bluuebunny/crossref_metadata_2025 huggingface dataset:
Pyroelectric enthalpimetric detection
Authors: D’Amico A. , Zemel J. N.
Published: July 2002

A pyroelectric enthalpimetric calorimeter for the H-O reaction was constructed using Pd and Au electrodes on a 2.5-cm-diam, 0.3-mm-thick LiTaO3 wafer. The counter (or common) electrode was of Au. The detector was placed in a chamber and various gases were admitted at pressures slightly above atmospheric. 100 ppm H2 in N2 was used as the hydrogen source and pure O2 as the oxygen source. A thermal signature was obtained on the Pd electrode that was confirmed by a thermocouple attached to the pyroelectric calorimeter. A delayed thermal signature was observed on the Au electrode resulting from thermal diffusion in the LiTaO3 substrate of the heat generated at the Pd electrode. The results on the H-O reaction are in general agreement with the literature on Pd metal oxide semiconductor (MOS) structures. The results demonstrate the utility of the pyroelectric calorimeter.
Dynamic model of trapping-detrapping in SiO2
Authors: Nissan-Cohen Y. , Shappir J. , Frohman-Bentchkowsky D.
Published: July 2002

The field and time dependence of charge carriers trapping under different charge injection conditions, is studied in this work, using the dc hot electron injection technique. It is shown that the trapping characteristics converge to field-dependent quasisaturation values. Variation of the trapping levels, due to change of the oxide field magnitude, are obtained in both directions and exhibit complete reversibility. These results, which cannot be explained by the first-order conventional trapping model, are consistent with a dynamic trapping-detrapping model. According to this model, quasisaturation of trapping characteristics is obtained when the trapping and detrapping processes are balanced. The occupation of the traps under steady-state conditions is therefore field dependent. The same model also describes the generation of positive charge under high-field injection conditions. This phenomenon is shown to be related to ionization of localized states in the SiO2 forbidden gap. The implications of the dynamic model suggest the need for a reevaluation of the present characterization methods for trapping in oxide layers.
Stability analysis of nonlinear coherent coupling
Authors: Daino B. , Gregori G. , Wabnitz S.
Published: July 2002

We analyze the nonlinear coupled differential equations for the nonlinear coherent coupler by extending the Stokes parameters formalism. The existence of bifurcation and instability phenomena is proved.
Oxidation temperature dependence of the dc electrical conduction characteristics and dielectric strength of thin Ta2O5 films on silicon
Authors: Oehrlein G. S.
Published: July 2002

Tantalum pentoxide thin films (60–80 nm thick) on silicon were prepared by thermal oxidation at 430–675 °C of electron-beam evaporated Ta. The tantalum layers had been deposited by electron-beam evaporation onto the Si substrates held at room temperature (RT) or heated to 150 °C during Ta evaporation. The dc conduction properties and the dielectic strength of the Ta2O5 films were studied employing Al/Ta2O5/Si capacitors. The smallest conductivity was found for Ta2O5 films formed from Ta deposited onto Si held at RT during evaporation and oxidized at 490 °C for 1 h in dry O2. This minimum conductivity corresponds to a leakage current of 1×10−7 A/cm2 at an applied field of 1 MV/cm (Al negative on p-type Si substrates). The dc conduction characteristics can be interpreted by assuming Poole–Frenkel conduction. For Ta2O5 formed from Ta which had been deposited onto Si substrates held at 150 °C during evaporation, the conductivity is smallest for a film which had been oxidized at 430 °C for 1 h (lowest oxidation temperature investigated). Ta2O5 films formed from Ta deposited onto Si substrates at RT exhibit lower conductivity than Ta2O5 films formed from Ta deposited onto heated (150 °C) Si substrates for a given oxidation temperature. The presence of water in the oxidizing ambient was found to greatly deteriorate the leakage characteristics of the Ta2O5 films. The dielectric strength of the Ta2O5 film for which the lowest conductivity was observed (oxidized at 490 °C) ranged from 2 to 4.5 MV/cm. A maximum in the breakdown distribution was found for a breakdown strength of 3–3.5 MV/cm.
Frequency and temperature dependence of elastic moduli of polymers
Authors: Lagakos N. , Jarzynski J. , Cole J. H. , Bucaro J. A.
Published: July 2002

The frequency and temperature dependence of the elastic moduli of a number of commercially available polymers has been studied in the temperature range of 0–35 °C and for frequencies 102–106 Hz. Away from transitions a significant new relationship has been obtained, i.e., the Young’s modulus of these polymers is proportional to log of frequency. Using this relationship, together with the low and high frequency data, transitions in some of the polymers were identified.
A p-n-p AlGaAs heterojunction bipolar transistor for high-temperature operation
Authors: Frost M. S. , Riches M. , Kerr T.
Published: July 2002

Successful operation of a p-n-p AlGaAs heterojunction bipolar transistor at temperatures up to 800 K (527 °C) has been achieved. The device was fabricated from molecular-beam epitaxy grown material and features an Al0.45Ga0.55As emitter and Al0.25Ga0.75As base and collector regions. At room temperature a common emitter current gain of 12 is obtained, which reduces to ∼3 at 800 K. Weak emission of light is seen from the base region with λ=7170 Å at 300 K, in agreement with the expected band gap for Al0.25Ga0.75As of 1.74 eV.
A reinvestigation of the etch products of silicon and XeF2: Doping and pressure effects
Authors: Houle F. A.
Published: July 2002

