β-Ga₂O₃ is a wide bandgap semiconductor (~4.8 eV) with properties suited to power electronics, truly solar-blind UV detection, and extreme environment applications. Additional bandgap tunability can be achieved through incorporation of Al into β-Ga₂O₃, leading to β-(Al,Ga)₂O₃ monoclinic phase alloys with the bandgap tuned from 4.8- 6 eV. This enables heterostructure designs such as modulation-doped electron channels, quantum wells, and superlattices.

Out of these possible structures, β-(Al_xGa_1-x)₂O₃/Ga₂O₃ heterostructure field-effect transistors (HFETs) have shown impressive and improving performance compared to Ga₂O₃ based metal semiconductor field effect transistor (MESFETs). For these heterostructures, other wide bandgap oxides can be used as gates dielectrics, and narrow energy bandgap conductive oxides can be utilized to reduce contact resistance between the ohmic contacts and semiconductor surface. For gate dielectrics on (Al_xGa_1-x)₂O₃, one must utilize oxide materials with a bandgap of 7 eV of higher to have adequate offsets in both the conduction and valence bands. In this study, we utilize x-ray photoelectron spectroscopy (XPS) to report on the determination of the band alignment of various insulating and semiconducting dielectrics onto (Al_xGa_1-x)₂O₃ at various aluminum concentrations. The dielectrics were deposited using atomic layer deposition (ALD) or sputtering. For SiO₂ and Al₂O₃, there are differences of up to 1 eV in band alignments on single crystal (Al_xGa_1-x)₂O₃, depending on whether they are deposited by sputtering or ALD. For the conductive oxides, AZO/ β-(Al_xGa_1-x)₂O₃ and ITO/ β-(Al_xGa_1-x)₂O₃ heterojunctions have a nested gap (type I) band offset. The valence and conduction band offsets are negative in both cases and can be used to enhance carrier transport across the heterointerface.