bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic launch process of performed fragments
Authors: O’Donoghue Padraic E. , Predebon William W. , Anderson Charles E.
Published: July 2002

It is shown through numerical simulations that the gap between performed fragments closes during explosive launch, trapping the detonation products until radial expansion of the fragments is sufficient to separate the fragments. Circumferential strains from the numerical calculations are in good agreement with the plastic strains from recovered fragments. Additionally, considerable insights into the dynamics of the explosive launch process are obtained by comparing spall failure and nucleation and growth of ductile voids in the recovered fragments against the numerical simulations.
Surface and size effects in magnetic phase transitions (invited)
Authors: Landau D. P. , Binder K.
Published: July 2002

Bulk phase transitions are now well understood for a wide range of magnetic systems. The modifications which occur when the system size is made finite or when surfaces are introduced form a topic of both theoretical and experimental interest. Not only may the transition be rounded and shifted but the properties of the surfaces may include new, independent behavior which is different from that deep in the bulk. We shall present results of Monte Carlo simulations which demonstrate these effects in simple lattice models.
Interferometric determination of optical anisotropy in fibers III multilayer fibers
Authors: Hamza A. A. , Sokkar T. Z. N. , Shahin M. M.
Published: July 2002

Mathematical formulas are modified for the shape of two-beam interference fringes crossing a cylindrical multilayer fiber to determine the refractive indices and birefringence at the boundary of layers across the fiber diameter. An equation is also derived to calculate the parts of cross-sectional areas of the fiber layers. A polarizing interference microscope is used to determine the variation of refractive indices and birefringence at any point across the diameter of polyester and nylon fibers. Two fibers are used for each measurement. The first fiber is of known mean refractive index and dispersion properties. The other fiber is of unknown values of refractive indices of fiber layers. The calculations are carried out with both line and area methods using the derived mathematical formulas. Illustrations are given by microinterferograms.
Detection of ultrasound using a tunneling microscope
Authors: Moreau A. , Ketterson J. B.
Published: July 2002

We have developed a scanning tunneling microscope capable of detecting high frequency (1.8–12.5 MHz) acoustic waves. Acoustic pulses arriving at a conducting surface are detected as a high frequency perturbation of the tunneling current. Near and below the rolloff frequency of the current to voltage amplifier, the instantaneous displacement of the surface can be reconstructed. For higher frequencies, the nonlinear (rectifying) properties of the tunneling behavior allow the detection of the envelope (and hence the time of arrival and total amplitude) of the acoustic burst. The technique’s sensitivity is comparable to that of optical detection schemes but offers much better lateral resolution.
On the internal current gain of buried N-body silicon-on-insulator N-metal-oxide-semiconductor field-effect-transistors
Authors: Gautier J.
Published: July 2002

We give a simplified expression of the internal current gain of buried N-body silicon-on-insulator N-metal-oxide-semiconductor field-effect-transistors. It applies only for the subthreshold region of operation, which is required for the bipolar action. The existence of a body effect is emphasized and explains the change of the current gain versus the body doping and the silicon film thickness. For a more accurate estimate of the current gain, it would be required to account for space charge hole recombination and high injection effect.
The number of contiguous neighbors of grains in polycrystalline materials
Authors: Thorvaldsen Andreas
Published: July 2002

The stochastic field approach to grain growth in polycrystalline materials permits to combine the stochastic element of the process with physical mechanisms causing grain growth without violating volume conservation, connectivity, and continuity. However, a stochastic field analysis requires knowledge of the number of nearest neighbors of the grains. Based on the assumption that the number of neighbors is determined by their relative size, the present article derives a linear relationship in two dimensions and a parabolic relationship in three dimensions between the number of contiguous neighbors and the grain radius. The models are found to describe simulated 2D and 3D Voronoi polygons as well as the observed number of neighbors–grain size relationships in polycrystalline aluminum.
Temperature fluctuations during the deposition of InSb films
Authors: Isai Masaaki , Yasuda Shun-ichi , Ogita Masami , Tanaka Ikuo
Published: July 2002

High-quality InSb films must be grown through the melting-solidification process. A fluctuation of the film temperature, especially in the early stages of the evaporation process gives rise to this melting-solidification process. The shape of the temperature fluctuation curves can be explained by the variation of deposition rate during the evaporation process. It was found that the fluctuation of the film temperature was closely related to the maximum value of the deposition rate. The optimum fluctuation curve of film temperature was established by selecting a suitable value of the maximum deposition rate. The maximum deposition rate of around 50 Hz/s gives the optimum fluctuation curve, which causes a large area of black spots. The effectiveness of the sensing position of the substrate temperature on the fluctuation of film temperature is also investigated.
Analysis of the current-voltage characteristic of the rf SQUID
Authors: Lethuillier P.
Published: July 2002

The electrical equations of a resonant circuit fed with a radio-frequency current and inductively coupled to a weakly superconducting loop have been derived. The current-voltage characteristic of the rf superconducting quantum interference device can then be calculated and studied as a function of the junction resistance or of the resonance frequency, enabling its behavior to be better understood. As shown by Kurkijärvi, the large positive slope of the steps and the rounding of the characteristic result from thermal noise. However, in order to explain quantitatively the experimental results concerning, namely, point contact weak links which are generally oxidized or which have several contact points, it is useful to remark that the connection between the two superconductors constituting the Josephson junction may be made from n resistive paths. The mean thermal energy of the fluctuating flux is then close to nkbT instead of kbT if the inductances linking the contact points are large enough, that is, on the order of 10−11 H.
p on n heterostructures in HgCdTe on GaAs analyzed by transmission electron microscopy
Authors: Krishnamurthy Mohan , Petroff P. M. , Arias J. M.
Published: July 2002

