Sergey K. Tolpygo

Ferromagnetic Josephson switching device with high characteristic voltage

We develop a fast magnetic Josephson junction (MJJ)—a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient single flux quantum (SFQ) circuits. We present experimental results for superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs with high characteristic voltage IcRn of >700 μV proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ IcRn product is only ∼30% lower than that of conventional junction co-produced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.

Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers

We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma oxidation, before and after rapid thermal postannealing of the completed structures at temperatures up to 550 °C. Postannealing at temperatures above 300 °C results in a significant decrease of the tunneling conductance of thermally grown barriers, while plasma-grown barriers start to change only at annealing temperatures above 450 °C. Fitting the experimental I-V curves of the junctions using the results of the microscopic theory of direct tunneling shows that the annealing of thermally grown oxides at temperatures above 300 °C results in a substantial increase of their average tunnel barriers height, from ∼1.8 eV to ∼2.45 eV, versus the practically unchanged height of ∼2.0 eV for plasma-grown layers. This difference, together with high endurance of annealed barriers under electric stress (breakdown field above 10 MV/cm) may enable all-AlOx and SiO2/A...

High quality YBa2Cu3O7 Josephson junctions made by direct electron beam writing

High‐Tc Josephson junctions have been fabricated by direct electron beam writing over YBa2Cu3O7 thin‐film microbridges, using scanning transmission electron microscope (STEM) with an accelerating voltage of 80–120 kV. Annealing at 330–380 K increases Tc and Ic of the junctions and makes them more stable. In the operating range of a few degrees below Tc, the junctions show 100% magnetic field modulation of the critical current, microwave‐induced Shapiro steps oscillating according to the resistively shunted junction (RSJ) model, and RSJ current‐voltage characteristics with IcRn product up to 0.5–0.6 mV at 75 K and 0.3 mV at 77 K.

Plasma process-induced damage to Josephson tunnel junctions in superconducting integrated circuits

It has been found that the critical current of Josephson junctions in superconducting integrated circuits may depend on the environment surrounding the junctions and on how a particular junction is connected (wired) to other junctions and circuit elements. This may cause large, pattern-dependent deviations of the junctions’ critical currents from design values and ultimately limit the yield and performance of superconducting digital integrated circuits. In particular, we have found a difference in the critical current of grounded and floating junctions, and a dependence of the critical current on the size of metal structures connected to the junction—the ‘antenna’ effect. Experimental data were obtained for Nb/AlOx /Nb Josephson junctions fabricated on 150 mm wafers by an 11-layer process for superconducting integrated circuits. The results are explained by plasma process-induced damage to ultra-thin tunnel barriers. The most damaging plasma processing fabrication steps are discussed. (Some figures in this article are in colour only in the electronic version)

Tc enhancement by low energy electron irradiation and the influence of chain disorder on resistivity and Hall coefficient in YBa2Cu3O7 thin films

Abstract The effect of electron irradiation with incident energy ( E ≤ 40 keV) lower than the threshold for displacement of in-plane atoms on T c , resistivity and Hall coefficient has been studied on fully oxygenated YBa 2 Cu 3 O 7−δ thin. films. In contrast to the known T c suppression at high-energy particle irradiation, an increase in T c has been found as well as a factor of two increase in both the T -linear resistivity slope and the Hall coefficient as a function of radiation fluence. It has been shown that the changes observed in transport properties can be explained as being caused by disruption of the conductivity of CuO chains by radiation-induced chain oxygen defects.

Tunable transformer for qubits based on flux states

Flux states of an RF SQUID are promising candidates for the implementation of quantum bits (qubits) for quantum computing. Accurate measurements of the flux states, especially in the time domain, require a controllable coupling between a qubit and a readout circuit (e.g., a dc SQUID magnetometer). Since the readout circuit can also be a source of decoherence, such a controllable coupling also allows one to control (minimize) the back action of the magnetometer. For this purpose we suggest a balanced, tunable transformer. This has a gradiometer configuration such that each arm is broken by a small loop containing two Josephson junctions. As a result the inductance of each arm can be adjusted by an applied bias flux. In the symmetrical case there is no coupling between the qubit and the magnetometer while a desired coupling can be achieved by changing the asymmetry of the arms. Similar transformers may be be useful for achieving controllable, lossless coupling that is required between qubits. Theoretical analysis of the noise back action of the transformer as well as preliminary experimental results are presented.

