Micro-Raman scattering profiling studies on HVPE-grown free-standing GaN

Free‐standing GaN of ∼330 μm thickness with low defect density was prepared by hydride vapor‐phase epitaxy (HVPE) on sapphire in a vertical atmospheric‐pressure reactor and a subsequent laser‐induced lift‐off process. The structural and optical properties of the material were assessed by various characterization techniques, like X‐ray diffraction, photo‐ and cathodoluminescence, spectroscopic ellipsometry, positron annihilation spectroscopy, and transmission electron microscopy. Here, we focus on μ‐Raman scattering profiling studies providing the vertical strain distribution and the evolution of the crystalline quality with increasing layer thickness. Profiles of the free‐carrier concentration are obtained from monitoring the LO‐phonon plasmon coupled mode. Comparative investigations are performed on the material before and after separation of the sapphire substrate. The GaN material presented here is well capable of serving as a substrate for further homoepitaxial strain‐relaxed and crack‐free growth needed for fabrication of high‐quality III‐nitride device heterostructures. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)