bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio study of elastic properties of Ir and Ir3X compounds
Authors: Chen Kuiying , Zhao L. R. , Tse John S.
Published: March 2003

Elastic constants and moduli of face-centered cubic Ir and its L12 intermetallic compounds Ir3X (X=Ti, Ta, Nb, Zr, Hf, V) have been determined using ab initio density functional theory calculations within the generalized gradient approximation. With the tetragonal, trigonal, and isotropical lattice distortions, elastic constants C11, C12, C44, and bulk modulus B are derived from the second derivative of the total energy as a function of volume. The calculated Young’s modulus E, shear modulus G, Poisson’s ratio ν, and the ratio RG/B of G over B are then used to examine mechanical properties of Ir and Ir3X compounds. By analyzing RG/B and Cauchy pressure C12–C44, the brittle-ductile behavior of the materials is assessed. Based on the modulus difference ΔG between the γ matrix (Ir) and γ′ precipitates (Ir3X), the γ′ strengthening effect in the γ matrix is studied.
Simulations of dynamical stabilization of Ag–Cu nanocomposites by ion-beam processing
Authors: Enrique R. A. , Nordlund K. , Averback R. S. , Bellon P.
Published: March 2003

Recent theoretical results indicate that ion-beam mixing can be used to synthesize nanocomposite structures from immiscible elements, relying on a self-organization phenomenon at steady state under irradiation. According to this modeling, self organization requires that the range of the forced atomic relocations in displacement cascades exceeds a critical value. Experimental evidence supporting the formation of nanocomposites by this mechanism has been found in the immiscible system Ag–Cu irradiated with 1 MeV Kr ions. To address this experimentally relevant model system, and to test the theoretical predictions, we study, by molecular dynamics (MD), the characteristics of irradiation mixing in a Ag–Cu alloy subjected to bombardment with 62 keV He, 270 keV Ne, 500 keV Ar, and 1 MeV Kr ions. The distribution of atomic relocations measured by MD is then used to perform lattice kinetic Monte Carlo (KMC) simulations of phase evolution, during which both thermal decomposition and irradiation mixing operate simultaneously. The KMC results show that, in the framework of this self-organization mechanism, a nanocomposite structure can be stabilized at steady state by irradiation with heavy ions (Ne, Ar, and Kr), but not with He ions. As the characteristic relocation range for He ions is half of that measured for the heavy ions, these results support the theoretical prediction of the existence of a critical relocation range for compositional patterning to take place under irradiation.
Modeling of heat transfer and fluid flow during gas tungsten arc spot welding of low carbon steel
Authors: Zhang W. , Roy G. G. , Elmer J. W. , DebRoy T.
Published: March 2003

The evolution of temperature and velocity fields during gas tungsten arc spot welding of AISI 1005 steel was studied using a transient numerical model. The calculated geometry of the weld fusion zone and heat affected zone and the weld thermal cycles were in good agreement with the corresponding experimental results. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection at various stages of the evolution of the weld pool and the role of various driving forces for convection in the liquid pool. The calculated cooling rates are found to be almost independent of position between the 1073 and 773 K (800 and 500 °C) temperature range, but vary significantly at the onset of solidification at different portions of the weld pool. During solidification, the mushy zone grew significantly with time until the pure liquid region vanished. The solidification rate of the mushy zone/solid interface was shown to increase while the temperature gradient in the mushy zone at this interface was shown to decrease as solidification of the weld pool progressed. Tracking these solidification parameters with time shows that the weld pool solidifies with decreasing interface stability, i.e., with a higher tendency to form dendrites towards the center of the weld.
Polarization-dependent intersubband absorption saturation and its effect on polarization selection in vertical cavity surface-emitting lasers
Authors: Ryvkin B. S. , Avrutin E. A. , Pessa M.
Published: March 2003

A mechanism of nonlinear dichroism of net gain in vertical-cavity surface-emitting lasers based on polarization-dependent saturation of intersubband absorption in p-contact layers is proposed and analyzed. Microscopic perturbative calculations are used to obtain approximate analytical expressions for the magnitude of the effect. It is shown that the proposed mechanism can play a significant part in polarization switching and bistability in vertical-cavity lasers.
Anisotropic photoluminescence from porous silicon layers made under polarized illumination: Origin of contradictory experimental observations
Authors: Koyama Hideki
Published: March 2003

A detailed study has been performed on the anisotropic photoluminescence of porous silicon (PSi) layers formed by the electrochemical anodization under polarized illumination. Emphasis is placed on the different effects of the photoelectrochemical dissolution process and the electroless photochemical counterpart on the direction of the anisotropy. Our experimental results indicate that the two dissolution mechanisms induce photoluminescence anisotropy in totally different directions: in photoelectrochemically etched PSi samples, a maximal degree of polarization is observed only when the polarization (Eexc) of the excitation light is parallel to the polarization (Eetch) of the illumination light, while in photochemically etched samples it is observed when Eexc is perpendicular to Eetch. The former and latter results are explained, respectively, by the thinning and disintegration of the Si nanostructures oriented parallel to Eetch during the etching treatments.
In-plane photovoltage and photoluminescence studies in sequentially grown GaInNAs and GaInAs quantum wells
Authors: Mazzucato S. , Balkan N. , Teke A. , Erol A. , Potter R. J. , Arikan M. C. , Marie X. , Fontaine C. , Carrère H. , Bedel E. , Lacoste G.
Published: March 2003

