Jun 19, 2009 - Superconducting NbTiN Nanowire Single Photon Detectors with Low Kinetic Inductance. Shigehito Miki, Masanori Takeda, Mikio Fujiwara. 1.
Applied Physics Express 2 (2009) 075002
Superconducting NbTiN Nanowire Single Photon Detectors with Low Kinetic Inductance Shigehito Miki, Masanori Takeda, Mikio Fujiwara1 , Masahide Sasaki1 , Akira Otomo, and Zhen Wang Kansai Advanced Research Center, National Institute of Information and Communications Technology, 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe 651-2492, Japan 1 National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan Received April 9, 2009; accepted May 25, 2009; published online June 19, 2009 We report on the development of superconducting nanowire single photon detectors (SNSPDs) by using NbTiN thin films on single crystal MgO substrates. NbTiN thin films showed fine crystal structure with closer lattice constant to MgO substrates than that of NbN thin films. The transition temperature of 3.5-nm-thick NbTiN films was comparable to, and the resistivity at 20 K was lower than NbN thin films. The kinetic inductance of NbTiN-SNSPDs was 25% lower than that of NbN-SNSPD, and system detection efficiency was 1.4% at a 1550 nm wavelength, which is comparable to NbN-SNSPDs. # 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.075002
n the fields of quantum information processing and sensing, superconducting nanowire single-photon detectors (SNSPDs) are expected as key components because of their infrared single-photon sensitivity, low dark count rates, excellent timing resolution, and short recovery times.1–5) So far, NbN ultrathin films have been used for nanowires because they have a relatively high superconducting critical temperature (Tc ) even at a thickness of several nanometers, which allows for intrinsically fast energy relaxation and high single photon sensitivity. NbNSNSPDs have shown favorable performances and have been successfully employed in quantum key distribution (QKD) experiments, boosting both transmission distances and key generation rates.6–10) However, further improvement in device performance is highly desirable and will broaden the impact of SNSPDs in QKD and other quantum information processing applications. In particular, significant effort is being put into increasing the detection efficiency (DE) and photon counting rate. A primary factor in limiting the counting rate is the large kinetic inductance (KI) of the nanowire Ldevice , which k determines the device recovery time as Ldevice =Zload k according to equivalent circuit model and systematic investigation.4) The KI is derived from the penetration depth of the superconducting thin films. Since is dependent on Tc and the resistivity of the films,11) applying superconducting material with a high Tc and low resistivity is a way to reduce the KI of SNSPDs. On the other hand, the DE is mainly determined by the product of the intrinsic photo-absorption coefficient of superconducting nanowires, the effective optical coupling coefficient to the active area, and the probability of electrical pulse generation after photon absorption. The probability of electrical pulse generation after photon absorption is considered to be limited by constrictions in the nanowire caused by defects in the superconducting material or introduced during nanofabrication.12) Therefore, superconducting thin films with fine crystal structural homogeneity would be useful in improving the DE of SNSPDs. In light of these conditions, we have been developing NbN-SNSPDs using single-crystal MgO substrates because single-crystal MgO has the same cubic lattice structure and a close lattice constant to NbN; this allows epitaxial growth from the initial layer and excellent superconductivity when compared to non-epitaxial NbN thin films on other substrates.13–15) Of course, further
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film improvement or finding other superconducting materials to replace NbN are important to further improve both the photon counting rate and DE. Niobium titanium nitride (NbTiN) is expected to be an adequate candidate for use in SNSPDs because of its similar crystal structure and superconductivity to NbN, and NbTiNSNSPDs prepared on Si substrates have already been reported recently.16) However, the DE stayed low (less than 0.5% at 960 nm wavelength), and further device performance improvements are required. Epitaxial NbTiN thin films prepared on MgO substrates may have better superconducting properties and a more favorable crystal structure for SNSPDs as compared to epitaxial NbN thin films. Therefore, we have carefully investigated the superconducting and crystal properties of NbTiN thin films, and have developed NbTiN-SNSPDs and verified their performance superiority to the NbN-SNSPDs. As with NbN thin films deposition,17) NbTiN thin films were prepared by a load lock reactive magnetron sputtering system which can achieve a background pressure of 1:5 107 Torr. A niobium titanium (Nb0:8 Ti0:2 ) target with 99.9% purity was used and the target size was 200 mm in diameter. Single-crystal MgO(100) substrates were used to promote the epitaxial growth of the films. The distance between the target and substrate was set to 50 mm. A discharge was performed at a total pressure of 2 m Torr in a mixture of argon (Ar) and nitrogen (N2 ) gases with purities of 99.9999% which were carefully controlled by mass-flow controllers. A direct current (DC) power supply was used to stabilize the discharge state,17,18) and the discharge bias current was set to always be 3.0 A. In order to find the optimum conditions for depositing NbTiN thin films, 150-nm-thick NbTiN films were deposited at various N2 gas mixture ratios between 19.4 and 30.5%. The substrates were intentionally not heated. Figure 1(a) shows the X-ray diffraction pattern for the films on MgO substrates, and Fig. 1(b) illustrates a correlation between Tc , the resistivity at 20 K (20 ), and the lattice constant (a0 ) of 150-nm-thick NbTiN films for a wide variety of N2 gas mixture ratios. For comparison, the properties of NbN thin films prepared by the same sputtering system are also shown in same figures. The films showed a strong (200) diffraction peak for both the NbTiN films and MgO substrates. The a0 of the films determined by the position of 2 for the (200) diffraction peaks were found to
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be from 0.437 to 0.443 nm as the N2 gas mixture ratio increased. The superconducting properties of the films depended upon the film’s lattice constant, and a maximum Tc of 15 K and minimum 20K of 65 cm were obtained from the films with an a0 of 0.438 nm; this film was deposited at the N2 gas ratio of 20.0%. This a0 is closer to that of MgO substrates (¼0:422 nm) than that of NbN thin films (¼0:447 nm), indicating that the lattice mismatch between the substrate and film is less than for NbN thin films. We believe that this good lattice matching must affect the initial growth layers of the films, leading to less defects and better uniformity in the ultrathin films. Figure 2 shows the Tc and 20K of NbTiN and NbN thins films as a function of film thickness. At thicknesses thicker than 100 nm, both Tc and 20K are similar or slightly worse than those for NbN thin films. However, at thicknesses less than 20 nm, although the Tc of NbTiN films is about 1 K lower, 20 is pretty lower than that of NbN thin films. It is reasonable to suppose these better superconductivities are due to the fine crystal structure in the initial growth layers resulting from better lattice matching to MgO compared to the NbN thin films.
