Electrical characteristics of metal/Si-Ge contacts

Abstract

Measurements and theoretical calculations have been made of the electrical properties of metal contacts to a polycrystalline Si-Ge alloy (80 at.% Si) for a wide variety of conditions. Thin-film contacts of W, Pb, and In have been formed on Si-Ge phosphorus doped in the range 2×1018−1×1020 cm−3. For P concentrations near 2×1019 cm−3 we have produced samples having several interfacial oxide thicknesses. We have heat treated W contacts to this material to simulate elevated-temperature aging effects of W thin-film contacts on thermopiles. The differential conductance for all these various contacts has been measured as a function of bias voltage for temperatures from 1.1 to 300 K. Calculations of the differential conductance of these contacts have been performed using a metal-insulator-semiconductor model that accounts for tunneling, thermionic field emission, and thermionic emission. The calculationg generally cover the ranges of temperature, bias voltage, doping level, and oxide thickness used in the experiments. Graphical summaries of these calculations are presented which include doping dependence of the zero-bias conductance ratio (300−4 K), the conductance values at several temperatures, and the location of the differential conductance minimum. Comparison of theory with experiment leads to the conclusion that the general features of the conductance characteristics can be predicted from the theory for certain ranges of oxide thickness and dopant concentration. Agreement appears to be best for thin oxides and P concentrations near 2×1019 cm−3, with both higher- and lower-doped contacts behaving qualitatively as if their effective doping levels were nearer this value. This effect has interpreted in terms of phosphorus depletion or enhancement mechanisms occurring in the Si-Ge depletion region during junction fabrication. When W contacts are hear treated after fabrication, agreement with theory improves. This believed to exhibit typical superconducting conductances at 1.1 K, thus showing that tunneling is, in fact, a major transport mechanism in Si-Ge/metal junctions.