Optical absorption in an amorphous silicon superlattice grown by molecular beam epitaxy

We have fabricated amorphous silicon (a-Si) superlattices, comprised of thin layers of a-Si separated by even thinner layers of SiO2 through ultra-high-vacuum molecular beam epitaxy and an ultraviolet ozone process. From measurements of the specular reflection spectrum at near normal incidence, and the regular transmittance spectrum at normal incidence, we have determined the spectral dependence of the optical absorption coefficient corresponding to the a-Si layers within such a superlattice deposited on sapphire. We contrast these results with those corresponding to thin films of a-Si deposited through ultra-high-vacuum molecular beam epitaxy and find that the optical absorption edge of the a-Si layers within the a-Si/SiO2 superlattices is sharper and occurs at higher energies as compared with the thin films of a-Si. We conjecture that both quantum confinement and impurities may be responsible for this effect.