The III-nitride material system with band gap ranging from 0.7eV to 6.2eV has substantial potential to develop high-efficiency solar cells. The III-nitride materials are grown by MOCVD on a lattice mismatched sapphire substrate (0001). This paper presents the generation of extended crystalline defects and their spatial distribution in the GaN and In0.12Ga 0.88N layers as a function of In0.12Ga0.88N thickness. The material is characterized by photoluminescence, and the primary peak intensity is observed to increase with thickness, up to 200 nm, but the intensity diminishes with further increase in thickness. Additional photoluminescence peaks are observed for In0.12Ga0.88N thicknesses greater than 100 nm. These observations are attributed to extended crystalline defects and are characterized by high resolution x-ray diffraction. A detailed analysis of these extended crystalline defects is presented based on rocking curves, symmetric and asymmetric reciprocal space maps. The crystalline defects are unavoidable during epitaxial growth, but knowledge of their generation process yields better control over them.