Ramp-edge Josephson Junctions Research Articles

Properties of ramp-edge Josephson junctions with a surface-modified barrier fabricated by high-rate PLD

YBa 2Cu 3O 7− δ (YBCO) ramp-edge Josephson junctions have been fabricated by pulsed laser deposition (PLD) with a relatively high deposition rate (∼50 nm/min). 200 nm thick YBCO films deposited on LaSrAlTaO x (LSAT) substrates in a PLD system with laser-beam scanning exhibited a typical T c of 88∼90.5 K. LSAT/YBCO bilayers were fabricated and then patterned to form a ramp structure. Electron cyclotron resonance ion bombardment and subsequent annealing in a various atmosphere were employed to form an amorphous layer and a recrystallized barrier layer on the ramp surface. Junction properties strongly depend on distance between the sample and the laser plume when upper layer YBCO is deposited. The junctions fabricated under optimum conditions exhibited resistively and capacitively shunted junction-like I– V curves with a typical I c R n product at 4.2 K of 1.0∼3.2 mV.

Angle-dependent Josephson current in high- Tc YBa 2Cu 3O 7− δ ramp-edge junctions

We report measurements on YBa 2Cu 3O 7− δ (YBCO)/PrBa 2Cu 3O 7− δ /YBCO asymmetric ramp-edge Josephson junctions whose ab-plane orientation relatively rotated by 45°. The temperature dependence of the maximum Josephson current I c is in good agreement with the d-wave theory taking the zero energy state into account. We discuss the crystal angle dependence of d-wave Josephson current on microwave irradiation properties and the comparison between several junctions with different crystal angles normal to the interface boundary.

Fabrication and characterization of Y-Ba-Cu-O and Nd-Ba-Cu-O ramp-edge junctions with an interface-modified barrier

The fabrication process for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) ramp-edge junctions with an interface-modified barrier has been studied. The conditions of electron cyclotron resonance (ECR) ion etching and following annealing treatment were optimized by evaluating the mean roughness of the ramp surface using atomic force microscope (AFM) observation. We could obtain resistively-shunted junction (RSJ) type current-voltage characteristics for the junctions with the counterelectrode YBCO layer deposited at temperatures lower than 720/spl deg/C, while a deposition temperature higher than 755/spl deg/C resulted in a high-J/sub c/ superconducting contact. The junctions exhibited an I/sub c/R/sub n/ product of 1.0-1.9 mV and magnetic field modulation of I/sub c/ more than 90% at 4.2 K. By applying the optimum etching condition to a NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (NBCO) base electrode and employing a slightly higher annealing and deposition temperature, YBCO/NBCO ramp-edge Josephson junctions with a similar I/sub c/R/sub n/ product were successfully obtained.

Fabrication of RS flip-flops using Y-Ba-Cu-O ramp-edge junctions and their operation

We fabricated single flux quantum RS flip-flop circuits by using ramp-edge Josephson junctions and tested their performance. The junctions were constructed with two Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) electrodes and a barrier layer. As barrier materials we used cobalt-doped YBCO. To insulate between YBCO layers we used Sr/sub 2/AlTaO/sub 6/ (SAT) thin films. We also fabricated RS flip-flop circuits by using YBCO ground planes to reduce the inductance of YBCO film. The fabricated Josephson junctions showed typical RSJ-like current-voltage (I-V) characteristics above 50 K. We could successfully demonstrate the correct operation of the RS flip-flop circuits at the temperatures near 50 K. We used a computer controlled digital measurement set-up to test the circuits. The RS flip-flop circuit fabricated on a ground plane showed more definite set and reset states on the voltage-flux (V-/spl phi/) modulation curves of the read SQUID than those of the circuit made without ground plane. This may be attributed to the shielding effect by a YBCO ground plane.

Investigation of ramp-type Josephson junctions with surface-modified barriers

We have investigated the properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ ramp-edge Josephson junctions with surface-modified barriers produced by Ar-ion irradiation followed by oxygen annealing. The fabricated junctions displayed RSJ-like I-V characteristics and excellent uniformity. The stray capacitance of the junctions was estimated from the ramp-edge structure. The junction capacitance was obtained by subtracting the stray capacitance from the shunting capacitance. We estimated the barrier thickness from the junction capacitance, and found that the critical current density of the junction increased exponentially with decreasing barrier thickness. The relative dielectric constant of the barriers ranged from 13 to 18.

