Carrier transport in poly crystalline ITO and ZnO:Al II: The influence of grain barriers and boundaries

摘要

ITO and ZnO:Al films have been deposited by magnetron sputtering from ceramic and metallic targets at different substrate temperatures and with different plasma excitation modes: DC and RF (13.56 and 27.12 MHz). Temperature-dependent conductivity and Hall measurements (down to 50 K) were used to determine the carrier concentrations N_D and the Hall mobilities μ. From the μ(N_D) dependences, which were fitted by a carrier transport model taking into account ionized impurity and grain barrier scattering, the trap densities at the grain boundaries were estimated. ITO films show much lower trap densities down to N_t ≈ 1.5·10~(12) cm~(-2), compared to N_t values up to 3·10~(13) cm~(-2) for ZnO:Al films. The temperature-dependent mobilities were fitted by a phenomenological model with a T-independent term and a metal-like contribution or a thermally-activated part due to grain barrier-limited transport. Seebeck coefficient measurements as a function of the carrier concentration give hints to different transport mechanisms in ITO and ZnO.