High quality gallium oxide (Ga₂O₃) thin films are deposited by remote plasma-enhanced atomic layer deposition (RPEALD) with trimethylgallium (TMG) and oxygen plasma as precursors. By introducing in-situ NH₃ plasma pretreatment on the substrates, the deposition rate of Ga₂O₃ films on Si and GaN are remarkably enhanced, reached to 0.53 and 0.46 Å/cycle at 250 °C, respectively. The increasing of deposition rate is attributed to more hydroxyls (–OH) generated on the substrate surfaces after NH₃ pretreatment, which has no effect on the stoichiometry and surface morphology of the oxide films, but only modifies the surface states of substrates by enhancing reactive site density. Ga₂O₃ film deposited on GaN wafer is crystallized at 250 °C, with an epitaxial interface between Ga₂O₃ and GaN clearly observed. This is potentially very important for reducing the interface state density through high quality passivation.