bluuebunny/crossref_metadata_2025 huggingface dataset:
Wall velocities, switching times, and the stabilization mechanism of 180° domains in congruent LiTaO3 crystals
Authors: Gopalan Venkatraman , Mitchell Terence E.
Published: July 2002

A systematic study of the kinetics of 180° domains as a function of external electric field is presented for Z-cut LiTaO3 single crystal wafers at room temperature using transient current measurements combined with nondestructive and real-time imaging of 180° domains by light microscopy. The switching time, wall velocity, and nucleation rate follow an exponential behavior with the applied field. A model is proposed which shows that the nucleation and sideways growth of domains play approximately equal parts in determining the switching time. A domain stabilization process occurs on the time scale of a few seconds even at electric fields where the switching time is milliseconds or less. We show that this stabilization process has a strong correlation to the internal fields in the crystal.
Electronic band structures and effective-mass parameters of wurtzite GaN and InN
Authors: Yeo Y. C. , Chong T. C. , Li M. F.
Published: July 2002

The electronic band structures of wurtzite GaN and InN are calculated by the empirical pseudopotential method (EPM) with the form factors adjusted to reproduce band features which agree with recent experimental data and accurate first-principles calculations. The electron and hole effective masses at the Γ point are obtained using a parabolic line fit. Further, using the effective-mass Hamiltonian and the cubic approximation for wurtzite semiconductors, band edge dispersion at the Γ point obtained using the k.p method is fitted to that calculated using the EPM by adjusting the effective-mass parameters. Thus, we derived important band structure parameters such as the Luttinger-like parameters for GaN and InN which will be useful for material design in wide-gap nitride-based semiconductor lasers employing InGaN. The results also showed that the cubic approximation is fairly successful in the analysis of valence band structures for wurtzite nitrides.
First principles calculations of structural properties of β-Si3−nCnN4 (n=0, 1, 2, 3)
Authors: Wang Cheng-Zhang , Wang En-Ge , Dai Qingyun
Published: July 2002

First principles calculations were performed for the structural properties of superhard materials, hexagonal β-Si3N4, β-C3N4, and two model structures of β-Si2CN4 and β-SiC2N4. Bulk modulus and lattice constants a and c have been obtained by fitting the total energy calculations to the Birch equation of state. As more C atoms are substituted for Si in β-Si3N4, the bulk modulus is observed to progressively increase up to 4.44 Mbar, comparable to that of diamond (4.43 Mbar), and both lattice constants a and c are reduced. For β-Si3N4 and β-C3N4, the lowest energy geometries have been searched by optimizing the internal structural parameters at the theoretical equilibrium lattice constants. The results are discussed and compared with available experiments.
On the current emitted by Taylor cone-jets of electrolytes in vacuo: Implications for liquid metal ion sources
Authors: Gamero-Castaño M. , Aguirre-de-Carcer I. , de Juan L. , Fernández de la Mora J.
Published: July 2002

Approximate scaling laws for the charge and size of the drops ejected from the apex of Taylor cones run in the cone-jet mode (electrospray) are now available for highly conducting electrolytes (10−4 S/m<K<1 S/m) electrosprayed at atmospheric pressure. In order to confirm that such laws do also apply to Taylor cones in vacuo, the current versus liquid flow rate curves I(Q) characteristic of a given liquid are investigated both in vacuum and in atmospheric pressure air. Although the sprays of drops differ profoundly in both cases, the two corresponding I(Q) curves are nearly identical for relatively involatile liquids such as tributyl phosphate. A discussion on the possible relation between the behavior of Taylor cones of electrolytes of organic liquids and liquid metal ion sources (K∼106 S/m) is attempted, yielding insights on the role of space charge. However, the electrical conductivity variable which dominates the behavior of liquid cones of electrolytes appears to be irrelevant in liquid metals.
Comparison of the effects of electron and proton irradiation on n+–p–p+ silicon diodes
Authors: Taylor S. J. , Yamaguchi M. , Yamaguchi T. , Watanabe S. , Ando K. , Matsuda S. , Hisamatsu T. , Kim S. I.
Published: July 2002

We have carried out an investigation of n+–p–p+ silicon diodes after irradiation with 1 MeV electrons and 10 MeV protons and subsequently after annealing. The effects upon the material and device parameters of samples irradiated with different particles are compared by expressing the particle fluence in terms of an effective absorbed dose of 1 MeV electrons. Although the spectrum of defects (observed by deep-level transient spectroscopy) introduced by 1 MeV electrons and 10 MeV protons was slightly different, the total defect introduction rate per effective 1 MeV electron dose was similar, as was the effect upon the device parameters. After irradiation with high fluences of electrons or protons, the effective carrier concentration in the base of the diodes was reduced dramatically, an effect referred to as “carrier removal.” The effects of carrier removal upon the device parameters, in particular, the series resistance and saturation current, are discussed in detail. In addition, the relative importance of different radiation-induced defects is compared.
Growth and small polaron properties of epitaxial La1−xCaxMnO3 thin films
Authors: Worledge D. C. , Miéville L. , Geballe T. H.
Published: July 2002

