Elastic moduli of scandium nitride (ScN) films are determined using a laser-based experimental method working with surface acoustic waves (SAWs). ScN, a semiconductor material with promising potential for various applications, crystallizes in the cubic rock salt (rs) structure. We investigate two samples of high-crystallinity ScN(111) films with thicknesses similar to 200 and similar to 300 nm, grown on Si(111) substrates by pulsed DC-magnetron co-sputtering and a sample with a fiber-textured ScN film ( similar to 800 nm) on Si(001). From the shape evolution of laser-generated acoustic pulses, SAW dispersion curves were obtained in a frequency range of 50-500 MHz. In order to take advantage of the anisotropy of the film and substrate materials, measurements were performed for 10-15 SAW wavevector directions, which could be defined with a precision of 0.2 degrees. Using perturbation theory with respect to the ratio of film thickness and SAW wavelength, two combinations of the three independent elastic constants of the high-crystallinity rs ScN films could be extracted from the measurement data. The surface roughness of the ScN films is accounted for with a simple model. Complete sets of the three elastic moduli were inferred in two different ways: (i) SAW dispersion data for the third sample were included in the extraction procedure; and (ii) the bulk modulus is set equal to a theoretical literature value. The extracted values for the three elastic constants are at variance with published theoretical results for single-crystal ScN. Possible reasons for these discrepancies are discussed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Wurtzite-type aluminum nitride (AlN) thin films exhibiting high thermal conductivity, large grain size, and low surface roughness are desired for both bulk acoustic wave and surface acoustic wave resonators. In this work, we use ammonia (NH3) assisted reactive sputter deposition of AlN to significantly improve these properties. The study shows a systematic change in the structural, thermal, and morphological properties of AlN grown in nitrogen (N2) and N2 + NH3 atmosphere. The study demonstrates that NH3 assisted AlN sputtering facilitates 2D growth. In addition, the study presents a growth model relating the 2D growth to improve the mobility of aluminum (Al) and nitrogen (N) ad-atoms in NH3 atmosphere. Consequently, the thermal conductivity and roughness improve by ≈76%, and ≈35%, while the grain size increases by ≈78%.

In this work, the value and the polarity of the spontaneous and piezoelectric polarization have been investigated, as the use of two different reference structures for wurtzite-type group-III nitrides, namely, the zinc-blende and the layered-hexagonal crystal lattice, have resulted in different predictions. It was found that although the differences in value and polarity of the polarization for heterostructures such as wurtzite Al1−xScxN/GaN lead to similar interface sheet charges, a significant mismatch is observed when polarization reversal is considered. The interface sheet charge predicted before and after the polarization reversal in the wurtzite Al1−xScxN layer on GaN using the zinc-blende lattice as a reference predominantly shows a change in sign. When using the layered-hexagonal lattice as a reference, not only is the same polarity of the interface sheet charge maintained after polarization reversal, but it is even 30 times larger. In this case, the giant and positive spontaneous polarization values for metal-polar Al1−xScxN extracted from the ferroelectric switching, as well as the alignment of the piezoelectric polarization to it, were observed to be consistent with the predictions referenced to the layered-hexagonal lattice. Thus, it is concluded that the layered-hexagonal reference is not only more suitable for predicting the ferroelectric properties of wurtzite Al1−xScxN but should also be the correct reference when considering polarization reversal in heterostructures. If the significant increase in the interface sheet charge after polarization reversal is experimentally detected, it will allow the design and fabrication of novel devices for future high-frequency and power electronics applications.

Since the discovery of the ferroelectric properties of AlScN, the range of possibilities for the development of novel electronic devices and technologies has expanded. However, the realization of all these technologies are based on the implementation of c-plane oriented AlScN films, as AlScN exhibits most desired physical properties along this axis. Thus, the deposition and characterization of c-plane oriented nitrides have been the main focus of recent research activities. However, non-polar a-plane oriented AlScN showed promising results regarding the field surface acoustic wave resonators for frequency filters. In this case, the c-axis is aligned parallel to the surface, which, however, increases the complexity of assessing and characterizing the in-plane properties. This work reports the experimental attempt of characterizing sputtered a-plane Al0.7Sc0.3N film on an r-plane Al2O3 substrate using ferroelectricity. In this context, in-plane ferroelectricity was demonstrated using an a-plane Al0.7Sc0.3N film, in which the current response of a surface acoustic wave resonator (SAW) to an applied voltage signal was measured. The ferroelectric properties were validated by estimating the ferroelectrically active cross-section based on the simulated electric field distribution using the finite-element method (FEM). The ferroelectrically active cross-section was also confirmed by measuring and simulating the frequency response of the admittance of the SAW resonators before and after ferroelectric switching. The derivation of this cross-section using both approaches revealed a remnant polarization value between 0.9 and 1.35 C/m2, which agrees with reported data for c-plane Al0.7Sc0.3N so far, confirming the high degree of c-axis orientation of the film along the surface.