The gas-phase etch products of silicon and XeF2 have been studied as a function of doping and XeF2 flux using molecular beam mass-phase spectrometry. In this experiment the product flux, not the incident reactant beam, is modulated, providing direct information on product identity and velocity, and indirect information on surface reaction kinetics. The data show that although the product ion distributions exhibit little change over the range of experimental conditions used, the neutral products from which the ions are formed vary significantly. More SiF4 desorbs from n-type than p-type silicon at constant flux. The balance of the products mainly consists of Si2F6 and SiF3, the relative amounts of which are flux dependent. These species have not been identified previously as etch products. Average product translational temperatures have also been determined, providing important information on surface interactions during etching. The stable molecules SiF4, Si2F6, and reflected (unreacted) XeF2 all have translational temperatures of approximately 270 K. The free radical SiF3 is much colder, having a temperature of about 200 K. Although etching of the two types of silicon is qualitatively similar, a quantitative comparison reveals intrinsic differences in their reactivity which are not attributable to field effects. The possible origin of these differences is discussed.
Doppler-broadened Hα line shapes in a rf low-pressure H2 discharge
Authors: Baravian G. , Chouan Y. , Ricard A. , Sultan G.
Published: July 2002

The presented results are concerned with the shapes of the Balmer-α line emitted by a low-pressure rf 13.56-MHz discharge in hydrogen. Optical emission analysis has been done using a monochromator with a high spectral resolution (0.04 Å). Essentially, the lines present a central nearly Gaussian-shape peak with a FWHM of about 0.3 Å emerging from a wide pedestal constituted by two plateaus with widths equal to 0.9 and 1.5 Å, respectively, and by large wings whose intensity quasilinearly decreases on both sides of the central wavelength. We explain these profiles by the Doppler broadening due to fast H atoms produced by the dissociative excitation and the dissociative ionization of the H2 molecules by electron impact generally followed by the excitation of the resulting atoms towards emitting states.
Magnetic environment model of ferromagnetism in dilute P
tCo alloys
Authors: Parra R. E. , López R. A.
Published: July 2002

In this model, the local moments of the platinum atoms are proportional to the moments of their near neighbor atoms while the cobalt moments are not affected by their local magnetic environment. The moment of the cobalt atom polarizes directly only its nearest neighbors and then this effect is transmitted to the whole platinum matrix producing the magnetization cloud. The interaction between clouds is responsible for the onset of ferromagnetism at low temperatures. Taking only one cloud and the value of the experimental susceptibility we calculated the self-energy of the cloud, the nonenhanced susceptibility and the exchange constant between Pt and Co atoms, JPtCo. Also with the model, we calculated the critical temperatures of ferromagnetism in the concentration region 0.66≤c≤6.0 at. % Co by the Monte Carlo method. The results obtained were compared with experimental values of the critical temperatures showing a good agreement. We conclude that the onset of ferromagnetism in these dilute alloys is due to interactions between magnetization clouds.
Phase equilibrium and stability of elastically stressed heteroepitaxial thin films
Authors: Johnson William C. , Chiang C. S.
Published: July 2002

The thermodynamic description and stability of constrained lattice-mismatched heteroepitaxial thin-film systems, in which the lattice parameter in the plane of the film is essentially fixed by that of the substrate, are examined for binary alloys and ternary III-V systems. It is shown that the crystallographic orientation of the substrate and the difference in the lattice parameter between the substrate and the thin films may significantly affect equilibrium phase compositions and volume fraction, and the relative stability of phases in two- and three-phase heteroepitaxial systems. Unlike the elastic effects usually associated with bulk coherent solids, stresses introduced by the substrate can shift the solvus line towards either the single-phase or two-phase regions, the magnitude of the shift depending strongly on both the crystallographic orientation and state of stress. Constrained thin-film heteroepitaxial systems are shown to comprise a special subset of coherent stressed solid systems for which the Gibbs phase rule governing the coexistence of phases is recovered and for which the common tangent construction to appropriate free-energy curves may be used to determine equilibrium phase compositions and stability.
Measurement of rotational temperature and dissociation in N2O glow discharges using i
n s
i
t
u Fourier transform infrared spectroscopy
Authors: Cleland T. A. , Hess D. W.
Published: July 2002

The technique of in situ Fourier transform infrared (FTIR) absorption spectroscopy has been used to determine rotational temperatures and the extent of dissociation of N2 O in a radio-frequency (rf) glow discharge. Measurements were made at 0.65 cm−1 resolution on 13.56-MHz plasmas at 500 mTorr, with an input flowrate of 40 sccm, and powers of 10 and 30 W. Temperature and dissociation information estimates are based upon analysis of P branch rotational lines of the 2ν1 harmonic and ν1 +ν3 combination band of the molecule. Line intensities are corrected for instrument-induced distortion. Under the conditions investigated, rotational temperatures are between 335 and 420 K, and dissociation ranges from 45% to 75%. Both rotational temperature and dissociation increase with rf power.
Magnetic properties of R-Fe-B and R-Fe-Co-Al-B magnets (R=Pr and Nd)
Authors: Jiang S. Y. , Chen H. Y. , Cheng S. F. , Boltich E. B. , Sankar S. G. , Laughlin D. E. , Wallace W. E.
Published: July 2002