We report on the epitaxial growth and properties of La1−xCaxMnO3 thin films deposited by pulsed laser ablation. We grew and characterized 11 thin films covering the entire doping range, from x=0 to x=1. A presence of oxygen during postdeposition annealing is shown to be required in order to reduce the resistivity of the samples and to obtain reproducible samples. The lattice constant and phonon frequency that appear in the Emin–Holstein, [D. Emin and T. Holstein, Ann. Phys. 53, 439 (1969).] adiabatic small polaron conductivity formula are reported. Resistivity data from 20 to 300 K are also reported.
Generalized inclusion model for the coercivity of soft magnetic materials
Authors: Lopez Diaz L. , Della Torre E.
Published: July 2002

The principal causes of coercivity in soft magnetic materials are the imperfections in the structure of the crystal, such as inclusion and grain boundaries, that pin domain walls. The models that have been proposed so far in order to explain this effect are only valid for a specific range of material parameters. In this paper, we present a two-dimensional micromagnetic model for Barkhausen coercivity due to inclusions that is valid for all physically realizable material parameters. The details of the interaction between Bloch walls and nonmagnetic inclusions are presented for some interesting special cases. One of the results the model predicts is that walls are stable when they are centered on the inclusion. Furthermore, much higher values of the coercivity are obtained than those predicted by previous models.
Polymerization and dissolution of fibrin under homogeneous magnetic fields
Authors: Iwasaka M. , Takeuchi M. , Ueno S. , Tsuda H.
Published: July 2002

We investigated whether or not a mutually compensating state of coagulation and fibrinolysis is changed by homogeneous magnetic fields. We used a superconducting magnet which produced magnetic fields of up to 14 T at its center. Fibrin polymerization over time, and the subsequent dissolution of the fibrin fiber network, were observed by measuring the optical absorbance of the mixture at 350 nm. A spectrophotometer with an external optical cell box in a superconducting magnet was used. We observed that the optical absorbance of the mixture at 350 nm increased during the fibrin-polymerization process, and decreased during the fibrinolytic processes. The optical absorbance was stable in the transient state between fibrin-polymerization and fibrinolytic processes. A magnetic field of 14 T increased the rate of the polymerization process by 55%–70% compared to the control group. On the other hand, the rate of the fibrinolytic process under a magnetic field at 14 T, increased by 27%–140% compared to the control. The results indicate that the magnetic orientation of fibrin fibers accelerated both the polymerization and the dissolution of fibrin fibers.
Microstructure and electrical properties of Sn nanocrystals in thin, thermally grown SiO2 layers formed via low energy ion implantation
Authors: Nakajima Anri , Futatsugi Toshiro , Nakao Hiroshi , Usuki Tatsuya , Horiguchi Naoto , Yokoyama Naoki
Published: July 2002

We have developed a simple technique for fabricating Sn nanocrystals in thin thermally grown SiO2 layers using low energy ion implantation followed by thermal annealing. The formed Sn nanocrystals have excellent size and depth uniformity. Their average diameter is 4.2 nm with a standard deviation of 1.0 nm. Our experimental results clearly reveal that a stable depth of Sn exists in the SiO2 layer at about 2 nm from the SiO2/Si interface. Most of the Sn nanocrystals reside near this stable depth. The I–V characteristics of the diode structure show a clear Coulomb blockade region of 0.12 V and a Coulomb staircase at 4.2 K. A Coulomb blockade region around 0 V was observed until reaching a temperature of 77 K. The features of these nanocrystals will open up new possibilities for the creation of novel devices.
Influence of strain and microstructure on magnetotransport in La0.7Ca0.3MnO3 thin films
Authors: Thomas K. A. , de Silva P. S. I. P. N. , Cohen L. F. , Hossain A. , Rajeswari M. , Venkatesan T. , Hiskes R. , MacManus-Driscoll J. L.
Published: July 2002

A La0.7Ca0.3MnO3 thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal LaAlO3, were subject to a series of six, short, controlled anneals. The oxygen content was purposely not changed in the films from the first anneal to subsequent anneals. After each anneal, the film microstructures were characterized to determine average grain size, lattice constants, nonuniform strain, and crystalline mosaic spread, and these parameters were correlated with the magnetotransport properties. For both sets of films, the influence of annealing was to both increase the temperature at which the maximum in the magnetoresistance occurs (Tm) and the maximum magnetoresistance (MR) value. The improvement in film properties occurred in conjunction with stress relaxation and improved crystallinity, as a result of grain growth. The MOCVD films showed poorer grain coupling and poorer epitaxy compared to the PLD films. These features did not significantly influence the absolute values of the resistivity, but did produce spin canting in the MOCVD film, as seen in magnetization and resistivity versus field data. The canting resulted in a lower Tm and depressed MR value for the MOCVD film which increased only marginally with annealing. The work highlights the importance of controlling microstructure for optimizing properties of colossal magnetoresistance films.
Intermixing in GaAsSb/GaAs single quantum wells
Authors: Khreis O. M. , Homewood K. P. , Gillin W. P. , Singer K. E.
Published: July 2002

Photoluminescence coupled with repetitive thermal annealing has been used to study the interdiffusion process in a 10 nm GaAs1−xSbx/GaAs single quantum well. The diffusion equations and the Schrödinger equation were solved numerically to obtain the composition profile and the n=1 electron to heavy-hole transition energies in the intermixed quantum well, respectively. The intermixing process was shown to obey Fick’s second law.
Soliton switching in three-core nonlinear directional fiber couplers
Authors: da Silva M. G. , Teles A. F. , Sombra A. S. B.
Published: July 2002

