Surface Origin of High Conductivities in Undoped In2O3 Thin Films

The microscopic cause of conductivity in transparent conducting oxides like ZnO, In2O3, and SnO2 is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for O-vacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In2O3 samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In2O3 can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O-2 partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In2O3 thin films.