We have investigated in-plane photovoltage (IPV) and photoluminescence (PL) in sequentially grown Ga0.8In0.2As/GaAs and Ga0.8In0.2N0.015As0.985/GaAs quantum wells. Temperature, excitation intensity, spectral and time dependent study of the IPV, arising from Fermi level fluctuations along the layers of the double quantum well structure, gives valuable information about the nonradiative centers and hence about the optical quality of the GaInNAs quantum well. It also provides information about the radiative transition energies in all the layers. In order to obtain either the trap activation energies and the detrapping rates of photogenerated carriers in the GaInNAs the IPV results are analyzed in terms of a theoretical model based on random doping fluctuations in nominally undoped multilayer structures. The PL results are analyzed in terms of the band anticrossing model to obtain the electron effective mass from the coupling parameter CNM.
THz radiation emission properties of multienergy arsenic-ion-implanted GaAs and semi-insulating GaAs based photoconductive antennas
Authors: Liu Tze-An , Tani Masahiko , Pan Ci-Ling
Published: March 2003

We compare the performance of THz photoconductive (PC) emitter antennas fabricated on multienergy arsenic ion implanted GaAs (multi-GaAs:As+) and semi-insulating GaAs. High damage threshold biasing (>60 kV/cm) and large saturation optical-pumping power (∼20 mW) for multi-GaAs:As+ based PC antennas are reported. Carrier mobility in the As ion implanted layer of GaAs:As+ was estimated to be about 150 cm2/V/s, which was comparable to that of low temperature GaAs.
Effective thermal conductivity estimates of particulate mixtures
Authors: Richter Christiaan , Viljoen Hendrik J. , van Rensburg N. F. J.
Published: March 2003

Several theories have been advanced to estimate effective thermal conductivities of particulate mixtures, but most theories have focused on the dilute case. A method is proposed to estimate the effective thermal conductivity coefficient of mixtures of arbitrary concentration. Earlier the authors developed a theory to determine the expected contact area between different species. This theory is employed to determine the Kapitza resistance of the heterogeneous mixture and forms part of an overall theory to estimate the effective thermal conductivity. Results are compared with other theoretical estimates and with experiments.
Enhancement of the effective tunnel mass in ultrathin silicon dioxide layers
Authors: Städele M. , Sacconi F. , Di Carlo A. , Lugli P.
Published: March 2003

Based on the results of three-dimensional atomistic tight-binding calculations, we argue that the effective tunnel mass of SiO2 employed as a fitting parameter in standard transfer-matrix multiple-scattering theory calculations increases strongly as the oxide thickness is decreased (we find more than 50% mass enhancement upon reduction of the oxide thickness from 4 to 1 nm). At least five factors, usually neglected in effective-mass-based calculations can contribute to this effect: the nonparabolicity of the complex bands in the gap of SiO2, the gradual (rather than abrupt) change of the electrostatic potential across the Si/SiO2 interface, a possible image force correction, the presence of native defects in the oxide, and the effective-mass approximation itself. Very good quantitative agreement between the theoretical predictions for the thickness dependence of the mass enhancement and corresponding results from transfer matrix fits to experimental currents is obtained if defect densities smaller than 1010 cm−2 and a small image force correction are assumed. Since the present findings imply significant errors (1–2 orders of magnitude) in tunnel currents through ultrathin oxides calculated with a single thickness-independent tunnel mass, an explicit parametrization of the thickness dependence for use in multiple-scattering calculations is suggested. For 4 nm thin oxides, we obtain a tunnel mass of 0.35 m0 (0.48 m0), if a parabolic (nonparabolic) dispersion of the complex bands in the band gap of the oxide is adopted. Furthermore, the mass at the conduction band bottom of SiO2 is found to be different from the tunnel mass and estimated to be 0.39 m0, in good agreement with previous measurements. The calculations also yield an estimate of the errors in oxide thicknesses obtained from current–voltage fitting, which are found to agree well with available experimental data.
Viscoelastic stress relaxation in film/substrate systems—Kelvin model
Authors: Ko Sun-Chien , Lee Sanboh , Hsueh Chun-Hway
Published: March 2003

Although the Maxwell model was widely applied to analyze viscoelastic behavior of film/substrate systems, Rafferty et al. [Appl. Phys. Lett. 54, 151 (1989)] found that the structural relaxation of silicon oxide films on silicon wafers during thermal annealing could be explained by the Kelvin model but not the Maxwell model. This result motivated us to investigate the relaxation of residual stresses due to viscoelastic deformation in a film/substrate system using the Kelvin model. The system can be a viscoelastic film on an elastic substrate or an elastic film on a viscoelastic substrate. The viscoelastic solutions of both cases can be obtained from the elastic solution using the Laplace transform. The stress relaxation rate increases with decreasing film-to-substrate thickness ratio for a viscoelastic film deposited on an elastic substrate. However, it shows the opposite trend for an elastic film deposited on a viscoelastic substrate. The present results are compared with those obtained from the Maxwell model.
Magnetic properties of Al, V, Mn, and Ru impurities in Fe–Co alloys
Authors: Reddy B. V. , Deevi S. C. , Khanna S. N.
Published: March 2003

