bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical and electrical properties of Mg-doped p-type AlxGa1−xN
Authors: Li J. , Oder T. N. , Nakarmi M. L. , Lin J. Y. , Jiang H. X.
Published: July 2002

Mg-doped AlxGa1−xN epilayers with Al content up to 0.27 were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). p-type conduction in these alloys has been achieved, as confirmed by variable temperature Hall-effect measurements. Emission lines of band-to-impurity transitions of free electrons with neutral Mg acceptors as well as localized excitons have been observed in the p-type AlxGa1−xN alloys. The Mg acceptor activation energies EA were deduced from photoluminescence spectra and were found to increase with Al content and agreed very well with those obtained by Hall measurements. From the measured activation energy as a function of the Al content, EA versus x the resistivity of AlxGa1−xN alloys with high Al contents can be deduced. Our results thus indicated that alternative methods for acceptor activation in AlGaN alloys with high Al contents must be developed. Our results have also shown that PL measurements provide direct means of obtaining EA, especially where this cannot be obtained accurately by electrical methods due to high resistance of Mg-doped AlxGa1−xN alloys with high Al content.
Thermal stability and degradation mechanism of NiFe/Cu giant magnetoresistance multilayer systems
Authors: Hecker M. , Tietjen D. , Schneider C. M. , Cramer N. , Malkinski L. , Camley R. E. , Celinski Z.
Published: July 2002

Ni 80 Fe 20 / Cu multilayers show large giant magnetoresistance (GMR) at low magnetic saturation fields. The GMR signal is known to degrade irreversibly at elevated temperatures. Clarification of the relevant deterioration mechanisms refines our basic understanding of the GMR effect and may help to improve the thermal stability of devices. We therefore investigated structural, transport, and magnetic properties of sputtered Ni80Fe20/Cu multilayers in the as-deposited state and after different anneals (up to 600 °C) by x-ray techniques, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect (MOKE). Multilayers with the second maximum of the antiferromagnetic (afm) coupling showed a sharp drop of the GMR at about 250 °C. The changes of the transport properties were associated with a series of structural alterations. These ranged from grain growth and defect reduction through texture sharpening and stress evolution up to the onset of interdiffusion. Interdiffusion changed the NiFe layer composition and the interface structure and finally caused layer intermixing with a loss of the former multilayer structure. Further insight into the magnetic behavior was gained from FMR and MOKE measurements, from which we determined the in-plane magnetic anisotropies, the strength of the afm coupling (bilinear and biquadratic), and the homogeneity of the layer magnetization as a function of the annealing temperature.
Examination of the relationship between the parameters of Barkhausen effect model and microstructure of magnetic materials
Authors: Lo C. C. H. , Lee S. J. , Kerdus L. C. , Jiles D. C.
Published: July 2002

A relationship between the parameters of a hysteretic-stochastic process model of the Barkhausen effect (BE) and the microstructural features of a series of ferritic/pearlitic steel samples has been identified. The root-mean-square values and pulse height distributions of the experimental and modeled BE signals showed similar dependence on the pearlite content. The correlation length parameter ξ of the model, which represents the range of interaction of domain walls with pinning sites, was found to obey ξ=AVfDf+BVpDp where Vf(Vp) and Df(Dp) are the volume fraction and grain size of ferrite (pearlite).
Extreme wave events in directional, random oceanic sea states
Authors: Onorato M. , Osborne A. R. , Serio M.
Published: July 2002

We discuss the effect of the directional spreading on the occurrence of extreme wave events. We numerically integrate the envelope equation recently proposed by Trulsen et al. [Phys. Fluids 12, 2432 (2000)] as a weakly nonlinear model for realistic oceanic gravity waves. Initial conditions for numerical simulations are characterized by the spatial JONSWAP power spectrum for several values of the significant wave height, steepness, and directional spreading. We show that by increasing the directionality of the initial spectrum the appearance of extreme events is reduced.
Oscillating two-stream instability in ionospheric heating experiments
Authors: Kuo S. P.
Published: July 2002

It is shown that oscillating two-stream instability (OTSI) can be excited in sizable spatial region below the reflection height of the O-mode heating wave in mid-latitude ionospheric heating experiments. Near the reflection height, the threshold field of OTSI increases drastically with the oblique propagation angle of its Langmuir sidebands. Such dependence limits the angular distribution of Langmuir sidebands in narrow cones around the geomagnetic field. Therefore, obliquely propagating Langmuir sidebands of OTSI prefer to be excited in their matching height regions, where sidebands satisfy the local dispersion relation and their matching height locations move downward from the reflection height as oblique propagation angles increase.
Direct observation of atomic disordering at the SrTiO3/Si interface due to oxygen diffusion
Authors: Shutthanandan V. , Thevuthasan S. , Liang Y. , Adams E. M. , Yu Z. , Droopad R.
Published: July 2002

