硼膜制备工艺、微观结构及其在硼化镁超导约瑟夫森结中的应用

MgB_2材料具备临界转变温度较高、相干长度大、临界电流和临界磁场高等优点,被认为有替代Nb基超导材料的潜力.研究了不同温度下以化学气相沉积法制备的硼(B)薄膜的微观结构.实验结果表明:较低温度沉积的B先驱薄膜为无定形B膜,可以与Mg蒸气反应生成MgB_2超导薄膜;当沉积温度高于550?C时,所得硼薄膜为晶型薄膜;以晶型硼薄膜为先驱膜在镁蒸气中退火,不能生成硼化镁超导薄膜.利用晶型B膜的这一特点,成功制备了以晶型硼薄膜为介质层的硼化镁超导约瑟夫森结.

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

Magnesium diboride is a binary compound with a simple AlB₂ type crystal structure and a high-T_c (nearly 40 K) superconductor. The rather high T_c 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 α-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 μm×100 μm 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.