Josephson Junction Structures Research Articles

Crystal analysis of grain boundaries in boron-doped diamond superconducting quantum interference devices operating above liquid helium temperature

Superconducting quantum interference devices (SQUIDs) are magnetometers with ultra-high sensitivity that have garnered attention owing to their potential application in flux qubits for quantum computing. The Josephson junction is an important component that determines the characteristics of a SQUID. Based on the superconductivity of heavily boron-doped diamond (111) homoepitaxial layers with a high critical temperature (Tc > 10 K), we propose two types of Josephson junction structures with discontinuous (111) boundaries. These structures allow the SQUID to operate above liquid helium temperature (4.2 K) with high reproducibility. We analyzed local misorientation and strain (i.e., compressive, tensile, and shear strain) at the boundary via electron backscatter diffraction. The Josephson junction characteristics were attributed to the weak link with discontinuous boundaries of diamond (111) sectors.

Anodization-based process for the fabrication of all niobium nitride Josephson junction structures.

We studied the growth and oxidation of niobium nitride (NbN) films that we used to fabricate superconductive tunnel junctions. The thin films were deposited by dc reactive magnetron sputtering using a mixture of argon and nitrogen. The process parameters were optimized by monitoring the plasma with an optical spectroscopy technique. This technique allowed us to obtain NbN as well as good quality AlN films and both were used to obtain NbN/AlN/NbN trilayers. Lift-off lithography and selective anodization of the NbN films were used, respectively, to define the main trilayer geometry and/or to separate electrically, different areas of the trilayers. The anodized films were characterized by using Auger spectroscopy to analyze compounds formed on the surface and by means of a nano-indenter in order to investigate its mechanical and adhesion properties. The transport properties of NbN/AlN/NbN Josephson junctions obtained as a result of the above described fabrication process were measured in liquid helium at 4.2 K.

Progress in high-linearity multi-element Josephson structures

This paper summarizes both theoretical and experimental studies aimed at synthesis of high-linearity multi-element Josephson structures. Both the dynamic range and the voltage response linearity are two conjugated characteristics that must be improved jointly. Increase in dynamic range is reasonably associated with increase of number of elements N in the Josephson-junction structures. To improve the voltage response linearity one can use special design of the array structures. The other way is based on use novel basic cell with is bi-SQUID capable of providing highly linear voltage response. Both the approaches were used in designs of the reported high-linearity multi-element Josephson structures.

Radiation from Flux Flow in Josephson Junction Structures

We derive the radiation power from a single Josephson junction (JJ) and from layered superconductors in the flux-flow regime. For JJ case, we formulate the boundary conditions for the electric and magnetic fields at the edges of the superconducting leads using the Maxwell equations in the dielectric media and find dynamic boundary conditions for the phase difference in JJ which account for the radiation. We derive the fraction of the power fed into JJ transformed into the radiation. In a finite-length JJ this fraction is determined by the dissipation inside JJ and it tends to unity as dissipation vanishes independently of mismatch of the junction and dielectric media impedances. We formulate also the dynamic boundary conditions for the phase difference in intrinsic JJs in highly anisotropic layered superconductors of the Bi2Sr2CaCu2O8 type at the boundary with free space. Using these boundary conditions, we solve equations for the phase difference in the linear regime of Josephson oscillations for rectangular and triangular lattices of Josephson vortices. In the case of rectangular lattice for crystals with the thickness along the c-axis much larger than the radiation wavelength, we estimate the radiation power per unit length in the direction of magnetic field at the frequency 1 THz as ∼N μW/cm for Tl2Ba2CaCu2O8 and ∼0.04 N μW/cm for Bi2Sr2CaCu2O8. For crystals with thickness smaller than the radiation wavelength, we found that the radiation power in the resonance is independent on number of layers and can be estimated at 1 THz as 0.5 W/cm (Tl2Ba2CaCu2O8) and 24 mW/cm (Bi2Sr2CaCu2O8). For rectangular lattice, due to superradiation regime, up to half of power fed into the crystal may be converted into the radiation. In the case of triangular or random lattice in the direction perpendicular to the layers, the fraction of power converted into the radiation depends on the dissipation rate and is much lower than for rectangular lattice in the case of high-temperature superconductors with nodes in the gap.

Effects of oxygen content on YBCO Josephson junction structures

The high degree of crystal stress and strain present at, and in the vicinity of, high angle grain boundary (GBJ), ramp edge Co doped SNS (Co-SNS), or interface engineered junctions (IEJ) can lead to localized and highly non-uniform regions of basal plane oxygen loss in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO). These oxygen inhomogeneities will, to a greater or lesser degree, affect or cause the superconducting weak link behaviour demonstrated by these types of junctions. In order to examine the impact of the localized oxygen micro/nanostructure on the weak link behaviour of these three junction technologies, we have utilized ozone anneals to provide a partial pressure of atomic oxygen far in excess of that produced by standard O/sub 2/ anneals.

