Semiconductor Devices

*Recent Advances in Nanophotonics - Fundamentals and Applications*

[49] Ji K, Yuan J, Li F, Shi Y,

2020;**8**:10233-10241

Ling X, Zhang X, et al. High-efficiency perovskite quantum dot solar cells benefiting from a conjugated polymerquantum dot bulk heterojunction connecting layer. Journal of Materials Chemistry A. 2020;**8**:8104-8112

[50] Rao H, Zhou M, Pan Z, Zhong X. Quantum dot material engineering boosting quantum dot sensitized solar cells efficiency over 13%. Journal of Materials Chemistry A.

[51] Wen S, Li Y, Zheng N, Raji IO, Yang C, Bao X. High-efficiency organic solar cells enabled by halogenation of polymers based on 2D conjugated benzobis(thiazole). Journal of Materials Chemistry A. 2020;**8**:13671-13678

**94**

**97**

**Chapter 6**

**Abstract**

Intermixing

techniques and give recent examples.

techniques, inter-diffusion, fabrication

a metal contact's endurance [1–4].

**1. Introduction**

the following:

substrate).

strate to surface).

*Thamer Tabbakh*

Diffusion and Quantum Well

Diffusion or intermixing is the movement of particles through space. It primarily occurs in every form of matter because of thermal motion. Atom diffusion and intermixing can also happen in crystalline semiconductors whereby the atoms that are diffusing and intermixing move from one side of the lattice to the adjacent one in the crystal semiconductor. Atom diffusion, which may also involve defects (including native and dopant), is at the core of processing of semiconductors. The stages involved in semiconductor processing are growth, followed by post-growth, and then the construction stage comes last. The control of every aspect of diffusion is necessary to accomplish the required goals, therefore creating a need for knowing what diffuses at any point in time. This chapter will briefly summarize the techniques that are in existence and are used to create diffused quantum wells (QWs). Also, it will outline the examples of QW semiconductor lasers and light-emitting diode (LED) by the utilization of inter-diffusion

**Keywords:** intermixing, semiconductors, diffusion, QWI, lasers, LED, intermixing

The demands of device technology have yielded the primary motivation for looking over atomic diffusion, as depicted by a semiconductor lattice. Since there has been a shrinking of the devices' physical dimensions, more problems have emerged concerning comprehending features of diffusion in more complex structures [1]. There is a link between some common problems with the deterioration of a doped structure, for instance, a superlattice or p-n junction, diffusion barrier, or

Knez pointed out four diffusion situations that are separate from each other, which can crop up in the post-processing of the substrate's surface layer. The layer can be thin like mercury telluride (HgTe) or cadmium telluride (CdTe) [4–6]. There are four different diffusion situations for the post-processing, which are

Secondly, there is surface component diffusion into the substrate (surface into

Thirdly, there is substrates' component diffusion into the surface layer (sub-

Firstly, there is components' lateral diffusion in the surface layer.

### **Chapter 6**

## Diffusion and Quantum Well Intermixing

*Thamer Tabbakh*

### **Abstract**

Diffusion or intermixing is the movement of particles through space. It primarily occurs in every form of matter because of thermal motion. Atom diffusion and intermixing can also happen in crystalline semiconductors whereby the atoms that are diffusing and intermixing move from one side of the lattice to the adjacent one in the crystal semiconductor. Atom diffusion, which may also involve defects (including native and dopant), is at the core of processing of semiconductors. The stages involved in semiconductor processing are growth, followed by post-growth, and then the construction stage comes last. The control of every aspect of diffusion is necessary to accomplish the required goals, therefore creating a need for knowing what diffuses at any point in time. This chapter will briefly summarize the techniques that are in existence and are used to create diffused quantum wells (QWs). Also, it will outline the examples of QW semiconductor lasers and light-emitting diode (LED) by the utilization of inter-diffusion techniques and give recent examples.

**Keywords:** intermixing, semiconductors, diffusion, QWI, lasers, LED, intermixing techniques, inter-diffusion, fabrication

### **1. Introduction**

The demands of device technology have yielded the primary motivation for looking over atomic diffusion, as depicted by a semiconductor lattice. Since there has been a shrinking of the devices' physical dimensions, more problems have emerged concerning comprehending features of diffusion in more complex structures [1]. There is a link between some common problems with the deterioration of a doped structure, for instance, a superlattice or p-n junction, diffusion barrier, or a metal contact's endurance [1–4].

Knez pointed out four diffusion situations that are separate from each other, which can crop up in the post-processing of the substrate's surface layer. The layer can be thin like mercury telluride (HgTe) or cadmium telluride (CdTe) [4–6]. There are four different diffusion situations for the post-processing, which are the following:

Firstly, there is components' lateral diffusion in the surface layer.

Secondly, there is surface component diffusion into the substrate (surface into substrate).

Thirdly, there is substrates' component diffusion into the surface layer (substrate to surface).

Fourthly is the diffusion barrier stationed between the substrate and surface layer.

The type of diffusion within the crystal lattice is called lattice diffusion, and it takes place by either substitutional or interstitial mechanisms. Interstitial lattice diffusion involves a diffusant like carbon in an iron combination diffusing in the middle of the lattice structure of one or more crystalline elements. On the other hand, substitutional lattice diffusion involves self-diffusion or inter-diffusion (where self-diffusion takes place in pure metals because atoms exchange location for the same type and there is no net mass transport, while inter-diffusion occurred in alloys which have net mass transport and atoms diffuse into different metals) whereby the movement of an atom is made possible by its substitution with another atom to replace it [6–10]. This diffusion is usually made possible by point vacancies' availability all over the crystal lattice. Diffusing particles relocate fast from one vacancy point to another, basically by random jumping termed as jump diffusion, as shown in **Figure 1**. Considering that the regularity of point vacancies multiplies in line with the Arrhenius equation, the frequency of diffusion crystal solid state improves with temperature [11–15].

The use of inter-diffusion of quantum wells (QWs) is an emerging technology that is significant for fabricating semiconductor lasers since it improves devices' optical and electrical properties [16]. Selective inter-diffusion is achievable by obscuring into the QW wafer's desired regions. Since the 1980s, there have been extensive investigations regarding inter-diffusion [16, 17]. It comprises disordering or intermixing of heterostructures that are quantum-confined like QWs and quantum dots (QDs). The thorough investigations are due to its potential to achieve monolithic integration of optoelectronic/photonic devices. Among the inter-diffusion techniques, there has been a consideration of impurity-free vacancy disordering (IFVD) as the technique that is most promising for device applications because of its simplicity and causes lesser residual damage to the sample [17–19].

During inter-diffusion, there will be a resultant modification of refractive index and electrical conductivity between the regions that are as-grown together with disordered ones. The technology allows a homogenous process that leads to the enhancement of the sideways electrical and optical restraint of laser semiconductors in such a manner that the bottom threshold current, as well as single operation that is lateral mode, is obtainable. Moreover, the QW's shape alters as a result of interdiffusion between QWs and barriers that are next to it. In turn, there is a modification of the sub-band energy in valence and conduction bands. Eventually, the inter-band transition energy is modified. Therefore, the inter-diffusion technique could be utilize the fabrication of QW lasers and LED for multiple wavelengths without using complicated epitaxial regrowth or etching processes. Other merits of utilizing inter-diffusion techniques include one, its simplicity. And there is also compatibility with existing semiconductor lasers' fabrication technologies [20–25].

**99**

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

site [25, 26]:

vacancy motion)

where \_ *dC dX*

as shown in **Figure 2**. Hence, from Eq. 5

**2. Diffusion mechanism and coefficient**

*P* = *z exp* (−

activation free energy for vacancy motion. Therefore, the diffusion coefficient [27] is

*<sup>D</sup>* = *zV*0 *a*<sup>2</sup> *exp* (

*D* = *D*<sup>0</sup> *exp* (

*J* = −*D*(

*D* = *D*<sup>0</sup> *exp* (

By taking the logarithm for Eq. 6, we can get

*lnD* = *lnD*0

*logD* = *logD*0

(8.31 J/mol K or 8.62 × 10–5 eV/atom K).

Eq. 3 can be rewritten as

For the vacancy diffusion mechanism, the probability for any atom in a solid to move is the product of the probability P of finding a vacancy in an adjacent lattice

where z is the coordination number (number of atoms adjacent to the vacancy), Gf is the free energy necessary to form the defects, T is the absolute temperature (K), KB is the Boltzmann constant, and the frequency of jumps (probability of thermal fluctuation needed to overcome the energy barrier for

*Rj* = *V0 exp* (

where *a* is the mean distance between atoms in a crystal lattice.

where D0 is the temperature-independent preexponential (m2

Porter and Easterling textbook and *Smithells Metals Reference Book* [1].

activation energy for diffusion (J/mol or eV/atom), and R is the gas constant

where D0 is a parameter of material (both matrix and diffusing species).

Thus, the diffusion coefficient is the measure of the mobility of disusing species:

\_ *dC dX*

is the concentration gradients (negative in the direction of diffusion),

\_ − *Qd*

−\_ *Q <sup>d</sup>*

\_ − *Q <sup>d</sup>*

From Eq. 8, Q d the activation energy for diffusion and D0 independent preexponential can be measured by estimating the logD0 versus 1/T or lnD0 versus 1/T as the Arrhenius plots (**Figure 3**). **Figures 1** and **2** and **Tables 1** and **2** were taken from

where Rj is the probability of such fluctuation or frequency of jumps, V0 is an attempt frequency related to the frequency of atomic vibrations, and Gm is the

> \_ −∆ *Gm*

\_ *Gf*

\_ −∆ *Gm*

*KBT* ) *exp* (<sup>−</sup>

\_ −∆ *Gm* − *Gf*

\_ *Gf*

*KBT*) (1)

*KBT* ) (2)

*KBT* ) (4)

) (5)

*RT* ) (6)

*RT* (7)

2.3*RT* (8)

/s), Qd is the

*KBT*) (3)

**Figure 1.**

*Atomic movement that results in atomic diffusion. (a) Interstitial diffusion, (b) self-diffusion or inter-diffusion, (c) vacancy diffusion.*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

Fourthly is the diffusion barrier stationed between the substrate and surface layer. The type of diffusion within the crystal lattice is called lattice diffusion, and it takes place by either substitutional or interstitial mechanisms. Interstitial lattice diffusion involves a diffusant like carbon in an iron combination diffusing in the middle of the lattice structure of one or more crystalline elements. On the other hand, substitutional lattice diffusion involves self-diffusion or inter-diffusion (where self-diffusion takes place in pure metals because atoms exchange location for the same type and there is no net mass transport, while inter-diffusion occurred in alloys which have net mass transport and atoms diffuse into different metals) whereby the movement of an atom is made possible by its substitution with another atom to replace it [6–10]. This diffusion is usually made possible by point vacancies' availability all over the crystal lattice. Diffusing particles relocate fast from one vacancy point to another, basically by random jumping termed as jump diffusion, as shown in **Figure 1**. Considering that the regularity of point vacancies multiplies in line with the Arrhenius equation, the frequency of diffusion crystal solid state improves with temperature [11–15].

The use of inter-diffusion of quantum wells (QWs) is an emerging technology that is significant for fabricating semiconductor lasers since it improves devices' optical and electrical properties [16]. Selective inter-diffusion is achievable by obscuring into the QW wafer's desired regions. Since the 1980s, there have been extensive investigations regarding inter-diffusion [16, 17]. It comprises disordering or intermixing of heterostructures that are quantum-confined like QWs and quantum dots (QDs). The thorough investigations are due to its potential to achieve monolithic integration of optoelectronic/photonic devices. Among the inter-diffusion techniques, there has been a consideration of impurity-free vacancy disordering (IFVD) as the technique that is most promising for device applications because of its simplicity and causes lesser residual damage to the sample [17–19]. During inter-diffusion, there will be a resultant modification of refractive index and electrical conductivity between the regions that are as-grown together with disordered ones. The technology allows a homogenous process that leads to the enhancement of the sideways electrical and optical restraint of laser semiconductors in such a manner that the bottom threshold current, as well as single operation that is lateral mode, is obtainable. Moreover, the QW's shape alters as a result of interdiffusion between QWs and barriers that are next to it. In turn, there is a modification of the sub-band energy in valence and conduction bands. Eventually, the inter-band transition energy is modified. Therefore, the inter-diffusion technique could be utilize the fabrication of QW lasers and LED for multiple wavelengths without using complicated epitaxial regrowth or etching processes. Other merits of utilizing inter-diffusion techniques include one, its simplicity. And there is also compatibility with existing semiconductor lasers' fabrication technologies [20–25].

*Atomic movement that results in atomic diffusion. (a) Interstitial diffusion, (b) self-diffusion or* 

**98**

**Figure 1.**

*inter-diffusion, (c) vacancy diffusion.*

### **2. Diffusion mechanism and coefficient**

For the vacancy diffusion mechanism, the probability for any atom in a solid to move is the product of the probability P of finding a vacancy in an adjacent lattice site [25, 26]:

\*\*z \{\Delta\}\*\*, \Delta\!b\*\*]: 
$$P = \operatorname{z-exp}\left(-\frac{G\_f}{K\_B T}\right) \tag{1}$$

where z is the coordination number (number of atoms adjacent to the vacancy), Gf is the free energy necessary to form the defects, T is the absolute temperature (K), KB is the Boltzmann constant, and the frequency of jumps (probability of thermal fluctuation needed to overcome the energy barrier for vacancy motion)

tion needed to overcome the energy barrier for

$$R\_j = V\_0 \exp\left(\frac{-\Delta \ G\_m}{K\_B T}\right) \tag{2}$$

where Rj is the probability of such fluctuation or frequency of jumps, V0 is an attempt frequency related to the frequency of atomic vibrations, and Gm is the activation free energy for vacancy motion.

Therefore, the diffusion coefficient [27] is

Within free energy for vacancy motion.

Therefore, the diffusion coefficient [27] is

$$D = zV\_0 a^2 \exp\left(\frac{-\Delta G\_m}{K\_B T}\right) \exp\left(-\frac{G\_f}{K\_B T}\right) \tag{3}$$

where *a* is the mean distance between atoms in a crystal lattice. Eq. 3 can be rewritten as

$$D = zV\_0 a^2 \exp\left(\frac{-\Delta \ G\_m}{K\_B T}\right) \exp\left(-\frac{G\_f}{K\_B T}\right) \tag{3}$$
 where  $a$  is the mean distance between atoms in a crystal lattice.

 Eq. 3 can be rewritten as

 
$$D = D\_0 \exp\left(\frac{-\Delta \ G\_m - G\_f}{K\_B T}\right) \tag{4}$$
 
$$D\_0 = \exp\left(\frac{-\Delta \ G\_m - G\_f}{K\_B T}\right) \tag{4}$$
 
$$D\_0 = \exp\left(\frac{-\Delta \ G\_m - G\_f}{K\_B T}\right) \tag{5}$$

where D0 is a parameter of material (both matrix and diffusing species). Thus, the diffusion coefficient is the measure of the mobility of disusing species:

$$J = -D\left(\frac{dc}{d\chi}\right) \tag{5}$$

where \_ *dC dX* is the concentration gradients (negative in the direction of diffusion), as shown in **Figure 2**.

Hence, from Eq. 5

\*\*L'normes, Ironu Lql.\*\* 
$$D = D\_0 \exp\left(\frac{-Q\_d}{RT}\right) \tag{6}$$

where D0 is the temperature-independent preexponential (m2 /s), Qd is the activation energy for diffusion (J/mol or eV/atom), and R is the gas constant (8.31 J/mol K or 8.62 × 10–5 eV/atom K).

By taking the logarithm for Eq. 6, we can get

Day tanking une programium no La ${}\_{0}$  o, we can get

$$lnD = lnD\_{0} \frac{-Q\_{d}}{RT} \tag{7}$$

$$
\cdots \quad \cdots \quad \bullet \quad RT \tag{8}
$$

$$
\log \mathcal{D} = \log D\_0 \frac{-Q\_d}{2.3RT} \tag{8}
$$

From Eq. 8, Q d the activation energy for diffusion and D0 independent preexponential can be measured by estimating the logD0 versus 1/T or lnD0 versus 1/T as the Arrhenius plots (**Figure 3**). **Figures 1** and **2** and **Tables 1** and **2** were taken from Porter and Easterling textbook and *Smithells Metals Reference Book* [1].

**Figure 2.** *The slope of particular point on the concentration gradient.*

**Figure 3.** *Arrhenius plots. Q d as the function of D0 diffusion temperature dependence [1].*


### **Table 1.**

*Examples of the temperature-independent preexponential and the activation energy for diffusion of some atoms in the case of interstitial diffusion mechanism.*

**101**

**Figure 4.**

substitutional atoms) [28–31].

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

*atoms in the case of vacancy diffusion mechanism.*

**Table 2.**

**Impurity D0 = (mm2**

**/S<sup>−</sup><sup>1</sup>**

Fe in FCC Fe 65 279 Fe in BCC Fe 410 246 Si in Si 180,000 460 Ni in Cu 230 242

*Examples for the temperature-independent preexponential and the activation energy for diffusion of some* 

**) Q d = (kJ/mol)**

In Eq. 7, it seems that the vacancy diffusion mechanism is slower than interstitial diffusion, as shown in **Figure 4** and **Tables 1** and **2** (self-diffusion or diffusion of

From Eq. 6, the big atoms cause more distortion and take more time to diffuse than the smaller atoms during the migration process as we can see from **Tables 1**

The fabrication of photonic integrated circuits (PICs) by employing an integration of lasers and transparent waveguides on a single epitaxially grown substrate demands the actual understanding and definition of regions possessing different bandgap energy characters. The approach to work out a solution to this

and **2**. Also the diffusion is slower in a close direction and lattices.

**3. Fabrication of quantum well intermixing (QWI)**

*Logarithm of the diffusion coefficient versus the reciprocal temperature [1].*

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*


**Table 2.**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

*The slope of particular point on the concentration gradient.*

*Arrhenius plots. Q d as the function of D0 diffusion temperature dependence [1].*

**/S<sup>−</sup><sup>1</sup>**

C in BCC Fe 1.1 87 C in FCC Fe 23 138 N in BCC Fe 0.74 77 N in FCC Fe 0.34 145 H in BCC Fe 0.12 15 H in FCC Fe 0.63 43

*Examples of the temperature-independent preexponential and the activation energy for diffusion of some* 

**) Q d = (kJ/mol)**

**Impurity D0 = (mm2**

*atoms in the case of interstitial diffusion mechanism.*

**100**

**Table 1.**

**Figure 3.**

**Figure 2.**

*Examples for the temperature-independent preexponential and the activation energy for diffusion of some atoms in the case of vacancy diffusion mechanism.*

**Figure 4.** *Logarithm of the diffusion coefficient versus the reciprocal temperature [1].*

In Eq. 7, it seems that the vacancy diffusion mechanism is slower than interstitial diffusion, as shown in **Figure 4** and **Tables 1** and **2** (self-diffusion or diffusion of substitutional atoms) [28–31].

From Eq. 6, the big atoms cause more distortion and take more time to diffuse than the smaller atoms during the migration process as we can see from **Tables 1** and **2**. Also the diffusion is slower in a close direction and lattices.

### **3. Fabrication of quantum well intermixing (QWI)**

The fabrication of photonic integrated circuits (PICs) by employing an integration of lasers and transparent waveguides on a single epitaxially grown substrate demands the actual understanding and definition of regions possessing different bandgap energy characters. The approach to work out a solution to this

problem can be categorized into intermixing and growth approaches. Among the growth approaches, the most popular ones are selective area growth approach and use of a plated substrate to etch-and-regrowth approach [29, 30, 32]. The former one allows for simultaneous epitaxy employing the use of different growth rates, which in turn allows for flexibility toward the growth of quantum wells with varying thicknesses [32–34]. In contrast to the former approach, the latter approach uses different quantum well thicknesses along with subsequent growth of material. Using impurities or vacancies toward the selective partial intermixing of quantum wells provides an alternative approach. The change in the shape of quantum well and thus the transition energies associated occur due to the intermixing of barrier material and quantum well material, which happens during a high-temperature annealing. The capability to identify and define regions that are not to be intermixed and which are to be intermixed acts as the key factor to the viability of the QWI approach. Intermixing method that does not demand epitaxial regrowth is identified to be more cost-effective and potentially simpler [35]. This is the main advantage of the intermixing method. In the following, the means of patterning non-intermixed and intermixed regions along with several QWI approaches are described.

### **4. Techniques utilized for QW intermixing**

There are three techniques of inter-diffusion that are in existence and are widely used. These are the inter-diffusion that is impurity-induced disordering (IID), vacancy diffusion that is IFVD, and laser-assisted disordering (LAD) laser-induced QW intermixing. The first technique uses impurities to accomplish inter-diffusion for the considerable alteration in electrical conductivity and refractive index. Its common utilization is in achieving sideways optical and electrical confinement in semiconductor lasers. In contrast, IFVD does not involve impurities in obtaining inter-diffusion such that there is the conservation of the electrical properties of the diffused QWs. Its typical use is in fabricating tuning LAD technique that has been tested and developed in the last three decades. This method is based on the direct writing of the laser beam into the structure [36–38].

### **5. Intermixing types and history**

Impurity-induced layer disordering (IILD) was the first quantum well intermixing technique to be ever demonstrated. In 1981, the affirmation of disordering of an AlAs-GaAs superlattice employing Zn (Zinc) as the active species was carried out by Laidig et al. [33, 34, 39]. In this affirmation, thermal annealing for several hours was conducted at a temperature of 600°C. As a result, it was identified that in a superlattice, different grades of intermixing can occur according to the anneal conditions used. The fabrication of lasers with emission wavelength (blue shifted) was conducted in 1983 employing this technique [33]. In 1984, it was made possible to laterally define the waveguide of a buried heterostructure employing stripe geometry QW laser devices using IILD [35, 40]. A year after, the first QW laser utilizing transparent facet windows was developed using IILD [33, 41, 42]. The refining of the intermixing method has been happening since then and has currently transformed into one of the best methods which are understood and employed in many commercial products; the most prominent of these includes high-power semiconductor lasers integrated with disordered facet windows. It should be noted that ion implantation can be utilized instead

**103**

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

dissimilar bandgap energies.

the measurements conducted.

intermixes with the adjoining barrier material [34].

of incorporating impurities (impurities include Si, Mg, or Zn) into the lattice utilizing the process of diffusion. Ion implantation possesses the primary benefit of not automatically incorporating heavy p-type or n-type doping while introducing the reactive species and of having a larger variety of species made available. On the flip side, high implant energies utilized have been identified to cause crystal damage which is not easily removable as in the case with other material systems (e.g., Si material systems). Both of the intermixing processes discussed above rely on the use of impurity atoms to intensify the Al-Ga self-diffusion process by employing different mechanisms. Although discussions and debates still exist around the exact nature relating to the process of intermixing, several experiments and authors have confirmed the unquestionable role of column-III vacancies and column-III interstitial types. A decade ago, methods such as VED, which are impurity-free intermixing methods, gained their popularity since they offered the possibility of intermixing without employing the doping process which prevent the absorption of the free carrier and without crystal damage created by implantation which would, on the other hand, be responsible for scattering loss. An As-rich ambient in a quartz ampoule was employed in the first experiments to prevent crystal surface damage by arsenic out-diffusion [36, 37]. In 1988, the use of an evaporated SiO2 encapsulant in order to improve the intermixing process was first demonstrated [38]. Soon after, the process of generating vacancies and thus supporting the process of intermixing became possible by employing other dielectrics such as SiON or SiN. In 1993, fluorides (such as SrF or AlF) were identified to prevent QWI in a more effective manner [39]. Essential for the development of optoelectronic devices and instruments, these materials identified allowed for the definition of a certain pattern with

The IILD process makes use of an Ar-based laser beam that is very highly focused in nature. To develop the AlGaAs-GaAs DFQW, the beam of laser marking a wavelength measurement of 488 nanometers (nm) is used to scan the sample which is heterostructure in nature and is also enclosed using a layer of Si-Si3N4, which is approximately 90 nm in thickness. The speed of scan employing the laser beam could be marked up to the highest value of 85 pds. The area in which the laser beam interacts will develop an enhanced cylindrical segment identifiable to the range of microns. The process of annealing is then initiated in order to guide the silicon into the required crystal, which will result in the local intermixing of the layers of crystal. On the other hand, to selectively intermix GaInAs over GaInAsP quantum well structure, pulsed photo-absorption-induced disordering (PAID) technique is employed, which was deliberated employing the utilization of time-resolved photoluminescence of high spatial resolution. As a consequence of the above-said process of intermixing, a reduction of approximately two orders of measure in the time of non-radioactive recombination was achieved, which was confirmed from

Impurity-induced, impurity-free (dielectric cap), implantation-induced, and laser-induced techniques are some of the QWI techniques that have been advanced. Out of these techniques, the use of impurity-free techniques is strongly advised since optical absorption occurs as a result of the process that the semiconductor waveguide being instituted to dopants which are electrically active in nature. In order to develop vacancies on the group III lattice site, the impurity-free vacancy disordering (IFVD) technique employs the utilization of dielectric caps, which are placed on the semiconductor's exterior surface [3, 7]. The vacancies happen to diffuse through the surface of the semiconductor resulting in solitary atoms bouncing among different lattice sites. Resultantly, it is found that the quantum well

### *Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

QWI approaches are described.

**4. Techniques utilized for QW intermixing**

writing of the laser beam into the structure [36–38].

