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IEEE International Superconductive Electronics Conference (ISEC)

Graphoepitaxial Josephson junctions and DC SQUIDs

Michael I. Faley, Doris Meertens, U. Poppe, Rafal E. Dunin‐Borkowski

Published Jul 1, 2013• 5 citations
Publisher

Abstract

We have prepared high-Tc superconducting heterostructures for DC SQUIDs with graphoepitaxial step edge Josephson junctions on single crystal MgO substrates and investigated them using complementary characterization techniques, including atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. Combined epitaxial (out-of-plane) and graphoepitaxial (in-plane) growth of YBa2Cu3O7-x (YBCO) films on the tilted surfaces of step edges on MgO substrates was observed. The high-Tc step-edge Josephson junctions demonstrated a good reproducibility and an IcRn product of about 600 μV at 77 K. The DC SQUIDs with step edge junctions showed voltage swings δVpp of up to about 60 μV at 77 K. High values of Rn of the graphoepitaxial step edge Josephson junctions can be advantageous for their applications as qubits.

DOI
10.1109/isec.2013.6604264

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Applied Physics Letters•Apr 26, 1993•24 citations
We have prepared high-Tc superconducting heterostructures for DC SQUIDs with graphoepitaxial step edge Josephson junctions on single crystal MgO substrates and investigated them using complementary characterization techniques, including atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. Combined epitaxial (out-of-plane) and graphoepitaxial (in-plane) growth of YBa2Cu3O7-x (YBCO) films on the tilted surfaces of step edges on MgO substrates was observed. The high-Tc step-edge Josephson junctions demonstrated a good reproducibility and an IcRn product of about 600 μV at 77 K. The DC SQUIDs with step edge junctions showed voltage swings δVpp of up to about 60 μV at 77 K. High values of Rn of the graphoepitaxial step edge Josephson junctions can be advantageous for their applications as qubits.
We fabricated Josephson junctions (JJs) between the d-wave superconductor (SC) YBa2Cu3O7-x (YBCO) and the s-wave SC Nb (ds-JJs) on graphoepitaxially buffered MgO substrates, studied ds-JJs at temperatures down to 30 mK and employed these JJs in π-loops. Current-voltage characteristics of ds-JJs oriented along the [100] axis of YBCO exhibited up to a factor of 200 higher critical current densities than ds-JJs oriented along the [110] axis of YBCO. The critical currentIcI_{c}Ic​and theIcRnI_{c}R_{n}Ic​Rn​product of [100]-oriented 3 μm wide ds-JJs are ∼70 μA and ∼200 μV, respectively, at 4.2 K. Rectangular arrays of up to 40 000 π-loops based on such ds-JJs were investigated using a low temperature scanning superconducting quantum interference device (SQUID) microscope. We observed ordering of spontaneously generated half integer magnetic flux quanta in the π-loops correlated with minute spurious background magnetic fields, as well as with configurations and mutual coupling of the π-loops. We manipulated the magnetic states of the π-loops by the local application of magnetic fields using nearby planar coils. This paper paves the way for the use of π-loops in computations that are based on annealing processes.
We report the development of a fabrication process of high-resistance step-edge Josephson junctions on sharp argon ion beam etched substrate steps. For a reproducible fabrication, the redeposition of material at the step during the etch process has to be avoided. This is achieved by an alternating angle of ion incidence parallel to the edge of the photoresist mask. The reproducible straight step profile exhibits an angle around and is reflected in reproducible electrical properties of the junctions. Critical current and asymptotic normal state resistance both scale with the junction width w . The current-voltage ( I - V ) characteristics depend on the ratio of film thickness t to step height h . For > 0.85 they are flux-flow-like; for < 0.8 they correspond to the resistively shunted junction (RSJ) model. Lower ratios of correspond to higher specific resistances resulting at 77 K typically in asymptotic normal state resistances between 5 and per junction, depending on w . At 77 K we achieve products of more than . The junctions are applied to dc superconducting quantum interference devices (dc-SQUIDs) with inductances between 25 and 35 pH. At 77 K transfer functions of more than are achieved. noise of the step-edge dc-SQUIDs is observed, but using bias current modulation it is entirely suppressed resulting in a spectral density of flux noise of measured at 1 Hz and 77 K for a SQUID with an inductance of 30 pH.
Autonomous single-layer dc superconducting quantum interference devices (SQUIDs) have been prepared from epitaxial, laser-deposited YBa2Cu3Ox films on step edge SrTiO3 and LaAlO3 substrates. For device patterning, a SiOx inhibit technique as well as conventional ethylenediaminetetraacetic wet etch was used. The Josephson junctions are of grain boundary type. Their widths are 5 μm. The SQUID hole is a square of about 5 μm; the SQUID inductance is estimated to be about 20 pH. We obtained a very regular, nonhysteretic flux to voltage modulation over more than 100 flux quanta (Φ0). The maximum voltage signal is of the order of 15 μV peak to peak and the maximum transfer function dV/dΦ at the appropriate flux bias is 50 μV/Φ0 at 77 K. The best value of the equivalent flux noise as measured in the flux locked loop mode is 1.4×10−5 Φ0/Hz1/2 at 1 Hz and 1×10−5 Φ0/Hz1/2 in the white noise region for f≳5 Hz. This results in an energy resolution εn,w(f≳5 Hz)=1×10−29 J/Hz in the white noise region and 2×10−29 J/Hz at 1 Hz.
We investigated full-epitaxial TiN/AlN/TiN Josephson junctions on MgO substrates for superconducting qubit applications. The critical temperature (TC) of TiN film is relatively low at around 5 K, but its lattice constant is about 0.424 nm, which is close to the lattice constant of MgO of 0.421 nm. TiN and AIN films were prepared by DC magnetron sputtering in a load-lock sputtering system with an ultra-high vacuum chamber. The deposition temperature was varied from ambient temperature to 1073 K. In XRD analysis, (200) peaks were observed in both the TiN single layer film and the TiN/AIN/TiN trilayer film. No other XRD peaks were observed in the single layer or trilayer films. The lattice constant of TiN was determined to be 0.4242 nm from XRD analysis, close to the value of 0.4212 nm for MgO. The 150 nm-thick single-layer TiN film on the MgO substrate showed a TCof 5.3 K and a resistivity of 3.5 μΩcm at 10 K. Based on these epitaxial TiN films, we fabricated TiN/AIN/TiN Josephson junctions and measured their current-voltage characteristics. At 1.9 K, the electrical parameters of junctions with JC= 50 A/cm2showed that the gap voltage and the ratio of Rsg/RNwere about 2.5 mV and 2.1, respectively.
We have fabricated Y1Ba2Cu3O7−x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions δV/δΦ of 20 μV/Φ0 at 77 K. We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the frequency dependent noise in our devices results from critical current fluctuations of the Josephson junctions. A bias current reversing technique is effective in suppressing this noise contribution. A flux noise Φn of 80 μΦ0/Hz1/2 at 1 Hz and 77 K was measured on a dc SQUID with an inductance Ls of 60 pH.