Epitaxial stabilization of a monoclinic phase in Y2O3 films on c-plane GaN

Nanometer-thick Y2O3 films were grown epitaxially on GaN (0 0 0 1) using molecular beam epitaxy (MBE). The structures of the oxide films were studied in situ by reflection high energy electron diffraction (RHEED) during the growth and ex situ by high resolution X-ray diffraction using synchrotron radiation. At atmospheric pressure, Y2O3 exists in either cubic or hexagonal structure. For the first time, the high-pressure monoclinic phase of Y2O3, stabilized by epitaxy, was prepared and preserved under atmospheric pressure. The electrical characterization carried out on the Y2O3/GaN metal-oxide-semiconductor (MOS) capacitors showed a leakage current density of ∼3.3×10−6 A/cm2 at 1.5 MV/cm, which remained almost the same after 800 °C annealing. A dielectric constant of ∼20 and a hysteresis of ∼250 mV were deduced from the capacitance-voltage (C-V) curves for the epitaxial monoclinic Y2O3.