bluuebunny/crossref_metadata_2025 huggingface dataset:
Broadband optical amplifier based on a conjugated polymer
Authors: Lawrence J. R. , Turnbull G. A. , Samuel I. D. W.
Published: July 2002

We demonstrate a compact, broadband optical amplifier using the conjugated polymer poly[2-methoxy-5-(2′,6′-dimethyloctyloxy)-paraphenylenevinylene] (OC1C10–PPV) in dilute solution. Gains of 30–40 dB in a wavelength range of 575–640 nm, corresponding to a 50 THz bandwidth, are observed due to the broad luminescence spectrum and large cross section for stimulated emission of the polymer. The variation in gain as a function of solution concentration and probe intensity is examined. For a 1 cm path length we observe a small signal gain of 44±1 dB, and deduce a stimulated emission cross-section for OC1C10–PPV of (5.3±0.6)×10−17 cm2.
Phase speciation by extended x-ray absorption fine structure spectroscopy
Authors: Frenkel Anatoly I. , Kleifeld Oded , Wasserman Stephen R. , Sagi Irit
Published: July 2002

The application of x-ray absorption spectroscopy methods to both materials and life sciences is well appreciated. However, the power of extended x-ray absorption fine structure (EXAFS) spectroscopy as a quantitative structural technique has largely been limited by its application to the microscopically homogeneous systems, in which the local environment around each absorbing atom in the sample is the same. The growing interest in time-resolved EXAFS studies of systems in physics, chemistry, biology, and materials science has reintroduced the requirement for an analytical tool to probe heterogeneous mixtures in situ. While long being recognized as a premiere technique for this role, EXAFS studies of mixtures have been particularly difficult due to the strong model dependence and correlations between parameters in the fit. To circumvent these drawbacks, we introduce two new techniques in EXAFS analysis: the principal component analysis and the residual phase analysis. Using a test case of a heterogeneous mixture of two organometallic Co compounds, we demonstrate that these new EXAFS modeling techniques, together with the existing one, the multiple datasets fit method are the most suitable and adequate methods for phase speciation. In addition, we discuss the application of these data analysis approaches to biological systems.
Discrete version of the SHE asymptotics: multigroup neutron transport equations
Authors: Goudon Thierry , Mellet Antoine
Published: July 2002

This paper is devoted to the derivation of multigroup diffusion equations from the Boltzmann equation. The limit system couples the energy levels from both zeroth-order term and diffusion currents.
A molecular-dynamics simulation study of the influence of attractive dispersion interactions on the phase behavior of rigid bead-necklace molecules
Authors: Tian Pu , Smith Grant D.
Published: July 2002

We have performed a molecular-dynamics simulation study of the influence of attractive dispersion interactions on the phase behavior of an ensemble of rigid bead–necklace molecules by gradually scaling the bead–bead interaction from a purely repulsive potential to a full Lennard-Jones potential. The system consisted of 600 bead–necklace molecules each composed of 11 interaction centers (beads). For all values of the attractive potential investigated the system manifested a nematic phase and a smectic A phase and both the isotropic–nematic and nematic–smectic A transitions were found to be weakly first order. The Gibbs–Duhem integration method was employed to trace the isotropic–nematic and nematic–smectic A(P,T) coexistence curves over the range of attractive interactions. With increasing attraction, the temperature–pressure range over which each phase was stable deceased to the advantage of the more ordered coexisting phase with a consequent narrowing of (P,T) space for the mesophases. Systems with weak attractions manifested transition strengths (entropy of transition) for the isotropic–nematic and nematic–smectic A transitions that were only weakly dependent upon temperature–pressure, while systems with stronger attractions exhibited transition strengths that decreased with increasing temperature. The phase behavior and thermodynamics of the isotropic–nematic and nematic–smectic A phase transitions for the systems with relatively strong dispersion interactions were found to be more representative of the behavior of real liquid crystalline materials than systems dominated by repulsive interactions.
The x-ray surface forces apparatus for simultaneous x-ray diffraction and direct normal and lateral force measurements
Authors: Golan Yuval , Seitz Markus , Luo Ci , Martin-Herranz Ana , Yasa Mario , Li Youli , Safinya Cyrus R. , Israelachvili Jacob
Published: July 2002

We describe the experimental setup and principles of operation of the second-generation x-ray surface forces apparatus that allows for the first time simultaneous x-ray scattering and direct force measurements.
Cardiac arrhythmias and circle mappingsa)
Authors: Arnold V. I.
Published: July 2002

The description of the Gelfand–Tsetlin model is presented as excerpts from the author’s diploma dissertation on the mappings of a circle to itself.
Interspike interval embedding of chaotic signals
Authors: Sauer Tim
Published: July 2002

According to a theorem of Takens [Lecture Notes in Mathematics (Springer-Verlag, Berlin, 1981), Vol. 898], dynamical state information can be reproduced from a time series of amplitude measurements. In this paper we investigate whether the same information can be reproduced from interspike interval (ISI) measurements. Assuming an integrate-and-fire model coupling the dynamical system to the spike train, there is a one-to-one correspondence between the system states and interspike interval vectors of sufficiently large dimension. The correspondence implies in particular that a data series of interspike intervals, formed in this manner, can be forecast from past history. This capability is demonstrated using a nonlinear prediction algorithm, and is found to be robust to noise. A set of interspike intervals measured from a simple neuronal circuit is studied for deterministic structure using a prediction error statistic.
Construction of the Jordan basis for the Baker map
Authors: Fox Ronald F.
Published: July 2002

