β-(AlxGa1−x)2O3/Ga2O3 (010) heterostructures grown on β-Ga2O3 (010) substrates by plasma-assisted molecular beam epitaxy

By systematically changing growth parameters, the growth of β-(AlxGa1−x)2O3/Ga2O3 (010) heterostructures by plasma-assisted molecular beam epitaxy was optimized. Through variation of the Al flux under O-rich conditions at 600 °C, β-(AlxGa1−x)2O3 (010) layers spanning ∼10% to ∼18% Al2O3 were grown directly on β-Ga2O3 (010) substrates. Nominal β-(AlxGa1−x)2O3 (010) compositions were determined through Al:Ga flux ratios. With x = ∼0.18, the β-(AlxGa1−x)2O3 (020) layer peak in a high-resolution x-ray diffraction (HRXRD) ω-2θ scan was barely discernible, and Pendellösung fringes were not visible. This indicated that the phase stability limit of Al2O3 in β-Ga2O3 (010) at 600 °C was less than ∼18%. The substrate temperature was then varied for a series of β-(Al∼0.15Ga∼0.85)2O3 (010) layers, and the smoothest layer was grown at 650 °C. The phase stability limit of Al2O3 in β-Ga2O3 (010) appeared to increase with growth temperature, as the β-(AlxGa1−x)2O3 (020) layer peak with x = ∼0.18 was easily distinguishable by HRXRD in a sample grown at 650 °C. Cross-sectional transmission electron microscopy (TEM) indicated that β-(Al∼0.15Ga∼0.85)2O3 (010) layers (14.4% Al2O3 by energy dispersive x-ray spectroscopy) grown at 650 °C were homogeneous. β-(Al∼0.20Ga∼0.80)2O3 (010) layers, however, displayed a phase transition. TEM images of a β-(Al∼0.15Ga∼0.85)2O3/Ga2O3 (010) superlattice grown at 650 °C showed abrupt layer interfaces and high alloy homogeneity.