Relationship between the hydroxyl termination and band bending at (¯201)β−Ga2O3 surfaces

Synchrotron x-ray photoelectron spectroscopy was used to explore the relationship between the hydroxyl termination and band bending at the (¯201) surface of β−Ga2O3 bulk single crystals. All as-received (¯201) surfaces were terminated with OH groups, with H/OH binding to surface Os/Gas atoms. Removal of this native OH termination produced a large upward shift in band bending of up to 1.0 eV, consistent with strong electron depletion and a semiconductor to insulator-like transition in the near-surface region. Simple surface treatments were used to control the size and stability of the band bending of as-received (¯201) surfaces by modifying the nature of the OH termination. NaOH (H2SO4) treatment consistently produced upward (downward) shifts in band bending and a significant increase (decrease) in the thermal stability of the OH termination that was associated with an increase in the relative density of Gas−OH (Os−H) species. Annealing in wet O2 (at 600 °C) produced an extremely stable OH termination and the strongest downward shift in band bending. These effects, combined with the relatively slow dissociation of H2O on bare (¯201) surfaces, allowed the preparation of surfaces with significant variations in band bending that may prove useful in optimizing the properties of β−Ga2O3 metal-semiconductor contacts and heterojunctions. A comparison of two methods used to determine the absolute band bending at semiconductor surfaces confirmed that bare β−Ga2O3(¯201) surfaces are characterized by strong upward band bending (∼0.5 to 1.0 eV) and an electron depletion layer that can be completely removed by the hydroxylation of the surface.