bluuebunny/crossref_metadata_2025 huggingface dataset:
Size of Cd(S,Se) quantum dots in glasses: Correlation between measurements by high-resolution transmission electron microscopy, small-angle x-ray scattering, and low-frequency inelastic Raman scattering
Authors: Champagnon Bernard , Andrianasolo Bruno , Ramos Aline , Gandais Madeleine , Allais Mireille , Benoit Jean-Pierre
Published: July 2002

The average sizes of Cd(S,Se) quantum dots in glasses have been measured for the same samples by using three methods: high-resolution transmission electron microscopy, low-frequency inelastic Raman scattering, and small-angle x-ray scattering. Three samples with crystal sizes of 5.5–8, 9–12, and 17 nm have been studied. For each method the interaction mode and the underlying assumptions in measuring the size are discussed. The correlation between the measurements is good and points out a narrow size distribution. The discrepancies observed for the smallest sizes are discussed with respect to the probe and the interactions used by each technique. They may be ascribed to structural effects.
Energy levels of GaSb grown by metalorganic chemical vapor deposition
Authors: Su Y. K. , Chen S. M.
Published: July 2002

Undoped GaSb epilayers were grown on GaSb and GaAs substrates using the metalorganic chemical vapor deposition (MOCVD) technique, and photoluminescence (PL). Hall effect measurements were used to characterize the undoped epilayers. For undoped GaSb epilayers, four energy levels revealed by PL spectra are shown to be due to acceptors, using KTdp/dEf va Ef-Ev data obtained from Hall effect results, where p is the hole concentration, Ef and Ev are the Fermi level and the top of valence band, respectively. The acceptor levels lie at 3, 58, 80, and 130 meV above the top of valence band, respectively. The donor level is 15 meV below the bottom of the conduction band. These acceptor levels exist in almost all of the undoped p-type GaSb epilayers grown at high temperature, while the donor level exists only in the undoped n-type GaSb epilayers grown by low-temperature MOCVD. It is found that these levels may be changed using the different growth temperatures.
Crystal distortion and weak ferromagnetism of Mn3+δGa1−xGex alloys
Authors: Niida Hiroshi , Hori Tomiei , Yamaguchi Yasuo , Nakagawa Yasuaki
Published: July 2002

The compound Mn3Ga (70–74 at.% Mn) has the hexagonal DO19-type structure, and a triangular antiferromagnetic spin structure with a weak ferromagnetic component below the Néel temperature of about 470 K. Below 170 K the hexagonal structure is distorted to an orthorhombic one, accompanied by a steep increase in the ferromagnetic component. X-ray-diffraction experiments and magnetization measurements have been made for a pseudobinary system Mn3+δGa1−xGex. It was found that the DO19-type structure is stable at 600 °C in the whole composition range 0≤x≤1.0 with appropriate values of δ. The Néel temperature of the triangular spin structure decreases with increasing x with a gentle slope. The crystal distortion temperature decreases more steeply and vanishes near x=0.6.
Inverse scattering algorithm applied to infrared thermal wave images
Authors: Crowther D. J. , Favro L. D. , Kuo P. K. , Thomas R. L.
Published: July 2002

We present an inverse scattering algorithm that makes possible the inversion of experimental thermal wave images of planar subsurface defects. The method is based on a Green’s function technique. It significantly improves the spatial resolution and removes the blurring which is otherwise characteristic of thermal wave images. The method has been applied to materials ranging from plastics (diffusivity ∼0.001 cm2/s) to aluminum alloys (diffusivity ∼0.5 cm2/s), and for depths ranging up to a few mm.
Enhanced luminescence due to impact ionization in photodiodes
Authors: Swoger J. H. , Kovacic S. J.
Published: July 2002

A simple mechanism explaining the enhanced luminescence of p-i-n structures and reach-though avalanche photodiodes approaching reverse-bias breakdown is proposed. The model presented predicts enhanced photon emission based on calculations of the spatial dependence of the impact ionization events. Experimentally, an enhancement of a factor of 100 in the optical efficiency is observed in p-i-n silicon photodiodes when biased in the gain regime. A series of measurements made on commercially available photodiodes are presented. Theoretical predictions are compared with the experimental photonic output versus internal electric field and current.
The specific volumes of Pd40Ni40P20 in the liquid, glass, and crystallized states
Authors: Chow K. C. , Wong S. , Kui H. W.
Published: July 2002

The specific volumes of Pd40Ni40P20 were measured in the liquid, glass, and crystallized states. It was found that in the molten state, for temperatures below ∼1000 K, the specific volume measurement was successful only if the specimens were properly purified. Such purification could be achieved by fluxing the molten specimen in dehydrated boron oxide. Relaxation effects are observed and they manifested themselves in the variation of the thermal expansion coefficient with the quenching rates of the glassy specimens. The intersection points of the specific volume curves of the liquid (both stable and metastable), glass and crystallized are 573±80 and 492±80 K (extrapolated values), respectively. The former agrees very well with the glass transition temperature 577 K, obtained by the calorimetry method, while the latter implies that the melt, in accord with free volume theory, will have collapsed to the glass before that temperature is reached.
Investigation of Pt/Ti bilayer metallization on silicon for ferroelectric thin film integration
Authors: Sreenivas K. , Reaney Ian , Maeder T. , Setter N. , Jagadish C. , Elliman R. G.
Published: July 2002

