Modification of ZnO:Al properties: post-deposition millisecond thermal processing vs direct growth at elevated substrate temperature

ZnO:Al (AZO) is a promising cost-efficient transparent conducting material for a number of applications. Thermal processing at millisecond time scale (very rapid thermal processing, vRTP) is an attractive approach of AZO film properties improvement. Present study focuses on systematic comparison of the properties of the films processed by vRTP and directly grown at various substrate temperatures. The AZO films were deposited both by reactive pulsed and non-reactive DC magnetron sputtering. The vRTP was performed using innovative low-cost high power diode laser arrays with microoptically designed line-shaped beam profiles (dwell time of 1 ms). Only the films grown without substrate heating were subjected to the vRTP. Even at air ambience, the optimized laser processing of the AZO films results in a decrease of the film electrical resistivity from (1-2)x10^-3 to less than 5x10^-4 Ohm cm. This is accompanied by a substantial, in some cases almost two-fold, increase of the free electron mobility and density, and increase of the film transmittance in the visible. According to TEM, ellipsometry, Raman spectroscopy and XANES, annealing of intra-grain oxygen-related point defects during vRTP has a main effect on the film electrical properties. Using deposition at elevated temperature, the AZO films with comparable electrical and optical properties were achieved only at Ts≥250°C.