The Jordan canonical form basis states for an invertible chaotic map, the Baker map, are constructed. A straightforwardly obtained recursion formula is presented for construction of the Jordan states and of the spectral decomposition of the Frobenius–Perron evolution operator. Comparison of this method with earlier, subdynamics techniques demonstrates that it is much more direct and simpler. The physical significance of the Jordan states is approached from the point of view of an entropy evolution equation. The method is also applied to the Bernoulli map, yielding its eigenstates more straightforwardly than done previously.
Entropy evolution for the Baker map
Authors: Fox Ronald F.
Published: July 2002

Gibbs entropy is invariant for the Baker map. A Jordan basis spectral decomposition of the Baker Frobenius-Perron operator suggests that any initial density evolves to the stationary density that has maximal entropy. This entropy conundrum is resolved by considering the difference between weak and strong convergence. A binary representation is used to make these points transparent.
Dynamics of a gravitational billiard with a hyperbolic lower boundary
Authors: Ferguson M. L. , Miller B. N. , Thompson M. A.
Published: July 2002

Gravitational billiards provide a simple method for the illustration of the dynamics of Hamiltonian systems. Here we examine a new billiard system with two parameters, which exhibits, in two limiting cases, the behaviors of two previously studied one-parameter systems, namely the wedge and parabolic billiard. The billiard consists of a point mass moving in two dimensions under the influence of a constant gravitational field with a hyperbolic lower boundary. An iterative mapping between successive collisions with the lower boundary is derived analytically. The behavior of the system during transformation from the wedge to the parabola is investigated for a few specific cases. It is surprising that the nature of the transformation depends strongly on the parameter values.
Magnetic properties of rare-earth-iron-boron permanent magnet materials
Authors: Sagawa M. , Fujimura S. , Yamamoto H. , Matsuura Y. , Hirosawa S.
Published: July 2002

Static magnetic measurements have been carried out on single crystals of Nd2Fe14B, Sm2Fe14B, and Y2Fe14B from 4.2 to 590 K. Values of K1 estimated from high field measurements at room temperature are 4.5, −12, and 1.1 MJ/m3 for Nd2Fe14B, Sm2Fe14B, and Y2Fe14B, respectively. Anisotropic behavior of the magnetization versus magnetic field curves in the basal plane has been observed for Sm2Fe14B, indicating large amplitude of the high order coefficients, K2 and K3. In Nd2Fe14B, the magnetization has been found to tilt from the c axis and simultaneously increase in magnitude. Average Fe moment is estimated to be 2.23 μB/Fe at 4.2 K from the saturation magnetization of Y2Fe14B.
Strain in evaporated Nb thin films
Authors: Murakami Masanori , Yogi Tadashi
Published: July 2002

Strain behavior of polycrystalline Nb thin films deposited onto oxidized silicon substrates using an electron gun in a high vacuum system was studied by an x-ray diffraction technique. The sign and magnitude of strain in Nb thin films were found to have a strong dependence on the film thickness. For films thinner than approximately 10 nm, the compressive strain was observed, which was considered to be due to diffusion of impurity atoms, most likely oxygen, into grain boundaries of Nb films within an extremely short distance from the film surface. For thicker films, the tensile strain was observed and a strong dependence of the strain (normal to the film surface) on crystal orientations of grains was observed. This dependence agreed very well with the one calculated using a biaxial stress model. The biaxial stress applied in the planar direction was calculated to be 1.0×1010 dyne/cm2 from the measured strain values. The present result indicates that the strain in the Nb films is not induced by temperature increase during the film preparation, but it is more likely due to grain growth during and/or immediately after the film deposition. Based on the biaxial stress model and strain distributions parallel and normal to the substrate planes and strain energies stored in grains with different crystal orientations were calculated.
Direct writing in self-developing resists using low-power cw ultraviolet light
Authors: Bjorkholm J. E. , Eichner L. , White J. C. , Howard R. E. , Craighead H. G.
Published: July 2002

Direct writing of micron-sized features in self-developing photoresists is demonstrated using less than 1 mW of cw ultraviolet light at 257 nm. No energy density or intensity thresholds for ablation are observed. Ablation depth depends only on the deposited energy density and is independent of the rate of deposition; that is, the self-developing process is reciprocal. We have determined that an environment containing oxygen is necessary for complete self-developing; in the absence of O2 the process is self-limiting. Patterns created in poly(methyl methacrylate) have been faithfully reproduced into silicon substrates using reactive-ion etching.
Collective focusing of a charge-neutral ion beam with warm electrons
Authors: Robertson Scott
Published: July 2002

A theoretical model is presented for the collective magnetic focusing of an intense ion beam which has been neutralized by a gas of warm electrons. This extends earlier work in which it was assumed that the electrons were cold and comoving. The field necessary to guide the beam at constant radius is found.
Low-frequency admittance measurements on the HgCdTe/Photox SiO2 interface
Authors: Tsau G. H. , Sher A. , Madou M. , Wilson J. A. , Cotton V. A. , Jones C. E.
Published: July 2002

