异质多层高温超导薄膜的研究

Heterojunctions of high-temperature superconductor (HTS)/insulating oxide, HTS/semiconductor or HTS/ magnetism usually exhibit a series of unique physical properties, which is not only helpful for us to understand some physical parameters of HTS but also can promote their future special application in the field of microelectronics. In this project, a novel heterostructure with coherent alternating layers will be prepared by using two kinds of HTS materials, YBCO and BSCCO, with different critical transition temperature Tc. When the number of alternating layers is two, three and even more, the corresponding multilayer heterostructures are called as HTS heterojunction, Josephson junction and superlattice, respectively. Thus, with the change of temperature, the as prepared multilayer heterostructure will display the different physical properties. Take the bilayer HTS heterostructure as an example, the heterostructure will exhibit a common characteristic like semiconductor heterojunction when the temperature is higher than the Tc of the two HTS films; while it will be a superconducting heterojunction when the temperature is lower than the Tc of the two HTS films. Besides, it will exhibit the characteristics of Schottky junction when the temperature is between the two Tc. The appropriate lattice mismatch between two superconducting phases will induce a certain elastic stress in the coherent interface of them without the loss of superconductivity, which maybe leads to the formation of barrier between the interface atoms. It is expected to overcome the difficulties in the preparation of high temperature superconducting Joseph junction due to their short coherence length. In addition, it is quite significant for the academic research and practical application to investigate the interfacial properties of the two superconducting materials, their transport characteristics of charge carrier and photovoltaic characteristics.

由高温超导和绝缘体、半导体、磁性体等材料构成的异质结具有一系列独特的物理性能，不仅有助于人们深入了解高温超导的一些物理参数，也可促进其在微电子领域的应用。该研究则拟将具有不同临界温度Tc的两种高温超导材料，如YBCO和BSCCO按照共格方式交替叠层生长，使其形成由高温超材料组成异质结（2层）、约瑟夫森结（3层）、甚至超晶格（多层）。这样，当温度变化时，所制备的多层膜体系会处于不同的物理状态，以异质结为例，在常温时，类似于普通的半导体异质结，当温度处于两个临界温度Tc之间时，类似于肖特基结，当两相均处于超导态时，就会形成全超导异质结。如果选择合适的晶格匹配，在不失去超导电性的同时，两相共格界面会保持一定的弹性应力，从而在两层之间形成原子尺度的势垒层，有望解决高温超导相干长度短、制备约瑟夫森结难的问题。进一步研究两相超导材料的界面特性，载流子的输运特性、光伏特性等都具有一定的学术意义及应用价值

高温超导异质结具有一系列独特的物理性能，制备出性能优异的高温超导异质结，不仅有利于人们深入了解高温超导的物理特性，也可开拓高温超导材料在微电子领域的应用。该课题首先研究了钇钡铜氧（YBCO）、钆钡铜氧（GdBCO）、铋锶钙铜氧（BSCCO-2212、BSCCO-2223）和镧铈铜氧（LCCO）这几种具有不同临界转变温度和不同超导电学特性的高温超导材料的制备工艺。在掌握了上述几种高温超导薄膜制备技术的基础上，实现了异质高温超导薄膜结构GdBCO/YBCO、BSCCO/YBCO、LCCO/YBCO、YBCO/BSCCO/YBCO或高温超导-氧化物-高温超导结构YBCO/Y2O3/YBCO、YBCO/Gd2O3/YBCO的共格交替叠层生长，探索了异质结的界面效应及由界面晶格失配、界面弹性应力等在异质超导界面间形成的原子尺度“势垒层”作为约瑟夫森结势垒的可行性，在全超导异质结构GdBCO/YBCO和YBCO/BSCCO/YBCO中观察到了直流约瑟夫森效应，在全超导异质结构LCCO/YBCO中观察到了光伏效应和超导转变时的光伏特性变化等现象。. 在上述研究过程中，课题组自主研究开发了高温超导薄膜的“感光溶胶-凝胶”微细加工方法，克服了多层超导或超导-氧化物-超导微细图形的套刻难题，实现了超导三维微细图形的制备，并制备了三维结构的YBCO-Y2O3-YBCO高温超导磁通变换器原型器件。. 最后，该课题还通过设计合理的La2-xGdxZr2O7或Ce1-xZrxO2梯度过渡层，结合感光溶胶-凝胶工艺，用全化学法在NiW基底上制备了图形化的YBCO高温超导薄膜，为高温超导柔性器件制备奠定了基础。