each one having an important function in the solar cell functioning and performance [8]. Figure 1 sketches the different semiconductor layers that compose typical thin-film solar cells. The absorber semiconductor or optically active material has an optical bandgap in 1.0–1.45 eV range, an absorber coefficient in the order of 104 cm�<sup>1</sup> , and a p-type conductivity. Table 1 lists the band structure parameters and melting point of more important absorber semiconductors.

The generation of electric current, called photocurrent, occurs in two steps: first, photons are absorbed from sunlight, which generates electron-hole pairs. Second, these pairs have to move inside the depletion zone of solar cell to be separated, generating photocurrent. In solar cells based on CdTe, this separation occurs in the p-n junction. As the solar cell generates electricity, it is characterized by the Shockley equation:

$$I = I\_{ph} - I\_0 \left(\mathcal{e}^{\frac{ql}{k\_B T}} - 1\right) \tag{1}$$

It has a bandgap of 1.42 eV (optimal for the solar spectrum), and it is a semiconductor of direct band transitions that allows thin film applications. Other property is its absorption coefficient

Semiconductor Eg (eV) Electron affinity (eV) Melting point (C)

CdTe 1.45 4.28 1092 a-Si 1.12 4.05 1414 GaAs 1.424 4.07 1230

CIGS 0.97–1.43 4.07 1400

Table 1. Properties of the principal semiconductor material with application in solar cells.

CZTS 1.45 4.5

advantage is the possibility of obtaining p- and n-type conductivity in the films, enabling formation of homojunctions. However, the material has some "disadvantages" when applied to solar cells; for example, it is highly resistive that it does not allow excellent carrier collection. Additionally, CdTe has a high work function of 5.7 eV, which affects the semiconductor-metal junction. To improve this junction, it is necessary to find a metal with a work function greater than that of the CdTe. Finally, the CdTe homojunction has a high surface recombination speed that does not allow the manufacture of devices with homojunctions [2]. As a result, CdTe solar cell devices have been commonly processed considering CdS/CdTe heterojunctions. The properties of each part of the solar cell will be explained later, and the deposit techniques for CdTe

For all of the reasons mentioned above, it is necessary to have a better understanding of all components of the solar cells, which would allow improvements in CdTe solar cell efficiency. In addition, since efficiency/cost ratio is an important figure of merit to be considered, it is mandatory to revise most commonly used processing routes for each component of CdTe solar cells for reducing processing cost. In this chapter, we present a review on CdTe solar cells where particular emphasis will be given to techniques used for depositing each component of the solar cell. A comparison between the different deposition techniques

The most known configuration of CdTe solar cells is the p-n junction. Some people have stated that the MIS-type configuration improves solar cell efficiency [9]; however, for any of the configurations, the structure of the solar cells is similar. CdTe solar cells are often fabricated

Glass=substrate=TCO=CdS=CdTe=Black contact

Figure 1 shows the configuration of CdTe solar cells where each one of the components will be

, which allows 90% absorption with a 1 μm thickness of the thin films. Other

CdTe Thin Films: Deposition Techniques and Applications

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133

of 104 cm<sup>1</sup>

films too.

is presented.

explained below.

1.1.1. Configuration of solar cells based on CdTe

considering the superstrate structure, which is presented below:

where kB is the Boltzmann constant, q is the electron charge, T is the temperature, and V is the voltage between terminals of the solar cells.

The solar cell is a semiconductor device; as mentioned earlier, the most used semiconductors for this application are cadmium tellurium (CdTe), amorphous silicon, gallium arsenide, CZTS, CIGS, and perovskites. Table 1 shows the most relevant properties for these semiconductor materials.

An important II-VI semiconductor is CdTe, which has been well studied and applied to solar cell devices. CdTe has excellent electrical and optical properties for its application to solar cells.


Table 1. Properties of the principal semiconductor material with application in solar cells.

inexpensive production technologies, which have led to the price reduction of the watt-hour generated by photovoltaic means. An interesting approach to the solar cells cost reduction

A thin-film solar cell comprises different semiconductor layers as will be described below, including the so-called absorber or active material. Among these, cadmium tellurium (CdTe), silicon (Si), gallium arsenide (GaAs), CZTS, CIGS, and perovskites are currently used to

The cadmium telluride (CdTe) semiconductor belonging to II-VI family has been studied for many years. The oldest studies on CdTe synthesis and applications date 1890–1920 decades. In the last 10 years, CdTe has been mainly studied as a polycrystalline thin film and as a quantum dot. As a thin film, it has been prepared by close space vapor transport (CSVT) [2], laser ablation [3], electrodeposition [4] and spray pyrolysis [5], and sputtering [6], and it has been mostly used as the absorber material of thin-film solar cells. More recent deposition techniques of CdTe are based on taking CdTe nanocrystals dispersed in water or organic solvents [5] and transform them into CdTe thin films by using very simple and cheap deposition techniques

As stated above, thin-film solar cells consist of various semiconductor materials, each one having an important function in the solar cell functioning and performance [8]. Figure 1 sketches the different semiconductor layers that compose typical thin-film solar cells. The absorber semiconductor or optically active material has an optical bandgap in 1.0–1.45 eV range, an absorber

The generation of electric current, called photocurrent, occurs in two steps: first, photons are absorbed from sunlight, which generates electron-hole pairs. Second, these pairs have to move inside the depletion zone of solar cell to be separated, generating photocurrent. In solar cells based on CdTe, this separation occurs in the p-n junction. As the solar cell generates electricity,

> qV kBT � 1

I ¼ Iph � I<sup>0</sup> e

where kB is the Boltzmann constant, q is the electron charge, T is the temperature, and V is the

The solar cell is a semiconductor device; as mentioned earlier, the most used semiconductors for this application are cadmium tellurium (CdTe), amorphous silicon, gallium arsenide, CZTS, CIGS, and perovskites. Table 1 shows the most relevant properties for these semicon-

An important II-VI semiconductor is CdTe, which has been well studied and applied to solar cell devices. CdTe has excellent electrical and optical properties for its application to solar cells.

, and a p-type conductivity. Table 1 lists the band structure

(1)

consists of using polycrystalline semiconductor thin films.

fabricate high-efficiency solar cells at industrial level [1].

such as dip-coating or spin-coating and an annealing process [7].

parameters and melting point of more important absorber semiconductors.

1.1. Solar cell operation: a brief

132 Coatings and Thin-Film Technologies

coefficient in the order of 104 cm�<sup>1</sup>

it is characterized by the Shockley equation:

voltage between terminals of the solar cells.

ductor materials.

It has a bandgap of 1.42 eV (optimal for the solar spectrum), and it is a semiconductor of direct band transitions that allows thin film applications. Other property is its absorption coefficient of 104 cm<sup>1</sup> , which allows 90% absorption with a 1 μm thickness of the thin films. Other advantage is the possibility of obtaining p- and n-type conductivity in the films, enabling formation of homojunctions. However, the material has some "disadvantages" when applied to solar cells; for example, it is highly resistive that it does not allow excellent carrier collection. Additionally, CdTe has a high work function of 5.7 eV, which affects the semiconductor-metal junction. To improve this junction, it is necessary to find a metal with a work function greater than that of the CdTe. Finally, the CdTe homojunction has a high surface recombination speed that does not allow the manufacture of devices with homojunctions [2]. As a result, CdTe solar cell devices have been commonly processed considering CdS/CdTe heterojunctions. The properties of each part of the solar cell will be explained later, and the deposit techniques for CdTe films too.

For all of the reasons mentioned above, it is necessary to have a better understanding of all components of the solar cells, which would allow improvements in CdTe solar cell efficiency. In addition, since efficiency/cost ratio is an important figure of merit to be considered, it is mandatory to revise most commonly used processing routes for each component of CdTe solar cells for reducing processing cost. In this chapter, we present a review on CdTe solar cells where particular emphasis will be given to techniques used for depositing each component of the solar cell. A comparison between the different deposition techniques is presented.

### 1.1.1. Configuration of solar cells based on CdTe

The most known configuration of CdTe solar cells is the p-n junction. Some people have stated that the MIS-type configuration improves solar cell efficiency [9]; however, for any of the configurations, the structure of the solar cells is similar. CdTe solar cells are often fabricated considering the superstrate structure, which is presented below:

$$\text{Class}/\text{substrate}/\text{TCO}/\text{CdS}/\text{CdTe}/\text{Black contact}$$

Figure 1 shows the configuration of CdTe solar cells where each one of the components will be explained below.

Of course, the laminar resistance and transparency are parameters that depend on the opposite of the thickness of the TCO, so the way to optimize a TCO as a function of its thickness is to use

> <sup>F</sup>:M: <sup>¼</sup> ð Þ Tave <sup>10</sup> Rsh

High resistance and transparent buffer layer between the TCO and the CdS is usually used in

• To decrease possible diffusion of atoms from the TCO to the rest of the films in the device

• To improve the surface morphology of TCO with respect to the roughness and pin-holes

• To ensure the deposition of CdS films with thicknesses lower than 100 nm with good

Since the CdTe homojunction did not work, it was required to replace the n-type CdTe. For this purpose, the material has to be transparent so that CdTe absorbs the greatest amount of light.

For the CdTe films, the material should be transparent and should have two properties: transmittance at least of 70% and the bandgap greater than that of the CdTe. The CdS has these two properties: its band gap is Eg = 2.45 eV and its transmittance is controlled by its thickness. The minimum thickness to guarantee the transmittance and an adequate morphology is around 100 nm; however, the best thickness of the CdS is 120 nm with the efficiency of 21% [14]. This material has thermal and chemical stability to CdTe thin-film deposition.

At present, CdS thin films are deposited by chemical bath [15]. The more recent woks present a variant to the chemical bath known as shallow where with this deposition technique the cost is reduced [16]. On the other hand, some works show an improvement of the efficiency of the solar cells when the CdS becomes intrinsic [17]; for this reason, the solar cells of CdTe are

The most important material of the solar cells is the active or absorbent material and the p part

• Its bandgap is the optimum for the absorption of the solar spectrum, i.e., it is Eg = 1.45 eV. Additionally, its bandgap presents direct transitions that allow fabrication of thin films.

sometimes considered as MIS configuration where the CdS is the intrinsic part.

of the heterojunction. The properties of the CdTe are as follows:

where Tave is the average transmittance and Rsh is the sheet resistance.

Properties that an adequate buffer layer must have are as follows:

(2)

135

CdTe Thin Films: Deposition Techniques and Applications

http://dx.doi.org/10.5772/intechopen.79578

the well-known figure of merit of Haccke [13]:

the CdTe solar cell technology.

surface coating of the films

The cadmium sulfide is the right one.

1.5. Cadmium telluride (CdTe)

1.4. Cadmium sulfide (CdS)

Figure 1. Configuration of the solar cell based on CdTe.

### 1.2. Glass—substrate

This component is important because all films will be deposited on the substrate. For this reason, the substrate has to endure the different deposition processes. The commonly used substrate is soda lime with 3 mm of thickness that endures high temperature (deposition of CdTe films) and presents chemical stability (deposition of CdS films). Other advantage of this substrate is its cost that is 5 USD/m<sup>2</sup> .

### 1.3. Transparent conducting oxide (TCO)

The two most used TCOs are SnO2:F [10] and ITO [11]. Like glass substrate, TCO thin film has to endure high temperature and present chemical stability because of configuration. Additionally, the TCO must comply with:


The most commonly used TCOs in solar cells based on CdTe are SnO2:F, ITO, Cd2SnO4, and ZnO:Al. Comparatively, ITO combines optical and electrical properties needed for good TCO very well; however, the abundance of indium in the upper continental crust is low (about 0.05 ppm). Due to its stronger mechanical and chemical stability at high temperature, together with the relative abundance of Sn, it makes SnO2:F one of the most used TCOs [12].

Properties that an adequate TCO must have are as follows:


Of course, the laminar resistance and transparency are parameters that depend on the opposite of the thickness of the TCO, so the way to optimize a TCO as a function of its thickness is to use the well-known figure of merit of Haccke [13]:

$$\text{F.M.} = \frac{\left(\text{T}\_{\text{ave}}\right)^{10}}{\text{R}\_{\text{sh}}} \tag{2}$$

where Tave is the average transmittance and Rsh is the sheet resistance.

High resistance and transparent buffer layer between the TCO and the CdS is usually used in the CdTe solar cell technology.

Properties that an adequate buffer layer must have are as follows:


### 1.4. Cadmium sulfide (CdS)

1.2. Glass—substrate

134 Coatings and Thin-Film Technologies

substrate is its cost that is 5 USD/m<sup>2</sup>

ally, the TCO must comply with:

• high bandgap (Eg > 4 eV)

• laminar resistance <sup>≈</sup> <sup>5</sup> <sup>Ω</sup>/□

• High transparence • Low sheet resistance

1.3. Transparent conducting oxide (TCO)

Figure 1. Configuration of the solar cell based on CdTe.

• transmittance between 80% and 90%

This component is important because all films will be deposited on the substrate. For this reason, the substrate has to endure the different deposition processes. The commonly used substrate is soda lime with 3 mm of thickness that endures high temperature (deposition of CdTe films) and presents chemical stability (deposition of CdS films). Other advantage of this

The two most used TCOs are SnO2:F [10] and ITO [11]. Like glass substrate, TCO thin film has to endure high temperature and present chemical stability because of configuration. Addition-

The most commonly used TCOs in solar cells based on CdTe are SnO2:F, ITO, Cd2SnO4, and ZnO:Al. Comparatively, ITO combines optical and electrical properties needed for good TCO very well; however, the abundance of indium in the upper continental crust is low (about 0.05 ppm). Due to its stronger mechanical and chemical stability at high temperature, together

with the relative abundance of Sn, it makes SnO2:F one of the most used TCOs [12].

Properties that an adequate TCO must have are as follows:

• High surface quality (low density of pinholes and low roughness)

.

Since the CdTe homojunction did not work, it was required to replace the n-type CdTe. For this purpose, the material has to be transparent so that CdTe absorbs the greatest amount of light. The cadmium sulfide is the right one.

For the CdTe films, the material should be transparent and should have two properties: transmittance at least of 70% and the bandgap greater than that of the CdTe. The CdS has these two properties: its band gap is Eg = 2.45 eV and its transmittance is controlled by its thickness. The minimum thickness to guarantee the transmittance and an adequate morphology is around 100 nm; however, the best thickness of the CdS is 120 nm with the efficiency of 21% [14]. This material has thermal and chemical stability to CdTe thin-film deposition.

At present, CdS thin films are deposited by chemical bath [15]. The more recent woks present a variant to the chemical bath known as shallow where with this deposition technique the cost is reduced [16]. On the other hand, some works show an improvement of the efficiency of the solar cells when the CdS becomes intrinsic [17]; for this reason, the solar cells of CdTe are sometimes considered as MIS configuration where the CdS is the intrinsic part.

### 1.5. Cadmium telluride (CdTe)

The most important material of the solar cells is the active or absorbent material and the p part of the heterojunction. The properties of the CdTe are as follows:

• Its bandgap is the optimum for the absorption of the solar spectrum, i.e., it is Eg = 1.45 eV. Additionally, its bandgap presents direct transitions that allow fabrication of thin films.

• CdTe has an absorption coefficient around αCdTe ≈ 104 cm�<sup>1</sup> whereby 90% of the photons are absorbed in 1 μm of the film.

obtained by these techniques are suitable for the operation of the solar cell, while for the CdTe films the best result was obtained when the film was deposited with physical technique.

CdTe Thin Films: Deposition Techniques and Applications

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137

The first part about thin-film deposition is the classification of the deposition techniques as physical and chemical techniques. The first technique starts with solid material which is sublimated to transport this gas and deposited on the substrate. This technique requires high vacuum, and in some cases ultrahigh vacuum. The second technique starts the deposition with reagents that by chemical reactions generate the material to deposit. The principal difference between these techniques is the use of the vacuum during the deposition of the film. For this reason, it is considered that the first technique is more expensive than the second one. Another difference is the control of the deposition speed, which is more precise with the physical technique. The consideration of the technique to be used is determined by the application of

the deposited film because the properties of the film depend on the technique used.

In the case of CdTe, at present, this film has been deposited by diverse techniques, for example, CSVT, sputtering, laser ablation, spray pyrolysis, and electrodeposition, each with different

The CSVT is a physical technique that consists of sublimating the material to transport the gas to deposit it on the substrate. One of the advantages is the close space, where the space to transport the gas is close whereby the control of the growth rate improves. With this technique,

The deposition system consists of two blocks, which can be made of graphite or metal. These blocks must be heated by halogen lamp or electrical resistance; for this reason, each block has temperature control. The block below is known as source and the block above is named as substrate. Between the blocks, a graphite boat is placed. The CdTe to deposit is placed inside it. In this case, some people used powder or tablet of CdTe, and finally the substrate is placed on the graphite boat. All system is in the vacuum chamber. Figure 3 shows a simple CSVT system. The mechanics of growth consist of creating a temperature grading between the block, where the source temperature is higher, in the case of the CdTe; the source temperature is around 500C. When the CdTe is sublimated, this is transported to the substrate that has a lower temperature causing the material to be deposited on the substrate. For this thin-film deposi-

One advantage of this technique is the morphology because authors have reported grain size

areas are more complicated because a uniform heating in the substrate block is necessary.

), but larger

around 5 μm [5]. Another advantage is that films are uniform in small area (1 inch<sup>2</sup>

The next section discusses the different techniques used to deposit CdTe film.

2. CdTe thin-film deposition

proprieties, and they are explained below.

2.1. Close spaced vapor transport (CSVT)

it is possible to obtain the films with thickness of 500 nm [2].

tion, high vacuum around 10<sup>6</sup> Torr is necessary.

• On the other hand, one property that is a disadvantage is its high resistivity and high work function that affects the semiconductor-metal union in the back contact.

Currently, CdTe thin film is deposited by physical techniques such as CSVT, sputtering, electrodeposition, and spray pyrolysis. It is known that the CdTe films need a thermal treatment of CdCl2 to improve the morphology and to reduce the recombination centers [18]. Other treatment is with fluorine [19].

### 1.6. Black contact

The ideal back contact meets two conditions: it has a p-type conductivity and it has a work function greater than that of the CdTe; these conditions are most important because they guarantee an ideal union in the band of the metal and the CdTe (see Figure 2). This part of the solar cell is deposited by coevaporation.

The most used back contact is Cu-Au [20] alloy, but this back contact is not ideal for the following reasons: first, these back contacts are, in general, Schottky barriers, and the other reason is diffusion of the copper atoms inside the solar cell, which degrades the device. This degradation occurs because the copper atoms create threads that connect the back contact with frontal contact, short-circuiting the solar cell; additionally, the solar cell is exposed to solar radiation whereby this problem increases [21]. Finally, the copper oxidizes with the environment; this problem is solved by depositing gold films on the copper, but the use of gold in industrial scale can be expensive. Other metal alternatives as back contact are molybdenum [22], nickel [20], etc. However, these metals are also expensive.

Another way to obtain a good back contact is by using compounds such as CuxTe [23–25] and Bi2Te3 [26] for creating a p+ region.

With points mentioned above, it is important to know the technology used in the deposition of each component of the solar cell. For example, the window materials are deposited by chemical techniques because these techniques are of lower cost and the properties of the films

Figure 2. Bending of bands in the metal-semiconductor junction: (a) the metal work function is greater than that of the semiconductor, and (b) the work function of the metal is less than that of the semiconductor.

obtained by these techniques are suitable for the operation of the solar cell, while for the CdTe films the best result was obtained when the film was deposited with physical technique.

The next section discusses the different techniques used to deposit CdTe film.

### 2. CdTe thin-film deposition

• CdTe has an absorption coefficient around αCdTe ≈ 104

• On the other hand, one property that is a disadvantage is its high resistivity and high

Currently, CdTe thin film is deposited by physical techniques such as CSVT, sputtering, electrodeposition, and spray pyrolysis. It is known that the CdTe films need a thermal treatment of CdCl2 to improve the morphology and to reduce the recombination centers [18]. Other

The ideal back contact meets two conditions: it has a p-type conductivity and it has a work function greater than that of the CdTe; these conditions are most important because they guarantee an ideal union in the band of the metal and the CdTe (see Figure 2). This part of

The most used back contact is Cu-Au [20] alloy, but this back contact is not ideal for the following reasons: first, these back contacts are, in general, Schottky barriers, and the other reason is diffusion of the copper atoms inside the solar cell, which degrades the device. This degradation occurs because the copper atoms create threads that connect the back contact with frontal contact, short-circuiting the solar cell; additionally, the solar cell is exposed to solar radiation whereby this problem increases [21]. Finally, the copper oxidizes with the environment; this problem is solved by depositing gold films on the copper, but the use of gold in industrial scale can be expensive. Other metal alternatives as back contact are molybdenum

Another way to obtain a good back contact is by using compounds such as CuxTe [23–25] and

With points mentioned above, it is important to know the technology used in the deposition of each component of the solar cell. For example, the window materials are deposited by chemical techniques because these techniques are of lower cost and the properties of the films

Figure 2. Bending of bands in the metal-semiconductor junction: (a) the metal work function is greater than that of the

semiconductor, and (b) the work function of the metal is less than that of the semiconductor.

work function that affects the semiconductor-metal union in the back contact.

are absorbed in 1 μm of the film.

the solar cell is deposited by coevaporation.

Bi2Te3 [26] for creating a p+ region.

[22], nickel [20], etc. However, these metals are also expensive.

treatment is with fluorine [19].

136 Coatings and Thin-Film Technologies

1.6. Black contact

cm�<sup>1</sup> whereby 90% of the photons

The first part about thin-film deposition is the classification of the deposition techniques as physical and chemical techniques. The first technique starts with solid material which is sublimated to transport this gas and deposited on the substrate. This technique requires high vacuum, and in some cases ultrahigh vacuum. The second technique starts the deposition with reagents that by chemical reactions generate the material to deposit. The principal difference between these techniques is the use of the vacuum during the deposition of the film. For this reason, it is considered that the first technique is more expensive than the second one. Another difference is the control of the deposition speed, which is more precise with the physical technique. The consideration of the technique to be used is determined by the application of the deposited film because the properties of the film depend on the technique used.

In the case of CdTe, at present, this film has been deposited by diverse techniques, for example, CSVT, sputtering, laser ablation, spray pyrolysis, and electrodeposition, each with different proprieties, and they are explained below.

### 2.1. Close spaced vapor transport (CSVT)

The CSVT is a physical technique that consists of sublimating the material to transport the gas to deposit it on the substrate. One of the advantages is the close space, where the space to transport the gas is close whereby the control of the growth rate improves. With this technique, it is possible to obtain the films with thickness of 500 nm [2].

The deposition system consists of two blocks, which can be made of graphite or metal. These blocks must be heated by halogen lamp or electrical resistance; for this reason, each block has temperature control. The block below is known as source and the block above is named as substrate. Between the blocks, a graphite boat is placed. The CdTe to deposit is placed inside it. In this case, some people used powder or tablet of CdTe, and finally the substrate is placed on the graphite boat. All system is in the vacuum chamber. Figure 3 shows a simple CSVT system. The mechanics of growth consist of creating a temperature grading between the block, where the source temperature is higher, in the case of the CdTe; the source temperature is around 500C. When the CdTe is sublimated, this is transported to the substrate that has a lower temperature causing the material to be deposited on the substrate. For this thin-film deposition, high vacuum around 10<sup>6</sup> Torr is necessary.

One advantage of this technique is the morphology because authors have reported grain size around 5 μm [5]. Another advantage is that films are uniform in small area (1 inch<sup>2</sup> ), but larger areas are more complicated because a uniform heating in the substrate block is necessary.

Figure 3. A simple CSVT system. This system generates a temperature gradient between the substrate and source where the temperature of the source is higher. For this temperature difference, the material is deposited on the substrate; the variable is substrate and source temperature.

