AFM analysis of step-edge Josephson junctions

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We report on an analysis of step-edge Josephson junctions with a variety of different electrical behavior I<sub>c</sub>'s and IV curve characteristic shapes. We investigated a correlation between the shape of the IV curve and the morphology of the step-edge YBCO film concentrating on the sharpness of the step. Using Atomic Force Microscopy (AFM), determination of the film's surface properties over the step were obtained. The steepness of the angle of the film over the step correlated with the behavior of the IV curve. When the average angle over the step was >23° the IV curve exhibited the resistively shunted junction [RSJ] shape. Less steep average angles <23° corresponded to flux flow-like IV curves

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For “SNS step and gap”, and for “step-edge grain boundary”, Josephson-junctions steep steps in the substrate are needed to get either a break in the YBa2Cu3O7-film (SNS) or to induce grain boundaries in the growth of the YBa2Cu3O7-film across the step (step-edge).We report on the influence of the profiles of ion-beam etched substrate steps in MgO and SrTiO3 on the growth of YBa2Cu3O7-films at the step-edges and on the electrical properties of the different Josephson-junction types. On MgO we have fabricated Josephson-junctions of the SNS-type (using Ag as “N”), while for the step-edge junctions SrTiO3 was used. Both types of junctions exhibit Josephson-effects at 77K. For the steps we use photoresist AZ 5214 E with steep edges as the mask. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis on differently fabricated steps are presented and compared to the electrical properties of patterned YBa2Cu3O7-films grown on them. Especially the role of material redeposited at the edge of a step during ion-beam etching is discussed. The reproducible step edge profiles are reflected in reproducible electrical properties of the Josephson-junctions and dc-SQUIDs (on chip and chip to chip).
The microstructure of YBa2Cu3O7 grown on steep steps in (001 pseudocubic LaAlO3 substrates was studied by high-resolution electron microscopy of cross-sectional and plan-view samples. Steps with angles of about 80° were obtained by ion milling. On the substrate plane, the films grew with the c-axis parallel to [001] while on the flank of a step the c-axis was parallel to the [100] direction of the substrate. As a result, two [010] tilt axis grain boundaries were formed at which the YBa2Cu3O7 lattice changed orientation by approximately 90°. In the upper grain boundary, a [100] tilt axis and, on the average, a (013) habit plane alternated with a [010] tilt axis and a (1 0 3) habit plane. This alternating structure was caused by twinning in the orthorhombic film structure. The lower grain boundaries were found to be rather irregular and consisted of a chain of (0 1 3)(0 1 3) and (0 1 0)(0 0 1) type segments exhibiting a tendency to tilt the whole habit plane toward the a-b plane of the flank film. Dislocations, stacking faults and misfit strains were also observed in or close to the boundaries. Grain boundary modeling indicated a good agreement with the experimental image and permitted us to determine the atomic plane of a boundary.
We discuss our multilayer YBCO process which uses step-edge junctions. Key issues are dielectric defect density, wiring layer critical current over edges of underlying features and junction critical current uniformity. We have demonstrated an average defect density of about 450/cm<sup>2</sup> over many wafers. Wiring critical current exceeds 1 mA/μm of line width. We have demonstrated SQUIDs at 77 K using this process.
We report the operation of and, exor, exnor, and inverting gates in a circuit architecture suitable for the set of all Boolean gates. Series arrays of YBa 2 Cu 3 O 7-x dc superconducting quantum interference devices with integrated coils exhibit sufficient modulation to allow low‐speed tests demonstrating gate logic, and lack of oscillation or latching. The best inverter operated (without load) at 36 μV, with 2 nW of gate dissipation.
We have fabricated RF and DC SQUIDS With step-edge Josephson junctions (SEJ) and YBa2Cu3O7 thin films. The low-frequency noise of these SQUIDs has been investigated as a function of temperature, bias current and magnetic field. Typically, the energy resolution in the white noise region of the DC SQUIDS with an inductance of 45 pH was 3 x 10(-30) J Hz-1 at 77.5 K. Depending on the bias current, the low-frequency spectrum could be of either 1/f type or Lorentzian type. The Lorentzian spectrum was generated by random telegraph signals (RTSS). An Arrhenius plot was obtained from the temperature dependence of the switching time of these RTSS. We have determined two barrier lengths of 0.23 eV and 1 eV of the two-level fluctuator (TLF) generating this RTS noise. From the magnetic-field dependence of the low-frequency noise we have found that the noise is periodic with the flux corresponding to the area of the SEJ. The results were discussed in terms of a model treating the SEJ as a multi-junction interferometer which forms a two-level fluctuator when the junction is placed inside a SQUID loop. The noise is generated by thermally activated switching of the SQUID between the two different states of the SQUID.
We have fabricated YBCO 90° grain boundary junctions on step edges in NdGaO<sub>3</sub> and in deposited dielectric (CeO<sub>2</sub> on YSZ and SrTiO<sub>3</sub> on MgO) in order to compare junction performance to our standard, LaAlO<sub>3</sub>. Average I<sub>c</sub>R<sub>n</sub> values at 77 K in the 300-400 μV range were measured for 2 μm step edge junctions on NdGaO<sub>3</sub>, LaAlO<sub>3</sub>, and SrTiO<sub>3</sub>/MgO. Junction I<sub>c</sub> is greatly reduced with the CeO<sub>2</sub>/YSZ system. I<sub>c</sub>R<sub>n</sub> values in the 300-400 μV range were measured at 65 K for 4 μm junctions.
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