bluuebunny/crossref_metadata_2025 huggingface dataset:
A numerical technique to describe acoustical scattering and propagation from an object in a waveguide
Authors: Norton Guy V. , Werby M. F.
Published: July 2002

The treatment of scattering from submerged objects in an unbounded environment is of considerable interest to both the academic and technological communities. Several approaches have yielded results for different classes of problems and have proven manageable for the free-environment case. Scattering in a confined environment is difficult to express in a form useful for calculation because the effects from the scattered object couple with the boundary effects. The purpose of this work is to propose a numerical scheme that will adequately describe scattering from realistic objects in a confined environment. Some realistic objects of interest are elongated spheroids and cylinders with rounded end caps. Boundary conditions of interest range from those associated with rigid objects to those associated with elastic shells. The object in a waveguide problem is examined in approximate numerical schemes rather than with attempts at exact solutions. The starting point will be the solution of the incident field in terms of normal modes. Next, transition matrix is used that relates the incident field to the scattered field. The transition matrix, which is obtained from Waterman’s extended boundary condition method, does not account for boundaries other than that of the object. The solution is then coupled with a waveguide solution to satisfy all boundary conditions. The method, an application of Huygens’ principle, couples the solutions and leads to a manageable solution of the problem. This method also satisfies all appropriate boundary conditions and yields a continuous solution throughout space. Details and examples of this method are presented.
Monte Carlo calculation of strained and unstrained electron mobilities in Si1−xGex using an improved ionized-impurity model
Authors: Kay L. E. , Tang T.-W.
Published: July 2002

An improved Monte Carlo (MC) model for ionized impurity scattering developed in a previous work [L. E. Kay and T.-W. Tang, J. Appl. Phys. 70 1475 (1991)] is used to perform a comprehensive study of majority- and minority-electron mobilities in the Si1−xGex material system for both strained and unstrained cases. This investigation includes calculation of low-field mobilities for wide ranges of doping and Ge mole fraction at both 300 and 77 K as well as high-field studies. A significant improvement in mobility (up to 50%) is observed for transport perpendicular to the growth plane in strained Si1−xGex as compared to the unstrained case. The magnitude of the improvement is dependent on doping (both concentration and type) and germanium content, and is somewhat larger at 77 K. High-field MC simulations show that some strained-mobility enhancement remains even at an electric field of 100 kV/cm. These studies also suggest there is a temperature-dependent Ge content for which mobility is maximized at higher dopings.
Origin of picosecond-pulse-induced, degenerate four-wave-mixing signals in KTa1−xNbxO3 crystals
Authors: Liu Huimin , Powell Richard C. , Boatner L. A.
Published: July 2002

Transient gratings have been produced in KTa1−xNbxO3 by picosecond-pulse, two-photon excitation using degenerate four-wave-mixing techniques. The excitation process has been characterized through fluorescence studies, and the fluorescence is attributed to the transition between an excited state and the ground state of B4+ ions that are produced in ABO3 perovskite crystals. Strong electron-phonon coupling gives rise to the luminescence quenching. The observed degenerate four-wave-mixing signal is shown to be due predominantly to a phase grating caused by a change in the susceptibility associated with the formation of Nb4+ or Ta4+ ions in the peak region of the grating. The grating signal intensity was found to be dependent on the concentration of niobium ions and the crossing angle of the two laser write beams.
The effects of pressure on the nucleation rate of an undercooled liquid
Authors: Lee Chun P. , Wang Taylor G.
Published: July 2002

The effects of pressure change on the homogeneous nucleation process are considered, including modeling of the dependence of the melt-crystal interfacial energy on the applied pressure.
Two-dimensional inverse heat conduction problem of estimating the time-varying strength of a line heat source
Authors: Neto A. J. Silva , Özişik M. N.
Published: July 2002

Inverse analysis utilizing the conjugate gradient method is used to estimate the timewise varying strength of a line heat source placed at a specified location in a rectangular region with insulated boundaries. No prior information was used on the functional form of the source strength with time. Transient temperature recordings taken at the boundaries of the region served as the simulated experimental data needed as input for the inverse analysis. In order to test the accuracy of the inverse analysis under most strict conditions, timewise variation of the source strength is chosen in the form of rectangular, triangular, and sinusoidal functions. Both the regular iterated and modified conjugate gradient method are used for solving the inverse problem. The modified conjugate gradient method often provided the answer with smaller number of iterations when the initial condition for the unknown function was available.
Stress induced martensitic transition in a molecular dynamics model of α-iron
Authors: Cheung Kin S. , Harrison Ralph J. , Yip Sidney
Published: July 2002

A structural transition bcc→hcp induced by uniaxial tensile stress is observed by molecular dynamics simulation using an interatomic potential function for α-iron derived by the embedded atom method. The transformation is reversible with hysteresis. The deformation mechanism for the forward transition is found to be different from that for the backward transition. The critical stress required for the transition decreases with increasing temperature, as is expected.
Anisotropy field distribution in amorphous ferromagnetic alloys from second harmonic response
Authors: García-Arribas A. , Barandiarán J. M. , Herzer G.
Published: July 2002

