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Superconductor Science and Technology

Terahertz direct-detection behavior of niobium nitride superconducting tunnel junctions above liquid helium temperature

Zheng Wang, Dong Liu, Shao-Liang Li, Jing Li, Sheng‐Cai Shi

Published May 23, 2014• 1 citation
Publisher

Abstract

Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 x 10(-13) W/root Hz around 5 K and 1.5 x 10(-12) W/root Hz at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.

DOI
10.1088/0953-2048/27/7/075003

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Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 x 10(-13) W/root Hz around 5 K and 1.5 x 10(-12) W/root Hz at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.
Terahertz direct detectors based on superconducting tunnel junctions (STJs) have the merits of short response time, large dynamic range and relatively high operating temperature, providing a good choice for terahertz detection. Compared with commonly used niobium STJs, niobium nitride (NbN) ones have a high transition temperature close to 16K. Hence they might be useful for terahertz detection operating at temperatures well above the LHe temperature of 4.2K. In this paper, we investigate the properties of NbN STJ detectors at different bath temperatures. Detailed experiment results are given.
We have developed submillimeter-wave direct detectors employing niobium-based superconducting tunnel junctions (STJs), with broadband spectral response, high sensitivity and imaging capability. Spectral response peaked at 650 GHz with a fractional bandwidth of 14 percent, which fits one of the important atmospheric windows for an astronomical project. We also confirmed the linearity of the detector response in the dynamic range greater than 10/sup 6/. The measured coupling efficiency was about 0.2, which was lower than the expected value of 0.5. The measured noise current of the detector was 10 fA//spl radic/Hz, which agreed with the shot noise from the residual leakage current of 100 pA at 0.3 K. Resultant noise equivalent power (NEP) is 1.6/spl times/10/sup -16/ W//spl radic/Hz, that is less than the background photon fluctuation limit for ground-based submillimeter-wave observations.
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.
In this paper, we report on the terahertz (THz) direct detection performance of two superconducting niobium nitride (NbN) hot-electron bolometers (HEBs) of different micro-bridge sizes. At the optimum bias point, the two HEB devices show similar thermal conductance (~80nW/K). The measured responsivity coincides well with the electrical responsivity calculated from the measured current-voltage (IV) curves at different bath temperatures. The noise equivalent power (NEP) of the two HEB devices is found to be about 10-12W/√Hz.
Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) with thermal, microwave (MW) and THz biasing have been studied at 0.65 THz and 4.2 K in detail. The current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the MW and THz biasing have about one order higher current responsivity than that of the thermal one. With the MW biasing, an NEP at the order of pW/√Hz is obtained by choosing the MW frequency and power appropriately.
Terahertz (THz) direct detectors based on superconducting niobium nitride hot electron bolometers with thermal, microwave and THz biasing methods have been studied at a frequency of 0.65 THz and at an operating temperature of 4.2 K. A low noise amplifier at room temperature is used to read out the current change of the detectors modulated by the chopping frequency at 732 Hz. The noise spectral density, current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the microwave and THz biasing methods have about one order higher current responsivity than that of the thermal one. With the microwave biasing, an optical NEP at the order of pW/√Hz which shows no deterioration is obtained by choosing the microwave frequency and power appropriately.