bluuebunny/crossref_metadata_2025 huggingface dataset:
High-pressure autoclave for multipurpose nuclear magnetic resonance measurements up to 10 MPa
Authors: Behr W. , Haase A. , Reichenauer G. , Fricke J.
Published: July 2002

High-pressure nuclear magnetic resonance (NMR) is an established method in NMR spectroscopy: on-line coupling of high-performance liquid chromatography with NMR, for example, reveals structural information which cannot be obtained with any other method. However, applications has been focused solely on high-pressure NMR spectroscopy, even though high-pressure NMR imaging allows in situ studies of processes such as the fluid exchange in porous media. A versatile high-pressure autoclave for NMR imaging is described in this article. The autoclave allows measurements in any horizontal NMR imager using magnetic field coil systems with an inside diameter of more than 70 mm. Any sample with a diameter up to 28 mm and a length of about 200 mm can be investigated. The autoclave is constructed for operating pressures up to 10 MPa and is temperature controlled between 10 and 60 °C. The materials of the high-pressure cell which are the thermoplastic polyetheretherketon (PEEK) for the pressure tube and brass (63% Cu, 37% Zn) for the caps also permit investigations with aggressive fluids such as supercritical carbon dioxide. Inlet and outlet valves allow replacement of fluids and pressure variations in the autoclave during the NMR measurement. FLASH NMR images of the fluid exchange of methanol for liquid carbon dioxide in silica alcogels at 6.5 MPa are presented in order to demonstrate possible applications.
Direct ultrasonic measurement of solid propellant ballistics
Authors: Di Salvo Roberto , Dauch Frédéric , Frederick Robert A. , Moser Marlow D.
Published: July 2002

This article illustrates the application of an ultrasonic pulse–echo technique to determine the burning rate of a composite solid propellant as a function of pressure. An evaluation of the measurement uncertainty of the method is also presented. Unlike the more traditional strand burner techniques, where dozens of constant pressure tests are necessary, the ultrasonic technique measures the burn surface position thousands of times per second as the pressure varies. This reduces the number of tests necessary to determine the ballistic characteristics of the propellant by an order of magnitude. This work presents new methods to characterize the changing speed of sound in the propellant and quantitative estimates of the measurement uncertainty in the burning rate measurement. The results of the uncertainty analysis showed that the measurement is accurate to around 4%. The propellant samples were tested in a closed-combustion vessel, under pressurization rates of up to 15.8 MPa/s. The data obtained with the closed-combustion-vessel tests using the ultrasonic method are experimentally congruent with tests conducted in strand burners in steady-state conditions.
Stop-motion illuminator for centrifuges
Authors: Moroz Pavel E.
Published: July 2002

The majority of laboratory centrifuges have no provisions for observation of the process of sedimentation and fractionation which goes on in the test tube in the darkness of a “black box.” Some centrifuges have a transparent cover or a magnifier in the cover to watch a blurred picture of the layers in the moving test tube which provides little information. The stop-motion effect with the test tube can a priori be based on either mechanical or electronic stroboscopy. To implement the mechanical stroboscopy, a standard microscope illuminator with a diaphragm is attached to the wall of the centrifuge. Through a hole in the wall the illuminator projects a cone of light with the diameter 0.1 mm at the top of the cone. The cone intersects with a 0.1 mm hole in the test-tube holder. The test tube is illuminated at the moment of intersection. Its content is observed through a magnifier on the cover of the centrifuge. The holder has a cut-out window to see the test tube. The flash at the hole–beam intersection is 0.5 μs, which is many times shorter than the duration of a xenon-tube stroboscopic flash (10–20 μs). This is the main reason why industrial stroboscopes cannot be used for stop-motion illumination in a centrifuge.
Laser vibration measurements and data processing for structural diagnostic on composite material
Authors: Castellini P. , Revel G. M.
Published: July 2002

In recent years, the identification and characterization of defects in mechanical and civil structures have been widely investigated by processing of vibration measurement results. In this field laser Doppler vibrometry (LDV) offers large potentials in terms of spatial resolution and reduced intrusivity. In this work LDV technique has been applied to measure delamination extension, depth, and location in composite materials. A lumped parameter model has been developed to describe the dynamic behavior of a delaminated composite panel with the aim of determining which kind of measurement data must be extracted for damage monitoring and to design efficient postprocessing algorithms for experimental LDV data. It is shown that the root-mean-square (rms) values computed in different frequency bands are indicators of the delamination depth. In fact, the vibration exhibits a higher rms value in higher frequency bands, if the defect is deeper. Following the model results, an experimental investigation by LDV has been performed on panels with known detachments. Accuracy of results has been checked by comparison with thermal tomography, which at present is one of the most used measurement techniques for monitoring the state of composite materials. Experimental results show the effectiveness of the model and the real applicability of the proposed technique. The presented methodology has proved to be efficient to determine the delamination depth. Also issues connected with structural excitation and measurement uncertainty are addressed.
Enhanced ion charge states in vacuum arc plasmas using a “current spike” method
Authors: Bugaev A. S. , Oks E. M. , Yushkov G. Yu. , Anders A. , Brown I. G.
Published: July 2002

