The effect of thermal annealing on the structural, chemical, electrical, and optical properties of Ga-doped ZnO (GZO) films was investigated in various ambients, such as oxygen, nitrogen, and forming gas (95% N-2 5% H,). The variation of these properties is thought to be related to the chemisorption and removal of adsorbed oxygen on the GZO film surface. The carrier concentration of the GZO films increased with heat treatment in the forming gas ambient. However, the carrier concentration of the GZO films decreased with heat treatment in oxygen and nitrogen. Annealing under forming gas, oxygen weakly absorbed on the GZO film was removed. Annealing under N- and O-rich conditions, chemisorption of oxygen was dominant. These results were clarified by comparatively analyzing the chemical states of oxygen on the surface of the GZO films. (C) 2007 Elsevier B.V. All rights reserved.