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PAPER
Special Section on Innovative Superconducting Devices and Their Applications
X-Ray Detection Using Superconducting Tunnel Junction Shaped Normal-Distribution-Function Tohru TAINO†a) , Tomohiro NISHIHARA† , Koichi HOSHINO† , Nonmembers, Hiroaki MYOREN† , Member, Hiromi SATO†† , Hirohiko M. SHIMIZU†† , Nonmembers, and Susumu TAKADA† , Member
SUMMARY A normal-distribution-function-shaped superconducting tunnel junction (NDF-STJ) which consists of Nb/Al-AlO x /Al/Nb has been fabricated as an X-ray detector. Current - voltage characteristics were measured at 0.4 K using three kinds of STJs, which have the dispersion parameters σ of 0.25, 0.45 and 0.75. These STJs showed very low subgap leakage current of about 5 nA. By irradiating with 5.9 keV X-rays, we obtained the spectrum of these NDF-STJs. They showed good energy resolution with small magnetic fields of below 3 mT, which is about one-tenth of those for conventional-shaped STJs. key words: superconducting tunnel junction, normal-distribution-function shape, the dispersion parameters σ, X-ray detector
1.
Introduction
Superconducting tunnel junctions (STJs) are expected to be next-generation X-ray detectors with high energy resolution and high counting rate. The theoretical energy resolution of Nb-based STJ X-ray detectors can be better than 5 eV for 5.9 keV X-rays [1]. A resolution of 29 eV was achieved using a Nb-based STJ at 0.2 K [2]. Nb-based STJ X-ray detectors can count at high counting rates of tens of thousands of counts per second because of their short pulse decay time of a few µsec [3]. When an STJ is used as an X-ray detector, we must supply a magnetic field to suppress the dc Josephson current and the Fiske step. The amplitude of magnetic field for suppressing the Josephson current and the Fiske step was about 30 mT. A reduction in the magnetic field is desired for the practical use of STJs. We have already proposed and demonstrated that normal-distribution-function-shaped STJs (NDF-STJs) are one of the solutions for realizing a simple and easy suppression of the dc Josepson current [4]. A NDF-STJ with dispersion parameter σ = 0.45 showed that the Josephson current was suppressed by a magnetic field of about 0.5 mT at 4.2 K. In this paper, we present the electronic and magnetic characteristics and the first measurement of X-ray detection using three kinds of NDF-STJs which have σ values of 0.25, 0.45 and 0.75.
Manuscript received August 8, 2006. Manuscript revised October 6, 2006. † The authors are with the Saitama University, Saitama-shi, 338-8570 Japan. †† The authors are with the RIKEN, Wako-shi, 351-0198 Japan. a) E-mail:
[email protected] DOI: 10.1093/ietele/e90–c.3.566
2.
NDF-STJs
The required magnetic field to suppress the dc Josephson current can be reduced by using NDF-STJs, because the magnetic field dependence of the Josephson current Ic is given by ⎤ ⎡ ⎛ √ ⎢⎢⎢ ⎜⎜ πB(2λL + t) Aσ ⎞⎟⎟2 ⎥⎥⎥ ⎟ ⎢ ⎜ Ic (B) = Ic (0) exp ⎢⎢⎣− ⎝⎜ (1) ⎠⎟ ⎥⎥⎥⎦ , φ0 where B is the applied magnetic field, λL is the London penetration depth of a superconductor, t is the thickness of tunnel barrier, A is the detector area, σ is the dispersion parameter and φ0 is the magnetic flux quantum. In this equation, Ic falls rapidly according to exp(−B)2 . As a result, the magnetic field required to suppress the Josephson current of an NDF-STJ is smaller than those for STJs of other shapes. The design parameters of NDF-STJ are determined by the detector area A and σ. We selected an A of 2,500 µm2 and the σ values of 0.25, 0.45 and 0.75. The fabrication of STJs is based on the Nb/Al multilayer technology in a facility at RIKEN. The thickness of the STJ structure was Nb(150 nm)/Al(70 nm)AlO x /Al(70 nm)/Nb(200 nm), deposited by dc magnetron sputtering without breaking the vacuum. Each layer was patterned by photolithography and reactive ion etching (RIE). The Al layers were applied as a trapping layer, which increases pulse height during photon irradiation. To reduce subgap leakage current, O2 plasma was employed after the formation of the NDF-STJs by RIE [5]. Figure 1 shows photographs of the fabricated NDF-STJs. In this figure, (a), (b) and (c) correspond to σ = 0.25, 0.45 and 0.75, respectively. The area of these NDF-STJs is A = 2,500 µm2 . The counterelectrode is connected to the wiring lines through a 5×5 µm2 contact hole. 3.