We present a new procedure to obtain the anisotropy distribution in a ferromagnetic amorphous alloy. The method is based on the existence of second-order harmonics in the differential magnetization around the anisotropy field. We derive a mathematical expression relating the strength of the second-harmonic component and the anisotropy distribution function. An experimental setup is described and the optimum operating conditions discussed. The apparatus is based on the detection of the voltage induced in a coil using a lock-in amplifier tuned to a frequency double to the exciting one. Results obtained on a stress annealed sample show the existence of well defined values of the anisotropy in addition to a continuous background.
Two-dimensional model for laser-induced materials damage: Effects of assist gas and multiple reflections inside the cavity
Authors: Kar A. , Rockstroh T. , Mazumder J.
Published: July 2002

A two-dimensional axisymmetric model for materials removal during laser irradiation is presented by taking into account the reflections of the laser beam at the wall of the laser-induced cavity. Also, the liquid metal flow due to the shear stress at the interface of the assist gas and the liquid metal is studied in this paper. The vaporization of the liquid metal and the melting of the solid substrate at the liquid-vapor and the solid-liquid interfaces, respectively, are modeled by applying the Stefan condition at various points on the interfaces. Maximum drilling rate is determined by considering that the laser energy is used only to melt and vaporize the material. The effects of various process parameters such as the laser power, laser beam radius, and the laser pulse-on time on the cavity depth, the recast layer thickness, and the cavity tapering are examined in this study. It is found that the laser-induced materials removal process can be described in terms of a single parameter called the ‘‘gross’’ laser intensity.
Reactive ion etching of SiGe alloys using CF2Cl2
Authors: Zhang Ying , Oehrlein Gottlieb S. , de Frésart Edouard , Corbett James W.
Published: July 2002

The dry etching characteristics of strained Si1−xGex, x≤0.22, thin films (≤1 μm thick) and elemental Si and Ge in CF2Cl2 reactive ion etching plasmas have been studied by measuring etch rates using in situ ellipsometry, etch profiles using scanning electron microscopy (SEM), and surface-chemical aspects by employing x-ray photoelectron spectroscopy (XPS). The etch rates of the Si1−xGex alloys increase as a function of Ge content and fall between the etch rates of pure Si and Ge. The etch rate ratios of Si1−xGex over Si and Ge over Si1−xGex are ∼1.5 and ∼2.4 at a Ge content of 22%, respectively. SEM photographs of the trench profiles in Si1−xGex alloys with either SiO2 or photoresist masks show directional etching characteristics of the CF2Cl2 reactive ion etching process. The chemically shifted intensities of the Si 2p(1/2, 3/2) and Ge 2p(3/2) core levels indicate a 1- or 2-monolayer-thick reaction layer on the Si1−xGex alloy surface as a result of CF2Cl2 reactive ion etching. Residual Cl is identified as the principle surface impurity by XPS. The first few monolayers of the plasma-exposed SiGe surface is enriched in Si. From a comparison of Si1−xGex etch characteristics with those of Si and Ge etched under identical conditions, we conclude that the behavior of Si1−xGex alloys is very similar to elemental Si.
Ion irradiation damage in n-type GaAs in comparison with its electron irradiation damage
Authors: Eisen F. H. , Bachem K. , Klausman E. , Koehler K. , Haddad R.
Published: July 2002

In an effort to attain a better understanding of the nature of the defects introduced in GaAs by irradiating it with energetic light ions; electron or proton irradiated n-type GaAs samples, cut from the same layer grown by molecular-beam epitaxy, have been studied by deep level transient spectroscopy. By comparing the spectra, including the effects of high electric fields, and by using results for annealed samples, it is possible to determine which of the traps reported in electron irradiated GaAs, most of which are believed to be arsenic interstitial-vacancy pairs, are present in the proton irradiated material. The traps identified in proton irradiated GaAs include most of those found in electron irradiated material, either after irradiation or after irradiation and annealing. The results indicate that two of these traps are associated with defects which are more complex than simple interstitial-vacancy pairs. Two traps were found in proton irradiated material which have not been observed in electron irradiated GaAs. One of these is nearly as abundant as the prominent E3 center observed in electron irradiated GaAs and is probably also not a simple pair. The deep level transient spectroscopy peak for this trap is not clearly separated from that of E3 in proton irradiated GaAs. The other trap is probably associated with a particular impurity present in the MBE grown sample layers.
Origin of coercivity in (Fe,Co)-based granular films
Authors: Tsoukatos A. , Wan H. , Hadjipanayis G. C. , Papaefthymiou V. , Kostikas A. , Simopoulos A.
Published: July 2002

The magnetic behavior of Co and Fe granular films was studied relative to their host matrix environment (BN, SiO2). The crystal structure of Co and Fe in the as-deposited samples is α-Co (hcp) and α-Fe (bcc) respectively, with particle sizes ranging between 3 and 9 nm. The coercivities measured ranged from a few tens to a few hundreds Oe, with the higher values observed for particles embedded in the oxygen based matrix.
Quantum tunneling in magnetic systems of various sizes (invited)
Authors: Barbara B. , Sampaio L. C. , Wegrowe J. E. , Ratnam B. A. , Marchand A. , Paulsen C. , Novak M. A. , Tholence J. L. , Uehara M. , Fruchart D.
Published: July 2002

Magnetic relaxation experiments constitute a unique method of determining the nature of fluctuations in dissipative magnetic systems. At high temperatures these fluctuations are thermal and strongly temperature dependent. At low temperatures, where quantum fluctuations dominate, magnetic relaxation becomes independent of temperature. Such behavior has been observed in many systems. In this review we emphasize the study of low temperature relaxation in ferromagnetic nanoparticles, layers, and multilayers (including ‘‘domain wall junctions’’), and large single crystals. The results of magnetic relaxation experiments are shown to agree with theoretical predictions of quantum tunneling of the magnetization. When dissipation becomes important, in large and complex systems, a time dependent WKB exponent needs to be introduced.
Kinetics of structure formation in electrorheological suspensions
Authors: Klingenberg D. J. , Zukoski C. F. , Hill J. C.
Published: July 2002

