bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of ferroelectric polymer-ceramic composites
Authors: Zewdie Habtamu , Brouers F.
Published: July 2002

A model is proposed to establish the relevant constitutional parameters of ferroelectric composite materials such as shape, composition, and connectivity which determine the piezoelectric, dielectric, and elastic properties of the composite. A proper Monte Carlo method is used to calculate the active volumetric fraction of ceramic. An equivalent circuit is proposed to calculate the effective dielectric constant. The effective medium expressions for the dielectric constant, elastic constant, and local field coefficients (LE, LD, LS, LT) are generalized to include shape effect. These results are used to calculate the piezoelectric coefficients. Recently, some authors have proposed a model and claimed to have established the degree of heterogeneity, determined by the scaled average grain diameter, to be the important constitutional parameter of a composite. We argue that the important constitutional parameter is the volumetric fraction of the ceramic. Our model is simpler, uses fewer parameters and its predictions compare better with experiment. The simplicity of the model can be exploited for predictions of the properties and design of multi-phase ferroelectric composites.
Photoluminescent properties and optical absorption of AlAs/GaAs disordered superlattices
Authors: Yamamoto Teiji , Kasu Makoto , Noda Susumu , Sasaki Akio
Published: July 2002

A disordered superlattice, a recently proposed artificially constructed material, is fabricated and photoluminescent properties and optical absorption are investigated. Disorder is intentionally introduced into the period of the superlattice in order to enhance its photoluminescence. The photoluminescent temperature dependences and the optical absorption spectra of Al0.5Ga0.5As bulk alloy, AlAs/GaAs ordered superlattice, and AlAs/GaAs disordered superlattice are studied and compared. The optical absorption spectra suggest that localized states are created in the band tail of the AlAs/GaAs disordered superlattice. The photoluminescence spectra of the disordered superlattice are strongly dependent on the localized states, and the temperature dependence of photoluminescence intensities obeys the same relation IPL∝[1+A exp(T/T0)]−1 as that reported for amorphous semiconductors.
The influence of interfacial exchange on the coercivity of acicular coated particle
Authors: Yang Jyh-Shinn , Chang Ching-Ray
Published: July 2002

The acicular coated magnetic particles are considered as magnetostatic interacting dual-component particles with interfacial exchange coupling. The calculations show that the coercivity varies with shell thickness and the relative magnetic properties between core and shell. When increasing the shell thickness, it is found that the initial slope of the increasing coercivity depends on the anisotropy energy of the shell, while the saturation level of the coercivity depends on the interfacial interaction. The calculated results show that a thin layer of shell is responsible for the increase of the coercivity. The best fitting curve for the available experimental data gives interfacial exchange coupling, A=5.87×10−8 erg/cm, and shell anisotropy constant, K=0.52×106 erg/cm3.
Noise measurement of YBa2Cu3O7−x and Ti2Ba2Ca2Cu3O10−x thin films
Authors: Zheng J. P. , Ying Q. Y. , Dong S. Y. , Kwok H. S. , Liou S. H.
Published: July 2002

The noise of YBa2Cu3O7−x and Tl2Ba2Cr2Cu3O10−x thin films in the frequency range from 0.5 Hz to 100 kHz was studied. In the normal state, it was found that 1/f noise dominated, with a magnitude strongly dependent on temperature. In the superconducting state, the noise was only observable at frequencies below 5 Hz with our present setup. Equilibrium thermal fluctuation noise was not observed in these films.
Spin-reorientation phenomenon in pseudobinary (Pr1−xRx)Co5 compounds (R=Sm, Gd, Dy, Tb, Ho, and Er) as determined by ac susceptibility measurements
Authors: Ma B. M. , Boltich E. B. , Sankar S. G. , Wallace W. E. , Malik S. K. , Tomy C. V.
Published: July 2002

Spin-reorientation transitions in (Pr1−xRx)Co5+δ, where R=Sm, Gd, Tb, Dy, Ho, and Er were determined by ac susceptibility measurements. The spin-reorientation temperature TSR of PrCo5 decreases from 105 K to lower temperatures when Pr is partially replaced by Sm, Gd, or Er. This decrease arises from different causes: the decrease of TSR in (Pr1−xGdx)Co5 results from the dilution of nonaxial anisotropy on the Pr sublattice, while the decrease of TSR in (Pr1−xSmx)Co5 and (Pr1−xErx)Co5+δ is due to the positive second-order Stevens coefficients of Sm and Er which yield uniaxial anisotropies. Rare earths of negative second-order Stevens coefficients such as R=Tb and Dy give rise to axial to conical and conical to planar transitions for x≳0.3 and 0.4 in (Pr1−xTbx)Co5+0.1x and (Pr1−xDyx)Co5+0.2x, respectively. Magnetic anisotropy phase diagrams of these systems are constructed accordingly.
Light-enhanced ferrite plating of Fe3−x
M
xO4 (M=Ni, Zn, Co, and Mn) films in an aqueous solution (invited)
Authors: Itoh T. , Hori S. , Abe M. , Tamaura Y.
Published: July 2002