Electrical and Magnetic Characteristics
To evaluate the fabricated NDF-STJs, their electrical and magnetic characteristics were measured in the temperature range from 4.2 to 0.4 K. We measured the current value as the subgap leakage current at 0.2 mV at each temperature. During I − V characteristics measurement, a magnetic field of about 30 mT was applied to suppress the dc Josephson current using a superconducting Helmholtz coil fabricated from NbTi wire. The
c 2007 The Institute of Electronics, Information and Communication Engineers Copyright
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Fig. 1 A photograph of the fabricated NDF-STJs. The area of three NDF-STJ is 2,500 µm2 . (a) is σ = 0.25, (b) corresponds to σ = 0.45 and (c) is σ = 0.75.
Fig. 3 Magnetic field dependence of the Josephson current of NDF-STJ of 2,500 µ2 m. The solid lines show the theoretical magnetic field. The circles, triangles and crosses show the measured values using NDF-STJs with σ = 0.25, 0.45 and 0.75, respectively.
the theoretical values calculated with Eq. (1). The circles, triangles and crosses show the values measured using NDFSTJs with σ = 0.25, 0.45 and 0.75, respectively. In this figure, the measured values are in good agreement with theoretical values. Moreover, the magnetic field required to suppress the Josephson current is smaller than 1 mT. STJs of other shapes usually require a magnetic field of more than 10 mT. These results show that a NDF-STJ can be utilized as a high-performance photon detector owing to its low subgap leakage current and its small required applied magnetic field. Fig. 2 Temperature dependence of the subgap leakage current. The solid line shows the calculation of BCS theory. The circles, triangles and crosses show measured values using the NDF-STJ with the σ values of 0.25, 0.45 and 0.75, respectively.
typical subgap leakage current of the NDF-STJs was 5 nA and the typical critical current density Jc of the fabricated STJ was 80 A/cm2 at 0.4 K. Figure 2 shows the temperature dependence of the subgap leakage current of three kinds of STJs. The vertical axis indicates the subgap current normalized by the subgap current measured at 4.2 K. Here, the subgap current at 4.2 K is determined by the tunnel current. The circles, triangles and crosses show values measured using the NDF-STJs with the σ values of 0.25, 0.45 and 0.75, respectively. The solid line shows the theoretical value predicted by BCS theory. From this figure, the temperature dependence of the subgap leakage current is found to correspond to that BCS theory. In particular, the NDF-STJ with σ = 0.45 is in good agreement with BCS theory. To confirm the promising characteristics of the NDFSTJ, the magnetic field characteristics are measured at 4.2 K. Figure 3 shows the magnetic field dependence of the Josephson current. The vertical and horizontal axes indicate normalized Josephson current and applied magnetic field using the Helmholtz coil, respectively. The three lines show
4.
