bluuebunny/crossref_metadata_2025 huggingface dataset:
Hole transport in tri-p-tolylamine-doped polymers
Authors: Borsenberger P. M.
Published: July 2002

Hole mobilities have been measured in tri-p-tolylamine-doped polymers as functions of the polymer composition, temperature, and electric field. Depending on the composition of the polymer, the mobilities vary by over two orders of magnitude. In all cases, the electric field and temperature dependencies can be described as exp(βE1/2) and exp−(T0/T)2, respectively. The effect of the host polymer on the mobility is largely related to the prefactor μ0. It is suggested that the dependence of μ0 on the polymer composition is related to the wave-function overlap factor, 2αR, and due to differences of the ionization potential of the host polymer.
On representation of fracture profiles by fractional integrals of a Wiener process
Authors: Kunin B. , Gorelik M.
Published: July 2002

An enhancement of the crack diffusion model of brittle fracture is proposed. In the model, certain probabilities are evaluated as averages over the set of all virtual crack paths, and the latter are modeled as realizations of a Wiener process. However, commonly observed fracture profiles are smoother. In fact, one can show that their fractal dimension is smaller than the value 3/2 for a Wiener process. Accordingly, we propose to represent fracture profiles by graphs of fractional integrals of a Wiener process. This allows one to employ additional fractographic information obtained from fracture profiles, e.g., their fractal dimension, in a predictive formalism, evaluating probabilities of failure.
Atomic layer growth of oxide thin films with perovskite-type structure by reactive evaporation
Authors: Iijima K. , Terashima T. , Bando Y. , Kamigaki K. , Terauchi H.
Published: July 2002

Epitaxial growth of BaTiO3 and SrTiO3 films by the reactive evaporation method was investigated using reflection high-energy electron diffraction (RHEED). The investigations were carried out using two growth methods: coevaporation and alternate evaporation of the metal elements in an oxygen atmosphere. Atomic layer growth was achieved by the alternate supply of Ba or Sr and Ti on the growing surface. In the case of coevaporation, epitaxial growth occurred in a two-dimensional unit-cell-by-unit-cell mode. The surface of each unit cell is terminated by a (TiO2) layer. Artificial superlattices of BaTiO3/SrTiO3 were fabricated by monitoring the film thickness with the RHEED oscillations.
A new model for the plasma anodization of silicon at constant current
Authors: Peeters Jozef , Li Li
Published: July 2002

A new physical model is described for the plasma anodization of Si. The model is constructed from the continuity equation for the charged oxidizing agent O−, with transport by field-imposed drift and by diffusion. It is argued that at constant total current, the field in the oxide layer is constant in space and in time. A loss term for O− ions is also incorporated in the model; the resulting gradual drop of the O− contribution to the total (constant) current at increasing depth into the oxide explains the observed decrease of the oxidation rate with time. The O− loss can occur, i.a., by detachment O−→O+e− or by two-step mechanisms resulting overall in 2O−→O2+2e−. The model predicts an exponential decay of O− in the oxide. At constant current the oxide width as a function of time is given by w=A ln(1+Bt), where A is the characteristic penetration distance of O− in the oxide and AB is the initial oxide growth rate, determined by the subsurface O− current density. The two-parameter model provides excellent fits to available experimental data; standard deviations are ∼1% of the final oxide width. From the parameters, numerical values are derived of underlying physical constants. A lower limit is also deduced for the O− loss rate constant.
Finite-size scaling behavior of ferromagnetic thin films
Authors: Huang F. , Mankey G. J. , Kief M. T. , Willis R. F.
Published: July 2002

We have used molecular-beam epitaxy to grow high-quality pseudomorphic Ni and Co1Ni9 films on Cu(001). From temperature-dependent surface magneto-optic Kerr effect measurements of these films, we have determined the finite-size scaling behavior of the Curie temperature of ultrathin films for a thickness range of n=2.5–16 monolayers (ML). The film thickness dependent Curie temperature for each of these ferromagnetic thin-film systems, TC(n), is described by a finite-size scaling formula: [TC(∞) − TC(n)]/TC(n) = [(n − n′)/n0]−1/ν, where TC(∞) is the bulk Curie temperature, n0=2.5±0.5 ML for Co films and 3.5±0.4 ML for Ni and Co1Ni9 films is the microscopic length scale, and ν=0.76±0.08 is the bulk correlation length exponent. An interesting result is that TC(n) extrapolates to zero in the single mononolayer limit, n′=1.
Fluxons in thin-film superconductor-insulator superlattices
Authors: Sakai S. , Bodin P. , Pedersen N. F.
Published: July 2002

In a system of thin alternating layers of superconductors and insulators the equations describing static and dynamic fluxon solutions are derived. The approach, represented by a useful compact matrix form, is intended to describe systems fabricated for example of niobium or niobium-nitride thin films; in the limit of ultrathin superconductor films it may give a model for describing fluxon motion in layered high-Tc superconductors. Numerical examples of current versus voltage curves to be expected in such an experiment are presented.
Hysteresis measurements on ferroelectric composites
Authors: Dias C. J. , Das-Gupta D. K.
Published: July 2002

