bluuebunny/crossref_metadata_2025 huggingface dataset:
Electroluminescent features of oligothiophenes dispersed as a dopant in host matrices
Authors: Suzuki Mutsumi , Fukuyama Masao , Hori Yoshikazu , Hotta Shu
Published: July 2002

The electroluminescent (EL) features of oligothiophenes dispersed as a dopant in the host matrices comprising tris(8-hydroxyquinoline)aluminum have been investigated. We chose the oligothiophenes that are substituted with phenyl or methyl groups at both the molecular terminals and possess various degrees of polymerization. Regarding both the phenyl- and methyl-substituted materials, the EL spectra are progressively red-shifted with the increasing number of thiophenes. Comparing these spectra with the photoluminescent spectra, we have found out that the EL arises mostly from the dopant molecules of the oligothiophenes. The emission is dominated by energy transfer from host matrices to the dopant molecules, leading to the enhanced device efficiencies. The specific effects of the phenyl- or methyl-substitution and the extension of the π-delocalization in the molecules are also discussed.
Interband transition distributions in the optical spectra of InAs/GaAs self-assembled quantum dots
Authors: Sheng Weidong , Leburton Jean-Pierre
Published: July 2002

The interband optical spectra of InAs/GaAs self-assembled quantum dots (SAD) are investigated with a three-dimensional eight-band k⋅p technique involving strain and piezoelectric effect. We show that the separation between hole states contributes to a significant fraction of the interband transition energy, thereby invalidating the two-dimensional harmonic oscillator model for the electronic structures of SADs. Moreover, aside from the threshold low energy peak which results from the strong ground state electron–hole transition, the major photoluminescence peaks observed experimentally are made of a significant number of equal-strength optical transitions.
High harmonic generation in relativistic laser–plasma interaction
Authors: Banerjee S. , Valenzuela A. R. , Shah R. C. , Maksimchuk A. , Umstadter D.
Published: July 2002

High harmonics generated due to the scattering of relativistic electrons from high intensity laser light is studied. The experiments are carried out with an Nd:Glass laser system with a peak intensity of 2×1018 W cm−2 in underdense plasma. It is shown that, at high intensities, when the normalized electric field approaches unity, in addition to the conventional atomic harmonics from bound electrons there is significant contribution to the harmonic spectrum from free electrons. The characteristic signatures of this are found to be the emission of even order harmonics, linear dependence on the electron density, significant amount of harmonics even with circular polarization and a much smaller spatial region over which these harmonics are produced as compared to the atomic case. Imaging of the harmonic beam shows that it is emitted in a narrow cone with a divergence of 2 to 3 degrees.
Effects of nanotube waviness on the modulus of nanotube-reinforced polymers
Authors: Fisher F. T. , Bradshaw R. D. , Brinson L. C.
Published: July 2002

Recent experimental results demonstrate that substantial improvements in the mechanical behavior of polymers can be obtained using very small amounts of carbon nanotubes as a reinforcing phase. Here, a method is developed to incorporate the typically observed curvature of the embedded nanotubes into traditional micromechanical methods for determination of the effective modulus of the nanotube-reinforced polymer. Using a combined finite element and micromechanical approach, it was determined that the nanotube curvature significantly reduces the effective reinforcement when compared to straight nanotubes. This model suggests that nanotube waviness may be an additional mechanism limiting the modulus enhancement of nanotube-reinforced polymers.
On the equality of Hausdorff and box counting dimensions
Authors: Mainieri Ronnie
Published: July 2002

By viewing the covers of a fractal as a statistical mechanical system, the exact capacity of a multifractal is computed. The procedure can be extended to any multifractal described by a scaling function to show why the capacity and Hausdorff dimension are expected to be equal.
Maintenance of chaos in a computational model of a thermal pulse combustor
Authors: In Visarath , Spano Mark L. , Neff Joseph D. , Ditto William L. , Daw C. Stuart , Edwards K. Dean , Nguyen Ke
Published: July 2002

The dynamics of a thermal pulse combustor model are examined. It is found that, as a parameter related to the fuel flow rate is varied, the combustor will undergo a transition from periodic pulsing to chaotic pulsing to a chaotic transient leading to flameout. Results from the numerical model are compared to those obtained from a laboratory-scale thermal pulse combustor. Finally the technique of maintenance (or anticontrol) of chaos is successfully applied to the model, with the result that the operation of the combustor can be continued well into the flameout regime.
On some dynamical diagrams of chemical reaction engineering
Authors: Aris Rutherford
Published: July 2002

A brief historical survey of some of the influential types of diagrams that have been used in chemical reaction engineering is given. These include the phase plane, the simple autocatalytic diagram, and the stroboscopic phase plane.
Pattern selection during electropolishing due to double-layer effects
Authors: Yuzhakov Vadim V. , Takhistov Pavlo V. , Miller Albert E. , Chang Hsueh-Chia
Published: July 2002

