Preparation Of MgB₂ High Temperature Superconducting Thin Films And SNSPD Enhanced Absorption Structure Design

Compared with traditional single photon detectors,superconducting nanowire single photon detectors（SNSPDs）working in the near-infrared band have many advantages such as high detection efficiency,low dark count,small time jitter,short recovery time,and wide response spectrum.At present,there are more than 20 kinds of superconducting thin film materials used in SNSPDs,but most SNSPDs are based on traditional low-temperature superconductors（LTS）operating in the ultra-low temperature range of 1.7 to5 K.The high cost of refrigeration by liquid helium has become the main disadvantage limiting the wide application in various fields.Therefore,it is of great significance to carry out research on superconducting nanowire single photon detectors based on high temperature superconducting materials（HTS）.Magnesium diboride（MgB₂）,as a binary intermetallic compound with the highest superconducting transition temperature（39 K）,has great potential application value in SNSPDs,which has attracted great attention from researchers.In this thesis,we prepared MgB₂ superconducting thin film by magnetron sputtering combined with rapid annealing process,and completed the design of SNSPD light absorption structure based on MgB₂superconducting thin film.The main research content is as follows.1.Preparation and optimization of MgB₂ superconducting thin films.We deposited MgB₂ thin films on single crystal Al₂O₃（1102）substrates by Mg,B double-target RF co-sputtering method,and then combined with slide rail tube furnace for rapid annealing to prepare MgB₂ superconducting thin films.By adjusting the process parameters such as sputtering power of Mg and B,substrate temperature,annealing temperature and time during the preparation process,and combining the characterization results of Mg,B atomic ratio,surface morphology and superconducting properties of the film with different process parameters,the effects of various process parameters on the properties of the film were studied.The final optimized process parameters are:substrate temperature of 400°C,sputtering working pressure of 1.5 Pa,sputtering power of B target is 140 W,sputtering power of Mg target is 25 W,sputtering for 1.5 h.The annealing temperature is 650°C,and the annealing time is 150 s.The prepared MgB₂ has achieved good superconducting properties,and its superconducting transition temperature is 26.59K.2.Design and optimization of SNSPD enhanced absorption structure based on MgB₂thin film.Since the MgB₂ thin film used in SNSPD is thin,its own light absorption is very low if no enhanced light absorption structure is integrated.So,based on the MgB₂thin film,we designed two enhanced absorption structures based on the metal mirror optical microcavity and the distributed Bragg reflection（DBR）optical microcavity to enhance the light absorption of the MgB₂ thin film.In order to further enhance the infrared absorption of MgB₂ thin films,an all-dielectric metasurface structure with extremely low optical loss is designed as an enhanced absorption structure.By optimizing the structural parameters,the light absorption efficiency of MgB₂ film at 1550 nm wavelength was increased to 85.3%.