Formation of homologous In₂O₃(ZnO)_m thin films and its thermoelectric properties

Homologous In₂O₃(ZnO)₅ thin films were produced on a synthetic quartz glass substrate by thermal annealing of magnetron sputtered In₂O₃-ZnO compound films. When the annealing temperature was increased to 700 °C, the sputtered In₂O₃-ZnO film with In₂O₃ microcrystalline changed to a c-oriented homologous In₂O₃(ZnO)₅ structure, for which the crystallization is suggested to begin from the surface and proceed along with the film thickness. The annealing temperature of 700 °C to form the In₂O₃(ZnO)₅ structure was substantially lower than temperatures of conventional solid state synthesis from In₂O₃ and ZnO powders, which is attributed to the rapid diffusional transport of In and Zn due to the mixing of In₂O₃ and ZnO in the atomic level for sputtered In₂O₃-ZnO compound films. The homologous structure collapsed at temperatures above 900 °C, which is attributed to (1) zinc vaporization from the surface and (2) a gradual increase of zinc silicate phase at the interface. This c-oriented layer structure of homologous In₂O₃(ZnO)₅ thin films along the film thickness allowed the thin film to reach a power factor of 1.3 × 10^-4 W/m K² at 670 °C, which is comparable with the reported maximum value for the textured In₂O₃(ZnO)₅ powder (about 1.6 × 10^-4 W/m K² at 650 °C).