Permanent magnets based on the compositions Pr15Fe79B6, Nd15Fe79B6, Pr15Fe62.5Co16Al1B5.5, and Nd15Fe62.5Co16Al1B5.5 have been fabricated employing conventional powder metallurgical processes. Proper heat treatment has been found to improve magnetic properties. For example, in magnets of composition Pr-Fe-Co-Al-B, the heat treatment optimized in this study improved Br and (BH)max by about 5% and 10%, respectively, in comparison with conventional heat treatment reported in the literature. The temperature coefficients of magnetic properties have also been improved through Co and Al substitutions.
Properties and mechanism of photochromism in reduced calcium aluminate glasses
Authors: Hosono Hideo , Asada Naoyuki , Abe Yoshihiro
Published: July 2002

Calcium aluminate glasses melted in graphite crucibles were found to show photochromism. Upon exposure to ultraviolet radiation, broad absorptions with an apparent peak of around 2 eV and a shoulder around 3.5 eV were induced, and after interruption of the light illumination both bands faded at room temperature. A distinct photobleaching effect was observed in the fading process. Two kinds of electron-paramagnetic-resonance signals, one symmetric and the other asymmetric, were induced near g=2 by exposure to radiation. The growing and fading behaviors of the symmetric and asymmetric components were parallel to those of the 2-eV and 3.5-eV band, respectively. It is strongly suggested that the photochromism originates from an electron trapped at the site of oxygen vacancy surrounded by Ca2+ ions produced in graphite crucibles during melting under strongly reducing conditions.
Influence of copper vapor on extinction in nitrogen arcs
Authors: Gleizes A. , Amry A. M. A. , Rahal A. M. , Vacquié S.
Published: July 2002

Spectroscopic measurements were made on a nitrogen nonblown arc in extinction at atmospheric pressure seeded with copper vapor produced by the anode. The experimentally determined axial temperatures and total line intensities were compared to theoretical values obtained from a model based on mass and energy conservation equations. This comparison validated the model and showed that for an initial current of 25 A, the concentration of copper decreases at 0.5 mm from the anode as soon as the electric field is cut but remains more or less constant for about 150 μs in the region of plasma at around 2.5 mm from the anode. This is mainly due to the copper diffusion mechanisms involved. The model allowed the influence of copper on the evolution of the conductance of a nitrogen arc during extinction to be calculated. In particular it was seen that a proportion of copper as low as 10−4 markedly changed the conductance of the arc. Moreover, when the conductance of pure nitrogen has dropped by a factor of 40 compared to the initial value, that of a mixture containing 99.9% nitrogen and 0.1% copper has only decreased by a factor of 4.
Magneto-optical properties of (TbBi)3Fe5O12 and its application to a 1.5 μm wideband optical isolator
Authors: Tamaki T. , Kaneda H. , Kawamura N.
Published: July 2002

A new composition of Bi-substituted terbium iron garnet (Tb3−xBixFe5O12) for a wideband optical isolator was investigated. The Faraday rotation of Tb3−xBixFe5O12 (x=0.25) at 1.5 μm band proved to be independent of wavelength λ. The value of the Faraday rotation of the crystal with x=0.25 was about −205 deg/cm at λ=1.5 μm. We have also found empirically that there is a simple relationship between the Faraday rotation coefficient θF(x) and Bi content x, given by θF(x)=−1540x+180 deg/cm. By using these crystals as a Faraday rotator, we have developed wideband optical isolators for a 1.5 μm band. Forward loss of 0.4 dB and backward loss of 34 dB were obtained in the wavelength region from 1.5 to 1.7 μm. The size of the isolator is 5 mm diameter×5 mm long and its weight is 0.4 g. Through an optical fiber transmission system with the isolator, we could transmit a stable and clear picture for the lights of 1.53 and 1.57 μm wavelength.
Optical doping of waveguide materials by MeV Er implantation
Authors: Polman A. , Jacobson D. C. , Eaglesham D. J. , Kistler R. C. , Poate J. M.
Published: July 2002

Implantation of MeV erbium ions into micron-thick silica and phosphosilicate glass films and 1200-Å-thick Si3N4 films is studied with the aim of incorporating the rare-earth dopant on an optically active site in the network. Implantation energies and fluences range from 500 keV to 3.5 MeV and 3.8×1015 to 9.0×1016 ions/cm2. After proper thermal annealing, all implanted films show an intense and sharply peaked photoluminescence spectrum centered around λ = 1.54 μm. The fluorescence lifetime ranges from 6 to 15 ms for the silica-based glasses, depending on annealing treatment and Er concentration. Silicon nitride films show lower lifetimes, in the range <0.2–7 ms. Annealing characteristics of all materials are interpreted in terms of annealing of ion-induced network defects. These defects are identified using photoluminescence spectroscopy at 4.2 K. Concentration quenching, diffusion and precipitation behavior of Er is also studied.
Magnetic properties of Pr-Fe-B alloy powders prepared by mechanical grinding
Authors: Noh T. H. , Jeung W. Y. , Kang I. K. , Shin S. H. , Lee J. J.
Published: July 2002