Fabrication of interface-controlled Josephson junctions using Sr/sub 2/AlTaO/sub 6/ insulating layers

We fabricated ramp-edge Josephson junctions with barriers formed by interface treatments instead of epitaxially grown barrier layers. A low-dielectric Sr/sub 3/AlTaO/sub 6/ (SAT) layer was used as an ion-milling mask as well as an insulating layer for the ramp-edge junctions. An ion-milled YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO)-edge surface was not exposed to solvent through all fabrication procedures. The barriers were produced by structural modification at the edge of the YBCO base electrode using high energy ion-beam treatment prior to deposition of the YBCO counter electrode. We investigated the effects of high energy ion-beam treatment, annealing, and counter electrode deposition temperature on the characteristics of the interface-controlled Josephson junctions. The junction parameters such as T/sub c/, I/sub c/, R/sub N/ were measured and discussed in relation to the barrier layer depending on the process parameters.

Effect of columnar defects in YBa2Cu3O7−x ramp-edge Josephson junctions

We investigated the transport properties in YBa2Cu3O7−x (YBCO) ramp-edge Josephson junctions before and after 1.3 GeV Pb-ion irradiation. The barrier of the junctions was formed by structural modification of the ramp-edge surface of the YBCO base electrode. After a dose of ∼2×1010 Pb ions/cm2, both critical current and normal resistance of the junctions increased by ∼30% and 10%, respectively, except near the transition temperature. We interpret such an enhancement of the critical current as a result of impeding Josephson vortex motion by pinned magnetic flux in the electrodes.

Fabrication of Sr 2AlTaO 6/YBCO interface-controlled junctions

We fabricated ramp-edge Josephson junctions with barriers formed by interface treatments instead of epitaxially grown barrier layers. A low-dielectric Sr 2AlTaO 6(SAT) layer was used as an ion-milling mask as well as an insulating layer for the ramp-edge junctions. An ion-milled YBA 2Cu 3O 7−x (YBCO)-edge surface was not exposed to solvent through all fabrication procedures. The barriers were produced by structural modification at the edge of the YBCO base electrode using high energy ion-beam treatment, prior to deposition of the YBCO counterelectrode. We investigated the effects of high energy ion-beam treatment, annealing, and counterelectrode deposition temperature on the characteristics of the interface-controlled Josephson junctions. The spreads in the critical current (I c) as well as the junction parameters such as T c, I c, R N were measured and discussed in relation to the interface states.

Current-phase relationship of YBa2Cu3O7−x ramp-edge Josephson junctions

The current-phase relation (CPR) of high-Tc ramp-edge Josephson junctions (YBa2Cu3O7−x/PrBa2Cu3O7/YBa2Cu3O7−x) was investigated experimentally in the temperature range 65<T<85 K. The investigated junctions were incorporated into a washer-shaped superconducting loop with inductance L≈80 pH forming a single-junction interferometer. The CPR was obtained from the measurement of the impedance of the phase-biased junction. We show that, for junctions with sufficiently small critical current Ic, the CPR is sinusoidal. In the case of larger critical current, deviations of the apparent CPR from sinusoidal dependence were found. These deviations can be described by thermal fluctuations of the magnetic flux in the loop. So we conclude that, for all investigated junctions, the “true” CPR is sinusoidal.

Microwave and DC-transport in YBa 2Cu 3O 7 ramp–edge Josephson junction arrays

The chip-to-chip and the on-chip spread of the transport properties like the critical current and the normal state resistance of Josephson junctions from high-temperature superconductors (HTS) are very important for many applications. To analyse the on-chip spread we prepare and investigate Josephson junction arrays with up to 30 Josephson junctions in series, where each Josephson junction can be measured individually. The preparation process consists of laser deposition, conventional photolithography and argon ion etching in a parallel-plate reactor. We use Josephson junctions in ramp–edge geometry on 1×1 cm 2 LaAlO 3-substrates. As barrier material we use PrBa 2Cu 3O 7. We measure current–voltage ( I– V) curves and the differential resistance. This way we determine the temperature-dependent spread of the electronic properties in the ramp–edge Josephson junctions. We analyse different contributions to the spread. Measurements under influence of RF-radiation illustrate the consequences of the observed spread in the critical current.

