Electronic Structure, Optical, And Electrical Properties Of In, Zn, And Ti-Based Transparent Conducting Oxides

The combination of high optical transparency at the visible wavelengths and high electrical conductivities in transparent conducting oxide films makes the class of materials be widely used in various fields, such as in solar cells, liquid crystal and high definition displays, electrochromic and smart windows, as well as in architectural energy-efficient windows.The structure, electrical transport properties, and optical properties of ITO, B- and Al-doped ZnO films were experimentally studied systematically. The band structures and optical properties of undoped and B-doped ZnO as well as undoped and Nb-doped anatase TiO2 were calculated respectively, using local-density approximation based on the density-functional theory.The amorphous ITO films with semiconductor behaviors were obtained when the substrate temperatures T≤100℃. The polycrystalline films deposited above 120℃( T≥120℃) exhibit metallic characteristics at higher temperatures ( > 100 K), and the temperature coefficients of resistivities become negative below 80 K. The magnetoconductivities of the polycrystalline films are all positive. The negative temperature coefficients of resistivities originate from the weak-localization effect and Coulomb interactions. The electron-electron interaction is the mainly inelastic scattering process in the polycrystalline films.The ZnO:B films deposited at 150℃exhibit metallic characteristics. The magnetoresistances of the films were studied and it is found that the negative components of the magnetoresistances originate from the weak-localization effect and the positive parts can be well described by the double-band model. The electron-electron collisions are the dominant inelastic scattering process. The nontrivial corrections in the temperature dependence of electrical conductivity mainly come from the Coulomb interactions at low temperatures in the films.The negative components of magnetoresistances observed in ZnO:Al films with different thickness do not come from the weak-localization effect, but from the scattering of conduction electrons due to localized magnetic moments.The first-principle calculations were performed with wurtzite ZnO and ZnO:B. For ZnO, the Fermi level locates in the band gap, indicating the semiconductor characteristic. The high dispersion of the bottom of the conduction band in ZnO suggests small effective mass of electron carriers. For ZnO:B, the Fermi level locates in the conduction band, indicating the metallic characteristic. The energy difference between the top of the valence and the Fermi level is larger than the band gap of the pure ZnO. The donor level locates in the conduction band and no impurity band was found in the band gap. This is consistent with the experimental observation that the carrier concentration is independent with temperature.The Fermi level of TiO2:Nb locates in the conduction, showing the metallic characteristic. The energy difference between the top of the valence and the Fermi level is larger than the band gap of the pure TiO2, which may explain the experimentally observed blueshift of absorption edge in anatase TiO2 films with Nb doping. According to the optical properties calculations, the fundamental absorption edge locates above the visible range. Absorption behaviors are observed both above and below the fundamental absorption edge and the absorption coefficient of the former is ~ 3 orders of magnitude weaker than that of the latter, indicating the intrinsic transparency at the visible wavelengths for TiO2:Nb films.