Comparison of Superconducting Nanowire Single ... - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 28, NO. 1, JANUARY 2018

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Comparison of Superconducting Nanowire Single-Photon Detectors Made of NbTiN and NbN Thin Films Xiaoyan Yang

, Member, IEEE, Lixing You , Member, IEEE, Lu Zhang, Chaolin Lv, Hao Li, Xiaoyu Liu, Hui Zhou, and Zhen Wang

Abstract—We fabricated superconducting nanowire singlephoton detectors (SNSPDs) made of NbTiN and NbN thin films in the same fabrication batch. Various parameters, such as critical temperature, resistivity, switching current, and kinetic inductance, were compared between the two types of SNSPDs. These data indicate that NbTiN SNSPD surpasses NbN SNSPD in many aspects, including a shorter recovery time due to smaller kinetic inductance, a lower timing jitter due to higher switching current, and better uniformity. Based on the measurement of 28 out of 121 NbTiN SNSPDs on a 2-in Si wafer, 71% of detectors show a system detection efficiency of over 50% at a dark count rate of 100 Hz, with the maximum efficiency being 77.5%. Index Terms—NbTiN films, NbN films, superconducting nanowire single-photon detectors (SNSPD).

I. INTRODUCTION IGH-PERFORMANCE superconducting nanowire single-photon detectors (SNSPDs) have been advancing a wide range of applications, including quantum information, quantum optics, free-space laser communication, and light detection and ranging [1]–[5]. This niche market has fostered five startup companies with SNSPDs as their core product. Several materials have been applied to fabricating high-performance SNSPDs, including NbN [6]–[8], NbTiN [9]–[12], WSi [13], MoSi [14], and NbSi [15]. NbN and NbTiN have similar superconducting transition temperatures and growth conditions and have been used not only for SNSPDs but also for other superconducting electronic devices including hot-electron bolometers. The key advantage of NbN and NbTiN is that SNSPDs

H

Manuscript received September 4, 2017; revised October 23, 2017; accepted November 16, 2017. Date of publication November 22, 2017; date of current version December 18, 2017. This work was supported in part by the National Key R&D Program of China under Grant 2017YFA0304000, in part by the National Natural Science Foundation of China under Grant 61501439, Grant 61671438, and Grant 1501442, in part by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant XDB04010200, and in part by the Science and Technology Commission of Shanghai Municipality under Grant 16JC1400402. This paper was recommended by Associate Editor C. Kilbourne. (Corresponding author: Lixing You.) The authors are with the State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro System and Information Technology and the Center for Excellence in Superconducting Electronics, Chinese Academy of Sciences, Shanghai 200050, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2017.2776288

made of these materials show a high detection efficiency when operated at temperatures of 2–4 K [6], [9], which are achievable by commercial compacted Gifford–McMahon cryocoolers. There are a few reports on the comparison of SNSPDs made of NbN and NbTiN films previously. For example, Miki et al. [16] compared electric characteristics of SNSPDs of two films deposited on MgO substrates. Dorenbos et al. [17] compared dark count rate (DCR) of SNSPDs of two films on silicon substrates. Thoen et al. [18], and Slysz et al. [19] studied and compared the homogeneity of NbN and NbTiN films on silicon and sapphire substrates. Nevertheless, the systematic comparisons for SNSPDs made of NbTiN and NbN films on silicon substrates have not been reported. In this paper, we deposited NbTiN and NbN films on two different 2-in Si wafers, fabricated SNSPDs, and compared their properties. The electrical properties include critical temperature (Tc ), switching current (Isw ), timing jitter (TJ), kinetic inductance (L k ), recovery time (τ ), system detection efficiency (SDE), and DCR. Although they had similar performances in terms of SDE and DCR, SNSPDs made of NbTiN showed better performance in regard to recovery time, TJ, and uniformity than those made of NbN. II. METHODS The NbTiN and NbN thin films were deposited onto 2-in double-side polished thermally oxidized Si substrates by reactive dc-magnetron sputtering in an Ar + N2 gas mixture under a total pressure of 0.27 Pa at room temperature. The base pressure is better than 2 × 10−5 Pa. The flow rates of Ar and N2 were set into 30 and 4 sccm using mass-flow controllers. The Nb:Ti ratio of the NbTi target is 1:1. The dc powers supplied during deposition are 600 and 570 W for NbTiN and NbN films, respectively. The dimension of both targets is 8 in with targetsubstrate distance of 50 mm. The film thicknesses are controlled via sputtering rate and sputtering time. As bulk materials, NbTiN and NbN have transition temperatures of 16–18 K [20], [21]; as ultrathin films fabricated on oxidized Si substrates, NbTiN usually has higher Tc values than NbN films with the same thickness based on our experience. To compare the SNSPDs made of NbTiN and NbN films, we empirically tuned the film thicknesses to 5 and 7 nm, respectively, such that they had similar Tc s. It is noted that the thickness mentioned here is the nominal thickness determined by the sputtering time. The precise