The stabilities of Pt/Ti bilayer metallizations in an oxidizing atmosphere have been investigated with several thicknesses of interfacial Ti-bonding layers. Reactions in the Pt/Ti/SiO2/Si interface were examined as a function of various annealing conditions in the temperature range 200–800 °C by using Rutherford backscattering spectrometry, Auger electron spectroscopy, x-ray diffraction, and transmission electron microscopy. Thermal treatment in oxygen was found to cause rapid oxidation of the Ti layer, accompanied by the migration of Ti into the Pt film. Diffusion of oxygen through the Pt grain boundaries was mainly responsible for the adverse reactions at the interface and loss of mechanical integrity. Thin Ti (10 nm) layers resulted in the depletion of the interfacial bonding layer causing serious adhesion problems, whereas thicker Ti films (100 nm) caused the formation of TiO2−x in the Pt-grain boundaries, ultimately encapsulating the Pt surface with an insulating TiO2 layer. Improved stability and adhesion in the Pt/Ti bilayer metallization compatible with ferroelectric thin film processing, were achieved by incorporating well reacted thin TiO2 layers in situ, and depositing Pt films at a high temperature.
Soft x-ray spectra of highly ionized elements with atomic numbers ranging from 57 to 82 produced by compact lasers
Authors: Zeng G. M. , Daido H. , Nishikawa T. , Takabe H. , Nakayama S. , Aritome H. , Murai K. , Kato Y. , Nakatsuka M. , Nakai S.
Published: July 2002

X-ray emission spectra in the spectral range of 2–13 nm from 19 kinds of material with high atomic numbers (lanthanum through lead) were recorded with a grazing incidence spectrometer equipped with a microchannel plate detector. There is an intense, narrow spectral band in these spectra which shifts toward shorter wavelength and becomes weak in intensity with increasing atomic number. The materials were irradiated either by a 4 J/35 ns slab Nd:glass laser or by a 0.5 J/8 ns Nd:YAG laser. The absolute photon intensities of the spectra were determined with an absolutely calibrated charge coupled device camera. The peak spectral brightness of the emission at the peak intensity of the spectral band for lanthanum plasma was estimated to be 2.1×1016 photons/s/mm2/mrad2 in 0.1% bandwidth. The origin of the narrow, intense spectral bands in the recorded spectra and their dependence on target materials and laser wavelength are interpreted.
Hysteresis reduction in NiFeCo/Cu multilayers exhibiting large low-field giant magnetoresistance
Authors: Hossain S. , Seale D. , Qiu G. , Jarratt J. , Barnard J. A. , Fujiwara H. , Parker M. R.
Published: July 2002

Giant magnetoresistance (GMR) has been observed in Ni66Fe16Co18/Cu multilayered uniaxial magnetic thin films prepared by dc magnetron sputtering. Both easy and hard axis loops saturate at very low applied fields. The MR ratio reaches ∼80% of its saturation value in less than 30 Oe. A maximum MR ratio of ∼10% has been observed in the as-deposited state in a structure containing only ten bilayers. Typical hysteresis values for magnetoresistance loops are ∼4 Oe in the as-deposited state. A gradual decrease of this hysteresis has been observed with annealing. An increase in the saturation field is also observed with annealing. Short-term annealing increases the magnitude of the magnetoresistance ratio; prolonged annealing causes a decrease. A field sensitivity of about 0.3% per Oe has been observed and dc magnetoresistance minor loops exhibit no measurable decrease in sensitivity.
Phase-locked oscillator optimization for arrays of Josephson junctions
Authors: Wiesenfeld K. , Benz S. P. , Booi P. A. A.
Published: July 2002

An overview of phase locking in two-dimensional (2D) arrays of identical Josephson junctions is presented. General design criteria are discussed for optimization of power and linewidth. A harmonic balance technique is used to derive an analytic expression for the fundamental power as a function of bias voltage for a single shunted tunnel junction with an external shunt resistor having parasitic inductance. A linear stability analysis is performed on the in-phase state of 2D arrays in the absence of any external load. Most excitation modes in the 2D array are damped, leading to stable phase locking between parallel junctions within each row; however, within the theoretical model, no mechanisms intrinsic to the array were found to induce phase locking between rows of junctions. The results of these calculations and their impact on and relevance to the design of phase-locked Josephson oscillators are discussed.
Activated reactive laser deposition of GeO2 films
Authors: Witanachchi Sarath , Wolf Paul J.
Published: July 2002

Amorphous GeO2 optical thin films were grown in an oxygen ambient on heated Si substrates using the technique of pulsed laser deposition. The application of a partially ionized oxygen plasma generated by passing the plume through a ring electrode facilitated stoichiometric film growth in low O2 partial pressures. Emission spectroscopy of the plume revealed an enhancement in the ionic and neutral excited Ge species. The concentration of excited neutral and ionic oxygen atoms also significantly increased when the ring electrode was activated at P(O2)≳10 mTorr. Coupling the results of the film property measurements with the emission studies suggested that the presence of O atoms near the substrate surface during film growth was more critical in promoting oxidation than the gas phase process in the plume. The low-pressure conditions that were utilized to deposit stoichiometric film growth identified the appropriate conditions to produce uniform films over a large area that may be suitable for waveguide fabrication.
Magnetic moment of α″-Fe16N2 films (invited)
Authors: Takahashi Migaku , Shoji H. , Takahashi H. , Nashi H. , Wakiyama T. , Doi M. , Matsui M.
Published: July 2002

