Phillips and D. P. Woody and G. J. Dolan and R. E. Miller. Bell Labs, Murray Hi1 1, N. J and R. A. Linke

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1 684 EEE TRANSACTONS ON MAGNETCS, VOL. MAC-~, NO. 1, JANUARY 1981 DAYEM-MARTN (SS TUNNEL JUNCTON)MXERS FOR LOW NOSE HETERODYNE RECEVERS Superconducting tin film tunnel junctions of 2 smal 1 area (.1 +1 um ) ae properties wic make tem suitable for ig frequency (>lo0 GHz) eterodyne receiers. Bot pair and single quasiparticle tunneling is present in tese deices, but it is found tat te mixing due to te pair effect is apparently excessiely noisy, wereas te single quasiparticle effect as a low noise caracter wic gies ope for near quantum limited performance. Te pysical effect inoled is poton assisted quasiparticle tunneling and was first obsered by Dayem and Martin. We ae made laboratory tests at 115 and 230 GHz wic gae single side band (SSB) mixer noise temperatures of 60 and 300 K respectiely. Also we ae fabricated a GHz receier for te Caltec Owens Valley Radio Obseratory wic as an oerall receier noise temperature of about 300 K (SSB). ntroduction Millimeter and Submillimeter Astronomical T. G. California nstitute Pillips and D. P. Woody of Tecnology, Pasadena,.CA and G. J. Dolan and R. E. Miller Bell Labs, Murray Hi1 1, N. J and R. A. Linke Bell Labs, Crawford Hill, N. J Lines Te region of te electromagnetic spectrum from a few GHz to a few tousand GHz contains many interesting narrow line transitions of bot atoms and molecules. Many species of atoms (e.g. H,C,O) and molecules (e.g. Hz, CO) exist in te clouds of gas witin our galaxy, and studies of tese species proide understanding of bot te pysics and cemistry of tis interstellar gas wic, of course, proides te material for forming stars. Typical temperatures for te dense interstellar clouds range from 5 to 100 K so tat energy leels can be populated for bot fine structure transitions of atoms and rotational transitions of molecules. Densities in te interstellar medium are low by laboratory standards, usually in te range lo3 - lo6 cm-3 for molecular clouds. So pressure broadening effects are negligible and linewidts are determined by Doppler effects caused, for instance, by turbulence oer large regions of te clouds. Linewidts, terefore, are small and spectral resolution of or better may be required. Heterodyne Receiers To aciee ig spectral resolution it is usual in te radio, millimeter and submillimeter-wae bands to employ eterodyne receiers in wic a local oscillator wae is coerently mixed wit te signal. Te mixer element is usually a diode type deice proiding non linear response. Te deice must be capable of carrying currents at te signal frequency. Te difference frequency between te signal and te local oscillator is aailable as an intermediate frequency (F) oer a somewat restricted range (usually less tan a GHz) depending on te capability of te low noise F amplifier. Suc a receier is caracterized by a noise temperature (TR) wic is contributed to by te noise generated in te mixer, wic is equal to or greater tan te quantum noise due to fluctuations in te local oscillator power, plus te noise of te F amplification cain wic appears as effectiely multiplied by te power conersion loss factor (L) of te mixer. TR = TM.t. LTF For te purposes of tis paper all numbers are quoted for single side band operation (SSB), wic is te usual spectroscopic mode. Noise temperature measurements are usually made wit wite noise, fixed temperature, loads applied to te front end of te receier, wic affect bot side bands equally. For small alues of te F it is often te case tat te double side band alues are one alf te SSB alues, since bot side bands conert power equally. n principle power loss to te conerted side band can be aoided (image suppression) by differential matcing if te F is large enoug, or by use of a two element front end. At ig frequencies tese tecniques are not usually practicable. Spectroscopy is performed by diiding te F band into cannels of widt A and rectifying and integrating te noise in eac cannel separately. All mixer receiers operating in te Rayleig-Jeans limit ( < k TR) proide a signal to noise ratio gien by te Dicke radiometer equation: SN = 3 Jat TR were TS is te equialent black body temperature of te signal and t is te integration time. Clearly it is ery important to reduce TR to a minimum. A1 1 radiometric deices are goerned by equation (Z), oweer not all are classical diode mixers. Quantum mixers, discussed below, and bolometer mixers, not discussed in tis article, ae properties wic make tem suitable for certain purposes. Figure 1 sows a grap for