Tunneling Spectroscopy in Degeneratep-Type Silicon

Tunneling in boron-doped $p$-type silicon metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) tunnel junctions has been studied at low temperatures by measuring the derivatives $\frac{\mathrm{dI}}{\mathrm{dV}}$ and $\frac{d^{2}I}{d{V}^2}$ of the current-voltage characteristics as functions of applied bias voltage $V$. The boron impurity concentration of the silicon crystals varied from 6.5 × ${10}^{18}$ to 2.3 × ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Junctions were prepared by evaporating metal contacts onto vacuum- or air-cleaved silicon surfaces. The general features of the tunneling conductance were found to be in qualitative agreement with existing theories of tunneling in semiconductors. Structure in the derivative data resulting from the interaction of tunneling electrons with silicon zone-center optical phonons and boron local-mode phonons has been observed. The optical-phonon line shapes in the most heavily doped MIS units are shown to compare well with the theoretical line shapes in which modifications in the bulk semiconductor states arising from electron—optical-phonon interactions in the semiconductor electrode have been included. The origin of the optical-phonon and local-mode-phonon structure in samples of lower doping is not fully understood.