Correlation between surface whisker growth and interfacial precipitation in aluminum thin films on silicon substrates

When subjected to thermal excursions, aluminum thin films on silicon substrates often show whisker or hillock growth on the film surface, along with formation of Si precipitates at the interface. This study demonstrates that the two effects are related, and that interfacial Si precipitation directly influences the growth of Al whiskers on the film surface during isothermal annealing at 300–550 °C. The density of whiskers and hillocks not only increases with increasing annealing temperatures where the film is under greater compressive stress, but also during longer hold times which should relieve the stress. At high temperatures and long annealing times, extensive Si precipitation, eventually leading to a bi-modal precipitate size distribution, occurs continuously at the interface. The total amount of Si precipitates far exceeds the solubility limit of Si in the Al thin film, and can generate enough compressive stress in the film to drive surface whisker growth. By continuously augmenting film stress, interfacial Si precipitation supplies the driving force for whisker/hillock formation on the Al-film surface.     

High resolution magneto-optical studies of the intermediate state in thin film superconductors

The structure of the intermediate state in superconducting lead films has been investigated as a function of magnetic field and film thickness. The detection system utilized the high specific Faraday rotation in thin films of a mixture of EuS and EuF₂ in combination with a polarizing microscope, yielding a resolution of about 1 μm. The thickness of the Pb films ranged between 0.7 and 9 μm, thus including the critical film thickness at which the transition from the intermediate state to the vortex state occurs. At low fields a liquid-like mixed state of multi-quanta flux tubes was observed which appeared to be stable up to increasing magnetic fields with decreasing film thickness. The diameter of these flux tubes varied approximately with the square root of the film thickness. At intermediate fields the intermediate state pattern was found to persist down to a film thickness of 0.7 μm, the smallest thickness investigated. The periodicity length of the intermediate state structure was in reasonable agreement with the non-branching model of Landau. Just below H_c, small superconducting domains were observed in increasing field, whereas long threads of superconducting material were formed abruptly in decreasing field. These superconducting threads were absent in the specimentsthinner than 1–2μ, being replaced by a liquid-like mixed state of superconducting tubes. After the passage of a sufficiently high electrical current through the specimen, the flux structure was found to be rearranged into long domains oriented predominantly perpendicular to the current, leading to current hysteresis effects. Finally, some dynamic observations were made during current induced flux flow.     

Patterns of magnetic flux penetration in superconducting films

Intermediate-state magnetic flux patterns have been observed in thin superconducting films of tin, lead, and indium in perpendicular applied fields of 0.1–100 Oe. The spatial variation in flux density was measured very close to the film with a high-resolution Hall probe which could be scanned in a plane parallel to the surface of the film. The patterns appear to contain both laminar and round normal regions, with the latter predominant. A model for the intermediate state has been developed in which the normal regions are macroscopic flux spots. Experimental results for the number density of these flux spots in 1.1–6.3--thick indium samples are in good agreement with the temperature, field, and thickness dependences of our model; indium samples thinner than 1 µ appear to enter the mixed state. In tin and lead, however, the flux patterns are strongly influenced by flux pinning. Appearance of the flux pattern at the critical field or temperature has been observed and correlated with distinctive features in the resistive transition. The extent of flux penetration into the sample was determined from measurement of the average field and compared for various field-temperature paths to the intermediate state. With the applied field reduced to zero, the magnetic field outside a current-carrying superconducting strip has been measured and indicates that the current distribution within the strip is consistent with the form predicted by Bowers.Supported in part by the NSF, ONR, and ARPA.     

Continuous vortex cutting in type II superconductors with longitudinal current

The consequences of stipulating translational symmetry for a type II superconductor to which longitudinal electric current and longitudinal magnetic field are applied are investigated. The magnetic flux lines must cut each other continuously in order to generate an electric field in this symmetry. We describe the steady state by two interpenetrating vortex lattices moving into and out of the sample. We find for the slab and cylinder geometries that cutting, crossjoining, and subsequent straightening of the flux lines reduce the electric field, as compared with the normal conducting state, by a factor which is of the order of one over the total number of flux lines in the cylinder. We conclude that the much larger voltages observed in cylinders of several millimeters diameter can be explained only by a breakdown of translational symmetry. With translational ¥mmetry, the voltage initially increases as the third power of the current. The resulting vortex configuration is force-free. The transverse flux component increases and the longitudinal component decreases from the axis to the surface, leading to a paramagnetic moment. The drift or oscillation velocity of the flux lines is reduced by the same factor as the electric field. We predict low-frequency oscillations of the vortices near the surface of thin superconducting wires.     