Ceramic/polymer composites made of calcium-modified lead titanate grains in the matrix of a copolymer of vinylidene fluoride and trifluorethylene, in a 65:35 volume fraction percentage, respectively, were produced in order to obtain mechanically strong ferroelectric films with improved piezo- and pyroelectric properties. Their poling behavior was studied using a modified ferroelectric hysteresis measurement setup that allowed for the measurement of the remanent polarization Pr and coercive field Ec, as a function of the peak of the sinusoidal applied field, the frequency of the excitation, and the poling temperature. The ranges of variation of the electric field, frequency, and temperature were from 5 to 25 MV/m, from 0.04 to 2 Hz, and from 50 to 120 °C, respectively. Estimates of the activation field and saturation polarization were calculated based on these measurements. The observed properties in the ferroelectric composites were then compared and correlated with the properties of the ceramic material.
Hydrogen introduction and hydrogen-enhanced thermal donor formation in silicon
Authors: Stein H. J. , Hahn SooKap
Published: July 2002

Hydrogen has been introduced from a rf plasma into Czochralski Si at 275 °C. Most of the hydrogen is trapped near the surface where it forms Si—H bonds, but a small fraction diffuses into the Si. This fraction enhances oxygen-related thermal donor (TD) formation rates in a diffusionlike profile during subsequent furnace anneals between 350 and 400 °C. A hydrogen concentration that is only a few percent of the oxygen concentration is sufficient to enhance the TD formation rate, indicative of a hydrogen-catalyzed process. Maximum concentrations for TDs after annealing at 400 °C exceed that for retained hydrogen. A mechanism of hydrogen diffusion through oxygen traps and correlated hydrogen-promoted oxygen diffusion is proposed to explain the enhanced TD formation rates.
Two- and three-dimensional calculation of remanence enhancement of rare-earth based composite magnets (invited)
Authors: Schrefl T. , Fischer R. , Fidler J. , Kronmüller H.
Published: July 2002

Micromagnetic calculations using a finite element technique rigorously describe the magnetic properties of novel, isotropic rare-earth-based composite magnets. Numerical results obtained for a composite material of Nd2Fe14B, SmCo5 or Sm2(Fe0.8Co0.2)17N2.8 and α-Fe particles show that remanence, coercivity, and coercive squareness sensitively depend on microstructural features. Interparticle exchange interactions enhance the remanence by about 60% with respect to noninteracting particles for a mean-grain size approaching the exchange length of the soft magnetic phase and a significant percentage of α-Fe. On the other hand, exchange interactions between the phases suppress the nucleation of reversed domains and thus preserve a high coercive field. Therefore, optimally structured, isotropic composite magnets show remarkably high energy products exceeding 400 kJ/m3.
Properties of amorphous diamond films prepared by a filtered cathodic arc
Authors: Lossy Richard , Pappas David L. , Roy Ronnen A. , Doyle James P. , Cuomo Jerome J. , Bruley John
Published: July 2002

Amorphous diamond films have been prepared by filtered cathodic arc deposition of carbon. The filtered arc is well suited for the growth of amorphous diamond, as it provides carbon ions with optimum kinetic energies at practical deposition rates. These films contain no hydrogen and are therefore structurally different from diamond-like carbon films generated by plasma chemical vapor deposition. Diamond-type bonding of carbon is quantitatively determined by electron energy loss spectroscopy, as an sp3 content up to 83% is measured. Data on the macroscopic properties are provided by optical transmittance, ellipsometry, Rutherford backscattering, elastic recoil scattering, and resistivity measurements. The films exhibit high optical transparency and an optical gap of 2.4 eV. Trends in the optical gap and refractive index as a function of deposition energy are consistent with semiconductor theory and indicates a change in the average bond length.
Thickness-dependent electrical characteristics of lead zirconate titanate thin films
Authors: Udayakumar K. R. , Schuele P. J. , Chen J. , Krupanidhi S. B. , Cross L. E.
Published: July 2002

Ferroelectric lead zirconate titanate thin films of morphotropic phase boundary composition were fabricated through the sol-gel spin-on technique to study the thickness dependence of electrical characteristics. At saturation, the films exhibited a dielectric constant of 1300, dissipation factor of 0.03, Curie temperature of 366 °C, remanent polarization of 36 μC/cm2, coercivity of around 30 kV/cm, and dielectric breakdown strength of over 1 MV/cm. The temperature dependence of permittivity showed an anomalous behavior with annealing temperature. The high temperature dielectric behavior of the films were distinguished by smeared and stunted dielectric maximum.
Comparative study of properties between a-GeC:H and a-SiC:H films prepared by radio-frequency reactive sputtering in methane
Authors: Saito N. , Yamaguchi T. , Nakaaki I.
Published: July 2002

Hydrogenated amorphous germanium–carbon (a-GeC:H) and silicon–carbon (a-SiC:H) films were deposited by reactive magnetron sputtering of Ge and Si targets in a methane argon gas mixture. The effect of rf power on the structural, optical, and electrical properties of the films was investigated. The carbon content in a-SiC:H films is larger than in a-GeC:H for the same deposition condition, and it decreases with increasing rf power. The intensity of the carbon-related bonds, the optical band gap, and the activation energy of dc conductivity of both films decreases with decreasing carbon content. The temperature dependence of dc conductivity of a-SiC:H exhibits activated-type conduction, whereas hopping conduction is predominant in a-GeC:H. Hydrogen concentration and H bonding ratio are examined, indicating that the termination of the dangling bond by hydrogen is more effective in a a-SiC:H films than a-GeC:H films.
Excitons in spatially separated electron–hole systems: A quantum Monte Carlo study
Authors: Wang E. G. , Zhou Yucai , Ting C. S. , Zhang Jianbo , Pang Tao , Chen Changfeng
Published: July 2002

