Influence of negative ion resputtering on Al-doped ZnO thin films prepared by mid-frequency magnetron sputtering

Al-doped ZnO (AZO) films were deposited on glass substrates by mid-frequency magnetron sputtering with a ceramic ZnO:Al₂O₃ (98 wt%:2 wt%) target. The origin of the high resistivity of the films at the substrate position facing the erosion area of the target was investigated. The results indicate a preferential resputtering of Zn atoms caused by the negative ions, which leads to an increase of the oxygen/metal ratio in the films. Then more Al oxides form and result in the decrease of Al_Zn (the main donor in the films) concentration in the films. Thus the free carrier concentration decreases badly. This is the main mechanism responsible for the high resistivity.