Local metallic conductivity of thin polyimide films as a result of “soft” electrical breakdown

The electrical breakdown of a thin polyimide film between metal electrodes has been investigated under conditions of strong confinement of the breakdown current. The result of this “soft” electrical breakdown is a local, highly conducting channel in the insulating film implanted in the polymer and consisting of a compound of carbon and metal from the electrodes. It is shown that the polymer channel is converted to the superconducting state by the superconducting properties of the metal from the electrodes. Pis’ma Zh. Tekh. Fiz. 23, 8–12 (July 26, 1997)     

Cryogenic thermoelectric (QVD) detectors: Emerging technique for fast single-photon counting and non-dispersive energy characterization

Abstract

”QVD”detectors are based on thermoelectric heat-to-voltage (Q V) conversion and digital (V D) readout. We have devised and analyzed the performance of QVD detectors with several different sensor designs that enable use of high thermoelectric figure of merit samples, be they of thin film, bulk crystal, or whisker form. Our first QVD devices had the well-studied material Au-Fe as thin film sensors. More recently, we have confirmed the literature reports of substantially higher Seebeck coefficient at cryogenic temperatures in lanthanum (cerium) hexaborides. We have also investigated the kinetic properties of La(Ce)B₆ crystals with different La – Ce ratios. Currently we are exploring prototype devices based on bulk single-crystalline sensors. These include a successfully tested candidate with a sharpend hexaboride sensor and small-size bismuth absorber – a whisker prototype. In theory, QVD sensors are competitive with superconducting tunnel junction (STJ) and transition edge sensor (TES) devices in energy resolution ability. However, QVD sensors ought to be able to respond at very much faster rates than these competitors; the lanthanum-cerium hexaboride sensors are expected to reach rates of 100 MHz counting rates for UV/optical photons. In addition to traditional astrophysical applications, these detectors can be applied to the tasks of quantum computing and communication.     

High-temperature superconducting dual-mode resonator on (100) MgO substrate

We report, for the first time to our knowledge, a clear resonant peak split in the range of 7.7–9.7 GHz in a perturbed dual-mode disk-type resonator (DMDR) made of YBa₂Cu₃O_7–x (YBCO) superconducting thin film on MgO substrate. Epitaxial YBCO superconducting thin films were grown on (100) MgO substrates by pulsed laser deposition technique. The critical temperature of superconducting thin film on MgO substrate was 85 K. Superconducting dualmode disk resonators were designed by microwave design software, EEsof, and patterned by photolithography and a wet-etch process. The unloaded quality factor (Q_UL) of the superconducting DMDR was found to be 1,312 at 77 K. We believe this type of DMDR can be utilized for dual-mode resonator-based filters for satellite communications.     

钇钡铜氧超导薄膜光诱导弛豫特性研究

采用TFA-MOD法在LaAlO3(LAO)单晶基底上制备了钇钡铜氧(YBCO)超导薄膜,利用X射线衍射对YBCO超导薄膜的结构进行了表征,并对其最佳沉积工艺进行了分析;研究了YBCO薄膜在不同温度和不同光功率下的光诱导特性.在光诱导下,YBCO薄膜出现了明显与电子体系相关的弛豫现象,说明薄膜的光致电阻变化与激光的功率...     

Intermodulation Measurements on High Temperature Superconducting Thin Films

A high temperature superconducting GdBa₂Cu₃O_7– thin film was characterized using a parallel plate resonator at 5.5 GHz and a dielectric puck resonator at 10 GHz, to observe the dependence of surface resistance on microwave power at the fundamental frequency, and the production of intermodulation products using a two-tone measurement setup, respectively. The electromagnetic field pattern of the dielectric puck-HTS thin film mode was modeled using the commercial software package MAFIA. The intermodulation product was compared with a simple RSJ model.     

Thermal performance of multilayer insulation around a horizontal cylinder

Thermal performance of multiplayer insulation (MLI) is affected by contact pressure between adjacent layers. In order to evaluate the thermal performance of the MLI fabricated in the horizontal cryostats of superconducting magnets, it is important to investigate the contact pressure in the MLI. In case of a horizontal cryostat, the MLI is wound around horizontal cylindrical surface and is compressed at the upper part of the cylinder due to the MLI self-weight. At first, a single thin film wound around the horizontal cylinder was analyzed to evaluate the contact pressure acting on the cylinder. The analysis has been extended to the multiply wound film around horizontal cylinder, in order to investigate the distribution of contact pressure between adjacent layers. By using experimental data obtained with a flat panel calorimeter, the results of this analysis have been applied to evaluate the thermal performance of MLI around a horizontal cylinder. And the non-dimensional contact pressure parameter P^* has been introduced as a useful parameter to evaluate and compare the thermal performance among different kinds of MLI.     

New research opportunities in superconductivity IV

This report surveys the considerable progress made over the last five years—such as the marketing of superconducting quantum interference devices (SQUIDs), cellular wireless filter systems, and high current leads—and assesses needs and opportunities in the areas of fundamental science, materials development, thin film and device applications, and wire and bulk applications. It examines the challenges facing high-temperature superconductivity: from the need to understand the mechanism of high-temperature superconductivity and the unusual “normal” state to the need for new instrumentation for material characterization. Advances in thin film and bulk materials are reviewed, and obstacles impeding the commercialization of HTS materials are examined. A report on the workshop on research needs and opportunities in superconductivity, held in Monterey, California, February 10–12, 1997.     