The complex admittance of an n-type Hg1−xCdxTe/Photox SiO2 interface with x=0.3 has been examined for frequencies ranging between 1 mHz and 4 MHz. The conductance method is used to decompose the total interface state density into three types of components: a valence-band tail, a conduction-band tail, and some well-resolved discrete states. The fixed charge density is low and there is no statistical broadening. The surface valence- and conduction-band edges are both found to be shifted upward in energy relative to their respective bulk values; moreover, the surface has converted to p type. The energy variation of the valence-band tail states response times follows a pattern characteristic of Shockley–Read recombination centers with a constant capture cross section, but the behavior of the conduction-band tail states is more complicated. Evidence is presented that the interface region has a higher Cd concentration than the bulk.
The electronic properties of plasma-deposited films of hydrogenated amorphous SiNx (0<x<1.2)
Authors: Lowe A. J. , Powell M. J. , Elliott S. R.
Published: July 2002

We present the results of a comprehensive series of measurements on glow-discharge (plasma) -deposited silicon nitride films SiNx:H, with x in the range 0<x<1.2. Optical spectroscopy in the visible and infrared regions is used to investigate the nature of the bonding and to assess the role of hydrogen. With increasing x, in the range x<0.7, an increase in the concentration of Si-H bonds results in an increase in the total hydrogen content; at higher x the rise in the N-H concentration produces a small increase in the hydrogen content, but even for these samples most of the hydrogen is bonded to silicon. The optical absorption edge due to band-gap transitions broadens with increasing x due to a change in the nature of the valence band from Si-Si bonds to N lone-pair states. Electrical conductivity at high fields and magnetic resonance measurements give information about the defects in the band gap. These results support the Robertson–Powell model in which the principal defect in the band gap of silicon nitride is the silicon dangling bond.
Theoretical analysis of soft x-ray generation in the plasma of a spherical pinch
Authors: Savic P. , Panarella E.
Published: July 2002

The production of soft x rays in the plasma of a spherical pinch is estimated, using the simple concepts of Bernstein and Comisar. It is shown that a relatively minor modification of the spherical pinch dynamics is required to account for the effect of radiation loss on the behavior of the plasma. The various time constants governing the plasma are examined and it is shown that of these, the confinement or ‘‘bounce’’ time, the radiation decay time, and the electron-ion equilibration time are the most important. A computer program has been developed which shows that, to retain most of the energy of the discharge in the electrons and thereby maximize the x-ray exposure, it is important to choose the correct time relationship between the central spark and the peripheral inductive discharge of the pinch.
Correlation between the cohesive energy and the onset of radiation-enhanced diffusion in ion mixing
Authors: Cheng Y.-T. , Zhao X.-A. , Banwell T. , Workman T. W. , Nicolet M-A. , Johnson W. L.
Published: July 2002

A correlation between the cohesive energy of elemental solids and the characteristic temperature Tc for the onset of radiation-enhanced diffusion during ion mixing is established. This correlation enables one to predict the onset of radiation-enhanced diffusion for systems which have not yet been investigated. A theoretical argument based on the current models of cascade mixing and radiation-enhanced diffusion is provided as a basis for understanding this observation.
Theory of resonant tunneling in a variably spaced multiquantum well structure: An Airy function approach
Authors: Brennan K. F. , Summers C. J.
Published: July 2002

A theoretical study of resonant tunneling in multilayered heterostructures is presented based on an exact solution of the Schrödinger equation under the application of a constant electric field. By use of the transfer matrix approach, the transmissivity of the structure is determined as a function of the incident electron energy. The approach presented herein is easily extended to many layer structures where it is more accurate than other existing transfer matrix or Wentzel–Kramers–Brillouin (WKB) models. The transmission resonances are compared to the bound-state energies calculated for a finite square well under bias using either an asymmetric square-well model or the exact solution of an infinite square well under the application of an electric field. The results show good agreement with other existing models as well as with the bound-state energies. The calculations were then applied to a new superlattice structure, the variably spaced superlattice energy filter, which is designed such that under bias the spatial quantization levels fully align. Based on these calculations, a new class of resonant tunneling superlattice devices can be designed.
Ultraviolet laser ablation of polyimide films
Authors: Srinivasan R. , Braren B. , Dreyfus R. W.
Published: July 2002

Pulsed laser radiation at 193, 248, or 308 nm can etch films of polyimide (DuPont KaptonTM). The mechanism of this process has been examined by the chemical analysis of the condensible products, by laser-induced fluorescence analysis of the diatomic products, and by the measurement of the etch depth per pulse over a range of fluences of the laser pulse. The most important product as well as the only one condensible at room temperature is carbon. Laser-induced fluorescence analysis showed that C2 and CN were present in the ablation plume. At 248 nm, even well below the fluence threshold of 0.08 J/cm2 for significant ablation, these diatomic species are readily detected and are measured to leave the polymer surface with translational energy of ∼5 eV. These results, when combined with the photoacoustic studies of Dyer and Srinivasan [Appl. Phys. Lett. 48, 445 (1986)], show that a simple photochemical mechanism in which one photon or less (on average) is absorbed per monomer is inadequate. The ablation process must involve many photons per monomer unit to account for the production of predominantly small (<4 atoms) products and the ejection of these fragments at supersonic velocities.
Refractive indices of polymer-dispersed liquid-crystal film materials: Epoxy-based systems
Authors: Vaz Nuno A. , Montgomery G. Paul
Published: July 2002

