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

本研究的目的是利用Y-Ba-Cu-O（YBCO）薄膜实现高灵敏度磁传感器squid（超导量子干涉器件），这是一种有前途的高质量器件材料。本文提出了“临界电流可控约瑟夫森结”的新概念。本研究于1991和1992学年进行，所得结果如下：1。采用单靶磁控溅射技术，成功地“原位”生长了YBCO薄膜。在（100）MgO衬底上生长薄膜时，生长温度越高，转变温度（Tc）越高。结晶度从a轴方向转变为c轴方向。当衬底温度为700℃时，得到Tc=77.5K的YBCO薄膜。该薄膜的XRD谱图仅显示出尖锐的（00l）峰，表明具有很高的c轴取向。用扫描隧道显微镜（STM）观察所得薄膜。随着生长温度的升高，材料表面由织构结构向螺旋结构转变。当YBCO薄膜在退火衬底上生长时，这些螺旋结构变得相当大。这可能是因为退火降低了基体表面螺位错的形核中心密度。然后在（100）SrTiO_3衬底上生长YBCO薄膜。当衬底温度为700℃时，得到Tc=82k的YBCO薄膜。这些薄膜的XRD谱图显示出a轴取向和c轴取向。YBCO薄膜被光刻成微桥，这是SQUD的关键结构。化学蚀刻没有改变Tc。利用“临界电流可控的约瑟夫森结”来实现新概念的dc-SQUID还有待进一步的研究。为明确YBCO的晶体生长机理和超导性获得基础性结果。

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.