Ion charge state distributions of vacuum arc ion sources are correlated to the arc operating voltage. An enhancement of ion charge state via an increase of the arc voltage can be achieved utilizing the transient processes that accompany an arc current spike. A current spike of 100–1000 A and several microseconds width was produced on top of the main arc current pulse (100 A, 250 μs). The ion charge state distribution was measured by charge-to-mass spectrometry. The measured charge state distributions were used as input data to the plasma model of partial local Saha equilibrium, giving the time-dependent electron temperature of the plasma at the freezing zone near the cathode spot.
Selection of the substrate metal best for thermal positive ionization
Authors: Kawano H. , Mine H. , Moriyama M. , Tanigawa M. , Zhu Y.
Published: July 2002

To select the substrate metal best for producing thermal positive ions from incident sample molecules, a beam of diatomic molecule (MX) such as LiCl, NaBr, or TlCl was directed with a constant flux (∼1012–1014 molecules cm−2 s−1) onto a polycrystalline thin wire (∼0.01–0.02 cm in diameter) of refractory metal (Nb, Mo, Ta, W, Re, Ir, or Pt) heated in vacuum (∼10−7–10−5 Torr), and the total emission current (∼10−10–10−8 A) of the positive ion (M+) was measured as a function of wire temperature (∼900–2300 K). The experimental data newly achieved were analyzed by our theoretical model to determine the ionization efficiency (β+) and also the work function (φ+) effective for the ionization. Among the seven metals, Re is concluded to be best for the ionization because Re is highest in φ+(∼7.5 eV), thereby making it possible to produce M+ efficiently (β+=1) and stably (within ±1%) from those 30 elements (M) whose ionization energy is less than ∼6.5 eV.
The interaction of slow highly charged ions on surfaces (invited)
Authors: Briand J.-P. , Giardino G. , Borsoni G. , Le Roux V. , Béchu N. , Dreuil S. , Tüske O. , Machicoane G.
Published: July 2002

The availability of highly charged ion sources (electron cyclotron resonance or electron beam ion source) led in the last decade to many new scientific discoveries in various fields of atomic, solid state, and plasma physics. This article will review some of the most exciting results obtained in the field of the interaction of highly charged ions on surfaces in fundamental physics (hollow atom properties, mechanisms of electron captures and losses above, below, or at surface interactions…) as well as in applied physics (surface modifications, lithography, etc.). The deceleration and monochromatization of the ion beams delivered by the ion sources will be discussed in the framework of their use in the study of the ion surface interactions.
A planar x-ray waveguide with a tunable air gap for the structural investigation of confined fluids
Authors: Zwanenburg M. J. , van der Veen J. F. , Ficke H. G. , Neerings H.
Published: July 2002

A tunable x-ray waveguide with an air gap as the guiding medium is presented. Discrete transverse-electric modes excited in the air gap propagate almost undisturbed. Filling the air gap with a fluid allows for studies of ordering phenomena in a confined geometry. Since the guided modes are mainly confined to the guiding layer, background scattering from the plates is very low. Starting from the propagation characteristics of the modes in the empty waveguide, requirements on the x-ray source and on the positioning accuracy of the plates are derived. The construction of the waveguide is described and measurements of the far-field angular distributions of intensity exiting the waveguide are presented which illustrate the waveguide’s properties.
Evidence from ion channeling images for the elastic relaxation of a Si0.85Ge0.15 layer grown on a patterned Si substrate
Authors: King P. J. C. , Breese M. B. H. , Smulders P. J. M. , Wilkinson A. J. , Booker G. R. , Parker E. H. C. , Grime G. W.
Published: July 2002

We demonstrate the ability of ion channeling analysis using a scanned, focused, 2 MeV proton beam from a nuclear microprobe to detect and quantify elastic relaxation in a Si1−xGex layer grown on a Si substrate. Channeling images of a sample consisting of a Si0.85Ge0.15 layer grown on a substrate patterned to produce 10 μm wide raised mesas were produced which revealed lattice plane bending of up to 0.25°, consistent with elastic relaxation of the epilayer. The channeling results are compared with those produced from electron backscattering diffraction.
Novel design of rapid single flux quantum logic based on a single layer of a high-T
c superconductor
Authors: Kaplunenko V. K. , Ivanov Z. G. , Stepantsov E. A. , Claeson T. , Holst T. , Sun Z. J. , Kromann R. , Shen Y. Q. , Vase P. , Freltoft T. , Wikborg E.
Published: July 2002

We suggest a new design of rapid single flux quantum (RSFQ) logic circuits which is based on a single superconducting layer and does not require a superconducting ground plane. Small inductances of about 10 pH, obligatory for RSFQ circuits, are formed as narrow slits of widths comparable to the London penetration depth (≂0.15 μm). The design allows us to decrease the geometric size of the RSFQ cell. Test circuits with YBaCuO grain boundary junctions on asymmetric 32° Y– ZrO2 bicrystals were used to measure the slit inductance per unit length and the mutual inductance of neighboring slits. A typical inductance of a 0.4 μm slit was found to be 0.7–0.8 pH/μm.
Cathodoluminescence and photoluminescence analysis of InxGa1−xAs/GaAs quantum well structures
Authors: Evoy S. , Redinbo G. F. , Craighead H. G.
Published: July 2002

Cathodoluminescence (CL) and continuous wave photoluminescence (cw-PL) were used to study carrier recombination in the InxGa1−xAs/GaAs system for x=0.1 and x=0.2. Ambipolar diffusion length values of LD=0.9±0.1 μm and of LD=0.5±0.1 μm are measured for x=0.1 and x=0.2, respectively, at 16 K. The CL analysis of dry-etched structures reveals a surface recombination velocity of S=107 cm/s for both material systems. A similar cw-PL study shows a dependence on laser excitation density, suggesting the saturation of sidewall traps when higher excitation densities are involved.
Optical behavior of alpha-C:N films
Authors: Wang X. , Martin P. J.
Published: July 2002

