MBE-grown single-crystal Gd2O3 epitaxially on GaN has exhibited excellent thermal stability, withstanding rapid thermal annealing (RTA) at 1100 degrees C. The high-K dielectric Gd2O3 thin film 10 nm thick has a monoclinic phase with a high degree of crystallinity, characterized by X-ray diffraction using synchrotron radiation. The orientation relationship is ((4) over bar 0 2)(Gd2O3) ||(0 0 0 1)(GaN) and < 0 1 0 >(Gd2O3) || < 2 (1) over bar (1) over bar 0 >(GaN). Interface between Gd2O3 and GaN remains atomically sharp after the RTA, as demonstrated using X-ray reflectivity and high-resolution transmission electron microscopy. Gd2O3/GaN metal-oxide-semiconductor capacitors show well-behaved capacitance-voltage characteristics with small dispersion and hysterisis, a high dielectric constant of similar to 17, and a low electrical leakage current density of 4.6 x 10(-9) A/cm(2) at 1 MV/cm. (c) 2008 Elsevier B.V. All rights reserved.