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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY,VOL. 9, No. 2, JUNE 1999

3765

HTS Mixers Based on the Josephson Effect and on the Hot-Electron Bolometric Effect
0. Harnack', St. Beuven, M. Darula, H. Kohlstedt Institute for Thin Film and Ion Technology, Research Center Juelich, 52425 Juelich, Germany

M. Tarasov, E. Stephansov, Z. Ivanov Institute of Applied Physics, Chalmers University of Technology, Sweden
Abstract-We report on our experimental studies of high-Tc Josephson mixers and high-Tc hot-electron bolometric (HEB) effect mixers. Mixers based on high-Tc bicrystal Josephson junctions have been fabricated, and noise, conversion efficiency, and receiver bandwidth measurements have been performed in the frequency range between 90 and 550 GHz. The dependence of the mixer performance on the operation temperature has been studied. High-Tc HEB mixers have been fabricated on MgO and sapphire substrates. We successfully sized the dimensions of the effective device volume down and managed to fabricate 50-60 nm long devices. In such short structures phonon diffusion into the normal metal electrodes should significantly improve the mixer performance.

voltage characteristics of a pumped junction can be utilized for low-noise heterodyne mixing. HTS Josephson mixers have been investgiated in a number of studies [1]-[3]. The mixer noise temperature of 1200 K at 4.2 K operating temperature has been measured at 345 GHz using a quasioptical coupled HTS step-edge Josephson mixer [11. We report on the noise and conversion performance of Wband waveguide Josephson mixers at operating temperatures above 20 K, based on bicrystal junctions on MgO substrates.
A. Fabrication

I.

INTRODUCTION

Mixers based on high-temperature superconductors (HTS) could be attractive candidates for millimeter and submillimeter wave receiver applications where operation temperatures above 20 K are required (e.g., earth remote-sensing observation from satellites). Low-temperature superconductive mixers utilizing quasiparticle tunneling are known to have a high sensitivity and low power consumption. However, all attempts to fabricate mixers based on quasiparticle tunneling in HTS junctions failed. Several reasons, such as short coherence length, nonconventional pairing mechanism, and sensitivity to processing, made it impossible to fabricate high quality tunnel junctions from HTS materials. The presently possible HTS mixers can utilize the Josephson effect or the Hot-Electron Bolometric (HEB) effect. We have investigated both types of mixers. 11. HIGH-T,JOSEPHSON MIXERS

Step-edge (SEJ) and bicrystal junctions (BCJ) are specific for high-T, superconductors. SEJs on MgO have been fabricated successfully [3], but problems with reproducebility and reliability have been found. It turned out that bicrystal junctions (BCJ) on MgO are much more stable and show higher I& products than SEJs. We have fabricated BCJs on 24" MgO bicrystal substrates (E, = 9.6). Pulsed laser deposition method was used for growing 50100 nm thin YBCO films. In addition, a 50 nm thick gold layer was deposited in-situ on top of the YBCO film.We used standard photolithographic processes and ion beam etching (IBE) in combination with mass spectroscopy (in order to control the etching process) to define the junction and the antenna structure (YBCO etching rate =lnm/min). The junctions were integrated into bow-tie and log-periodic antennas. In order to remove the shunting gold layer from the top of the 1 pm-wide bridge, we lithographically defined a window across the bridge and milled the gold using an etching rate of 4 d m i n . The HTS bridges were passivated by evaporating a SiO, layer.
B. Mixer Noise and Conversion Measurements

A HTS Josephson mixer consists of a grain boundary junction, which is intrinsically shunted due to the microscopic nature of the boundary. The nonlinear currentManuscript received September 15, 1998. This work was supported in part by the German Ministry of Education and Research (BMBF,13N732116) and by the European Community under Grant No. IC15-CT96-0806 (MCO-Copemicus project). 'Present address: Jet Propulsion Laboratory, M/S 168-314,4800 Oak Grove Drive, Pasadena, CA 91 109, USA.