Kinetics of structure formation in electrorheological (ER) suspensions are characterized by the time required to form percolating fibers following application of an external electric field. Experimental results are compared with predictions from simulations using an electrostatic polarization model for ER suspensions. Results from the two approaches agree, with large response times at small concentrations, decreasing to 10 ms at large concentrations and field strengths of 2 kV/mm.
Microstructural evaluation of strained multilayer InAsSb/InSb infrared detectors by transmission electron microscopy
Authors: Chadda Saket , Datye Abhaya , Dawson L. Ralph
Published: July 2002

InSb/InAsSb strained layer superlattices (SLS) were grown on (001) InSb substrates by molecular beam epitaxy at 425 °C. The active device consisted of an InAs0.15Sb0.85/InSb superlattice region embedded within a p-i-n junction. The large lattice mismatch between the active device and the substrate required the growth of a buffer. InAs0.15Sb0.85/InSb SLS, where the average As content was gradually increased, was used as a buffer. The buffer structure was varied to probe its microstructural effect on the capping device. Three distinct approaches (A, B, and C) were used to grow the buffer. Approach A was a four-step buffer where the average content of As in the superlattice was increased in four equal composition steps. This approach led to a crystal with an extensive network of threading dislocations and microcracks. Approach B was to change the average composition in five equal composition steps, thereby decreasing the misfit at the interfaces between composition steps. This led to a decrease in the threading dislocation density but microscopic cracks were still evident. The last approach (C) was to employ migration enhanced epitaxy (MEE) for the growth of the five-step buffer. Samples grown by employing MEE revealed no microcracks but they contained a high density of unusual ‘‘wiggly’’ dislocations at the buffer/device interface. Detailed microstructural analysis by transmission electron microscopy is presented.
Determination of thermal diffusivity of low-diffusivity materials using the mirage method with multiparameter fitting
Authors: Rantala J. , Wei Lanhua , Kuo P. K. , Jaarinen J. , Luukkala M. , Thomas R. L.
Published: July 2002

A method to determine anisotropic thermal diffusivities parallel to the surface of a solid sample is presented. The measurement method is based on the mirage effect and the data are analyzed using multiparameter least-squares regression fitting. The specimens studied were of medium and low diffusivity: rare earths, ceramics, and polymers. Before this study, the low-diffusivity limit for the applicability of the mirage method was about 0.02 cm2/s, and therefore this technique has not been suitable for polymeric materials. In this work the limit has been reduced to 5×10−4 cm2/s by improving the measurement apparatus and the data analysis method. The thermal diffusivities obtained agree with the values obtained using the flash method within ±20%. The agreement with published values is good, taking into account that for ceramics and polymers the literature gives only the correct order of magnitude due to the differences in the manufacturing process. Even though the sensitivity of the measurement decreases with the decreasing diffusivity, the thermal diffusivity of bulk polymers can still be determined within an accuracy of 10%. For polymer foils the method has been found to be reliable when the foil thickness exceeds 100 μm.
Magnetomechanical damping in FeSiB amorphous wires
Authors: Atalay S. , Squire P. T.
Published: July 2002

Magnetomechanical damping of amorphous wires has been measured as a function of the surface torsional strain (frequency∼1–4 Hz) at different applied tensile stresses and applied magnetic fields. The wire used in these measurements has a composition of Fe77.5B15Si7.5 with a diameter of 125 μm. The variation of magnetomechanical damping as a function of torsional strain shows a maximum as previously observed in ferromagnetic materials. It has been found that the main mechanism responsible for magnetomechanical damping is magnetomechanical hysteresis associated with irreversible motion of 90° domain walls. The influence of macroeddy and microeddy currents is negligible at the very low frequency of measurement. The large casting stress of ≳200 MPa was reduced by furnace annealing at zero field to about 2 MPa. Further annealing induced a small amount of surface crystallization. The effects of annealing, magnetic field and tensile stress on the magnetomechanical properties of amorphous wires are discussed in the light of existing models. It is shown that a model based on the distribution of internal stress alone is unable to explain all the features of the results. A model incorporating the distribution of domain wall energy is successful in explaining the results if a correction is made for the significant strain dependence of shear modulus.
Mössbauer study of intercalation modified compounds R2Fe17 (R=Y, Sm)
Authors: Chen X. , Ryan D. H. , Altounian Z. , Liao L. X.
Published: July 2002

Mössbauer studies were carried out at 77 K in two series of R2Fe17 compounds with a magnetic (Sm) and nonmagnetic (Y) rare-earth, intercalated with H, C, and N. The increase in hyperfine field is largest at the 12j(18f) sites for R2Fe17H3.7 and R2Fe17N2.3. While the lattice expansion in R2Fe17C2 is similar to that in the nitrides and hydrides, the small change in hyperfine fields at the 12j(18f) and 12k(18h) sites indicates that the presence of neighboring carbon largely cancels the moment increase associated with the volume increase. For R2Fe17 carbonitrides, a single, sharp magnetic transition indicates a uniform compound. However, Mössbauer spectra suggest the existence of both C-rich and N-rich precipitates in the carbonitrides whose average hyperfine fields and isomer shifts scale with the nitrogen to carbon ratio.
Ferromagnetic resonance studies of Fe16N2 films with a giant magnetic moment
Authors: Takahashi H. , Mitsuoka K. , Komuro M. , Sugita Y.
Published: July 2002

