Optimization of oxygen and pressure of ZnO:Al films deposited on PMMA substrates by facing target sputtering

ZnO:Al (ZAO) thin films have been deposited on PMMA substrates using facing target sputtering system at room temperature. The dependence of the properties on the oxygen partial pressure and the sputtering pressures was investigated. With increased oxygen partial pressure, the resistivity increases sharply, while the optical band gap and the carrier concentration decrease. The intrinsic band gap of 3.40 eV was obtained according to the Burstein-Moss (BM) widening. The lowest resistivity of 2.4 × 10^-3 Ω cm and the figure of merit value of 3.42 × 10^-3 Ω^-1 were achieved when the oxygen partial pressure and sputtering pressure was 0% and 0.7 Pa, respectively. For ZAO films prepared at various sputtering pressure, carrier concentration changes slightly whereas Hall mobility increases almost linearly with increasing grain size. The carrier transport of the films is mainly limited by grain boundary scattering.