Another disadvantage is the manipulation of the sample because the vacuum is broken between each deposition; for this reason, this technique cannot be industrialized.

This technique is excellent in small laboratory and could be harnessed if uniform heating in the blocks is guaranteed; for this, the halogen lamp is better than electrical resistance.

### 2.2. Sputtering

Sputtering technique is a physical technique which sublimates the material of the target; for this, the target is bombarded by energetic ions. The ions are obtained from plasma that is generated inside the system; this system has a vacuum chamber. When the ions strike on the target, these change momentum with the atom of the target. When the ions strike with energy greater than the binding energy, the atoms of the target are ejected; this process is named sputtering. The variables to consider in this process are energy of ions, incident angle, mass of ions, and mass of the atom in the target. Commonly, these ions are obtained from an ionized gas, which can be argon. Figure 4 shows a simple sputtering system.

The advantage is the control of the growth speed; as same as CSVT, in this case, it is possible to obtain ultrathin films. Arhlesh Gupta reported films with thickness of 600 nm and efficiency of 9.4% [6]. In this case, the grain size reported is around 2 μm [27]. At present, the efficiency is 14% [28].

In general, with this technique, it is possible to obtain "ultrathin films" or transparent CdTe films that are the current trend, semitransparent technology. An advantage is the industrialization of the technique. Other advantage is the possibility to use flexible substrate in this process.

### 2.3. Laser ablation

Laser ablation is also a physical technique. It is similar to CSVT because the process of deposition is done by sublimation of the material. The difference is the process of the sublimation because in this case the material is sublimated with a laser beam. To get this, the laser beam energy needs to be "small" so that the incident photons are absorbed by the material, and this can be sublimated. This deposit process is controlled with the pulse and intensity of the laser beam. Additionally, this technique is important because it can be used in the industry. Figure 5 shows a simple laser ablation system. One advantage of this technique is that the

Figure 5. Simple ablation laser system. The beam laser impacts in the target that absorbs the energy to sublimate; this gas

Figure 4. A simple sputtering system. The plasma is generated by electric shock with the used gas. The target is bombed to release ions from it; these ions are deposited on the substrate. The variable is the electric current and the kind of gas.

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substrate maintains a low temperature being possible to use flexible substrate.

is deposited in the substrate. The variable is intensity of the laser beam.

Figure 4. A simple sputtering system. The plasma is generated by electric shock with the used gas. The target is bombed to release ions from it; these ions are deposited on the substrate. The variable is the electric current and the kind of gas.

Another disadvantage is the manipulation of the sample because the vacuum is broken

Figure 3. A simple CSVT system. This system generates a temperature gradient between the substrate and source where the temperature of the source is higher. For this temperature difference, the material is deposited on the substrate; the

This technique is excellent in small laboratory and could be harnessed if uniform heating in the

Sputtering technique is a physical technique which sublimates the material of the target; for this, the target is bombarded by energetic ions. The ions are obtained from plasma that is generated inside the system; this system has a vacuum chamber. When the ions strike on the target, these change momentum with the atom of the target. When the ions strike with energy greater than the binding energy, the atoms of the target are ejected; this process is named sputtering. The variables to consider in this process are energy of ions, incident angle, mass of ions, and mass of the atom in the target. Commonly, these ions are obtained from an ionized

The advantage is the control of the growth speed; as same as CSVT, in this case, it is possible to obtain ultrathin films. Arhlesh Gupta reported films with thickness of 600 nm and efficiency of 9.4% [6]. In this case, the grain size reported is around 2 μm [27]. At present, the efficiency is

In general, with this technique, it is possible to obtain "ultrathin films" or transparent CdTe films that are the current trend, semitransparent technology. An advantage is the industrialization of the technique. Other advantage is the possibility to use flexible substrate in this

Laser ablation is also a physical technique. It is similar to CSVT because the process of deposition is done by sublimation of the material. The difference is the process of the sublimation because in this case the material is sublimated with a laser beam. To get this, the laser

between each deposition; for this reason, this technique cannot be industrialized.

blocks is guaranteed; for this, the halogen lamp is better than electrical resistance.

gas, which can be argon. Figure 4 shows a simple sputtering system.

2.2. Sputtering

variable is substrate and source temperature.

138 Coatings and Thin-Film Technologies

14% [28].

process.

2.3. Laser ablation

Figure 5. Simple ablation laser system. The beam laser impacts in the target that absorbs the energy to sublimate; this gas is deposited in the substrate. The variable is intensity of the laser beam.

beam energy needs to be "small" so that the incident photons are absorbed by the material, and this can be sublimated. This deposit process is controlled with the pulse and intensity of the laser beam. Additionally, this technique is important because it can be used in the industry. Figure 5 shows a simple laser ablation system. One advantage of this technique is that the substrate maintains a low temperature being possible to use flexible substrate.

Since 1994, there has been a report about this technique used to grow CdTe films, where solar cells have 3% of solar cell efficiency [3]. At present, films with a thickness between 1.8 and 3 μm and an average grain size of 300 nm have been reported [29].

Another effect reported is the change in the bandgap of the CdTe because it is formed by

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For this technique, CdTe solution is necessary; usually, this solution is a colloidal system. For this colloidal system, the stabilizing agent or surfactant is important because it stabilized the CdTe molecule; for this reason, one surfactant is necessary, which, in addition to the above, is easy to remove in the deposition process. The most used surfactant is TGA

The TGA surfactant offers excellent protection to CdTe molecule and it is easy to remove, but the problem with this stabilization agent is when it is removed, the CdTe molecule gets broken and then this solution cannot be used for material deposition. The second surfactant, oleic acid, protects CdTe molecule too, but it is difficult to remove it, this involves a chemical process with ferrozine and an additional process [32]. Ammonium hydroxide can be an excellent surfactant because it protects the CdTe molecule, supports with the pH solution, and is easily removed.

Drop-casting is a chemical technique, which is reported as a spray pyrolysis, but the problem

On the other hand, it was possible to manufacture solar cells under the following configuration:

Glass=ZnO=CdTe=Au � Cu

The CdS was omitted for this device but it has 8.8% of efficiency [28]. With this technique, it is

Electrodeposition is a chemical technique in which an electric current is used in an electrolytic solution with the objective that there is an ion movement toward the cathode. When the ions arrive on the cathode then the material is deposited. The simple deposition system is shown in

With this technique, Mathew et al. manufactured solar cells. They reported efficiencies between 8.6 and 11% [4]. Another thing they reported is the morphology; they obtained film with 500 nm of size grain. This morphology is not appropriate for the electrical application such as solar cells. This report is important because it is a disadvantage of the chemical techniques; the morphology obtained by these techniques is not the best to make solar cells but the reported efficiencies are acceptable. This topic is important and will be discussed later. This technique has several advantages; for example, with this technique, it is possible to use flexible substrates. Another advantage is that the substrate is not heated [4], and finally, this

All of the above are a short summary about the work around CdTe. It is important to know about the advantages and disadvantages of these techniques. Generally, these techniques have

The authors did not report efficiency, but they obtained photovoltaic effect [5, 32].

possible to obtain large deposit areas and this technique can be industrialized.

Figure 7. We can observe that the deposition system is really simple.

technique can deposit larger areas and, therefore, can be industrialized.

two differences: the cost and the properties obtained.

with this technique is the formation of cracks [33].

colloidal particles [5].

[30] and oleic acid [31].

2.5. Electrodeposition

### 2.4. Spray pyrolysis

The spray pyrolysis is a chemical technique used mainly to deposit TCO film; this is a simple technique where the material, that is in the solution, is pulverized by the pressure of a gas (argon, air, nitrogen, etc.). For this process, it is important to control the flow of the solution and the pressure of the gas. The pulverized solution is sprayed on the hot substrate to obtain the film. At present, spray pyrolysis has two different systems, which are differentiated by the kind of pulverization. The one described previously is named pneumatic and is shown in Figure 6, and the other system is named ultrasonic as it uses an ultrasonic system to pulverize; this system is not shown because there is no report on CdTe film deposited by this method.

One previous step to use this system is synthesis of the solution, because it is important to prevent oxidation on the surface. For this, the use of ammonium is excellent as it supports the protection of the ions. In this case, some authors use colloidal system of CdTe with reports on nanoparticle around 20 and 60 nm, and with this solution, they can deposit films with 500 nm thickness; the authors reported photovoltaic effect, but they did not manufacture solar cell.

Figure 6. Spray pyrolysis system. The solution moves through the tubes to meet the air in the nozzle to produce the spray, which arrives on the substrate that is heated to deposit the material. The variables are concentration solution, pressure, and the substrate temperature.

Another effect reported is the change in the bandgap of the CdTe because it is formed by colloidal particles [5].

For this technique, CdTe solution is necessary; usually, this solution is a colloidal system. For this colloidal system, the stabilizing agent or surfactant is important because it stabilized the CdTe molecule; for this reason, one surfactant is necessary, which, in addition to the above, is easy to remove in the deposition process. The most used surfactant is TGA [30] and oleic acid [31].

The TGA surfactant offers excellent protection to CdTe molecule and it is easy to remove, but the problem with this stabilization agent is when it is removed, the CdTe molecule gets broken and then this solution cannot be used for material deposition. The second surfactant, oleic acid, protects CdTe molecule too, but it is difficult to remove it, this involves a chemical process with ferrozine and an additional process [32]. Ammonium hydroxide can be an excellent surfactant because it protects the CdTe molecule, supports with the pH solution, and is easily removed. The authors did not report efficiency, but they obtained photovoltaic effect [5, 32].

Drop-casting is a chemical technique, which is reported as a spray pyrolysis, but the problem with this technique is the formation of cracks [33].

On the other hand, it was possible to manufacture solar cells under the following configuration:

$$\text{Glass/ZnO/CdTe/Au} - \text{Cu}$$

The CdS was omitted for this device but it has 8.8% of efficiency [28]. With this technique, it is possible to obtain large deposit areas and this technique can be industrialized.

### 2.5. Electrodeposition

Since 1994, there has been a report about this technique used to grow CdTe films, where solar cells have 3% of solar cell efficiency [3]. At present, films with a thickness between 1.8 and

The spray pyrolysis is a chemical technique used mainly to deposit TCO film; this is a simple technique where the material, that is in the solution, is pulverized by the pressure of a gas (argon, air, nitrogen, etc.). For this process, it is important to control the flow of the solution and the pressure of the gas. The pulverized solution is sprayed on the hot substrate to obtain the film. At present, spray pyrolysis has two different systems, which are differentiated by the kind of pulverization. The one described previously is named pneumatic and is shown in Figure 6, and the other system is named ultrasonic as it uses an ultrasonic system to pulverize; this system is not shown because there is no report on CdTe film deposited by this method.

One previous step to use this system is synthesis of the solution, because it is important to prevent oxidation on the surface. For this, the use of ammonium is excellent as it supports the protection of the ions. In this case, some authors use colloidal system of CdTe with reports on nanoparticle around 20 and 60 nm, and with this solution, they can deposit films with 500 nm thickness; the authors reported photovoltaic effect, but they did not manufacture solar cell.

Figure 6. Spray pyrolysis system. The solution moves through the tubes to meet the air in the nozzle to produce the spray, which arrives on the substrate that is heated to deposit the material. The variables are concentration solution, pressure,

3 μm and an average grain size of 300 nm have been reported [29].

2.4. Spray pyrolysis

140 Coatings and Thin-Film Technologies

and the substrate temperature.

Electrodeposition is a chemical technique in which an electric current is used in an electrolytic solution with the objective that there is an ion movement toward the cathode. When the ions arrive on the cathode then the material is deposited. The simple deposition system is shown in Figure 7. We can observe that the deposition system is really simple.

With this technique, Mathew et al. manufactured solar cells. They reported efficiencies between 8.6 and 11% [4]. Another thing they reported is the morphology; they obtained film with 500 nm of size grain. This morphology is not appropriate for the electrical application such as solar cells. This report is important because it is a disadvantage of the chemical techniques; the morphology obtained by these techniques is not the best to make solar cells but the reported efficiencies are acceptable. This topic is important and will be discussed later.

This technique has several advantages; for example, with this technique, it is possible to use flexible substrates. Another advantage is that the substrate is not heated [4], and finally, this technique can deposit larger areas and, therefore, can be industrialized.

All of the above are a short summary about the work around CdTe. It is important to know about the advantages and disadvantages of these techniques. Generally, these techniques have two differences: the cost and the properties obtained.

large area because all of them need vacuum chamber and heating to the substrate in some cases. Sputtering is the best technique in this case because this is feasible, but this technique has long deposition time. For the CSVT, the difficulty is in escalation to large areas. Another disadvantage of this technique is the time expenses due to the vacuum that the process requires. The chemical technique eliminates this steep, but the efficiency of the solar cells is

All techniques have advantages and disadvantages; in summary, the physical techniques are of high cost, and the manufacturing process is not continuing because of vacuum chamber, while chemical techniques are of lower cost, but the efficiency is lower. For this, it is necessary to establish an efficiency/cost ratio, which should be high. The difference in efficiency between physical and chemical techniques; the physical technique guarantees grain size greater than that obtained through the chemical technique, which affects the electrical properties of the material. When the grain size increases, the intergranular barrier potential decreases, allowing better diffusion of the charged carriers toward the p-n union and enhancing thereby the

Table 2 shows the comparison of all the techniques used to deposit CdTe films. In this table, the kind of technique, the pressure used, the temperature used in the process, the process time, the grain size and thickness obtained, the efficiency, and finally if the technique is scaling to large area or industrialization are given. And also, if the technique is not scaled, the reason is

The objective of this chapter is to summarize the different techniques used for depositing thin films of CdTe. All the techniques mentioned above are innovative because it is possible to obtain thin films with different properties by these techniques; for example, physical techniques are used to manufacture conventional solar cells, while chemical techniques can be

contribution of these photogenerated carriers to current density.

reduced.

P0: atmospheric pressure.

Technique Type Vacuum

(Torr)

Substrate temperature Tmax (c)

CSVT Physical 10<sup>6</sup> >400 600 3 5 5000 12 No

Sputtering Physical 10<sup>6</sup> No 600 2 2 2000 14 Yes Laser ablation Physical 10<sup>6</sup> No 600 2 3 300 3 No

Spray pyrolysis Chemical P0 <350 350 1.5 0.5 500 8 Yes Electrodeposition Chemical P0 No 100 1 2 500 11 Yes

Process time (h) Thickness (μm)

Grain size (nm)

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Efficiency (%)

Scaling

143

(system)

(efficiency)

( C)

Table 2. Differences between all the techniques used to deposit CdTe films.

written.

4. Conclusion

Figure 7. Simple deposition system of electrodeposition technique, the solution is an electrolyte that is deposited by electric current and moved from the anode to the cathode where the material is deposited; the variables are electrolyte concentration and amperage.

### 3. Comparison of techniques

As mentioned in the beginning, the CdTe is an important material because it is applied to solar cells or renewable energy. The thought around CdTe has changed over time. The first idea about solar cell was "with CdTe solar cells the world will be saved," after that the idea was "the best efficiency," and at present, the idea is "the best efficiency and cost ratio," which are the most important because of the presence of technological limitations; for example, in the CdTe case, the efficiency stops for a few years around 16% because of technological limitations.

On the other hand, manufacturing of CdTe solar cells consists of several processes; if the cost is reduced for these processes, then the cost of the solar cells would be reduced. The best way to do it is to implement chemical techniques; in the case of TCO and CdS, this is possible. But in the CdTe case, this is more complicated because this film was deposited by physical technique. The main reason is the relationship of efficiency with the morphology of the films. The best morphology is obtained by physical technique; the disadvantage is the difficulty of scaling to


Table 2. Differences between all the techniques used to deposit CdTe films.

large area because all of them need vacuum chamber and heating to the substrate in some cases. Sputtering is the best technique in this case because this is feasible, but this technique has long deposition time. For the CSVT, the difficulty is in escalation to large areas. Another disadvantage of this technique is the time expenses due to the vacuum that the process requires. The chemical technique eliminates this steep, but the efficiency of the solar cells is reduced.

All techniques have advantages and disadvantages; in summary, the physical techniques are of high cost, and the manufacturing process is not continuing because of vacuum chamber, while chemical techniques are of lower cost, but the efficiency is lower. For this, it is necessary to establish an efficiency/cost ratio, which should be high. The difference in efficiency between physical and chemical techniques; the physical technique guarantees grain size greater than that obtained through the chemical technique, which affects the electrical properties of the material. When the grain size increases, the intergranular barrier potential decreases, allowing better diffusion of the charged carriers toward the p-n union and enhancing thereby the contribution of these photogenerated carriers to current density.

Table 2 shows the comparison of all the techniques used to deposit CdTe films. In this table, the kind of technique, the pressure used, the temperature used in the process, the process time, the grain size and thickness obtained, the efficiency, and finally if the technique is scaling to large area or industrialization are given. And also, if the technique is not scaled, the reason is written.

### 4. Conclusion

3. Comparison of techniques

concentration and amperage.

142 Coatings and Thin-Film Technologies

limitations.

As mentioned in the beginning, the CdTe is an important material because it is applied to solar cells or renewable energy. The thought around CdTe has changed over time. The first idea about solar cell was "with CdTe solar cells the world will be saved," after that the idea was "the best efficiency," and at present, the idea is "the best efficiency and cost ratio," which are the most important because of the presence of technological limitations; for example, in the CdTe case, the efficiency stops for a few years around 16% because of technological

Figure 7. Simple deposition system of electrodeposition technique, the solution is an electrolyte that is deposited by electric current and moved from the anode to the cathode where the material is deposited; the variables are electrolyte

On the other hand, manufacturing of CdTe solar cells consists of several processes; if the cost is reduced for these processes, then the cost of the solar cells would be reduced. The best way to do it is to implement chemical techniques; in the case of TCO and CdS, this is possible. But in the CdTe case, this is more complicated because this film was deposited by physical technique. The main reason is the relationship of efficiency with the morphology of the films. The best morphology is obtained by physical technique; the disadvantage is the difficulty of scaling to

The objective of this chapter is to summarize the different techniques used for depositing thin films of CdTe. All the techniques mentioned above are innovative because it is possible to obtain thin films with different properties by these techniques; for example, physical techniques are used to manufacture conventional solar cells, while chemical techniques can be used to manufacture transparent technology because their deposition time is low. In other words, the technique used depends on the applications of the thin films.

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substrates. Solar Energy. 2004;77:831-838. DOI: 10.1016/j.solener.2004.06.020

At present, three different technologies are used in solar cells based on CdTe: rigid (conventional), semitransparent, and flexible. The technology determines the technique to be used. Additional, the configuration of the solar cell changes when the technology changes; in conventional and semitransparent technology, the configuration is superstrate, while in flexible technology, the configuration is substrate. This change on the substrate is due to application; in the case of the first two technologies, the substrate used is soda lime that resists high temperature, and in the last case, the substrate used is a polymer that can resist the temperatures used to deposit CdTe films, which can be done by sputtering, electrodeposition, or laser ablation. Besides, CdTe films could be deposited by spray pyrolysis, but it is necessary to adjust deposit parameters. In the case of the semitransparent technology, it is necessary to have a low growth speed; the most suitable techniques are sputtering, laser ablation, and spray pyrolysis. In the case of the CSVT, it is necessary to adjust the deposition gradient. The conventional technology and technique can be used, but the deposit time has to be considered; that is, it is not reasonable to take a day to deposit a film.

The advantage of each technique depends on the technology used; that is, advantage and disadvantage cannot be stated if the technology is not mentioned. In the conventional technology, the low growth speed is a disadvantage, while this is an advantage in the semitransparent technology.

### Author details

Antonio Arce-Plaza<sup>1</sup> \*, Fernando Sánchez-Rodriguez<sup>2</sup> , Maykel Courel-Piedrahita<sup>3</sup> , Osvaldo Vigil Galán<sup>4</sup> , Viviana Hernandez-Calderon<sup>4</sup> , Sergio Ramirez-Velasco<sup>4</sup> and Mauricio Ortega López<sup>5</sup>

\*Address all correspondence to: aarce312@gmail.com

1 Escuela Superior de Ingeniería y Arquitectura Unidad Zacatenco, Instituto Politécnico Nacional (ESIAZ-IPN), CDMX, Mexico

2 Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Sinaloa (UAS), Culiacan, Sinaloa, Mexico

3 Centro Universitario de los Valles (CUValles), Universidad de Guadalajara, Ameca, Jalisco, Mexico

4 Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional (ESFM-IPN), CDMX, Mexico

5 SEES, Electrical Engineering Department, Center for Research and Advanced Studies of the National Polytechnic Institute, Mexico City, Mexico

### References

used to manufacture transparent technology because their deposition time is low. In other

At present, three different technologies are used in solar cells based on CdTe: rigid (conventional), semitransparent, and flexible. The technology determines the technique to be used. Additional, the configuration of the solar cell changes when the technology changes; in conventional and semitransparent technology, the configuration is superstrate, while in flexible technology, the configuration is substrate. This change on the substrate is due to application; in the case of the first two technologies, the substrate used is soda lime that resists high temperature, and in the last case, the substrate used is a polymer that can resist the temperatures used to deposit CdTe films, which can be done by sputtering, electrodeposition, or laser ablation. Besides, CdTe films could be deposited by spray pyrolysis, but it is necessary to adjust deposit parameters. In the case of the semitransparent technology, it is necessary to have a low growth speed; the most suitable techniques are sputtering, laser ablation, and spray pyrolysis. In the case of the CSVT, it is necessary to adjust the deposition gradient. The conventional technology and technique can be used, but the deposit time has to be considered; that is, it is not

The advantage of each technique depends on the technology used; that is, advantage and disadvantage cannot be stated if the technology is not mentioned. In the conventional technology, the low growth speed is a disadvantage, while this is an advantage in the semitransparent

, Maykel Courel-Piedrahita<sup>3</sup>

, Sergio Ramirez-Velasco<sup>4</sup> and

,

\*, Fernando Sánchez-Rodriguez<sup>2</sup>

, Viviana Hernandez-Calderon<sup>4</sup>

1 Escuela Superior de Ingeniería y Arquitectura Unidad Zacatenco, Instituto Politécnico

2 Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Sinaloa (UAS),

4 Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional (ESFM-IPN),

3 Centro Universitario de los Valles (CUValles), Universidad de Guadalajara, Ameca, Jalisco,

5 SEES, Electrical Engineering Department, Center for Research and Advanced Studies of the

\*Address all correspondence to: aarce312@gmail.com

National Polytechnic Institute, Mexico City, Mexico

Nacional (ESIAZ-IPN), CDMX, Mexico

words, the technique used depends on the applications of the thin films.

reasonable to take a day to deposit a film.

technology.

Author details

Antonio Arce-Plaza<sup>1</sup>

Osvaldo Vigil Galán<sup>4</sup>

Mauricio Ortega López<sup>5</sup>

144 Coatings and Thin-Film Technologies

Culiacan, Sinaloa, Mexico

Mexico

CDMX, Mexico


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[26] Vigil-Galán O, Cruz-Gandarilla F, Fandiño J, Roy F, Sastré-Hernández J, Contreras-Puente G. Physical properties of Bi2Te3 and Sb2Te3 films deposited by close space vapor transport. Semiconductor Science and Technology. 2009;24:1-6. DOI: 10.1088/0268-1242/24/2/025025

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[28] Gupta A, Compaan AD. All-sputtered 14% CdS∕CdTe thin-film solar cell with ZnO: Al transparent conducting oxide. Applied Physics Letters. 2004;85:684. DOI: 10.1063/1.1775289

[29] Pandey SK, Tiwari U, Raman R, Prakash C, Vamsirishna VD, Zimik K. Growth of cubic and hexagonal CdTe thin films by pulsed laser deposition. Thin Solid Films. 2005;473:54-57.

