Influences of substrate types and heat treatment conditions on structural and thermoelectric properties of nanocrystalline Bi2Te3 thin films formed by DC magnetron sputtering

We deposited Bi2Te3 thin films on different substrates under various heat treatment conditions to improve the thermoelectric properties of these films by changing the crystal orientation and crystallite size. The films were deposited through direct-current magnetron sputtering. We used four substrates (glass, alumina, sapphire, and polyimide) and three heat treatment conditions; these conditions included non-substrate heating, substrate heating during film deposition, and thermal annealing after film deposition. The in-plane electrical conductivity and Seebeck coefficient were measured near 300 K, and in-plane thermal conductivity was calculated using models to estimate the power factor (PF) and dimensionless figure of merit (ZT). Among the heat treatment conditions, the most noticeable difference in the substrates appeared during thermal annealing after film deposition. In particular, the Bi2Te3 film deposited on the glass substrate obtained an almost perfect crystal orientation with an average crystallite size of 50 nm; thus, it exhibited a relatively high electrical conductivity of 1025 S/cm, Seebeck coefficient of -163 mu V/K, and thermal conductivity of 1.2 W/(m.K). Consequently, the PF and estimated dimensionless ZT were 27.3 mu W/(cm.K-2) and 0.70, respectively. Therefore, we successfully deposited Bi2Te3 thin films with high thermoelectric properties by selecting the appropriate substrate and heat treatment conditions.