**5. Intermixing types and history**

problem can be categorized into intermixing and growth approaches. Among the growth approaches, the most popular ones are selective area growth approach and use of a plated substrate to etch-and-regrowth approach [29, 30, 32]. The former one allows for simultaneous epitaxy employing the use of different growth rates, which in turn allows for flexibility toward the growth of quantum wells with varying thicknesses [32–34]. In contrast to the former approach, the latter approach uses different quantum well thicknesses along with subsequent growth of material. Using impurities or vacancies toward the selective partial intermixing of quantum wells provides an alternative approach. The change in the shape of quantum well and thus the transition energies associated occur due to the intermixing of barrier material and quantum well material, which happens during a high-temperature annealing. The capability to identify and define regions that are not to be intermixed and which are to be intermixed acts as the key factor to the viability of the QWI approach. Intermixing method that does not demand epitaxial regrowth is identified to be more cost-effective and potentially simpler [35]. This is the main advantage of the intermixing method. In the following, the means of patterning non-intermixed and intermixed regions along with several

There are three techniques of inter-diffusion that are in existence and are widely

Impurity-induced layer disordering (IILD) was the first quantum well intermixing technique to be ever demonstrated. In 1981, the affirmation of disordering of an AlAs-GaAs superlattice employing Zn (Zinc) as the active species was carried out by Laidig et al. [33, 34, 39]. In this affirmation, thermal annealing for several hours was conducted at a temperature of 600°C. As a result, it was identified that in a superlattice, different grades of intermixing can occur according to the anneal conditions used. The fabrication of lasers with emission wavelength (blue shifted) was conducted in 1983 employing this technique [33]. In 1984, it was made possible to laterally define the waveguide of a buried heterostructure employing stripe geometry QW laser devices using IILD [35, 40]. A year after, the first QW laser utilizing transparent facet windows was developed using IILD [33, 41, 42]. The refining of the intermixing method has been happening since then and has currently transformed into one of the best methods which are understood and employed in many commercial products; the most prominent of these includes high-power semiconductor lasers integrated with disordered facet windows. It should be noted that ion implantation can be utilized instead

used. These are the inter-diffusion that is impurity-induced disordering (IID), vacancy diffusion that is IFVD, and laser-assisted disordering (LAD) laser-induced QW intermixing. The first technique uses impurities to accomplish inter-diffusion for the considerable alteration in electrical conductivity and refractive index. Its common utilization is in achieving sideways optical and electrical confinement in semiconductor lasers. In contrast, IFVD does not involve impurities in obtaining inter-diffusion such that there is the conservation of the electrical properties of the diffused QWs. Its typical use is in fabricating tuning LAD technique that has been tested and developed in the last three decades. This method is based on the direct

**102**

of incorporating impurities (impurities include Si, Mg, or Zn) into the lattice utilizing the process of diffusion. Ion implantation possesses the primary benefit of not automatically incorporating heavy p-type or n-type doping while introducing the reactive species and of having a larger variety of species made available. On the flip side, high implant energies utilized have been identified to cause crystal damage which is not easily removable as in the case with other material systems (e.g., Si material systems). Both of the intermixing processes discussed above rely on the use of impurity atoms to intensify the Al-Ga self-diffusion process by employing different mechanisms. Although discussions and debates still exist around the exact nature relating to the process of intermixing, several experiments and authors have confirmed the unquestionable role of column-III vacancies and column-III interstitial types. A decade ago, methods such as VED, which are impurity-free intermixing methods, gained their popularity since they offered the possibility of intermixing without employing the doping process which prevent the absorption of the free carrier and without crystal damage created by implantation which would, on the other hand, be responsible for scattering loss. An As-rich ambient in a quartz ampoule was employed in the first experiments to prevent crystal surface damage by arsenic out-diffusion [36, 37]. In 1988, the use of an evaporated SiO2 encapsulant in order to improve the intermixing process was first demonstrated [38]. Soon after, the process of generating vacancies and thus supporting the process of intermixing became possible by employing other dielectrics such as SiON or SiN. In 1993, fluorides (such as SrF or AlF) were identified to prevent QWI in a more effective manner [39]. Essential for the development of optoelectronic devices and instruments, these materials identified allowed for the definition of a certain pattern with dissimilar bandgap energies.

The IILD process makes use of an Ar-based laser beam that is very highly focused in nature. To develop the AlGaAs-GaAs DFQW, the beam of laser marking a wavelength measurement of 488 nanometers (nm) is used to scan the sample which is heterostructure in nature and is also enclosed using a layer of Si-Si3N4, which is approximately 90 nm in thickness. The speed of scan employing the laser beam could be marked up to the highest value of 85 pds. The area in which the laser beam interacts will develop an enhanced cylindrical segment identifiable to the range of microns. The process of annealing is then initiated in order to guide the silicon into the required crystal, which will result in the local intermixing of the layers of crystal. On the other hand, to selectively intermix GaInAs over GaInAsP quantum well structure, pulsed photo-absorption-induced disordering (PAID) technique is employed, which was deliberated employing the utilization of time-resolved photoluminescence of high spatial resolution. As a consequence of the above-said process of intermixing, a reduction of approximately two orders of measure in the time of non-radioactive recombination was achieved, which was confirmed from the measurements conducted.

Impurity-induced, impurity-free (dielectric cap), implantation-induced, and laser-induced techniques are some of the QWI techniques that have been advanced. Out of these techniques, the use of impurity-free techniques is strongly advised since optical absorption occurs as a result of the process that the semiconductor waveguide being instituted to dopants which are electrically active in nature. In order to develop vacancies on the group III lattice site, the impurity-free vacancy disordering (IFVD) technique employs the utilization of dielectric caps, which are placed on the semiconductor's exterior surface [3, 7]. The vacancies happen to diffuse through the surface of the semiconductor resulting in solitary atoms bouncing among different lattice sites. Resultantly, it is found that the quantum well intermixes with the adjoining barrier material [34].

### **6. QW intermixing in industries**

From the development of individually addressable laser arrays of higher density to laser-based products within extreme power ranges, monolithic integration platform which is highly innovative and known as quantum well intermixing (QWI) is reshaping methods in which laser diodes are used to solve the ever-increasing optoelectronic requirements. This is particularly important since laser systems which are QWI-enabled are found to deliver far better performance characteristics in factors of power output, luminosity, yield, and dependability.

The QWI is utilized to develop passive waveguides to the interior of the laser cavities adjoining to each facet. It is identified that excellent electro-optical performance is achieved owing to the incorporation of the passive waveguides, especially referring to high-power, single-mode function. An idiosyncratic attribute of this approach is that it allows for the mass production of huge numbers of lasers in parallel, on the very same chip, with very superior efficiency since the passive waveguides are adequately long enough to relax mechanical-related cleaving tolerances. The QWI technologies can be largely employed in many other applications, owing to their farthest versatility. Some of the areas in which the extremely versatile nature of the QWI technology could be utilized to its maximum potential include monolithic photonic integrated circuits (PICs) and in the comprehension of the broad area and stack lasers which provide atypical high-power characteristics and dependability. PICs mainly find their application in broadband optical systems, optoelectronic signal processing systems, microwave photonics, and biophotonics.

QWI gains its importance since it is an integration technique that permits the tampering of the properties of a semiconductor quantum well structure, after its growth. The quantum well intermixing technique combines active and passive components on the very same chip. To manufacture complex laser diodes, laser diode array systems, and photonic integrated circuits (PICs) in a manufacturing environment, intense proprietary QWI technology is utilized. The result of this process is the development of next-generation laser technology which can easily be utilized for a variety of applications [31, 41, 42].

The evolution of the next-generation systems is driven today by the latest innovations in laser diode technology. Intense is providing laser products with far better brightness, improved lifetimes, and increased dependability by employing modernized semiconductor design and patented QWI technology. The ways in which lasers are providing viable solutions to mission-critical problems are revolutionized by the quantum well intermixing method developed by the company, innovatively by producing integrated chips at efficient levels and yields which was unidentified in the industry before.

### **7. Real applications and fabrication using QWI**

In this section, we will summarize some of the laser and light-emitting diode (LED) QWI applications that have been fabricated and tested by our group at the University of Central Florida (UCF) cleanroom facility [14, 43]. These experiments will show the important role of the intermixing and how it can be used for the integrated devices. We will start with the laser followed by the LED.

### **7.1 Laser diodes**

When quickly heated at higher degrees and topped using SiNx and SiOyNx films of various constitutions, quantum well frameworks InGaAsP are interlinked to

**105**

*layer.*

**Figure 5.**

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

red, and their output is recorded.

men as shown in **Figure 5** [14, 17, 32, 40, 41].

*7.1.1 Result*

different degrees. Laser diodes are fabricated with shifted samples of both blue and

could be considered a crucial strategy toward the development of consolidated optoelectronic circuits and instruments. The bandgap energy of the substance can be controlled with stability over a wide spectral range by monitoring the intensity of the intermixing process. The lasers generated on a single monolithic substratum may, therefore, have wavelengths of output which differ widely. The correct combination of the encrusted films may vary the wavelength to either blue or red. The narrow-field semiconductor regrowth procedures have not been very successful in repeatedly producing high-yielding optoelectronic products. Others have documented many techniques for the after-growth combination of QW. In the analysis, we selected a method of induced disorder by impurity-free vacancy that works by rapid thermal annealing (RTA) of QW specimen coated by SiNx or SiOyNx. The range of intermingling could be precisely controlled by changing the dielectric layer capping constitution. Employing this method, we were able to manufacture multiple lasers using a single sample of the InGaAsP multiple quantum well framework, which has been covered by various SiOyNx configurations in different parts and annealed at 800°C for 30 s. Slope efficiencies, threshold currents, and laser diodes that are manufactured in the separate section are then carefully defined based on their lasing wavelengths. Such output properties are then juxtaposed with that of the laser diode made employing the primary as-grown multiple quantum well speci-

Selective area mixing of semiconductor-based multiple quantum wells (MQWs)

Increasing the ratio between NH3 and SiH4 to N2O during the SiOyNx film growth has been found to result in a higher refractive index. It is noted that wavelengths (lasing) of the instruments manufactured on intermixed specimens are identified to be shifted to lower frequencies (red shift). At the same time, the capping film refractive index throughout RTA is higher than the value of 1.95 (refractive index). In comparison, the instruments covered with films having a refractive index lower than 1.95 in value show lasting wavelengths changed blue to higher frequencies. Accordingly, the absolute value of the laser spectrum is experiencing a red shift with a larger ratio in SiNx film and blue shift with a smaller ratio in SiOyNx film as shown in **Figure 6**.

Laser diode made from an as-grown multiple quantum well specimen acted as a base standard and is identified to have a lasing wavelength of 1556 nm. In **Figure 7**, all the fabricated laser diodes are shown with the accompanying spectra. The

*Schematic of the InGaAsP MQW laser diode with InP substrate as substrate layer and InGaAs as capping* 

### *Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

different degrees. Laser diodes are fabricated with shifted samples of both blue and red, and their output is recorded.

Selective area mixing of semiconductor-based multiple quantum wells (MQWs) could be considered a crucial strategy toward the development of consolidated optoelectronic circuits and instruments. The bandgap energy of the substance can be controlled with stability over a wide spectral range by monitoring the intensity of the intermixing process. The lasers generated on a single monolithic substratum may, therefore, have wavelengths of output which differ widely. The correct combination of the encrusted films may vary the wavelength to either blue or red. The narrow-field semiconductor regrowth procedures have not been very successful in repeatedly producing high-yielding optoelectronic products. Others have documented many techniques for the after-growth combination of QW. In the analysis, we selected a method of induced disorder by impurity-free vacancy that works by rapid thermal annealing (RTA) of QW specimen coated by SiNx or SiOyNx. The range of intermingling could be precisely controlled by changing the dielectric layer capping constitution. Employing this method, we were able to manufacture multiple lasers using a single sample of the InGaAsP multiple quantum well framework, which has been covered by various SiOyNx configurations in different parts and annealed at 800°C for 30 s. Slope efficiencies, threshold currents, and laser diodes that are manufactured in the separate section are then carefully defined based on their lasing wavelengths. Such output properties are then juxtaposed with that of the laser diode made employing the primary as-grown multiple quantum well specimen as shown in **Figure 5** [14, 17, 32, 40, 41].

### *7.1.1 Result*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

in factors of power output, luminosity, yield, and dependability.

utilized for a variety of applications [31, 41, 42].

**7. Real applications and fabrication using QWI**

From the development of individually addressable laser arrays of higher density to laser-based products within extreme power ranges, monolithic integration platform which is highly innovative and known as quantum well intermixing (QWI) is reshaping methods in which laser diodes are used to solve the ever-increasing optoelectronic requirements. This is particularly important since laser systems which are QWI-enabled are found to deliver far better performance characteristics

The QWI is utilized to develop passive waveguides to the interior of the laser cavities adjoining to each facet. It is identified that excellent electro-optical performance is achieved owing to the incorporation of the passive waveguides, especially referring to high-power, single-mode function. An idiosyncratic attribute of this approach is that it allows for the mass production of huge numbers of lasers in parallel, on the very same chip, with very superior efficiency since the passive waveguides are adequately long enough to relax mechanical-related cleaving tolerances. The QWI technologies can be largely employed in many other applications, owing to their farthest versatility. Some of the areas in which the extremely versatile nature of the QWI technology could be utilized to its maximum potential include monolithic photonic integrated circuits (PICs) and in the comprehension of the broad area and stack lasers which provide atypical high-power characteristics and dependability. PICs mainly find their application in broadband optical systems, optoelectronic signal processing systems, microwave photonics, and biophotonics. QWI gains its importance since it is an integration technique that permits the tampering of the properties of a semiconductor quantum well structure, after its growth. The quantum well intermixing technique combines active and passive components on the very same chip. To manufacture complex laser diodes, laser diode array systems, and photonic integrated circuits (PICs) in a manufacturing environment, intense proprietary QWI technology is utilized. The result of this process is the development of next-generation laser technology which can easily be

The evolution of the next-generation systems is driven today by the latest innovations in laser diode technology. Intense is providing laser products with far better brightness, improved lifetimes, and increased dependability by employing modernized semiconductor design and patented QWI technology. The ways in which lasers are providing viable solutions to mission-critical problems are revolutionized by the quantum well intermixing method developed by the company, innovatively by producing integrated chips at efficient levels and yields which was unidentified in

In this section, we will summarize some of the laser and light-emitting diode (LED) QWI applications that have been fabricated and tested by our group at the University of Central Florida (UCF) cleanroom facility [14, 43]. These experiments will show the important role of the intermixing and how it can be used for the

When quickly heated at higher degrees and topped using SiNx and SiOyNx films of various constitutions, quantum well frameworks InGaAsP are interlinked to

integrated devices. We will start with the laser followed by the LED.

**6. QW intermixing in industries**

**104**

the industry before.

**7.1 Laser diodes**

Increasing the ratio between NH3 and SiH4 to N2O during the SiOyNx film growth has been found to result in a higher refractive index. It is noted that wavelengths (lasing) of the instruments manufactured on intermixed specimens are identified to be shifted to lower frequencies (red shift). At the same time, the capping film refractive index throughout RTA is higher than the value of 1.95 (refractive index). In comparison, the instruments covered with films having a refractive index lower than 1.95 in value show lasting wavelengths changed blue to higher frequencies. Accordingly, the absolute value of the laser spectrum is experiencing a red shift with a larger ratio in SiNx film and blue shift with a smaller ratio in SiOyNx film as shown in **Figure 6**.

Laser diode made from an as-grown multiple quantum well specimen acted as a base standard and is identified to have a lasing wavelength of 1556 nm. In **Figure 7**, all the fabricated laser diodes are shown with the accompanying spectra. The

### **Figure 5.**

*Schematic of the InGaAsP MQW laser diode with InP substrate as substrate layer and InGaAs as capping layer.*

### **Figure 6.**

*The absolute values of all lasers' spectrum as a value of the refractive index of the film for different capping layer combinations. The blue shift is associated with SiOyNx films, while the red shift is associated with SiNx.*

### **Figure 7.**

*Laser spectrum of all fabricated devices. It shows the blue and red shifted from the as-grown ones. This figure is taken from [14].*

highest blue-shifted wavelength of laser noted is 1392 nm (164 nm change compared to as-grown laser), and 1687 nm (131 nm change as compared to as-grown laser) is the most excellent noted red-shifted wavelength of the laser. Throughout this review, it is discovered that the laser light is not emitted by a system manufactured utilizing a noncapped RTA manufactured multiple quantum well sample. Thus, uncapped regions of the MQW specimen were found to have sustained irreparable harm during thermal annealing [14].

**Figure 8** shows the output power curve (L-I curve) for all intermixed laser devices. The laser that fabricated using the most intermixed MQWs had the lowest output power, while the as-grown laser diode has the highest output power. Therefore, as we intermixed more, we create more losses that affect the device efficiency.

### **7.2 LED and modulators on InGaAsP**

Using a controllable technique for the red and blue shifting of bandgap energy of the quantum well, we were able to develop LED sources that reach a broad

**107**

**Figure 9.**

**Figure 8.**

frequency spectrum along with all-optical modulator intensity instruments. Through using an impurity-free vacancy diffusion method, they show bandgap adjustment of multiple quantum well structures of InGaAsP. By utilizing SiO2, SiOyNx, and SiNx capping layers, and by regulating the related oxygen and nitrogen

*absolute PL spectrum for selected data points. This figure and caption were taken from [43].*

*Measured absolute value of the PL shift of the RTA-treated samples from that of the as-grown wafer for different dielectric film capping, in respect to the refractive index of the film. The blue shift is associated with SiOyNx films for different ratios of NH3/N2O, while the red shift refers Si-rich compositions. The inset shows the* 

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

*The fabricated lasers diode as function of threshold current L-I curve.*

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

*Laser spectrum of all fabricated devices. It shows the blue and red shifted from the as-grown ones. This figure is* 

*The absolute values of all lasers' spectrum as a value of the refractive index of the film for different capping layer combinations. The blue shift is associated with SiOyNx films, while the red shift is associated with SiNx.*

**Figure 8** shows the output power curve (L-I curve) for all intermixed laser devices.

Using a controllable technique for the red and blue shifting of bandgap energy

of the quantum well, we were able to develop LED sources that reach a broad

highest blue-shifted wavelength of laser noted is 1392 nm (164 nm change compared to as-grown laser), and 1687 nm (131 nm change as compared to as-grown laser) is the most excellent noted red-shifted wavelength of the laser. Throughout this review, it is discovered that the laser light is not emitted by a system manufactured utilizing a noncapped RTA manufactured multiple quantum well sample. Thus, uncapped regions of the MQW specimen were found to have sustained

The laser that fabricated using the most intermixed MQWs had the lowest output power, while the as-grown laser diode has the highest output power. Therefore, as we

intermixed more, we create more losses that affect the device efficiency.

irreparable harm during thermal annealing [14].

**7.2 LED and modulators on InGaAsP**

**106**

**Figure 7.**

**Figure 6.**

*taken from [14].*

**Figure 8.** *The fabricated lasers diode as function of threshold current L-I curve.*

### **Figure 9.**

*Measured absolute value of the PL shift of the RTA-treated samples from that of the as-grown wafer for different dielectric film capping, in respect to the refractive index of the film. The blue shift is associated with SiOyNx films for different ratios of NH3/N2O, while the red shift refers Si-rich compositions. The inset shows the absolute PL spectrum for selected data points. This figure and caption were taken from [43].*

frequency spectrum along with all-optical modulator intensity instruments. Through using an impurity-free vacancy diffusion method, they show bandgap adjustment of multiple quantum well structures of InGaAsP. By utilizing SiO2, SiOyNx, and SiNx capping layers, and by regulating the related oxygen and nitrogen

### *Recent Advances in Nanophotonics - Fundamentals and Applications*

content, a significant modification of the bandgap energy toward the red and blue portions of the spectrum is identified. The subsequent degree of tuning, with bandto-band wavelength emissions of up to 120 nm red shift and 140 nm blue shift, was analyzed using photoluminescence at room temperature, following the emission spectra acquired from LED semiconductor instruments manufactured on this framework. The intensity modulator instruments are made along with compatible LED sources for the chosen frequency, designed to achieve minimal material losses and modulation of residual amplitude as shown in **Figure 9** [43].

The fabricated LED has been integrated with transparent intensity modulator as shown in **Figure 10**. The intensity modulator is based on a Mach-Zehnder

**Figure 10.** *Schematic for the integrated LED with MZI intensity modulator [43].*

**109**

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

the passing current as shown in **Figure 11**.

band-to-band blue shift of 140 nm.

**Acknowledgements**

into the core of the waveguide.

**8. Conclusion**

interferometer (MZI) where the phase control is achieved by injecting electrons

As the light source from the LED passes through the MM-MZI device, the outpower changes. The result has been recorded and evaluated as the function of

In this chapter, we have studied and compared the different methods for diffusion of atoms into both surface and internal layers. Also, we have shown the variety of QWIs that change and modify the refractive index and energy bandgap of QW's structures. There are several QWI techniques accessible, and each technique has specific characteristics that are useful under various circumstances. Very likely, more than one process will be used to produce a semiconductor chip. Among the techniques used for this purpose, owing to their capacity to preserve the electrical properties of the QW structure and its strong selectivity throughout the spatial domain, triggered disordering of MQWs by using impurity-free vacancy diffusion process gained much interest. A selective area QWI procedure is used that includes vacancy diffusion via the fast-thermal strengthening of the sample which is capped by silicon dioxide or different silicon oxynitride coatings. Prior to the fast-thermal annealing of the specimen, it is identified that the bandgap energy of the intermixed QW system can be efficiently managed by varying the dielectric capping film composition. As an illustration, for laser, we displayed the implications of intermixing of laser diodes based on InGaAsP QWs. By adjusting the proportion of mixed films, it was possible to adjust the lasing wavelength to the red or blue shift regions. Using an impurity-free vacancy diffusion method, we illustrated bandgap adjustment of several quantum well structures of InGaAsP, which was then used for the LED applications. By utilizing SiO2, SiOyNx, and SiNx capping films and by regulating the corresponding oxygen and nitrogen levels, a significant alteration of the bandgap energy toward the red and blue segments of the spectrum was achieved. The resultant level of adjustment was noted, red shift up to 120 nm and

I want to give special thanks to my professor and advisor Prof. Patrick Lickamwa for his help and advice through all my PhD. Also, I would like to thank UCF and CREOL for letting me use their cleanroom facility. Finally, many thanks go to King

Abdulaziz City for Science and Technology for their supports.

**Figure 11.** *The output power as function of injected current for the integrated device.*

### *Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

interferometer (MZI) where the phase control is achieved by injecting electrons into the core of the waveguide.

As the light source from the LED passes through the MM-MZI device, the outpower changes. The result has been recorded and evaluated as the function of the passing current as shown in **Figure 11**.

### **8. Conclusion**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

and modulation of residual amplitude as shown in **Figure 9** [43].

content, a significant modification of the bandgap energy toward the red and blue portions of the spectrum is identified. The subsequent degree of tuning, with bandto-band wavelength emissions of up to 120 nm red shift and 140 nm blue shift, was analyzed using photoluminescence at room temperature, following the emission spectra acquired from LED semiconductor instruments manufactured on this framework. The intensity modulator instruments are made along with compatible LED sources for the chosen frequency, designed to achieve minimal material losses

The fabricated LED has been integrated with transparent intensity modulator as shown in **Figure 10**. The intensity modulator is based on a Mach-Zehnder

*Schematic for the integrated LED with MZI intensity modulator [43].*

*The output power as function of injected current for the integrated device.*

**108**

**Figure 11.**

**Figure 10.**

In this chapter, we have studied and compared the different methods for diffusion of atoms into both surface and internal layers. Also, we have shown the variety of QWIs that change and modify the refractive index and energy bandgap of QW's structures. There are several QWI techniques accessible, and each technique has specific characteristics that are useful under various circumstances. Very likely, more than one process will be used to produce a semiconductor chip. Among the techniques used for this purpose, owing to their capacity to preserve the electrical properties of the QW structure and its strong selectivity throughout the spatial domain, triggered disordering of MQWs by using impurity-free vacancy diffusion process gained much interest. A selective area QWI procedure is used that includes vacancy diffusion via the fast-thermal strengthening of the sample which is capped by silicon dioxide or different silicon oxynitride coatings. Prior to the fast-thermal annealing of the specimen, it is identified that the bandgap energy of the intermixed QW system can be efficiently managed by varying the dielectric capping film composition. As an illustration, for laser, we displayed the implications of intermixing of laser diodes based on InGaAsP QWs. By adjusting the proportion of mixed films, it was possible to adjust the lasing wavelength to the red or blue shift regions. Using an impurity-free vacancy diffusion method, we illustrated bandgap adjustment of several quantum well structures of InGaAsP, which was then used for the LED applications. By utilizing SiO2, SiOyNx, and SiNx capping films and by regulating the corresponding oxygen and nitrogen levels, a significant alteration of the bandgap energy toward the red and blue segments of the spectrum was achieved. The resultant level of adjustment was noted, red shift up to 120 nm and band-to-band blue shift of 140 nm.

### **Acknowledgements**

I want to give special thanks to my professor and advisor Prof. Patrick Lickamwa for his help and advice through all my PhD. Also, I would like to thank UCF and CREOL for letting me use their cleanroom facility. Finally, many thanks go to King Abdulaziz City for Science and Technology for their supports.

*Recent Advances in Nanophotonics - Fundamentals and Applications*

### **Author details**

Thamer Tabbakh King Abdulaziz City for Science and Technology, National Center for Nanotechnology and Semiconductor, KACST, Riyadh, Saudi Arabia

\*Address all correspondence to: t.tabbakh@gmail.com

© 2020 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.

**111**

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

[1] Gale WF, Totemeier TC. Smithells Metals Reference Book. Elsevier; 2003 InGaAsP multiple quantum wells with constant P/As ratio. Applied Physics

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[17] Tabbakh T, LiKam WP. Dual wavelength single waveguide laser diode fabricated using selective area quantum well intermixing. Optik.

[18] Teng J et al. Dual-wavelength laser source monolithically integrated with Y-junction coupler and isolator using quantum-well intermixing. IEEE Photonics Technology Letters.

[19] Leon R et al. Effects of interdiffusion on the luminescence of InGaAs/GaAs quantum dots. Applied Physics Letters.

[12] Oh Y, Kang T, Kim T.Photoluminescence and photoreflectance from GaAs/AlAs multiple quantum wells. Journal of Applied Physics.

[13] Poole P et al. Defect diffusion in ion implanted AlGaAs and InP: Consequences for quantum well

intermixing. Journal of Applied Physics.

[14] Tabbakh T, LiKam WP. Blue and red shifted, partially intermixed InGaAsP quantum well semiconductor laser diodes. In: 2017 IEEE Photonics

[15] Alferness R et al. Broadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57-nm tuning range. Applied Physics Letters.

[16] Sun H et al. Characterization of selective quantum well intermixing in 1.3 μm GaInNAs/GaAs structures.

[2] Hilliard J, Averbach B, Cohen M. Self and interdiffusion in aluminum-zinc alloys. Acta Metallurgica. 1959;**7**(2):86-92

[3] Bachrach R, Bauer R. Surface reactions and interdiffusion. Journal of Vacuum Science and Technology.

[4] Pape IJ, Wa PLK, David JPR, Claxton PA, Robson PN, Sykes D. Diffusion-induced disordering of Ga/ sub 0.47/In/sub 0.53/As/InP multiple quantum wells with zinc. Electronics

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2002;**8**(4):870-879

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[5] Aimez V et al. Low-energy ionimplantation-induced quantum-well intermixing. IEEE Journal of Selected Topics in Quantum Electronics.

[6] Stephenson GB. Deformation during interdiffusion. Acta Metallurgica.

[7] Koch TL et al. Tapered waveguide InGaAs/InGaAsP multiple-quantumwell lasers. IEEE Photonics Technology

[8] O'neill M et al. Reduction of the propagation losses in impurity disordered quantum well waveguides. Electronics

intermixing. Semiconductor Science and Technology. 1993;**8**(6):1136

[10] Charbonneau S et al. Quantumwell intermixing for optoelectronic integration using high energy ion implantation. Journal of Applied Physics. 1995;**78**(6):3697-3705

Letters. 1990;**26**(19):1613-1615

[9] Marsh JH. Quantum well

[11] Hamoudi A et al. Cation interdiffusion inter-in InGaAsP/

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**References**

### **References**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

**110**

**Author details**

Thamer Tabbakh

King Abdulaziz City for Science and Technology, National Center for Nanotechnology and Semiconductor, KACST, Riyadh, Saudi Arabia

© 2020 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,

\*Address all correspondence to: t.tabbakh@gmail.com

provided the original work is properly cited.