Fabrication of natural-barrier ramp-edge Josephson junctions

We have studied the effect of process parameters on the natural-barrier ramp-edge Josephson junctions using YBa2Cu3Ox electrodes, in order to improve uniformity and reproducibility of junctions. The natural barrier is formed during an etching process and an annealing process. The junction properties are controlled by an annealing temperature and an annealing pressure. Also the junction characteristics depend on the ramp-edge angle. After ramp structuring, a bulge of the underelectrode is observed by AFM. The IcRn products of the junctions with the bulge of the ramp surface reach 2 mV at 4.2 K. We speculate that the etching condition is a very important parameter in the junction fabrication.

YBaCuO/Co-doped PrBaCuO/YBaCuO ramp-edge junctions and their application to flip–flop circuits

We fabricated YBa 2 Cu 3 O 7− x /PrBa 2 Cu 2.8 Co 0.2 O 7− y /YBa 2 Cu 3 O 7− x ramp-edge Josephson junctions and measured their current–voltage ( I – V ) characteristics. Critical current ( I c ) and conductance ( G ) were evaluated as a function of the barrier layer thickness, and both of them exhibited a nearly exponential dependence. The decay parameters for I c and G were estimated to be 1.1 and 1.5 nm, respectively. We also examined superconducting quantum interference devices (SQUIDs) using this type of junction as a preliminary test for realizing oxide superconductive integrated circuits with high-speed operation. DC SQUIDs with direct-coupling control lines and no ground plane were fabricated and their loop inductance was evaluated. The obtained SQUID inductance ranged from 8 to 38 pH for a 4- μ m-wide, 3- to 12- μ m-long hole in a rectangular SQUID loop. In addition, a flip–flop circuit based on such a dc SQUID was fabricated. Both flux trapping and flux detrapping in the SQUID loop hole were confirmed when 1-ms-wide current pulses were injected into the SQUID loop.

Electromigration study of SNS ramp edge Josephson junctions

We report on the effects of electromigration of basal plane oxygen vacancies on SNS ramp edge Josephson junctions where the N-layer is YBa 2 Cu 2.79 Co 0.21 O 7− δ , a doped version of the YBCO electrodes. Through the application of a 4–10 mA (∼2–5 MA/cm 2 ) current bias at room temperature, the basal plane oxygen order and content in the N and S layers were improved. This is demonstrated by an increase in I c R n from <5 μ V, to as much as 205 μ V. The implications of these results on SNS junction fabrication, and the nature of tunneling in such devices are discussed.

Design and fabrication of a voltage divider utilizing high-T/sub c/ ramp-edge Josephson junctions with a ground plane

Circuit operation of a voltage divider utilizing YBa/sub 2/Cu/sub 3/O/sub x//Co-doped PrBa/sub 2/Cu/sub 3/O/sub y//YBa/sub 2/Cu/sub 3/O/sub x/ ramp-edge Josephson junctions with a ground plane was demonstrated for the voltage region of 0-400 /spl mu/V at 12.5 K, where the maximum voltage corresponds to /spl sim/200 GHz internal Josephson oscillation. This voltage was around the critical current-normal resistance (I/sub c/R/sub n/) product of 420 /spl mu/V. In the design of the voltage divider, a transmission line model was adopted for an inductance loop because of the large dielectric constant of the insulation layer interposed between the ground plane and the wiring serving as a loop inductance. In the fabrication of the circuit, a SrTiO/sub 3//Co-doped PrBa/sub 2/Cu/sub 3/O/sub y//SrTiO/sub 3/ multilayer insulation and four-directional ramp-edge junction process using a photoresist reflow technique were adopted.

Very large YBa/sub 2/Cu/sub 3/O/sub 7/-Josephson-junction-arrays

Arrays with YBa/sub 2/Cu/sub 3/O/sub 7/-Josephson junction ramp-edge geometry and on bicrystal substrates were investigated. The ramp-edge Josephson junctions were prepared in arrays of three, five and seven junctions in series and in a mixed array with a very large number of up to two million on one chip. First measurements are presented. On bicrystal substrates Josephson junctions were prepared up to 105 Josephson junctions in series. They show scaled current-voltage characteristics and Shapiro-steps under influence of microwave radiation.

