Thermoelectric Properties of Oxygen-Tuned ALD-Grown [Ca2CoO3]0.62[CoO2] Thin Films

Thin films of the p-type thermoelectric misfit-layered oxide, [Ca2CoO3](0.62)[CoO2], were prepared for the first time by means of the atomic layer deposition (ALD) technique using Ca(thd)(2), Co(thd)(2), and O-3 as precursors. As-deposited films were amorphous; however, with heat treatment in an O-2 gas flow, well-crystallized highly c-axis-oriented [Ca2CoO3](0.62)[CoO2] films were obtained. The oxygen content of the O-2-annealed film was further controlled through a reductive N-2-annealing. Because the degree of reduction was dependent on the annealing temperature, the choice of N-2-annealing temperature provided us with a tool for precise tuning of the oxygen content. With decreasing oxygen content, the lattice parameter (c) and the Seebeck coefficient (S) were found to increase. The room-temperature S values were 113 and 128 mu V/K for the oxygen-richest sample and the most-reduced sample, respectively.