High quality, uni-polar, epitaxial AlN with minimum oxygen content promises excellent surface acoustic wave and bulk acoustic wave resonator characteristics such as high electromechanical coupling coefficient and power handling capabilities, which is particularly useful for RF filter applications. By systematically varying the growth temperature, the study investigates its impact on the oxygen levels, defect states, and crystallographic texture of the AlN thin films using a combination of atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, spectroscopic ellipsometry, scanning transmission electron microscopy, as well as room temperature and temperature dependent I–V measurements. The research demonstrates that the films grown at a temperature of 700°C exhibit the most favorable results. These films exhibit the lowest oxygen levels, possess epitaxial growth, and display the highest crystalline quality (XRD AlN 0002 ω−FWHM=1.3°). Additionally, these films demonstrate a significant reduction in sub-bandgap absorption. By comparing the cathode current measured during deposition, we suggest that the presence of an impurity layer formed during idle time between depositions as a possible source of oxygen in the sputter chamber. In addition, the study presents a possible model to explain the mixed polarity observed in AlN and proposes various ways to achieve uni-polar AlN on silicon substrates.

The direct impact of structural quality on the ferroelectric properties of hexagonal Al1–xScxN with an Sc-content of x = 0.3 was investigated using dynamic hysteresis measurements, high-resolution x-ray diffraction (HRXRD), and atomic force microscopy. The films investigated were deposited on p-doped (001)-Si substrates by reactive pulsed DC magnetron sputtering under different gas mixtures to vary the structural quality and surface morphology between samples. Misoriented grains were identified as ferroelectrically inactive, as these grains resulted in an underestimation and distortion of the ferroelectric quantities. In fact, a high amount of misoriented volume was found to have a significant effect on the coercive electric field, as this is mainly determined by the crystal strain in the ferroelectric [0001]-oriented regions, independent of its origin. Furthermore, it was concluded that the crystal quality does not have a pronounced effect on the coercive field strength. Conversely, the polarization in the film is mainly determined by the crystal quality, as a difference of 1° in the HRXRD FWHM of the ω-scan resulted in a 60% loss of polarization. The amount of polarization was influenced to a lesser extent by the misoriented grains since the ferroelectric volume of the layers was only slightly overestimated. This reveals that optimizing reproducible and transferable properties, such as crystal quality and surface morphology, is more reasonable, as the film with the lowest misoriented volume and the highest degree of c-axis orientation showed the highest polarization.

A‐plane Al0.7Sc0.3N) thin films are grown on r‐plane Al2O3() substrates using reactive pulsed‐DC magnetron sputter epitaxy. This is the first report of successful synthesis of nonpolar epitaxial AlScN films with a high scandium concentration (30%). The influence of different sputtering conditions, such as magnetron power, temperature, and process gas flow rates, is investigated. The film characteristics are also compared on different substrate offcuts. Controlling the diffusion of adatoms on surface of the substrate is found to have the highest influence on film quality. The X‐ray diffraction measurements confirm in‐plane oriented AlScN) layers and the final optimized films show significant improvement in rocking curve full width at half maximum (ω‐FWHM) of reflection. Corresponding atomic force microscopy (AFM) measurements show mean root square surface roughness (R q < 0.4 nm) nearing atomically smooth levels. The optimized films also exhibit anisotropic growth characteristics. A growth model for a‐plane AlScN has been proposed based on the growth parameters of the film.