Polymer-dispersed liquid crystal (PDLC) films are potentially useful in applications requiring electrically controllable light transmission. In these applications, both a high on-state transmittance and a strong off-state attenuation are often needed over a wide operating temperature range. These transmittance characteristics depend strongly on the refractive indices of the materials in the PDLC films. We have measured the temperature dependent refractive indices of typical PDLC film materials and the temperature dependent electro-optic transmittance of a PDLC film composed of liquid crystal microdroplets dispersed in an epoxy matrix. We show that our refractive index measurements can account for all the features in the measured transmittance characteristics and discuss several methods for controlling refractive indices to optimize electro-optic transmittance over an extended temperature range. We have also measured the room temperature refractive indices of mixtures of epoxy resins and hardeners as a function of composition. We discuss the problems associated with predicting the refractive indices of such mixtures in terms of either the volume fractions or mole fractions of the mixture components. These considerations are important in matching refractive indices of droplets and matrix materials to maximize on-state transmittance. The refractive indices of epoxy matrix materials increase monotonically with time during their chemical cure. The measured time dependence can be described by a simple model in which the concentrations of the reacting resin and hardener each decay exponentially in time with their own characteristic time constants while the concentration of the cured polymer increases. Finally, we relate the measured rates of index change with temperature to the coefficients of volume expansion of PDLC film materials; the results are used to discuss the mechanical stability of PDLC films.
Electro-optical properties of guest-host nematic liquid-crystal displays
Authors: Ong Hiap Liew
Published: July 2002

Analytic expressions have been obtained for transmissive and reflective intensities for an absorbing guest-host nematic liquid-crystal display as functions of the liquid-crystal material parameters (elastic constants, dielectric constants, refractive indices, and absorption coefficients), pretilt angle, cell thickness, applied voltage, and viewing angle in the principal viewing plane. Excellent agreement between the theoretical and experimental results is demonstrated.
Crystallization and texturing in rapidly quenched Nd2Fe14B1 and Nd15Fe77B8
Authors: Tu Guo-hau , Altounian Z. , Ryan D. H. , Ström-Olsen J. O.
Published: July 2002

The crystallization and texturing in amorphous Nd2 Fe14 B and Nd15 Fe77 B8 were investigated. The crystallization kinetics for both compositions were similar with an activation energy of 3.5 eV, an Avrami exponent of 3.4, and crystallization enthalpy of 5.1 kJ/mol. Extreme texturing, with the c axis perpendicular to the ribbon plane, is observed in the slow quench materials. Similar, but less pronounced, effects are seen in the recrystallized alloys. In some cases surface oxidation was found to give the samples the appearance of texture due to a combination of Nd2 O3 and alpha-iron reflections.
The formation of an amorphous silicide by thermal reaction of sputter-deposited Ti and Si layers
Authors: Raaijmakers Ivo J. M. M. , Reader Alec H. , Oosting Piet H.
Published: July 2002

The initial reaction in amorphous Si-Ti-amorphous Si trilayers was investigated with Auger electron spectroscopy, transmission electron microscopy, and x-ray diffraction. It was clearly demonstrated that at temperatures not exceeding 450 °C an amorphous Ti-Si alloy is formed. At temperatures of 500 °C and higher, crystalline TiSi2 with the ZrSi2 (C49) structure was found. The growth kinetics of the amorphous silicide could not be described by a simple diffusion controlled process. It was shown that at 400 °C the thickness of the amorphous silicide is limited to approximately 18 nm. Since it was found that the growth of the amorphous phase is accompanied by excessive Kirkendall void formation, it is proposed that these voids eventually suppress the growth of amorphous silicide. The composition of the amorphous phase was determined to be between TiSi0.9 and TiSi1.2, i.e., close to the composition of the monosilicide.
Analog simulation of superconducting loops containing one or two Josephson junctions
Authors: Blackburn James A. , Wu Binruo , Smith H. J. T.
Published: July 2002

Analog circuits are described which are capable of electronically simulating the static and dynamic behavior of superconducting loops containing one or two Josephson junctions when bias currents or magnetic fields are applied. Time-dependent flux entry into or out of the ring can, in either system, be observed by monitoring appropriate node voltages within the simulator circuits. The various dynamical modes observed in earlier numerical simulations are accurately reproduced. A theoretical analysis of the two-junction configuration identifies certain important criteria which determine which of these different states the system will adopt.
Segregation of impurities in pulsed-laser-melted carbon
Authors: Steinbeck J. , Braunstein G. , Dresselhaus G. , Dresselhaus M. S. , Venkatesan T. , Jacobson D. C.
Published: July 2002

The segregation of 73Ge and 75As in pulsed-laser-melted carbon has been investigated. Both 73Ge and 75As were implanted into highly oriented pyrolytic graphite at a fluence of 1.0×1015 cm−2 at several energies. The implanted graphite was subsequently irradiated with a 30-ns pulsed ruby laser with laser pulse energy densities above the melt threshold for graphite. The distribution of impurities was measured before and after laser irradiation using Rutherford backscattering spectrometry to determine the redistribution of impurities resulting from diffusion in liquid carbon and segregation at the liquid-solid interface. Numerical calculations were then used to determine the diffusivity of the impurities in liquid carbon and the nonequilibrium segregation coefficient of Ge and As in carbon.
Mechanical properties of a new class of metallic glasses based on aluminum
Authors: Shiflet G. J. , He Y. , Poon S. J.
Published: July 2002

