RF sputter-deposited aluminum-oxide films as high quality artificial tunnel barriers

We report on the results of fabricating artificial tunnel barriers by the simple method of direct sputtering of an aluminum-oxide (Al2O3) target. These barriers have been studied by preparing tunnel junctions of the form: Cu/AlOx/C.E., with aluminum-oxide layers ranging in thickness from 8 to 20 Å and counter-electrodes (C.E.) of Cu, Pb and PbBi. We have found that barriers with exceptionally large effective barrier heights, in excess of 2eV, and low zero-bias conduction, less than 0.4% leakage, can be reliably obtained by this method. We discuss the fabrication of these barriers and the systematics of both the effective (WKB) barrier shape, and superconducting and normal-state tunneling characteristics as a function of the measured barrier film thickness. X-ray photoelectron spectroscopy (XPS) studies on sputter-deposited aluminum-oxide films indicate the single composition, namely Al2O3. This method should prove universally applicable for depositing very high quality tunnel barriers.