The RF setup consists of a mixer block for 80 GHz to 120 GHz with mechanical tuners, assembled inside a cryostat. A simple beam splitter was used to combine the signal and the local oscillator (LO) signal. For experiments at frequencies between 430 GHz and 550 GHz the junctions were mounted into a quasioptical setup using an extended hyperhemisphere MgO lens. The intermediate frequency (IF) signal was amplified by a cooled amplifier or by an external room-

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Fig. 1: IF power measurement at 90 GHz (LO) and 20 K using the standard hotkold technique. The lowest receiver noise temperature was 2900 K. Fig. 2: Dependence of the receiver noise temperature and the conversion efficiency on the operating temperature at 90 GHz for the BCJ mixer which was described in section I1 B.

temperature amplifier at 1.4 GHz followed by a 65 dB amplifier stage and limited by a bandpass (Af=400 MHz). We calibrated the noise temperature of the IF chain by using a heated 50 SZ load attached to the cold plate. A coaxial switch for connecting the mixer or the load to the IF system was used. The calibration yield IF noise temperatures (TiJ of 5 K for the cooled amplifier and 130 K for the roomtemperature amplifier. The noise measurements were done by using the Y-factor method (300 U77 K loads). From the related IF power levels we can determine the receiver noise temperature T,. The mixer noise temperature T,, and the conversion efficiency q are related to T,, by T,= T,,,+q-' Ti, Fig. 1 shows an IF power measurements in the waveguide setup with external pumping at 90 GHz. Receiver noise temperatures of about 2900 K at operating temperatures of 20 K and at an IF of 1.4 GHz were measured. The critical current (I,) of this BCJ on MgO was 50 pA at 20 K and the normal resistance (R,,) was 14 R. The best conversion efficiency, not corrected for the IF impedance mismatch, was approximately -10 dB. The noise contribution by the room-temperature amplifier was relatively high (about 1300 K). The estimated mixer noise temperature is about 1600 K at 20 K. The absorbed local oscillator power of 4 nW was calculated from the value of the suppressed I,[4].

normalized frequency R (q ot Q-'), where R=o/o,, o is the signal frequency, and oc=4nIc~e/h the characteristic freis quency. Since q drops linear with T, the nonlinear increase of T, above 40 K should be connected to a degradation of the mixer performance. This is most likely due to a change in the excess noise mechanism in the junction [SI.

D. Receiver Bandwidth Measurement
In order to clarify the mode of side-band operation a receiver bandwidth measurement has been performed in the range 87-93 GHz. In this experiment the mechanical tuners have been adjusted for 90 GHz operation, the operating frequency has been changed after this, and the LO level has been adjusted for constant pumping at the same time. The measured characteristics of TN and q shows that we can assume rather double-side-band (DSB) operation than single-side-band (SSB) operation, since the receiver performance is more or less constant within 90 GHzfl.6GHz (Af=400 h4Hz). However, there is a clear evidence that we have a quite resonant tuning and about 2-3dI3 signal reduction in the lower side band. An optimization of the impedance matching between mixer and waveguide could improve the broad-band receiver operation.
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Fig. 2 displays the measured dependence of the receiver noise temperature and the conversion efficiency on the operating temperature between 20 K and 60 .K. The conversion efficiency was calculated in respect to the IF calibration described in section I1 B. The receiver noise temperature increases approximately linear in the temperature range from 20 K up to 35 K and increases dramatically at operating temperatures above 40 K. The conversion efficiency decreases linear with increasing temperature in the whole range. As expected from theory, the reduction can be well described by the increase of the

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E. Noise Measurements at 430 GHz and 550 GHz
Similar measurements were performed in a quasioptical setup. A 1 pm wide bicrystal junction with a current voltage characteristic without any excess current at 4.2 K was used. The mixer noise temperature at 4.2 K and 430 GHz measured in the fundamental mixing mode was 1200 K for the junction shunted by a low-inductance resistive shunt, and 1400 K for the junction with no shunt [6]. Measurements at 546 GHz gave noise temperatures of about 1100 K. HTS HEB mixer have been investigated in [11]-[13]. An optimized fabrication technique, which is obviously the most important issue, was described in [ll]. However, there exist still only a few data on the noise performance of these devices. Karasik et al. showed that low-noise mixing (-2000 K SSB) could be achieved, if relaxation mechanism like phonon diffusion could be made dominant by reducing the device length (L) down to a few hundred nm [lo]. Here we report on the fabrication and device tests of sub-pm HEB mixers on MgO and sapphire substrates.

