Preparation And Study Of MgB₂ Superconducting Films And Josephson Junctions

MgB₂ is a binary alloy compound with the highest superconducting transition temperature?Tc³9K?.The wide energy gap,no weak connectivity,and very short electron phonon interaction time make it a promising application in the fabrication of superconducting devices.Therefore,the study of MgB₂ is of great significance not only in the superconducting mechanism,but also in the application of superconducting devices.High quality Mg B₂ films are the basis for the fabrication of MgB₂ superconducting devices.At present,post-annealing method and one-step method are used to prepare MgB₂ films.The annealing time of the former method is too long,which reduces the preparation efficiency.The surface roughness of the film obtained using the latter method is large,which is not conducive to the preparation of MgB₂ superconducting devices.In this paper,MgB₂ thin films were prepared by electron beam annealing Mg-B precursor film.The surface morphology,microstructure characters,electrical and magnetic properties of the MgB₂ films,as well as the preparation conditions of electron beam annealing of MgB₂ films,were systematically studied.On this basis,the MgB₂ ultrathin films and MgB₂ Josephson junctions were prepared and studied.These studies lay the foundation for the application of MgB₂ in superconducting devices and superconducting integrated circuits.The main achievements of this paper are as follows:The effect of annealing temperature on the superconducting transition temperature of MgB₂ films was investigated.The results showed that the transition temperature Tc?0?reaches its maximum value of 33 K when the electron beam annealing temperature was about 600?.By changing the annealing current and time with fixed temperature,the optimum annealing conditions were determined as follows: the beam spot radius was 20 mm,the annealing current was 5mA and the annealing time was 1s.The effect of the content of Mg in the precursor film on the properties of the films was studied.When the excess Mg was 40%,the superconducting properties of the films were the best.The Tc?0?was 34.2K,and the critical current density was about 6.5×105A/cm2 at 20 K and 0T.The main pinning mechanism of MgB₂ films was proved to be grain boundary pinning according to magnetic measurements.Small surface roughness of the MgB₂ thin film is of fundmental importance for fabricating superconducting devices.The effect of Mg and B deposition rates on the surface roughness of Mg-B precursor film was investigated.The AFM images showed that the deposition rate of B had no effect on the surface roughness of Mg-B precursor film,while the deposition rate of Mg was the main factor affecting the roughness of the precursor film.The surface roughness of Mg film decreased firstly and then increased with the increase of Mg deposition rate.When the deposition rate of Mg was 0.9?/s,the surface roughness of Mg film was the smallest.When the deposition rates of Mg and B were 0.9?/s and 2?/s,respectively,the Mg-B precursor film with a surface roughness of 1.78 nm was obtained.This Mg-B precursor film satisfies the conditions of preparing Mg B₂ Josephson junction.The effect of cycle thickness on the surface roughness was studied.The cross-section morphology of the film showed that bubbles appeared on the surface of MgB₂ films when the cycle thickness was large.The results of EDS showed that the bubble was caused by the thick Mg layer in the precursor film.The bubbles were eliminated by diminishing the cycle thickness of the films,and the superconducting properties of the films were further improved.When the ratio of Mg to B was 1/2,the cycle thickness was 12.5nm and the film thickness was 100 nm,the superconducting transition temperature Tc?0?was 33.4K,the critical current density was 3.8×106A/cm2 and the roughness was 2.7nm.The film could be operated in the working temperature range?20-25K?of the MgB₂ based superconducting device.Superconducting ultrathin films are the basis for fabricating HEB and SSPD.The effect of thickness on the superconducting properties of MgB₂ films was studied.When the thickness was reduced from 100 nm to 10 nm,the transition temperature Tc hardly changed.But when the film thickness continued to decrease,Tc decreased sharply.High resolution transmission electron microscopy images confirmed that there were a large number of amorphous regions in the thin films,which reduced the effective current cross-sectional area,and led to the decrease of the critical current density.The Mg B₂ ultrathin film with a thickness of 5nm was obtained.The Tc?0?was 21.5K,and the critical current density at 15 K and 2T was 104A/cm2.Josephson junctions with a special structure were prepared.This structure was the improvement of the conventional sandwich-type Josephson junction.Different from the ordinary sandwich structure,the upper superconducting layer was deposited after the grooves were etched on the barrier layer.The transition from SIS junction to SNS junction can be controlled by changing the etching depth.The SNS type MgB₂ Josephson junction with a barrier thickness of 100 nm,a etching depth of 150 nm and a groove area of 10 × 10 ?m2 was obtained.The superconducting energy gap width was 1.13 meV and the critical current density was 1.2×105A/cm2 at 5K and 0T.Josephson junctions with this structure can work at 500 GHz,which is comparable to the experimental results of Nb based superconducting devices.