Theoretical studies on the magnetic properties of impurities in Fe–Co alloys have been carried out using a molecular-orbital approach within a gradient corrected density functional formalism. The defected alloy is modeled by a large cluster and the calculations on the ordered alloy are used to show that a cluster containing 67 atoms can provide quantitative information on the local magnetic moment. It is found that although bulk Al, V, and Ru are nonmagnetic, all the impurities carry finite moments. While Al and V impurities couple antiferromagnetically, Ru impurities couple ferromagnetically to the host sites. It is shown that the observed composition dependence of the rate of increase of magnetic moment of FexCo1−x alloys upon addition of Mn impurities is due to the change in the magnetic moment of Mn impurities with composition. The reasons for this change and the possibility of stabilizing a higher Mn moment at all concentrations are discussed.
Infrared fingerprints and periodic formation of nanoparticles in Ar/C2H2 plasmas
Authors: Kovačević Eva , Stefanović Ilija , Berndt Johannes , Winter Jörg
Published: March 2003

The formation of dust particles in argon diluted C2H2 plasmas was studied by means of Fourier transform infrared absorption spectroscopy and mass spectroscopy. The detection limit for infrared absorption was significantly improved by the use of a multipass technique. Measuring the intensity of the Rayleigh/Mie scattering of the infrared signal we found a periodicity of dust formation/vanishing (period of about 35 min in our experimental conditions). The fast disappearance of the dust from the plasma region at the end of every period is the evidence of a narrow particle size distribution, as confirmed by secondary electron micrographs of the collected powder. Characteristic infrared absorption features have their origin in absorption within the dust particles. Besides the strong presence of aliphatic hydrocarbons characteristic for amorphous hydrocarbon films, a significant amount of aromatic structures was detected. Heavy positive ions measured by ion-mass spectroscopy originate from polyacetilenic (C2nH2) and aromatic compounds. Time resolved mass spectra gave insight into the plasma response to the dust formation.
Frequency dispersion of complex permeability in Mn–Zn and Ni–Zn spinel ferrites and their composite materials
Authors: Tsutaoka Takanori
Published: March 2003

Complex permeability spectra μ*=μ′−iμ″ for two types of spinel ferrites (Ni–Zn ferrite and Mn–Zn ferrite) and their composite materials have been investigated. The contribution of domain-wall and natural resonance to the permeability spectra was estimated by the numerical fitting of actual measurement data to a simple formula. Frequency dispersion type of each component, relaxation or resonance, can be estimated from one of the fitting parameters, damping factor. In sintered Mn–Zn ferrite, domain-wall contribution is dominant and gyromagnetic spin resonance or relaxation-type magnetization rotation is large in Ni–Zn ferrite. However, relaxation character is dominant in both Mn–Zn and Ni–Zn ferrite composite materials. In composite materials, the permeability value can be scaled by the ferrite particle content using a simple model concerning demagnetizing field. This analysis is useful in designing the permeability spectra of ferrite composite materials.
Simulations of liquid-crystal Fabry–Perot etalons by an improved 4×4 matrix method
Authors: Huang Yuhua , Wu Thomas X. , Wu Shin-Tson
Published: March 2003

The optical performances of a liquid crystal (LC) Fabry–Perot (FP) etalon in various configurations were simulated by a faster 4×4 matrix method. Results show that the FP with a 90° twisted nematic LC is polarization insensitive if the applied voltage exceeds ∼3X of the Freedericksz transition threshold. This method can well depict the transmission spectra of the FP cavity and, therefore, is useful for optimizing the FP tunable filters for telecommunications.
Magnetic anisotropy derived from interparticle interactions in CoPtCr/SiO2 granular thin film longitudinal media
Authors: Tateno Yasuo , Katakura Hitoshi , Igari Takahiro , Uchiyama Hiroshi
Published: March 2003

The magnetic anisotropy derived from interparticle interactions in a CoPtCr/SiO2 granular thin film longitudinal medium has been experimentally studied. In order to clarify the effect of magnetostatic interactions between particles in the medium, the angular dependence of the interaction fields has been investigated. The interaction fields in the medium were obtained from the remanent magnetization curves measured in several directions in the plane normal to the medium plane by compensating for the demagnetizing fields at each measurement direction. It has been shown that the interparticle interactions in the medium exhibit clear anisotropy, which is related to the magnetostatic interactions. As a result, the intrinsic magnetic anisotropy of the medium has been quantitatively separated into two contributions, the magnetic interaction anisotropy between particles and the anisotropy of each particle. The magnetic anisotropy caused by the interparticle interactions in the medium plays an important role in the thermal stability of the magnetization in the recorded media, in the minor-loop magnetizing process and in the recording process of the medium when using a ring-type head.
Confocal micro-Raman spectroscopy of single biological cells using optical trapping and shifted excitation difference techniques
Authors: Xie Changan , Li Yong-qing
Published: March 2003