The structure of p on n HgCdTe heterostructures grown by molecular beam epitaxy on GaAs(211) B substrates have been analyzed by transmission electron microscopy. The HgCdTe absorber layer is grown on a CdTe buffer and doped n type with indium. The p-type arsenic dopant is incorporated in the heterostructure during growth into a HgTe/CdTe cap superlattice and annealed ex situ to form the random alloy. The active p-n junction is formed below the as-grown p-n interface as a result of As diffusion. We compare the device performance with the microstructural defects in the various regions of the device layers and show the importance of dislocations near the p-n junction as well as in the n layer in influencing device quality.
Correlation of room temperature photoluminescence to structural properties of ZnSSe/ZnSe superlattices grown by metalorganic vapor phase epitaxy
Authors: Heuken M. , Scholl M. , Schneider A. , Söllner J. , Woitok J.
Published: July 2002

ZnS0.1Se0.9/ZnSe strained layer superlattices with up to 120 periods were grown by metalorganic vapor phase epitaxy. We demonstrate the sensitivity of room temperature photoluminescence of these structures for the assessment of the dependence of structural properties on growth conditions. Low temperature photoluminescence (PL) and x-ray diffraction data confirm the results of the room temperature PL measurements. In optimized samples showing highly efficient blue luminescence (2.71 eV) at 300 K heavy-hole and light-hole free exciton recombinations at 11 K and higher-order satellite reflections in the x-ray diffraction profiles were observed.
Heterojunctions of InP with amorphous hydrogenated silicon
Authors: Wu S. , Haneman D.
Published: July 2002

We report the fabrication and properties of p-InP:n-amorphous silicon heterojunctions. The good rectification ratio of 1000:1 at 0.5 V, and existence of straight-line capacitance curves, indicated a sharp interface of good quality. Optical internal photoemission measurements under various bias conditions showed that the band offsets were 0.06 and 0.37 eV for the valence and conduction bands, respectively. Photovoltaic action resulted in open circuit voltages of over 0.9 V at air mass 2 solar illumination and efficiencies of over 9%.
GaAs micrometer-sized dot imaging by Raman microscopy
Authors: Wang P. D. , Cheng C. , Torres C. M. Sotomayor , Batchelder D. N.
Published: July 2002

We have studied III-V semiconductor dot samples with Raman microscopy. The samples were fabricated by electron beam lithography and dry etching. The non-resonant Raman scattering can provide direct information on the structure alteration and associated phonon bands. Direct Raman band imaging reveals the uniformity of the GaAs micrometer-sized dot arrays.
Pulsewidth analysis in a mode locked internally frequency doubled Ti: sapphire laser
Authors: Reddy K. P. J.
Published: July 2002

Theoretical analysis of internal frequency doubling in actively mode locked broadband solid state lasers is presented. The analysis is used to study the dependence of mode locked pulsewidth on the second harmonic conversion efficiency, the modulation depth, and the tuning element bandwidth in an AM mode locked Ti: sapphire laser. The results are presented in the form of graphs.
Magnetic switching, relaxation, and domain structure of a Co/Si(111) film
Authors: Baird M. J. , Bland J. A. C. , Gu E. , Ives A. J. R. , Schumann F. O. , Hughes H. P.
Published: July 2002

We have used scanning magneto-optic Kerr effect (MOKE) microscopy to investigate the magnetic relaxation of a polycrystalline hcp 125 Å Co/Si(111) film with planar uniaxial anisotropy, on time scales between 10 and 2400 s and with a spatial resolution of 15 μm. In a static magnetic field slightly less than the coercive field and applied along the easy axis direction, domains develop and the magnetization reversal proceeds via displacements of 180° domain walls. Microscopic images of this metastable state allow the 180° domains to be identified by calibration of the MOKE signal with respect to that for the saturated magnetization states. The 180° reversed domains are observed to grow in the direction of the field in the form of narrow fingers, extending via short Barkhausen jumps, randomly spaced in time over the entire time-scale range investigated, with typical distances between pinning sites of the order of microns. This reversal behavior is qualitatively similar to that reported for Au/Co perpendicular anisotropy films a few monolayers thick.
Influence of pressure on nitrogen incorporation in ultraviolet chemical vapor deposited SiO2 films
Authors: Debauche C. , Licoppe C. , Ossart P. , Devine R. A. B. , Rochet F.
Published: July 2002