The mechanical properties (i.e., tensile fracture strength and Young’s modulus) of eight different alloys of a new class of metallic glasses containing up to 90 at. % aluminum are reported along with crystallization temperatures of these alloys. The Al90Fe5Ce5 material has a tensile fracture strength of 940 MPa (1 MPa=145 psi), while two others (Al87Fe8.7Gd4.3 and Al87Ni8.7Y4.3) exceed 800 MPa. Young’s modulus measurements for three of these exceed 60 GPa with a high value of 66 GPa for the Al90Fe5Ce5 glass. These unusually high strengths of the aluminum glasses can be of significant importance in obtaining high-strength, low-density materials.
A Stirling engine with a liquid working substance
Authors: Swift G. W.
Published: July 2002

A theoretical and experimental study of a Stirling engine using a liquid (propylene) as its working substance is presented. This is the first experimental work on liquid Stirling engines (or Malone engines) since the 1920s. Liquid-Stirling technology appears advantageous to conventional freon-based Rankine-cycle technology in many applications. The theory developed here is simple, intuitive, and modular because of the nature of liquid working fluids and because the temperature spanned by liquid Stirling engines is not large. The experimental engine is extremely versatile, and can operate as either a heat pump or prime mover. Extensive measurements with it are in substantial agreement with the simple theory.
Selenium and zinc doping in Ga0.5In0.5P and (Al0.5Ga0.5)0.5In0.5P grown by metalorganic chemical vapor deposition
Authors: Ikeda M. , Kaneko K.
Published: July 2002

Electrical properties of Se- and Zn-doped Ga0.5In0.5P and (Al0.5Ga0.5)0.5In0.5P grown by atmospheric pressure metalorganic chemical vapor deposition under a wide range of growth conditions were investigated using van der Pauw–Hall measurements at room temperature. The dopants were hydrogen selenide and dimethylzinc. The samples were prepared so that parasitic conduction in the GaAs substrate just adjacent to the ternary or quaternary layers could be eliminated from the Hall measurement. The carrier concentration of GaInP and AlGaInP increased as the 0.8±0.1th power of the feed amount of dopants for both conductivity types. At a growth temperature around 680 °C, the hole concentration tended to saturate near the 1018 cm−3 level as the amount of dimethylzinc being fed increased. The carrier concentration decreased with increasing growth temperature, with apparent activation energies of 0.95 eV for Se doping and 1.9 eV for Zn doping. The group-V to group-III feed ratio had a weak influence on the carrier concentration. On the other hand, the Hall mobility of the layers grown under the various growth conditions remained almost constant: the electron mobilities of Se-Ga0.5In0.5P and Se-(Al0.5Ga0.5)0.5In0.5P within the carrier concentration range of 1017 <n<1018 cm−3 were 950–700 and ∼100 cm2/V s, respectively. The hole mobilities of Zn-Ga0.5In0.5P and Zn-(Al0.5Ga0.5)0.5In0.5P within the carrier concentration range of 1017 <p<1018 cm−3 were ∼34 and ∼16 cm2/V s, respectively.
Electron density in quantum well diodes
Authors: Jogai B. , Huang C. I. , Bozada C. A.
Published: July 2002

A self-consistent calculation of the electron density and conduction-band edge is presented. The time-independent Schrödinger and Poisson equations are solved simultaneously under a high applied bias for structures with thick, lightly doped spacer layers. It is shown that strongly localized states occur in the well and accumulation regions. These states are capable of trapping a substantial amount of charge, which in turn can drastically change the shape of the band edge, and therefore need to be included in studies of the resonant tunneling problem in which the Coulomb interaction must be accounted for.
Stable damping associated with linear viscous motion of the interface in a multiphase Al-Zn alloy
Authors: Zhu Xianfang
Published: July 2002

In the present paper, the characteristics of low-frequency stable damping are studied in a multiphase eutectoid Al-Zn alloy. Experimental results show that, within a low-temperature and low-strain-amplitude range, the damping evidently manifests itself by linear viscous characteristics, namely, it obeys a law Q−1=(B/fn)exp(−nH/kT), where H is the real process activation energy, B and n (=0.21) are two experimental parameters, and k is the Boltzmann constant. A value of H=0.74 eV is obtained, which is closely related to the viscosity during interface motion. Accordingly, a linear viscous interface motion mode is put forward and can explain the experimental results as well.
SiO diffusion during thermal decomposition of SiO2
Authors: Sasse H.-E. , König U.
Published: July 2002

Chemically prepared thin oxide films were decomposed in an ultrahigh vacuum at temperatures of between 760 and 850 °C. By measuring the remaining oxide thickness as a function of time we could extract the diffusion coefficient of SiO in SiO2. This was determined to be D(cm2/s)=1.48×10−3exp(−3.2 eV/kT). The results are discussed in respect to an appropriate heat cleaning of wafers in a Si-molecular-beam epitaxial system.
The carrier effects on the change of refractive index for n-type GaAs at λ=1.06,1.3, and 1.55 μm
Authors: Huang H. C. , Yee S. , Soma M.
Published: July 2002

