Search

We have performed resistance measurements on thin films of the high-temperature superconductor YBa2Cu3O7 (YBCO) in applied magnetic fields to above 200 T (2 MegaOersted) at temperatures as low as 2.5 K. The fields are produced by an explosively driven flux-compression system. We can see a particularly clear onset, without replotting the data, of the “hydrodynamic” flow of vortices probably because of the very fast increasing field. The low-temperature “critical field” for the field parallel to the c-axis of the sample is 135 T. Our data in the other direction are still preliminary. We discuss possible interpretation of our results.

We measure the complex conductivity of thin superconducting niobium films using coherent time‐domain terahertz spectroscopy. Both real and imaginary part of the conductivity are measured simultaneously without referring to the Kramers–Kronig relations. We also measure the superconducting band gap directly from the onset of absorption and compare our results with theory.

We measure directly both the real and imaginary parts of the sub-band-gap conductivity \ensuremath{\sigma} in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ in the 15--80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ frequency range using coherent time-domain terahertz spectroscopy. We observe a peak in the real part ${\mathrm{\ensuremath{\sigma}}}_{1}$ similar to the coherence peak expected in an s-wave BCS superconductor. The absence of such a peak in NMR relaxation-rate experiments suggests that its origin lies in a strongly temperature-dependent inelastic-scattering rate rather in coherence factors.

The surface resistance and surface reactance of high-quality YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films were determined from 10 GHz to 500 GHz using time-domain propagation measurements of picosecond electrical pulses on coplanar transmission lines. For the surface resistance, the authors find a square law dependence with frequency almost up to 500 GHz and a crossover of the losses of gold and YBa/sub 2/Cu/sub 3/O/sub 7/ films at 77 K at roughly 100 GHz. The pulse propagation is successfully modeled using the Mattis-Bardeen theory for the frequency-dependent conductivity of YBa/sub 2/Cu/sub 3/O/sub 7/. Good quantitative agreement of the surface resistance and surface reactance with this theory is obtained. The potential of YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films and equivalent gold and superconducting films for high-speed applications is studied theoretically.

The design and fabrication of high-T/sub c/ chirp-response tapped delay line filters are discussed. The key components in this development were long delay lines with more than 10 ns of delay, impedance transformers, and backward-wave couplers. All of the typical transmission line geometries were examined, including microstrip, coplanar and stripline. Designs were developed using microwave CAD routines and superconducting niobium delay line prototypes. Tapped delay line chirp filters with up to 12 ns of total delay have been successfully fabricated in YBaCuO.

We have demonstrated soft x-ray projection lithography using radiation at wavelengths of 14 nm and 37 nm with a commercially available 20X reduction Schwarzschild camera. Line widths as small as 0.05 microns have been printed. The resolution obtained was essentially diffraction limited. Iridium coated mirrors were used with 37 nm radiation and Mo/Si multilayer coated mirrors with 14 nm radiation. A 1:1 magnification Offner Ring-field system with iridium coated mirrors has been used with 42 nm radiation. This optic has imaged line widths as small as 0.2 microns, which is close to the diffraction limit for this system. Transmission masks were used for all these experiments and the radiation was obtained from an undulator in the Vacuum Ultraviolet Synchrotron Storage Ring at Brookhaven National Laboratory.

Determining the cross-sectional doping profile of very small metal–oxide–semiconductor field effect transistors and specifically the direct measurement of their channel length is necessary for true channel engineering to be possible. Scanning capacitance microscopy (SCM) has generated unprecedented images of the cross-sectional doping profiles of very small transistors. The bias voltage dependence of these images has motivated us to investigate the SCM technique in greater detail. Using electrical simulations, we have focused on the pn junction to establish the qualitative and quantitative relationship between the bias voltage and the pn junction location. The ability to confidently interpret the images produced with SCM will allow us to improve simulation models, trouble-shoot process flow, and determine the effective channel length of semiconductor devices.

