Role of initial nucleation in molecular‐beam epitaxy of GaN on lattice‐matched ZrB 2 substrates

Abstract

Nitrogen-polar GaN was grown on lattice-matched ZrB2 substrates by a two-step rf-MBE process with conventional low-temperature GaN (LT-GaN) used as the nucleation layer. For nucleation in slightly Ga-rich conditions, a streaky RHEED pattern was maintained from the very beginning and persisted throughout subsequent high-temperature growth. However, despite the streaky RHEED pattern the initial nucleation was evidently not 2D as inferred from AFM and x-ray results. For nucleation in slightly N-rich conditions, spots appeared in the RHEED in the early stages of growth but a streak pattern was restored with high-temperature GaN growth. Tradeoffs were identified in optimizing the LT-GaN nucleation process. Slightly Ga-rich nucleation conditions offered the best x-ray characteristics with ω-scan FWHM values of 400-500 and 720-800 arcsec for the (0002) and (104) reflections, respectively. Short-length atomic steps (up to ∼200 nm) were present on the surfaces of such layers, but the morphology showed a sub-grain structure. Slightly N-rich nucleation conditions yielded remarkably smooth films surfaces (rms 0.5 nm over 10 µm2) with uninterrupted atomic steps up to several µm long, but inferior x-ray ω-scans (∼50% larger FWHM values). (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)