Comparative study of ZnO:Al films on Si and glass prepared by rf magnetron sputtering at room temperature

A comparative study of aluminium-doped zinc oxide (ZnO:Al) films deposited by rf magnetron sputtering at room temperature on Si and glass has been performed. In these experiments, two types of substrates (Si wafer and glass) and two Ar pressures (0.31 and 3.23 Pa) were used. The structural and optical characteristics of the ZnO:Al films grown on Si and glass substrates were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and double-beam spectrophotometry. The XRD patterns show that, on both substrates, the ZnO films have a dominant [002] peak. The SEM investigation of the film surfaces revealed that the microstructure depends strongly on the substrate type. Transmission measurements showed that, for visible wavelengths, the ZnO films on glass have an average transmission of over 84%, independent of the Ar pressure. The direct optical bandgap of ZnO:Al is 3.31 and 3.34 eV, respectively, for Ar pressures of 0.31 and 3.23 Pa. Furthermore, to evaluate the films´ photovoltaic characteristics, ZnO:Al/n-Si heterojunction solar cells were fabricated by sputtering ZnO:Al films at room temperature onto n-Si wafers. The fill factor of the cells was lower than that obtained at 250°C substrate temperature, which is due to the relatively high sheet resistance of ZnO:Al films deposited at room temperature.