Nb Superconducting Tunnel Junctions Research Articles

Terahertz high-sensitivity SIS mixer based on Nb–AlN–NbN hybrid superconducting tunnel junctions

The terahertz band, a unique segment of the electromagnetic spectrum, is crucial for observing the cold, dark universe and plays a pivotal role in cutting-edge scientific research, including the study of cosmic environments that support life and imaging black holes. High-sensitivity superconductor–insulator–superconductor (SIS) mixers are essential detectors for terahertz astronomical telescopes and interferometric arrays. Compared to the commonly used classical Nb/AlO x /Nb superconducting tunnel junction, the Nb/AlN/NbN hybrid superconducting tunnel junction has a higher energy gap voltage and can achieve a higher critical current density. This makes it particularly promising for the development of ultra-wideband, high-sensitivity coherent detectors or mixers in various scientific research fields. In this paper, we present a superconducting SIS mixer based on Nb/AlN/NbN parallel-connected twin junctions (PCTJ), which has a bandwidth extending up to 490 GHz–720 GHz. The best achieved double-sideband (DSB) noise temperature (sensitivity) is below three times the quantum noise level.

1-THz low-noise SIS mixer with a double-dipole antenna

A quasi-optical mixer containing two Nb/Al/AlOx/Nb superconducting tunnel junctions integrated into a NbTiN/SiO2/Al microstrip line is studied experimentally in the 800–1000 GHz frequency range. The mixer is developed as an optional front end of the heterodyne receiver operating in frequency band 3 or 4 and incorporated into the HIFI module of the Herschel space-borne telescope. The double-dipole antenna of the mixer is made of NbTiN and Al films; the quarter-wavelength reflector, of a Nb film. The mixer is optimized for the IF band of 4–8 GHz. The double-sideband noise temperature TRX measured at 935 GHz is 250 K at a mixer temperature of 2 K and an IF of 1.5 GHz. Within 850–1000 GHz, TRX remains below 350 K. The antenna pattern is symmetrical with a sidelobe level below −16 dB.

Giant shot-like voltage noise in high current density Nb–AlO x–Nb superconducting tunnel junctions

The flat voltage noise component of current biased, high-transparency Nb/AlO x /Nb superconducting tunnel junctions has been investigated at frequencies up to 70 kHz. Several aspects of the analyzed phenomena suggest the presence of current noise effects induced by the discreteness of the charge carriers. At subgap voltages, where excess currents occur, a behavior coherent with a multiple Andreev reflection-assisted transport through the tunnel barrier has been found. However, the measured charge values exceed any theoretical prediction.

Detection of heavy ions by a superconducting tunnel junction

Instantaneous switching to the voltage state of a superconducting tunnel junction (STJ) following a decrease in the critical current induced by a heavy ion beam was observed by introducing 40Ar particles with a kinetic energy of 95 MeV/nucleon into an Nb/Al–AlO x /Nb STJ. The particle detection by using this rapid switching of STJ could realize fast timing measurements at the level of ps.

Electron and phonon effects in superconducting tunnel detectors of x-radiation

Nb/Al/AlOx/Nb superconducting tunnel junctions were investigated in the role of x-ray detectors. Amplitude spectra of pulses arising upon irradiation of tunnel junctions of different sizes by 55Mn x-radiation were recorded at a temperature T=1.4 K. We also analyzed the temporal shape of the pulses. We considered the influence of diffuse motion of nonequilibrium quasiparticles, the inverse tunneling effect, and exchange of 2Δ phonons between electrodes, on the characteristics of the tunnel detectors. It is shown that phonon processes can bring about changes in the amplitude, duration, and polarity of the signal.