[1] Gale WF, Totemeier TC. Smithells Metals Reference Book. Elsevier; 2003

[2] Hilliard J, Averbach B, Cohen M. Self and interdiffusion in aluminum-zinc alloys. Acta Metallurgica. 1959;**7**(2):86-92

[3] Bachrach R, Bauer R. Surface reactions and interdiffusion. Journal of Vacuum Science and Technology. 1979;**16**(5):1149-1153

[4] Pape IJ, Wa PLK, David JPR, Claxton PA, Robson PN, Sykes D. Diffusion-induced disordering of Ga/ sub 0.47/In/sub 0.53/As/InP multiple quantum wells with zinc. Electronics Letters. 1988;**24**(15):910-911

[5] Aimez V et al. Low-energy ionimplantation-induced quantum-well intermixing. IEEE Journal of Selected Topics in Quantum Electronics. 2002;**8**(4):870-879

[6] Stephenson GB. Deformation during interdiffusion. Acta Metallurgica. 1988;**36**(10):2663-2683

[7] Koch TL et al. Tapered waveguide InGaAs/InGaAsP multiple-quantumwell lasers. IEEE Photonics Technology Letters. 1990;**2**(2):88-90

[8] O'neill M et al. Reduction of the propagation losses in impurity disordered quantum well waveguides. Electronics Letters. 1990;**26**(19):1613-1615

[9] Marsh JH. Quantum well intermixing. Semiconductor Science and Technology. 1993;**8**(6):1136

[10] Charbonneau S et al. Quantumwell intermixing for optoelectronic integration using high energy ion implantation. Journal of Applied Physics. 1995;**78**(6):3697-3705

[11] Hamoudi A et al. Cation interdiffusion inter-in InGaAsP/ InGaAsP multiple quantum wells with constant P/As ratio. Applied Physics Letters. 1995;**66**(6):718-720

[12] Oh Y, Kang T, Kim T.Photoluminescence and photoreflectance from GaAs/AlAs multiple quantum wells. Journal of Applied Physics. 1995;**78**(5):3376-3379

[13] Poole P et al. Defect diffusion in ion implanted AlGaAs and InP: Consequences for quantum well intermixing. Journal of Applied Physics. 1995;**78**(4):2367-2371

[14] Tabbakh T, LiKam WP. Blue and red shifted, partially intermixed InGaAsP quantum well semiconductor laser diodes. In: 2017 IEEE Photonics Conference (IPC)

[15] Alferness R et al. Broadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57-nm tuning range. Applied Physics Letters. 1992;**60**(26):3209-3211

[16] Sun H et al. Characterization of selective quantum well intermixing in 1.3 μm GaInNAs/GaAs structures. Journal of Applied Physics. 2003;**94**(3):1550-1556

[17] Tabbakh T, LiKam WP. Dual wavelength single waveguide laser diode fabricated using selective area quantum well intermixing. Optik. 2017;**140**:592-596

[18] Teng J et al. Dual-wavelength laser source monolithically integrated with Y-junction coupler and isolator using quantum-well intermixing. IEEE Photonics Technology Letters. 2000;**12**(10):1310-1312

[19] Leon R et al. Effects of interdiffusion on the luminescence of InGaAs/GaAs quantum dots. Applied Physics Letters. 1996;**69**(13):1888-1890

[20] Alahmadi Y, LiKam WP. Effects of selective area intermixing on InAlGaAs multiple quantum well laser diode. Semiconductor Science and Technology. 2019;**34**(2):025010

[21] Beall R et al. Gallium arsenide and related compounds. In: 1988 Inst. Phys. Conf. Ser. 96. Bristol: Institute of Physics; 1989

[22] Forouhar S et al. InGaAs/InGaAsP/ InP strained-layer quantum well lasers at approximately 2 μm. Electronics Letters. 1992;**28**(15):1431-1432

[23] May-Arrioja D et al. Intermixing of InP-based multiple quantum wells for integrated optoelectronic devices. Microelectronics Journal. 2009;**40**(3):574-576

[24] May-Arrioja D et al. Intermixing of InP-based multiple quantum wells for photonic integrated circuits. In: AIP Conference Proceedings. American Institute of Physics; 2008

[25] Zucker J et al. Large blueshifting of InGaAs/InP quantum-well band gaps by ion implantation. Applied Physics Letters. 1992;**60**(24):3036-3038

[26] Lazarus D. Diffusion in metals. In: Solid State Physics. Elsevier; 1960. pp. 71-126

[27] Shewmon P. Diffusion in Solids. Springer; 2016

[28] Jost W. Diffusion in Solids, Liquid, Gases. New York: Academic Press Inc; 1960. p. 73

[29] Chang L, Koma A. Interdiffusion between GaAs and AlAs. Applied Physics Letters. 1976;**29**(3):138-141

[30] Lee M et al. Intermixing behavior in InGaAs/InGaAsP multiple quantum wells with dielectric and InGaAs capping layers. Applied Physics A. 2001;**73**(3):357-360

[31] Liu C-C et al. Intermixing in InGaAs/AlGaAs quantum well structures induced by the interdiffusion of Si impurities. 2020

[32] Tabbakh T, LiKam WP. Intermixed InGaAsP MQW tunable laser diode suitable for probing surface plasmon resonance optical sensor. In: Nanoengineering: Fabrication, Properties, Optics, and Devices XV. International Society for Optics and Photonics; 2018

[33] Laidig W et al. Disorder of an AlAs-GaAs superlattice by impurity diffusion. Applied Physics Letters. 1981;**38**(10):776-778

[34] Holonyak N Jr, Laidig WD, Camras MD, Coleman JJ, Dapkus PD. Applied Physics Letters. 1981;**39**:102

[35] Sasaki T, Kitamura M, Mito I. Selective metalorganic vapor phase epitaxial growth of InGaAsP/InP layers with bandgap energy control in InGaAs/ InGaAsP multiple-quantum well structures. Journal of Crystal Growth. 1993;**132**(3-4):435-443

[36] Qiao Z et al. Monolithic fabrication of InGaAs/GaAs/AlGaAs multiple wavelength quantum well laser diodes via impurity-free vacancy disordering quantum well intermixing. IEEE Journal of the Electron Devices Society. 2017;**5**(2):122-127

[37] Skogen EJ et al. Monolithically integrated active components: A quantum-well intermixing approach. IEEE Journal of Selected Topics in Quantum Electronics. 2005;**11**(2):343-355

[38] Ralston J et al. Roomtemperature exciton transitions in partially intermixed GaAs/AlGaAs superlattices. Applied Physics Letters. 1988;**52**(18):1511-1513

[39] Beauvais J et al. Suppression of bandgap shifts in GaAs/AlGaAs

**113**

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

[40] Tabbakh T, LiKam WP. Quantum well intermixed tunable wavelength single stripe laser diode in active photonic platforms IX. In: International Society for Optics and Photonics. 2017

[41] Tabbakh T, LiKam WP. Tunable laser diode using partially intermixed InGaAsP multiple quantum well. In: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII, Vol. 10519. International Society for Optics and

[42] Kumar R et al. Realization of high-quality InGaAs/GaAs quantum dot growth on Ge substrate and improvement of optical property through ex-situ ion implantation. Journal of Luminescence. 2020:117208

[43] Aleahmad P et al. Controllable red and blue bandgap energy shifted LEDs and modulators on InGaAsP quantum well platform. In: Integrated Optics: Devices, Materials, and

Technologies XX. International Society

for Optics and Photonics. 2016

quantum wells using strontium fluoride caps. Electronics Letters.

1992;**28**(17):1670-1672

Photonics. 2018

*Diffusion and Quantum Well Intermixing DOI: http://dx.doi.org/10.5772/intechopen.92440*

quantum wells using strontium fluoride caps. Electronics Letters. 1992;**28**(17):1670-1672

*Recent Advances in Nanophotonics - Fundamentals and Applications*

[31] Liu C-C et al. Intermixing in InGaAs/AlGaAs quantum well

of Si impurities. 2020

[32] Tabbakh T, LiKam WP.

Optics and Photonics; 2018

1981;**38**(10):776-778

1993;**132**(3-4):435-443

2017;**5**(2):122-127

2005;**11**(2):343-355

[38] Ralston J et al. Room-

1988;**52**(18):1511-1513

[33] Laidig W et al. Disorder of an AlAs-GaAs superlattice by impurity diffusion. Applied Physics Letters.

[34] Holonyak N Jr, Laidig WD, Camras MD, Coleman JJ, Dapkus PD. Applied Physics Letters. 1981;**39**:102

[35] Sasaki T, Kitamura M, Mito I. Selective metalorganic vapor phase epitaxial growth of InGaAsP/InP layers with bandgap energy control in InGaAs/ InGaAsP multiple-quantum well structures. Journal of Crystal Growth.

[36] Qiao Z et al. Monolithic fabrication of InGaAs/GaAs/AlGaAs multiple wavelength quantum well laser diodes via impurity-free vacancy disordering quantum well intermixing. IEEE Journal of the Electron Devices Society.

[37] Skogen EJ et al. Monolithically integrated active components: A quantum-well intermixing approach. IEEE Journal of Selected Topics in Quantum Electronics.

temperature exciton transitions in partially intermixed GaAs/AlGaAs superlattices. Applied Physics Letters.

[39] Beauvais J et al. Suppression of bandgap shifts in GaAs/AlGaAs

structures induced by the interdiffusion

Intermixed InGaAsP MQW tunable laser diode suitable for probing surface plasmon resonance optical sensor. In: Nanoengineering: Fabrication, Properties, Optics, and Devices XV. International Society for

[20] Alahmadi Y, LiKam WP. Effects of selective area intermixing on InAlGaAs multiple quantum well laser diode. Semiconductor Science and Technology.

[21] Beall R et al. Gallium arsenide and related compounds. In: 1988 Inst. Phys. Conf. Ser. 96. Bristol: Institute of

[22] Forouhar S et al. InGaAs/InGaAsP/ InP strained-layer quantum well lasers at approximately 2 μm. Electronics Letters. 1992;**28**(15):1431-1432

[23] May-Arrioja D et al. Intermixing of InP-based multiple quantum wells for integrated optoelectronic devices. Microelectronics Journal.

[24] May-Arrioja D et al. Intermixing of InP-based multiple quantum wells for photonic integrated circuits. In: AIP Conference Proceedings. American

[25] Zucker J et al. Large blueshifting of InGaAs/InP quantum-well band gaps by ion implantation. Applied Physics Letters. 1992;**60**(24):3036-3038

[26] Lazarus D. Diffusion in metals. In: Solid State Physics. Elsevier; 1960.

[27] Shewmon P. Diffusion in Solids.

[28] Jost W. Diffusion in Solids, Liquid, Gases. New York: Academic Press Inc;

[29] Chang L, Koma A. Interdiffusion between GaAs and AlAs. Applied Physics Letters. 1976;**29**(3):138-141

[30] Lee M et al. Intermixing behavior in InGaAs/InGaAsP multiple quantum wells with dielectric and InGaAs capping layers. Applied Physics A.

2019;**34**(2):025010

Physics; 1989

2009;**40**(3):574-576

Institute of Physics; 2008

pp. 71-126

Springer; 2016

1960. p. 73

**112**

2001;**73**(3):357-360

[40] Tabbakh T, LiKam WP. Quantum well intermixed tunable wavelength single stripe laser diode in active photonic platforms IX. In: International Society for Optics and Photonics. 2017

[41] Tabbakh T, LiKam WP. Tunable laser diode using partially intermixed InGaAsP multiple quantum well. In: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII, Vol. 10519. International Society for Optics and Photonics. 2018

[42] Kumar R et al. Realization of high-quality InGaAs/GaAs quantum dot growth on Ge substrate and improvement of optical property through ex-situ ion implantation. Journal of Luminescence. 2020:117208

[43] Aleahmad P et al. Controllable red and blue bandgap energy shifted LEDs and modulators on InGaAsP quantum well platform. In: Integrated Optics: Devices, Materials, and Technologies XX. International Society for Optics and Photonics. 2016

**115**

**1. Introduction**

**Chapter 7**

**Abstract**

Development and

understanding of the improvement phenomenon.

Characterization of High-Quality

The scope of this chapter is to introduce a highly efficient HfO2 atomic layer deposition (ALD) process with superior interface defect characteristics that can be applied on high-mobility III-V substrates. For a long time, the major academic research of III-V metal-oxide-semiconductor (MOS) studies was mainly oriented on searching for the suitable high-k dielectric, and among the reported high-k/ III-V MOS studies, Al2O3 and AlN have demonstrated the most promising results. However, usually, the dielectrics with higher dielectric constant suffered from more defective interface quality including the HfO2, which should be overcome to meet the intensive operation voltage scaling requirements. In order to protect the interface of the HfO2/III-V MOS, the exposed III-V surface has to be carefully treated before, while, and after the whole high-k deposition process. For this purpose, the effect of isopropyl alcohol precursor and in situ cyclic nitrogen plasma treatment on the HfO2 ALD process at III-V substrates was thoroughly investigated. Remarkable interface state density levels with strong inversion behavior were achieved, which have not been observed at the previous HfO2/InGaAs studies. Also, detailed analysis of the interface characteristics was investigated to broaden the

**Keywords:** high-k oxides, hafnium oxide (HfO2), atomic layer deposition (ALD), III-V channel, indium gallium arsenide (InGaAs), metal-oxide-semiconductor (MOS)

Over the past decades, the semiconductor foundry business has gone through a dynamic transformation. Recently, the foundries are leading the process development race at 10 nm [1, 2] and even to 7 nm [3, 4] and will continue to do so. However, the traditional physical scaling of advanced MOSFETs in conjunction with Dennard's scaling rules has become extremely challenging as to increase the drive currents for faster switching speeds at lower supply voltages is largely at the expense of large leakage current in extremely scaled device [5]. As a result, even with the huge R&D investments, the semiconductor firms gradually lagged the advertised on-chip feature sizes demonstrated in the scaling roadmap, and finally the end of Moore's law has been declared with the end of the 2016 International Technology Roadmap of Semiconductors (ITRS) [5, 6]. Also, the emergence of internet of things (IoT) and big data applications has driven a necessity of abundant

HfO2/InGaAs MOS Interface

*Sukeun Eom, Min-woo Kong and Kwang-seok Seo*

### **Chapter 7**

## Development and Characterization of High-Quality HfO2/InGaAs MOS Interface

*Sukeun Eom, Min-woo Kong and Kwang-seok Seo*

### **Abstract**

The scope of this chapter is to introduce a highly efficient HfO2 atomic layer deposition (ALD) process with superior interface defect characteristics that can be applied on high-mobility III-V substrates. For a long time, the major academic research of III-V metal-oxide-semiconductor (MOS) studies was mainly oriented on searching for the suitable high-k dielectric, and among the reported high-k/ III-V MOS studies, Al2O3 and AlN have demonstrated the most promising results. However, usually, the dielectrics with higher dielectric constant suffered from more defective interface quality including the HfO2, which should be overcome to meet the intensive operation voltage scaling requirements. In order to protect the interface of the HfO2/III-V MOS, the exposed III-V surface has to be carefully treated before, while, and after the whole high-k deposition process. For this purpose, the effect of isopropyl alcohol precursor and in situ cyclic nitrogen plasma treatment on the HfO2 ALD process at III-V substrates was thoroughly investigated. Remarkable interface state density levels with strong inversion behavior were achieved, which have not been observed at the previous HfO2/InGaAs studies. Also, detailed analysis of the interface characteristics was investigated to broaden the understanding of the improvement phenomenon.

**Keywords:** high-k oxides, hafnium oxide (HfO2), atomic layer deposition (ALD), III-V channel, indium gallium arsenide (InGaAs), metal-oxide-semiconductor (MOS)

### **1. Introduction**

Over the past decades, the semiconductor foundry business has gone through a dynamic transformation. Recently, the foundries are leading the process development race at 10 nm [1, 2] and even to 7 nm [3, 4] and will continue to do so. However, the traditional physical scaling of advanced MOSFETs in conjunction with Dennard's scaling rules has become extremely challenging as to increase the drive currents for faster switching speeds at lower supply voltages is largely at the expense of large leakage current in extremely scaled device [5]. As a result, even with the huge R&D investments, the semiconductor firms gradually lagged the advertised on-chip feature sizes demonstrated in the scaling roadmap, and finally the end of Moore's law has been declared with the end of the 2016 International Technology Roadmap of Semiconductors (ITRS) [5, 6]. Also, the emergence of internet of things (IoT) and big data applications has driven a necessity of abundant computing and memory resources that requires always-on and high-performance ultralow-power devices to generate data instantly. Several device architectures and novel materials based on both analytical and experimental academic research were proposed in the metal-oxide-semiconductor field-effect transistor (MOSFET) technology. Among the viable technologies, the compound semiconductor especially the III-V materials have stood out to be a promising channel candidate for the future highly scaled CMOS application.

The light effective mass of III-V materials compared to the Si even in the highly strained case leads to a higher electron mobility and a higher injection velocity, which should translate into a great turn-on performance even at a lower operation voltage (VDD) level down to 0.5 V. Moreover, there is already a mature industry that uses III-V high electron mobility transistors (HEMTs) for high-frequency applications [7, 8], and it provides excellent techniques such as InGaAs and InAs quantum well (QW) FETs [9, 10]. However, most of these III-V compound semiconductors have smaller bandgaps, which have great impact on the band-to-band tunneling leakage currents. In addition, according to Yan's model [11], the higher permittivity of these materials may worsen the short channel effects (SCE). In spite of the demerits that may limit the scalability, the benefits are much more attractive which makes the III-V channel technology a powerful beyond CMOS solution. However, the use of III-V compound semiconductors has been reluctant to the industry because of its high-cost manufacturing process and CMOSincompatible process. Naturally, it brought out a strong motivation of research of III-V hetero-integration on a Si platform. The main obstacle of III-V on Si integration research is that as huge lattice constant mismatch exists between those two materials, growing epitaxial films directly on Si without defect is difficult [12]. Accordingly, different approaches have been developed, and among them, direct wafer bonding [13, 14] and aspect ratio trapping (ART) [15, 16] technologies have projected the most promising results.

Consequently, the remaining issue toward the practical realization of III-V materials is its defective interface quality which has been the major drawback compared to Si [17–21]. The poor native oxide quality compared with SiO2 is challenging even more with III-V materials. The III-V compound semiconductors are typically composed of binary, ternary, or even quaternary material by covalent bonding, and more complex elements mean a much richer population of possible oxides for the III-V materials [22]. These native oxides are not thermodynamically stable and very leaky that rise serious issues of creation of significant surface states on the oxidesemiconductor interface and huge trap-assisted gate leakage current [17, 19]. At the early stage of research, GaAs MOSFET suffered from high density of interface states (over 3 orders compared to Si) hindering inversion mode operation.

In order to overcome the defective interface problem, many research groups conducted extensive research effort with a search for a perfect gate dielectric that suits the III-V substrate [23, 24]. The study of atomic layer deposition (ALD) high-k dielectric led to a successful integration of high-k gate dielectrics on III-V substrate, and recent research is mainly focused on the development and interface characterization of ALD high-k and III–V compound semiconductor. To evaluate the objective III-V metal-oxide-semiconductor (MOS) characteristics, it is important to understand the trapping mechanism and know what kind of measurement is required. For Si, the primary defects are the well-known Pb centers, which are dangling bonds at the immediate interface with the dielectric [25]. However, for the III-V material, the anti-sites and interstitials are the critical defect centers [17], and the small DOS of the III-V materials is also a weak point [26]. These differences lead to different trapping mechanism, and unlike Si MOS, the III-V MOS gate stack often exhibits a particular C-V phenomenon typically known as the frequency dispersion effect [27].

**117**

studies work.

after the pretreatment process.

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

lation capacitance increases as the measurement frequency decreases.

process is responsible for the strong temperature dependency.

The features of the frequency dispersion effect are threefold. First, large inversionlike hump occurs even at high measurement frequency, which could not be an actual inversion characteristic theoretically. Secondly, the C-V curve horizontally shifts to the negative direction as the measurement frequency decreases. Finally, the accumu-

The large interfacial trap densities (Dit) that reside within high-k dielectric and III-V substrate are mostly responsible for the explained features [28]. The high Dit especially the near mid-gap states act as generation recombination centers that attribute to the inversion hump phenomenon in the weak inversion regime. In addition, the large donor-like Dit near the conductance band (for n-type substrate) induces a substantial surface charge that needs to be compensated by larger gate biases resulting in a horizontal shift in the C-V curve. Detailed discussions are well explained through both theoretical and experimental research [27, 28]. The accumulation capacitance increase, however, is quite difficult to be explained only by the interface traps. There have been numerous publications on this particular accumulation dispersion behavior, and discussion led to an explanation of a carrier transport model from the crystalline semiconductor into the border traps, which are defects within the bulk of the dielectric [29]. The capture and emission process occur at border traps with the interaction of conduction band electrons resulting in discrepancy of accumulation capacitance, and the thermal barrier in capture

Among the reported high-quality insulator/InGaAs interface studies, the direct deposition of hafnium oxide (HfO2) on InGaAs substrate has generally led to poor electrical characteristics, and there are only few studies aimed at improving the intrinsic HfO2/InGaAs interface quality [30, 31]. These studies also target only in pretreatments, which is vulnerable during oxide deposition. Meanwhile, O3 and H2O are the most common oxidants employed in HfO2 ALD. However, one of the disadvantages of H2O-based ALD is high-concentration hydroxyl groups in the films, which degrades the dielectric interface during the post deposition annealing process [32]. In addition, sufficiently long purge time is needed because H2O tends to physisorb on the surface strongly, especially at low temperature. To solve this problem, O3 is used as one of the most promising alternative oxidants in ALD process, due to its strong oxidization and high volatility. However, O3 is known to oxidize the III-V surface during the initial deposition cycles which will neglect the prior surface treatments that easily cause the formation of inferior native oxides [33]. The excess interfacial oxidation of the InGaAs surface initiated by the use of ozone is widely reported in the previous studies. H2O oxidant also is not totally free from surface oxidation [34]. Therefore, the research on alternative oxidation sources is necessary for the HfO2/InGaAs MOS studies to make the effort made in the pretreatment

**2. Development of IPA-based ALD HfO2 on n-type InGaAs substrates**

Looking into the oxidant candidates, isopropyl alcohol (IPA) is known to be irresponsive to the semiconductor surface during the initial ALD cycles [35], and as most pretreatment studies are aimed at removing the native oxides of the III-V surface, the IPA oxidant will be able to efficiently suppress the surface oxidation

In order to study the effect of using IPA oxidant, O3 was used as the reference to compare. The basic cycle of the HfO2 deposition is consisted of a TEMAH precursor pulse and an oxidant (O3 or IPA) exposure with N2 purging process between the precursor injection and oxidant process. The temperature of the IPA precursor was

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

### *Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

The features of the frequency dispersion effect are threefold. First, large inversionlike hump occurs even at high measurement frequency, which could not be an actual inversion characteristic theoretically. Secondly, the C-V curve horizontally shifts to the negative direction as the measurement frequency decreases. Finally, the accumulation capacitance increases as the measurement frequency decreases.

The large interfacial trap densities (Dit) that reside within high-k dielectric and III-V substrate are mostly responsible for the explained features [28]. The high Dit especially the near mid-gap states act as generation recombination centers that attribute to the inversion hump phenomenon in the weak inversion regime. In addition, the large donor-like Dit near the conductance band (for n-type substrate) induces a substantial surface charge that needs to be compensated by larger gate biases resulting in a horizontal shift in the C-V curve. Detailed discussions are well explained through both theoretical and experimental research [27, 28]. The accumulation capacitance increase, however, is quite difficult to be explained only by the interface traps. There have been numerous publications on this particular accumulation dispersion behavior, and discussion led to an explanation of a carrier transport model from the crystalline semiconductor into the border traps, which are defects within the bulk of the dielectric [29]. The capture and emission process occur at border traps with the interaction of conduction band electrons resulting in discrepancy of accumulation capacitance, and the thermal barrier in capture process is responsible for the strong temperature dependency.

Among the reported high-quality insulator/InGaAs interface studies, the direct deposition of hafnium oxide (HfO2) on InGaAs substrate has generally led to poor electrical characteristics, and there are only few studies aimed at improving the intrinsic HfO2/InGaAs interface quality [30, 31]. These studies also target only in pretreatments, which is vulnerable during oxide deposition. Meanwhile, O3 and H2O are the most common oxidants employed in HfO2 ALD. However, one of the disadvantages of H2O-based ALD is high-concentration hydroxyl groups in the films, which degrades the dielectric interface during the post deposition annealing process [32]. In addition, sufficiently long purge time is needed because H2O tends to physisorb on the surface strongly, especially at low temperature. To solve this problem, O3 is used as one of the most promising alternative oxidants in ALD process, due to its strong oxidization and high volatility. However, O3 is known to oxidize the III-V surface during the initial deposition cycles which will neglect the prior surface treatments that easily cause the formation of inferior native oxides [33]. The excess interfacial oxidation of the InGaAs surface initiated by the use of ozone is widely reported in the previous studies. H2O oxidant also is not totally free from surface oxidation [34]. Therefore, the research on alternative oxidation sources is necessary for the HfO2/InGaAs MOS studies to make the effort made in the pretreatment studies work.

### **2. Development of IPA-based ALD HfO2 on n-type InGaAs substrates**

Looking into the oxidant candidates, isopropyl alcohol (IPA) is known to be irresponsive to the semiconductor surface during the initial ALD cycles [35], and as most pretreatment studies are aimed at removing the native oxides of the III-V surface, the IPA oxidant will be able to efficiently suppress the surface oxidation after the pretreatment process.

In order to study the effect of using IPA oxidant, O3 was used as the reference to compare. The basic cycle of the HfO2 deposition is consisted of a TEMAH precursor pulse and an oxidant (O3 or IPA) exposure with N2 purging process between the precursor injection and oxidant process. The temperature of the IPA precursor was

*Recent Advances in Nanophotonics - Fundamentals and Applications*

highly scaled CMOS application.

projected the most promising results.

computing and memory resources that requires always-on and high-performance ultralow-power devices to generate data instantly. Several device architectures and novel materials based on both analytical and experimental academic research were proposed in the metal-oxide-semiconductor field-effect transistor (MOSFET) technology. Among the viable technologies, the compound semiconductor especially the III-V materials have stood out to be a promising channel candidate for the future

The light effective mass of III-V materials compared to the Si even in the highly strained case leads to a higher electron mobility and a higher injection velocity, which should translate into a great turn-on performance even at a lower operation voltage (VDD) level down to 0.5 V. Moreover, there is already a mature industry that uses III-V high electron mobility transistors (HEMTs) for high-frequency applications [7, 8], and it provides excellent techniques such as InGaAs and InAs quantum well (QW) FETs [9, 10]. However, most of these III-V compound semiconductors have smaller bandgaps, which have great impact on the band-to-band tunneling leakage currents. In addition, according to Yan's model [11], the higher permittivity of these materials may worsen the short channel effects (SCE). In spite of the demerits that may limit the scalability, the benefits are much more attractive which makes the III-V channel technology a powerful beyond CMOS solution. However, the use of III-V compound semiconductors has been reluctant to the industry because of its high-cost manufacturing process and CMOSincompatible process. Naturally, it brought out a strong motivation of research of III-V hetero-integration on a Si platform. The main obstacle of III-V on Si integration research is that as huge lattice constant mismatch exists between those two materials, growing epitaxial films directly on Si without defect is difficult [12]. Accordingly, different approaches have been developed, and among them, direct wafer bonding [13, 14] and aspect ratio trapping (ART) [15, 16] technologies have

Consequently, the remaining issue toward the practical realization of III-V materials is its defective interface quality which has been the major drawback compared to Si [17–21]. The poor native oxide quality compared with SiO2 is challenging even more with III-V materials. The III-V compound semiconductors are typically composed of binary, ternary, or even quaternary material by covalent bonding, and more complex elements mean a much richer population of possible oxides for the III-V materials [22]. These native oxides are not thermodynamically stable and very leaky that rise serious issues of creation of significant surface states on the oxidesemiconductor interface and huge trap-assisted gate leakage current [17, 19]. At the early stage of research, GaAs MOSFET suffered from high density of interface states

(over 3 orders compared to Si) hindering inversion mode operation.

