Nucleation of GaN on sapphire substrates at intermediate temperatures by Hydride Vapor Phase Epitaxy

A novel approach to deposit GaN layers directly on a sapphire substrate by Hydride Vapor Phase Epitaxy is presented. The two-step deposition process includes the growth of GaN nucleation layers at intermediate temperatures in the range of 750-900 degrees C and subsequent high-temperature overgrowth at about 1040 degrees C. Closed and non-closed nucleation layers with a thickness of up to 2 mu m were produced and characterized by scanning and transmission electron microscopy, micro-Raman spectroscopy and X-ray diffraction. A growth temperature of 780 degrees C is found to be optimal with respect to density and size distribution of nucleation islands. Raman measurements performed on the nucleation layers reveal nearly zero residual stress indicating effective stress relaxation on cooling down from growth temperature. The results of first overgrowth experiments demonstrate the possibility to grow 10 mu m thick, crack-free GaN layers of high crystalline quality on the nucleation layers. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)