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IEEE Transactions on Magnetics

All-NbN nanobridges as Josephson mixers

Toshiaki Matsui, Z. Wang, K. Hamasaki, T. Yamashita, M. Endo

Published Mar 1, 1989• 5 citations
Publisher

Abstract

Using RIE (reactive in etching) and lift-off techniques, all-NbN nanobridges with gap structure in the I-V (current-voltage) curves have been reproducibly constructed for testing as millimeter-wave mixers. The nanobridges were constructed on NbN/MgO/NbN edge junctions. They were characterized by measurements of their I-V curve and dI/dV versus V and by mixing experiments at 105.9 GHz. These devices had high normal-state resistance, and showed almost ideal Josephson response to external magnetic flux and to millimeter-wave irradiation. The I-V curve remained nonhysteretic from T/sub c/ to 4.2 K, and millimeter-wave-induced steps in the I-V curve were observed up to approximately 3 mV. The cooling of the NbN nanobridges was quite good due to their small size and VTB geometry and the high thermal conductivity of the MgO films. Hence the Josephson response of these devices to millimeter waves was comparatively less limited by self-heating.>

DOI
10.1109/20.92473

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Josephson-Effect Mixers using Shunted Nb and NbN Tunnel Junctions

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We present test results of a 100 GHz waveguide receiver using resistively-shunted Nb and NbN tunnel junctions. These junctions were fabricated with a process similar to that used previously with Nb junctions [1], which produces non-hysteretic devices with normal-state resistances of 30-50 Ohms, and IcRiz products of 0.5-0.75 mV. The best receiver temperature obtained to date with NbN is 270 K (DSB) with 6-7 dB of conversion loss. A receiver temperature of 390 K (DSB) was previously obtained with Nb junctions; these devices showed qualitative agreement with the predictions of the simple DC and RF current-biased RSJ model. The improvement in receiver performance is probably due to improved RF coupling of the junctions, and due to the higher IcRn products of NbN. However, these NbN devices display different pumped I-V curves and IF power curves than predicted by the RSJ model. We show that these effects can be caused by the strong interaction of the junction with the embedding circuit, and conclude that an accurate model of the impedance presented to the junction will have to be included in the RSJ simulations to compare with the observed performance. Fourth International Symposium on Space Terahertz Technology Page 443 Introduction We are studying the performance of Josephson-effect mixers using resistivelyshunted tunnel junctions in order to develop a better understanding of their behavior, the sources of noise and its limitation, and to determine how to optimize their performance. Also, we wish to compare the observed behavior and performance with the theoretical predictions of the so-called RSJ (resistively shunted junction) model, so that there will be a guide to the expected performance for different frequency regimes and device parameters. This work may be important as high-Tc SNS devices become available [P.A. Rosenthal and E.N. Grossman, these proceedings] with large Ic h?„ products, for they provide the possibility of using Josephson mixers at frequencies in the terahertz range, above the range where SIS mixers are currently available. In contrast, due to the extremely short coherence lengths of the high-Tc materials, junctions suitable for quasiparticle (SIS) mixers may not be feasible with these materials Device Fabrication We have applied the technique used for creating non-hysteretic Nb Josephson devices[1] to NbN/MgO/NbN tunnel junctions. First the junction tfilayer is fabricated, then the junction areas are patterned by electron-beam lithography and subsequently etched using reactive-ion etching (RJE) in the standard method [3]. After planarization of the etched area with evaporated SiO, a AuGe resistor is fabricated on top of the SiO, and the junction and resistor are then contacted with another layer of NbN. A photomicrograph of a completed Nb junction is shown in Figure la. Since the bottom of the etched trilayer lies under the resistor, it acts as a ground plane, and the resistor structure can be analyzed as a lossy transmission line. The impedance of this short microstrip structure is purely real, even up to frequencies comparable to the gap frequency of the superconductor, thus providing the desired broadband shunt. However, the section of Page 444 Fourth International Symposium on Space Terahertz Technology superconductor which contacts the junction and resistor creates a non-negligible parasitic capacitance when compared to the very small area tunnel junctions used, and therefore the area of this contact pad must be kept to a minimum. From a knowledge of the geometry and thickness of the insulator (Si0), we estimate this parasitic capacitance to be 7-8 fF. Shunting of the junction is necessary to reduce the McCumber beta parameter (Pc = 2eIcRN C Ih) of the unshunted tunnel junctions. Junctions with /3, values greater than one have hysteretic I-V curves [4], and their dynamics can be complicated, including the possibility of chaos. Reduction of 13, is accomplished by resistive shunting, which reduces /3 by the square of the ratio of the initial junction resistance to the final resistance of the parallel combination of the junction and the resistive shunt. However, this shunting has the undesirable effect of reducing the 1,Rn product of the junction. For the NbN junctions used, the critical current density was about 331cA/cm 2 , and the specific capacitance is estimated to be about 150/F 1 itm [5], giving an unshunted of about 12. Thus the optimal shunted junction for these parameters would have a shunt resistance of about one-fourth of R,„ and thus an IcR7, reduced from the unshunted value of 3.0 mV to 0.8 mV. In order to maintain high enough resistances for the final device, very small (0.3pm on a side) area junctions must be used, and the parasitic capacitance will be comparable to the junction capacitance. This further reduces the final IeR, product attainable. The I-V curve of a typical device is shown in Figure lb. This device had a critical current of 11 microamps and a normal state resistance of 39 Ohms, giving an IcR, product of 430 microvolts. The IV curve is nonhysteretic and follows the RSJ model. Mixer Modelling Mixer performance can be predicted using the RSJ model, in a method similar to that used by Taur[6], but extended to the case of finite junction capacitance. The

