OPTICAL-ABSORPTION EVIDENCE OF QUANTUM CONFINEMENT IN SI CAF2 MULTILAYERS GROWN BY MOLECULAR-BEAM EPITAXY/

We have investigated the physical properties of nanocrystalline Si/CaF 2 multilayers grown by molecular beam epitaxy, which exhibit efficient visible luminescence at room temperature. X-ray diffraction under gra zing incidence as well as transmission electron microscopy demonstrate the periodicity of the multilayers. Extended X-ray absorption fine st ructure measurements show that the dimensions of the Si grains within the Si layers do not exceed 1.5 nm. We report on the optical absorptio n coefficient deduced from transmission measurements performed on samp les deposited on CaF2 substrates. The resulting optical pseudogap pres ents 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.