Electrical properties and structure of transparent conductive oxide films deposited by pulsed reactive magnetron sputtering

Sn-doping of InOx and Al-doping of ZnO is extensively used to create transparent thin film electrodes. However, the mechanisms of donor impurity incorporation and its electrical activation, especially in relation to the film structure and phase composition, are not properly understood. In order to have a deeper insight into these processes, InOx, InOx:Sn, ZnO and ZnO:Al films were grown by reactive pulsed magnetron sputtering from In, In:Sn, Zn and Zn:Al targets, respectively, at substrate temperatures ranging from 40 °C to 580 °C, and characterized by spectroscopic ellipsometry, Hall effect measurements, X-ray diffraction (XRD) and, in case of ZnO and ZnO:Al films, by X-ray absorption near edge spectroscopy (XANES). For InOx:Sn, the film crystallinity always improves with increasing substrate temperature or during isothermal annealing, with the electrical resistivity decreasing. This is explained by Sn donor activation during the film amorphous-to-crystalline transition. In contrast, the electrical resistivity of ZnO:Al films shows a clear minimum at a certain substrate temperature, which correlates with a maximum in crystallinity (grain size). Here, the lower resistivity is due to an increased density and free electron mobility. Increasing the Zn/O2 flux ratio decreases this optimum temperature from 350 °C to 250 °C. At higher temperature, the film electrical properties and crystalline quality decrease due to the formation of a new metastable phase, which is identified as homologous (ZnO)3(Al2O3) by XANES and XRD for low and high Zn/O2 ratio, respectively. The optimization of the oxygen partial pressure, the substrate temperature and the Al concentration allows to minimize the ZnO:Al resistivity down to 2.3-2.5x10^-4 Ohm cm at a significantly improved free electron mobility.