Inelastic Electron Tunneling in Al-Al-Oxide-Metal Systems

Abstract

A systematic study of inelastic electron tunneling in Al-Al-oxide-metal thin-film junctions in the bias range 30-500 mV has been made. Most measurements were performed on twin junctions made by evaporating two different metal films across the same oxidized Al base film. The experimental results of this paper can be divided into three groups. The first of these is a reidentification of some vibrational bands that appear in the tunneling spectra of Al-oxide systems. A prominent peak that appears between 115 and 120 mV in these spectra has been identified as being due to a vibrational mode of an alumina hydrate. In addition, a new OH mode has been found at about 75 mV in these systems. The experimental results that constitute the second group are the effects on the tunneling spectra of Al-Al-oxide systems due to the metal used as the top electrode. By studying the twin junctions described above, it has been found that the intensity of vibrational bands due to molecular impurities relative to the intensity of vibrational bands due to oxide modes is correlated with the ionic radius of the metal used as the top electrode. This correlation is interpreted as being caused by a size-dependent penetration of the top electrode into the oxide layer of the junction. The last group of experimental results involves changes in tunneling spectra of Al-Pb junctions produced by applying a DC potential across the samples at room temperature. These changes are interpreted as being due to the movement of dissolved gases in the Pb electrode of the junctions.