The refractive index n, extinction coefficient k, and optical band gap Eopt of nitrogen incorporated amorphous carbon films α-C:N have been investigated by spectroscopic ellipsometry. The effect of the incorporated nitrogen on the bonding configuration of carbon atoms has been discussed in term of the optical behavior of the films. The results show that nitrogen incorporation into the films stabilizes the tetrahedral bonds of carbon atoms, resulting in lower values of both n and k. However, if the nitrogen incorporation is accomplished by means of excessive N+ ion bombardment, the formed α-C:N films possess zero gap and are more ‘‘graphitelike.’’
Stable optical storage and high-order diffraction in a liquid-crystal polymer film by two-wave mixing
Authors: Wei X. , Yan X. Z. , Zhu D. R. , Mo D. , Liang Z. X. , Lin W. Z.
Published: July 2002

Optical storage with long-term stability and up to third-order diffraction have been observed in a liquid-crystal polymer film by two-wave mixing. After turning off the pump beam, the diffraction of the probe beam remains. While the probe beam is turned off, the intensities of the diffraction beams of the pump beam does not decrease significantly. A brilliant hologram spot is observed on the film during the experiments and the trace of the spot can be maintained over 120 days without obvious change.
Stable and efficient electroluminescence from a porous silicon-based bipolar device
Authors: Tsybeskov L. , Duttagupta S. P. , Hirschman K. D. , Fauchet P. M.
Published: July 2002

A complete process compatible with conventional Si technology has been developed in order to produce a bipolar light-emitting device. This device consists of a layer of light-emitting porous silicon annealed at high temperature (800–900 °C) sandwiched between a p-type Si wafer and a highly doped (n+) polycrystalline Si film. The properties of the electroluminescence (EL) strongly depend on the annealing conditions. Under direct bias, EL is detected at voltages of ∼2 V and current densities J∼1 mA/cm2. The maximum EL intensity is 1 mW/cm2 and the EL can be modulated by a square wave current pulse with frequencies ν≥1 MHz. No degradation has been observed during 1 month of pulsed operation.
Direct observation of vibrational modes in Hg1−xCdxTe
Authors: Li Biao , Chu J. H. , Ye H. J. , Guo S. P. , Jiang W. , Tang D. Y.
Published: July 2002

The vibrational modes in Hg1−xCdxTe epitaxial films and HgTe/CdTe superlattices have been investigated by means of far-infrared (FIR) transmittance spectra at temperatures from 4.2 to 300 K. The observations were put forward in the spectral range from 20 to 350 cm−1, emphasizing the wave-number region at the low-frequency side of the reststrahlen absorption band. A series of single-phonon and two-phonon modes as well as impurities- and defects-induced vibrational modes are observed. FIR transmission seems to be a good probe for characterization of the perfection of Hg1−xCdxTe films.
Investigations of vertical cavity surface emitting laser by photoreflectance spectroscopy
Authors: Berger P. D. , Bru C. , Benyattou T. , Guillot G. , Chenevas-Paule A. , Couturier L. , Grosse P.
Published: July 2002

We report photomodulated reflectance results on vertical cavity surface emitting laser structures. Photoreflectance spectra have been recorded under normal incidence at different temperatures between 9 and 300 K. The structure used is a λ cavity grown in the AlGaAs-based system emitting at a wavelength near 800 nm. We show that photoreflectance is a unique noninvasive tool to measure accurately the quantum well transition and the cavity mode alignment: both features can be distinguished very well. Furthermore, this technique offers the opportunity to determine the electric field within the undoped region from Franz–Keldysh oscillations, and gives the Al composition of the barrier material in the cavity.
Optical gain and lasing in self-assembled InP/GaInP quantum dots
Authors: Moritz A. , Wirth R. , Hangleiter A. , Kurtenbach A. , Eberl K.
Published: July 2002

Optical gain measurements have been performed on self-assembled InP quantum dots embedded in GaInP. The dots are formed during the growth of InP on GaInP by molecular beam epitaxy due to the strong lattice mismatch and are overgrown with GaInP afterwards. The optical gain spectra show two peaks which could be clearly identified by their polarization properties as being due to the quantum dots and the wetting layer, respectively. As expected from the results of the gain measurements, optical pumping of cleaved samples at room temperature leads to lasing of either the quantum dots or the wetting layer, depending on the experimental conditions.
Passivation of GaAs surface by sulfur glow discharge
Authors: Hou Xiaoyuan , Chen Xiying , Li Zheshen , Ding Xunmin , Wang Xun
Published: July 2002

A new sulfur passivation technique, the sulfur vapor glow discharge, has been developed to form a thick sulfide layer on GaAs surface. By using Auger electron spectroscopy and x-ray photoelectron spectroscopy measurements, the main composition of the passivation layer is found to be gallium sulfide without the existence of unstable As–S bonds. The stability of the passivation effect is demonstrated by the nondecaying behavior of the photoluminescence intensity of the GaAs passivation surface under the illumination of the laser beam with very high intensity.
Optical near-field photocurrent spectroscopy: A new technique for analyzing microscopic aging processes in optoelectronic devices
Authors: Richter A. , Tomm J. W. , Lienau Ch. , Luft J.
Published: July 2002