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thickness can be determined by either X-ray reflectivity (XRR) or transmission electron microscopy [22], [23]. A total of 121 SNSPDs are fabricated for each film on 2-in wafers in the same process with the same structure. The fabrication processes included electron-beam lithography (EBL), UVstepper lithography, reactive-ion etching, and others. The active area of the detector has a conventional meandered nanowire structure with a width of 70 nm, a space of 90 nm, and a diameter of 15 μm. A multilayer film of SiO/Ti/Au was e-beam evaporated atop the nanowires, together with the SiO2 /Si layer beneath the nanowire, which form an optical cavity to enhance light absorption of SNSPD when SNSPDs are illuminated from the backside of the substrate. Besides, the SiO2 layer on the backside of the Si wafer also works as the antireflection coating to reduce the reflection. Additionally, we also fabricated straight nanowires with different lengths using the same process parameters to evaluate the uniformities of the films. A lensed single-mode fiber was aligned with the active area of the SNSPD by backside optical coupling. The packaged detectors were mounted inside a cryostat based on the compacted Gifford–McMahon cryocooler and the working temperature was usually about 2.3 K. The device is connected to a room-temperature bias tee. An isolated voltage source in series with a resistor (20 k) provides stable current bias to the detector. Bias current is fed to the device through the dc port of the bias tee and high-frequency response pulses of SNSPD are extracted from the ac port of the tee and amplified via an ultrawideband amplifier. The amplified pulse signals can be read by an oscilloscope or counted by a photon counter. The laser source is connected in series to one or two variable attenuators and a polarization controller, and then to the fiber connector fixed on the cryostat, which ultimately connects to the device package inside. There are two fiber connectors between the packaged SNSPD and the laser source, one inside and the other outside the cryostat. The two fiber connectors produce an extra system loss of 0.5–1 dB. In other words, the measured SDE may increase by 12–25% if fiber-connection loss is avoided.

III. COMPARISON BETWEEN SNSPDS MADE OF NBTIN AND NBN FILMS A. Critical Temperature and Resistivity Fig. 1 shows the resistivity–temperature (ρ–T) curves of SNSPDs made of 5-nm-thick NbTiN film and 7-nm-thick NbN film. Tc s, defined as the midpoint of the ρ–T transition, were measured to be 8.4 and 8.1 K for NbTiN and NbN, respectively, consistent with our expectation. However, the resistivities just above Tc are considerably different. The resistivity at 20 K (ρ20K ) of NbTiN is about half of that of NbN, and these values are given as 156.3 and 293.5 μ·cm, respectively. The lower ρ20K of NbTiN is due to the addition of Ti, which increases metallic electrical-conduction properties [24]. The lower resistivity is beneficial for decreasing the kinetic inductance of the SNSPD, and therefore reducing the recovery time of photon response.

Fig. 1. Resistivity versus temperature of SNSPDs made of NbTiN and NbN thin films.

Fig. 2. Timing jitter versus switching current of the SNSPDs (each plotted symbol represents the timing jitter of one device; the red area is the Isw range of NbN SNSPDs and the blue area is the Isw range of NbTiN SNSPDs). The dashed curve is provided to guide the eye.