The effect of ball milling on the magnetic properties of the Pr15Fe80B5 alloy, whose as-cast structure was composed of matrix phase Pr2Fe14B and Pr-rich and α-Fe phases, has been investigated by measuring iHc, thermomagnetic curves, and x-ray diffraction (XRD) patterns. By ball milling using a planetary ball mill up to 96 h, iHc decreased with the amorphization of the matrix and Pr-rich phases. However, subsequent heat treatments gave rise to the marked increase of iHc. The alloy, after 48 h of ball milling and subsequent 600 °C annealing possessed iHc of 17.4 kOe, Br of 5.8 kG, and (BH)max of 6.5 MGOe. This alloy showed sharp diffraction peaks of Pr2Fe14B phase, while the α-Fe phase peak, which persisted through the ball milling process, almost disappeared. The high iHc of the Pr-Fe-B alloy, subjected to optimum milling and subsequent annealing, was attributed to a very fine Pr2Fe14B phase precipitation from the amorphous state and to the effective solution of the α-Fe phase into the matrix phase. Also, a small addition of Cu to the Pr15Fe80B5 alloy was found to increase the coercivity. Further, the alloy milled for 12 h and annealed at 600 °C for 1 h showed iHc of 19.7 kOe, Br of 7.0 kG, and (BH)max of 10.2 MGOe.
Kinetics of xerographic discharge by surface charge injection
Authors: Young Ralph H.
Published: July 2002

Using a novel and very simple method, a complete formal solution is presented for the kinetics of discharge (surface potential versus time) of a xerographic photoreceptor under the following conditions. (i) The discharge is effected entirely by charge injected or generated at one surface. The kinetics of injection is arbitrary. Included as special cases are instantaneous injection (e.g., under flash exposure) and injection at a rate proportional to the quantum efficiency of charge generation (e.g., under exposure at constant intensity). (ii) Charge transport is well described by a mobility law without diffusion, trapping, or dispersion. In the case of instantaneous injection explicit analytical solutions are given for constant, power-law, exponential, and Poole–Frenkel mobility laws and general relationships are pointed out among solutions for different thicknesses, initial or final potentials, or parameters in the mobility law. These properties can be used to diagnose whether an experimentally observed discharge is limited by the kinetics of injection or transport. They also form the basis of a graphical method for predicting the course of any discharge with instantaneous injection, using a single master plot for the appropriate mobility law. Along the way, several nonstandard formulas relating the surface potential to the space-charge density are discussed.
Analysis of transient photoluminescence measurements on GaAs and AlGaAs double heterostructures
Authors: Marvin Dean C. , Moss Steven C. , Halle Linda F.
Published: July 2002

The analysis of transient photoluminescence measurements and extraction of carrier recombination lifetimes in GaAs and AlGaAs double heterostructures is discussed. In contrast to recently reported claims, it is demonstrated that even in regions where the measured decay curves show single exponential behavior, the slopes do not, in general, correspond to any single physical carrier lifetime such as the minority-carrier lifetime. A series of measurements over a range of incident optical intensities is required to extract such lifetimes.
Photothermal description of polymer ablation: Absorption behavior and degradation time scales
Authors: Cain Stephen R. , Burns F. C. , Otis Charles E. , Braren Bodil
Published: July 2002

A photothermal model of ablation is used to investigate the time scales for polymer degradation by UV laser light. In the absence of a significant incubation effect, strong absorbers (e.g., polyimide) are expected to decompose roughly three orders of magnitude faster than weak absorbers (e.g., polymethylmethacrylate), owing to the higher surface temperature attained during the absorption. This difference in the time scales reflects processes taking place at significantly different temperatures and should extrapolate to the overall ablation process. The very short calculated time scales (femtoseconds for polyimide and picoseconds for polymethylmethacrylate) indicate that polymer decomposition occurs rapidly compared to the actual ejection of material. For multipulsed ablation experiments, significant incubation modification tends to increase the absorbance of weak absorbers, making the effect less marked than in the ideal case. Incubation effects are attributed to nonablative decomposition that occurs at cooler temperatures (<1000 K).
Free carrier absorption in semiconducting quantum wells for confined LO phonon scattering
Authors: Bhat J. S. , Kubakaddi S. S. , Mulimani B. G.
Published: July 2002

A theory of free carrier absorption is given for GaAs/GaAlAs quantum wells when the carriers are scattered by confined LO phonons. The Huang and Zhu approximation for the confined phonon modes is used. The absorption coefficient is calculated and is found to have, broadly, features similar to the calculations with bulk phonons. The well width dependence of absorption is found to be opposite to that exhibited by scattering rates.
Aging study of nickel-copper-manganite negative temperature coefficient thermistors by thermopower measurements
Authors: Castelan P. , Ai Bui , Loubiere A. , Rousset A. , Legros R.
Published: July 2002

Ceramic nickel-copper-manganite based negative temperature coefficient thermistors are unstable under thermal constraint, which increases the resistivity of these devices. This unstability can be reduced by the addition of a small amount of barium. These ceramics have a spinel structure, and the conduction takes place by a hopping process involving [Mn3+]/[Mn4+] octahedral ions. This study by the use of thermopower measurements points out that the resistivity change under thermal constraint is principally due to the disproportionation of [Mn3+]/[Mn4+] ions in the octahedral sites. Furthermore, it seems that some nickel ions migrate from octahedral to tetrahedral sites.
A new method of auto-separating thermally stimulated current
Authors: Lei Qing-quan , Wang Xuan , Fan Yong
Published: July 2002

A new method of auto-separating thermally stimulated current is presented. This method allows an easy, accurate, and quick resolution of the overall thermally stimulated current (TSC) spectrum. By comparing the individual TSC curves, plotted by microcomputer, with those obtained by the peak cleaning technique, it is believed that this method is a viable alternative to the traditional complicated test methods, and may find wide use in the separation of thermally stimulated current curves.
Surface reaction kinetics of gas-phase diamond growth
Authors: Kondoh Eiichi , Ohta Tomohiro , Mitomo Tohru , Ohtsuka Kenichi
Published: July 2002