The potential of optical near-field photocurrent spectroscopy for analyzing microscopic aging processes in optoelectronic devices is demonstrated. The technique combines the subwavelength spatial resolution of near-field optics with tunable laser excitation, allowing for selective investigation of specific parts of the device structure. Experiments on GaAs/(AlGa)As high power laser diodes before and after accelerated aging provide direct visualization of defect growth within the p-i-n junction and information on aging-enhanced recombination processes close to the laser facet. The effect of wave guiding of the exciting light on the image formation is discussed. The nondestructiveness makes this technique a particularly attractive method for in situ analysis in high power laser diodes.
Investigation of the temperature dependence of the field emission current of polycrystalline diamond films
Authors: Glesener J. W. , Morrish A. A.
Published: July 2002

This letter reports on the measurement of the field emission current dependence of doped polycrystalline diamond (PCD) films on temperature. The motivation for this type of measurement was to assess the thermal stability of PCD field emitters and gain some insight into a possible emission mechanism. Between room temperature and 300 °C, for a fixed electric field, the emission current from B doped films was found to remain constant. Heavily doped nitrogen films were also examined and the emission current was found to increase exponentially with an activation energy of 0.16 eV.
Ballistic electron emission microscopy in liquid helium using low dimensional collector electrodes
Authors: Eder C. , Smoliner J. , Strasser G. , Gornik E.
Published: July 2002

We report ballistic electron emission microscopy (BEEM) studies which show that a GaAs/AlGaAs heterostructure provides a very efficient collector electrode at 4.2 K. The enhanced thermal resolution at low temperature is utilized to determine the BEEM current threshold behavior very accurately. We find that diffusive scattering processes dominate the carrier transport across the Au/GaAs interface. Quantum wire regions on laterally structured samples are clearly identified with BEEM. The BEEM current is strongly enhanced on top of the quantum wires.
Luminescence, absorption, and site symmetry of Ce activated SrGa2S4 phosphors
Authors: Warren W. L. , Vanheusden K. , Rodriguez M. A. , Seager C. H. , Tallant D. R. , Rack P. D. , Holloway P. H. , Wagner B. K. , Summers C. J. , Yocom P. N.
Published: July 2002

Ce activated SrGa2S4 blue emitting phosphors were examined using electron paramagnetic resonance and x-ray diffraction to determine the local environment and oxidation state of the Ce ion. Photothermal deflection and photoluminescence spectroscopies were applied to determine absorption and emission characteristics. It is found that the majority of the Ce3+ ions substitute for the eightfold coordinated antiprismatic Sr2+ cations in the structure. Our results show that the Ce ions exhibit C3h symmetry; this suggests that they are ninefold coordinated. This coordination may arise from a nearby S interstitial, energetically favorable because of charge compensation. To achieve this symmetry, only minor rearrangements of the eight S atoms in the antiprismatic configuration are required.
Determination of the activation energy for the heterogeneous nucleation of misfit dislocations in Si1−xGex/Si deposited by selective epitaxy
Authors: Wickenhäuser S. , Vescan L. , Schmidt K. , Lüth H.
Published: July 2002

Si 0.84 Ge 0.16 /Si heterostructures with variable finite lateral dimensions (10–300 μm) and different layer thicknesses grown by selective low pressure chemical vapor deposition epitaxy at a temperature of 700 °C were investigated with regard to relaxation by formation of misfit dislocations. While in small structures only nucleation and propagation occur, the dislocation–dislocation interaction (mainly multiplication) becomes more and more important in larger structures. Therefore it was possible to separate the three different mechanisms which play a role in relaxation, i.e., nucleation, propagation, and multiplication, and to study them independently. From the analysis of the misfit dislocations at the initial stage of relaxation it was possible to determine the nucleation site density and an activation energy of 2.8 eV for the heterogeneous nucleation of misfit dislocations.
Growth of CdTe/MnTe tilted and serpentine lattices on vicinal surfaces
Authors: Hartmann J. M. , Charleux M. , Rouvière J. L. , Mariette H.
Published: July 2002

We have grown by atomic layer epitaxy CdTe/MnTe tilted and serpentine superlattices. These heterostructures are formed by depositing in the step-flow growth mode fractional monolayer superlattices (CdTe)m(MnTe)n, with p=m+n∼1, onto 2 °A and 2 °B Cd0.95Zn0.05Te vicinal substrates. Transmission electron microscopy images reveal a good in-plane CdTe/MnTe separation and a uniform short-range superlattice period. The very existence of those superlattices imply that Te-based vicinal surfaces present a regular array of monomolecular steps, with no important step meandering and no step bunching.
Morphology of epitaxial TiN(001) grown by magnetron sputtering
Authors: Karr Brian W. , Petrov I. , Cahill David G. , Greene J. E.
Published: July 2002

The evolution of surface morphology and microstructure during growth of single crystal TiN(001) is characterized by in situ scanning tunneling microscopy and postdeposition plan-view transmission electron microscopy. The TiN layers are grown on MgO at 650<T<750 °C using reactive magnetron sputter deposition in pure N2. The surface morphology is dominated by growth mounds with an aspect ratio of ≃0.006; both the roughness amplitude and average separation between mounds approximately follow a power law dependence on film thickness, tα, with α=0.25±0.07. Island edges show dendritic geometries characteristic of limited step-edge mobility at the growth temperature.
Control of the growth and domain structure of epitaxial SrRuO3 thin films by vicinal (001) SrTiO3 substrates
Authors: Gan Q. , Rao R. A. , Eom C. B.
Published: July 2002

