bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of in situ ionizing radiation effects on Raman and photoluminescence intensity of high OH, low OH silica, and fluoride core fibers
Authors: Bilodeau T. G. , Ewing K. J. , Nau G. M. , Aggarwal I. D.
Published: July 2002

An in situ study of the effects of ionizing radiation on the strength of the Raman and photoluminescence signal of high OH, low OH, and fluoride core fibers has been performed with 514.5 nm laser excitation. The fibers were irradiated with a 60Co source at a constant dose rate of 560 rads/h. The high OH fiber displayed a much slower decay of the fiber Raman intensity than the other two fibers during irradiation. The fluoride fiber exhibited the quickest decline in Raman signal with the intensity dropping by a factor of 1000 in less than 20 min. The Raman intensity of the low OH silica fiber recovered to greater than 90% of its pre-irradiation value after a post-irradiation photoanneal with 488 nm laser light. The silica fibers displayed an increase in intensity of a broad photoluminescence feature centered at 650 nm. However the fiber photoluminescence intensity remained much weaker than the Raman intensity throughout the irradiations.
Characteristics of surface acoustic wave propagation in III-V semiconductor quantum well structures
Authors: Thompson C. , Weiss B. L.
Published: July 2002

The propagation of surface acoustic waves (SAWs) in AlGaAs/GaAs and InGaAs/InP quantum well structures is modeled using a Laguerre polynomial approach to determine the effects of the quantum well parameters (layer thickness, layer composition, and number of layers) on the induced potential, electric field, particle displacement, and strain. These characteristics show that variations of the electric field within regions of the structure of interest for device applications can be brought about, and are of use for the design of devices based on SAW interactions in quantum well structures.
Calculations of high-frequency response of two-dimensional hot electrons in GaAs quantum wells
Authors: Sarkar S. K. , Ghosh P. K. , Chattopadhyay D.
Published: July 2002

Small-signal ac mobility of two-dimensional (2D) degenerate hot electrons in a GaAs square quantum well is studied on a heated drifted Fermi–Dirac distribution function for the carriers. Carrier energy loss via longitudinal optic (LO) phonons, and the momenta losses via LO, acoustic and ionized impurity scatterings are considered. The ac mobility is found to drop off with frequency beyond 100 GHz although the drift velocity starts to lag behind the applied field at a lower frequency. The ac mobility μac and the phase angle φ increase with the rise of the channel width and the 2D carrier concentration. However, the 3-dB cut-off frequency f3 dB is found to decrease with increasing channel width. The values of μac and f3dB are higher but those of φ are lower at 300 K than that at 77 K.
Modeling the electrical characteristics of n-channel 6H–SiC junction-field-effect transistors as a function of temperature
Authors: McLean F. B. , Tipton C. W. , McGarrity J. M. , Scozzie C. J.
Published: July 2002

The electrical characteristics of buried-gate, n-channel junction-field-effect transistors (JFETs) fabricated in epitaxial layers grown on 6H–SiC wafers have been measured as a function of temperature, from 218 to 773 K (−55 to 500 °C). The data are in good agreement with predictions of a model that uses standard abrupt-junction, long-channel JFET device equations for which the carrier concentration is calculated based on a two-level ionization structure for the nitrogen donor. An inverse power-law dependence of carrier mobility on temperature is assumed based on recent measurements of Hall mobility in epitaxial films of comparable doping. The only free parameter of the model is the compensation density, which is chosen by fitting the calculated saturated drain current to the measured value at room temperature. There are some deviations between the calculated and measured I–V characteristics at both temperature extremes (218 and 773 K), which are attributed to increased substrate resistivity at 218 K and to increased gate leakage current at 773 K.
Poole–Frenkel conduction in polycrystalline diamond
Authors: Gonon P. , Boiko Y. , Prawer S. , Jamieson D.
Published: July 2002

High-field electrical conduction has been studied in undoped polycrystalline diamond over a wide temperature range. The current increases exponentially with the electric field with an exponential factor which increases linearly with the inverse of temperature. The activation energy of the conductivity is found to be strongly field dependent and to decrease linearly with the electric field. The experimental data support a Poole–Frenkel conduction with overlapping centers. The centers are found to be located at around 1.1 eV from the band edge with a density of about 2×1017 cm−3.
Magnetic force microscopy study of submicron track width recording in thin-film media
Authors: Luo Yansheng , Lam Terence T. , Zhu Jian-Gang , Tong Hua-Ching , Rottmayer Robert
Published: July 2002

The magnetic force microscopy (MFM) technique is used to investigate the writing properties of a set of thin-film heads with track widths ranging from 2 to 0.5 μm. MFM images show that track edge percolation occurs at lower densities than on-track intertransition percolation. Track edge percolation results in track edge fluctuations and effective track width reduction. As the head track width is reduced to the near-micron or submicron ranges, the track edges become dominant portions of the track and consequently cause severe degradation of the recording tracks. Track edge percolation is caused by a poor edge field gradient and is possibly enhanced by pole tip corner saturation. In order to achieve high-density narrow track recording, high moment writing heads become necessary.
Magnetic and transport properties of Na doped SrRuO3 and CaRuO3
Authors: Shepard M. , Cao G. , McCall S. , Freibert F. , Crow J. E.
Published: July 2002

