The correlation between the shape and the morphology of ramps prepared by ion-beam etching of YBa 2 Cu 3 O 7− δ thin films, and the properties of ramp-type junctions were investigated in detail. We examined the influence of different fabrication parameters on the YBa 2 Cu 3 O 7− δ ramps by atomic force microscopy. Ramp-type junctions were fabricated using PrBa 2 Cu 2.9 Ga 0.1 O 7− δ as barrier material. We observed a strong influence of the shape of the ramp on the homogeneity and, thus, on the transport properties of the junctions. Furthermore, we observed that the roughness of the ramps is strongly influenced by the voltage of the ion-beam during etching. Best results are achieved when an additional wet cleaning step by bromine solution in ethanol is introduced prior to the deposition of the barrier and the top electrode. As a result from our optimization, the on-chip spread of the junction critical current was reduced to 11%.

Influence of ramp shape and morphology on the properties of YBa2Cu3O7−δ-ramp-type junctions

Ramp-type Josephson junctions with highly doped PrBa2Cu3-xGaxO7-δ barrier layers (x=0.7, 1.0) have been investigated by high-resolution electron microscopy. A Ga-rich intergrowth and Ga diffusion in the ion-milled SrTiO3 substrate are observed. The Ga segregation is responsible for the deviating electrical behavior as expected from extrapolation of low doping levels (x=0.1-0.4). The Ga diffusion in the ion-milled substrate, and possibly in the base electrode, may hamper Josephson junction fabrication using substituted REBa2Cu3O7-delta materials (RE=rare earth)

/pdf/ga-segregation-in-dyba2cu3o7-d-prba2cu3-xgao7-d-dyba2cu3o7-d-44w6togrlx.pdf

Ga segregation in DyBa2Cu3O7-δ/PrBa2Cu3-xGaO7-δ/DyBa2Cu3O7-δ ramp-type Josephson junctions

We studied the morphology of ramps in REBa/sub 2/Cu/sub 3/O/sub 7/ (REBCO) epitaxial films on SrTiO/sub 3/ substrates, fabricated by RF magnetron sputter deposition and pulsed laser deposition (PLD), by Atomic Force Microscopy (AFM) and High Resolution Electron Microscopy (HREM). The ramps were fabricated by Ar ion beam etching using different masks of standard photoresist and TiN. AFM-studies on ramps in sputter deposited films show a strong dependence, i.e., formation of facets and ridges, on the angle of incidence of the ion beam with respect to the substrate surface as well as the rotation angle with respect to the crystal axes of the substrate. Ramps in pulsed laser deposited films did not show this dependence. Furthermore, we studied the effect of an anneal step prior to the deposition of barrier layers (i.e. PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7/) on the ramp. First results show a crystallization of the ramp surface, resulting in terraces and a non-homogeneous growth of the barrier material on top of it. The thickness variations, for thin layers of barrier material, can even become much larger than expected from the amount of deposited material and are dependent on the deposition and anneal conditions. HREM studies show a well-defined interface between barrier layer and electrodes. The angle of the ramp depends on the etch rate of the mask and REBCO and on the angle of incidence of the ion beam. Hard masks, like TIN, have a much lower etch rate compared to photoresist, resulting in an angle of the ramp comparable to the angle of incidence and, subsequently, in a low etching rate on the ramp.

/pdf/characterization-of-ramp-type-yba-sub-2-cu-sub-3-o-sub-7-vg97inyqld.pdf

Characterization of ramp-type YBa/sub 2/Cu/sub 3/O/sub 7/ junctions by AFM

We have developed a new technique for etching of high-temperature superconducting YBa 2 Cu 3 O 7 (YBCO) thin films using C 60 etching mask. The minimal pattern size 0.5 μm was reached in the process of the ion beam etching. Wet chemical etching in Br–ethanol using C 60 mask was successfully realized. In the case of the chemical etching as a mask we have a 400 nm thick patterned C 60 layer due its insolubility in Br–ethanol. The submicron patterning of the YBCO films, thick up to 600 nm, was realized using Ar ion beam etching through the patterned C 60 film. In this case, the C 60 mask, 600–800 nm thick, was created by lift off technique in PMMA electron resist.

C60 films as etching masks for creation of micrometer and submicrometer YBa2Cu3O7 structures

Who would have imagined a supercurrent flowing between two phase coherent electrodes at 100 K? Josephson junctions (JJs) based on high temperature superconductors (HTS) can actually sustain large supercurrents at such unprecedented high temperatures. They are as well the first example of a wide class of devices based on exotic materials. In this chapter, we examine the properties of different types of HTS-based junctions and discuss notions on fabrication which is useful to classify junction properties.

High Critical Temperature Superconductor Josephson Junctions and Other Exotic Structures

The dependence of the ramp geometry on high- or low-angle ion-beam etching, used to structure ramp-type Josephson junctions based on high- superconductors, is investigated by cross-section transmission electron microscopy. The surface quality, interfaces and crystal defects are analysed by high-resolution electron microscopy. Technical difficulties to reproducibly obtain the desired slope angle and shape make high-angle ion-beam etching less interesting although the surface quality is comparable and no systematic differences in electrical properties are observed.

/pdf/low-or-high-angle-ar-ion-beam-etching-to-create-ramp-type-4ii9e0qe5t.pdf

Low- or high-angle Ar ion-beam etching to create ramp-type Josephson junctions

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Low- or high-angle Ar ion-beam etching to create ramp-type Josephson junctions