Step Edge Junctions in YBa2Cu3O7-δ High-Tc Superconductors
Doctoral thesis, 1996
This thesis describes experimental work on GdBa2Cu3O7-.delta. (GBCO) thin films, YBa2Cu3O7-.delta. (YBCO) step edge junctions, properties of the grain boundary, and Josephson flux-flow transistors based on YBCO step edge junctions. A home made dc magnetron sputtering system was used for the fabrication of GBCO thin films. Epitaxial films on various substrates with the critical temperature Tc of about 90 K and the critical current density jc of above 106 A/cm2 at 77 K were routinely obtained.
A process based on an amorphous carbon (a-C) film mask and electron beam lithography was developed for making YBCO step edge junctions. Because of the very low ion milling rate of the a-C film and the high resolution of e-beam lithography, sharp and straight step edges were successfully fabricated on LaAlO3 (LAO) substrates. YBCO thin films patterned to narrow strips across the step edge gave high quality Josephson junctions. Their current-voltage (I-V) curves could well be described by the resistively shunted junction (RSJ) model. Their critical current as a function of the applied magnetic field Ic(B) showed Fraunhofer-like patterns.
By intentionally fabricating wavy step edges, we found that junctions across the wavy step edge showed neither flux-flow type, nor RSJ type I-V curves. The magnetic field modulation of Ic was very small even at 77 K (less than 5%), indicating large excess current and nonuniform current distribution. High resolution cross-sectional transmission electron microscopy studies showed that the nucleation and growth of the YBCO film on wavy step edges were much more complicated as compared to that on straight step edges, e.g., second phase particles usually appeared in the YBCO film along the wavy step edge region.
Properties of the grain boundary were investigated. Electromagnetic resonances, i.e. Fiske and flux-flow resonances, were observed from the I-V curves. The existence of electromagnetic resonances suggested that the barrier was dielectric. Three different methods, i.e. the McCumber constant ßc, Fiske and flux-flow resonances, were used to estimate the shunting capacitance of the junction. The capacitance value per unit area was found to be between 12-35 fF/.my.m2 for junctions on LAO substrates. Tunneling experiments on step edge junctions gave strong evidence that the grain boundary consisted of an insulating layer. Gap-like structures were observed from the I-V curves and dI/dV vs V curves. The superconducting gap value .DELTA., derived from the peak-to-peak values on the dI/dV vs V curve varied between 24 to 32 meV. If interpreted in the context of a maximum gap, this would imply 2.DELTA./kBTc=6-8.
As an example of applications, Josephson flux-flow transistors were designed and fabricated based on step edge junctions. The critical current Icc, and the response voltage V at a fixed bias current Ib, were measured as a function of the control current Icr through a control line inductively coupled to the junction. With an asymmetric in-line geometry, a current gain g=.DELTA.Ic/.DELTA.Icr of 17, and a transresistance rm=.DELTA.V/.DELTA.Icr of about 9 .OMEGA., were achieved at 4.2 K.
Josephson flux-flow transistor
step edge junction