Hofherr, M.

Holzapfel, B.

Hänisch, J.

Hübers, H.-W.

Il\u27in, K.

Judin, V.

Müller, A.-S.

Probst, P.

Scheuring, A.

Semenov, A.

Siegel, M.

Wünsch, S.

English

KITopen

Superconducting YBa2Cu3O7−δ Thin Film Detectorsfor Picosecond THz PulsesP. Probst · A. Scheuring · M. Hofherr · S. Wünsch ·K. Il’in · A. Semenov · H.-W. Hübers · V. Judin ·A.-S. Müller · J. Hänisch · B. Holzapfel · M. SiegelAbstract Ultra-fast THz detectors from superconducting YBa2Cu3O7−δ (YBCO)thin films were developed to monitor picosecond THz pulses. YBCO thin films wereoptimized by the introduction of CeO2 and PrBaCuO buffer layers. The transitiontemperature of 10 nm thick films reaches 79 K. A 15 nm thick YBCO microbridge(transition temperature—83 K, critical current density at 77 K—2.4 MA/cm2) em-bedded in a planar log-spiral antenna was used to detect pulsed THz radiation ofthe ANKA storage ring. First time resolved measurements of the multi-bunch fillingpattern are presented.Keywords High-temperature superconductor · YBCO · Thin-film technology ·Pulsed-laser deposition · THz detector · Picosecond synchrotron pulsesP. Probst () · A. Scheuring · M. Hofherr · S. Wünsch · K. Il’in · M. SiegelInstitut für Mikro- and Nanoelektronische Systeme, Karlsruher Institut für Technologie,Hertzstrasse 16, 76187 Karlsruhe, Germanye-mail: petra.probst@kit.eduA. Semenov · H.-W. HübersDLR e.V. (German Aerospace Center), Institut für Planetenforschung, Rutherfordstrasse 2,12489 Berlin-Adlershof, GermanyH.-W. HübersInstitut für Optik und Atomare Physik, TU Berlin, Hardenbergstr. 36, 10623 Berlin, GermanyV. Judin · A.-S. MüllerLaboratorium für Applikationen der Synchrotronstrahlung, Karlsruher Institut für Technologie,Engesser Straße 15, 76131 Karlsruhe, GermanyJ. Hänisch · B. HolzapfelLeibniz Institute for Solid State and Materials Research Dresden, IFW Dresden, Helmholtzstraße 20,01069 Dresden, Germany1 IntroductionSynchrotron radiation in electron storage rings is emitted when accelerated electronspass deflecting magnets. Due to the radio frequency (RF) system which is used toaccelerate the electrons, they are grouped in bunches and the radiation is emittedin the form of short pulses. Coherent synchrotron radiation (CSR) is generated atwavelengths comparable or larger than the length of the electron bunch. Very shortelectron bunches, which were realized at the synchrotron source ANKA [1], enablepulses of THz CSR with the duration down to 1 ps. To analyze these very short pulsesultra-fast THz detectors are required.Due to the requirement of ultimate speed, neither room-temperature THz detectors[2–4] nor cooled semiconducting composite bolometers are suitable for this applica-tion. Superconducting hot-electron bolometers (HEB) are promising candidates forfast direct detection of THz radiation [5–7]. YBCO microbridges have shown evenfaster photoresponse to THz radiation [8] as well as at optical through infrared wave-lengths [9–12]. At optical wavelengths the relaxation time of the detector was foundto be in the range of only a few picoseconds due to strong electron-phonon interac-tion [9, 12]. Another great advantage of high-transition-temperature YBCO detectorsis the possibility to operate devices at temperatures above 77 K, thus low-cost cry-ocoolers can be used in applications.In this paper, we report on the technological challenges in the development ofultra-thin antenna-coupled YBCO microbridges of only a few unit-cells in thicknesswith critical temperatures above 77 K. We present the results of first measurementsof THz pulses by a 15 nm YBCO detector operated at 77 K at the ANKA