NbN–Nb–Al superconducting tunnel junctions as photon counting detectors

Asymmetric NbN–Nb–Al–AlOx-Al–Nb superconducting tunnel junctions have been investigated as photon counting detectors at x-ray and ultraviolet (UV)-visible wavelengths. The inclusion of a thin NbN passivation layer on the top electrode of the devices in place of the natural niobium oxides has reduced the quasiparticle loss rates, thereby enhancing the probability of multiple tunnel processes. As a consequence, the detector responsivity has increased from 900e−/eV, up to values in excess of 2000e−/eV in the temperature range 0.3–0.8 K. Such a responsivity level has allowed single photon counting performance at wavelengths as long as 700 nm and at operating temperatures as high as 830 mK. The devices show a linear response in the UV-visible range, while at 6 keV the expected nonlinearities in the energy response and moderate energy resolution similar to that found in Nb–Al junctions are observed.

Effect of the Critical Current Density and the Junction Size on the Leakage Current of Nb/Al–AlOx/Nb Superconducting Tunnel Junctions for Radiation Detection

Nb/Al–AlOx /Nb superconducting tunnel junctions (STJ's) designed for X-ray detection have been fabricated. The behavior of the low-temperature subgap leakage current, which severely limits the energy resolution obtained in such devices, is investigated. From trends in the dependence of the leakage currents on the critical current density and the size of the STJ, as well as from the low-temperature current-voltage characteristics, and an analysis of the base electrode surface morphology, it is concluded that physical defects in the barrier region are the most probable cause of the leakage currents. Suggestions are given for optimization of the device processing.

Characteristics of Nb/Al-AlOx/Al/Nb tunnel junction for X-ray of 5 to 15keV

This paper provides detailed response characteristics of a Nb/Al-AlOx/Al/Nb superconducting tunnel junction for X-ray of 5keV to 15keV investigated by using synchrotron radiation.

High quality Nb/Al–AlOx/Nb superconducting tunnel junctions for radiation detection

Nb/Al–AlOx/Nb tunnel junctions designed for x-ray detection have been manufactured and characterized at temperatures down to 0.35 K. At a bias voltage of 0.5 mV, a leakage current of the order of 0.1 pA/μm2, and a dynamic resistance exceeding 106 times the normal state resistance, were achieved in junctions with dimensions up to 200×200 μm2 and critical current densities of order 200 A/cm2. The combination of high critical current density, low leakage current, and large area represents an important step towards the development of superconducting tunnel junction detectors with improved spectroscopic performance.

Characterization of a 680–760 GHz SIS waveguide mixer

A heterodyne waveguide receiver employing 1 µm2 Nb superconducting tunnel junctions with on chip integrated tuning structures is characterized from 680–760 GHz. Several different types of integrated tuning structures are investigated. Lowest DSB receiver noise temperatures of 310 K at 709 GHz and 400 K at 720 GHz are measured. Analysis of the data shows that the loss of the superconducting tuning structures has a major influence on the overall receiver performance. A 25% reduction in receiver noise temperature is observed if the mixer is cooled from 4.2 K to 2 K, which we attribute to the reduced loss of the superconducting microstrip lines at lower temperatures. The calculated performance of the different tuning structures is shown to be in good agreement with the actual receiver noise measurements.

Effect of Al overlayer thickness on the leakage current of radiation detectors using Nb/Al-AlOx/Nb superconducting tunnel junctions

Nb/Al-AlOx/Nb superconducting tunnel junctions for the application of radiation detectors were fabricated using dc magnetron sputtering. The surface morphology of polycrystalline Nb films was observed using an atomic force microscope. Junctions with various Al film thicknesses were fabricated. The leakage current of the junctions is found to decrease as the Al thickness increases, indicating an improvement of the Al film coverage over the Nb surface. X rays were detected with the junctions prepared and the low leakage property across the junction makes possible x-ray detection with various size junctions up to 200×200 μm2.

Nb/Al/AlOx/Nb superconducting tunnel junctions as x-ray detectors

Nb/Al/AlOx/Nb junctions have been shown to be effective x-ray detectors that are robust to thermal cycling. We compare results from two junctions, one with a fine-grained base-layer electrode and counterelectrode and the other with an epitaxial base-layer electrode and a fine-grained counterelectrode. For a 6 keV x-ray, at 0.4 K, the epitaxial sample had a FWHM resolution of 157 eV and the fine-grained junction had a FWHM resolution of 300 eV. The differences between the junctions will be discussed.

