Abstract
Ellipsometry on a series of Co-Si interfaces, for which the equivalent polycrystalline Co deposits differ by only a few monolayers, demonstrates that a large decrease in refractive index of the glassy surface layer occurs quite sharply at Co deposits slightly less than the amount necessary to effect the semiconductor-metal transition observed with dc temperature coefficient-of-resistance measurements at 100 K. This observed behavior of the optical constants of the glassy layer can be interpreted by using a simple model for the optical-frequency polarizability of a localized electron state with a phenomenological localization parameter, which decreases with increasing Co deposit.