Polycrystalline spinel ferrite films were plated on glass substrates in an aqueous solution by the thin liquid-film method combined with an electric lamp, which was named ‘‘light-enhanced ferrite plating.’’ By irradiating the substrate surface with a Xe lamp (450 W/cm2), the deposition rate of Fe3O4 films increased by a factor of ∼5–10 (from 30 to 150–320 nm/min) compared to that obtained without the light irradiation. The Fe3O4 film prepared in a reaction solution with a small FeCl2 concentration had a structure (i.e., grain size ∼200 nm, and columnar structure perpendicular to the film surface) similar to that observed when prepared without the light irradiation. However, the grain size increased and the columnar structure disappeared as the FeCl2 concentration increased. The deposition rate of Fe3−xMxO4 (M=Ni, Mn, Co, and Zn) was 50–100 nm/min and increased by a factor of 2–5 compared to that without the light irradiation. The saturation magnetization of the all films prepared with the light irradiation agrees with the value of the bulk samples.
Structural and magneto-optical properties of Tb/Fe and (Tb/Fe)/Si3N4 multilayers
Authors: Pierre F. , Boher P. , Kergoat R. , Houdy Ph. , Ferré J. , Pénissard G.
Published: July 2002

Tb/Fe and (Tb/Fe)/Si3N4 multilayer films with layers in the nanometric range have been prepared using a reactive diode rf sputtering system. Their structural and magneto-optical properties have been investigated to determine the stack parameters, the crystallization states, the direction of easy magnetization, and the Kerr angle values. The multilayer structure can be observed even when terbium layers are about 0.2 nm thick. Iron crystallization appears to depend on the terbium layer thickness. For Tb layer thicknesses greater than 0.37 nm, iron is amorphous when thinner than 3 nm. It spontaneously crystallizes when the layer becomes thicker. When Tb layer thickness is smaller than 0.37 nm, Fe crystallization is always detected: It occurs in the whole stack through the incomplete Tb layers. Kerr rotation measurements show a magnetic anisotropy depending on the Tb/Fe thickness ratio. Indeed, the direction of easy magnetization was found to be in the plane of the samples for ratio values of about 0.23 and 1.5, and rather perpendicular for ratio values of about 1. A Kerr angle of 1.2° has been obtained on a (Tb/Fe)/Si3N4 multilayer film in a nonsaturated magnetic state (λ=632.8 nm).
Intersubband photocurrent from the quantum well of an asymmetrical double-barrier structure
Authors: Liu H. C. , Aers G. C. , Buchanan M. , Wasilewski Z. R. , Landheer D.
Published: July 2002

A detailed study of intersubband photocurrents from the single quantum well of an asymmetrical double-barrier resonant tunneling structure is reported. Measured current-voltage characteristics are compared with a self-consistent calculation. The calculation also gives the density of electrons accumulated in the quantum well, and the lower and the upper resonant state escape times. Photocurrents from the quantum well are induced by 9 and 10.6 μm photons using a CO2 laser, which are directly proportional to the electron densities in the well. The observed photocurrents are much larger in the bias polarity where the thicker barrier is on the collector side consistent with theory. Our experiments show directly that a substantial accumulation of electrons occurs even for undoped quantum wells.
Beam and hot-spot formation in a low impedance line driven vacuum spark discharge
Authors: Wyndham E. , Chuaqui H. , Favre M. , Soto L. , Choi P.
Published: July 2002

Observations of a vacuum spark discharge are presented using a coaxial line driver. A 120 ns, 1.5 Ω coaxial line is used to give peak discharges of 90 kA. The usual line spark gap is shorted out giving a new mode of operation. The discharge is initiated once the rising sinusoidal voltage is applied by focusing a Nd:YAG laser onto the cathode front surface, peak current is reached at 250 ns after this. Reproducible hot spot formation is observed at this time. Holographic interferometry combined with time and space resolved x-ray observations show emission from a dense anode plasma as well as from the dense plasma column in which hot spot forms at peak current.
Relationship between electrical properties and structure in uniaxially oriented polycrystalline silicon films
Authors: Hasegawa S. , Arai M. , Kurata Y.
Published: July 2002

Undoped polycrystalline Si (poly-Si) films were prepared on a fused quartz substrate as a function of the rf power (0–30 W) and deposition temperature (620–770 °C) by a plasma-enhanced chemical vapor deposition method. The preferential orientation to a random, 〈100〉 or 〈110〉 crystal axis can be selected by changing some of these preparation conditions. Electron spin resonance (ESR) and the dependence of the dark conductivity σd on the measurement temperature T have been investigated to examine how the structure of the grain boundary depends on the preferential orientation (P.O.) and affects the electrical properties. An enhancement in the degree of P.O. is found to lead to a reduction in the local ESR spin density within the boundary region and g factor. Also, for the films with a random or weak P.O., σd as a function of 1/T exhibits a kink around room temperature, and the conduction in a low-T range is interpreted in terms of the model of variable range hopping within the boundary region. These results and the characteristics of thin-film transistors fabricated on the poly-Si films are discussed in terms of the structural change of the boundary region.
The experimental determination of the temperature dependence of the total emissivity of GaAs using a new temperature measurement technique
Authors: Timans P. J.
Published: July 2002

Temperature measurement and control are key issues in many technologically important areas of semiconductor processing, including rapid thermal annealing, molecular-beam epitaxy, and plasma processing. As a result, the development of accurate techniques for the characterization of thermal cycles is a problem of great practical interest. In this study, the total emissivity of specimens of GaAs was determined using a technique which combines isothermal electron-beam heating with a temperature measurement method which exploits the temperature dependence of the band gap of GaAs. Emission spectra from the GaAs specimens were recorded for a range of heating power densities. These spectra display a maximum near the semiconductor absorption edge, because the blackbody radiation rises with increasing wavelength but the spectral emissivity decreases rapidly once the photon energy falls below the band gap. The temperature was determined by fitting the Planck radiation function to the high-energy side of the maximum. This allowed a self-consistent determination of the temperature dependence of the position of the absorption edge. The results were used to calibrate a second set of experiments in which a corresponding set of reflection spectra were recorded. The reflection spectra exhibit a large change in reflectivity at the absorption edge, because light starts being reflected from the back surface of the wafer as well as from the front when it becomes transparent.