The magnetic susceptibility χ(T) (2≤T≤400 K) and electrical resistivity ρ(T) (2≤T≤300 K) of undoped and Na doped SrRuO3 and CaRuO3 single crystals were measured. These two perovskites are orthorhombic with space group Pbnm. While both of the undoped systems exhibit metallic conductivity, SrRuO3 is ferromagnetic with Tc≤160 K, but CaRuO3 is paramagnetic. When a small concentration of Na (≤12%) replaces Sr in the structure, SrRuO3 becomes an insulator with a depressed ferromagnetic response. In contrast, Na doped CaRuO3 (≤5%) remains metallic with the appearance of antiferromagnetism below 70 K. The dramatic changes in magnetic and transport properties resulting from Na replacement of Sr and Ca reflects the very subtle differences driving the magnetism in SrRuO3 and CaRuO3.
Influence of Al capping layers on growth, topography, and magnetic properties on MnBi thin films
Authors: Rüdiger U. , Fumagalli P. , Dworak P. , Schirmeisen A. , Güntherodt G.
Published: July 2002

Bi/Mn bilayers have been deposited on fused-quartz substrates at room temperature and annealed at elevated temperatures of 300–380 °C. To investigate the influence of capping layers on MnBi crystallite size and magnetic properties, some of the Bi/Mn bilayers were protected by Al and SiOx layers. The coercive fields of the resulting MnBi films without a protective layer reach values of up to 1.25 T. In case of depositing an Al capping layers prior to annealing, the coercive fields are decreasing strongly showing coercive fields in the range of 0.6 T. In contrast, using SiOx as a capping layer, the Kerr hysteresis loops show a nonlinearity near the coercive field indicating an inhomogeneous film. The change in the coercive field is explained by the influence of capping layers on the MnBi crystallite size during annealing.
Bifurcations in highly magnetostrictive amorphous wires
Authors: Navarro E. , García-Beneytez J. M. , Vázquez M. , Hernando A.
Published: July 2002

The aim is to correlate the magnetoelastic resonance phenomena in Fe-based amorphous wires with physics of bifurcations and chaos. The simultaneous application of a dc axial HZ and ac circular Hφ magnetic fields, with respect to the wire axis, gives rise to the appearance of nonlinear phenomena. The pickup signal, related with the time derivative of axial magnetization, is analyzed through Fourier analysis. For exciting frequencies ω much higher than the characteristic resonance value, it is possible to observe bifurcation behavior characterized by the appearance of signal contributions at a certain values of ω: ω/2, ω/4, ω/8, ω/16, ω/32,... . Such behavior is also checked through the determination of the phase space of the system.
Investigation of resonance light absorption and rectification by subnanostructures
Authors: Lin Guang H. , Abdu Reyimjan , Bockris John O’M.
Published: July 2002

In this article, we report the first experimental observation of resonance light absorption by a fabricated subnanostructure and high frequency rectification in the range corresponding to that of visible light. The subnanostructure consists of a parallel dipole antenna array. A resonance peak signal of short circuit current was observed. This peak signal appeared only at the polarization of incident light parallel to the dipole element. The short circuit peak signal was also observed at a photon energy less than that of the optical band gap. The peak angle exhibited the predicted relation with the wavelength of the incident light. These experimental observations open an avenue to the fabrication of a high efficiency solar cell, as well as a new kind of optical sensor.
Threading dislocations in silicon layer produced by separation by implanted oxygen process
Authors: Prieur E. , Guilhalmenc C. , Härtwig J. , Ohler M. , Garcia A. , Aspar B.
Published: July 2002

Threading dislocations in the silicon layer in three different types of the silicon on insulator samples produced by standard and improved separation by implanted oxygen (SIMOX) processes were investigated by synchrotron x-ray topography, scanning electron microscopy (SEM), and optical microscopy. The densities and Burgers vectors of the dislocations were first determined nondestructively by synchrotron x-ray topography. Then the line directions of the same dislocations were determined by SEM after chemical Secco etching. Some of these results were compared with results obtained from optical microscopy of Secco etched samples. The threading dislocations in the Si layer were found to occur mainly in pairs with densities of the order of 105 cm−2 in standard SIMOX samples and of the order of 104 cm−2 in improved SIMOX samples. These dislocations have an edge character. Other features of these dislocations, such as the distances between two dislocations forming a pair, orientations of these pairs, and dislocations that change their line direction, are also discussed.
The properties of co-sputtered YBa2Cu3O7 and CeO2 thin films
Authors: Cukauskas E. J. , Allen L. H.
Published: July 2002

We have investigated the morphology and superconducting properties of off-axis co-sputtered thin films of YBa2Cu3O7 and CeO2. The composite films were made with CeO2 concentrations up to approximately 43%. Morphology investigations using x-ray diffraction and scanning electron microscopy suggest that films with less than 29% CeO2 may consist of a distorted YBCO crystal structure with Ce being incorporated into the lattice. Films with more than 29% consist of a new material phase. Tc and resistance ratio decreased with increasing CeO2 concentration. Degradation of Tc after photoresist processing was observed for films with higher amounts of CeO2. However, this degradation was reversed and Tc improved beyond that of the as-deposited film by processing the films with an oxygen plasma etch. The temperature dependence of the critical current near Tc showed two power law dependence regions with a crossover near 0.99 Tc for films having low concentrations of CeO2. This crossover shifted to lower temperatures for films with higher amounts of CeO2. Ic’s were decreased by small applied magnetic fields, and for some samples response was observed which was greater than 0.5 mA/G and increased with decreasing temperature. For a sample having 26% CeO2, a 40% reduction in critical current for a 2 G applied field was observed. These magnetic field responses indicate that this material may be a promising candidate for the development of Josephson vortex flow devices.
A model of an aligned nematic droplet for small angle light scattering study
Authors: Huang Z. , Chidichimo G. , Nicoletta F. P. , De Simone B. C. , Caruso C.
Published: July 2002

