Tunneling in artificial ${\mathrm{Al}}_2$${\mathrm{O}}_3$ tunnel barriers and ${\mathrm{Al}}_2$${\mathrm{O}}_3$-metal multilayers

We report on the material and electron-tunneling properties of thin ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ films deposited onto room-temperature substrates by rf magnetron sputtering of a pressed aluminum oxide target in a pure argon atmosphere. X-ray photoelectron spectroscopy shows the films to be composed of >99% ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$. The electrical properties have been investigated by tunneling in junctions of the form: Cu/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$/counterelectrodes with counterelectrodes of Cu, Pb, and Pb-Bi. The Cu/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ bilayers were deposited in situ with ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ thicknesses ranging from 8 to 20 Å. These junctions have been found to exhibit excellent tunneling characteristics including low zero-bias conduction (typically below 1% at 4.2 K) and large effective barrier heights (typically above 1 eV). We have observed full, clean superconducting gap structure, and strong, clear phonon structure for junctions with Pb-Bi counterelectrodes. The expected exponential rise of junction resistance with increasing barrier thickness was observed, giving an average barrier height of 1.65 eV and an effective tunneling length of 0.82 Å. We observe a steplike increase in junction yield as barrier thickness exceeds 12–15 Å, the dependence of which has been successfully modeled as a statistical buildup of barrier molecules on the base-electrode surface. We have also investigated the tunneling characteristics of metal/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$/intermediate metal/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$/metal multilayer junctions wherein the properties of the intermediate metal films can be studied and have provided a confirmation of single-electron tunneling effects.