We report the effect of both miscut angle (α) and miscut direction (β) of vicinal substrates on the epitaxial growth and domain structure of isotropic metallic oxide SrRuO3 thin films. The thin films have been grown on vicinal (001) SrTiO3 substrates with α up to 4.1° and β up to 37° away from the in-plane [010] axis. Single-crystal epitaxial (110)o SrRuO3 thin films were obtained on vicinal SrTiO3 substrates with a large miscut angle (α=1.9°, 2.1°, and 4.1°) and miscut direction close to the [010] axis. Decreasing the substrate miscut angle or aligning the miscut direction close to the [110] axis (β=45°) resulted in an increase of 90° domains in the plane. The films grown on vicinal substrates displayed a significant improvement in crystalline quality and in-plane epitaxial alignment as compared to the films grown on exact (001) SrTiO3 substrates. Atomic force microscopy revealed that the growth mechanism changed from two-dimensional nucleation to step flow growth as the miscut angle increased.
Mechanically detected nuclear magnetic resonance image of a multilayer system at normal pressure
Authors: Schaff Arnd , Veeman Wiebren S.
Published: July 2002

Magnetic resonance force microscopy (MRFM) is a new microscopy technique that combines the spin selective detection of nuclear or electron spin resonance with the high spatial resolution of atomic force microscopy. In this letter we present MRFM measurements on a three-layer system in the micron range. We detected the one-dimensional distribution of proton spins in a sample that consisted of two-ammonium sulphate layers and an intermediate layer of sodium chloride. The experiments were done at room temperature and normal pressure, in contrast to earlier MRFM measurements that have been carried out in vacuum at room temperature or low temperatures.
Finite-element modelling of magnetostrictive bending of a coated cantilever
Authors: Watts R. , Gibbs M. R. J. , Karl W. J. , Szymczak H.
Published: July 2002

Finite element modelling was applied to the bending behaviour of a cantilever coated with a magnetostrictive film. The model is verified by comparison with accepted analytical solutions, then extended to the more realistic case in which one end of the cantilever is rigidly clamped across the width. It is shown that the central deflection of the end of the cantilever is a function of the geometric aspect ratio. An empirical correction factor is suggested to account for this variation.
Growth of single phase GaAs1−xNx with high nitrogen concentration by metal–organic molecular beam epitaxy
Authors: Qiu Y. , Nikishin S. A. , Temkin H. , Faleev N. N. , Kudriavtsev Yu. A.
Published: July 2002

High quality layers of GaAs1−xNx were grown on (001)GaAs by metal–organic molecular beam epitaxy. The growth conditions, and especially the nitrogen to arsenic flux ratio, were carefully explored to assure epitaxial crystal growth. We show well behaved and reproducible growth of single phase GaAs1−xNx with the GaN mole fraction as high as x=0.10. The nitrogen content of epitaxial layers was determined directly by secondary ion mass spectroscopy and high resolution x-ray diffraction.
Orientation-controlled organic electroluminescence of p-sexiphenyl films
Authors: Yanagi H. , Okamoto S.
Published: July 2002

A multilayered electroluminescent device was constructed with epitaxially oriented films of p-sexiphenyl with its molecular axis lying or standing next to the substrate surface. The films with the lying and standing orientations were prepared by vapor deposition onto the KCl (001) surface kept at 20 and 150 °C, respectively. After successive depositions of electron-transport layer and Al cathode, the films were removed from the KCl substrate and transferred on an indium tin oxide coated glass anode. The cell with lying molecules emitted a higher electroluminescence with a narrowed spectrum at remarkably low driving voltages, as compared to one with standing molecules. This electroluminescent behaviors depend upon anisotropic distribution of the polarized emission light as well as efficiency of the carrier transport in the ordered molecular arrays with different orientation modes.
Optical investigation of growth mode of Ge thin films on Si(110) substrates
Authors: Arai J. , Ohga A. , Hattori T. , Usami N. , Shiraki Y.
Published: July 2002

A unique growth mode of Ge on Si(110) substrates was clarified by photoluminescence (PL) spectroscopy. A spectral redshift and an increase of the relative no-phonon intensity were found for PL from the two-dimensional Ge layer on Si(110) compared to that on Si(100). These results likely arise from nonuniformity in the Ge layer thickness owing to the step-bunched Si(110) surface and resultant exciton localization. The two-dimensional to three-dimensional growth mode changeover was observed as evidenced by emergence of broad PL from Ge islands. In contrast to Ge on Si(100) PL from the wetting layer was found to show continuous redshift with increasing Ge coverage even after Ge island formation.
Experimental observation of conductance transients in Al/SiNx:H/Si metal-insulator-semiconductor structures
Authors: Dueñas S. , Pelaez R. , Castan E. , Pinacho R. , Quintanilla L. , Barbolla J. , Martil I. , Gonzalez-Diaz G.
Published: July 2002

Room temperature conductance transients in the SiNx:H/Si interface are reported. Silicon nitride thin films were directly deposited on silicon by the low temperature electron-cyclotron-resonance plasma method. The shape of the conductance transients varies with the frequency at which they are obtained. This behavior is explained in terms of a disorder-induced gap-state continuum model for the interfacial defects. A perfect agreement between experiment and theory is obtained proving the validity of the model.
The persistent photoconductivity effect in modulation Si δ-doped pseudomorphic In0.2Ga0.8As/GaAs quantum well structure
Authors: Babinski Adam , Li G. , Jagadish C.
Published: July 2002