An aligned nematic droplet in the axial configuration is modeled as a hollow spheric scatterer for studying its small angle light scattering characters. When the thickness of the shell is reduced by an external field, calculations by anomalous diffraction approximation suggest that scattering intensity would decrease and scattering peak would drift to a smaller angle. Measured angular scattering intensities show good agreement with calculations. As the shell part originates from anchoring of the nematic molecules to the surrounding medium, angular scattering intensity can be associated with correlation length.
Electron densities and temperatures in a diamond-depositing direct-current arcjet plasma
Authors: Brinkman E. A. , Stalder K. R. , Jeffries J. B.
Published: July 2002

Langmuir probe measurements of electron density and temperature are made in the plume of a dc arcjet reactor. A dc arc is struck in an argon or argon/hydrogen mixture at 6 atm pressure and expands through a converging/diverging nozzle into the reactor with a pressure of 25 Torr. Methane and methane/nitric oxide are added in the diverging nozzle, and diamond film grows on a substrate in the gas plume. Electron temperatures of 1–2 eV are significantly hotter than the neutral gas temperature in the plume. Electron densities range from 1010 to 1013 cm−3, well above the Saha equilibrium for the gas temperature and pressure and well below the equilibrium for the electron temperature.
Edge effects in photorefractive thin films
Authors: Wang Qingnan , Hodari Apriel K. , Temple Doyle A.
Published: July 2002

We have used a two-dimensional charge transport model to study edge effects in photorefractive thin films. Our result shows the presence of a large surface charge layer that causes saturation of trap-limited-field in the large fringe spacing limit. Quadratic electro-optic materials were used as an example to quantitatively study the contribution of the surface-charge-layer field to the total photorefractive grating. We clearly demonstrate that the device performance can be dominated by either surface charge or bulk trap charges.
The role of chain folding in the crystallization kinetics of nucleated polypropylene: An elementary model
Authors: Pieruccini Marco
Published: July 2002

A simple model is proposed to derive the Gibbs potential change associated with the formation of secondary nuclei in the crystallization of systems consisting of isotactic polypropylene nucleated with small amounts of indigo. A basic assumption is that the average basal surface tension of critical nuclei lowers as an effect of chain–nucleant interaction. The actual fold length of individual stem pairs may be lower than critical due to chain–nucleant encounters. Two limiting cases are considered, namely, when the fold lengths of adjacent stem pairs are independent of one another (random fold), and when they are not (persistence). Corresponding expressions of the nucleation free enthalpy are used to analyze available data of crystallization kinetics. Both scenarios describe the physical situation well, most likely because under the actual experimental conditions critical nuclei consist, on average, of one stem pair. The random fold scheme, however, is more informative as it provides (at least in principle) a method for determining both the basal and lateral contributions to surface tension through an analysis of the crystallization kinetics.
Structural perfection of (001) CeO2 thin films on (11_02) sapphire
Authors: Zaitsev A. G. , Ockenfuss G. , Guggi D. , Wördenweber R. , Krüger U.
Published: July 2002

Large-area (001) oriented epitaxial CeO2 films with extremely high crystalline perfection characterized by x-ray diffraction rocking curves of the (002) CeO2 reflection with a full width at half maximum (FWHM) Δω⩽0.013° and thickness dependent oscillations in the Bragg-Brentano x-ray diffraction spectra were deposited via rf-magnetron sputtering on (11_02) sapphire. Pole figure measurements of the space symmetry confirmed that the examined sharp reflections belong to CeO2 and no other phases like CeAlO3 are present. The improvement of the crystalline quality was obtained by optimization of the high-pressure sputter deposition process and the use of large-area substrates. The [100] CeO2 axis was slightly tilted with respect to the [11_02] sapphire axis by 0.185°. Subsequently sputter-deposited high-Tc YBa2Cu3O7−x thin films revealed structural properties characterized by FWHM <0.06° of the (005) θ-2θ peaks and by FWHM of the (005) peaks rocking curves of Δω=0.3°.
Characterization of multilayers by Fourier analysis of x-ray reflectivity
Authors: Voorma H. J. , Louis E. , Koster N. B. , Bijkerk F. , Spiller E.
Published: July 2002