Deep sub-micron stud-via technology of superconductor VLSI circuits

A fabrication process has been developed for fully planarized Nb-based superconducting interlayer connections (vias) with minimum size down to 250 nm for superconductor very large scale integrated (VLSI) circuits with 8 and 10 superconducting layers on 200-mm wafers. Instead of etched contact holes in the interlayer dielectric it employs etched and planarized Nb pillars (studs) as connectors between adjacent wiring layers. Detailed results are presented for one version of the process that utilizes Nb/Al/Nb trilayers for each wiring layer instead of single Nb wiring layers. Nb studs are etched in the top layer of the trilayer to provide vertical connections between the wires etched in the bottom layer of the trilayer and the next wiring layer that is also deposited as a Nb/Al/Nb trilayer. This technology makes possible a dramatic increase in the density of superconducting digital circuits by reducing the area of interconnects with respect to presently utilized etched contact holes between superconducting layers and by enabling the use of stacked vias. Results on the fabrication and size dependence of electric properties of Nb studs with dimensions near the resolution limit of 248-nm photolithography are presented in the normal and superconducting states. Superconducting critical current density in the fabricated stud-vias is about 0.3 A μm−2 and approaches the depairing current density of Nb films.

Electrical stress effect on Josephson tunneling through ultrathin AlOx barrier in Nb/Al/AlOx/Nb junctions

The effect of dc electrical stress and breakdown on Josephson and quasiparticle tunneling in Nb/Al/AlOx/Nb junctions with ultrathin AlOx barriers typical for applications in superconductor digital electronics has been investigated. The junctions’ conductance at room temperature and current-voltage (I-V) characteristics at 4.2 K have been measured after the consecutive stressing of the tunnel barrier at room temperature. Electrical stress was applied using current ramps with increasing amplitude ranging from 0 to ∼1000Ic corresponding to voltages across the barrier up to ∼0.65 V, where Ic is the Josephson critical current. A very soft breakdown has been observed with polarity-dependent breakdown current (voltage). As the stressing progresses, a dramatic increase in subgap conductance of the junctions, the appearance of subharmonic current steps, and a gradual increase in both the critical and the excess currents as well as a decrease in the normal-state resistance have been observed. The observed changes i...

Tunneling properties of barriers in Nb/Al/AlO/sub x//Nb junctions

We have measured DC I-V curves of niobium-trilayer (Nb/Al/AlO/sub x//Nb) junctions with barriers thermally grown within a broad range of oxygen exposure E=Pt, from 2/spl times/10/sup 5/ to 2/spl times/10/sup 9/ Pa-s, and for applied electric fields ranging from zero all the way up to the breakdown - typically, above 10 MV/cm. The data can be reasonably well fitted by the direct theory assuming trapezoidal barrier profile and using the numerical solution of the Schrodinger equation. (The traditional WKB approximation gives considerable errors for barriers so thin and sharp.) The fitting has shown that with the increase of oxygen exposure, the effective oxide thickness d/sub ef//spl equiv/(m/m/sub 0/)/sup /spl alpha//d where m is the effective mass of the tunneling electron and (/spl alpha//spl ap/0.51) grows from 0.83 to 1.08 nm, while the average barrier height grows from 1.7 to 1.9 eV, and the zero-voltage conductance G/sub 0/ continues to drop as E/sup -1/2/ through all the studied exposure range.

Critical currents and Josephson penetration depth in planar thin‐film high‐Tc Josephson junctions

The temperature dependence of the critical current in planar high‐Tc Josephson junctions fabricated in YBa2Cu3O7 thin films by focused electron irradiation has been studied. It is shown that in the range of critical current densities spanning more than five orders of magnitude and temperature range 0.1≤T/Tc≤1, the critical current density jc varies as (1−T/Tc)2. The T dependence of the critical current, however, is affected by the transition from the narrow junction to the wide junction limit as jc increases. An expression for the Josephson penetration depth in thin‐film coplanar structures is derived, and magnetic field penetration depth in junction banks is extracted from the Ic(T) dependences.