storagering.2 YBCO Thin-Film Microbridge DetectorThe YBCO thin films were fabricated using the pulsed-laser deposition (PLD) tech-nique. Pulses from a KrF excimer laser (wavelength 248 nm) with an energy densityof 1 J/cm2 were focused on a rotating stoichiometric YBCO target. The substrateholder was situated at a distance of 50 mm from the target in on-axis position andwas heated up during film deposition.The YBCO PLD process was optimized for the development of sensitive mi-crobolometers. To improve the responsivity the volume of the microbolometer hasto be minimized. Therefore, the deposition process was optimized in order to reducethe thickness of the YBCO films and to simultaneously keep the transition tempera-tures well above 77 K. To fabricate detectors for the THz frequency range with lowdielectric losses, the YBCO films were grown on sapphire substrates. However, dueto the crystalline mismatch between sapphire and YBCO and the diffusion of alu-minum into the YBCO film at high deposition temperatures, it is not possible to growhigh-quality superconducting YBCO films directly on sapphire. Buffer layers have tobe used in the deposition process. A standard buffer material for YBCO on sapphireis cerium oxide (CeO2). A CeO2 buffer layer with a film thickness between 8 nm and50 nm was deposited at a substrate temperature of 820°C and an oxygen pressure ofFig. 1 (Color online)Dependence of the transitiontemperature on the YBCO filmthickness for as-deposited filmswith different buffer layers:50 nm thick CeO2 buffer layer(squares), 8 nm CeO2 layer(triangles), PBCO buffer layers(circles). The dashed lines are toguide the eyepO2 = 0.9 mbar. YBCO films with thicknesses between 10 and 100 nm were then de-posited in situ on top of the CeO2 layer at the same temperature with pO2 = 0.7 mbar.After the YBCO deposition the oxygen pressure was increased to 900 mbar and thesubstrate temperature was ramped down to 550°C with a rate of 10°C/min. The tem-perature was kept constant at 550°C for ten minutes for annealing of the YBCO film.Afterwards the heater was ramped down to 400°C before switching off and coolingdown exponentially to room temperature. The vacuum chamber was then evacuatedand a 140 nm Au layer was grown in situ by PLD.The as-deposited films were characterized by measuring the temperature depen-dence of the resistance. The transition temperature Tc was defined as the temperatureat which the resistance decreases below 1% of the normal state resistance above thetransition. The dependence of Tc on the YBCO film thickness for the 50 nm thickCeO2 buffer layer is shown in Fig. 1. For YBCO film thicknesses above 30 nm Tcis nearly constant and well above 77 K. For films thinner than 30 nm Tc decreasessignificantly. This can be explained by the above-mentioned lattice mismatch leadingto oxygen deficiency in the YBCO film, which results in a reduction of Tc [13] downto 25 K for 15 nm thick films. The Tc of films with d < 30 nm has been improvedby a decrease of the CeO2 layer thickness. Due to a shorter deposition time, the thin-ner CeO2 films have less droplets leading to a smoother film surface. The YBCOfilms grow more homogenous on these surfaces which results in an improvement ofthe crystalline quality. The optimum was found at a CeO2 film thickness of approx-imately 8 nm resulting in an increase of Tc up to 74 K for a 18 nm YBCO film (seeFig. 