A Si substrate purposely grown for Si-based optoelectronic applications is described in this letter. The structure contains a built-in Si/SiO2 Bragg reflector which is prepared by multiple separation-by-implanted-oxygen technique, where in situ low energy oxygen ion implantation is performed on molecular beam deposited Si to create alternating Si/SiO2 epitaxial bilayers. The quality of the top Si layer was confirmed to be epitaxy-ready after cross-sectional transmission electron microscopy. Maximum reflectance at near normal incidence was over 90% for bilayer periods of 4 to 5.