Improvement of ZnO TCO film growth for photovoltaic devices by reactive plasma deposition (RPD)

Reactive plasma deposition (RPD) is a technique for depositing a thin film on a substrate using a pressure–slope type plasma ion gun. This method offers the advantage of low-ion damage, low deposition temperature, large area deposition and high growth rates. Ga-doped zinc oxide (ZnO) thin film was grown on a moving glass substrate by RPD. Evaporation of very small quantity of tungsten from anode electrode by plasma collision lets the resistivity of grown ZnO transparent conductive oxide (TCO) film to increase. However, no reduction of carrier concentration was observed but only reduction of carrier mobility. It indicates that reduction of evaporation of tungsten from anode electrode induces increase of carrier mobility without any increase of carrier concentration. After installation of an anode cooling system in order to avoid the tungsten evaporation, increase of the mobility (37 cm2/Vsec) was observed and the lowest resistivity (2.0×10−4 Ω cm) film was obtained from large size grown ZnO TCO of 200×200 mm at low growth temperature of 200 °C with high growth rate of 24 μm/h.