We extend our earlier study of nanoscale pattern formation during electropolishing [Nanotechnology 7, 360 (1996); Phys. Rev. B 56, 12 608 (1997)]. The patterns are attributed to preferential adsorption of organic molecules on the convex portion of the electrode due to its enhanced electric field. This local enhancement occurs because of the effect of surface curvature on the double-layer potential drop. By allowing for transport correction to the double-layer potential drop at thermodynamic equilibrium, we estimate this anodic overpotential to be in the realistic mV range and hence verify the Debye–Hückel approximation used in our model. This small anodic overpotential suggests that pattern formation is a generic electropolishing phenomenon whose only requirement is that the polarizability of the organic additive relative to water must lie within a range specified by our theory. We verify this prediction experimentally with a variety of electrolyte solutions. The voltage ranges for specific hexagonal and ridge patterns are well correlated by our model with only a single parameter.
Four-tap shift-register-sequence random-number generators
Authors: Ziff Robert M.
Published: July 2002

Correlations in the generalized feedback shift-register random-number generator are shown to be greatly reduced when the number of feedback taps is increased from two to four (or more) and the tap offsets are made large. Simple formulas for producing maximal-cycle four-tap rules from available primitive trinomials are given, and explicit three- and four-point correlations are found for some of those rules. Several generators are also tested using a simple but sensitive random-walk simulation that relates to a problem in percolation theory. While virtually all two-tap generators fail this test, four-tap generators with offsets greater than about 500 pass it, have passed tests carried out by others, and appear to be good multipurpose high-quality random-number generators. © 1998 American Institute of Physics.
First-order Raman scattering in MgO microcrystals
Authors: Ishikawa K. , Fujima N. , Komura H.
Published: July 2002

Raman scattering is observed in MgO microcrystals with three different particle sizes (105, 103, and 102 Å). The second-order Raman lines in the bulk crystals of MgO disappear in all the microcrystals, while new peaks emerge at 280, 446, and 1088 cm−1 except for specimen prepared by grinding a large single crystal. Raman lines with lower frequencies, which are enhanced with decrease of the particle size, are discussed as due to the first-order Raman scattering forbidden in the bulk crystals. The assignment is confirmed in terms of photon density of states in microcrystals. Another line with the highest frequency is coincident with a previously reported Raman line whose origin has been attributed to a surface phonon mode.
Hysteresis models from the mathematical and control theory points of view
Authors: Mayergoyz I. D.
Published: July 2002

A new approach to Preisach’s model which emphasizes the phenomenological nature of this model and its mathematical generality is discussed. This approach enables the formulation of necessary and sufficient conditions under which actual hysteresis nonlinearities can be represented by Preisach’s model.
Molecular beam epitaxial growth of GaAs on Si(211)
Authors: Uppal Parvez N. , Kroemer Herbert
Published: July 2002

The molecular beam epitaxial growth of GaAs on Si(211) has been investigated. Theoretical considerations had suggested the (211) orientation to be particularly suitable for the nucleation and growth of a zincblende-type compound semiconductor on a diamond-type elemental one. The experimental results support the theoretical prediction. Morphologies of thin (≤0.1 μm) (211) layers are substantially better than for (100) layers, which nucleate poorly and require large layer thicknesses (≂1μm) to yield good morphologies. When the (211) layer growth is initiated with a thin (GaAs/Al, Ga)As superlattice buffer (0.1 μm), consisting of 10 periods of 5+5 nm, the (211) morphology rivals that of GaAs(100) homoepitaxial growth. Chemical etching studies as well as transmission electron microscope investigations show the layers to have the (211)B orientation and to be free of antiphase domains, both as predicted. The (211) layers show strong photoluminescence at 4 K. Not intentionally doped layers are n type, with electron concentrations on the order of 2×1016 cm−3, except in the immediate vicinity of the Si interface. Some of the conduction parallel to the layer plane appears to take place on the Si side of the interface, where a two-dimensional electron gas seems to be present. The n-GaAs/p-Si heterojunctions themselves show excellent rectification characteristics and photosensitivity. It is concluded that the (211) orientation is almost certainly preferable to the (100) orientation for the growth of GaAs on Si.
Interferometric studies of the pressure of a confined laser-heated plasma
Authors: Griffin R. D. , Justus B. L. , Campillo A. J. , Goldberg L. S.
Published: July 2002

We have determined the temporal history of the pressure of a confined low-temperature plasma generated under conditions of modest laser intensity (0.05–1 GW/cm2) by utilizing the target as a thin witness plate to interferometrically measure the particle velocity at its rear surface. The observed data are in agreement with a simple ideal gas model describing the behavior of the plasma. A 150-ns duration laser-driver pulse can deliver a multikilobar pressure ramp to a condensed phase sample for dynamic high-pressure studies.
Photoluminescence from GaAs quantum wells under high electric fields
Authors: Pan Janet L. , Höpfel Ralph A. , Shah Jagdeep
Published: July 2002

The room-temperature photoluminescence spectra of n-modulation doped GaAs–AlGaAs multiple quantum wells have been studied at high parallel electric fields in order to obtain the distribution function of the two-dimensional electron system under high field conditions. The spectra show thermalized energy distributions with electron temperatures of ∼600 K in the range where current instabilities indicate negative differential conductivity. This temperature is well below the value expected for bulk GaAs, and can be understood on the basis of real-space transfer of hot electrons into AlGaAs.
Transport number measurements during plasma anodization of Si, GaAs, and ZrSi2
Authors: Perrière J. , Siejka J. , Rémili N. , Laurent A. , Straboni A. , Vuillermoz B.
Published: July 2002

