Effect of substrate temperature on Al-doping of ZnO films

Al-doping of ZnO is routinely used to create transparent electrodes, although the mechanisms of incorporation and electrical activation of the doping impurity are not well understood. In order to have a deeper insight into these processes, polycrystalline ZnO and ZnO:Al films were grown by reactive pulsed magnetron sputtering of Zn and Zn:Al targets, respectively. The substrate temperatures (Ts) were spanning in the 40-580 °C range. The films were characterized by spectroscopic ellipsometry, Hall effect measurements, X-ray diffraction (XRD) and X-ray absorption near edge spectroscopy (XANES). The electrical resistivity of ZnO:Al films shows a clear minimum at Ts=350 °C, which correlates with a maximum in crystallinity (grain size). The lower resistivity is due to the increased density and mobility of free electrons according to the Hall effect measurements. At Ts>350 °C, the ZnO:Al film crystallinity significantly deteriorates, while the crystallinity of undoped ZnO films grown at identical conditions always improves by increasing Ts. XANES results show that the poorer film crystallinity and higher resistivity at high Ts can be related to AlOx segregation. Further, the electrical properties correlate with changes in the O(1s) absorption edge, whereas the Zn(2p) edge shows no modification with respect to undoped ZnO films. This result suggests that the Al incorporation increases charge carrier density due to oxygen vacancy formation in ZnO lattice.