The effect of post-growth annealing of epitaxial gallium phosphide grown on silicon substrates using gas-source molecular-beam epitaxy is described. The epitaxial layers were grown at substrate temperatures ranging from 250 to 550 degrees C. After optimizing the growth temperature, the prepared films were thermally annealed. Two thermal annealing methods are compared: annealing at a constant temperature and step-graded annealing, in which the temperature is raised by a constant rate per unit time The effect of the thermal annealing on the crystal structure was studied by characterizing the epilayer using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM), It was found that after thermal annealing, the epilayer exhibited a reconstructed RHEED pattern, and its quality was much improved. (C) 2015 Elsevier B.V. All rights reserved.