**3. Incorporation of oxygen into AlN crystal lattice**

Generally, thin films are found to be defective due to the presence of impurities. A common defect in AlN thin films is the presence of a substitutional defect - N3− ions of the AlN crystal lattice are replaced by O2− ions - according to **Figure 8**.

This occurs because the characteristics (ionic radius and electronegativity) of oxygen and nitrogen are quite similar. The ionic radius of the oxygen is rO = 0.140 nm and of the nitrogen rN = 0.146 nm gives a difference of ionic radii Δr = 4.11%. The electronegativity of oxygen is ξO = 3.5, of aluminum ξAl = 1.6, of nitrogen ξN = 3.0, and therefore the differences in electronegativity with respect to Al are ΔξO-Al = 1.9 (ionic covalent bonds) and ΔξN-Al = 1.4 (polar covalent bonds). The impurity atoms give rise to stress in the AlN (100) crystalline lattice since the ionic radius difference between N and O is very small (Δr<15%) [18]. Otherwise, the impurity atoms will create substantial distortions in the crystalline lattice and may form a new phase. Also, during the process of thin film growth, there is a strong competition between oxygen and nitrogen because the high electropositivity of aluminum and high electronegativity of the oxygen favors the formation of an intermediate compound (Al2O3), instead of material with substitutional defect.

### **4. Growth of AlN films**

#### **4.1 Deposition techniques of AlN films**

Aluminum nitride (AlN) films can be deposited on various substrate types via physical and chemical deposition methods as shown in **Figure 9**. Typical deposition methods include sputtering processes, metalorganic chemical vapor deposition (MOCVD), Molecular-beam epitaxy (MBE), and pulsed laser deposition (PLD).

Sputtering methods are the most used to grow AlN films due to their advantages such as low temperature, low cost, and flexibility. The sputtering deposition allows

**Figure 9.** *Overview of the main methods for deposition of AlN films.*

the development of devices and sensors on different types of substrates including polymeric substrates. Recently, Cunha et al. reported growth of highly (100)-oriented aluminum nitride (AlN) thin films on (100) Si substrate, in poor vacuum systems, by radio frequency magnetron sputtering (**Figure 10a**). High-quality films with good stoichiometry Al/N (≈1:1) and low oxygen concentration (<10%) were produced by varying the target-substrate distance and the deposition time, whereas the temperature, the nitrogen flow, RF power, and sputtering pressure were fixed [17].

Ghosh et al. discussed the growth of both the undoped and doped AlN films on GaN/Sapphire templates in an MBE chamber (**Figure 10b**) using a plasma-assisted process called PAMBE (plasma-assisted molecular beam epitaxy). The authors concluded that employing an excimer laser annealing with optimized power and frequency rather than the conventional thermal annealing can be a potential alternative route toward improving the structural and electrical properties of AlN layers [19].

An AlN interfacial passivation layer prepared in an MOCVD system (**Figure 10c**) was reported by Aoki et al. The proposed of this study was to present a route for the fabrication and optimization of GaAs metal–oxide–semiconductor (MOS) structures comprising an Al2O3 gate oxide, deposited via atomic layer deposition (ALD) and using an AlN interfacial passivation layer [20].

Pulsed-laser-deposited AlN films were produced by Vispute et al. using the system shown in **Figure 10d** at substrate temperatures ranging from 25°C (room temperature) to 1000°C. The AlN films were employed in the fabrication of device-quality AlN heterostructures grown on SiC for high-temperature electronic devices [21].

In relation to emerging advanced AlN deposition methods, atomic layer deposition (ALD) can be highlighted. Amorphous AlN films obtained by ALD were investigated by Parkhomenko et al. using trimethylaluminum and monomethylhydrazine as the precursors at a deposition temperature of 375–475°C. The ALD AlN films exhibited an oxygen content of as low as 4%. In addition, they were compact, continuous and with mechanical properties comparable to those of AlN films produced by other techniques [22]. ALD of AlN on different SiC surfaces with different crystallographic orientation was also investigated recently [23]. For all layers, the surface morphology

*Development and Applications of Aluminum Nitride Thin Film Technology DOI: http://dx.doi.org/10.5772/intechopen.106288*

**Figure 10.**

*Examples of systems used to grow AlN films: (a) sputtering [17], (b) MBE [19], (c) MOCVD [20] and (d) PLD [21].*

and the chemical composition results showed that the ALD AlN films exhibit good characteristics films for surface acoustic wave (SAW) devices. The same authors also reported the morphological evolution of ALD AlN films on 4H-SiC substrates [24].

The growth of AlN films has been exploited on different substrates to form both a buffer layer and a sensing layer (**Figure 11**). The AlN buffer layer is used to improve the growth and properties of other thin films, such as GaN, SiC, ZnO, and diamond among others, for several applications, whereas AlN sensing layers are used mainly in piezoelectric devices, for example SAW (surface acoustic wave) sensors.

