Integration time dependence of tunnel noise and energy resolution of superconducting tunnel junctions

Abstract

Superconducting tunnel junctions (STJs) offer the capability of photon counting with intrinsic energy resolving power. This resolving power is ultimately limited by the variance on the number of charge carriers generated in the photon absorption process (Fano limit) and the variance on the number of tunnelled charge carriers (tunnel limit). In addition, the performance can be degraded by electronic noise related to the read-out of the devices and by spatial nonuniformities in the response across the detector area.The present generation of our Ta-Al STJs is such that their spectroscopic performance in the UV/visible is limited by tunnel noise. This noise contribution is usually considered a device constant, (which may only vary marginally with bias conditions) and evaluated for infinite integration time. It can be shown, however, that the tunnel noise contribution is strongly time dependent and can be reduced by almost an order of magnitude for a properly chosen integration time.In this paper we present the experimental demonstration and numerical simulations of this time dependence on a series of Ta-Al STJs with different pulse decay times. The experimental results are in qualitative agreement with the simulations, but do not quite achieve the predicted performance. For the optimum configuration, an effective tunnel noise contribution of ~70% of the conventional tunnel limit is found.

Energy-dependent kinetic model of photon absorption by superconducting tunnel junctions

“…Over the last few years both the quality of the devices fabricated and the understanding of the relevant physical processes have improved greatly. Notable amongst the latter are the proximity effect which determines the properties of the bi-layer electrodes commonly used 3,4,5 , the various diffusion and loss mechanisms which limit the energy resolution of an STJ 6,7 and the details of the quasiparticle interactions through which the overall performance of the detector is modelled 8,9,10 . In this paper we describe a major advance in the treatment of quasiparticle dynamics, which is essential for modeling the latest generation of low gap, multi-tunneling STJs designed to operate at mK temperatures.…”