B. Switching Current and Timing Jitter Due to the nonideal fabrication process and film quality, the switching currents (Isw ) of the SNSPDs have certain distributions. For NbTiN SNSPDs, the range of Isw is 19–24 μA, while that for NbN SNSPDs is only 8–13 μA. With a higher Isw , an SNSPD can be biased at a higher current, thus producing a response signal with higher amplitude. Therefore, a lower TJ can be obtained [25]. Fig. 2 shows TJ versus Isw , with the TJ measured under a bias of about 0.93 Isw . The measurement method for TJ was described in a previous publication [25]. It can be seen from Fig. 2 that the TJs of the NbTiN detectors are much lower than those of NbN detectors, which are 38–46 ps for NbTiN SNSPDs and 68–110 ps for NbN SNSPDs. C. Kinetic Inductance and Recovery Time Fig. 1 shows that the resistivity for NbTiN is roughly half of that of NbN, meaning that the kinetic inductance of the NbTiN SNSPD should also be smaller than that of NbN SNSPD. When T Tc , using [26], [27] LK =

l ρ20K · · 1.76π k wd Tc

(1)

(where is Planck’s constant, k is the Boltzmann constant, and l, w, and d are the length, width, and thickness of the SNSPD, respectively), we obtained L k s of 785 and 1530 nH for the NbTiN

YANG et al.: COMPARISON OF SNSPDs MADE OF NbTiN AND NbN THIN FILMS

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Fig. 4. Critical currents of straight nanostrips versus the length of nanostrips made of NbTiN and NbN films. The inset shows a scanning electron microscopic image of the nanostrip of 2 μm; the red line is the working nanostrip and the eight other isolated nanostrips assist in the EBL proximity effect correction.

Fig. 3. (a) Phase of S11 as a function of frequency for the NbTiN and NbN SNSPDs (the solid lines are measured data and the dashed lines are curves fitted using the expression arg [( jωL − 50 )/( jωL + 50 ]), and (b) electrical pulses of the NbTiN and NbN SNSPDs (the recovery time τ is defined as the time at 1/e of the magnitude).

and NbN SNSPDs, respectively. Moreover, we measured L k by measuring the phase of the reflection coefficient S11 , using the network analyzer [28]. Because the SNSPD impedance has no resistive component when in the superconducting state, S11 of the device is expressed as ( jωL − 50 )/( jωL + 50 ). L k is obtained by fitting the data to this expression with L as the free parameter. Fig. 3(a) shows the phase of S11 as a function of the frequency for both devices; it can be seen that L k = 910 nH for NbTiN SNSPD and L k = 1993 nH for NbN SNSPD. The ratio of L k NbTiN /L k NbN is similar, though the values are higher than those calculated by (1). The deviation of L k can be attributed to the overestimated film thickness in calculation compared to its actual thickness due to the existence of the oxide layer. Fig. 3(b) shows the averaged response pulses (the amplifier gain is 300). The recovery times τ , defined as the time at 1/e of the magnitude, are 17 and 32 ns for NbTiN and NbN SNSPDs, respectively, which are also consistent with the simple estimation τ ≈ L k /Z load (where Z load is 50 ), according to the simple equivalent-circuit model [26]. D. Homogeneity In order to compare the homogeneities of the two films, we also fabricated straight nanostrips with different lengths to avoid

possible current-crowding effects in the meandered nanowires (see the inset of Fig. 4 as an example) [29]. The width was fixed at 100 nm and the lengths were 100 nm, 400 nm, 2 μm, and 15 μm. We measured Isw s of 4–6 samples with the same geometric parameters at different locations on the 2-in wafer and the data are shown in Fig. 4. The relative fluctuation of Isw for NbTiN nanostrips seems to be smaller than that of the NbN nanostrips with the same length for all the lengths. We use coefficient of variation (standard deviation divided by average value) to estimate the relative fluctuation of Isw , which are 5.5% (NbTiN) and 10.5% (NbN) for the 100-nm-long nanostrips. It proves better homogeneity of NbTiN film on the wafer scale. With the increase in the length, we see a gradual decrease in the switching current for both films, indicating the existence of nonuniformity/defects in their active areas. If we compare the decrease rate of Isw over the length defined by I¯sw (100 nm)/ I¯sw (15 μm), the value of NbTiN is 1.1, which is also smaller than that of NbN (1.3), thereby proving that the local uniformity of NbTiN is superior to that of NbN as well. As a result, the NbTiN film shows better uniformity than NbN not only on the micrometer scale, but also on the whole 2-in wafer scale. IV. STATISTICAL ANALYSIS OF THE NBTIN SNSPDS ON THE 2-IN WAFER The above-mentioned comparisons indicate that NbTiN SNSPDs may have better uniformity and performance in factors such as jitter and recovery time than do NbN SNSPDs. In order to examine the uniformity of NbTiN SNSPDs, we measured 28 out of 121 detectors evenly spaced over the whole 2-in wafer since the measurements for all the detectors over the wafer would be extremely time-consuming. The measured properties include the sheet resistances at room temperature (R300K ), current–voltage (I–V) curves, and the SDE and DCR dependences on the bias current. Fig. 5 shows the distribution of R300K on the wafer. The data show that most of R300K are around 400–440 , which indicates good uniformity of NbTiN film thickness. However, there is large fluctuation for the samples at the edge of the wafer. Fig. 6(a) shows the I–V curves of the 28 detectors and the inset shows the switching current of the SNSPDs. The Isw s of most