1).The crystalline mismatch was further reduced by the introduction of an additionalbuffer layer made from PrBa2Cu3O7−δ (PBCO). The lattice mismatch of approxi-mately 1% between YBCO and PBCO allows for growing of very thin superconduct-ing YBCO films with high crystalline quality. To protect the YBCO thin film duringthe patterning process, a PBCO protection layer was deposited on top of the YBCOfilm. The introduction of the 25 nm PBCO buffer layer combined with the PBCOprotection layer of the same thickness resulted in Tc = 79 K of YBCO films withthickness of only 10 nm (see Fig. 1).The multi-layers described above were patterned by several electron-beam lithog-raphy steps. The active, detecting area of the YBCO film was opened by removingFig. 2 (Color online)Current-voltage characteristic ofa YBCO detector measured at77 K in LN2. The straight line isthe tangent drawn through theworking point where thedifferential resistance is 50 the Au layer by wet etching with an I2-KI solution. Ion milling was used to patternthe Au antenna. To reduce oxygen loss in the YBCO layer during the ion milling,active sample cooling was employed. A broadband log-spiral antenna was used tocover the wide spectral band (0.2 through 2 THz) of ANKA [1]. The planar antennawas embedded into a coplanar readout line. The detectors were YBCO microbridges(4.5 µm wide, 1.5 µm long made from a 15 nm thick film) with a normal-state resis-tance of approximately 120  that was measured right before the superconductingtransition. The transition temperature of the microbridges reached 83 K which wasalmost the same as for non-structured films. A typical current-voltage characteristic(CVC) which was measured at 77 K in liquid nitrogen is shown in Fig. 2. The ob-served critical current Ic = 1.6 mA corresponds to a uniform critical current densityof about jc = 2.4 MA/cm2. The s-shape of the CVC above 2 mA is caused by dis-sipative movement of magnetic vortices. Switching of the detector into the normalstate around 6 mA occurs via formation of a normal domain across the whole YBCOmicrobridge.3 Experimental Environment and ResultsThe YBCO detectors were mounted in a cryocooler allowing one to vary the oper-ation temperature between 50 K and 300 K. The synchrotron radiation entered thecryocooler through a polyethylene window. The operating temperature was 77 K.The operating point was adjusted by current bias within the resistive transition be-tween the normal and the superconducting state where the DC differential resistancewas close to 50  (Fig. 2). The bias was arranged via a room-temperature bias-tee.The signal was amplified with a room temperature amplifier and fed into a samplingoscilloscope.The experiments were performed at ANKA, the synchrotron light source at KITwhich was operated at 1.3 GeV electron energy with a dedicated magnetic opticsfor short bunches (“low-alpha mode”). Short CSR pulses down to 1 ps with a fre-quency bandwidth of f = 0.1–2 THz [1] are achieved at ANKA by this low alphamode. The time-delay between two adjacent electron bunches is 2 ns correspondingto the 500 MHz frequency of the RF system. The typical filling pattern of the ANKAFig. 3 (a) Measured detector signal of a 15 nm YBCO THz detector over time. The distance between twotrains is 20 ns (50 MHz). In (b) one train with 33 bunches is depicted in detailstorage ring consists of 3 trains separated by a 20 ns gap. Each train consists of 33bunches. The revolution frequency of the whole pattern is 2.7 MHz. Figure 3a showsthe YBCO detector response to the radiation produced by these three trains. In Fig. 3bthe response to only one train is depicted in detail that demonstrates the successfuldetection of all 33 bunches.We have developed a technology process for antenna-coupled superconductingultra-thin YBCO microbridges. A