The energy band alignment of the atomic-layer-deposited In2O3/β-Ga2O3 (2¯01) interface is evaluated by X-ray photoelectron spectroscopy. The X-ray diffraction pattern reveals that the In2O3 film grown at 160 °C is amorphous, while it becomes polycrystalline at a higher deposition temperature of 200 °C. The bandgaps, determined by reflection electron energy loss spectroscopy, are 4.65, 3.85, and 3.47 eV for β-Ga2O3, polycrystalline In2O3, and amorphous In2O3, respectively. Both amorphous and polycrystalline In2O3/β-Ga2O3 interfaces have Type I alignment. The conduction and valence band offsets at the polycrystalline (amorphous) In2O3/β-Ga2O3 interface are 0.35 and 0.45 eV (0.39 and 0.79 eV), respectively. These observations suggest that polycrystalline In2O3 as an intermediate semiconductor layer is beneficial to the barrier reduction of metal/Ga2O3 contact.

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The energy band offsets between indium–gallium–zinc oxide (IGZO) and β-Ga2O3 were examined by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The Ga 2p spectra from the heterojunction contributed by the upper IGZO film and the β-Ga2O3 substrate were deconvoluted into two sub-peaks with the binding energy difference of 0.3 eV, in good agreement with the theoretical model. Meanwhile, the bandgaps of IGZO and β-Ga2O3 were measured to be 3.44 ± 0.1 and 4.64 ± 0.1 eV from the ultraviolet–visible (UV–vis) transmittance spectra. The valence and conduction band offsets between the IGZO and β-Ga2O3 were consequently determined to be 0.49 ± 0.05 and 0.71 ± 0.1 eV, respectively. These findings reveal that IGZO is an attractive intermediate semiconductor layer (ISL) for reducing the electron barrier height at metal/Ga2O3 interfaces.