Ultraviolet induced chemical vapor deposition was used to deposit silicon dioxide dielectrics on III–V materials at low temperature. Auger electron spectroscopy and nuclear reaction analysis measurements show that the nitrogen concentration in the layers decreases continuously with the total pressure. These results are in complete agreement with infrared transmission spectroscopic and ellipsometric measurements. The number and the nature of the paramagnetic defects measured by electron spin resonance are also shown to be dependent upon the deposition pressure. Bridging nitrogen (O3≡Si–N–Si≡O3) and oxygen-like-vacancy centers (E′1) defects are observed in small quantities (≊1016 cm−3), while overcoordinated N defects are observed in concentrations up to 1018 cm−3, depending upon deposition pressure. Such SiO2 films were used in the processing of metal-insulator InP structures. Improvement of the electrical properties also occurs when the total pressure is increased, in agreement with expectations founded on the electron spin resonance results.
Study of defects in chemical vapor deposited diamond films by cross-sectional cathodoluminescence
Authors: Cremades A. , Domínguez-Adame F. , Piqueras J.
Published: July 2002

Cathodoluminescence (CL) in the scanning electron microscope has been used to study the upper surface and cross-sectional samples of chemical vapor deposited diamond films. The CL emission is mainly localized at the grain boundaries of the columnar grains. The concentration of dislocation related radiative centers is higher in boundaries parallel to the growth axis than in boundaries parallel to the sample surface. The opposite occurs with the concentration of centers related to the presence of nitrogen.
Reverse-biased performance of a molecular-beam-epitaxial-grown AlGaAs/GaAs high-power optothyristor for pulsed power-switching applications
Authors: Zhao J. H. , Burke T. , Weiner M. , Chin A. , Ballingall J. M.
Published: July 2002

The reverse-biased performance of a molecular-beam-epitaxy-grown high-power optothyristor has been systematically characterized for pulsed power-switching applications. The device has a P+N-SI-PN+ thyristor-like structure with the bipolar junctions formed by AlGaAs. The semi-insulating (SI) GaAs used is liquid-encapsulated-Czochralski grown, undoped, and 650 μm in thickness. It is found that the reverse-biased optothyristor can be triggered by a light-emitting diode operated at 10−5 W, and miniature semiconductor lasers can trigger the switch with 132 A current using only a 1-mm-diam optical aperture. The reverse switching di/dt and the maximum peak current are reported as a function of blocking voltage. The effects of bipolar junctions on both sides of the SI-GaAs are also reported by comparing the bulk photoconductive current with the optothyristor switched current. It is shown that a laser beam of 0.05 μJ can be used to trigger on and switch about the same current as a 0.3 μJ laser beam, suggesting the possibility of integrating miniature semiconductor lasers and the optothyristors on the same chip to form a portable, compact, high-power solid-state pulser.
Luminescence in crosslinked polyethylene at elevated temperatures
Authors: Bamji S. S. , Bulinski A. T. , Suzuki H. , Matsuki M. , Iwata Z.
Published: July 2002

Electrical treeing is often responsible for the breakdown of insulating materials used in power apparatus such as high-voltage transformers, cables, and capacitors. Insulation, such as crosslinked polyethylene (XLPE), used in underground high-voltage cables usually operates at temperatures above ambient. This paper describes the characteristics of luminescence, emitted prior to electrical tree inception, at a crosslinked polyethylene-semiconducting material (XLPE-semicon) interface held above room temperature. Use of a sensitive light detection system showed that XLPE subjected to elevated temperatures emits luminescence even without voltage application. This light was attributed to thermoluminescence which decreased with the decrease in the concentration of the crosslinking by-products present in the polymer. The spectra of thermoluminescence were only in the visible range. On the other hand, electroluminescence occurred when the XLPE-semicon interface was held above room temperature and subjected to high electric stress. This light did not depend on the concentration of the crosslinking by-products and the spectra of electroluminescence were in the visible and the ultraviolet ranges. It is proposed that XLPE-semicon interface held at elevated temperature and subjected to long-term voltage application initially emits both thermoluminescence and electroluminescence. As the crosslinking by-products exude out of the polymer, thermoluminescence decreases with time and ultimately ceases, but electroluminescence occurs as long as the voltage is applied to the polymer. Although the intensity of electroluminescence emitted at high temperature was lower than that emitted at ambient, sufficient ultraviolet radiation was emitted. The ultraviolet radiation could photodegrade the polymer and lead to electrical tree inception.
Ultraviolet filters using liquid crystal molecules
Authors: Wu Shin-Tson
Published: July 2002

A simple ultraviolet filter utilizing the absorption mechanism of a liquid crystal compound at its isotropic state is demonstrated. Such liquid crystal filter absorbs ultraviolet radiation below the cutoff wavelength and transmits visible and near infrared light with high efficiency. The cutoff wavelength is mainly determined by the liquid crystal material employed, but can be fine-tuned by varying the absorbing layer thickness. These devices are very simple to fabricate and can be scaled up to large sizes. Practical application for large screen projection display is emphasized.
Extensive test of the three-port quantum mixer theory on 345 GHz superconductor-insulator-superconductor mixers
Authors: Honingh C. E. , Wezelman J. J. , Dierichs M. M. T. M. , de Lange G. , Schaeffer H. H. A. , Klapwijk T. M. , de Graauw Th.
Published: July 2002