[30] Liu J, Shi Z, Yu Y, Yang R, Zuo S. Water-soluble multicolored fluorescent CdTe quantum dots: Synthesis and application for fingerprint developing. Journal of Colloid and Inter-

[31] Kolny-Olesiak J, Kloper V, Osovsky R, Sashchiuk A, Lifshitz E. Synthesis and characterization of brightly photoluminescent CdTe nanocrystals. Surface Science. 2007;601:2667-2670.

[32] Arce Plaza A. Síntesis de CdTe coloidal para potencial uso en celdas solares de películas

[33] Hernández Vásquez C, Albor Aguilera ML, González Trujillo MA, Flores Márquez JM, Jiménez Olarte D, Gallardo Hernández S, Cruz Orea A. Enhancement of CdS/CdTe solar cells by the interbuilding of a nanostructured Te-rich layer. Materials Research Express.

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[12] Gordon RG. Criteria for choosing transparent conductors. MRS Bulletin. 2000;25:52-57. DOI:

[13] Haacke G. New figure of merit for transparent conductors. Journal of Applied Physics.

[14] Green MA, Hishikawa Y, Warta W, Dunlop ED, Levi DH, Hohl-Ebinger J, Ho-Baillie AWH. Solar cell efficiency tables (version 51). Progress in Photovoltaics. 2017;25. DOI:

[15] Tanushevski A, Osmani H. CdS thin films obtained by chemical bath deposition in presence of fluorine and the effect of annealing on their properties. Chalcogenide Letters. 2018;15

[16] More PV, Hiragond CB, Jadhav A, Kush P, Sapra S, Khanna PK. Instant synthesis of white light-emitting Cd Chalcogenide Nanoclusters using homogenization method. Chemistry-

[17] Avendano A, Jesus A. New Applications for CDTE/CDS Heterojunctions: The Prospects

[18] Berg M, Kephart JM, Munshi A, Sampath WS, Ohta T, Chan C. Local electronic structure changes in polycrystalline CdTe with CdCl2 treatment and air exposure. Applied Mate-

[19] Abdul-Manaf NA, Dharmadasa. Development of CdTe thin film solar cells for military applications. Defence S&T Technical Bulletin. 2017;10:129-141. http://shura.shu.ac.uk/id/

[20] Bastola E, Subedi KK, Bhandari KP, Ellingson RJ. Solution-processed nanocrystal based thin films as hole transport materials in cadmium telluride photovoltaics. Materials

[21] Corwine CR, Pudov AO, Gloeckler M, Demtsu SH, Sites JR. Copper inclusion and migration from the back contact in CdTe solar cells. Solar Energy Materials & Solar Cells. 2004;

[22] Moustafa M, AlZoubi T. Effect of the n-MoTe2 interfacial layer in cadmium telluride solar

[23] Wu X, Zhou J, Duda A, Yan Y, Teeter G, Asher S, Metzger WK, Demtsu S, Wei S-H, Noufi R. Phase control of CuxTe film and its effects on CdS/CdTe solar cell. Thin Solid Films.

[24] Zhou J, Wu X, Duda A, Teeter G, Demtsu SH. The formation of different phases of CuxTe and their effects on CdTe/CdS solar cells. Thin Solid Films. 2007;515:7364-7369. DOI:

[25] Arce-Plaza A. Obtención de contactos de tipo CuxTe en celdas solares de CdT e mediante ataques químicos de ácidos nítrico-fosfórico y evaporación de Cu. http://www.repositor-

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**Chapter 8**

Provisional chapter

**Impact of the Glancing Angle Deposition on the Yttria-**

DOI: 10.5772/intechopen.81905

Yttria-stabilized zirconia (YSZ) is the most common material used as a thermal barrier in several engineering applications. The majority of films produced by physical vapor deposition (PVD) techniques use normal incidence and lead to the columnar growth normal to the substrate. The typical columnar structure of sputter-deposited films is largely influenced, among other parameters, by pressure, temperature, thickness, and the ion-toatom ratio incident at the substrate or substrate bias voltage. Another important experimental parameter used to modify the film properties is the direction of the incident flux of the depositing species with respect to the substrate surface. In this chapter an oblique angle deposition (OAD) approach was used to grow YSZ with tilted columnar structures, to study the impact of this deposition technique on the microstructure, morphology, and, correspondingly, the thermal conductivity of YSZ films, in order to improve the insulator potential of these thin films. Additionally, in the chapter, we present a detailed description of the oblique angle deposition (OAD) technique and double-layer model used for determination of the effective thermal conductivity of YSZ samples grown over thick substrates.

Keywords: physical vapor deposition, yttria-stabilized zirconia (YSZ), oblique angle

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

deposition (OAD), thermal conductivity, double-layer model

**Stabilized Zirconia Growth and Their Thermal Barrier**

Impact of the Glancing Angle Deposition on the

Yttria-Stabilized Zirconia Growth and Their

**Coating Properties**

Gustavo Zambrano

Gustavo Zambrano

Abstract

Cesar Amaya, John Jairo Prıas-Barragan,

Cesar Amaya, John Jairo Prıas-Barragan,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81905

Julio Cesar Caicedo, Jose Martin Yañez-Limon and

Julio Cesar Caicedo, Jose Martin Yañez-Limon and

Thermal Barrier Coating Properties

#### **Impact of the Glancing Angle Deposition on the Yttria-Stabilized Zirconia Growth and Their Thermal Barrier Coating Properties** Impact of the Glancing Angle Deposition on the Yttria-Stabilized Zirconia Growth and Their Thermal Barrier Coating Properties

DOI: 10.5772/intechopen.81905

Cesar Amaya, John Jairo Prıas-Barragan, Julio Cesar Caicedo, Jose Martin Yañez-Limon and Gustavo Zambrano Cesar Amaya, John Jairo Prıas-Barragan, Julio Cesar Caicedo, Jose Martin Yañez-Limon and Gustavo Zambrano

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81905

#### Abstract

Yttria-stabilized zirconia (YSZ) is the most common material used as a thermal barrier in several engineering applications. The majority of films produced by physical vapor deposition (PVD) techniques use normal incidence and lead to the columnar growth normal to the substrate. The typical columnar structure of sputter-deposited films is largely influenced, among other parameters, by pressure, temperature, thickness, and the ion-toatom ratio incident at the substrate or substrate bias voltage. Another important experimental parameter used to modify the film properties is the direction of the incident flux of the depositing species with respect to the substrate surface. In this chapter an oblique angle deposition (OAD) approach was used to grow YSZ with tilted columnar structures, to study the impact of this deposition technique on the microstructure, morphology, and, correspondingly, the thermal conductivity of YSZ films, in order to improve the insulator potential of these thin films. Additionally, in the chapter, we present a detailed description of the oblique angle deposition (OAD) technique and double-layer model used for determination of the effective thermal conductivity of YSZ samples grown over thick substrates.

Keywords: physical vapor deposition, yttria-stabilized zirconia (YSZ), oblique angle deposition (OAD), thermal conductivity, double-layer model

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### 1. Introduction

Yttria-stabilized zirconia (YSZ) coating systems are widely used for the thermal, oxidation, and hot corrosion protection of high-temperature components in gas turbine and diesel engines [1], and, additionally, the electrolyte YSZ is the standard ionic conductor [2] used in fuel cells, being a brittle material due to its high hardness [3]. This thermal and oxidation protection must be achieved without incurring excessive thermomechanical loading of the coating system and the metal component to which it is applied.

addition of rare earth oxides (REO). Klemens et al. [20] conclude that co-doping with REO can solve the problems concerning the high-temperature (tetragonal) phase stability of ZrO2 and important decreases in thermal conductivity can be achieved by approaching values obtained by APS (k = 0.8 W/mK). The second way is to manipulate the microstructure of the coating, which basically involves including fields of stresses and interfaces to the interior of the material, in such a way that they act as centers of dispersion of the phonons. In the study conducted by Soyez et al. [21] on YSZ nanocrystalline, the dependence of the thermal conductivity with the grain size in nanocrystals of YSZ, for films with thicknesses of 0.5 and 1.2 μm and yttria compositions between 8 and 15 mol.%, was observed. Another approach to the effect of the structure variation to micro- and nanoscale is to use multilayers, since the value of the coating thermal resistance in the form of multilayers is the sum in a series of the thermal resistances of the interfaces where the interaction between the phonon and the scattering centers that are in them occurs. For applications at high temperatures, coatings can be designed in nanostructured multilayer sequences. For example, studies have been conducted on multilayer systems of Al2O3/YSZ obtained via EB-PVD and multilayer YSZ/SiO2 [22] obtained via ion beam-PVD, but the thermal conductivity measurements obtained show no significant decreases, regardless of the materials, the technique, and the number of layers used [23]. In addition to the grain size and the generation of interfaces using multilayer systems, the effect of the thickness of the coating on its thermal conductivity must be taken into account. In the model proposed by Nicholls [24], two characteristic zones of the ceramic coatings obtained by the EB-PVD technique are presented: the internal zone of fine grain and the external zone of coarse grain. The thermal conductivity of the internal zone of fine grain is much lower than the thermal conductivity of the external zone, due to a greater density of grain boundaries as well as to numerous oblique columnar limits in the internal zone, since there is a multiple nucleation and subsequent growth of the columnar microstructure. Thus, the thermal conductivity in this area is dominated by the dispersion of phonons with defects and grain boundaries in this part of the coating, resulting in a low thermal conductivity of around 1.0 W/m K at room temperature. In this way, the total thermal conductivity of the coating will be the result of the combined effect of these two zones, so if the total thickness of the coating is equal to the thickness of the internal zone, the total thermal conductivity will be equal to that of this area; therefore, it will be lower. Obtaining this type of TBC microstructure can be achieved using the "shuttering" method, which consists in the periodic interruption of the vapor flow of atoms. Using this technique Wolfe et al. [25] reported a 10% decrease in the thermal conductivity of 8YSZ coatings deposited by EB-PVD. Summarized, creating imperfections within the network, the phonons free path

Impact of the Glancing Angle Deposition on the Yttria-Stabilized Zirconia Growth and Their Thermal Barrier…

http://dx.doi.org/10.5772/intechopen.81905

151

could change producing a greater dispersion and decreasing the thermal conductivity.

In the same way, another important experimental parameter used to modify the micro- and nanostructure is to change the direction of the incident flux of the depositing species respecting the substrate surface. Most of films produced by PVD techniques use normal incidence and lead to the columnar growth normal to the substrate. Depositions at oblique angles or sculptured thin films were first reported in 1959 [26] and later by others [27–29] and are often referred to as glancing angle deposition (GLAD). The structure is achieved when the substrate is tilted and forms a high angle between the material flux and substrate surface. In this way, in the microstructure, the column growth direction follows the orientation of the material flux,

For the thermal, oxidation, and hot corrosion protection of high-temperature components, the multifunctional requirements of these coatings dictate the use of a coating system consisting of three separate layers: a porous, 7–8 wt.% yttria-stabilized zirconia (7–8YSZ) thermal barrier coating (TBC) which provides thermal insulation, a thermally grown (α-alumina) oxide (TGO) layer which inhibits oxygen transport to the component, and a low-sulfur platinum aluminide or MCrAlY (where M is Ni or Co) bond coat [4, 5]. Usually, the TBC thick layer (the thickness is typically of hundreds of microns) is deposited either by air plasma spray (APS) [6] or electron beam physical vapor deposition (EB-PVD) [7]. Low-pressure plasma spray (LPPS) [5] or pack cementation [8] is used to apply the bond coat. Oxidation of the bond coat prior to or during deposition of the TBC layer (and later during service) forms the (1 mm) TGO layer. However, for other applications the YSZ TBC thin films can be deposited by sputtering and pulsed laser deposition (PLD). In both cases, the typical columnar structure of sputter or pulsed laser deposited films is largely influenced, among other parameters, by pressure, temperature, thickness, and the ion-to-atom ratio incident at the substrate or substrate bias voltage [9–11]. This morphology is expected to follow the structure zone model of Thornton [12] and could be overcome by an increase of the deposition temperature.

YSZ is the currently preferred TBC layer material for gas turbine engine applications because of its low thermal conductivity, k, its relatively high (compared to other ceramics) thermal expansion coefficient, and its good erosion resistance [13]. The low thermal conductivity of bulk YSZ is a result of the low intrinsic thermal conductivity of zirconia and the addition of yttria [14]. An yttria concentration in the range of 6–8 wt.% is generally used since this composition maximizes spallation life due to the formation of the metastable t´ phase [13]. This phase yields a complex microstructure which resist crack propagation and transformation into the monoclinic phase (4% volume change) upon cooling. The result is a thermomechanically tough TBC layer with a room temperature, grain size dependent, and thermal conductivity of 2.2–2.6 W/m K in the densest (bulk) form [15]. The thermal protection and spallation lifetimes of YSZ TBC layers produced via different deposition techniques differ significantly. TBC coatings produced by APS have a thermal conductivity in the range of 0.8–1.0 W/m K at 25C [1, 14, 16]. This is significantly lower than the 1.5–1.9 W/m K reported for EB-PVD coatings at 25C [1, 17]. On the other hand, for YSZ thin films obtained by different techniques, the thermal conductivity values depend on the grain size, and these are in the range between 0.6 and 1.8 W/m K for grain sizes between 10 and 100 nm [18, 19].

Usually, there are two ways to reduce the thermal conductivity of YSZ TBC obtained by physical vapor deposition (PVD). The first one is the addition of dopants, in this case the 1. Introduction

150 Coatings and Thin-Film Technologies

Yttria-stabilized zirconia (YSZ) coating systems are widely used for the thermal, oxidation, and hot corrosion protection of high-temperature components in gas turbine and diesel engines [1], and, additionally, the electrolyte YSZ is the standard ionic conductor [2] used in fuel cells, being a brittle material due to its high hardness [3]. This thermal and oxidation protection must be achieved without incurring excessive thermomechanical loading of the

For the thermal, oxidation, and hot corrosion protection of high-temperature components, the multifunctional requirements of these coatings dictate the use of a coating system consisting of three separate layers: a porous, 7–8 wt.% yttria-stabilized zirconia (7–8YSZ) thermal barrier coating (TBC) which provides thermal insulation, a thermally grown (α-alumina) oxide (TGO) layer which inhibits oxygen transport to the component, and a low-sulfur platinum aluminide or MCrAlY (where M is Ni or Co) bond coat [4, 5]. Usually, the TBC thick layer (the thickness is typically of hundreds of microns) is deposited either by air plasma spray (APS) [6] or electron beam physical vapor deposition (EB-PVD) [7]. Low-pressure plasma spray (LPPS) [5] or pack cementation [8] is used to apply the bond coat. Oxidation of the bond coat prior to or during deposition of the TBC layer (and later during service) forms the (1 mm) TGO layer. However, for other applications the YSZ TBC thin films can be deposited by sputtering and pulsed laser deposition (PLD). In both cases, the typical columnar structure of sputter or pulsed laser deposited films is largely influenced, among other parameters, by pressure, temperature, thickness, and the ion-to-atom ratio incident at the substrate or substrate bias voltage [9–11]. This morphology is expected to follow the structure zone model of Thornton [12] and could be

YSZ is the currently preferred TBC layer material for gas turbine engine applications because of its low thermal conductivity, k, its relatively high (compared to other ceramics) thermal expansion coefficient, and its good erosion resistance [13]. The low thermal conductivity of bulk YSZ is a result of the low intrinsic thermal conductivity of zirconia and the addition of yttria [14]. An yttria concentration in the range of 6–8 wt.% is generally used since this

phase yields a complex microstructure which resist crack propagation and transformation into the monoclinic phase (4% volume change) upon cooling. The result is a thermomechanically tough TBC layer with a room temperature, grain size dependent, and thermal conductivity of 2.2–2.6 W/m K in the densest (bulk) form [15]. The thermal protection and spallation lifetimes of YSZ TBC layers produced via different deposition techniques differ significantly. TBC coatings produced by APS have a thermal conductivity in the range of 0.8–1.0 W/m K at 25C [1, 14, 16]. This is significantly lower than the 1.5–1.9 W/m K reported for EB-PVD coatings at 25C [1, 17]. On the other hand, for YSZ thin films obtained by different techniques, the thermal conductivity values depend on the grain size, and these are in the range between 0.6

Usually, there are two ways to reduce the thermal conductivity of YSZ TBC obtained by physical vapor deposition (PVD). The first one is the addition of dopants, in this case the

phase [13]. This

composition maximizes spallation life due to the formation of the metastable t´

coating system and the metal component to which it is applied.

overcome by an increase of the deposition temperature.

and 1.8 W/m K for grain sizes between 10 and 100 nm [18, 19].

addition of rare earth oxides (REO). Klemens et al. [20] conclude that co-doping with REO can solve the problems concerning the high-temperature (tetragonal) phase stability of ZrO2 and important decreases in thermal conductivity can be achieved by approaching values obtained by APS (k = 0.8 W/mK). The second way is to manipulate the microstructure of the coating, which basically involves including fields of stresses and interfaces to the interior of the material, in such a way that they act as centers of dispersion of the phonons. In the study conducted by Soyez et al. [21] on YSZ nanocrystalline, the dependence of the thermal conductivity with the grain size in nanocrystals of YSZ, for films with thicknesses of 0.5 and 1.2 μm and yttria compositions between 8 and 15 mol.%, was observed. Another approach to the effect of the structure variation to micro- and nanoscale is to use multilayers, since the value of the coating thermal resistance in the form of multilayers is the sum in a series of the thermal resistances of the interfaces where the interaction between the phonon and the scattering centers that are in them occurs. For applications at high temperatures, coatings can be designed in nanostructured multilayer sequences. For example, studies have been conducted on multilayer systems of Al2O3/YSZ obtained via EB-PVD and multilayer YSZ/SiO2 [22] obtained via ion beam-PVD, but the thermal conductivity measurements obtained show no significant decreases, regardless of the materials, the technique, and the number of layers used [23]. In addition to the grain size and the generation of interfaces using multilayer systems, the effect of the thickness of the coating on its thermal conductivity must be taken into account. In the model proposed by Nicholls [24], two characteristic zones of the ceramic coatings obtained by the EB-PVD technique are presented: the internal zone of fine grain and the external zone of coarse grain. The thermal conductivity of the internal zone of fine grain is much lower than the thermal conductivity of the external zone, due to a greater density of grain boundaries as well as to numerous oblique columnar limits in the internal zone, since there is a multiple nucleation and subsequent growth of the columnar microstructure. Thus, the thermal conductivity in this area is dominated by the dispersion of phonons with defects and grain boundaries in this part of the coating, resulting in a low thermal conductivity of around 1.0 W/m K at room temperature. In this way, the total thermal conductivity of the coating will be the result of the combined effect of these two zones, so if the total thickness of the coating is equal to the thickness of the internal zone, the total thermal conductivity will be equal to that of this area; therefore, it will be lower. Obtaining this type of TBC microstructure can be achieved using the "shuttering" method, which consists in the periodic interruption of the vapor flow of atoms. Using this technique Wolfe et al. [25] reported a 10% decrease in the thermal conductivity of 8YSZ coatings deposited by EB-PVD. Summarized, creating imperfections within the network, the phonons free path could change producing a greater dispersion and decreasing the thermal conductivity.

In the same way, another important experimental parameter used to modify the micro- and nanostructure is to change the direction of the incident flux of the depositing species respecting the substrate surface. Most of films produced by PVD techniques use normal incidence and lead to the columnar growth normal to the substrate. Depositions at oblique angles or sculptured thin films were first reported in 1959 [26] and later by others [27–29] and are often referred to as glancing angle deposition (GLAD). The structure is achieved when the substrate is tilted and forms a high angle between the material flux and substrate surface. In this way, in the microstructure, the column growth direction follows the orientation of the material flux, typically performed by directional deposition techniques, such as PVD. Several studies have been conducted to elucidate the influence of the thin-film microstructure grown by PVD under GLAD technique on the morphology and structure [30, 31] of different thin-film materials, as well as on their mechanical, [3] electrical [31], and optical properties [33, 34]. On the other hand, previously Hass et al. [35] conducted a study with a TBC layer deposited by electron beam evaporation technique (EB-PVD), but placing the substrate inclined with respect to the vapor flux, to obtain a zigzag-shaped pore microstructure that greatly reduced the thermal conductivity.

columns, where some grow at the expense of the adjacent ones. The randomness of the columns results in inhomogeneous properties of the film in the plane parallel to the substrate, while the competition between the growing columns makes the films nonuniform in the direction along the normal to the substrate. Since the shading effect is the main mechanism, the higher values of α lead to a more pronounced porosity. This occurs because the shading effect generates areas where the vapor flow cannot directly reach the nuclei of atoms on the surface and, therefore, the shadow effect is widely favored [36] leading to a porous columnar microstructure of isolated grains and inclined toward the vapor source. Therefore, the columns do not grow parallel to the direction of the incident vapor flow, and the microstructure tilt can be generated by changing the substrate inclination angle. Figure 2 shows a schematic illustra-

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To achieve a glancing angle deposition, the geometry of a conventional sputtering setup was modified similar to some studies reported by others [31]. For this study, the deposition angle of the substrate was fixed at 45o with respect to the incident flux. Additionally, in-plane rotations of 180o were performed to obtain a "zigzag"-like morphology and stop the deposition when the rotation was completed. Figure 3(a) and (b) shows the experimental setup used in the present study for the in-plane rotations of 180<sup>o</sup> to obtain "zigzag"-like growth morphology

Figure 2. Schematic illustrations of the growth mechanism of micro-columnar structures by OAD: (a) nucleation, (b)

Figure 3. (a) Experimental setup used for the in-plane 180o rotations to obtain "zigzag"-like growth morphology. (b) Scanning electron microscopy (SEM) cross-sectional view of YSZ thin film grown under this configuration at a period of

tion of the growth mechanism of micro-columnar structures.

onset of self-shadowing, and (c) micro-columnar growth.

n = 1.

In this chapter we present the sculpturing of YSZ thin films using radio-frequency (r.f.) magnetron sputtering under oblique incidence with respect to the normal substrate surface.

### 2. Glancing angle deposition technique

In the last decades, the physical deposition in vapor phase (PVD) of films and coatings at different incident angles of the steam flow has arisen as an alternative for the control of the morphology, distribution, and shape of the pores present throughout the thickness of the coating as a result of a "shading" effect which is influenced by the angle of incidence of the atoms arriving on the substrate. All this leads to a challenge from the technological point of view focused on obtaining an oblique angle deposit in situ, with sufficient versatility and reproducibility that allows obtaining coatings with "customized" microstructures for various applications. This approach is commonly referred to as glancing angle deposition (GLAD) or oblique angle deposition (OAD), which in this case results in YSZ coatings with an inclined columnar microstructure. In order to obtain this microstructure, the substrate is inclined at an angle (α) with respect to the vapor incident flow on the substrate plane, as it can be seen schematically in Figure 1.

When the coatings are deposited on substrates of low roughness, the films obtained by OAD consist of randomly distributed columns with strong competition between the growing

Figure 1. Geometry of oblique angle deposition.

columns, where some grow at the expense of the adjacent ones. The randomness of the columns results in inhomogeneous properties of the film in the plane parallel to the substrate, while the competition between the growing columns makes the films nonuniform in the direction along the normal to the substrate. Since the shading effect is the main mechanism, the higher values of α lead to a more pronounced porosity. This occurs because the shading effect generates areas where the vapor flow cannot directly reach the nuclei of atoms on the surface and, therefore, the shadow effect is widely favored [36] leading to a porous columnar microstructure of isolated grains and inclined toward the vapor source. Therefore, the columns do not grow parallel to the direction of the incident vapor flow, and the microstructure tilt can be generated by changing the substrate inclination angle. Figure 2 shows a schematic illustration of the growth mechanism of micro-columnar structures.

typically performed by directional deposition techniques, such as PVD. Several studies have been conducted to elucidate the influence of the thin-film microstructure grown by PVD under GLAD technique on the morphology and structure [30, 31] of different thin-film materials, as well as on their mechanical, [3] electrical [31], and optical properties [33, 34]. On the other hand, previously Hass et al. [35] conducted a study with a TBC layer deposited by electron beam evaporation technique (EB-PVD), but placing the substrate inclined with respect to the vapor flux, to obtain a zigzag-shaped pore microstructure that greatly reduced the thermal

In this chapter we present the sculpturing of YSZ thin films using radio-frequency (r.f.) magnetron sputtering under oblique incidence with respect to the normal substrate surface.

