Growth of Hf and HfN on GaN by molecular beam epitaxy

Hf and HfN thin films were grown on n-type GaN(<000(1)over bar>) by, molecu lar beam epitaxy using a custom built Hf electron beam source and an ammoni a leak. The films were characterized by reflection high-energy electron dif fraction (RHEED) and atomic force microscopy. It was found that epitaxial g rowth of Hf is possible even at room temperature. GaN films varying in thic kness from 0.6 to 1.8 mu m were grown on c-plane sapphire, using ammonia as a precursor, to serve as substrates. Then the films:were annealed in ammon ia 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 deg rees C, both with and without ammonia incident on the sample. Deposition of pure Hf at room: temperature revealed an epitaxial two-dimensional (2D) (a lthough with some three-dimensional character) RHEED pattern-with sixfold s ymmetry. The surface is reconstructed with a (root 2 x root 2) R30 degrees structure. We propose chat the pattern is rotated 30 degrees with respect t o that pf the substrate GaN because of a HfN interlayer between the GaN and Hf layers. When the films were annealed in vacuum up to 730 degrees C, the 2D pattern became more transmission like. If Hf was deposited at substrate temperatures of 350 degrees C and higher, a diffraction pattern correspond ing to that of a polycrystalline material was observed. (C) 1999 American V acuum Society. [S0734-2101(99)18804-3].