Predictions of the three-port model of the quantum theory of mixing are compared with measured results on 345 GHz superconductor-insulator-superconductor waveguide mixers. Single Nb-Al2O3-Nb tunnel junctions or two or four identical junctions in series are used as mixing elements. Two different waveguide mixerblocks, one with two tuners and another with one tuner, are used. In addition a single junction with integrated tuning stub is analyzed. Embedding impedances are obtained from fits to the pumped I-V curves for all three types of mixing elements. In all cases the dependence of mixer conversion and mixer noise on bias voltage, pump power, and embedding impedance is well described by the three-port model. The measured mixer gain is lower than the calculated gain by a factor of 0.35–0.65, independent of the type of mixer. The use of an additional integrated tuning element does not change this factor. It is concluded that an excess noise power equivalent with a blackbody source of 40–65 K must be added to the mixer noise to account for the absolute value of the observed noise power.
Hot implantation of protons into GaAs: Transmission electron microscopy
Authors: Schober T. , Friedrich J.
Published: July 2002

Implantation of hydrogen at 600 °C into GaAs at 8 keV and at high doses resulted in the formation of a two-layer structure. The topmost layer roughly extended in depth to the range of the protons and exhibited a high concentration of loops, hydrogen bubbles, decorated dislocations, and faults. Occasional microsplits on {110} were observed. The deeper layer contained pressurized hydrogen platelets on {111}. These results are critically compared with previous results obtained with cold implanted and subsequently annealed GaAs. The present observations are also discussed in relation to the theoretical treatment by d’Olieslaeger [J. Mater. Sci. 23, 2697 (1988)]. Estimates of the importance of arsine production are also presented.
Effect of ion energy on Sn donor activation and defect production in molecular beam epitaxy GaAs doped with Sn ions during growth
Authors: Thompson J. H. , Jones G. A. C. , Ritchie D. A. , Linfield E. H. , Churchill A. C. , Smith G. W. , Houlton M. , Lee D. , Whitehouse C. R.
Published: July 2002

The production of electrically active defect centers in molecular beam epitaxy (MBE) GaAs irradiated with low energy (50–500 eV) Sn ions during growth has been investigated as a function of ion energy. GaAs was doped n type during growth with a specially designed Sn focused ion beam column mounted on a MBE growth chamber. The 77 and 300 K Hall mobility and carrier concentration of the GaAs depended strongly on the ion energy, thus providing a sensitive measure of the concentration of ion-induced acceptorlike defect centers. The material was found to be nonconducting for ion energies greater than 200 eV, while a systematic decrease in the acceptor concentration, and consequent increase in the mobility, was observed as the ion energy was decreased below this value. A peak mobility of 90 000 cm2 V−1 s−1 at a carrier concentration of 1×1014 cm−3 was achieved (at 60 K) which is in excess of that obtained in other reports of ion-doped GaAs. A similar dependence on ion energy was found in the 4.2 K photoluminescence spectra of the ion-doped GaAs, characterized by the appearance of a broad acceptor level peak and a decrease in the overall luminescent intensity with increasing ion energy. The lowest energy doped samples provided high quality spectra with narrow linewidths comparable with thermal (Si) doped material. Using the above data, a mechanism is proposed for the production of acceptor centers in this system.
Thermal pulse study of the polarization distributions produced in polyvinylidene fluoride by corona poling at constant current
Authors: Giacometti José A. , DeReggi Aimé S.
Published: July 2002

A thermal pulse study of the polarization profiles in samples of 12-μm-thick, biaxially oriented polyvinylidene fluoride after corona poling under approximately constant-current conditions, using a modified corona triode in atmospheric air, is reported. An electrical characterization of the corona triode is also reported to show how it may be operated in the constant-current mode. Samples poled without electrode on the corona-exposed surface show polarization distributions sensitive to the corona polarity, with polarization depletion on the corona side of the samples when the corona is positive. Polarization-reversal experiments show switching inhomogeneities with a pronounced dependence on the initial corona polarity. The above observations are consistent with a simple model in which positive charges from the positive corona partially penetrate the sample during poling and cause an inhomogeneous reduction of the poling field.
Enhancement of intersubband Stark effects in L-valley step quantum wells for infrared modulation and voltage tunable detection
Authors: Xie H. , Wang W. I. , Meyer J. R. , Hoffman C. A. , Bartoli F. J.
Published: July 2002

We propose a new type of optical modulator employing two-step Ga1−xAlxSb/Ga1−yAlySb/Ga1−zAlzSb L-valley quantum wells to enhance the Stark shifts of the intersubband transition energy and therefore to achieve large absorption spectral changes with applied bias. Due to the effective-mass anisotropy of electrons in the L-valleys and the tilted growth direction with respect to the valleys, this novel structure can intrinsically absorb normal incidence light. Under an electric field of 50 kV/cm, a blue shift of the absorption peak from 10.9 to 9.8 μm was found from our calculations in a Ga0.7Al0.3Sb/Ga0.5Al0.5Sb/Ga0.4Al0.6Sb structure with a Ga0.7Al0.3Sb well width of 25 Å and a Ga0.5Al0.5Sb step width of 25 Å. The ability to absorb normally incident light and to achieve significant Stark effects with bias makes this structure an attractive choice for such high-speed optoelectronic devices as vertical infrared light modulators and voltage tunable photodetectors.
Micromagnetic structure of domains in Co/Pt multilayers. II. Investigations of domain structures in written marks
Authors: Ploessl R. , Chapman J. N. , van Kesteren H. W. , Hoffmann H.
Published: July 2002

