High quality GaN was grown on 200 mm Si (111) substrates by using AlN and 3 step-graded AlxGa1-xN as the buffer layer in a metalorganic chemical vapor deposition system. We have investigated the influence of NH3 preflow time on the threading dislocation density (TDD) of AlN, AlGaN buffer layers and GaN layers. It was observed that the compressive stress introduced into the buffer layer and GaN is dependent on the nitridation time. The lowest TDD for GaN obtained in our samples was similar to 1 x 10(9) cm(-2) for screw type and 3.2 x 10(9) cm(-2) for edge type dislocations, as obtained from atomic force microscopy and further confirmed by high resolution X-ray diffraction analysis. The threading dislocations generated in the first buffer layer (A1N) during its nucleation are found to influence the TDD in the subsequent layers. Samples without an intentional nitridation step exhibit higher TDD compared to the samples with optimal nitridation time. Longer nitridation time also leads to poor crystalline quality likely because of amorphous SiNx formation at the interface.