This work investigates defect formation and evolution associated with the deposition of GaP layers on precisely oriented Si(0 01) substrates. The GaP layers were grown with thicknesses ranging from similar to 37 nm to similar to 2 mu m at a growth rate of 0.52 mu m/hr using molecular beam epitaxy (MBE). The crystallinity of thin (37-nm) MBE-grown GaP layers was also compared with thin GaP layers grown by migration-enhanced epitaxy (MEE). The MBE growth procedure was shown to postpone relaxation of the epitaxial GaP layers up to a thickness of similar to 250 nm. Detailed analysis of high-resolution X-ray diffraction patterns and comparison with cross-sectional transmission electron micrographs clarified the defect formation mechanism. Thin GaP layers showed very low defect densities except for anti-phase boundaries, whereas substantial threading defects predominated in the thicker, noticeably relaxed structures.