X-Ray Detection Experiments
An X-ray detection experiment was carried out by irradiating 5.9 keV X-rays on to the fabricated NDF-STJs. A 5 × 5 mm2 chip consisting of a number of NDF-STJs was mounted on a copper plate. The plate was attached to the cold stage of a 3 He cryostat. An X-ray source (55 Fe) was attached to the surface of the cryostat. A detection signal was observed through a charge-sensitive preamplifier at room temperature. The typical pulse height of detected signals was about 300 mV, as shown in Fig 4. A rise time of 5 µsec and a decay time of 200 µsec were obtained using the NDF-STJ with σ = 0.75. The number of tunneling quasiparticles due to X-ray irradiation was calculated to be 3.6 × 106 . The typical pulse height of the NDF-STJs without an Al trapping layer was about 60 mV. The Al trapping layer works as a multitrapping layer for tunneling quasi-particles. Pulse height spectra were measured using the NDFSTJs under various applied magnetic fields. A typical energy resolution was about 55 eV using the NDF-STJ with σ = 0.75. Figure 5 shows the magnetic field dependence of the energy resolution using the three NDF-STJs. The magnetic field was applied using the Helmholtz coil in a 3 He cryostat. In this figure, the circles, triangles and crosses show values measured using NDF-STJs with σ = 0.25, 0.45 and 0.75,
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Acknowledgement The authors wish to express thanks to Drs. S. Shiki, M. Kurakado, S. Ariyoshi and C. Otani of RIKEN for their various discussions and support. We also thank Drs. H. Nakagawa, K. Kikuchi and M. Aoyagi for the fabrication process at the early stages of this work. References
Fig. 4 Detected signal waveform by irradiating 5.9 keV X-ray using NDF-STJ with σ = 0.75.
[1] N. Rando, A. Peacock, C. Foden, and A.V. Dordrecht, “X-ray characteristics of a niobium superconducting tunnel junction with a highly transmissive tunnel barrier,” J. Appl. Phys., vol.73, pp.5098–5104, 1993. [2] C.A. Mears, S.E. Labov, M. Frank, M.A. Lindeman, L.J. Hiller, H. Netel, and A.T. Barfknecht, “Analysis of pulse shape from a high-resolution superconducting tunnel junction X-ray spectrometer,” Nucl. Instr. and Meth., vol.370, pp.53–56, 1996. [3] M. Frank, L.J. Hiller, J.B. le Grand, C.A. Mears, S.E. Labov, M.A. Lindeman, H. Netel, and D. Chow, “Energy resolution and high count rate performance of superconducting tunnel junction x-ray spectrometers,” Rev. Sci. Instrum., vol.69, no.1, pp.25–31, 1998. [4] K. Kikuchi, H. Myoren, T. Iizuka, and S. Takada, “Normaldistribution-function-shaped Josephson tunnel junction,” Appl. Phys. Lett., vol.77, no.22, pp.3660–3661, 2000. [5] H. Sato, Y. Takizawa, W. Ootani, T. Ikeda, T. Oku, C. Otani, H. Watanabe, K. Kawai, H. Miyasaka, H. Kato, H.M. Shimizu, H. Nakagawa, H. Akoh, M. Aoyagi, and T. Taino, “Improved fabrication method for Nb/Al/AlO x /Al/Nb superconducting tunnel junctions as X-ray detectors,” Jpn. J. Appl. Phys., vol.39, pp.5090–5094, 2000.
Fig. 5 Magnetic field dependence of the energy resolution using three NDF-STJs of 2,500 µ2 m. The circles, triangles and crosses show measured values using NDF-STJ with parameter σ = 0.25, 0.45 and 0.75, respectively.
respectively. As shown in Fig. 5, the STJ with σ = 0.25 is better than the other NDF-STJs at 30 mT. On the other hand, the STJ with σ = 0.75 is better than the other NDF-STJs at about 5 mT. This difference in energy resolution has not been analyzed yet. However, the NDF-STJs showed good energy resolution with small magnetic field. 5.
Conclusion
We first measured X-ray detection using three NDF-STJs which had the σ values of 0.25, 0.45 and 0.75. The fabricated NDF-STJs showed very low subgap leakage current, and the magnetic field required to suppress the Josephson current was about 10 times smaller than those for conventional STJs, which are square and diamond shaped. By irradiating 5.9 keV X-rays, the spectrum of these STJs showed good energy resolution with small magnetic field compared with STJs of other shapes. The energy resolution of the NDF-STJs showed the same value as for conventionalshaped STJs. We confirmed that NDF-STJs have a large advantage, particularly for suppressing the Josephson current because of the small applied magnetic fields. We are extending the application of the present NDF-STJs a terahertz wave detector.
