| Al oxide tunneling barriers were modified by exposure to a vapor of n-octadecyltrimethoxysilane which forms Self-Assembled Monolayers (SAMs). The dynamic conductance dI/dV of the modified Al oxide barrier between Al and Pb electrodes were measured at 4.2 K. Quasi linear conductance backgrounds are observed up to 200 mV with a strength that increases with increasing exposure time from 10 to 60 min. A saturation effect is observed around 200 mV. Beyond 200 mV dynamic conductance shows a parabolic behavior indicative of elastic tunneling from an asymmetric barrier. The linear background is attributed to inelastic tunneling from a continuum of excitations. Inelastic processes are additional tunneling channels in parallel with conventional elastic tunneling. A single excitation in the barrier region gives a step increase in the conductance curve, dI/dV, when the inelastic tunneling channel opens up at {dollar}Vsb{lcub}c{rcub}=hbaromegasb{lcub}c{rcub}/e.{dollar} We examine in detail the inelastic tunneling contribution due the vibrational dipole moment of a single molecule adsorbed on the surface of the barrier. This inelastic tunneling model is extended to multiple excitations. The inelastic tunneling process due to multiple excitations gives a linearly increasing background in tunneling conductance. Similar quasi-linear increase in the tunneling junction conductance of high T{dollar}sb{lcub}c{rcub}{dollar} superconductors as well as in several conventional superconductors indicates that such inelastic tunneling processes in the barrier region are common. |