The stability of epitaxially grown single crystal SrTiO3(001) thin films on Si(100) substrates was studied as a function of temperature under vacuum and oxygen-rich environments using Rutherford backscattering spectrometry in channeling geometry, nuclear reaction analysis, and x-ray photoelectron spectroscopy. During vacuum annealing, it was found that interfacial silica formed due to diffusion of oxygen from the film to Si. This was further accompanied by the atomic disordering of Sr, Ti, and O sublattices in the film due to reduction reactions. Although the interfacial degradation process is similar during heating in oxygen environment, no disordering of the film was observed.
Enhanced electron mobility and high order fractional quantum Hall states in AlAs quantum wells
Authors: De Poortere E. P. , Shkolnikov Y. P. , Tutuc E. , Papadakis S. J. , Shayegan M. , Palm E. , Murphy T.
Published: July 2002

We report improvements in the quality of two-dimensional (2D) electrons in modulation-doped AlAs quantum wells, leading to electron mobilities as high as 31 m2/Vs, a ten-fold increase over the mobility of previous AlAs samples. Confirming the quality of our quantum wells, developing fractional quantum Hall states are observed in the first Landau level at high order filling factors ν=2/5, 3/5, 3/7, 4/7, and 4/9, and at larger fillings, at ν=4/3, 5/3, 7/3, 8/3, 8/5, and 11/3. Thanks to the much larger effective mass and g factor of AlAs 2D electrons compared to GaAs, and to the possible multivalley occupancy of AlAs 2D electrons, our improved systems help bring a wider scope to the investigation of low-disorder, interacting 2D electrons.
Investigation of rescaled range analysis, the Hurst exponent, and long-time correlations in plasma turbulence
Authors: Gilmore M. , Yu C. X. , Rhodes T. L. , Peebles W. A.
Published: July 2002

A detailed investigation of rescaled range (R/S) analysis to search for long-time correlations (via the Hurst exponent, H) in plasma turbulence is presented. In order to elucidate important issues related to R/S analysis, structure functions (SFs), one of several techniques available for calculating H, are also applied, and comparisons between the two methods are made. Time records of both simulated data and fluctuation reflectometry data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] are analyzed. It is found that the R/S method can be used to accurately determine H, provided a long enough data record is used, and that H is an indicator of persistence in the data. In addition, subtleties of the correct application of both methods are discussed, and potential advantages of SFs are pointed out.
CaCu
3
Ti
4
O
12
:
One-step internal barrier layer capacitor
Authors: Sinclair Derek C. , Adams Timothy B. , Morrison Finlay D. , West Anthony R.
Published: July 2002

There has been much recent interest in a so-called “giant-dielectric phenomenon” displayed by an unusual cubic perovskite-type material, CaCu3Ti4O12; however, the origin of the high permittivity has been unclear [M. A. Subramanian, L. Dong, N. Duan, B. A. Reisner, and A. W. Sleight, J. Solid State Chem. 151, 323 (2000); C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Science 293, 673 (2001); A. P. Ramirez, M. A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, and S. M. Shapiro, Solid State Commun. 115, 217 (2000)]. Impedance spectroscopy on CaCu3Ti4O12 ceramics demonstrates that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. The giant-dielectric phenomenon is therefore attributed to a grain boundary (internal) barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure. This barrier layer electrical microstructure with effective permittivity values in excess of 10 000 can be fabricated by single-step processing in air at ∼1100 °C. CaCu3Ti4O12 is an attractive option to the currently used BaTiO3-based materials which require complex, multistage processing routes to produce IBLCs of similar capacity.
Radiative recombination during ambipolar carrier transport by surface acoustic waves in GaAs quantum wells
Authors: Zhang S. K. , Santos P. V. , Hey R.
Published: July 2002

We report on the defect-assisted radiative recombination of photogenerated electrons and holes in GaAs quantum wells (QWs) during the transport by surface acoustic waves (SAWs). The studies were performed by detecting the spatial distribution of the photoluminescence (PL) with a resolution of a few micrometers. Under a SAW, a high PL intensity is observed on spatially localized spots along the SAW propagation path. This high PL intensity is attributed to recombination of the carriers, which are transported by the SAW, induced by charged defects located on or close to the QW plane.
Phonon wave-packet dynamics at semiconductor interfaces by molecular-dynamics simulation
Authors: Schelling P. K. , Phillpot S. R. , Keblinski P.
Published: July 2002

We directly observe phonon wave packets of well-defined frequency and polarization scattering at a coherent semiconductor interface using molecular-dynamics simulations. We find that in the low-frequency limit the transmission coefficients of both longitudinal and transverse acoustic phonons agree well with those predicted by the continuum-level based acoustic mismatch model. However, the transmission coefficients rapidly decrease close to the cutoff frequency, a result that can be understood within a simple one-dimensional discrete atomic-chain model. We also find that the transmission coefficient for transverse acoustic phonons depends strongly on the relative orientation of the polarization and the Si–Si bonds in the diamond lattice structure.
Epitaxial growth of TaN thin films on Si(100) and Si(111) using a TiN buffer layer
Authors: Wang H. , Tiwari Ashutosh , Kvit A. , Zhang X. , Narayan J.
Published: July 2002

We have deposited high-quality epitaxial B1 NaCl-structured TaN films on Si(100) and Si(111) substrates with TiN as the buffer layer, using pulsed laser deposition. Our method exploits the concept of lattice-matching epitaxy between TiN and TaN and domain-matching epitaxy between TiN and silicon. X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy (Z-contrast) experiments confirmed the single-crystalline nature of the films with cube-on-cube epitaxy. The stoichiometry of the TaN films was determined to be nitrogen deficient (TaN0.95±0.05) by Rutherford backscattering. Resistivity of the TaN films was found to be ∼220 μΩ cm at room temperature with a temperature coefficient of resistivity of −0.5 μΩ K−1.
New patterns of polymer blend miscibility associated with monomer shape and size asymmetry
Authors: Dudowicz Jacek , Freed Karl F. , Douglas Jack F.
Published: July 2002

