Influence of AlN buffer layer on molecular beam epitaxy growth of wurtzite Al1−xScxN thin films

Wurtzite–Al1−xScxN thin films deposited by solid-state alloying of AlN with ScN exhibit high piezoelectric coefficient and large band gap that makes it a promising material for a variety of applications in piezo-electronics, electronic, acoustoelectric devices, etc. Research on epitaxial Al1−xScxN growth in wurtzite crystal structure is still at an early stage and achieving high scandium (Sc) concentrations in epitaxial films without any phase separation or secondary phase formation is still a critical challenge. Moreover, as most of the reports of wurtzite–Al1−xScxN growth thus far relies on low-vacuum growth techniques, such as magnetron sputtering that are prone to large impurities and contaminants detrimental for device applications, high-vacuum deposition techniques, such as molecular beam epitaxy method needs to be developed. In this paper, we report the epitaxial growth of wurtzite–Al1−xScxN on sapphire (Al2O3) substrates under different Sc fluxes using ultra-high vacuum plasma-assisted molecular beam epitaxy. To prevent ScN phase separation, a 30 nm AlN buffer layer is deposited in situ on GaN epilayers as well as Al2O3 substrates that result in phase-pure wurtzite–Al1−xScxN thin films without any phase separation or secondary phase formation. The structural and compositional analyses performed with high-resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS), reveal epitaxial wurtzite–Al1−xScxN growth with 0001 orientations on (0001) Al2O3 substrates and the presence of cubic ScN. Demonstration of phase-pure Al1−xScxN on AlN buffer layers will enable the development of devices with improved efficiencies.