Observation of anomaly in tunneling through Si--Pb junctions

We have observed that the normalized tunnel conductance $\frac{{\left(\frac{dI}{dV}\right)}_S}{{\left(\frac{dI}{dV}\right)}_N}$ of Si-Pb junctions shows a strong dependence on the electron concentration of the bulk Si electrodes. The phonon-induced structure in the tunneling density of states of superconducting Pb is enhanced in the data with Pb positively biased and is suppressed in the data with Pb negatively biased. These enhancement and suppression effects increase with decreasing electron concentration in Si.