Since the fabrication of the first superconducting nanowire single-photon detector (SNSPD) by Gol’tsman et al. [1], SNSPDs have merged as the most prevalent single-photon detectors due to their high system detection efficiency (SDE) [2], remarkably low dark count rate (DCR) [3], low timing jitter [4], ultrashort recovery time [5], and high photon count rate (PCR) [6]. Moreover, SNSPDs as advanced technologies exhibit the high potential for various applications, such as quantum optics experiments [7], quantum key distribution (QKD) for quantum communication [8], fluorescent lifetime analysis for medical applications [9], space-to-ground optical communication [10], and integrated waveguide nanophotonic circuits [11].

High internal detection efficiency (IDE) and operating temperature have always been the permanent pursuit for SNSPD; however, it is difficult to optimize the two features simultaneously [12]. The restriction stems from the internal mechanism of single photon detection by the superconducting materials. At the same time, it remains challenging for widely used NbN materials to fabricate such devices due to the crystal defects and large energy gap [13]. For this, researchers have developed novel superconductors based on amorphous materials, which have been demonstrated as promising candidates for SNSPDs [14,15]. Compared to the polycrystalline materials, the amorphous films show clear advantages in terms of tolerance and scalability. The nanowires fabricated by using amorphous materials appear to suffer from fewer constrictions, along with the high yield on the Si wafers [16]. The disordered structure offers a high degree of homogeneity and significant compatibility in the optical cavity. Moreover, the films with low energy gap are noted to be sensitive to midinfrared wavelengths [17]. Therefore, SNSPDs based on amorphous films have the potential to emerge as the desirable choice to detect the wavelengths from near-infrared to midinfrared.

According to the Bardeen–Cooper–Schrieffer relation 2Δ=3.53kBTc [18], low energy gap results from low T_c, and vice versa. Based on this, the single-photon detectors fabricated from amorphous superconductors generally operate at low temperatures. For instance, the highest system detection efficiency (SDE) of 93% was achieved for the SNSPDs fabricated from amorphous WSi operated at 0.12 K at the National Institute of Standards and Technology (NIST) in 2013 [2], indicating that WSi-SNSPDs need to run below 1 K to achieve the optimal performance. In contrast, though the superconducting energy gaps of the MoSi thin films are comparable to that of WSi, their Tc value is higher than 4 K [15]. Thus, the MoSi film becomes as a promising amorphous material for SNSPDs, which can be operated at T>2K temperature.

Recently, MoSi-SNSPDs have presented an excellent performance, with a detection efficiency higher than 95% at 0.7 K [19]. Besides, it has been demonstrated that a 1 μm-wide MoSi superconducting strip is capable of single-photon detection. A large active area (400μm×400μm) of the microwire single-photon detector was fabricated, which exhibited the saturated photon counts at 0.3 K [16], suggesting that the MoSi film was competent for the fabrication of SNSPD with high efficiency and large area fabrication. As mentioned above, the single-photon detectors fabricated from amorphous superconductors generally operate at low temperatures to achieve the optimal performance, which hinders their development from the fundamental research phase to the practical application.

In this study, we fabricated SNSPDs with high-performance for both high T_c and high optical absorption coefficient of Mo0.8Si0.2 films.

In this study, the 6.5 nm-thick Mo0.8Si0.2 film has been grown on the polished and thermally oxidized silicon wafer by employing the DC magnetron sputtering from an alloy target (4-in. diameter, 1 in. = 2.54 cm) with the stoichiometry of Mo:Si=0.8:0.2. Before the film deposition, the sample is cleaned with Ar ion milling with the beam currents of 300 mA for 10 s and subsequently delivered to the chamber, followed by vacuum-pumping to 10−6Pa. The sputtering process is performed at room temperature with 33 SCCM (standard cubic centimeter per minute) Ar gas, 45° throttle valve and 2 mTorr (1 Torr = 133.322 Pa) chamber pressure. Generally, the alloy target is presputtered for 5 min to clean the surface and stabilize the deposition conditions. The thickness of the film depends on the deposition rate (0.9 nm/s) and time.

Considering the poor antioxidant ability of the MoSi film, 4 nm-thick Nb5N6 grown on the film is carried out in-situ to prevent oxidation. The Nb5N6 film is deposited by an RF magnetron sputtering with RF power of 400 W in a 1:4 Ar and N2 at 20 mTorr chamber pressure. Based on our previous experimental study, the chemical composition and crystalline structure of Nb5N6 have been analyzed by using an Auger electron energy spectrometer (AES) and X-ray diffractometer (XRD). The results indicate that the sample has a Nb/N ratio of 5:6, along with a hexagonal structure [20]. Further, the Nb5N6 film shows semiconducting electronic properties, in which the resistance rises as the temperature decreases, reaching >100,000ohm at 4.2 K [21]. Thus, it is safe to suggest that the capping layer has no adverse impact on the superconducting film at low temperature. Simultaneously, the presence of the Nb₅N₆ capping layer also protects the film from degradation during the follow-up fabrication.

