Metalorganic chemical vapor deposition (MOCVD) of a crack-free, mirror surface of GaN on 150-mm-diameter (111) Si substrate was performed using a horizontal MOCVD system. We used the combination of an AlGaN/AlN nucleation layer with an AlN/GaN strained superlattice structure (SLS) for strain control. A good mirror surface morphology was obtained over the entire GaN surface. Transmission electron microscopy (TEM) showed that screw dislocations were terminated at the interface of the GaN top layer and SLS. A pit density of 4 x 10(9) cm(-2) was determined by atomic force microscopy, the mean thickness of the GaN top layer was approximately 0.4 mu m, and the uniformity (1 sigma) was 4.37%. Asymmetrical reciprocal-lattice space mapping (RSM) measurement and TEM observation showed that the GaN film was fully relaxed. Relaxation occurs at both the interface of the SLS and AlN buffer layer and the interface of the GaN top layer and SLS. (c) 2006 Elsevier B.V. All rights reserved.