We present an analytical investigation of the propagation and the switching of fundamental solitons in a three-core nonlinear fiber coupler with a variational method using the Lagrangian density formulation. The analytical solutions were obtained directly from the coupled nonlinear Schrödinger equations. The transmission characteristics of first order solitons obtained by the analytical procedure agree well with the results from numerical analysis. Three different geometries for the nonlinear three-core directional coupler were studied. Taking into account the length of the coupler and the critical power a comparative study between these geometries was done.
Structure and bond length distribution in Hf2Fe and Hf2Co intermetallic compounds
Authors: Cekić B. , Ivanović N. , Rakočević Z. , Koteski V. , Manasijević M. , Koički S.
Published: July 2002

The grain structure and interatomic distances in annealed Hf2Fe, as well as in nonannealed and annealed Hf2Co polycrystalline isostructural phases, with unit cells of the Ti2Ni type, are studied using scanning tunneling microscopy (STM) in air. In contrast to x-ray powder diffraction data, refined according to the Rietveld profile method, STM measurements show discrepancies between compounds, caused by the peculiarities of their electronic structure. The results obtained give better insight into the extensive electric field gradient measurements performed in this class of compounds, which could not be properly explained by the existing crystallographic data alone.
I–V characteristics of resonant tunneling devices: Difference equation method
Authors: Krishnamurthy Srinivasan , Chen A.-B. , Sher A.
Published: July 2002

We have developed a full-band-structure-based theory to calculate the transmission coefficients through nanostructures of semiconductor quantum wells and barriers. This method, based on the solution to a difference equation, is efficient, numerically stable, and accurate. The I–V characteristics of realistic resonant tunneling diodes are calculated, and this method enables us to follow the contribution to the spatially resolved current from channels mediated by different valleys. We have carried out a systematic study of I–V characteristics in GaAs/AlAs double-barrier resonant structures using a second-neighbor tight-binding Hamiltonian. Our calculations clearly show that the energy separation between the quantized Γ state in GaAs and X state in AlAs plays a crucial role in determining the peak-to-valley current ratio. We further show that the quantized states and peak-to-valley current ratio are sensitively dependent on the tight-binding parameters.
Relation of coercivity to net magnetization in ferrimagnetically ordered Co/Gd multilayers
Authors: Chizhik A. B. , Gnatchenko S. L. , Merenkov D. N. , Baczewski L. T. , Wawro A. , Szymczak H. , Gamari-Seale H.
Published: July 2002

Magnetization reversal processes in ferrimagnetically ordered Co/Gd multilayers were studied by means of the magneto-optical Kerr effect. Temperature dependencies of coercive field of the multilayers have been determined in the wide temperature region 25<T<300 K. An increase of temperature can cause both an enhancement and diminution of coercivity in the Co/Gd multilayers examined. Different temperature behavior of coercivity is explained by taking into account the temperature variation of net magnetization in the ferrimagnetically ordered Co/Gd multilayers.
Characterization of Si+ ion-implanted SiO2 films and silica glasses
Authors: Guha Soumyendu
Published: July 2002

We report here electron spin resonance (ESR), Raman scattering, photoluminescence (PL), and absorption studies of Si+ ions implanted into silica glasses and thermally grown SiO2 films on Si wafers. The aim is to understand the defect formation and luminescence mechanism as the films were annealed at several temperatures. In as-grown Si-implanted films, paramagnetic defects in the form of Si–E′ and nonbridging oxygen hole centers (NBOHC) were detected by ESR. A luminescence band, centered at 650 nm, was observed in these samples. The E′ and NBOHC defects were annealed out in samples annealed above 500 °C, but Si dangling bonds in the form of Pb centers were observed. In films annealed above 800 °C, Si nanocrystals of varying size between 1 and 5 nm were formed, as detected by transmission electron microscopy. A red PL band (>700 nm) and a Si-phonon band at 510 cm−1, gradually evolved as a function of anneal temperature. The observation of the Si-phonon mode with a 15 cm−1 shift in the peak position from the bulk affirms the presence of Si clusters in the annealed ion-implanted SiO2 films and silica glasses. We attribute this red PL band to excitonic recombination in Si nanocrystals. The peak position of this PL band was found to shift with excitation wavelength. The shift of the PL spectra as a function of excitation energy was interpreted as the emission from Si particles of certain sizes that are excited by the incident energy. An energy band gap distribution due to a size distribution was used to model the PL spectra at each excitation energy.
Raman probing of thermal damage depth profile in annealed GaAs
Authors: Pizani P. S. , Campos C. E. M.
Published: July 2002

Raman scattering has been used to get information on the depth profile of both lattice damage and solid arsenic formation in gallium arsenide annealed at different temperatures. Measurements of Raman peak intensity ratio of GaAs transverse to longitudinal optical phonons as a function of exciting wavelength with a different penetration depth of the light gave information on the depth profile of the chemical and structural disorder in the gallium arsenide lattice. Moreover, measurements of the arsenic A1g and Eg phonon mode frequencies as a function of temperature showed that the arsenic formed in the vicinity of the surface is crystalline and submitted to very high temperature-dependent tensile stresses, with values ranging from 1.5 GPa at 300 K until 2.8 GPa at 10 K. On the other hand, the analysis of selection rules, phonon frequency, and linewidth indicates a strain-free and misoriented GaAs matrix.
Effect of bismuth content on the properties of Sr0.8BixTa1.2Nb0.9O9+y ferroelectric thin films
Authors: Tsai Huei-Mei , Lin Pang , Tseng Tseung-Yuen
Published: July 2002

