We report on the association of Ultra High Vaccum Chemical Vapor Deposition (UHVCVD) and Molecular Beam Epitaxy (MBE) to achieve III-V (GaP) integration on Si/Si(100) substrates. We first demonstrate that a very good flatness (0.3 nm) can be obtained when growing directly GaP on a chemically prepared Si substrate. X-ray diffraction pole figure however demonstrates that a residual amount of micro-twins originating from the hetero-interface still remains. Silicon homoepitaxial buffer layer is then optimized in the UHVCVD chamber on different Si substrates misorientation (+/-0.15-6°-off) . A flat, clean and bistepped Si surface is achieved during the homoepitaxial growth on 6°-off silicon substrates. Samples are then transferred under UHV conditions to the MBE chamber to perform GaP overgrowth. Keeping the same III-V overgrowth conditions, influence of silicon homoepitaxial buffer layer on micro-twins generation is determined quantitatively using Synchrotron X-Ray Diffraction. We finally demonstrate that growing a flat, clean and bistepped silicon buffer layer on a 6°-off substrate, and transferring it under UHV to the MBE chamber for GaP overgrowth reduces significantly the amount of anisotropic defects generated in the GaP epilayers.