Iron-disilicide precipitates were formed by two different processes; (i) Fe+ ion implantation in Si and subsequent dry oxidation and (ii) Fe+ ion implantation at the interface of SiO2/Si and subsequent annealing. These samples were characterized by Rutherford backscattering spectrometry (RBS), transmission electron microscope (TEM), X-ray diffraction (XRD), photothermal deflection spectroscopy (PDS) and photoluminescence (PL). The implanted Fe concentrated at the SiO2/Si interface and formed beta -FeSi2 precipitates by both processes. However, part of the implanted Fe still remained in SiO2 and possibly formed Fe precipitates through the process (ii). It is notable that a clear interface has formed through the plc,cess (i) whereas a defect rich region was produced in Si near the Si/SiO2 interface by the process (ii). Weak optical absorption above 0.8 eV corresponding to the band gap of beta -FeSi2 was confirmed for both processes. It is noted that sharp PL was observed at 0.81 eV at 5 K for the sample (i) whereas broad PL was observed for the sample (ii). The origin of the FL was discussed from the: point of view of direct transition of strained beta -FeSi2, a shallow acceptor level in a non-intentionally doped p-type beta -FeSi2 and defect levels, in particular dislocations in Si.