Fig. 3. Sheet inductance of NbTiN and NbN-SNSPDs at each nanowire thickness. The width and nanowire pitch of all the devices in the figure are 100 and 200 nm respectively. NbTiN-SNSPD devices showed about a 25% lower sheet inductance than NbN-SNSPDs.
By utilizing these NbTiN ultrathin films, nanowire devices were fabricated through the same process as NbNSNSPDs described elsewhere,13) and we measured the KI of devices Ldevice by observing the phase of a reflected k microwave signal versus the frequency using a network analyzer.12,14) Figure 3 shows the sheet inductance (¼ Ldevice l=w, where l is nanowire total length and w is k nanowire width) of NbTiN- and NbN-SNSPDs at each thickness. The width and nanowire pitch of all devices in the figure are 100 and 200 nm, respectively. The thicknesses of each device were controlled by the film deposition rate and time; they were measured by transmission electron micrograph (TEM) observations after fabrication to define the thickness precisely. There were almost no differences between the supposed and measured thicknesses; this means that the thickness of the NbTiN films can be controlled as we contemplate. The KI of our NbN-SNSPDs shown here have already been described elsewhere13,14) and showed lower KI values than those reported in refs. 12, 16, and 19. For example, the Ldevice in ref. 16 was 500 nH for 10 10 m2 k device with 6 nm-thick and 100 nm-wide nanowire, while same design of our NbN device obtained from measured sheet inductance becomes much lower values of 160 nH. As shown in the figure, the sheet inductances of NbTiNSNSPDs were about 25% lower than those of NbN-SNSPDs; this results from NbTiN films having comparable Tc to and lower 20 than NbN films. These results lead that NbTiNSNSPD can achieve a faster response speed than our NbNSNSPDs at the same design. We also measured the DE of NbTiN-SNSPD at the 1550nm wavelength. The device sensitive area, nanowire-width, and nanowire pitch in this time were designed to be a 15 15 m2 square, 100 nm, and 160 nm, respectively. The thickness of the NbTiN films used for the nanowire was 4.5 nm, and the Tc of the devices was 9.6 K. The Ic of the device was 35.0 A (Jc of 8 106 A/cm2 , which is similar in value to NbN devices13)); this is large enough to a produce output signal with a high signal-to-noise ratio for certain counting. The measured Ldevice was 390 nH, corresponding k to 7.8 ns recovery time and 72 MHz counting rate. The NbTiN-SNSPD was installed into the fiber-coupled SNSPD system with a Gifford MacMahon (GM) cryo cooler, as described elsewhere in detail.15) Briefly, this system can be cooled down to 2.9 K with a thermal fluctuation of 10 mK.
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a practically available DE of 1.4% at a nominal dark count rate of 100 Hz. As a final note, the superconducting properties of NbTiN thin films would be controllable by changing the composition ratio of the NbTi target, allowing for further optimization of the superconducting properties to improve both the KI and DE.
