Non-polar a-plane GaN layers were grown directly on r-plane sapphire substrates without a buffer layer. This study examined the effect of initial growth pressure on crystalline anisotropy and quality of a-plane GaN grown by metalorganic vapor phase epitaxy. The initial growth pressure was varied from 200 to 600 mbar for the nucleation layer and the second growth pressure was fixed at 100 mbar to obtain a smooth surface through lateral growth. X-ray diffraction, in situ monitoring of reflectance spectra, atomic force microscopy and photoluminescence showed that pressure during the growth of the nucleation layer played an important role in growing a-plane GaN with reduced crystalline anisotropy and improved crystalline quality. Crystalline anisotropy decreased and crystalline quality improved with increasing initial growth pressure. Increase in pressure during the growth of the nucleation layer increased 3D island growth and recovery time via a growth mode transition from 3D growth to quasi-2D lateral growth. (C) 2010 Elsevier B.V. All rights reserved.