Growth of Hf and HfN on GaN by molecular beam epitaxy

Hf and HfN thin films were grown on n-type GaN(0001) by molecular beam epitaxy using a custom built Hf electron beam source and an ammonia leak. The films were characterized by reflection high-energy electron diffraction (RHEED) and atomic force microscopy. It was found that epitaxial growth of Hf is possible even at room temperature. GaN films varying in thickness from 0.6 to 1.8 μm were grown on c-plane sapphire, using ammonia as a precursor, to serve as substrates. Then the films were annealed in ammonia as the temperature was lowered in order to produce a N termination. Hf was then deposited on top of the GaN at temperatures between 20 and 730 °C, both with and without ammonia incident on the sample. Deposition of pure Hf at room temperature revealed an epitaxial two-dimensional (2D) (although with some three-dimensional character) RHEED pattern with sixfold symmetry. The surface is reconstructed with a (√2 × √2) R30° structure. We propose that the pattern is rotated 30° with respect to that of the substrate GaN because of a HfN interlayer between the GaN and Hf layers. When the films were annealed in vacuum up to 730 °C, the 2D pattern became more transmission like. If Hf was deposited at substrate temperatures of 350 °C and higher, a diffraction pattern corresponding to that of a polycrystalline material was observed.