Electronic structure and surface band bending of Sn-doped β−Ga2O3 thin films studied by x-ray photoemission spectroscopy and ab initio calculations

The bulk and surface electronic structures of Sn-doped $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films have been studied by soft and hard x-ray photoemission spectroscopy (soft PES at 1486.6 eV and HAXPES at 5920 eV). The experimental spectra are compared with density functional theory calculated density of states in the valence band and conduction band. Excellent agreement was found between experimental spectra and calculated density of states by taking into account the photoionization cross section of different orbitals involved in the valence and conduction bands. The electronic states derived from Ga $4s$ character are selectively enhanced by HAXPES. This allows us to infer that the states at the conduction band and bottom of the valence band contain pronounced Ga $4s$ character. The occupation of the lower conduction band in degenerately Sn-doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is clearly observed by HAXPES, which allows for direct measurement of Burstein-Moss shift and band-gap renormalization as a function of Sn doping. A comparison of the valence band spectra of Sn-doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ films with Si-doped samples suggests that Sn doping has different effects on the electronic structure than Si doping. An in-gap electronic state is observed for Sn-doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, which is attributed to self-compensating $\mathrm{S}{\mathrm{n}}^{2+}$ related defects. Furthermore, a larger band-gap renormalization is found in Sn-doped samples, because the Sn $5s$ dopant orbital mixes strongly with the host Ga $4s$ derived conduction band. Finally, a comparison of the valence band and core-level spectra excited with soft and hard x rays allows us to identify an upward band bending at the surface region of Sn-doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ films.