Tohru Taino received the B.E. degree in engineering from Nagaoka University of Technology, Niigata, Japan in 1996, and the M.E. and D.E. degrees in engineering from Kyushu University, Fukuoka, Japan in 1999 and 2002, respectively. In 2002, he joined as a contract researcher of the Image Information Division, RIKEN. Since 2002, he has been as a research associate in the Department of Electrical and Electronic System, Faculty of Engineering, Saitama University, Saitama, Japan. His research interests are in superconducting electronics and systems. Dr. Taino is a member of the Japan Society of Applied Physics and the Atomic Energy Society of Japan.
Tomohiro Nishihara receive the B.E. and M.E. degrees in engineering from Saitama University, Saitama, Japan in 2000 and 2002. Since 2002, he has been with Fuji heavy Industries Ltd.
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Koichi Hoshino receive the B.E. and M.E. degrees in engineering from Saitama University, Saitama, Japan in 2001 and 2003. Since 2003, he has been with G.E. Yokogawa medical system Ltd.
Hiroaki Myoren received the B.E. degree in physical electronics and the M.E. and the D.E. degrees in advanced materials engineering from Hiroshima University, Higashi-Hiroshima, Japan, in 1985, 1987, and 1992, respectively. In 1989, he joined the Faculty of Engineering, Hiroshima University, as a Research Associate. In 1992, he joined the Research Institute of Electrical Communication, Tohoku University, Sendai, Japan, as a Research Associate. He also joined the Low Temperature Division, Faculty of Applied Physics, University of Twente, Enschede, The Netherlands, in 1996, and 1997, as a Research Fellow. Since 1998, he has been as an Associate Professor of Department of Electrical and Electronic System, Faculty of Engineering, Saitama University, Saitama, Japan. His research interests include superconducting electronics and materials, especially in high-Tc superconductor devices. Dr. Myoren is a member of the Japan Society of Applied Physics.
Hiromi Sato received the B.S., the M.S. and the D.S. degrees in applied physics from Tokyo Institute of Technology, Tokyo, Japan, in 1991, 1993, and 1997, respectively. In 1997, he joined the Cosmic Radiation Laboratory, RIKEN, Wako, Saitama, Japan as a Special Postdoctoral Researcher. Since 2001, he has been as a researcher of RIKEN. His Research interests include development of superconducting tunnel junctions as radiation detectors. Dr. Sato is a member of the Physical Society of Japan, the Japan Society of Applied Physics and the Japanese Society for Neutron Science.
Hirohiko M. Shimizu received the B.S., M.S. and D.S. degrees in physics from Kyoto University, Kyoto, Japan, in 1986, 1988 and 1992, respectively. In 1992, he joined National Laboratory for High Energy Physics (High Energy Accelerator Research Organization), Tsukuba, Ibaraki, Japan as a research associate. In 1995, he joined RIKEN, Wako, Saitama, Japan as a research scientist. In 2000, he was promoted to a head and a senior scientist of Image Information Division of RIKEN. Since 2005, he has been as a professor, Neutron Division, KEK, Tsukuba, Ibaraki, Japan. His major research interests include nuclear and particle physics, astrophysics. Dr. Shimizu is a member of the Physical Society of Japan and the Japanese Society for Neutron Science.
Susumu Takada received the B.S. and M.S. degrees in electrical engineering from Yokohama National University, Japan, in 1967, 1969, respectively, and the Ph.D. degree in electronic engineering from Tohoku University, Sendai, Japan, in 1979. He joined the Electrical Laboratory (ETL), Tsukuba, Japan, in 1969, where initially he was engaged in research on surface acoustic wave devices. Since 1977 he has been engaged in Josephson integrated circuit technology, expect for one year from 1981 to 1982, where he worked as a visiting researcher at Max-Planck-Institute for Physics and Astrophysics, Munich, Germany. From 1981 to 1988 he worked in the special section on Josephson computer technology. He was the chief of the superconductivity Electronics Section and led the ETL work on Josephson integrated devices. Since 1996, he has been as a Professor in the Department of Electrical and Electronic System, Faculty of Engineering, Saitama University, Saitama, Japan. Dr. Takada is a member of the Japan Society of Applied Physics, and the Institute of Electrical Engineers of Japan.
