Fabricated Josephson Junctions Research Articles

Characterization and reduction of capacitive loss induced by sub-micron Josephson junction fabrication in superconducting qubits

Josephson junctions form the essential non-linearity for almost all superconducting qubits. The junction is formed when two superconducting electrodes come within ∼1 nm of each other. Although the capacitance of these electrodes is a small fraction of the total qubit capacitance, the nearby electric fields are more concentrated in dielectric surfaces and can contribute substantially to the total dissipation. We have developed a technique to experimentally investigate the effect of these electrodes on the quality of superconducting devices. We use λ/4 coplanar waveguide resonators to emulate lumped qubit capacitors. We add a variable number of these electrodes to the capacitive end of these resonators and measure how the additional loss scales with the number of electrodes. We then reduce this loss with fabrication techniques that limit the amount of lossy dielectrics. We then use these techniques for the fabrication of Xmon qubits on a silicon substrate to improve their energy relaxation times by a factor of 5.

Atomically Thin Al2O3 Films for Tunnel Junctions

Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.

Development of Nb/Al–AlOx/Nb Tunnel Junction for Large-Scale Integrated Digital Circuits

We present the latest development of an Nb/Al-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb Josephson junction fabrication process for superconducting integrated circuits at SIMIT. Our fabrication process has four niobium superconducting layers and one Ti/Pd resistor layer with 1.0 Ω/square sheet resistor for junction shunting and circuit biasing. Critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the junctions is controlled from 30 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to 10 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> by exposing aluminum layer to pure or mixed O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> atmosphere. Correlation between measured J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and exposure conditions has been established. Average quality factor (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sg</sub> /R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> ) of the junctions in our process is larger than 20 with subgap leakage factor (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) beyond 30 mV. Critical current uniformity is obtained according to the switching current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) distribution of a 1000-JJ array. Measured standard deviation sigma of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> is less than 4% over the 100-mm wafer. Plasma resonance in a large Josephson junction is used to measure specific capacitance (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) of the junction. Sheet inductances of superconducting niobium strip-lines are extracted by a SQUID diagnose circuit and the measured results show good agreement with our design values.

YBa2Cu3O7-x bicrystal Josephson junctions with high IcRn-products and wide-ranging resistances for THz applications

Josephson junctions with high characteristic voltages $I_{c}R_{n}$ and resistances $R_{n}$ ranging from subohm values to several hundred ohms are required for various terahertz (THz) applications. Due to optimization of technology, [100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions with an average $I_{c}R_{n}$ (5 K) value of 7 mV at an $R_{n}$ -range from 0.4 to 400 $\Omega$ have been fabricated on (320) bicrystal NdGaO3 substrates. Classical and frequency-selective detection of THz radiation have been demonstrated with the fabricated Josephson junctions. The main characteristics of Josephson oscillators, spectrum analyzers, admittance spectrometers, and classical detectors have been evaluated within this range of junction parameters, and promising results have been obtained.

Fabrication of sandwich-type MgB2/Boron/MgB2 Josephson junctions with rapid annealing method

Sandwich-type MgB2/Boron/MgB2 Josephson junctions were fabricated using magnetron sputtering system. The rapid-anneal process was adopted to replace traditional way of annealing, trying to solve the problem of interdiffusion and oxidation with multilayer films. The boron film was used as barrier layer to avoid the introduction of impurities and improve reproducibility of the junctions. The bottom MgB2 thin films deposited on c-plane sapphire substrate exhibits a critical temperature TC of 37.5 K and critical current density JC at 5 K of 8.7 × 106 A cm−2. From the XRD pattern, the bottom MgB2 thin film shows c-axis orientation, whereas the top MgB2 became polycrystalline as Boron barrier layer grown thicker. Therefore, all junction samples show lower TC than single MgB2 thin film. The junctions exhibit excellent quasiparticle characteristics with ideal dependence on temperature and Boron barrier thickness. Subharmonic gap structure was appeared in conductance characteristics, which was attributed to the multiple Andreev reflections (MAR). The result demonstrates great promise of this new fabrication technology for MgB2 Josephson junction fabrication.

