Thermoelectric Performance of ZnO:Al/ZnO:(Al,In) Quantum Well Multilayer Structures as a Function of Indium Composition and Band-Gap Offset at High Operating Temperatures

This study investigates the thermoelectric (TE) power factor for radiofrequency (RF)-sputtered -type ZnO:Al/ZnO:(Al,In) multilayer quantum wells (QW) and the dependence on band-gap offset at high operating temperatures. The structures are 50 periods of 10-nm-thick barrier and well layers. The TE power factors for all films are promising (> 20 x 10(-4) W/m(2) K); however, a decline in electronic transport is evident for each of the doped wells over a specific range of operating temperatures. This supports the need for optimum doping levels for a specific operating temperature range; e.g., 2 at.%, 5 at.%, and 8 at.% indium are optimum at 300 K to 600 K, 600 K to 725 K, and > 725 K, respectively. A model has also been developed to calculate the QW electronic transport as a function of temperature and band-gap offset, which is subsequently related to doping concentration. This model confirms a loss in electrical conductivity due to charge carriers gaining enough kinetic energy to overcome the QW barrier, which results in multilayer hopping across well-barrier interfaces. In addition, film microstructure and interface roughness have been determined by x-ray and spectroscopy techniques.