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

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 (13.5 6 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 mu. From the mu(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 similar or equal to 1.5 center dot 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. (C) 2008 Published by Elsevier B.V.