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Fig. 5. Distribution of the sheet resistances of the SNSPD at room temperature (300 K) on the wafer.

Fig. 7. (a) Bias-dependent SDE and DCR for 28 detectors; (b) SDE100 distribution map on the wafer with data derived from (a); and (c) SDE100 and SDEmax versus Isw for the 28 SNSPDs (The blue and red dashed circles are the area of the devices of SDE100 = 50% ± 20% and SDEmax = 60% ± 15%, respectively).

Fig. 6. (a) I–V curves for the 28 out of 121 NbTiN SNSPDs on the 2-in wafer. The inset zooms in on the switching currents; and (b) Isw distribution map on the wafer with data derived from (a).

of the detectors are in the range of 19–23 μA, except for one with an abnormally high value (24.4 μA). The Isw distribution on the wafer derived from the I–V curves is shown in Fig. 6(b). A total of 21 out of 28 detectors (75%) have Isw in the range of 21.5 ± 1 μA. Similarly to R300K , the uniformity of Isw of the middle parts is better than that of the edge parts. The SDE of SNSPD (ηSDE ) can be expressed by ηSDE = ηoc × ηabs × ηint , where ηoc is the optical coupling efficiency,

ηabs is the absorption efficiency of the nanowires, and ηint is the intrinsic detection efficiency, which describes the pulse generation probability of the nanowire when a photon is absorbed. ηint is related to not only Tc /gap energy, but also the bias current normalized to the switching current. Besides, the process imperfection (uniformity) also influences SDE. So it is hard to tell the most important parameter determining SDE of SNSPD. By examining SDE characteristics experimentally, we may have some information on the uniformity of the SNSPDs over the wafer directly. Fig. 7(a) shows the SDE and DCR properties of the 28 SNSPDs. Generally speaking, all the SDE − Ib curves and the DCR − Ib curves have similar features, albeit with different values. The highest SDEs of most detectors are in the range of