In order to overcome the defective interface problem, many research groups conducted extensive research effort with a search for a perfect gate dielectric that suits the III-V substrate [23, 24]. The study of atomic layer deposition (ALD) high-k dielectric led to a successful integration of high-k gate dielectrics on III-V substrate, and recent research is mainly focused on the development and interface characterization of ALD high-k and III–V compound semiconductor. To evaluate the objective III-V metal-oxide-semiconductor (MOS) characteristics, it is important to understand the trapping mechanism and know what kind of measurement is required. For Si, the primary defects are the well-known Pb centers, which are dangling bonds at the immediate interface with the dielectric [25]. However, for the III-V material, the anti-sites and interstitials are the critical defect centers [17], and the small DOS of the III-V materials is also a weak point [26]. These differences lead to different trapping mechanism, and unlike Si MOS, the III-V MOS gate stack often exhibits a particular C-V phenomenon typically known as the frequency dispersion effect [27].

**116**

maintained at 4°C. The vapor pressure of IPA at 4°C is around 10 mmHg, which is four times smaller than that at the room temperature [36]. It is important to control the excessive vapor pressure because it leads to a longer purge time, which disables an efficient ALD cycle.

The ALD characteristics of HfO2 using O3 and IPA oxidants are shown in **Figure 1**. Oxidant pulse times were 1 and 3 s for O3 and IPA, respectively, which

**Figure 1.**

*Comparison of the O***3***- and IPA-based HfO***2** *ALD characteristics: (a) deposition rate vs. oxidant time, (b) deposition rate vs. deposition temperature, and (c) growth per cycle rate.*

**119**

**Figure 2.**

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

were chosen to meet the saturation requirement of ALD. Both oxidants had similar saturated deposition rate of 0.1 nm/cycle. Noticeable difference was observed in the temperature windows of oxidant type. While stable deposition rate of O3 oxidant was maintained in a large temperature range, saturated deposition rate of IPA oxidant was only observed in a small temperature range around 320°C. In low temperatures, low deposition rate is due to insufficient reaction which is originated from low reactivity of IPA. Also, in high temperatures above 320°C, thermal decomposition of Hf precursor occurs, and it hinders the self-limiting characteristics of ALD. Therefore, the deposition temperatures of HfO2 ALD were chosen to be 230 and 320°C for O3 and IPA, respectively. Moreover, the film thickness per ALD cycles is presented. It is observed that the linear deposition rate per cycle is obtained for both oxidants and a thicker interface layer thickness appears to be existed for the O3 oxidant due to is strong reactivity (**Figure 2**).

Based on the ALD characteristics, the HfO2/Si MOS capacitors are fabricated on the Si substrate. All samples underwent standard Si cleaning steps that consisted of SPM- and HF-based cleaning and 400°C 10 min annealing after the dielectric deposition. The C-V and forward gate leakage characteristics are measured and discussed. First of all, the C-V hysteresis difference is notable. As anticipated, the C-V hysteresis significantly decreases by employing the IPA oxidant. Powerful

*Comparison of O3- and IPA-based HfO2/Si MOS capacitors: (a) C-V and (b) leakage-E plot.*

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

### *Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

were chosen to meet the saturation requirement of ALD. Both oxidants had similar saturated deposition rate of 0.1 nm/cycle. Noticeable difference was observed in the temperature windows of oxidant type. While stable deposition rate of O3 oxidant was maintained in a large temperature range, saturated deposition rate of IPA oxidant was only observed in a small temperature range around 320°C. In low temperatures, low deposition rate is due to insufficient reaction which is originated from low reactivity of IPA. Also, in high temperatures above 320°C, thermal decomposition of Hf precursor occurs, and it hinders the self-limiting characteristics of ALD. Therefore, the deposition temperatures of HfO2 ALD were chosen to be 230 and 320°C for O3 and IPA, respectively. Moreover, the film thickness per ALD cycles is presented. It is observed that the linear deposition rate per cycle is obtained for both oxidants and a thicker interface layer thickness appears to be existed for the O3 oxidant due to is strong reactivity (**Figure 2**).

Based on the ALD characteristics, the HfO2/Si MOS capacitors are fabricated on the Si substrate. All samples underwent standard Si cleaning steps that consisted of SPM- and HF-based cleaning and 400°C 10 min annealing after the dielectric deposition. The C-V and forward gate leakage characteristics are measured and discussed. First of all, the C-V hysteresis difference is notable. As anticipated, the C-V hysteresis significantly decreases by employing the IPA oxidant. Powerful

**Figure 2.** *Comparison of O3- and IPA-based HfO2/Si MOS capacitors: (a) C-V and (b) leakage-E plot.*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

an efficient ALD cycle.

maintained at 4°C. The vapor pressure of IPA at 4°C is around 10 mmHg, which is four times smaller than that at the room temperature [36]. It is important to control the excessive vapor pressure because it leads to a longer purge time, which disables

The ALD characteristics of HfO2 using O3 and IPA oxidants are shown in **Figure 1**. Oxidant pulse times were 1 and 3 s for O3 and IPA, respectively, which

**118**

**Figure 1.**

*Comparison of the O***3***- and IPA-based HfO***2** *ALD characteristics: (a) deposition rate vs. oxidant time,* 

*(b) deposition rate vs. deposition temperature, and (c) growth per cycle rate.*

oxidation ability of ozone may induce undesired interfacial oxide at the Si interface forming defective hafnium silicate leading to a large hysteresis, while IPA-based HfO2 appears to be negligible on this effect [37]. The dielectric constants of IPAbased and O3-based HfO2 extracted by the thickness series method are 19.4 and 17.6, respectively [38]. While the C-V results report promising potential of IPA oxidant in ALD HfO2, the leakage properties suggest a different aspect. Leaky forward gate leakage especially in the medium gate voltage range of the IPA-based HfO2 is presented compared to the O3-based HfO2. It is well known that at this gate bias range, the dominant leakage mechanism is by the Poole-Frenkel tunneling, which is a conduction method of electron tunneling from a metal electrode to traps in a nearby insulator layer, followed by detrapping of the electrons from the traps by virtue of a lowered potential well due to an applied electric field [39]. It usually implies the bulk quality of dielectric; in short, the larger the leakage in this E-field is, the more inferior the gate insulator is. It is speculated that by using the IPA oxidant, the bulk quality may be inferior than using the O3 oxidant in ALD HfO2. This might affect the further scaling down potential and the border trap density in ALD HfO2 application on InGaAs substrate [40].

**Figure 3.**

*Multifrequency C-V responses of (a) O3- and (b) IPA-based HfO2/In0.53Ga0.47As MOS capacitors; insets are the hysteresis at 1 MHz.*

**121**

**Figure 4.**

*(a) O3-HfO2 and (b) IPA-HfO2.*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

By using the developed O3- and IPA-based HfO2 dielectrics, HfO2/InGaAs MOS capacitors were successfully demonstrated [41]. The multifrequency

(1 kHz–1 MHz) C-V characteristics of HfO2/n-In0.53Ga0.47As MOS capacitors using the O3 and IPA oxidants are presented in **Figure 3**. The C-V curves of the O3-based HfO2 ALD showed a large inversion hump in the negative bias range, which is attributed to the large density of interface defect states near the mid-gap trap level. In contrast, those of the IPA-based HfO2 ALD showed a notable suppression of the inversion hump behavior. In addition, by employing the IPA oxidant, the effective oxide thickness (EOT) has decreased. We hypothesize that the reduced inversion hump and decrease of the EOT originate from the suppression of unintentional interfacial oxides by the use of the IPA oxidant. Detailed material characteristics analysis was conducted and proved the hypothesis to be convincing [41]. To our knowledge, it is the first successful demonstration of HfO2 deposition using IPA at

Despite the advantages of using the IPA oxidant, frequency dispersion at the accumulation region slightly increased from 3.3 to 4.7% per decade. In **Figure 4**, these values were used to estimate the border trap densities (Nbt) by using a distrib-

*Border trap estimation of HfO2/InGaAs MOS capacitors by using the distributed oxide bulk trap model* 

eV<sup>−</sup><sup>1</sup>

. Also, larger C-V hysteresis and severely degraded

was extracted

uted bulk-oxide trap model, and increased Nbt of 1.1 × 1020 cm−<sup>3</sup>

eV<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

InGaAs substrate.

compared to 6.7 × 1019 cm−<sup>3</sup>

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

By using the developed O3- and IPA-based HfO2 dielectrics, HfO2/InGaAs MOS capacitors were successfully demonstrated [41]. The multifrequency (1 kHz–1 MHz) C-V characteristics of HfO2/n-In0.53Ga0.47As MOS capacitors using the O3 and IPA oxidants are presented in **Figure 3**. The C-V curves of the O3-based HfO2 ALD showed a large inversion hump in the negative bias range, which is attributed to the large density of interface defect states near the mid-gap trap level. In contrast, those of the IPA-based HfO2 ALD showed a notable suppression of the inversion hump behavior. In addition, by employing the IPA oxidant, the effective oxide thickness (EOT) has decreased. We hypothesize that the reduced inversion hump and decrease of the EOT originate from the suppression of unintentional interfacial oxides by the use of the IPA oxidant. Detailed material characteristics analysis was conducted and proved the hypothesis to be convincing [41]. To our knowledge, it is the first successful demonstration of HfO2 deposition using IPA at InGaAs substrate.

Despite the advantages of using the IPA oxidant, frequency dispersion at the accumulation region slightly increased from 3.3 to 4.7% per decade. In **Figure 4**, these values were used to estimate the border trap densities (Nbt) by using a distributed bulk-oxide trap model, and increased Nbt of 1.1 × 1020 cm−<sup>3</sup> eV<sup>−</sup><sup>1</sup> was extracted compared to 6.7 × 1019 cm−<sup>3</sup> eV<sup>−</sup><sup>1</sup> . Also, larger C-V hysteresis and severely degraded

### **Figure 4.**

*Border trap estimation of HfO2/InGaAs MOS capacitors by using the distributed oxide bulk trap model (a) O3-HfO2 and (b) IPA-HfO2.*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

ALD HfO2 application on InGaAs substrate [40].

oxidation ability of ozone may induce undesired interfacial oxide at the Si interface forming defective hafnium silicate leading to a large hysteresis, while IPA-based HfO2 appears to be negligible on this effect [37]. The dielectric constants of IPAbased and O3-based HfO2 extracted by the thickness series method are 19.4 and 17.6, respectively [38]. While the C-V results report promising potential of IPA oxidant in ALD HfO2, the leakage properties suggest a different aspect. Leaky forward gate leakage especially in the medium gate voltage range of the IPA-based HfO2 is presented compared to the O3-based HfO2. It is well known that at this gate bias range, the dominant leakage mechanism is by the Poole-Frenkel tunneling, which is a conduction method of electron tunneling from a metal electrode to traps in a nearby insulator layer, followed by detrapping of the electrons from the traps by virtue of a lowered potential well due to an applied electric field [39]. It usually implies the bulk quality of dielectric; in short, the larger the leakage in this E-field is, the more inferior the gate insulator is. It is speculated that by using the IPA oxidant, the bulk quality may be inferior than using the O3 oxidant in ALD HfO2. This might affect the further scaling down potential and the border trap density in

**120**

**Figure 3.**

*the hysteresis at 1 MHz.*

*Multifrequency C-V responses of (a) O3- and (b) IPA-based HfO2/In0.53Ga0.47As MOS capacitors; insets are* 

leakage currents at positive bias are noticed. Based on these results, an inferior quality of the HfO2 film for using IPA oxidant was predicted which should be resolved for reliable use of the IPA oxidant.

In order to improve the weak IPA-based HfO2 bulk quality, the study of origin in HfO2 defect is necessary. One of the main concerns in the replacement of SiO2 to HfO2 is that compared to SiO2, HfO2 generally suffers from high defect densities leading to several issues such as large carrier trapping, mobility degradation due to coulombic scattering in the channel surface, and threshold voltage shifts in gate stress conditions [42]. To be specific, the threshold voltage shift issue was not a new phenomenon that suddenly happened with use of HfO2. In immature SiO2 MOSFETs, it is widely known that the extrinsic contaminations in SiO2 with alkali ions induce this similar phenomenon [43]. However, with HfO2, it appeared to be caused by the high defect concentrations, which originated from a more fundamental problem, not an extrinsic defect. Consequently, many researches were devoted to HfO2 physical model simulation in order to identify the type of defects and their energy levels, and by these physical studies, researchers hoped to learn how the deposition and processing conditions can be optimized to minimize these defect origins [42, 44, 45].

Based on computational calculations, it is identified that oxygen vacancies in HfO2 are both the principal trap and main cause of the discussed issues, and its formation energy and energy levels were also calculated [44]. Hence, in order to reduce the defect densities, experiments regarding deposition and post processing conditions were aimed to remove or passivate these defects, with an oxygen-rich ambient. However, in many cases, it only worked to some extent and led to new issues of excessive oxidation leaving oxygen interstitials and oxygen diffusion to the interface [46].

Additionally, due to the low density of states of III-V semiconductors, III-V substrates are heavily influenced to border traps that could severely worsen the device performance resulting in poor reliability properties. Therefore, not only the interface but also the bulk characteristics of HfO2 should be considered in III-V MOS studies, and improvement in both qualities is definitely important.

One of the most effective methods to improve the inherent properties of HfO2 is the incorporation of nitrogen to passivate oxygen vacancies, and it has been extensively utilized in many recent studies [47–51]. Significant improvement in the electrical characteristics of various high-k gate dielectrics by nitrogen incorporation has been demonstrated, and it is found that interfacial layer growth is effectively suppressed [49] and there is lower boron penetration with nitrogen incorporation [51]. Also, lower leakage current density in HfOxNy is widely reported due to suppression of oxygen vacancy traps [50]. It has been reported that nitrogen incorporation in HfO2 can be achieved by several methods mostly by nitrogen ambient plasmabased nitridation [47, 50] or ammonia (NH3) ambient high-temperature annealing treatment [52, 53]. For Si-based MOS studies, the later approach is known to be very powerful for achieving good uniformity of nitrogen incorporation and excellent interface quality due to the absence of plasma damage. However, in order to successfully apply nitrogen incorporation technology on III-V substrate, the low thermal budget of III-V compound semiconductor always has to be considered, and high-temperature annealing treatment should be ruled out for nitrogen incorporation study in III-V MOS. In the other hand, although plasma-based nitridation technology offers low thermal budget capacity, most studies generally suffers from several issues such as nonuniform nitrogen distribution throughout dielectric, plasma-induced damage due to high-power plasma for dielectric penetration, and high energy potential nitrogen species substituting well-combined Hf-O bonds. Post deposition plasma treatments have recently been suggested for InGaAs MOS

**123**

**Figure 6.**

*the hysteresis at 1 MHz.*

**Figure 5.**

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

*ALD cycle sequence of the developed HfOxNy processes on InGaAs substrates.*

*Multifrequency C-V responses of (a) O3- and (b) IPA-based HfON/In0.53Ga0.47As MOS capacitors; insets are* 

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

**Figure 6.**

*Multifrequency C-V responses of (a) O3- and (b) IPA-based HfON/In0.53Ga0.47As MOS capacitors; insets are the hysteresis at 1 MHz.*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

for reliable use of the IPA oxidant.

origins [42, 44, 45].

interface [46].

leakage currents at positive bias are noticed. Based on these results, an inferior quality of the HfO2 film for using IPA oxidant was predicted which should be resolved

In order to improve the weak IPA-based HfO2 bulk quality, the study of origin in HfO2 defect is necessary. One of the main concerns in the replacement of SiO2 to HfO2 is that compared to SiO2, HfO2 generally suffers from high defect densities leading to several issues such as large carrier trapping, mobility degradation due to coulombic scattering in the channel surface, and threshold voltage shifts in gate stress conditions [42]. To be specific, the threshold voltage shift issue was not a new phenomenon that suddenly happened with use of HfO2. In immature SiO2 MOSFETs, it is widely known that the extrinsic contaminations in SiO2 with alkali ions induce this similar phenomenon [43]. However, with HfO2, it appeared to be caused by the high defect concentrations, which originated from a more fundamental problem, not an extrinsic defect. Consequently, many researches were devoted to HfO2 physical model simulation in order to identify the type of defects and their energy levels, and by these physical studies, researchers hoped to learn how the deposition and processing conditions can be optimized to minimize these defect

Based on computational calculations, it is identified that oxygen vacancies in HfO2 are both the principal trap and main cause of the discussed issues, and its formation energy and energy levels were also calculated [44]. Hence, in order to reduce the defect densities, experiments regarding deposition and post processing conditions were aimed to remove or passivate these defects, with an oxygen-rich ambient. However, in many cases, it only worked to some extent and led to new issues of excessive oxidation leaving oxygen interstitials and oxygen diffusion to the

Additionally, due to the low density of states of III-V semiconductors, III-V substrates are heavily influenced to border traps that could severely worsen the device performance resulting in poor reliability properties. Therefore, not only the interface but also the bulk characteristics of HfO2 should be considered in III-V

One of the most effective methods to improve the inherent properties of HfO2 is the incorporation of nitrogen to passivate oxygen vacancies, and it has been extensively utilized in many recent studies [47–51]. Significant improvement in the electrical characteristics of various high-k gate dielectrics by nitrogen incorporation has been demonstrated, and it is found that interfacial layer growth is effectively suppressed [49] and there is lower boron penetration with nitrogen incorporation [51]. Also, lower leakage current density in HfOxNy is widely reported due to suppression of oxygen vacancy traps [50]. It has been reported that nitrogen incorporation in HfO2 can be achieved by several methods mostly by nitrogen ambient plasmabased nitridation [47, 50] or ammonia (NH3) ambient high-temperature annealing treatment [52, 53]. For Si-based MOS studies, the later approach is known to be very powerful for achieving good uniformity of nitrogen incorporation and excellent interface quality due to the absence of plasma damage. However, in order to successfully apply nitrogen incorporation technology on III-V substrate, the low thermal budget of III-V compound semiconductor always has to be considered, and high-temperature annealing treatment should be ruled out for nitrogen incorporation study in III-V MOS. In the other hand, although plasma-based nitridation technology offers low thermal budget capacity, most studies generally suffers from several issues such as nonuniform nitrogen distribution throughout dielectric, plasma-induced damage due to high-power plasma for dielectric penetration, and high energy potential nitrogen species substituting well-combined Hf-O bonds. Post deposition plasma treatments have recently been suggested for InGaAs MOS

MOS studies, and improvement in both qualities is definitely important.

**122**

devices [54]; however, no effort was made to improve the nitridation technology regarding the discussed issues.

As a result, in order to improve the film quality of HfO2, a cyclic nitrogen lowpower plasma step was added within the ALD cycles to passivate oxygen vacancies uniformly without causing damage or surface degradation. Through this technology with a combination of IPA oxidant, achievement of improvement in both interface and bulk quality of high-k/InGaAs MOS properties is expected. The detailed information of the ALD sequence is depicted in **Figure 5**. Every cycle consisted of sequential precursor pulse steps and a gas stabilization step followed by 5 s of 50 W N2 plasma step with purge steps between pulse steps. It is discovered that there is a trade-off relationship of plasma condition. The plasma power should be enough for effective passivation although it may degrade the substrate by radiation damage. Through the developed ALD sequence, with adequate plasma condition, the HfO2 layer is improved without having influence in the substrate.

By using the proposed nitridation technology, HfOxNy/InGaAs MOS capacitors are fabricated showing promising results as shown in **Figure 6** [41]. A significant suppression of the frequency dispersion was observed upon nitrogen incorporation in every gate bias range. The inversion humps and flat band voltage shift were effectively reduced for all samples, which imply that the defective interface states

### **Figure 7.**

*Border trap estimation of HfON/InGaAs MOS capacitors by using the distributed oxide bulk trap model (a) O3-HfON and (b) IPA-HfON.*

**125**

**Figure 8.**

*the IPA-based HfOxNy.*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

near the mid-gap level can be treated with nitrogen incorporation. It is hypothesized that oxygen diffusion through the oxygen vacancies of HfO2, which results in the formation of As-Ga anti-sites, was greatly reduced, as oxygen vacancies were effectively passivated with nitrogen [17]. Therefore, nitrogen may block further oxygen diffusion, thereby preventing surface oxidation, which could occur not only during but also after dielectric deposition. Furthermore, the frequency dispersion in the accumulation region greatly reduced to 2.1 and 3.2% per decade for O3- and IPA-based ALD, respectively. These values are comparable to suppressed dispersion values in low-EOT gate stacks, which imply excellent reliability quality of dielectric stacks on III-V substrate [55]. As the proposed nitridation technology is aimed to treat inferior bulk qualities of HfO2, it showed greater impact on IPA-based HfO2. The inversion behavior was observed for the IPA-based ALD HfO2 which has not been reported from the former HfO2/InGaAs MOS studies, and it will be further

**3. Characterization of IPA-based ALD HfO2 on n- and p-type InGaAs** 

method as shown in **Figure 8**. The combination of IPA oxidant and PA-ALD HfOxNy with standard interface treatments resulted in a reduced Dit level of

Based on the n-type MOS results, the Dit was extracted using the conductance

that the inversion behavior observed in the C-V curves is due to the significant midgap Dit decrease, which is consistent with the previously reported studies. As the mid-gap Dit is known to correlate to the As-Ga anti-site defect and the ozone-based HfOxNy lacked inversion characteristics with high mid-gap Dit, these defects might be the reason why the inversion behavior is difficult to be achieved in most studies. Also, these defects might be formed in the initial ALD steps through the oxidant exposure. Also, in **Figure 9**, we have benchmarked our results, comparing them

*The Dit distribution of the fabricated III-V MOS capacitors showing great reduction in the mid-gap Dit with* 

at Ec − Et = 0.3 eV. Based on the Dit distribution, it is evident

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

discussed (**Figure 7**).

**substrates**

4.5 × 1011 eV<sup>−</sup><sup>1</sup>

cm<sup>−</sup><sup>2</sup>

### *Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

near the mid-gap level can be treated with nitrogen incorporation. It is hypothesized that oxygen diffusion through the oxygen vacancies of HfO2, which results in the formation of As-Ga anti-sites, was greatly reduced, as oxygen vacancies were effectively passivated with nitrogen [17]. Therefore, nitrogen may block further oxygen diffusion, thereby preventing surface oxidation, which could occur not only during but also after dielectric deposition. Furthermore, the frequency dispersion in the accumulation region greatly reduced to 2.1 and 3.2% per decade for O3- and IPA-based ALD, respectively. These values are comparable to suppressed dispersion values in low-EOT gate stacks, which imply excellent reliability quality of dielectric stacks on III-V substrate [55]. As the proposed nitridation technology is aimed to treat inferior bulk qualities of HfO2, it showed greater impact on IPA-based HfO2. The inversion behavior was observed for the IPA-based ALD HfO2 which has not been reported from the former HfO2/InGaAs MOS studies, and it will be further discussed (**Figure 7**).

### **3. Characterization of IPA-based ALD HfO2 on n- and p-type InGaAs substrates**

Based on the n-type MOS results, the Dit was extracted using the conductance method as shown in **Figure 8**. The combination of IPA oxidant and PA-ALD HfOxNy with standard interface treatments resulted in a reduced Dit level of 4.5 × 1011 eV<sup>−</sup><sup>1</sup> cm<sup>−</sup><sup>2</sup> at Ec − Et = 0.3 eV. Based on the Dit distribution, it is evident that the inversion behavior observed in the C-V curves is due to the significant midgap Dit decrease, which is consistent with the previously reported studies. As the mid-gap Dit is known to correlate to the As-Ga anti-site defect and the ozone-based HfOxNy lacked inversion characteristics with high mid-gap Dit, these defects might be the reason why the inversion behavior is difficult to be achieved in most studies. Also, these defects might be formed in the initial ALD steps through the oxidant exposure. Also, in **Figure 9**, we have benchmarked our results, comparing them

### **Figure 8.**

*The Dit distribution of the fabricated III-V MOS capacitors showing great reduction in the mid-gap Dit with the IPA-based HfOxNy.*

*Recent Advances in Nanophotonics - Fundamentals and Applications*

regarding the discussed issues.

devices [54]; however, no effort was made to improve the nitridation technology

the HfO2 layer is improved without having influence in the substrate.

As a result, in order to improve the film quality of HfO2, a cyclic nitrogen lowpower plasma step was added within the ALD cycles to passivate oxygen vacancies uniformly without causing damage or surface degradation. Through this technology with a combination of IPA oxidant, achievement of improvement in both interface and bulk quality of high-k/InGaAs MOS properties is expected. The detailed information of the ALD sequence is depicted in **Figure 5**. Every cycle consisted of sequential precursor pulse steps and a gas stabilization step followed by 5 s of 50 W N2 plasma step with purge steps between pulse steps. It is discovered that there is a trade-off relationship of plasma condition. The plasma power should be enough for effective passivation although it may degrade the substrate by radiation damage. Through the developed ALD sequence, with adequate plasma condition,

By using the proposed nitridation technology, HfOxNy/InGaAs MOS capacitors are fabricated showing promising results as shown in **Figure 6** [41]. A significant suppression of the frequency dispersion was observed upon nitrogen incorporation in every gate bias range. The inversion humps and flat band voltage shift were effectively reduced for all samples, which imply that the defective interface states

**124**

**Figure 7.**

*(a) O3-HfON and (b) IPA-HfON.*

*Border trap estimation of HfON/InGaAs MOS capacitors by using the distributed oxide bulk trap model* 

### **Figure 9.**

*Benchmarking the mid-gap Dit values of the proposed high-k ALD compared to the best results in the field of III-V MOS. The filled circles represent C-V curves with inversion behavior.*

to the best results ever reported in the field of III-V MOS device studies [56–61]. Extraordinary mid-gap Dit values are achieved with low CET values with the proposed technology. Especially, while other studies mostly suffer from insufficient dielectric constant of the IL, our work employs HfO2 as an IL, which has merit in terms of the EOT scaling.

In addition, with conductance method in the measurement frequency range of 1 kHz–1 MHz, the n-type MOS capacitor results can only provide information of Dit distribution near the conduction band. In order to estimate the total Dit distribution throughout the bandgap in InGaAs, p-type InGaAs MOS was fabricated and analyzed. The p-type InGaAs MOS capacitors are fabricated in the same process flow of n-type MOS capacitors.