In the last decades, the physical deposition in vapor phase (PVD) of films and coatings at different incident angles of the steam flow has arisen as an alternative for the control of the morphology, distribution, and shape of the pores present throughout the thickness of the coating as a result of a "shading" effect which is influenced by the angle of incidence of the atoms arriving on the substrate. All this leads to a challenge from the technological point of view focused on obtaining an oblique angle deposit in situ, with sufficient versatility and reproducibility that allows obtaining coatings with "customized" microstructures for various applications. This approach is commonly referred to as glancing angle deposition (GLAD) or oblique angle deposition (OAD), which in this case results in YSZ coatings with an inclined columnar microstructure. In order to obtain this microstructure, the substrate is inclined at an angle (α) with respect to the vapor incident flow on the substrate plane, as it can be seen

When the coatings are deposited on substrates of low roughness, the films obtained by OAD consist of randomly distributed columns with strong competition between the growing

conductivity.

152 Coatings and Thin-Film Technologies

schematically in Figure 1.

Figure 1. Geometry of oblique angle deposition.

2. Glancing angle deposition technique

To achieve a glancing angle deposition, the geometry of a conventional sputtering setup was modified similar to some studies reported by others [31]. For this study, the deposition angle of the substrate was fixed at 45o with respect to the incident flux. Additionally, in-plane rotations of 180o were performed to obtain a "zigzag"-like morphology and stop the deposition when the rotation was completed. Figure 3(a) and (b) shows the experimental setup used in the present study for the in-plane rotations of 180<sup>o</sup> to obtain "zigzag"-like growth morphology

Figure 2. Schematic illustrations of the growth mechanism of micro-columnar structures by OAD: (a) nucleation, (b) onset of self-shadowing, and (c) micro-columnar growth.

Figure 3. (a) Experimental setup used for the in-plane 180o rotations to obtain "zigzag"-like growth morphology. (b) Scanning electron microscopy (SEM) cross-sectional view of YSZ thin film grown under this configuration at a period of n = 1.

material studied in this work. In general, the thermal conductivity of a material may depend on the temperature; however, in this work, all measurements of the thermal conductivity in

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Among the techniques used for the determination of thermal transport, parameters are the techniques in steady-state and transient or frequency-dependent techniques. In the first case, the thermal conductivity is directly determined, and in the second case, the thermal diffusivity (α) is measured, through which the thermal conductivity (k) can be estimated if the density (r) and the specific heat (cp) of the sample under study are known, by means of the expression α = k/rcp.

Within the transient and frequency-dependent techniques are the laser-flash technique [38], thermal lens spectroscopy [39], photoacoustic spectroscopy in its different modalities of open cell [40], closed cell [41], two-beam cell [42], etc.; as well as the photoelectric spectroscopy [43]. These techniques are very appropriate in the case of bulk samples, but they have their instrumental complication and limitations in the case of thin coatings. The techniques of 3w [44] and thermoreflectance [45] are the most used in the determination of the thermal conductivity of coatings and thin films, for which a sophisticated instrumentation is required in the case of thermoreflectance and additionally adequate preparation of the small metallic elements deposited on the material to

In order to find the thin film's thermal conductivity value, a hot plate technique as an appropriate thermal conductivity measurement system was used [46]. In this technique, it was assumed that the transfer of heat is by conduction through the YSZ film; the thermal conductivity measurement experimental setup is shown in Figure 5. The sample is placed on the heater, which increases the temperature to 373 K, where it remains stable, until that the heat reservoir comes into contact with the sample and then the heat is transferred from the heater to the heat reservoir through the sample; this variation is sensed using solid-state sensors.

Considering the heat energy conservation law in the thermal system as presented in Figure 5, it is possible to obtain the thermal power differences between heater and heat reservoir as following:

PH � PR ¼ 0 (1)

be studied to heat and temperature monitoring in the case of the 3w technique.

Figure 5. Thermal conductivity measurement experimental setup [46].

YSZ were made at room temperature.

Figure 4. (a) 3D model of the designed device. (b) Illustrative diagram of the device inside the vacuum deposition chamber.

and the SEM cross-sectional view of YSZ thin film grown under this configuration at a period of n = 1, respectively.

However, in practice the substrate rotation must be controlled by a mechanism that allows the transmission of movement inside the vacuum chamber and at the same time be operative for the deposit conditions such as pressure, temperature, bias voltage, etc. To achieve this goal, it was necessary to design a mechanism that would allow to transmit this movement but without modifying the location of the substrate surface with respect to the flow of evaporated material, since if this occurs, the substrate would be in an area that would be outside of the material flow affecting the deposition rate and the shading effect. To solve this problem, a device was designed [37] based on a cylinder with three axes and two bearings that can withstand high temperatures. On the other hand, to provide greater stability during the substrate holder movement, a tie is added consisting of a threaded rod which passes through axis 3 and is coupled to a sheet on the upper part of the device, which is not in contact with axis 1 (see Figure 4a). Figure 4b illustrates the device inside the vacuum deposition chamber, indicating the arrangement that allows the application of a polarizing voltage to the substrate (bias voltage).

A detailed description of the experimental procedure and deposition parameters used for 8 mol.% YSZ TBC film growth was previously reported by Amaya et al. [19]. To obtain the "zigzag" structure, initially the period (n) like the repetition unit composed by two layers was defined, each grown with an angle of +45o and �45o , respectively, and the spatial period (Λ), the bilayer thickness. We systematically varied n (1, 2, 10, 30, 50, and 70), keeping the total thickness (3.50 μm) of the multilayer constant. For this reason, the spatial period will be smaller when n increases.

### 3. Thermal conductivity determination

High thermal conductivity materials are widely used in heat dissipation applications, and materials with low thermal conductivity are used as thermal insulators, for example, the YSZ material studied in this work. In general, the thermal conductivity of a material may depend on the temperature; however, in this work, all measurements of the thermal conductivity in YSZ were made at room temperature.

Among the techniques used for the determination of thermal transport, parameters are the techniques in steady-state and transient or frequency-dependent techniques. In the first case, the thermal conductivity is directly determined, and in the second case, the thermal diffusivity (α) is measured, through which the thermal conductivity (k) can be estimated if the density (r) and the specific heat (cp) of the sample under study are known, by means of the expression α = k/rcp.

Within the transient and frequency-dependent techniques are the laser-flash technique [38], thermal lens spectroscopy [39], photoacoustic spectroscopy in its different modalities of open cell [40], closed cell [41], two-beam cell [42], etc.; as well as the photoelectric spectroscopy [43]. These techniques are very appropriate in the case of bulk samples, but they have their instrumental complication and limitations in the case of thin coatings. The techniques of 3w [44] and thermoreflectance [45] are the most used in the determination of the thermal conductivity of coatings and thin films, for which a sophisticated instrumentation is required in the case of thermoreflectance and additionally adequate preparation of the small metallic elements deposited on the material to be studied to heat and temperature monitoring in the case of the 3w technique.

In order to find the thin film's thermal conductivity value, a hot plate technique as an appropriate thermal conductivity measurement system was used [46]. In this technique, it was assumed that the transfer of heat is by conduction through the YSZ film; the thermal conductivity measurement experimental setup is shown in Figure 5. The sample is placed on the heater, which increases the temperature to 373 K, where it remains stable, until that the heat reservoir comes into contact with the sample and then the heat is transferred from the heater to the heat reservoir through the sample; this variation is sensed using solid-state sensors.

Considering the heat energy conservation law in the thermal system as presented in Figure 5, it is possible to obtain the thermal power differences between heater and heat reservoir as following:

$$P\_H - P\_R = 0\tag{1}$$

Figure 5. Thermal conductivity measurement experimental setup [46].

and the SEM cross-sectional view of YSZ thin film grown under this configuration at a period

Figure 4. (a) 3D model of the designed device. (b) Illustrative diagram of the device inside the vacuum deposition chamber.

However, in practice the substrate rotation must be controlled by a mechanism that allows the transmission of movement inside the vacuum chamber and at the same time be operative for the deposit conditions such as pressure, temperature, bias voltage, etc. To achieve this goal, it was necessary to design a mechanism that would allow to transmit this movement but without modifying the location of the substrate surface with respect to the flow of evaporated material, since if this occurs, the substrate would be in an area that would be outside of the material flow affecting the deposition rate and the shading effect. To solve this problem, a device was designed [37] based on a cylinder with three axes and two bearings that can withstand high temperatures. On the other hand, to provide greater stability during the substrate holder movement, a tie is added consisting of a threaded rod which passes through axis 3 and is coupled to a sheet on the upper part of the device, which is not in contact with axis 1 (see Figure 4a). Figure 4b illustrates the device inside the vacuum deposition chamber, indicating the arrangement that allows the

A detailed description of the experimental procedure and deposition parameters used for 8 mol.% YSZ TBC film growth was previously reported by Amaya et al. [19]. To obtain the "zigzag" structure, initially the period (n) like the repetition unit composed by two layers was

the bilayer thickness. We systematically varied n (1, 2, 10, 30, 50, and 70), keeping the total thickness (3.50 μm) of the multilayer constant. For this reason, the spatial period will be

High thermal conductivity materials are widely used in heat dissipation applications, and materials with low thermal conductivity are used as thermal insulators, for example, the YSZ

, respectively, and the spatial period (Λ),

application of a polarizing voltage to the substrate (bias voltage).

defined, each grown with an angle of +45o and �45o

3. Thermal conductivity determination

of n = 1, respectively.

154 Coatings and Thin-Film Technologies

smaller when n increases.

where these thermal powers are given by:

$$P\_H = \frac{(T\_\odot - T)KA}{l} \quad P\_R = cM\frac{dT}{dt} \tag{2}$$

where TC and T are the heater temperature and the variation of temperature, respectively; K is the thermal conductivity of the material; l and A are the thickness and area of the sample, respectively; and M and c are the mass and the specific heat of the heat reservoir, respectively.

Equating these thermal powers and solving the first-order differential expression, the temperature evolution of the process can be described, given by

$$
\Delta T\_R = T\_H e^{-\dagger\_{\tau}} + T\_S \tag{3}
$$

where ΔTR, TH, and TS are the temperature variation in the heat reservoir, the temperature in the heater, and the temperature in the sample, respectively, and t and τ are the time and the inverse of the slope, which is directly related to the thermal conductivity of the sample. Therefore, the thermal conductivity of the material can be calculated as

$$K = \frac{lcM}{\tau A} \tag{4}$$

Then, expression (7) can be written as

measured by using expression (4).

4.1. Film morphology

lT KT

Figure 6. Schematic representation of the heat flow through the YSZ/glass double-layer sample.

<sup>¼</sup> lYSZ KYSZ þ lGlass KGlass

finally, from expression (8) the thermal conductivity in YSZ film can be determined as.

KYSZ <sup>¼</sup> lYSZ

4. Effect of glancing angle deposition on the YSZ film properties

Here, lT and KT are the total thickness and the total thermal conductivity, respectively. And

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Expression (9) requires knowing the thermal conductivity value of glass substrate, previously

To determine the thermal conductivity of the 8YSZ coatings, these were deposited on glass substrates of known thermal conductivity values by using the procedure described by expression (9).

Cross-sectional images were carried out in a JEOL JSM-6490LV™ scanning electron microscopy. To show the effect of the oblique angle deposition on the microstructure of the thin films, a series of cross-sectional SEM images (Figure 7) were obtained by cleaving the samples

ð Þ� lT=KT ð Þ lGlass=KGlass ½ � (9)

(8)

157

The effective thermal conductivity in the samples as film growth over substrates can be obtained by using the double-layer method [47, 48]. In this method, the total thermal resistance is the sum of the film and substrate thermal resistances, and knowing the sample geometry and thermal conductivity of the substrate, it is possible to calculate the effective value of the thermal conductivity in the film as was proposed by Mansanares et al. [47, 48].

The thermal conductivity of YSZ over glass substrates can be determined by considering the double-layer model as follows.

Figure 6 shows a schematic representation of the total thermal resistance by the superposition of thermal resistances between YSZ film and glass substrate. This representation can be written as the sum of thermal resistances [46–48]:

$$R\_T = R\_{YSZ} + R\_{\text{Glass}} \tag{5}$$

Here, RT, RYSZ, and RGlass are the total, YSZ film, and glass thermal resistances, respectively. Each thermal resistance depends on geometry (its thickness and area A) and thermal conductivity of the sample; then, expression (5) is given by [46–48]:

$$R\_T = \frac{l\_{YSZ}}{K\_{YSZ}} A + \frac{l\_{Glas}}{K\_{Glas}} A\tag{6}$$

where lYSZ, lGlass and KYSZ, KGlass are the YSZ and glass thicknesses and thermal conductivities, respectively. The total thermal resistance also is

$$\frac{l\_T}{K\_T \ A} = \frac{l\_{\rm YSZ}}{K\_{\rm YSZ} \ A} + \frac{l\_{\rm Glass}}{K\_{\rm Glass} \ A} \tag{7}$$

Impact of the Glancing Angle Deposition on the Yttria-Stabilized Zirconia Growth and Their Thermal Barrier… http://dx.doi.org/10.5772/intechopen.81905 157

Figure 6. Schematic representation of the heat flow through the YSZ/glass double-layer sample.

Then, expression (7) can be written as

where these thermal powers are given by:

156 Coatings and Thin-Film Technologies

double-layer model as follows.

as the sum of thermal resistances [46–48]:

tivity of the sample; then, expression (5) is given by [46–48]:

respectively. The total thermal resistance also is

ature evolution of the process can be described, given by

PH <sup>¼</sup> ð Þ TC � <sup>T</sup> KA

<sup>l</sup> PR <sup>¼</sup> cM dT

where TC and T are the heater temperature and the variation of temperature, respectively; K is the thermal conductivity of the material; l and A are the thickness and area of the sample, respectively; and M and c are the mass and the specific heat of the heat reservoir, respectively. Equating these thermal powers and solving the first-order differential expression, the temper-

where ΔTR, TH, and TS are the temperature variation in the heat reservoir, the temperature in the heater, and the temperature in the sample, respectively, and t and τ are the time and the inverse of the slope, which is directly related to the thermal conductivity of the sample.

<sup>K</sup> <sup>¼</sup> lcM

The effective thermal conductivity in the samples as film growth over substrates can be obtained by using the double-layer method [47, 48]. In this method, the total thermal resistance is the sum of the film and substrate thermal resistances, and knowing the sample geometry and thermal conductivity of the substrate, it is possible to calculate the effective value of the

The thermal conductivity of YSZ over glass substrates can be determined by considering the

Figure 6 shows a schematic representation of the total thermal resistance by the superposition of thermal resistances between YSZ film and glass substrate. This representation can be written

Here, RT, RYSZ, and RGlass are the total, YSZ film, and glass thermal resistances, respectively. Each thermal resistance depends on geometry (its thickness and area A) and thermal conduc-

KYSZ <sup>A</sup> <sup>þ</sup>

where lYSZ, lGlass and KYSZ, KGlass are the YSZ and glass thicknesses and thermal conductivities,

KYSZ <sup>A</sup> <sup>þ</sup>

lGlass

lGlass

RT <sup>¼</sup> lYSZ

lT KT <sup>A</sup> <sup>¼</sup> lYSZ

ΔTR ¼ THe

Therefore, the thermal conductivity of the material can be calculated as

thermal conductivity in the film as was proposed by Mansanares et al. [47, 48].

dt (2)

�t=<sup>τ</sup> <sup>þ</sup> TS (3)

<sup>τ</sup><sup>A</sup> (4)

RT ¼ RYSZ þ RGlass (5)

KGlass <sup>A</sup> (6)

KGlass <sup>A</sup> (7)

$$\frac{l\_T}{K\_T} = \frac{l\_{YSZ}}{K\_{YSZ}} + \frac{l\_{Glas}}{K\_{Glas}} \tag{8}$$

Here, lT and KT are the total thickness and the total thermal conductivity, respectively. And finally, from expression (8) the thermal conductivity in YSZ film can be determined as.

$$K\_{\rm YSZ} = \frac{l\_{\rm YSZ}}{[(l\_T/K\_T) - (l\_{\rm Glass}/K\_{\rm Glass})]} \tag{9}$$

Expression (9) requires knowing the thermal conductivity value of glass substrate, previously measured by using expression (4).

To determine the thermal conductivity of the 8YSZ coatings, these were deposited on glass substrates of known thermal conductivity values by using the procedure described by expression (9).

### 4. Effect of glancing angle deposition on the YSZ film properties

#### 4.1. Film morphology

Cross-sectional images were carried out in a JEOL JSM-6490LV™ scanning electron microscopy. To show the effect of the oblique angle deposition on the microstructure of the thin films, a series of cross-sectional SEM images (Figure 7) were obtained by cleaving the samples parallel to the grown "zigzag" structure. Due to YSZ non-conductive nature, a 5-nm gold layer was deposited to avoid charge accumulation during the measurements. A cross-sectional SEM image of an 8YSZ thin film grown with the conventional PVD geometry is shown as a reference in Figure 7(a), where we can identify the parallel columnar structure normal to the surface, according to the Thornton diagram [12].

By applying an inclination of 45 between the vapor flow and the surface normal, the microstructure depicted in Figure 7(b) was obtained. The columnar structure is preserved but tilted toward the direction of the plasma plume with a column width similar to the reference sample. By turning the sample by 180 after half of the deposition, the column growth direction was also turned in-plane by 180 (Figure 7(c)). This "zigzag" structure is repeated without any degradation of the well-aligned columnar structure, as seen in Figure 7(d). In Figures 7(c) and 7(d), we observe that the thicknesses of the column growth to the left are larger than to the right. This is because the thickness of the column growth depends greatly on the sample position in the sample holder (see Figure 3a).

However, when the number of repetitions increased to 10 (n = 10) and more (n = 30), then the "zigzag" structure appears within the columns, but the columns itself practically are not inclined, as we see in Figure 8(a–d). In our case, the total coating thickness for all repetitions remains approximately constant (close to 3.5 μm), and for this reason, the time between repetitions is very short when n increases, for hence the columns do not reach to tilt as a whole.

4.2. Transmission electron microscopy analysis

nanostructure within the columns [49].

(HRTEM) mode [50].

In order to analyze the samples further, transmission electron microscopy (TEM) images recorded in cross-sectional lamellae of an 8YSZ thin film were carried out. Lamellae were prepared by focused ion beam (FIB) technique using a FEI Helios NanoLab 600i. TEM images were taken in a dedicated Hitachi HF-3300 (I2TEM-Toulouse) microscope operated at 300 kV. This microscope is equipped with a cold field emission gun and an image-aberration corrector (B-COR from CEOS), achieving a spatial resolution of 80 pm, in high-resolution TEM

Figure 8. SEM cross-sectional views of YSZ thin films grown for (a) n = 10 and (b) n = 30; (c) and (d) provide details of the

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Figure 9(a) displays an overview image for an 8YSZ multilayer with n = 10. We can see the substrate-multilayer interface and the zigzag microstructure. The total thickness of the multilayer is around 3.5 μm. In Figure 9(b), we show a magnified area of the TEM image of Figure 9(a) around the kink (dashed lines) of the zigzag structure. Apparently, all columns continue to grow through the kink without any evidence of a discontinuous crystal structure as we can see in the high-resolution transmission electron microscope (HRTEM) micrographs (Figure 9(c) and (d)) corresponding to the zones labeled with red numbers 1 and 2 in Figure 9(b). In addition, the contrast diffraction observed in such TEM images reveals that the oblique growth method induces the inclination of the crystallographic planes at α = 45 and α = 45 in the "thin" and

Figure 7. SEM cross-sectional views of YSZ thin films grown under (a) perpendicular incidence of the vapor flow to the substrate and (b)–(d) 45<sup>o</sup> of incidence [49].

Impact of the Glancing Angle Deposition on the Yttria-Stabilized Zirconia Growth and Their Thermal Barrier… http://dx.doi.org/10.5772/intechopen.81905 159

Figure 8. SEM cross-sectional views of YSZ thin films grown for (a) n = 10 and (b) n = 30; (c) and (d) provide details of the nanostructure within the columns [49].

### 4.2. Transmission electron microscopy analysis

parallel to the grown "zigzag" structure. Due to YSZ non-conductive nature, a 5-nm gold layer was deposited to avoid charge accumulation during the measurements. A cross-sectional SEM image of an 8YSZ thin film grown with the conventional PVD geometry is shown as a reference in Figure 7(a), where we can identify the parallel columnar structure normal to the

By applying an inclination of 45 between the vapor flow and the surface normal, the microstructure depicted in Figure 7(b) was obtained. The columnar structure is preserved but tilted toward the direction of the plasma plume with a column width similar to the reference sample. By turning the sample by 180 after half of the deposition, the column growth direction was also turned in-plane by 180 (Figure 7(c)). This "zigzag" structure is repeated without any degradation of the well-aligned columnar structure, as seen in Figure 7(d). In Figures 7(c) and 7(d), we observe that the thicknesses of the column growth to the left are larger than to the right. This is because the thickness of the column growth depends greatly on the sample

However, when the number of repetitions increased to 10 (n = 10) and more (n = 30), then the "zigzag" structure appears within the columns, but the columns itself practically are not inclined, as we see in Figure 8(a–d). In our case, the total coating thickness for all repetitions remains approximately constant (close to 3.5 μm), and for this reason, the time between repetitions is very short when n increases, for hence the columns do not reach to tilt as a whole.

Figure 7. SEM cross-sectional views of YSZ thin films grown under (a) perpendicular incidence of the vapor flow to the

surface, according to the Thornton diagram [12].

158 Coatings and Thin-Film Technologies

position in the sample holder (see Figure 3a).

substrate and (b)–(d) 45<sup>o</sup> of incidence [49].

In order to analyze the samples further, transmission electron microscopy (TEM) images recorded in cross-sectional lamellae of an 8YSZ thin film were carried out. Lamellae were prepared by focused ion beam (FIB) technique using a FEI Helios NanoLab 600i. TEM images were taken in a dedicated Hitachi HF-3300 (I2TEM-Toulouse) microscope operated at 300 kV. This microscope is equipped with a cold field emission gun and an image-aberration corrector (B-COR from CEOS), achieving a spatial resolution of 80 pm, in high-resolution TEM (HRTEM) mode [50].

