Study On Superconducting Nb Metal Films Fabricated By Pulsed Laser Deposition

As a commonly used low-temperature superconducting material,the metal Nb has a bulk superconducting transition temperature(Tc)of 9.3 K and a similar film Tc.Because of its good conductivity,stable chemical property,high mechanical strength,and a coherence length,the Nb thin films are widely used in medicine,biology,energy,and so on.In recent years,low-temperature superconducting devices have realized terahertz wave bands Detection,so that they have broad application prospects in astronomy,atmosphere,physics and other aspects.Then,how to achieve large-scale production of high-performance Nb metal superconducting thin films has become the focus of the current research.The preparation of Nb films mainly used pulsed laser deposition technology(PLD).This technology can fabricate Nb films flexibly and conveniently,however the resulting films are lack of well-organized crystalline structure.At the same time,the low ionization rate of the target usually leads to a small amount of unnecessary particle residual on the surface of the film.Therefore,the Tc value of the obtained film can only reach 8.4 K.This work optimized the pulsed laser deposition system and the preparation technology.Meanwhile,the Tc of the prepared Nb superconducting thin films reached 8.6 K and their highly crystalline property was studied.The main contents of this work are as follows:(1)A single-beam pulsed laser deposition system was designed to only have one focusing mirror which has low cost,small size,and reduced energy loss.A Nd:YAG solid-state laser was applied as the light source and the incident laser beam into the chamber is angled at 45°.In addition,to simplify the preparation process,the optical path bracket was placed at an angle of 45° instead of adjusting the optical components of the optical path.(2)A dual-beam pulsed laser deposition system was proposed to improve the single-beam system.A long-wavelength laser is introduced into the traditional single-beam pulsed laser deposition system.By tuning the two beams in time and space,the enhanced plasma component controllability results in the improved ionization rate of the target and the crystallinity and uniformity of the film.(3)The Nb films fabricated by the optimized pulsed laser deposition system were characterized by XRD,SEM,AFM,XPS,and PPMS.The effect of the three process parameters(laser energys,substrate temperatures and target-substrate distance)of the film crystal structure was investigated on surface morphology,internal stress,chemical composition,and electrical properties.The results showed that the crystalline quality of the film is gradually improved with increasing the laser energy.Besides,the influence of the substrate temperature and the distance between the target and the substrate on the film crystallization was studied.After optimization,the final prepared Nb film has low oxidation rate and excellent quality as follows:the internal stress of the film was successfully reduced from-1.6 GPa to-0.3 GPa;the pure Nb content in the film was increased from 25.57%to 29.36%;the Tc value is increased from 8.4 K to 8.6 K.