Resistive superconducting transition of ?-type BEDT-TTF organic superconductors in a magnetic field

The superconducting transition of the organic compounds [kappa]-(BEDT-TTF)[sub 2]X is studied by resistive measurement in a magnetic field up to 10 T applied normal to the conducting plane. For the salts with X = Cu[N(CN)[sub 2]]Br and X = CuCN[N(CN)[sub 2]] the transition shows fan-shaped broadening caused by superconductivity fluctuation. For the X = Cu(NCS)[sub 2] salt the resistivity shows a peak in the transition region in a magnetic field below 4 T. This phenomenon is suppressed in defect-reduced samples for intralayer conduction. We discuss this peak in relation to the thermal fluctuation on the Josephson junction structures in this salt. 16 refs., 3 figs.

Comparison of anodization spectroscopy with SIMS and RBS measurements for the characterization of Nb/AlAlO x/NB Josephson junction structures

The thin tunneling barrier in a Nb/AlAlO x /Nb was characterized by anodization spectroscopy, SIMS and RBS. From the anodization spectroscopy we can get the film thickness from the voltage span and, most importantly, also the sharpness of the Nb/AlO x and Al/Nb interfaces around the tunneling barrier. Comparison of the anodization profiles with SIMS depth profiles and RBS spectra shows that the anodization spectroscopy can be used to characterize the junction quality.

Secondary ion mass spectrometry study for Josephson junction with Nb/AlOx-Al/Nb structure

We studied the interface changes of niobium (Nb) Josephson junction structures (Nb/Al/Nb and Nb/AlOx-Al/Nb) using secondary ion mass spectrometry (SIMS). Using a cesium (Cs+) primary beam, CsM+ ions (M=H, O, OH, Nb, and Al) were monitored for cesium bombardment. In the SIMS depth profile, the CsH+ and CsO+ intensity indicated accumulation at junction interfaces, where these intensities were larger for the Al/Nb base electrode interface than for the Nb counterelectrode/Al interface. We think a layer of absorbed water vapor with a thickness a few nm, is formed on the junction interface, and is attributed to chamber outgassing. In the Nb/Al/Nb structure, the annealing changes for the Al/Nb base electrode interface were smaller than those for the Nb counterelectrode/Al interface. We confirmed that the absorbed water vapor layer plays an important role in the junction structure, that is, it acts as a barrier for grain boundary diffusion between Nb and Al.

Niobium-based integrated circuit technologies

Metallurgical and electrical properties of Nb and NbN films for use as Josephson junction electrodes and wiring layers are investigated. The crystallographic and superconducting properties necessary for Nb-based integrated circuit processes are clarified. Tunnel barrier structures of NbN-Nb oxide-NbN (Pb alloy) and Nb-Al oxide-Nb Josephson junctions have been analyzed and correlated with junction characteristics and critical current uniformity. It was found that the surface structure of a base electrode should be smooth to ensure that Josephson junctions have low leakage current and uniform critical current distribution. New types of Josephson junctions with artificial tunnel barriers such as amorphous Si or Mg oxide are reviewed. A variety of Josephson junction structures or processes have been developed for Nb-based Josephson integrated circuits in order to improve circuit performance. These include junction miniaturization, planarization, and stacked junction structures. These structures are mainly intended for Nb-Al oxide-Nb Josephson circuits. The Nb-Al oxide-Nb Josephson junction technology is by far the most advanced and has been used in logic and memory circuits, for example a 4-bit*4-bit parallel multiplier, a Josephson logic gate array, a 16-bit arithmetic logic unit, a 4-bit microprocessor, and 1-kb and 4-kb memory circuits.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Auger electron spectroscopy, transmission electron microscopy, and scanning electron microscopy studies of Nb/Al/Nb Josephson junction structures

Al films deposited on Nb can be oxidized and used to make Josephson junction devices. We studied the structure of Al films deposited under ‘‘warm’’ (estimated to be near 200 °C) and ‘‘cold’’ (near room temperature) conditions because the cold films produced better Josephson junction devices. For the warm case, the Al film was composed of islands with open channels between them, which we attribute to a high mobility of the Al atoms that lowers the island nucleation density. The Nb surface was extremely flat, which we ascribe to the high surface atom mobility at the higher deposition temperature. The cold Al film was of uniform thickness which can be explained by a high island nucleation density. The cold Nb films had an undulating surface, caused by the lack of surface atom mobility during deposition. There was no evidence of Al-Nb interdiffusion, even after postdeposition heating to 300 °C. Auger spectroscopy, transmission electron microscopy, and scanning electron microscopy were needed to obtain these definitive conclusions.