Surface reaction kinetics for diamond growth by chemical vapor deposition (CVD) was investigated experimentally. The temperature dependence for the growth rate exhibited surface-reaction controlled kinetics with the activation energy of about 23 kcal/mol below 900 °C. The dependence of the growth rate on the other CVD parameters and the corresponding calculated concentration of H and CH3 gave the growth rate (GR) expression GR= kX0HX1CH3, where k and Xi are a rate constant and the mole fraction of species i, respectively. A decrease in the activation energy for apparent diamond-film growth with increasing nondiamond carbon content was also discussed in relation to the role of gaseous H atoms. A dramatic decrease of the activation energy occurred at high pressure (300 Torr) deposition, since the growth kinetics changed to the diffusion controlled growth.
Magnetic properties of colloidal silica: Potassium silicate gel/iron nanocomposites
Authors: Shull R. D. , Kerch H. M. , Ritter J. J.
Published: July 2002

Homogeneous composites of Fe and silica containing 5–40 wt % Fe have been prepared by infiltration of ferric nitrate solution into a colloidal silica: potassium silicate gel at room temperature. Previous electron microscopy, small angle neutron scattering, and nitrogen sorption (BET) measurements on such gels have shown they are comprised of an interconnected silica matrix, having pores ranging from 2–800 nm in size. Mössbauer effect and magnetization data show the Fe is in a paramagnetic environment with weak antiferromagnetic interactions. Curie–Weiss analysis indicated negative intercepts of the temperature axis for all the samples and that the magnetic moment per Fe atom (μFe) decreased as the Fe content increased at a rate of ∼0.0058 μB/% Fe. For most of the samples μFe≊2.1 μB. By contrast, in silica gel/Fe nanocomposites formed by the polymerization of an aqueous solution of tetraethoxysilane and iron nitrate, the Fe possessed magnetic moments varying from 3.9 to 2.9 μB. It is suggested that the difference is a result of the different pore structures of the gels, and not due to the different methods (e.g., postmatrix infiltration as compared to premixing prior to matrix polymerization) used to introduce the Fe.
An approximate formula for the relaxation time of a single domain ferromagnetic particle with uniaxial anisotropy and collinear field
Authors: Cregg P. J. , Crothers D. S. F. , Wickstead A. W.
Published: July 2002

Recently there have been renewed efforts to solve the Fokker–Planck equation resulting from Brown’s model [Phys. Rev. 130, 1677 (1963)] of single domain ferromagnetic particle relaxation for the case of uniaxial anisotropy and zero external field. In particular, the usefulness of a simple analytic formula for the relaxation time based on the reciprocal of the lowest non-zero eigenvalue resulting from this model has been stressed. Here we suggest an improved analytic formula which we extend to the case of applied collinear field and compare it with the exact numerical solution and previous analytic expressions. The results are presented in terms of the pre-factor dependence which is commonly taken as constant. The formula exhibits good agreement with the exact results throughout the full range of anisotropy and reduced external field values.
Physical and optical properties of Cu nanoclusters fabricated by ion implantation in fused silica
Authors: Magruder R. H. , Haglund R. F. , Yang L. , Wittig J. E. , Zuhr R. A.
Published: July 2002

Cu clusters of nanometer dimensions were created by implantation of Cu ions into pure fused silica substrates at energies of 160 keV. The sizes and size distributions of the Cu clusters were measured by transmission electron microscopy, and were found to be determined by the ion-beam current during implantation. Optical-absorption spectra of these materials show the size-dependent surface plasmon resonance characteristic of noble-metal clusters. There are also significant size-dependent effects in both the nonlinear index of refraction and two-photon absorption coefficients. The distinctive variations in linear and nonlinear optical properties with Cu nanocluster sizes and size distributions affords potentially interesting possibilities for using these materials in nonlinear optical devices.
Effect of gate voltage on hot-electron and hot phonon interaction and transport in a submicrometer transistor
Authors: Majumdar A. , Fushinobu K. , Hijikata K.
Published: July 2002

This paper studies the effects of gate voltage on heat generation and transport in a metal–semiconductor field effect transistor made of gallium arsenide (GaAs) with a gate length of 0.2 μm. Based on the interactions between electrons, optical phonons, and acoustic phonons in GaAs, a self-consistent model consisting of hydrodynamic equations for electrons and phonons is developed. Concurrent study of the electrical and thermal behavior of the device shows that under a source-to-drain bias at 3 V and zero gate bias, the maximum electron temperature rise in this device is higher than 1000 K whereas the lattice temperature rise is of the order of 10 K, thereby exhibiting nonequilibrium characteristics. As the gate voltage is decreased from 0 to −2 V the maximum electron temperature increases due to generation of higher electric fields whereas the maximum lattice temperature reduces due to lower power dissipation. The nonequilibrium hot-electron effect can reduce the drain current by 15% and must be included in the analysis. More importantly, it is found that the electron temperature rise is nearly independent of the thermal package conductance whereas the lattice temperature rise depends strongly on it. In addition, an increase of lattice temperature by 100 K can reduce the drain current by 25%.
Study of the spatial distribution of backscattered electrons from a gold target with a new Monte Carlo simulation
Authors: Yasuda M. , Kawata H. , Murata K.
Published: July 2002