High resolution detection of x-rays with a Nb-based STJ detector

A high energy-resolution of 88 eV has been achieved for 5.9-keV x-rays with a large area (178×178 µm2) Nb/Al-AlOx/Al/Nb superconducting tunnel junction (STJ) detector, which is stable at room temperature and robust to thermal cycles. The energy resolution is higher than those of semiconductor detectors. The resolution and the short shaping-time-constant, 2 µs, of the main amplifier used to obtain the energy resolution indicate that STJ detectors can be developed as nuclear radiation detectors with high energy resolution even for high rate radiations. Besides, a theoretical limit of energy resolution due to the statistical fluctuation of signal charges is discussed.

A New Fabrication Process of Superconducting Nb Tunnel Junctions with Ultralow Leakage Current for X-Ray Detection

A new fabrication process is presented to make Nb/Al/AlOx/Nb superconducting tunnel junctions with ultralow subgap leakage current at low temperatures below 4.2 K for X-ray detection. A Nb anodization process has been adopted to make electric insulation at the periphery of an ultrathin AlOx tunnel barrier through use in combination with the conventional SiO self-aligned insulation process. The temperature dependence of the subgap current of the junctions fabricated by the present process is in good agreement with that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory, while junctions made by the conventional process show a weak temperature dependence of the subgap current. In addition, the Vm value of the junction fabricated by the present process was found to be more than 3000 mV at 1.2 K. It has been clarified that the anodization process introduces improvements to make the subgap leakage current extremely small in the temperature range from 4.2 K down to 1.2 K.

X-ray detection with Nb/AlO x/Nb superconducting tunnel junctions and energy diffusion effect on energy resolution

Nb/AlO x /Nb superconducting tunnel junctions with low leakage current were fabricated. The ultra low leakage current was maintained after 100 thermal cycles. X-rays were detected with three sizes of junctions on several wafers. The pulse height spectra show the improvement of energy resolution with the increase in junction area. Using the diffusion length of excited electrons in the superconductor, the effect of dispersion of excited electron from the junction area to the electric lead and trap of excited electrons at the film edge (energy diffusion effect) was evaluated. Calculated results explain the characteristics of the experimental results. It is concluded that a large junction area is desirable to reduce the energy diffusion effect on the energy resolution.

A low noise 410-495 GHz Nb/Al/sub 2/O/sub 3//Nb SIS waveguide mixer

The noise and gain of a heterodyne waveguide mixer using Nb/Al/sub 2/O/sub 3//Nb superconducting tunnel junctions were measured in the 400-500-GHz frequency range. Three different arrays of two junctions in series are analyzed. The minimum receiver noise temperature is 120 K DSB at 480 GHz, measured with an array having integrated tuning stubs. The authors compare data of the pumped I-V curves with the Werthammer-Tucker theory and demonstrate an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs. a mixer noise temperature of 90+or-30 K and a DSB mixer gain of -12.5+or-0.6 dB were measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows good qualitative agreement, indicating the applicability of this theory to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage is 220 K at 495 GHz. This frequency is 85% of the reduced gap frequency, indicating that Nb superconductor insulator-superconductor (SIS) mixers can be used up at least the gap frequency of 680 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of Nb/AlO x/Nb superconductor detectors with X-ray and pulse light

We have detected 5.9 keV X-rays with Nb/AlO x ,/Nb superconducting tunnel junctions which are resistant to thermal cycling. The signal charge from the superconductor detector was about 180 times larger than that from a Si detector. An experimental method making use of pulse light sources of various wave lengths was proposed to determine the fundamental limit of energy resolution which comes from the statistical fluctuation of signal charges. Our results indicate the high potential of superconductor detectors as radiation detectors and also as optical sensors.