Compensation independence of anomalous metal-semiconductor tunneling near the Mott transition

A temperature-dependent minimum in the conductance $\frac{\mathrm{dI}}{\mathrm{dV}}=G\left(V\right)\mathrm{}$ is observed in tunneling into compensated Si:Sb near the metallic transition which, unexpectedly, does not shift from $V=0\mathrm{}$ for substantial compensation $K=\frac{N_A}{N_D}$. A temperature dependence $G\left(0\right)\mathrm{}\propto \exp \left(\frac{-B}{T^{\frac{1}{4}}}\right)$ is observed below 4.2 K in nonmetallic samples, which indicates variable-range assisted tunneling into localized states. These results discriminate against a direct density-of-states interpretation of the conductance minimum. It is suggested that the effective electron-phonon coupling indicated by $G\left(0\right)\mathrm{}\propto \exp \left(\frac{-B}{T^{\frac{1}{4}}}\right)$ at $V=0\mathrm{}$ enables an electron injected with energy $\mathrm{eV}\gg \mathrm{kT}$, but still in the range of Anderson localization, to emit phonons and thus reach final states in the energy range $0<E<eV$, relative to the Fermi energy. One may thus expect $G\left(eV\right)\propto \int 0^{\mathrm{eV}}N\left(E\right)\mathrm{} \mathrm{dE}$, consistent with $G\left(0\right)\mathrm{}$ a minimum, independent of $K$, as observed.