Surface oxygen vacancy origin of electron accumulation in indium oxide

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 In(2)O(3) 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.