| Electron tunneling through very thin oxides (('(TURN)) 20 (ANGSTROM)) on degenerate silicon has been studied at room, liquid nitrogen and liquid helium temperatures. Metal, silicon and silicon dioxide vibrational modes have been identified by the method of inelastic tunneling spectroscopy at liquid helium temperatures. Variations of the tunneling conductance and location of the silicon phonon peaks with orientation and carrier type of the semiconductor, and the shape of the lead superconducting gap in the tunneling conductance, centered about zero bias, show that the current flow is predominantly by direct, band-to-band, momentum-conserving tunneling. Variation of the phonon mode intensities and the tunneling conductance background were found to correspond with differences in oxide growth method and with the type of metal electrode. Steam grown oxides were found to be more reproducible tunneling barriers than dry oxides. However, phonon assisted tunneling was found to be enhanced in the steam oxides relative to the dry oxides. The origin of the enhanced electron-phonon coupling potential in the steam oxides was found to be annealed out by RF plasma annealing. Aluminum and lead metal films were used as a cover electrodes. The reaction of aluminum with the grown oxide was found to change the structure of the oxide, causing a lower tunneling current with a given oxide thickness. Although a pronounced process enhancement of the tunneling spectra have been observed, the density of the responsible structural defects is probably too low for their vibrational modes to be detected without elaborate processing of the spectra. |