The spatial distribution of backscattered electrons is studied for an Au target with a new Monte Carlo simulation including the discrete energy loss process. The new model is based on the Mott cross section for elastic scattering and the Vriens cross section for inelastic scattering. Some results are compared with the ones by the old Monte Carlo simulation which is based on the screened Rutherford cross section and the Bethe law. The new model results in a larger peak near the incident point and a more gradual decrease of the background part of the radial distribution. The incident energy dependence of the spatial distribution of backscattered electrons is shown for 0° and 45° incident angles. The calculated spatial distribution of low-loss backscattered electrons indicates the possibility of very high resolution even in the backscattered electron image. The exit angle dependence of the penetration depth of backscattered electrons is also studied with the new simulation.
In situ x-ray measurements of the initial epitaxy of Fe(001) films on MgO(001)
Authors: Lairson B. M. , Payne A. P. , Brennan S. , Rensing N. M. , Daniels B. J. , Clemens B. M.
Published: July 2002

Small- and large-angle x-ray scattering from epitaxial Fe(001) on an MgO(001) surface has been measured as a function of film thickness, using grazing incidence x-ray scattering. Small-angle scattering shows that for Fe thicknesses less than 15 monolayers, the Fe is islanded with Fe[110]∥MgO[100]. For deposition at 360 °C, the Fe lattice parameter increases toward the MgO surface net spacing with increasing thickness in the 1–10 monolayer coverage regime, and then relaxes back toward the bulk Fe lattice parameter at greater thicknesses. Agglomeration of the islands results in changes in the Fe lattice parameter and in the high-angle peak widths. Prior to agglomeration, the measured in-plane lattice parameter versus thickness is described by a pairwise site interaction between the island and the substrate interface nets. Strain relaxation subsequent to agglomeration is described by continuum elasticity theory.
Influence of structural defects on carrier recombination and current gain in an InGaAs/AlGaAs/GaAs heterojunction phototransistor
Authors: Lin H. T. , Rich D. H. , Sjölund O. , Ghisoni M. , Larsson A.
Published: July 2002

We have studied the influence of structural defects on the spatial variation of radiative and nonradiative recombination in an InGaAs/AlGaAs/GaAs resonant cavity enhanced npn heterojunction phototransistor (HPT) structure using cathodoluminescence (CL) and electron beam-induced current (EBIC) imaging. Absorber layers of InGaAs/GaAs multiple quantum wells (MQWs) are used to provide the photosensitivity for light with wavelengths that are transparent to the GaAs substrate. The current gain of the HPT under an applied bias voltage depends on the hole accumulation efficiency in the base and therefore on the hole lifetime. Strain relaxation-induced misfit dislocations in the MQWs are found to create regions of enhanced nonradiative recombination thereby reducing locally the hole accumulation efficiency and current gain. The reduction in the local EBIC signal caused by the dark line defects is less than ∼20%, suggesting that misfit dislocations in this sample have a relatively small impact on overall device performance. EBIC and CL are found to be excellent complementary probes of the hole accumulation efficiency and relative recombination rates, owing to the high spatial resolution (∼1 μm) of excitation of the electron beam used in these techniques. The temperature dependence of the EBIC and spectrally integrated CL images is examined for 85≤T≤300 K, and reveals that nonradiative recombination in the vicinity of misfit dislocations is predominantly thermally activated with observed spatial variations in lifetime and activation energy.
Experimental investigation and characterization of the departure from local thermodynamic equilibrium along a surface-wave-sustained discharge at atmospheric pressure
Authors: Calzada M. D. , Moisan M. , Gamero A. , Sola A.
Published: July 2002

Surface-wave-sustained discharges (SWDs) form a particular class of high frequency (HF) discharges: their HF sustaining field is provided by a traveling wave that transfers energy as it propagates along the discharge column, yielding a plasma column with an axially decreasing electron density. SWDs have proved to be ideal for investigating experimentally and theoretically both the HF field and discharge aspects of HF plasma sources at reduced gas pressure. In this article, SWDs are utilized at atmospheric pressure to gain insight into the departure from thermodynamic equilibrium (TE) of HF sustained discharges. This departure is found to increase significantly as the electron density decreases along the plasma column whereas the gas temperature and the power absorbed per electron do not vary axially. The two-temperature plasma model provides an adequate description of this departure from TE.
Anomalous shock initiation of detonation in pentaerythritol tetranitrate crystals
Authors: Dick J. J.
Published: July 2002

The anomalous, low-stress, shock initiation of detonation observed in earlier studies of pentaerythritol tetranitrate single crystals was examined in more detail experimentally. Time-resolved particle-velocity histories were obtained for [110], [001] and [100] orientations of single-crystal pentaerythritol tetranitrate explosive for shock input stresses of 4–7 GPa using laser interferometry instrumentation. At about 4.2 GPa an elastic-plastic, two-wave structure was noted in [110] and [001] orientations, and a single shock wave for [100] orientation. The two-wave structure provides an explanation for the anomalous shock initiation sensitivity and intermediate velocity transition previously observed in [110] orientation at this stress level. It also explains details of fluorescent emission histories from [110] and [001] crystals previously measured. The orientation-dependent results are consistent with the model of steric hindrance to shear at the molecular level. Fits to the elastic Hugoniot data in [110] and [001] orientations are given as well as a revised fit for the bulk Hugoniot.
A miniature dc current sensor based on magnetoimpedance
Authors: Valenzuela R. , Freijo J. J. , Salcedo A. , Vázquez M. , Hernando A.
Published: July 2002