The multifrequency C-V measurements of IPA-based PA-ALD HfOxNy/n-In0.53Ga0.47As MOS capacitors compared to the HfO2 (O3) sample is presented in **Figure 10**. Compared to the reference, the optimized HfON process exhibits significant frequency dispersion suppression with steeper C-V slope. This result is comparable to the previously reported high-quality p-type InGaAs MOS results. It is assumed that the interface improvement mechanism is similar to the previous n-type MOS analysis.

Based on the results, the Dit distribution within the InGaAs bandgap is extracted with the conductance method shown in **Figure 11**. The Dit level at the exact mid-gap energy level (Eg/2 = 0.375 eV) is around 8 × 1011 eV−<sup>1</sup> cm<sup>−</sup><sup>2</sup> . This value is still low for reported III-V MOS interface, and it is suggested that based on the Dit distribution, the accumulation mode n-channel III-V devices are favorable than the inversion mode p-substrate III-V devices because the overall Dit levels are much lower at the conduction band area.

Moreover, temperature-dependent conductance method was performed in order to analyze the mid-gap Dit level thoroughly. High-temperature (350, 400 and 450 K) multifrequency C-V analysis was conducted on HfON/InGaAs MOS capacitors. The C-V results of each measurement temperature are shown in **Figure 12**.

As the measurement temperature increases, the inversion response gets stronger at higher frequencies compared to the room temperature-measured results. Also,

**127**

**Figure 11.**

*capacitors.*

**Figure 10.**

*substrates.*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

while the dispersion at the accumulation region seems to be similar, there was a significant impact on the inversion hump phenomenon which is the interface trap characteristic. The measurement noise at higher temperature and lower frequencies was also noted. When the measurement temperature reaches around 450 K, the strong inversion response occurs even at 1 MHz, and it interferes with the interface trap-related conductance peak making the deconvolution process impossible.

*The total Dit distribution within the In0.53Ga0.47As bandgap, which is extracted from the n- and p-type MOS* 

*C-V characteristics of IPA-based PA-ALD HfOxNy (left) and O3-based HfO2 (right) on p-type In0.53Ga0.47As* 

The Dit distribution was estimated from the temperature-dependent conductance technique as shown in **Figure 13**. As the measurement temperature increases, the

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

**Figure 10.**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

*III-V MOS. The filled circles represent C-V curves with inversion behavior.*

to the best results ever reported in the field of III-V MOS device studies [56–61]. Extraordinary mid-gap Dit values are achieved with low CET values with the proposed technology. Especially, while other studies mostly suffer from insufficient dielectric constant of the IL, our work employs HfO2 as an IL, which has merit in

*Benchmarking the mid-gap Dit values of the proposed high-k ALD compared to the best results in the field of* 

In addition, with conductance method in the measurement frequency range of 1 kHz–1 MHz, the n-type MOS capacitor results can only provide information of Dit distribution near the conduction band. In order to estimate the total Dit distribution throughout the bandgap in InGaAs, p-type InGaAs MOS was fabricated and analyzed. The p-type InGaAs MOS capacitors are fabricated in the same process

The multifrequency C-V measurements of IPA-based PA-ALD HfOxNy/n-In0.53Ga0.47As MOS capacitors compared to the HfO2 (O3) sample is presented in **Figure 10**. Compared to the reference, the optimized HfON process exhibits significant frequency dispersion suppression with steeper C-V slope. This result is comparable to the previously reported high-quality p-type InGaAs MOS results. It is assumed that the interface improvement mechanism is similar to the previous

Based on the results, the Dit distribution within the InGaAs bandgap is extracted with the conductance method shown in **Figure 11**. The Dit level at the exact mid-gap

reported III-V MOS interface, and it is suggested that based on the Dit distribution, the accumulation mode n-channel III-V devices are favorable than the inversion mode p-substrate III-V devices because the overall Dit levels are much lower at the

Moreover, temperature-dependent conductance method was performed in order to analyze the mid-gap Dit level thoroughly. High-temperature (350, 400 and 450 K) multifrequency C-V analysis was conducted on HfON/InGaAs MOS capacitors. The C-V results of each measurement temperature are shown in **Figure 12**.

As the measurement temperature increases, the inversion response gets stronger at higher frequencies compared to the room temperature-measured results. Also,

cm<sup>−</sup><sup>2</sup>

. This value is still low for

**126**

terms of the EOT scaling.

**Figure 9.**

flow of n-type MOS capacitors.

energy level (Eg/2 = 0.375 eV) is around 8 × 1011 eV−<sup>1</sup>

n-type MOS analysis.

conduction band area.

*C-V characteristics of IPA-based PA-ALD HfOxNy (left) and O3-based HfO2 (right) on p-type In0.53Ga0.47As substrates.*

**Figure 11.**

*The total Dit distribution within the In0.53Ga0.47As bandgap, which is extracted from the n- and p-type MOS capacitors.*

while the dispersion at the accumulation region seems to be similar, there was a significant impact on the inversion hump phenomenon which is the interface trap characteristic. The measurement noise at higher temperature and lower frequencies was also noted. When the measurement temperature reaches around 450 K, the strong inversion response occurs even at 1 MHz, and it interferes with the interface trap-related conductance peak making the deconvolution process impossible.

The Dit distribution was estimated from the temperature-dependent conductance technique as shown in **Figure 13**. As the measurement temperature increases, the

### **Figure 12.**

*C-V characteristics of IPA-based PA-ALD HfOxNy on n-type InGaAs substrate measured at (a) 350 K, (b) 400 K, and (c) 450 K.*

deeper energy range could be measured. Similar Dit profile was observed showing a peak energy level around the exact mid-gap level (~0.375 eV). The peak Dit value is slightly higher than the previously estimated value which would be the effect of

**129**

depicted.

**Figure 13.**

*dependent conductance method.*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

enhanced thermal broadening of trap response in higher temperatures. The differences between the Dit profile estimation can be summarized as follows: While using p-MOS capacitors, a larger energy level range is observable with no thermal broadening of the trap response due to the fixed measurement temperature. On the other hand, using a temperature-dependent method has a thermal broadening issue but

*The total Dit distribution within the In0.53Ga0.47As bandgap, which is extracted from the temperature-*

It was noted that the inversion behavior of IPA-based HfOxNy ALD is attributed

Both C-V profiles have shown inversion-like behavior in the negative bias region.

In **Figure 15**, to verify true inversion characteristics of IPA-based HfOxNy, the minority carrier response was investigated based on the extraction of the transition frequency, wm, which is known to be a characteristic of a strong inverted surface for III-V MOS capacitors [62]. It is known that at the transition frequency, the –wdC/ dw and Gm/w share the same peak magnitude in the strong inversion gate bias. Notably, –wdC/dw and Gm/w share the same peak magnitude at the same transition frequency of 4 kHz which suggests that IPA-based HfOxNy exhibits true inversion behavior. The true inversion behavior of hafnium oxide-based dielectrics on InGaAs

substrate has not been reported yet which implies significant potential.

to the mid-gap Dit level decrease. However, in order to verify true inversion characteristics, more analyses must be investigated. In **Figure 14**, the conductance profile of sample O3-based HfO2 and IPA-based HfOxNy InGaAs MOS capacitors are

However, clear difference is observed between the conductance profiles. While huge and Gaussian conductance profiles in the negative bias regions are observed for O3-based HfO2, smaller Gaussian conductance peaks are observed in depletion region, and distinct from these peaks, saturated conductance profiles are observed for IPA-based HfOxNy. Therefore, it is concluded that the inversion-like behavior in O3-based HfO2 is attributed from the huge and broad conductance peaks that reflect high mid-gap Dit levels, while inversion behavior in IPA-based HfOxNy might be

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

only requires one sample for characterization.

attributed from real true minority carrier inversion.

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

### **Figure 13.**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

deeper energy range could be measured. Similar Dit profile was observed showing a peak energy level around the exact mid-gap level (~0.375 eV). The peak Dit value is slightly higher than the previously estimated value which would be the effect of

*C-V characteristics of IPA-based PA-ALD HfOxNy on n-type InGaAs substrate measured at (a) 350 K,* 

**128**

**Figure 12.**

*(b) 400 K, and (c) 450 K.*

*The total Dit distribution within the In0.53Ga0.47As bandgap, which is extracted from the temperaturedependent conductance method.*

enhanced thermal broadening of trap response in higher temperatures. The differences between the Dit profile estimation can be summarized as follows: While using p-MOS capacitors, a larger energy level range is observable with no thermal broadening of the trap response due to the fixed measurement temperature. On the other hand, using a temperature-dependent method has a thermal broadening issue but only requires one sample for characterization.

It was noted that the inversion behavior of IPA-based HfOxNy ALD is attributed to the mid-gap Dit level decrease. However, in order to verify true inversion characteristics, more analyses must be investigated. In **Figure 14**, the conductance profile of sample O3-based HfO2 and IPA-based HfOxNy InGaAs MOS capacitors are depicted.

Both C-V profiles have shown inversion-like behavior in the negative bias region. However, clear difference is observed between the conductance profiles. While huge and Gaussian conductance profiles in the negative bias regions are observed for O3-based HfO2, smaller Gaussian conductance peaks are observed in depletion region, and distinct from these peaks, saturated conductance profiles are observed for IPA-based HfOxNy. Therefore, it is concluded that the inversion-like behavior in O3-based HfO2 is attributed from the huge and broad conductance peaks that reflect high mid-gap Dit levels, while inversion behavior in IPA-based HfOxNy might be attributed from real true minority carrier inversion.

In **Figure 15**, to verify true inversion characteristics of IPA-based HfOxNy, the minority carrier response was investigated based on the extraction of the transition frequency, wm, which is known to be a characteristic of a strong inverted surface for III-V MOS capacitors [62]. It is known that at the transition frequency, the –wdC/ dw and Gm/w share the same peak magnitude in the strong inversion gate bias. Notably, –wdC/dw and Gm/w share the same peak magnitude at the same transition frequency of 4 kHz which suggests that IPA-based HfOxNy exhibits true inversion behavior. The true inversion behavior of hafnium oxide-based dielectrics on InGaAs substrate has not been reported yet which implies significant potential.

**Figure 14.** *The conductance profiles for (a) O3-HfO2 and (b) IPA-HfON ALD.*

**131**

**Author details**

Sukeun Eom\*, Min-woo Kong and Kwang-seok Seo Seoul National University, Seoul, Republic of Korea

provided the original work is properly cited.

\*Address all correspondence to: djatnrms90@snu.ac.kr

© 2020 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,

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface*

In order to achieve both low EOT and low Dit, a highly advanced gate stack, prepared by using an IPA oxidant in the PA-ALD of HfOxNy on In0.53Ga0.47As substrates, was proposed and showed the most outstanding results. A cyclic nitrogen low-power plasma step was added within the ALD cycles to passivate the oxygen vacancies uniformly without causing damage or surface degradation in comparison to the post deposition nitridation technology. Remarkable midgap Dit levels with strong inversion characteristics were achieved which has not been reported in the previous HfO2/InGaAs interface studies. The improved interface characteristics can be attributed to both low surface oxidation ability of IPA and suppression of oxygen diffusion by effective nitrogen passivation to oxygen vacancies in HfO2. The proposed ALD HfOxNy was fully characterized by investigating different dopant types and measurement temperatures. The results show comprehensive understanding on the interface defect density distribution. It is suggested that not only surface treatments but also the development of an advanced HfO2 ALD process has a great impact on the quality of the III-V MOS interface and the IPA-based HfON interfacial layer might have great potential in

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

**4. Conclusions**

future technology node.

**Figure 15.** *–wdC/dw and Gm/w profiles for IPA-HfON/InGaAs MOS.*

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

### **4. Conclusions**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

**130**

**Figure 15.**

**Figure 14.**

*–wdC/dw and Gm/w profiles for IPA-HfON/InGaAs MOS.*

*The conductance profiles for (a) O3-HfO2 and (b) IPA-HfON ALD.*

In order to achieve both low EOT and low Dit, a highly advanced gate stack, prepared by using an IPA oxidant in the PA-ALD of HfOxNy on In0.53Ga0.47As substrates, was proposed and showed the most outstanding results. A cyclic nitrogen low-power plasma step was added within the ALD cycles to passivate the oxygen vacancies uniformly without causing damage or surface degradation in comparison to the post deposition nitridation technology. Remarkable midgap Dit levels with strong inversion characteristics were achieved which has not been reported in the previous HfO2/InGaAs interface studies. The improved interface characteristics can be attributed to both low surface oxidation ability of IPA and suppression of oxygen diffusion by effective nitrogen passivation to oxygen vacancies in HfO2. The proposed ALD HfOxNy was fully characterized by investigating different dopant types and measurement temperatures. The results show comprehensive understanding on the interface defect density distribution. It is suggested that not only surface treatments but also the development of an advanced HfO2 ALD process has a great impact on the quality of the III-V MOS interface and the IPA-based HfON interfacial layer might have great potential in future technology node.

### **Author details**

Sukeun Eom\*, Min-woo Kong and Kwang-seok Seo Seoul National University, Seoul, Republic of Korea

\*Address all correspondence to: djatnrms90@snu.ac.kr

© 2020 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.

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integration path for gate-first UTB III-V-on-insulator MOSFETs with silicon, using direct wafer bonding and donor wafer recycling. In: 2012 International Electron Devices Meeting. IEEE; 2012. DOI: 10.1109/IEDM.2012.6479088

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[19] Hinkle CL, Milojevic M, Brennan B, Sonnet AM, Aguirre-Tostado FS, Hughes GJ, et al. Detection of Ga suboxides and their impact on III-V passivation and Fermi-level pinning. Applied Physics Letters. 2009;**94**(16):162101. DOI: 10.1063/1.3120546

[20] Shahrjerdi D, Tutuc E, Banerjee SK. Impact of surface chemical treatment on capacitance-voltage characteristics of GaAs metal-oxidesemiconductor capacitors with Al2O3 gate dielectric. Applied Physics

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low-noise 0.1 mu m T-gate InAlAs-InGaAs-InP HEMT. IEEE Microwave and Guided Wave Letters. 1991;**1**(5): 114-116. DOI: 10.1109/75.89081

[9] Lin J, Antoniadis DA, del

Alamo JA. Sub-30 nm InAs Quantum-Well MOSFETs with self-aligned metal contacts and Sub-1 nm EOT HfO2 insulator. In: 2012 International Electron Devices Meeting. IEEE; 2012. DOI: 10.1109/IEDM.2012.6479149

[10] Kim D-H, del Alamo JA. Logic Performance of 40 nm InAs HEMTs. In: 2007 IEEE International Electron Devices Meeting. IEEE; 2007. DOI: 10.1109/IEDM.2007.4419018

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[13] Daix N, Uccelli E, Czornomaz L, Caimi D, Rossel C, Sousa M, et al. Towards large size substrates for III-V co-integration made by direct wafer bonding on Si. APL Materials.

[14] Czornomaz L, Daix N, Caimi D, Sousa M, Erni R, Rossell MD, et al. An

2014 Aug;**2**(8):086104. DOI:

10.1063/1.4893653

[11] Yan R-H, Ourmazd A,

[12] Cirlin GE, Dubrovskii VG, Soshnikov IP, Sibirev NV,

[8] Lai R, Mei XB, Deal WR, Yoshida W, Kim YM, Liu PH, et al. Sub 50 nm InP HEMT Device with Fmax Greater than 1 THz. In: 2007 IEEE International Electron Devices Meeting. IEEE; 2007. DOI: 10.1109/IEDM.2007.4419013

[1] Cho H-J, Oh HS, Nam KJ, Kim YH, Yeo KH, Kim WD, et al. Si FinFET based 10nm technology with multi Vt gate stack for low power and high performance applications. In: 2016 IEEE Symposium on VLSI Technology. IEEE; 2016. DOI: 10.1109/VLSIT.2016.7573359

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[3] Tsai T-H, Sheen R-B, Chang C-H, Staszewski RB. A 0.2GHz to 4GHz Hybrid PLL (ADPLL/Charge-Pump-PLL) in 7NM FinFET CMOS Featuring 0.619PS Integrated Jitter and 0.6US Settling Time at 2.3MW. In: 2018 IEEE Symposium on VLSI Circuits. IEEE; 2018. DOI: 10.1109/VLSIC.2018.8502274

[4] Jeong WC, Kwon DJ, Nam KJ, Rim WJ, Jang MS, Kim HT, et al. True 7nm Platform Technology featuring Smallest FinFET and Smallest SRAM cell by EUV, Special Constructs and 3rd Generation Single Diffusion Break. In: 2018 IEEE Symposium on VLSI Technology. IEEE; 2018. DOI: 10.1109/

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MM.2017.4241347

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substrate. Microelectronic Engineering.

2005;**80**:30-33. DOI: 10.1016/j.

[54] Chang P-C, Luc Q-H, Lin Y-C, Lin Y-K, Wu C-H, Sze SM, et al. InGaAs QW-MOSFET performance improvement using a PEALD-AlN passivation layer and an *In-situ* NH3Post

in HfO2 for ultrathin equivalent oxide thickness. Journal of Applied Physics. 2013;**113**(4):044103. DOI:

combined inductively-coupled

10.1016/j.apsusc.2015.05.066

TED.2003.821707

10.1063/1.2709948

10.1063/1.4775817

mee.2005.04.033

*DOI: http://dx.doi.org/10.5772/intechopen.92424*

[41] Eom S-K, Kong M-W, Kang M-J, Lee J-G, Cha H-Y, Seo K-S. Enhanced interface characteristics of PA-ALD HfOxNy/InGaAs MOSCAPs using IPA oxygen reactant and cyclic N2 plasma. IEEE Electron Device Letters. 2018;**39**(11):1636-1639. DOI: 10.1109/

[42] Gavartin JL, Muñoz Ramo D, Shluger AL, Bersuker G, Lee BH. Negative oxygen vacancies in HfO2 as charge traps in high-k stacks. Applied Physics Letters. 2006;**89**(8):082908.

[43] Greeuw G, Verwey JF. The mobility of Na+, Li+, and K+ ions in thermally grown SiO2 films. Journal of Applied Physics. 1984;**56**(8):2218-2224. DOI:

[44] Trivedi AR, Ando T, Singhee A, Kerber P, Acar E, Frank DJ, et al. A simulation study of oxygen vacancyinduced variability in HfO2/metal gated SOI FinFET. IEEE Transactions on Electron Devices. 2014;**61**(5):1262-1269.

DOI: 10.1109/TED.2014.2313086

[45] Xiong K, Robertson J, Clark SJ. Passivation of oxygen vacancy states in HfO2 by nitrogen. Journal of Applied Physics. 2006;**99**(4):044105. DOI:

[46] Lee BH, Kang L, Nieh R, Qi W-J, Lee JC. Thermal stability and electrical characteristics of ultrathin hafnium oxide gate dielectric reoxidized with rapid thermal annealing. Applied Physics Letters. 2000;**76**(14):1926-1928.

LED.2018.2870176

DOI: 10.1063/1.2236466

10.1063/1.334256

10.1063/1.2173688

DOI: 10.1063/1.126214

DOI: 10.1063/1.3079409

[47] Dalapati GK, Sridhara A,

[48] Jin CG, Yang Y, Zhang HY, Huang TY, Wu MZ, Zhuge LJ, et al. Controllable nitrogen incorporation in

Wong ASW, Chia CK, Chi DZ. HfOxNy gate dielectric on p-GaAs. Applied Physics Letters. 2009;**94**(7):073502.

*Development and Characterization of High-Quality HfO2/InGaAs MOS Interface DOI: http://dx.doi.org/10.5772/intechopen.92424*

[41] Eom S-K, Kong M-W, Kang M-J, Lee J-G, Cha H-Y, Seo K-S. Enhanced interface characteristics of PA-ALD HfOxNy/InGaAs MOSCAPs using IPA oxygen reactant and cyclic N2 plasma. IEEE Electron Device Letters. 2018;**39**(11):1636-1639. DOI: 10.1109/ LED.2018.2870176

*Recent Advances in Nanophotonics - Fundamentals and Applications*

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oxidation of In0.53Ga0.47As(100) through ultra-thin atomic layer deposited Al2O3. Applied Physics Letters. 2013;**103**(25):251602. DOI:

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[36] Parks GS, Barton B. Vapor pressure data for isopropyl alcohol and tertiary butyl alcohol. Journal of the American Chemical Society. 1928;**50**(1):24-26.

[37] Swerts J, Peys N, Nyns L, Delabie A, Franquet A, Maes JW, et al. Impact of precursor chemistry and process conditions on the scalability of ALD HfO[sub 2] gate dielectrics. Journal of the Electrochemical Society. 2010;**157**(1):G26. DOI:

Ritter D. Determination of the dielectric

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[40] Simoen E, Lin DH-C, Alian A, Brammertz G, Merckling C, Mitard J, et al. Border traps in Ge/III–V channel devices: Analysis and reliability aspects. IEEE Transactions on Device and Materials Reliability. 2013;**13**(4):444-455. DOI: 10.1109/

precursors. Applied Physics Letters. 2008;**93**(3):031902. DOI:

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10.1063/1.4902114

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constant of InGaAs based gate stacks by a modified thickness series method. Applied Physics Letters. 2014;**105**(20):203506. DOI:

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[29] Yuan Y, Yu B, Ahn J, McIntyre PC, Asbeck PM, Rodwell MJW, et al. A distributed bulk-oxide trap model for Al2O3 InGaAs MOS devices. IEEE Transactions on Electron Devices. 2012;**59**(8):2100-2106. DOI: 10.1109/

[30] Chobpattana V, Son J, Law JJM, Engel-Herbert R, Huang C-Y, Stemmer S. Nitrogen-passivated dielectric/InGaAs interfaces with sub-nm equivalent oxide thickness and low interface trap densities. Applied Physics Letters. 2013;**102**(2):022907.

[31] Kent T, Tang K, Chobpattana V, Negara MA, Edmonds M, Mitchell W,

preparation on low temperature HfO2 ALD on InGaAs (001) and (110) surfaces. The Journal of Chemical Physics. 2015;**143**(16):164711. DOI:

[32] Rafí JM, Zabala M, Beldarrain O, Campabadal F. Effect of processing

characteristics of atomic layer deposited Al2O3 and HfO2 films. ECS Transactions. 2010;**28**(2):213. DOI: 10.1149/1.3372577

[33] García H, Castán H, Dueñas S, Bailón L, Campabadal F, Beldarrain O, et al. Electrical characterization of atomic-layer-deposited hafnium oxide films from hafnium

tetrakis(dimethylamide) and water/ ozone: Effects of growth temperature, oxygen source, and postdeposition annealing. Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films. 2013;**31**(1):01A127. DOI:

conditions on the electrical

et al. The influence of surface

10.1063/1.3520431

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DOI: 10.1063/1.4776656

10.1063/1.4934656

**134**

10.1116/1.4768167

[42] Gavartin JL, Muñoz Ramo D, Shluger AL, Bersuker G, Lee BH. Negative oxygen vacancies in HfO2 as charge traps in high-k stacks. Applied Physics Letters. 2006;**89**(8):082908. DOI: 10.1063/1.2236466

[43] Greeuw G, Verwey JF. The mobility of Na+, Li+, and K+ ions in thermally grown SiO2 films. Journal of Applied Physics. 1984;**56**(8):2218-2224. DOI: 10.1063/1.334256

[44] Trivedi AR, Ando T, Singhee A, Kerber P, Acar E, Frank DJ, et al. A simulation study of oxygen vacancyinduced variability in HfO2/metal gated SOI FinFET. IEEE Transactions on Electron Devices. 2014;**61**(5):1262-1269. DOI: 10.1109/TED.2014.2313086

[45] Xiong K, Robertson J, Clark SJ. Passivation of oxygen vacancy states in HfO2 by nitrogen. Journal of Applied Physics. 2006;**99**(4):044105. DOI: 10.1063/1.2173688

[46] Lee BH, Kang L, Nieh R, Qi W-J, Lee JC. Thermal stability and electrical characteristics of ultrathin hafnium oxide gate dielectric reoxidized with rapid thermal annealing. Applied Physics Letters. 2000;**76**(14):1926-1928. DOI: 10.1063/1.126214

[47] Dalapati GK, Sridhara A, Wong ASW, Chia CK, Chi DZ. HfOxNy gate dielectric on p-GaAs. Applied Physics Letters. 2009;**94**(7):073502. DOI: 10.1063/1.3079409

[48] Jin CG, Yang Y, Zhang HY, Huang TY, Wu MZ, Zhuge LJ, et al. Controllable nitrogen incorporation in HfO2 films by modulating capacitivelycombined inductively-coupled plasmas. Journal of Physics D: Applied Physics. 2013;**46**(48):485206. DOI: 10.1088/0022-3727/46/48/485206

[49] Lee YB, Oh I-K, Cho EN, Moon P, Kim H, Yun I. Characterization of HfO N thin film formation by in-situ plasma enhanced atomic layer deposition using NH3 and N2 plasmas. Applied Surface Science. 2015;**349**:757-762. DOI: 10.1016/j.apsusc.2015.05.066

[50] Kang CS, Cho H-J, Choi R, Kim YH, Kang CY, Rhee SJ, et al. The electrical and material characterization of hafnium oxynitride gate dielectrics with TaN-gate electrode. IEEE Transactions on Electron Devices. 2004;**51**(2):220-227. DOI: 10.1109/ TED.2003.821707

[51] Yu X, Zhu C, Yu M. Impact of nitrogen in HfON gate dielectric with metal gate on electrical characteristics, with particular attention to threshold voltage instability. Applied Physics Letters. 2007;**90**(10):103502. DOI: 10.1063/1.2709948

[52] Dai M, Wang Y, Shepard J, Liu J, Brodsky M, Siddiqui S, et al. Effect of plasma N2 and thermal NH3 nitridation in HfO2 for ultrathin equivalent oxide thickness. Journal of Applied Physics. 2013;**113**(4):044103. DOI: 10.1063/1.4775817

[53] Cheng C-C, Chien C-H, Chen C-W, Hsu S-L, Yang M-Y, Huang C-C, et al. Impact of post-deposition-annealing on the electrical characteristics of HfOxNy gate dielectric on Ge substrate. Microelectronic Engineering. 2005;**80**:30-33. DOI: 10.1016/j. mee.2005.04.033

[54] Chang P-C, Luc Q-H, Lin Y-C, Lin Y-K, Wu C-H, Sze SM, et al. InGaAs QW-MOSFET performance improvement using a PEALD-AlN passivation layer and an *In-situ* NH3Post remote-plasma treatment. IEEE Electron Device Letters. 2017;**38**(3): 310-313. DOI: 10.1109/LED. 2017.2656180

[55] Vais A, Franco J, Martens K, Lin D, Sioncke S, Putcha V, et al. A new quality metric for III–V/high-k MOS gate stacks based on the frequency dispersion of accumulation capacitance and the CET. IEEE Electron Device Letters. 2017;**38**(3):318-321. DOI: 10.1109/ LED.2017.2657794

[56] Hoshii T, Lee S, Suzuki R, Taoka N, Yokoyama M, Yamada H, et al. Reduction in interface state density of Al2O3/InGaAs metal-oxidesemiconductor interfaces by InGaAs surface nitridation. Journal of Applied Physics. 2012;**112**(7):073702. DOI: 10.1063/1.4755804

[57] Suzuki R, Taoka N, Yokoyama M, Lee S, Kim SH, Hoshii T, et al. 1-nm-capacitance-equivalent-thickness HfO2/Al2O3/InGaAs metal-oxidesemiconductor structure with low interface trap density and low gate leakage current density. Applied Physics Letters. 2012;**100**(13):132906. DOI: 10.1063/1.3698095

[58] Lee S, Chobpattana V, Huang C-Y, Thibeault BJ, Mitchell W, Stemmer S, et al. Record Ion (0.50 mA/um at VDD = 0.5 V and Ioff = 100 nA/um) 25 nm-gate-length ZrO2/InAs/InAlAs MOSFETs. In: 2014 Symposium on VLSI Technology (VLSI-Technology): Digest of Technical Papers. IEEE; 2014. DOI: 10.1109/VLSIT.2014.6894363

[59] Chang C-Y, Ichikawa O, Osada T, Hata M, Yamada H, Takenaka M, et al. Impact of La2O3 interfacial layers on InGaAs metal-oxide-semiconductor interface properties in Al2O3/La2O3/ InGaAs gate stacks deposited by atomiclayer-deposition. Journal of Applied Physics. 2015;**118**(8):085309. DOI: 10.1063/1.4929650

[60] Luc Q, Cheng S, Chang P, Do H, Chen J, Ha M, et al. Effects of in-situ plasma-enhanced atomic layer deposition treatment on the performance of HfO2/In0.53Ga0.47As metal-oxide-semiconductor fieldeffect transistors. IEEE Electron Device Letters. 2016;**1**:1. DOI: 10.1109/ LED.2016.2581175

**Chapter 8**

**Abstract**

various fields.