measured mixer noise temperatures (TM, SSB), equation (1), for arious deices at frequencies up to GHz. Te lowest noise temperatures at ig frequencies are found for nsb bolometer mixers. Tese are bulk, single crystal deices, wic do not suffer from embedding network difficulties (i.e. tey ae easily computed resistie and reactie impedance components in a waeguide mount), oweer tey ae ery limited F bandwidts due to te intrinsic electronic relaxation time of te material. Superconducting Mixers Te lowest noise temperatures in te GHz range are found for superconducting mixer deices. Tis is primarily because te small alue of te superconducting gap ( -1 mv rater tan -1 V for a semiconductor) produces only a small current flow at typical ~ $ EEE

2 685 Y t- 2 o o K SCHOTTKY X 15K SCHOTTKY 0 SUPER SCHOTTKY 0 nsb BOLOMETER MXER + ~ 0 JOSEPHSON A SS QUASPARTCLE i- + + ox 0 00 X 0 0 A?y contrast we find 18, as do oters 19, tat te mixing effect associated wit single quasiparticle tunneling exibits noise wic is largely local oscillator poton noise, wit some extra contribution from sot noise due to certain unwanted currents. Tese 'Dayem- Martin' effect deices are likely to proe most effecti e for radi oastronomy. At ig frequencies ( > 100 GHz) te conersion efficiency is reduced from te teoretical ae by parasitic impedances in te microwae circuit. Te traditional tecnique for mounting diodes in a waeguide inoles contacting te diode wit a wire spring or 'wisker', running along te waeguide electric field direction. Tis is aoided in tin film superconducting mixers since te wole deice is formed litograpically on a substrate wic can contain te contact leads and F coke structure (see Figure 2). Te deice designer ten as control oer te microwae circuit, wic sould a1 low te reduction of conersion losses due to parasitics. O' o2 u (GHz) 3 Fig. 1. SSB mixer noise temperatures for seeral different types of mi llimeter and submi 11 imeter et- erodyne receiers. Data for 300 k Scottky mixers are from references [Z- 51. Data for 15 k Scottky mixers are from references [6-81. Data for super Scottky mixers are from references [9 and lo]. Data for nsb bolometer mixers are from reference [ll]. Data for Josepson junction mixers are from references [ Data for SS quasiparticle mixing are from [ impedances of n, so reducing sot noise in te deice. Also, te onset of current at te gap bias often takes place in a oltage range<e so tat quantum effects are important. n principle all deices could ae quantum noise limited performance, but in practice it is difficult to reduce unwanted currents (not associated wit local oscillator induced effects) to zero. A furter adantage of superconducting mixer deices is tat tey may sow conersion gain rat.er tan loss, so eliminating receier noise due to F amplifier noise. From equation (1) it is clear tat one,only as two aspects of te deice to optimize, te mixer noise and te conersion efficiency. Howeer, some types of deice, wic at first seemed promising, ae been eliminated due to difficulties in one of tese areas. An example is te Josepson effect mixer wic as persistently exibited an excess noise penomenon. Tis wil be clearly seen in te data below. Fig, 2. Substrate containing an SS junction wit dc and F connection and RF coke structure mounted across a 14 eigt WR-8 waeguide. Finally, of great importance in te deelopment of superconducting mixer deices, tere now exists a quantum mecanical teory due to Tucker 2o wic predicts te performance of Dayem-Martin mixers. Tis teory allows comparison wit experiment oer a range of conditions; e.g. te mixing may be examined for arious alues of bias suc tat single or multiple poton effects come into play. Tis flexibility sould allow te deice designer to separate out te parasitic effects from tose intrinsic to te deice. Quasiparticle Tunneling Mixer Deices Types of Deices At te present tere appear to be tree types of deice wi c exibit quasi parti cle current mi xi ng. Te first to be inestigated was te Super-Scottky Diode 'l. Basically tis only differs from te Scottky diode by replacement of te normal metal electrode by a superconductor and by te increase of semiconductor dopants to gie a narrower depletion