Variational and diffusion quantum Monte Carlo simulations of excitons in the type-II semiconductor quantum well system In1−xGaxAs/GaSb1−yAsy are performed for a wide range of composition (0,0)≤(x,y)≤(0.62,0.64) and well width 25 Å ≤L≤200 Å. The exciton binding energy is found to increase with the decrease of the layer thickness and with the increase of the composition. The calculated results suggest that a novel semiconductor–excitonic insulator (Bose condensate)–semimetal transition should be observable in this system.
Nanocomposite Sm2Co17/Co permanent magnets by mechanical alloying
Authors: Chen S. K. , Tsai J. L. , Chin T. S.
Published: July 2002

Mechanically alloyed magnets SmCox (x=6.0–11.0) have been prepared using starting materials of commercial SmCo5 and Co powders. X-ray diffraction data indicate that the equilibrium Sm2Co17 phase can be achieved in a nanocomposite Sm2Co17/Co system at x=8.5–11.0, while a Sm2Co17/SmCo5 structure occurs at x=6.0–8.0. The best magnetic properties obtained at x=10.0 are as follows: (BH)max=10.2 MGOe, Br=9.4 kG, Hc=4.0 kOe, and S=0.82. Wohlfarth’s remanence analysis indicates that the high remanence can be attributed to the intergranular interaction between neighboring cobalt and Sm2Co17 grains.
A gigahertz-range electromagnetic wave absorber with wide bandwidth made of hexagonal ferrite
Authors: Matsumoto Morihiko , Miyata Yoshimori
Published: July 2002

A thin electromagnetic wave absorber has been investigated; it is composed of hexagonal ferrite and retains high permeability even in the gigahertz-range. The grid-type wave absorber composed of the hexagonal ferrite was examined to broaden the bandwidth in a gigahertz-range by introducing free space in the direction perpendicular to the electric field, which decreased total permittivity. Absorbers made of Ba3Co2Fe24O41 (Co2Z) and Ba2(Zn0.8Mn0.2)2Fe12O22 [(Zn0.8Mn0.2)2Y] showed wide bandwidths of 1.8–5 GHz and 1.5–4.5 GHz (≳15 dB), respectively, when the free space was 37% (v/v) and the thickness of the ferrite was about 5 mm. Increasing the free space led to a wider bandwidth: 1.7–9 GHz was achieved with a 50% (v/v) of free space when the thickness of the (Zn0.8Mn0.2)2Y was 7 mm.
A two-dimensional molecular dynamics simulation of thin film growth by oblique deposition
Authors: Dong Liang , Smith Richard W. , Srolovitz David J.
Published: July 2002

Atomistic, molecular dynamics simulations are employed to investigate the relationship between film microstructure and deposition conditions (substrate temperature, deposition kinetic energy, and deposition angle). Increasing substrate temperature and deposition kinetic energy leads to fewer voids, smaller voids, smoother surfaces, and higher film density. As the deposition angle increases, the film microstructure changes from a dense film, with few voids, to a microstructure in which nearly colinear tracks of elongated voids form and, finally, to a highly porous structure of well-formed columns. The angle along which the voids are elongated and the orientation of the void tracks are the same and increase monotonically with the deposition angle (the column angles follow the same trend as the deposition angle). Void formation, void alignment into tracks, and the columnar structure are all attributable to shadowing effects, which become more pronounced with increasing deposition angle. The variation of the column/void track angle β with deposition angle α fits well with the classical tangent law at low angles, but is overpredicted by the tangent law at α≳60°, consistent with experiment. The column angle β decreases slowly with increasing deposition kinetic energy due to increased surface mobility.
A shallow state in molecular beam epitaxial grown CuGaSe2 film detectable by 1.62 eV photoluminescence
Authors: Yamada Akimasa , Fons Paul , Niki Shigeru , Shibata Hajime , Obara Akira , Makita Yunosuke , Oyanagi Hiroyuki
Published: July 2002

Cu-rich CuGaSe2 films grown on [001] oriented GaAs substrates by molecular beam epitaxy show a remarkable low temperature photoluminescence emission peaked at 1.620 eV. This emission can be attributed to a free-to-bound recombination accompanying a donor-to-acceptor pair recombination. The ionization energies of these states are found to be 3.4 and 108 meV, respectively, via temperature dependent photoluminescence. The 108 meV state is attributed to a donor and the 3.4 meV state to an acceptor state observed.
On the nature of the luminescence emitted by a polypropylene film after interaction with a cold plasma at low temperature
Authors: Massines F. , Tiemblo P. , Teyssedre G. , Laurent C.
Published: July 2002

The light emitted by an insulating material once its surface has been submitted to a cold plasma, plasma-induced luminescence is investigated on polypropylene films at low temperature. An analysis of the integral and wavelength resolved light is carried out. The kinetical evolution of the spectrum after plasma interaction are reported and analyzed. Investigation of the photo- and chemiluminescence spectra of the material shows that plasma-induced luminescence has three spectral components, each having different excitation mechanisms and thus different kinetics. The fastest is due to the ultraviolet irradiation during plasma contact, the second is dominated by radical chemistry producing carbonyl groups, and the third, with the slowest kinetics, is due to carrier recombination on the most conjugated chromophores which are preferentially charged by the plasma. To confirm the interpretation of plasma induced luminescence spectrum, the first results concerning short and long plasma interaction are considered.
Modeling the temperature response of 4H silicon carbide junction field-effect transistors
Authors: Scozzie C. J. , McLean F. B. , McGarrity J. M.
Published: July 2002