YBCO microbridge detector with only 15 nm filmthickness and a critical temperature above 77 K was successfully tested at ANKA.The multi-bunch filling pattern of the storage ring was resolved in time via its syn-chrotron radiation. The measurements have demonstrated the successful use of theYBCO microbridge detector for monitoring THz radiation at a synchrotron sourceand—in combination with advanced readout techniques—the potential for picosec-ond time-resolution of THz synchrotron pulses.Acknowledgements This work was funded by the German Federal Ministry of Education and Research(Grant No. 05K2010).References1. A. Plech, S. Casalbuoni, B. Gasharova, E. Huttel, Y.-L. Mathis, A.-S. Müller, K. Sonnad, A. Bartels,R. Weigel, in Proceedings of PAC09, vol. TU5RFP026 (2009), p. 12. E.R. Brown, Solid-State Electron. 48(10–11), 2051 (2004)3. D B. Rutledge, D.P. Neikirk, D. Kasilingam, in Infrared and Millimeter Waves. Millimeter Compo-nents and Techniques, Part II, vol. 11 (1984)4. D.P. Neikirk, W.W. Lam, D.B. Rutledge, Int. J. Infrared Millim. Waves 5(3), 245 (1984)5. A D. Semenov, G.N. Gol’tsman, R. Sobolewski, Supercond. Sci. Technol. 15, R1 (2002)6. H.-W. Hübers, A. Semenov, K. Holldack, U. Schade, G. Wüstefeld, G. Gol’tsman, Appl. Phys. Lett.87, 184103 (2005)7. A D. Semenov, H.-W. Hübers, K.S. Ilin, M. Siegel, V. Judin, A.-S. Müller, in Proceedings of 34thInternational Conference on Infrared, Millimeter, and Terahertz Waves, 2009 (IRMMW-THz 2009)(2009), p. 18. P. Probst, A. Scheuring, M. Hofherr, D. Rall, S. Wünsch, K. Il’in, M. Siegel, A. Semenov, A. Pohl,H.-W. Hübers, V. Judin, A.-S. Müller, A. Hoehl, R. Müller, G. Ulm, Appl. Phys. Lett. 98, 043504(2011)9. M. Lindgren, M. Currie, C.A. Williams, T.Y. Hsiang, P.M. Fauchet, R. Sobolewski, S. Moffat,R. Hughes, J.S. Preston, F. Hegmann, IEEE J. Sel. Top. Quantum Electron. 2(3), 668 (1996)10. M. Danerud, D. Winkler, M. Lindgren, M. Zorin, V. Trifonov, B. Karasik, G.N. Gol’tsman, E. Ger-shenzon, J. Appl. Phys. 76(3), 1902 (1994)11. A D. Semenov, I.G. Goghidze, G.N. Gol’tsman, A.V. Sergeev, E.M. Gershenzon, Appl. Phys. Lett.63(5), 681 (1993)12. A D. Semenov, R.S. Nebosis, Y.P. Gousev, M.A. Heusinger, K.F. Renk, Phys. Rev. B, Condens. MatterMater. Phys. 52(1), 581 (1995)13. K.S. Il’in, M. Siegel, J. Appl. Phys. 92, 361 (2002)      Repository KITopen  Dies ist ein Postprint/begutachtetes Manuskript.  Empfohlene Zitierung:  Probst, P.; Scheuring, A.; Hofherr, M.; Wünsch, S.; Il’in, K.; Semenov, A.; Hübers, H.-W.; Judin, V.; Müller, A.-S.; Hänisch, J.; Holzapfel, B.; Siegel, M. Superconducting YBa₂Cu₃O7−δ Thin Film Detectors for Picosecond THz Pulses. 2012. Journal of low temperature physics, 167 doi: 10.554/IR/1000031889    Zitierung der Originalveröffentlichung:  Probst, P.; Scheuring, A.; Hofherr, M.; Wünsch, S.; Il’in, K.; Semenov, A.; Hübers, H.-W.; Judin, V.; Müller, A.-S.; Hänisch, J.; Holzapfel, B.; Siegel, M. Superconducting YBa₂Cu₃O7−δ Thin Film Detectors for Picosecond THz Pulses. 2012. Journal of low temperature physics, 167 (5-6), 898–903.  doi:10.1007/s10909-012-0499-5               Lizenzinformationen: KITopen-Lizenz 

Superconducting YBa₂Cu₃O$_{7-δ}$ Thin Film Detectors for Picosecond THz Pulses

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Superconducting YBa₂Cu₃O $_{7-δ}$ Thin Film Detectors for Picosecond THz Pulses

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Superconducting YBa₂Cu₃O7−δ_{7-δ}7−δ​ Thin Film Detectors for Picosecond THz Pulses

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Superconducting YBa₂Cu₃O $_{7-δ}$ Thin Film Detectors for Picosecond THz Pulses