3 686 layer suitable for use as a tunneling barrier. As a microwae deice it suffers from te problems of wisker contacts. A second type is te SN structure. Here te tunnel junction is a tin film structure wit one normal and one superconducting electrode. An early attempt to uti 1 ize tese deices as microwae detectors2* was unsuccessful, in part due to metallurgical difficulties. Howeer, for reasons tat will appear below, at frequencies > 300 GHz it may be tat tese deices will be required. Te tird, and currently most popular structure, is te SS deice. nitial uses of tese at 115 GHz 18 and 36 GHz l9 sowed efficient mixing, low noise temperatures and confirmed tat quasiparticle currents were inoled. Subsequently it as become clear tat te poton assisted tunneling obsered bydayem and 1 Martin and explained by Tien and Gordon 23 is te dominant feature of te deice. Tis effect is manifest as steps in te -V caracteristic wen local oscillator power is applied, spaced AV = e from te gap structure. t is easily seen in te SS system because tere are two sarp density of states structures conoled, compared wit just one for te SN or Super Scottky deices. SS deices also sow Josepson steps, of course, spaced AV = 2e from bias. Bot SS and SN structures can be formed as arrays of deices and at least two inestigations ae been reported. 16,18 Construction of Deices n our inestigation junctions fabricated by te metods of Dolan l8 ae been employed. Electron beam litograpy is used to generate masks from wic potoresist bridge structures are made. An example of suc a bridge structure is sown in figure 3. Figure 4 Fig. 4. (a) An SS junction made using te potoresist structure sown in Fig. 3. (b) An SS Fig. 3. Scanning electron microscope picture of a junction wose area is near te lower limit suspended potoresist bridge structure used to acieable by tis tecnique. (c) A series fabricate small junctions array area of 20 SS junctions.

4 sows examples of eaporated structures made wit tis tecnique. By eaporating past te bridge at different angles, oerlapping structures are formed. 2 Bot ery small area junctions ( N -111 m ) and arrays of junctions can be made in tis way. Tunneling and Mixing Caracteristics 1. Results at 115 GHz. -V caracteristics for an SS deice, wit Pb + 4% in electrodes, are sown in figure 5a. Te poton assisted tunneling steps,become clearly isible wit increasing local oscillator (LO) power. Two steps are obserable aboe and below te gap bias oltage, but bel ow about 1.5 mv bias te caracteristics are confused by te presence of Josepson currents. n C 5 X 40C 2-20c as displayed in 5b. Knowing te F amplifier noise temperature and two black body load temperatures a1 lows a measurement, at eac bias point, of te mixer temperature and conersion loss, using a somewat more sopisticated ersion of equation (1). Tese quantities are plotted in figures 5c and d, and are SSB alues. For tis particular junction te peak responsiity, minimum noise temperature and optimum conersion loss all occur on te single poton assisted tunneling step below te gap. Some of our junctions ae sown sligtly improed performance on te second step. Te best alues for TW and L found at 115 GHz ae been about 60 K and 7 db respectiely. t is noticeable in figures 5b,c and d tat te noise increases dramatically and te system performance worsens in te region at low bias were te Josepson currents are dominant. Tis effect is more detrimental at iger frequencies. 2. Results at 230 GHz Measurements at 230 GHz were carried out on te same type of junction as used at 115 GHz. Te waeguide arrangement was somewat simplified because of te greater construction difficulties at suc frequencies. n fact a circular fundamental waeguide mode was used, rater tan te rectangular reduced (14) eigt system employed at 115 GHz. Figure 6a sows te -V caracteristics wit and witout LO power, te only significant cange being te doubling of te Dayem-Martin step widt. Te region below 1.5 mv, for wic te Josepson currents are dominant in tese junctions, now protrudes almost into te region of te first step. Figure 6b sows te dramatic increase in F noise at tat oltage. We ae attempted to deal LL a a a t-( dbm-, -47 dbml\ (a 1 - MAGNETC FELD OFF ---- MAGNETC FELD ON V (mv) Fig. 5. (a) Current s. oltage caracteristics for as SS junction wit different 115 GHz LO power leels applied. (b) F output power wit ot and cold load signals applied. (c) SSB mixer noise temperature. (d) SSB mixer conersion loss. Mixing response is indicated in figure 5b. For an LO power of about lo-* watts te sensitiity is rougly optimum as can be seen from te cures of 5a. Wen te waeguide is coupled to an LO source of tat power and also to a black body load (by a means described in te receier description below) te down conerted power coupled to te F is a function of bias oltage, Fig. 6. (a) Current s. oltage Caracteristics for a junction wit and witout 230 GHz LO power applied. (b) F output power wit ot and cold load signals applied. Te effect of applying a magnetic field is sown by te dased lines.

