Highly non-stoichiometric amorphous oxide semiconductors: the structure and electronic properties of defects in a-InOx

Oxygen non-stoichiometry in crystalline In2O3 leads to formation of oxygen vacancies that play key role in carrier generation and transport in this transparent conducting oxide and have been well-studied both theoretically and experimentally. In contrast to the crystalline oxide, the structure and electronic properties of oxygen defects in amorphous indium oxide (a-InOx) are not understood and render to be fundamentally different due to the lack of lattice sites and periodicity as well as an increased number of degrees of freedom in disordered materials. Strikingly, the observed carrier concentration in a-InOx is two orders of magnitude higher than that in the crystalline oxide. Moreover, given the ionic nature of indium-oxygen bonding, it is possible to grow highly non-stoichiometric amorphous oxides that helps tune the properties over an extremely wide range.
Here, we employ ab-initio Molecular Dynamics simulations and density-functional calculations to study the structural and electronic properties of a-InOx. Based on a thorough analysis of the interatomic distances, coordination, distortions, charge localization, and defect state energies, we derive a microscopic model of defects in disordered oxides and discuss their effect on the electrical and optical properties.