The laser ultrasound (LU) technique has been used to determine dispersion curves for surface acoustic waves (SAW) propagating in AlScN/Al2O3 systems. Polar and non-polar Al0.77Sc0.23N thin films were prepared by magnetron sputter epitaxy on Al2O3 substrates and coated with a metal layer. SAW dispersion curves have been measured for various propagation directions on the surface. This is easily achieved in LU measurements since no additional surface structures need to be fabricated, which would be required if elastic properties are determined with the help of SAW resonators. Variation of the propagation direction allows for efficient use of the system's anisotropy when extracting information on elastic properties. This helps to overcome the complexity caused by a large number of elastic constants in the film material. An analysis of the sensitivity of the SAW phase velocities (with respect to the elastic moduli and their dependence on SAW propagation direction) reveals that the non-polar AlScN films are particularly well suited for the extraction of elastic film properties. Good agreement is found between experiment and theoretical predictions, validating LU as a non-destructive and fast technique for the determination of elastic constants of piezoelectric thin films.

This work reports on the growth of 1 µm nonpolar a-plane Al0.7Sc0.3N(112¯0) thin films on an r-plane sapphire Al2O3(11¯02) via magnetron sputter epitaxy. The electro-acoustic properties of the film structures were characterized using surface acoustic wave (SAW) resonators. Measured electrical responses were found to be strongly anisotropic in terms of the wave propagation direction. We identified a sagittal polarized Rayleigh wave mode with large coupling (keff2= 3.7%), increased phase velocity (v= 4825 m/s), as well as high quality factor (Q > 1000) for SAW propagation along the c-axis [0001] and normalized thicknesses h/λ=0.2. Finite element method simulations using electro-acoustic properties of Al0.7Sc0.3N obtained from the density functional theory reproduce our experimental results.

This work reports on the growth of 1 mu m nonpolar a-plane Al0.7Sc0.3N(1120) thin films on an r-plane sapphire Al2O3(1102) via magnetron sputter epitaxy. The electro-acoustic properties of the film structures were characterized using surface acoustic wave (SAW) resonators. Measured electrical responses were found to be strongly anisotropic in terms of the wave propagation direction. We identified a sagittal polarized Rayleigh wave mode with large coupling (k(eff)(2) = 3.7%), increased phase velocity (v = 4825 m/s), as well as high quality factor (Q> 1000) for SAW propagation along the c-axis [0001] and normalized thicknesses h/lambda= 0.2. Finite element method simulations using electro-acoustic properties of Al0.7Sc0.3N obtained from the density functional theory reproduce our experimental results. (C) 2022 Author(s).

In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial Al0.68Sc0.32N(0001) films on Al2O3(0001) substrates were studied using numerical and experimental methods. In order to find optimal propagation directions for higher-order SAW modes, phase velocity dispersion branches of Al0.68Sc0.32N on Al2O3 with Pt mass loading were computed for the propagation directions <112¯0> and <11¯00> with respect to the substrate. Experimental investigations of phase velocities and electromechanical coupling were performed for comparison with the numerical results. Simulations carried out with the finite element method and a Green function approach allowed identification of each wave type, including Rayleigh, Sezawa, and shear-horizontal wave modes. For the propagation direction <11¯00>, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling (keff2=1.6%) and phase velocity (vphase=6km/s). We demonstrated that selecting wave propagation in <11¯00> with high mass density electrodes results in increased electromechanical coupling without significant reduction in phase velocities for the Sezawa wave mode. An improved combination of metallization, Sc concentration x, and SAW propagation direction is suggested that exhibits both high electromechanical coupling (keff2>6%) and high velocity (vphase=5.5km/s) for the Sezawa mode.

In this article, we report on the synthesis and characterisation of fluorinated epitaxial films of BaFeO2F via low-temperature fluorination of thin films of BaFeO2.5+d grown by pulsed laser deposition. Diffraction measurements show that fluoride incorporation only results in a contraction of the film perpendicular to the film surface, where clamping by the substrate is prohibitive for strong in-plane changes. The fluorinated films were found to be homogenous regarding the fluorine content over the whole film thickness, and can be considered as single crystal equivalents to the bulk phase BaFeO2F. Surprisingly, fluorination resulted in the change of the tetragonal distortion to a nearly cubic symmetry, which results in a lowering of anisotropic orientation of the magnetic moments of the antiferromagnetically ordered compound, confirmed by Mössbauer spectroscopy and magnetic studies.