The reflection spectra provide a convenient method of temperature measurement because the intensity of the reflected light can be much larger than that of the emitted light. The large signal levels should permit temperature measurements to be made with good spatial and temporal resolution, even for samples at low temperatures. The temperature measurements can be performed with a precision of about 2 °C, and in this study the accuracy was ±5 °C. As a result of these measurements the total hemispherical emissivity of the GaAs specimens was determined over the temperature range from 350 to 630 °C. The values for the total emissivity of GaAs reported in this paper are believed to be the first direct, experimental measurements of this quantity. It was found that the total hemispherical emissivity of 485-μm-thick specimens rises from 0.08 at 350 °C to 0.28 at 630 °C. The electron-beam heating method used in this work presents distinct advantages for this type of measurement because there is little background light in the apparatus, the energy coupling to the specimen is not affected by its optical properties, and the surroundings remain cool. The power input can be accurately determined from the electron-beam energy and current and the scan area, and hence the total emissivity can be determined in a straightforward manner.
The dependence of contamination particle traps on wafer material and topography
Authors: Geha Sam G. , Carlile Robert N. , O’Hanlon John F. , Selwyn Gary S.
Published: July 2002

In recent years a number of workers have observed, using laser light-scattering methods, contamination particles suspended in rf process plasmas. Some recent studies show that the regions occupied by the particles appear finite and well defined, e.g., a ring. In this paper, a tuned Langmuir probe has been used to measure the plasma potential of these regions. Five cases are considered, where a case is distinguished by a disk of material A placed upon a larger diameter disk of material B, which in turn is placed upon an aluminum electrode driven by the rf power. It is consistently found that the regions occupied by particles are at a larger potential than the surrounding or ambient plasma. These regions are electrostatic traps. It is found that the incremental increase in potential as well as the geometry of the trap is dependent on materials A and B. For example, the trap configurations found are: case 1, silicon on graphite: ring and disk traps; case 2, silicon on silicon: ring trap; case 3, 304 stainless steel on silicon: disk trap; case 4, aluminum on silicon: bowl-shaped trap; case 5, silicon on aluminum: T-shaped or mushroom-shaped trap. Case 5 also contains a ring-shaped antitrap, i.e., a region where the potential is less than the ambient potential. The scenario proposed is that traps are system generated and depend on the design of the tool and the electrode. Particles subsequently flow into the traps. Particles may distort the trap boundaries as the density of trapped particles increases. However, particles do not determine the basic configuration (e.g., disk) of a trap, nor do they determine the plasma potentials within the trap or the surrounding ambient plasma region. In nearly all cases the trap and antitrap regions appear to be bounded by a double layer. Measurements of the plasma-sheath interface show that it generally follows the topography of the driven electrode. Explanations are presented for many of the observed phenomena.
Optical emissions from electron-impact-excited tetra-ethoxysilane
Authors: Ducrepin M. , Dike J. , Siegel R. B. , Tarnovsky V. , Becker K.
Published: July 2002

The optical emission spectrum in the spectral region 200–800 nm produced by electron impact on tetra-ethoxysilane (TEOS) under controlled single-collision conditions is analyzed. Absolute emission cross sections (at an impact energy of 100 eV) of 0.7±0.2×10−20 cm2 for the Si (3p4s 1P0→3p2 1S) line at 390.6 nm, 2.4±0.5×10−20 cm2 for the CH(A 2Δ→X 2Π) band centered around 430 nm, and cross sections below 0.5×10−20 cm2 for the hydrogen Balmer-α, -β, and -γ lines at, respectively, 656.4, 486.1, and 434.0 nm are measured. Comparatively high onset energies of 56.2±2.0 eV (Si) and 30.7±2.0 eV (CH) were found which when combined with the small cross sections indicate that the single-step dissociative excitation of TEOS by electron impact cannot be expected to play a major role in the collision processes that dominate the plasma chemistry of TEOS-containing processing plasmas.
Spectral origins of giant Faraday rotation and ellipticity in Bi-substituted magnetic garnets
Authors: Dionne Gerald F. , Allen Gary A.
Published: July 2002

