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High-Tc voltage amplifier based on single flux quantum processing and operating up to 77 K

V. K. Kaplunenko, Huai-ren Yi, Bengt J. Nilsson, T. Claeson, E. Wikborg

Published Jan 1, 1995• 0 citations
OpenAlex

Abstract

A voltage amplifier based on single flux quantum processing is fabricated using only a single layer of YBa2Cu3O7 (YBCO) film deposited on a LaAlO3 substrate. Josephson step edge junctions are formed on two steps and show a reasonable spread of parameters. The maximal output voltage of the amplifier is defined by the IcRn product of the junctions and was 0.8 mV at 10 K and 16 V at 77 K. The circuit allows improved SQUID performance and can be used as an rf amplifier

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Applied Physics Letters•May 25, 1998•11 citations
A voltage amplifier based on single flux quantum processing is fabricated using only a single layer of YBa2Cu3O7 (YBCO) film deposited on a LaAlO3 substrate. Josephson step edge junctions are formed on two steps and show a reasonable spread of parameters. The maximal output voltage of the amplifier is defined by the IcRn product of the junctions and was 0.8 mV at 10 K and 16 V at 77 K. The circuit allows improved SQUID performance and can be used as an rf amplifier
We have designed, fabricated and successfully tested three SFQ elements: a toggle flip-flop, a SFQ voltage amplifier, and an RS-flip-flop. The design uses a single superconducting layer and it is optimized by repeated computer simulations and repeated inductance calculations. The inductances are made as narrow slits of 0.5 /spl mu/m width. Their values are calculated numerically taking into account material parameters derived from experiment. The YBCO junctions are either of step edge type on LaAlO/sub 3/ substrates or located on two closely spaced bi-crystal grain boundaries on a novel YSZ tricrystal substrate. Carbon masks were used to form steps on LaAlO/sub 3/ substrates and for the YBCO patterning. Voltage amplifier shows operation up to 60 K, and both flip-flops were observed to operate at temperatures up to 30 K with parameters close to the simulated values. The operation frequency of a T-flip-flop cell of about 150 GHz was achieved and a new measurement technique of the margins of an RS trigger is suggested.
We have prepared high-Tc superconducting heterostructures for DC SQUIDs with graphoepitaxial step edge Josephson junctions on single crystal MgO substrates and investigated them using complementary characterization techniques, including atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. Combined epitaxial (out-of-plane) and graphoepitaxial (in-plane) growth of YBa2Cu3O7-x (YBCO) films on the tilted surfaces of step edges on MgO substrates was observed. The high-Tc step-edge Josephson junctions demonstrated a good reproducibility and an IcRn product of about 600 μV at 77 K. The DC SQUIDs with step edge junctions showed voltage swings δVpp of up to about 60 μV at 77 K. High values of Rn of the graphoepitaxial step edge Josephson junctions can be advantageous for their applications as qubits.
We have fabricated step edge superconductor-normal metal-superconductor microbridges using YBa2Cu3O7−x(YBCO) and noble metals with critical current-normal resistance (IcRN) products as high as 10 mV and normal resistances up to 38 Ω. Our fabrication process achieves high values of the IcRN product by exploiting the anisotropy in the properties of epitaxial YBCO films, allowing contact only between normal metal and superconductor through the crystalline axes which support the largest Josephson coupling. This results in a dramatic increase in the normal resistance of a junction without decreasing its critical current. We discuss the role of the superconductor-normal metal boundary resistance on the junction electrical properties. We have coupled submillimeter wave rf currents quasioptically into junctions integrated at the feeds of noble metal planar log periodic antennas and have induced up to 7 Shapiro steps in the current-voltage characteristics with a 760 GHz beam from a far infrared laser.
Autonomous single-layer dc superconducting quantum interference devices (SQUIDs) have been prepared from epitaxial, laser-deposited YBa2Cu3Ox films on step edge SrTiO3 and LaAlO3 substrates. For device patterning, a SiOx inhibit technique as well as conventional ethylenediaminetetraacetic wet etch was used. The Josephson junctions are of grain boundary type. Their widths are 5 μm. The SQUID hole is a square of about 5 μm; the SQUID inductance is estimated to be about 20 pH. We obtained a very regular, nonhysteretic flux to voltage modulation over more than 100 flux quanta (Φ0). The maximum voltage signal is of the order of 15 μV peak to peak and the maximum transfer function dV/dΦ at the appropriate flux bias is 50 μV/Φ0 at 77 K. The best value of the equivalent flux noise as measured in the flux locked loop mode is 1.4×10−5 Φ0/Hz1/2 at 1 Hz and 1×10−5 Φ0/Hz1/2 in the white noise region for f≳5 Hz. This results in an energy resolution εn,w(f≳5 Hz)=1×10−29 J/Hz in the white noise region and 2×10−29 J/Hz at 1 Hz.
YBa2Cu3O7-x(YBCO) ramp-edge junction technologies have been developed for fabricating high-operating-temperature SFQ(Single Flux Quantum) circuits. The Ic(critical current) spread that is the key parameter for integrating junctions could be decreased to 8% for a 1000-junction array that was formed on a groundplane. The background technologies that enabled this uniform junction characteristic are precise control of the counter-electrode deposition-conditions and deposition of the extremely smooth multiplayer that showed the roughness as small as 2 nm. Based upon these ramp-edge junction technologies, oxide SFQ-circuit-elements were fabricated and their logic operations were investigated. SFQ circuit elements such as a confluence buffer, a set-reset flipflop, and a toggle flipflop have been fabricated and their logic operations were measured by using an SFQ-dc converter. Both toggle flipflops that have one output and two outputs have been fabricated. These circuits could be operated up to the temperature of near 60 K under finite operating margins. These experimental results enable us to further develop fuctional and much more integrated SFQ circuits.
Basic characteristics of a simple single-flux-quantum (SFQ) logic gate using high-Tc material and Josephson junction (NdBa2Cu3O7−δ and focused ion beam junction) have been investigated. The logic gate consists of an rf-superconducting quantum interference device (rf-SQUID) and a dc-SQUID. In the logic gate, elementary SFQ logic operations, such as generating SFQ (dc/SFQ) and providing simultaneous readout (SFQ/dc), have been confirmed up to 50 K. The temperature dependencies of the output voltage level and the critical current-normal resistance (IcRn) product were compared, and the decreasing tendency of the output voltage level for increasing temperature was found to be more rapid than that of the IcRn product.