A dc electric current sensor, based on the magnetoimpedance phenomenon, is presented. The basic element of this sensor is a nonmagnetotstrictive, amorphous ferromagnetic wire of composition (Co0.94Fe0.06)72,5B15Si12.5, in the as-cast state. The dc current to be monitored flows through a solenoid, and the magnetic field produced by this solenoid on the wire leads to a controlled decrease of its impedance response. This impedance dependence on the axial field is used to measure the dc current. An important feature of this sensor is its reduced dimensions, since the whole device (sensing wire, solenoid, ac current generator) fits into a 2×2×1 cm3 volume. A discussion on its thermal stability, reproducibility, and wire contact preparation is presented.
Localized absorption effects during 351 nm, pulsed laser irradiation of dielectric multilayer thin films
Authors: Papernov S. , Schmid A. W.
Published: July 2002

A correlation is attempted between atomic-force-microscopy-mapped, postmortem damage morphologies in model thin films irradiated near threshold at 351 nm and finite-element, numerical simulations of the damage process in such multilayer films. This comparison permits deriving lower bounds on the temperature field at and near the absorbing defect, necessitating revisions to any damage model based solely on thermal conduction as the energy dissipation mechanism.
Electric breakdown in deuterium and hydrogen at low pressures
Authors: Armstrong R. J. , Bennett T. K.
Published: July 2002

The electrical breakdown of plane parallel steel electrodes (10 cm diam) is studied in a Pyrex-walled vacuum chamber (minimum pressure less than 8×10−6Torr). For clean gases, H2 and D2, and with electrode gaps from 0.5 to 8 cm, the breakdown at pressures from 0.3 to 0.5 Torr is consistent with a secondary process of electron release by ion bombardment. The breakdown curves for the two gases, when scaled on both axes, overlap to a considerable extent.
Effect of TiC additions to the microstructure and magnetic properties of Nd9.5Fe84.5B6 melt-spun ribbons
Authors: Kramer M. J. , Li C. P. , Dennis K. W. , McCallum R. W. , Sellers C. H. , Branagan D. J. , Lewis L. H. , Wang J. Y.
Published: July 2002

Rapidly solidified samples of Nd9.5Fe84.5B6 with and without 3 at. % TiC were prepared by melt spinning and melt extraction and then annealed in vacuum (3×10−6 Torr) at temperatures from 600 to 750 °C. For alloys melt spun under similar conditions, the overquenched state was achieved at wheel speeds >10 m/s for the TiC added alloy while >20 m/s was necessary without TiC. The overquenched samples contained a smaller fraction of α-Fe in smaller grains than the undercooled samples where Fe dendrites formed near the free surface during solidification. These Fe dendrites were not removed by annealing. In addition, large orientated 2-14-1 grains nucleated on the Fe dendrites. This combination is detrimental to the magnetic properties. The addition of TiC results in improved control of the microstructure over a larger fraction of the ribbon volume enhancing the magnetic properties.
Phase separation and ordering in InGaN alloys grown by molecular beam epitaxy
Authors: Doppalapudi D. , Basu S. N. , Ludwig K. F. , Moustakas T. D.
Published: July 2002

In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700–750 °C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures (650–675 °C) revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725 °C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725 °C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice strain in the alloy.
Structural transformations of Bi2CuO4 induced by mechanical deformation
Authors: Chen X. L. , Liang J. K. , Liu Y. , Lan Y. C. , Zhang Y. L. , Ma Y. , Che G. C. , Liu G. D. , Xing X. Y. , Qiao X. Y.
Published: July 2002

Structural transformation of α-Bi2CuO4 (a=8.4996 Å, c=5.8172 Å, space group P4/ncc) during ball milling was investigated by powder x-ray diffraction and thermal analysis. It is shown that α-Bi2CuO4 first transforms into an amorphous phase; and then transforms into a new phase (designated by β-Bi2CuO4) with milling time. The new phase appears to be isostructural to (La,Sr)2CuO4 and has a tetragonal cell with cell parameters a=3.869 and c=13.83 Å. It is a metastable phase which can transform back into α-Bi2CuO4 at 650 °C. No superconductivity was observed above 5 K for both as-milled samples and oxygen-annealed β-Bi2CuO4.
Surface-acoustic-wave properties of MgO-doped LiNbO3 single crystals measured by line-focus-beam acoustic microscopy
Authors: Kushibiki J. , Kobayashi T. , Ishiji H. , Jen C. K.
Published: July 2002

Basic acoustic properties of five MgO-doped LiNbO3 rectangular parallelpiped specimens, having X-, Y-, and Z-cut planes, with different MgO dopant concentrations ranging from 0 to 13 mol % are investigated by line-focus-beam acoustic microscopy. The investigation is conducted to characterize the optical-use MgO:LiNbO3 crystals and wafers and to establish large-diameter crystal growth conditions. Leaky-surface-acoustic-wave (LSAW) velocities are measured on each crystalline plane of each specimen as a function of the wave propagation direction and are compared with the measured chemical composition ratios of Mg/Li/Nb, densities, and lattice constants. It is shown that, as the dopant concentrations increase in the experimental composition region, the LSAW velocities increase almost linearly for all the surfaces and all the propagation directions. The LSAW velocities are linearly proportional to the lattice constants, but inversely proportional to the densities.
Strain-balanced Si/SiGe short period superlattices: Disruption of the surface crosshatch
Authors: Hartmann J. M. , Gallas B. , Zhang J. , Harris J. J. , Joyce B. A.
Published: July 2002

