5—10μm波段超导单光子探测器设计与研制

Design and fabrication of superconducting single-photon detector operating in 5–10μm wavelength band

高性能的中长波单光子探测器在红外天文和军事国防领域具有重要的研究价值,也是单光子探测技术领域的研究难点.超导纳米线单光子探测器在近红外波段已经展示出优异的性能,但如何进一步提高器件的探测截止波长λ_(c)是一个受到广泛关注的话题.本文探讨了一种通过超导无序调控辅助提高λ_(c)的方法,设计并制备出工作波段为5-10μm的超导单光子探测器.理论分析表明,增大衡量无序强度的主要评价因子即薄膜方块电阻R_(s),将有利于增大λ_(c),如当纳米线宽保持在30 nm且R_(s)>380Ω/square时,可使得λ_(c)>10μm.实验测得R_(s)约为320Ω/square的Mo_(0.8)Si_(0.2)红外器件在6μm波长上可以获得完全饱和的量子效率.此外,当器件工作在0.9I_(SW)(I_(SW)为纳米线超导转变电流)的偏置电流下时,在10.2μm波长上的量子效率达到53%.

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 wavelengthλ_(c)is a topic of widespread concern.In this paper,the method of improvingλ_(c)by regulating the superconducting disorder is discussed,and a detector with an operating wavelength band of 5–10μm 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 wavelengthλ_(c)can be obtained based on the phenomenological quasiparticle diffusion model.Furthermore,the disorder-dependent superconducting phase transition temperature T_(o),superconducting energy gapΔ,and electron thermalization timeτare also considered,in order to obtain more precise results.Theoretical analysis suggests that the increase in the sheet resistance R_(o),which evaluates the disorder strength,will help to increaseλ_(c).For example,when the nanowire width is kept at 30 nm and R_(s)>380Ω/square,it can be deduced thatλ_(c)is larger than 10μm.Experimentally,the active area of the device consists of a straight superconducting nanowire with a length of 10μm 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 Mo_(0.8)Si_(0.2)mid infrared SNSPD,which has a cutoff wavelengthλ_(c)no more than 5μm,the effective strength of the disorder-the film sheet resistance R_(o)=248.6Ω/square.For comparison,the sheet resistance,which is controlled by the film thickness,increases to about 320Ω/square in this experiment.It is demonstrated that the Mo_(0.8)Si_(0.2)detector with R_(s)~320Ω/square can achieve saturated quantum efficiency at a wavelength of 6μm.Furthermore,53%quantum efficiency at a wavelength of 10.2μm can be obtained when the detector works at a bias current of 0.9I_(SW)(I_(SW) 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 T_(o) and I_(SW) 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.