In contrast to the thermal oxidation of Si where molecular oxygen is the sole moving species, the plasma anodization involves the movements of anions (oxygen or its compounds) or cations (Si or its compounds) or both. A similar situation is observed during the plasma anodization of refractory metal silicides (ZrSi2) or even GaAs. To get some insights on the mechanisms of plasma anodization, the transport number of cations t+ in oxides grown by anodization in oxygen plasma has been investigated using Xe atom markers. Rutherford backscattering techniques were used to determine the change in Xe marker position related to the change in oxide thickness. Results showed that t+≂0 for SiO2 growth indicating oxide formation solely by oxygen ion movement while for GaAs oxide, the t+ value obtained (t+≠0) evidences cation and oxygen anion migration during the growth. For ZrSi2 anodization, the position of the Xe markers remains unchanged during oxide formation yielding t+≂0, while the analysis of the backscattering spectra shows a Zr oxide enrichment in the near surface region of the oxide mixture. This suggests a relative movement of Zr cations with respect to Si cations. The implications of these findings on the microscopic transport mechanisms of ions under high electric field are presented.
Elastic relationships in layered composite media with approximation for the case of thin films on a thick substrate
Authors: Townsend P. H. , Barnett D. M. , Brunner T. A.
Published: July 2002

A general theory is presented for the elastic interactions in a composite plate of layers with different relaxed planar dimensions. Solutions are obtained for the case of equivalent elastic properties and for the case of different elastic properties among the layers. Approximations for the case of thin films on a substrate lead to the governing equations for stresses in multilayered thin-film systems. Finally, the experimental determination of the planar substrate strain, which is first order in the film/substrate thickness ratio, is presented as a rigorous test of the correctness of the theory.
Nanosecond-pulse breakdown of gas-insulated gaps
Authors: Kawada Yukimitsu , Shamoto Sumikazu , Hosokawa Tatsuzo
Published: July 2002

Breakdown characteristics (50% probability of flashover voltage versus pressure-length characteristics) by impulse voltages with half-peak widths, 30 and 74 ns, respectively, were investigated at uniform field gaps in the atmospheric gases such as air, N2, and O2. Our investigation shows that under impulse voltages with short durations such as 30 and 74 ns, which seem to have an influence on streamer formation, Paschen’s law is valid at gap lengths l=1 and 2 cm, and streamer criteria are satisfied within an electron developing distance, 0.048–0.18 cm, from the cathode.
Diluted magnetic semiconductors
Authors: Furdyna J. K.
Published: July 2002

We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half-filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++-Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susceptibility of DMS in the paramagnetic range; we describe a somewhat empirical picture of the spin-glasslike freezing in the AII1−xMnxBVI alloys, and its relationship to the short range antiferromagnetic order revealed by neutron scattering; and we point out some not yet fully understood questions concerning spin dynamics in

DMS revealed by electron paramagnetic resonance. We then discuss the sp-d exchange interaction between the sp band electrons of the AII1−xMnxBVI alloy and the 3d5 electrons associated with the Mn atoms. Here we present a general formulation of the exchange problem, followed by the most representative examples of its physical consequences, such as the giant Faraday rotation, the magnetic-field-induced metal-to-insulator transition in DMS, and the properties of the bound magnetic polaron. Next, we give considerable attention to the extremely exciting physics of quantum wells and superlattices involving DMS. We begin by describing the properties of the two-dimensional gas existing at a DMS interface. We then briefly describe the current status of the AII1−xMnxBVI layers and superlattices (systems already successfully grown; methods of preparation; and basic nonmagnetic properties of the layered structures). We then describe new features observed in the magnetic behavior of the quasi-two-dimensional ultrathin DMS layers; and we discuss the exciting possibilities which the sp-d exchange interaction offers in the quantum-well situation. Finally, we list a number of topics which involve DMS but which have not been explicitly covered in this review such as elastic properties of DMS, DMS-based devices, and the emerging work on diluted magnetic semiconductors other than the AII1−xMnxBVI alloys—and we provide relevant literature references to these omitted topics.
Processing effects on the tribological characteristics of reactively sputtered chromium oxide (Cr2O3) overcoat films
Authors: Kao A. S. , Doerner M. F. , Novotny V. J.
Published: July 2002

The mechanical and tribological characteristics of thin chromium oxide films are investigated by controlling the process parameters in reactive deposition and subsequent annealing. From this, the correlation between the deposition process, film structure, and properties is established. Substrate heating and oxygen partial pressure are found to be the critical parameters that alter the oxygen concentration and crystallization of the films which, in turn, affect their stress, hardness, and wear resistance. The chromium oxide film deposited at 150 °C in pure argon and annealed at 300 °C shows a hardness of 25 GPa, which is near the bulk hardness of Cr2O3, and exhibits a good wear resistance with adhesive wear being the dominant wear mechanism. Reactive deposition at 25 °C or with excessive oxygen leads to films with nonstoichiometric composition, which hinders their crystallization upon low-temperature annealing. As a result, the hardness is reduced, and the wear resistance deteriorates several orders of magnitude with a concomitant transition of the wear mechanism from adhesive into an abrasive regime.
Influence of growth striations on para-ferroelectric phase transitions: Mechanism of the formation of periodic laminar domains in LiNbO3 and LiTaO3
Authors: Chen Jun , Zhou Qun , Hong Jin-feng , Wang Wen-shang , Ming Nai-ben , Feng Duan , Fang Chen-gao
Published: July 2002