This study investigates the effect of bismuth content on the ferroelectric properties of Sr0.8BixTa1.2Nb0.9O9+y (SBTN, x=1.7, 2.0, 2.5, 2.7, 2.9, and 3.2) thin film capacitors. SBTN films are in situ grown on Pt/SiO2/Si substrates by using two-target off-axis radio-frequency magnetron sputtering at a substrate temperature of 600 °C. The films are crystallized with a high (115) diffraction intensity and exhibit a columnar microstructure. Experimental results indicate that the root mean square surface roughness of the film increases with an increase of the bismuth content. In addition, the ferroelectric properties of the films heavily rely on the bismuth content. Moreover, the 440-nm-thick Sr0.8Bi2.5Ta1.2Nb0.9O9+y films exhibit maximum remanent polarization (2Pr) of 52 μC/cm2 and minimum coercive field (2Ec) of 28 kV/cm at an applied voltage of 1.5 V. X-ray photoelectron spectral studies reveal that except for Bi+3, no lower valence state bismuth exists in the Sr0.8Bi2.5Ta1.2Nb0.9O9+y film and bismuth substituted in the strontium site still remains in its +3 valence state.
Raman studies of delta-doped semiconductors and quantum wells
Authors: Chang Yia-Chung , Yao Huade , Mohiuddin M.
Published: July 2002

Theoretical and experimental studies of the Raman spectra of delta-doped bulk GaAs and GaAs–AlxGa1−xAs multiple quantum wells are presented. Intersubband plasmon modes are calculated with an energy-dependent effective-mass theory, which takes into account the band nonparabolicity. The screened external potential due to impurity and electron charge distribution including the exchange and correlation effects are calculated self consistently within the local density approximation. The calculated Raman spectra are in agreement with experimental data with a reasonable assumption of doping profile.
Thermal stability of Si/Si1−x−yGexCy/Si quantum wells grown by rapid thermal chemical vapor deposition
Authors: Liu C. W. , Tseng Y. D. , Chern M. Y. , Chang C. L. , Sturm J. C.
Published: July 2002

The thermal stability of Si/Si1−x−yGexCy/Si quantum wells was studied by high resolution x-ray diffraction, Fourier transform infrared spectroscopy, and defect etching. There are different pathways of strain relaxation in this material system, depending on the annealing temperature. The lattice structure of Si1−x−yGexCy was as stable as the Si1−xGex alloys at an annealing temperature of 800 °C for 2 h. At an annealing temperature of 900 °C for 2 h, the structures of both Si1−x−yGexCy and Si1−xGex started to relax. The addition of C enhanced the Ge outdiffusion in Si1−x−yGexCy, compared to that of Si1−xGex. For the annealing temperatures of 950 and 1000 °C for 2 h, the Si1−xGex continued to relax with the decrease of strain in the quantum wells, but the Si1−x−yGexCy relaxed with the increase of the strain due to the formation of SiC precipitates. Misfit dislocation formation was observed in the Si1−x−yGexCy alloys with initial thicknesses below the critical thickness after annealing at 1000 °C for 2 h. This relaxation is probably caused by the SiC precipitation, since SiC precipitates can reduce the strain compensation and, therefore, decrease the critical thickness.
Effect of Ti-capping thickness on the formation of an oxide-interlayer-mediated-epitaxial CoSi2 film by ex situ annealing
Authors: Kim Gi Bum , Kwak Joon Seop , Baik Hong Koo , Lee Sung Man
Published: July 2002

A modified oxide mediated epitaxy process using a single deposition and ex situ annealing by Ti capping has been suggested in this study. It has been shown that in the case of pure Co on SiOx-covered Si, the reaction between Co and Si did not occur up to 800 °C during ex situ annealing. However, Co silicidation occurred in the case of Ti-capped Co on SiOx-covered Si. The crystalline nature of CoSi2 formed in this case strongly depends on the Ti capping thickness. When a thin Ti capping layer of thickness less than 5 nm was used, Ti oxidation occurred nonuniformly, and the morphology of the surface Ti oxide layer was very rough. This caused an exposure of Co to the oxygen in the ambient, resulting in the formation of polycrystalline CoSi2 due to the suppressed Co diffusion towards the Si substrate. In the case of Ti capping thickness being more than 10 nm, however, a uniform Ti oxide surface layer, which blocks the incoming oxygen retarding Co diffusion, was formed, and it led to uniform Co diffusion into the Si substrate, resulting in epitaxial CoSi2.
Dynamics of photoexcited carriers in molecular beam epitaxy grown semimagnetic epilayers in magnetic fields
Authors: Debnath M. C. , Souma I. , Shirado E. , Mitsu H. , Sato T. , Shen J. X. , Oka Y.
Published: July 2002

Time-resolved photoluminescence has been measured on molecular beam epitaxy grown Cd1−xMnxTe epilayer samples. The photoluminescence intensity of magnetic polarons exhibits a very long emission tail. The observed phenomenon is attributed to the conversion of the dark excitons to dipole allowed transitions by a spin-flip process. The magnetic field dependence of the two-exponential decay property is discussed within the frame of giant Zeeman splitting, the spin-flip scattering, and the formation of the magnetic polarons.
Radiation from an alternating current high-pressure mercury discharge: A comparison between experiments and model calculations
Authors: Hartel G. , Schöpp H. , Hess H. , Hitzschke L.
Published: July 2002

