Contains reports on two research projects.Defense Advanced Research Projects Agency Contract MDA 972-90-C-002

Hu, Qing

Orlando, Terry P.

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Chapter 2. Superconducting Electronic DevicesChapter 2. Superconducting Electronic DevicesAcademic and Research StaffProfessor Qing Hu, Professor Terry P. Orlando, Dr. John Melngailis, Dr. Ivan V. ZitkovskyGraduate StudentsPartha Saha, Rolf WyssTechnical and Support StaffBarbara A. King2.1 High Tc SuperconductingSQUIDs and MixersSponsorDefense Advanced Research Projects AgencyContract MDA 972-90-C-0021Project StaffProfessor Qing Hu, Professor Terry P. OrlandoMany superconducting analog devices have beendemonstrated to have higher sensitivities, speed,and frequency limits and lower power dissipationthan competing semiconductor devices. Amongthe superconducting devices, the most successfulones are SQUIDs (Superconducting QUantumInterference Devices) and mixers.Similar to its optical analog, the Mach-Zehnderinterferometer, a SQUID is an interferometer com-posed of two waves of superconducting electrons.By modulating the relative phases of the twowaves using a small magnetic field, a SQUIDmagnetometer can achieve the sensitivity of afaction of magnetic quantum flux. Supercon-ducting mixers use the high nonlinearity of thecurrent-voltage characteristics of superconductingtunnel junctions to perform high-efficiency mixing.Currently, superconducting mixers have thehighest sensitivities which are only limited byHeisenberg's uncertainty principle.The discovery of superconductors with a super-conducting transition temperature higher thanliquid nitrogen temperature (high-Tc superconduc-tors) has opened up exciting new possibilities inelectronic device technology. The high temper-ature version of the superconducting devices men-tioned above will have a much wider range ofapplications wherever refrigeration is a problem.However, the benefit of higher operating temper-ature has posed a new challenge which is notencountered in conventional superconductingdevices. The key element for both SQUIDs andmixers is the so-called Josephson junction. Thisjunction is formed by two superconductors weaklycoupled together electrically. Heisenberg's uncer-tainty principle dictates that higher transition tem-perature will result in a shorter superconductingcoherence length. This is the length scale atwhich superconductivity vanishes at an interfacialboundary. Consequently, high-Tc Josephson junc-tions require that the weaklink regions are muchsmaller than those for low Tc devices.In collaboration with Dr. Melngailis' group, wehave invented several novel ways of making highT, superconducting devices using focused ionbeams (FIB). FIBs can be used to either lith-ographically pattern high-To superconducting filmsor to directly pattern the films or the substrates toform Josephson junctions. Currently, we are opti-mizing our process to improve the quality of theJosephson junctions.This work is being performed with the cooperationof the AT&T Bell Laboratories in Murray Hills,New Jersey.2.2 Millimeter Wave andInfrared SuperconductingReceiversSponsorDefense Advanced Research Projects AgencyContract MDA 972-90-C-0021Project StaffProfessor Qing HuAlthough there is great potential for applications inremote sensing and communication, millimeterwave and far-infrared frequencies remain one ofthe most underdeveloped frequency ranges. Thisis because the millimeter wave and far-infrared fre-quency range falls between the two other fre-quency ranges in which conventional107Chapter 2. Superconducting Electronic Devicessemiconductor devices are usually operated. One isthe microwave frequency range, and the other isthe near-infrared and optical frequency range.Semiconductor devices which utilize the classicaldiffusive transport of electrons, such as diodes andtransistors, have a high frequency limit. This limitis set by the time it takes for electrons to travel acertain distance. Currently, electron mobility andthe smallest feature size which can be fabricatedby lithography limit the frequency range to below100 GHz. It is not likely that this limit can bepushed much higher. Semiconductor devicesbased on quantum mechanical interband transi-tions, however, are limited to frequencies higherthan those corresponding to the semiconductorenergy gap, which is higher than 10 THz for mostbulk semiconductors. Therefore, a large gap existsfrom 100 GHz to 10 THz in which very fewdevices are available.The gap energies of conventional superconductorssuch as Nb are in the range of 100 GHz to 2 THz.This coincidence makes superconducting devicesnatural candidates for millimeter and submillimeterwave applications. In addition, superconductingdevices usually have higher sensitivities and speedand lower power dissipation than semiconductordevices. Two types of superconducting devices arestudied in this project: coherent heterodynereceivers and incoherent radiation detectors. Theformer has potential applications in communi-cation as well as radiation detection, while thelatter is usually used for remote sensing.At millimeter wave frequencies, superconductingradiation detectors have their current responsivityapproaching the quantum efficiency e/hw, that is,a transport of one electron for one incomingphoton. Such efficiencies have been achievedonly at much higher frequencies by semiconductorphotoconductive detectors. Therefore, supercon-ducting radiation detectors may find wide-rangeapplications in remote sensing and far-infraredspectroscopy.At millimeter and submillimeter wavelengths, thesuperconducting coherent receivers have their sen-sitivities limited only by the zero-point fluctuationof vacuum. Such receivers have been used widelyin astrophysical studies. More applications are fea-sible in space-based communication and far-infrared spectroscopy, which requires ultimatesensitivity.This work is being performed with the cooperationof the MIT Lincoln Laboratory, IBM Corporation'sThomas J. Watson Research Center at YorktownHeights, New York, and the AT&T Bell Laborato-ries in Murray Hills, New Jersey.108 RLE Progress Report Number 133

Superconducting Electronic Devices

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Superconducting Electronic Devices

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