Distribution of screening currents in thin superconducting films

A theoretical analysis is made of the magnetic field screening in thin superconducting films in the Meissner state. Simple approximations are derived for the current density distribution over the film thickness which are everywhere analytical and satisfy the initial equations with a high degree of accuracy. This approximation can be used to determine the demagnetization factor of the film and to estimate the magnetic field for entry of the first vortices into the sample. Pis’ma Zh. Tekh. Fiz. 25, 84–88 (October 26, 1999)     

Thermal state of superconducting films on wide substrates

The stationary thermal state of a long superconducting film on a wide substrate is analyzed analytically. Expressions describing voltage-current characteristics and temperature-current dependence of the film are derived for the flux creep, flux flow and normal regimes. It is shown how the flux creep influences the conditions of the thermal stability. In particular, it is found that the bistability of the thermal state can appear in this regime. Under the boiling crisis, the temperature-current dependence for a film differs markedly from that for a wire and is characterized by a smooth temperature increase with the current. A “mixed” regime is analyzed where the flux flow and normal states exist simultaneously with the boundary between them parallel to the film axis.     

Characterization of GdBa2Cu3O7−x superconducting thin films by a new optical interference fringe method

GdBa₂Cu₃O_7–x superconducting thin films, grown on yttrium-stabilized ZrO₂ (YSZ) and LaAlO₃ (LAO) substrates, were investigated by a new optical interference fringe method. The results show that (i) two sets of interference fringe pattern are observed on the samples grown on LAO substrates when the film thickness is less than 250 nm, one of them coming from the thin film and the other from the substrate; (ii) the fringe patterns vary slightly on different regions of the samples, which means that the perfection of the thin films is non-uniform; and (iii) defects such as grain boundaries and twin lamellae are observed in some samples. The relationship between the fringes and the degree of perfection of the sample is discussed according to the experimental results.     

Current and Temperature Dependent Matching Fields in Nb Film of Rectangular Array of Holes

Nanoengineered periodic array of holes on superconducting thin films have a great interest due to their excellence for the studies of the vortex pinning mechanisms in the type-II superconductors. Rectangular array of holes has been fabricated over a microbridge of Nb superconducting thin film by e-beam lithography. Rectangular array of holes have two type of scenario, at low magnetic fields matching effects are sharp and narrow while at high fields wide and shallower. In this work, we study the matching pinning effect by the artificial hole array in superconducting Nb thin films. We observed that as the inter distance between holes is decreased; the sharp matching effects become dominant and vice versa.     

Receiver protecting device based on microstrip structure with high-temperature superconductor film

New design of an effective device for protection against high-power electromagnetic pulses has been created based on a pair of noninteracting microstrip resonators, which are coupled in the working frequency band via a third resonator based on a thin film of high-temperature superconductor (HTSC) occurring in the superconducting state. Under the action of an electromagnetic pulse with the power above a certain threshold, the HTSC film element passes from the superconducting to normal (high-resistivity) state, thus breaking the coupling between resonators. This leads to power limitation at the device output due to a strong signal reflection from the input.     

Localization processes near the superconductor-insulator transition in Bi2Sr2 − xLaxCuOy nanoscale thin films

Thin film Bi₂Sr_2 − xLa_xCuO_y (x = 0.6) was deposited onto SrTiO₃ by using DC magnetron sputtering. The structural characterization was carried by X-ray diffraction and the transport properties were carried by resistivity and Hall Effect measurements. The underdoped system near superconductor-insulator transition (SIT) was performed by partial substitution of Sr with x = 0.6 La. By varying the oxygen content in very small amounts through a vacuum anneal process, a highly precise hole-doping of thin film was obtained and the same film is changed from initial superconducting state to strongly insulating underdoped state. More than 14 doping states in the vicinity of SIT were performed and studied by electrical resistivity as function of temperature. The thermally activated behavior, log (1/T) behavior or electrical resistivity and VRH localization processes were evidenced function of doping and temperature.     

Design for a repetitive superconducting opening switch

This paper details a design study of a superconducting opening switch suitable, for example, for discharging the stored energy of a superconducting inductor into a load at room temperature in a short time (～ 1 μ s). In order to achieve a reasonable repetition rate of ～ 10 Hz, the superconducting element must be a thin film and consideration is given to the presently attainable properties of such films. Based on this, the design is made specific to 10 000 V and 10 000 A. Savings in size and cost would occur with modest increases in the critical current density and/or normal state resistivity of candidate film materials.     

The Pressure Coefficients of the Superconducting Order Parameters at the Ground State of Ferromagnetic Superconductors

We investigated the pressure coefficients of the superconducting order parameters at the ground state of ferromagnetic superconductors based on the microscopic single band model by Linder et al. The superconducting gaps (i) similar to the ones seen in the thin film of A2 phase in liquid ³He and (ii) with the line node were used. This study shows that we would be able to estimate the pressure coefficients of the superconducting and magnetic order parameters at the ground state of ferromagnetic superconductors.     

Thickness Effect on Sheet Resistance in BSCCO Mixed Crystal Thin Film

In the Bi2212/Bi2201 mixed crystal thin films with same Bi2212 molar percentage of 70mol%, the superconducting transition takes place in the specimens thicker than about 44. SI transition is brought about by the domain size effect in this mixed crystal thin film. The film thickness also gives a remarkable influence on T _c in a dissipative state.