The concentration distributions of Y dopings in LiTaO3 across the growth striations in as-grown crystals are obtained by means of x-ray energy dispersive spectrum analysis in a scanning transmission electron microscope at room temperature. The results show that it is the concentration gradient of the dopings which determines the configurations of ferroelectric domains. The mechanism for the formation of the periodic laminar ferroelectric domains (PLFDs) is explained theoretically by considering the influence of the distributions of the dopings on the para-ferroelectric phase transitions in LiTaO3 and LiNbO3 crystals. It is shown that the coupling between the order parameter and the conjugate external field, which results from the ambipolar diffusion or from the strain-induced polarization mechanism associated with the dopings and their compensated point defects, is just the physical reason for the formation of such PLFDs in these two materials.
Theoretical models of the electrical discharge machining process. I. A simple cathode erosion model
Authors: DiBitonto Daryl D. , Eubank Philip T. , Patel Mukund R. , Barrufet Maria A.
Published: July 2002

A simple cathode erosion model for the electrical discharge machining (EDM) process is presented. This point heat-source model differs from previous conduction models in that it accepts power rather than temperature as the boundary condition at the plasma/cathode interface. Optimum pulse times are predicted to within an average of 16% over a two-decade range after the model is tuned to a single experimental point. A constant fraction of the total power supplied to the gap is transferred to the cathode over a wide range of currents. A universal, dimensionless model is then presented which identifies the key parameters of optimum pulse time factor (g) and erodibility (j) in terms of the thermophysical properties of the cathode material. Compton’s original energy balance for gas discharges is amended for EDM conditions. Here it is believed that the high density of the liquid dielectric causes plasmas of higher energy intensity and pressure than those for gas discharges. These differences of macroscopic dielectric properties affect the microscopic mechanisms for energy transfer at the cathode. In the very short time frames of EDM, our amended model uses the photoelectric effect rather than positive-ion bombardment as the dominant source of energy supplied to the cathode surface.
Room-temperature reaction of a Ni/Bi2Sr2CaCu2O8 interface
Authors: Kumar P. S. Asoka , Mahumuni Shailaja , Kulkarni Pramada , Mulla I. S. , Chandrachood M. , Sinha A. P. B. , Nigavekar A. S. , Kulkarni S. K.
Published: July 2002

A Ni/Bi2Sr2CaCu2O8 reactive interface has been investigated by x-ray and ultraviolet photoelectron spectroscopy. Nickel reacts with the substrate disrupting BiO bonds along with CuO bonds. This behavior has been explained on the basis of thermodynamics. A complex NiBiSrCaCuO phase formed at the interface, sets up the diffusion barrier for oxygen withdrawal from the substrate at a thickness of ∼10 Å of nickel.
10-nm resolution by magnetic force microscopy on FeNdB
Authors: Grütter P. , Jung Th. , Heinzelmann H. , Wadas A. , Meyer E. , Hidber H.-R. , Güntherodt H.-J.
Published: July 2002

Magnetic force microscopy (MFM) images of rapidly quenched FeNdB are presented. The magnetic domain structure as observed by MFM consists of elongated, polyhedral-shaped domains imaged by measuring forces smaller than 10−9 N at distances ranging from 20 to more than 200 nm. The domain transition regions, which are quite sharp and well defined, often show a double-peaked structure with a peak-to-peak distance of 10 nm. At force sensor tip-to-sample separations larger than 90 nm only a single peak is observable. It has to be assumed that the relevant effective magnetic volume of the force sensing tip is considerably smaller than the geometric dimensions as determined by scanning electron microscopy in order to understand this 10-nm resolution.
Domain-wall dynamics and Barkhausen effect in metallic ferromagnetic materials. II. Experiments
Authors: Alessandro Bruno , Beatrice Cinzia , Bertotti Giorgio , Montorsi Arianna
Published: July 2002

Barkhausen effect (BE) phenomenology in iron-based ferromagnetic alloys is investigated by a proper experimental method, in which BE experiments are restricted to the central part of the hysteresis loop, and the amplitude probability distribution, P0(Φ̇), and power spectrum, F(ω), of the B flux rate Φ̇ are measured under controlled values of the magnetization rate İ and differential permeability μ. It is found that all of the experimental data are approximately consistent with the law P0(Φ̇)∝Φ̇c̃−1 exp(−c̃Φ̇/〈Φ̇〉), where all dependencies on İ and μ are described by the single dimensionless parameter c̃>0. Also the parameters describing the shape of F(ω) are found to obey remarkably simple and general laws of dependence on İ and μ. The experimental results are interpreted by means of the Langevin theory of domain-wall dynamics proposed in a companion paper. The theory is in good agreement with experiments, and permits one to reduce the basic aspects of BE phenomenology to the behavior of two parameters describing the stochastic fluctuations of the local coercive field experienced by a moving domain wall.
Heavily doped GaAs:Se. I. Photoluminescence determination of the electron effective mass
Authors: Szmyd D. M. , Porro P. , Majerfeld A. , Lagomarsino S.
Published: July 2002