An alternating current (ac) high-pressure mercury arc has been experimentally investigated, and the results have been compared with model calculations. In the model, only radial dependencies are considered, and a careful treatment of the radiation transport is included. The absolutely measured side-on radiance in the visible and near ultraviolet spectral range can now be quantitatively reproduced by this model starting from the measured arc current. Agreement between the measured and calculated radial temperature profile can be obtained only by taking into account the time-dependent behavior of the investigated ac arc. The calculated field strengths agree with the measured ones only if more recent values of momentum transfer cross sections for the calculation of the electrical conductivity of mercury have been used. The time-dependent pressure in the discharge is determined via the electrical conductivity using Ohm’s law, the radial temperature distribution, and the electrical field strength.
Porous silicon strain during in situ ultrahigh vacuum thermal annealing
Authors: Buttard D. , Dolino G. , Faivre C. , Halimaoui A. , Comin F. , Formoso V. , Ortega L.
Published: July 2002

In situ synchrotron radiation measurements of porous silicon (PS) strain have been performed during ultrahigh vacuum (UHV) thermal annealing. For a p+ sample, the initial lattice expansion shifts toward a contraction above 270 °C in relation with hydrogen desorption. For a p− sample, the strain variation is similar to that of a p+ one, but with effects five times larger: after hydrogen desorption, the contraction strain is large (>1.5%) and inhomogeneous. In both cases, most of these strains are elastic as an HF etch re-establishes the initial expansion with a narrow diffraction peak. For p+ samples, the lattice constant exhibited a slow variation during subsequent exposure to air due to a slow oxidation of the annealed porous samples. The origin of these strain variations is discussed in relation with the presence of hydrogen or oxide coverage. The observation of similar variations in other PS properties is also discussed. Finally, the absence of a strain effect during the introduction of water vapor in UHV is discussed as possibly due to a contamination of the PS sample by residual water during the long time passed under UHV at high temperature.
Theoretical investigation of incomplete ionization of dopants in 6H–SiC metal-oxide-semiconductor capacitors
Authors: Raynaud C. , Autran J.-L.
Published: July 2002

The effect of incomplete ionization of dopants on the capacitance–voltage (C–V) characteristics in 6H–SiC metal-oxide-semiconductor (MOS) capacitors has been investigated. The present exploration is based on the one-dimensional numerical solving of Poisson’s equation using the finite-difference method with a nonuniform mesh size. The results obtained from this approach are in good agreement with the first analytical results reported by Bouillon and Skotnicki in the case of indium impurities in silicon [IEEE Electron Device Lett. 19, 19 (1996)]. For p-type 6H–SiC material, characterized by deeper doping impurity energies (principally aluminum and boron), simulations predict the occurrence of a “kink effect” on the C–V curves, depending on temperature and doping level, that could lead to experimental errors in the extraction of MOS device parameters, including effective doping level, interface trap density, and fixed oxide charge.
Noise of a single electron transistor on a Si3N4 membrane
Authors: Hakonen P. J. , Ikonen J. M. , Parts Ü. , Penttilä J. S. , Roschier L. R. , Paalanen M. A.
Published: July 2002

We have investigated the influence of electron-beam writing on the creation of charge trapping centers which cause 1/f noise in single electron transistors (SET). Two Al/AlOx/Al devices were compared: one where the SET is on a {100} silicon wafer covered by a 120-nm-thick layer of Si3N4, and another one in which the Si was etched away from below the nitride membrane before patterning the SET. The background charge noise was found to be 1×10−3e/Hz at 10 Hz in both devices, independent of the substrate thickness.
Electron microprobe quantification: A new model based on electrons rather than on mass
Authors: Pingitore Nicholas E. , Donovan John J. , Jeanloz Raymond
Published: July 2002

Conventional models for reduction of electron probe microanalysis (EPMA) data are based improperly on mass. A unique set of electron microprobe measurements shows no significant differences between the intensities of characteristic x rays emitted from natural and enriched isotope pairs of Cu, Ni, and Mo. This observation and fundamental physical precepts both indicate that mass does not affect the generation of characteristic x rays, the basis of the EPMA technique. We offer instead a mass-independent, atomic-number based approach, which features electron fractions, the proportion of electrons in a compound contributed by each element. In this model, the initial estimate of the intensity of characteristic x rays emitted by each element in a compound is the intensity of the emission from each pure element times its electron fraction (instead of its mass fraction) in the compound. Published characteristic x-ray data, culled to provide 655 measurements of binary compounds which require only minor corrections (less than 2% each) for fluorescence and absorption, yield initial estimates of compositions closer to the measured values when calculations are based on electron fractions rather than on mass fractions. Thus, when fitted to an appropriate correction scheme, the electron-fraction model ultimately should yield a simplified and physically based method of data reduction which will simplify and improve the accuracy of electron microprobe quantitation.
Effects of preirradiation and thermal annealing on photoinduced defects creation in synthetic silica glass
Authors: Saito K. , Ikushima A. J. , Kotani T. , Miura T.
Published: July 2002

