A Si substrate purposely grown for Si‐based optoelectronic applications is described in this letter. The structure contains a built‐in Si/SiO2Bragg reflector which is prepared by multiple separation‐by‐implanted‐oxygen technique, whereinsitulow energy oxygen ion implantation is performed on molecular beam deposited Si to create alternating Si/SiO2epitaxial 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. ©1996 American Institute of Physics.