The initiation of GaN growth by molecular beam epitaxy on GaN composite substrates

We report on an investigation of the initial stages of growth by molecular beam epitaxy (MBE) of GaN layers on composite substrates consisting of hydride vapour phase epitaxial (HVPE) GaN films on SiC substrates. In situ reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) were used to monitor the surface, reconstruction of the GaN and to measure the structure of the grown surface, respectively. A 1 x 3 streaky RHEED pattern was observed on the HVPE GaN layers from room temperatures up to 750 degrees C. This corresponds to a (root 3 x root 3)R30 degrees reconstruction of the GaN surface. At a substrate temperature of 600-650 degrees C, growth by MBE of GaN layers on the HVPE GaN substrate leads to a change of the RHEED pattern from 1 x 3 to hexagonal 2 x 2 during the first few minutes of epitaxy. RHEED oscillations were observed at the beginning of growth at 400 degrees C, MBE growth at higher temperatures similar to 730-750 degrees C significantly enhances the flatness of the surface of the GaN layer. In this case, RHEED patterns were more intense and streaky than those seen for GaN layers grown at lower temperatures. The surface of the MBE GaN layers grown at high temperatures were atomically flat within the sensitivity of our AFM system. (C) 1999 Elsevier Science B.V. All rights reserved.