We report on the study of single biological cells with a confocal micro-Raman spectroscopy system that uses optical trapping and shifted excitation Raman difference technique. A tunable diode laser was used to capture a living cell in solution, confine it in the confocal excitation volume, and then excite the Raman scattering. The optical trapping allows us to lift the cell well off the cover plate so that the fluorescence interference from the plate can be effectively reduced. In order to further remove the interference of the fluorescence and stray light from the trapped cell, we employed a shifted excitation Raman difference technique with slightly tuned laser frequencies. With this system, high-quality Raman spectra were obtained from single optically trapped biological cells including E. coli bacteria, yeast cells, and red blood cells. A significant difference between control and heat-treated E. coli B cells was observed due to the denaturation of biomolecules.
Mitigation of comminution effects of explosives by particulate materials
Authors: Edwards M. R. , Palmer M. E.
Published: March 2003

Brisance effects were measured by the crushing of a copper cone by the stress wave set up by an explosive. The crushing effect was mitigated by the presence of layers of particulate material. The mitigation was principally achieved by the energy used in consolidating the particulate material. This was a function of the particle size of the sand used and was greatest for a fine-grained sand. For military explosives relative brisance effects measured by cone crushing were similar to those calculated by formulas derived from analysis of the chemistry of the explosive, but results for the lower detonation velocity nitromethane explosive gave values somewhat higher than those derived from chemistry-based formulas.
Molecular-beam epitaxy of Ge on GaAs(001) and Si capping
Authors: Goldfarb I. , Azar J. L. , Grisaru A. , Grunbaum E. , Nathan M.
Published: March 2003

Epitaxial quality of Ge layers on GaAs(001), as well as the quality of the Si capping layers, were investigated in situ by reflection high-energy electron diffraction during growth and, subsequently, by scanning electron and scanning probe microscopies. Ge was grown on the (1×1)-GaAs(001) surface prepared by oxide desorption at 580 °C in an As-free ultrahigh vacuum; its morphology, varying from reasonably flat layers with only atomic scale roughness to relatively large three-dimensional asperities, was found to crucially depend on the GaAs surface quality and growth temperature. The data presented in this work also account for the apparent discrepancies between various groups regarding the Ge/GaAs reconstruction; our detailed analysis proves that, at least under the experimental conditions described herein, it is a mixture of (1×2) and (2×1), rather than a (2×2) or c(2×2). Smooth Si growth was mainly impeded by a large lattice mismatch with the underlying Ge, initially replicating the morphology of the Ge layer and eventually forming discrete three-dimensional islands and continuous undulations. The study shows that flat epitaxial Si capping of GaAs should be possible by employing graded silicon–germanium buffers.
Single to polycrystalline transition in silicon growth by ion-assisted deposition at low temperatures
Authors: Nerding M. , Oberbeck L. , Wagner T. A. , Bergmann R. B. , Strunk H. P.
Published: March 2003

We investigate the microstructure of thin silicon films produced at temperatures below 450 °C by ion-assisted deposition. Our transmission electron microscopy investigations show that in this temperature regime epitaxial growth breaks down at a critical film thickness beyond which the growth changes to polycrystalline or amorphous. The critical thickness increases with an increase in temperature. We use this effect that is well known from molecular beam epitaxy to analyze and quantify growth on silicon substrates with different surface normal orientations at various temperatures. Our results indicate that epitaxy works by far the best for the 〈001〉 surface normal orientation. The critical epitaxial thickness hepi decreases in the sequence hepi(001)≫hepi(113)>hepi(011)>hepi(111). The respective activation energies of 0.6, 2.1, 1.2, and 1.4 eV are evaluated from the temperature dependence.
Growth process and properties of silicon nitride deposited by hot-wire chemical vapor deposition
Authors: Stannowski B. , Rath J. K. , Schropp R. E. I.
Published: March 2003

Hot-wire chemical vapor deposition (HWCVD) is a promising technique for the deposition of silicon nitride layers (a-SiNx:H) at low temperatures. In contrast to the commonly used plasma-enhanced chemical vapor deposition, no ion bombardment is present in HWCVD, which makes it particularly attractive for the deposition of passivation layers on structures that are sensitive to the impact of energetic ions. We deposit hot-wire a-SiNx:H from a mixture of silane and ammonia at substrate temperatures in the range of 300–500 °C. Layers deposited with an ammonia/silane gas-flow ratio of R=30 are close to stoichiometry (N/Si=1.33) with a hydrogen content around 10 at. %. Such films have been implemented in hot-wire a-Si:H thin-film transistors. Deposition with R>30 did not result in an increase of the N content, but led to more porous films. Infrared spectroscopy revealed that moisture penetrates these layers and that oxygen is incorporated in the network under air exposure. Cross-sectional transmission electron microscopy images show that these layers contain spherical voids with diameters of several nanometers. In contrast, films deposited with a lower gas-flow ratio (R<30) are inert and do not contain these voids. Both types of films show columnar growth. To better understand the deposition process, we used deuterated silane (SiD4) as a source gas. We found that no deuterium was incorporated in the films. This gives rise to the assumption that SiH4(SiD4) is cracked effectively at the filaments. We infer that the ammonia species are scarcely dissociated at the filaments but rather in the gas phase by the atomic hydrogen (deuterium) originating from the dissociated silane. The abundance of atomic hydrogen in the gas phase is crucial for the breakup of ammonia and the incorporation of N in the film.
Study of the electrical activation of Si+-implanted InGaAs by means of Raman scattering
Authors: Hernández S. , Cuscó R. , Blanco N. , González-Dı́az G. , Artús L.
Published: March 2003

