Surface origin of high conductivities in undoped In ₂O ₃ thin films

The microscopic cause of conductivity in transparent conducting oxides like ZnO, In ₂O ₃, and SnO ₂ 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 In ₂O ₃ samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In ₂O ₃ 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 In ₂O ₃ thin films.