Niobium nano-SQUIDs based on sub-micron tunnel junction fabricated by three-dimensional Focused Ion Beam sculpting

A three dimensional nano-SQUID (Superconducting Quantum Interference Device) has been realized in a vertical configuration (with the loop in the same plane of Josephson Tunneling Junctions, JTJs). The loop area is 0.25 μm2 corresponding to a modulation period of about 5 mT, the square JTJs have a side length of 0.3 μm. Josephson junction's fabrication is carried out combining optical lithography to pattern trilayer and three dimensional (3D) Focused Ion Beam (FIB) sculpting technique to define the junctions' and the loop's areas. Two different ion etching processes were performed, perpendicular and parallel to the multilayer, resulting in a precise 3D structure. Finally, a standard anodization was performed to eliminate the unstructured surface material generated by the high energetic ion beam assuring high quality junctions. Electric transport characteristics of the nanodevice measured at T = 4.2 K are reported, in particular the current-voltage characteristics and critical current vs external magnetic field. The high modulation depth of the critical current (up to 70% of the Ic at zero magnetic flux) and the device reliability are very encouraging in view of nanoscience applications.

Josephson Junctions Made of YBa2Cu3O7-x Superconducting Nanofilms

Abstract Conventional Josephson junctions are used to build superconducting quantum interference devices (SQUIDs) which are the most sensitive magnetic field sensors known. Today, SQUIDS are widely used in many different areas like medical and biological research and diagnostics, geophysics, non-destructive evaluation of materials and structures, or fundamental physics research. In recent years the highly anisotropic and unique intrinsic properties of YBa2Cu3O7- x superconducting nano-films allowed fabrication of a special type of Josephson junctions, so-called π-junctions, which are excellent candidates as building block for fast, low-dissipative superconducting digital electronics or quantum computation. For those reasons fabricating high quality and reliably Josephson junctions has been one of the priority areas of research in superconductivity. There are several well-established technologies to fabricate Josephson junctions made of YBa2Cu3O7-x superconducting nano-films. Here I will briefly describe two of the most important ones used to fabricate both conventional Josephson junctions and π-junctions.

SNS Junctions for AC Josephson Voltage Standards

Abstract We developed and fabricated series arrays of overdamped SNS Josephson junctions for AC Josephson voltage standards (S: superconductor, N: normal conductor). The junctions are based on NbxSi1-x barriers allowing nearly independent tuning of critical current density and characteristic voltage by varying niobium content and thickness of the barrier. We optimised the junction barrier deposition for the co-sputtered NbxSi1-x so that we are able to fabricate Josephson junction series arrays with up to 69,632 junctions for output voltages up to 10 V under 70 GHz microwave irradiation. The width of the constant-voltage steps at 10 V exceeds 1 mA for a typical critical current of about 3 mA.

Measurement of dielectric losses in amorphous thin films at gigahertz frequencies usingsuperconducting resonators

We have developed a reliable method for the direct measurement of dielectriclosses in thin films using a special design of lumped element superconductingresonators. The method presented allows obtaining quantitative values for the losses inthe film volume as well as the metal/dielectric interfaces. Numerous differentresonator geometries were designed and measured in order to study the lossesin dielectric thin films usually used for Josephson junction fabrication, such asNb2O5, SiOand SiNx, at low temperatures and low gigahertz frequencies. The results show that for suchamorphous materials, the bulk losses clearly exceed the interface losses. Furthermore, thefrequency dependence of the losses in this working regime was studied for the first time.Our results are in good agreement with the universal law describing the frequencydependence of dielectric losses and suggest that the losses are due to many-bodyinteracting dipoles, which is an important fact for the theoretical modelling of two-levelfluctuators. We also investigated the losses in multi-layer films and find that the results arein good agreement with the theoretical expectations, which allows optimizationof the dielectric multi-layers used in Josephson junction and qubit fabrication.

Temperature-dependent current-voltage characteristics of niobium SNIS Josephson junctions

Motivated by a search for a suitable technology to fabricate Josephson junctions with a tunable damping regime, we performed a systematic study of the temperature effect on the critical current in Nb/Al-AlOx-Nb heterostructures with a nanometer-thick Al interlayer. For Al layer thicknesses ranging from 40 to 110 nm, we have observed a transition from hysteretic (below 4.2 K) to non-hysteretic (above 4.2 K) current-voltage curves. Measured supercurrent-vs-temperature characteristics which significantly differ from those of traditional SIS and SNS devices are interpreted in terms of the superconducting proximity effect between Al and Nb films. Thermal stability and good reproducibility of our junctions are demonstrated.