The g factor and 4π Ms for epitaxially grown Fe16N2(001)/In0.2Ga0.8As(001) films have been investigated by ferromagnetic resonance along with Fe films for comparison. Angular dependence of the resonance fields in the film plane of Fe16N2 films had four-fold symmetry, which was attributed to the in-plane anisotropy. The g factor for Fe16N2 films was about 2.0, which means that the magnetic moment originates mainly from spin. Thus, nothing unusual is seen about the g factor. The g factor for Fe films was about 2.1, which is very similar to the value reported previously. 4πMs values for Fe16N2 and Fe films were 2.8×104 and 2.1×104 G, respectively, which agree well with the previous data obtained by a vibrating sample magnetometer. This confirmed that Fe16N2 has a giant magnetic moment. Torque magnetometer measurements showed that Fe16N2 films have a larger perpendicular anisotropy of 7.8×106 erg/cm3, which can originate from its bct structure.
Photoluminescence, photoluminescence excitation, and resonant Raman spectroscopy of disordered and ordered Ga0.52In0.48P
Authors: DeLong M. C. , Mowbray D. J. , Hogg R. A. , Skolnick M. S. , Hopkinson M. , David J. P. R. , Taylor P. C. , Kurtz Sarah R. , Olson J. M.
Published: July 2002

Ordering in the CuPt structure is known to significantly reduce the band gap of Ga0.52In0.48P as well as induce a number of unusual details in its optical properties, including long, excitation-intensity-dependent lifetimes and an excitation-intensity-dependent emission energy. We report photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman measurements performed on ordered and disordered Ga0.52In0.48P. The dominant high energy emission process at low temperature in disordered Ga0.52In0.48P is established to be excitonic, but the exciton trapping energy is not unique. PLE from ordered Ga0.52In0.48P shows significant tailing of electronic states into the band gap and a ‘‘band edge’’ which depends on detection energy. The dominant radiative process in ordered Ga0.52In0.48P is not excitonic. A large increase in the Stokes shift between the absorption edge (band gap) and PL emission peak occurs when the material orders. Hence, low temperature PL is determined to be a particularly poor measure of band gap. Resonant Raman scattering is used to study optical phonons and their coupling to electronic states. We find that the resonance enhancement at the band edge occurs via localized excitons.
Radiation-induced reverse dark currents in In0.53Ga0.47As photodiodes
Authors: Shaw G. J. , Messenger S. R. , Walters R. J. , Summers G. P.
Published: July 2002

The reverse dark current-voltage (dark I-V) curves of InGaAs photodiodes have been measured as a function of temperature following irradiation with 1-MeV electrons. Prior to irradiation, the I-V curves are well described by a diffusion term alone indicating that the junctions are of good quality. Irradiation produces a large increase in the generation current which can be modelled as resulting from a single defect center with an energy Ec−0.29 eV. Such a defect center called E2 has been detected using deep level transient spectroscopy.
Texture, microstructure, and magnetic properties of Fe-Co alloy films formed by sputtering at an oblique angle of incidence
Authors: Ono H. , Ishida M. , Fujinaga M. , Shishido H. , Inaba H.
Published: July 2002

A study was made of the structure and magnetic properties of Fe-Co alloy films produced by dc magnetron sputtering at an oblique angle of incidence. The films obtained in this manner showed uniaxial magnetic anisotropy in the film plane, with an easy axis of magnetization perpendicular to the incident direction of the sputtered particles. The anisotropy field increased with increasing sputtering gas pressure and with increasing Co content at high pressures. The structure of the film surfaces also changed with sputtering Ar gas pressure, and elongated grains were observed in the intermediate Ar pressure range. The texture was essentially isotropic in the film plane over the whole pressure range. It is suggested that the observed uniaxial anisotropy was due mainly to a magnetoelastic effect.
Characteristics of low-temperature and low-energy plasma-enhanced chemical vapor deposited SiO2
Authors: Hsieh S. W. , Chang C. Y. , Hsu S. C.
Published: July 2002

Deposition of high quality silicon dioxides was obtained at low temperatures by a specially designed plasma chemical vapor deposition (PECVD) system. The system employed a long quartz tube as a reactor and a set of three vertically standing plates for glow discharge. The deposited oxide, which employed a large amount of helium (He) to dilute silane (SiH4) and nitrous oxide (N2O) reactants, was observed to possess excellent qualities, as compared to those of thermally grown oxides. The chemical composition used for obtaining electrical integrities of the PECVD oxides was investigated. Additionally, the effects of post-metallization annealing on the oxides were investigated in detail. Oxides deposited at a substrate temperature of 250 °C were observed to possess a low interface trap state density (Dit) of only 3×1010 cm−2 eV−1 and low total trapped charge density (Qtotal) of 5.8×1016 cm−3. Those, however, which were deposited at 350 °C, have more stable electrical characteristics under current/voltage bias-temperature stress, but their Dit and Qtotal are around 2×1011 cm−2 eV−1 and 3.4×1016 cm−3, respectively. An atom-bonding model was proposed in this present work, according to physical, chemical, and electrical analyses in accounting for phenomena of charge-trapping and also in upgrading the electrical integrity of the deposited oxides. The applicability of these low-temperature oxides toward fabricating hydrogenated amorphous-silicon thin film transistor (a-Si:H TFT’s) was investigated. Adequate electrical performances of the TFT’s with a high on/off current ratio of more than 106 and high field effect mobility (μFET) of around 0.6 cm2/V s were obtained.
Preparation of Bi2Sr2CaCu2Oy films on alumina substrates with a CuAl2O4 buffer layer
Authors: Lee Kiejin , Song Insang , Park Gwangseo
Published: July 2002

