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 degrees-off). A flat, clean and bistepped Si surface is achieved during the homoepitaxial growth on 6 degrees-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 degrees-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. (C) 2013 Elsevier B.V. All rights reserved.