Raman scattering has been used to study the lattice recovery and electrical activation of Si+-implanted In0.53Ga0.47 As achieved by rapid thermal annealing. The degree of crystallinity recovery of totally amorphized samples is studied for annealing temperatures between 300 and 875 °C. A good degree of recovery is achieved for an annealing temperature of 600 °C. Higher annealing temperatures are required to electrically activate the Si donors. The observed LO phonon-plasmon coupled modes allow us to monitor the electrical activation by means of Raman scattering. We find that electrical activation sets in for annealing temperatures around 700 °C, and gradually increases up to an annealing temperature of 875 °C. The optimal conditions for the rapid thermal annealing are found to be 875 °C for 10 s.
Equation of state for detonation product gases
Authors: Nagayama Kunihito , Kubota Shiro
Published: March 2003

A thermodynamic analysis procedure of the detonation product equation of state (EOS) together with the experimental data set of the detonation velocity as a function of initial density has been formulated. The Chapman–Jouguet (CJ) state [W. Ficket and W. C. Davis, Detonation: Theory and Experiment (University of California Press, Berkeley 1979)] on the p-ν plane is found to be well approximated by the envelope function formed by the collection of Rayleigh lines with many different initial density states. The Jones–Stanyukovich–Manson relation [W. Ficket and W. C. Davis, Detonation: Theory and Experiment (University of California Press, Berkeley, 1979)] is used to estimate the error included in this approximation. Based on this analysis, a simplified integration method to calculate the Grüneisen parameter along the CJ state curve with different initial densities utilizing the cylinder expansion data has been presented. The procedure gives a simple way of obtaining the EOS function, compatible with the detonation velocity data. Theoretical analysis has been performed for the precision of the estimated EOS function. EOS of the pentaerithrytoltetranitrate explosive is calculated and compared with some of the experimental data such as CJ pressure data and cylinder expansion data.
Real-time lock-in imaging by a newly developed high-speed image-processing charge coupled device video camera
Authors: Nishikata Kentarou , Kimura Yoshihide , Takai Yoshizo , Ikuta Takashi , Shimizu Ryuichi
Published: March 2003

Real-time lock-in imaging with high-frequency modulation (up to 16 kHz) was successfully performed at high spatial resolution (640×480 pixels) using a newly developed high-speed image-processing charge coupled device (CCD) video camera. To achieve high-resolution lock-in imaging, the high-speed image-processing CCD video camera incorporates a high-definition TV image sensor that uses a high-definition frame-interline-transfer (FIT) CCD architecture. A novel FIT-CCD driving method, in which vertical CCD shift registers are utilized as a temporary frame memory, enables lock-in imaging at a modulation frequency in excess of the video frame rate (30 Hz). Furthermore, since the high-speed image-processing CCD video camera has a function for subtracting images taken with high-frequency modulation, a lock-in image with no background contrast can be observed in real time. The phase detection function for lock-in imaging, when coupled with real-time image processing using the high-speed image-processing CCD video camera, makes it possible to dynamically observe the phase distribution in a two-dimensional area. As a demonstration of the real-time lock-in imaging, the wave pattern that appears on the surface of water was successfully visualized by a real-time lock-in imaging system that uses the high-speed image-processing CCD video camera.
Biological effects of millimeter-wave radiation: A high-throughput screening system
Authors: Pooley David T. , Gibson Colin , Stewart William R. , Magee John , Ellison Brian N. , Lloyd David
Published: March 2003

This paper reports on the design and evaluation of a high-throughput assay system for investigating the biological effects of microwave/millimeter wave radiation. The approach presented here differs significantly from others with the deployment of a continuous culture device and sample handling technology that allows the rapid presentation of test samples in a uniform physiological state. Segmented flow of the sample passes through a fundamental mode millimeter-wave exposure cell that is, in this prototype, well characterized between 26 and 40 GHz. A sensitive bioluminescence-based reporter monitored the temporal evolution in each assay segment using optical detector arrays mounted at three observation points, i.e., before the sample entered the cell and during millimeter-wave exposure and postexposure. Using this approach, the continuous monitoring of bioluminescence may be used on different time scales to measure cell energy metabolism, gene expression and growth. Exposed and control samples were monitored and any combination of stimulus parameters, namely, radiation frequency, intensity, and polarization (or any modulation of these) that induced a response that exceeded a noise threshold of the system could be further investigated automatically in real time, for example, with increased frequency resolution and at different power levels. Environmental parameters such as the temperature and magnetic and electric fields were carefully controlled. This sensitive “active” search system may have application in industry, biomedical research, and environmental health.
Porosity measurement by comparison of air volumes
Authors: Leclaire P. , Umnova O. , Horoshenkov K. V. , Maillet L.
Published: March 2003