5 688 wit te problem by applying a magnetic field to te junction. Tis as te effect of reducing te oltage range in wic te junction sows ysteresis. Te dased cure of 6a sows negligible ysteresis compared wit te cure for no magnetic field. As anticipated, 6b sows a reduced range for te excess noise. Howeer, in tese particular experiments te field on te junction is not sufficiently omogeneous and as smeared te gap (sal, resulting in reduced mixing sensitiity (6b). t can be remarked ere tat a series array of junctions may allow a way out of tis problem. A disadantage for arrays at ig frequency is tat te series inductance between junctions must be kept small, wic accounts for te complicated structure of figure 4c. A complete solution to te Josepson current problem would be to use SN deices, of course. Our best single side band performance to date for SS deices at 230 GHz is TM < 300 K. As yet, we ae not accurately measured te conersion loss at tis iger frequency GHz Astronomical Receier A receier to mount at te focus of a 10 m telescope at Caltec's Owens Valley Radio Obseratory, using an SS mixer element, is now nearing completion. A simplified block diagram of te receier is sown in figure 7. Tis figure also demonstrates te metod of measurement of te receier and mixer performance parameters. Te parts witin te box are in acuum, bolted to te cold plate of a liquid elium cryostat. Some measurements ae been made at 4.2 k, but te best results are generally found at lower temperatures. HOT AND COLD LOADS l=gggj LENS,"b BEAMSPLTTER % DELECTRC GHz.F. - D.C. BAS Fig. 7. Scematic diagram of 115 GHz receier. Fig. 8. Picture of 115 GHz receier to be used for astronomical obserations. Te principal elements are; A, LO feedorn; B, 1% dielectric beamsplitter; C, mixer feedorn; D, mixer block; E, backsort tuner; F, 30 db directional coupler for coupling test signals into te F line; G, 1.4 GHz cooled GaAs FET F amplifier; H, F output port.

6 Te dielectric beam splitter a1 lows te signal to pass freely and couples in only -1% of te LO power. Tis is possible because of to low LO power requirement for te SS mixer. Figure 8 is a potograp of te receier dewar wit radiation sields remoed. A prototype ersion of tis receier, built at Bell Labs, was used in June 1979 to determine its suitability for astronomical operation. No major problems wer encountered. Te table below indicates performance figures for te prototype and current receier. Te current receier as a system temperature of 300 K at 115 GHz, wic is comparable wit or better tan contemporary cooled Scottky diode receiers. Te table also indicates wat would result if te best measured indiidual component figures could be acieed in te final configuration. Tis is a matter of selection and correct matcing. Finally, te teory column sows te expectations from te teory of Tucker. 