In order to determine the value of the intrinsic magnetic moment of the α ″ phase, the films of nitrogen-martensite with various N content were fabricated under various reactive sputtering conditions. The magnetic moment of (α ″+α′)-Fe16N2 films is discussed in connection with the change of the unit-cell volume of the bct structure and the degree of N site ordering in nitrogen-martensite. As a result, it is found that (1) the same structure as bulk α ″-Fe16N2 is realized in the present films, (2) the saturation magnetization σs of the α′ phase increases about 4% with increasing unit-cell volume of the α′ phase, (3) the degree of N site ordering from α′ to α ″-Fe16N2 does not much affect σs, and (4) the experimentally obtained maximum value of σs for the (α ″+α′)-Fe16N2 film was 232 emu/g. The intrinsic value of σs in the α ″ phase (in the perfectly ordered state) is proposed to be no more than 240 emu/g at 300 K.
Experimental investigation of thermal conduction normal to diamond-silicon boundaries
Authors: Goodson K. E. , Käding O. W. , Rösler M. , Zachai R.
Published: July 2002

Passive chemical-vapor-deposited diamond layers have the potential to improve thermal conduction in electronic microstructures because of their high thermal conductivities. The thermal resistances for conduction normal to the boundaries of diamond layers, which must be small in order to realize this potential, have not been measured. This research develops two independent experimental methods that measure the total thermal resistance for conduction normal to diamond layers thinner than 5 μm on silicon substrates, yielding an upper bound for the thermal resistance of the diamond-silicon boundary. The data for layers as thin as 0.2 μm agree with predictions that account for phonon scattering on layer boundaries and couple the local scattering rate in the diamond to the grain size. The agreement lends support to the conclusion that the effective diamond-silicon boundary resistance is dominated by a highly localized volume resistance in the diamond near the interface.
Changes in the densities of dislocations on distinct slip systems during stress relaxation in thin aluminium layers: The interpretation of x-ray diffraction line broadening and line shift
Authors: Vermeulen A. C. , Delhez R. , de Keijser Th. H. , Mittemeijer E. J.
Published: July 2002

Stress relaxation at room temperature in polycrystalline aluminium layers, deposited onto silicon wafers, was explained by processes in which changes in the dislocation structure play a dominant role. Applying x-ray diffraction, information was obtained simultaneously about the macrostress (from line position) and the dislocation structure (from line broadening) without destroying the specimen and without disturbing the stress relaxation process. A method has been developed to determine the dislocation configuration from the direction-dependent line broadening. The method is based on an analytical expression for the integral breadth due to microstrain from sets of straight and parallel edge and/or screw dislocations on the specific slip systems. Analysis of the x-ray-diffraction measurements shows unequal densities and unequal changes of densities of dislocations with the Burgers vector parallel and with the Burgers vector inclined with respect to the surface of the layer. The stress relaxation at room temperature is primarily achieved by glide of dislocations with inclined Burgers vectors. The time behavior of the macrostress appears to be correlated with the change of the dislocation density. A model was developed to describe the dependency of the decrease of macrostress on the decrease of the dislocation density.
An isotropic two band model for hot electron transport in silicon: Including electron emission probability into SiO2
Authors: Jin Gyoyoung , Dutton Robert W. , Park Young-June , Min Hong-Shick
Published: July 2002

An isotropic two band model is proposed for electrons in silicon, that has the same density of states and magnitude of group velocity as those of the full band structure based on empirical pseudopotential method calculations. The band model and transport parameters are calibrated through extensive comparisons with Monte Carlo simulation results and various experiments related to electron transport in silicon. Specifically, the drift velocity, impact ionization coefficient, quantum yield, 2p core level line intensity and broadening from x-ray photoemission spectroscopy have all been used in calibration. Through the study of electron emission at the Si/SiO2 interface in metal-oxide-semiconductor structures using the Monte Carlo method, it is demonstrated that the model has good accuracy in modeling high field transport phenomena in silicon.
Passive Q-switch at 1.53 μm using divalent uranium ions in calcium fluoride
Authors: Stultz Robert D. , Camargo Marly B. , Birnbaum Milton
Published: July 2002

Using a U2+:CaF2 passive Q-switch for the Er:glass laser, 20 ns, 3 mJ pulses were obtained. This is the shortest duration passive Q-switched pulse obtained for this laser without intracavity focusing and without optical damage. The measured U2+:CaF2 absorption cross section and relaxation lifetime are compared with those of U2+:SrF2 and U2+:BaF2.
Reduced intergranular magnetic coupling in Pd/Co multilayers
Authors: Liu W. H. , Fleming S. , Lairson B. M.
Published: July 2002

The ideal magnetic switching mechanism for many types of data storage, including hard disk recording, is isolated domain coherent rotation. However, in typical Pd/Co multilayers with high coercivity, the dominant switching mechanism is domain wall motion, which causes noise in the readback signal. We have investigated the addition of elements, such as carbon, into Pd/Co multilayers to reduce the coupling between adjacent magnetic domains. This reduction of magnetic coupling reduces the length scale over which incoherent switching occurs. Kelly–Hankel plots and the switching radius ratio have been used to compare interactions and domain sizes of oxidized and carbon segregated Pd/Co multilayers. Both oxidation and carbon segregation reduce the coupling between domains, but the multilayers nevertheless remain more strongly coupled than perpendicular alloy films.
Undercooling of bulk liquid silicon in an oxide flux
Authors: Shao Y. , Spaepen F.
Published: July 2002