Electrical permittivity theory has been applied to the interpretation of data describing the anomalously high Faraday rotation and ellipticity in the Y3−xBixFe5O12 ferrimagnetic system. By the use of exact forms of the basic phenomenological equations for the off-diagonal tensor element ε1 = ε′1 + iε″1, the measured Bi3+ influence on the separate component ε′1 and ε″1 spectra from 1 to 5 eV has been closely fitted to theory by the superposition of three Fe3+ (diamagnetic) electric dipole transitions occurring at 2.6, 3.15, and 3.9 eV, respectively. The strong transition at 3.15 eV that is believed to originate from Fe-O6 molecular complexes of the octahedral sublattice may also have a smaller companion peak in the vicinity of 5 eV. Transition bandwidths and excited-state splittings determined from the matching of theory to experiment both indicate that strong Bi covalent interactions exist with the Fe-O6 and Fe-O4 complexes. These results also confirm that the enhanced magneto-optical effects in the longer-wavelength region (λ∼1 μm) are dominated by the tail of the ε′1 diamagnetic peak at 3.15 eV.
Bias-voltage- and bias-light-dependent high photocurrent gains in amorphous silicon Schottky barriers
Authors: Rubinelli F. A. , Hou J. Y. , Fonash S. J.
Published: July 2002

Experimental results of very large, long-wavelength photocurrent gains in amorphous silicon-based Schottky barrier structures are reported. It is shown that these occur for devices in the space-charge current regime operated at forward bias voltages past the flatband condition. The analysis of microelectronic and photonic structures computer program is used to show that these high gains are due to hole trapping and the resulting modulation of the virtual cathode barrier potential at the ohmic contact. As demonstrated, these gains are bias voltage, bias light, monochromatic light intensity, material property, and device thickness dependent. This type of photocurrent gain is another photogating effect and is much different than the usual bulk photoconductive gain.
Spontaneous alloying of copper atoms into gold clusters at reduced temperatures
Authors: Yasuda H. , Mori H. , Komatsu M. , Takeda K.
Published: July 2002

The alloying behavior of copper atoms into gold clusters at temperatures lower than 300 K was examined using a 200 kV transmission electron microscope equipped with a double source evaporator in the specimen chamber. Isolated gold clusters of about 4 nm in mean diameter were prepared on a supporting film and were cooled down to 245, 215, 165, or 125 K. Copper atoms were then deposited onto the same film. Upon the deposition of copper, the gold clusters quickly changed into completely or partially alloyed clusters, depending upon the substrate temperature. The copper diffusivity estimated from such spontaneous alloying was many orders of magnitude faster than that in bulk gold. A possible mechanism behind the spontaneous alloying is discussed.
The chemisorption and reaction of diethylsilane on silicon (100) and (111) surfaces
Authors: Lapiano-Smith D. A. , Himpsel F. J. , Terminello L. J.
Published: July 2002

Soft x-ray photoemission has been utilized to investigate the chemisorption and subsequent reaction of diethylsilane on Si(111) and (100) surfaces. We show that diethylsilane chemisorbs dissociatively to form Si-CH2CH3 surface species on Si(111) and Si(100) at room temperature. These species are identified by two very sharp peaks observed in the valence band spectra of both surfaces, positioned at 17.9 and 14.3 eV binding energy. Si 2p and C 1s core level spectra were measured following exposures of Si(100) and (111) substrates as a function of surface temperature. C 1s data show that carbon, in some form, exists on the Si surface following exposures at every temperature from room temperature to about 600 °C. While only –CH2CH3 ethyl groups are observed on the surface at room temperature, these species appear to partially dehydrogenate above 300 °C producing a mixture of –CH2CH3 groups and other intermediate carbonaceous species. At a growth temperature of about 400 °C for the technologically important (100) surface, the intermixing of elemental carbon with Si begins. At higher temperatures, we observe the continued degradation of diethylsilane to produce a Si+C alloy on the surface at 600 °C. Our results indicate that diethylsilane has potential as a candidate for SiC formation by chemical vapor deposition techniques.
Phase transitions during solid-state formation of cobalt germanide by rapid thermal annealing
Authors: Ashburn Stanton P. , Öztürk Mehmet C. , Harris Gari , Maher Dennis M.
Published: July 2002

Phase transitions that involve solid-state reactions between cobalt and thin films of germanium have been investigated. Germanides are formed by reacting Co (300 Å thick) with thin layers of Ge (∼2000 Å thick) deposited on silicon substrates. Germanium was deposited on Si by rapid thermal chemical-vapor deposition and cobalt was deposited onto Ge by evaporation. The Co/Ge/Si stacked structure samples were then rapid thermally annealed at atmospheric pressure in an inert ambient consisting of Ar. Using x-ray-diffraction spectroscopy, Co5Ge7 and CoGe2 are identified as the phases which form at 300 and 425 °C respectively. The sheet resistance was found to be a strong function of the annealing temperature and a minimum resistivity of approximately 35 μΩ cm is obtained after annealing at 425 °C. The minimum resistivity material corresponds to the CoGe2 phase with an orthorhombic crystal structure. Above 600 °C, the resistivity increases due to an instability of the solid-phase reaction between Co and thin Ge layers deposited on Si. This instability is attributed to rapid Co diffusion at the temperatures which are required to form CoGe2 along with structural defects in the Ge layer.
Self-consistent calculations of an electron-wave Y-branch switch
Authors: Palm Thomas
Published: July 2002

