Effects of hydrogen ambient and film thickness on ZnO:Al properties

Undoped ZnO and ZnO:Al (0.1, 0.2, 0.5, 1.0, and $2.0wt.%$ $Al2O3$⁠) films were deposited by rf magnetron sputtering. Controlled incorporation of $H2$ in the Ar sputtering ambient for films grown at substrate temperatures up to $200°C$ results in mobilities exceeding $50cm2V−1s−1$ when using targets containing 0.1 and $0.2wt.%$ $Al2O3$⁠. Temperature-dependent Hall measurements show evidence of phonon scattering as the dominant scattering mechanism in these lightly Al-doped films, while ionized impurity scattering appears increasingly dominant at higher doping levels. A combination of compositional and structural analysis shows that hydrogen expands the ZnO lattice normal to the plane of the substrate and desorbs from ZnO at $∼250°C$ according to temperature-programmed desorption and annealing experiments.