Thin films of aluminum oxide (Al2O3) on GaN substrates were grown by the process of low-temperature liquid-phase deposition (LPD), in which aluminum sulfate with crystallized water [Al-2(SO4)(3)center dot 18H(2)O] and sodium bicarbonate [NaHCO3] were used as the precursors. The pH value of the growth solution plays an important role in the deposition process. The best quality of the oxide thin film was obtained at the pH value of 3.80, while the growth rate was 35 nm/ h at the optimized concentration values of Al-2(SO4)(3) = 0.0834 mol/ L and NaHCO3 = 0.211 mol/L and at the temperature of 30 degrees C. The films were characterized by means of X-ray photoelectron spectroscopy, Auger electron spectroscopy, and atomic force microscopy. It was found that the leakage current density of 50 nm thin Al2O3 oxide film was between 10(-4) and 10(-5) A/ cm(2) at a negative electric field of 1 MV/cm, with the breakdown electric field being greater than 10 MV/cm. After annealing the oxide at 750 S C for 30 min, the leakage current density was lowered to the value of 10(-6) - 10(-7) A/cm(2) at the negative electric field of 1 MV/cm. The oxide-semiconductor interface state density as calculated from the capacitance-voltage curve was 3.89 x 10(11) cm(-2) eV(-1). (c) 2007 The Electrochemical Society.