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The input signal can be introduced through an single mode (SM) optical fiber with a core diameter of 9 m, and the fiber-end was placed on the face of the device sensitive area with gap of 40 m. Using simple Gaussian beam estimation, 99% of the input light can be irradiated into the 15 15 m2 device area. We measured the system DE of a single photon at the 1550-nm wavelength including whole system optical losses and the dark count rate. Figure 4 shows the system DE as a function of dark count rate, biasing the dc current from 0.95 to 0:99Ic at each point. The system DE at a 100 Hz dark count rate of the device was about 1.4%, which is comparable in value to our NbN (1– 3%14,15)) devices. This is a first report that the available system DE value at telecommunication wavelength can be obtained by using NbTiN nanowire. We believe that a reason of high DE is our NbTiN thin films showed high Tc and high quality at 4.5-nm-thick nanowire, which is thinner enough to have sensitivity in telecom wavelength, in contrast to ref. 16. Although we have not yet confirmed its clear competitive edge against NbN-SNSPDs in terms of the system DE, we believe that NbTiN-SNSPDs can demonstrate superior DE performance through further design optimization and systematic investigation because NbTiN ultrathin films must have fewer defects in the films due to their fine crystal structure, as described already. In conclusion, we have reported on the superconducting and crystal properties of NbTiN thin films and the performance of NbTiN-SNSPDs. As results of careful optimization and investigation of NbTiN films, we found that NbTiN ultra-thin films have several favorable features for use in SNSPDs: a fine crystal structure, a better lattice matching to MgO substrates, and a lower resistivity than NbN thin films. We succeeded in fabricating NbTiNSNSPDs with 25% lower KI than NbN-SNSPDs and with
1) G. Gol’tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski: Appl. Phys. Lett. 79 (2001) 705. 2) A. Korneev, V. Matvienko, O. Minaeva, I. Milostnaya, I. Rubtsova, G. Chulkova, K. Smirnov, V. Voronov, G. Gol’tsman, W. Slysz, A. Pearlman, A. Verevkin, and R. Sobolewski: IEEE Trans. Appl. Supercond. 15 (2005) 571. 3) A. Pearlman, A. Cross, W. Słysz, J. Zhang, A. Verevkin, M. Currie, A. Korneev, P. Kouminov, K. Smirnov, B. Voronov, G. Gol’tsman, and R. Sobolewski: IEEE Trans. Appl. Supercond. 15 (2005) 579. 4) A. J. Kerman, E. A. Dauler, W. E. Keicher, J. K. W. Yang, K. K. Berggren, G. Gol’tsman, and B. Voronov: Appl. Phys. Lett. 88 (2006) 111116. 5) K. M. Rosfjord, J. K. W. Yang, E. A. Dauler, A. J. Kerman, V. Anant, B. M. Boronov, G. N. Gol’tsman, and K. K. Berggren: Opt. Express 14 (2006) 527. 6) R. H. Hadfield, J. L. Habif, J. Schlafer, and R. E. Schwall: Appl. Phys. Lett. 89 (2006) 241129. 7) H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, and Y. Yamamoto: Nat. Photonics 1 (2007) 343. 8) A. Tanaka, M. Fujiwara, S. W. Nam, Y. Nambu, S. Takahashi, W. Maeda, K. Yoshino, S. Miki, B. Baek, Z. Wang, A. Tajima, M. Sasaki, and A. Tomita: Opt. Express 16 (2008) 11354. 9) T. Honjo, S. W. Nam, H. Takesue, Q. Zhang, H. Kamada, Y. Nishida, O. Tadanaga, M. Asobe, B. Baek, R. Hadfield, S. Miki, M. Fujiwara, M. Sasaki, Z. Wang, K. Inoue, and Y. Yamamoto: Opt. Express 16 (2008) 19118. 10) D. Stucki, N. Walenta, F. Vannel, R. T. Thew, N. Gisin, H. Zbinden, S. Gray, C. R. Towery, and S. Ten: to be published in New J. Phys. 11) Z. Wang, A. Kawakami, Y. Uzawa, and B. Komiyama: J. Appl. Phys. 79 (1996) 7837. 12) A. J. Kerman, E. A. Dauler, J. K. W. Yang, K. M. Rosfjord, V. Anant, K. Berggren, G. N. Gol’tsman, and B. M. Voronov: Appl. Phys. Lett. 90 (2007) 101110. 13) S. Miki, M. Fujiwara, M. Sasaki, and Z. Wang: IEEE Trans. Appl. Supercond. 17 (2007) 285. 14) S. Miki, M. Fujiwara, M. Sasaki, B. Baek, A. J. Miller, R. H. Hadfield, S. W. Nam, and Z. Wang: Appl. Phys. Lett. 92 (2008) 061116. 15) S. Miki, M. Fujiwara, M. Sasaki, and Z. Wang: to be published in IEEE Trans. Appl. Supercond. 16) S. N. Dorenbos, E. M. Reiger, U. Perinetti, V. Zwiller, T. Zijlstra, and T. M. Klapwijk: Appl. Phys. Lett. 93 (2008) 131101. 17) S. Miki, Y. Uzawa, A. Kawakami, and Z. Wang: Electron. Commun. Jpn., Part II 85 (2002) No. 77. 18) P. Yagoubov, G. Gol’tsman, B. Voronov, L. Seidman, V. Siomash, S. Cherednichenko, and E. Gershenzon: Proc. 7th Int. Symp. Space Terahertz Technology, 1996, p. 290. 19) R. H. Hadfield, P. A. Dalgarno, J. A. O’Connor, E. Ramsay, R. J. Warburton, E. J. Gansen, B. Baek, M. J. Stevens, R. P. Mirin, and S. W. Nam: Appl. Phys. Lett. 91 (2007) 241108.
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