The electrical characteristics of 4H-SiC depletion-mode junction field-effect transistors (JFETs) have been measured over an extended temperature range from 218 to 673 K. A basic model has been applied to predict I–V characteristics for SiC JFETs over this extended temperature range using the standard abrupt-junction long-channel JFET equations. The model employs a two-level donor ionization structure using ionization energies of 0.050 and 0.080 eV and assumes a two-step inverse power law dependence of mobility on temperature based on recently published Hall measurement data. The modeled I–V characteristics are in good agreement with the experimental data over the temperature range from 273 to 673 K. The deviations between the experimental data and the response model at the temperature extremes are attributed to increased substrate resistivity at 218 K and increased device leakage currents at 673 K.
Thermostability of Sm2(FeGa)17Cy prepared by gas-solid reaction (GSR)
Authors: Cao L. , Handstein A. , Gebel B. , Schäfer R. , Müller K.-H.
Published: July 2002

The gas-solid-reaction (GSR) was used to introduce interstitial carbon atoms into Sm2Fe17−xGax compounds with x=0, 0.5, 1, and 2. For this process, powders made from homogenized ingots were annealed at 500 °C under methane for different times. The thermostability increases for small amounts of Ga and the investigation shows that Sm2Fe16.5Ga0.5Cy is stable up to 750 °C. In the case of Sm2Fe15Ga2Cy carburized for 6 h (y=2.0) and 18 h (y=2.2), the x-ray diffraction patterns show the Th2Zn17-type structure only. After annealing at 800 °C for 20 min the 6 h carburized sample shows a small amount of α-Fe and other phases and there is a large Fe content after annealing at 850 °C. For an 18 h carburized sample, less Fe and no other phases have be seen after annealing at 800 °C, i.e., the material is nearly single phase. The result that longer carburization times stabilize the Th2Zn17-type structure could also be manifested by Kerr microscopy. A comparison with mechanically alloyed Sm2Fe15Ga2C2 powders prepared with Sm excess shows that those are very stable up to 900 °C. The density of fine-grained Sm2Fe17−xGaxCy could be increased by hot pressing, but the degree of compaction and the phase purity very sensitively depend on the Ga content.
The dielectric response as a function of temperature and film thickness of fiber-textured (Ba,Sr)TiO3 thin films grown by chemical vapor deposition
Authors: Basceri Cem , Streiffer S. K. , Kingon Angus I. , Waser R.
Published: July 2002

The temperature- and field-dependent permittivities of fiber-textured Ba0.7Sr0.3TiO3 thin films grown by liquid-source metalorganic chemical vapor deposition were investigated as a function of film thickness. These films display a nonlinear dielectric response under conditions representative of those encountered in dynamic random access memories or other integrated capacitor applications. This behavior has the exact form expected for a classical nonlinear, nonhysteretic dielectric, as described in terms of a power series expansion of the free energy in the polarization as in the Landau–Ginzburg–Devonshire approach. Curie–Weiss-like behavior is exhibited above the bulk Curie point (∼300 K), although the ferroelectric phase transition appears frustrated. Small-signal capacitance measurements of films with different thicknesses (24–160 nm) indicate that only the first term in the power series expansion varies significantly with film thickness or temperature. Possible origins for this thickness dependence are discussed.
Elastic properties of zinc-blende and wurtzite AlN, GaN, and InN
Authors: Wright A. F.
Published: July 2002

Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions. Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies. These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzite c/a ratio under hydrostatic pressure. Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases.
Crystal-field splitting of energy levels of rare-earth ions Dy3+(4f9) and Yb3+(4f13) in M(II) sites in the fluorapatite crystal Sr5(PO4)3F
Authors: Gruber John B. , Zandi Bahram , Merkle Larry
Published: July 2002

We have calculated a set of crystal-field parameters, Bnm, for Dy3+ through Yb3+ ions in M (II) sites in the crystal Sr5(PO4)3F (SFAP). The Bnm parameters were derived from lattice-sum calculations and from analyses of the optical spectra reported for Ho3+, Er3+, and Tm3+ individually doped into SFAP. These parameters were used to predict starting values, which were then semiempirically fitted for the crystal-field splitting of the multiplet manifolds of the sextet states of Dy3+(4f9), HJ6, and FJ6 and the doublet states of Yb3+(4f13) having manifolds F7/22 and F5/22. The results are compared with an analysis of the absorption and fluorescence spectra of Dy3+ and Yb3+ individually doped into SFAP. The rms deviation between 44 calculated and observed Stark levels of Dy3+ in Dy:SFAP is 10 cm−1. Without fitting of any parameters, the predicted crystal-field splitting of the Yb3+ multiplet manifolds is in reasonable agreement with the observed spectra for Yb:SFAP.
On the origin of the magneto-optical effects in Li, Mg, Ni, and Co ferrite
Authors: Fontijn W. F. J. , van der Zaag P. J. , Metselaar R.
Published: July 2002

