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The incorporation of Er in crystalline Si during MBE on Si(100) at 400-700/spl deg/C in vacuum (8/spl times/10/sup -9/ torr) has been investigated by SIMS, RBS, TEM and luminescence spectroscopy. The Er concentration was varied from 8/spl times/10/sup 18/ to 4/spl times/10/sup 19/ cm/sup -3/. The co-doping with Er, O/sub 2/ and C was happening during the MBE growth, but the doping levels of the impurities-co-activators of luminescence were lower than the Er concentration. Unintended co-doping of epitaxial layers with O/sub 2/ and C resulted in the formation of Er-O- and Er-C-related optically active centers with narrow linewidths. The simultaneous formation of these sharp lines has been observed in the samples grown at substrate temperatures /spl les/500/spl deg/C. At the higher temperatures, wide SiO/sub 2/-like Er-related lines dominated in the photoluminescence (PL) spectra. The maximum of Er-related PL intensity was observed at an Er concentration of /spl sim/2/spl times/10/sup 19/ cm/sup -3/. In this sample, plate-like ErSi/sub 2/ precipitates distributed over the layer and sphere-like Er precipitates located at the interface were observed. A study of the temperature dependence of the Er-related PL intensity revealed two quenching mechanisms with activation energies of 22 and 170 meV.