Drops of molten silicon surrounded by a SiO2–BaO–CaO flux were undercooled at 350 K below their melting temperature. This undercooling is 75 K greater than the largest one reported so far for bulk silicon. To account for this result as well as the nucleation data from laser-melted thin films, classical nucleation theory requires a crystal-melt interfacial tension with a positive temperature coefficient.
Electron inelastic mean free path and stopping power modelling in alkali halides in the 50 eV–10 keV energy range
Authors: Boutboul T. , Akkerman A. , Breskin A. , Chechik R.
Published: July 2002

A model for calculating the electron inelastic mean free path and stopping power in insulators in the 50 eV–10 keV energy range is presented. Both valence and core electron contributions have been considered. The valence part has been estimated following the dielectric theory modified to include the energy gap; the core contribution has been evaluated on the basis of the classical binary encounter theory. Inelastic mean free path and stopping power calculations based on this model have been performed for several alkali halides: LiF, NaCl, KCl and CsI. They are compared to existing experimental data and Penn model’s predictions for the mean free path and to Bethe’s values for the stopping power; a fair agreement is found for incident electron energies higher than 100-200 eV.
Transmission electron microscopy of Terfenol-D crystals
Authors: Holden A. P. , Lord D. G. , Grundy P. J.
Published: July 2002

Magnetic domain and microstructure observations are presented from samples of pseudo-single-crystal Terfenol-D examined by transmission electron microscopy (TEM). This ternary alloy is of significant technological interest since it exhibits the highest known magnetostriction to anisotropy ratio near room temperature. Specimens for TEM studies in (110), (111), and (112) orientations have also shown regions of unusual diffraction contrast in bright field which appears to be very sensitive to specimen tilt. Lorentz mode TEM has subsequently shown such regions to correspond exactly with magnetic domains. This contrast is attributed to the high magnetostrictive strain causing a local distortion of the lattice and thus a local deviation from the Bragg condition. This conclusion has been investigated and supported by TEM observations with the samples cooled below the spin reorientation temperature. When this transition is reached the diffraction contrast in bright field is considerably decreased and cannot be made to vary by tilting the specimen. The latter experiments also indicate that the change from 〈111〉 to 〈100〉 easy axis is not a well-defined one but, rather, that the spin reorientation is a sluggish change. High-resolution lattice images show the coherency of the twin boundaries.
Spectroscopic study of cobalt-related optical centers in synthetic diamond
Authors: Lawson Simon C. , Kanda Hisao , Watanabe Kenji , Kiflawi Isaac , Sato Yoichiro , Collins Alan T.
Published: July 2002

This article presents evidence that cobalt forms a series of optically active defect centers in diamond grown by high-temperature, high-pressure synthesis. Photoluminescence (PL) studies reveal that the newly observed vibronic systems with zero-phonon energies at 1.989, 2.135, 2.207, 2.277, 2.367, and 2.590 eV appear only in samples grown using a cobalt-containing solvent–catalyst. Results of an annealing study, carried out in the temperature range 1500 to 1800 °C, establish that many of the new bands appear during the temperature regime of nitrogen aggregation. It is therefore proposed that nitrogen forms complexes with cobalt to produce optically active centers, in a manner analogous to that of nickel point defects in diamond. Detailed radiative decay time measurements and temperature dependence measurements show that all but one of the bands which are here associated with nitrogen–cobalt complexes have long radiative decay times (∼100 μs), and this again is a characteristic of the PL centers arising from nickel–nitrogen complexes. All of the vibronic bands observed by PL may also be produced by electron-beam excitation (cathodoluminescence). In this case it is necessary to use a low beam current density (≤10 mA cm−2), otherwise the spectra are dominated by emission from optical centers with much shorter decay times (∼20 ns). Only one vibronic band, with a zero-phonon line at 1.852 eV, has been detected in absorption measurements, and the center responsible for this system does not give rise to luminescence.
Investigation of bias enhanced nucleation of diamond on silicon
Authors: Gerber J. , Sattel S. , Ehrhardt H. , Robertson J. , Wurzinger P. , Pongratz P.
Published: July 2002

The process of bias enhanced nucleation of microwave chemical vapor deposited diamond on silicon has been extensively characterized using plasma diagnostics, scanning and transmission electron microscopy (TEM), Raman spectroscopy, and x-ray diffraction. The nucleation kinetics were measured as a function of bias voltage, methane partial pressure, and substrate temperature. The nucleation is found to be transient in character, with a delay time followed by an exponential increase in nucleation density with time, and finally a saturation. The ion flux and ion energy distribution was measured by a retarding field probe. The nucleation density was found to reach a maximum at a bias at which the ion energy distribution has a maximum of 80 eV, independent of the substrate temperature. This is taken as strong evidence that nucleation enhancement involves ion subplantation. The Raman spectra and x-ray diffraction suggests that the films during nucleation consist primarily of sp2 bonded noncrystalline carbon. The presence of the (0002) interlayer graphitic peak suggests that the carbon is primarily graphitic. The diamond nuclei form in this matrix. TEM shows mainly amorphous hillocks being formed on the substrates by bias enhanced nucleation. Diffraction patterns and high resolution TEM reveal the presence of β-SiC and also a small number of diamond particles.
Parameter space representation of liquid crystal display operating modes
Authors: Kwok H. S.
Published: July 2002

