Interaction of niobium counter electrodes with aluminum oxide and rare-earth oxide tunnel barriers

Abstract

With the use of rare-earth oxide and aluminum oxide barriers, low-leakage-current tunnel junctions can be made if the oxidation takes place in the presence of some humidity, and if the counter electrode is not niobium. It is proposed that when these oxides form in the presence of ${\mathrm{H}}_{2}$O, the pores of the oxides become sealed by surface O-H groups, by which process the oxide film becomes a good insulator and remains as such if the counter electrode is nonreactive. When niobium, which is a very reactive metal, is deposited as a counter electrode, the first arriving Nb atoms form NbO (metallic) with the O-H groups in the pores, thereby opening up the pores and leading to tunnel barriers with shorts. The proposed mechanism is supported by heats-of-decomposition arguments, previously reported inelastic electron tunneling spectra of Al-Al oxide-Pb, Ho-Ho oxide-Pb, and Er-Er oxide-Pb junctions as well as new in situ x-ray photoelectron spectroscopy studies made on evaporated yttrium films. When freshly deposited yttrium films were oxidized in 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$-Torr(UHV) pure oxygen, ${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$ formed and the spectrum showed only a single O $1s$ peak. Upon deposition of 5-\AA{} Nb, no interaction was detected between the yttrium oxide and niobium. If, however, the freshly deposited yttrium films were oxidized in air or water vapor, the O $1s$ signal showed a second, higher-energy peak representative of O-H groups. After the depositing of niobium on this yttrium oxide hydroxide, the first-arriving Nb atoms formed a NbO compound while the second O $1s$ peak representative of O-H groups disappeared from the spectrum.