A reexamination of the magneto-optical (MO) spectra of Li0.5Fe2.5O4, MgFe2O4, NiFe2O4, and CoFe2O4 is presented. Thus far the MO spectra of these compounds have been explained by either orbital promotion processes, oxygen to iron charge transfer transitions or crystal field transitions and, in the case of CoFe2O4, a single intervalence charge transfer (IVCT) transition. For Li0.5Fe2.5O4 and MgFe2O4, the major transitions in the MO–Kerr spectra between 0.5 and 5.0 eV are assigned to intersublattice charge transfer (ISCT) transitions. The MO–Kerr spectrum of NiFe2O4 is more complex as in this case both Fe3+ and Ni2+ contribute to the spectrum. In NiFe2O4 an additional IVCT transition is observed at 2.9 eV. The complex MO spectrum of CoFe2O4 could be resolved using the dielectric tensors of Co2+Alx3+Fe2−x3+O4 (x=0, 0.1, 0.6, and 1) and rigorously fitting both the diagonal and the off-diagonal elements of the dielectric tensor with one set of transitions which was consistent for all degrees of Al3+ substitution. Thus, in these ferrites a consistent assignment of all the major transitions in the 2.0 to 5.0 eV energy range to either IVCT or ISCT transitions is made.
Microstructure of FePt/Pt magneticthin films with high perpendicular coercivity
Authors: Hong M. H. , Hono K. , Watanabe M.
Published: July 2002

Microstructures of 50 nm thick FePt magnetic thin films deposited on Pt buffer layers with various thicknesses (0–500 nm) grown on a Fe seeded MgO (100) substrate have been studied by transmission electron microscopy to correlate them with magnetic properties. High density of planar defects such as twin and antiphase boundary are present in the FePt films. The twins observed in these films are not the {011} twins which are commonly observed in the bulk FePt magnet, but they are the {111} twins. The density and morphology of these twins drastically change depending on the composition of the FePt thin films as well as the thickness of the Pt buffer layer, while that of the antiphase boundary does not show noticeable changes. In the Pt buffer layer, a high density of dislocations is present in order to reduce the elastic strain due to a large lattice mismatch imposed between the Pt layer and the MgO substrate (about 9%). When the thickness of the Pt buffer layer is increased to 500 nm, the Pt and FePt layers show a polycrystalline microstructure having a grain size ranging from 0.3 to 2.0 μm. The origin of the magnetic hardness is discussed based on these microstructural observation results.
A consistent interpretation of the magneto-optical spectra of spinel type ferrites (invited)
Authors: Fontijn W. F. J. , van der Zaag P. J. , Feiner L. F. , Metselaar R. , Devillers M. A. C.
Published: July 2002

Through a systematic investigation of the complete dielectric tensor, between 0.5 and 5.0 eV, of Fe3O4 and of related spinel ferrites, i.e., MgFe2O4, Li0.5Fe2.5O4, NiFe2O4, and CoFe2O4, we have established that intervalence charge transfer and intersublattice charge transfer transitions dominate the optical and magneto-optical spectrum (between 0.5 and 5.0 eV) of all spinel ferrites of the general composition MexFe3−xO4. In all cases examined the same set of intersublattice charge transfer transitions was observed. These are the only transitions observed in the cases where Me is a nonmagnetic ion (Mg2+, Li+). In the cases where Me is a magnetic ion (Fe2+, Ni2+, Co2+) additional intervalence charge transfer transitions are observed. CoFe2O4 is the only spinel ferrite with a major contribution of crystal field transitions to the magneto-optical spectrum. The observed presence of only two intense crystal field transitions in specifically CoFe2O4 is explained. The observed relative strengths of these two transitions in CoFe2O4, in which remarkably the upper transition at 1.82 eV is more intense than the lower transition at 0.83 eV is also explained in a crystal field analysis.
Measurement of the electric screening field in Bi12TiO20
Authors: Cedilnik G. , Esselbach M. , Kiessling A. , Kowarschik R. , Kamshilin A. A. , Shamonina E. , Ringhofer K. H.
Published: July 2002

In Bi12TiO20 with an applied ac voltage the coupling for two-wave mixing can reach an extreme value at the transient moment tp when the internal electric field is zero. This is because of the changing polarization state of the pump wave during field reversal. The internal field is found to be the sum of the applied field and a time developing screening field arising from charge redistribution. Therefore, at the moment tp the amount of the screening field is equal to the value of the applied field and thus can be measured. Also tracking of the temporal course of the electric screening field is possible. The buildup of screening fields leads to an intensity dependence of tp for a given shape of the ac voltage.
Magnetoconductance in quantum waveguides with inhomogeneous magnetic fields
Authors: Gu Ben-Yuan , Wang Jian
Published: July 2002

We discuss the properties of magnetotransport of electrons in quantum waveguides (QWs) in the presence of laterally inhomogeneous magnetic fields perpendicular to the QW plane. The inhomogeneous magnetic fields can be produced by the deposition, on top of a heterostructure, of ferromagnetic stripes with magnetization perpendicular and parallel to the two-dimensional electron gas. It is found that the magnetoconductance in such a device as a function of the Fermi energy of electrons exhibits square-wave-like oscillations that are strongly dependent on the geometrical arrangement and magnetic configurations in the QWs. Different dispersions can be observed. From analyses of the magnetic effective potential, the dispersion relations can be understood well. In some magnetic modulations, the energy spectrum exhibits a bump superimposed upon every bulk Landau level. It is these oscillatory structures in dispersions that lead to square-wave-like shaped modulation in magnetoconductance, which may serve as an energy filter of electrons.
Magnetic coupling in amorphous Co–Er films
Authors: Benjelloun J. , Lassri H. , Driouch L. , Krishnan R. , Omri M. , Ayadi M.
Published: July 2002

