Transmission electron microscopy and atomic force microscopy analysis of Nb-Al-AlOx-Nb superconducting tunnel junction detectors

Abstract

The performance of photon detectors based on superconducting tunnel junctions are related to their current - voltage (I-V) curve characteristics and, ultimately, to the quality of the thin tunnel barriers (of order 1 nm) which separate the two superconducting thin films. Both the optimization of the spectroscopic performance of these detectors and the development of a reproducible and high yield fabrication route, require a better understanding of barrier quality and growth techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide valuable tools for the investigation of the barrier region and for the control of the quality of the different thin films and related interfaces. In this paper, the results of a TEM and AFM evaluation of Nb-Al-AlOx-Nb tunnel junctions are reported, together with their interpretation on the basis of the I-V curve performance at low temperature (T≥0.3 K). Thickness disuniformities of the Al plus AlOx overlayer and evidence of barrier defects have been found, which may place constraints on the spectroscopic performance of such devices. Through the use of TEM it has also been possible to confirm the epitaxial nature of the Nb base electrode. The junction counter electrode however appears to be polycrystalline, with a columnar morphology and an average grain width of 40 nm. The overall structure of the various layers may well place constraints on the tunneling characteristics of the device.

Asymmetric response of superconducting niobium-tunnel-junction x-ray detectors

“…It is well known that the polycrystalline niobium films have a columnar structure with grain sizes of 30-80 nm. 34,35 The columnar structure can cause phonon inelastic scattering within B in consideration of the ⌳. It is therefore reasonable that the effective lifetimes are shortened by the fast phonon decay enhanced at the grain boundaries.…”

Quantum Computing with Superconducting Electronics

“…We could with reason choose an extremely large value for R by using the BCS theory prediction that the leakage current goes exponentially to zero as T goes to zero. But we wish to use a much more conservative estimate of future junction quality and choose a resistance ratio of 10^ (ratios greater than 10** have been experimentally achieved [6]) multiplied by our intended normal resistance RN of 500 Q. Then the quantum dephasing time is thirty seconds.…”