W. Y. Lee

Superconducting Tl-Ca-Ba-Cu-O thin films with zero resistance at temperatures of up to 120 K

We have prepared superconducting Tl‐Ca‐Ba‐Cu‐O thin films on a variety of substrates with transition temperatures as high as ≂120 K, confirmed by sharp onsets of substantial Meissner and shielding signals at the same temperatures. The properties of the films are found to depend sensitively on the post‐annealing conditions. Highly textured c‐axis‐oriented films comprised mostly of Tl2Ca1Ba2Cu2Ox, Tl1Ca2Ba2Cu3Ox, and Tl2Ca2Ba2Cu3Ox were synthesized by varying the annealing procedure with corresponding maximum superconducting transition temperatures of ≂100, 110, and 120 K, respectively.

Low-noise thin-film TlBaCaCuO dc SQUIDs operated at 77 K

We have made a series of single‐level dc superconducting quantum interference devices (SQUIDs) from 4‐μm‐thick TlBaCaCuO films with large grain sizes and operated them in liquid nitrogen. Although device characteristics could not be precisely controlled, some devices had white‐noise levels that approached thermally limited noise above ∼1000 Hz. In addition, devices with 5 and 80 pH loop inductances had 1/ f noise levels at 10 Hz of 2×10−29 and 5×10−29 J/Hz, respectively. The noise levels at these frequencies are comparable to commercial rf SQUIDs operating in liquid helium, but the hysteresis of the voltage‐flux characteristic of the high Tc SQUIDs remains large.

Weak link behavior of grain boundaries in Nd‐, Bi‐, and Tl‐based cuprate superconductors

We have studied single‐grain‐boundary junctions in the neodymium‐, bismuth‐, and thallium‐based cuprate superconductors and find that they behave as weak links, qualitatively similar to the YBa2Cu3O7−δ superconductor. In general, the grain boundary critical current is determined by flux flow for small misorientation angles and by Josephson junctionlike coupling for large angles. The latter is verified by the observation of voltage oscillations with an external magnetic field in superconducting quantum interference devices built using single‐grain‐boundary junctions of these materials. The commonality of behavior of grain boundaries in all of the high temperature cuprate superconductors suggests that the weak link is most likely associated with the structure of the grain boundary and the evidence points increasingly to dislocations, which describe the topology of the boundary.

High Tc YBa2Cu3O7−x thin films on Si substrates by dc magnetron sputtering from a stoichiometric oxide target

Thin films of YBa2Cu3O7−x were deposited on Si substrates at 600–700 °C by dc magnetron sputtering from a stoichiometric oxide target. Resistivity measurement results indicate that these films are superconducting with a zero resistance Tc as high as 76 K, without further high‐temperature post‐annealing treatments. These films give both core and valence‐band x‐ray photoemission, and x‐ray diffraction spectra similar to those for superconducting films prepared with a high‐temperature post‐annealing step. No significant diffusion of Si from the substrate into the film was detected for the films deposited at 650 °C or lower, according to depth profiles obtained using secondary ion mass spectrometry.

Low‐temperature formation of epitaxial Tl2Ca2Ba2Cu3O10 thin films in reduced O2 pressure

Epitaxial Tl2Ca2Ba2Cu3O10 films on (100) LaAlO3 are prepared by post‐annealing 2Tl:2Ca:2Ba:3Cu precursor films at 830–860 °C in ≂0.03–0.15 atm of O2. These films (0.2–1.1 μm thickness) are smooth and shiny to the eye, and have a sharp zero resistance and onset diamagnetic transition temperature of 117–121 K. Transport critical current densities of 1.6×106 A/cm2 at 77 K and ≂105 A/cm2 at 100 K are obtained for a 0.38‐μm‐thick film in magnetic fields up to 100 Oe. Strong flux pinning at low temperatures is inferred from the weak‐field dependence of the critical current density calculated from magnetic hysteresis loops. At 5 K, the best film has a magnetic critical current density of 9×106 A/cm2 in zero field, decreasing gradually to 1.5×106 A/cm2 in a 5‐T field.