A numerical Kramers–Kronig analysis is used to calculate the refractive-index change Δn caused by the injection/depletion of free carriers in various doped n-type GaAs. The analysis makes use of a carrier-related absorption spectrum, which is established by using quantum theory as well as empirical relations and is confirmed by the experimental absorption data in the literature. We obtain the Δn values for various doping concentrations ND and carrier concentrations N at three wavelengths; λ=1.06, 1.3, and 1.55 μm. The Δn value is positive for low-N region, and increases gradually to its maximum which is around 10−4 for λ=1.06 μm. Thereafter, Δn decreases rapidly to 0 as N increases. The linear relation between Δn and N, as predicted by the Drude theory, only happens when N is beyond a certain value. In this region, the Δn value may attain to −10−2 at N=5×1018 cm−3 for λ=1.55 μm. Because of this significant Δn value and its linear relation with N, the free-carrier induced index-change effect may find the applications in integrated optics and optical probing.
Crystallization, field-induced phase transformation, thermally induced phase transition, and piezoelectric activity in P(vinylidene fluoride-TrFE) copolymers with high molar content of vinylidene fluoride
Authors: Koga Keiko , Nakano Nobuko , Hattori Takeshi , Ohigashi Hiroji
Published: July 2002

Field-induced phase transformation and thermally induced phase transition in vinylidene fluoride (VDF) and trifluoroethylene copolymers with VDF content higher than 82 mol% and their ferroelectric and piezoelectric properties have been studied by means of x-ray diffraction, DSC, D-E hysteresis, and piezoelectric resonance. It is found that copolymers with VDF content ranging from ∼82 to 90 mol% crystallize under ordinary pressure into thick lamellar single crystals composed of a mixture of α, β, and possibly γ phases. Application of a strong ac electric field transforms these mixed phase crystals into β-phase crystals completely. The β-phase crystals are ferroelectric and exhibit strong piezoelectric activity stable up to the melting temperature (160–180  °C depending on VDF content); the Curie point coincides with the melting point. Unpoled, mixed phase film has a paraelectric or rotational phase below the melting temperature, in which thick lamellar single crystals are grown extensively, whereas β-phase film prepared by mechanical drawing has no paraelectric phase below the melting point. Definite ferroelectric-to-paraelectric phase transition in P(VDF-TrFE) is inferred to occur only in thick lamellar crystals grown in the paraelectric phase. A phase diagram of P(VDF-TrFE) is described.
Epitaxial relations between i
n s
i
t
u superconducting YBa2Cu3O7−x thin films and BaTiO3/MgAl2O4/Si substrates
Authors: Hwang D. M. , Ramesh R. , Chen C. Y. , Wu X. D. , Inam A. , Hegde M. S. , Wilkens B. , Chang C. C. , Nazar L. , Venkatesan T. , Miura S. , Matsubara S. , Miyasaka Y. , Shohata N.
Published: July 2002

I n situ superconducting YBa2Cu3O7−x films with Tc0 up to 87 K and Jc, 77 K up to 6×104 A/cm2 were prepared on Si substrates with MgAl2O4 and BaTiO3 double-buffer layers. The epitaxial relations between various layers were established by transmission electron microscopy. The MgAl2O4 layer is heavily faulted. The subsequent BaTiO3 layer stops most of the faults, provides a template for the YBa2Cu3O7−x growth, and partially screens off the stress due to different thermal expansion coefficients. The microstructure of the YBa2Cu3O7−x layer is very similar to that of the films deposited directly on SrTiO3, exhibiting a homogeneous heavily faulted single-crystal-like structure free from secondary phases and grain boundaries. The slight degradation of the transport properties is attributed to residual thermal stress.
Properties of diamond membranes for x-ray lithography
Authors: Windischmann Henry , Epps Glenn F.
Published: July 2002

Microwave plasma chemical vapor deposition was used to deposit diamond films on polished silicon and then the substrate was back-etched to form taut free-standing membranes up to 75 mm diam. Raman spectroscopy and x-ray diffraction verify that the films are diamond. Transmission electron microscopy and x-ray diffraction reveal that the films are fine-grained polycrystalline diamond with an average grain size between 15 and 110 nm. The films contain microcrystalline graphite and nondiamond carbon as indicated by the broad Raman bands at 1355 and 1560 cm−1, respectively. The measured x-ray transmission of a 3.25-μm-thick membrane at the Cu Kα line is 99.2% while the optical transmission at the He-Ne laser line is 58% for a 1-μm membrane, uncorrected for reflection losses from both surfaces, absorption, and scatter. The transmission in the ir is at the theoretical limit, 71%. The surface roughness of a typical 3-μm-thick membrane is about 30 nm on the side away from the silicon substrate and 20 nm on the side adjacent to the substrate. The surface roughness scales inversely with methane fraction in the supply gas due to decreasing grain size. The typical membrane stress is 0.03–0.15 GPa. The films contain between 0.8% and 7.0% hydrogen, as determined by 15N nuclear reaction analysis. No correlation was observed between hydrogen content in the film and deposition temperature; however, a direct correlation was observed with methane fraction. The average in-plane distortion is 70 nm for a 1.5-μm-thick diamond mask with 0.2-μm-thick tungsten absorber. No measurable out-of-plane distortion was observed for evaporated gold absorber metallization while tungsten produced 0.8 μm distortion.
Target magnetic-field effects on deposition rate in rf magnetron sputtering
Authors: Furuya Akinori , Hirono Shigeru
Published: July 2002