A method for measuring the volume of a solid with arbitrary shape and the open porosity (volume fraction of the open pores) of air-saturated porous materials is proposed. The experimental principle is inspired from Beranek’s method [J. Acoust. Soc. Am. 13, 248 (1942)] based on the application of the ideal gas law to the gas surrounding the solid. The originality of the proposed method lies in the comparison of the volume of a measurement chamber containing the sample with that of an empty reference chamber. During a measurement, the reduction of volume in the measurement chamber due to the introduction of the solid is compensated by increasing the volume of a piston connected to the measurement chamber. An important aspect of this experimental system is that the influence of temperature can be considerably reduced as nearly the same variations of temperature apply simultaneously to the measurement and the reference chambers. A preliminary calibration experiment without sample in the measurement chamber is carried out in order to equalize the volumes of the two chambers. The calibration can be performed once and for all, thus minimizing the number of operations in the measurement process. The porosities of several materials are measured and compared to reference values. The precision and possible future improvements are discussed.
Conformal irradiation by proton beam scanning and multilayer energy filter
Authors: Sakae Takeji , Nohtomi Akihiro , Tsunashima Yoshikazu , Terunuma Toshiyuki , Sato Masaru , Hayakawa Yoshinori , Maruhashi Akira
Published: March 2003

Using a new energy filter, three-dimensional conformal irradiation for proton therapy can be realized by monoenergetic beam scanning. It is a big advantage that the method does not necessitate energy scanning from the viewpoint of shortening the scanning time for cancer treatment of a moving organ. The new filter can yield an irradiation field where the width of the spread-out Bragg peak is adjusted to the target. Present work shows evidence of three-dimensional conformal irradiation by the new filter. Instead of the intensity modulation necessary for conformal irradiation by beam scanning, irradiation-time modulation is performed in scanning using constant beam current. As the result, a conformal field around a step-shaped virtual target in a solid phantom is realized by the filter.
Response compensation of thermistors: Frequency response and identification of thermal time constant
Authors: Tagawa M. , Kato K. , Ohta Y.
Published: March 2003

The purpose of this study is to improve the accuracy of general-purpose temperature sensors (commercially available thermistors) in fluctuating temperature measurement and to extend their applicability. First, heat transfer models reflecting the internal structures of plate-type and spherical thermistors were proposed to obtain theoretical frequency response to temperature fluctuations. Second, parameters dominating the response characteristics of the thermistor were extracted, and it was shown that the thermistor response can be approximated by a first-order system. Finally, a flexible response compensation technique for the thermistor was constructed, the development of which has been very difficult because the thermal time constant varies greatly depending on the flow velocity and physical properties of the fluid surrounding the thermistor. The technique utilized a two-thermocouple probe method—rational identification of thermal time constants and digital compensation for response delay—proposed by Tagawa et al. [Rev. Sci. Instrum. 69, 3370 (1998)], and greatly improved the accuracy of fluctuating temperature measurement by the thermistor. Although the level of the improvement depends on the fluid type measured and the signal-to-noise ratio of temperature signals, the present response compensation technique worked successfully. As a result, the compensated thermistor became 5 to 50 times as fast as the uncompensated (original) one.
Energy evaluation of cavitation bubble generation and shock wave emission by laser focusing in liquid nitrogen
Authors: Tomita Y. , Tsubota M. , An-naka N.
Published: March 2003

The present article describes the energy evaluation of bubble generation and shock wave emission by laser focusing in liquid nitrogen at 78 K. By adjusting the ambient pressures to 164.9 kPa which corresponds to an overpressure, Δp, of 58.8 kPa, the threshold energy for bubble generation can be investigated. When the energy level of a laser beam at the focus exceeds an irradiance threshold, optical breakdown occurs, followed by a series of high-speed phenomena such as plasma formation, shock wave emission, and vapor bubble generation. It is found that at a threshold laser energy there is some probability of a cavitation bubble being generated whose extent tends to decrease with increasing Δp. For an incident laser energy larger than the energy capable of forming cavities with 100% probability, a cavitation bubble is inevitably generated and its maximum size tends to increase with the incident laser energy. The cavitation bubble energy EB reaches about 26% of the consumed laser energy, which is the energy substantially dissipated at the focus, fluctuating between 20% and 30% and independent of the overpressure. The shock wave energy Es is estimated by solving a nonlinear wave equation and calculated as about 35% of the consumed laser energy. Consequently, for liquid nitrogen, the conversion rate of absorbed laser energy to mechanical energy, EB+Es, is calculated as 55%–65%.
Elastic and piezoelectric fields in a substrate AlN due to a buried quantum dot
Authors: Pan E. , Yang B.
Published: March 2003