Polymer blends are formulated by mixing polymers with different chemical structures to create new materials with properties intermediate between those of the individual components. While Flory–Huggins (FH) theory explains some basic trends in blend miscibility, the theory completely neglects the dissimilarity in monomer structures that is central to the fabrication of real blends. We systematically investigate the influence of monomer structure on blend miscibility using a lattice cluster theory (LCT) generalization of the FH model. Analytic calculations are rendered tractable by restricting the theoretical analysis to the limit of incompressible and high molecular weight blends. The well-known miscibility pattern predicted by FH theory is recovered only for a limited range of monomer size and shape asymmetries, but additional contributions to the LCT entropy and internal energy of mixing for polymers with dissimilarly shaped monomers lead to three additional blend miscibilty classes whose behaviors are quite different from the predictions of classical FH theory. One blend miscibility class (class IV) exhibits a remarkable resemblance to the critical behavior of polymer solutions. In particular, the theta temperature for class IV blends is near a molecular weight insensitive critical temperature for phase separation, the critical composition is highly asymmetric, and the correlation length amplitude is significantly less than the chain radius of gyration. Experimental evidence for these new blend miscibility classes is discussed, and predictions are made for specific blends of polyolefins that should illustrate these new patterns of blend miscibility.
Quantum effects on the structure of water at constant temperature and constant atomic density
Authors: Badyal Y. S. , Price D. L. , Saboungi M.-L. , Haeffner D. R. , Shastri S. D.
Published: July 2002

To explore quantum effects on the structure of liquid water, we have carried out high-energy x-ray diffraction measurements of quantum differences both under isothermal conditions at 24.5 °C and under isochoral conditions at 0.0997 atom/Å3. The measured isothermal difference correlation function is approximately equivalent to an isochoric temperature differential (ITD) of 5.5 °C, reflecting the tendency of quantum effects to introduce more disorder into the liquid. The measured isochoral correlation function is about three times higher in amplitude and is approximately equivalent to an ITD of 19 °C. Since the isochoral measurements for H2O and D2O were made at temperatures 13.5 °C apart, the isothermal and isochoral functions are roughly consistent. The discrepancies are discussed with reference to data on the pressure dependence in the literature. They are comparable with differences in results obtained with different potential functions in path-integral molecular dynamics simulations. The present results lend further validity to the notion that quantum effects on the structure of liquid water are similar to those of thermal disorder, as long as the effects of differences in the equation-of-state of H2O and D2O are taken into account.
Wet chemical nitridation of GaAs (100) by hydrazine solution for surface passivation
Authors: Berkovits V. L. , Ulin V. P. , Losurdo M. , Capezzuto P. , Bruno G. , Perna G. , Capozzi V.
Published: July 2002

A mild wet nitridation procedure using hydrazine-based solutions has been developed for GaAs (100) surface passivation. Both x-ray photoelectron spectroscopy and spectroscopic ellipsometry show that this nitridation procedure results in a very thin, coherent, and homogeneous GaN layer that is very stable in air. Photoluminescence data show a strong enhancement of the intensity as compared to that of an as-cleaned GaAs sample, indicating that this nitrided layer provides both chemical and electronic passivation of GaAs surfaces. The chemical mechanism of nitridation is discussed.
Efficiency of the π conjugation in a novel family of α,α′-bisphenyl end-capped oligothiophenes by means of Raman spectroscopy
Authors: Moreno Castro C. , Ruiz Delgado M. C. , Hernández V. , Hotta S. , Casado J. , López Navarrete J. T.
Published: July 2002

The degree of π conjugation in a novel series of molecular materials containing a central oligothiophene moiety of variable length, with its end α positions capped by phenyl groups (PnTP; n=1–4) is closely inspected by means of Fourier-transform infrared and Fourier-transform Raman spectroscopies in the neutral state as solids. Density functional theory quantum chemical calculations were performed for each co-oligomer, at the B3LYP/6-31G** level, to obtain the optimized molecular geometry and force field. The thermal stability of these oligothiophenes has been also investigated, at the molecular level, by recording infrared and Raman spectra at different temperatures between −170 °C and 160 °C.
Enhanced electroluminescence using polystyrene as a matrix
Authors: He Gufeng , Li Yongfang , Liu Jie , Yang Yang
Published: July 2002

Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH–PPV) blends with polystyrene (PS) were used as emitting layers in polymer light-emitting diodes. Studies of photoluminescence and electroluminescence (EL) of the blends indicate that interchain interactions were tremendously suppressed due to the dilution effect. The device of MEH–PPV/PS (50/50) shows much higher EL efficiency compared to pure MEH–PPV devices. Since there is neither energy transfer nor charge transfer involved in MEH–PPV/PS blends, the observed efficiency enhancement is mainly attributed to the suppressed interchain species, which are responsible for the low photoluminescence yields. In addition, the addition of PS into MEH–PPV improves the thermal stability of polymer thin films and reduces the sensitivity of device performance to processing conditions.
Noise in chaotic data: Diagnosis and treatment
Authors: Schreiber Thomas , Kantz Holger
Published: July 2002

