Surface reaction dependence of molecular beam epitaxy grown aluminum on various orientations of β-Ga2O3

An orientational dependence on the interfacial reaction between aluminum and (010), (001), and (2̄01) β-Ga2O3 substrates is addressed. Electron microscopy and x-ray diffraction were used to assess the interface crystallinity, thickness, and chemical composition of the interfacial layers. At the interface, amorphous aluminum oxide is observed in all three samples with a thicknesses of 3.5 nm for (010) β-Ga2O3 and 2 nm for (001) β-Ga2O3 and (2̄01) β-Ga2O3. Aluminum oxide is formed at the interface as a result of a chemical reaction that reduces the Ga2O3 surface when aluminum is deposited. We propose that in Al on (010) β-Ga2O3, in which the thickest interfacial oxide layer is observed, diffusional pathways of consecutive octahedral Ga sites perpendicular to the interface promote increased interdiffusion in the out-of-plane direction. In contrast, the (001) β-Ga2O3 and (2̄01) β-Ga2O3 substrates exhibit alternating rows of tetrahedral and octahedral Ga sites parallel to the interface, where the rows of tetrahedral Ga sites act as increased energy barriers that impede interdiffusion of Al and β-Ga2O3. The orientational dependence of metal-oxide interlayers in β-Ga2O3 can impact electronic and thermal transport, pointing to the importance of understanding the impact of β-Ga2O3 orientation on interfacial properties.