Residual stresses in Ga-doped ZnO films formed using a reactive deposition system were found to be compressive. The stresses decreased with film thickness, for example, 1.5 GPa in a 30 nm thick film and 0.2 GPa in a 500 nm thick film. The stress in each of the films was analyzed by heat-cycle testing between room temperature and 500 degrees C in flowing Ar. The stress component in films thinner than ca. 100 nm was mainly caused by strains due to crystalline irregularities. The major component in thicker films was thermal stress. This correlates with the fact that the Zn and O layers that make up the wurtzite crystalline structure became regularly stacked away from the interface in the vertical direction from the substrate surface. The compressive stresses in the Ga-doped ZnO films decreased remarkably at around 400 degrees C during heating in the first heating cycle step. This phenomenon was caused by volume shrinkage resulting from the volatilization of Zn. Furthermore, the volatilization was accompanied by the displacement of Ga, which occupied Zn sites in the wurtzite structure. As a result, the resistivities of the Ga-doped ZnO films were increased by about three orders of magnitude. (C) 2008 The Electrochemical Society.