A prominent limiting factor in the analysis of chaotic time series are measurement errors in the data. We show that this influence can be quite severe, depending on the nature of the noise, the complexity of the signal, and on the application one has in mind. Theoretical considerations yield general upper bounds on the tolerable noise level for dimension, entropy and Lyapunov estimates. We discuss methods to detect and analyze the noise present in a measured data set. We show how the situation can be improved by nonlinear noise reduction.
Indecomposable continua in dynamical systems with noise: Fluid flow past an array of cylinders
Authors: Sanjuan Miguel A. F. , Kennedy Judy , Grebogi Celso , Yorke James A.
Published: July 2002

Standard dynamical systems theory is based on the study of invariant sets. However, when noise is added, there are no bounded invariant sets. Our goal is then to study the fractal structure that exists even with noise. The problem we investigate is fluid flow past an array of cylinders. We study a parameter range for which there is a periodic oscillation of the fluid, represented by vortices being shed past each cylinder. Since the motion is periodic in time, we can study a time-1 Poincaré map. Then we add a small amount of noise, so that on each iteration the Poincaré map is perturbed smoothly, but differently for each time cycle. Fix an x coordinate x0 and an initial time t0. We discuss when the set of initial points at a time t0 whose trajectory (x(t),y(t)) is semibounded (i.e., x(t)>x0 for all time) has a fractal structure called an indecomposable continuum. We believe that the indecomposable continuum will become a fundamental object in the study of dynamical systems with noise.
Numerical modeling of self-propagating polymerization fronts: The role of kinetics on front stability
Authors: Solovyov Stanislav E. , Ilyashenko Victor M. , Pojman John A.
Published: July 2002

Frontal propagation of a highly exothermic polymerization reaction in a liquid is studied with the goal of developing a mathematical model of the process. As a model case we consider monomers such as methacrylic acid and n-butyl acrylate with peroxide initiators, although the model is not limited to these reactants and can be applied to any system with the similar basic polymerization mechanism. A three-step reaction mechanism, including initiation, propagation and termination steps, as well as a more simple one-step mechanism, were considered. For the one-step mechanism the loss of stability of propagating front was observed as a sequence of period doubling bifurcations of the front velocity. It was shown that the one-step model cannot account for less than 100% conversion and product inhomogeneities as a result of front instability, therefore the three-step mechanism was exploited. The phenomenon of superadiabatic combustion temperature was observed beyond the Hopf bifurcation point for both kinetic schemes and supported by the experimental measurements. One- and two-dimensional numerical simulations were performed to observe various planar and nonplanar periodic modes, and the results for different kinetic schemes were compared. It was found that stability of the frontal mode for a one-step reaction mechanism does not differ for 1-D and 2-D cases. For the three-step reaction mechanism 2-D solutions turned out to be more stable with respect to the appearance of nonplanar periodic modes than corresponding 1-D solutions. Higher Zeldovich numbers (i.e., higher effective activation energies or lower initial temperatures) are necessary for the existence of planar and nonplanar periodic modes in the 2-D reactor with walls than in the 1-D case.
Period-doubling behavior in frontal polymerization of multifunctional acrylates
Authors: Masere Jonathan , Stewart Felicia , Meehan Timothy , Pojman John A.
Published: July 2002

Front dynamics in the frontal polymerization of two multifunctional acrylate monomers, 1,6-hexanediol diacrylate (HDDA) and trimethylolpropane ethoxylate triacrylate (TMPTA), with Lupersol 231 [1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane] as the initiator, are studied. In most frontal polymerization systems, the dynamics are associated with a planar front propagating through the sample. However, in some cases, front behavior can be altered: the front becomes nonplanar characterized by complex patterns like spin modes and pulsations. To determine how these periodic and aperiodic modes arise, reactant solutions consisting of HDDA diluted with diethyl phthalate (DEP) and TMPTA diluted with dimethyl sulfoxide (DMSO) were used in the study. In the study we reveal frontal behavior characteristic of period-doubling behavior, a doubling of spin heads that degenerate into an apparently chaotic mode. Also, a pulsating symmetric mode has been observed. These observations have a striking similarity to observations made in studies of self-propagating high-temperature synthesis (SHS) in which the addition of an inert diluent afforded a rich variety of dynamical behavior. The degree of cross-linking has also been found to be a bifurcation parameter. The energy of activation of multifunctional acrylate polymerization is a strong function of the degree of polymerization. By adding a monoacrylate (benzyl acrylate: BzAc), such that the front temperature was invariant, we observed a period-doubling bifurcation sequence through changes in the energy of activation, which has not been previously reported.
Contact forces in a granular packing
Authors: Radjai Farhang , Roux Stéphane , Moreau Jean Jacques
Published: July 2002

