Tunneling in artificial Al2O3 tunnel barriers and Al2O3-metal multilayers.

Abstract

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}$${\mathrm{O}}_{3}$ thicknesses ranging from 8 to 20 A\r{}. These junctions have been found to exhibit excellent tunneling characteristics including low zero-bias conduction (typically below 1% at 4.2 K) and large effective barrier heights (typically above 1 eV). We have observed full, clean superconducting gap structure, and strong, clear phonon structure for junctions with Pb-Bi counterelectrodes. The expected exponential rise of junction resistance with increasing barrier thickness was observed, giving an average barrier height of 1.65 eV and an effective tunneling length of 0.82 A\r{}. We observe a steplike increase in junction yield as barrier thickness exceeds 12--15 A\r{}, the dependence of which has been successfully modeled as a statistical buildup of barrier molecules on the base-electrode surface. We have also investigated the tunneling characteristics of metal/${\mathrm{Al}}_{2}$${\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.