An analysis is presented of the detailed effects of varying the write parameters in Co/Pt multilayer films. Domains written thermomagnetically by laser modulation have been imaged using the modified differential phase contrast mode of Lorentz electron microscopy. The effects of different laser powers and bias fields were investigated. While the domain size increases with laser power, the bias field is found to have a profound effect on domain regularity. The periodicity of the irregular circumference of the domains written at high-bias fields closely resembles the mean repeat in the ac-demagnetized state suggesting that it arises as a relaxation phenomenon.
Magnetic properties of praseodymium-substituted iron garnet films
Authors: Azevedo Antonio , Cinbis C. , Kryder M. H.
Published: July 2002

We have investigated the growth conditions, the magnetic properties and low-field ferromagnetic resonance (FMR) properties of praseodymium-substituted garnet films. Films of general composition Y3−x−yPrxLuyFe5O12, were grown by liquid phase epitaxy onto [111]-oriented Gd3Ga5O12 substrates. Vibrating sample magnetometry, torque magnetometry and FMR were used to measure saturation magnetization and the cubic and induced anisotropy constants. It is shown that the uniaxial anisotropy constant Ku and the first order magnetocrystalline anisotropy constant K1, have a strong dependence upon the praseodymium content. The addition of praseodymium in the garnet films improves the soft magnetic properties. The effect of the doping by praseodymium on the saturation magnetization is insignificant at room temperature. In-plane coercivity is about 0.1 Oe.
Dynamical scaling in fragmentation
Authors: Coutinho Kaline , Adhikari Sadhan K. , Gomes M. A. F.
Published: July 2002

The dynamics of a fragmentation model is examined from the point of view of numerical simulation and rate equations. The model includes effects of temperature. The number n(s,t) of fragments of size s at time t is obtained and is found to obey the scaling form n(s,t)∼ s−τtwsγe−ρtf(s/tz) where f(x) is a crossover function satisfying f(x)≂1 for x≪1 and f(x)≪1 for x≫1. The dependence of the critical exponents τ, w, γ, and z on space dimensionality d is studied from d=1 to 5. The result of the dynamics on fractal and nonfractal objects as well as on square and triangular lattices is also examined.
Electron coincidence spectroscopy studies of secondary and Auger electron generation mechanisms
Authors: Drucker Jeff , Scheinfein M. R. , Liu J. , Weiss J. K.
Published: July 2002

Electron coincidence spectroscopy in an ultrahigh vacuum scanning transmission electron microscope has been used to study the generation pathways for secondary (SE) and Auger electrons (AE) excited by high-energy incident electrons. Energy and momentum transfer of inelastically scattered 100 keV primary electrons have been correlated with energy selected SE and AE for both thin 〈111〉 oriented Si crystals and amorphous C films. Coincidence spectra from the valence excitation region indicate that bulk plasmon decay is not the primary production channel for SE in Si(111) and that SE result partially from the decay of ionizations from deep in the valence band. Energy deposition by the primary beam is responsible for SE production at excitation energies above the valence region. At most one SE is emitted from the entrance surface of a thin film for each inelastically scattered 100 keV primary electron. An enhancement in both the SE yield and generation probability is observed at the C K ionization edge. Correlations between energy loss electrons in the vicinity of the C K ionization edge and energy selected SE near the C KLL AE energy show a very sharp threshold in the generation probability. High-momentum transfer (spatially localized) inelastic scattering events are more efficient at creating SE than low-momentum transfer events. The high-spatial resolution obtained in SE images is explained using the Heisenberg uncertainty principle and the scattering angle dependence of the SE generation probability.
An energy- and spatial-dependent effective mass approach for resonant interband tunneling devices
Authors: Houng M. P. , Wang Y. H. , Chen H. H.
Published: July 2002

A theoretical study of resonant interband tunneling in GaAs δ-doped inducing homostructure and polytype GaSb/AlSb/InAs heterostructure is presented in this article. The resonant interband tunneling in such homo- and heterostructures is modeled by an energy- and spatial-dependent effective mass equation incorporating the general transfer matrix method. The present formalism is based on the envelope function approximation as done to date, but contains two significant improvements: a more realistic treatment of the spatial and energy dependence of effective-mass and bandstructures; and the avoidance of plane-wave, Airy function or Wentzel–Kramers–Brillouin approximations for calculating the envelope function in favor of direct numerical evaluation. The transmission coefficients of the unipolar and bipolar resonant interband tunneling structures are calculated and the symmetric and asymmetric multiple quantum well resonant interband tunneling structures are also discussed.
Transmission-line-matrix modeling of grain-boundary diffusion in thin films
Authors: Gui Xiang , Dew Steven K. , Brett Michael J. , de Cogan Donard
Published: July 2002

The use of the transmission-line-matrix (TLM) method for analyzing the grain boundary and interfacial diffusion problems in thin films is demonstrated. The method employed has a variable mesh and automatic time-stepping capability, and is highly versatile in treating complex structures and incorporating various boundary conditions. The present model takes account of separate diffusion coefficients in grain boundaries, grains, interfaces, and the substrate. The combined diffusion problems are solved as a function of position and time, and the concentration distributions are presented as clearly visible isometric projections and contour plots. The results are compared with those of semi-infinite samples (Whipple’s analysis [Philos. Mag. 45, 1225 (1954)]) and idealized thin-film systems (Gilmer and Farrell’s analysis [J. Appl. Phys. 47, 3792 (1976)]), with the emphasis being placed on the differences due to the treatment of concentration flux along the interface between the film and the substrate and into the substrate. The TLM method is shown to have considerable potential in practical applications for diffusion processes in thin films.
Conflicting values of the third-order nonlinear permittivity of α-quartz
Authors: Hruska Carl K.
Published: July 2002

