Epitaxial calcium oxide films deposited on gallium nitride surfaces

Solid solutions of rocksalt oxides are proposed for lattice-matched dielectrics in gallium nitride (GaN) electronics. This article explores the epitaxial growth of the rocksalt oxide calcium oxide (CaO) by molecular beam epitaxy on gallium nitride surfaces. As a possible end member to a rocksalt oxide solid solution, it is important to understand the processing space that allows for epitaxial CaO growth. Exposing the GaN surface to the oxidant flux prior to the metal flux is shown to be critical in eliminating polycrystalline growth. The effect of deposition temperature, metal flux, and oxidant flux on the film's epitaxial crystalline quality is also examined. Optimal epitaxial quality is found for growth temperatures of 600 °C and near oxygen pressures of 10-6 Torr. Thermal stability of the CaOGaN interface is experimentally evaluated. No reaction phases are observed by x-ray diffraction up to 850 °C in air and > 1100 °C in a reducing atmosphere. However, CaO films are found to be extremely reactive with H2 O, forming Ca (OH)2 within a few hours when exposed to ambient atmosphere at room temperature.