The effects of water partial pressure (P-H2O) on electrical and optical properties of Ga-doped ZnO films grown by DC magnetron sputtering were investigated. With increasing P-H2O, the resistivity (p) of the films grown in pure Ar gas (Ar-films) significantly increased due to the decrease in both free carrier density and Hall mobility. The transmittance in the wavelength region of 300-400 nm for the films also increased with increasing P-H2O. However, no significant P-H2O dependence of the electrical and optical properties was observed for the films grown in H-2/Ar gas mixture (H-2/Ar-films). Secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) analysis revealed that hydrogen concentration in the Ar-films increased with increasing PH,0 and grain size of the films decreases with increasing the hydrogen concentration. These results indicate that the origin of the incorporated hydrogen is attributed to the residual water vapor in the coating chamber, and that the variation of rho and transmittance along with P-H2O of the films resulted from the change in the grain size. On the contrary, the hydrogen concentration in H-2/Ar-films was almost constant irrespective Of P-H2O and the degree of change in the grain size of the films versus P-H2O was much smaller than that of Ar-films. These facts indicate that the hydrogen primarily comes from H-2 gas and the adsorption species due to H-2 gas preferentially adsorb to the growing film surface over residual water vapor. Consequently, the effects of P-H2O on the crystal growth are reduced. (C) 2004 Elsevier B.V. All rights reserved.