Defect States in the Electronic Structure of Ga$_2$O$_3$ 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 Ga$_2$O$_3$ samples in a depth-resolved fashion. Specifically, we investigate a Ga$_2$O$_3$ thin film, as used in Cu(In,Ga)Se$_2$-based thin-film solar cells, as well as a β-Ga$_2$O$_3$ 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-Ga$_2$O$_3$ single crystal spectra can be well described by the DFT calculations. In contrast, the Ga$_2$O$_3$ thin film and Ar$^+$-ion treated β-Ga$_2$O$_3$ 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. ... mehrThis 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.