Enhancement of the Quality of Stacked-type High-Tc Josephson Junctions by Doping and Development of the Integration Technology for These Junctions

本研究的目的是建立用新的掺杂方法制备叠层型高tc约瑟夫森结的技术，并提高该技术在电路中的应用。实验结果如下：影响结性能恶化的因素研究：除了通过掺杂Pr和Ga抑制针孔外，我们还研究了影响散射和结性能恶化的因素。研究发现，在形成势垒的过程中，表面形貌的变化与结的性质有关，我们得到了具有过阻尼特性和30K下高临界电流密度的结。这些结的散射很小，是我们所获得的最高质量的结。La掺杂使YBCO薄膜平面化：我们用YBCO中的部分Y或Ba代替la3，证实了YBCO薄膜中的空穴减少。用透射电子显微镜研究势垒结构：用透射电子显微镜观察影响结性能的势垒层周围微观结构。在高氩离子加速电压下形成势垒时，势垒层周围明显可见Y_2O_3相。考虑到加速电压与结临界电流密度等的关系，更高的临界电流密度对应更薄、更均匀的势垒。顶电极材料的研究：为了抑制阻挡层中的针孔，我们尝试了YbBa_2Cu_3O_<7-x> (YbBCO)作为顶电极，YBCO可以在比YBCO更低的温度下获得薄膜。YBCO顶电极结的散射比YBCO顶电极结的散射小。YbBCO可以有效地获得良好的结性能。

The purpose of this research is to establish the fabrication technique of stacked-type high-Tc Josephson junctions by the new doping method and to enhance the technique for application to circuits. Following results were obtained.1.Investigation of the factors to worsen the junction properties : In addition to the suppression of pinholes by doping of Pr and Ga, we investigate the factors influencing scattering and deterioration of junction properties. It was found that the change in the surface morphology by the etching for forming the barrier was related to the junction properties, and we obtained junctions with overdamped characteristics and high critical current density at 30K. These junctions showed small scattering, and were of highest quality we ever obtained.2.Planarization of YBCO films by La doping : We confirmed that holes in YBCO films decreased by substituting part of Y or Ba in YBCO for La.3.Investigation of the barrier structure by TEM : Microstructure around the barrier layer which affects junction properties by transmission electron microscopy (TEM). Y_2O_3 phases were clearly observed around the barrier layer when the barrier was formed using high Ar ion acceleration voltage. Taking the relationship between the acceleration voltage and the junction critical current density, etc. into account, higher critical current density would correspond to thinner and more uniform barrier.4.Investigation of the material for the top electrode : In order to suppress the pinholes in the barrier layer, we tried YbBa_2Cu_3O_<7-x> (YbBCO) of which film can be obtained at lower temperature than YBCO as the top electrode. Junctions with YbBCO top electrode showed scattering smaller than those with YBCO top electrode. YbBCO may be effective to obtain good junction properties.