Figure 9(a) displays an overview image for an 8YSZ multilayer with n = 10. We can see the substrate-multilayer interface and the zigzag microstructure. The total thickness of the multilayer is around 3.5 μm. In Figure 9(b), we show a magnified area of the TEM image of Figure 9(a) around the kink (dashed lines) of the zigzag structure. Apparently, all columns continue to grow through the kink without any evidence of a discontinuous crystal structure as we can see in the high-resolution transmission electron microscope (HRTEM) micrographs (Figure 9(c) and (d)) corresponding to the zones labeled with red numbers 1 and 2 in Figure 9(b). In addition, the contrast diffraction observed in such TEM images reveals that the oblique growth method induces the inclination of the crystallographic planes at α = 45 and α = 45 in the "thin" and

micrographs. From the TEM image in Figure 9b, this angle was calculated as 75.94 for the +45

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For the microstructural analysis, X-ray diffraction (XRD) measurements were performed using a powder diffractometer Panalytical X'Pert PRO™ with a Cu Kα radiation source (λ = 1.54184 Å). Figure 10a and 11a present the 8YSZ XRD patterns recorded in Bragg– Brentano geometry, for thin films deposited at a normal incident angle (α = 0) and at different repetition numbers of the "zigzag" structure (from n = 1 to n = 70), respectively. The XRD pattern of the 8YSZ film deposited at a normal incident angle (α = 0) shows a mixture of two phases: tetragonal zirconium yttrium oxide (Zr0.94Y0.06O1.88) and monoclinic baddeleyite (ZrO2) (JCPDF #01–089-9068 and #01–070-8739 cards, respectively). The preferred orientation on (002) plane of tetragonal phase is detected when n = 1, and its relative diffracted intensity increases gradually with n (Figure 10a). Moreover, (211) also shows a significantly preferred orientation of samples with n = 1, 2, and 10 "zigzag" arrays. The presence of silicon reflections of the substrate is very clear in the XRD pattern of the film growth at a normal incident angle and those with n = 1, 2, and 70. Samples with a great number of n (i.e., n = 30, 50, and 70) evolve toward a mono-axial preferential orientation on (002) plane; there is then a concomitant loss of orientation from pyramidal facets of {112} form. This latter behavior is a probable consequence of the increment of structural defects—disordered vacancies on Zr sites that affect

Figure 10. (a) XRD patterns of 8YSZ thin films deposited at a normal incident angle (α = 0) and at different repetition

numbers of the "zigzag" structure. (b) Schematic crystallographic simulation of the "zigzag" structure [49].

orientation.

4.3. X-ray diffraction microstructural analysis

Figure 9. TEM micrographs for FIB lamella of a "zigzag"-structured YSZ thin film. (a) Low-magnification TEM image of the "zigzag" structure for n = 10. (b) Zoom around the dashed line in the low-magnification micrograph (a). HRTEM micrographs for the thickest region (c) labeled 1 in (b) and the thinnest (d) for the region labeled 2 in (b). Fast Fourier transform (FFT) pattern for thick (e) and thin (f) layers [49].

"thick" layers of the zigzag structure, respectively. Moreover, crystal diffraction patterns (Figure 9 (e) and (f)) obtained by applying a fast Fourier transform (FFT) on Figures 9(c) and 5(d), respectively, show that the microstructure of the YSZ films has a textured crystalline structure where some crystal planes are well defined in the HRTEM image. The angle that forms the twin can be determined from the angles that form the pyramidal faces to each other, in the images of TEM micrographs. From the TEM image in Figure 9b, this angle was calculated as 75.94 for the +45 orientation.

### 4.3. X-ray diffraction microstructural analysis

"thick" layers of the zigzag structure, respectively. Moreover, crystal diffraction patterns (Figure 9 (e) and (f)) obtained by applying a fast Fourier transform (FFT) on Figures 9(c) and 5(d), respectively, show that the microstructure of the YSZ films has a textured crystalline structure where some crystal planes are well defined in the HRTEM image. The angle that forms the twin can be determined from the angles that form the pyramidal faces to each other, in the images of TEM

Figure 9. TEM micrographs for FIB lamella of a "zigzag"-structured YSZ thin film. (a) Low-magnification TEM image of the "zigzag" structure for n = 10. (b) Zoom around the dashed line in the low-magnification micrograph (a). HRTEM micrographs for the thickest region (c) labeled 1 in (b) and the thinnest (d) for the region labeled 2 in (b). Fast Fourier

transform (FFT) pattern for thick (e) and thin (f) layers [49].

160 Coatings and Thin-Film Technologies

For the microstructural analysis, X-ray diffraction (XRD) measurements were performed using a powder diffractometer Panalytical X'Pert PRO™ with a Cu Kα radiation source (λ = 1.54184 Å). Figure 10a and 11a present the 8YSZ XRD patterns recorded in Bragg– Brentano geometry, for thin films deposited at a normal incident angle (α = 0) and at different repetition numbers of the "zigzag" structure (from n = 1 to n = 70), respectively. The XRD pattern of the 8YSZ film deposited at a normal incident angle (α = 0) shows a mixture of two phases: tetragonal zirconium yttrium oxide (Zr0.94Y0.06O1.88) and monoclinic baddeleyite (ZrO2) (JCPDF #01–089-9068 and #01–070-8739 cards, respectively). The preferred orientation on (002) plane of tetragonal phase is detected when n = 1, and its relative diffracted intensity increases gradually with n (Figure 10a). Moreover, (211) also shows a significantly preferred orientation of samples with n = 1, 2, and 10 "zigzag" arrays. The presence of silicon reflections of the substrate is very clear in the XRD pattern of the film growth at a normal incident angle and those with n = 1, 2, and 70. Samples with a great number of n (i.e., n = 30, 50, and 70) evolve toward a mono-axial preferential orientation on (002) plane; there is then a concomitant loss of orientation from pyramidal facets of {112} form. This latter behavior is a probable consequence of the increment of structural defects—disordered vacancies on Zr sites that affect

Figure 10. (a) XRD patterns of 8YSZ thin films deposited at a normal incident angle (α = 0) and at different repetition numbers of the "zigzag" structure. (b) Schematic crystallographic simulation of the "zigzag" structure [49].

of columnar twinned crystals is broken by profusion of defects as nano-porous, the relative intensity of (211) diminishes. This packing efficiency loss is clearly related to the occurrence of Si peaks in n = 70 XRD pattern. On the other hand, the enhancement of the intensity of twin plane (002) is a function of the number of tilt changes or "zigzag" structures. Accordingly,

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Thus, in our experiment, we obtain a polysynthetic twin. When α = 0, neither preferred

To determine the thermal conductivity of the 8YSZ coatings, these were deposited on glass substrates of known thermal conductivity values by using the procedure described in paragraph 3. Figure 12(a) and (b) present the typical temperature evolution curves for the films deposited at a normal incident angle (α = 0) and at n = 10 number repetition of "zigzag" structure. In these curves, we see excellent agreement between the experimental data (black circles) and the fit to Eq. (3) (red curves). For the temperature evolution curves, three measurements for each sample

From this fit, it is possible to obtain τ, which is directly related to the thermal conductivity (K) of the sample through Eq. (4) via procedure established by expression (9). Then, the effective thermal conductivity in the samples as film growth over glass substrates is obtained by using

In Figure 12(c) summary of thermal conductivity behavior in all 8YSZ samples, for different values of α and n, is presented. For α = 0, the thermal conductivity (K) presents a value of 0.74 0.05 W/mK, similar to that reported by Amaya et al. [19] of 0.57 0.06 W/mK. Both values are for 8YSZ coatings grown via r.f. sputtering at equal deposition conditions with density, thermal diffusivity, and specific heat separately determined. As shown in Figure 12(c), for 8YSZ coatings deposited with "zigzag" structure, the thermal conductivity (κ) drastically decreases in an order of magnitude when the number of bilayers n increases. However, for

Figure 12. Typical temperature evolution curves for the films deposited at (a) normal incident angle (α = 0) and (b) n = 10 number repetition of "zigzag" structure. (c) Thermal conductivity evolution for α (0) (normal incident angle), α<sup>1</sup> (45), α<sup>2</sup>

patterns show a dependence on the number of twin planes present in the film.

orientation nor silicon-dependent 8YSZ epitaxial growth has been observed.

were carried out, and the tolerance temperature of the measurements was 1 K.

the double-layer method according to Eq. (9) [47, 48].

n = 70, the thermal conductivity starts to increase.

(45), and n ranging from 1 to 70 repetitions of the "zigzag" structure [49].

4.4. Thermal conductivity behavior

Figure 11. (a) XRD experimental data for films deposited at a normal incident angle (α = 0), n = 1 and n = 2 repetition numbers of the "zigzag" structure. (b) The schematic simulated crystallographic structure obtained from the experimental data [49].

directly the intensity of (211) reflection—as well as the profusion of both nano-porous and microstructural dislocations; thus, subsequent vertical columnar arrangement in the stacking arises, with the corresponding enhancement of the intensity on (002) diffraction peak.

Taking into account the predominance of the tetragonal structure and the multiaxial model of preferred orientations, as well as the "zigzag" features observed in SEM and TEM microphotographs for samples obtained from inclined experiments, a crystallographic simulation of the "zigzag" structure is proposed and graphically shown in Figure 10b and 11b. The proposed microstructural model is based on the key role of a specific substrate tilt (e.g., α = 45) in promoting the growth of the tetragonal {211} bipyramidal facets on one side of the {001} planes, for example, (002) plane. The (002) is the twin plane in a contact twin where individual crystals are related by an inversion (i.e., 1). The growing direction is [001], and the surface of the twinned crystals is formed by {211} facets. In accordance with this scheme, when the tilt angle of the substrate is changed by 90 (from +45 to 45), the {001} planes act as twin boundaries, with the subsequent growth of the pyramidal faces in a parallel but opposite direction. It is worth to note that a difference in length between both arms of twinned crystals is due to a different time of deposition.

The arrangement of the "zigzag" structures within the same level or, in other words, along with the direction [010], consists of a stacking of contiguous {211} facets. That is to say, the elbows are inter grooved along the direction of the bisector of the angle between two facets of the {211} form, for example, (2–11) ^ (211) in Figure 11b, where complete bipyramidal morphologies are represented. The elbow angle results in 145.44o and the (002) ^ (211) is 72.72. When the stacking of columnar twinned crystals is broken by profusion of defects as nano-porous, the relative intensity of (211) diminishes. This packing efficiency loss is clearly related to the occurrence of Si peaks in n = 70 XRD pattern. On the other hand, the enhancement of the intensity of twin plane (002) is a function of the number of tilt changes or "zigzag" structures. Accordingly, patterns show a dependence on the number of twin planes present in the film.

Thus, in our experiment, we obtain a polysynthetic twin. When α = 0, neither preferred orientation nor silicon-dependent 8YSZ epitaxial growth has been observed.

### 4.4. Thermal conductivity behavior

directly the intensity of (211) reflection—as well as the profusion of both nano-porous and microstructural dislocations; thus, subsequent vertical columnar arrangement in the stacking

Figure 11. (a) XRD experimental data for films deposited at a normal incident angle (α = 0), n = 1 and n = 2 repetition numbers of the "zigzag" structure. (b) The schematic simulated crystallographic structure obtained from the experimental

Taking into account the predominance of the tetragonal structure and the multiaxial model of preferred orientations, as well as the "zigzag" features observed in SEM and TEM microphotographs for samples obtained from inclined experiments, a crystallographic simulation of the "zigzag" structure is proposed and graphically shown in Figure 10b and 11b. The proposed microstructural model is based on the key role of a specific substrate tilt (e.g., α = 45) in promoting the growth of the tetragonal {211} bipyramidal facets on one side of the {001} planes, for example, (002) plane. The (002) is the twin plane in a contact twin where individual crystals are related by an inversion (i.e., 1). The growing direction is [001], and the surface of the twinned crystals is formed by {211} facets. In accordance with this scheme, when the tilt angle of the substrate is changed by 90 (from +45 to 45), the {001} planes act as twin boundaries, with the subsequent growth of the pyramidal faces in a parallel but opposite direction. It is worth to note that a difference in length between both arms of twinned crystals

The arrangement of the "zigzag" structures within the same level or, in other words, along with the direction [010], consists of a stacking of contiguous {211} facets. That is to say, the elbows are inter grooved along the direction of the bisector of the angle between two facets of the {211} form, for example, (2–11) ^ (211) in Figure 11b, where complete bipyramidal morphologies are represented. The elbow angle results in 145.44o and the (002) ^ (211) is 72.72. When the stacking

arises, with the corresponding enhancement of the intensity on (002) diffraction peak.

is due to a different time of deposition.

data [49].

162 Coatings and Thin-Film Technologies

To determine the thermal conductivity of the 8YSZ coatings, these were deposited on glass substrates of known thermal conductivity values by using the procedure described in paragraph 3. Figure 12(a) and (b) present the typical temperature evolution curves for the films deposited at a normal incident angle (α = 0) and at n = 10 number repetition of "zigzag" structure. In these curves, we see excellent agreement between the experimental data (black circles) and the fit to Eq. (3) (red curves). For the temperature evolution curves, three measurements for each sample were carried out, and the tolerance temperature of the measurements was 1 K.

From this fit, it is possible to obtain τ, which is directly related to the thermal conductivity (K) of the sample through Eq. (4) via procedure established by expression (9). Then, the effective thermal conductivity in the samples as film growth over glass substrates is obtained by using the double-layer method according to Eq. (9) [47, 48].

In Figure 12(c) summary of thermal conductivity behavior in all 8YSZ samples, for different values of α and n, is presented. For α = 0, the thermal conductivity (K) presents a value of 0.74 0.05 W/mK, similar to that reported by Amaya et al. [19] of 0.57 0.06 W/mK. Both values are for 8YSZ coatings grown via r.f. sputtering at equal deposition conditions with density, thermal diffusivity, and specific heat separately determined. As shown in Figure 12(c), for 8YSZ coatings deposited with "zigzag" structure, the thermal conductivity (κ) drastically decreases in an order of magnitude when the number of bilayers n increases. However, for n = 70, the thermal conductivity starts to increase.

Figure 12. Typical temperature evolution curves for the films deposited at (a) normal incident angle (α = 0) and (b) n = 10 number repetition of "zigzag" structure. (c) Thermal conductivity evolution for α (0) (normal incident angle), α<sup>1</sup> (45), α<sup>2</sup> (45), and n ranging from 1 to 70 repetitions of the "zigzag" structure [49].

to this, either the interfaces or twist boundaries of the same material can play a critical role in nanoscale thermal transport [51, 52]. In this sense, we propose a phenomenological interpretation to explain the reduction in thermal conductivity when the value of (n) for the "zigzag" columns increases, with respect to the thermal conductivity of the coating deposited with a growth of

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Figure 13-I shows the parallel thermal flux incidence with respect to the growth of columns for the 8YSZ coatings deposited with the normal direction of the incident flux of the species, with respect to the substrate surface. In contrast, Figure 13-II(A) at n < 10 samples indicates that when coatings are deposited under an oblique angle, a "zigzag" structure appears creating inclined crystallographic interfaces. Taking the column as an individual element, (α = 0), the heat flow is distributed in a longitudinal direction due to the growth direction that is normal to the substrate; therefore, the phonon scattering is due to the presence of the oxygen vacancies. When the oblique angle deposition takes place (n = 1), there is a change in the growth direction of the column, and as a result of this, an interface is generated, with which the heat carriers interact, scattering the heat flow into two components, a vertical component and a lateral component, due to the inclusion of the zigzag microstructure. This makes the mean free path of phonons to diminish due to the interaction of heat carriers with these interfaces, thus

However, at n ≥ 10 (Figure 13-II(B)), the zigzag structure appears within the columns, which are practically not inclined but emerge the formation of nanopores between columns, and crystallographic interfaces contribute to greater phonon dispersion. Finally, increased thermal conductivity for n = 70 may be related to the fact that the total coating thickness for all repetitions remains approximately constant (close to 3.5 μm). For this reason, the time between repetitions is quite short, and the columns do not reach tilt as a whole, increasing the nanopore

The oblique angle deposition is a powerful technique to modify the microstructure of PVD coatings deposited by sputtering in the micro- and nanometer scale, with high reproducibility and repeatability as the period (n) varied. For low values of repetition number (n = 10), the "zigzag" structure of 8YSZ coatings can be repeated without any degradation. However, when n increases from 10 to 70, the well-aligned columnar structure with "zigzag" microstructure has a columnar diameter refinement and the columns in the nanometer range growth perpendicular to the substrate surface, changing the orientation of porosity in the coatings. From the XRD and TEM analyses, it was established that twins are being produced to improve the thermal insulation properties. In fact, the thermal conductivity study allows to establish that the k value is strongly influenced by the "zigzag" microstructure of the PVD coatings, with a decrease of the thermal conductivity in an order of magnitude, when the columns change from normal growth orientation (α = 0) with respect to the substrate surface to a microstructure in a "zigzag" pattern with n = 50 repetitions, showing the potential of growing YSZ thin films in

columns normal to the substrate surface.

decreasing the thermal conductivity of the sample.

size; therefore, the mfp of phonons begins to increase again.

5. Conclusions

Figure 13. Schematic illustrations of (I) microstructure and porosity of PVD coatings and (II) influence of the "zigzag" microstructure on the YSZ coating thermal conductivity [49].

This thermal conductivity behavior of YSZ TBC is consistent with that obtained by Hass et al. [35], placing the substrate inclined with respect to the vapor flux to obtain a similar "zigzag" microstructure.

Heat thermal transport at nanometric scale is produced by phonons. Phonons have a wide variation in frequency and an even larger variation in their mean free paths (mfps). However, the bulk of the heat is often carried by phonons of a large wave vector, and they have mfps of 1–100 nm at room temperature.

Thereby, in many systems similar to those studied here, the scale of the phonon scattering centers has the same scale as the mfps of phonons, sometimes comparable to phonon wavelength. Due to this, either the interfaces or twist boundaries of the same material can play a critical role in nanoscale thermal transport [51, 52]. In this sense, we propose a phenomenological interpretation to explain the reduction in thermal conductivity when the value of (n) for the "zigzag" columns increases, with respect to the thermal conductivity of the coating deposited with a growth of columns normal to the substrate surface.

Figure 13-I shows the parallel thermal flux incidence with respect to the growth of columns for the 8YSZ coatings deposited with the normal direction of the incident flux of the species, with respect to the substrate surface. In contrast, Figure 13-II(A) at n < 10 samples indicates that when coatings are deposited under an oblique angle, a "zigzag" structure appears creating inclined crystallographic interfaces. Taking the column as an individual element, (α = 0), the heat flow is distributed in a longitudinal direction due to the growth direction that is normal to the substrate; therefore, the phonon scattering is due to the presence of the oxygen vacancies. When the oblique angle deposition takes place (n = 1), there is a change in the growth direction of the column, and as a result of this, an interface is generated, with which the heat carriers interact, scattering the heat flow into two components, a vertical component and a lateral component, due to the inclusion of the zigzag microstructure. This makes the mean free path of phonons to diminish due to the interaction of heat carriers with these interfaces, thus decreasing the thermal conductivity of the sample.

However, at n ≥ 10 (Figure 13-II(B)), the zigzag structure appears within the columns, which are practically not inclined but emerge the formation of nanopores between columns, and crystallographic interfaces contribute to greater phonon dispersion. Finally, increased thermal conductivity for n = 70 may be related to the fact that the total coating thickness for all repetitions remains approximately constant (close to 3.5 μm). For this reason, the time between repetitions is quite short, and the columns do not reach tilt as a whole, increasing the nanopore size; therefore, the mfp of phonons begins to increase again.

### 5. Conclusions

This thermal conductivity behavior of YSZ TBC is consistent with that obtained by Hass et al. [35], placing the substrate inclined with respect to the vapor flux to obtain a similar "zigzag"

Figure 13. Schematic illustrations of (I) microstructure and porosity of PVD coatings and (II) influence of the "zigzag"

Heat thermal transport at nanometric scale is produced by phonons. Phonons have a wide variation in frequency and an even larger variation in their mean free paths (mfps). However, the bulk of the heat is often carried by phonons of a large wave vector, and they have mfps of

Thereby, in many systems similar to those studied here, the scale of the phonon scattering centers has the same scale as the mfps of phonons, sometimes comparable to phonon wavelength. Due

microstructure.

164 Coatings and Thin-Film Technologies

1–100 nm at room temperature.

microstructure on the YSZ coating thermal conductivity [49].

The oblique angle deposition is a powerful technique to modify the microstructure of PVD coatings deposited by sputtering in the micro- and nanometer scale, with high reproducibility and repeatability as the period (n) varied. For low values of repetition number (n = 10), the "zigzag" structure of 8YSZ coatings can be repeated without any degradation. However, when n increases from 10 to 70, the well-aligned columnar structure with "zigzag" microstructure has a columnar diameter refinement and the columns in the nanometer range growth perpendicular to the substrate surface, changing the orientation of porosity in the coatings. From the XRD and TEM analyses, it was established that twins are being produced to improve the thermal insulation properties. In fact, the thermal conductivity study allows to establish that the k value is strongly influenced by the "zigzag" microstructure of the PVD coatings, with a decrease of the thermal conductivity in an order of magnitude, when the columns change from normal growth orientation (α = 0) with respect to the substrate surface to a microstructure in a "zigzag" pattern with n = 50 repetitions, showing the potential of growing YSZ thin films in glancing angle deposition, as an effective method to improve the thermal insulator property of this material.

[4] Wortman DJ, Nagaraj BA, Duderstadt EC. Thermal barrier coatings for gas turbine use.

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167

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### Acknowledgements

This research was supported by funds for internal calls for projects at the Universidad del Valle 2013 (CI 7923) and by the Center of Excellence for Novel Materials (CENM) and ASTIN-SENA, Colombia. The authors would like to thank Dr. Pedro Prieto from the CENM and Dr. Luis Yate, Platform Manager of CIC biomaGUNE at Donostia-San Sebastián, Spain, for the AFM analysis.

### Author details

Cesar Amaya1,2, John Jairo Prıas-Barragan<sup>3</sup> , Julio Cesar Caicedo<sup>4</sup> , Jose Martin Yañez-Limon<sup>5</sup> and Gustavo Zambrano1 \*

\*Address all correspondence to: gustavo.zambrano@correounivalle.edu.co

1 Department of Physics, Universidad del Valle, Cali, Colombia

2 Development of Materials and Products Research Group, CDT-ASTIN SENA, Cali, Colombia

3 Interdisciplinary Institute of Sciences and Electronic Instrumentation Technology Program, Universidad del Quindío, Armenia, Colombia

4 Tribology, Powder Metallurgy and Processing of Solid Recycled Research Group, Universidad del Valle, Cali, Colombia

5 Department of Materials Science and Engineering, Cinvestav-Unidad Querétaro, Querétaro, Mexico

### References


[4] Wortman DJ, Nagaraj BA, Duderstadt EC. Thermal barrier coatings for gas turbine use. Materials Science and Engineering. 1989;A121:443

glancing angle deposition, as an effective method to improve the thermal insulator property of

This research was supported by funds for internal calls for projects at the Universidad del Valle 2013 (CI 7923) and by the Center of Excellence for Novel Materials (CENM) and ASTIN-SENA, Colombia. The authors would like to thank Dr. Pedro Prieto from the CENM and Dr. Luis Yate, Platform Manager of CIC biomaGUNE at Donostia-San Sebastián, Spain, for the AFM

, Julio Cesar Caicedo<sup>4</sup>

, Jose Martin Yañez-Limon<sup>5</sup>

this material.

analysis.

Colombia

Mexico

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Author details

and Gustavo Zambrano1

Cesar Amaya1,2, John Jairo Prıas-Barragan<sup>3</sup>

Universidad del Quindío, Armenia, Colombia

Universidad del Valle, Cali, Colombia

\*

\*Address all correspondence to: gustavo.zambrano@correounivalle.edu.co

2 Development of Materials and Products Research Group, CDT-ASTIN SENA, Cali,

4 Tribology, Powder Metallurgy and Processing of Solid Recycled Research Group,

3 Interdisciplinary Institute of Sciences and Electronic Instrumentation Technology Program,

5 Department of Materials Science and Engineering, Cinvestav-Unidad Querétaro, Querétaro,

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[3] Fujikane M, Setoyama D, Nagao S, Nowak R, Yamanaka S. Nanoindentation examination of yttria-stabilized zirconia (YSZ) crystal. Journal of Alloys and Compounds. 2007;431

[2] Skinner SJ, Kilner JA. Oxygen ion conductors. Materials Today. 2003;6(3):30-37

1 Department of Physics, Universidad del Valle, Cali, Colombia

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[38] Baba T, Ono A. Improvement of the laser flash method to reduce uncertainty in thermal diffusivity measurements. Measurement Science and Technology. 2001;12:2046-2057

[39] Mayen Mondragon R, Yàñez-Limòn JM. Study of blue phases transition kinetics by thermal lens spectroscopy in cholesteryl nonanoate. The Review of Scientific Instruments.

