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 (13.56 and 27.12 MHz). Temperature-dependent conductivity and Hall measurements (down to 50 K) were used to determine the carrier concentrations ND and the Hall mobilities μ. From the μ(ND) 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 Nt ≈ 1.5 · 1012 cm− 2, compared to Nt values up to 3.1013 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.