Optical properties ofin situEr-doped Si epilayers grown by molecular beam epitaxy at low temperatures (<450 °C), using oxygen and fluorine as codopants, are studied using photoluminescence (PL) and electroluminescence (EL) spectroscopies. Sharp and intense Er-related PL is observed at low temperatures from the as-grown Si epilayers with Er concentrations up to5×1019 cm−3.The structure of the dominant optically active Er centers is shown to be dependent of the codopants used and can essentially be modified by postgrowth thermal annealing. The chemical nature of the codopants as well as postgrowth treatments have only a minor effect on the thermal quenching of Er-related emissions. Thermal quenching of the Er-related PL and EL is shown to occur with a similar activation energy suggesting the same quenching mechanism may be involved for both processes. The observed higher EL efficiency at elevated temperatures is tentatively attributed to the higher concentration of excited Er ions under impact ionization conditions (EL).