This paper describes film deposition rates for rf planar magnetron sputtering when the magnetic field parallel to the target surface is varied. Magnetic-field strength is a key parameter to control deposition rate in rf magnetron sputtering but applying a high magnetic field does not increase the deposition rate. Self-bias voltage also decreases as the target magnetic field increases and plasma impedance involving the ion sheath region and glow discharge region is inductive. Plasma impedance measurements lead to the conclusion that a deteriorated deposition rate in the high magnetic field occurs due to decreased self-bias voltage resulting from an increased sheath capacitance.
Photoacoustic and photothermal beam deflection as a probe of laser ablation of materials
Authors: Sell Jeffrey A. , Heffelfinger David M. , Ventzek Peter L. G. , Gilgenbach Ronald M.
Published: July 2002

Photoacoustic and photothermal laser-beam deflection were applied as diagnostics of the pulsed ultraviolet (UV) laser ablation of a polymer polyethyleneterephthalate. Here, a continuous-wave (cw) laser beam is passed parallel to the sample, but displaced from it by a few hundred micrometers. A density gradient caused by the pulsed UV laser heating or ablation of the sample deflects the cw laser beam. This deflection is measured directly using a position-sensitive detector. A quantitative model of the photothermal deflection at low fluence was developed which fits the data very well. This enabled a new method of measuring the thermal diffusivity of the fluid in contact with the sample. Distortion of the photothermal and photoacoustic signal as the excimer fluence is raised through the ablation threshold allowed the determination of the threshold. Also, the velocity of the ablation products was measured through a time-of-flight analysis and found to be dependent on the laser fluence used, the nature of the gas above the sample, and the distance above the sample at which the velocity is measured. The beam deflection in a vacuum is used to measure the ablation product velocity.
Deep impurity levels and diffusion coefficient of manganese in silicon
Authors: Nakashima H. , Hashimoto K.
Published: July 2002

Manganese-related deep levels in n- and p-type silicon have been investigated by deep level transient spectroscopy and Hall effect. Two electron traps of Ec−(0.12±0.01) eV and Ec−(0.41±0.01) eV, and a hole trap of Ev+(0.32±0.01) eV are found in manganese-doped silicon. The energy levels of these traps correspond to the transitions between four charge states (Mn−, Mn0, Mn+, Mn++ ) of interstitial manganese. An additional donor-type electron trap of Ec−(0.51±0.02) eV is observed in the n-type samples, and the trap can be tentatively assigned to substitutional manganese. Furthermore, an electron trap of Ec−(0.50±0.02) eV is observed for n+p junction samples diffused with manganese in boron-doped p-type silicon. The trap is attributed to the manganese-boron complex, which is formed owing to the pairing reaction of interstitial manganese and substitutional boron. From the investigation of the pairing reaction, the diffusion coefficient DMn of interstitial manganese is determined in the temperature range 14–90 °C. It can be represented by the expression DMn=2.4×10−3 exp(−0.72/kT)cm2 s−1.
Heat accumulation function and optimization of heat engines
Authors: Lampinen Markku J. , Vuorisalo Jari
Published: July 2002

The second law of thermodynamics is presented in terms of the heat accumulation function. The meaning of the heat accumulation functions is illustrated by a practical example. The optimization problem of heat engines is formulated with the aid of the heat accumulation function at a very general level. As an example, we have studied a heat engine process where the heat input and output changes are limited by the finite heat capacity of the combustion air. We have obtained formulas for estimating the maximum efficiency for heat engines as a function of the ratio of the combustion heat and the heat capacity of the combustion air. Representative numerical examples are presented.
Interaction of Co with Si and SiO2 during rapid thermal annealing
Authors: Chen W. D. , Cui Y. D. , Hsu C. C. , Tao J.
Published: July 2002

The interaction of Co with Si and SiO2 during rapid thermal annealing has been investigated. Phase sequence, layer morphology, and reaction kinetics were studied by sheet resistance, x-ray diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. With increasing annealing temperature, Co film on Si(100) is transformed sequentially into Co2Si, CoSi, and finally CoSi2 which corresponds to the minimum of sheet resistance. No evidence of silicide formation was observed for Co/SiO2 annealed even at the high temperature of 1050 °C.
Laser photodecomposition of sintered YBa2Cu3O6+x: Ejected species population distributions and initial kinetic energies for the laser ablation wavelengths 351, 248, and 193 nm
Authors: Wiedeman L. , Helvajian H.
Published: July 2002

