Defect States in the Electronic Structure of Ga₂O₃ Transparent Conductive Oxide Surfaces

Defect states are crucial in many electronic devices. Oxygen vacancies, for example, are common in transparent conductive oxides and can both be beneficial (e.g., as a dopant to enhance conductivity) as well as detrimental (e.g., leading to reduced device performance by recombination at interfaces). Using soft and hard X-ray photoelectron spectroscopy (PES and HAXPES) with excitation photon energies ranging from 0.1 to 6.3 keV, we study the electronic surface structure of differently processed Ga₂O₃ samples in a depth-resolved fashion. Specifically, we investigate a Ga₂O₃ thin film, as used in Cu-(In,Ga)-Se₂-based thin-film solar cells, as well as a β-Ga₂O₃ single crystal before and after a defect-inducing Ar⁺-ion treatment. Spectra calculations based on density functional theory (DFT) are used to explain the PES and HAXPES valence band signatures as a function of the excitation photon energy. The β-Ga₂O₃ single crystal spectra can be well described by the DFT calculations. In contrast, the Ga₂O₃ thin film and Ar⁺-ion treated β-Ga₂O₃ show significant spectral broadening of the valence band features and additional spectral intensity close to the valence band maximum, which we assign to defect states. This additional intensity varies as a function of probing depth, suggesting that these defects are mostly localized at the surface. We also discuss the importance of DFT-based spectra calculations to verify the proper determination of valence band maxima using a linear extrapolation.