We discuss a new method to characterize multilayer structures with grazing-incidence reflectivity measurements using hard x-ray radiation, such as Cu-Kα or Mo-Kα radiation. The method is based on the analysis of the reverse Fourier transforms of the reflectivity at the Bragg peaks in q-space, the reflectivity data being obtained from an angular scan (θ−2θ). This method is faster than curve fitting of the reflectivity data, results in an accurate value of the density and thickness of both materials, and needs no pre-assumptions about the material composition and the parameters of the multilayer. The method makes a distinction between interface roughness and layer thickness errors, and is independent of measurement of the critical angle. A minor disadvantage is that only an average value of the layer thickness is determined, rather than the individual layer thicknesses. As an example our method is used to analyze small-angle reflectivity measurements of Mo/Si and Co/C multilayers. The parameters thus obtained are used to model a structure, which is subsequently used to predict the near-normal incidence reflectivity at soft x-ray wavelengths. The accuracy thus found corresponds to a relative error of 5%. The densities of a Mo/Si multilayer are determined independently with critical angle measurements to verify the values obtained from the grazing incidence reflectivity measurements. The results of the analysis of the Co/C multilayer are compared to values obtained using a conventional method based on the kinematical theory. However, the conventional method requires additional data of soft x-ray reflectivity measurements. The new method yields the same values for the multilayer parameters but does not require the extra soft x-ray measurements.
Measurement of the scattered light magneto-optical Kerr effect from plasmon-resonant Ag particles near a magnetic film
Authors: Pufall M. R. , Berger A. , Schultz S.
Published: July 2002

The magneto-optical Kerr effect (MOKE) of light scattered from plasmon-resonant Ag particles placed near a magnetic sample is investigated. Ag particles were deposited on sputtered NiFe films with an SiO2 spacer layer of known thickness. The MOKE signal in the scattered light was found to vary strongly with the observation angle. For s-polarized incident light, no MOKE rotation was found in the normal direction, and the rotation signal switched sign as the observation direction passes through the surface normal direction. Conversely, p-polarized incident light generated a nonzero MOKE signal in the normal direction. In general, the MOKE signal increased for decreasing spacer thickness, and in addition the angular distribution of the MOKE changed, for both s- and p-incident polarizations. Implications for MOKE near-field optical microscopy using plasmon-resonant Ag particles as probes are discussed.
Finite element micromagnetic simulations with adaptive mesh refinement
Authors: Tako K. M. , Schrefl T. , Wongsam M. A. , Chantrell R. W.
Published: July 2002

The finite element method has recently been employed in investigating magnetization processes in materials with an irregular microstructure. In order to improve the accuracy of the solution, a method based on an adaptive refinement scheme is clearly desirable. Error control is essential if accuracy of results is to be optimal. We have applied an adaptive scheme to a system of interacting two-dimensional polycrystalline grains. The error estimator is based on the divergence and curl of both the magnetization and demagnetizing field. It is shown that increasing levels of discretization improve the micromagnetic solution, and the spatial localization of the additional nodes demonstrates the importance of the adaptive meshing technique.
Heat conduction of (111) Co/Cu superlattices
Authors: Tsui Frank , Chen Baoxing , Wellman Joanne , Uher Ctirad , Clarke Roy
Published: July 2002

We report the observation of a large negative magnetothermal resistance in (111) Co/Cu superlattices grown by molecular beam epitaxy (MBE) techniques. The observed field dependence is proportional to that of the electrical resistance, in accordance with the Wiedemann–Franz law. The Lorentz number deduced from the measurements is (2.7±0.3)×10−8 V2/K2. The magnetothermopower also shows a similar correlation with resistivity. These findings reveal that large-angle elastic scattering of conduction electrons, arising from a spin-dependent density of states at the Fermi level, is the dominant process responsible for the observed large magnetotransport effects. In zero field, both electrons and phonons contribute to the thermal conduction of the MBE-grown Co/Cu system, at a ratio of about 1:2 near 300 K becoming nearly equal below 150 K.
Reduced threshold current density due to excitonic optical gain in the presence of dislocations and surface states in tensile strained ZnCdSe quantum wire lasers
Authors: Huang W. , Jain F.
Published: July 2002

Higher binding energies (30–60 meV) in II–VI wide-gap materials result in large exciton densities, making optical transitions due to excitons dominant over free electrons and holes. Optical gain and threshold current densities in ZnCdSe based multiple quantum wire lasers are computed including the effect of strain. It is found that the tensile-strained quantum wires yield lower threshold current densities than compressive strained or unstrained structures. The calculated threshold current density for a defect free tensile-strained ZnCdSe–ZnMgSSe quantum wire laser, realized on an InP substrate, has been computed to be 58 A/cm2. The exciton transitions assist in lowering the threshold current density which is adversely affected by the presence of dislocations and surface states. It is found that the threshold current density would increase to 435 A/cm2 assuming 1017 cm−3 trapping levels due to dislocations and surface states. However, taking into account the exciton transitions, the threshold current density is reduced to 79 A/cm2 when assuming the same trapping level density.
In situ monitoring of the surface reconstructions on InP(001) prepared by molecular beam epitaxy
Authors: Ozanyan K. B. , Parbrook P. J. , Hopkinson M. , Whitehouse C. R. , Sobiesierski Z. , Westwood D. I.
Published: July 2002

