The effects of growth temperature and reactor pressure on the microstructural and optical properties of InAlGaN quaternary epilayers grown on thick GaN buffer by metalorganic chemical vapor deposition were investigated. The addition of Al atoms at the optimized growth temperature of an InGaN layer resulted in deep level emission and in pyramidal shaped surface pits due to island growth caused by the low mobility of the deposited Al atoms in spite of the reduction of in-plane misfit strain and full width at half maximum from high-resolution X-ray diffraction. As the growth temperature increased and reactor pressure decreased, the broad deep level emission and the density of surface pits were greatly reduced and the band-edge emission of InAlGaN epilayers was intense. Surface pits originating from the vertex of threading dislocations under high misfit strain were greatly affected by lowering the growth temperature while the band-edge emission strongly depended on reactor pressure. Therefore, we found that the reduced reactor pressure and the increased growth temperature enhanced lateral growth and were effective in growing the InAlGaN epilayers of optically and micro structurally good quality. (C) 2004 Elsevier B.V. All rights reserved.