Softwaretechnik-Trends•Apr 1, 1993•1 citation
Using RIE (reactive in etching) and lift-off techniques, all-NbN nanobridges with gap structure in the I-V (current-voltage) curves have been reproducibly constructed for testing as millimeter-wave mixers. The nanobridges were constructed on NbN/MgO/NbN edge junctions. They were characterized by measurements of their I-V curve and dI/dV versus V and by mixing experiments at 105.9 GHz. These devices had high normal-state resistance, and showed almost ideal Josephson response to external magnetic flux and to millimeter-wave irradiation. The I-V curve remained nonhysteretic from T/sub c/ to 4.2 K, and millimeter-wave-induced steps in the I-V curve were observed up to approximately 3 mV. The cooling of the NbN nanobridges was quite good due to their small size and VTB geometry and the high thermal conductivity of the MgO films. Hence the Josephson response of these devices to millimeter waves was comparatively less limited by self-heating.>
The testing of NbN(g) nanobridges with Al direct shunt resistors for use as millimeter-wave Josephson mixers is discussed. The NbN(g)/Al nanobridges have a nearly sinusoidal current-phase relation, high resistance, well-defined gap structure at approximately 4 mV, large I/sub J/ R/sub N/ products of approximately 3 mV, and nonhysteresis I-V curves. By means of the Al direct shunt, the effective noise temperature T/sub N//sup eff/ of the NbN(g) nanobridges, which was calculated by transition-state theory, was reduced to 4.2 K as a result of reducing the self-heating and noise rounding effects. The NbN(g)/Al nanobridges show almost ideal Josephson response to millimeter-wave radiation at 106 GHz. The well-pronounced Josephson steps were observed up to approximately 4 mV. The observed systematic decreasing of the step heights with increasing the RF current can be accounted for by the effect of the thermal noise if an effective noise temperature of T/sub N/=15 K (at i/sub RF/=3) is assumed. The IF peaks in the Josephson mixing were obtained up to the bias voltage of approximately 1.2 mV. The NEP was about 3*10/sup -20/ W/Hz at 4.2 K.
All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness ofcRNproducts of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.
Abstract To reduce the self‐heating effect and thermal noise in bridge‐type Josephson mixers, all‐NbN nanobridges with an Al “direct shunt” resistor were fabricated. This paper describes the millimeter‐wave response and mixing properties of the NbN(g)/Al nanobridge‐type Josephson mixers in the 100‐GHz frequency. The effective noise temperature TNeff of the nanobridges was decreased to the bath temperature (liquid helium temperature, 4.2 K) by the Al “direct shunt” resistor and the millimeter‐wave induced steps were clearly observed at higher voltage (∼4 mV) than in the nanobridge without the Al “direct shunt” resistor. The millimeter‐wave current dependence of the height of the millimeter‐wave induced steps was analyzed quantitatively, taking into account the thermal noise in the RSCJ model. The simulated result agreed with the experimental result very well. The systematic decrease of the induced step height with an increase of millimeter‐wave current was explained as the increase of effective noise temperature due to absorption of millimeter‐wave power. In the 100‐GHz Josephson mixing experiment, a large IF power output was obtained at a voltage up to about 1.6 mV. The effective noise power NEP and conversion efficiency were about 3 × 10−20 W/Hz and 0.2, respectively.
Niobium variable thickness bridges are being fabricated for testing as millimeter wave mixers. Bridges have been fabricated on fused quartz, silicon and gallium arsenide substrates and characterized by measurements of the I−V curve, and dV/dI versus V and by mixing experiments at 80‐100 GHz. The bridges on Si and GaAs exhibit small Josephson steps but the response of all the bridges is limited by self‐heating, and mixing appears to be primarily bolometric. However the cooling of the bridges of Si is much better than the others.
To realize antenna-coupled Josephson detectors for microwave and millimeter-wave radiation, planar-type Nb nanobridge Josephson junctions were fabricated. Nb thin films whose thickness, the root mean square roughness and the critical temperature were 20.0 nm, 0.109 nm and 8.4 K, respectively were deposited using a DC magnetron sputtering at a substrate temperature of 700°C. Nanobridges were obtained from the film using 80-kV electron beam lithography and reactive ion-beam etching in CF 4 (90%) + O2 (10%) gases. The minimum bridge area was 65 nm wide and 60 nm long. For the nanobridge whose width and length were less than 110 nm, an I-V characteristic showed resistively-shunted-junction behaviour near the critical temperature. Moreover, Shapiro steps were observed in the nanobridge with microwave irradiation at a frequency of 6 – 30 GHz. The Nb nanobridges can be used as detectors in the antenna-coupled devices.
