Fine Scale Microstructures of Josephson Junctions in YBa2Cu3O7-δ Thin Film Structures
Doctoral thesis, 2000
This work concerns the microstructure of YBa2Cu3O7-.delta. (YBCO) and other related thin films in multilayer structures, studied by mainly transmission electron microscopy (TEM). High temperature superconducting thin films are frequently used in applications, such as Josephson junctions, where deposition and growth of multilayers and the patterning processes are key factors for successful devices. Crucial issues are the effect of substrate morphology, interaction between different layers and the influence of the individual layers' surfaces on the subsequent deposited layers.
The nucleation and growth of YBCO deposited on patterned step edges in (001) LaALO3 (LAO) substrates have been characterised. A local 90° orientation tilt of the c-axis oriented YBCO thin films was introduced and grain boundaries evolved. The crucial parameters influencing the onset of each boundary morphology; nucleation probability and minimisation of surface energy were identified. A change of the deposition method, showed that it is possible to influence the grain boundary evolution by controlling the deposition parameters. Nucleation of YBCO along an intentionally made wavy step edge profile was studied by TEM and scanning electron microscopy (SEM). A variation in nucleation rate along the wave was determined. The film evolution of the YBCO film was more complex than that on straight set edges.
Nucleation and growth of multilayer structures, consisting of YBCO/isolating layer/YBCO have been characterised using TEM and electron energy loss spectroscopy (EELS). Two different methods of fabricating patterned bottom YBCO layers have been studied, i.e. a common process of ion milling the deposited YBCO bottom layer and an alternative planarizing process, where the substrate was patterned prior the YBCO deposition. Two types of insulating layer, single STO layer and multilayer (alternating SrTiO3 (STO) and PrBa2Cu3O7-.delta. (PBCO) layers), were investigated. An insulating multilayer resulted in less secondary phases and pinholes in the layers compared to a single STO layer.
Ramp-edge Josephson junctions with two different barrier layers were studied; a 25 nm thick laser ablated Ga doped PBCO layer and a layer made by a process, where the problems associated with the deposition of the barrier layer are avoided, the fabrication of interface-engineered ramp-edge junctions (IEJs). In ramp-edge junctions the ramp angle must not exceed 40° in order to deposit an epitaxial high quality top YBCO layer. Characterisation of ramp-edge junctions with deposited Ga doped PBCO as barrier layer showed that the barrier layer nucleated and grew epitaxially with a uniform thickness on the ion-milled YBCO edges. Planar IEJs have been characterised using high resolution TEM together with image simulation, energy dispersive X-ray spectrometry (EDS) and EELS.
thin film evolution
transmission electron microscopy