This communication presents a comparison of three values of third-order nonlinear permittivity of α-quartz that have been obtained independently by three different methods. Each method produces a value of a different order of magnitude, in the range between 10−22 and 10−20 F/m, and with no uniformity in sign among them. This apparently represents the most pronounced conflict existing today among third-order nonlinear electromechanical constants of quartz obtained from various sources. No explanation for the conflict is readily available.
Neutron scattering investigation of the structure of semiconductor-doped glasses
Authors: Banfi GianPiero , Degiorgio Vittorio , Speit Burkhard
Published: July 2002

A small-angle neutron scattering (SANS) study of the structure of II-VI semiconductor crystallites in a semiconductor-doped glass is presented. The scattered intensity I(k) exhibits a peak at a nonzero scattering vector and decreases to zero as k goes to zero. The data are interpreted with a simple phenomenological model, based on local mass conservation, which describes a dilute gas of crystallites surrounded by depletion zones. We show that SANS allows a quick and accurate measurement of the average size and of the volume fraction of the crystallites, and we give values of both quantities for a commercial series of glasses.
Enhanced refractive index change in asymmetrical quantum wells with an applied electric field
Authors: Davé Digant P.
Published: July 2002

We theoretically calculate the electric field induced refractive change index for the ground to first excited state transition in the conduction band of asymmetrical quantum wells. We show that there is a significant change in stark shifts and the refractive index with asymmetry potential. Comparing the results for a square, graded, and step well, it is found that larger refractive index change can be achieved in a step well by a proper choice of the step potential.
Defects and ion redistribution in implant-isolated GaAs-based device structures
Authors: Pearton S. J. , Ren F. , Chu S. N. G. , Abernathy C. R. , Hobson W. S. , Elliman R. G.
Published: July 2002

Implant isolation of thick GaAs based epitaxial structures using either multiple energy keV ions or a single MeV ion implantation is becoming more popular for devices such as heterojunction bipolar transistors or quantum well lasers. We report examples of both types of isolation schemes, using keV F+ and H+ ions, or MeV O+ ions. Post-implant annealing at temperatures in the range 500–600 °C is needed to maximize the resistivity of the implanted material, but this causes redistribution of both F and H (but not O) and accumulation of hydrogen at strained or ion-damaged interfaces. The amount of hydrogen motion is sufficient to cause concerns about dopant passivation occurring in the initially masked, active regions of the devices. The resistance of the ion-implanted regions is stable for periods of ≥50 days at 200 °C, and is controlled by deep level point defects which pin the Fermi level near mid gap.
Effect of proton irradiation and annealing on the diffusion length in indium phosphide
Authors: Hakimzadeh Roshanak , Vargas-Aburto Carlos , Bailey Sheila G. , Williams Wendell S.
Published: July 2002

We report on the measurement of the diffusion length damage coefficient (KL) and the annealing characteristics of the minority carrier diffusion length (Ln) in Czochralski-grown zinc-doped indium phosphide (InP), with a carrier concentration of 1×1018 cm−3. In measuring KL, irradiations were made using 0.5 MeV protons with fluences ranging from 1×1011 to 3×1013 cm−2. Pre-and post-irradiation electron-beam induced current measurements allowed for the extraction of Ln, from which KL was determined. In studying the annealing characteristics of Ln, irradiations were made with 2 MeV protons with a fluence of 5×1013 cm−2. Post-irradiation studies of Ln with time at room temperature, and with minority carrier photoinjection and forward-bias injection were carried out. The results showed that recovery under Air Mass Zero photoinjection was complete. Ln was also found to recover under forward-bias injection, where recovery was found to depend on the value of the injection current. However, no recovery of Ln after proton irradiation was observed with time at room temperature, in contrast to the behavior of 1 MeV electron-irradiated InP solar cells reported previously.
Holographic recording in Ge1−xSnxSe2 glassy films
Authors: Islam D. , Changkakoti Rupak , Manivannan Gurusamy , Lessard Roger A.
Published: July 2002

Thin films of Ge1−xSnxSe2 glasses were studied for self-developing holograms. The recording property of the films was found to depend on the tin fraction x. The peak values of the diffraction efficiency of recorded holograms decrease as the tin content of the films increase. For films with x≥0.20 the recording property is nearly nonexistent. The dependence of the peak diffraction efficiency on the tin fraction x is correlated to that of photodarkening in these glasses.
Evidence of an off-center position of thulium in GaAs and excitonic excitation of the Tm3+ emission
Authors: Dörnen A. , Haase D. , Hiller C. , Pressel K. , Weber J. , Scholz F.
Published: July 2002

