Relationship between the hydroxyl termination and band bending at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mover accent="true"><mml:mn>2</mml:mn><mml:mo>¯</mml:mo></mml:mover><mml:mn>01</mml:mn></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mi>β</mml:mi><mml:mtext>−</mml:mtext><mml:mi mathvariant="normal">G</mml:mi><mml:msub><mml:mi mathvariant="normal">a</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> surfaces
Rodrigo M. Gazoni, Liam R. Carroll, Jonty I. Scott, Simon Astley, D. A. Evans, Alison J. Downard, Roger J. Reeves, Martin Allen
Abstract
Synchrotron x-ray photoelectron spectroscopy was used to explore the relationship between the hydroxyl termination and band bending at the $(\overline{2}01)$ surface of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ bulk single crystals. All as-received $(\overline{2}01)$ surfaces were terminated with OH groups, with H/OH binding to surface ${\mathrm{O}}_{\mathrm{s}}/\mathrm{G}{\mathrm{a}}_{\mathrm{s}}$ 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 $(\overline{2}01)$ surfaces by modifying the nature of the OH termination. NaOH (${\mathrm{H}}_{2}\mathrm{S}{\mathrm{O}}_{4}$) 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 $\mathrm{G}{\mathrm{a}}_{\mathrm{s}}\text{\ensuremath{-}}\mathrm{OH}$ (${\mathrm{O}}_{\mathrm{s}}\text{\ensuremath{-}}\mathrm{H}$) species. Annealing in wet ${\mathrm{O}}_{2}$ (at 600 \ifmmode^\circ\else\textdegree\fi{}C) produced an extremely stable OH termination and the strongest downward shift in band bending. These effects, combined with the relatively slow dissociation of ${\mathrm{H}}_{2}\mathrm{O}$ on bare $(\overline{2}01)$ surfaces, allowed the preparation of surfaces with significant variations in band bending that may prove useful in optimizing the properties of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ metal-semiconductor contacts and heterojunctions. A comparison of two methods used to determine the absolute band bending at semiconductor surfaces confirmed that bare $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}(\overline{2}01)$ surfaces are characterized by strong upward band bending (\ensuremath{\sim}0.5 to 1.0 eV) and an electron depletion layer that can be completely removed by the hydroxylation of the surface.