Tailoring stress relaxation for dopant-free ZnO thin films with high thermoelectric power factor

In this study, the effects of stress relaxation on the thermoelectric properties (carrier concentration n, Hall mobility mu(H), weighted mobility mu(W), density-of-state mass m(d)*, Seebeck coefficient S, and thermopower factor PF) of undoped ZnO films were rationalized in terms of native defects (V-O-related defects and Zn-i-related donors) induced through the deposition temperature (T-D) during the sputtering process. All investigated ZnO films exhibited compressive stress and tended to become less compressive with increasing T-D. The stress relaxation at high T-D resulted in improved film crystallization and decreased native defect concentration, thus significantly enhancing m(d)* through the reduction of intrinsic lattice defects, while less carriers were trapped and scattered by defects. Therefore, n and mu increased simultaneously (by 28 times and one order of magnitude, respectively), markedly enhancing the PF of dopant-free ZnO films.