Reflection anisotropy spectroscopy (RAS) and reflection high-energy electron diffraction (RHEED) were applied to study clean InP(001) surfaces prepared by molecular beam epitaxy (MBE). At phosphorus beam equivalent pressures (BEPs) between 3.5×10−7 and 3.5×10−6 mbar and substrate temperature (Ts) falling from 590 to 150 °C, (2×4), (2×1), (2×2), and c(4×4) RHEED patterns are observed. The main RAS features, observed at 1.7–1.9 and 2.6–2.9 eV are assigned to In and P dimers, respectively. The above reconstruction sequence is associated closely with transformations identified in RAS signatures that are induced by progressively increasing the P surface coverage. The RAS results also imply the existence of (2×4)α and (2×4)β phases. A surface-phase diagram for MBE-grown (001) InP, in the whole range of Ts and phosphorus BEPs is proposed.
Field mapping characterization for axially magnetized, superconducting cylinders in the remanent critical state: theory and experiment
Authors: Klupsch Th. , Strasser Th. , Habisreuther T. , Gawalek W. , Gruss S. , May H. , Palka R. , Mora Serrano F. J.
Published: July 2002

Based on the Bean model with a constant critical current density jc, novel and very precise, analytical approximations for the axial induction component Bz(r) inside and outside of a finite, axially magnetized, circular cylinder with arbitrary aspect ratio in the fully penetrated, remanent critical state are presented. Starting from any mapped field profile the possibility to determine the material parameter jc by means of a global optimum fit procedure is demonstrated. Some characteristic results for melt-textured yttrium barium copper oxide material are outlined. We found that jc determined by vibrating sample magnetometry (VSM) exceeds that from mapping at least by about 50%. This systematic discrepancy is discussed on the basis of two independent effects resulting in a local, space-dependent critical current density: the B-dependence of jc and material inhomogeneities. The first effect becomes important for sample radii in the 1-cm-range, although the Bz-profile remains axially symmetric and well fitted by the Bean model. The second one may lead to asymmetric Bz-profile deformations connected with a decreasing fitting significance. For the first and second effect we estimate the considerable ratio of the absolute maximum to absolute minimum of the local, critical current density of 2 and 4, respectively, which explains the observed discrepancies between the effective jc from mapping and VSM measurements.
Methodology for evaluation of mobility-lifetime product by spectroscopy measurements in CdZnTe spectrometers
Authors: Ruzin A. , Nemirovsky Y.
Published: July 2002

A novel methodology is presented for extraction of the semiconductor electron and hole mobility-lifetime products by x- and γ-ray spectroscopy analysis. The methodology is based on the analysis of spectroscopy results, namely the maximal and the average charge collection efficiencies as discussed below, for different photon energies at various bias voltages. The methodology enables the evaluation of both electron and hole mobility-lifetime products without the need to use α particle measurements. The evaluation is carried out by a single parameter fitting of the models analyzed in this study. Mobility-lifetime products for CdZnTe substrates grown by Bridgman and high pressure Bridgman (HPB) methods are reported and compared. Typical values of μe⋅τe=2⋅10−3 and μh⋅τh=1⋅10−5 cm2/V are extracted for HPB grown CdZnTe with 10% Zn concentration. Values of μe⋅τe=9⋅10−4 and μh⋅τh=1⋅10−7 cm2/V are obtained for CdZnTe with 10% Zn concentration grown by the Bridgman method.
Radiation induced formation of color centers in PbWO4 single crystals
Authors: Nikl M. , Nitsch K. , Baccaro S. , Cecilia A. , Montecchi M. , Borgia B. , Dafinei I. , Diemoz M. , Martini M. , Rosetta E. , Spinolo G. , Vedda A. , Kobayashi M. , Ishii M. , Usuki Y. , Jarolimek O. , Reiche P.
Published: July 2002

The changes in the absorption spectra induced by Co60 irradiation and high temperature annealing was studied for selected PbWO4 crystals. Based on radiation and annealings induced absorption spectra, four color centers are proposed, which could be responsible for shaping the absorption spectrum of PbWO4 crystals below 3.6 eV, namely Pb3+, O−, F, and F+ centers. Thermoluminescence (TSL) glow curves above room temperature were also studied on these samples to determine further the trap levels resulting from irradiation. Correlation between the results obtained by TSL and Co60 irradiation induced absorption changes is discussed.
Characterization and modelling of the strain fields associated with InGaAs layers on V-grooved InP substrates
Authors: Gupta A. , Weatherly G. C. , Cassidy Daniel T. , Bruce D. M.
Published: July 2002

A series of lattice-matched or compressively strained InGaAs layers were grown by gas source molecular beam epitaxy on V grooved InP substrates, with grooves bounded by {211}A or {111}B facets. The stress field associated with the layers was measured by the degree of polarization technique, and compared to the predictions of analytical or finite element models. Good agreement was found for the {211}A V grooves, but both the nominally lattice-matched and compressively strained layers grown on {111}B  V grooves displayed similar degree of polarization maps. Analytical electron microscopy demonstrated that the {211}A  V-groove samples had the targeted composition, but the {111}B  samples showed much higher In/Ga ratios at the bottom of the groove than the expected values. Indium enrichment at the bottom of the groove led to defect formation there, and left the V groove of both the lattice-matched and compressively strained samples under a net compressive force.
Hydrogen passivation of nitrogen in 6H–SiC
Authors: Theys B. , Gendron F. , Porte C. , Bringuier E. , Dolin C.
Published: July 2002