Low-temperature formation of epitaxial Tl sub 2 Ca sub 2 Ba sub 2 Cu sub 3 O sub 10 thin films in reduced O sub 2 pressure

Epitaxial Tl2Ca2Ba2Cu3O10 films on (100) LaAlO3 are prepared by post‐annealing 2Tl:2Ca:2Ba:3Cu precursor films at 830–860 °C in ≂0.03–0.15 atm of O2. These films (0.2–1.1 μm thickness) are smooth and shiny to the eye, and have a sharp zero resistance and onset diamagnetic transition temperature of 117–121 K. Transport critical current densities of 1.6×106 A/cm2 at 77 K and ≂105 A/cm2 at 100 K are obtained for a 0.38‐μm‐thick film in magnetic fields up to 100 Oe. Strong flux pinning at low temperatures is inferred from the weak‐field dependence of the critical current density calculated from magnetic hysteresis loops. At 5 K, the best film has a magnetic critical current density of 9×106 A/cm2 in zero field, decreasing gradually to 1.5×106 A/cm2 in a 5‐T field.

Noise and hysteresis in flux‐locked TlBaCaCuO SQUIDs

Measurements of TlBaCaCuO‐based dc superconducting quantum interference devices (SQUIDs) performed using a practical feedback circuit gave a flat frequency response and an energy sensitivity of 6×10−30 J/Hz at 5 kHz and 77 K with an 80 pH SQUID. Flux‐locked loop operation was demonstrated to be strongly effective in eliminating the magnetic field hysteresis of the SQUID. The SQUIDs were operated in the flux‐locked loop at temperatures over 90 K and the 1/f noise was found to decrease with increasing temperature near 77 K.

Field‐assisted bonding below 200 °C using metal and glass thin‐film interlayers

Bonding between similar or dissimilar surfaces with metal and glass thin‐film interlayers as well as bulk glass plates using an electric field assisted bonding technique at 160 °C or less is described. This low‐temperature field‐assisted bonding is achieved using, e.g., few μm thick glass films rf magnetron sputter deposited from Na or Li silicate glasses, and 0.05–1.0 μm thick Al, Sn, Mg, and Hf films. The bond strength thus achieved is found to be greater than the cohesive strength of the glass used. Results from x‐ray photoelectron spectroscopy analyses confirm the migration across the glass layer and the plating out on the cathode surfaces of mobile ions such as Na+ or Li+ during field‐assisted bonding. The migration and plating out of mobile ions can be detected in 10 s after the potential is applied and whether actual bonding between the metal and glass occurs or not. The possible bonding mechanism based on these XPS results is proposed.

Effect of oxygen pressure on Tl2Ca2Ba2Cu3O10±δ formation

We investigated the formation of Tl2Ca2Ba2Cu3O10±δ as a function of oxygen pressure and temperature. Our results explain some previous difficulties in forming single‐phase Tl2Ca2Ba2Cu3O10±δ and suggest new routes for making bulk and thin‐film Tl‐Ca‐Ba‐Cu‐O superconductors. In particular, we find that the temperature needed to form Tl2Ca2Ba2Cu3O10±δ can be greatly reduced by using low oxygen pressures during synthesis.

High-current-density ITOx/NiOx thin-film diodes

Thin-film diodes 0.2–4×10−5 cm2 in size were fabricated on Al2O3–TiC ceramic substrates from junctions between p-type NiOx and n-type ITOx thin films sputter deposited at ambient temperature. These diodes show a room-temperature turn-on voltage of 0.3–0.6 V and a dc forward current density exceeding 104 A/cm2 at an applied voltage of ≃1.5 V. The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of these diodes can be described by an exponential dependence of I on the applied V and by a linear I/C2 dependence on the reverse applied V, respectively, consistent with those predicted by the various models proposed for the mechanisms for charge-carrier transport in abrupt anisotype heterojunctions. The ideality factor at low-applied V (⩽0.3 V), the junction built-in potential, and the carrier concentration deduced from these I–V and C–V data are ≃1.5–2, 0.20–0.24 V, and 1.90–1.97×1018/cm3, respectively.