A semi-classical model for studying split-gate devices in three dimensions is described. The model, useful at low bias voltages, finds the charge distribution self-consistently in a classical approximation using Poisson’s equation and a Thomas-Fermi electron distribution. This gives a potential in which the Schrödinger equation is solved for electrons near the Fermi level. The transmission coefficients for electrons then give the conductance. The model is used to evaluate the performance of an electron Y-branch switch. This device uses a control voltage to switch an incoming current between two drain leads. A single-mode switch using 80 nm gate separation, needs only 40 mV switching voltage. Multi-mode operation is also demonstrated using 200 nm wide leads. Here 1 V is needed for 10 dB current reduction in one branch.
Enhanced electrical properties of ferroelectric Pb(Zr0.5, Ti0.5)O3 thin films grown with low-energy oxygen ion assistance
Authors: Hu H. , Krupanidhi S. B.
Published: July 2002

Electrical properties pertinent to memory applications of ferroelectrics are systematically characterized for multi-ion-beam reactive sputter deposited Pb(Zr0.5, Ti0.5)O3 thin films. The processing dependence of electrical properties of the films offers an opportunity for property modification. Low-energy oxygen ion bombardment assistance is useful in this aspect. Under optimal bombardment conditions (ion/atom ratios between 1.0 and 1.3; ion/energies ranging from 60 to 80 eV), many electrical properties can be beneficially modified. Relative to the nonbombarded films, the bombarded films exhibit increases in remanent polarization and resistivity, and reductions in coercive field. Bombardment-enhanced properties also include switching characteristics, dielectric responses, switching endurance, polarization retention, dc current-voltage behavior, and time-dependent dielectric breakdown strength. The experimental results are explained in correlation to processing and structure of the films. The roles of preferential orientation, crystallinity, degree of imperfection, grain size, and grain boundaries of the films are emphasized.
Doping effect of buckminsterfullerene in poly(2,5-dialkoxy-p-phenylene vinylene)
Authors: Morita Shigenori , Kiyomatsu Shinji , Yin Xiao Hong , Zakhidov Anvar A. , Noguchi Takanobu , Ohnishi Toshihiro , Yoshino Katsumi
Published: July 2002

Photoluminescence has been markedly quenched and photoconductivity has been enhanced by more than one order of magnitude upon introduction of several mol% of buckminsterfullerene (C60) to poly(2,5-dialkoxy-p-phenylene vinylene) (RO-PPV), especially at excitations about 2.2 eV, corresponding to the band gap energy of RO-PPV and also in bands at 1.8 and 3.5 eV, which correspond to optical excitation of C60 molecules, suggesting that photo-induced charge transfer occurs between RO-PPV and C60. On the other hand, absorption spectrum and electrical conductivity of RO-PPV have been scarcely influenced by doping of small amount of C60, suggesting that the ground state charge transfer between C60 and RO-PPV is not effective, contrary to the case of poly(3-hexylthiophene). These results are discussed by taking relative electronic energy states of RO-PPV and C60 into consideration. The photo-excited exciton-polaron (Ex-P) in RO-PPV is interpreted to migrate along about 100 monomer units along a polymer main chain in its lifetime and dissociates when encountered with C60. These unique doping characteristics of C60 in RO-PPV are not dependent on the alkyl chain length, contrary to the case of poly(3-alkylthiophene).
Measurement of interface strength by the modified laser spallation technique. I. Experiment and simulation of the spallation process
Authors: Yuan J. , Gupta V.
Published: July 2002

Part I of this paper presents the modified laser spallation technique for measuring the tensile strength of planar interfaces, using a Doppler displacement interferometer. In this technique, a laser-produced compressive stress pulse in the substrate, reflecting from the coating’s free surface pulls the interface in tension and leads to its failure if the tensile amplitude is high enough. The current technique is an improvement over the previous one, since the interface stress is determined directly by recording the coating or substrate free-surface velocities using a laser displacement interferometer. The recorded surface velocity is related to the interface stress via an elastic wave equation simulation. The process of coating spallation is investigated, and the effect of the stress pulse profile and the coating and substrate characteristics on the interface tensile stress is studied using the simulation. Several interface stress charts are given for wider applicability of the modified technique.
Control of reactive plasmas in a multicusp plasma source equipped with a movable magnetic filter
Authors: Fukumasa Osamu , Naitou Hiroshi , Sakiyama Satoshi
Published: July 2002

With the use of both a movable magnetic filter and a plasma grid, plasma parameters (H2-CH4 or Ar-CH4 plasmas) are spatially well controlled. At any filter position, plasma parameters change steeply across the magnetic filter. Then, a plasma source is divided into the two parts, i.e., the source plasma region (high density plasma with energetic electrons) and the diffused plasma region (low density and low-temperature plasma without energetic electrons). Plasma parameters in the diffused plasma are well controlled by changing the plasma grid potential. The role of the magnetic filter (i.e., preferential reflection of energetic electrons) is well clarified by computer simulation. The relation between plasma parameters and some species of neutral radicals is also briefly discussed.
Growth-temperature dependence of Co layer strain and magnetic anisotropy in epitaxial Co/Au superlattices
Authors: Kingetsu Toshiki , Sakai Katsura
Published: July 2002

We studied the variation of the in-plane lattice constant of Co layers as a function of layer thickness in Co/Au superlattices. The samples were grown at various temperatures by molecular-beam epitaxy and were investigated using reflection high-energy electron diffraction. It was found that the strains of Co layers are larger in a sample grown at a higher temperature. It might be thought that larger strains possibly introduce larger magnetoelastic anisotropy, contributing to the perpendicular magnetic anisotropy of the superlattices. However, magnetization measurements show that the magnetic anisotropy energy is smaller in the sample grown at a higher temperature. Possible causes of this discrepancy are discussed.
Effect of ion angular distributions on microloading in oxygen reactive-ion etching of submicrometer polymer trenches
Authors: Janes Joachim , Pilz Wolfgang
Published: July 2002