A systematic study of the photoluminescence (PL) of Se-doped n-type GaAs grown by metalorganic chemical vapor deposition is reported. A new method is presented to determine the electron effective mass of n+-direct-gap semiconductors from the PL spectrum. GaAs samples with electron densities from 1015 to 8×1018 cm−3 were investigated over the temperature range of 13 to 353 K. The PL spectra of n+-GaAs are analyzed using a physical model which for the first time explains in a consistent manner both the energy of the peak and the full width at half-maximum, and accounts for the electron density. An accurate fit of the PL spectra is obtained by invoking band-to-band transitions without k selection. The electron exchange and correlation interactions account for all the observed band shrinkage, which reaches 48 meV for n=8.0×1018 cm−3. No significant density of band-tail states is observed. The Fermi energy is obtained directly from the PL fitting and is used with the measured Hall electron density n to determine the energy-dependent effective mass m*. An increase in m* beyond the value expected from the nonparabolicity of the conduction band in pure GaAs is observed. The mass at the minimum of the conduction band m*0 increases from 0.0636 me for pure GaAs to 0.073 me for n=8.0×1018 cm−3, where me is the free electron mass. The increase in m*0 is given empirically as m*0/me =0.0635+2.06×10−22n+1.16×10−40n2. The change in m* is interpreted as a distortion of the conduction band produced by the impurities. The small dilation of the lattice, 0.0035% at n=8.0×1018 cm−3, indicates that this distortion is not mechanical, but electrical in nature. Also, the PL linewidth is abnormally small for samples in which inhomogeneities are believed to exist.
An ecological optimization criterion for finite-time heat engines
Authors: Angulo-Brown F.
Published: July 2002

An endoreversible Carnot-type heat engine is studied under the usual restrictions: no friction, working substance in internal equilibrium (endoreversibility), no mechanical inertial effects, and under Newton’s cooling law for heat transfer between working fluid and heat reservoirs. A monoparametric family of straight lines which is isoefficient is found; i.e., all points (engine configurations) that belong to same line have the same efficiency. Along each line the power output divided by entropy production is a constant. From these properties and by using some dissipated quantities, relationships are obtained between reversible work and finite-time work and between reversible efficiency and finite-time efficiency. An ‘‘ecological’’ criterion is proposed for the best mode of operation of this heat engine. It consists in maximizing a function representing the best compromise between power and the product of entropy production and the cold reservoir temperature. The corresponding efficiency results almost equal to the average of the Carnot and the Curzon and Ahlborn [Am. J. Phys. 43, 22 (1975)] efficiencies.
Stacked structures of passive magnetic bearings
Authors: Yonnet Jean-Paul , Lemarquand Guy , Hemmerlin Sophie , Olivier-Rulliere Elisabeth
Published: July 2002

The stiffness of magnetic bearings composed of only two permanent magnet rings is limited. To increase this stiffness, stacked structures are used. Conventional stacking is obtained by placement of the magnetizations in opposition. This article presents another type of stacking with rotating magnetization direction (RMD), which improves the stiffness by about a factor of 2 in comparison with the conventional stack.
Anisotropic thermal conductivity in chemical vapor deposition diamond
Authors: Graebner J. E. , Jin S. , Kammlott G. W. , Bacon B. , Seibles L. , Banholzer W.
Published: July 2002

The thermal conductivity of thick-film diamond prepared by chemical vapor deposition (CVD) has been measured with heat flowing in a direction perpendicular to the plane of the film. A laser flash technique with fast infrared detection has been devised for measurement of thin samples with high conductivity. The conductivity perpendicular to the plane is observed to be at least 50% greater than with heat flowing parallel to the plane. This anisotropy is attributed to low-quality grain boundaries in the columnar microstructure. The observed dependence of the thermal conductivity on microstructure has important implications for thermal management of microelectronic devices with CVD diamond.
Effects of band nonparabolicity on two-dimensional electron gas
Authors: Altschul V. Ariel , Fraenkel A. , Finkman E.
Published: July 2002

Energy band nonparabolicity is present in the majority of materials used in heterojunctions, quantum wells, and superlattices. In the present work we derive a simplified dispersion relation for electrons using the k⋅p model. We demonstrate that this dispersion relation can account for nonparabolicity in both narrow and wide gap isotropic semiconductors. We then calculate the carrier effective mass and densities of states for both two- and three-dimensional electron systems. Finally, we derive a simple analytical relation between the carrier concentration and the Fermi energy in a nonparabolic two-dimensional electron gas. Agreement with a numerical model is demonstrated, while the traditional, parabolic approximation results in a large error. The simplicity of the new approximation allows an intuitive understanding of the nonparabolicity effect in two-dimensional systems. Therefore, the new approach should be useful for design, characterization, and modeling of quantum semiconductor devices with nonparabolic energy bands.
CF and CF2 actinometry in a CF4/Ar plasma
Authors: Kiss L. D. B. , Nicolai J.-P. , Conner W. T. , Sawin H. H.
Published: July 2002

