Dependence of Ga-doped ZnO thin film properties on different sputtering process parameters: Substrate temperature, sputtering pressure and bias voltage

Abstract

This paper reports on the effects of different sputtering deposition process parameters (substrate temperature, sputtering pressure and bias voltage) on the electrical, optical, structural and morphological properties of gallium-doped ZnO (ZnO:Ga) of ~1μm thick. These highly transparent and conductive films were deposited on glass surfaces by d.c. pulsed magnetron sputtering from a GZO (ZnO(95.5):Ga2O3(4.5)) ceramic target in an argon atmosphere. X-ray diffraction experiments show that all films have a hexagonal wurtzite structure with the [001] preferred crystallographic direction, and the morphology of the films (obtained from scanning electron microscope analysis) is sensitive to the process parameters. All ZnO:Ga films have an average transmittance above 80% in the visible region, and the lowest electrical resistivity of 3.03×10−4Ω·cm was achieved for the sample submitted to the lowest bias voltage (−40V), which corresponds to a carrier concentration and a carrier mobility of 6.99×1020cm−3 and 29.49cm2V−1s−1, respectively. A high substrate temperature, high sputtering pressure and low negative bias voltage (within the range of studied parameters) proved to be very promising on obtaining optimized ZnO:Ga films, ensuring suitable properties for application as transparent electrodes in photovoltaic cells.