As MOSFET channel lengths are scaled to below 100 nm, the determination of the effective channel length, L/sub 0/ becomes increasingly important. We have studied cross-sectioned MOSFETs with gate lengths down to 90 nm using scanning capacitance microscopy (SCM), and show the first images of these state-of-the-art devices. Using a device simulator we have quantitatively established the relation between L/sub 0/ and the SCM response in the channel region, allowing us to determine L/sub 0/ from the SCM measurements. We have explored the ultimate resolution attainable using this technique; experimentally, using very sharp probe tips and with numerical simulations.

With the increased scaling down of MOSFET dimensions, delineation of the p-n junction becomes increasingly important. We have studied cross-sectioned 0.15 micron process MOSFETs using a newly developed technique, scanning capacitance microscopy (SCM), and report the first direct measurement of the channel length for these state-of-the-art devices. Using a device simulator we have quantitatively established the connection between the dopant profile and the SCM response while scanning across a p-n junction. This allows us to determine the position of the p-n junction from the SCM measurements.

Four-pole microstrip bandpass filters have been fabricated using both postannealed and in-situ-grown thin films of Y-Ba-Cu-O. The 4-GHz, 3% bandwidth filters exhibit, at 77 K, a passband insertion loss as low as 0.3 dB, compared with the 2.8-dB loss of similar gold metallic filters at the same temperature. These results demonstrate that complex passive microwave devices can be designed and implemented using high-temperature superconducting (HTS) material. More aggressive filter designs utilizing many more poles will result in much greater performance advantages for an HTS filter technology over a normal metal technology. Ultimately, ultrasharp-skirt filters can be produced, which would allow for more closely spaced communication channels and more efficient use of microwave communication bands than is currently possible with conventional filters.

A projection optical system with 20:1 reduction has been used with radiation at ∼36 nm to evaluate resists for use in soft‐x‐ray projection lithography. The high absorption of soft x rays by carbon‐based polymers requires that an imaging resist layer be very thin. The sensitivities and contrasts of several such resists are reported. By incorporating a thin imaging layer into a trilayer resist scheme, we have exposed, developed, and transferred features as small as 0.2 μm into silicon.

Passive microwave devices are expected to be among the first devices made from high‐temperature superconductors to be inserted into system applications outside of a laboratory environment. The deposition and patterning of YBa2Cu3O7−x (YBCO) thin films has progressed sufficiently that a variety of high‐quality passive microwave devices can now be fabricated on LaAlO3 substrates. Demonstrated microwave devices include tapped‐delay‐line transversal filters with multigigahertz bandwidths, untapped delay lines, and narrowband (2% or less) microstrip filters. Potential system applications at operating temperatures of 77 K and below are discussed. The YBCO microwave devices described here demonstrate the potential performance advantages that superconducting thin‐film devices offer for high‐frequency, wide‐bandwidth, high‐performance analog signal processing systems.

The authors give a revised effective mass of the cooper pairs and its temperature dependence for the YBCO high{minus}Tc superconductors. (AIP)

In recent years, a variety of techniques have been developed to generate and detect ultrafast electromagnetic transients with bandwidths up to 1 THz1. The spectrum of these transients spans part of the microwave, submillimeter wave and far-infrared range of the electro-magnetic spectrum. The spectroscopic benefits include the enormous spectral bandwidth covered, the coherent detection that enables to extract both real and imaginary part of the dielectric response function, and a sensitivity that far exceeds the sensitivity of conventional far-infrared spectrometers in the same spectral range.

Free standing microstructures become necessary when transparent materials are unavailable as supporting layers in transmission components. In this work we use electron beam lithography and reactive ion etching in crystalline silicon membranes to fabricate open stencil patterns for two novel applications: transmission reticles containing resolution test patterns for projection soft x‐ray lithography at λ=36 nm; and a Fresnel zone plate for focusing beams of cold neutral atoms. Various schemes are described for making components with minimum zone plate feature sizes down to 0.23 μm and grating features below 0.1 μm. We also report on several materials and processing issues which limit feature sizes and distortions in these microstructures.