We present the results of a systematic numerical investigation of force distributions in granular packings. We find that all the main features of force transmission previously established for two-dimensional systems of hard particles hold in three-dimensional systems and for soft particles, too. In particular, the probability distribution of normal forces falls off exponentially for forces above the mean force. For forces below the mean, this distribution is either a decreasing power law when the system is far from static equilibrium, or nearly uniform at static equilibrium, in agreement with recent experiments. Moreover, we show that the forces below the mean do not contribute to the shear stress. The subnetwork of the contacts carrying a force below the mean thus plays a role similar to a fluid surrounding the solid backbone composed of the contacts carrying a force above the mean. We address the issue of the computation of contact forces in a packing at static equilibrium. We introduce a model with no local simplifying force rules, that allows for an exact computation of contact forces for given granular texture and boundary conditions.
A novel method for determining the anchoring energy function at a nematic liquid crystal-wall interface from director distortions at high fields
Authors: Yokoyama H. , van Sprang H. A.
Published: July 2002

We show that the anchoring energy function, i.e., the anisotropic part of the interfacial free energy, at a nematic liquid crystal-wall interface can be determined uniquely without a numerical fitting procedure, when the integrated birefringence of a liquid crystal cell with a thickness much larger than the extrapolation length is measured as a function of an electric or magnetic field well above the Freedericksz threshold. The precision of the present method is closely argued, showing that the resulting anchoring energy function is reasonably insensitive to the uncertainties in the material parameters and in the cell thickness. As an example, the anchoring energy function at the interface between 5CB(pentylcyanobiphenyl) and an obliquely evaporated SiO was determined for the first time, by measuring the birefringence and the capacitance of a 56-μm-thick cell up to 150 V rms at 0.23 °C below the clearing temperature. A saturation of the field-induced distortion was clearly observed at about 100 V rms. The anchoring energy function was found to be well fitted by a function of the form (1)/(2) Ea sin2 θ+ (1)/(4) E1 sin4 θ, where θ is the angle between the boundary director and the substrate, with Ea ∼4.0×10−5 J/m2 and E1∼−1.8×10−5 J/m2.
Al/TiW reaction kinetics: Influence of Cu and interface oxides
Authors: Olowolafe J. O. , Palmstro/m C. J. , Colgan E. G. , Mayer J. W.
Published: July 2002

The reaction kinetics of Al with Ti22W78 alloys has been investigated under vacuum annealing conditions. In particular, the effects of Cu in Al and venting during deposition of TiW were studied. It was observed that Cu did not play any significant role in the kinetics of the interdiffusion of Al and TiW. During the reaction process at temperatures around 500 °C, Ti accumulated on the surface of the samples with or without Cu in Al. The Ti accumulation is diffusion limited with an activation energy of 2.4 eV. Interface oxides are believed to be primarily responsible for the stability of Al/TiW metallization.
Noise characteristics of a dc SQUID with a resistively shunted inductance
Authors: Enpuku K. , Muta T. , Yoshida K. , Irie F.
Published: July 2002

Noise characteristics of a dc SQUID with an inductance shunted by a damping resistance are studied numerically. It is shown that the damping resistance improves considerably the resolution of the SQUID in the case of large β, where β=2LI0/Φ0, I0 is a critical current, L is a loop inductance and Φ0 is the flux quantum. The energy resolutions for β=4 and β=10 are only about 2 and 4 times larger than that for β=1, respectively. Furthermore, the ranges of both the bias current and the external flux, where good resolution is obtained, become very wide compared with the conventional SQUID. Therefore, the SQUID with the damping resistance can be used for large β (or L) without the significant degradation of the resolution, and will much improve the coupling properties between the SQUID and the input circuitry. The numerical simulation results are also compared with analytical ones, and a reasonable agreement is obtained.
Raman microprobe determination of local crystal orientation
Authors: Hopkins J. B. , Farrow L. A.
Published: July 2002

The analysis of local crystal orientation with micron spatial resolution is of prime importance in understanding the dynamics of crystal growth processes with potential for device applications. Examples of such processes are laser annealing, lateral epitaxial growth over oxides, and molecular-beam-epitaxial growth over structured substrates. We present here a nondestructive technique for obtaining such information based on polarization selective Raman scattering from near-diffraction limited O(1 μm) regions of the sample. Depth resolution is limited by the optical penetration depth at the laser wavelength, which is on the order of 1.0 μm for Si at visible wavelengths. By rastering the sample with respect to the probe laser beam, maps of the local crystal orientation can be obtained. Abrupt changes in crystal orientation at grain boundaries are readily apparent. This paper details the experimental apparatus and the method for extracting local crystal orientation information for the particular case of Si. Experiments on single crystals of known orientation are presented which confirm our theoretical predictions and serve to calibrate the technique. Practical examples of mapping of laser annealed silicon-on-oxide structures are given. The analysis shows the loss of the lateral epitaxial seed, with many grains exhibiting 〈110〉 surface normals, in agreement with qualitative results obtained from a destructive selective chemical-etching procedure. Signal intensities are such that orientation information at a single point can be recorded in less than 30 s, thus providing the potential for a real time in situ monitor of slow growth processes.
Electrical response of BaTiO3 ceramics to the shock-induced ferroelectric-paraelectric transition
Authors: Mashimo Tsutomu , Toda Kazumasa , Nagayama Kunihito , Goto Tsuneaki , Syono Yasuhiko
Published: July 2002