High-Tc Bi2Sr2CaCu2Oy films have been prepared using the surface diffusion process, with the screen printing of Bi2O3, SrCO3, and CaCO3 (Bi:Sr:Ca=2:2:2) mixed powders on Cu-deposited alumina substrates. Through the heating at temperatures above 400 °C, CuAl2O4 buffer layers were formed via an interaction between the Cu layer and alumina. The Bi2Sr2CaCu2Oy films, heat treated at 860 °C for 30 min in air, have a zero resistance at 72 K. The x-ray diffraction, scanning electron microscope, and energy dispersive x-ray analysis studies show that the Bi2Sr2Ca1Cu2Oy films are strongly c-axis oriented along the direction normal to the alumina substrate, and the CuAl2O4 buffer layer acted as a barrier to suppress the interdiffusion of Al ions into the superconducting films.
Re-emission of iron originally gettered by oxygen precipitates in a silicon wafer
Authors: Aoki Masaki , Hara Akito
Published: July 2002

We studied the thermal stability of Fe originally gettered by oxygen precipitates. After low-temperature annealing for gettering, we annealed at higher temperatures. We found that the Fe concentration increases faster in gettered wafers than as-grown wafers. This shows that Fe was re-emitted from the bulk defect region faster than surface Fe precipitates decomposed. The Fe concentration increases with annealing time to the solid solubility level at the temperature used. These results suggest that gettering and the re-emission of Fe are two parts of a reversible reaction.
Observations on the surface and bulk luminescence of porous silicon
Authors: Jiang D. T. , Coulthard I. , Sham T. K. , Lorimer J. W. , Frigo S. P. , Feng X. H. , Rosenberg R. A.
Published: July 2002

Using the x-ray excited optical luminescence technique, we have investigated the soft x-ray induced photoluminescence of porous silicon in the optical region (200–900 nm) and the Si K-edge x-ray absorption fine structures of porous silicon in the near edge region. It is found that while porous silicon prepared at low current density (20 mA/cm2 for 20 min) exhibits a single broad luminescence band, porous silicon prepared at high current density (200 mA/cm2 for 20 min) exhibits three optical luminescence channels; i.e., in addition to the broad peak characteristic of all porous silicon, there are at least two additional optical luminescence channels at shorter wavelengths, one with modest intensity at ∼460 nm and the other a weak and very broad peak at ∼350 nm. These optical channels have been used to monitor the Si K-edge absorption of porous silicon in the near edge structure region. Analysis of the data shows that while the band at ∼627.5 nm corresponds to the bulk emission, the other channels are of a surface origin. These observations and their implications are discussed.
Critical solidification behavior of undercooled Ag-Cu alloys
Authors: Walder S. , Ryder P. L.
Published: July 2002

Melts of Ag, Cu, and four different Ag-Cu alloys, including the eutectic composition, with masses between 1 and 2.5 g were undercooled by the glass flux method, and the temperature of the specimen was measured with a pyrometer during cooling and recalescence. For each composition the maximum recalescence rate was determined as a function of the undercooling. Comparison of the results for Ag and Cu with published measurements of the dendrite growth velocities in these metals showed that the maximum recalescence rate provides a good semiquantitative measure of the growth rate. For the Ag-Cu alloys there is a critical solidification temperature—close to T0—at which the maximum recalescence rate rises abruptly by about two orders of magnitude, accompanied by significant changes in the microstructure. It is shown that this behavior can be explained theoretically on the basis of a transition from diffusion-controlled growth of the eutectic or the primary dendrites to dendritic growth of a supersaturated phase, provided that interface kinetics and growth-rate-dependent partition coefficients are taken into account.
Experimental scaling laws for mass-ablation rate, ablation pressure in planar laser-produced plasmas with laser intensity, laser wavelength, and target atomic number
Authors: Dahmani Faiz
Published: July 2002

Layered-target experiments at 1.06 μm for carbon and silicon materials have been carried out to measure mass-ablation rate ṁ and ablation pressure Pa as a function of absorbed laser intensity Ia, laser wavelength λL, and target atomic number Z at irradiances of 1013–1015 W/cm2. The results can be put in the forms ṁ(kg/s cm2)≂55 [Ia(W/cm2)/1014]1/3λL−4/3(μm) Z3/8 and Pa(Mbar)≂7.4 [Ia(W/cm2)/1014]2/3λL−2/3(μm) Z3/16. The experimental data are compared with one-dimensional hydrodynamic calculations (code medusa) using different values of electron heat-flux limitation. An indication of laser intensity, target atomic number dependent on flux inhibition was found: 0.06≤f≤0.08 for silicon targets and f=0.015–0.03 for carbon targets. These flux limitations are discussed in terms of a small lateral transport and an eventual presence of intense magnetic fields.
Analysis of a novel phenomenon in a solidlike structure in ovalbumin aqueous colloids using the Yukawa potential
Authors: Matsumoto Takayoshi , Inoue Hiroshi
Published: July 2002

