Stoichiometry control mechanisms for bias-sputtered zinc-oxide thin films

We have deposited transparent, conducting ZnO thin films of resistivity 3 × 10−3 Ω∙cm by the technique of DC planar-magnetron sputtering in a reactive Ar–O2 atmosphere, incorporating a reactive gas baffle and substrate RF discharge. The substrate discharge was found to increase the oxygen content of the growing film. Films at low values of the RF-induced substrate self-bias voltage were characterized by a brown colour, resistivities of about 4 × 10−2 Ω∙cm, and composition ZnO0.8. Films at higher bias voltages of −80 V were clear with resistivities of 3 × 10−3 Ω∙cm and a composition approaching stoichiometric ZnO. The oxidation of the films by the RF discharge was shown to occur through preferential resputtering and re-evaporation of excess zinc and by activation and ion-plating of oxygen species. Resputtering and re-evaporation rates were found to be enhanced above that expected for bulk Zn, owing to the loosely bound nature of surface adatoms during film growth.