Artificial oxide barriers for NbN tunnel junctions

Abstract

Superconducting tunnel junctions have been prepared with NbN-base electrodes, oxidized Al or Mg tunnel barriers, and NbN or Pb counterelectrodes. The tunnel barriers were formed either by thermal oxidation at room temperature or by subjecting the thin overlayers of Al or Mg to a low-energy ion beam in an argon-5% oxygen background. High-quality junctions with Pb counterelectrodes were produced by either method. However, for junctions with NbN counterelectrodes deposited at room temperature, the thermal oxidation resulted in shorts and the ion-beam oxidation resulted in low-leakage junction. X-ray photoelectron spectroscopy measurements of the NbN artificial-oxide bilayers showed that the ion-beam treatment increased the aluminum oxide thickness by the minimum detectable increment, approximately 0.2 nm, and increased the MgO thickness by 1 nm. The superconducting energy gap inferred for NbN counterelectrodes was typically half as large as the gap of the NbN base. Limitations on the gap values of NbN counterelectrodes grown on these barriers were established by measuring the energy gap of films as thin as 7.5 nm.