Relative ground state CF and CF2 concentrations have been measured in a 13.56-MHz rf CF4/Ar discharge using both laser-induced fluorescence (LIF) and actinometric techniques to assess the validity of actinometry for CF and CF2 over a limited parameter space of pressure and power. Relative measurements of the CF ( A2Σ − X2Π) system and the CF2 ( A1B1 − X1A1) system were made by LIF. Actinometric values were calculated from relative measures of the plasma-induced emission (PIE) intensity of the CF* ( B2Δ− X2Π) at 202.4 nm, CF2* ( A1B1 − X1A1) at 251.9 nm, and Ar* [4s′(1/2)°−4p′(1/2)] at 750.4 nm. Both LIF and PIE signals were spatially averaged over the bulk of the plasma. Steady-state actinometric and LIF measurements were compared for CF4/5% Ar discharges at pressures in the range of 500 to 1000 mTorr and nominal powers in the range of 20 to 100 W. Dynamic actinometric and LIF measurements of CF were made by modulating the discharge power and monitoring the CF transient at one set of conditions, 500-mTorr pressure and 70-W nominal power. Our results indicate that actinometric measurements of CF and CF2 correlate well with relative CF and CF2 LIF measurements under the studied conditions.
Hysteresis measurements of remanent polarization and coercive field in polymers
Authors: Dickens B. , Balizer E. , DeReggi A. S. , Roth S. C.
Published: July 2002

An experimental method is described which allows estimation of remanent polarization and coercive field without assuming functional forms for the capacitive and electrical resistance terms. The method can be used to measure polarization in specimens with voltage-dependent conductivity (often arising from the presence of ions in the specimens), voltage-dependent capacitance, and significant amounts of space charge. It consists of: (1) performing bipolar current/voltage hysteresis loops to allow a steady state of remanent polarization and space charge to build up in the specimen, and (2) following a bipolar loop with two or more unipolar loops in which the polarization changes in the first unipolar loop. Both sinusoidal and linear time-dependent applied voltages may be used. Automatic data processing of hysteresis loops is described for cases in which specimen behavior may be considered to be ideal.
n-CdSe/p-ZnTe based wide band-gap light emitters: Numerical simulation and design
Authors: Wang M. W. , Phillips M. C. , Swenberg J. F. , Yu E. T. , McCaldin J. O. , McGill T. C.
Published: July 2002

The only II-VI/II-VI wide band-gap heterojunction to provide both good lattice match and p- and n-type dopability is CdSe/ZnTe. We have carried out numerical simulations of several light emitter designs incorporating CdSe, ZnTe, and Mg alloys. In the simulations, Poisson’s equation is solved in conjunction with the hole and electron current and continuity equations. Radiative and nonradiative recombination in bulk material and at interfaces are included in the model. Simulation results show that an n-CdSe/p-ZnTe heterostructure is unfavorable for efficient wide band-gap light emission due to recombination in the CdSe and at the CdSe/ZnTe interface. An n-CdSe/MgxCd1−xSe/p-ZnTe heterostructure significantly reduces interfacial recombination and facilitates electron injection into the p-ZnTe layer. The addition of a MgyZn1−yTe electron confining layer further improves the efficiency of light emission. Finally, an n-CdSe/MgxCd1−xSe/MgyZn1−yTe/p-ZnTe design allows tunability of the wavelength of light emission from green into the blue wavelength regime.
Composite conduction in ion-implanted polymers
Authors: Wang Yongqiang , Bridwell L. B. , Giedd R. E.
Published: July 2002

The temperature dependence of the dc conductivity for polyamide-imide films, implanted with 50 keV As ions to different fluences, has been studied. Our high-resolution data reveal a two-component conductivity that depends on both one-dimensional variable range hopping (VRH) and three-dimensional VRH. For low fluence levels (5×1015 ions/cm2), the one-dimensional VRH is dominant while at higher fluences (1×1017 ions/cm2), the three-dimensional VRH dominates. These materials become highly disordered and form hard carbon materials along the ion track. The conductivity may be explained by 1D VRH along the ion track at low fluences while at higher fluences, regions of carbon rich material form three-dimensional structures where 3D VRH exists. This rather simple model and its composite conductivity can explain the exact curvature of the temperature dependent conductivity, while single VRH models and percolation models cannot.
Critical currents in normal conductors
Authors: Dolinsky Yu. , Elperin T.
Published: July 2002

It is shown that in a normal conductor the value of a current that generates a magnetic pressure equal to the critical pressure may be considerably lower than it is generally considered. The lower value of the critical current occurs due to the shift of the whole curve of phase equilibrium in the presence of strong electric current. This shift arises due to the additional work performed against ponderomotive forces which prevents from the formation of the nucleus of a phase with a lower value of electric conductivity. In case of the Van der Waals model of the critical state the value of the critical current calculated taking into account the shift of the phase equilibrium curve is by factor 2–3 less than the critical current determined when this effect is neglected.
A theory of nonlinear spin wave interactions in magnetic films
Authors: Slavin A. N. , Rojdestvenski I. V.
Published: July 2002

A theory of nonlinear interactions of spin waves (SW) propagating in thin magnetic films is developed. We use the classical Hamiltonian formalism for SW in an unbounded ferromagnetic medium in combination with the theory of dipole-exchange SW spectrum for magnetic films to derive a nonlinear Schrödinger equation for the SW envelope which gives a good qualitative description of the formation of the envelope soliton experimentally observed in yttrium-iron garnet (YIG) films.
Energy levels and upconversion fluorescence in trivalent thulium-doped yttrium scandium aluminum garnet
Authors: Gruber John B. , Seltzer Michael D. , Hills Marian E. , Stevens Sally B. , Morrison Clyde A.
Published: July 2002

