Tuning the properties of complex transparent conducting oxides: Role of crystal symmetry, chemical composition, and carrier generation

The electronic properties of single- and multication transparent conducting oxides (TCOs) are investigated using first-principles density-functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In₂O₃, α, and β-Ga₂O₃, rock salt and wurtzite ZnO, and layered InGaZnO₄ reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electronic configuration of the cation(s), specifically, the type of orbital(s)---s, p, or d---which form the conduction band; and (iii) the strength of the hybridization between the cation's states and the p states of the neighboring oxygen atoms. The results not only explain the experimentally observed trends in the electrical conductivity in the single-cation TCO, but also demonstrate that multicomponent oxides may offer a way to overcome the electron localization bottleneck which limits the charge transport in wide band-gap main-group metal oxides. Further, the advantages of aliovalent substitutional doping---an alternative route to generate carriers in a TCO host---are outlined based on the electronic band structure calculations of Sn, Ga, Ti, and Zr-doped InGaZnO₄. We show that the transition metal dopants offer a possibility to improve conductivity without compromising the optical transmittance.