Design and fabrication of superconducting single-photon detector operating in 5-10 mu m wavelength band

High-performance mid-wave and long-wave infrared single-photon detectors not only have significant research value in the fields of infrared astronomy and defense technology, but also are the challenges in the field of single-photon detection technology. Superconducting nanowire single-photon detectors (SNSPDs) have shown excellent performances in the near-infrared band. However, how to further improve the cutoff wavelength lc is a topic of widespread concern. In this paper, the method of improving lc by regulating the superconducting disorder is discussed, and a detector with an operating wavelength band of 5-10 mu m is designed and fabricated. The studies show that the multiplication and diffusion behaviors of the quasiparticles always occur during the photon detection events, although the microscopic photodetection mechanism of SNSPD still lacks a perfect theoretical explanation. Therefore, the theoretical analysis mainly considers the influence of the quasiparticles in this paper, and the mathematical formula of the detection cutoff wavelength lc can be obtained based on the phenomenological quasiparticle diffusion model. Furthermore, the disorder-dependent superconducting phase transition temperature Tc, superconducting energy gap Delta, and electron thermalization time tth are also considered, in order to obtain more precise results. Theoretical analysis suggests that the increase in the sheet resistance Rs, which evaluates the disorder strength, will help to increase lambda(c). For example, when the nanowire width is kept at 30 nm and Rs > 380 W/square, it can be deduced that lambda(c) is larger than 10 mu m. Experimentally, the active area of the device consists of a straight superconducting nanowire with a length of 10 mu m and a width of 30 nm, so that it can effectively reduce the probability of the defects on the nanowire and avoid the current crowding effect. We fabricate a 30 nm-wide Mo0.8Si0.2 mid infrared SNSPD, which has a cutoff wavelength lc no more than 5 mu m, the effective strength of the disorder-the film sheet resistance Rs = 248.6 W/square. For comparison, the sheet resistance, which is controlled by the film thickness, increases to about 320 W/square in this experiment. It is demonstrated that the Mo0.8Si0.2 detector with R-s 320 W/square can achieve saturated quantum efficiency at a wavelength of 6 mu m. Furthermore, 53% quantum efficiency at a wavelength of 10.2 mu m can be obtained when the detector works at a bias current of 0.9ISW (ISW is the superconducting transition current), and it can theoretically reach a maximum value of 92% if the compression of switching current is excluded. Therefore, it can be predicted that the disorder regulation may become another efficient approach to designing high-performance mid-wave and long-wave infrared SNSPDs, in addition to the optimization of the superconducting energy gap and the cross section of superconducting nanowire. However, the continuous increase in the disorder will cause both the superconducting phase transition temperature Tc and ISW of the detector to decrease from the viewpoint of detector fabrication and application. This downward trend is especially pronounced when the nanowire width is ultranarrow, which is not conducive to the signal readout of the detector. Thus, exploring the optimal disorder regulation technology and balancing the relationship among the operating temperature, the signal-to-noise ratio, and the cutoff wavelength will have key scientific and application value for the development of high-performance mid-wave and long-wave infrared SNSPDs.