High kappa Ga2O3(Gd2O3) dielectric was deposited on n-type GaN (0001) using molecular beam epitaxy (MBE). TiN/Ga2O3(Gd2O3)/ GaN metal-oxide-semiconductor (MOS) diodes have exhibited a negligible frequency dispersion, low leakage currents (similar to 10(-8) A/cm(2)), and a low interfacial density of states (D-it) of 10(11) cm(-2) eV(-1) at the midgap. Well-behaved capacitance-voltage (C-P) curves with accumulation and depletion behaviors were shown, with a dielectric constant of 14.7. Forming gas annealing at 600 degrees C has reduced the frequency dispersion in the C-V curves. A sharp oxide/semiconductor interface was shown by high-resolution transmission electron microscopy (HR-TEM). (c) 2007 Published by Elsevier B.V.