Effects of preirradiation by ArF-excimer laser and thermal annealing on photoinduced defects creation in synthetic silica glass were investigated by optical absorption measurements. It has been confirmed that the durability for ArF-excimer laser irradiation can be improved by the preirradiation followed by annealing. The absorption spectra strongly suggest that a dominant role of the preirradiation should be to assist relaxations of weak parts in the glass such as strained Si–O–Si bonds. Furthermore, the photoinduced defects creation can be divided into two processes, and the preirradiation only suppresses the photoinduced defects creation in the first process.
Current and heat conduction during a pulsed electrical discharge
Authors: Zielinski Alexander E. , Niles Steven , Powell John D.
Published: July 2002

A technique has been developed to measure the surface temperature of an electrical conductor during a pulsed electrical discharge. The technique of two-color interferometry (radiometry) is used to make the temperature measurements. This article explains the technique and the issues encountered in performing such measurements on a conducting rod. In addition, a theoretical model for the current and heat transport for the pulsed conductor has been developed. Predicted and experimental results are compared. The measured thermal data support the assumption of a slightly nonuniform distribution in the current. This nonuniformity is suspected of causing the rod to fail prematurely. Values for the resistivity and specific heat are determined for temperatures as great as 470 °C.
Electron ejection from MgO thin films by low energy noble gas ions: Energy dependence and initial instability of the secondary electron emission coefficient
Authors: Moon Kyoung Sup , Lee Jihwa , Whang Ki-Woong
Published: July 2002

Low energy ion-induced secondary electron emission from the surface of thin (500–5000 Å) polycrystalline MgO films has been investigated with various noble gas ions at energies ranging from 45 to 300 eV. The dependence of secondary electron emission coefficient γi on the type and energy of ions is reported and interpreted in terms of electron ejection mechanisms. As-deposited MgO films showed an initial fluctuation in the secondary emission current, which upon annealing or after a certain ion bombardment time irreversibly disappeared.
Enhancement of characteristic red emission from SrTiO3:Pr3+ by Al addition
Authors: Okamoto Shinji , Kobayashi Hiroshi , Yamamoto Hajime
Published: July 2002

The mechanism of enhancement on the red emission efficiency from SrTiO3:Pr3+ (1D2→3H4 emission of Pr3+) by Al addition has been investigated. The emission intensity at room temperature for the phosphor, SrTiO3:Pr3+ synthesized with 23 mol % Al is about 200 times higher than Al-free samples. Photoluminescence (PL) spectra at low temperatures show the progress of several red emission lines of the D21→H43 transition with an increase in Al molar ratio. The lines are probably due to formation of Al-associated Pr3+ centers. The Al-associated Pr3+ PL does not exhibit thermal quenching up to room temperature. Temperature dependent PL spectra below 100 K and PL excitation spectra reveal that the energy transfer occurs from photoexcited carriers in SrTiO3 host to PL centers. The PL intensity, x-ray diffraction measurements, and high-resolution transmission electron microscope images indicate the improvement of crystallinity of SrTiO3:Pr3+ with Al addition, resulting in a decrease in defects and probably an increase in the energy transfer to radiative centers. These observations imply that both the increase of the Al-associated Pr3+ luminescence centers and the improvement of the crystallinity result in the enhancement of the D21→H43 emission.
An allotropic transformation induced by mechanical alloying
Authors: Chatterjee P. P. , Pabi S. K. , Manna I.
Published: July 2002

This study concerns a hitherto unknown bcc→fcc allotropic transformation in Nb induced by the mechanical alloying of Nb80Al20. This metastable transformation is preceded by a gradual increase in the lattice parameter of bcc–Nb. The stored excess energy in nanocrystalline bcc–Nb may be responsible for the bcc→fcc phase transition.
Cr/Al and Cr/Al/Ni/Au ohmic contacts to n-type GaN
Authors: Papanicolaou N. A. , Edwards A. , Rao M. V. , Mittereder J. , Anderson W. T.
Published: July 2002

In this work we investigate the performance of Cr/Al and Cr/Al/Ni/Au ohmic contacts on n-type GaN. Annealing of the contacts was achieved by using a low temperature conventional quartz tube furnace in an Ar ambient and a new vacuum annealing technique using a tungsten strip heater. Low specific contact resistivity (ρc) metallizations were achieved with furnace annealing at considerably lower temperatures (550–600 °C) than those typically required for GaN contacts by halogen lamp rapid thermal annealing (∼900 °C). Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. For the Cr/Al bilayer on GaN with n doping of 1018 cm−3, minimum specific contact resistivities of 1.6×10−4 Ω cm2 and 2.3×10−5 Ωcm2 were achieved for furnace annealing and vacuum annealing, respectively. Our experiments showed that, when Cr was used as a contact material, the simultaneous presence of Cr and Al was necessary in order to obtain the best possible ohmic properties. Furthermore, the Cr/Al contacts maintained good stability at elevated temperatures. The Cr/Al/Ni/Au system offers lower ρc values and even greater temperature stability than the Cr/Al system when annealed in the 800–1200 °C temperature range. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer had the effect of increasing the surface roughness after annealing.
Structure, exchange interactions and magnetic anisotropy of Ho2Co17−xSix compounds
Authors: Wang Jing-Yun , Shen Bao-Gen , Zhang Shao-Ying , Zhan Wen-Shan , Zhang Li-Gang
Published: July 2002

