Metal oxides are typically insulating materials that can be made conductive through aliovalent doping and/or non-stoichiometry. Recent studies have identified conductive states at surfaces and interfaces of pure oxide materials; high electron concentrations are present, resulting in a high-mobility two-dimensional electron gas. We demonstrate for In2O3 that the energy required to form an oxygen vacancy decreases rapidly towards the (111) surface, where the coordination environment is lowered. This is a general feature of metal oxide systems that can result in a metal-insulator transition where donors are produced at chemically reduced extended defects.
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I would like to acknowledge useful discussions with P. D. C. King, C. R. A. Catlow, and A. A. Sokol. Access to the HECToR supercomputer was facilitated through membership of the UK’s HPC Materials Chemistry Consortium, which is funded by EPSRC (Grant No. EP/F067496).