Si/SiO2 superlattices were grown by molecular beam epitaxy using in situ oxidation by the rf-plasma source. Transmission electron microscopy shows high uniformity of the grown layers. Optical band gap obtained from the absorption measurement changes from 1.2 eV for thick samples to 2.3 eV for thin samples, indicating quantum confinement in the Si layers. Room temperature photoluminescence (PL) was observed in the spectral range 1.9-2.3 eV. Two different mechanisms of PL were found. In the Si layer thickness range 1-2 nm the luminescence arises from the quantum confined energy states in the ultrathin silicon layers. For Si layers with thickness more than 3 nm oxygen related defects are the main origin of PL.