Crystallization of free standing bulk GaN by HVPE

Gallium nitride was crystallized on 2 inch MOVPE GaN/sapphire substrates by Hydride Vapor Phase Epitaxy. A stable growth has been achieved in long duration (> 10 h) processes at growth rates bigger than 100 mu m/h. As a result, entirely transparent and colorless bulk crystals with thickness exceeding 2 mm were obtained. The cracks in the thick HVPE GaN layers deposited on the MOVPE GaN/sapphire substrates can appear especially during cooling of the system after crystallization. It is shown that the formation of cracks at cooling is dependent on the gradients in the layer thickness The relaxation of strains in the resulting crystal coupled to the substrate leads to the self separation of GaN from sapphire. (At present 30 x 30 x 2 mm free standing bulk GaN crystals are obtained). The GaN crystals are characterized by defect selective etching (DSE) and X-ray diffraction. The density of threading dislocations (measured by DSE of (0001) surface) decreases with the thickness of the HVPE layer and becomes lower than 10(7) cm(-2) in the layers thicker than app. 1 mm. The X-ray rocking curves for (0002) reflection (slit 0.5 x 0.1 mm) are in the range of 80-95 arcsec. However, larger scans reveal bending of crystallographic {0001} planes. The behavior of these "deformed" free standing crystals used as substrates for HVPE re-growth is also analyzed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.