Amorphous Co100−XErX thin films have been prepared by rf sputtering and their magnetic properties have been studied. The mean field theory has been used to explain the temperature dependence of the magnetization. High-field magnetization studies performed at 4.2 K in magnetic fields up to 5.5 T on amorphous Co100−XErX alloys have revealed, for samples with stoichiometry close to that of a compensated ferrimagnet, a magnetic behavior that is characteristic of a noncollinear magnetic structure of the Er and Co sublattices. From the noncollinear regime the exchange interactions between the Co and Er magnetic sublattices have been accurately evaluated.
Photoelectron bremsstrahlung spectrum in synchrotron radiation excited total reflection x-ray fluorescence analysis of silicon wafers
Authors: Shin N. S. , Koo Y. M. , Chang C. H. , Padmore H.
Published: July 2002

When synchrotron radiation is used as an excitation source, the total reflection x-ray fluorescence analysis of surface contamination on silicon wafer has an extremely low background intensity that determines the minimum detection limit. In this article, the background spectrum originating from the photoelectron bremsstrahlung is calculated using the Monte Carlo method. The doubly differential electron bremsstrahlung cross sections obtained from the Born approximation modified by the Elwert factor and with the use of the form factor approach for screening are used instead of empirical formulas. In addition to the bremsstrahlung spectrum produced from the silicon wafer, the bremsstrahlung intensity that photoelectrons, which escape from the silicon wafer, produce in the filter attached to the detector is also calculated in accordance with the usual synchrotron radiation excited total reflection x-ray fluorescence experimental conditions. The calculated photoelectron bremsstrahlung spectra are compared with experimental results and the conditions for a lower minimum detection limit are discussed.
Raman characterization of germanium nanocrystals in amorphous silicon oxide films synthesized by rapid thermal annealing
Authors: Choi W. K. , Ng V. , Ng S. P. , Thio H. H. , Shen Z. X. , Li W. S.
Published: July 2002

Raman characterization of germanium (Ge) nanocrystals embedded in amorphous silicon oxide (a-SiO2) films synthesized by rapid thermal annealing (RTA) has been carried out. The samples were prepared by cosputtering Ge and SiO2 targets using a rf magnetron sputtering machine. Ge nanocrystals can only be obtained from samples sputtered with six pieces of Ge attached to the SiO2 target. For samples annealed at different RTA temperatures, the Raman spectra indicated a transition from amorphous to nanocrystalline Ge when annealed between 600 and 750 °C. The spectra were analyzed in terms of phonon confinement model and the estimated nanocrystal size was between 20 and 66 Å. A minimum annealing time of 160 s at 750 °C was necessary for Ge nanocrystal formation. Strong visible broadband photoluminescence was observed from the nanocrystals and the photoluminescence showed a blueshift with decrease in the nanocrystal size. The effect of compressive stress on nanocrystal growth was examined by varying the rampup and rampdown rates of the RTA process. The compressive stress was shown to affect the growth of the nanocrystals.
Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals
Authors: Paillard V. , Puech P. , Laguna M. A. , Carles R. , Kohn B. , Huisken F.
Published: July 2002

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.
Emission characteristics of two-dimensional organic photonic crystal lasers fabricated by replica molding
Authors: Meier M. , Dodabalapur A. , Rogers J. A. , Slusher R. E. , Mekis A. , Timko A. , Murray C. A. , Ruel R. , Nalamasu O.
Published: July 2002

We report on the far-field emission characteristics of two-dimensional photonic crystal-based organic waveguide lasers. The photonic crystals possess square vein, triangular, and honeycomb symmetries. The two-dimensional gratings are fabricated by employing soft lithographic methods. The far-field pattern that we observe is a result of out-of-plane diffractive coupling of the laser emission generated in the plane of the waveguide. This emission pattern offers a convenient and powerful way to evaluate the nature of laser action in such resonators. In devices which possess defects/breaks in the periodicity of the two-dimensional grating, laser emission generated in-plane is scattered in the plane of the waveguide. This phenomenon is the photonic crystal analogue of Kikuchi scattering in electronic crystals.
Coevaporated Cu–In films as precursors for solar cells
Authors: Gossla M. , Metzner H. , Mahnke H.-E.
Published: July 2002

We investigate Cu–In thin films used as precursors for the production of CuInS2 and Se2 solar-cell absorber material via reactive annealing. The films are produced by coevaporation of Cu and In onto glass substrates and are characterized by means of Rutherford backscattering and x-ray diffraction (XRD). The interplay of phase composition, morphology, and surface topography is studied as a function of chemical composition, substrate temperature, and annealing processes. The analysis of the XRD data is based on known crystallographic data for the phases Cu7In3 (δ phase), Cu16In9 (η′ phase), Cu11In9, and In. Refined crystallographic data for CuIn2 are presented, and the low-temperature modification of Cu16In9 (η-phase) is investigated by means of bulk powder samples. These data and the inclusion of texture effects allow us to perform a complete RIETVELD type analysis of the Cu–In precursors. It is shown that, in contrast to sequentially evaporated films, all known Cu–In equilibrium phases can be formed during film deposition. These are Cu7In3, Cu16In9, Cu11In9, and CuIn2. Moreover, it is found that single-phase films of all these phases can be produced. Film roughness is shown to increase with deposition temperature and In content. The results presented offer new prospects for sulfurization and selenization processes in solar-cell production.
Atomic force microscopy study of laser induced phase transitions in Ge2Sb2Te5
Authors: Weidenhof V. , Friedrich I. , Ziegler S. , Wuttig M.
Published: July 2002