A parameter space approach is used to represent the operation of all liquid crystal displays. The Jones matrix nematic is used to analyze the static optical properties of liquid crystal displays with any twist angle, cell thickness, natural birefringence, and polarizer angles. All the usual display modes such as 90° TN, ECB, OMI, SBE, and STN can be visualized on a simple parameter space diagram. Besides its pedagogical values, this formulation has the advantage of simplicity in showing the physics and operating conditions of supertwist as well as low twist liquid crystal displays. It can also be used to analyze novel situations such as reflective displays which do not require any back polarizer.
A refined scheme for the reduction of threading dislocation densities in InxGa1−xAs/GaAs epitaxial layers
Authors: MacPherson G. , Goodhew P. J.
Published: July 2002

A scheme is presented for the reduction of threading dislocation densities in InxGa1−xAs/GaAs epitaxial single layers by accurate control of layer thickness. The model developed differs from previous models since the InxGa1−xAs growth is restricted to less than ten times the Matthews and Blakeslee critical thickness (hc) where the asymmetry in the [110] and [1̄10] dislocation densities is the greatest. Beyond this thickness it is shown that the removal or annihilation of threading dislocations in the epilayer is more than offset by the introduction of new threading dislocations from spiral and Frank–Read-type sources. Maintaining epilayer thickness below this thickness ensures that the majority of misfit dislocations generated lie predominantly in only one of the 〈110〉 directions, reducing the likelihood of blocking with orthogonal dislocations, thereby increasing the mean free path from that expected in higher density dislocation arrays. Etch pit densities show that the threading dislocation density can be reduced by up to a factor of 10 below that found in the substrate, with the added benefit of reducing the inhomogenities in the distribution of threading dislocations. Atomic force microscopy shows that the surface quality of these layers remains high with an absence of striations.
High temperature behavior of Pt and Pd on GaN
Authors: Duxstad K. J. , Haller E. E. , Yu K. M.
Published: July 2002

We report on the thermal stability of Pt and Pd thin films on GaN up to 800 °C. We have found that for Pt thin films submicron size metal spheres form at temperatures as low as 600 °C and the film turns into discontinuous islands at 725 °C and above. Pd begins to form islands above 700 °C and areas of the film begin to delaminate due to thermally generated compressive stress. The general behavior of these films is that found for most metal films on ceramics, suggesting that metals on GaN do not react as they typically do on common elemental and compound semiconductors. This ceramic-like behavior must be considered when determining which metals may act as good electrical contacts for GaN, especially for high temperature applications or alloyed contacts.
Magnetic, structural, and spin dynamical properties of La1−xCaxMnO3
Authors: Lynn J. W. , Erwin R. W. , Borchers J. A. , Santoro A. , Huang Q. , Peng J.-L. , Greene R. L.
Published: July 2002

Neutron scattering has been used to study the magnetic order, spin dynamics, and structural properties of La1−xCaxMnO3 in the ferromagnetic regime (0<x<12). For x=13(TC=250 K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (<0.04 meV) dispersion relation E=Dq2 and DT=0≈155 meV Å2. However, an anomalous diffusive component develops above ∼200 K and dominates the fluctuation spectrum as T→TC. A magnetic field strongly reduces this quasielastic scattering, shifting the spectral weight into the spin wave component of the fluctuation spectrum.
Critical temperature and ion flux dependence of amorphization in GaAs
Authors: Brown R. A. , Williams J. S.
Published: July 2002

The formation of amorphous layers in GaAs during ion bombardment at elevated temperatures, where dynamic annealing of radiation-induced defects is substantial, is shown to be extremely sensitive to the implantation temperature. For example, we have found that a temperature change of only 6 °C can change the residual damage from small clusters barely visible by conventional transmission electron microscopy and Rutherford backscattering to a thick amorphous layer. The temperature at which this occurs is strongly dependent upon the ion flux.
Complementary application of electron microscopy and micro-Raman spectroscopy for microstructure, stress, and bonding defect investigation of heteroepitaxial chemical vapor deposited diamond films
Authors: Michler J. , von Kaenel Y. , Stiegler J. , Blank E.
Published: July 2002

The evolution and interdependence of microstructure, stress, and bonding defects of heteroepitaxial diamond films deposited on silicon substrates has been investigated by applying scanning electron microscopy, transmission electron microscopy (TEM), and micro-Raman spectroscopy to the same places in the films. For this purpose, TEM plane-view specimens were prepared and the same grains in the electron transparent areas were characterized by all three methods that allowed crystalline defects and their relation to spectral features of the Raman spectrum to be identified. To the authors’ knowledge, this is the first successful complementary application of these methods to diamond films. Concerning microstructure evolution, dislocations in the silicon substrate and a residual plastic deformation of the silicon wafer prove that plastic deformation of the silicon substrate had occurred with the presence of mechanical stress during deposition. Evolutionary selection of randomly oriented, highly defective diamond grains observed at a film thickness of 300 nm leads to a textured film at 4 μm (an intermediate state) consisting of truncated pyramids with defect-free {001} growth sectors, bounded by four {111} growth sectors which exhibit a high density of twins and stacking faults. During further growth, merging of {001} growth sectors begins and apart from the formation of low-angle grain boundaries, the formation of partial wedge disclinations takes place, partly accommodating the misorientation between grains by elastic deformation. The latter process is shown to be more favorable than the formation of low-angle grain boundaries below a certain misorientation. Merging of grains introduces a high number of dislocations and mechanical stress into the {001} growth sectors. The comparison of the Raman spectra with electron micrograph images shows that the G band of the Raman spectrum originates exclusively from grain boundaries having an associated {111} growth sector. Very localized luminescence sources have been detected, not correlating to microstructure elements. Stress inhomogeneities measured within single grains and an earlier observed transition of the biaxial stress state in the film plane to a more complicated stress state after grain merging is shown to originate from disclinations.
Binding energy of impurity states in spherical quantum dots with parabolic confinement
Authors: Bose Chayanika
Published: July 2002