Absorption spectra of Tm3+ ions in yttrium scandium aluminum garnet are reported between 1.9 and 0.25 μm at 4 K. Laser-excited fluorescence was observed at 4 K from Tm3+(4f12) multiplet manifolds 1D2, 1G4, 3F2, and 3H4 to the ground-state manifold 3H6. Emission from 1D2 and 1G4 also includes transitions to Stark levels in manifolds 3F4 and 3H5. Upconversion excited fluorescence was observed between 1D2 and 3F4 at 10 K. Analysis of the fluorescence emission confirms assignments given to individual Stark levels based on an analysis of the absorption spectra. A crystal-field splitting calculation was carried out in which a parameterized Hamiltonian (including Coulombic, spin-orbit, and crystal-field terms in D2 symmetry) was diagonalized for all multiplets of the Tm3+(4f12) configuration. The rms deviation between 42 experimental and calculated Stark levels was 8 cm−1.
A simulation of the multiple trapping model for continuous time random walk transport
Authors: Vasudevan V. , Vasi J.
Published: July 2002

A multiple trapping model is used to study the dispersive transport of holes in insulators. The one-dimensional Poisson and continuity equations are solved numerically along with the trap rate equations that model multiple trapping. The transient current due to a pulse of radiation is obtained as a function of the spread in the trap energy levels and the trap density distribution. The main properties of continuous time random walk transport, namely universality and superlinear dependence of the transit time on the electric field and oxide thickness, are verified.
Magnetic resonance detection and imaging using force microscope techniques (invited)
Authors: Züger O. , Rugar D.
Published: July 2002

We describe the principles and imaging characteristics of a new type of high resolution magnetic resonance microscopy. Magnetic resonance is detected by measuring a small oscillating magnetic force acting between the spins in a sample and a nearby ferromagnetic particle. The oscillating magnetic force is generated by polarizing the spins in the magnetic field and then modulating the sample magnetization using magnetic resonance techniques. The oscillating magnetic force is detected by sensing the angstrom-scale vibration of a micromechanical cantilever on which the sample is mounted. High spatial resolution is achieved as a result of the narrowness of the magnetic resonance spectral response and the large magnetic field gradient generated by the ferromagnetic particle. Electron paramagnetic resonance images are presented that demonstrate axial resolution on the order of 1 μm and lateral resolution on the order of 5 μm. Submicron resolution can be expected with straightforward technical improvements.
Clustering in erbium-doped silica glass fibers analyzed using 980 nm excited-state absorption
Authors: Quimby R. S. , Miniscalco W. J. , Thompson B.
Published: July 2002

A nonsaturable component to the 980 nm excited state absorption (ESA) is found in Er3+-doped silica glass fibers. This anomalous behavior of the ESA provides a way of quantitatively characterizing the degree of clustering in these glasses. It is found that the degree of clustering is greater for higher Er3+ concentrations, and is significantly reduced with the addition of aluminum to the glass. With this method it is also found to be possible to distinguish between true clustering and the occurrence of closely spaced Er3+ pairs in a random distribution.
Semiconductor molecular-beam epitaxy at low temperatures
Authors: Eaglesham D. J.
Published: July 2002

Low-temperature molecular-beam epitaxy (MBE) in semiconductors is reviewed, with a focus on limited thickness epitaxy (LTE), the regime where crystalline growth over an epitaxial thickness hepi is followed by a transition to amorphous deposition. The goal is to summarize the main results on this phenomenon, make the connection to other results on low-temperature MBE, and present the large body of unpublished data on hepi. Since the problem is still not fully understood, all available data that have a bearing on the understanding of the effect are outlined. The scientific questions and practical problems that have driven interest in low-temperature growth are outlined, and the phenomenon of LTE and the dependence of hepi on the growth conditions are described. The LTE effect is apparently general, but Si(100) is the model system for which most data are available. Breakdown of epitaxy follows a universal curve that is inconsistent with continuous nucleation of the amorphous phase, implying that growth is truly thickness dependent. The epitaxial thickness is thermally activated in substrate temperature T as hepi=h0 exp(−Eact/kBT), with h0 following a weak ln(R) or R1/4 dependence on deposition rate R. hepi is also strongly influenced by lattice mismatch strain, residual H in the ultrahigh vacuum, and annealing during growth interrupts. Possible mechanisms for LTE are discussed, with particular emphasis on the roles played by H and kinetic roughening, and the key experiments distinguishing these mechanisms are described. Finally, an attempt is made to draw up the best current picture of the phenomenon. It is concluded that roughening provides the fundamental limit to epitaxy at low temperature, but with H contamination playing an important part in controlling surface diffusion: outstanding problems include the rate dependence and the details of the roughening behavior.
Empirical depth profile simulator for ion implantation in 6Hα-SiC
Authors: Ahmed S. , Barbero C. J. , Sigmon T. W. , Erickson J. W.
Published: July 2002