F. Discussion
The sensitivity of Josephson mixers is limited by excess noise, which includes intrinsic junction noise and noise generated by internal Josephson oscillations. Lkharev et al. predicted a minimum noise temperature of T,= 10.5T(o/w,)2 for normalized frequencies Q > 1 [7]. A more optimistic prediction of T, = 6T was given for signal frequencies o equal to o,(R = 1). Schoelkopf et al. have shown that the linewidth of the Josephson oscillations is comparable with the frequency at bias points close to the critical current [5]. This causes an excess noise floor at low voltages. Their simulations, based the resistively shunted junction model (RSJ), predict a minimum DSB mixer noise temperature of T, = 20T for Q = 0.5 assuming a fluctuation parameter y = 0.01, which is defined by y = 4nekBT/hI,, the ratio of the thermal energy, k,T, to the Josephson coupling energy, Ich/4ne. In the experiment described in section I1 B, y was about 0.015 and Q c 0.5. Under these conditions we would expect a DSB T,,, of about 400 K at 20 K, which is about four times lower than the measured value of 1600 K. However, intrinsic material losses and additional sources of noise which are not modeled yet, could be responsible for higher mixer noise. 111. HIGH-T,, HOT-ELECTRON BOLOMETRIC MIXERS Another type of the mixer, which is most promising for operation in the THz regime, is the superconductive hotelectron bolometer (HEB) [8],[9]. In contrast to the Josephson mixer, HEB mixer are not limited by the energy gap. Other advantages of HEB mixers are the real device impedance and the insensitivity against magnetic noise. HEB devices usually consist of a ultra small volume of thin superconducting film which is connected to two normal metal electrodes. Absorbed rf power generates non-equilibrium ,,hot" electrons, which result in an increase of the resistance of the device and a related voltage response. Depending on intrinsic relaxation mechanisms, HEB can be fast enough to follow the IF oscillation of several GHz. Karasik et al. published a detailed theoretical analysis of the performance of HEB mixers based on YBCO [lo].

A. Fabrication of Sub-p YBCO HEB Mixers
For the fabrication of HEB mixers, thin YBCO films of thickness of about 20-40 nm have been grown on MgO and sapphire substrates using the laser deposition method. In the case of sapphire we applied a 30 nm thin CeO, buffer layer between the substrate and the YBCO film. The thin YBCO films have been protected by in-situ sputtered gold layer. Inductive T,-measurements showed critical temperatures of 89-90 K for YBCO films on sapphire substrates and 84-86 K for films on MgO substrates. The transition widths were in the range between 0.5 and 1 K. After patterning the microbridges (width of the bridge w-1 pm), integrated into bow-tie antennas for waveguide measurements, a small-area window (50 nm to 500 nm length and 1 pm width) was defined across the bridge by using electron beam lithography. After removing the gold layer from the window by using IBE, a SiOx protection layer was applied by evaporation. We observed no significant degradation of the critical current density (JJ and the R(T) characteristics.

B. DC Characteristics of Sub-pm YBCO HEB Mixers
Fig. 4 shows a SEM picture of the central part of our HEB mixer which has a device length of about 100 nm. The DC

Fig. 4: Sub-pm YBCO HEB structure on MgO. The goldNBCO sandwich electrodes are visible on the left and right hand side. The uncovered YBCO strip is located in the center between both electrodes, L=100 nm, w=lpm.


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IV. CONCLUSION
We demonstrated that low-noise mixing is possible with HTS Josephson mixers at temperatures above 20 K. The measured mixer noise is about four times higher than predicted by the RSJ model. The required LO power was in the range of a few nW at 90 GHz. We successfully fabricated HEB mixers with device lengths between 50 nm and 0.5 pm and demonstrated heterodyne mixing at 90 GHz. V. ACKNOWLEDGMENT We would like to thank D. Diehl and P. Zirnmermann from Radiometer Physics GmbH, K. Jacobs and B. Vowlnkel from KOSWniversity of Cologne, and E. Tong for useful help and fruitful discussions. We also are grateful for comments by B. S. Karasik and W. R. McGrath from JPL.

