Reactive sputtering of ZnO:Al thin films from rotatable dual metallic targets

In this study, aluminum doped zinc oxide (ZnO:Al) films were prepared by reactively sputtering from rotatable dual metallic targets, which were controlled by a plasma emission monitoring (PEM) system. The influences of different sputtering conditions including different discharge powers, working pressures and working points (or oxygen partial pressure) on ZnO:Al films are investigated systematically. It is found that the deposition rate strongly relies on the discharge power. However, different PEM intensities lead to different deposition rates, which are related with oxygen partial pressure as well as the sputtering properties to oxide (ZnO) and metal (zinc) materials. In addition, the oxygen partial pressure at different PEM intensities strongly influences the electrical and optical properties as well as morphologies and etching behaviors of ZnO:Al films. High rate ZnO:Al films with good electrical and optical properties as well as proper surface structures are achieved at the transition mode region. High deposition rate of up to 100 nm m/min and high carrier mobility of up to 41 cm2/Vs are achieved, which demonstrates the great advantage for such a cost-efficient sputtering technique. The surface structure of high rate ZnO:Al films can be modified further by etching with a novel two-step etching method and good device performance has been achieved when they applied in silicon thin film solar cells.