Experimental Studies to Realize Josephson Junctions and Qubits in Cuprate and Fe-based Superconductors

Cuprate superconductors are very promising in terms of Josephson junction device because of the large energy scale of high-T c superconductivity. In particular, fabrication of qubits attracts lots of attention because of its primary importance for future computer technology. We will present some of our recent activities pointing to this purpose. (1) We succeeded in fabricating Josephson junction of cuprate without making any extra oxide barrier layer, rather putting Fe islands on the small strip of cuprate superconductors. This is very promising, since the fabrication of good Josephson junction was unsuccessful so far. Together with the introduction of the data, we discuss the possible mechanism of the weak-link fabrication in this structure. (2) We investigated the switching events in the I-V characteristics of the intrinsic Josephson junctions of Bi-cuprate superconductor, where macroscopic quantum tunneling (MQT) observation is well established recently. In addition to confirm the MQT for the first switching at 1 K in the multiple-branched current-voltage characteristics, we found that the temperature independence of the switching distribution for the second switching up to higher temperatures (10 K) is not due to the trivial Joule heating. We discuss the mechanism of the phenomena, including the possibility of MQT. (3) New Fe-based superconductors are also promising in terms of the application of superconductivity, since the anisotropy looks rather weak, in contrast to cuprates. We will introduce our trial to fabricate epitaxial thin films as the initial step to fabricate Josephson junction of this material.

Superconducting Yttrium Barium Copper Oxide Thin Films Growing on the Strained Substrates

The article is devoted to formation of superconducting thin films on the single-crystal substrate where areas with different values of the critical current density are, that is needed for fabrication of superconducting devices. The method is based on an establishment of elastic mechanical stresses on the substrate crystal under the nanosecond focused pulsed laser irradiation. On the irradiated substrate the superconducting thin film having auxiliary elastic stresses not till the area is over the irradiated section of the substrate is grown. At the same time the critical film current density is suppressed for required values are used to fabricate Josephson junctions. Observations for a long time demonstrate superconducting transport film properties are not varied significantly during the maintenance.

Fabrication of submicron YBCO Josephson junctions by a sample mosaic navigationassisted laser etching process

Taking advantage of a New Wave 213 nm laser system, equipped withhigh-precision submicron resolution sample mosaic navigation, we are ableto laser-etch and fabricate planar submicron-size Josephson junctions onYBa2Cu3O7−x thin films successfully. The entire process of laser etching and sample navigation is softwarecontrolled. The widths were measured by atomic force microscopy (AFM), and they range from 0.7 to1.4 µm. We report onthe observed I–V characteristics and Shapiro steps, which confirm the Josephson effect in these junctions.The measured critical current dependence on temperature shows a linear relationship formicron-size constrictions. In the case of submicron constrictions, the dependence is anexponential decay type, consistent with diffusive long S–N–S junction behaviour. Itis believed that the observed behaviour can be ascribed to laser heating of theconstriction material, changing the superconducting phase to the normal one.

Mechanism of operation of Josephson junctions made from HT c materials by ion modification

In this paper, existing attempts to fabricate Josephson junctions from HT c materials by ion modification published in the literature are summarized. A computer simulation with these experimental data for every case was made calculating the critical temperature depth profiles of the modified material. It is shown that the experimental results are consistent with our calculation and the Josephson behavior is observed in the expected temperature range.

Fabrication and Optical Properties of Ultrathin Ferromagnet/Superconductor Metallic Bilayers

Ferromagnet/superconductor (F/S) bilayers composed of ultrathin metallic films demonstrate new proximity effects and show a peculiar behavior of the superconductor-order parameter. In terms of applications, F films provide additional anchoring of the S film vortex lattice, resulting in increased critical current. By changing composition and the thickness ratio of the F/S bilayer, one can modify the quasiparticle dynamics through an effective enhancement or suppression of the quasiparticle lifetime under nonequilibrium conditions. The latter effect should lead to custom-designed superconducting detectors, with the sensitivity and photoresponse speed tailored to a given application. We present fabrication details of nanometer-thickness NiCu/Nb bilayers of different F/S ratios. The NiCu/Nb structures have been implemented to fabricate Josephson junctions and tested using femtosecond optical spectroscopy. The time-resolved photoresponse experiments allowed us to measure the quasiparticle relaxation dynamics in our bilayers. In addition, the photoresponse temperature dependence studies revealed the spatial evolution of the superconductor-order parameter across the bilayer. Comparison between the bilayer data and the single-material data is also presented.