The etch rate of the resist by energetic reactive ions (O+, O+2) from a 13.56 MHz plasma was studied as a function of the aspect ratio. Using a trilevel sequence, submicrometer grooves with widths of 500, 200, 100, and 50 nm are etched with an oxygen rf plasma in a parallel-plate reactor. Pattern transfer into the polymer photoresist proceeds via a thin silicon nitride mask. With increasing aspect ratio a decrease in etch rate is observed for increasing etch time resulting in a diminished etch depth for small width grooves. This effect is commonly called microloading and its origin is subject to many discussions. With the assumption that lower local etch rates are caused by the characteristics of particle fluxes entering the structures it is necessary to consider the ion energy distributions and the ion angular distributions of O+ and O+2 emerging from the oxygen rf plasma bombarding the surface to be structured.

The measurement of ion angular impact energy distributions of O+ and O+2 is performed in an apparatus simulating a parallel-plate rf reactor. Through a small orifice in the rf-powered electrode ions are sampled and are analyzed in a quadrupole mass spectrometer equipped with an energy filter. Tilting the detection system with the vertex lying in the orifice, mass selected ion energy distributions are measured at each angle. O+2 energy distributions consist of the well-known peaked structures caused by charge-exchange collisions in the plasma sheath. The angular distributions are characterized by mace-shaped structures with an angular width of about 2° accompanied by wings with intensity maxima at about ±2° for most of the experimental conditions considered. O+ energy distributions consist of bimodal split high-energy peaks and continuous intensities between 50 eV and the maximal energy defined by the potential drop across the sheath. The angular distributions of the high-energy O+ ions have widths between 2° and 6° while the continuous part of the ion energy distributions exhibits angular widths between 7° and 11°.

The phenomena observed in the ion energy distributions and ion angular distributions are a result of scattering mechanisms of ions with O2 parent gas molecules in the plasma sheath. The experimental ion angular distributions are used to model the effect of lower local etch rates. Assuming that direct impingement of particles onto the bottom surface of a structure is the dominant ablation mechanism it is possible to correlate the effect of lower local etch rates with the angularly broadened flux of O+ ions.
The effect of electric fields on time-resolved photoluminescence spectra in semiconductors
Authors: Rosenwaks Y. , Nozik A. J. , Yavneh I.
Published: July 2002

We present a rigorous analysis of the effects of electric fields on time-resolved photoluminescence in semiconductors. The results show that the effect of the field alone on the photoluminescence decay can be distinguished from that of field-enhanced surface recombination if the carrier injection levels, the surface recombination velocity at zero field, and the band bending in the dark are within certain limits. When these experimental conditions are met it is possible to extract the recombination and/or transfer velocity of free carriers in the presence of electric fields.
Electric-field-induced polarization current studies in guest–host polymers
Authors: Zimmerman K. , Ghebremichael F. , Kuzyk M. G. , Dirk C. W.
Published: July 2002

We apply polarization current measurements to study dipole reorientation of nonlinear chromophores that are embedded in a polymer matrix. In particular, we apply a step voltage to a thin film of poly(methyl methacrylate) that is doped with a well-known dipolar chromophore, Disperse Red 1, or an apparently centrosymmetric squarylium chromophore, and measure the isothermal temporal behavior of the current at several temperatures. By correlating the current with electric-field-induced second-harmonic generation, we find that at very early times, polarization current dominates, while at later times, charge hopping mechanisms probably dominate. For the centrosymmetric squarylium chromophore dopant, we find an early-time current suggesting polarization decay with no corresponding second-harmonic signal. This suggests that the squarylium molecule has a dipole moment that is perpendicular to the dominant axis of the polarizability tensor and is consistent with the presence of the cis isomer. A time–temperature–voltage complimentarity relationship is also observed.
Pulsed anodic oxides for III-V semiconductor device fabrication
Authors: Grove M. J. , Hudson D. A. , Zory P. S. , Dalby R. J. , Harding C. M. , Rosenberg A.
Published: July 2002

A simple procedure for the rapid formation of uniform native oxides on various III-V semiconductor materials is described. A pulsed applied potential drives an anodic oxide formation process on the semiconductor immersed in a glycol:water:acid solution. Uniform oxides up to 2000 Å thick can be grown in a few minutes at room temperature and used to define areas for current injection into the semiconductor. AlGaAs diode lasers fabricated with 50-μm-wide current stripes defined by pulsed anodic oxide had threshold current densities substantially lower than lasers fabricated with 50-μm-wide stripes defined by chemical-vapor-deposited SiO2.
Enhanced photorefractive performance in a photorefractive polymeric composite
Authors: Orczyk Maciek E. , Swedek Bogdan , Zieba Jaroslaw , Prasad Paras N.
Published: July 2002

