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IEEE Transactions on Applied Superconductivity

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

Xiaoyan Yang, Lixing You, Lu Zhang, Chaolin Lv, Hao Li, Xiaoyu Liu, Hui Zhou, Zhen Wang

Published Nov 22, 2017• 39 citations
Publisher

Abstract

We fabricated superconducting nanowire single-photon 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%.

DOI
10.1109/tasc.2017.2776288

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We fabricated superconducting nanowire single-photon 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%.
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.
Superconducting nanowire single photon detectors (SNSPDs) based on Si substrates have demonstrated excellent performance, such as high efficiency, low dark count rate, short reset time and low timing jitter. But due to the lattice mismatch between NbN and Si substrate, the performance of film is limited. To this end, Nb5N6layer is fabricated as the buffer layer between the NbN film and Si substrate, and that will reduce the mismatch and optimize superconducting properties of NbN films. The 6nm-thick film with a 65 nm-thick buffer layer shows the zero resistance critical temperature (Tc0) of 13.27 K, which is 5 K higher than that without buffer. Based on the NbN film on the buffered substrate, we fabricated SNSPD devices demonstrate critical current (IC) of 65 μA which is 5 ~ 6 times higher to the detectors without buffer. Besides, the experiment results prove that the buffer layer reduces the kinetic inductance, which fasten the recovery of nanowire.
Superconducting nanowire single photon detectors (SNSPD) offer excellent performance for infrared single photon detection, combining high efficiency, low timing jitter, low dark count rates and high photon counting rates. Promising application areas for SNSPDs include quantum key distribution, space-to-ground communications and single photon remote sensing [1]. SNSPDs are typically made with ultrathin niobium nitride (NbN) films with thickness 4 nm and a superconducting transition temperature above 9 K. NbN offers high performance in the near infrared but their sensitivity drops at wavelengths beyond 2 um. There is growing interest in potential photon counting applications in the mid infrared domain (for example remote sensing of greenhouse gases in the atmosphere [2]). One way to overcome the wavelength limit in NbN SNSPDs is to use films with a lower superconducting energy gap [3]. Here we report on the study of SNSPDs fabricated with thin films of titanium nitride (TiN). We compare TiN films deposited by atomic layer deposition (ALD) and by magnetron sputtering. The TiN films range in thickness from 5 to 60 nm, with superconducting transition temperatures from ~1 K to 3.5 K. We have analyzed the films via transmission electron microscopy and variable angle spectroscopic ellipsometry. We characterize TiN SNSPDs performance from near to mid-infrared at wavelengths (1-4 um) with fast optical parametric oscillator (OPO) source. We compare the performance of TiN SNSPDs to devices based on other lower gap materials: MoSi, NbTiN, WSi. [1] Natarajan et al Superconductor Science and Technology 25 063001 (2012) [2] Abshire et al Laser Applications to Chemical, Security and Environmental Analysis, (Optical Society of America, 2008) paper LMA4 [3] Verma et al Applied Physics Letters 105 022602 (2014)
We have developed a practical superconducting nanowire single photon detector (SNSPD) system with our superconducting thin films and devices fabrication, optical coupling packaging, and cryogenic technology. The SNSPD devices were fabricated from high-quality niobium nitride (NbN) ultra-thin films that were epitaxially grown on single-crystal MgO substrates, and optical cavity structure were applied for improvement of photo absorption efficiency. The SNSPD system was developed based on a compact Gifford-McMahon (GM) cryocooler, in which SNSPD can operate continuously at temperature of 2.9 K +/- 10 mK with low power consumption without any liquid cryogens. The system detection efficiencies at 1550 nm wavelength (at a 100 Hz dark count rate) were found to be 2.6% for device without optical cavity and 21% for device with optical cavity. The response speed of devices showed maximally 200 MHz with 5 × 5 mm square device, and timing jitter of 100 ps full width at a half maximum (FWHM) was observed.
Superconducting Nanowire Single Photon Detectors (SNSPD) are very fast, low noise devices, that detect single photons over a broad spectral range with practical quantum efficiency. The quantum efficiency drops at a certain deposited energy abruptly, the so called cutoff energy or the corresponding cutoff wavelength. The most common material to fabricate these nanowires is niobium nitride (NbN). A promising material, that has not been used for this purpose before is tantalum nitride (TaN). Many material properties of TaN and NbN are similar, but TaN has a smaller energy gap. This should make the device more sensitive to photons with lower energies. In this thesis, SNSPDs made of NbN and TaN were characterized, the quantum efficiencies were measured and the cutoff criteria were determined and compared. With the use of TaN the cutoff wavelength could be increased significantly. Measurements of quantum efficiency and dark-count rates at several temperatures and operation conditions are presented and discussed. Furthermore, the cutoff criteria are determined and compared between the materials mentioned as well as the experimental results are compared to the theoretical predictions of the detection model.
We investigated the suitability of AlN as a buffer layer for NbN superconducting nanowire single-photon detectors (SNSPDs) on GaAs. The NbN films with a thickness of 3.3-20 nm deposited onto GaAs substrates with an AlN buffer layer demonstrate a higher critical temperature, critical current density, and lower residual resistivity in comparison to films deposited onto bare substrates. Unfortunately, the thermal coupling of the NbN film to the substrate weakens. SNSPDs made of 4.9-nm-thick NbN films on buffered substrates (in comparison to detectors made from NbN films on bare GaAs) demonstrate three orders of magnitude lower dark count rates and about ten times higher detection efficiency at 900 nm being measured at 90% of the critical current. The system timing jitter of SNSPDs on buffered substrates is 72 ps, which is 36 ps lower than those on bare substrate. However, a weaker thermal coupling of NbN nanowire to the buffered substrate leads to a latching effect at bias currents > 0.97 IC.