Uniformity Analysis Of Ultrathin NbN Films And Nanowires For Superconducting Nanowire Single Photon Detectors

Superconducting nanowire single photon detectors(SNSPDs)has the best overall performance among all single photon detectors owing to its high efficiency,low dark count,high detection rate,low time jitter and wide spectrum.It has been applied in many fields like quantum communication,high-speed deep space communication,laser ranging,biological fluorescence,etc.The development of all these applications requires further improvement of SNSPD,expecting it to achieve higher detection efficiency,larger detection area,wider array size and photon number resolution.Compared to a small-size device,it is much more difficult to prepare a large-array or large-size SNSPD with high efficiency because of lower consistency and yield,thus restricting the further application of SNSPD.NbN is one of the earliest and most mature materials used for SNSPD.There are two main methods for direct preparation of high performance NbN films-preparation at either ambient or heated substrates.Superconducting nanowire with super large ratio of length to width is the core component of SNSPD.The uniformity of these nanowires directly affects key parameters of SNSPD,like the detection efficiency,dark counts and time jitter.It is also a significant factor contributing to the uniformity and the product yield of final devices.Researchers so far have focused on the influence of meander homogeneity of devices and barely discussed the uniformity of films and heterogeneity caused by etching process.Using a magnetron sputtering system,the effects of different gas ratio,sputtering target distance,pumping rate and sputtering temperature on the voltage and current during film preparation were compared between 4 and 8 inch Nb targets,respectively.The superconducting transition temperature(Tc0)and critical current density(Jc)of the films prepared under different conditions were characterized by liquid helium dewar and physical property measurement system(PPMS),and the optimized preparation process of ambient and heating NbN films was determined.We have prepared high quality superconducting NbN thin films on MgO(100),Si(100)and SiOx/Si substrates on both ambient and heated substrates.By means of X Ray Diffractometer(XRD),Transmission Electron Microscopy(TEM),Auger Electron Spectroscopy(AES)and Atomic Force Microscopy(AFM),we have studied the crystal structure,chemical composition and surface roughness of NbN thin films.On this basis,in order to study the uniformity of NbN films and nanowires from the angle of element distribution,we prepared 6 nm thick NbN thin films on the ambient and heated SiOx/Si substrates.The nanowires of 100 nm to 600 nm were obtained by Electron Beam Lithography(EBL)and Reactive Ion Etching(RIE).With the help of Auger Electron Spectroscopy(AES)and MATLAB,we got the distribution of element C,N,Nb,O and Si,gathered the related information about homogeneity of nanowires and analyzed it statically.The results show that the Tc0 of 6nm thick NbN films achieved at room temperature and on heated SiOx/Si substrates can reach 7.2K and 9.5K respectively.The superconducting transition temperature of NbN film can be greatly improved by heating the substrate.Based on image processing and statistical analysis,we proposed a method to evaluate the uniformity of thin films and nanowires.For film samples,the distribution uniformity in the selected areas can be characterized for different elements;for nanowire samples,the line width uniformity and the line edge uniformity of the nanowire lines can be characterized in both transverse and vertical dimensions.This evaluation method will provide an intuitive criterion for optimizing the processing technique of SNSPD in order to improve the consistency and product yield.Further studies will be carried out in following three aspects.First,we will make further efforts to optimize the performance of NbN films and then use the heated NbN film to prepare the SNSPD devices.Second,WSi films and nanowires based on it will be brought into uniformity experiment comparison.Third,by improving the resolution of AES element imaging,the accuracy of the uniformity analysis is supposed to be optimized in order to collect more information about the distribution of elements on both films and nanowires.