Optical absorption evidence of quantum confinement in Si/CaF2 multilayers grown by molecular beam epitaxy

We have investigated the physical properties of nanocrystalline Si/CaF2 multilayers grown by molecular beam epitaxy, which exhibit efficient visible luminescence at room temperature. X-ray diffraction under grazing incidence as well as transmission electron microscopy demonstrate the periodicity of the multilayers. Extended X-ray absorption fine structure measurements show that the dimensions of the Si grains within the Si layers do not exceed 1.5 nm. We report on the optical absorption coefficient deduced from transmission measurements performed on samples deposited on CaF2 substrates. The resulting optical pseudogap presents a large blue shift for decreasing Si layer thickness. The latter results are consistent with quantum confinement of carriers in the low-dimensional Si structures.