#### **4.2 Growth of AlN buffer layer**

Several studies have been devoted to the growth of AlN buffer layers to be used in different applications as those illustrated in **Figure 12**.

Zhang et al. discussed the growth of sputtered highly oriented AlN films on Si (100) and Si (111) substrates to use them as a proper buffer layer for epitaxial growth of gallium nitride (GaN) films. It was observed that the AlN (0001) films grown on Si (100) exhibit large strain due to the large lattice mismatch between these materials, whereas the AlN films grown on Si (111) have strain dependent on the discharge

#### **Figure 11.**

*Illustration of AlN structure, thin film deposition and thin film application as sensing layer and buffer layer.*

#### **Figure 12.**

*Examples of applications of AlN buffer layers.*

power in sputtering. Therefore, they concluded that the orientation of the Si substrates and the discharge power impact greatly the strain of sputtered AlN films [25].

The growth of sputtered AlN buffer layer on Si (111) was also reported by Núñez-Cascajero et al. In their paper, they addressed the use of AlN as a buffer layer for the development of AlInN/p-Si heterojunction solar cells. For this, it was investigated the influence of power applied to the Al target on the properties of AlN on Si (111). They found that the presence of the AlN buffer layer leads to an improvement of the structural quality of the Al0.37In0.63N and that the solar cells based on this material show good rectifying behavior in the dark [26].

The growth of amorphous SiC thin films on AlN buffer layers deposited on glass and Si substrates was reported by Wang et al. It evaluated the effect of AlN buffer layer thickness on the morphological and mechanical properties of the SiC. Overall, their results indicated that the AlN buffer layer can effectively improve the adhesion strength of SiC thin films [27]. In another study focused on a comparison among substrates for the development of SiC thin film piezoresistive sensors, Fraga et al. evaluated the piezoresistive properties of SiC films grown on AlN/Si [28]. It evidenced the importance of growing SiC film on AlN in order to develop piezoresistive sensors for high-temperature applications.

In addition to silicon, other substrates are being used in the deposition of AlN buffer layer by sputtering. In a recent paper published in the Journal Materials Science in Semiconductor Processing, the effects of the use of sputtered AlN buffer layer on

#### *Development and Applications of Aluminum Nitride Thin Film Technology DOI: http://dx.doi.org/10.5772/intechopen.106288*

the carrier transport properties of p-NiO/n-InN heterojunction diode were investigated. In this study, AlN films were grown Al2O3 substrates with varying N2 flow rates in the sputtering process. In order to analyze the reasons for the deterioration of the device characteristics, the influence of AlN buffer layer on I–V characteristics of the heterojunction diode was studied in the temperature range of 30–110°C. A good performance was observed for the heterojunction diode fabricated [29]. Chen et al. also investigated sputtered AlN films on sapphire substrates. However, their focus was to release the film stress using a post-deposition rapid thermal annealing (RTA) at 700–900°C for 5 min. The Raman spectra showed that the in-plane tensile stress of deposited AlN films is released by the RTA [30].

Regarding the use of AlN films as a buffer layer for diamond growth, Mandal et al. carried out the growth of thick (>100 μm) CVD diamond layers on AlN with low thermal boundary resistance between diamond and AlN. In their study, they used a metalorganic chemical vapor deposition (MOCVD) system to grow a 250-nm-thick AlN layer on 150 mm Si substrates. It was highlighted that diamond/AlN could be used for thermal management of GaN high-power devices [31]. Most recently, Forsberg et al., a high sensitivity infrared spectroscopy with a diamond waveguide on aluminum nitride [32].

Zinc oxide (ZnO) films have been also grown on AlN buffer layer for electronic device applications. A recent paper evaluated the temperature-dependent electrical transport properties of n-ZnO/AlN/p-Si heterojunction diodes [33]. Both AlN and ZnO films were deposited by RF magnetron sputtering. Results showed that the use of AlN buffer layer improved electrical and structural characteristics because the AlN between ZnO and Si lowers the mismatch in thermal expansion coefficient/lattice. In a previous study, Xiong et al. exploited the growth of ZnO films on a 150 nm AlN buffer layer on -sapphire substrates. It was noted that c-plane ZnO growth on c-plane sapphire by PLD at slight rough surface morphology of AlN buffer layer can result in a significant variation of ZnO crystallinity [34].

#### **4.3 Growth of AlN active sensing layer**

The excellent piezoelectric properties of AlN films have motivated studies on the development of sensors based on these materials. Tonisch et al. reported the piezoelectric properties of polycrystalline AlN thin films on Si (111) substrates for MEMS applications [35]. Reactive dc-sputtering and metalorganic chemical vapor deposition (MOCVD) were used to deposit the AlN films. The piezoelectric coefficient d33eff of AlN thin films was measured using two techniques: piezoresponse force microscopy and an interferometric technique. The value of the effective piezoelectric coefficient d33 for the prepared AlN thin films remained as high as 5.1 pm/V even for lower degrees of texture.