**137**

Surface-Enhanced Raman

*Samir Kumar, Prabhat Kumar, Anamika Das*

Scattering of light by molecules can be elastic, Rayleigh scattering, or inelastic, Raman scattering. In the elastic scattering, the photon's energy and the state of the molecule after the scattering events are unchanged. Hence, Rayleigh scattered light

inelastic scattering, the frequency of monochromatic light changes upon interaction with the vibrational states, or modes, of a molecule. With the advancement in the laser sources, better and compact spectrometers, detectors, and optics Raman spectroscopy have developed as a highly sensitive technique to probe structural details of a complex molecular structure. However, the low scattering cross section (1031) of Raman scattering has limited the applications of the conventional Raman spectroscopy. With the discovery of surface-enhanced Raman scattering (SERS) in 1973 by Martin Fleischmann, the interest of the research community in Raman spectroscopy as an analytical method has been revived. This chapter aims to familiarize the readers with the basics of Raman scattering phenomenon and SERS. This chapter will also discuss the latest developments in the SERS and its applications in

does not contain much information on the structure of molecular states. In

**Keywords:** Raman scattering, surface-enhanced Raman spectroscopy,

Scattering of light by molecules can be elastic, *Rayleigh scattering*, or inelastic, *Raman scattering*. In the elastic case, the photon's energy and the state of the molecule after the scattering events are unchanged. Hence, Rayleigh scattered light does not contain much information on the structure of molecular states [1]. In inelastic scattering, the frequency of photons of monochromatic light changes upon interaction with the vibrational states, or modes, of a molecule. The effect was postulated theoretically by Smekal et al. in 1923 but was first discovered experimentally by C.V. Raman in 1928 in an experiment using the sun as a light source [2–4].

• *Stokes* process: An incident photon *hν<sup>0</sup>* excites a molecular vibration *h*ν*vib* and is thus scattered with the corresponding difference in energy *h*(ν<sup>0</sup> ν*vib*) (red shift).

enhancement factor, nanoparticles, 2D materials

In Raman scattering, two inelastic processes can occur:

**1. Introduction: Raman scattering**

Scattering: Introduction

and Applications

*and Chandra Shakher Pathak*

[61] Kim SK, Geum D-M, Shim J-P, Kim CZ, Kim H, Song JD, et al. Fabrication and characterization of Pt/ Al2O3/Y2O3/In0.53Ga0.47As MOSFETs with low interface trap density. Applied Physics Letters. 2017;**110**(4):043501. DOI: 10.1063/1.4974893

[62] O'Connor É, Cherkaoui K, Monaghan S, Sheehan B, Povey IM, Hurley PK. Effect of forming gas annealing on the inversion response and minority carrier generation lifetime of n and p-In0.53Ga0.47As MOS capacitors. Microelectronic Engineering. 2015;**147**:325-329. DOI: 10.1016/j. mee.2015.04.103

### **Chapter 8**

*Recent Advances in Nanophotonics - Fundamentals and Applications*

[60] Luc Q, Cheng S, Chang P, Do H, Chen J, Ha M, et al. Effects of in-situ plasma-enhanced atomic layer deposition treatment on the performance of HfO2/In0.53Ga0.47As metal-oxide-semiconductor fieldeffect transistors. IEEE Electron Device Letters. 2016;**1**:1. DOI: 10.1109/

[61] Kim SK, Geum D-M, Shim J-P, Kim CZ, Kim H, Song JD, et al.

Fabrication and characterization of Pt/ Al2O3/Y2O3/In0.53Ga0.47As MOSFETs with low interface trap density. Applied Physics Letters. 2017;**110**(4):043501.

LED.2016.2581175

DOI: 10.1063/1.4974893

[62] O'Connor É, Cherkaoui K, Monaghan S, Sheehan B, Povey IM, Hurley PK. Effect of forming gas annealing on the inversion response and minority carrier generation lifetime of n and p-In0.53Ga0.47As MOS capacitors.

Microelectronic Engineering. 2015;**147**:325-329. DOI: 10.1016/j.

mee.2015.04.103

remote-plasma treatment. IEEE Electron Device Letters. 2017;**38**(3):

[55] Vais A, Franco J, Martens K, Lin D, Sioncke S, Putcha V, et al. A new quality metric for III–V/high-k MOS gate stacks based on the frequency dispersion of accumulation capacitance and the CET. IEEE Electron Device Letters. 2017;**38**(3):318-321. DOI: 10.1109/

310-313. DOI: 10.1109/LED.

2017.2656180

LED.2017.2657794

10.1063/1.4755804

10.1063/1.3698095

[56] Hoshii T, Lee S, Suzuki R,

Taoka N, Yokoyama M, Yamada H, et al. Reduction in interface state density of Al2O3/InGaAs metal-oxidesemiconductor interfaces by InGaAs surface nitridation. Journal of Applied Physics. 2012;**112**(7):073702. DOI:

[57] Suzuki R, Taoka N, Yokoyama M,

1-nm-capacitance-equivalent-thickness HfO2/Al2O3/InGaAs metal-oxidesemiconductor structure with low interface trap density and low gate leakage current density. Applied Physics Letters. 2012;**100**(13):132906. DOI:

[58] Lee S, Chobpattana V, Huang C-Y, Thibeault BJ, Mitchell W, Stemmer S, et al. Record Ion (0.50 mA/um at VDD = 0.5 V and Ioff = 100 nA/um) 25 nm-gate-length ZrO2/InAs/InAlAs MOSFETs. In: 2014 Symposium on VLSI Technology (VLSI-Technology): Digest of Technical Papers. IEEE; 2014. DOI:

10.1109/VLSIT.2014.6894363

[59] Chang C-Y, Ichikawa O, Osada T, Hata M, Yamada H, Takenaka M, et al. Impact of La2O3 interfacial layers on InGaAs metal-oxide-semiconductor interface properties in Al2O3/La2O3/ InGaAs gate stacks deposited by atomiclayer-deposition. Journal of Applied Physics. 2015;**118**(8):085309. DOI:

Lee S, Kim SH, Hoshii T, et al.

**136**

10.1063/1.4929650

## Surface-Enhanced Raman Scattering: Introduction and Applications

*Samir Kumar, Prabhat Kumar, Anamika Das and Chandra Shakher Pathak*

### **Abstract**

Scattering of light by molecules can be elastic, Rayleigh scattering, or inelastic, Raman scattering. In the elastic scattering, the photon's energy and the state of the molecule after the scattering events are unchanged. Hence, Rayleigh scattered light does not contain much information on the structure of molecular states. In inelastic scattering, the frequency of monochromatic light changes upon interaction with the vibrational states, or modes, of a molecule. With the advancement in the laser sources, better and compact spectrometers, detectors, and optics Raman spectroscopy have developed as a highly sensitive technique to probe structural details of a complex molecular structure. However, the low scattering cross section (1031) of Raman scattering has limited the applications of the conventional Raman spectroscopy. With the discovery of surface-enhanced Raman scattering (SERS) in 1973 by Martin Fleischmann, the interest of the research community in Raman spectroscopy as an analytical method has been revived. This chapter aims to familiarize the readers with the basics of Raman scattering phenomenon and SERS. This chapter will also discuss the latest developments in the SERS and its applications in various fields.

**Keywords:** Raman scattering, surface-enhanced Raman spectroscopy, enhancement factor, nanoparticles, 2D materials

### **1. Introduction: Raman scattering**

Scattering of light by molecules can be elastic, *Rayleigh scattering*, or inelastic, *Raman scattering*. In the elastic case, the photon's energy and the state of the molecule after the scattering events are unchanged. Hence, Rayleigh scattered light does not contain much information on the structure of molecular states [1]. In inelastic scattering, the frequency of photons of monochromatic light changes upon interaction with the vibrational states, or modes, of a molecule. The effect was postulated theoretically by Smekal et al. in 1923 but was first discovered experimentally by C.V. Raman in 1928 in an experiment using the sun as a light source [2–4]. In Raman scattering, two inelastic processes can occur:

• *Stokes* process: An incident photon *hν<sup>0</sup>* excites a molecular vibration *h*ν*vib* and is thus scattered with the corresponding difference in energy *h*(ν<sup>0</sup> ν*vib*) (red shift). • *Anti-Stokes* process: The photon acquires vibrational energy and is scattered with a higher energy *h*(ν<sup>0</sup> + ν*vib*) (blue shift).

This shift provides information about vibrational, rotational, and other lowfrequency transitions in molecules. Raman spectroscopy can be used to study solid, liquid, and gaseous samples.

### **1.1 Classical theory of the Raman effect**

Raman scattering can be explained using the molecular polarizability [5]. If a molecule is placed in an electric field, electrons and nuclei get displaced. Due to the separation of charged species, an electric dipole moment is induced in the molecule, and it is said to be polarized. If *E* is the strength of the electric field and *μ* is the magnitude of the induced dipole moment, then

$$
\mu = aE \tag{1}
$$

*μ* ¼ *E*<sup>0</sup> cos 2*πυt α*<sup>0</sup> þ

*Surface-Enhanced Raman Scattering: Introduction and Applications*

*μ* ¼ *α*0*E*<sup>0</sup> cos 2*πυt* þ *E*<sup>0</sup>

*∂α ∂q*

quency is modulated by the vibrational frequency of the bond.

must experience a change in polarizability during a vibration *<sup>∂</sup><sup>α</sup>*

*E*0 <sup>2</sup> *qmax*

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

**1.2 Quantum theory of the Raman effect**

or

or

**Figure 1.**

**139**

*vibration energy levels [7].*

*μ* ¼ *α*0*E*<sup>0</sup> cos 2*πυt* þ

*∂α ∂q*

*∂α ∂q*

cos 2½ �þ *π υ*ð Þ � *<sup>υ</sup>vib <sup>t</sup>*

The first term in Eq. (9) represents Rayleigh scattering and occurs at the excitation frequency *ν*. The second and third terms correspond to Stokes (*ν* � *νvib*) and anti-Stokes (*ν* + *νvib*) scattering. In both inelastic scatterings, the excitation fre-

Besides, from Eq. (9), the molecules that have Raman-active vibration modes

Raman scattering can be easily understood in terms of the quantum theory of radiation. In the quantum model, the molecules exist in quantized energy levels corresponding to possible stationary states of the molecule. When radiation having an energy *hν* incident on a sample, it is considered that the photons undergo collisions with the molecules. When the collision is elastic, the photons will be

*Energy level diagram for Rayleigh and Raman scattering, where* ΔE *=* hνvib *represents the difference in*

density in the molecule must distort from its typical shape (inducing a dipole). Molecules with symmetrical bends and stretches, therefore, are generally better Raman scatterers. So, for a molecule to be Raman active, its molecular rotation or

vibration must cause a change in a component of molecular polarizability.

*qmax* cos 2*πυvibt*

*E*0 <sup>2</sup> *qmax*

(7)

*qmax* cos 2*πυ<sup>t</sup>* cos 2*πυvibt* (8)

*∂α ∂q*

*∂q*

cos 2½ � *π υ*ð Þ <sup>þ</sup> *<sup>υ</sup>vib <sup>t</sup>*

, i.e., the electron

(9)

where *α* is the polarizability of the molecule. If a sample is subjected to an electromagnetic wave of frequency *ν*, the electric field experienced by each molecule of the sample varies as

$$E = E\_0 \cos 2\pi vt\tag{2}$$

where *E*<sup>0</sup> is the amplitude of the electromagnetic wave. From Eq. (1)

$$
\mu = aE\_0 \cos 2\pi\nu t\tag{3}
$$

Thus, Eq. (3) implies that interaction of electromagnetic radiation of frequency *ν* induces a molecular dipole moment that oscillates and emits radiation of the same frequency, and this is the classical explanation of Rayleigh scattering. However, the ability to perturb the local electron cloud of a molecular structure depends on the relative location of the individual atoms; hence, the polarizability is a function of the instantaneous position of the constituent atoms. So, the polarizability changes with small displacement from equilibrium position (i.e., molecular vibration) and is given by

$$a = a\_0 + \left(q - q\_{eq}\right)\frac{\partial a}{\partial q} \tag{4}$$

where *α<sup>0</sup>* is equilibrium polarizability and *qeq* and *q* are bond lengths at equilibrium position and any instant, respectively. If a molecule executes simple harmonic motion, the displacement can be represented as

$$q - q\_0 = q\_{\text{max}} \cos 2\pi v\_{vib} t \tag{5}$$

where *νvib* is the vibrational frequency of a molecule and *qmax* is the maximum separation distance between atoms relative to their equilibrium position. Substituting Eq. (5) into Eq. (4) gives

$$a = a\_0 + \left(\frac{\partial a}{\partial q}\right) q\_{\text{max}} \cos 2\pi \nu\_{vib} t \tag{6}$$

Substituting Eq. (6) into Eq. (3) gives

*Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

$$\mu = E\_0 \cos 2\pi\nu t \left[ a\_0 + \left(\frac{\partial a}{\partial q}\right) q\_{\text{max}} \cos 2\pi\nu\_{vib} t \right] \tag{7}$$

or

• *Anti-Stokes* process: The photon acquires vibrational energy and is scattered

This shift provides information about vibrational, rotational, and other lowfrequency transitions in molecules. Raman spectroscopy can be used to study solid,

Raman scattering can be explained using the molecular polarizability [5]. If a molecule is placed in an electric field, electrons and nuclei get displaced. Due to the separation of charged species, an electric dipole moment is induced in the molecule, and it is said to be polarized. If *E* is the strength of the electric field and *μ* is the

where *α* is the polarizability of the molecule. If a sample is subjected to an electromagnetic wave of frequency *ν*, the electric field experienced by each mole-

where *E*<sup>0</sup> is the amplitude of the electromagnetic wave. From Eq. (1)

Thus, Eq. (3) implies that interaction of electromagnetic radiation of frequency *ν* induces a molecular dipole moment that oscillates and emits radiation of the same frequency, and this is the classical explanation of Rayleigh scattering. However, the ability to perturb the local electron cloud of a molecular structure depends on the relative location of the individual atoms; hence, the polarizability is a function of the instantaneous position of the constituent atoms. So, the polarizability changes with small displacement from equilibrium position (i.e., molecular vibration) and is

*α* ¼ *α*<sup>0</sup> þ *q* � *qeq*

where *α<sup>0</sup>* is equilibrium polarizability and *qeq* and *q* are bond lengths at equilibrium position and any instant, respectively. If a molecule executes simple harmonic

where *νvib* is the vibrational frequency of a molecule and *qmax* is the maximum separation distance between atoms relative to their equilibrium position. Substitut-

> *∂α ∂q*

*∂α*

*∂q*

*q* � *q*<sup>0</sup> ¼ *qmax* cos 2*πυvibt* (5)

*qmax* cos 2*πυvibt* (6)

(4)

*μ* ¼ *αE* (1)

*E* ¼ *E*<sup>0</sup> cos 2*πυt* (2)

*μ* ¼ *αE*<sup>0</sup> cos 2*πυt* (3)

with a higher energy *h*(ν<sup>0</sup> + ν*vib*) (blue shift).

*Recent Advances in Nanophotonics - Fundamentals and Applications*

liquid, and gaseous samples.

cule of the sample varies as

given by

**138**

**1.1 Classical theory of the Raman effect**

magnitude of the induced dipole moment, then

motion, the displacement can be represented as

Substituting Eq. (6) into Eq. (3) gives

*α* ¼ *α*<sup>0</sup> þ

ing Eq. (5) into Eq. (4) gives

$$\mu = a\_0 E\_0 \cos 2\pi \nu t + E\_0 \left(\frac{\partial \alpha}{\partial q}\right) q\_{\text{max}} \cos 2\pi \nu t \,\,\cos 2\pi \nu\_{\text{vib}} t \tag{8}$$

or

$$\mu = a\_0 E\_0 \cos 2\pi \nu t + \frac{E\_0}{2} q\_{\text{max}} \left( \frac{\partial a}{\partial q} \right) \cos \left[ 2\pi (\nu - \nu\_{\text{vib}}) t \right] + \frac{E\_0}{2} q\_{\text{max}} \left( \frac{\partial a}{\partial q} \right) \cos \left[ 2\pi (\nu + \nu\_{\text{vib}}) t \right] \tag{9}$$

The first term in Eq. (9) represents Rayleigh scattering and occurs at the excitation frequency *ν*. The second and third terms correspond to Stokes (*ν* � *νvib*) and anti-Stokes (*ν* + *νvib*) scattering. In both inelastic scatterings, the excitation frequency is modulated by the vibrational frequency of the bond.

Besides, from Eq. (9), the molecules that have Raman-active vibration modes must experience a change in polarizability during a vibration *<sup>∂</sup><sup>α</sup> ∂q* , i.e., the electron density in the molecule must distort from its typical shape (inducing a dipole). Molecules with symmetrical bends and stretches, therefore, are generally better Raman scatterers. So, for a molecule to be Raman active, its molecular rotation or vibration must cause a change in a component of molecular polarizability.

### **1.2 Quantum theory of the Raman effect**

Raman scattering can be easily understood in terms of the quantum theory of radiation. In the quantum model, the molecules exist in quantized energy levels corresponding to possible stationary states of the molecule. When radiation having an energy *hν* incident on a sample, it is considered that the photons undergo collisions with the molecules. When the collision is elastic, the photons will be

### **Figure 1.**

*Energy level diagram for Rayleigh and Raman scattering, where* ΔE *=* hνvib *represents the difference in vibration energy levels [7].*

deflected unchanged, but it is also possible that during the collision, energy is exchanged between the photon and molecule, and as a result, the molecule can gain or lose energy *ΔE*, where *ΔE=hνvib* represents a difference in the vibrational or rotational energy levels of that molecule [6]. In quantum mechanical terms, the scattering can be considered as an excitation to a virtual state lower in energy than a real electronic state. When the molecule gains energy *ΔE*, the photon will be scattered with energy *hν* � *hνvib*, and the scattering is known as Stokes'scattering. Conversely, if the molecule loses energy *ΔE*, the scattered photon will have energy *hν + hνvib*, and this type of scattering is known as anti-Stokes'scattering. Generally, Stokes' radiation is stronger than the anti-Stokes' radiation. **Figure 1** illustrates the energy level diagram for scattering [7].

where *Ae*ð Þ *ω<sup>e</sup>* and *As*ð Þ *ω<sup>s</sup>* are electromagnetic surface-averaged intensity enhancement factors, *Ie* excitation light intensity, *Nsur* the number of adsorbed

From Eq. (10), it follows that the Raman intensity can be enhanced in three

i. by increasing the number of molecules that are on the metal surface

iii. by increasing the electromagnetic surface averaged intensity enhancement

Experiments have proved that by increasing the surface roughness, the number of absorbed molecules was changed only a few times, leaving us with the last two possibilities. They are electromagnetic (EM) and chemical contributions to the

CE requires the probe to be chemically bound to the SERS substrate. The CE can be grouped into three contributions to the chemical mechanism: (i) a resonance Raman (RR) effect due to the incident light matching an electronic transition in the

dent light is in resonance with a metal-molecule or molecule-metal transition. (10– 10<sup>4</sup> contribution), (iii) a nonresonant chemical (CHEM) effect due to ground-state orbital overlap between the molecule and the metal (≤10–100 contribution) [21]. RR is a molecular resonance mechanism that arises from the incident light being

*Schematic diagram of the four-step process of the photon-driven charge transfer model for a molecule adsorbed*

resonant with a molecule, and without a metal surface, this leads to resonance

–10<sup>6</sup> contribution), (ii) a charge-transfer (CT) effect where the inci-

*<sup>d</sup><sup>Ω</sup>* the solid angle of collection optics.

molecules excited by the light, and *<sup>d</sup><sup>σ</sup>*

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

factors.

molecule (10<sup>3</sup>

**Figure 2.**

**141**

*on an electrode.*

enhancement of Raman signal.

**2.1 Chemical enhancement mechanism**

compared to the smooth surface;

ii. by increasing the Raman cross section; and

*Surface-Enhanced Raman Scattering: Introduction and Applications*

ways:

### **2. Surface-enhanced Raman spectroscopy**

One of the limitations of the Raman effect is that it is a very weak phenomenon. About one in 10<sup>7</sup> photons undergo Raman scattering. Therefore the Raman signal is very low from low concentrations of the analyte or poor Raman scatterers. Sometimes the high fluorescence from the molecule obscures the Raman signals. Surfaceenhanced Raman spectroscopy (SERS) is all about amplifying Raman signals from molecules, by several orders of magnitude [8]. SERS is a technique where molecules undergo much higher scattering efficiencies when adsorbed on metal colloidal nanoparticles or rough metal surfaces. The SERS effect was discovered in 1974 by Fleischmann et al. [9]. The group discovered an anomalously large enhancement of the Raman signal of pyridine in the presence of a roughened silver electrode. The enhancement was initially attributed to greater than expected, or fractal-like, surface area, but subsequent reports showed that the anomalous intensity could not be accounted for by increased surface area and was, in fact, a new phenomenon, giving rise to the idea of the SERS cross-section [10, 11]. However, while SERS has become a large and extremely active field of study, there is still a debate on the exact details of its mechanism and its magnitude [12, 13].

Since then, several enhancement mechanisms were proposed in the early days of SERS. However, only two mechanisms are now broadly accepted, i.e., electromagnetic (EM) theory and chemical enhancement (CE) theory [8, 14, 15]. The electromagnetic models treat the molecule as a point dipole which responds to the enhanced local fields at or near the metal surface [16]. These enhanced fields, in turn, arise from roughness features that couple the incident field to surface plasmons [17]. On the other hand, chemical models attribute SERS intensity to modification of the molecular polarizability by interaction with the metal with ensuing molecular resonances, giving rise to enhancements such as those associated with resonance Raman scattering [18]. CE theory depends on the chemical interaction between probe molecules and the noble metal and is said to contribute only a maximum of about two to three orders of magnitude [19]. Both of these enhancements work simultaneously but are yet to be fully understood because of the difficulties in investigating the enhancements separately. Considering that the Raman signal is proportional to the square of dipole moment, ð Þ *P* ¼ *αE* , both enhancement mechanism influences can be viewed as one changes the local electric field (*E*), and the second changes polarizability (*α*) near the analyzed molecule. Another way to understand the enhancement of SERS is by looking at the SERS intensity components [20].

$$I\_{\rm serr} = I\_{\varepsilon} N\_{\rm sur} \mathcal{Q} A\_{\varepsilon}(o\_{\varepsilon}) A\_{\varepsilon}(o\_{\varepsilon}) \frac{d\sigma}{d\Omega},\tag{10}$$

### *Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

deflected unchanged, but it is also possible that during the collision, energy is exchanged between the photon and molecule, and as a result, the molecule can gain or lose energy *ΔE*, where *ΔE=hνvib* represents a difference in the vibrational or rotational energy levels of that molecule [6]. In quantum mechanical terms, the scattering can be considered as an excitation to a virtual state lower in energy than a real electronic state. When the molecule gains energy *ΔE*, the photon will be scattered with energy *hν* � *hνvib*, and the scattering is known as Stokes'scattering. Conversely, if the molecule loses energy *ΔE*, the scattered photon will have energy *hν + hνvib*, and this type of scattering is known as anti-Stokes'scattering. Generally, Stokes' radiation is stronger than the anti-Stokes' radiation. **Figure 1** illustrates the

*Recent Advances in Nanophotonics - Fundamentals and Applications*

One of the limitations of the Raman effect is that it is a very weak phenomenon. About one in 10<sup>7</sup> photons undergo Raman scattering. Therefore the Raman signal is very low from low concentrations of the analyte or poor Raman scatterers. Sometimes the high fluorescence from the molecule obscures the Raman signals. Surfaceenhanced Raman spectroscopy (SERS) is all about amplifying Raman signals from molecules, by several orders of magnitude [8]. SERS is a technique where molecules undergo much higher scattering efficiencies when adsorbed on metal colloidal nanoparticles or rough metal surfaces. The SERS effect was discovered in 1974 by Fleischmann et al. [9]. The group discovered an anomalously large enhancement of the Raman signal of pyridine in the presence of a roughened silver electrode. The enhancement was initially attributed to greater than expected, or fractal-like, surface area, but subsequent reports showed that the anomalous intensity could not be accounted for by increased surface area and was, in fact, a new phenomenon, giving rise to the idea of the SERS cross-section [10, 11]. However, while SERS has become a large and extremely active field of study, there is still a debate on the exact details

Since then, several enhancement mechanisms were proposed in the early days of SERS. However, only two mechanisms are now broadly accepted, i.e., electromagnetic (EM) theory and chemical enhancement (CE) theory [8, 14, 15]. The electro-

magnetic models treat the molecule as a point dipole which responds to the enhanced local fields at or near the metal surface [16]. These enhanced fields, in turn, arise from roughness features that couple the incident field to surface plasmons [17]. On the other hand, chemical models attribute SERS intensity to modification of the molecular polarizability by interaction with the metal with ensuing molecular resonances, giving rise to enhancements such as those associated with resonance Raman scattering [18]. CE theory depends on the chemical interaction between probe molecules and the noble metal and is said to contribute only a maximum of about two to three orders of magnitude [19]. Both of these enhancements work simultaneously but are yet to be fully understood because of the difficulties in investigating the enhancements separately. Considering that the Raman signal is proportional to the square of dipole moment, ð Þ *P* ¼ *αE* , both enhancement mechanism influences can be viewed as one changes the local electric field (*E*), and the second changes polarizability (*α*) near the analyzed molecule. Another way to understand the enhancement of SERS is by looking at the SERS

*Isers* ¼ *IeNsurΩAe*ð Þ *ω<sup>e</sup> As*ð Þ *ω<sup>s</sup>*

*dσ*

*<sup>d</sup><sup>Ω</sup>* , (10)

energy level diagram for scattering [7].