The structural and magnetic characteristics of Ho2Co17−xSix (x=0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) compounds have been investigated by means of x-ray diffraction and magnetic measurements. Powder x-ray diffractograms show that all samples are single phase and have the hexagonal Th2Ni17-type or the rhombohedral Th2Zn17-type structure. The substitution of Si for Co leads to a monotonous decrease in unit cell volume v and Curie temperature TC. The compensation points are observed for the samples with x=2.5 and 3.0, and the compensation temperatures shift to higher temperatures with increasing Si content. The intersublattice-molecular-field coefficient, nHo–Co, has been determined on the basis of magnetization curves at the compensation temperature. It has been found that the Si substitution for Co has little influence on Ho–Co exchange interaction. The magnetic phase diagram has been determined from the temperature dependence of magnetization and the XRD patterns of magnetically oriented powder samples. The spin-reorientation temperatures of Ho2Co17−xSix compounds are found to decrease with Si content for x<2.5 and then increase for x>2.5, attaining a minimum value of 237 K at x=2.5.
Creation of high resistivity GaN by implantation of Ti, O, Fe, or Cr
Authors: Cao X. A. , Pearton S. J. , Dang G. T. , Zhang A. P. , Ren F. , Wilson R. G. , Van Hove J. M.
Published: July 2002

Implantation of n- and p-type GaN with Ti+, O+, Fe+, or Cr+ was found to produce defect levels which pinned the Fermi level in these materials at EC−(0.20–0.49) eV (n type) or EV+0.44 eV (p type). Maximum sheet resistances of ∼1012 Ω/□ (n type) and ∼1010 Ω/□ (p type) were obtained after implantation and annealing in the range of 300–600 °C. At higher annealing temperatures, the sheet resistance decreased to near the unimplanted values (3×104 Ω/□ in p type, 7×102 Ω/□ in n type). The evolution of the sheet resistance with annealing temperature is consistent with damage-related trap sites removing carriers from the conduction or valence bands.
Asymmetric ionic adsorption and cell polarization in liquid crystals
Authors: Barbero G. , Evangelista L. R. , Olivero D.
Published: July 2002

A model to determine the steady-state distribution of ionic charges in a liquid sample, in the shape of a slab, whose limiting surfaces are supposed to adsorb positive ions with different adsorption energies, is proposed. It is used to explain the asymmetric electro-optical response in a liquid-crystal cell recently observed. The ionic adsorption gives rise to a difference of potential between the surfaces which can be responsible for the existence of an internal field. It changes the effective voltage thresholds for the reorientation of the liquid crystal when external fields are applied.
The modeling of excimer laser particle removal from hydrophilic silicon surfaces
Authors: Wu X. , Sacher E. , Meunier M.
Published: July 2002

We summarize experimental results on the successful removal of submicron-sized polystyrene latex, carboxylate-modified latex, SiO2, and Al2O3 particles from hydrophilic silicon surfaces by excimer laser, using both dry and steam cleaning; the cleaning and damage thresholds have also been determined for these particles. Adhesion and removal models for an ideal sphere particle, that include van der Waals forces, hydrogen bonding, and thermoelastic effects, theoretically explain the laser cleaning results. Two models of the removal force due to the explosive evaporation of liquid film have been calculated and compared. The effects of both asperities on the particle surface and particle aggregation have also been considered. The results of the calculations show that even those surface asperities which are small compared with the particle dimension can cause a large reduction in both adhesion and thermoelastic removal forces. The theoretical predictions are consistent with the experimental observations.
Far-infrared electroluminescence from parabolic quantum well superlattices excited by resonant tunneling injection
Authors: Maranowski K. D. , Gossard A. C.
Published: July 2002

We present an investigation of intersubband emission at far-infrared wavelengths from semiconductor superlattices with parabolically graded quantum wells. Light emission is produced by sequential resonant tunneling injection of electrons into higher energy levels of the quantum wells and subsequent radiative decay. The current versus voltage curves of these devices exhibit negative differential resistances characteristic of the sequential resonant tunneling injection. A single, narrow emission peak is observed from the superlattices with parabolic quantum wells demonstrating radiative decay through multiple evenly spaced energy levels. When a chirped superlattice acting as an electron energy filter replaces the barrier in each period of the structure, clearer resonances are observed in the current–voltage characteristics and more efficient injection is achieved.
Dynamic probe of pyroelectricity of nonlinear optical polymer and theoretical analysis of its temporal response
Authors: Zhang G. J. , Yoshida J. , Sugita T.
Published: July 2002

The light pulsing technique was used for detecting the pyroelectricity of poled nonlinear optical polymer. We provided a simple experiment setup and an analytical model for theoretical analysis. Based on the results of our experiment, according to a model that was given in this study, we have calculated the temperature field distribution in the sample caused by the flash lamp irradiation in the polymer by means of numerical analysis, and further, the pyroelectric signal response was obtained. The model could be used for precisely investigating the response from a finite duration of light pulse. This numerical analytical method applied to the thermal pulse method yields unique relations between the time-dependent pyroelectric response and time-dependent temperature distribution for a thin nonlinear optical polymer film. The experimental results compared quite favorably with theoretical analysis.
Wavelength dependence of photoreduction of Ag+ ions in glasses through the multiphoton process
Authors: Kondo Yuki , Inouye Hideyuki , Fujiwara Seiji , Suzuki Toshio , Mitsuyu Tsuneo , Yoko Toshinobu , Hirao Kazuyuki
Published: July 2002