A solidlike structure in ovalbumin (OA) aqueous colloids was investigated at various ionic concentrations of the phosphate buffer solution (0–1.5 M) and was analyzed using the Yukawa potential. Rigidity and yield stress due to the solidlike structure increase with increasing ionic concentration and decrease passing through a maximum. This is novel behavior which has never been reported. This behavior is very different from the behavior of the usual (conventional) colloids. From the theoretical consideration by the Yukawa potential, it turns out that the novel behavior of the OA colloid is caused by the fact that the dispersing particles in the colloid, i.e., dimeric molecules composed of the OA globular molecules, are extremely small compared with polymer particles and lattices, etc. That is, the theoretical curve of rigidity versus κ (Debye–Hückel parameter) shifts to a region of large values of κ with decreasing particle radius. A phase diagram is shown in the logarithmic relationship between the OA concentration c and the concentration of buffer solution cb. The boundary between the liquid phase and the solid phase can be represented by a straight line with a slope of −1 over wide ranges of c and cb.
A Monte Carlo simulation of anisotropic electron transport in silicon including full band structure and anisotropic impact-ionization model
Authors: Kunikiyo T. , Takenaka M. , Kamakura Y. , Yamaji M. , Mizuno H. , Morifuji M. , Taniguchi K. , Hamaguchi C.
Published: July 2002

The physics of electron transport in bulk silicon is investigated by using a newly developed Monte Carlo simulator which improves the state-of-the-art treatment of hot carrier transport. (1) The full band structure of the semiconductor was computed by using an empirical-pseudopotential method. (2) A phonon dispersion curve was obtained from an adiabatic bond-charge model. (3) Electron-phonon scattering was computed by using a rigid pseudo-ion model. The calculated scattering rate is consistent with the full band structure and the phonon dispersion curve of silicon, thus leaving no adjustable parameters such as deformation potential coefficients. (4) The impact-ionization rate was calculated by using Fermi’s golden rule directly from the full band structure. We took into account the dielectric function depending on both wave vector and transition energy in the numerical calculation of the rate. The impact-ionization rate obtained in the present study strongly depends on both wave vector and band index of the conduction electron, which is ignored by the traditional Keldysh formula. (5) In the simulator, the final state of a scattering electron is determined in such a way as to conserve both energy and momentum in scattering processes. The simulated results, under the steady-state conditions as well as under the nonequilibrium conditions, are presented and compared with experimental results. Special attention is focused on anisotropic transport during velocity overshoot. Quantitative agreement between calculated and experimental results confirms the validity of the newly developed Monte Carlo simulator and the physical models that were used.
Growth and properties of YBa2Cu3O7−x/(SrTiO3/PrGaO3)/YBa2Cu3O7−x trilayers: Optimization of the insulation
Authors: Brorsson G. , Olsson E. , Nilsson P. Å. , Claeson T.
Published: July 2002

YBa2Cu3O7−x/insulator/YBa2Cu3O7−x trilayers with a single PrGaO3 (PGO) layer or a SrTiO3 (STO)/PGO multilayer as the insulator were prepared by laser deposition. The PGO layers contained pinholes extending down to the bottom layer, causing superconducting microshorts. The pinholes were eliminated by introducing thin STO films, both as buffer layers and in the PGO itself. Insulation resistivity values of 2×109 Ω cm were measured at 77 K for areas up to 1500 μm2 in these trilayers. For larger areas the insulation decreased by orders of magnitude. This was caused by local disruption of the insulator layer, possibly induced by contaminations or defects in the substrate. The particles observed on the bottom layer were in general continuously covered by the multilayer insulator, and did not affect the insulation. The high-quality insulator layer was found to limit the oxidation of the bottom layer, due to a slower rate of oxygen diffusion. The structure of the STO/PGO layers depended on the thickness of the individual films. Voids started to form in the PGO above a critical thickness of 40 nm. The voids lowered the dielectric constant of the insulator, and a value of 18.5±4.5 was measured at room temperature for a STO/PGO multilayer insulator. The voids could be eliminated by decreasing the thickness of the individual PGO films.
Indium nitride thin films prepared by radio-frequency reactive sputtering
Authors: Maruyama Toshiro , Morishita Tomonori
Published: July 2002

Indium nitride thin films were obtained by the reactive sputtering method. The metallic indium target was sputtered by nitrogen gas with rf magnetron sputtering equipment. The stoichiometric indium nitride film showed high Hall mobility (363 cm2 (V s)−1) and low carrier concentration (5.98×1018 cm−3). To suppress the nitrogen vacancies, the reevaporation of atomic nitrogen from the substrate must be suppressed by lowering the substrate temperature and the resputtering of atomic nitrogen must be minimized by adjusting the total pressure close to the upper limit of the effective pressure for deposition.
Generation of deep levels in silicon under posthydrogen-plasma thermal anneal
Authors: Nam C. W. , Ashok S.
Published: July 2002

Si wafers subject to short-time (4–12 min), low-temperature atomic hydrogen cleaning in an electron-cyclotron-resonance plasma system have been annealed subsequently in the temperature range 300–750 °C for 20 min. While only a small broad peak is discernible immediately after hydrogenation, several pronounced and distinct majority-carrier trap levels appear in deep-level transient spectroscopy measurements of subsequently fabricated Schottky diodes on both n- and p-type Si samples annealed at 450 °C and above. The concentrations peak at anneal temperatures around 500 °C and drop substantially beyond 750 °C. This phenomenon appears to be unrelated to the presence of oxygen in Si and is of potential importance in silicon processing technology.
Photoluminescence from Si(001) films doped with 100–1000 eV B+ ions during deposition by molecular beam epitaxy
Authors: Noël J.-P. , Rowell N. L. , Greene J. E.
Published: July 2002