By photoluminescence and by Zeeman spectroscopy we study the characteristic 4f luminescence transition 3H5 → 3H6 at 1.0 eV of thulium in gallium arsenide which has been reported recently. It turns out that optically active Tm3+, which is present in mainly one specific type of center, does not occupy a simple substitutional lattice site. The results show a considerable tetragonal crystal field. The excitation mechanism of the 1.0-eV luminescence is investigated by photoluminescence excitation. The 3H5 → 3H6 is pumped most efficiently by trapping of free excitons.
Monte Carlo modeling of high-field transport in III-V heterostructures
Authors: Thobel J. L. , Baudry L. , Bourel P. , Dessenne F. , Charef M.
Published: July 2002

A Monte Carlo model of parallel high-field transport in III-V heterostructures is presented. Special features of the model are the following: only two-dimensional electron states are considered, the possible existence of secondary wells inside the barriers is accounted for, and nonparabolicity effect and quantization of satellite valleys are included. The wave functions and eigenenergies are calculated by self-consistent resolution of Poisson and Schrödinger equations. The effect of nonparabolicity on dispersion relations is determined at first order by a perturbation method. First, the simple case of an infinite GaAs square well is investigated as a test for the model, then more realistic heterostructures are considered. A study of a modulation-doped pseudomorphic AlxGa1−xAs/In0.15Ga0.85As structure shows that the electric field induces a significant repopulation of the doped AlGaAs layer. When x=0.32, this real-space transfer is strongly correlated with the intervalley transitions toward X valley states. For In0.52Al0.48As/In0.53Ga0.47As the situation is quite different and a good confinement in the InGaAs well is preserved even at high fields owing to the large band offset in the L valley. This study demonstrates a complicated influence of band structure on electron transport in heterostructures.
Influence of the screening model and wave-function shape on impurity scattering mobility in quantum wire
Authors: Baudry L. , Thobel J. L. , Charef M. , Dessenne F. , Bourel P.
Published: July 2002

A theoretical investigation of impurity scattering limited mobility in a quantum wire is presented. The general method of mobility calculation is presented and some approximations are made in order to simplify the problem and reduce the amount of computations. Cylindrical symmetry of the wire is assumed and the carriers are assumed to be confined by an infinite barrier. The mobility has been computed using the exact wave functions and the results are compared with those obtained using constant or Dirac wave-function approximations. It is shown that the mobility is strongly influenced by the shape of the wave function. The screening effect is introduced using two different models. At high temperatures both models yield similar results whereas at low temperature a significant discrepancy is observed. The influence of important parameters such as wire dimension, carrier density, and temperature on impurity scattering mobility is investigated. A comparison of impurity scattering mobility in quantum well and quantum wire is also presented. Finally, it is shown that the electron-transport properties in quantum wires may be strongly enhanced by the use of a large spacer.
Flux trapping in superconducting thin films in weak magnetic fields
Authors: Geng Q. , Goto E.
Published: July 2002

Magnetic-field distribution measurements over a patterned superconducting strip line sample were conducted using a superconducting quantum interference device pickup coil, showing that, in the range of 500 μG–50 mG of perpendicular magnetic field B⊥,i, the superconducting films record previous magnetic histories precisely. The magnetic-field distribution with a field B⊥,i applied at all times is identical to one with no field applied at any time. A calculation based on the flux trapping model explains these results indicating that all the magnetic fluxes penetrate the superconducting thin films.
Cesium coating of thin-film zinc sulphide cold cathodes
Authors: Carkner Don , Kitai Adrian , Young Richard
Published: July 2002

Avalanche breakdown in polycrystalline ZnS thin films can produce cold cathode hot electron emission. The physical processes behind this phenomenon are not well understood, nor is the nature of the electron energy distribution inside the ZnS during breakdown. Lowering the work function of the emission surface by evaporation of elemental cesium is discussed. The increase in emission current is compared to that from a tungsten filament and photoemission measurements are made, with results suggesting a non-Boltzman distribution of electron energies in the ZnS. The time profile of the emission current is investigated. A preliminary measurement of the energy distribution of the emission current is obtained with a retarding grid setup.
Analysis of the properties of radio frequency ion plated Cu films based on the calculation of the ion energy available per film atom
Authors: Khandagle M. J. , Gangal S. A. , Karekar R. N.
Published: July 2002

During plasma assisted film deposition (PAD), the ion energy available per film atom plays an important role in film growth and morphology development. Consequently, it influences the film properties such as stress, adhesion, and dc resistivity. Calculation of the ion energy available per film atom is a little difficult in PAD because of a lack of data on the flux and energy distribution of ions (IED) incident on the film during deposition. In this article, the nature of the IED, in a rf ion plating system used for Cu film deposition, is determined with the help of a theoretical model available in literature and the plasma parameters obtained from Langmuir probe data. Then the ion energy available per Cu film atom (ECu) is calculated to be in the range of 0.5–5 eV/atom. This is further used to explain the variation of film morphology and dc resistivity with the ion plating process variables, viz., pressure, Cu evaporation rate and substrate bias. This range of ECu is insufficient to influence the film thickness significantly via resputtering. The film thickness is influenced to a much greater degree by the evaporation rate due to collisional scattering of the film vapor in the relatively high pressure used during ion plating. The film morphology is shown to improve with increase in the ECu but shows the presence of micro cracks at pressure higher than 2 mTorr, probably due to increased neutral gas adsorption. The data on ECu and morphology is then used to show that the dc resistivity of the films is influenced by grain boundary scattering and partial oxidation during deposition.
High-frequency capacitive effects in resonant tunneling diodes
Authors: Lu X. J. , Rhodes D. , Perlman B. S.
Published: July 2002