Persistent conductivity effect in modulation Si δ-doped In0.2Ga0.8As/GaAs quantum well (QW) structure grown by metal organic vapor phase epitaxy was examined using Hall effect and magnetotransport measurements in magnetic fields up to 12 T at T=1.7 K. No measurable electron density was found in the QW after cooling down the sample in the dark and the electron density in the V-shaped δ-doped potential well (V-QW) of the modulation Si δ-doped layer was two times lower than the electron density of the same Si δ-doped layer in GaAs. The illumination resulted in the increase of electron density in the V-QW at the beginning and consequently in the population of the ground subband in the InGaAs QW. Due to parallel conduction, a nonmonotonic dependence of Hall density as a function of illumination time was observed. The total electron density in the modulation doped InGaAs/GaAs heterostructure after the illumination became approximately equal to the electron density in the Si δ-doped layer in GaAs.
Enhancement of optical gain of semiconductors embedded in three-dimensional photonic crystals
Authors: Vlasov Yu. A. , Luterova K. , Pelant I. , Hönerlage B. , Astratov V. N.
Published: July 2002

The three-dimensional photonic crystals used in this study were synthetic opals, composed of submicron silica spheres, close-packed in a face-centered cubic structure with a period of 200 nm, that exhibit photonic stopbands around 600 nm. We present measurements of the optical gain of CdS quantum dots (QDs) embedded inside the interstitials between the silica spheres. Unlike the usual gain spectra of CdS QDs in glass matrices, which display maximum gain at energies of the first quantum-confined transitions, for QDs embedded in photonic crystals the gain maximum is shifted toward the high-frequency edge of the photonic stopband (2.2 eV) far below the absorption edge of the semiconductor (2.5 eV). Studies of temperature, intensity, and orientation dependencies of the gain spectra allow one to ascribe the observed effect to gain enhancement caused by multiple coherent Bragg scattering of light in the periodic photonic crystal.
Intraband absorption in n-doped InAs/GaAs quantum dots
Authors: Sauvage S. , Boucaud P. , Julien F. H. , Gérard J.-M. , Thierry-Mieg V.
Published: July 2002

We have investigated the intraband absorption within the conduction band of InAs/GaAs quantum dots. The islands obtained by self-organized epitaxy are modulation doped with a silicon planar doping 2 nm below the dot layer plane. The dots exhibit infrared absorption polarized along the growth axis in the midinfrared spectral range. The absorption is maximum around 150 meV with a large broadening around 130 meV. This broadening is attributed to size fluctuations within the one dot layer plane and the consequent variation of the electron confinement energy with the dot size. The magnitude of the absorption along the growth axis for the one dot layer plane is ≈2.5×10−2% which corresponds to an equivalent absorption cross section σz≈3.1×10−15 cm2. We show that the intraband absorption can also be clearly observed using a photoinduced infrared absorption technique with the doped quantum dots.
Current carrying capability of multifilamentary (BiPb)2Sr2Ca2Cu3Ox tapes determined from transport and magnetization measurements
Authors: Reimann C. , Waldmann O. , Müller P. , Leghissa M. , Roas B.
Published: July 2002

The magnetic field dependence of the critical current IC(B) and the remanent magnetization of several multifilamentary (BiPb)2Sr2Ca2Cu3Ox tapes were measured. The low field behavior of IC(B) is interpreted by the self-field effect, whereas at higher fields, the field dependence for all tapes was found to be identical except for a constant factor. Although self-field transport critical current densities jC0 differed widely for the tapes, the intergranular critical current densities determined from remanent magnetization were found to be equal. The results indicate that the reduction of jC0 by macroscopic defects is an important factor, whereas locally the filaments are of high quality.
Magnetic tunnel junctions with in situ naturally-oxidized tunnel barrier
Authors: Tsuge H. , Mitsuzuka T.
Published: July 2002

Al/Fe/Al 2 O 3 /CoFe/Al junctions with dimensions of 2×2–40×40 μm2 were prepared on a 2 in. Si wafer using conventional photolithography and ion-beam etching. The junction trilayers were deposited sequentially without breaking vacuum and a tunnel barrier was in situ naturally oxidized. The resultant junction resistance scaled linearly with the junction area over all dimensions used. Normalized resistance of less than 1.5×10−5 Ω cm2 was obtained in maintaining a magnetoresistance (MR) ratio of about 5%. The resistance values are much smaller than ever reported and close to those required for an MR head device. The MR ratio exhibited no significant change up to at least 1×103 A/cm2 with increasing junction current density.
An optical method for studying carrier diffusion in strained (InP)2/(GaP)2 quantum wires
Authors: Tang Y. , Rich D. H. , Moy A. M. , Cheng K. Y.
Published: July 2002

The carrier transport in strain-induced laterally ordered (InP)2/(GaP)2 quantum wire (QWR) samples was examined with a noncontact Haynes–Shockley diffusion measurement which utilized time-resolved scanning cathodoluminescence. An anisotropy in ambipolar diffusion along the [110] and [11̄0] directions (perpendicular and parallel to the QWRs, respectively) was observed. The temperature dependence of this anisotropy was measured, revealing that carrier diffusion along the QWR direction is thermally activated.
Trapping and recombination dynamics of low-temperature-grown InGaAs/InAlAs multiple quantum wells
Authors: Chen Yue , Prabhu S. S. , Ralph Stephen E. , McInturff Dave T.
Published: July 2002

