Optimizing the discharge voltage in magnetron sputter deposition of high quality Al-doped ZnO thin films

Abstract

The role of negative ions in the sputter deposition of oxides can be termed negative in the sense that they typically cause radiation-induced structural damage during film growth. For magnetron sputtered Al-doped ZnO (AZO) films, efforts have been made to alleviate the radiation damage by decreasing the discharge voltage |Vd|, with ∼100 V being the lower limit explored. Here, the authors report initial results of depositing highly conductive and transparent AZO films by reducing |Vd| down to 40 V. The deposition was performed by sputtering an AZO target using dense magnetron discharges, in which an 81 MHz radio frequency power was superimposed onto a DC power applied to the cathode. The authors found an optimal |Vd| window within which high quality AZO films, with a <4 × 10−4 Ω cm resistivity, >30 cm2/(V s) Hall mobility, and >88% visible transmittance, were obtained at relatively high deposition rates (>30 nm/min). Based on the corresponding structural features, i.e., a full mass density coupled with a slight c-axis contraction along the out-of-plane direction, energetic negative ions were identified to have dominated in the structural evolution. The |Vd| window was then interpreted in terms of two competitive kinetic processes simultaneously caused by the energetic negative ions: void reduction via ballistic relocation of atoms as well as residual radiation-induced damage. The findings thus reveal a positive role (i.e., concomitant densification) played by the negative ions in growing high quality fully dense AZO films, which has been overshadowed by the excessive radiation damage induced by ions with relatively higher energies.