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 μ_H, weighted mobility μ_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 μ increased simultaneously (by 28 times and one order of magnitude, respectively), markedly enhancing the PF of dopant-free ZnO films. [ABSTRACT FROM AUTHOR]