We have observed a long-lived residual photoconductivity in low-temperature-grown (LT) InGaAs. These results have significant consequences for devices comprised of LT-InGaAs, other defect moderated materials, and standard-temperature-grown InGaAs. Our investigation utilizes time-resolved terahertz conductivity to quantify the trapping and recombination rates of LT Be-doped In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells and bulk InGaAs. It is found that Be doping reduces the residual photoconductivity and increases the initial electron trapping rate. These results are in contrast to those observed via transient absorption studies, which suggest that these systems have returned to equilibrium after the initial transient. Furthermore, a 600 °C anneal increases both the trapping and recombination rate in all Be-doped samples.
Structure and polarization in epitaxial ferroelectric PbZr0.52Ti0.48O3/YBa2Cu3O7−x/Nd:YAlO3 thin films
Authors: Grishin A. M. , Yamazato M. , Yamagata Y. , Ebihara K.
Published: July 2002

We fabricate epitaxial PbZr0.52Ti0.48O3/YBa2Cu3O7−x submicron film ferroelectric/superconductor heterostructures on the single-crystal YAlO3+1%Nd2O3 substrate by the pulsed laser deposition technique. Frequency independent low loss tan δ=0.04 and dielectric constant of 950, high electric resistivity ρ (150 kV/cm)=6×1011Ω cm, remnant polarization of 32 μC/cm2, no visible fatigue after 107 short bipolar pulses switching indicate excellent electrical performance of the new capacitor structure. The slight crystallite polar axis misalignment and depolarizing effect were found to be responsible for the shape of the apparent polarization loop. The only fitting parameter depolarizing coefficient N=2.37×10−4 gives the best fit between theory and experimental data and corresponds to prolate ellipsoidal shaped crystallites with the length-to-diameter ratio of 140.
Optical retardation of rub-induced scratches in a polyimide-treated substrate
Authors: Kim Jong-Hyun , Rosenblatt Charles
Published: July 2002

Optical retardation measurements were performed on rubbed polyimide-coated substrates used for liquid-crystal cells. Based on measurements with and without a refractive index matching fluid, the retardation component due to the rub-induced scratches was extracted, and found to dominate the total retardation at higher rubbing strengths. Additionally, atomic force microscope measurements were performed to examine the rub-induced surface topography. The retardation calculated from the topography measurements was found to be in agreement with the optical results.
Device quality submicron arrays of stacked sidewall quantum wires on patterned GaAs (311)A substrates
Authors: Nötzel Richard , Jahn Uwe , Niu Zhichuan , Trampert Achim , Fricke Jörg , Schönherr Hans-Peter , Kurth Thomas , Heitmann Detlef , Däweritz Lutz , Ploog Klaus H.
Published: July 2002

Three-dimensional arrays of vertically stacked sidewall quantum wires are fabricated by molecular beam epitaxy on GaAs (311)A substrates patterned with 500-nm-pitch gratings. The cathodoluminescence spectra at low temperature are dominated by the emission from the quantum wires with narrow linewidth accompanied by a very weak emission from the connecting thin quantum wells due to localization of excitons at random interface fluctuations. When the carriers in the quantum well become delocalized at elevated temperature, only the strong emission from the quantum-wire array is observed revealing perfect carrier capture into the quantum wires without detectable thermal repopulation of the quantum well up to room temperature. Thus, unpreceded device quality of this quantum-wire structure is demonstrated.
Nanosize diamond formation promoted by direct current glow discharge process: Synchrotron radiation and high resolution electron microscopy studies
Authors: Gouzman I. , Hoffman A. , Comtet G. , Hellner L. , Dujardin G. , Petravic M.
Published: July 2002

Diamond nucleation on Si(100) surfaces can be promoted by a dc-glow discharge process, using a CH4/H2 gas mixture. However, the phase composition and structure of the carbon film deposited during the dc-glow discharge pretreatment are still unclear. In the present work, we report on a combined study of near edge x-ray absorption fine structure (NEXAFS), and high resolution scanning electron microscopy (HR-SEM) of this film as a function of substrate temperature. NEXAFS measurements of the films deposited by the dc-glow discharge process render unambiguous evidence of diamond phase formation in the 880–900 °C substrate temperature range. It is determined from HR-SEM measurements that in this temperature range, nanosize diamond particles are formed. At lower and higher substrate temperatures the NEXAFS results indicate the predominant formation of graphitic carbon. The changes in the film composition as a function of substrate temperature during the dc-glow discharge process is expressed in terms of relative graphitic character (RGC) of the precursor film.
Improvement of Ta diffusion barrier performance in Cu metallization by insertion of a thin Zr layer into Ta film
Authors: Kwak Joon Seop , Baik Hong Koo , Kim Jong-Hoon , Lee Sung-Man
Published: July 2002

In order to increase the failure temperature of Ta diffusion barrier for Cu, we investigated the effect of insertion of a thin Zr layer into Ta film with/without ion bombardment on Ta diffusion barrier performance in Cu metallization. The insertion of a thin Zr layer into Ta film improved barrier properties significantly when the Ta/Zr/Ta barrier layers were deposited with concurrent ion bombardment. The significant improvement of Ta diffusion barrier properties by insertion of a thin Zr layer into Ta film with ion bombardment was attributed to the densification of grain boundaries in Ta/Zr/Ta films and the formation of an intermixing layer between Ta and Zr by ion bombardment, followed by the reduction of fast diffusion of Cu through Ta/Zr/Ta films.
Photon generation by silicon diodes in avalanche breakdown
Authors: Akil N. , Kerns S. E. , Kerns D. V. , Hoffmann A. , Charles J-P.
Published: July 2002

