**1. Introduction**

Already for decades, piezoelectric thin film AlN has been of interest for its excellent dielectric properties as well as its chemical and temperature stability and has been widely used in piezoelectric MEMS (microelectromechanical systems) sensors and actuator elements [1–4]. In 2009, Akiyama et al. first reported that the piezoelectric coefficient of AlN could be significantly increased by doping with Sc [5,6]. Since then, AlScN has attracted great attention and has become a promising piezoelectric material for MEMS applications [7–11]. Multiple studies on AlScN-based MEMS magnetroelectric sensors [12], MEMS energy harvesters [13], MEMS quasistatic mirrors [14], and acoustic wave resonators [15–21] have been reported.

In most piezoelectric MEMS devices, AlN or AlScN thin films are grown on metallic seed layers such as molybdenum (Mo) or platinum (Pt) to ensure good *c*-axis orientation, where full width at half maximum (FWHM) values of the rocking curve measurements are typically larger than 1.3◦ for AlN and larger than 1.6◦ for Al0.73Sc0.27N [22–26]. However, metallic seed layers cannot be used for, e.g., piezoelectric thin-film-based surface acoustic wave (SAW) devices [21], optical waveguides [27], or MEMS actuators with doped silicon used as bottom electrode [28]. For these applications, the piezoelectric layer (AlN or AlScN) has to be grown directly on substrates such as (100) silicon (Si), silicon dioxide (SiO2) or epitaxial polysilicon (poly-Si), but still a high degree of *c*-axis orientation of AlN or AlScN is required. For AlN the deposition on various Si-based substrates has been studied by

**Citation:** Su, J.; Fichtner, S.; Ghori, M.Z.; Wolff, N.; Islam, M.R.; Lotnyk, A.; Kaden, D.; Niekiel, F.; Kienle, L.; Wagner, B.; et al. Growth of Highly c-Axis Oriented AlScN Films on Commercial Substrates. *Micromachines* **2022**, *13*, 783. https:// doi.org/10.3390/mi13050783

Academic Editor: Nam-Trung Nguyen

Received: 24 April 2022 Accepted: 13 May 2022 Published: 17 May 2022

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Jiao et al. in [29] and SiO<sup>2</sup> was found to be most suitable substrate for AlN *c*-axis growth. Due to its higher piezoelectric response compared to AlN, AlScN films have attracted more interest, thus becoming a focus in piezoelectric MEMS research. Although there is success in growing AlScN films with low Sc concentrations on different nonmetallic substrates (on high-resistivity (100) Si [21], low-resistivity boron-doped (001) Si [17,30], SiC [31]), higher Sc concentrations present an increasingly difficult challenge for the growth of AlScN films with exclusive *c*-axis orientation [10].

In this work, we present a largely substrate-independent method to grow wurtzite-type AlScN films with exclusive *c*-axis orientation even for high Sc concentrations. Throughout this paper, AlScN with 27% Sc is chosen as a balance between high piezoelectric coefficient and robust deposition process. Hereafter, AlScN is used synonymously with Al0.73Sc0.27N. AlN and AlScN films are deposited directly on (100) Si, SiO<sup>2</sup> and poly-Si substrates, using process parameters established for the growth on metallic nucleation layers. The microstructure and *c*-axis texture quality of AlN and AlScN films are investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution X-ray diffraction (XRD). The evaluation of the surface morphology and rocking curve XRD scans reveals that AlN films show a high degree of *c*-axis orientation on all investigated substrates. This confirms that sputter deposited AlN is able to realize a good texture with low distortions on many substrates, largely independent of the underlying texture [32–36]. In contrast, AlScN films grown directly on various nonmetallic substrates exhibit a high density of misaligned grains. The superior crystalline quality of AlN films motivates the approach reported herein to grow high quality AlScN on nonmetallic substrates using AlN as the nucleation layer.