The electrical response of BaTiO3 ceramics to the strong shock compression well above the Hugoniot-elastic limit or ferroelectric-paraelectric transition pressure is studied to separate and identify the causes of shock-induced electrical response of ferroelectric materials. Measurements of voltage and current histories under shock compression are performed by using poled and nonpoled specimens, assemblies of both parallel and normal modes, and explosive systems. In order to consider the effects due to elastic-plastic deformation and shock-induced conduction, the Hugoniot compression curve and electrical conductivities are also measured by using nonpoled specimens. The Hugoniot-elastic limit stress is determined to be 4.8–6.4 GPa for porous specimens with 1.7–4.4% porosity, and the shock velocity (Us) and particle velocity (Up) Hugoniot can be closely described as Us =4.29+1.66Up km/s in the plastic region. The electrical conductivity values at 12- and 29-GPa peak stresses are measured to be 2.1×10−3 and 4.8×10−2 S/cm, respectively. Both voltage histories for the parallel and normal modes at 12- and 29-GPa peak stresses show similarly increasing signals with time. Although the signals of the 29-GPa peak stress show large internal losses due to shock-induced conduction, these signals are reasonably analyzed by a simple analytical model based on the ferroelectric-paraelectric transition. The relative dielectric constant values under shock compression calculated by the model from the measured voltage histories are in the range (5–71)×103, which is consistent with those measured under static compression. Current histories for the parallel and normal modes at the 12-GPa peak stress show a pulse-shaped signal and a constant-current-shaped signal, respectively, whose total integrated charges are in agreement with those due to the initial remanent polarization, with the losses less than 20%. It is concluded that the measured electrical responses of poled BaTiO3 ceramics to the shock compression well above the Hugoniot-elastic limit are caused not by piezoelectricity, but chiefly by the shock-induced ferroelectric-paraelectric transition, in this study.
A time-of-flight study of the neutral species produced by nanosecond laser etching of CuCl at 308 nm
Authors: van Veen G. N. A. , Baller T. , de Vries A. E.
Published: July 2002

A time-of-flight (TOF) study of the particles leaving a CuCl target after irradiation by 15 ns laser pulses at 308 nm is performed. It is shown that the ejected species are Cl, Cu, CuCl, Cu2Cl, Cu2Cl2, and Cu3Cl3. The majority of the products consists of CuCl. The TOF spectra can be fitted by the sum of two contributions: a Maxwell–Boltzmann (MB) and a Gaussian-type (G) distribution. The MB distribution has a temperature of T=6000 K for all masses. The average energy and the standard deviation in the energy of the G contributions are typical for every individual product. The results strongly suggest that the MB contribution is due to a single photon-induced process, whereas the G contribution originates from a multiphoton and/or a multistep process.
New crystalline soft magnetic alloy with high saturation magnetization (invited)
Authors: Hayashi K. , Hayakawa M. , Ishikawa W. , Ochiai Y. , Matsuda H. , Iwasaki Y. , Aso K.
Published: July 2002

Extensive study was made on Fe-base crystalline alloys in search for new soft magnetic materials with high saturation magnetization by means of sputtering technique. It revealed that newly found FeGaSi and FeAlGe alloy systems possessed excellent soft magnetic properties with remarkably higher saturation magnetization. These new alloy films, annealed in vacuum at 500 °C for 1 h, show the magnetic properties of Bs=13 and 15 kG, Hc=0.09 and 0.20 Oe, and μ1 MHz=2000 and 1800 for the FeGaSi and FeAlGe alloy films of 2 μm in thickness, respectively. These values are superior to those of FeAlSi (sendust) alloy which is a typical soft magnetic material with high saturation magnetization. Results of the Co-added systems of FeCoGaSi and FeCoAlGe alloys are also presented. Discussions are given on various systems of Fe base crystalline alloy films studied, which are classified into four types of groups with respect to magnetic softness.
Atomic force microscope–force mapping and profiling on a sub 100-Å scale
Authors: Martin Y. , Williams C. C. , Wickramasinghe H. K.
Published: July 2002

A modified version of the atomic force microscope is introduced that enables a precise measurement of the force between a tip and a sample over a tip-sample distance range of 30–150 Å. As an application, the force signal is used to maintain the tip-sample spacing constant, so that profiling can be achieved with a spatial resolution of 50 Å. A second scheme allows the simultaneous measurement of force and surface profile; this scheme has been used to obtain material-dependent information from surfaces of electronic materials.
Substrate effects on the properties of Y-Ba-Cu-O superconducting films prepared by laser deposition
Authors: Venkatesan T. , Chang C. C. , Dijkkamp D. , Ogale S. B. , Chase E. W. , Farrow L. A. , Hwang D. M. , Miceli P. F. , Schwarz S. A. , Tarascon J. M. , Wu X. D. , Inam A.
Published: July 2002

Superconducting thin films of Y1Ba2Cu3Ox with superconducting transition temperature (Tc) near 90 K have been prepared by a laser deposition technique. We show that films prepared on sapphire, lithium niobate, and strontium titanate under identical processing conditions exhibit different electrical characteristics. The film surfaces, interfaces, and crystallinity have been studied by a number of analytical techniques. We conclude that the substrate influences the film properties primarily in three ways: the thermal expansion mismatch introduces cracks in the film, the interface reaction changes the film composition, and the substrate lattice influences the crystallographic orientation of the film.
Nonstationary spin glass dynamics from susceptibility and noise measurements
Authors: Refregier Ph. , Ocio M. , Hammann J. , Vincent E.
Published: July 2002

