LAYER-THICKNESS DEPENDENCE OF THE CONDUCTIVE PROPERTIES OF MO SI MULTILAYERS/

We report new measurements of the conductance and superconducting tran sition temperature of a set of Mo/Si multilayers, as a function of the metal layer thickness (from 7-85 Angstrom) for a constant semiconduct or layer thickness of 22 Angstrom. Unlike previously reported measurem ents, we do not observe oscillations in either the resistivity, resist ivity ratio, or the superconducting transition temperature with the me tal layer thickness. Rather, we observe monotonic variations in the tr ansport properties as the metal layer thickness increases. The sheet c onductance and its change between 10 and 300 K both vary approximately linearly with the metal layer thickness, above a threshold thickness. The conductance starts to grow with metal layer thickness at approxim ately 10 Angstrom, whereas the temperature coefficient of resistance c hanges sign at approximately 25 Angstrom, exhibiting a Mooij correlati on with a crossover resistivity of 125 mu Omega cm. The observed tempe rature dependence of the conductance rules out localization as the ori gin of the negative temperature coefficient of resistance. The conduct ance data are analyzed using a simple phenomenological model involving transport in interfacial and metallic layers, whose relative contribu tion to the conductance depends on the metal layer thickness and the t emperature. The model is applied to separate two competing contributio ns that determine the overall temperature dependence of the conductanc e. We attribute the differences between our measurements and previous measurements to differences in bulk metallic conductivities and interf ace morphologies, due to differences in thermal evaporation versus spu ttering fabrication processes. Our results show that the level and nat ure of disorder is an important ingredient in any theory that explains the cause of the observed oscillations.