The time-varying charge buildup in the quantum well region of a resonant tunneling diode (RTD) and related capacitive effects are calculated using the scattering matrix method developed. The small signal analysis of admittance shows a prominent influence from the capacitive effect, due to the dynamic shifting of the resonant energy levels. The model developed is helpful for understanding the RTD’s high-frequency behavior and device applications.
Electrical and magnetic studies of (Cu/Zn)-bonded Sm2Fe17MxNy magnets (M=B or C)
Authors: Yao Y. D. , Kuo P. C. , Chang W. C. , Liu C. J.
Published: July 2002

Samples of (Cu/Zn)-bonded and unbonded Sm2Fe17MxNy with M=B or C (x=0, 0.25, and 0.5; y<3) were fabricated. All the samples, besides those with B, show single Curie temperature TC and with Sm2Fe17-type crystal structure; however, multiphase structure and double TC were observed in all the samples with B. For all the heating runs the electrical resistivity roughly above 600 K increases abruptly for all Zn-bonded samples; and decreases abruptly for all Cu-bonded samples. After these high temperature runs, the residual electrical resistivity increases for all Zn-bonded samples, and decreases for all Cu-bonded samples. The effects of Cu segregation and Zn reaction with samples are identified by the EPMA analyses.
Crystal structure and low-temperature magnetic properties of melt-spun Sm2Co7B3 compounds
Authors: Yang C. J. , Lee W. Y. , Choi S. D.
Published: July 2002

Low-temperature magnetic properties and crystal structures of melt-spun Sm2Co7B3 compound were characterized. The magnetic measurements in the temperature range 77–450 K indicated that a spin reorientation took place at about 150–160 K. A huge anisotropy was observed (Ha=135 kOe at 300 K, 725 kOe at 77 K) for Sm2Co7B3, although the magnetic moment is rather low. The crystal structure of the Sm2Co7B3 compound was analyzed in detail by Rietveld analysis of powder diffraction for the first time, and revealed that B(4h) atoms are not placed in the Sm(2e) layer but in between the Sm(2e) and Co(6i1) layers.
Successive Bloch line write operation in a 1-μM bubble material
Authors: Matsuyama K. , Ohyama T. , Asada H. , Taniguchi K.
Published: July 2002

The successive vertical Bloch line (VBL) write operation has been performed in an as-grown 1-μm bubble material with a flank wall writing scheme. An unwinding VBL pair was nucleated by a domain expanding pulse current (200-mA amplitude, 200-ns pulse width, 8-ns rising edge) applied through a hair-pin type conductor (gap:2.6 μm, width:4 μm). The VBL position was controlled by a local in-plane magnetic field produced by a conductor current of 30 mA and a uniform in-plane magnetic field of 10 Oe. The above operating parameters were adjusted by measuring the collapse field of the stripe domain as a function of the parameters. The increase of bubble collapse field from 442 to 513 Oe in accordance with the number of write operations indicated a successful write operation.
Temperature dependence of the hyperfine field distributions in the Fe93.5−xNdxZr6.5 (x=0, 2) amorphous alloys
Authors: Nicolaides G. K. , Pissas M. , Niarchos D. , Taylor R. D. , Rao K. V.
Published: July 2002

The 112 K spin-glass Fe93.5Zr6.5 and the reentrant Fe91.5Nd2Zr6.5 amorphous alloys have been studied at various temperatures with Mössbauer spectroscopy technique. At around T=135 K for the Fe93.5Zr6.5 and at around T=255 K for the Fe91.5Nd2Zr6.5, six broad lines appear, which are not well resolved. This broadening is ascribed to the magnitude distribution of the magnetic moments of the Fe atoms. From the fitting of the Mössbauer spectra at T=4.2 K, the mean hyperfine field 〈H〉 and the Ecart-type standard deviation σH were estimated, and are (i) for the Fe93.5Zr6.5 sample, 〈H〉≊205 kOe, and σH≊77 kOe; (ii) for the Fe91.5Nd2Zr6.5 sample, 〈H〉≊228 kOe and σH≊70 kOe. These values are found to be relatively consistent with the expected degree of magnetic frustration of the correspondingly Fe moments, according to the Fe content of these alloys.
Structural and magnetic properties of rf-sputtered iron nitrides using NH3 (abstract)
Authors: Xiao John Q. , Chien C. L.
Published: July 2002

Thin films of iron nitrides have attracted much attention due to their superior magnetic properties. Conventional rf reactive sputtering using N2 gas encounters a number of difficulties, including mixed phases, which are alleviated by the use of NH3 gas. By controlling the partial pressure of NH3, we have been able to fabricate all stable Fe nitrides of ζ-Fe2N, ε-Fe2–3N, and γ′-Fe4N in single-phase films. The structure has been determined using an x-ray diffractometer. The magnetic properties have been investigated using VSM, SQUID magnetometers, and Mössbauer spectroscopy. All nitrides except Fe2N have high saturation moments with a large in-plane anisotropy. The saturation moment decreases with increasing nitrogen concentration. The Mössbauer results, in excellent agreement with those from bulk samples, are consistent with structural results and magnetization measurements.