We present test results of a 100 GHz waveguide receiver using resistively-shunted Nb and NbN tunnel junctions. These junctions were fabricated with a process similar to that used previously with Nb junctions [1], which produces non-hysteretic devices with normal-state resistances of 30-50 Ohms, and IcRiz products of 0.5-0.75 mV. The best receiver temperature obtained to date with NbN is 270 K (DSB) with 6-7 dB of conversion loss. A receiver temperature of 390 K (DSB) was previously obtained with Nb junctions; these devices showed qualitative agreement with the predictions of the simple DC and RF current-biased RSJ model. The improvement in receiver performance is probably due to improved RF coupling of the junctions, and due to the higher IcRn products of NbN. However, these NbN devices display different pumped I-V curves and IF power curves than predicted by the RSJ model. We show that these effects can be caused by the strong interaction of the junction with the embedding circuit, and conclude that an accurate model of the impedance presented to the junction will have to be included in the RSJ simulations to compare with the observed performance. Fourth International Symposium on Space Terahertz Technology Page 443 Introduction We are studying the performance of Josephson-effect mixers using resistivelyshunted tunnel junctions in order to develop a better understanding of their behavior, the sources of noise and its limitation, and to determine how to optimize their performance. Also, we wish to compare the observed behavior and performance with the theoretical predictions of the so-called RSJ (resistively shunted junction) model, so that there will be a guide to the expected performance for different frequency regimes and device parameters. This work may be important as high-Tc SNS devices become available [P.A. Rosenthal and E.N. Grossman, these proceedings] with large Ic h?„ products, for they provide the possibility of using Josephson mixers at frequencies in the terahertz range, above the range where SIS mixers are currently available. In contrast, due to the extremely short coherence lengths of the high-Tc materials, junctions suitable for quasiparticle (SIS) mixers may not be feasible with these materials Device Fabrication We have applied the technique used for creating non-hysteretic Nb Josephson devices[1] to NbN/MgO/NbN tunnel junctions. First the junction tfilayer is fabricated, then the junction areas are patterned by electron-beam lithography and subsequently etched using reactive-ion etching (RJE) in the standard method [3]. After planarization of the etched area with evaporated SiO, a AuGe resistor is fabricated on top of the SiO, and the junction and resistor are then contacted with another layer of NbN. A photomicrograph of a completed Nb junction is shown in Figure la. Since the bottom of the etched trilayer lies under the resistor, it acts as a ground plane, and the resistor structure can be analyzed as a lossy transmission line. The impedance of this short microstrip structure is purely real, even up to frequencies comparable to the gap frequency of the superconductor, thus providing the desired broadband shunt. However, the section of Page 444 Fourth International Symposium on Space Terahertz Technology superconductor which contacts the junction and resistor creates a non-negligible parasitic capacitance when compared to the very small area tunnel junctions used, and therefore the area of this contact pad must be kept to a minimum. From a knowledge of the geometry and thickness of the insulator (Si0), we estimate this parasitic capacitance to be 7-8 fF. Shunting of the junction is necessary to reduce the McCumber beta parameter (Pc = 2eIcRN C Ih) of the unshunted tunnel junctions. Junctions with /3, values greater than one have hysteretic I-V curves [4], and their dynamics can be complicated, including the possibility of chaos. Reduction of 13, is accomplished by resistive shunting, which reduces /3 by the square of the ratio of the initial junction resistance to the final resistance of the parallel combination of the junction and the resistive shunt. However, this shunting has the undesirable effect of reducing the 1,Rn product of the junction. For the NbN junctions used, the critical current density was about 331cA/cm 2 , and the specific capacitance is estimated to be about 150/F 1 itm [5], giving an unshunted of about 12. Thus the optimal shunted junction for these parameters would have a shunt resistance of about one-fourth of R,„ and thus an IcR7, reduced from the unshunted value of 3.0 mV to 0.8 mV. In order to maintain high enough resistances for the final device, very small (0.3pm on a side) area junctions must be used, and the parasitic capacitance will be comparable to the junction capacitance. This further reduces the final IeR, product attainable. The I-V curve of a typical device is shown in Figure lb. This device had a critical current of 11 microamps and a normal state resistance of 39 Ohms, giving an IcR, product of 430 microvolts. The IV curve is nonhysteretic and follows the RSJ model. Mixer Modelling Mixer performance can be predicted using the RSJ model, in a method similar to that used by Taur[6], but extended to the case of finite junction capacitance. The