Nonstationary dynamics has been observed in very-low-frequency (10−2–10−1 Hz) ac susceptibility and noise measurements on the thiospinel CdCr1.7 In0.3 S4 spin glass. The time relaxation of the ac susceptibility yields a large decrease of the out-of-phase component χ″ towards a finite stationary value. The relaxation of the spectral density of the magnetization derived from zero-field noise measurements is in remarkable agreement with the behavior of χ″. The observed time dependence compares well with the predictions from the thermoremanent magnetization decay analyses. In the range of times of χ″ measurements, the linear-response theory applies to the nonstationary dynamics.
Bulk and thick films of the superconducting phase YBa2Cu3O7−y made by controlled precipitation and sol-gel processes
Authors: Barboux P. , Tarascon J. M. , Greene L. H. , Hull G. W. , Bagley B. G.
Published: July 2002

The synthesis of high-temperature superconducting oxides using solution chemistry has been investigated and physical properties are compared to ceramics obtained by conventional solid-state reactions. We report on controlled precipitation and sol-gel processes, both of which produce materials with particle sizes smaller than 5 μm. We find that the superconducting properties of the high Tc ceramics are affected by their manner of preparation, such that the transitions are slightly lower in temperature, but sharper, for samples made by solution rather than solid-state chemistry. The ability to prepare stable viscous gels provides an opportunity for obtaining large areas of superconducting coatings. For thick films on alumina or silicon substrates, contamination from the substrate is shown to be a problem. Finally, we observe that the sol-gel process lowers the synthesis temperature by 100 °C.
Reactive sticking coefficients for silane and disilane on polycrystalline silicon
Authors: Buss Richard J. , Ho Pauline , Breiland William G. , Coltrin Michael E.
Published: July 2002

Reactive sticking coefficients (RSCs) were measured for silane and disilane on polycrystalline silicon for a wide range of temperature and flux (pressure) conditions. The data were obtained from deposition-rate measurements using molecular beam scattering and a very low-pressure cold-wall reactor. The RSCs have nonlinear Arrhenius temperature dependencies and decrease with increasing flux at low (710 °C) temperatures. Several simple models are proposed to explain these observations. The results are compared with previous studies of the SiH4/Si(s) reaction and low-pressure chemical vapor deposition-rate measurements.
Optical charge injection into a gallium arsenide acoustic charge transport device
Authors: Beggs B. C. , Young L. , Johnson R. R.
Published: July 2002

Transport of photoelectrons by the 〈110〉 propagating surface acoustic wave (SAW) on (100)-cut gallium arsenide was observed using the acoustic charge transport (ACT) principle reported by Hoskins, Morkoϑ, and Hunsinger [Appl. Phys. Lett. 41, 332 (1982)]. By directly modulating an AlGaAs light-emitting diode, pulses of near-infrared radiation (λp=730 nm) were applied to semitransparent (25 nm thick) chromium windows on the device surface. Four such windows separated by 250 μm were positioned along the otherwise opaque Schottky channel plate of the ACT device. Electron-hole pairs generated by incident radiation were separated by the electrostatic field in the depleted n-type channel region. Calculations suggested that the separation time would be small enough to reduce radiative recombination and allow enough electrons to collect in each acoustically defined well passing beneath the injection window for detection at the output. Electrons, separated from the holes, were subsequently bunched and carried along by the electric field coupled to the SAW. Output pulse delays for injection over each window were in agreement with theory taking into account the window positions and the SAW velocity (2864 m/s). Quantum efficiency, defined as the number of electrons collected at the output per incident photon on the chromium window, was approximately 0.09. Operated at 360 MHz, the experimental device had a charge transfer efficiency in excess of 0.992. It is believed that the injection technique described may be of use in future optical signal processing or imaging applications using ACT.
Evaluation of cylinder-impact test data for constitutive model constants
Authors: Johnson Gordon R. , Holmquist Tim J.
Published: July 2002

This paper examines the usefulness of cylinder-impact test data to determine constants for various computational constitutive models. The Johnson–Cook and Zerilli–Armstrong constitutive models are evaluated by comparing model predictions to tension, torsion, and cylinder-impact test data. Then the cylinder-impact test data are used to determine constitutive model constants for various forms of these models. Under bounded conditions of strains and strain rates, this approach can produce useful results. It can also produce very erroneous results if not used properly.
Critical thickness for pseudomorphic growth of Si/Ge alloys and superlattices
Authors: van de Leur R. H. M. , Schellingerhout A. J. G. , Tuinstra F. , Mooij J. E.
Published: July 2002

A model, based on energy considerations, to determine the critical thickness for pseudomorphic growth of Si/Ge alloys and superlattices is introduced. Comparison with experiments shows a good agreement between the theory and experiment. It is also shown that in films which are initially dislocation-free, the energy barrier for the nucleation of dislocation loops can be crossed by thermal activation if the film is thicker than the critical value provided by the model.
Molecular-beam-epitaxial growth and characterization of In2Te3
Authors: Golding T. D. , Boyd P. R. , Martinka M. , Amirtharaj P. M. , Dinan J. H. , Qadri S. B. , Zahn D. R. T. , Whitehouse C. R.
Published: July 2002