We characterize the photorefractive behavior of a photorefractive multicomponent polymer composite of PVK-TCP:C60:DEANST. Efficient plasticization of the host polymeric matrix and utilization of a nonlinear chromophore with a large dipole moment provide a large poling-induced electro-optic coefficient. Diffraction efficiencies as high as 40% and asymmetric net two-beam coupling gain coefficients in excess of 130 cm−1, surpassing those of known inorganic single-crystalline photorefractive media, are reported.
Magnetic force microscopy of Co-Pd multilayers with perpendicular anisotropy
Authors: Barnes J. R. , O’Shea S. J. , Welland M. E. , Kim J.-Y. , Evetts J. E. , Somekh R. E.
Published: July 2002

The domain structure of sputtered Co-Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain-wall pinning which is not considered in the model. It is estimated that the characteristic length of the films is 55 Å and the domain-wall energy is 14 mJ/m2.
High-brightness InGaN/AlGaN double-heterostructure blue-green-light-emitting diodes
Authors: Nakamura Shuji , Mukai Takashi , Senoh Masayuki
Published: July 2002

High-brightness InGaN/AlGaN double-heterostructure blue-green-light-emitting diodes with a luminous intensity of 2 cd were fabricated by increasing an indium mole fraction of the InGaN active layer up to 0.23. Both Zn and Si were codoped into the InGaN active layer to afford relatively stronger luminescence. The blue-green emission intensity of room-temperature photoluminescence became maximum when the electron carrier concentration of the InGaN active layer was around 1×1019 cm−3. Donor-acceptor pair recombination is a dominant emission mechanism of the InGaN active layer. The external quantum efficiency was as high as 2.4% at a forward current of 20 mA at room temperature. The peak wavelength and the full width at half-maximum of the electroluminescence were 500 and 80 nm, respectively.
Role of dislocation blocking in limiting strain relaxation in heteroepitaxial films
Authors: Gillard V. T. , Nix W. D. , Freund L. B.
Published: July 2002

The residual strain following relaxation in a variety of Si1−xGex heteroepitaxial films grown on (001) Si wafers has been compared with the values of residual strain predicted by the theory based on the incremental movements of isolated threading dislocation segments. It is found that for very thin films (40–500 nm) the measured residual strains after relaxation are significantly higher than the values predicted by this theory. For thicker films, the residual strains are very close to the predicted values. The effect of the interactions of parallel dislocations on the residual strain are investigated using the model developed by Willis, Jain, and Bullough [Philos. Mag. A 62, 115 (1990)] for a two-dimensional array of dislocations. It is found that the experimental data cannot be explained by this model since it predicts even lower values of residual strain than the model based on isolated threading segments. The residual strains are also compared with predictions based on Freund’s treatment of the blocking of a moving threading segment by an orthogonal misfit dislocation present in its path [J. Appl. Phys. 68, 2073 (1990)]. It is found that Freund’s blocking criterion gives a very good account of the residual strain in Si1−xGex films. Blocking of threading dislocations by other misfit dislocations appears to play an important role in the late stage of strain relaxation in these structures and thus may limit the possibility of obtaining fully relaxed films with low threading dislocation densities.
Monte Carlo histogram calculation of the critical exponents of an FexMg1−xCl2 Ising model (invited)
Authors: Hernández Laura , Diep H. T.
Published: July 2002

In a previous work, a realistic three-dimensional 3-D Ising model of FeCl2 was studied. It consists of triangular lattice planes stacked along the c axis of the crystal, with the following interactions: a nearest neighbor (nn) ferromagnetic one competing with a next-nearest-neighbor (nnn) antiferromagnetic interaction, which causes some degree of frustration in the plane, the planes being weakly coupled by an antiferromagnetic interaction. In fact, the latter is a superexchange interaction, and we have taken into account all the equivalent superexchange paths in the sense of the Anderson rule, which amounts to 12 neighbors in each one of the adjacent planes. In this work we present the preliminary results of a study of the critical exponents of this model using the Monte Carlo histogram method and finite size scaling. The interest of such study lies in the layered structure of this system, which might have an effect on its effective dimensionality at the transition point, and also in the effects of frustration, as well as those of superexchange interactions on the critical exponents. We also show the results obtained by the same method, on the critical exponents of the diluted system in the zero and low magnetic field.
Magnetic properties of Sm2(Fe,V)17Ny coarse powder
Authors: Suzuki Shunji , Suzuki Shinya , Kawasaki Masahito
Published: July 2002

Magnetic properties and crystal structures of Sm2(Fe,V)17Ny alloys were studied. Crystal structures of Sm2Fe17−xVx alloys varied from Th2Zn17 (x=0–1) to TbCu7 (x=1.5), and ThMn12-type (x=2) as a function of vanadium content (x). The alloys were nitrogenated at 823 K for 4 h, and in all cases resulted in the expansion of the crystal lattice. The nitrogenated and annealed Sm2Fe15.5V1.5Ny coarse powders with 20–74 μm particles showed HcJ of 342 kA/m (4.3 kOe) without fine grinding. It is supposed that one of the reasons of the high coercivity is due to the microstructures of the particles.
Structural properties of amorphous silicon carbide films by plasma-enhanced chemical vapor deposition
Authors: Choi W. K. , Chan Y. M. , Ling C. H. , Lee Y. , Gopalakrishnan R. , Tan K. L.
Published: July 2002