Micron- and submicron-size changes induced by local laser heating in thin films of Ge2Sb2Te5 are studied by atomic force microscopy (AFM). This material is presently used for rewritable data storage that employs phase change recording. Reversible switching between the amorphous and crystalline states, which is accompanied by a considerable change in optical properties and film density, is exploited to store bits of information. The density change can be detected by AFM, which we use here to study the amorphization (writing) and recrystallization (erasure) of single bits. Both processes have been measured as a function of modification pulse power and duration. Morphology changes can be detected even if the phase change film is covered by a thin protective layer. AFM allows a precise determination of the bit size and bit depth, which characterizes the progress of the phase change in the direction of the surface normal. The present setup allows the correlation of the change in optical reflectance ΔR caused by a specific laser pulse to the bit topography. This enables a detailed study of the mechanisms of bit writing and erasure.
Interfacial reaction and wetting behavior in eutectic SnPb solder on Ni/Ti thin films and Ni foils
Authors: Kim P. G. , Jang J. W. , Lee T. Y. , Tu K. N.
Published: July 2002

Ni-based under bump metallization (UBM) is of interest in low cost flip chip technology primarily due to a slower chemical reactions with high-Sn solders such as eutectic SnPb as compared to Cu-based UBM. We studied wetting behaviors and interfacial reactions of the eutectic 63Sn–37Pb on Ni foils and Ni/Ti thin films using transmission electron microscopy (TEM), scanning electron microscopy, and energy dispersion x-ray analysis. Wetting angle, morphology of solder surface, and the rate of consumption of Ni have been studies as a function of reflow time at the temperatures of 200, 220, and 240 °C. From the TEM analysis, we found that Ni forms a single layer of scallop-type Ni3Sn4 compound with the eutectic SnPb. During the isothermal annealing, we observed the spalling of Ni3Sn4 compound from the Ni/Ti thin films. The spalling phenomenon is similar to that of Cu6Sn5 from the Cu/Cr thin films, yet the rate is slower. The spalling of Ni–Sn compound eventually caused dewetting of the molten solder from the Ti surface.
Preparation and magnetic properties of Ba–Co2Z and Sr–Zn2Y ferrites
Authors: Chen In-Gann , Hsu Shu-Hau , Chang Y. H.
Published: July 2002

We have prepared Ba–Co2Z and Sr–Zn2Y ferrites by mixing SrCO3 (BaCO3), Co2O3 (ZnO), and Fe2O3 and calcinating in air and O2. The effects of sintering atmosphere (O2 and Ar) and sintering temperatures (1373–1573 K) on the structural, magnetic, and electrical properties have been measured. The Ba–Co2Z sample sintered at 1573 K under a flowing O2 exhibits superior dc electrical resistivity (∼108 Ω-cm), high initial permeability (u′∼21), and low loss factor (F∼1.6×10−3) at 20 MHz. The related XRD and SEM/EDS analyses of these samples have also been studied to correlate these properties. These results indicate that significant modification of magnetic properties up to 20 MHz can be achieved with the addition of the quaternary component and proper control of the processing oxygen atmosphere.
Temperature and gate voltage dependence of hole mobility in polycrystalline oligothiophene thin film transistors
Authors: Horowitz Gilles , Hajlaoui Mohsen E. , Hajlaoui Riadh
Published: July 2002

We have performed current–voltage measurement on polycrystalline sexithiophene (6 T) thin film transistors at temperatures ranging from 10 to 300 K. A method is developed to extract the carrier mobility from an analysis of the transfer characteristics. In particular, data are corrected for contact resistance. The carrier mobility is found to increase quasilinearly with gate voltage at room temperature. The dependence becomes superlinear at low temperatures. The temperature dependence shows three domains. For 100 K<T<300 K, the mobility is thermally activated with an activation energy of around 0.1 eV. The activation energy reduces to 5 meV for 25 K<T<100 K. Finally, the mobility is practically temperature independent for temperatures lower than 25 K. The data are explained by a model where charge transport is limited by a high concentration of traps at grain boundaries. At high temperatures, charge transfer at boundaries occurs via thermionic emission, while tunnel effect takes place at low temperatures. The energy distribution of traps is determined, and various features predicted by the model are outlined.
Occurrence of ferromagnetic shape memory alloys (invited)
Authors: Wuttig Manfred , Liu Luohong , Tsuchiya Koichi , James Richard D.
Published: July 2002

This article presents an overview of the necessary conditions for the formation of shape memory alloys (SMAs) and how these must overlap with the occurrence of ferromagnetism so that ferromagnetic shape memory alloys (MSMAs) may be formed. The electronic, elastic, and geometric conditions permit an understanding of the occurrence of Cu- and Fe-based SMAs as well as NiMnGa Heusler MSMAs except that the naively determined critical electron concentration for the latter, 7.3, disagrees with the values of 1.5 and 8.6 accepted for the Hume–Rothery phases and certain Fe-based systems.
Effect of bias voltage waveform on ion energy distribution
Authors: Rauf Shahid
Published: July 2002