Light emission from a p-n diode biased in controlled avalanche breakdown has been measured over the photon energy range 1.4–3.4 eV. Previously published models are compared with measured data to associate specific mechanisms with avalanche photon emission in silicon. A multimechanism model fitting the measured spectrum is presented.
Two-dimensional electron-gas density in AlXGa1−XN/GaN heterostructure field-effect transistors
Authors: Maeda Narihiko , Nishida Toshio , Kobayashi Naoki , Tomizawa Masaaki
Published: July 2002

We have calculated maximum two-dimensional electron-gas densities in AlXGa1−XN/GaN heterostructure field-effect transistors with wurtzite crystal structures in (0001) orientation, by self-consistently solving Schrödinger’s and Poisson’s equations, taking the piezoelectric effect into account. In order to obtain a guideline for increasing electron densities in the devices, we have examined dependences of the maximum electron densities on both Al compositions of AlXGa1−XN layers and lattice relaxations at the heterointerfaces. The maximum electron density was found to depend more strongly on the lattice relaxation than on the Al composition, which determines the conduction-band discontinuity. Controlling the lattice relaxation is shown to be crucial for obtaining high electron densities in the devices.
Photovoltage and photoreflectance spectroscopy of InAs/GaAs self-organized quantum dots
Authors: Sun B. Q. , Lu Z. D. , Jiang D. S. , Wu J. Q. , Xu Z. Y. , Wang Y. Q. , Wang J. N. , Ge W. K.
Published: July 2002

We present a detailed study of the interband excitonic transitions of InAs/GaAs self-organized quantum dots (QDs) based on photovoltage (PV), photoreflectance (PR) and photoluminescence (PL) spectroscopy. At room temperature, the interband absorption transitions of QDs have been observed by using PV spectrum, which clearly exhibits four well-resolved excitonic absorption peaks. The absorption line shape is Gaussian-like. Furthermore, the corresponding excitonic transitions are also observed in PR experiment at 77 K. The first derivative of a Gaussian profile can fit the experimental data well.
Force-detected magnetic resonance in a field gradient of 250 000 Tesla per meter
Authors: Bruland K. J. , Dougherty W. M. , Garbini J. L. , Sidles J. A. , Chao S. H.
Published: July 2002

We report the detection of slice-selective electron spin resonance with an external magnetic field gradient comparable to local interatomic gradients, using the techniques of magnetic resonance force microscopy. An applied microwave field modulated the spin-gradient force between a paramagnetic DPPH sample and a micrometer-scale ferromagnetic tip on a force microscope cantilever. A sensitivity equivalent to 184 polarized electron moments in a one-Hertz detection bandwidth was attained. We mapped the tip magnetic field with a resonant slice thickness of order one nanometer, thereby demonstrating magnetic resonance on length scales comparable to molecular dimensions.
Zeeman splitting of shallow donors in GaN
Authors: Mireles Francisco , Ulloa Sergio E.
Published: July 2002

The Zeeman splitting of the donor spectra in cubic and hexagonal GaN is studied using an effective mass theory approach. Soft-core pseudopotentials were used to describe the chemical shift of the different substitutional dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with typically 1 meV higher binding in the hexagonal phase. Carbon is found to produce the largest donor binding energy. The ionization levels and excited states are in excellent agreement with Hall and optical measurements, and suggest the presence of residual C in recent experiments.
High-temperature continuous-wave operation of λ∼8 μm quantum cascade lasers
Authors: Slivken S. , Matlis A. , Jelen C. , Rybaltowski A. , Diaz J. , Razeghi M.
Published: July 2002

We report single-mode continuous-wave operation of a λ∼8 μm quantum cascade laser at 140 K. The threshold current density is 4.2 kA/cm2 at 300 K in pulsed mode and 2.5 kA/cm2 at 140 K in continuous wave for 2 mm long index-guided laser cavities of 20 μm width. Wide stripe (W∼100 μm), index-guided lasers from the same wafer in pulsed operation demonstrate an average T0 of 210 K with other wafers demonstrating a T0 as high as 290 K for temperatures from 80 to 300 K. This improvement in high-temperature performance is a direct result of three factors: excellent material quality, a low-loss waveguide design, and a low-leakage index-guided laser geometry.
Visible cathodoluminescence of GaN doped with Dy, Er, and Tm
Authors: Lozykowski H. J. , Jadwisienczak W. M. , Brown I.
Published: July 2002

We reported the observation of visible cathodoluminescence of rare-earth Dy, Er, and Tm implanted in GaN. The implanted samples were given isochronal thermal annealing treatments at a temperature of 1100 °C in N2 or NH3, at atmospheric pressure to recover implantation damages and activated the rare-earth ions. The sharp characteristic emission lines corresponding to Dy3+, Er3+, and Tm3+ intra-4fn-shell transitions are resolved in the spectral range from 380 to 1000 nm, and observed over the temperature range of 8.5–411 K. The cathodoluminescence emission is only weakly temperature dependent. The results indicate that rare-earth-doped GaN epilayers are suitable as a material for visible optoelectronic devices.
Intersubband electroluminescence in InAs/GaSb/AlSb type-II cascade structures
Authors: Ohtani K. , Ohno H.
Published: July 2002

Intersubband electroluminescence in InAs quantum wells embedded in InAs/GaSb/AlSb type-II cascade structures is reported. The observed emission energy is in good agreement with calculation based on the multiband k⋅p theory. Dominant polarization of the emitted light is perpendicular to the quantum well layers. Difference in the spectrum shape between intersubband and interband cascade transitions is also presented.