We have investigated the wavelength dependence of the photoreduction of Ag+ ions in glass irradiated by visible femtosecond pulses. These pulses, issued at wavelengths ranging from 400 to 800 nm, were nonresonant with the glass absorption. In this article, a relationship between threshold powers, wavelengths, and linear and nonlinear refractive indices is described. The nonlinear refractive index of Ag+-doped glass was measured by an optical Kerr shutter method. The wavelength dependence of threshold powers of the photoreduction is explained by considering linear and nonlinear refractive indices. The mechanism of the photoreduction is also discussed.
Excimer laser induced diffusion in magnetic semiconductor quantum wells
Authors: Howari H. , Sands D. , Nicholls J. E. , Hogg J. H. C. , Stirner T. , Hagston W. E.
Published: July 2002

Studies of pulsed laser annealing (PLA) of CdTe/CdMnTe quantum well structures are made in order to examine depth dependent effects in laser irradiated semiconductors. Since diffusion coefficients are strongly dependent on the temperature, depth resolution is achieved because the diffusion of Mn from the barriers into the quantum wells is depth dependent. Multiple quantum well (MQW) structures of CdTe/CdMnTe were annealed with single pulses from an XeCl laser at 308 nm. At a threshold of 90 mJ cm−2 two new emission bands are observed that are attributed to the diffusion of Mn from barrier layers to QWs. The diffusion associated with these bands, measured as the integrated product of the diffusion constant and time, is found to be 300 and 30 Å2. Calculations of the temperature, reached within the surface following PLA, using an analytical solution of the heat diffusion equation coupled with known high temperature diffusion coefficients predict the diffusion to decrease by one order of magnitude within one period at the top of the MQW stack. It is suggested that at the threshold surface melting occurs and that these emission bands arise from the QWs immediately beneath the melt front. The diffusion of Mn ions into the QWs is confirmed by magneto-optical data. A further emission band occurs at this same threshold with a Mn concentration above that of the concentration in the barrier layers of the MQW stack. This emission is attributed tentatively to the segregation of the Mn ion within the molten region following recrystallization.
Microscopic analysis of voltage noise operation mode in SiGe/Si bipolar heterojunctions: Influence of the SiGe strained layer
Authors: Martı́n M. J. , Pardo D. , Velázquez J. E.
Published: July 2002

A Monte Carlo analysis of bipolar transport and voltage fluctuations in a p+-Si/n-Si0.7Ge0.3 heterojunction and in a p+n Si homojunction under different operation regimes is presented. Comparison of the spectral density of voltage fluctuations at low frequency, SV(x,0), between both structures reveals the strong effect of the SiGe layer on the noise behavior in the heterojunction. Alloy scattering hinders the electron mobility enhancement expected from the removal of valley degeneracy in the SiGe layer. Despite this mobility reduction, the greater accumulation of carriers in the low-doped region supported by the valence band discontinuity reduce SV(x,0) in the heterojunction for a given average voltage. This study also reveals the impact of hot carriers on noise performance in the quasisaturation regime of both diodes.
Enhancement of diamond nucleation using the solid-liquid-gas interface energy
Authors: Baranauskas Vitor , Tosin Marcelo C. , Peterlevitz Alfredo C. , Ceragioli Helder , Durrant Steven F.
Published: July 2002

We demonstrate the enhancement of diamond nucleation through the use of the equilibrium forces at the solid-liquid-gas interface on a substrate wetted with droplets of an oil of low vapor pressure. Such a process is shown to produce well-faceted grains with densities of roughly 107 nuclei cm−2 (boundary), 105 nuclei cm−2 (oil-coated area), and 104 nuclei cm−2 (uncoated area) without the need for scratching or seeding the substrate. Diamond deposition was undertaken on silicon using ethanol and hydrogen in the feed of a hot-filament chemical vapor deposition reactor. The oil-covered regions, in addition to showing higher nucleation densities, have the merit that the intergrain spaces are covered with diamond structures, while the parts uncovered with oil exhibit intergrain spaces covered with diamond-like carbon.
Polishing-related optical anisotropy of semi-insulating GaAs studied by reflectance difference spectroscopy
Authors: Chen Y. H. , Wang Z. G. , Qian J. J. , Yang Z.
Published: July 2002

The strong in-plane optical anisotropy of (001) semi-insulating GaAs, which comes from the submicron region under the surface, has been observed by reflectance difference spectroscopy. The optical anisotropy can be explained by the anisotropic strain that is introduced by the asymmetric distribution of 60° dislocations during surface polishing. The simulated spectra reproduce the line shape of the experimental ones. The simulations show that the anisotropic strain is typically about 2.3×10−4.
Electron paramagnetic resonance studies on microcrystalline silicon prepared by sputtering method
Authors: Ehara Takashi , Ikoma Tadaaki , Akiyama Kimio , Tero-Kubota Shozo
Published: July 2002

Dangling bond (DB) defects in unhydrogenated microcrystalline silicon (μc-Si) prepared by rf sputtering have been studied. Raman spectra and x-ray diffraction indicate that the μc-Si fraction has been formed at the Ar sputtering pressure higher than 26.6 Pa while only amorphous silicon (a-Si) has been produced at the lower pressure. The electron paramagnetic resonance (EPR) spectrum in the μc-Si film is broad and unsymmetrical with the average g value of g=2.006 compared with that of a-Si (g=2.0055). The X- and Q-band EPR measurements suggest that the line shape is mainly governed by the inhomogeneous broadening due to the g anisotropy, indicating relatively large distribution of the structure of the DB defects.