An overview of the prior work on the superconducting properties of the MoSi films has been presented in Table 1. The optimization of the film sputtering recipes to operate the devices at high temperatures has been presented. As expected, the Tc obtained in this study is higher than the other films, allowing the detectors to operate at T>2K.Table 1.

Comparison of the Different Stoichiometric Ratios of the MoSi Films in Terms of Bulk Tc, Complex Refractive Index, and FDTD Simulation Results (1550 nm Wavelength)^a

The photon counting measurement is carried out with continuous tunable laser at 1550 nm, and the input photon flux is attenuated by the two serial variable attenuators to a level of 106 photons/s, calibrated with a highly precise light power meter (Thorlabs-PM100D). The SNSPDs are illuminated by using a single module fiber coupled with the input light into detector. Figure 6(b) exhibits the photon count rate (PCR) as a function of the bias current (IB) at different operating temperatures. Notably, the device has a saturated count rate ranging from 75 mK to 2.2 K. The saturation of the PCR versus IB curves indicates that the detector quantum efficiency is also saturated [17]. The observed behavior suggests that the device reaches its 100% internal detection efficiency [28]. Figure 6(c) demonstrates the internal detection efficiency of the 50 nm-width SNSPD, which reveals a sigmoidal dependence on the normalized bias current, with saturation at 0.8ISW, where the DCR value is below 1 cps. To extract the saturation level of the detector, the bias dependence of IDE is fitted with an empirical sigmoid function [29]. The red solid line in Fig. 6(c) represents the fitting curve, where the maximum IDE reaches 100%.

Notably, as a proof of the array detector, a 4-pixel array device with 90 nm-width, a period of 200 nm, and an active area of 40μm×40μm has explicitly been fabricated. Figure 6(d) shows the normalized photon counts as a function of the bias current for the 4-pixel device operating at 2.2 K (inset depicting the I-V curve of one of the pixels). The ISW is noted to be 9.5 μA with the hysteresis current of 1 μA, which is higher than the waveguide-integrated nanowire of 3.3 μA reported by Häußler [11], resulting in a higher signal-to-noise ratio. As demonstrated in the previous literature studies [28], the Mo0.75Si0.25−SNSPD with a nanowire width of 110 nm and an active area of 16μm×16μm exhibited saturated internal efficiency at 2.3 K. The nanowire geometric defects and constrictions are proportional to the length, and the length of the nanowires is also proportional to the active area for the same size (width and pitch) of the nanowires. Therefore, it is extremely challenging to achieve the saturated efficiency at high temperatures for large active areas. In this study, the 4-pixel array device with 90 nm-width nanowires and an active area of 40μm×40μm has explicitly been fabricated, exhibiting the saturated intrinsic detection efficiency at a temperature of 2.2 K.

The I-V curves of the 4 pixels are noted to be similar for the switching current ranging from 9.0 μA to 9.5 μA. We have calculated the depairing current of SNSPD according to the equations presented by Korneeva et al. [30]. Herein, The Idep (2.2 K) is 14.5 μA; we found the ratio of ISW/Idep to be in the range of 0.62–0.65. This ratio supports the fact of the little constrictions during array SNSPD fabrication. Specifically, it is seen that the photon count of each pixel exhibits a saturation plateau, indicating the close-to-unity internal detection efficiency. The photon count of each pixel exhibits the maximum count rate of 105s−1 with <103cps dark counts at 2.2 K. Recently, the superconducting microwire was used for single photon detection with the aim of achieving saturated internal detection efficiency over a large area. The results exhibited the saturated internal detection efficiency for 1550 nm wavelengths at 0.3 K [16]. Such detectors rely on a very low temperature (mK) to achieve their excellent performance, thus, limiting the application for single photon detection. Considering that the cryogenic requirements are less complicated and less costly at 2.2 K than at <1K, it is beneficial to have devices with optimal saturation efficiency at high temperatures, which debases the demand for the cryogenic systems and simplifies the integration of device measurement.

In conclusion, this work demonstrates the amorphous Mo0.8Si0.2 film exhibiting a high optical absorption coefficient in the visible to midinfrared range. The SNSPD fabricated from the Mo0.8Si0.2 film with 6.5 nm-thickness and 50 nm-width demonstrates the saturated intrinsic detection efficiency with sub 1 Hz dark count rate at a temperature of 0.2 K. Particularly, a uniform 4-pixel SNSPD has been successfully developed, exhibiting a saturation plateau for the photon counts at a temperature of 2.2 K. As a whole, the SNSPD based on Mo0.8Si0.2 films exhibits excellent advantages in operation temperature, spectral sensitivity, and internal detection efficiency.