Using the Green’s function solution in an anisotropic and linearly piezoelectric half space developed recently by the authors, this article studies the elastic and piezoelectric fields in substrate AlN due to a buried quantum dot (QD). Two different growth orientations are considered: One is the AlN (0001) growing along the (0001) axis, and the other is the AlN (1000) growing along the polar direction [i.e., a direction normal to (0001) axis]. For an InN QD, modeled as a concentrated source, with a volume va=4πR3/3 where R=3 nm, and the typical values of misfit strain, at a depth h=10 nm below the surface, the following features have been observed: (1): on the surface of substrate AlN (0001), the hydrostatic strain, piezoelectric potential, and vertical and horizontal electric fields are rotationally symmetric with respect to the z axis. However, these quantities are not rotationally symmetric on the surface of substrate AlN (1000); (2): a hydrostatic strain as large as 0.01 on the surface of the AlN (1000) and as large as 0.008 on the surface of AlN (0001) can be reached, both of which are larger than that on the surface of the substrate GaAs due to a QD with the same volume at the same depth; (3): the piezoelectric potential on the surface of substrate AlN (0001) is much larger than that on the surface of AlN (1000), i.e., 0.8 versus 0.3 V; and (4): large horizontal and vertical electric fields, in the order of 108 V/m, can be induced on the surface of AlN, about two orders of magnitude larger than that on the surface of the substrate GaAs due to a QD with the same volume at the same depth.
Raman investigations of disorder in InN thin films grown by reactive sputtering on GaAs
Authors: Qian Z. G. , Shen W. Z. , Ogawa H. , Guo Q. X.
Published: March 2003

We have used Raman scattering to study the disorder in InN thin films deposited by reactive sputtering on GaAs substrates at low temperature of 100 °C. The asymmetric Raman line shape at the range of 400–700 cm−1 can be well fitted by the combination of a spatial correlation model and a Lorentzian, which are attributed to disordered A1 (LO) and E2 (high) modes for hexagonal InN, respectively. We have shown that the correlation length is growth condition dependent and can be directly related to the crystalline quality. These results further correlate very well with the x-ray diffraction measurements. The relationships between the line-shape parameters and the correlation length have been revealed. We have also analyzed an InN thin film on sapphire substrate grown by the same method for comparison.
Beryllium implantation induced deep level defects in p-type 6H–silicon carbide
Authors: Chen X. D. , Ling C. C. , Fung S. , Beling C. D. , Gong M. , Henkel T. , Tanoue H. , Kobayashi N.
Published: March 2003

Beryllium implantation into p-type 6H–SiC and subsequent thermal annealing have been performed. The deep level defects induced by this beryllium-implantation process have been investigated using deep level transient spectroscopy. Four deep levels labeled BEP1, BEP2, BEP3, and BEP4 were detected in the temperature range 100–500 K. The prominent hole trap BEP1 with an energy level at EV+0.41 eV agrees well with the acceptor level of beryllium determined by Hall effect measurements. The remaining hole traps with energy levels at EV+0.60 eV, EV+0.76 eV, and EV+0.88 eV, respectively, are proposed to be beryllium-implantation induced defects or complexes.
Influences of nitridation on tunneling barrier change and charge trapping caused by electrical stress
Authors: Tan Shyue Seng , Chen T. P. , Ang C. H.
Published: March 2003

By analyzing the change in the Fowler–Nordheim tunneling current as a result of the modulations of both the cathode field and the barrier shape at the tunneling interface caused by electrical stress, the charge trapping modulating the cathode field as well as the barrier change resulting from the charge trapping within the tunneling distance are determined quantitatively. The influence of nitridation on the barrier change and the charge trapping is examined. It is found that nitridation reduces the barrier change and the charge trapping significantly.
Composition dependence of the ultraviolet absorption edge in lithium tantalate
Authors: Bäumer Ch. , David C. , Tunyagi A. , Betzler K. , Hesse H. , Krätzig E. , Wöhlecke M.
Published: March 2003

Comprehensive preparations of lithium tantalate crystals with compositions ranging from the congruent to stoichiometric ones have been carried out. Vapor transport equilibration treatments were used to determine the composition of the samples with an absolute accuracy of 0.05 mol %. This absolute determination of the composition can serve as the basis for convenient relative methods where an easily measurable physical property allows a simple determination of the composition. As an example, we present a study of the fundamental absorption edge in the near-ultraviolet region.
Electron energy loss studies of dislocations in GaN thin films
Authors: Bangert U. , Gutiérrez-Sosa A. , Harvey A. J. , Fall C. J. , Jones R.
Published: March 2003

We present studies of the effects of dislocations in epitaxial GaN films on the low loss and the core loss electron energy loss spectrum. Electron microscopy samples were prepared in cross-sectional and plan-view geometry and investigations carried out in a dedicated cold field emission scanning transmission electron microscope. We consider the relative dislocation signal strength in both orientations on grounds of geometric considerations, and from comparison with the experimental signal, deduce that the scattering cross section for bulk and dislocation related scattering in the core loss energy regime are similar. The low loss results suggest that the scattering cross sections for dislocation related scattering in the band-gap regime are significantly smaller than for bulk scattering processes, also the localization of scattering events in this energy regime is an order of magnitude less than in core loss spectroscopy. The experimental low loss spectra provide evidence for dislocation related energy states below band edge, in accordance with predictions from calculated low loss spectra. Core excitation losses show dislocation related changes in the s−pz hybridized states. A peak in the energy range around 2.5 eV, which is not related to energy states introduced by the dislocation cores but presumably due to point defects, increases in strength for measurements towards the thin film surface, and in the vicinity of dislocations.
Optical and electrical characterization of plasma polymerized pyrrole films
Authors: Kumar D. Sakthi , Nakamura Kenji , Nishiyama Satoko , Ishii Shigeru , Noguchi Hiromichi , Kashiwagi Kunihiro , Yoshida Yasuhiko
Published: March 2003

