We have studied growth and self-separation of bulk GaN on c-oriented Al2O3 using low temperature (LT) GaN buffer layers. By studying the X-ray diffraction (XRD) signature for the asymmetric and symmetric reflections versus the LT-GaN thickness and V/III precursor ratio, we observe that the peak width of the reflections is minimized using a LT buffer thickness of similar to 100-300 nm. It was observed that the VAIL precursor ratio has a strong influence on the morphology. In order to obtain a smooth morphology, the Will precursor ratio has to be more than 17 during the growth of the buffer layer. By using an optimized LT buffer layer for growth of a 20 mu m thick GaN layer, we obtain a XRD peak with a full width at half maximum of similar to 400 and similar to 250 arcs for (002) and (105) reflection planes, respectively, and with a dark pit density of similar to 2.2 x 10(8) cm(-2). For layers thicker than 1 mm, the GaN was spontaneously separated and by utilizing this process, thick free freestanding 2 '' GaN substrates were manufactured. (C) 2012 Elsevier B.V. All rights reserved.