[40] Alvarado-Gil JJ, Zelaya-Angel O, Sanchez-Sinencio F, Yáñez Limón JM, Vargas H, Figueroa JCC, et al. Photoacoustic monitoring of processing conditions in cooked tortillas:

[41] Rosencwaig A, Gersho A. Theory of the photoacoustic effect of solids. Journal of Applied

[42] Bento AC, Dias DT, Olenka L, Medina AN, Baesso ML. On the application of the photoacoustic methods for the determination of thermo-optical properties of polymers.

[43] Mandelis A, Zver MM. Theory of photopyroelectric spectroscopy of solids. Journal of

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[46] Prías-Barragán JJ, Muñoz-Gómez AP, Ariza-Calderón H. System for Measuring Thermal

Measurements of thermal diffusivity. Journal of Food Science. 1995;60:438-442

nano y micrométricas mediante técnicas PVD. Co-patent 14-185631; 2016

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[18] Kabacoff LT. Thermally sprayed nano-structured thermal barrier coatings. NATO Workshop on Thermal Barrier Coatings, Aalborg, Denmark, AGARD-R-823; 1998, paper 12

[19] Amaya C, Caicedo JC, Yáñez-Limón JM, Vargas RA, Zambrano G, Gomez ME, et al. A non-destructive method for determination of thermal conductivity of YSZ coatings depos-

[20] Klemens PG, Gell M. Thermal conductivity of thermal barrier coatings. Materials Science

[21] Soyez G, Eastman JA, Thompson LJ, Bai GR, Baldo PM. Grain-size-dependent thermal conductivity of nanocrystalline yttria-stabilized zirconia films grown by metal-organic

[22] Lee S-M, Matamis G, Cahill DG. Thin-film materials and minimum thermal conductivity.

[23] An K, Ravichandran KS, Dutton RE, Semiatin SL. Microstructure, texture, and thermal conductivity of single‐layer and multilayer thermal barrier coatings of Y2O3-stabilized ZrO2 and Al2O3 made by physical vapor deposition. Journal of the American Ceramic

[24] Nicholls JR, Lawson KJ, Johnstone A, Rickerby DS. Low thermal conductivity EB-PVD

[25] Wolfe DE, Singh J, Miller RA, Eldridge JI, Zhu DM. Tailored microstructure of EB-PVD 8YSZ thermal barrier coatings with low thermal conductivity and high thermal reflectivity

[26] Knorr TG, Hoffman RW. Dependence of geometric magnetic anisotropy in thin iron films.

[27] Robbie K, Sit JC, Brett MJ. Advanced techniques for glancing angle deposition. Journal of

[28] Malac M, Egerton R. Observations of the microscopic growth mechanism of pillars and helices formed by glancing-angle thin-film deposition. Journal of Vacuum Science &

[29] Lakhtakia A, Messier R. Sculptured thin films: Nanoengineered morphology and optics.

[30] Takadoum J, Lintymer J, Gavoille J, Martin N. Chromium multilayered thin films with

[31] Lintymer J, Gavoille J, Martin N, Takadoum J. Glancing angle deposition to modify microstructure and properties of sputter deposited chromium thin films. Surface and

for turbine applications. Surface and Coating Technology. 2005;190:132-149

thermal barrier coatings. Materials Science Forum. 2001;369(372):595

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**Chapter 9**

Provisional chapter

**Ti-Al-N-Based Hard Coatings: Thermodynamical**

Ti-Al-N-Based Hard Coatings: Thermodynamical

Florent Uny, Elisabeth Blanquet,

Florent Uny, Elisabeth Blanquet,

http://dx.doi.org/10.5772/intechopen.79747

Abstract

oxidation resistance

1. Introduction

Frédéric Schuster and Frédéric Sanchette

Frédéric Schuster and Frédéric Sanchette

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Background, CVD Deposition, and Properties. A Review**

DOI: 10.5772/intechopen.79747

For several decades, the increasing productivity in many industrial domains has led to a significant and ever-increased interest to protective and hard coatings. In this context, titanium-aluminum nitrides were developed and are now widely used in a large range of applications, due to their high hardness, good thermal stability, and oxidation resistance. This chapter reviews the thermodynamical characteristics of the Ti-Al-N system by reporting the progress made in the description of the Ti-Al-N phase diagram and the main mechanical and chemical properties of Ti1xAlxN-based coatings. As a metastable phase, the existence of the fcc-Ti1xAlxN depends on particular process parameters, allowing stabilizing this desirable solid solution. The influence of process parameters, with a particular interest for chemical vapor deposition (CVD) methods, on morphology and crystallographic structure is then described. The structure of Ti1xAlxN thin films depends also on the aluminum content as well as on the annealing temperature, due to the spinodal nature of the Ti-Al-N system. These changes of crystallographic structure can induce an improvement of the hardness, oxidation resistance, and wear behavior of these coatings. The main mechanical and chemical properties of physical vapor deposition

Keywords: TiAlN coatings, CVD, aluminum content dependence, mechanical properties,

Hard coatings started to be used at an industrial scale in the 1970s [1]. It was then titanium nitride (TiN) obtained by thermal CVD and deposited on tungsten carbide tools in order to

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

(PVD) and CVD Ti1xAlxN-based coatings are also described.

Background, CVD Deposition, and Properties. A Review


#### **Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review** Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review

DOI: 10.5772/intechopen.79747

Florent Uny, Elisabeth Blanquet, Frédéric Schuster and Frédéric Sanchette Florent Uny, Elisabeth Blanquet, Frédéric Schuster and Frédéric Sanchette

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79747

#### Abstract

[47] González de la Cruz G, Gurevich YG, Logvinov GN, Muñoz Aguirre N. Superficies Vacío.

[49] Amaya C, Prías-Barragán JJ, Aperador W, Hernández-Landaverde MA, Ramírez-Cardona M, Caicedo JC, et al. Thermal conductivity of yttria-stabilized zirconia thin films with a

[50] Snoeck E, Houdellier F, Taniguch Y, Masseboeuf A, Gatel C, Nicolai J, et al. Off-axial aberration correction using a B-COR for lorentz and HREM modes. Microscopy and

[51] Cahill DG, Ford WK, Goodson KE, Mahan GD, Majumdar A, Maris HJ, et al. Nanoscale

[52] Cahill DG, Braun PV, Chen G, Clarke DR, Fan S, Goodson KE, et al. Nanoscale thermal

[48] Mansanares AM, Bento AC, Vargas H, Leite NF. Physical Review B. 1995;42:4477

zigzag microstructure. Journal of Applied Physics. 2017;121:245110

thermal transport. Journal of Applied Physics. 2003;93:793

transport. II. 2003–2012. Applied Physics Reviews. 2014;1:011305

2000;10:40

170 Coatings and Thin-Film Technologies

Microanalysis. 2014;20(S3):932

For several decades, the increasing productivity in many industrial domains has led to a significant and ever-increased interest to protective and hard coatings. In this context, titanium-aluminum nitrides were developed and are now widely used in a large range of applications, due to their high hardness, good thermal stability, and oxidation resistance. This chapter reviews the thermodynamical characteristics of the Ti-Al-N system by reporting the progress made in the description of the Ti-Al-N phase diagram and the main mechanical and chemical properties of Ti1xAlxN-based coatings. As a metastable phase, the existence of the fcc-Ti1xAlxN depends on particular process parameters, allowing stabilizing this desirable solid solution. The influence of process parameters, with a particular interest for chemical vapor deposition (CVD) methods, on morphology and crystallographic structure is then described. The structure of Ti1xAlxN thin films depends also on the aluminum content as well as on the annealing temperature, due to the spinodal nature of the Ti-Al-N system. These changes of crystallographic structure can induce an improvement of the hardness, oxidation resistance, and wear behavior of these coatings. The main mechanical and chemical properties of physical vapor deposition (PVD) and CVD Ti1xAlxN-based coatings are also described.

Keywords: TiAlN coatings, CVD, aluminum content dependence, mechanical properties, oxidation resistance

### 1. Introduction

Hard coatings started to be used at an industrial scale in the 1970s [1]. It was then titanium nitride (TiN) obtained by thermal CVD and deposited on tungsten carbide tools in order to

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

improve not only the lifetime of tools but also their behavior during machining. Physical vapor deposition technologies (PVD) were used in the mid-1980s for this type of application. "Vide et Traitement Holding" started this activity in France in 1984 with cathodic arc deposition technology, now widely used in this field. Titanium and chromium nitrides were the first materials on the market in the field of coatings for cutting tools or molds. The evolution of deposition reactors and research increasingly active on this issue has led to coatings with multilayer architecture. Thus, titanium carbonitride (TiCN), whose layered architecture is performed by modulating the introduction of reactive gases, led, in the early 1990s, to a significant change of machining performances under severe conditions. The evolution of both machining techniques, high-speed machining for example, and materials to be machined has led to new alloy developments such as Ti1�xAlxN whose thermal stability and oxidation resistance, in particular, are better than that of TiN. Ti1�xAlxN films are thus now industrially used in a wide range of applications, from hard and barrier coatings on cutting tools or molds [2] to electronic devices or optical coatings [3]. Since the late 1990s, new architectures have been introduced on the hard coatings market. Indeed, the nanostructuring of deposited materials has allowed an increase of hardness while keeping Young's modulus and internal residual stresses at relatively low values. These nanostructured coatings are essentially divided into two kinds: at first, the heterostructures (superlattice, nanolayered films), stacks of two materials with thicknesses of nanometer size, and secondly, the nanocomposites consisting of transition metal nitride crystallites of a nanoscale size in a matrix (nanocrystalline or amorphous) of a nonmetal covalent nitride [4]. This review chapter is focused mainly on the Ti1�xAlxN material coatings. Thermodynamical basis and influence of the elaboration processes, especially the thermal CVD, on the structure of the coatings are discussed. The main properties (hardness, tribological properties, and oxidation resistance) of these coatings are finally described regarding their dependence on Al content and deposition temperature. The overall mechanisms related to these properties are also discussed. Information about other properties as thermal and electrical conductivity, hydrogen permeation, or thermal expansion, not presented here in details, should be found in [5–9].

### 2. Thermodynamical background

#### 2.1. Phase thermodynamics basis

Phase stability (or metastability) is generally described using the change in Gibb's free energy G as a function of the concentration c. When ð Þ ∂2G =∂c2 ≥ 0, the system is thermodynamically stable (or metastable). If ð Þ ∂2G =∂c2 < 0, the system is unstable and will decompose to form the stable phases. Actually, for a two-phase system (or more), the free energy of the system is described using a mixing term ΔGm. This free energy of mixing essentially defines the interactions between the atoms and so the ability of the system to form a solid solution or to segregate, i.e., to decompose into two distinct phases. This decomposition should be made by two different ways, depending on the value of the system's free energy. Figure 1 shows the decomposition domains for a mixing of two elements A and B associated to the free energy curve at the temperature T. The curves 1 and 2 delimit the domain of the binodal decomposition. Below the curve 2, the decomposition will be spinodal, and above the curve 1, the solid solution exists, regardless of the amount of the element B in the mixture. In the same way, on the G curve (G vs. chemical composition, Figure 1), a demixing domain is identified by the presence of a "hill" limited by two inflection points. Between these two points (points b and d on Figure 1), the demixing will be spinodal. Between the inflection points and the two minima of free energy (domains a-b and c-d on Figure 1), demixing will occur by nucleation-growth mechanism [10]. This G curve is associated to the demixing curves of the TiN-AlN system.

Figure 1. Schematic representation of a typical free energy curve at a temperature T with the associated decomposition domain. The two domains between the points "a-b" and "c-d" will show a nucleation-growth decomposition, while the

Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review

http://dx.doi.org/10.5772/intechopen.79747

173

Thermodynamic calculations have mainly aimed at stabilizing, experimentally, the ternary compounds as Ti3AlN, Ti3Al2N2, and particularly Ti2AlN, because of its Mn+1AXn nature (where M is a transition metal, A is an A-group element, and X could be C or N and 1 < n < 3) [11–14]. Many authors have reported phase diagrams with rather good agreement with

2.2. Ti-Al-N system

"b-d" domain is related to the spinodal decomposition [10].

2.2.1. Stable diagram

Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review http://dx.doi.org/10.5772/intechopen.79747 173

Figure 1. Schematic representation of a typical free energy curve at a temperature T with the associated decomposition domain. The two domains between the points "a-b" and "c-d" will show a nucleation-growth decomposition, while the "b-d" domain is related to the spinodal decomposition [10].

curve at the temperature T. The curves 1 and 2 delimit the domain of the binodal decomposition. Below the curve 2, the decomposition will be spinodal, and above the curve 1, the solid solution exists, regardless of the amount of the element B in the mixture. In the same way, on the G curve (G vs. chemical composition, Figure 1), a demixing domain is identified by the presence of a "hill" limited by two inflection points. Between these two points (points b and d on Figure 1), the demixing will be spinodal. Between the inflection points and the two minima of free energy (domains a-b and c-d on Figure 1), demixing will occur by nucleation-growth mechanism [10]. This G curve is associated to the demixing curves of the TiN-AlN system.

#### 2.2. Ti-Al-N system

improve not only the lifetime of tools but also their behavior during machining. Physical vapor deposition technologies (PVD) were used in the mid-1980s for this type of application. "Vide et Traitement Holding" started this activity in France in 1984 with cathodic arc deposition technology, now widely used in this field. Titanium and chromium nitrides were the first materials on the market in the field of coatings for cutting tools or molds. The evolution of deposition reactors and research increasingly active on this issue has led to coatings with multilayer architecture. Thus, titanium carbonitride (TiCN), whose layered architecture is performed by modulating the introduction of reactive gases, led, in the early 1990s, to a significant change of machining performances under severe conditions. The evolution of both machining techniques, high-speed machining for example, and materials to be machined has led to new alloy developments such as Ti1�xAlxN whose thermal stability and oxidation resistance, in particular, are better than that of TiN. Ti1�xAlxN films are thus now industrially used in a wide range of applications, from hard and barrier coatings on cutting tools or molds [2] to electronic devices or optical coatings [3]. Since the late 1990s, new architectures have been introduced on the hard coatings market. Indeed, the nanostructuring of deposited materials has allowed an increase of hardness while keeping Young's modulus and internal residual stresses at relatively low values. These nanostructured coatings are essentially divided into two kinds: at first, the heterostructures (superlattice, nanolayered films), stacks of two materials with thicknesses of nanometer size, and secondly, the nanocomposites consisting of transition metal nitride crystallites of a nanoscale size in a matrix (nanocrystalline or amorphous) of a nonmetal covalent nitride [4]. This review chapter is focused mainly on the Ti1�xAlxN material coatings. Thermodynamical basis and influence of the elaboration processes, especially the thermal CVD, on the structure of the coatings are discussed. The main properties (hardness, tribological properties, and oxidation resistance) of these coatings are finally described regarding their dependence on Al content and deposition temperature. The overall mechanisms related to these properties are also discussed. Information about other properties as thermal and electrical conductivity, hydrogen permeation, or thermal expansion,

Phase stability (or metastability) is generally described using the change in Gibb's free energy G as a function of the concentration c. When ð Þ ∂2G =∂c2 ≥ 0, the system is thermodynamically stable (or metastable). If ð Þ ∂2G =∂c2 < 0, the system is unstable and will decompose to form the stable phases. Actually, for a two-phase system (or more), the free energy of the system is described using a mixing term ΔGm. This free energy of mixing essentially defines the interactions between the atoms and so the ability of the system to form a solid solution or to segregate, i.e., to decompose into two distinct phases. This decomposition should be made by two different ways, depending on the value of the system's free energy. Figure 1 shows the decomposition domains for a mixing of two elements A and B associated to the free energy

not presented here in details, should be found in [5–9].

2. Thermodynamical background

2.1. Phase thermodynamics basis

172 Coatings and Thin-Film Technologies

#### 2.2.1. Stable diagram

Thermodynamic calculations have mainly aimed at stabilizing, experimentally, the ternary compounds as Ti3AlN, Ti3Al2N2, and particularly Ti2AlN, because of its Mn+1AXn nature (where M is a transition metal, A is an A-group element, and X could be C or N and 1 < n < 3) [11–14]. Many authors have reported phase diagrams with rather good agreement with

phase must spontaneously decompose into fcc-TiN and fcc-AlN. Then, with increasing time or temperature, fcc-AlN forms its hcp-AlN stable phase by germination and growth. This

Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review

http://dx.doi.org/10.5772/intechopen.79747

175

Considering that fcc-Ti1xAlxN solid solution is the result of the mixing of fcc-TiN and fcc-AlN, the thermodynamical modeling of this system is based on an equilibrium between these two phases with a total solubility. Anderbouhr et al. proposed thus a metastable ternary diagram of the Ti-Al-N system, showing the presence of the fcc-Ti1xAlxN phase on the whole range of aluminum content [20, 21]. Thermodynamic values for the lattice stability of fcc-AlN were based on experimental study led by Stolten et al. [17, 22]. However, experimental studies show that it is possible to deposit single-phased fcc-Ti1xAlxN with a maximum x ≈ 0.7 for PVD processes as magnetron sputtering [23, 24] or cathodic arc [25, 26] and up to x ≈ 0.9 for coatings deposited by thermal CVD [27] or PECVD [27–29] (with x defined as the Al/(Al + Ti) molar ratio). Above these values (x = 0.7 for PVD techniques and x = 0.9 for LPCVD), the deposition of a hexagonal phase occurs (hcp-Ti1 xAlxN for PVD techniques and hcp-AlN for CVD processes). Thus, it seems to be difficult to correlate the modeling of the Ti-Al-N ternary phase diagram and these evolutions of

Based on experimental results, a pseudobinary diagram (Figure 3) was proposed by Cremer et al. [30, 31] and confirmed by further experiments. This diagram is now widely accepted by many researchers. Although this diagram comes from experimental PVD data from films obtained by magnetron sputtering up to 700C, other data for higher temperatures were extrapolated and are not in agreement with the current results, where deposition of an fcc-Ti1xAlxN single-phased film was obtained by LPCVD and PECVD up to x = 0.9 [27, 28].

Figure 3. Metastable pseudobinary TiN-AlN diagram based on experimental results from films obtained by magnetron

implies that fcc-TiN and fcc-AlN are thermodynamically immiscible (see Figure 2).

structure with aluminum content.

sputtering up to 700C [31] (from experimental study in [30]).

Figure 2. Pseudobinary TiN-AlN stable phase diagram [16].

experimental results. Especially, Chen and Sundman achieved a quite complete modeling up to 2500C showing the evolution of the stability of these ternary compounds with temperature and establishing a thermodynamical database with a good reliability compared to experiments [15]. The pseudobinary diagram of the Ti-Al-N system is shown Figure 2 [16]. It is clearly shown that mixing of AlN and TiN results in a biphased domain composed of the stable fcc-TiN and hcp-AlN on the entire range of composition. It is only from 1250C that a little solubility of Al appears in the TiN structure.

#### 2.2.2. Metastable diagram

As said previously, the thermodynamic aspects of the Ti-Al-N system do not predict the existence of the solid solution, the solubility of Al in TiN is very low. It implies that the experimentally observed Ti1xAlxN solid solution is a metastable phase, i.e., it is kinetically favorable to stabilize it rather than the stable phases. The metastable Ti1xAlxN phases can exist under two structures: an fcc-solid solution (also known as B1 structure), considered as the mixing of the stable fcc-TiN and the metastable fcc-AlN, and an hcp-solid solution (B4 structure), considered as the mixing of the metastable hcp-TiN and the stable hcp-AlN. The deposition kinetic then hinders the demixing and allows to stabilize these solid solutions [17, 18].

In order to better understand and discuss about the existence of the Ti1xAl xN solid solution, a lot of thermodynamical studies, and more recently ab initio calculations, have been realized for several decades. These calculations show that the shape of the ΔGm curve clearly leads to demixing. More, the system is typically spinodal [19], i.e., the Ti1xAlxN phase must spontaneously decompose into fcc-TiN and fcc-AlN. Then, with increasing time or temperature, fcc-AlN forms its hcp-AlN stable phase by germination and growth. This implies that fcc-TiN and fcc-AlN are thermodynamically immiscible (see Figure 2).

Considering that fcc-Ti1xAlxN solid solution is the result of the mixing of fcc-TiN and fcc-AlN, the thermodynamical modeling of this system is based on an equilibrium between these two phases with a total solubility. Anderbouhr et al. proposed thus a metastable ternary diagram of the Ti-Al-N system, showing the presence of the fcc-Ti1xAlxN phase on the whole range of aluminum content [20, 21]. Thermodynamic values for the lattice stability of fcc-AlN were based on experimental study led by Stolten et al. [17, 22]. However, experimental studies show that it is possible to deposit single-phased fcc-Ti1xAlxN with a maximum x ≈ 0.7 for PVD processes as magnetron sputtering [23, 24] or cathodic arc [25, 26] and up to x ≈ 0.9 for coatings deposited by thermal CVD [27] or PECVD [27–29] (with x defined as the Al/(Al + Ti) molar ratio). Above these values (x = 0.7 for PVD techniques and x = 0.9 for LPCVD), the deposition of a hexagonal phase occurs (hcp-Ti1 xAlxN for PVD techniques and hcp-AlN for CVD processes). Thus, it seems to be difficult to correlate the modeling of the Ti-Al-N ternary phase diagram and these evolutions of structure with aluminum content.

Based on experimental results, a pseudobinary diagram (Figure 3) was proposed by Cremer et al. [30, 31] and confirmed by further experiments. This diagram is now widely accepted by many researchers. Although this diagram comes from experimental PVD data from films obtained by magnetron sputtering up to 700C, other data for higher temperatures were extrapolated and are not in agreement with the current results, where deposition of an fcc-Ti1xAlxN single-phased film was obtained by LPCVD and PECVD up to x = 0.9 [27, 28].

experimental results. Especially, Chen and Sundman achieved a quite complete modeling up to 2500C showing the evolution of the stability of these ternary compounds with temperature and establishing a thermodynamical database with a good reliability compared to experiments [15]. The pseudobinary diagram of the Ti-Al-N system is shown Figure 2 [16]. It is clearly shown that mixing of AlN and TiN results in a biphased domain composed of the stable fcc-TiN and hcp-AlN on the entire range of composition. It is only from 1250C that a little

As said previously, the thermodynamic aspects of the Ti-Al-N system do not predict the existence of the solid solution, the solubility of Al in TiN is very low. It implies that the experimentally observed Ti1xAlxN solid solution is a metastable phase, i.e., it is kinetically favorable to stabilize it rather than the stable phases. The metastable Ti1xAlxN phases can exist under two structures: an fcc-solid solution (also known as B1 structure), considered as the mixing of the stable fcc-TiN and the metastable fcc-AlN, and an hcp-solid solution (B4 structure), considered as the mixing of the metastable hcp-TiN and the stable hcp-AlN. The deposition kinetic then hinders the demixing and allows to stabilize these solid solutions [17, 18].

In order to better understand and discuss about the existence of the Ti1xAl xN solid solution, a lot of thermodynamical studies, and more recently ab initio calculations, have been realized for several decades. These calculations show that the shape of the ΔGm curve clearly leads to demixing. More, the system is typically spinodal [19], i.e., the Ti1xAlxN

solubility of Al appears in the TiN structure.

Figure 2. Pseudobinary TiN-AlN stable phase diagram [16].

2.2.2. Metastable diagram

174 Coatings and Thin-Film Technologies

Figure 3. Metastable pseudobinary TiN-AlN diagram based on experimental results from films obtained by magnetron sputtering up to 700C [31] (from experimental study in [30]).