**2. Surface-enhanced Raman spectroscopy**

of its mechanism and its magnitude [12, 13].

intensity components [20].

**140**

where *Ae*ð Þ *ω<sup>e</sup>* and *As*ð Þ *ω<sup>s</sup>* are electromagnetic surface-averaged intensity enhancement factors, *Ie* excitation light intensity, *Nsur* the number of adsorbed molecules excited by the light, and *<sup>d</sup><sup>σ</sup> <sup>d</sup><sup>Ω</sup>* the solid angle of collection optics.

From Eq. (10), it follows that the Raman intensity can be enhanced in three ways:


Experiments have proved that by increasing the surface roughness, the number of absorbed molecules was changed only a few times, leaving us with the last two possibilities. They are electromagnetic (EM) and chemical contributions to the enhancement of Raman signal.

### **2.1 Chemical enhancement mechanism**

CE requires the probe to be chemically bound to the SERS substrate. The CE can be grouped into three contributions to the chemical mechanism: (i) a resonance Raman (RR) effect due to the incident light matching an electronic transition in the molecule (10<sup>3</sup> –10<sup>6</sup> contribution), (ii) a charge-transfer (CT) effect where the incident light is in resonance with a metal-molecule or molecule-metal transition. (10– 10<sup>4</sup> contribution), (iii) a nonresonant chemical (CHEM) effect due to ground-state orbital overlap between the molecule and the metal (≤10–100 contribution) [21].

RR is a molecular resonance mechanism that arises from the incident light being resonant with a molecule, and without a metal surface, this leads to resonance

### **Figure 2.**

*Schematic diagram of the four-step process of the photon-driven charge transfer model for a molecule adsorbed on an electrode.*

Raman scattering. RR involves the formation of a surface complex involving the metal and the analyte, leading to a change in the properties of the molecule (such as the possibility of resonance Raman scattering). The RR effect is typically thought of as a molecular property, and it has been included as a SERS mechanism since the presence of the metal surface can alter where this resonance lies. The CT effect only appears when the molecule and metal are close enough to allow for a sufficient overlap of their wave functions. In this mechanism, tunneling of electrons between the metal and adsorbate molecules takes place. Due to the transfer of an electron from metal to molecule or from molecule to metal, a negative ion is formed. Enhancement occurs when the energy of the negative ion is resonant with the incident photon. This mechanism is explained by considering the molecule and metal system as a whole. It is considered that the Fermi level of the metal layer lies between the molecular ground level and one or more excited states of the molecule. The charge transfer mechanism is short-ranged (0.1–0.5 nm) and strongly dependent on the geometry, bonding, and the molecule energy level [22]. The CHEM effect is the least studied and most difficult to quantify experimentally due to its small contribution to the overall enhancement. The formation of metal-molecular complexes mainly causes the CHEM effect due to chemical bonding [23]. This modifies the ability of the dipole to radiate energy, i.e., it can effectively oppose or amplify the dipole amplitude (**Figure 2**).

transparency of these metals in the ultraviolet region can be explained by the fact that they have a lot of free electrons. Electrons of such metals as Al, Cu, Au, and Ag

The surface plasmon frequency *ωsp* in small spherical metal nanoparticle includes the frequency of the volume plasma *ω<sup>p</sup>* and permittivity of the surround-

*<sup>ω</sup>sp* <sup>¼</sup> *<sup>ω</sup><sup>p</sup>*

Hence at the resonant frequency *ωsp* ¼ *ω*, from Eqs. (11) and (12),

ð Þ 1 þ 2*ε<sup>d</sup>*

From Eq. (12), it follows that the permittivity of metal should have a negative value. Few metals such as Cu, Ag, and Au exhibit strong visible light plasmon resonance, whereas other transition metals show only broadband in the ultraviolet region. Ag, in particular, is suitable for SERS applications in the visible and near IR because it has a tiny imaginary component in this region and thus is less "lossy"

When monochromatic radiation of frequency *ν<sup>0</sup>* and electric filed *E* interacts with a molecule, it induces a Raman dipole oscillating at a frequency *μ* ¼ *αE*. The

the frequency detected as Raman signal in far-field. The same phenomenological description can be applied to SERS. However, the presence of nanostructured metal

a. The electromagnetic field at the metallic surface can be dramatically increased and may result in a possible *local field enhancement*.

environment and may result in a possible *radiation enhancement*.

b. The radiation properties of the Raman dipole, *μ*, are affected by the metallic

Raman spectrum can give rich information of analyte molecules, and SERS due to its higher signal intensity make it possible to detect analyte molecules in very low concentration, which enhances its practical applications [21]. This technique has a large number of applications in various fields, including trace chemical detection [21, 36], such as dye molecules [37–39], food additives [40, 41], pesticide trace detection [42–44], bioanalysis [45–49], and explosive detection [50, 51]. The detection of a trace amount of hazardous chemicals is also in high demand because of the increasing threat from toxic environments and unreliable food safety [52]. Melamine is a chemical compound and has been widely used in milk and pet food as an additive to increase protein percentage. However, since 2007, melamine, with its contaminant cyanuric acid, has become prominent because of the milk scandal. As a facile and simple spectroscopy technique, SERS has been used to detect melamine content [53, 54]. Apart from this, SERS has been widely used for bioanalysis, i.e., in the detection of biomolecules [55], cancer diagnosis [56, 57], urine component detection [58, 59], and in vivo molecular probing in live cells [60, 61], which play an important role in the life science for health care or treatment. Biomolecules, such

oscillating Raman dipole radiates a power proportional to *|μ|*

surface alters the effects in the following ways [31–35]:

**3. Applications of Raman spectroscopy**

as DNA, can also be detected using SERS [62, 63].

**143**

1 2

*ε<sup>m</sup>* ¼ �2*ε<sup>d</sup>* (12)

(11)

<sup>2</sup> at frequency *ν* and is

are not completely free but partially bounded.

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

*Surface-Enhanced Raman Scattering: Introduction and Applications*

ing dielectric [29]:

than other metals [30].

### **2.2 Electromagnetic mechanism**

In theory, EM enhancement is analyte independent, while CE is probedependent and requires some chemical interaction to the metal surface [24]. Most of SERS enhancements are due to the EM enhancement mechanism and are a direct consequence of the roughness present on the noble metal surfaces [25]. The nanostructure can be formed on the substrate itself or by depositing noble metal nanoparticles. These metal nanoparticles can interact with the excitation light because of unique properties caused by their low dimensions (10–100 nm). The small size of the metal nanoparticles makes a special kind of light-induced electric polarization possible for their surface electrons. Collective oscillations of these electrons, driven by the alternating electric field of the light wave, are called surface plasma oscillations. At a particular frequency, plasmon oscillations are resonant with light; then electric field intensity and Raman scattering from the molecules attached to the nanostructures are enhanced [8, 26]. A locally strong light-induced electric field of plasmons in metal nanoparticle causes the increase of *Ae*ð Þ *ω<sup>e</sup>* and *As*ð Þ *ω<sup>s</sup>* factors. It is because nanoparticles work as a kind of optical antenna, redistributing and concentrating light energy near a nanoparticle. As a result, the cross-section of the light scattering processes, including Raman scattering, can be much larger than the geometrical cross-section of the metal nanoparticle.

The surface electron oscillations in metal nanoparticles can be derived from the classical Drude model, describing metal as a lattice of ions immersed into the "gas" consisting of the free electrons [27, 28]. In a static electric field, the internal field of the metals, generated by the displacement of free electrons, shield the external electric field. As a result, the external electrostatic field cannot create the electric field inside the metal. As a result, if electrostatic fields are applied to the metals, their dielectric permittivity is ambiguous. The dielectric permittivity is the measure of how much the electric field inside a material differs from that of a vacuum. However, when a high-frequency electric field is applied, the free electrons inside the metal cannot completely follow in time with the high-frequency oscillations of the electric field. It creates a situation where at very high frequencies, metal can pass the electric field from the incident light, i.e., behave as a dielectric. The high

*Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

Raman scattering. RR involves the formation of a surface complex involving the metal and the analyte, leading to a change in the properties of the molecule (such as the possibility of resonance Raman scattering). The RR effect is typically thought of as a molecular property, and it has been included as a SERS mechanism since the presence of the metal surface can alter where this resonance lies. The CT effect only appears when the molecule and metal are close enough to allow for a sufficient overlap of their wave functions. In this mechanism, tunneling of electrons between the metal and adsorbate molecules takes place. Due to the transfer of an electron from metal to molecule or from molecule to metal, a negative ion is formed. Enhancement occurs when the energy of the negative ion is resonant with the incident photon. This mechanism is explained by considering the molecule and metal system as a whole. It is considered that the Fermi level of the metal layer lies between the molecular ground level and one or more excited states of the molecule. The charge transfer mechanism is short-ranged (0.1–0.5 nm) and strongly dependent on the geometry, bonding, and the molecule energy level [22]. The CHEM effect is the least studied and most difficult to quantify experimentally due to its small contribution to the overall enhancement. The formation of metal-molecular complexes mainly causes the CHEM effect due to chemical bonding [23]. This modifies the ability of the dipole to radiate energy, i.e., it can effectively oppose or

*Recent Advances in Nanophotonics - Fundamentals and Applications*

In theory, EM enhancement is analyte independent, while CE is probedependent and requires some chemical interaction to the metal surface [24]. Most of SERS enhancements are due to the EM enhancement mechanism and are a direct consequence of the roughness present on the noble metal surfaces [25]. The nanostructure can be formed on the substrate itself or by depositing noble metal nanoparticles. These metal nanoparticles can interact with the excitation light because of unique properties caused by their low dimensions (10–100 nm). The small size of the metal nanoparticles makes a special kind of light-induced electric polarization possible for their surface electrons. Collective oscillations of these electrons, driven by the alternating electric field of the light wave, are called surface plasma oscillations. At a particular frequency, plasmon oscillations are resonant with light; then electric field intensity and Raman scattering from the molecules attached to the nanostructures are enhanced [8, 26]. A locally strong light-induced electric field of plasmons in metal nanoparticle causes the increase of *Ae*ð Þ *ω<sup>e</sup>* and *As*ð Þ *ω<sup>s</sup>* factors. It is because nanoparticles work as a kind of optical antenna, redistributing and concentrating light energy near a nanoparticle. As a result, the cross-section of the light scattering processes, including Raman scattering, can be

much larger than the geometrical cross-section of the metal nanoparticle.

The surface electron oscillations in metal nanoparticles can be derived from the classical Drude model, describing metal as a lattice of ions immersed into the "gas" consisting of the free electrons [27, 28]. In a static electric field, the internal field of the metals, generated by the displacement of free electrons, shield the external electric field. As a result, the external electrostatic field cannot create the electric field inside the metal. As a result, if electrostatic fields are applied to the metals, their dielectric permittivity is ambiguous. The dielectric permittivity is the measure of how much the electric field inside a material differs from that of a vacuum. However, when a high-frequency electric field is applied, the free electrons inside the metal cannot completely follow in time with the high-frequency oscillations of the electric field. It creates a situation where at very high frequencies, metal can pass the electric field from the incident light, i.e., behave as a dielectric. The high

amplify the dipole amplitude (**Figure 2**).

**2.2 Electromagnetic mechanism**

**142**

transparency of these metals in the ultraviolet region can be explained by the fact that they have a lot of free electrons. Electrons of such metals as Al, Cu, Au, and Ag are not completely free but partially bounded.

The surface plasmon frequency *ωsp* in small spherical metal nanoparticle includes the frequency of the volume plasma *ω<sup>p</sup>* and permittivity of the surrounding dielectric [29]:

$$
\alpha\_{\rm sp} = \frac{\alpha\_{\rm p}}{(\mathbf{1} + \mathbf{2}\varepsilon\_d)^{\frac{1}{2}}} \tag{11}
$$

Hence at the resonant frequency *ωsp* ¼ *ω*, from Eqs. (11) and (12),

$$
\varepsilon\_m = -2\varepsilon\_d \tag{12}
$$

From Eq. (12), it follows that the permittivity of metal should have a negative value. Few metals such as Cu, Ag, and Au exhibit strong visible light plasmon resonance, whereas other transition metals show only broadband in the ultraviolet region. Ag, in particular, is suitable for SERS applications in the visible and near IR because it has a tiny imaginary component in this region and thus is less "lossy" than other metals [30].

When monochromatic radiation of frequency *ν<sup>0</sup>* and electric filed *E* interacts with a molecule, it induces a Raman dipole oscillating at a frequency *μ* ¼ *αE*. The oscillating Raman dipole radiates a power proportional to *|μ|* <sup>2</sup> at frequency *ν* and is the frequency detected as Raman signal in far-field. The same phenomenological description can be applied to SERS. However, the presence of nanostructured metal surface alters the effects in the following ways [31–35]:


### **3. Applications of Raman spectroscopy**

Raman spectrum can give rich information of analyte molecules, and SERS due to its higher signal intensity make it possible to detect analyte molecules in very low concentration, which enhances its practical applications [21]. This technique has a large number of applications in various fields, including trace chemical detection [21, 36], such as dye molecules [37–39], food additives [40, 41], pesticide trace detection [42–44], bioanalysis [45–49], and explosive detection [50, 51]. The detection of a trace amount of hazardous chemicals is also in high demand because of the increasing threat from toxic environments and unreliable food safety [52]. Melamine is a chemical compound and has been widely used in milk and pet food as an additive to increase protein percentage. However, since 2007, melamine, with its contaminant cyanuric acid, has become prominent because of the milk scandal. As a facile and simple spectroscopy technique, SERS has been used to detect melamine content [53, 54]. Apart from this, SERS has been widely used for bioanalysis, i.e., in the detection of biomolecules [55], cancer diagnosis [56, 57], urine component detection [58, 59], and in vivo molecular probing in live cells [60, 61], which play an important role in the life science for health care or treatment. Biomolecules, such as DNA, can also be detected using SERS [62, 63].

### **3.1 Raman spectroscopy for transition metal dichalcogenides (TMDs)**

Transition metal dichalcogenides, as the names suggest, are a class of material that is made up of the transition metals (M = Mo, W, Ta, Pt) and chalcogenides (X = S, Se, Te). The unit cell of bulk MX2 consists of X–M–X units, where one M plane is sandwiched between two X planes. Depending upon how these units are stacked, different kinds of polytypes are formed, for example:

stacking of layers has a rotational (or translational) freedom with reference to an axis perpendicular to (or also along) a 2D plane, giving them non-uniqueness); (2) weak interlayer interaction, which is much smaller than the intralayer interactions; and (3) observation of inner layers (the physical properties of each layer within the stacked TMDs are not selectively accessible except for the outermost layer). Other than resolving these challenges, Raman spectroscopy can identify functional groups, structural damage, unwanted by-products, and chemical modifications introduced during synthesis, processing, or its placement on the various substrates during device fabrication. It is spectroscopy, which is nondestructive, quick, and noninvasive for characterizing the TMD materials with high selectivity [64–66]. In general, the Raman spectroscopy has been widely used to determine the layers of the TMDs. For all the layered materials, which also includes TMDs, there are typically two categories of Raman vibrations. One is intralayer vibrations, which occur within a layer and normally appear in the high-frequency region of the spectra. The second category of Raman vibrations is observed due to the relative motion of the layers. These vibrations give the interlayer Raman modes, which are

*Surface-Enhanced Raman Scattering: Introduction and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

normally observed in the low-frequency region of the spectra (<100 cm<sup>1</sup>

layer number (N) dependence of the peak position and width for *E*<sup>1</sup>

MoS2 is shown in **Figure 4(a)**. The A1g and *E*<sup>1</sup>

shows opposite trends, as shown in **Figure 4(b)**.

with increasing numbers of layers for MoS2.

<sup>2</sup>*<sup>g</sup>* and E1u [71].

The position of (*E*<sup>1</sup>

pected behavior of *E*<sup>1</sup>

between *E*<sup>1</sup>

**Figure 4.**

*between E*<sup>1</sup>

*(A1g)/I(E*<sup>1</sup>

*(E*<sup>1</sup>

**145**

In the layered TMDs, to determine the number of layers N for few-layer TMDs, high-frequency intralayer Raman modes can be used [67, 68]. For example, the

However, these two modes would decrease in frequency from 2L to 1L based on the linear chain model (only van der Waals interactions are included). This unex-

also exist [68–70], which is also disclosed by the anomalous Davydov splitting

Molina-Sanchez et al. [72] carefully examined and reported the relationship between the number of monolayers and the Raman active modes (A1g and *E*<sup>1</sup>

They demonstrated that the weak interlayer interaction is the leading cause of the frequency increase (i.e., for A1g) with the number of layers. Moreover, the decrease

*(a) Raman spectra of NL-(N = 1–8, 10, 14 and 18) and bulk MoS2. The two gray-dashed lines indicate Pos*

<sup>2</sup>*<sup>g</sup> and A1g as a function of 1/N. For 1* ≤ *N* ≤ *5, the linear fitting gives Δω(A–E) =25.8–8.4/N. (c)*

<sup>2</sup>*<sup>g</sup> E1 2g) and Pos(A1g) in bulk MoS2. (b) Frequency (ω) of E*<sup>1</sup>

<sup>2</sup>*g), and peak width summarized in (d) [66, 73].*

*Polarized Raman spectra of 1–5L and bulk WS2, with the frequencies of E*<sup>1</sup>

<sup>2</sup>*<sup>g</sup>*) and (A1g) with decreasing thickness from the bulk to 1 L

<sup>2</sup>*<sup>g</sup>* suggests that interactions other than van der Waals forces

).

<sup>2</sup>*<sup>g</sup>* and A1g of NL-

<sup>2</sup>*<sup>g</sup>*).

<sup>2</sup>*<sup>g</sup>* modes undergo blue and redshifts

<sup>2</sup>*<sup>g</sup> and A1g and the frequency difference (Δω)*

<sup>2</sup>*<sup>g</sup> and A1g frequency difference, I*


The schematic diagram of these polytypes is given in **Figure 3**. In recent years, Raman spectroscopy is adopted to address the challenges in the characterization of these TMDs due to: (1) Many possible structures (generally the

### **Figure 3.**

*Structural representation of 1T, 2H, and 3R TMC polytypes and their corresponding metal atom coordination. The side and top view of layered forms are shown [61].*

### *Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

stacking of layers has a rotational (or translational) freedom with reference to an axis perpendicular to (or also along) a 2D plane, giving them non-uniqueness); (2) weak interlayer interaction, which is much smaller than the intralayer interactions; and (3) observation of inner layers (the physical properties of each layer within the stacked TMDs are not selectively accessible except for the outermost layer). Other than resolving these challenges, Raman spectroscopy can identify functional groups, structural damage, unwanted by-products, and chemical modifications introduced during synthesis, processing, or its placement on the various substrates during device fabrication. It is spectroscopy, which is nondestructive, quick, and noninvasive for characterizing the TMD materials with high selectivity [64–66].

In general, the Raman spectroscopy has been widely used to determine the layers of the TMDs. For all the layered materials, which also includes TMDs, there are typically two categories of Raman vibrations. One is intralayer vibrations, which occur within a layer and normally appear in the high-frequency region of the spectra. The second category of Raman vibrations is observed due to the relative motion of the layers. These vibrations give the interlayer Raman modes, which are normally observed in the low-frequency region of the spectra (<100 cm<sup>1</sup> ).

In the layered TMDs, to determine the number of layers N for few-layer TMDs, high-frequency intralayer Raman modes can be used [67, 68]. For example, the layer number (N) dependence of the peak position and width for *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* and A1g of NL-MoS2 is shown in **Figure 4(a)**. The A1g and *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* modes undergo blue and redshifts with increasing numbers of layers for MoS2.

The position of (*E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>*) and (A1g) with decreasing thickness from the bulk to 1 L shows opposite trends, as shown in **Figure 4(b)**.

However, these two modes would decrease in frequency from 2L to 1L based on the linear chain model (only van der Waals interactions are included). This unexpected behavior of *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* suggests that interactions other than van der Waals forces also exist [68–70], which is also disclosed by the anomalous Davydov splitting between *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* and E1u [71].

Molina-Sanchez et al. [72] carefully examined and reported the relationship between the number of monolayers and the Raman active modes (A1g and *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>*). They demonstrated that the weak interlayer interaction is the leading cause of the frequency increase (i.e., for A1g) with the number of layers. Moreover, the decrease

### **Figure 4.**

**3.1 Raman spectroscopy for transition metal dichalcogenides (TMDs)**

stacked, different kinds of polytypes are formed, for example:

*Recent Advances in Nanophotonics - Fundamentals and Applications*

hexagonal symmetry, H stands for hexagonal)

rhombohedral symmetry, R stands for rhombohedral)

The schematic diagram of these polytypes is given in **Figure 3**.

symmetry, T stands for trigonal)

**Figure 3.**

**144**

*The side and top view of layered forms are shown [61].*

Transition metal dichalcogenides, as the names suggest, are a class of material that is made up of the transition metals (M = Mo, W, Ta, Pt) and chalcogenides (X = S, Se, Te). The unit cell of bulk MX2 consists of X–M–X units, where one M plane is sandwiched between two X planes. Depending upon how these units are

1.1T (one X–M–X unit in the unit cell, octahedral coordination, tetragonal

2.2H (two X–M–X layers per repeat unit, trigonal prismatic coordination,

3.3R (three X–M–X layers per repeat unit, trigonal prismatic coordination,

In recent years, Raman spectroscopy is adopted to address the challenges in the characterization of these TMDs due to: (1) Many possible structures (generally the

*Structural representation of 1T, 2H, and 3R TMC polytypes and their corresponding metal atom coordination.*

*(a) Raman spectra of NL-(N = 1–8, 10, 14 and 18) and bulk MoS2. The two gray-dashed lines indicate Pos (E*<sup>1</sup> <sup>2</sup>*<sup>g</sup> E1 2g) and Pos(A1g) in bulk MoS2. (b) Frequency (ω) of E*<sup>1</sup> <sup>2</sup>*<sup>g</sup> and A1g and the frequency difference (Δω) between E*<sup>1</sup> <sup>2</sup>*<sup>g</sup> and A1g as a function of 1/N. For 1* ≤ *N* ≤ *5, the linear fitting gives Δω(A–E) =25.8–8.4/N. (c) Polarized Raman spectra of 1–5L and bulk WS2, with the frequencies of E*<sup>1</sup> <sup>2</sup>*<sup>g</sup> and A1g frequency difference, I (A1g)/I(E*<sup>1</sup> <sup>2</sup>*g), and peak width summarized in (d) [66, 73].*

in the *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* phonon frequency is associated with a stronger dielectric screening of the long-range Coulomb interaction (which is induced by the effective charges resulting from the relative displacement between Mo and S atoms) in a few layers and in bulk. Thus, it is expected to exhibit an anomalous frequency trend in which the A1g mode increases in frequency with an increasing number of layers, while the *E*1 <sup>2</sup>*<sup>g</sup>* mode decreases.

conductive PEDOT:PSS film exhibits a narrower band. This change is similar to that of PEDOT:PSS film treated with ethylene glycols reported by Xia et al. [81]. These vibrational modes correspond to the stretching vibrations of C<sup>α</sup> *=* C<sup>β</sup> on the five-

*Surface-Enhanced Raman Scattering: Introduction and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

The band at 1440 cm<sup>1</sup> is associated with the Cα]C<sup>β</sup> symmetric vibration. The band near about 1368 cm<sup>1</sup> is associated with the CβdC<sup>β</sup> stretching. Raman peaks located at 1508 and 1568 cm<sup>1</sup> are associated with the Cα]C<sup>β</sup> asymmetric stretching vibrations. The band at 1540 cm<sup>1</sup> has been related to the splitting of these asymmetrical stretching vibrations [79, 81–84]. Two kinds of resonant structures have been proposed for PEDOT, namely, benzoid and quinoid structure. For coil conformation, benzoid structure is the favorite structure, and quinoid structure is the favorite structure for linear and expanded-coil structure. Both benzoid and quinoid resonant structures exist simultaneously in pristine PEDOT:PSS film. The benzoid structure may be transformed into the quinoid structure after DMSO treatment so that quinoid structure becomes dominant in the highly conductive PEDOT:PSS film.

The conducting PEDOT:PSS films are vastly used in optoelectronic devices.

"SERS substrates" are any nanostructured metallic platform that supports plasmon resonance and amplifies Raman signals [85]. Herein, SERS substrates are

a. **Random morphology SERS substrates** include roughened electrodes, metallic silver and gold colloids, metal-island film on planer substrate, and

b. **Ordered or periodic metallic SERS substrates** include arrays of regular morphology metallic nanotextures created on planar substrates using nanolithography and other physical vapor deposition techniques.

Random morphology SERS substrates are inhomogeneous and are not highly reproducible [86]. Roughened electrodes are the most primitive SERS substrate and were discovered by Fleischmann et al. [9]. These substrates are typically created by running the redox cycle in an electrochemical cell containing a metallic salt solution. Such substrates have gained popularity due to an ability to adjust electrode potential to understand the charge transfer phenomenon between adsorbate and metallic surface [87]. Regardless, the importance of this substrate is decreasing substantially

Among the random morphology SERS substrates, silver or gold colloids are the most common substrates used in both early and more recent studies. Since colloids are easy to produce in a laboratory and tend to generate large enhancement factors, most researchers are still involved in colloid-based SERS rather than more sophisticated substrates [88, 89]. Metallic colloids are also of historic significance related to SERS development, as the first single-molecule SERS detection was reported using colloid substrates [90]. In colloid-based methods, nanoparticle size and geometry can be controlled by altering experimental conditions. One of the most popular methods for controlling nanoparticle morphology stems from the polyol synthesis of silver nanocubes by Sun and Xia [78]. In addition to the nanocubes, various groups have produced octahedra and cuboctahedra [91] and octapods [92].

member ring of PEDOT.

**4. SERS substrates**

classified into two broad distinctions:

other related substrates.

**4.1 Random morphology SERS substrates**

due to relatively low enhancement factors.

**147**

In the case of the WS2, the proximity of the 2LA(M) and *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* peaks makes it challenging to measure the accurate difference between the A1g and *E*<sup>1</sup> <sup>2</sup>*<sup>g</sup>* peaks and increases the chance of error in determining the frequency shift of both the modes. To resolve this problem, Berkdemir et al. [74] reported a new method based on the ratio of the intensities of I2LA and IA1g peaks. In this method, the authors reported that the absolute intensity of the 2LA(M) mode increases with decreasing the number of layers, while the intensity of the A1g displays the opposite behavior [74]. The behavior of the A1g mode with a decreasing number of layers presumably results from weaker interlayer contributions to the phonon restoring forces.

