Transparent and conductive thin films of ZnO for photovoltaic applications prepared by RF magnetron sputtering

High-quality zinc oxide (ZnO) films were deposited by RF magnetron sputtering from a zinc oxide target. The material obtained had a low resistivity (8*10^-4 Omega -cm) and an average transmission higher than 90% over the 400-800-nm range. It was found that the properties of ZnO prepared by RF magnetron sputtering are strongly dependent on deposition conditions; in particular the conductivity is markedly dependent on the substrate-to-target distance. It is speculated that the growing surface of the film is subject to resputtering due to the bombardment of energetic neutral O atoms. The target-to-substrate separation is then probably on the order of the mean free path of the O atoms, so that the flux of O atoms to the growing surface can be optimized by changing the distance to the target. The degree of resputtering influences the degree of nonstoichiometry of the material and thus its conductive properties. The polycrystalline films appear to have a textured surface and an ability to withstand plasma conditions, both very attractive properties for use in amorphous silicon solar cell structures.