Realization of Josephson junctions with electrically controlled critical current and its application to dc-SQUIDs.

本研究的目的是利用有望获得高质量器件材料的Y-BaCuO(YBCO)薄膜实现高灵敏度的超导量子干涉器件。本文提出了“电控临界电流约瑟夫森结”的概念，并在1991-1992学年进行了这方面的研究，取得了如下结果：1.用单靶磁控溅射法“原位”生长了YBCO薄膜。当薄膜生长在(100)MgO衬底上时，生长温度越高，转变温度(T_C)越高。结晶度由a轴取向变为c轴取向。当衬底温度为700ﾟC时，获得了T_c=77.5K的YBCO薄膜。X射线衍射图显示该薄膜只有很强的c轴取向的(00l)峰。研究发现，随着生长温度的升高，薄膜表面由织构结构向螺旋结构转变。当在退火衬底上生长YBCO薄膜时，这些螺旋结构变得相当大。这可能是因为退火降低了衬底表面螺位错的形核中心密度。然后，在(100)SrTiO3衬底上生长了YBCO薄膜。当衬底温度为700ﾟC时，获得了T_c=82K的YBCO薄膜。薄膜的X-射线衍射谱同时显示出a轴和c轴的取向。4.采用光刻工艺将YBCO薄膜刻画成微桥结构，这是SQUD的关键结构。超导超导薄膜的T_c不受化学腐蚀的影响，利用“电控临界电流约瑟夫森结”实现DC-SQUID的新概念还有待进一步研究。为弄清YBCO晶体生长和超导电性的机理所作的努力。

The purpose of this study is to realize high sensitivity magnetic sensor SQUIDs (Superconducting QUantum Interference Devices) using Y-Ba-Cu-O(YBCO) thin films which are promissing high quality device material. Here a new conception "Josephson junctions with electrically controlled critical current" is proposed.This study was done during 1991 and 1992 academic years and the obtained results are as follows :1.YBCO thin films were successfully grown "in situ" by single target magnetron sputtering.2. when films were grown on (100)MgO substrates, the higher the growth temperature was, the higher the transition temperature(Tc) became. And cristallinity changed from a-axis orientation to c-axis one. YBCO thin films of Tc=77.5K was obtained when substrate temperature was 700ﾟC.XRD pattern of this film showed only sharp (00l) peaks indicating very high c-axis orientation.3.Obtained thin films were observed by scanning tunneling microscope(STM). And it was found that the surface changed from textured structures to spiral ones as the growth temperature increased. These spiral structures became fairly large when YBCO films were grown on annealled substrates. This may because the density of nucleation centers of screw dislocations at substrate surface was reduced by annealing. Then, YBCO thin films were grown on (100)SrTiO_3 substrates. When substrate temperature was 700ﾟC, YBCO thin films of Tc=82k were obtained. XRD pattern of these films showed both a-axis orientation and c-axis one.4.YBCO thin films were patterned by photolithography process into microbridges, which are the key structures of SQUD.Tc did not changed by chemical etching.Further study will be done to realize new conception dc-SQUID utilizing "Josephson junctions with electrically controlled critical current". The effort to obtain fundamental results for making clear the mechanism of crystal growth and superconductivity of YBCO.