N-doped 6H–SiC wafers have been annealed in hot hydrogen for two doping levels, corresponding to n- (n∼1017 cm−3) and n+-type (n∼1019 cm−3) material. Electron spin resonance shows little passivation of N by hydrogen in n-type material, where secondary-ion-mass spectroscopy shows better penetration of deuterium. Both observations are accounted for in terms of Fermi-level control of the hydrogen charge state.
Oxidation and roughening of silicon during annealing in a rapid thermal processing chamber
Authors: Mohadjeri B. , Baklanov M. R. , Kondoh E. , Maex K.
Published: July 2002

Silicon wafers were annealed at 1100 °C in nitrogen flowing ambient, using a conventional rapid thermal processing system operating at atmospheric pressure. Subsequently, silicon oxidation and the resulting surface morphology following chemical etching of the oxide film were studied. According to atomic force microscopy analysis, a significant amount of roughening occurs on the silicon surface after the high temperature anneal in nitrogen. However, by adding 10% oxygen to the nitrogen ambient, the average surface roughness was reduced by a factor of ∼3. Ellipsometry measurements were performed to study silicon oxidation following annealing. It was shown that the ellipsometric characteristics of the surface after removal of the oxide layers can be correlated to generation of surface roughness. A close agreement between the experimental results and theoretical calculations was obtained with respect to the oxidation induced surface roughening.
Free versus localized exciton in GaAs V-shaped quantum wires
Authors: Lomascolo M. , Ciccarese P. , Cingolani R. , Rinaldi R. , Reinhart F. K.
Published: July 2002

We present a systematic investigation of continuous wave and transient photoluminescence in GaAs V-shaped quantum wires as a function of temperature, aimed to the understanding of the radiative recombination mechanism of the ground level (localized versus free-exciton recombination). Exciton localization is observed at low temperatures. Free-exciton polariton transitions are monitored at intermediate temperatures through the square root temperature dependence of the decay time. Exciton localization energy, density of localization centers and exciton intrinsic lifetime have been determined from the theoretical analysis of the transient photoluminescence, thus providing a simple quantitative method for the assessment of sample quality.
Electron transport across metal/discotic liquid crystal interfaces
Authors: Boden N. , Bushby R. J. , Clements J. , Movaghar B.
Published: July 2002

Electron transport across micron thick films of columnar hexagonal discotic liquid crystal phases homeotropically aligned between metal electrode surfaces has been studied both experimentally and theoretically. These molecules are unique in their combination of charge transport along individual molecular columns with liquidlike self-organization. Typical of organic insulators, a high resistance Ohmic regime is evident at fields of less than 0.05 MV cm−1, due to a low concentration of chemical impurities (n<109 cm−3), and a space-charge injection regime at higher fields. Breakdown fields are reasonably high: in hexakis(hexyloxy)triphenylene they reach ∼5 MV cm−1 at room temperature. Our results show that triphenylene-based discotics form an excellent class of highly ordered optically transparent insulators. At high temperatures and high fields the current is injection controlled and exhibits typical tunneling and space charge limited, nonlinear I–V characteristics. Dramatic jumps in injection currents are observed at phase transitions. The change at the crystalline to liquid crystalline phase transition is mainly due to more efficient “wetting” of the electrode surface in the liquid crystalline phase, whilst at the liquid crystalline to isotropic phase transition it arises from the enhancement in the molecular mobility. The concepts of semiconducting gaps, band mobilities, and carrier injection rates are extended to these new materials. The experimental observations are interpreted in a framework which takes into account the important role played by liquidlike dynamics in establishing the microscopic structural order in, what is, otherwise a highly anisotropic and weakly bonded “molecular crystal.”
Deep levels and minority carrier lifetime in proton irradiated silicon pin diode
Authors: Sasaki T. , Nishizawa J. , Esashi M.
Published: July 2002

Deep levels induced by MeV-proton irradiation in n−(ν) region of Si p-ν-n diodes were investigated by photocapacitance method under constant capacitance condition. Electrical property, dosage dependence, spatial distribution, and annealing behavior of the deep levels were studied. Carrier lifetime reduction in pin diodes by the deep level was examined by current–voltage characteristics and impedance measurements. From proton dosage dependence and annealing effect, the decreasing behavior of minority carrier lifetime with increasing deep level density was revealed.
The role of structural relaxation in the plastic flow of metallic glasses
Authors: Khonik V. A. , Kitagawa K. , Mikhailov V. A. , Vinogradov A. Yu.
Published: July 2002

The role of structural relaxation in the plastic flow behavior of metallic glasses is analyzed both theoretically and experimentally. The characteristic time of structural relaxation is calculated as a function of glass thermal prehistory. It is revealed that heating above the room temperature by several tens of Kelvins results in a sharp, by several orders of magnitude, decrease of this time. It is argued that localized “inhomogeneous” dislocation-like flow occurs on loading if the characteristic time of structural relaxation is much greater than the characteristic loading time, while “homogeneous” viscous deformation is observed in the opposite case. Precise measurements of acoustic emission in a Co-based metallic glass being loaded at different temperatures and strain rates are employed for verification of this statement. It is shown that the inhomogeneous → homogeneous flow transition occurs at temperatures somewhat higher than T=400 K, and the transition temperature increases by ≈ 40 K as the strain rate increases by two orders of magnitude. Theoretical estimations show that for the inhomogeneous flow the characteristic time of structural relaxation in the loaded state is indeed much greater than the characteristic loading time. It is concluded that the kinetics of structural relaxation determines the flow mode of metallic glasses in a unique manner. The kinetically “frozen” structural relaxation gives rise to a crystalline-like localized flow under load while intensive structural relaxation facilitates a viscous glass-like behavior.
Dynamic stress bridging in granular material
Authors: Bardenhagen S. G. , Brackbill J. U.
Published: July 2002