#### 2.2.3. The spinodal nature of the Ti-Al-N system

As previously enounced, thermodynamical and ab initio calculations have clearly shown the spinodal nature of the Ti-Al-N system. They also demonstrated that this spinodal decomposition occurs in a wide range of temperature. In their study concerning demixing behavior of the fcc-Ti1�xAlxN structure, Mayrhofer et al. [32] have notably found that the free energy curve of the TiN-AlN mixture shows the presence of the spinodal decomposition up to 4000�C (Figure 4). Thus, the existence of the Ti1�xAlxN solid solution is thermodynamically not possible up to this temperature. However, recent calculations of a metastable phase diagram taking into account the influence of the lattice vibrations due to temperature on free energy of the fcc-Ti1�xAlxN structure showed a considerable reduction of maximum temperature of the demixing domain [33]. Nevertheless, this calculated temperature (≈2600�C) is still higher than the common temperature range used for deposition of coatings by CVD methods (600–1800�C).

Calculations are also focused on the chemical spinodal nature of Ti-Al-N in order to explain why the solid solution fcc-Ti1�xAlxN remains stable up to 700�C. Actually, spinodal decomposition is distinguished from nucleation and growth by the spontaneous isostructural demixing [19]. The total free energy evolutions, associated to the spinodal decomposition, depend on several factors, as shown in the Eq. (1) [34]:

$$
\Delta G = \frac{1}{2} \frac{d^2 G}{d\varepsilon^2} \Delta c^2 + \frac{K}{\lambda^2} \Delta c^2 + \frac{E}{1-\nu} \eta^2 V\_m \Delta c^2 \tag{1}
$$

decomposition, the second term describes the compositional fluctuations inside the material and represents then the driving force for the spinodal decomposition, and the last term is related to the strain energy associated to the formation of the two domains resulting from spinodal decomposition. During spinodal decomposition, the compositional fluctuations inside the material are amplified to form two isostructural phases with coherent or incoherent interfaces [35]. However, the formation of these interfaces is accompanied by strain energy, related to the lattice parameter misfit between the two phases formed after demixing (about 200–500 mJ/m2 for a coherent interface and 500–1000 mJ/m<sup>2</sup> for incoherent interfaces). According to the Hilliard and Cahn theory [34, 36], if this strain energy is sufficiently large, it is able to balance the driving force for spinodal decomposition (i.e., compositional fluctuations) and then, able to hinder spinodal decomposition. Thus, Zhang et al. show that spinodal decomposition in the Ti-Al-N system occurs exclusively if coherent domains are formed [34, 36]. This phenomenon is responsible of the age-hardening effect observed in Ti1xAlxN coatings. The relationship between the spinodal decomposition parameters (especially the wavelength of the compositional fluctuations) and the microstructure of the Ti-Al-N coatings was recently studied by mean of the Calphad technique with good agreement with the experimen-

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Alling et al. [38] studied the effect of pressure on the stability of the B1 structure. On one hand, they found that contact pressure of 20 GPa gives a weak increase of the isostructural mixing enthalpy for the fcc structure and then enhances slightly the spinodal decomposition. On the other hand, the pressure leads to a significant reduction of the mixing enthalpy of fcc and hcp structures and thus should delay the formation of the detrimental hexagonal structure for Alrich fcc-Ti1xAlxN during operations as machining, where coatings are subjected to high

As a metastable phase, the fcc-Ti1xAlxN solid solution tends to return to its stable state when temperature increases. The annealing process under vacuum thus leads to decomposition of the solid solutions, firstly by spinodal decomposition to form the two coherent fcc domains and then by nucleation of hcp-AlN. The spinodal decomposition is otherwise not always observed during annealing experiments and stable phases fcc-TiN and hcp-AlN can be directly formed, depending on annealing process [28]. Furthermore, the observation of the two cubic TiN and AlN phases, as a result of the spinodal decomposition, can be difficult due to the close position of some fcc-AlN X-ray diffraction responses with those of fcc-TiN. Recent studies on the coefficient of thermal expansion of Ti1xAlxN and CrAlN coatings have shown that the presence of the B4 structure (hcp-Ti1xAlxN or hcp-AlN) in the as-deposited state promotes the

The decomposition process is, as oxidation and hardness, highly dependent to the aluminum content of the as-deposited coatings. Studies show that films with high aluminum contents, and thus having a higher demixing energy than that of films with low aluminum contents, tend to decompose at lower temperature [23, 28]. However, depending on annealing parameters, the strain energy needed to the formation of the two cubic domains can retard the

further formation of these hexagonal phases during annealing [9].

tal results obtained in the literature [37].

temperatures and pressures.

2.2.4. Thermal stability

where Δc is the compositional variation c � c0, λ, the period of the compositional fluctuations, K, a constant that depends on the bonding energy of the atoms, E, the Young's modulus, ν, the Poisson's ratio, η, the lattice mismatch per unit of compositional variations η ¼ ð Þ 1=a ð Þ da=dc , and Vm, the molar volume. The first term refers to the change in free energy associated to the

Figure 4. ΔG curve of the TiN-AlN mixture as a function of the AlN content for different temperatures [32].

decomposition, the second term describes the compositional fluctuations inside the material and represents then the driving force for the spinodal decomposition, and the last term is related to the strain energy associated to the formation of the two domains resulting from spinodal decomposition. During spinodal decomposition, the compositional fluctuations inside the material are amplified to form two isostructural phases with coherent or incoherent interfaces [35]. However, the formation of these interfaces is accompanied by strain energy, related to the lattice parameter misfit between the two phases formed after demixing (about 200–500 mJ/m2 for a coherent interface and 500–1000 mJ/m2 for incoherent interfaces). According to the Hilliard and Cahn theory [34, 36], if this strain energy is sufficiently large, it is able to balance the driving force for spinodal decomposition (i.e., compositional fluctuations) and then, able to hinder spinodal decomposition. Thus, Zhang et al. show that spinodal decomposition in the Ti-Al-N system occurs exclusively if coherent domains are formed [34, 36]. This phenomenon is responsible of the age-hardening effect observed in Ti1xAlxN coatings. The relationship between the spinodal decomposition parameters (especially the wavelength of the compositional fluctuations) and the microstructure of the Ti-Al-N coatings was recently studied by mean of the Calphad technique with good agreement with the experimental results obtained in the literature [37].

Alling et al. [38] studied the effect of pressure on the stability of the B1 structure. On one hand, they found that contact pressure of 20 GPa gives a weak increase of the isostructural mixing enthalpy for the fcc structure and then enhances slightly the spinodal decomposition. On the other hand, the pressure leads to a significant reduction of the mixing enthalpy of fcc and hcp structures and thus should delay the formation of the detrimental hexagonal structure for Alrich fcc-Ti1xAlxN during operations as machining, where coatings are subjected to high temperatures and pressures.

### 2.2.4. Thermal stability

2.2.3. The spinodal nature of the Ti-Al-N system

176 Coatings and Thin-Film Technologies

several factors, as shown in the Eq. (1) [34]:

range used for deposition of coatings by CVD methods (600–1800�C).

<sup>Δ</sup><sup>G</sup> <sup>¼</sup> <sup>1</sup> 2 d2 G dc<sup>2</sup> <sup>Δ</sup><sup>c</sup> 2 þ K <sup>λ</sup><sup>2</sup> <sup>Δ</sup><sup>c</sup> 2 þ

As previously enounced, thermodynamical and ab initio calculations have clearly shown the spinodal nature of the Ti-Al-N system. They also demonstrated that this spinodal decomposition occurs in a wide range of temperature. In their study concerning demixing behavior of the fcc-Ti1�xAlxN structure, Mayrhofer et al. [32] have notably found that the free energy curve of the TiN-AlN mixture shows the presence of the spinodal decomposition up to 4000�C (Figure 4). Thus, the existence of the Ti1�xAlxN solid solution is thermodynamically not possible up to this temperature. However, recent calculations of a metastable phase diagram taking into account the influence of the lattice vibrations due to temperature on free energy of the fcc-Ti1�xAlxN structure showed a considerable reduction of maximum temperature of the demixing domain [33]. Nevertheless, this calculated temperature (≈2600�C) is still higher than the common temperature

Calculations are also focused on the chemical spinodal nature of Ti-Al-N in order to explain why the solid solution fcc-Ti1�xAlxN remains stable up to 700�C. Actually, spinodal decomposition is distinguished from nucleation and growth by the spontaneous isostructural demixing [19]. The total free energy evolutions, associated to the spinodal decomposition, depend on

where Δc is the compositional variation c � c0, λ, the period of the compositional fluctuations, K, a constant that depends on the bonding energy of the atoms, E, the Young's modulus, ν, the Poisson's ratio, η, the lattice mismatch per unit of compositional variations η ¼ ð Þ 1=a ð Þ da=dc , and Vm, the molar volume. The first term refers to the change in free energy associated to the

Figure 4. ΔG curve of the TiN-AlN mixture as a function of the AlN content for different temperatures [32].

E 1 � ν η2 VmΔc

<sup>2</sup> (1)

As a metastable phase, the fcc-Ti1xAlxN solid solution tends to return to its stable state when temperature increases. The annealing process under vacuum thus leads to decomposition of the solid solutions, firstly by spinodal decomposition to form the two coherent fcc domains and then by nucleation of hcp-AlN. The spinodal decomposition is otherwise not always observed during annealing experiments and stable phases fcc-TiN and hcp-AlN can be directly formed, depending on annealing process [28]. Furthermore, the observation of the two cubic TiN and AlN phases, as a result of the spinodal decomposition, can be difficult due to the close position of some fcc-AlN X-ray diffraction responses with those of fcc-TiN. Recent studies on the coefficient of thermal expansion of Ti1xAlxN and CrAlN coatings have shown that the presence of the B4 structure (hcp-Ti1xAlxN or hcp-AlN) in the as-deposited state promotes the further formation of these hexagonal phases during annealing [9].

The decomposition process is, as oxidation and hardness, highly dependent to the aluminum content of the as-deposited coatings. Studies show that films with high aluminum contents, and thus having a higher demixing energy than that of films with low aluminum contents, tend to decompose at lower temperature [23, 28]. However, depending on annealing parameters, the strain energy needed to the formation of the two cubic domains can retard the spinodal decomposition and then can increase the stability of the coating. Typically, the decomposition of the solid solution occurs at temperatures between 700 and 900C, depending on the aluminum content in the as-deposited coating [23, 39]. LPCVD Ti1xAlxN coatings with aluminum content of about x = 0.8 deposited by Endler in 2008 have shown a particularly high stability, with the first signs of decomposition at 1200C [27]. The reasons for this exceptional stability under vacuum conditions are not clearly defined. An increase of the thermal stability of Ti1xAlxN coatings can be achieved by adding elements as Si, Hf, or Ta [40–42].

of Ti0.05Al0.95N coatings, composed of an alternate of fcc-TiN/hcp-AlN nanolamellae embedded in a Al-rich Ti1xAlxN was thus found by Keckes et al. in 2013 [44]. More recently, nanocomposite structures constituted by Ti-rich fcc-Ti1xAlxN/Al-rich fcc-Ti1xAlxN nanolam-

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In the case of PVD, the deposition of a single-phase hcp-Ti1xAlxN is observed for aluminum contents above x ≈ 0.7. This phase is generally found to be detrimental for mechanical properties and oxidation resistance but some researchers found a better oxidation resistance for these

These microstructural changes are summarized in Figure 5. The structure obtained for different deposition processes (PVD, PECVD, and thermal CVD), associated to their out-of-equilibrium level, is defined as a function of the aluminum content in the Ti1xAlxN coatings. As fcc-Ti1xAlxN is a metastable phase, we could expect that higher aluminum content should be obtained for the process with the highest out-of-equilibrium level, so for PVD processes. However, as described above, higher aluminum content in the fcc phase is obtained for LPCVD and PECVD processes. Atmospheric pressure CVD (APCVD) process, near to the thermodynamic equilibrium, leads to relatively low aluminum contents in the fcc-Ti1xAlxN

Vacuum cathodic arc deposition (CAD) process [50–53] is probably the most widely used PVD technique on an industrial scale to prepare protective hard Ti1xAlxN-based coatings on cutting tools and forming molds [50, 54, 55]. The high ionization levels of cathodic arc

Figure 5. Crystallographic structures of Ti1xAlxN coatings obtained for APCD, LPCVD, PECVD, and PVD processes according to the aluminum content. "fcc" is related to the fcc-Ti1xAlxN solid solution; "fcc + hcp" refers to a mix of fcc-Ti1xAlxN + hcp-Ti1xAlxN for PVD coatings and to fcc-TiN + hcp-AlN for CVD and PECVD coatings; "hcp" refers to the

ellae (for 0.73 < x < 0.82) were also deposited in an industrial CVD facility [44, 47–49].

hcp-Ti1xAlxN coatings deposited by unbalanced magnetron sputtering [23].

3.2. Physical vapor deposition (PVD) of Ti1xAlxN coatings

hcp-Ti1xAlxN solid solution (data from [23, 27, 43, 46]).

phase (about x = 0.4) [43].

## 3. Deposition of Ti1xAlxN coatings

#### 3.1. Crystallographic structure

As briefly described earlier in the thermodynamical description of the Ti-Al-N system, asdeposited Ti1xAlxN coatings have commonly three distinct structures: fcc, hcp, and a mix of fcc and hcp. These crystallographic structures are obtained for both PVD and CVD coatings and show a strong dependence on the aluminum content. For lower aluminum content, aluminum atoms are considered to replace titanium atoms in the fcc-TiN structure and form the single-phase fcc-Ti1xAlxN. This phase is the desired one on tools for cutting operations, needing high hardness, high thermal stability, and good oxidation resistance.

By increasing aluminum content in the films, a biphased structure appears. These phases are cubic (fcc) and hexagonal (hcp), but their compositions depend on the deposition process. Concerning PVD processes, for which condensation of a vapor on a cold substrate occurs, the cooling rate are so high that deposition of metastable materials is expected. A mixture of fcc-Ti1xAlxN and hcp-Ti1xAlxN can be observed, as reported by Chen et al. [23] for magnetron sputtered coatings deposited at 500C and a working pressure of 0.4 Pa. These results are in agreement with the metastable diagram established by Cremer et al. (Figure 3). Concerning the thermal CVD process, performed at high temperatures and then closer to the thermodynamic equilibrium, a mixture of the stable phases fcc-TiN and hcp-AlN is generally reported [23, 27]. The presence of fcc-AlN, in addition to the fcc-TiN and hcp-AlN phases, was also found by Wagner et al. for APCVD coatings, forming a three-phase structure. The fcc-AlN formation was attributed to the spinodal decomposition of the fcc-Ti1xAlxN coatings, where an excess of Al atoms in the fcc-Ti1xAlxN solid solution leads to an increase of the demixing energy and then cause the formation of fcc-AlN [43]. The aluminum content threshold, at which this crystallographic transition occurs, depends on the deposition techniques and process parameters (up to about x = 0.7 for PVD and up to x = 0.9 for CVD and PECVD). Anyway, the presence of a mixture of these two phases is generally detrimental for both mechanical properties and oxidation resistance of the films [7, 25, 26], except if a nanocomposite is formed [44, 45]. This mixing of hcp and fcc structures remains up to x = 1 for coatings deposited by CVD and PECVD processes [27, 44–46]. It should be noted, however, that the achievable microstructures for LPCVD processes are a little bit more complex and very dependent of process parameters. As denoted above, even though fcc-Ti1xAlxN solid solutions were found up to x = 0.9, the presence of nanocomposite structures was recently reported. The deposition of Ti0.05Al0.95N coatings, composed of an alternate of fcc-TiN/hcp-AlN nanolamellae embedded in a Al-rich Ti1xAlxN was thus found by Keckes et al. in 2013 [44]. More recently, nanocomposite structures constituted by Ti-rich fcc-Ti1xAlxN/Al-rich fcc-Ti1xAlxN nanolamellae (for 0.73 < x < 0.82) were also deposited in an industrial CVD facility [44, 47–49].

In the case of PVD, the deposition of a single-phase hcp-Ti1xAlxN is observed for aluminum contents above x ≈ 0.7. This phase is generally found to be detrimental for mechanical properties and oxidation resistance but some researchers found a better oxidation resistance for these hcp-Ti1xAlxN coatings deposited by unbalanced magnetron sputtering [23].

These microstructural changes are summarized in Figure 5. The structure obtained for different deposition processes (PVD, PECVD, and thermal CVD), associated to their out-of-equilibrium level, is defined as a function of the aluminum content in the Ti1xAlxN coatings. As fcc-Ti1xAlxN is a metastable phase, we could expect that higher aluminum content should be obtained for the process with the highest out-of-equilibrium level, so for PVD processes. However, as described above, higher aluminum content in the fcc phase is obtained for LPCVD and PECVD processes. Atmospheric pressure CVD (APCVD) process, near to the thermodynamic equilibrium, leads to relatively low aluminum contents in the fcc-Ti1xAlxN phase (about x = 0.4) [43].

## 3.2. Physical vapor deposition (PVD) of Ti1xAlxN coatings

spinodal decomposition and then can increase the stability of the coating. Typically, the decomposition of the solid solution occurs at temperatures between 700 and 900C, depending on the aluminum content in the as-deposited coating [23, 39]. LPCVD Ti1xAlxN coatings with aluminum content of about x = 0.8 deposited by Endler in 2008 have shown a particularly high stability, with the first signs of decomposition at 1200C [27]. The reasons for this exceptional stability under vacuum conditions are not clearly defined. An increase of the thermal stability

As briefly described earlier in the thermodynamical description of the Ti-Al-N system, asdeposited Ti1xAlxN coatings have commonly three distinct structures: fcc, hcp, and a mix of fcc and hcp. These crystallographic structures are obtained for both PVD and CVD coatings and show a strong dependence on the aluminum content. For lower aluminum content, aluminum atoms are considered to replace titanium atoms in the fcc-TiN structure and form the single-phase fcc-Ti1xAlxN. This phase is the desired one on tools for cutting operations,

By increasing aluminum content in the films, a biphased structure appears. These phases are cubic (fcc) and hexagonal (hcp), but their compositions depend on the deposition process. Concerning PVD processes, for which condensation of a vapor on a cold substrate occurs, the cooling rate are so high that deposition of metastable materials is expected. A mixture of fcc-Ti1xAlxN and hcp-Ti1xAlxN can be observed, as reported by Chen et al. [23] for magnetron sputtered coatings deposited at 500C and a working pressure of 0.4 Pa. These results are in agreement with the metastable diagram established by Cremer et al. (Figure 3). Concerning the thermal CVD process, performed at high temperatures and then closer to the thermodynamic equilibrium, a mixture of the stable phases fcc-TiN and hcp-AlN is generally reported [23, 27]. The presence of fcc-AlN, in addition to the fcc-TiN and hcp-AlN phases, was also found by Wagner et al. for APCVD coatings, forming a three-phase structure. The fcc-AlN formation was attributed to the spinodal decomposition of the fcc-Ti1xAlxN coatings, where an excess of Al atoms in the fcc-Ti1xAlxN solid solution leads to an increase of the demixing energy and then cause the formation of fcc-AlN [43]. The aluminum content threshold, at which this crystallographic transition occurs, depends on the deposition techniques and process parameters (up to about x = 0.7 for PVD and up to x = 0.9 for CVD and PECVD). Anyway, the presence of a mixture of these two phases is generally detrimental for both mechanical properties and oxidation resistance of the films [7, 25, 26], except if a nanocomposite is formed [44, 45]. This mixing of hcp and fcc structures remains up to x = 1 for coatings deposited by CVD and PECVD processes [27, 44–46]. It should be noted, however, that the achievable microstructures for LPCVD processes are a little bit more complex and very dependent of process parameters. As denoted above, even though fcc-Ti1xAlxN solid solutions were found up to x = 0.9, the presence of nanocomposite structures was recently reported. The deposition

of Ti1xAlxN coatings can be achieved by adding elements as Si, Hf, or Ta [40–42].

needing high hardness, high thermal stability, and good oxidation resistance.

3. Deposition of Ti1xAlxN coatings

3.1. Crystallographic structure

178 Coatings and Thin-Film Technologies

Vacuum cathodic arc deposition (CAD) process [50–53] is probably the most widely used PVD technique on an industrial scale to prepare protective hard Ti1xAlxN-based coatings on cutting tools and forming molds [50, 54, 55]. The high ionization levels of cathodic arc

Figure 5. Crystallographic structures of Ti1xAlxN coatings obtained for APCD, LPCVD, PECVD, and PVD processes according to the aluminum content. "fcc" is related to the fcc-Ti1xAlxN solid solution; "fcc + hcp" refers to a mix of fcc-Ti1xAlxN + hcp-Ti1xAlxN for PVD coatings and to fcc-TiN + hcp-AlN for CVD and PECVD coatings; "hcp" refers to the hcp-Ti1xAlxN solid solution (data from [23, 27, 43, 46]).

discharges and high ion energy can provide advantages such as enhanced adhesion required for mechanical applications involving high loads. CAD allows deposition of a wide range of hard compounds as nitrides or carbonitrides. However, CAD is also often associated to macrodroplet generation that degrades the surface roughness of coatings. Droplets' density in the films, which is not redhibitory for cutting operations, has been significantly reduced thanks to the development of high-performances cathodes. The current trend is to develop advanced nanostructured hard coatings in order to enhance properties as hardness, toughness, and oxidation resistance [54, 55]. The deposition of Ti1xAlxN coatings by various PVD methods was hugely documented since several decades. Then, this review will focus mainly on CVD coatings and particularly on the thermal CVD process.

only AlCl3 and its dimer Al2Cl6 species. Thus, the direct chlorination of TiAl has to be realized at temperatures above 700�C. The nitrogen source was initially N2, for the deposition on carbide tools. NH3 is now commonly used, notably due to the development of microelectronics industry. The lower stability of NH3 compared to that of N2 allows to react with TiCl4 and

Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review

As previously said, the formation of the fcc-Ti1�xAlxN solid solution is envisioned to be the result of the codeposition of the metastable fcc-AlN and stable fcc-TiN. The reaction process

Other intermediate reactions forming complex molecules should also occur in the reactor and in the exhaust system but their formation seems mainly to occur for APCVD process

Thermal CVD deposition is highly dependent on the total pressure, partial pressure of precursors, as well as on the deposition temperature. An overall description of the effects of these

• High temperatures during deposition promote surface diffusion and adatom mobility,

Figure 6. Summary of the typical microstructure of CVD coatings with regards to the deposition temperature and the

• In the same way, a low partial pressure (or a low supersaturation) allows better surface mobility of atoms and then leads to coarse grains. Epitaxial coatings are then obtained for high deposition temperatures and low partial pressure. High partial pressure and low

temperature rather give nanosized grains or amorphous coatings.

parameters could be summarized as follows [59, 68] and shown in Figure 6:

6TiCl<sup>4</sup> þ 8NH<sup>3</sup> ! 6TiN þ 24HCl þ N<sup>2</sup> (2)

AlCl<sup>3</sup> þ NH<sup>3</sup> ! AlN þ 3HCl (3)

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using NH3 is considered to result mainly from the reactions (2) and (3):

AlCl3 at lower temperatures.

3.3.2. Morphology/microstructure

leading to coarsening of the grains.

supersaturation [59].

[66, 67].

### 3.3. Chemical vapor deposition (CVD) of Ti1xAlxN coatings

Thermal CVD deposition process is based on thermodynamic aspects, fluid mechanics, related to the transport of gaseous precursors toward the substrates and kinetics of deposition, allowing to synthetize metastable phases. Adjusting these parameters allows reaching a large range of microstructure and morphologies. For example, the stabilization of the metastable fcc-Ti1xAlxN is the result of a low mobility of species at the substrate surface. Thus, low temperatures, high partial pressure, and low total pressure are required. Processes as LPCVD and PECVD are now widely used in industrial plants for deposition of aluminum-rich Al-Ti-N coatings. However, MOCVD processes remain rare, because of the highly volatile nature and the high cost for preparing metalorganic precursors. Thus, regarding the lack of extended literature on MOCVD Ti1xAlxN coatings, the following parts will mainly focus on thermal CVD and PECVD processes. Anyway, studies on MOCVD processes and specific precursors are available in [21, 56–58].

