We have observed, in the tunneling characteristics of InAs-oxide-Pb junctions, a structure corresponding to a step decrease in conductance in both bias polarities at the emission threshold for LO phonons in InAs. This unusual phonon structure has been predicted by Appelbaum and Brinkman and by Caroli et al. for electron-phonon interactions confined to an electrode. When the junction conductance is decomposed into an even part $G^E$ and an odd part $G^O$, a dip in $G^O$ as well as a step decrease in $G^E$ are observed at the LO-phonon energy. The dependence of this phonon structure on the electron density, $n$, in InAs is characterized by the effect of $n$ on $\Delta G^E$, the change in $G^E$, and $\Delta G^O$, the change in $G^O$ at the phonon energy. It is found that, for $n$ varying from 2.1×${10}^{16}$/${\mathrm{cm}}^3$ to 6.3×${10}^{18}$/${\mathrm{cm}}^3$, $\Delta G^O$ remains independent of $n$ and $\Delta G^E$ decreases with increasing $n$, following $n^{\frac{1}{3}}$.

• Received 15 May 1975

DOI:https://doi.org/10.1103/PhysRevB.12.5853