### **3.2 Application of Raman spectroscopy in photovoltaics**

Conducting polymers are widely used in organic light-emitting diodes, heterojunction diodes, organic thin-film transistors, solar cells, actuators, sensors, etc. [75–79]. Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) is extensively used conducting polymer because of its high conductivity, excellent thermal stability, transparency, structural stability. PEDOT:PSS polymer is a promising candidate as a transparent electrode for optoelectronic devices. The solvent treatment of PEDOT:PSS films may affect the conformation of the polymer. The structure of the PEDOT chain changes from benzoid to quinoid structure after solvent treatment [75, 80]. The effect of the conformation of the PEDOT chains in the PEDOT:PSS film before and after the dimethyl sulfoxide (DMSO) treatment was studied by Raman spectroscopy. **Figure 5** shows the Raman spectra of PEDOT: PSS with different concentrations of DMSO. The most obvious change was observed for the strongest band between 1400 and 1500 cm<sup>1</sup> . The highly

**Figure 5.** *Raman spectra of PEDOT:PSS films with 0–8 vol.% DMSO.*

*Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

conductive PEDOT:PSS film exhibits a narrower band. This change is similar to that of PEDOT:PSS film treated with ethylene glycols reported by Xia et al. [81]. These vibrational modes correspond to the stretching vibrations of C<sup>α</sup> *=* C<sup>β</sup> on the fivemember ring of PEDOT.

The band at 1440 cm<sup>1</sup> is associated with the Cα]C<sup>β</sup> symmetric vibration. The band near about 1368 cm<sup>1</sup> is associated with the CβdC<sup>β</sup> stretching. Raman peaks located at 1508 and 1568 cm<sup>1</sup> are associated with the Cα]C<sup>β</sup> asymmetric stretching vibrations. The band at 1540 cm<sup>1</sup> has been related to the splitting of these asymmetrical stretching vibrations [79, 81–84]. Two kinds of resonant structures have been proposed for PEDOT, namely, benzoid and quinoid structure. For coil conformation, benzoid structure is the favorite structure, and quinoid structure is the favorite structure for linear and expanded-coil structure. Both benzoid and quinoid resonant structures exist simultaneously in pristine PEDOT:PSS film. The benzoid structure may be transformed into the quinoid structure after DMSO treatment so that quinoid structure becomes dominant in the highly conductive PEDOT:PSS film. The conducting PEDOT:PSS films are vastly used in optoelectronic devices.

### **4. SERS substrates**

in the *E*<sup>1</sup>

<sup>2</sup>*<sup>g</sup>* mode decreases.

*E*1

**Figure 5.**

**146**

<sup>2</sup>*<sup>g</sup>* phonon frequency is associated with a stronger dielectric screening of the

<sup>2</sup>*<sup>g</sup>* peaks makes it

. The highly

<sup>2</sup>*<sup>g</sup>* peaks and

long-range Coulomb interaction (which is induced by the effective charges resulting from the relative displacement between Mo and S atoms) in a few layers and in bulk. Thus, it is expected to exhibit an anomalous frequency trend in which the A1g mode increases in frequency with an increasing number of layers, while the

In the case of the WS2, the proximity of the 2LA(M) and *E*<sup>1</sup>

*Recent Advances in Nanophotonics - Fundamentals and Applications*

**3.2 Application of Raman spectroscopy in photovoltaics**

challenging to measure the accurate difference between the A1g and *E*<sup>1</sup>

increases the chance of error in determining the frequency shift of both the modes. To resolve this problem, Berkdemir et al. [74] reported a new method based on the ratio of the intensities of I2LA and IA1g peaks. In this method, the authors reported that the absolute intensity of the 2LA(M) mode increases with decreasing the number of layers, while the intensity of the A1g displays the opposite behavior [74]. The behavior of the A1g mode with a decreasing number of layers presumably results from weaker interlayer contributions to the phonon restoring forces.

Conducting polymers are widely used in organic light-emitting diodes, heterojunction diodes, organic thin-film transistors, solar cells, actuators, sensors, etc. [75–79]. Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) is extensively used conducting polymer because of its high conductivity, excellent thermal stability, transparency, structural stability. PEDOT:PSS polymer is a promising candidate as a transparent electrode for optoelectronic devices. The solvent treatment of PEDOT:PSS films may affect the conformation of the polymer. The structure of the PEDOT chain changes from benzoid to quinoid structure after solvent treatment [75, 80]. The effect of the conformation of the PEDOT chains in the PEDOT:PSS film before and after the dimethyl sulfoxide (DMSO) treatment was studied by Raman spectroscopy. **Figure 5** shows the Raman spectra of PEDOT:

PSS with different concentrations of DMSO. The most obvious change was

observed for the strongest band between 1400 and 1500 cm<sup>1</sup>

*Raman spectra of PEDOT:PSS films with 0–8 vol.% DMSO.*

"SERS substrates" are any nanostructured metallic platform that supports plasmon resonance and amplifies Raman signals [85]. Herein, SERS substrates are classified into two broad distinctions:


### **4.1 Random morphology SERS substrates**

Random morphology SERS substrates are inhomogeneous and are not highly reproducible [86]. Roughened electrodes are the most primitive SERS substrate and were discovered by Fleischmann et al. [9]. These substrates are typically created by running the redox cycle in an electrochemical cell containing a metallic salt solution. Such substrates have gained popularity due to an ability to adjust electrode potential to understand the charge transfer phenomenon between adsorbate and metallic surface [87]. Regardless, the importance of this substrate is decreasing substantially due to relatively low enhancement factors.

Among the random morphology SERS substrates, silver or gold colloids are the most common substrates used in both early and more recent studies. Since colloids are easy to produce in a laboratory and tend to generate large enhancement factors, most researchers are still involved in colloid-based SERS rather than more sophisticated substrates [88, 89]. Metallic colloids are also of historic significance related to SERS development, as the first single-molecule SERS detection was reported using colloid substrates [90]. In colloid-based methods, nanoparticle size and geometry can be controlled by altering experimental conditions. One of the most popular methods for controlling nanoparticle morphology stems from the polyol synthesis of silver nanocubes by Sun and Xia [78]. In addition to the nanocubes, various groups have produced octahedra and cuboctahedra [91] and octapods [92].

El-Sayed et al. have contributed to the control of particle morphology, yielding a variety of interesting and useful structures [93–95]. Since the size, shape, and material of the particles govern the resulting plasmonic resonance characteristics, significant effort has been exerted in the control of plasmon resonance via core-shell and alloyed particles, to which the Halas group has a large contribution [96–98].

enhancement when compared with metal-island film substrates [113]. They demonstrated two different methods of substrate fabrication by e-beam lithography. In the first method, regular fields of nanoparticles are produced by the lift-off technique. A silver layer is evaporated on the structured resist, and the resist is removed afterward. In the second method, gratings or crossed gratings are transferred into a silicon wafer with a thermal oxide surface layer by reactive ion etching (RIE). Then, the e-beam resist is removed, and finally, a silver layer is evaporated. Hatab et al. have demonstrated significant SERS enhancement factors exceeding 1011, resulting from a new configuration of elevated gold bowtie nano-antenna arrays with optimized array periodicity [114]. A process combining nanofabrication steps of pattern definition by EBL, metal deposition, lift-off, and RIE arranged in a particular sequence was used to fabricate the elevated gold bowtie arrays on Si wafers. The elevated bowties allow the manifestation of intrinsic plasmonic coupling effects in suspended nanocavities, or the tip-to-tip nanogaps, from structures that are not in physical contact with a substrate. This configuration results in up to two orders of

*Surface-Enhanced Raman Scattering: Introduction and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

additional magnitude enhancement in SERS response compared to that of

high cost of the processes used in their fabrication.

**4.3 SERS from silver columnar film**

**149**

nonelevated bowtie arrays. The diversity of designs is endless when fabrication with EBL is considered. However, these techniques, while excellent at making SERS substrates with defined characteristics, are hampered by the slow, serial nature and

The SERS enhancement strongly depends on the substrate. As already discussed earlier, various techniques have been proposed and identified for the fabrication of the SERS substrate. However, only a few methods are available to develop uniform, reproducible, robust, stable, and cost-effective SERS substrates. Recently, silver columnar thin films fabricated by glancing angle deposition (GLAD) have been identified as high sensitivity SERS active substrates [115–122]. A remarkable SERS enhancement factor with applications in sensing the biomolecules at very low concentrations has been observed on the silver nanorod arrays [123]. To understand the SERS mechanism and attain a maximum possible enhancement, large numbers of studies have been performed on the Ag nanorod (AgNR) arrays. In an interesting study, Chaney et al. have investigated the SERS response as a function of the nanorod length using trans-1,2-bis(4-pyridyl)ethane (BPE) as a probe molecule at an excitation wavelength of 785 nm [124]. They found that the SERS intensity increases dramatically with nanorod length. Zhou et al. fabricated aligned, singlecrystalline AgNRs on planar Si substrates by GLAD technique, with sample substrate cooled by liquid nitrogen in the e-beam deposition system [125]. They were successful in detecting aqueous solution of10<sup>12</sup> molL<sup>1</sup> Rhodamine 6G by the porous Ag film with nanorods. They also deposited AgNRs on Ag, Al, Si, and Ti thin films with a thickness of 100 and 400 nm, respectively, to achieve layers with different reflectivities. The SERS intensity of the AgNRs grown on Ag thin film was found to be higher than others, and the SERS intensity of the AgNRs on Al film was larger than that on Ti film, and the AgNRs on Si film showed the minimum SERS intensity. They concluded that the larger the under-layer reflectivity, the larger the SERS performance of substrate. So, the pre-deposition of Ag layer under AgNRs can be an effective way to promote the SERS performance of AgNRs. Zhang et al. have made AgNRs in film grow into periodic patterns at a micro-nano scale, and they showed that the AgNR film with periodic patterns exhibits better SERS performance than Ag film with nanorods arranged randomly as before [126]. He et al. also reported a new scalable strategy based on dynamic shadowing growth (DSG) to fabricate large-scale chiral Swiss roll nanostructures. They developed a chiral

SERS substrates have also been fabricated by depositing nanoparticles onto different surfaces. These simple deposition approaches include micro pipetting [99], soaking [100], screen-printing [101], filtration [102], and inkjet printing [103]. However, a major concern with these simple deposition processes is that the hotspots are generated and distributed randomly over the substrate. The differences in metallic particle sizes and their shapes due to the differences in preparation recipes can lead to several orders of magnitude difference in the SERS enhancement factor.

In order to obtain more consistent hotspots from nanoparticles, researchers have explored both self-assembly [99] and directed assembly techniques, such as the Langmuir-Blodgett techniques [91], to create regular arrays of nanoparticles. However, these techniques also introduce more complexity to the fabrication process.

### **4.2 Periodic or uniform SERS substrate**

Although metallic colloidal particles are known for their high SERS EF and possibility to accomplish SERS spectra of a single molecule, it is often challenging to reproduce or routinely deliver such a high performing SERS feature. To overcome this issue, a few alternatives have been introduced on engineering periodic arrays of metallic nanostructured SERS substrates. Nanosphere lithography (NSL), developed by Van Duyne et al., is one of the most extensively used nano-fabrication procedures used in understanding SERS phenomena and performing plasmonicbased sensing [104–106]. It involves the assembly of polystyrene nanospheres into a regular array, using this as a mask to create periodic nanostructures, sometimes called "metal island films," by the evaporation of Ag or Au through the gaps created by the packing of the nanospheres, followed by removal of the nanospheres. A variation on this method, called "metal film over nanospheres," is to evaporate a metal film directly onto the nanosphere template, using closely-packed nanospheres to pattern the substrate surface itself [107]. Electron beam lithography (EBL) is another most widely used conventional nanofabrication technique in designing uniform and controlled morphology SERS substrates [108, 109]. In the literature, there are many SERS-active surface designs prepared with EBL [110, 111]. These designs are mainly periodic arrays of simple nano-structures, and generally, the relation between the LSP resonance wavelength and SERS signal enhancement is studied. For example, Le Ru et al. have taken SERS measurements, from periodic gold dot, square, and triangle arrays, from "Rhodamine 6G" [112]. They demonstrated that the localized plasmon resonances, which are at the origin of visible-NIR extinction spectra and the SERS effect, can be tuned to any desired wavelength by varying the particle shape/size and spacing, thus tuning the Raman amplification. In a very similar study, Gunnarsson et al. studied similar Ag structures on a silicon wafer for the same molecule and reported that better results are obtained than nano-roughened Ag film [110]. They investigated the size and geometry dependence of the SERS effect on supported particles, by manufacturing artificial structures by modern nanofabrication techniques. Arrays of 100–200 nm silver particles of different shapes were prepared on a Si wafer by electron beam lithography. Kahl et al. have shown that the SERS measurements of "Rhodamine 6G" on gold periodic nano-dot arrays and grating structures resulted in order of magnitude better SERS

*Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

enhancement when compared with metal-island film substrates [113]. They demonstrated two different methods of substrate fabrication by e-beam lithography. In the first method, regular fields of nanoparticles are produced by the lift-off technique. A silver layer is evaporated on the structured resist, and the resist is removed afterward. In the second method, gratings or crossed gratings are transferred into a silicon wafer with a thermal oxide surface layer by reactive ion etching (RIE). Then, the e-beam resist is removed, and finally, a silver layer is evaporated. Hatab et al. have demonstrated significant SERS enhancement factors exceeding 1011, resulting from a new configuration of elevated gold bowtie nano-antenna arrays with optimized array periodicity [114]. A process combining nanofabrication steps of pattern definition by EBL, metal deposition, lift-off, and RIE arranged in a particular sequence was used to fabricate the elevated gold bowtie arrays on Si wafers. The elevated bowties allow the manifestation of intrinsic plasmonic coupling effects in suspended nanocavities, or the tip-to-tip nanogaps, from structures that are not in physical contact with a substrate. This configuration results in up to two orders of additional magnitude enhancement in SERS response compared to that of nonelevated bowtie arrays. The diversity of designs is endless when fabrication with EBL is considered. However, these techniques, while excellent at making SERS substrates with defined characteristics, are hampered by the slow, serial nature and high cost of the processes used in their fabrication.

### **4.3 SERS from silver columnar film**

El-Sayed et al. have contributed to the control of particle morphology, yielding a variety of interesting and useful structures [93–95]. Since the size, shape, and material of the particles govern the resulting plasmonic resonance characteristics, significant effort has been exerted in the control of plasmon resonance via core-shell and alloyed particles, to which the Halas group has a large contribution

*Recent Advances in Nanophotonics - Fundamentals and Applications*

SERS substrates have also been fabricated by depositing nanoparticles onto different surfaces. These simple deposition approaches include micro pipetting [99], soaking [100], screen-printing [101], filtration [102], and inkjet printing [103]. However, a major concern with these simple deposition processes is that the hotspots are generated and distributed randomly over the substrate. The differences in metallic particle sizes and their shapes due to the differences in preparation recipes can lead to several orders of magnitude difference in the SERS enhancement

In order to obtain more consistent hotspots from nanoparticles, researchers have explored both self-assembly [99] and directed assembly techniques, such as the Langmuir-Blodgett techniques [91], to create regular arrays of nanoparticles. However, these techniques also introduce more complexity to the fabrication process.

Although metallic colloidal particles are known for their high SERS EF and possibility to accomplish SERS spectra of a single molecule, it is often challenging to reproduce or routinely deliver such a high performing SERS feature. To overcome this issue, a few alternatives have been introduced on engineering periodic arrays of metallic nanostructured SERS substrates. Nanosphere lithography (NSL), developed by Van Duyne et al., is one of the most extensively used nano-fabrication procedures used in understanding SERS phenomena and performing plasmonicbased sensing [104–106]. It involves the assembly of polystyrene nanospheres into a regular array, using this as a mask to create periodic nanostructures, sometimes called "metal island films," by the evaporation of Ag or Au through the gaps created by the packing of the nanospheres, followed by removal of the nanospheres. A variation on this method, called "metal film over nanospheres," is to evaporate a metal film directly onto the nanosphere template, using closely-packed nanospheres to pattern the substrate surface itself [107]. Electron beam lithography (EBL) is another most widely used conventional nanofabrication technique in designing uniform and controlled morphology SERS substrates [108, 109]. In the literature, there are many SERS-active surface designs prepared with EBL [110, 111]. These designs are mainly periodic arrays of simple nano-structures, and generally, the relation between the LSP resonance wavelength and SERS signal enhancement is studied. For example, Le Ru et al. have taken SERS measurements, from periodic gold dot, square, and triangle arrays, from "Rhodamine 6G" [112]. They demonstrated that the localized plasmon resonances, which are at the origin of visible-NIR extinction spectra and the SERS effect, can be tuned to any desired wavelength by varying the particle shape/size and spacing, thus tuning the Raman amplification. In a very similar study, Gunnarsson et al. studied similar Ag structures on a silicon wafer for the same molecule and reported that better results are obtained than nano-roughened Ag film [110]. They investigated the size and geometry dependence of the SERS effect on supported particles, by manufacturing artificial structures by modern nanofabrication techniques. Arrays of 100–200 nm silver particles of different shapes were prepared on a Si wafer by electron beam lithography. Kahl et al. have shown that the SERS measurements of "Rhodamine 6G" on gold periodic nano-dot arrays and grating structures resulted in order of magnitude better SERS

[96–98].

factor.

**148**

**4.2 Periodic or uniform SERS substrate**

The SERS enhancement strongly depends on the substrate. As already discussed earlier, various techniques have been proposed and identified for the fabrication of the SERS substrate. However, only a few methods are available to develop uniform, reproducible, robust, stable, and cost-effective SERS substrates. Recently, silver columnar thin films fabricated by glancing angle deposition (GLAD) have been identified as high sensitivity SERS active substrates [115–122]. A remarkable SERS enhancement factor with applications in sensing the biomolecules at very low concentrations has been observed on the silver nanorod arrays [123]. To understand the SERS mechanism and attain a maximum possible enhancement, large numbers of studies have been performed on the Ag nanorod (AgNR) arrays. In an interesting study, Chaney et al. have investigated the SERS response as a function of the nanorod length using trans-1,2-bis(4-pyridyl)ethane (BPE) as a probe molecule at an excitation wavelength of 785 nm [124]. They found that the SERS intensity increases dramatically with nanorod length. Zhou et al. fabricated aligned, singlecrystalline AgNRs on planar Si substrates by GLAD technique, with sample substrate cooled by liquid nitrogen in the e-beam deposition system [125]. They were successful in detecting aqueous solution of10<sup>12</sup> molL<sup>1</sup> Rhodamine 6G by the porous Ag film with nanorods. They also deposited AgNRs on Ag, Al, Si, and Ti thin films with a thickness of 100 and 400 nm, respectively, to achieve layers with different reflectivities. The SERS intensity of the AgNRs grown on Ag thin film was found to be higher than others, and the SERS intensity of the AgNRs on Al film was larger than that on Ti film, and the AgNRs on Si film showed the minimum SERS intensity. They concluded that the larger the under-layer reflectivity, the larger the SERS performance of substrate. So, the pre-deposition of Ag layer under AgNRs can be an effective way to promote the SERS performance of AgNRs. Zhang et al. have made AgNRs in film grow into periodic patterns at a micro-nano scale, and they showed that the AgNR film with periodic patterns exhibits better SERS performance than Ag film with nanorods arranged randomly as before [126]. He et al. also reported a new scalable strategy based on dynamic shadowing growth (DSG) to fabricate large-scale chiral Swiss roll nanostructures. They developed a chiral

conical Swiss roll nanostructure by helically stacking Ag films on a SiO2 frustum with SiO2 films as insulating layers [127]. They also showed that the chiral dichroism (CD) spectral feature can be tuned by changing the bead diameter. They achieved a broadband CD response in visible to near-IR region by making the bead diameter a few hundred nanometers. Mark et al. combined the low-temperature shadow deposition with nanoscale patterning to fabricate nanocolloids with anisotropic three-dimensional shapes, feature sizes down to 20 nm [128]. They first deposited a uniform hexagonal array of Au nanodots deposited onto a Si wafer by micellar nanolithography. Then they deposited material onto the substrate by physical vapor deposition at grazing incidence. To reduce the adatoms' mobility and reduce the diffuse during growth, they cooled the substrate. So, by combining the uniform nano-seeding and low-temperature growth, they fabricated various complex hybrid nanostructures of many materials like from Al2O3, Ti, and Cu. GLAD has also emerged as a powerful tool for the fabrication of 3D chiral plasmonic nanostructures. Titus et al. investigated the optical properties of Ti-doped Ag helices in the visible and near-infrared ranges using transmission ellipsometry and spectroscopy fabricated by GLAD [129]. Nair et al. reported the fabrication of wafer-scale 3D chiral nanoplasmonic substrates with different dielectric templates, namely, silica, magnesium fluoride, and titanium dioxide using GLAD [130]. They have also investigated the effect of interparticle separation on the chiroptical response of chiral nanohelices [131].

Hence, we can see that the development of fabrication and application of substrates for SERS is driven by nanotechnology and the development of high-end fabrication processes. Increasingly SERS substrates with high sensitivity and reproducibility are invented by electrochemical deposition, physical vapor deposition of the metal film, metal nanoparticle colloids, and so forth and applied into various fields, such as detection of pollutants at trace level, surface analysis, biomolecule, and bacteria detection. With the development of SERS substrate, advancement in Raman spectrometers, and tip-enhanced Raman scattering (as the combination of SERS and atomic force microscopy), SERS is becoming increasingly popular as a detection and diagnostic tool.

**Author details**

University, Kyoto, Japan

\*, Prabhat Kumar<sup>2</sup>

*Surface-Enhanced Raman Scattering: Introduction and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.92614*

Academy of Sciences, Prague, Czech Republic

provided the original work is properly cited.

University of the Negev, Midreshet Ben-Gurion, Israel

\*Address all correspondence to: drsamirkumar2017@gmail.coom

1 Department of Micro Engineering, Graduate School of Engineering, Kyoto

2 Department of Thin Films and Nanostructures, Institute of Physics of the Czech

3 Department of Paramedical Sciences, Guru Kashi University, Bhatinda, India

4 Department of Solar Energy and Environmental Physics, Ben-Gurion National Solar Energy Center, Jacob Blaustein Institutes for Desert Research, Ben-Gurion

© 2020 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,

, Anamika Das3 and Chandra Shakher Pathak<sup>4</sup>

Samir Kumar<sup>1</sup>

**151**

*Surface-Enhanced Raman Scattering: Introduction and Applications DOI: http://dx.doi.org/10.5772/intechopen.92614*

### **Author details**

conical Swiss roll nanostructure by helically stacking Ag films on a SiO2 frustum with SiO2 films as insulating layers [127]. They also showed that the chiral dichroism (CD) spectral feature can be tuned by changing the bead diameter. They achieved a broadband CD response in visible to near-IR region by making the bead diameter a few hundred nanometers. Mark et al. combined the low-temperature shadow deposition with nanoscale patterning to fabricate nanocolloids with anisotropic three-dimensional shapes, feature sizes down to 20 nm [128]. They first deposited a uniform hexagonal array of Au nanodots deposited onto a Si wafer by micellar nanolithography. Then they deposited material onto the substrate by physical vapor deposition at grazing incidence. To reduce the adatoms' mobility and reduce the diffuse during growth, they cooled the substrate. So, by combining the uniform nano-seeding and low-temperature growth, they fabricated various complex hybrid nanostructures of many materials like from Al2O3, Ti, and Cu. GLAD has also emerged as a powerful tool for the fabrication of 3D chiral plasmonic nanostructures. Titus et al. investigated the optical properties of Ti-doped Ag helices in the visible and near-infrared ranges using transmission ellipsometry and spectroscopy fabricated by GLAD [129]. Nair et al. reported the fabrication of wafer-scale 3D chiral nanoplasmonic substrates with different dielectric templates, namely, silica, magnesium fluoride, and titanium dioxide using GLAD [130]. They have also investigated the effect of interparticle separation on the chiroptical

*Recent Advances in Nanophotonics - Fundamentals and Applications*

Hence, we can see that the development of fabrication and application of substrates for SERS is driven by nanotechnology and the development of high-end fabrication processes. Increasingly SERS substrates with high sensitivity and reproducibility are invented by electrochemical deposition, physical vapor deposition of the metal film, metal nanoparticle colloids, and so forth and applied into various fields, such as detection of pollutants at trace level, surface analysis, biomolecule, and bacteria detection. With the development of SERS substrate, advancement in Raman spectrometers, and tip-enhanced Raman scattering (as the combination of SERS and atomic force microscopy), SERS is becoming increasingly popular as a

response of chiral nanohelices [131].

detection and diagnostic tool.

**150**

Samir Kumar<sup>1</sup> \*, Prabhat Kumar<sup>2</sup> , Anamika Das3 and Chandra Shakher Pathak<sup>4</sup>

1 Department of Micro Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan

2 Department of Thin Films and Nanostructures, Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic

3 Department of Paramedical Sciences, Guru Kashi University, Bhatinda, India

4 Department of Solar Energy and Environmental Physics, Ben-Gurion National Solar Energy Center, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel

\*Address all correspondence to: drsamirkumar2017@gmail.coom

© 2020 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.

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*Recent Advances in Nanophotonics - Fundamentals and Applications*

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Raman spectromicroscopy (μRS). Journal of Raman Specroscopy. 2015;**46**:

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632-635. DOI: 10.1002/jrs.4710

[40] Granger JH, Schlotter NE,

10.1039/C5CS00828J

022811-101227

analchem.6b04324

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[42] Wang P, Wu L, Lu Z, Li Q, Yin W,

nanotentacle SERS substrate for rapid sampling and reliable detection of pesticide residues in fruits and

vegetables. Analytical Chemistry. 2017; **89**:2424-2431. DOI: 10.1021/acs.

[44] Kumar S, Goel P, Singh JP. Flexible and robust SERS active substrates for conformal rapid detection of pesticide residues from fruits. Sensors and Actuators B: Chemical. 2017;**241**: 577-583. DOI: 10.1016/j.snb.2016.10.106

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C6RA06163J

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[36] Zhang L, Xu J, Mi L, Gong H, Jiang S, Yu Q. Multifunctional

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## *Edited by Mojtaba Kahrizi and Parsoua A. Sohi*

This volume brings together several recent research articles in the field of nanophotonics. The editors have arranged the chapters in three main parts: quantum devices, photonic devices, and semiconductor devices. The chapters cover a wide variety of scopes in those areas including principles of plasmonic, SPR, LSPR and their applications, graphene-based nanophotonic devices, generation of entangled photon and quantum dots, perovskite solar cells, photo-detachment and photoionization of two-electrons systems, diffusion and intermixing of atoms in semiconductor crystals, lattice and molecular elastic and inelastic scattering including surface-enhanced Raman Scattering and their applications. It is our sincerest hope that science and engineering students and researchers could benefit from the new ideas and recent advances in the field that are covered in this book.

Published in London, UK © 2020 IntechOpen © Yavuz Meyveci / iStock

Recent Advances in Nanophotonics - Fundamentals and Applications

Recent Advances in

Nanophotonics

Fundamentals and Applications

*Edited by Mojtaba Kahrizi and Parsoua A. Sohi*