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IEEE Transactions on Applied Superconductivity

Large-Area Niobium Titanium Nitride Superconducting Microstrip Single-Photon Detector Fabricated Using a Photolithography Process

Masahiro Yabuno, Fumihiro China, Shigehito Miki, Hirotaka Terai

Published Feb 24, 2023• 9 citations
Publisher

Abstract

In this paper, we present a large-area niobium titanium nitride (NbTiN) superconducting microstrip single-photon detector (SMSPD) fabricated using a photolithography process. We fabricated a 1-μm-wide NbTiN-SMSPD covering 420 × 420 μm2active area with a filling factor of 20% using an i-line stepper. The detector is cooled down to approximately 2.2 K, and its performance is evaluated at the wavelengths from visible to near-infrared. The detector operates stably without a shunt resistor and shows single-photon sensitivity for photons with a wavelength of 1550 nm. The bias current dependence of the detection count rate is measured at the wavelengths from 406 to 1550 nm. The saturation of the detection count rate with increasing the bias current was successfully observed at wavelengths below 850 nm, suggesting an internal detection efficiency approaching 100%. Furthermore, we evaluated the timing jitter performance using the femtosecond pulse laser with a wavelength of 1550 nm. Consequently, we achieved a system timing jitter of 85 ps for a light spot diameter of 100 μm.

DOI
10.1109/tasc.2023.3249130

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In this paper, we present a large-area niobium titanium nitride (NbTiN) superconducting microstrip single-photon detector (SMSPD) fabricated using a photolithography process. We fabricated a 1-μm-wide NbTiN-SMSPD covering 420 × 420 μm2active area with a filling factor of 20% using an i-line stepper. The detector is cooled down to approximately 2.2 K, and its performance is evaluated at the wavelengths from visible to near-infrared. The detector operates stably without a shunt resistor and shows single-photon sensitivity for photons with a wavelength of 1550 nm. The bias current dependence of the detection count rate is measured at the wavelengths from 406 to 1550 nm. The saturation of the detection count rate with increasing the bias current was successfully observed at wavelengths below 850 nm, suggesting an internal detection efficiency approaching 100%. Furthermore, we evaluated the timing jitter performance using the femtosecond pulse laser with a wavelength of 1550 nm. Consequently, we achieved a system timing jitter of 85 ps for a light spot diameter of 100 μm.
The effective and convenient detection of single photons via advanced detectors with a large active area is becoming significant for quantum and classical applications. This work demonstrates the fabrication of a superconducting microstrip single-photon detector (SMSPD) with a millimeter-scale active area via the use of ultraviolet (UV) photolithography. The performances of NbN SMSPDs with different active areas and strip widths are characterized. SMSPDs fabricated by UV photolithography and electron beam lithography with small active areas are also compared from the aspects of the switching current density and line edge roughness. Furthermore, an SMSPD with an active area of 1 mm × 1 mm is obtained via UV photolithography, and during operation at 0.85 K, it exhibits near-saturated internal detection efficiency at wavelengths up to 800 nm. At a wavelength of 1550 nm, the detector exhibits a system detection efficiency of ∼5% (7%) and a timing jitter of 102 (144) ps, when illuminated with a light spot of ∼18 (600) µm in diameter, respectively.
Generally, a superconducting nanowire single-photon detector (SNSPD) is composed of wires with a typical width of ∼ 100 nm . Recent studies have found that superconducting strips with a micrometer-scale width can also detect single photons. Compared with the SNSPD covering the same area, the superconducting microstrip single-photon detector (SMSPD) has smaller kinetic inductance, higher working current, and lower requirements in fabrication accuracy, providing potential applications in the development of ultralarge active area detectors. However, the study of SMSPD is still in its infancy, and the realization of its high-performance and practical use remains an open question. This study demonstrates a NbN SMSPD with a nearly saturated system detection efficiency (SDE) of ∼ 92.2 % at a dark count rate of ∼ 200 cps , a polarization sensitivity of ∼ 1.03 , and a minimum timing jitter of ∼ 48 ps at the telecom wavelength of 1550 nm when coupled with a single-mode fiber and operated at 0.84 K. Furthermore, the detector’s SDE is over 70% when operated at a 2.1 K closed-cycle cryocooler.
We present high performance fiber-coupled niobium titanium nitride superconducting nanowire single photon detectors fabricated on thermally oxidized silicon substrates. By applying a double sided cavity structure, simulated absorptance in the nanowire meander reached 97% at 1550 nm wavelength. The fabricated device showed a system detection efficiency (DE) of 74% at 1550 nm, dark count rate of 100 c/s, and a full width at half maximum timing jitter of 68 ps under a bias current of 18.0 μA with a practical Gifford-McMahon cryocooler system.
Niobium nitride superconducting nanowire single-photon detectors were fabricated on thermally oxidized silicon substrates with large active areas of 30 μm× 30 μm.To achieve non-constricted detectors,we improved the film growth and electron beam lithography process to fabricate uniform 100-nm wide Nb N nanowires with a fill factor of 50%.The devices showed 72.4% system detection efficiency(SDE) at 100-Hz dark count rate(DCR) and 74-ps timing jitter,measured at the fiber communication wavelength of 1550 nm.The highest SDE which is 81.2% when the DCR is ～ 700 c/s appears at the wavelength of 1650 nm.
We report the fabrication and characterization of a niobium titanium nitride (NbTiN)-based superconducting nanowire single-photon detector (SSPD or SNSPD) on the Si waveguide toward the realization of waveguide-coupled SSPD. The device consists of a silicon optical waveguide fabricated using a silicon-on-insulator (SOI) substrate and a 100-nm-wide NbTiN nanowire constructed on the waveguide, with an inductor to prevent latch operation. For the fabricated device, the observed superconducting critical temperature and critical current were 7.85 K and 34.4 μA at 0.3 K, respectively. We investigated the optical response of the fabricated device and found that the device exhibited a single-photon response by measuring of the input power dependence of the output count rate. Furthermore, the fabricated device showed a clear bias current dependence of the output counts similar to conventional SSPDs, and the pulse-generation probability is estimated to be 90.1% based on two assumptions: a sigmoidal model to the detection efficiency and the device has a 100% pulse-generation probability at the saturated region.
Niobium Titanium nitride (NbTiN) based superconducting nanowire single photon detectors (SNSPDs) are known for their high performance across a wide spectral range, from the X-ray to the mid-infrared. Nonetheless, fabrication challenges and performance degradation attributable to surface oxidation and lack of uniformity in films thinner than 5 nm remain a significant barrier for achieving high-quality detectors. In this work, we study the influence of a Silicon capping layer on film properties and on the performance of SNSPDs. A Silicon capping layer effectively suppresses oxidation and increases the superconducting transition temperature. This enables superconductivity in films as thin as 3 nm at 3 K, increases critical current in patterned nanowires and significantly extends the saturation plateau from the visible to the near infrared (up to 2050 nm): These detectors maintain sub-50 ps timing jitter, even for nanowires as wide as 250 nm and with detection areas of 20x20μm2. Our results establish that thinner films protected by a capping layer allow for the fabrication of wider wires, decreasing nanofabrication challenges and extending the operating temperature range for efficient single photon detection.