Silicon carbide is a semiconductor material suitable for a variety of specialized devices. Implantation profiles of 30–300 keV B, Al, N, and As in 6Hα-SiC are reported. The profiles, measured by secondary-ion-mass spectrometry, are fit with Pearson-IV curves which require knowledge of the first four moments of the distribution. The moments of the impurity distributions are extracted from the experimental data and fit to simple functions of the ion energies. Thus, an accurate implantation depth profile simulator, based on experimental data for the common dopants in 6Hα-SiC, is developed. This method results in a more accurate implant simulator than is obtained using conventional first-principles calculations, primarily due to channeling considerations.
Analytical thermal model of ultraviolet laser ablation with single-photon absorption in the plume
Authors: Tokarev V. N. , Lunney J. G. , Marine W. , Sentis M.
Published: July 2002

An analytical thermal model of ultraviolet (UV) laser ablation has been proposed. The approximation used to describe the absorption of the incident beam in excimer laser induced ablation plasma, for the case of single-photon absorption σeff(t)=const. (where σeff is the effective absorption cross section of ejected species) gives analytical results on UV laser ablation kinetics. In particular, the changing (increasing or decreasing) slope of the curve of etch depth, z, versus logarithmic fluence, ln E, is explained.
Enhancement of luminescence by pulse laser annealing of ion-implanted europium in sapphire and silica
Authors: Can N. , Townsend P. D. , Hole D. E. , Snelling H. V. , Ballesteros J. M. , Afonso C. N.
Published: July 2002

Sapphire (Al2O3) and silica samples have been implanted with 400 keV europium ions at fluences between 5×1014 and 1×1016 ions cm−2. As-implanted, samples show luminescence at 622 nm, and although the intensity may be increased by furnace anneals up to 1000 °C, higher temperatures, to 1200 °C, result in less emission, as the impurity ions form precipitate clusters. This problem can be avoided by the use of pulsed laser anneals which dissociate the clusters and quench in atomically dispersed ions. The luminescence intensity has been increased by factors of 95 and 85 for sapphire and silica, respectively, relative to the initial implanted signal. On comparing with furnace anneals at 1200 °C, the pulsed laser annealing is more effective, by factors of up to 45 times. Data for pulsed excimer and CO2 lasers are compared. Both types of laser appear to remove the ion-implanted radiation damage, but in the case of silica, higher luminescence performance was obtained with the excimer anneals. There was no evidence for diffusion of the implanted europium, as assessed by Rutherford backscattering spectrometry.
Infrared-absorption spectroscopy of Si(100) and Si(111) surfaces after chemomechanical polishing
Authors: Pietsch G. J. , Chabal Y. J. , Higashi G. S.
Published: July 2002

The mechanism of silicon stock removal in chemomechanical polishing (CMP) is studied by characterizing surface chemical species with infrared-absorption measurements and the corresponding degree of hydrophobicity with contact angle measurements immediately after CMP. Surface properties and stock removal rates are found to depend strongly on the pH of the silica slurry used in this ‘‘syton polishing’’ technique. At the peak of the removal rate [pH∼11 for both Si(100) and Si(111)], the surfaces have the highest hydrophobicity and the highest hydrogen coverage. Si(111) has an ideal monohydride termination, while Si(100) is characterized by a variety of hydrides (mono-, di-, and trihydrides), suggesting different morphologies for the surfaces: atomically flat domains on Si(111) and rougher areas on Si(100). Away from the optimum slurry pH (at lower stock removal rates), a higher concentration of hydroxyl groups is observed, increasing the surface hydrophilicity. At all pH, some oxidation occurs beneath the H-terminated Si surface, as evidenced by a characteristic frequency shift of oxygen-backbonded hydrides. The mechanisms of stock removal are considered in view of these observations for the different ranges of slurry pH. In particular, at the highest removal rates, an interplay of surface oxidation, removal of oxidized silicon, and subsequent H termination is suggested. Based on the spectroscopic characterization of surface morphologies, the relevance of CMP to prepare atomically smooth silicon surfaces is discussed.
Anisotropic Nd–Fe–B bonded magnets made from HDDR powders (invited)
Authors: Takeshita T. , Morimoto K.
Published: July 2002

Anisotropic Nd–Fe–B magnet powders can be produced by the hydrogenationdecomposition-desorption-recombination (HDDR) process from Nd–Fe–B–Co–M (M=Ga, Zr, Nb, Hf, and Ta) alloys. The present status of those HDDR powders and the bonded magnets made from them are reviewed with regards to the powder particle size dependence of their magnetic properties, their magnetic thermal stability, and their magnetization behavior. The results of a mechanistic study on the recombination step are also presented. The magnetic properties of the anisotropic HDDR powder depend relatively little on the powder particle size. Bonded magnets with a density of ∼6.20 g/cm3 and a BHmax of 18.5–20.5 MGOe can be produced from anisotropic HDDR powders with particle sizes of below 300 μm diam. The temperature coefficient of the intrinsic coercive force iHc, of the bonded magnet is −0.55%/°C in the temperature range from 25 to 100 °C. The magnetization force needed for full magnetization of the bonded magnet is about twice the coercive force of the magnet, indicating that the magnetization mechanism is different from that of the rapidly solidified isotropic Nd–Fe–B magnet. In the early stage of the recombination step of the HDDR process (1 min desorption), three phases are produced, i.e., α-(Fe,Co), spherical NdH2 and rimlike Nd2(Fe,Co)14B surrounding the NdH2 particle. Further desorption makes the Nd2(Fe,Co)14B phase grow.