Alfred E. Lee

Low‐loss substrate for epitaxial growth of high‐temperature superconductor thin films

A perovskite‐like single‐crystal substrate material has been investigated that simultaneously permits epitaxial growth of 1‐2‐3 superconductor films and possesses desirable rf properties of low dielectric constant and loss tangent. The lattice constant of 3.792 A provides a lattice match to within 1% of the a axis of 1‐2‐3. Sputtered films of erbium‐barium‐copper‐oxide have been produced on (100) LaAlO3 substrates that exhibit sharp resistive transitions at 90 K (ΔT=1K), bulk superconductivity as determined by ac susceptibility measurements, and nearly single‐crystal growth as evidenced by x‐ray diffraction and high‐resolution scanning electron microscopy. The high‐frequency dielectric properties of LaAlO3 were experimentally investigated at several temperatures. The low‐frequency dielectric constant was measured to be 15 and the microwave loss tangent ranged from 6×10−4 at room temperature to 5×10−6 at 4 K.

Epitaxially grown sputtered LaAlO3 films

We have grown crystalline thin films of LaAlO3 using off‐axis rf sputtering from a single stoichiometric target. The films grow epitaxially on SrTiO3 and LaAlO3  (100) substrates as well as on YBa2Cu3O7 thin films. We report on the growth conditions used to make these films, the properties of the films, and the properties of bilayer and trilayer structures containing both LaAlO3 and YBa2Cu3O7 films. Transmission electron microscopy cross‐sectional and x‐ray diffraction analyses indicate that all the constituent films in the multilayers grow epitaxially and that the interfaces between the films are sharply defined. Preliminary transport measurements on these multilayers show that LaAlO3 can be used for dielectric layers in a variety of high‐temperature superconductor electronic circuits.

Characteristics of high performance YBa2Cu3O7 step-edge junctions

Step‐edge Josephson junctions are engineered grain boundary junctions fabricated using standard lithographic and film deposition techniques. We report a systematic study of 180 YBa2Cu3O7 step‐edge junctions and identify a fabrication technique which results in a 90% yield of working junctions with critical current spreads from 30% to 50% (1σ/Ic‐ave)over the entire substrate. Technically useful critical current values at 65 K can be obtained by adjusting YBa2Cu3O7 film thickness. IcRn values, approximately independent of film thickness, are ∼1 mV at 4.2 K and ∼0.1 mV at 65 K. Most junctions exhibit ideal electrical behavior in accordance with the RSJ model.

Novel monolithic phase shifter combining ferroelectrics and high temperature superconductors

Abstract We describe a monolithic phase shifter which combines the low loss of high temperature superconductivity (HTS) with the variable dielectric properties of a ferroelectric material SrTiO3. Phase shifts greater than 28 degrees per wavelength were observed around 30 K. Benefits of the combination of HTS and ferroelectric materials are described. The compatibility of YBa2Cu3O7-x and other ferroelectric materials is discussed.

Co-doped-YBCO SNS junctions for superconductive integrated circuits

We have established a high temperature superconductor SNS junction fabrication process and are reproducibly fabricating junctions whose behavior is rooted in the physics of the proximity effect. SNS edge junctions are being fabricated using YBa/sub 2/Cu/sub 2.8/Co/sub 0.2/O/sub 7-/spl delta// (Co-YBCO) as the normal barrier, and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) as the superconductor. In the small junction regime, the junctions have RSJ characteristics with I/sub c/R/sub n//spl sim/ 50 /spl mu/V at 77 K. Junction normal resistance values are consistent with known values of the barrier bulk resistivity and junction geometry, indicating that the interface resistance between YBCO and Co-YBCO is not significant. We have demonstrated I/sub c/ and R/sub n/ 1-/spl sigma/ spreads of 33% and 17%, respectively, for 6-junction strings. Barrier thickness and/or morphology variations appear to significantly contribute to these spreads.<<ETX>>

Superconducting Ba/sub 1-x/K/sub x/BiO/sub 3/ thin film by in-situ sputtering

Superconducting Ba/sub 1-x/K/sub x/BiO/sub 3/ thin films with T/sub c/ (R=0) as high as 29.2 K have been grown using off-axis RF magnetron sputtering. Films are routinely deposited with zero resistance between 25 K and 29 K. High pressure during deposition is considered to be responsible for higher-T/sub c/ films. The films grow epitaxially with

Capacitively shunted, hysteretic YBa/sub 2/Cu/sub 3/O/sub 7/ step-edge junctions

Hysteretic YBa/sub 2/Cu/sub 3/O/sub 7/ step-edge junctions on LaAlO/sub 3/ substrates have been fabricated by shunting intrinsically overdamped junctions with a monolithic capacitor. By comparing the I-V curves of junctions fabricated on the same substrate with and without capacitor counterelectrodes, the authors are confident that the observed hysteresis is due to the shunting capacitor. The capacitor consists of a dielectric layer (SrTiO/sub 3/ or LaAlO/sub 3/), deposited on the YBa/sub 2/Cu/sub 3/O/sub 7/ directly over the step-edge junction and an Ag counterelectrode. Capacitor counterelectrodes ranging in area from 10 mu m*30 mu m to 200 mu m*220 mu m have been investigated. Dielectric layers several tens of nanometers thick have been used. The inferred beta /sub c/ values are as large as 10 at 4 K and decrease with increasing temperature. At 65 K, beta /sub c/ of 1.3 was observed. The measured beta /sub c/ values are smaller than one would naively calculate. These differences are attributed to the usual limitations of lumped-element circuit analysis and resistive losses.<<ETX>>

Dielectric properties of SrTiO/sub 3/ thin films at low temperature

The authors have fabricated YBCO (or Ag)/SrTiO/sub 3//YBCO capacitors for low-frequency measurements and SrTiO/sub 3//YBCO/LaAlO/sub 3/ coplanar-waveguide end-coupled resonators for high-frequency measurements of the SrTiO/sub 3/ dielectric constant at low temperatures. From the capacitor measurements, values for the dielectric constant ranging from approximately 200 to 800 were obtained for films of various thicknesses. This behavior of the thin-film dielectric constant was found to differ from that of the bulk crystal.<<ETX>>

High temperature performance of HTS step-edge DC SQUIDs

YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) step edge DC superconducting quantum interference devices (SQUIDs) have been developed which exhibit characteristics suitable for near-term incorporation into high-temperature superconductive (HTS) circuitry. Step-edge junction DC SQUIDs for series array interferometer logic (SAIL) digital applications exhibit resistively shunted junction (RSJ) properties, 65 K I/sub c/ values of approximately 150 mu A, 65 K I/sub c/R/sub n/ values up to approximately 300 mu V, and large I/sub c/ modulation (>50%) in accordance with a standard DC SQUID model. 65 K SQUID switching voltages of approximately 100 mu V have been demonstrated and are sufficient for near-term applications of SAIL digital circuitry operating on a cryocooler platform.<<ETX>>

Multisource-sputtered ErBa2Cu3O7−x films on single-crystal and buffered crystalline substrates

High-quality superconducting films of erbium-barium-copper-oxide have been produced in a multisource sputtering system on a variety of substrates, including buffered sapphire and silicon wafers as well as various single-crystal materials. Fine-grained polycrystalline films with narrow (<4 K) resistive transitions have been grown on a number of different substrates. The use of erbium in the 12–3 compound leads to improved film morphology and to more forgiving conditions for establishing of the superconducting phase. Sputtered neutral mass spectroscopy (SNMS) yields useful information about the compositional profile of the films.