Technology for YBa/sub 2/Cu/sub 3/O/sub 7/ SNS- and SIS-Josephson junctions

Ramp-edge Josephson junctions from the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7/ require a multilayer preparation process. In order to increase the reproducability of the junction characteristics we investigate the subprocesses using statistical methods for the design of experiments. The optimization of the process parameters enables us to prepare ramp-edge Josephson junctions with PrBa/sub 2/Cu/sub 3/O/sub 7/ as barrier material which exhibit tighter barriers. Adapting the growth conditions for YBa/sub 2/Cu/sub 3/O/sub 7/ on MgO films we prepared Josephson junctions with a 10 nm MgO barrier.

Preparation of ramp-edge Josephson junctions with natural barriers

We have demonstrated ramp-edge Josephson junctions using high temperature superconductors without depositing artificial barriers. We use a surface barrier formed naturally during an etching process by an Ar ion beam. The resistivity of the barrier, which is evaluated in the Nb/Au/YBCO structures, changes over 4 orders in accordance with the gas pressure in the vacuum annealing executed before the deposition of the counter-electrode. All the junctions having YBCO/barrier/YBCO structure exhibit RSJ-like current-voltage characteristics over the entire temperature range of operation. Fraunhofer-like modulation patterns are observed with a very small amount of excess current even at low temperatures except for large junctions. The temperature dependence of J/sub c/ and R/sub u/A are similar to those in HTS grain boundary junctions. Since the junction parameters are controlled by the total pressure during the vacuum annealing, the junctions made through this procedure have a potential for circuit applications.

Effect of ozone anneals on YBa/sub 2/Cu/sub 3-x/Co/sub x/O/sub 2/ thin films

We report on the effect of 200-500C ozone anneals on the resistivity versus temperature (RT) and Raman spectra characteristics of YBa/sub 2/Cu/sub 3-x/Co/sub x/O/sub 2/ (Co-YBCO) thin films, with x=0.3, 0.5, 0.75, and 1.0. Cold wall anneals are conducted at /spl sim/1 aim. of a flowing O/sub 2//O/sub 3/ mixture with /spl sim/2% O/sub 3/ by weight. The enhanced partial pressure of atomic oxygen, relative to canonical O/sub 2/ anneals, provided by ozone to the surface of Co-YBCO films, leads to enhanced oxygenation. This is demonstrated by the observation of an increase in T/sub c/ (25 K to 75 K for x=0.3, 0 to 38 K for x=0.5, and 0 to a T/sub c/(onset) of 19 K for x=0.75) and improvement in Raman spectra data following ozone anneals. This high degree of oxygenation is unstable under O/sub 2/ anneals at temperatures as low as 200C. We attribute this to the fact that the partial pressure of oxygen above well oxygenated Co-YBCO is greater than 1 atm. at /spl sim/200C. We will discuss the implication of these results for the stability and uniformity of SNS ramp edge Josephson junctions which employ Co-YBCO N-layers.

The effect of microstructure on the electrical properties of YBCO interface-engineered Josephson junctions

The microstructure of interface engineered ramp-edge Josephson junctions in YBCO has been studied by transmission electron microscopy (TEM). The junctions are distinctly different from the structures found in heterophase SNS junctions. This structural difference can be directly related to the improved electrical properties of interface engineered junctions. The plasma treatment used in the interface engineering method forms a continuous barrier layer of 2 to 3 nm in thickness, much thinner than the deposited heterophase layers in SNS junctions. The layer, which separates the two YBCO layers, is crystalline, but has a crystal structure different from YBCO. The main factors responsible for the improved junction properties and their good reproducibility are: small barrier thickness, small thickness variation, weak interfacial strain fields, flat and nearly defect-free barrier/YBCO interfaces and good epitaxy of the second YBCO layer with few defects. Possible mechanisms for the formation of the large junction resistance are discussed.

Characterisation of Interface-Engineered Ramp-Edge Josephson Junctions by Transmission Electron Microscopy

Interface-engineered ramp-edge junctions (IEJ) have been characterised by high-resolution transmission electron microscopy, X-ray analysis and electron energy loss spectroscopy. The IEJ are prepared by plasma etching an ion-milled YBa 2 Cu 3 O 7-δ (YBCO) ramp-edge and subsequently a top YBCO layer is deposited. The results show that the YBCO at the plasma-etched interface has a modified structure. The modified barrier structure has a pseudo cubic unit cell which is rotated 45° around the [001] axis with respect to the underlying YBCO.