Multichannel Theory of Inelastic Electron Tunneling in Normal Metal-Insulator-Metal Junctions

A theory of inelastic electron tunneling due to phonons in normal metal-insulator-metal junctions is developed based on a multichannel tunneling model. The results are in good agreement with experiment and show that shifts in the bulk phonon energies observed by inelastic tunneling arise largely from kinematical factors. The observed harmonics are shown to arise due to two-phonon processes involving phonons at both sides of the barrier. A phonon polarization selection rule is shown to be operative in such tunneling processes. The model presented in this paper can readily be generalized to other types of inelastic excitations observed in tunnel junctions.