The binding energy of shallow hydrogenic impurities in spherical quantum dots (QDs) with parabolic confinement is calculated, using a variational approach within the effective mass approximation. The binding energy is computed for GaAs QD as a function of the dot size for different impurity positions, and also as a function of the impurity position for different dot sizes. The results show that the impurity binding energy increases with the reduction in the dot dimension. The binding energy is also found to depend on the location of the impurity, and the same is the maximum for the on-center impurity.
Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas
Authors: Zhou D. , Gruen D. M. , Qin L. C. , McCauley T. G. , Krauss A. R.
Published: July 2002

The transition from microcrystalline to nanocrystalline diamond films grown from Ar/H2/CH4 microwave plasmas has been investigated. Both the cross-section and plan-view micrographs of scanning electron microscopy reveal that the surface morphology, the grain size, and the growth mechanism of the diamond films depend strongly on the ratio of Ar to H2 in the reactant gases. Microcrystalline grain size and columnar growth have been observed from films produced from Ar/H2/CH4 microwave discharges with low concentrations of Ar in the reactant gases. By contrast, the films grown from Ar/H2/CH4 microwave plasmas with a high concentration of Ar in the reactant gases consist of phase pure nanocrystalline diamond, which has been characterized by transmission electron microscopy, selected area electron diffraction, and electron energy loss spectroscopy. X-ray diffraction and Raman spectroscopy reveal that the width of the diffraction peaks and the Raman bands of the as-grown films depends on the ratio of Ar to H2 in the plasmas and are attributed to the transition from micron to nanometer size crystallites. It has been demonstrated that the microstructure of diamond films deposited from Ar/H2/CH4 plasmas can be controlled by varying the ratio of Ar to H2 in the reactant gas. The transition becomes pronounced at an Ar/H2 volume ratio of 4, and the microcrystalline diamond films are totally transformed to nanocrystalline diamond at an Ar/H2 volume ratio of 9. The transition in microstructure is presumably due to a change in growth mechanism from CH3⋅ in high hydrogen content to C2 as a growth species in low hydrogen content plasmas.
Annealing and recrystallization of amorphous silicon carbide produced by ion implantation
Authors: Höfgen A. , Heera V. , Eichhorn F. , Skorupa W.
Published: July 2002

The annealing behavior of amorphous SiC layers produced by MeV Si implantation into 6H–SiC has been investigated systematically by means of step height measurements, x-ray diffraction, and optical microscopy. Two annealing stages are found. Each of them causes a specific densification of the amorphous layer. At temperatures between 250 and 700 °C both the rapidity and the low activation energy (184 meV) of the densification suggest that defect annealing processes are responsible for densification. Partial crystallization and changes of the amorphous network structure can be excluded as a possible reason for low temperature densification. Annealing at temperatures above 700 °C is characterized by a combination of defect annealing and recrystallization. The crystallization kinetics is analyzed in terms of the Johnson–Mehl–Avrami theory. It is shown that the crystallization mode changes with increasing temperature from nucleated growth at 800 °C to epitaxial growth at 1000 °C. The recrystallization generates stress in the layer which leads to surface cracking if the layer exceeds a critical thickness.
A reciprocal theorem for effective conductivities of two-dimensional composites containing multiply coated cylinders of arbitrary cross section
Authors: Lu Shih-Yuan
Published: July 2002

A reciprocal theorem for the effective conductivities of two-dimensional (2D) rectangular arrays of multiply coated cylinders of arbitrary cross section is derived. The theorem is expressed as: σeffx(σ0,σ1,σ2,…,σn,1)×σeffy(1/σ0,1/σ1,1/σ2,…,1/σn,1)=1, where σeffx and σeffy are the reduced effective conductivities of the array in the x and y directions, respectively, and σ0 and σi (i=1,n) are the reduced conductivities of the cylinder and coating layer i, respectively, with the matrix assigned the reduced conductivity of one. This theorem is also valid for a class of 2D composites containing aligned, randomly distributed, multiply coated cylinders of arbitrary cross section, whose heat conduction behavior can be described by an equivalent inclusion model.
Photoacoustic study of the effect of hydroxyl ion on thermal diffusivity of γ alumina
Authors: Sankara Raman S. , Nampoori V. P. N. , Vallabhan C. P. G. , Ambadas G. , Sugunan S.
Published: July 2002

