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. (C) 2009 Elsevier B.V. All rights reserved.