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Shot noise in NbN SIS junctions suitable for THz radiation detection

Pieter Dieleman, Herman van de Stadt, T. M. Klapwijk, Matthias Schicke, K. H. Gundlach

Published Jan 1, 1997• 1 citation
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Abstract

We study the behavior of NbN SIS junctions as radiation detectors with emphasis on the shot noise generated at voltages below the gap voltage. The intrinsically large subgap current of NbN junctions is carried by pinholes with a conduction attributed to multiple Andreev reflection, leading to transported charges q much greater than e. Using this charge enhancement mechanism we explain the junction shot noise characteristics in the unpumped case as well as in the pumped case. The measured mixer noise temperature in the pumped case is more than twice that calculated with standard Tucker theory. The measured double side band noise temperatures are 1450 K at 600 GHz and 2800 K at 950 GHz.

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Global Symposium on Millimeter-Waves•Apr 1, 2008•3 citations
We study the behavior of NbN SIS junctions as radiation detectors with emphasis on the shot noise generated at voltages below the gap voltage. The intrinsically large subgap current of NbN junctions is carried by pinholes with a conduction attributed to multiple Andreev reflection, leading to transported charges q much greater than e. Using this charge enhancement mechanism we explain the junction shot noise characteristics in the unpumped case as well as in the pumped case. The measured mixer noise temperature in the pumped case is more than twice that calculated with standard Tucker theory. The measured double side band noise temperatures are 1450 K at 600 GHz and 2800 K at 950 GHz.
We study the behavior of NbN SIS junctions as radiation detectors with emphasis on the shotn.oise generated at voltages below the gap voltage. The intrinsically large subgap current of NbN junctions is carried by pinholes with a conduction attributed to multiple Andreev reflection, leading to transported charges q >> e. Using this charge enhancement mechanism we explain the junction shotnoise characteristics in the unpumped case as well as in the pumped case. The measured mixer noise temperature in the pumped case is more than twice that calculated with standard Tucker theory. Measured double side band noise temperatures are 1450 K and 3300 K at 600 and 850 GHz respectively. Main limitations to the receiver sensitivity are the loss in the aluminum circuit and the subgap current induced shotnoise.
The electrical and shot-noise characteristics of high-current-density niobium superconductor–insulator–superconductor (SIS) tunnel junctions suitable for heterodyne detection of THz radiation are studied. It is found that a significant part of the current at voltages V<2Δ/e is carried via barrier defects (pinholes). Due to the transport mechanism in these pinholes, the shot noise is considerably increased. The data presented clarify an often-observed discrepancy between measured and expected heterodyne mixer noise temperatures and predict a 25% increase in the receiver noise temperature of Nb SIS heterodyne detectors at 1 THz.
With sensitivity approaching the quantum limit, superconductor-insulator-superconductor (SIS) mixers play an important role in radio astronomy and atmospheric science at millimeter and submillimeter wavelengths. As one of the intrinsic noise sources in superconducting tunneling junctions, shot noise is still not well understood, particularly for those of relatively high energy gap (e.g., NbN/AlN/NbN). In this paper, we mainly study the multiple Andreev reflection (MAR) enhanced shot noise of two different NbN SIS junctions (i.e., junction array and long junction) as well as their temperature dependence. Barrier strength Z is taken into consideration for theoretical analysis of effective charge of the MAR effect based on the Blonder-Tinkham-Klapwijk (BTK) theory with the Andreev clusters method. It has been found that the effective charge of the MAR effect is inversely proportional to temperature, the barrier strength Z and transparency T are proportional to temperature. Detailed measurement results and analysis will be presented.
We have theoretically and experimentally investigated submillimeter-wave receiver performances employing NbN/ALN/NbN tunnel junctions with high current density and high energy. gap. The receiver noise based on parameters for our typical NbN SIS junction were analyzed using Tucker's quantum theory of mixing, which showed that DSB receiver noise temperatures of below 3hv/K B can be achieved by using the NbN SIS junctions for frequencies up to 2 THz. An experimental receiver has been designed and tested for the 1 THz band. The mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, a NbN twin-slot antenna, and NbN tuning circuits. The prepared NbN/A1N/NbN junction size was about 0.5 !mut), and the current density was about 30 kA/cm 2 (o)CJRN = 11 at 1 THz). Although the junctions showed good dc I-V characteristics with a small sub-gap leakage current, the receiver noise temperature measured by the standard Y-factor method was about 2700 K(DSB) at 761 GHz, which is much larger than the theoretical prediction. The relative high noise performance may be caused by the large RF losses in the tuning circuit of polycristalline NbN on the SiO.
We measured shot noise and submillimeter-wave response in a superconducting NbN tunnel junction that had a subharmonic gap structure on the current-voltage (I-V) curve. We found that the observed effective charge, defined from the noise-current ratio, tends to a steplike function of voltage. In the presence of submillimeter-wave radiation of frequency v, novel step structures spaced by hv/2e below and above the half-gap voltage clearly appeared on the I-V curve, overlapping the ordinary photon-assisted tunneling steps spaced by hv/e. Observation of these features provides clear evidence that coherent multiple Andreev reflection processes occur in the NbN tunnel junction with low barrier transparency.
Superconducting SIS (Superconductor-Insulator-Superconductor) tunnel junctions are promising application prospect in the incoherent detection of astronomy observation. In this paper, the direct detection performances of Nb and NbN based superconducting SIS junctions, which cover about 250 GHz and 140 GHz centered at 600 GHz respectively, are characterized at a bath temperature of 4.2 K. The experiments show that Nb and NbN based SIS junctions have a current shot noise of 2.5×10-12and 3.1×10-12A/radicHz, a current responsivity of 303 and 187 A/W (corresponding to the quantum efficiency of 0.67 and 0.43) respectively. Thus their respective NEPs are 8.4×10-15and 1.7×10-14W/√Hz.