The effect of the chemisorbed hydroxyl groups on the thermal diffusivity of γ alumina is determined by evaluating the thermal diffusivity at various degassing temperatures and by doping it with rare earth oxide using photoacoustic technique. The thermal diffusivity is found to decrease with the increase in degassing temperature as well as with the increase in the doping concentration of rare earth oxide. This decrease has been attributed to the loss of hydroxyl ion from the γ-Al2O3.
Experimental study of the self-focusing process temporal behavior in photorefractive Bi12TiO20
Authors: Fressengeas N. , Wolfersberger D. , Maufoy J. , Kugel G.
Published: July 2002

A systematic experimental study of the photorefractive self-focusing process in Bi12TiO20 is conducted and its results are successfully compared to previous and new theoretical work.
Modeling the optical constants of hexagonal GaN, InN, and AlN
Authors: Djurišić Aleksandra B. , Li E. Herbert
Published: July 2002

Optical constants of hexagonal GaN (in the range 1.5–10 eV), InN (in the range 2–10 eV), and AlN (in the range 6–20 eV) for E⊥c are modeled using a modification of Adachi’s model of optical properties of semiconductors. Model parameters are determined using the acceptance-probability-controlled simulated annealing method. The employed model uses an adjustable broadening function instead of the conventional Lorentzian one. The broadening can vary over a range of functions with similar kernels but different wings. Therefore, excessive absorption inherent to Lorentzian broadening due to the large wings of a Lorentz function can be reduced, yielding better agreement with experimental data. As a result, excellent agreement with experimental data is obtained; the relative rms errors for the real part of the index of refraction are below 2% for all three materials, and, for the imaginary part, below 5% for GaN and below 3% for InN and AlN.
Step response characterization of nonlinear diffusion of electromagnetic fields
Authors: Mayergoyz Isaak D. , Serpico Claudio
Published: July 2002

It is demonstrated that, under some natural conditions, nonlinear diffusion of electromagnetic fields in a magnetically nonlinear conducting cylinder of arbitrary cross-section admits “step response characterization.” This means that if the flux through the cylinder is measured (or computed) for a step change in the magnetic field at the boundary, then these measured (or computed) data allows one to predict the flux for monotonically increasing (or decreasing) magnetic fields at the boundary.
Giant magnetoresistive sensors for rotational speed control
Authors: Freitas P. P. , Costa J. L. , Almeida N. , Melo L. V. , Silva F. , Bernardo J. , Santos C.
Published: July 2002

This article demonstrates spin valve (sv) sensor applications as rotational speed control devices, in a Wheatstone bridge configuration. The sensor architecture uses two active self-biased sv elements (dimensions 400×6 μm2) in a CoZrNb flux guide. The other two spin valves are inactivated for external fields inferior to 150 Oe. Inactivation is achieved by increasing the ferromagnetic coupling of the inactive spin valves above 200 Oe, by roughening the surface of the underlying oxide layer (12 Å average roughness). When excited by a magnetized wheel with 60 N-S poles rotating at 360 rpm, the sensor yields a 400 mV pp square wave output with rise/fall times corresponding to an angular resolution of 0.17°. Amplitude is independent of speed (0–3000 rpm), and of sensor to wheel separation (0.5⩽d(mm)⩽2.0). Bridge output decreases 26% when temperature is increased to 150 °C. Bridge offset of 350 mV is caused by the resistance differences between the active and inactive sv elements.
Deposition, microstructure and properties of sputtered copper films containing insoluble molybdenum
Authors: Chu J. P. , Lin T. N.
Published: July 2002

The microstructure and properties of Cu films containing insoluble Mo in as-deposited and annealed conditions have been studied. Using magnetron sputtering, Cu films with Mo concentrations up to 25 at. % have been deposited. The Cu-Mo films consist of nonequilibrium supersaturated solid solutions of Mo in Cu and have nanocrystalline microstructures. Upon heating, most of the films studied undergo three major transition events: recovery, crystallite growth and coalescence, and grain growth. Recovery occurs at ∼220–250 °C due to the release of strain energies stored during deposition, while the growth and coalescence of crystallites at ∼490 °C are likely driven by the crystallite-boundary reduction. Upon further annealing at above ∼670 °C, the microstructure alters considerably in most of the films, resulting in extensive growth of crystallites and grains. Rather high strain energies are thought to cause the precipitation of Cu particles in the molybdenum-rich Cu-Mo films (e.g., Cu-25 at. % Mo) during annealing at a temperature as low as 200 °C. The fine structure observed in the 800 °C-annealed Cu-25 at. % Mo film suggests that the extensive crystallite/grain growth is effectively impeded by the presence of molybdenum, signifying a good property of thermal stability. The film’s resistivity and hardness properties correlate well with the microstructure and are governed by the impurity effect of molybdenum. Low-molybdenum Cu-Mo films yield relatively low resistivity, and this is attributable to the improved film microstructure. Hardness results indicate that the strengthening of the films is mainly due to the fine structure and presence of molybdenum.
A comparison of projected thermoelectric and thermionic refrigerators
Authors: Nolas G. S. , Goldsmid H. J.
Published: July 2002

Expressions for the coefficient of performance and the cooling power of a thermionic refrigerator are derived taking into account the losses associated with thermal radiation and the need for a so-called passive conductor in the electrical circuit. Specific calculations are made for a thermionic device with a work function of 0.3 eV and it is shown that the performance is generally superior to that of a thermoelectric refrigerator made from materials with a dimensionless figure of merit ZT equal to 4.
Pyroelectric hysteresis loop at ferroelectric phase transition
Authors: Itskovsky M. A.
Published: July 2002

