Influence of low sputtering pressure on structural, electrical and optical properties of Al-doped zinc oxide thin films

Aluminum-doped zinc oxide thin films were deposited without intentional heating by radio-frequency magnetron sputtering. The sputtering pressure varied from 0.02Pa to 0.32Pa while the deposition power was kept at 240W for all depositions. The structural properties of as-deposited films were analyzed by X-ray diffraction and scanning electron microscopy, indicating that the deposited films have a strong preferred c-axis (002) orientation perpendicular to the substrate regardless of sputtering pressure. The minimum resistivity of 6.4×10−4Ωcm is obtained at 0.05Pa, which is mainly influenced by the hall mobility, rather than carrier concentration. The highest transmittance could be ~80% on average in the visible range under various working pressures, and the largest bandgap achieved is about 3.82eV. The ultraviolet emission peaks in photoluminescence spectra are centered at about 360nm. A new mechanism is proposed to explain the dependence of the electrical and optical properties on structural evolution of deposited films.