Plasma polymerization of pyrrole was carried out in the presence and absence of iodine, and the resulting films were characterized by optical and electrical means. Their infrared spectra were very similar to each other, suggesting that iodine was neither bonded in any manner to, nor strongly interacting with, the pyrrole polymer chains. Based on their infrared spectra, a chemical structure was proposed for the plasma-polymerized pyrrole (PPPy) film. An analysis of the electronic spectra gave band gap energies of 1.3 and 0.8 eV for the undoped and doped PPPy films, respectively. In line with this result, the current–voltage characteristics of the two types of polymer films revealed that the conductivity of the doped PPPy film was approximately two times greater than that of the undoped one. An investigation of the scanning electron micrographs led us to conclude that iodine had changed the surface morphology of the PPPy film, resulting in the small increase in conductivity. A detailed analysis of the conduction mechanism disclosed that the conduction mechanism in the undoped PPPy film is a Schottky-type mechanism.
Elemental surface analysis at ambient pressure by electron-induced x-ray fluorescence
Authors: Feldman Jason E. , Wilcox Jaroslava Z. , George Thomas , Barsic David N. , Scherer Axel
Published: March 2003

The development of a portable surface elemental analysis tool, based on the excitation of characteristic x rays from samples at ambient pressure with a focused electron beam is described. This instrument relies on the use of a thin electron transmissive membrane to isolate the vacuum of the electron source from the ambient atmosphere. The major attributes of this instrument include rapid (several minutes) spectrum acquisition, nondestructive evaluation of elemental composition, no sample preparation, and high-to-medium (several hundreds μm) spatial resolution. The instrument proof-of-principle has been demonstrated in a laboratory setup by obtaining energy dispersive x-ray spectra from metal and mineral samples.
Observation of enhanced soft x-ray emission using nitrogen clusters ionized by intense, femtosecond laser
Authors: Mocek Tomáš , Park Jong Ju , Kim Chul Min , Kim Hyung Taek , Lee Dong Gun , Hong Kyung Han , Nam Chang Hee
Published: March 2003

We report soft x-ray spectra (5–18 nm) from a cryogenically cooled nitrogen gas jet irradiated by 28-fs-laser pulses at an intensity of ∼7×1016 W/cm2. While the time-integrated spectrum obtained at room temperature showed exclusively transitions in N4+ ions, spectral lines from N5+ and N6+ charge states appeared with cooling. Rapid, nonlinear increase of x-ray emission on lines from N4+, N5+, and N6+ has been observed by lowering the preexpansion gas temperature.
Swift heavy ion induced growth of nanocrystalline silicon in silicon oxide
Authors: Chaudhari Prajakta S. , Bhave Tejashree M. , Kanjilal D. , Bhoraskar S. V.
Published: March 2003

Recystallization of nanocrystalline silicon in silicon oxide has been initiated with swift heavy ion irradiation. 100 MeV Ni ions from pelletron were used for irradiating the thin films of silicon oxide (SiOx) at fluences varying from 1×1012 to 5×1013 ions/cm2. Phase separation between silicon and silicon oxide is seen to be responsible for the photoluminescence spectrum peaking around 350 and 610 nm. This spectral nature is understood on the basis of defects and interface states in SiOx matrix and silicon nanocrystals, respectively. The formation of silicon nanocrystals resulting from the phase separation has been confirmed from the complimentary evidence of change in the refractive index, Fourier transform infrared spectroscopy, and energy despersive x-ray analysis. High electronic loss associated with the 100 MeV Ni ions is thought to be responsible for the recrystallization, and rearrangement of silicon.
Optimizing the growth of 1.3 μm InAs/InGaAs dots-in-a-well structure
Authors: Liu H. Y. , Hopkinson M. , Harrison C. N. , Steer M. J. , Frith R. , Sellers I. R. , Mowbray D. J. , Skolnick M. S.
Published: March 2003

The structural and optical properties of GaAs-based 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized in terms of different InGaAs and GaAs growth rates, the amount of InAs deposited, and In composition of the InGaAs quantum well (QW). An improvement in the optical efficiency is obtained by increasing the growth rate of the InGaAs and GaAs layers. A transition from small quantum dots (QDs), with a high density (∼5.3×1010 cm−2) and broad size distribution, to larger quantum dots with a low dot density (∼3.6×1010 cm−2) and narrow size distribution, occurs as the InAs coverage is increased from 2.6 to 2.9 monolayers. The room-temperature optical properties also improve with increased InAs coverage. A strong dependence of the QD density and the QD emission wavelength on the In composition of InGaAs well has been observed. By investigating the dependence of the dot density and the high-to-width ratio of InAs islands on the matrix of InGaAs strained buffer layer (SBL), we show that the increasing additional material from wetting layer and InGaAs layer into dots and the decreasing repulsive strain field between neighboring islands within substrate are responsible for improving QD density with increasing In composition in InGaAs SBL. The optical efficiency is sharply degraded when the InGaAs QW In composition is increased from 0.15 to 0.2. These results suggest that the optimum QW composition for 1.3 μm applications is ∼15%. Our optimum structure exhibits a room temperature emission of 1.32 μm with a linewidth of 27 meV.