Josephson-Effect Mixers using Shunted Nb and NbN Tunnel Junctions

Abstract

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