Pulsed laser deposition of LaNiO 3 and YBa 2Cu 3O 7− δ/LaNiO 3 on SrTiO 3 buffered (100) MgO

Multilayer structure YBa 2Cu 3O 7− δ (YBCO)/LaNiO 3 (LNO)/SrTiO 3 (STO)/MgO has been carefully studied in the hope that it can be used for applications such as quasiparticle injection or Josephson junction fabrication. Deposited by pulsed laser, LNO on STO-buffered (100) MgO has low resistivity, good metallic characteristics, and very smooth surface (with a mean-square-root roughness of less than 3 nm). After the deposition of LNO, YBCO film can be grown in situ on it, showing good superconducting properties with a critical temperature above 81 K and a narrow transition width below 1.5 K. X-ray diffraction analyses have indicated that all the deposited films are highly c-axis-oriented.

A rapid single flux quantum 1 bit arithmetic logic unit constructed with a half-addercell

We have designed, fabricated, and tested a rapid single flux quantum (RSFQ) 1bit arithmetic logic unit (ALU) block. The circuit consists of three DC currentdriven SFQ switches and a half-adder. We successfully tested the half-addercell at clock frequency up to 20 GHz. The switches were commutating outputports of the half-adder to produce AND, OR, XOR, or ADD functions. For ahigh-speed test, we attached two switches at the input ports of the half-adderto control the high-speed input data by low-frequency pattern generators. Theoutput in this measurement was an eye-diagram. Using this set-up, the circuit wassuccessfully tested up to 20 GHz. The chip was fabricated using a standard HYPRES1 kA cm−2 Nb Josephson junction fabrication process.

Josephson junctions fabricated by focused ion beam from ex situ grown MgB 2 thin films

We prepared MgB 2 thin films on SrTiO 3 (1 0 0) and Al 2O 3 (1 1̄ 0 2) substrates by e-beam evaporation of MgB 2 pellet. The films were deposited at room temperature and post-annealed at 900 °C in Mg vapour for 5–30 min. Superconducting transition temperatures were observed between 22 and 30 K. Structure and surface morphology of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The films grown on Al 2O 3 substrates are c-axis oriented while a film grown on SrTiO 3 substrate is aligned with the (1 0 1) direction normal to the substrate planes. The films have grain sizes of about 70 nm. The films were patterned into 4 and 8 μm wide microbridges. The microbridges were observed to carry large critical current densities of approximately 1 MA/cm 2 at 6.7 K. Focused ion beam (FIB) was used on the bridges in order to fabricate Josephson junctions. A cut 50 nm in width was made across the microbridges followed by an in situ platinum (Pt) deposition into the cut made. SNS-like weak-link junctions were formed in the process.

Estimation of surface resistance for epitaxial NbN films in the frequency range of 0.1-1.1 THz

To improve the performance of superconducting analog devices at frequencies exceeding the Nb gap frequency, we developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers. To evaluate the RF performance of the trilayers, we fabricated Josephson junctions with an epitaxial NbN/MgO/NbN microstrip resonator. A Josephson junction was placed in the center of the resonator and it was used as both an oscillator and a detector. The width and length of the microstrip resonator were 10 and 1000 /spl mu/m, respectively. Current steps of up to 1.9 mV resulting from coupling with the resonator were observed in the I-V characteristics. We found that the height of the current steps depended on the loss of the resonator. By assuming that the sum of the dielectric loss and the radiation loss is much smaller than the conductor loss, we estimated the surface resistance of the epitaxial NbN thin films. The surface resistance of the epitaxial NbN films was estimated to be approximately 2.6-88 m/spl Omega/ at 0.1-1.1 THz.

Novel in-situ fabricated Josephson junctions: Trilayer on a substrate slope

We demonstrate a high-temperature superconductor (HTS) Josephson junction geometry using only in situ interfaces and with the current flowing in the a-b plane of the HTS. The trilayer on a substrate slope (TOSS) junction is a HTS-barrier-HTS structure deposited in situ on top of a pre-etched slope in the substrate. We present initial results on the fabrication and testing of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// TOSS junctions with a Ga-doped PrBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// barrier. These devices display resistively shunted junction like I-V characteristics with characteristic voltages up to 5 mV at 4.2 K. The TOSS junction concept is of interest for fundamental studies of interfaces in HTS and can also be applied to an integrated circuit technology.