### 3.3.1. Gaseous reactions

Although the deposition of Ti1xAlxN coatings is widely performed by PVD and PECVD processes, it is less common by thermal CVD processes. The good understanding of the process needs to take into account a lot of parameters linked to thermal homogeneity and distribution of gases. A detailed description of the fluid mechanics and thermic mechanisms inside the reactor related to chemical vapor deposition systems is given in [59–63]. Here, we briefly describe the gases production and reactions leading to the formation of coatings in the Ti-Al-N system.

Deposition of TiN and AlN by thermal CVD is generally performed using metal chlorides as TiCl4 and AlCl3. Gaseous TiCl4 is often generated with a bubbler by adjusting vapor pressure in the TiCl4 tank but systems regulating directly the liquid flow of TiCl4 also exist. AlCl3 is typically generated by in situ chlorination by passing HCl through Al platelets. Anderbouhr et al. [64, 65] have also demonstrated the possibility to deposit Ti1xAlxN coatings from TiCl3 and AlCl3 generated by passing Cl2, HCl, or TiCl4 on Ti-Al bulk alloys. However, the chlorination conditions (temperature and pressure) could generate unwanted species, and particularly solid <TiCl2> that precipitates below about 700C [64]. This range of temperature is in the common range used for generation of aluminum chlorides (300–400C), in order to generate only AlCl3 and its dimer Al2Cl6 species. Thus, the direct chlorination of TiAl has to be realized at temperatures above 700�C. The nitrogen source was initially N2, for the deposition on carbide tools. NH3 is now commonly used, notably due to the development of microelectronics industry. The lower stability of NH3 compared to that of N2 allows to react with TiCl4 and AlCl3 at lower temperatures.

As previously said, the formation of the fcc-Ti1�xAlxN solid solution is envisioned to be the result of the codeposition of the metastable fcc-AlN and stable fcc-TiN. The reaction process using NH3 is considered to result mainly from the reactions (2) and (3):

$$6TiCl\_4 + 8NH\_3 \to 6TiN + 24HCl + N\_2 \tag{2}$$

$$AlCl\_3 + NH\_3 \to AlN + \text{3HCl} \tag{3}$$

Other intermediate reactions forming complex molecules should also occur in the reactor and in the exhaust system but their formation seems mainly to occur for APCVD process [66, 67].

### 3.3.2. Morphology/microstructure

discharges and high ion energy can provide advantages such as enhanced adhesion required for mechanical applications involving high loads. CAD allows deposition of a wide range of hard compounds as nitrides or carbonitrides. However, CAD is also often associated to macrodroplet generation that degrades the surface roughness of coatings. Droplets' density in the films, which is not redhibitory for cutting operations, has been significantly reduced thanks to the development of high-performances cathodes. The current trend is to develop advanced nanostructured hard coatings in order to enhance properties as hardness, toughness, and oxidation resistance [54, 55]. The deposition of Ti1xAlxN coatings by various PVD methods was hugely documented since several decades. Then, this review will focus mainly on CVD

Thermal CVD deposition process is based on thermodynamic aspects, fluid mechanics, related to the transport of gaseous precursors toward the substrates and kinetics of deposition, allowing to synthetize metastable phases. Adjusting these parameters allows reaching a large range of microstructure and morphologies. For example, the stabilization of the metastable fcc-Ti1xAlxN is the result of a low mobility of species at the substrate surface. Thus, low temperatures, high partial pressure, and low total pressure are required. Processes as LPCVD and PECVD are now widely used in industrial plants for deposition of aluminum-rich Al-Ti-N coatings. However, MOCVD processes remain rare, because of the highly volatile nature and the high cost for preparing metalorganic precursors. Thus, regarding the lack of extended literature on MOCVD Ti1xAlxN coatings, the following parts will mainly focus on thermal CVD and PECVD processes. Anyway, studies on MOCVD processes and specific precursors

Although the deposition of Ti1xAlxN coatings is widely performed by PVD and PECVD processes, it is less common by thermal CVD processes. The good understanding of the process needs to take into account a lot of parameters linked to thermal homogeneity and distribution of gases. A detailed description of the fluid mechanics and thermic mechanisms inside the reactor related to chemical vapor deposition systems is given in [59–63]. Here, we briefly describe the gases production and reactions leading to the formation of coatings in the

Deposition of TiN and AlN by thermal CVD is generally performed using metal chlorides as TiCl4 and AlCl3. Gaseous TiCl4 is often generated with a bubbler by adjusting vapor pressure in the TiCl4 tank but systems regulating directly the liquid flow of TiCl4 also exist. AlCl3 is typically generated by in situ chlorination by passing HCl through Al platelets. Anderbouhr et al. [64, 65] have also demonstrated the possibility to deposit Ti1xAlxN coatings from TiCl3 and AlCl3 generated by passing Cl2, HCl, or TiCl4 on Ti-Al bulk alloys. However, the chlorination conditions (temperature and pressure) could generate unwanted species, and particularly solid <TiCl2> that precipitates below about 700C [64]. This range of temperature is in the common range used for generation of aluminum chlorides (300–400C), in order to generate

coatings and particularly on the thermal CVD process.

are available in [21, 56–58].

180 Coatings and Thin-Film Technologies

3.3.1. Gaseous reactions

Ti-Al-N system.

3.3. Chemical vapor deposition (CVD) of Ti1xAlxN coatings

Thermal CVD deposition is highly dependent on the total pressure, partial pressure of precursors, as well as on the deposition temperature. An overall description of the effects of these parameters could be summarized as follows [59, 68] and shown in Figure 6:

Figure 6. Summary of the typical microstructure of CVD coatings with regards to the deposition temperature and the supersaturation [59].


• The total pressure acting mostly on the diffusion in the boundary layer, a low total pressure promotes diffusion in the boundary layer and the mass transport to the surface, according to the Grove's model [59]. Then, a higher supersaturation should be achieved by lowering the working pressure.

microstructures like agglomerate-like grains, lenticular-shaped grains, or star-shaped grains

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In the case of Ti1xAlxN, a strong dependence to Al content and process parameters is observed. Thus, Anderbouhr et al. have deposited Ti1xAlxN films by LPCVD in a laboratory scale unit at temperatures from 600–1000C and a pressure of 1130 Pa [64]. They found fine columnar morphology (Figure 7a) up to x = 0.2 and glassy morphology for the highest aluminum contents (x ≈ 0.7). Wagner et al. have deposited Ti1xAlxN up to x = 0.72 in an industrial thermal CVD unit and at atmospheric pressure [43]. As it should be seen in Figure 7b, they also found columnar growth for low Al contents (single-phased fcc-Ti1xAlxN structure) but coarse agglomerate-like structure for highest value of x (biphased fcc-TiN/hcp-AlN structure). These results confirm the influence of working pressure on grain coarsening: the LPCVD process allows achieving higher supersaturation on the surface of the growing film and then leads to a grain refinement. These coatings have generally no preferential orientation. However, the recent deposition of Ti0.18Al0.82N nanocomposites reveals the particular formation of "cubes," growing with the [111] direction perpendicular to the substrate surface (see Figure 7c). XRD analysis revealed the presence of both Al-rich fcc-Ti1xAlxN and Ti-rich fcc-Ti1xAlxN solid solutions. Further TEM analysis revealed the presence of coherent Ti-rich fcc-Ti1xAlxN/Al-rich fcc-Ti1xAlxN nanolamellae. The lattice accommodation between these lamellae was attributed to the self-adjustment of the Ti, Al, and N concentration near the interface [48]. PECVD coatings also show a grain refinement with increasing aluminum incorporation [28, 46, 77]. Other parameters have to be taken into account in order to describe coatings' morphologies obtained by PECVD processes. An increase of the gas inlet distance seems to lead to a fine-grain morphology. This evolution should be related to the increase of the aluminum content and the drop of the nitrogen content in the coatings. The higher gas inlet distance leads to substoichiometric coatings (only 35 at% of nitrogen in the coatings instead of the 50 at% for stoichiometric coatings) and showing a mixture of fcc-Ti1xAlxN and hcp-Ti1xAlxN phases [46]. Discharge voltage does not seem to influence directly the morphology

Chemical composition is one of the most important characteristics in Ti1xAlxN coatings due to its strong influence on the crystallographic structure. Since the crystallographic structure is highly dependent on the aluminum content, researches led on Ti1xAlxN coatings focused mainly on the optimization of the aluminum content in the films, particularly in the fcc structure, allowing to reach the particular mechanical and chemical properties of Ti1xAlxN

Although the deposition of Ti1xAlxN coatings is widely performed by PVD processes, it is less common by PECVD and thermal CVD processes. Thus, PVD fcc-Ti1xAlxN singlephased coatings are limited to x = 0.67–0.7. As an example, the single-phase fcc-Ti1xAlxN was obtained up to x = 0.67 in films deposited by magnetron sputtering [23], up to 0.65 by

were reported [70, 72–76].

of coatings but influences the aluminum content.

films.

3.3.3.1. Aluminum content

3.3.3. Chemical composition and crystallographic structure

Other parameters as distance of the substrate from gas inlet or discharge voltage for PECVD techniques have also to be taken into account to optimize the morphology/microstructure of the coatings.

TiN coatings obtained by CVD processes show generally columnar growth, according to the Van der Drift model [69], and resulting from the competition between different crystalline orientations. In fact, growth of grain is thermodynamically more favorable along some orientations: the grains having these favorable orientations perpendicularly to the substrate surface will be favored and will lead to column formation [69]. The preferential orientation depends strongly on the process parameters (gas ratios, deposition temperature, partial pressure of each precursors, etc.). As an example, for TiN coatings deposited at atmospheric pressure, Cheng et al. found, at high temperature and high N2 partial pressure, a (200) texture while they found a (110) texture at low temperature and high N2 partial pressure [70, 71]. In the same way, the control of process parameters allows to achieve a wide range of morphologies [68]. Thus,

Figure 7. Microstructure of Ti1xAlxN coatings for different aluminum content: (a) columnar structure of Ti0.92Al0.08N deposited on silicon substrate by LPCVD [65]; (b) agglomerate-like structure of Ti0.23Al0.72N deposited on WC/Co substrate by APCVD [43]; (c) nanocomposite fcc-Ti1xAlxN/Al-rich fcc-Ti1xAlxN arrangement forming cubes having the (111) planes parallel to the surface [48].

microstructures like agglomerate-like grains, lenticular-shaped grains, or star-shaped grains were reported [70, 72–76].

In the case of Ti1xAlxN, a strong dependence to Al content and process parameters is observed. Thus, Anderbouhr et al. have deposited Ti1xAlxN films by LPCVD in a laboratory scale unit at temperatures from 600–1000C and a pressure of 1130 Pa [64]. They found fine columnar morphology (Figure 7a) up to x = 0.2 and glassy morphology for the highest aluminum contents (x ≈ 0.7). Wagner et al. have deposited Ti1xAlxN up to x = 0.72 in an industrial thermal CVD unit and at atmospheric pressure [43]. As it should be seen in Figure 7b, they also found columnar growth for low Al contents (single-phased fcc-Ti1xAlxN structure) but coarse agglomerate-like structure for highest value of x (biphased fcc-TiN/hcp-AlN structure). These results confirm the influence of working pressure on grain coarsening: the LPCVD process allows achieving higher supersaturation on the surface of the growing film and then leads to a grain refinement. These coatings have generally no preferential orientation. However, the recent deposition of Ti0.18Al0.82N nanocomposites reveals the particular formation of "cubes," growing with the [111] direction perpendicular to the substrate surface (see Figure 7c). XRD analysis revealed the presence of both Al-rich fcc-Ti1xAlxN and Ti-rich fcc-Ti1xAlxN solid solutions. Further TEM analysis revealed the presence of coherent Ti-rich fcc-Ti1xAlxN/Al-rich fcc-Ti1xAlxN nanolamellae. The lattice accommodation between these lamellae was attributed to the self-adjustment of the Ti, Al, and N concentration near the interface [48]. PECVD coatings also show a grain refinement with increasing aluminum incorporation [28, 46, 77]. Other parameters have to be taken into account in order to describe coatings' morphologies obtained by PECVD processes. An increase of the gas inlet distance seems to lead to a fine-grain morphology. This evolution should be related to the increase of the aluminum content and the drop of the nitrogen content in the coatings. The higher gas inlet distance leads to substoichiometric coatings (only 35 at% of nitrogen in the coatings instead of the 50 at% for stoichiometric coatings) and showing a mixture of fcc-Ti1xAlxN and hcp-Ti1xAlxN phases [46]. Discharge voltage does not seem to influence directly the morphology of coatings but influences the aluminum content.

### 3.3.3. Chemical composition and crystallographic structure

Chemical composition is one of the most important characteristics in Ti1xAlxN coatings due to its strong influence on the crystallographic structure. Since the crystallographic structure is highly dependent on the aluminum content, researches led on Ti1xAlxN coatings focused mainly on the optimization of the aluminum content in the films, particularly in the fcc structure, allowing to reach the particular mechanical and chemical properties of Ti1xAlxN films.

### 3.3.3.1. Aluminum content

• The total pressure acting mostly on the diffusion in the boundary layer, a low total pressure promotes diffusion in the boundary layer and the mass transport to the surface, according to the Grove's model [59]. Then, a higher supersaturation should be achieved

Other parameters as distance of the substrate from gas inlet or discharge voltage for PECVD techniques have also to be taken into account to optimize the morphology/microstructure of

TiN coatings obtained by CVD processes show generally columnar growth, according to the Van der Drift model [69], and resulting from the competition between different crystalline orientations. In fact, growth of grain is thermodynamically more favorable along some orientations: the grains having these favorable orientations perpendicularly to the substrate surface will be favored and will lead to column formation [69]. The preferential orientation depends strongly on the process parameters (gas ratios, deposition temperature, partial pressure of each precursors, etc.). As an example, for TiN coatings deposited at atmospheric pressure, Cheng et al. found, at high temperature and high N2 partial pressure, a (200) texture while they found a (110) texture at low temperature and high N2 partial pressure [70, 71]. In the same way, the control of process parameters allows to achieve a wide range of morphologies [68]. Thus,

Figure 7. Microstructure of Ti1xAlxN coatings for different aluminum content: (a) columnar structure of Ti0.92Al0.08N deposited on silicon substrate by LPCVD [65]; (b) agglomerate-like structure of Ti0.23Al0.72N deposited on WC/Co substrate by APCVD [43]; (c) nanocomposite fcc-Ti1xAlxN/Al-rich fcc-Ti1xAlxN arrangement forming cubes having the

by lowering the working pressure.

(111) planes parallel to the surface [48].

the coatings.

182 Coatings and Thin-Film Technologies

Although the deposition of Ti1xAlxN coatings is widely performed by PVD processes, it is less common by PECVD and thermal CVD processes. Thus, PVD fcc-Ti1xAlxN singlephased coatings are limited to x = 0.67–0.7. As an example, the single-phase fcc-Ti1xAlxN was obtained up to x = 0.67 in films deposited by magnetron sputtering [23], up to 0.65 by cathodic arc deposition [78, 79] and up to 0.71 by high-power impulse magnetron sputtering (HiPIMS) [80]. This limit is consistent with the metastable solubility limit predicted by thermodynamic and ab initio calculations. In contrast, CVD processes allow to deposit fcc structure with higher aluminum contents. Even though the possibility to deposit fcc-Ti1xAlxN at atmospheric pressure (APCVD) and at relatively low temperatures (<700C) was demonstrated by Wagner et al. [43], the morphological aspects of the coatings and the associated bad mechanical properties for high aluminum contents lead to some limitations for APCVD processes. It seems thus difficult to deposit performant Ti1xAlxN coatings by APCVD with an aluminum content higher than 0.4. However, LPCVD development has led to achieve deposition of aluminum-rich coatings with good mechanical properties and having aluminum contents higher than that reached at atmospheric pressure. Anderbouhr et al. have thus shown the possibility to deposit the fcc-Ti1xAlxN solid solution up to x ≈ 0.7 by a LPCVD process using titanium and aluminum chlorides [64, 65]. Later, Endler et al. obtained fcc-Ti1xAlxN up to x = 0.9 at 800C and pressure < 10 KPa [27]. Fcc-Ti1xAlxN coatings with x ≈ 0.8 and deposited at pressure below 50 mbar are notably industrially used [81]. These CVD coatings were deposited with AlCl3/TiCl4 gas ratio > 1, due to the higher stability of AlCl3 than that of TiCl4. An increase of the AlCl3/TiCl4 ratio leads to a rise of the aluminum content in the coating [27, 43, 64]. As shown in Figure 8, the same dependence is observed for PECVD coatings [28, 29]. As specified previously, discharge voltage and gas inlet distance from the substrate strongly influence the chemical composition. Increasing discharge voltage favors aluminum incorporation in the coatings [28, 46]. In the same way, increasing distance from gas inlet strongly increases the aluminum content and, as a consequence, leads to granular microstructure [46].

fcc-TiN structure. They found that the aluminum distribution giving the lowest quantity of Ti-Al bonds led to high solubility limit. Their results showed notably an increase of the solubility limit from x = 0.64 to x = 0.74 only by changing the atomic distribution of Al and Ti. Thus, they concluded that the distribution of the aluminum atoms in the lattice is one of the factors

Ti-Al-N-Based Hard Coatings: Thermodynamical Background, CVD Deposition, and Properties. A Review

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185

Another factor that should influence properties of the coatings is the chlorine incorporation after CVD deposition. Concerning CVD process, for which the temperature is the key factor, higher deposition temperatures promote precursor dissociation and thus lead to a decrease of the chlorine content in the coatings. However, a rise of the aluminum precursor flow rate leads to an increase of the Cl content in the film [43, 77, 83]. Concerning PECVD process, an increase of the discharge voltage leads to a better dissociation of species and thus reduces Cl content in the coatings [28, 46]. In the same way, increasing gas inlet distance leads to a higher dissociation of Al precursor and then to decrease the Cl content [46]. Oxygen is also generally identified as a detrimental element for the properties of the coatings. However, while several studies describe the mechanisms responsible for the oxide formation (the oxygen diffusion notably) during annealing of the Ti1xAlxN coatings, no studies were found concerning the effect of the oxygen content on the properties of the coatings (except

Ti1xAlxN coatings are now extensively used for machining applications, where good mechanical and chemical properties are needed in order to ensure the stability of the coatings in inservice conditions. These properties are strongly dependent on the chemical composition of the coating and the working temperature, which could lead to a change in structure of the coating. Properties as hardness and oxidation resistance are then reviewed with respect to the alumi-

Hardness of the coating is a key property for increasing the wear resistance and improving the tool lifetime of the cutting tools. The hardness of coatings is generally characterized by mean of nanohardness tests. Among the mechanical properties of Ti1xAlxN coatings, the hardness is only indirectly related to the aluminum content via the crystallographic structure. Thus, an increase of the aluminum content in the fcc-Ti1xAlxN phase leads to an increase of the hardness. The main mechanism is solid solution hardening: the substitution of Ti by Al atoms in the fcc-TiN leads to a lattice distortion and thus limits the motion of

num content in the fcc-Ti1xAlxN phase and deposition temperature.

explaining the high solubility limit obtained for PECVD and LPCVD coatings.

3.3.3.2. Impurities

the oxide scale formation).

4.1. Mechanical properties

4.1.1.1. Al-content dependence of hardness

4.1.1. Hardness

4. Properties of Ti1xAlxN coatings

The reason for the huge gap between the metastable solubility limits of aluminum in the fcc-TiN structure obtained for different deposition processes (around 0.7 for PVD, 0.9 for LPCVD and PECVD methods) is only poorly discussed in literature. However, Mayrhofer et al. [82] led ab initio calculations to study the influence of the distribution of Al atoms substituting Ti in the

Figure 8. Evolution of the aluminum content in the Ti1xAlxN coatings vs. AlCl3/TiCl4 ratio for PECVD coatings deposited at 500C and about 670 Pa [83].

fcc-TiN structure. They found that the aluminum distribution giving the lowest quantity of Ti-Al bonds led to high solubility limit. Their results showed notably an increase of the solubility limit from x = 0.64 to x = 0.74 only by changing the atomic distribution of Al and Ti. Thus, they concluded that the distribution of the aluminum atoms in the lattice is one of the factors explaining the high solubility limit obtained for PECVD and LPCVD coatings.

### 3.3.3.2. Impurities

cathodic arc deposition [78, 79] and up to 0.71 by high-power impulse magnetron sputtering (HiPIMS) [80]. This limit is consistent with the metastable solubility limit predicted by thermodynamic and ab initio calculations. In contrast, CVD processes allow to deposit fcc structure with higher aluminum contents. Even though the possibility to deposit fcc-Ti1xAlxN at atmospheric pressure (APCVD) and at relatively low temperatures (<700C) was demonstrated by Wagner et al. [43], the morphological aspects of the coatings and the associated bad mechanical properties for high aluminum contents lead to some limitations for APCVD processes. It seems thus difficult to deposit performant Ti1xAlxN coatings by APCVD with an aluminum content higher than 0.4. However, LPCVD development has led to achieve deposition of aluminum-rich coatings with good mechanical properties and having aluminum contents higher than that reached at atmospheric pressure. Anderbouhr et al. have thus shown the possibility to deposit the fcc-Ti1xAlxN solid solution up to x ≈ 0.7 by a LPCVD process using titanium and aluminum chlorides [64, 65]. Later, Endler et al. obtained fcc-Ti1xAlxN up to x = 0.9 at 800C and pressure < 10 KPa [27]. Fcc-Ti1xAlxN coatings with x ≈ 0.8 and deposited at pressure below 50 mbar are notably industrially used [81]. These CVD coatings were deposited with AlCl3/TiCl4 gas ratio > 1, due to the higher stability of AlCl3 than that of TiCl4. An increase of the AlCl3/TiCl4 ratio leads to a rise of the aluminum content in the coating [27, 43, 64]. As shown in Figure 8, the same dependence is observed for PECVD coatings [28, 29]. As specified previously, discharge voltage and gas inlet distance from the substrate strongly influence the chemical composition. Increasing discharge voltage favors aluminum incorporation in the coatings [28, 46]. In the same way, increasing distance from gas inlet strongly increases the aluminum content and, as a consequence, leads

The reason for the huge gap between the metastable solubility limits of aluminum in the fcc-TiN structure obtained for different deposition processes (around 0.7 for PVD, 0.9 for LPCVD and PECVD methods) is only poorly discussed in literature. However, Mayrhofer et al. [82] led ab initio calculations to study the influence of the distribution of Al atoms substituting Ti in the

Figure 8. Evolution of the aluminum content in the Ti1xAlxN coatings vs. AlCl3/TiCl4 ratio for PECVD coatings

to granular microstructure [46].

184 Coatings and Thin-Film Technologies

deposited at 500C and about 670 Pa [83].

Another factor that should influence properties of the coatings is the chlorine incorporation after CVD deposition. Concerning CVD process, for which the temperature is the key factor, higher deposition temperatures promote precursor dissociation and thus lead to a decrease of the chlorine content in the coatings. However, a rise of the aluminum precursor flow rate leads to an increase of the Cl content in the film [43, 77, 83]. Concerning PECVD process, an increase of the discharge voltage leads to a better dissociation of species and thus reduces Cl content in the coatings [28, 46]. In the same way, increasing gas inlet distance leads to a higher dissociation of Al precursor and then to decrease the Cl content [46]. Oxygen is also generally identified as a detrimental element for the properties of the coatings. However, while several studies describe the mechanisms responsible for the oxide formation (the oxygen diffusion notably) during annealing of the Ti1xAlxN coatings, no studies were found concerning the effect of the oxygen content on the properties of the coatings (except the oxide scale formation).