We report studies of the molecular-beam-epitaxial growth of In2Te3. The unique structure of In2Te3, with 1/3 of the In sublattice sites vacant, is of fundamental interest for molecular-beam-epitaxial growth dynamics. We show that thin-film (500–7000 Å) single-crystal In2Te3 can be grown successfully on InSb(100) homoepitaxial layers at substrate temperatures of 300–350 °C and Te/In flux ratios of 3/2 to 5/2. Epitaxy has been monitored by reflection high-energy electron diffraction and the stoichiometry of the grown layers assessed by Auger spectroscopy and energy dispersive x-ray analysis. Raman studies of the layers are presented and compared with a bulk In2Te3 standard. Crystal structure has been determined by x-ray diffraction using Weissenburg and oscillation photographs, confirming that the layers have a fcc crystal structure with a lattice parameter of 18.50 Å, in excellent agreement with the bulk value. Band-gap measurements have been performed on the layers by photoreflectance. We report a value for the α-In2Te3 band gap of 1.19 and 1.31 eV at 300 and 77 K, respectively. Molecular-beam-epitaxial growth of InSb and CdTe on epitaxial In2Te3 films for fabrication of InSb/In2Te3/InSb and InSb/In2Te3/CdTe multilayers has been studied. Auger depth profiling of the resulting layers shows severe intermixing into the In2Te3. These results are supported by thermodynamic considerations of the InSb-In2Te3 interface.
Thermal conductivity of metallic glasses
Authors: Choy C. L. , Leung W. P. , Ng Y. K.
Published: July 2002

The thermal diffusivity D of three metallic glasses Fe80B20, Fe40Ni40P14B6, and Fe32Ni36Cr14P12B6 has been measured from 160 to 500 K by the laser-flash radiometry method and the thermal conductivity K has been calculated from these data. The accuracy of D and K is 6% and 8%, respectively. The electronic contribution (Ke) to the thermal conductivity, evaluated by using the Wiedemann–Franz law, increases almost linearly with increasing temperature. The phonon thermal conductivity Kph follows the temperature dependence exhibited by all amorphous dielectrics, i.e., Kph is roughly proportional to the phonon specific heat, thus giving an approximately constant phonon mean free path. When the metalloid atom B is replaced by P or the number of chemical components in the alloy is increased, both Ke and Kph decrease, but the drop in Kph is much more significant. The phonon mean free paths for Fe80B20 and Fe40Ni40P14B6 are 14 and 12 Å, respectively, which are substantially higher than the values (5–8 Å) for amorphous insulators.
Magnetic properties of Bi3Fe5O12 garnet
Authors: Okuda T. , Katayama T. , Kobayashi H. , Kobayashi N. , Satoh K. , Yamamoto H.
Published: July 2002

Nonthermodynamical garnet Bi3Fe5O12 was synthesized by direct epitaxial growth via vapor phase using a reactive ion beam sputtering technique. The lattice constant was 12.631 Å. The saturation magnetization of the film grown onto a substrate (a=12.495 Å) was 1500 G at room temperature. The uniaxial anisotropy energy was +7.2×104 erg/cm3. The conversion electron Mössbauer spectroscopy measurement indicates that the internal fields of 24d and 16a sites in the nearly lattice-matched film were 420 and 490 kOe, respectively. The internal field of the 24d site was larger by 30 kOe than that in YIG. The angle between the direction of the internal field and the film normal was 34.8°.
Nematic liquid crystal clad tapered optical fiber with temperature sensing properties
Authors: Veilleux C. , Black R. J. , Lapierre J. , Reeves L. W.
Published: July 2002

We discuss the thermal and optical properties of the lyotropic liquid crystal decylammonium chloride/water/NH4Cl (DACl-LLC) from −40 to 80 °C. Combined use of DACl-LLC and highly tapered monomode optical fibers allows sensing of temperature (from room temperature up to 65 °C).
The quantum transmitting boundary method
Authors: Lent Craig S. , Kirkner David J.
Published: July 2002

A numerical algorithm for the solution of the two-dimensional effective mass Schrödinger equation for current-carrying states is developed. Boundary conditions appropriate for such states are developed and a solution algorithm constructed that is based on the finite element method. The utility of the technique is illustrated by solving problems relevant to submicron semiconductor quantum device structures.
Observation of magnetic resonance modes of Fe layers coupled via intervening Cr (invited)
Authors: Krebs J. J. , Lubitz P. , Chaiken A. , Prinz G. A.
Published: July 2002

Multiple-frequency (2–14 GHz) ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes of a large set of single-crystal Fe/Cr/Fe(001) sandwiches grown by molecular-beam epitaxy. The FMR data reveal two resonance modes which have complicated frequency dependencies for those samples that show antialigned Fe layers in zero applied field. Such alignment is only found for samples with 12 Å<t(Cr)<25 Å. These samples also have dramatic discontinuities in their M-vs-H curves and magnetoresistance curves. Angle-dependent 35-GHz FMR was used to determine the anisotropy and effective magnetization of these samples when they were magnetically aligned. We show that the detailed FMR and M-vs-H behavior can be quantitatively explained by an AF coupling term of the form J M̂1⋅M̂2 and find that J has a t(Cr) dependence peaked about 16 Å.