Drastic improvement of oxide thermoelectric performance using thermal and plasma treatments of the InGaZnO thin films grown by sputtering

Single-crystal InGaO₃(ZnO)_m thin films with periodic superlattice structures suitable for transparent thermoelectric applications were fabricated using a commercially available c-plane sapphire substrate, an epitaxial ZnO buffer layer, a thermal treatment at 900 °C, and an Ar plasma treatment. The introduction of the epitaxial ZnO buffer layer led to a significant reduction in the lattice mismatch at the interface with the InGaO₃(ZnO)_m films. The sandwich structure of the ZnO/InGaZnO/ZnO resulted in an increase in the ZnO content in the superlattice InGaO₃(ZnO)_m thin films. With respect to thermoelectric properties, the formation of a perfect, layered structure induced an increase in the Seebeck coefficient and, at the same time, a decrease in the thermal conductivity. After complete crystallization, the Ar plasma treatment resulted in a considerable decrease in the electrical resistivity without microstructural changes and without a large decrease in the thermal conductivity. As a result, the thermoelectric properties using n-type oxide semiconductors were dramatically improved.