We have studied the effects of inserting Si/Si0.6Ge0.4 strain-balanced superlattices (SLs) into Si0.8Ge0.2 (001) virtual substrates. The SiGe SL layer thickness chosen was larger than the critical thickness for elastic relaxation and generated numerous hemicylindrical features oriented along the 〈100〉 directions. These features lead, when covered by Si0.8Ge0.2, to a disruption of the well-ordered surface crosshatch along the 〈110〉 directions, and to a significant lowering of the surface roughness. There is also evidence for some filtering of the threading dislocations by the SL.
Double tunnel junctions for magnetic random access memory devices
Authors: Inomata K. , Saito Y. , Nakajima K. , Sagoi M.
Published: July 2002

Optimum junction resistance and minimum tunnel magnetoresistance (TMR) ratio required for high density and high performance magnetoresistive random access memory (MRAM) devices with a TMR cell plus field effect transistor (FET) switch architecture are discussed by taking into account the variation of FET resistance causing noise. This implies that a TMR ratio over 25% at a 400 mV bias voltage and junction resistance of several tens of kilo-ohms for TMR cells are required with a signal voltage of 30 mV and a sense current of 10 μA, which leads to about 10 ns read time. This large magnetoresistance ratio at the elevated bias voltage requires low bias voltage dependence of TMR for the MRAM devices. In order to try to meet this requirement, double tunnel junctions were fabricated which possess the central ferromagnetic layer consisting of a thin discontinuous layer of hard ferromagnetic Co80Pt20 nanoparticles and insulating Al2O3 prepared by alternate sputtering of Co80Pt20 and Al2O3 targets. The maximum TMR obtained was 20.5% at room temperature for FeCo top and bottom electrodes without annealing. Bias voltage dependence of the (NiFe/CoFe)/1.5 nm Al2O3/discontinuous CoPt/2.6 nm Al2O3/(CoFe/NiFe) double tunnel junctions were revealed to be small compared to that of single junctions, the barrier of which was also fabricated by sputtering of an Al2O3 target.
CoFe/IrMn exchange biased top, bottom, and dual spin valves
Authors: Anderson Geoff , Huai Yiming , Miloslawsky Lena
Published: July 2002

The present work is a comparative study of CoFe/IrMn exchange biased top, bottom, and dual spin valves. For as-deposited top spin valves the exchange bias energy Jk was determined to be 0.14 erg/cm2 for IrMn 50 Å. The exchange field (Hex) decreases rapidly upon heating, resulting in a blocking temperature (Tb) of 250 °C. For bottom IrMn based spin valves the exchange bias energy is much higher (Jk=0.28 erg/cm2) while ΔR/R (6.9%) is reduced with respect to top spin valves (>8.0%). This is interpreted as showing that Hex is dominantly dependent on fcc(111) crystalline texture while ΔR/R is sensitive to both crystalline texture and interface roughness. Dual spin valves based on the above results showed two exchange loops due to the difference in the exchange bias energy between the top and bottom pinning layers.
Capacitance–voltage measurements on plasma enhanced chemical vapor deposited silicon nitride films
Authors: Basa D. K. , Bose M. , Bose D. N.
Published: July 2002

Silicon nitride films of varying composition have been deposited with nitrogen dilution onto p-type silicon substrates at 250 °C by plasma enhanced chemical vapor deposition technique. Careful and detailed capacitance–voltage (C–V) measurements have been undertaken in the metal-insulator-semiconductor configuration. Silicon-rich films are found to exhibit large symmetric hysteresis loops in the C–V curve while the nitrogen-rich films display much smaller asymmetric hysteresis loops. Furthermore, the minimum interface state density is observed to decrease with the increase in nitrogen to silicon ratio. In this study we have observed that the concentration of both electron as well as hole traps are much lower for the nitrogen-rich films.
The temperature dependence of coercivity in nanocrystalline Nd–Fe–B–(TiC) magnets
Authors: Merkulova G. Ya. , Margulies L. , Dennis K. W. , McCallum R. W.
Published: July 2002

TiC additions to Nd2Fe14B have been shown to enhance the control of microstructure and greatly modify the solidification behavior of the alloy. The temperature dependence of the coercivity, Hci, of the Nd2Fe14B+xTiC(0<x<6 at. %) is important for both technical applications and a comprehensive understanding of the coercivity mechanism. In addition, by studying the onset of coercivity as a function of the annealing temperature of amorphous ribbons the activation energy for crystallization may be determined. Amorphous melt-spun ribbons with x=0, 2, 3, and 6 were annealed isothermally in an Ar atmosphere at temperatures near, above and below the crystallization temperature. The activation energies Ea were determined by fitting the dependence of the coercivity as a function of annealing time and temperature to the Arrenius’ equation and found to be 18–25 kcal. The coercivity distribution functions of micro volumes of magnets were created using demagnetization and reversible magnetization curves that allowed the correlation of the coercive force with nanostructure. Maximum Hci of up to 15 kOe (3 at. % TiC) is achieved for an average grain size of 20–40 nm. The temperature dependencies of coercivity of stoichiometric Nd2Fe14B ribbons without and with additions TiC have similar character in temperature range (250–400 K) for all samples. The coercivities for nanocrystalline Nd2Fe14B magnets with TiC are more than for stoichiometric material.