In this study, 1.61-mu m-thick GaN/graded AlGaN/AlN epilayers were prepared on Si substrates by using metalorganic chemical vapor deposition. To determine whether the insertion of a SixNy interlayer enhances the epitaxial quality of the GaN layer, various numbers of SixNy interlayers were inserted at different locations, and the effects were investigated. In addition to analyzing crystal quality, epilayer stress was investigated in detail by performing Raman, photoluminescence, and curvature measurements. Inserting SixNy into the AlGaN layer resulted in a deterioration in epitaxial quality and the appearance of serious surface cracks because of the concentration of Al atoms near pits, which enabled new and bigger centers of defects to form. A crack-free surface and relatively high crystal quality were achieved by inserting a SixNy interlayer into a GaN epilayer; the resultant improvement in crystal quality was attributed to the bending and annihilation of dislocations. For a sample in which SixNy was not inserted, Raman and photoluminescence measurements revealed that the stresses were 0.535 and 0.741 GPa, respectively. The addition of a single SixNy interlayer into the GaN epilayer significantly reduced these stresses to 0.279 and 0.556 GPa, respectively. (C) 2014 Elsevier By. All rights reserved,