A theory of a nonlinear pyroelectric effect in a phase transition region of a ferroelectric thin film due to harmonic temperature changes with large amplitudes reaching tens of K has been developed. Polarization during the temperature cycle–pyroelectric hysteresis loop is obtained at various frequencies, amplitudes, thermal, and electric relaxation times. It is proved that ferroelectric film can be maintained in a monodomain state during the cycle. Applicability of a nonlinear pyroelectric cycle to solar energy conversion (ferroelectric solar cell) is discussed.
10 Gbit/in.2 longitudinal media on a glass substrate (invited)
Authors: Li Jinshan , Mirzamaani Mohammad , Bian Xiaoping , Doerner Mary , Duan Shanlin , Tang Kai , Toney Michael , Arnoldussen Tom , Madison Michael
Published: July 2002

This article reports on the properties of the media prepared on glass substrates which were used in IBM’s 10 Gbit/in.2 demonstration. In order to support a linear density of 315 kbpi and a track density of 33 ktpi, the remanant coercivity Hcr and remanant moment thickness product Mrt of the magnetic layer were 3450 Oe and 0.37 memu/cm2, respectively. The media used a NiAl seed layer, a CrV underlayer, a Co alloy magnetic layer, and a carbon overcoat protection layer. The magnetic film had a grain size of 12 nm as observed by transmission electron microscopy. The preferred orientation (PO) of the magnetic layer was (101̄0). This PO enables one to sustain high coercivities at low values of Mrt. It is observed that the c-axis in-plane texture of the magnetic layer is critical to achieve a low noise medium. Using a focused-ion-beam (FIB) trimmed giant magnetoresistance head and conventional partial response maximum likelihood channel, the on-track-error rates were measured at the 10−10 level.
Magnetic domains in thin films with perpendicular anisotropy: An extensive study
Authors: Samson Y. , Marty A. , Hoffmann R. , Gehanno V. , Gilles B.
Published: July 2002

Thin films of tetragonal ordered alloys grown by molecular beam epitaxy provide a unique template for the study of magnetic configurations in thin films with perpendicular anisotropy. These configurations have been investigated by magnetic force microscopy over a wide thickness range, for large and weak perpendicular anisotropies, in both equilibrium and out of equilibrium states. FePd samples ordered into the tetragonal L10 structure have been obtained by simultaneous evaporation of Fe and Pd on Pd(100). These samples exhibit large perpendicular anisotropy. Magnetic domains have been imaged as a function of increasing thickness from 1.4 to 100 nm. Both an exponential decrease of the domain size at low thickness and a slow increase at higher thickness have been observed, allowing quantitative confirmation of the theoretical evolution expected. Due to this exponential decrease of their equilibrium size, the domains appearing at the beginning of growth exhibit a large size, well above the equilibrium one when the layer thickness is thereafter increased. The growth of the layer then leads to highly out of equilibrium magnetic configurations, where the domain shape evolves through wall undulation at low thickness and thereafter through a fingering instability. This fingering instability preserves large parts of the initial domains, allowing a quantitative estimation of the strain to which they are submitted. Samples grown with a low chemical order exhibit weak perpendicular anisotropy. The magnetization then lies into the plane below a critical thickness. Above this thickness, the images confirm the square root dependence of the period of the stripe structure upon thickness. Stripes as small as 37 nm have been observed.
Limiting Si/SiO2 interface roughness resulting from thermal oxidation
Authors: Lai L. , Irene E. A.
Published: July 2002

The changes effected by oxidation on purposely roughened and initially smooth Si surfaces are followed via atomic force microscopy and spectroscopic ellipsometry and a technique called spectroscopic immersion ellipsometry. Initially, rough and smooth Si surfaces yield opposite roughening trends upon thermal oxidation. Rough surfaces become smoother, and smooth surfaces become rougher, ultimately yielding a limiting roughness of about 0.3 nm root-mean square. A consideration of the distribution of surface roughness features plus the thermodynamics of small features are used to explain these trends. It is also reported that the changes of interface roughness are primarily the result of the oxidation reaction and not from the high temperatures.
Femtosecond laser ablation of frozen acetone for deposition of diamond-like carbon films
Authors: Okoshi Masayuki , Higuchi Satoshi , Hanabusa Mitsugu
Published: July 2002

790 nm, 130 fs ultrashort laser pulses generated by a Ti: sapphire laser system were used to ablate frozen acetone and deposit diamond-like carbon (DLC) films on Si substrate. The film was identified as DLC by Raman spectra. The sp3 bonding fraction in the film was estimated to be 41% from an electron energy loss spectroscopy measurement. Black, conductive films were deposited from frozen acetylene target. Frozen methanol did not produce any deposit. Optical emission was measured to compare the plumes generated by different targets.
An investigation of reversible magnetization in NdFeB
Authors: Crew D. C. , McCormick P. G. , Street R.
Published: July 2002

Magnetization reversal processes in sintered and melt-quenched NdFeB materials have been investigated by measurements of reversible magnetization. For isotropic and anisotropic sintered NdFeB and anisotropic hot pressed die upset NdFeB (MQIII) reversible magnetization is the result of bowing of pinned domain boundary walls. In contrast in isotropic melt quenched NdFeB (MQI) reversible magnetization appears to be the result of the rotation of domain magnetization vectors and not domain wall bowing. The differences in the reversible magnetization processes of the four materials investigated are determined.