Heteroepitaxy and anisotropy of nonpolar m-plane α-(AlxGa1-x)2O3 films

Abstract

In this work, high quality single crystalline α-Ga₂O₃ epilayers were successfully grown on m-plane ($10\overline{1}0$) sapphire substrates by optimizing the heater temperature (700-850 °C) and chamber pressure (0.2-1.0 mTorr). Based on these optimized conditions, α-(Al_xGa_1-x)₂O₃ epilayers were subsequently epitaxially grown on the m-plane sapphire. The epilayers' smooth surface morphology and high crystalline quality were confirmed via atomic force microscopy and X-ray diffraction methods. Analyzes of atomic force microscopy also indicated anisotropic growth, with the stripe-shaped surface morphology along the [0001] crystallographic orientations. Further verification through XRD with varying incident angles and polarization-dependent transmittance spectra established the anisotropy characteristics of α-(Al_xGa_1-x)₂O₃ epilayers. Due to differences in surface energy and epitaxial growth rates along various crystallographic orientations, the surface of the m-plane α-(Al_xGa_1-x)₂O₃ epilayers develops basal plane stacking faults along the [0001] crystallographic orientations, resulting in a stripe-shaped surface morphology. Furthermore, orientation-dependent defect formation and strain accumulation cause variations in the electronic band structure, contributing to the observed optical anisotropy in the α-(Al_xGa_1-x)₂O₃ epilayers.