YANG et al.: COMPARISON OF SNSPDs MADE OF NbTiN AND NbN THIN FILMS

40–80%. The background DCRs of all detectors are on the level of 10 Hz. We define SDE100 as the SDE at a DCR of 100 Hz, and the highest SDE100 is 77.5% at 22.2 μA. Considering the existing loss of the two fiber connectors (0.5–1 dB), the SDE over 85% can be obtained for the optimized system by replacing the connectors with a fused fiber. Fig. 7(b) shows the SDE100 -distribution map on the wafer for the 28 SNSPDs. The data are derived from Fig. 7(a). Most of the SDEs are between 50% and 70%, with several higher than 70% or lower than 40%. Since we did not see the higher SDE100 for the detectors with higher Isw , it indicates that the fluctuations of SDE are related to film uniformity instead of fabrication processing. On the other hand, DCR is more sensitive to the homogeneity of the nanowire as the section having the narrowest width generates most of the dark counts, while the SDE is an average of all the nanowires. For comparison, we present the Isw dependences of both SDE100 and SDEmax in Fig. 7(c), in which SDEmax is the measured maximum detection efficiency. Clearly, most of the SDE100 data are located in the range of SDE100 = 50% ± 20%, with Isw = 21.5 ± 2 μA, showing in a blue circle. If we set 50% as the threshold for SDE100 , the yield is 71%, which increases to 86% for an SDE100 threshold of 40%. For SDEmax , most of the values are located in the range of SDEmax = 60% ± 15%, showing in a red smaller circle. The smaller fluctuation is attributed to the partially saturated SDE − Ib curves shown in Fig. 7(a). We note that the SDEmax could be limited by the switching current, which is related to the readout circuit sometimes [30]. V. CONCLUSION We fabricated SNSPDs out of 5-nm-thick NbTiN and 7-nmthick NbN films on 2-in wafers and compared their properties. Due to the smaller resistivity and higher current density of NbTiN, the NbTiN SNSPDs showed smaller recovery time and lower jitter. Moreover, NbTiN has better uniformity, which is helpful for improving the yields of the SNSPDs. We statistically studied the SDE of 28 NbTiN SNSPDs on one 2-in wafer. The yields are 71% (86%) with SDE thresholds of 50% (40%). REFERENCES [1] H. L. Yin et al., “Measurement-device-independent quantum key distribution over a 404 km optical fiber,” Phys. Rev. Lett., vol. 117, Nov. 2016, Art. no. 190501. [2] R. H. Hadfield, M. J. Stevens, R. P. Mirin, and S. W. Nam, “Singlephoton source characterization with twin infrared-sensitive superconducting single-photon detectors,” J. Appl. Phys., vol. 101, 2007, Art. no. 103104. [3] T. Kobayashi et al., “Frequency-domain Hong–Ou–Mandel interference,” Nature Photon., vol. 10, pp. 441–444, 2016. [4] H. Hemmati et al., “Overview and status of the lunar laser communications demonstration,” Proc. SPIE, vol. 8246, 2012, Art. no. 82460C. [5] A. McCarthy et al., “Kilometer-range, high resolution depth imaging via 1560 nm wavelength single-photon detection,” Opt. Express, vol. 21, pp. 8904–8915, Apr. 2013. [6] W. Zhang et al., “NbN superconducting nanowire single photon detector with efficiency over 90% at 1550 nm wavelength operational at compact cryocooler temperature,” Sci. China Phys., Mech. Astron., vol. 60, 2017, Art. no. 120314.

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Xiaoyan Yang (M’17) received the M.S. degree in microelectronics and solidstate electronics from Shanghai University, Shanghai, China, in 2008, and the Ph.D. degree in microelectronics and solid-state electronics from the Chinese Academy of Sciences, Beijing, China, in 2015. From 2010 to 2011, she was a Guest Researcher with the National Institute of Information and Communications Technology, Kobe, Japan. Currently, she is working with the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China. Her current research interests include the design and experimental characterization of superconducting nanowire single-photon detectors.

Lixing You (M’09) received the B.S., M.S., and Ph.D. degrees in physics from Nanjing University, Nanjing, China, in 1997, 2001, and 2003, respectively. From 2003 to 2005, he was a Postdoctoral Researcher with the Department of Microtechnology and Nanoscience, Chalmers University of Technology, G¨oteborg, Sweden. From 2005 to 2006, he was a Postdoctoral Researcher with the Condensed Matter Physics and Devices Group, University of Twente, Twente, The Netherlands. Since 2006, he has been a Guest Researcher with the Electromagnetics Division, National Institute of Standards and Technology, Boulder, CO, USA. Since September 2007, he has been a Research Professor with the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China. His research interests include superconductive electronics, including micro-/nano-superconductive devices and high-frequency applications.

Lu Zhang, biography not available at the time of publication. Chaolin Lv, biography not available at the time of publication. Hao Li, biography not available at the time of publication. Xiaoyu Liu, biography not available at the time of publication. Hui Zhou, biography not available at the time of publication.

Zhen Wang received the Ph.D. degree in electrical engineering from the Nagaoka University of Technology, Nagaoka, Japan, in 1991. From 1991 to 2013, he was with the National Institute of Information and Communications Technology (NICT), Kobe, Japan. He is currently a Research Professor with the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai, China. His research interests include superconducting electronics, including superconducting devices and physics, superconducting superconductor–insulator–superconductor terahertz mixers, and photon detectors. Dr. Wang is a Fellow of NICT.