Preparation, microstructure of B film and its applications in MgB2 superconducting Josephson junction

Abstract

Magnesium diboride is a binary compound with a simple AlB2 type crystal structure and a high-Tc (nearly 40 K) superconductor. The rather high Tc value and the specific properties make it a potential material for electronic applications. The key structure for the application is a Josephson junction. The growth of tri-layer structure consisting of MgB2 film and tunneling barrier layer is a key technology for a Josephson junction. Boron is a kind of good insulating medium. Preparation of MgB2/B/MgB2 tri-layer structures by chemical vapor deposition (CVD) method is investigated. The experimental results indicate that the depositing temperature will influence the microstructure of boron film significantly and different crystal structures of boron films are obtained at different temperatures.The boron film is an amorphous film while the deposition temperature is lower than 500 ℃, and the amorphous B film can be transformed into MgB2 superconducting film by annealing in Mg vapor. For precursor B films deposited at 470 ℃ and 500 ℃, the critical temperatures of the relevant MgB2 films are 39.8 K and 38.5 K, respectively. As the deposition temperature is higher than 550 ℃, the boron film becomes crystallized, and increasing deposition temperature will increase the crystallinity of the B film as can be seen from the samples deposited at 550 ℃, 600 ℃, 650 ℃ and 680 ℃. The boron film turns out to be of -phase crystalline texture, which is verified by X-ray diffraction and scanning electron microscope. What is more, the crystalline boron film is a kind of inert film, and it does not react with Mg in Mg vapor, thus it cannot be transformed into superconducting film in the subsequent annealing steps. By utilizing the property of the crystallized boron film, a square-shaped Josephson junction with a size 100 m100 m of MgB2/B/MgB2 structure is prepared. The thickness of boron dielectric layer is about 10 nm, and the DC Josephson effect is observed by the I-V measurement of the junction. Compared with other tri-layer structure based on MgB2 material, such as the MgB2/MgO/MgB2, the structure in which B film serves as a barrier layer eliminates the oxygen and can be fabricated in-situ easily by CVD method, and reliable Josephson junctions can be expected by such a technology.