In the same year, the temperature dependence of the piezoelectric coefficient d33 of sputtered AlN film on a polycrystalline silicon/silicon dioxide/silicon wafer measured at temperatures up to 300°C was reported by Kano et al. It was observed that the piezoelectric coefficient d33 has a constant value at temperatures ranging between 20°C and 300°C [36].

A more recent paper shows that the high (002) orientation AlN films have uniform piezoelectric performance [37]. In this study, AlN thin films were grown on Pt/Ti/ SiO2/Si (100) substrates by an optimized magnetron sputtering process. A highfrequency SAW device (fo = 4.47 GHz) was constructed based on optimal AlN films. In another study on optimization of AlN growth, Cao et al. optimized DC magnetron sputtering, the process by controlling the distance target-substrate in the deposition of AlN thin films deposited on Si and Pt substrates [38]. A (002) AlN preferred


#### **Table 1.**

*Piezoelectric device types based on sputtered AlN films. Adapted from [42].*

orientation was obtained with DTS = 4 cm. They concluded that under optimum conditions, the as-deposited AlN films show uniform piezoelectric properties and favorable read and write performance [38].

The piezoelectric properties of sputtered Sc-doped AIN polycrystalline films on 200 mm Si wafers were also evaluated in order to use them as active layers for high frequency (GHz range) acoustic resonators [39]. The piezoelectric activity of the as-deposited AlScN films were improved after a 15 min post-deposition annealing at 600°C, leading to a 20% increase in the electromechanical coupling factor [39].

The influence of He implantation on piezoelectric properties of epitaxial AlN thin films were discussed recently [40]. It was noted that while He implantation induces uniaxial strain, it decreases d33 due to implantation-induced N site disorder [40].

Pressure gradient sputtering (PGS) has also been used to grow AlN films. The piezoelectric constant (d33) of the AlN grown by the PGS method was higher than that of the conventional method indicating which the PGS technique has an advantage in the growth of AlN films with highly c-axis oriented and a single dielectric domain [41].

In September 2021, an open research knowledge graph (ORKG) comparison devoted to aluminum nitride films and their applications in piezoelectric devices was published [42]. **Table 1** summarizes the characteristics of some piezoelectric devices based on AlN films.

### **5. SAW sensors based on AlN films**

In recent years, much progress has been carried out on surface acoustic wave (SAW) sensors and applications have been reported in the fields of microfluidics, chemical, biomedical, and mechanical as illustrated in **Figure 13**.

*Development and Applications of Aluminum Nitride Thin Film Technology DOI: http://dx.doi.org/10.5772/intechopen.106288*

#### **Figure 13.**

*Some relevant applications of SAW sensors.*

In the literature, various AlN-based sensors are reported. In 1991, Odintzov MA et al. evaluated sputtered AlN films for SAW sensors. AlN films were grown on glass substrates by reactive RF magnetron sputtering and their piezoelectric properties were analyzed. The obtained film was used to implement a SAW temperature sensor. Moreover, it discussed the importance of obtaining polycrystalline AlN-oriented films with perfect crystallization [43].

A SAW-based sensor pressure with a sensibility of 0.33 MHz/bar based on AlN deposited on free-standing diamond substrates was developed by Rodríguez-Madrid. The influence of the piezoelectric film thickness on the SAW response was evaluated. Optimized AlN thin films of 300 nm were used to fabricate one-port SAW resonators operating in the 10–14 GHz frequency range, which were used as SAW pressure sensors [44].

A 3 μm-thick AlN/Sapphire-based SAW resonators with high-quality factors for high-temperature applications (up to 600°C) were fabricated by Streque et al. The quasi-synchronous resonators proposed remained well-tuned for temperatures up to 400°C, and show high-quality factors, as high as 3400 at 400°C [45].

A new two-step growth process integrating metalorganic chemical vapor deposition (MOCVD) and physical vapor deposition (PVD) technologies were proposed to grow AlN films on Si substrates by Xinyan et al. [46]. High-quality AlN-based FBARs wafers were obtained, showing that the Q-factor of FBARs with two-step grown AlN precedes FBARs with one-step grown PVD AlN by 57.6% [46].

### **6. Conclusion**

As shown in this book chapter, the excellent physical, chemical, dielectric, thermal and mechanical properties of aluminum nitride (AlN) thin films have stimulated great interest in these materials and their wide applications. Until today, sputtering processes are the most used to grow AlN films. In recent years, the ALD technique, an outstanding process in nanotechnology, has been explored to grow AlN films. This

book chapter also presented various applications for AlN films as a buffer layer and as a sensing layer. Among the applications, the use of AlN sensing layers is the most discussed in the literature due to their excellent piezoelectric properties and the good performance of SAW sensors based on these materials. In summary, this book chapter presents the key aspects of aluminum nitride thin film technology and its applications in electronic devices and sensors.
