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The role of water (H2O) vapor in reducing background impurity concentration in epitaxial Ga2O3 thin films grown by metalorganic chemical vapor deposition (MOCVD) was studied. Adding H2O vapor to oxygen was found to increase the surface roughness and decrease the carrier concentration but have no effect on the structural quality and RT electron mobility of both the UID and doped films. The H2O vapor promotes the growth of the stable (110) and 1 ¯ 10 facets elongated along the [001] direction. Using [H2O] of 250 ppm, the β-Ga2O3 films showed a RMS roughness of >13 nm, which is much higher than that measured for similar films grown without H2O (RMS = 0.8 nm). The electron mobility of the UID Ga2O3 layers grown with and without water vapor was ∼150 cm2/V s, while the carrier concentration dropped by ∼2.5× after increasing the [H2O] to 6000 ppm. Similarly, for the lightly Si doped films, the RT carrier concentration dropped from 5.4 × 1015 cm−3 ([H2O] = 0 ppm) to 3.4 × 1015 cm−3 ([H2O] = 250 ppm), leaving the mobility unaffected. The results offer a new method of using water vapor assisted growth of Ga2O3 films with low RT carrier concentration that are required for the realization of high-performance electronic power devices with high breakdown voltages. [ABSTRACT FROM AUTHOR]