Temperature-dependent photoluminescence (PL) measurements have been used to characterize 5-μm-thick Si(001) layers doped with low-energy 11B+ ions (EB+=100, 500, and 1000 eV) during molecular beam epitaxy (MBE) at growth temperatures Ts=500, 650, and 800 °C. Films deposited at 800 °C with EB+=100 and 500 eV exhibited spectra comparable to bulk Si with narrow intense B1TO exciton recombination peaks together with multiple-exciton B2TO and B3TO peaks as well as free-exciton FETO, BTO+Or1, BTO(2h), B1TA, and B1LO peaks showing that the films are of very high quality. The overall luminescence intensity was found to decrease with decreasing Ts and increasing EB+. PL spectra from films grown at the lowest temperature, Ts=500 °C, were quite different in appearance with very weak bound-exciton peaks and additional features, I3 and I3TA, near 1040 meV due to residual ion-induced damage which increased in intensity with increasing EB+. Compared to As+ ion doping, lower ion energies and/or higher growth temperatures are required to avoid residual ion-induced damage in B+ ion-doped MBE Si(001). Post-annealing experiments showed no detectable residual ion-induced lattice damage in B+-doped, Ts=500 °C, films after 15 min at 800 °C.
Spectroscopic and laser investigations of Mn5+:Sr5(VO4)3F
Authors: Merkle Larry D. , Guyot Yannick , Chai Bruce H. T.
Published: July 2002

Mn:Sr5(VO4)3F has been investigated as a laser material for operation near 1200 nm. Absorption and emission spectra are consistent with Mn5+ substitution into the distorted tetrahedral V5+ sites of this apatite crystal. Emission is almost entirely from the 1E state, resulting in a room-temperature fluorescence lifetime of 475 μs. Together with a peak stimulated emission cross section of about 1×10−19 cm2 and strong, broad absorption bands, this is attractive for lamp- or diode-pumped laser operation. However, the 1E emission makes Mn:Sr5(VO4)3F a three-level system. Laser operation has been demonstrated at room temperature, but the efficiency is lower than expected. Single-pass gain measurements and saturable absorption and fluorescence data indicate that the primary difficulty is excited-state absorption at the pump wavelength. This absorption is very broad, and is probably due to 1T1 and 1T2 excited states.
Tight-binding calculation of ZnSe/Ge superlattices: Electronic structure and optical property
Authors: Wang E. G. , Chen Changfeng , Ting C. S.
Published: July 2002

The results of a detailed tight-binding calculation of the electronic structure and optical properties of (ZnSe)n/(Ge2)m (110) superlattices are presented for a wide range of n,m≤20. It is found that the fundamental energy gap increases with decreasing superlattice period due to spatial quantum confinement effects. For all reasonable values (ranging from 0.2 to 2.0 eV) of the valence band discontinuity used in the calculation, no interface states are found in the thermal gap of the (ZnSe)n/(Ge2)m (110) (n,m≤20) system. An indirect–direct band gap transition is found to be driven by increasing valence band offset. The optical properties of the superlattices are investigated by calculating the absorption coefficient. The results are analyzed according to the calculated electronic structure and compared with those of the corresponding bulk materials.
Tunneling conduction in Co-cluster/tetraoctylammonium bromide/poly(phenyl-p-phenylenevinylene) nanocomposites
Authors: Hohl G.-F. , Baranovskii S. D. , Becker J. A. , Hensel F. , Quaiser S. A. , Reetz M. T.
Published: July 2002

A system of nanometer sized cobalt clusters surrounded by dielectric shells has been investigated by electrical dc conductivity measurements. The cobalt clusters have a diameter of 36 Å and are surrounded by an approximately 12-Å-thick layer of tetra-octyl-ammonium bromide surfactant molecules and poly(phenyl-p-phenylenevinylene). The conductivity σ shows a temperature dependence ln(σ)∝(T0/T)1/2 in the range 100<T<240 K. At T≳240 K, the temperature dependence of the conductivity demonstrates some peculiarities. Differential scanning analysis suggests that at 240<T<350 K phase transitions of the composite material take place. The observed temperature dependence of the conductivity σ for T<240 K is typical for hopping conduction in granular metallic systems. A theoretical analysis shows that the data are consistent with a hopping model in which the separation between grains is a random variable uncorrelated with the activation energy. The observed nonlinear dependence of the conductivity on the electric field F is interpreted in the framework of the concept of the effective temperature, which suggests that the influence of T and F on σ can be parameterized by a single quantity Teff(T,F). Comparison between the theoretical results and experimental data provides a suitable expression for Teff(T,F).
Temperature dependence of strain in ZnSe(epilayer)/GaAs(epilayer)
Authors: Thomas R. J. , Rockwell Benjamin , Chandrasekhar H. R. , Chandrasekhar Meera , Ramdas A. K. , Kobayashi M. , Gunshor R. L.
Published: July 2002