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Fig. 5: DC Characteristics of a YBCO HEB, L=0.05 pm, w=lpm, d=30nm; The dotted lines show the IVCs at different input levels at 90 GHz.

characteristics of a similar device on MgO with a smaller device length of 50-60 nm is displayed in Fig. 5 (film thickness 30 nm, device width 1 pm). The R, was about 10 and the I, was 6.5 mA at 67 K. A thermal hysteresis which is related to self-heating of the device was visible at temperatures below 76 K. The dotted curves displays the response of the pumped device at different levels at 90 GHz and at 67 K. Comparing the dissipated electrical power along the constant resistance line and the applied microwave power, we estimated input losses of about 15 dB.

VI. REFERENCES
[l] H. Shimakage, Y. Uzawa, M. Tonouchi, Z. Wang, IEEE Trans. on Appl. Supercond., 5, p. 2801, 1997 [2] J. Chen, H. Myoren, K. Nakajima, T. Yamashita, P. H. Wu, Appl. Phys. Lett., 71 (5), 1997 [3] 0. Hamack. M. Darula, H. Andoh, S. Beuven, H. Kohlstedt, Proc. of jth Int. Workshop on THz Electronics, Grenoble 1997 [4] K. K. Likharev, "Dynamics of Josephson Junctions and Circuits", Gordon and Breach Science Publishers, New York, 1986 [5] R. I. Schoelkopf, Ph.D. dissertation, CalTech., 1995 (unpublished) [6] M.Tarasov, EStepantsov, ZIvanov, O.Hamack, M.Darula, S.Beuven, H.Kohlstedt, in prep. for Appl. Supercond. Conference, 1998 [7] K. K. Likharev, V. V. Migulin, "Josephson effect millimeter range receivers", Radio Engin. and Elect. Phys., v. 25, June 1980, pp. 1-18 [8] E. M. Gershenzon et al. "Millimeter and Submillimeter Range Mixer based on Electronic Heating of Supe-conducting Films in the Resistive State", Sverhprovodimost' (KIAE), vol. 3(10), pp. 21432160, October 1990 [9] D. E. Prober, "Superconducting Terahertz Mixer using a Transitionedge Microbolometer", Appl. Phys. Lett., vol. 62(17), pp. 21 19, 1993 [lo] B.S. Karasik, W. R. McGrath, and M. C. Gaidis, "Analysis of a highTc hot-electron superconducting mixer for terahertz applications", J. Appl. Phys., 81 (3), 1997 [ll] M. I. Bums, A. W. Kleinsasser, K. A. Delin, R. P. Vasquenz, B. S. Karasik, W. R. McGrath and M. C. Gaidis, "Fabrication of High-T, Hot-Electron Bolometric Mixers for Terahertz Applications", Proc. of 8. Ini. Symp. on Space THz Techn., Boston, 1997 [12] Yu. P. Gousev et al., "Coupling of THz radiation to a high-Tc supercond. hot electron bolometer mixer", Appl. Phys. Lett., 69 (9,1996 [13] C.-T. Li et al, "Low power submillimeter-wave mixing and responsivity properties of YBaCuO hot-electron bolometers", Appl. Phys. Lett., 71 (ll), 1997 [14] 0. Hamack, S. Beuven, M. Darula, H. Kohlstedt, M. Tarasov, E. Stepantsov, Z. Ivanov, Proc. of 6th IEEE International Conference on Teraheriz Elecironics, Leeds, Sept. 1998

C. Heterodyne Mixing and Conversion Efficiency
Heterodyne mixing experiments have been performed at temperatures around 67 K and at 90 GHz. Fig. 6 displays the
result of a mixing experiment using two monochromatic

sources at 90 GHz (signal) and 91.4 GHz (LO). The overall conversion efficiency at 1.4 GHz was measured to be -25 dB.The same value was measured at an IF of 3 GHz, which is slightly better (5dB) than earlier experiments with longer devices (Lwl-2 pm) have shown [14]. This might be already an evidence that phonon diffusion plays a significant role in
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