Band bending and surface defects in β-Ga2O3

Surface band bending and surface defects on the UV-transparent conducting oxide β-Ga₂O₃ (100) are studied with hard x-ray photoemission spectroscopy and scanning tunneling microscopy. Highly doped β-Ga₂O₃ shows flat bands near the surface, while the bands on nominally undoped (but still n-type), air-cleaved β-Ga₂O₃ are bent upwards by $\gtrsim 0.5$ eV. Negatively charged surface defects are observed on vacuum annealed β-Ga₂O₃, which also shows upward band bending. Density functional calculations show oxygen vacancies are not likely to be ionized in the bulk, but could be activated by surface band bending. The large band bending may also hinder formation of ohmic contacts.