Enhanced power factor of Indium co-doped ZnO:Al thin films deposited by RF sputtering for high temperature thermoelectrics

An improvement in the thermoelectric power factor of Al doped ZnO has been achieved by means of co-doping with indium using a dual magnetron sputtering system. The concentration of indium in the film was varied from 0 to 10 atomic % by varying the RF power of the In target, with the ZnO:Al target fixed at 100 W. It has been found that the films with In concentrations at or below 5 at.% have no significant change in microstructure, and yet a marked improvement in thermopower. At higher doping levels, the Seebeck coefficient continues to increase, however poly-crystallinity is induced in the ZnO matrix which results in a considerable decrease in electrical conductivity. This factor ultimately has a negative impact on the materials power factor. Taking into account the films studied, (ZnO)Al.03In.02 exhibited the best thermoelectric properties with an electrical conductivity of 5.88 × 102 S/cm and a Seebeck coefficient of −220 μV/K at 975 K, resulting in a power factor is 22.1 × 10−4 Wm−1 K−2, which is three times greater than for the film with no In doping. Film microstructure, composition, and thermal stability were investigated using X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy.

Research highlights▶ Highly crystalline films were deposited through a co-sputtering process using both In and Al-doped ZnO targets. ▶ (ZnO)Al.03In.02 films exhibited the best thermoelectric properties with an electrical resistivity of 2.1 × 10−3 Ω cm and Seebeck coefficient of −220 μV/K at 975 K, resulting in a power factor is 22.1 × 10−4 Wm−1 K−2, which is three times greater than for the film with no In doping. ▶ Atomic concentrations of greater than 5% indium have resulted in poly-crystalline films with electrical resistivity values two orders of magnitude higher than those with 0 and 2% doping. ▶ Films appear to have excellent thermal stability after being exposed to various heat treatments up to 1150 °C and analyzing the indium and aluminum diffusion.