Verlag des Forschungszentrums Jülich
JUEL-3525 Influence of the Fabrication Process on the Transport Properties of Ramp-Type Junctions
with PrBa2Cu2.9Ga0.1O7[delta]Barrier Josephson
Junctions are the most important active elements in high-temperature-superconductor (HTS)
electronics. For the fabrication of HTS digital circuits, a Josephson junction design is
necessary that is compatible with multilayer technology and has a high design flexibility.
Ramp-type junctions are especially good candidates to fulfill these requirements. However,
arbitrary positioning of ramp-type junctions on a chip is essential for effective circuit
layouts. Position-independent ramp quality can be achieved by rotating the sample during
ion-beam etching of the ramp. In this work, a fabrication process for ramp-type junctions
for digital circuits was developed for the first time. Ramp-type junctions were fabricated
using PrBa2Cu2.9Ga0.1O7-[delta] (PBCGO) as the
barrier material. The gallium substitution into the PBCO leads to an increase of the
compounds resistivity and, therefore, of the junctions normal resistance, RN.
The effect of processing parameters upon the morphology and profile of ramps ion-beam
etched in YBa2Cu3O7-[delta] /SrTiO3 bilayers
was investigated in detail. At the same time, the influence of the resulting ramp quality
on the junction electrical properties as well as the transport mechanisms for the critical
current, Ic, and the normal resistance were studied. Independent or the tilt
angle during ion milling, the profile of the unoptimized ramp showed two parts: a slope
and a tail. A uniform slope of 30° could eventually be obtained by post-baking of the
resist mask and a subsequent ion-beam etching with a tilt angle of the ion-beam of 30°
with respect to the substrate normal. Junctions with improved ramps showed current-voltage
characteristics and a temperature dependence of RN typical for resonant tunneling of
quasiparticle current through the barrier. Junctions with unoptimized ramps, exhibited an
order of magnitude lower values of RN, and a metallic temperature dependence of
RN. This could be explained tentatively by contributions from metallic chains
in thinnest regions of the PBCGO barrier located on the tail of a ramp. The morphology of
the ramp surface is strongly depending upon ion-beam etching parameters. A roughness of
15nm peak-to-peak after the annealing process could be reduced to 3-4nm peak-to-peak by
reducing the voltage at the end of the milling process and a subsequent wet chemical
etching process in bromine-ethanol that removes almost all of the amorphized material
which could otherwise recrystallize in the following annealing process. By optimizing the
morphology of the ramp the spread of the junction Ic could be reduced from
about [sigma]=88% on-chip to a best value of about 11% on-chip. Investigations by
Transmission Electron Microscopy showed that PBCGO-barriers grew with a mixture of a- and
c-orientation, leading to inhomogeneous current distributions. The homogeneity of the
current distribution could be improved by using PrBa2Cu3O7-[delta]-barriers
instead of PBCGO-barriers. The magnetic field dependence of Ic showed a Fraunhofer-like
pattern, indicating a more homogeneous current distribution than in the case of gallium
substitution in the barrier material. In conclusion, the optimization of the quality of
the ramp lead to uniform profile and low roughness of ramps. This in turn resulted in more
homogeneous current distribution in junction cross-section and reduced spreads of the
junctions Ic. This represents a useful contribution to the development of
ramp-type junctions for advanced multilayered circuit technology required for digital
applications. In best junctions, IcRN-products of 8mV at 4.2K and
50[my]V at 77K could be achieved.
Horstmann, Cornelia
Einfluß des Herstellungsprozesses auf die Transporteigenschaften von Rampenkontakten mit PrBa2Cu2.9Ga0.1 07-delta-Barrieren
114 S., 1998
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