A study of biaxial strain as a function of temperature in a ZnSe epilayer grown on a GaAs substrate is presented. The strains are determined by measuring the heavy- and light-hole related excitonic transitions via photomodulated spectroscopy. The strain is found to increase with increasing temperature. The data are compared with a calculation using a previously determined elastic constant and thermal expansion coefficients. The temperature dependence determined here allows a comparison of various other optical measurements performed at different temperatures.
Further studies on ultrasonic excitation in an acoustic superlattice
Authors: Zhu Yong-yuan , Zhu Shi-ning , Qin Yi-qiang , Ming Nai-ben
Published: July 2002

This article analyzes theoretically the ‘‘in-line field’’ scheme and the ‘‘crossed-field’’ scheme for acoustic excitation of an acoustic superlattice for both resonators and transducers. The electrical impedance and the admittance are deduced and the electromechanical coupling coefficients are calculated.
Three-dimensional epitaxy: Thermodynamic stability range of coherent germanium nanocrystallites in silicon
Authors: Balasubramanian S. , Ceder G. , Kolenbrander K. D.
Published: July 2002

We study the stability range of coherent Ge quantum dots with an epitaxial Si shell. The critical radius is evaluated as a function of Si shell thickness and Ge nanocrystallite radius by comparing the energy of the system in the coherent and incoherent state. We find that the system is coherent up to a Ge nanocrystallite radius of about 100 Å, irrespective of the Si shell thickness. Nanocrystallites of radii larger than 270 Å lose coherency by the generation of perfect dislocation loops. In nanocrystallites of intermediate radii (between 100 and 270 Å), the coherency is lost by the introduction of partial dislocation loops enclosing a stacking fault. As the shell thickness decreases, the critical radius increases.
Kinetic Ising systems as models of magnetization switching in submicron ferromagnets
Authors: Richards Howard L. , Sides Scott W. , Novotny Mark A. , Rikvold Per Arne
Published: July 2002

Experimental techniques, such as magnetic force microscopy (MFM), have recently enabled the magnetic state of individual submicron particles to be resolved. Motivated by these experimental developments, we use Monte Carlo simulations of two-dimensional kinetic Ising ferromagnets to study the magnetic relaxation in a negative applied field of a grain with an initial magnetization m0=+1. The magnetostatic dipole–dipole interactions are treated to lowest order by adding to the Hamiltonian a term proportional to the square of the magnetization. We use droplet theory to predict the functional forms for some quantities, which can be observed by MFM. One such quantity is the probability that the magnetization is positive, which is a function of time, field, grain size, and grain dimensionality. The relaxation is characterized by the number of droplets larger than a field-dependent critical size, which form during the switching process. Our simulations of the kinetic Ising model are in excellent agreement with droplet-theoretical predictions. The qualitative agreement between experiments and our simulations of switching in individual single-domain ferromagnets indicates that the switching mechanism in such particles may involve local nucleation and subsequent growth of droplets of the stable phase.
Use of Kramers–Kronig transforms for the treatment of admittance spectroscopy data of p-n junctions containing traps
Authors: León C. , Martin J. M. , Santamaría J. , Skarp J. , González-Díaz G. , Sánchez-Quesada F.
Published: July 2002

The use of Kramers–Kronig transforms is proposed for the treatment of admittance spectroscopy data of junctions when significant shunt conductance or series resistance is present. An algorithm has been implemented to calculate the transformations numerically and the validity of the method developed has been tested using simulated data. Two experimental systems, p-n junctions into InP made by ion implantation, and atomic-layer-epitaxy-grown CdS/CdTe heterojunctions, have been characterized using this procedure.
Characterization of semiconductor heterojunctions using internal photoemission
Authors: Chen Ing-Shin , Jackson T. N. , Wronski C. R.
Published: July 2002

Internal photoemission is a photoelectric characterization technique commonly used to study semiconductor heterojunction characteristics. Previous analyses of internal photoemission including those of Fowler, Kane, and Powell have been well accepted and applied; however, the quantum effects were not fully taken into account. We examined the quantum effects on internal photoemission spectra and found that the photoemission yield, Y(hν), is proportional to (hν−ET)γ, where hν is the photon energy, ET is the threshold energy, and γ is a constant determined by the emitter density of states, the transmission probability, and the collector conduction band density of states, respectively.
Interaction of dopants with a host GaAs lattice: The case of low-temperature grown molecular beam epitaxial GaAs(Si)
Authors: O’Hagan S. P. , Missous M. , Mottram A. , Wright A. C.
Published: July 2002

Superlattice structures of alternately undoped and [Si]=1×1019 cm−3 doped GaAs have been grown by molecular beam epitaxy at a substrate temperature of 250 °C. X-ray diffraction profiles give an average value of lattice constant in the structures in between the values measured in single, nominally undoped and [Si]=1×1019 cm−3 doped epilayers grown under identical conditions. Transmission electron microscopy dark-field (004) beam images of the as-grown structures contain bands of strain contrast and hybrid diffraction/imaging reveals periodic variations in the position of the higher-order Laue zone lines in the superlattice structures. We believe the results signify a modulation of the lattice constant along the growth direction: the lattice constant being smaller in the doped regions than in the undoped regions due to a reduction of excess As concentration at this high doping level. On annealing, dense bands of large As precipitates appear in the undoped regions with bands of fewer, smaller precipitates in the doped regions. Hall-effect measurements on the as-grown structures yield electron concentrations comparable with those measured in heavily Si-doped, low-temperature grown single epilayers, and the mobilities are indicative of band conduction in the doped regions of the superlattices.