An investigation of the structural properties of hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films prepared by the plasma-enhanced chemical vapor deposition of silane and acetylene has been undertaken using a combination of infrared (IR), Raman, and x-ray photoelectron spectroscopy (XPS) measurements. The compositions of the silicon, carbon, and hydrogen in the films were found to be dependent on the preparation conditions. From the IR results, it is found that the Si—H bond decreases and the C—H bond increases as the film’s carbon increases. The Raman spectra showed that while the Si—Si and C—C bonds can be detected in silicon-rich and carbon-rich samples, respectively, the Si—C band can only be observed in a-Si0.7C0.3:H and a-Si0.5C0.5:H. The XPS results showed that the stoichiometry calculation from the flow rates of the reacting gases was good for a-Si0.7C0.3:H but not for a-Si0.3C0.7:H. Reactive ion etching of the a-Si1−xCx:H films showed that the etch rate was dependent on the films’ carbon concentration and films prepared with acetylene as source gas were more resistive to etching compared to that prepared by butadiene.
Optical properties of silicon nitride films deposited by hot filament chemical vapor deposition
Authors: Deshpande Sadanand V. , Gulari Erdogan , Brown Steven W. , Rand Stephen C.
Published: July 2002

Silicon nitride films were deposited at low temperatures (245–370 °C) and high deposition rates (500–1700 Å/min) by hot filament assisted chemical vapor deposition (HFCVD). Optical properties of these amorphous silicon nitride thin films have been extensively characterized by absorption, photoluminescence (PL), photoluminescence excitation, and electroluminescence measurements. The optical band gap of the films was varied between 2.43 and 4.74 eV by adjusting the flow rate of the disilane source gas. Three broad peaks at 1.8, 2.4, and 3.0 eV were observed in the PL spectra from these films. A simple qualitative model based on nitrogen and silicon dangling bonds adequately explains the observed PL features. The photoluminescence intensity observed in these films was 8–10 times stronger than films deposited by plasma enhanced chemical vapor deposition, under similar conditions. The high deposition rates obtained by HFCVD is believed to introduce a large number of these optically active defects.
A spectroscopic ellipsometry study of cerium dioxide thin films grown on sapphire by rf magnetron sputtering
Authors: Guo S. , Arwin H. , Jacobsen S. N. , Järrendahl K. , Helmersson U.
Published: July 2002

Variable angle spectroscopic ellipsometry (VASE) has been used in the photon energy range 1.25–5.0 eV to study the structure and optical properties of cerium dioxide (CeO2) films. Both amorphous and highly oriented crystalline films were grown on sapphire by rf magnetron sputtering. The crystallinity, chemical structure, and surface morphology of the films were studied by x-ray diffraction, x-ray photoelectron spectroscopy, and atomic force microscopy, respectively. The measured VASE spectra on a series of films with different thicknesses were analyzed by using multiple optical models. In this way, the complex refractive index N=n+ik of CeO2, the film thicknesses, and the surface roughness of the different films could be determined. The ellipsometrically deduced refractive index spectrum was observed to be strongly dependent on the film structure. Highly oriented crystalline CeO2 films exhibited a higher refractive index and a higher band gap energy than the amorphous film. The surface roughness of the crystalline films increased with film thickness. The optical dispersion relations have also been analyzed by line-shape fitting with a modified Lorentz oscillator model.
Direct observation of a thin reacted layer buried at Al/SiO2 interfaces
Authors: Miura Y. , Hirose K.
Published: July 2002

An Al/SiO2 interfacial structure is investigated by cross-sectional transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) before and after post-metallization annealing. An interfacial reacted layer with uniform thickness of 2.5 nm is observed by TEM for samples annealed at 450 °C for 1 h. Further reaction is suppressed at higher annealing temperatures up to the eutectic point (577 °C) of an Al-Si system. XPS analysis of the interfacial structure is performed by removing the unreacted metallic Al layer selectively from the surface. The XPS spectra of the reaction products show that the interface has a layered structure of Al/Al2O3/Si/SiO2. This is discussed in relation to thermodynamic stability.
Epitaxial YBa2Cu3O7−δ /BaxSr1−xTiO3 heterostructures on silicon-on-sapphire for tunable microwave components
Authors: Boikov Yu. A. , Ivanov Z. G. , Kiselev A. N. , Olsson E. , Claeson T.
Published: July 2002

Epitaxial trilayer heterostructures of YBa2Cu3O7−δ/BaxSr1−xTiO3/YBa2Cu3O7−δ were grown on silicon-on-sapphire buffered by a double layer of CeO2/Y–ZrO2. Such structures may be considered for tunable microwave filters. The top and bottom YBa2Cu3O7−δ films were well c-axis oriented, free from microcracks and had superconducting transitions Tc’s in the range 86–90 K. A thin antidiffusion layer of SrTiO3 (d≊70 Å) between YBa2Cu3O7−δ and BaxSr1−xTiO3 (x=0.25–0.9) promoted better crystallinity and higher Tc of the top superconducting film. An Ag/BaxSr1−xTiO3/YBa2Cu3O7−δ capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan δ of the BaxSr1−xTiO3 layer. Maximum values of the permittivity of the BaxSr1−xTiO3 layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the BaxSr1−xTiO3 (x=0.25–0.75) layers depended strongly (≊20%) on an applied voltage (±2.5 V) at temperatures around 77 K. The tan δ was much higher in films than in bulk crystals.