Through numerical experiments we investigate stress localization in granular material with application to plastic-bonded explosives. The results shed light on static versus dynamic loading, and on the role of an interstitial material between grains. Under static loading, we observe stress localization and bridging in a lattice of polydisperse cylinders in two dimensions. Under dynamic loading, we observe a two wave structure with stress fingers propagating ahead of a normal compaction wave. When an interstitial material is added (a polymeric binder), force fluctuations are substantially reduced under both static and dynamic loading. Under dynamic loading, the structure of the stress wave depends upon the material properties of the binder. As the binder stiffness increases, stress fingering decreases and is replaced by a planar compaction wave.
Hysteresis loop areas in kinetic Ising models: Effects of the switching mechanism
Authors: Sides S. W. , Rikvold P. A. , Novotny M. A.
Published: July 2002

Experiments on ferromagnetic thin films have measured the dependence of the hysteresis loop area on the amplitude and frequency of the external field, A=A(H0,ω), and approximate agreement with numerical simulations of Ising models has been reported. Here we present numerical and theoretical calculations of A in the low-frequency regime for two values of H0, which bracket a temperature and system-size dependent crossover field. Our previous Monte Carlo studies have shown that the hysteretic response of the kinetic Ising model is qualitatively different for amplitudes above and below this crossover field. Using droplet theory, we derive analytic expressions for the low-frequency asymptotic behavior of the hysteresis loop area. In both field regimes, the loop area exhibits an extremely slow approach to an asymptotic, logarithmic frequency dependence of the form A∝−[ln(H0ω)]−1. Our results are relevant to the interpretation of data from experiments and simulations, on the basis of which power-law exponents for the hysteresis-loop area have been reported.
Noncontact temperature measurements of diamond by Raman scattering spectroscopy
Authors: Cui J. B. , Amtmann K. , Ristein J. , Ley L.
Published: July 2002

The possibility of determining the temperature of diamond by noncontact Raman spectroscopy is assessed critically. The intensity ratio of Stokes to anti-Stokes lines is shown to be ill suited for temperatures above ∼750 K. Employing the temperature coefficient of the Raman line position, on the other hand, turns out to be a straightforward and highly reliable means to measure diamond temperatures between 300 and 2000 K with an accuracy of ±10 K. A prerequisite for the application of this method is an empirically developed formula which describes the temperature coefficient of the Raman active phonon frequency with high accuracy. Examples of temperature measurements on single crystal diamond and diamond films grown by chemical vapor deposition are given. The application of this procedure to the temperature measurement of silicon and germanium is demonstrated.
Photoluminescence on ordered GaxIn1−xAsyP1−y
Authors: Oelgart G. , Knauer A. , Oster A. , Weyers M.
Published: July 2002

Photoluminescence of Ga0.54In0.46As0.12P0.88 layers grown by metalorganic vapor phase epitaxy on differently oriented GaAs substrates has been investigated. Valence-band splitting due to symmetry breaking in ordered quaternary alloys was demonstrated by room temperature photoluminescence polarization spectroscopy. Single- and two-variant ordering was identified by high resolution x-ray diffraction on samples grown on (001) GaAs substrates misoriented 2° off to {11̄1} B and on just oriented ones, respectively. The low temperature photoluminescence spectra consist of moving and nonmoving components, typical for ordered material. The blueshift of the moving component versus the pump level for the 0° samples is stronger than that for the 2°B samples. The full width at half maximum of the moving component increases with increasing injection level for the 0° samples, whereas it decreases for 2°B samples. We suppose that this different behavior of the low temperature photoluminescence observed on single- and two-variant samples is related to different domain structures.
Binding energy of biexcitons in GaAs–AlxGa1−xAs quantum wells
Authors: Liu Jian-Jun , Kong Xiao-Jun , Liu Ying
Published: July 2002

The energy of a biexciton in a GaAs–AlxGa1−xAs quantum well is calculated variationally by use of a two-parameter trial wave function. The calculated binding energy, relative to two well-separated excitons, is greater than that given previously by Kleinman [Phys. Rev. B 28, 871 (1983)]. Our binding energy takes its greatest value when the well width is approximately 10 Å. The ratio of the binding energy of a biexciton to that of an exciton for a range of well thickness (5–300 Å) is found to be from 0.127 to 0.284, which agrees fairly well with previous experimental results. The results of our approach are compared with those of earlier theories.
Low thermal diffusivity measurements of thin films using mirage technique
Authors: Wong P. K. , Fung P. C. W. , Tam H. L.
Published: July 2002