Ion energy distribution (IED) is one of the primary factors governing the etching or deposition characteristics in plasma-aided microelectronics manufacturing processes. This article explores the influence of rf bias voltage waveform and frequency on the IED. It is demonstrated that the sheath voltage above the wafer is reasonably similar to the rf voltage on the biased substrate. Since the IED correlates well with the sheath voltage if the ion transit time through the sheath is smaller than the rf time period, the IED can be controlled by means of the rf bias voltage. The voltage waveform controls the shape of the distribution while the frequency determines its width. The sinusoidal waveform leads to a distribution that peaks at high energies and gradually decreases with decreasing energy. Square wave results in a sharp step in the IED at high energies, the width of which can be controlled by means of the blocking capacitance. The triangular waveform generates a constant IED over a considerable range of energy. It is also demonstrated that, by utilizing the correlation between the IED and applied voltage waveform, one can design voltage waveforms that produce distributions with specific features.
Vibrational energy levels of formaldehyde
Authors: Romanowski Hubert , Bowman Joel M. , Harding Lawrence B.
Published: July 2002

Vibrational energies for nonrotating H2CO and D2CO are calculated using unadjusted and adjusted ab initio quartic force fields in normal mass-weighted coordinates. Converged energies are obtained using uncoupled anharmonic oscillator and vibrational self-consistent field basis sets and are compared with experiment. Strong ‘‘Fermi-like’’ resonances are found involving the CH symmetric and antisymmetric stretches.
On the short time dynamics of dense polymeric systems and the origin of the glass transition: A model system
Authors: Kolinski Andrzej , Skolnick Jeffrey , Yaris Robert
Published: July 2002

In order to model the short time (and distance) scale motions for dense polymeric systems, we have performed dynamic Monte Carlo simulations of chains on a diamond lattice at considerably greater densities than those done previously. Chain dynamics were simulated by a random sequence of three- and four-bond kink motions and end moves. For times shorter than the chain diffusion time, the single bead autocorrelation function g(t) exhibits three distinct regimes: a short time Rouse-like regime where g(t)∼t1/2; a mid-region where g(t)∼t β, followed by a longer time, Rouse-like regime where g(t)∼t1/2. There is a smooth crossover from Rouse-like dynamics, β=1/2, at low density to smaller values of β at higher density, and β=0 at the glass transition density (φG =0.92±0.01). It is shown that the major motion of the chains is transverse to the chain contour rather than along the chain. The observed motion is successfully analyzed in terms of the motion of defects (holes) through the sample. It is shown that the glass transition at φG =0.92 is caused by the shutting down of the orientation changing four-bond motions.
Molecular dynamics simulation of polymer liquid and glass. I. Glass transition
Authors: Rigby David , Roe Ryong-Joon
Published: July 2002

Molecular dynamics simulation of bulk liquid and glass of long-chain molecules has been performed. The system consists of linear chains of up to 50 spherical segments, each subject to forces due to bond stretching, bending, and torsion, and to nonbonded interaction, according to a truncated Lennard-Jones potential, between segments in neighboring chains and between segments separated by more than three bonds along the chain. The parameters are chosen to mimic polymethylene, the segment representing a CH2 unit. Behaviors suggestive of liquid-to-glass transition were exhibited by (i) cessation of trans–gauche conformational transitions, (ii) changes in the temperature coefficients of the density and internal energy, and (iii) effective vanishing of the segmental self-diffusion coefficient. The ‘‘freezing in’’ of these properties occurs at decreasing temperatures in the order given above, indicating the decreasing size of domains of cooperative motion required. The dependence of the transition temperature on the chain length and on the flexibility of the chain (effected by switching of the torsional potential off) obtained is in accord with experimental observations. Below the transition temperature the system behavior depends on the path through which the state was reached, suggesting that simulation of relaxation effects could be achieved in longer runs.
Polarization effects in the valence and inner-shell spectra of poly(di-n-hexylsilane)
Authors: McCrary V. R. , Sette F. , Chen C. T. , Lovinger A. J. , Robin M. B. , Stöhr J. , Zeigler J. M.
Published: July 2002

The polarized absorption spectra of oriented films of phase-I and phase-II poly(di-n-hexylsilane), PDHS, have been determined in the visible/UV and in the near-edge regions of the C 1s and Si 1s x-ray ionization potentials. The phase-I absorption band at 360 nm is strongly polarized parallel to the direction of the Si chain, whereas the analog of this band in phase II (317 nm) is only weakly polarized in the same direction; this is consistent with partial trans→gauche isomerization of the Si chain in phase II and delocalization of the excitation in part onto disordered n-hexyl groups. Polarized absorption in the vicinity of the Si 1s ionization limit reveals strong polarization of the discrete and continuum transitions in both phase I and phase II, whereas the equivalent spectra taken in the vicinity of the C 1s limit show strong polarization in phase I but little or no polarization in phase II. The x-ray spectra are interpreted as showing that the Si chain and the n-hexyl groups of the polymer are ordered in phase I and are essentially perpendicular to one another, whereas in phase II the n-hexyl chains are disordered while the Si backbone remains relatively fixed in space. The Si 1s electron-yield spectra also display EXAFS interferences which have been transformed to yield Si–Si and Si–C distances in the polymer of 2.37±0.02 and 1.81±0.03 Å, respectively. The various discrete and continuum transitions are assigned according to an orbital model, and the visible/UV thermochromism earlier reported for PDHS is now observed in the Si 1s x-ray near-edge spectrum as well.