20 T F LSSB Table 1 SS DAYEM-MARTN RECEVER AT SGHZ ACHEVED ON NOW N TELESCOPE LAB JUNE 1979 (C..T.) (B.T.L.) 60 K 15 K 8.5db 8 db TM,ssB 95 K BOK TREC,SSB 400 K 300 K Acknowledgements BEST ACHEVED (C..T. B.T.L.) K THEORY 40 K 6 db GAN 60 K 45 K (105 K) <30 K nitial aspects of tis work were carried out in collaborations wit R. C. Dynes and J. H. Magerlein. Conersations wit T. A. Fulton, J. M. Rowel1 and T. C. L. G. Sollner ae been most elpful. Work at Caltec was carried out wit support from NSF grants # AST and AST References A. H. Dayem and R. J. Martin, "Quantum nteraction of Microwae Radiation wit Tunneling Between Superconductors," Pys. Re. Lett., 8, 246 (1962). A. R. Kerr, "Low-Noise Room Temperature and Cryogenic Mixers for GHz,' EEE Trans. Microwae Teory Tec., MTT-23, 781 (1975). E. R. Carlson, M. V. Scneider and T. F. McMaster, "Subarmoni cal ly Pumped Mi 11 imeter-wae Mixers,' EEE Trans. Microwae Teory Tec., MTT-26, 706 (1 978). N. R. Erickson, "A 0.9 mn Heterodyne Receier for Astronomical Obserations," EEE nternational Microwae Symp. E, 438 (1978) M. R. Fetterman, P. E. Tannenwald, B. J. Clifton, C. D. Parker, W. 0. Fitzgerald and N. R. Erickson, "Far R Heterodyne Radiometric Measurements wit Quasi-Optical Scottky Diode Mixers," Appl. Pys. Lett. 2, 151 (1978). E. Kolberg, "Scottky Barrier Diode Mixers," URS Symp. on Mi 1 1 i meter Tecno1 ogy in Radi oas tronomy, Grenoble (1980). R. A. Linke, M. V. Scneider and A. Y. Co, "Cryogenic Millimeter-Wae Receiers Using Molecular Beam Epitaxy Diodes," EEE Trans. Microwae Teory 'and Tec. MTT -26, 935 (1978). S. Weinreb and J. Payne, Personal communi cation. M. McColl, M. F. Bottjer, A. B. Case, R. J. Pederson, A. H. Siler and J. R. Tucker, "Te Super-Scottky Diode at 30 GHz," EEE Trans. Magn. MAG-15, 468 (1979). W. Wilson, Personal communication. T. G. Pillips, Unpublised. Y. Taur, J. H. Classen and P. L. Ricards, "Josepson Junctions as Heterodyne Detectors,1 EEE Trans. Microwae Teory Tec. MTT-22, 1005 (1976). Y, Taur and A. R. Kerr, "Low Noise Josepson Mixers at 115 GHz Using Recyclable Point Contacts," 775 (1978). Appl. Pys. Lett., x, T. de Graauw, Personal communication. T. M. Sen, P. L. Ricards, R. E. Harris and F. L. Lloyd, "Conersion Gain in MM-Wae Quasiparticle Heterodyne Mixers,' Appl. Pys. Lett. 36, 777 (1 980). S. Rudner, M. J. Feldman, E. Kolberg and T. Claeson, "SS Quasiparticle Mixing wit Long Antenna Coupled Arrays," Proc.. C. Squid 1 Conf., Tis work. G. J. Dolan, T. G. Pillips and D. P. Woody, "Low Noise 115 GHz Mixing in Superconducting Oxide- 347 (1 979). 689 Barrier Tunnel Junctions," Appl. Pys. Lett. 3, P. L. Ricards, T. M. Sen, R. E. Harris and F. L. Lloyd, "Quasiparticle Heterodyne Mixing in SS Tunnel Junctions," Appl. Pys. Lett. 34, 345 (1979). J. R. Tucker, "Quantum Limi ted Detection in Tunnel Junction Mixers," EEE J. Quantum Electron. QE-15, 1234 (1979). M. Mc Coll, M. 3. Mi 1 lea and A. H. Si ler, "Te Superconductor-Semiconductor Scottky Barrier% Diode Detector," Appl. Pys. Lett., 23, 263 (1973). T. G. Pillips, R. C. Dynes and J. M. Magerlein, Unpublised. P. K. Tien and J. P. Gordon, "Multipoton Process Obsered in te nteraction of Microwae Fields wit te Tunneling between Superconducting Films," Pys. Re. 129, 647 (1963).