Light emission at room temperature from Si/CaF2 multilayers

We have synthesized, by Molecular Beam Epitaxy (MBE), Si/CaF2 multilayers which are optically active at room temperature. The photoluminescence spectra present a blue shift far decreasing Si layer thickness, in analogy to those obtained from porous silicon when the porosity is increased. We find a critical dependence of the photoluminescence efficiency on the thickness of the Si layers. We compare the experimental results to ab initio calculations of the band structure of Si/CaF2 multilayers which predict the band gap opening and the presence of confined and interface states leading to a quasi-direct band gap.