Circular Fresnel zone plates, apodized linear zone plates, and a variety of apertures have been fabricated in smooth germanium films deposited on boron doped silicon membranes. Zone widths as small as 60 nm have been fabricated in absorbing layers from 0.25 to 0.5 μm thick. These results were achieved using electron beam lithography in a trilevel resist followed by reactive ion etching to transfer the patterns. These optical elements are key components in a soft x‐ray ‘‘focalometer’’ currently being tested at the NSLS at Brookhaven National Laboratories. The focalometer is a system which can be used to determine the imaging properties of an optical element. In this work it has been used to evaluate the imaging properties of a convergent mirror both before and after a multilayer reflective coating was applied. Results of knife edge scans of images formed by a spherical mirror using 14 nm radiation are reported. The multilayer coating applied to the mirror is found to have only a small effect on these scans.

We demonstrate nearly diffraction-limited printing of 0.2-microm features, using soft x rays of approximately 36-nm wavelength. An open-stencil transmission mask with minimum features of 4 microm was imaged by a twentyfold-reduction Schwarzschild-type objective onto silicon wafers coated with various e-beam resists. Implications for soft-x-ray projection lithography are discussed.

Batch leaching experiments were performed in which the initial amounts of chalcopyrite and ferric chloride were selected to ensure that the oxidant was significantly depleted over the course of an experiment. Solution samples were analyzed for Cu(II) and Fe(III) by visible spectrophotometry and for total copper and total iron by atomic absorption, making it possible to measure changes in the solution component concentrations as leaching progressed. For selected samples, the solution potential was also measured. In all experiments, the Cu(II) concentration passed through a maximum and, simultaneously, the Cu(I) concentration increased very sharply. An acceleration in the total rate of leaching was normally observed at the same time. Early in a leach, the solution potential was too high for the reduction of Cu(II) to Cu(I) to take place at the time of the increase in the overall leaching rate, however, the solution potential dropped sharply during a span of a few hours, reaching a value low enough that reduction of cupric ion became possible. The amount of Cu(I) present at the completion of a leach was dependent on the total chloride concentration of the system. The highest Cu(I)/Cu ratios were observed in systems with the highest chloride concentrations. The ultimate extent of CuFeS2 leaching was dependent on the initial FeCl3 and total chloride concentrations; the FeCl3 was virtually completely consumed and the total chloride concentration controlled the extent to which Cu(II) was reduced by reaction with chalcopyrite.

The microbiological degradation of a mixture of alkylpyridines in groundwater maintained under aerobic and anaerobic conditions was examined. A marked difference was observed between the aerobic and anaerobic degradation rates. In the presence of a soil inoculum and under aerobic conditions, the residual alkylpyridine concentrations generally approached zero concentration within 10 to 31 days of incubation, whereas under anaerobic conditions the concentrations of residual alkylpyridines only decreased between 40 and 80% over a 33 day incubation period. Biodegradation rates under aerobic conditions were greatly affected by the specific ring substitution of structural isomers within a given weight class. A similar effect was not observed for anaerobic degradation rates.

The fate and effects of ocean dumped sewage sludge and acid iron waste were studied at two mid-continental shelf sites off the Delaware-Maryland coast. Materials accumulated in sediment, especially in winter, and tended to persist in topographic low areas. Ambient levels in sediments, of metals, organic carbon, and organohalogens were determined, and statistically significant areas of impaction delineated. Uptake of certain metals in scallops and clams was indicated. The mahogany clam, Arctica islandica, may have suffered mortalities due to dumping activities. Contamination of bottom sediments by sewage indicator bacteria was demonstrated. Studies of impact on bottom communities are continuing.

It is now possible using scanning capacitance microscopy to directly obtain a two-dimensional profile of the doping in MOSFETs with gate lengths greater than 60 nm. We have accomplished this using highly doped, ultrasharp silicon cantilevers with a tip diameter of less than 10 nm. These tips provide images with very little voltage dependence (over a 1-2 V range), and improved resolution ( 30-50 nm resolution.

We have studied cross-sectioned n- and p-metal-oxide-semiconductor field effect transistors with gate lengths approaching 60 nm using a scanning capacitance microscope (SCM). In a homogeneous semiconductor, the SCM measures the depletion length, determining the dopant concentration. When imaging a real device there is an interaction between the probe tip and the built-in depletion of the p-n junction. With the help of a device simulator, we can understand the relation between the SCM images and the position of the p-n junction, making the SCM a quantitative tool for junction delineation and direct measurement of the electrical channel length.