Mirage technique is proved to be powerful in measurements of thermal diffusivity. Its contactless nature makes it suitable for delicate samples such as thin films and single crystals. However, as the damping of the thermal wave profile increases progressively upon the decrease in thermal diffusivity of the medium, mirage technique becomes more difficult to be applied to low thermal diffusivity measurements. Moreover influences from substrate signals make analysis difficult when the samples are thermally thin. Recently a thermal-wave-coupling method for mirage signal analysis [P. K. Wong, P. C. W. Fung, H. L. Tam, and J. Gao, Phys. Rev. B 51, 523 (1995)] was reported for thermal diffusivity measurements of thin film down to 60 nm thick. In this article we apply the thermal-wave-coupling method to thin films of low thermal diffusivity, especially polymer films. A new lower limit of thermal diffusivity measurable by mirage technique has been reached.
Measurement of the nonlinear refractive index of polydiacetylene using Michelson interferometry and z-scan
Authors: Quintero-Torres R. , Thakur M.
Published: July 2002

We have performed a detailed measurement of the sign of the nonlinear refractive index of PTS-polydiacetylene using Michelson interferometry and z-scan at 1.06 μm wavelength. The measurement was performed using 60 ps laser pulses at 10 Hz repetition rate that avoided any significant thermal contribution. The sign of n2 was found to be negative. The negative sign was explained considering saturation of the exciton absorption for off-resonant pumping. The saturation far outweighs two photon resonances in determining sign and magnitude of n2 in the off-resonant domain within the wavelength range 700–1064 nm.
Ion implantation in confined, pinching metallic plasmas
Authors: Demokan O. , Marji E.
Published: July 2002

A series of preliminary experiments have been carried out to obtain a metallic arc discharge, in the form of an annular cylindrical column, carrying a current in the axial direction. To assert this geometry, a ring-shaped cathode is adopted and an arc formation is triggered by heating the cathode externally. The plasma, initially in the form of a cylindrical shell, is seen to be confined and pinched by its own magnetic field. The resulting, radially inward flux is observed to produce efficient and multidirectional implantation on targets placed near the axis, even without biasing.
Can charge fluctuations explain particle growth in low-pressure plasmas?
Authors: Lee Kangtaek , Matsoukas Themis
Published: July 2002

Particles in low-pressure plasmas carry a substantial negative charge and are subject to strong repulsion. However, the instantaneous particle charge undergoes fluctuations whose magnitude and duration can be substantial. Excursions of the charge from the mean value alters the particle behavior and results in interactions that are substantially different from the average. We calculate the rate of particle agglomeration taking into account the statistical distribution of the particle charge. We find that effect of fluctuations is to always increase the rate of agglomeration, especially for small particles. The collision cross section of larger particles is enhanced significantly, but the overall collision rate remains too low. Under these conditions, growth is fueled by small particles and is predicted to result in narrow size distributions.
Reactions at amorphous SiC/Ni interfaces
Authors: Edelstein A. S. , Gillespie D. J. , Cheng S. F. , Perepezko J. H. , Landry K.
Published: July 2002

When multilayer samples of polycrystalline Ni and amorphous SiC are heated, the sequence of phase formation initiates with a dissolution of Ni into the amorphous phase and is followed first by the formation of NiSi and then Ni2Si. Multilayer samples of a-SiC/Ni with modulation wavelengths of 83.9 nm and with the ratio of the thickness of the SiC layer to the Ni layer equal to 3.8 retain a multilayer structure even after they undergo two phase transitions. When annealing causes reactions to occur, the surface roughness, measured by atomic force microscopy (AFM), increases and was correlated with similar increases in the interface width. AFM measurements may provide a convenient way to observe reactions at buried interfaces.
Photoluminescence quenching in Si1−xGex/Si multiple quantum wells grown with atomic hydrogen
Authors: Balchin G. A. , Amirtharaj P. M. , Silvestre C. , Thompson P.
Published: July 2002

We compare the photoluminescence spectra from a series of Si1−xGex/Si (0.1⩽x⩽0.3) multiple quantum well (MQW) samples grown with atomic hydrogen to a series of similar samples grown without atomic hydrogen. All of the samples were grown at 710 °C. We observe intense quantum confined photoluminescence in the Si1−xGex/Si MQW samples grown without atomic hydrogen. No quantum confined photoluminescence was observed in the Si1−xGex/Si MQW samples grown with atomic hydrogen. This was unexpected, since quantum confined photoluminescence has been observed in the hydrogen-assisted growth of Si1−xGex/Si quantum well structures grown at lower temperatures by other researchers. We believe that this is caused by defects, introduced into the samples during growth with atomic hydrogen, which lead to efficient nonradiative centers that compete with the radiative centers. These defects lead to a reduction of the photoluminescence of the samples grown with atomic hydrogen. Since hydrogen is used as a surfactant to reduce the deleterious effects due to Ge segregation, understanding its role is an important step in SiGe materials development.
Anisotropy and magnetization processes in Co-rich amorphous wires
Authors: Rouabhi M. , Cochrane R. W. , Ménard D. , Britel M. , Ciureanu P. , Yelon A.
Published: July 2002

Longitudinal magnetization curves and giant magnetoimpedance (GMI) analyses are combined to investigate the anisotropy and magnetization processes of dc-current-self-annealed Co68.15Fe4.35Se12.5B15 wires. The magnetization curves show an almost anhysteretic behavior with small Barkhausen jumps occurring at a critical field H*. This unusual shape is in very good agreement with the high resolution GMI response to the field, which reflects the circumferential bias permeability. Magnetization curves yield information on the bulk properties, while GMI allows the investigation of the surface region of the sample. The anisotropy constant is calculated using initial-magnetization curves and the resonance condition, yielding interesting conclusions concerning the inhomogeneity of the wires.