We present experimental results of photoablated product and kinetic energy (KE) distributions from the ultraviolet laser ablation of a sintered YBa2Cu3O6+x wafer at the laser wavelengths 351, 248, and 193 nm. Data is presented which spans the laser fluence range beginning at the threshold for species ejection (50 mJ/cm2) to nearly that required for the formation of an above surface plasma (800 mJ/cm2). The goal of this experiment was to measure changes in the photophysical process as the incident laser fluence was increased above the threshold value. Our results show, that near the laser threshold fluence, the photoejected products consist of small atomic and oxide species. An unforeseen result for the UV laser wavelengths used, is the lack of CuO+/CuO and free oxygen (O+/O, O+2/O2) in the mass spectrum. In addition, the product distributions are dependent on the laser wavelength. Measured also, at laser threshold fluence, are the nascent photoejected cation kinetic energy distribution. Here our results show that the KE distribution is independent of both the laser fluence and the wavelength. The mean kinetic energy, 〈KE〉, exceeds 3 eV and cannot be explained by a thermal excitation process. With increasing laser fluence (50% above threshold), we detected the photodissociation of the ejected oxide species, and the appearance of the O− anion. Unlike the KE of the cation species, the O− kinetic energy is nearly thermal (<1 eV). With additional increases in the laser fluence, we measure the photoejection of slow KE neutral species and the simultaneous KE enhancement of the laser ablated ions (KE≳30 eV). At the highest laser fluences used in this experiment, it was noted that atomic cluster formations were enhanced. These compounds are presumably formed in the expanding ablated plume. In summary, our results show that, at threshold laser fluences, the photoejection process is via non thermal excitation. With increasing laser fluence the ejected species mass spectrum includes products from the plume photolysis and the plume chemistry.
Catalytic control requirements for the stable operation of the closed-cycle, transversely excited atmospheric CO2 laser
Authors: Hokazono Hirokazu , Kobayashi Naoki , Obara Minoru
Published: July 2002

Using a comprehensive theoretical CO2 laser model, we calculated the amount of the catalytic conversion from CO to CO2, which is required for the stable and long-life operation of the closed-cycle, transversely excited atmospheric CO2 laser. The amount of the catalytic conversion was evaluated by the fractional conversion (η) defined as η=([CO]in−[CO]out)/[CO]in, where [CO]in and [CO]out are the CO molecules number density at the entrance and exit of the CO2 regenerator in the laser chamber, respectively. For the laser gas mixture of CO2/N2/He=15/15/70(%) and the output coupler reflectivity of 80%, an η of the CO2 regenerator greater than 0.22 is theoretically required to maintain the 95% level of the initial laser output energy at an input energy density and a laser gas clearing ratio CR of 176.1 J/ℓ and 2.0, respectively.
Laser ablation of ZnSe using pulsed 193-nm irradiation
Authors: Simpson J. , Williams J. O.
Published: July 2002

Plasma jets have been produced by the pulsed laser ablation of bulk ZnSe targets. Electrical measurements show that a sudden increase in ion yield occurs at a threshold pulse energy density of 210 mJ cm−2 pulse−1. The plasmas obtained do not exhibit overall electrical neutrality, but have a net positive charge, while the ZnSe target develops a corresponding negative charge. The initial ion translational kinetic energies are also found to be very low, <1 eV. These observations suggest that the ablation process is the result of a buildup of photogenerated holes at the semiconductor surface sufficient to result in disintegration of the lattice and rapid vaporization at low thermal energies.
Magnetism and heavy fermion-like behavior in the RBiPt series
Authors: Canfield P. C. , Thompson J. D. , Beyermann W. P. , Lacerda A. , Hundley M. F. , Peterson E. , Fisk Z. , Ott H. R.
Published: July 2002

Members of the RBiPt (R=Ce–Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on representatives. The high temperature resistance uniformly changes from that of a small-gap semiconductor or semimetal seen in NdBiPt to that of a heavy-fermion metal seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K2 mole. Further, the lighter rare earth members show an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.
Comment on ‘‘Spreading of liquid droplets on cylindrical surfaces: Accurate determination of contact angle’’ [J. Appl. Phys. 7
0, 493 (1991)]
Authors: Carroll B. J.
Published: July 2002

This communication outlines the theory for calculation of contact angle from the shape of a liquid drop adhering to a thin fiber. Particular attention is paid to a recent paper on this subject [H. D. Wagner, J. Appl. Phys. 67, 1352 (1990)], in which an alternative computational procedure is described. Comparison of this and the previously published procedure is made.
Structural identification of the silicon and nitrogen dangling-bond centers in amorphous silicon nitride
Authors: Warren William L. , Rong F. Christopher , Poindexter Edward H. , Gerardi Gary J. , Kanicki Jerzy
Published: July 2002

We report the observation of both silicon and nitrogen paramagnetic defect centers using X-band and Q-band electron spin resonance microwave excitation frequencies. By using two different microwave frequencies along with a computer analysis of the resonance lineshapes, we have been able to confirm and extend earlier observations regarding the chemical identity of these paramagnetic defects. Specifically, we provide additional evidence that the silicon dangling bond, i.e., K center, is an unpaired electron on a silicon atom bonded to three nitrogen atoms in stoichiometric silicon nitride. We further demonstrate that the g tensor of the K center exhibits very little anisotropy and that the lineshape is broadened primarily by hyperfine interactions of the nitrogens bonded to the silicon atom. We also confirm that the recently observed nitrogen dangling-bond resonance in silicon nitride is indeed due to a hyperfine interaction with a nitrogen nucleus. This improved understanding of these two important paramagnetic defects may be of importance in eliminating or perhaps exploiting their chemical properties.
Strained GaAs layers grown on GaAs substrates with an intermediate GaAs1−xPx buffer layer
Authors: Strobl G. , Freundlich A. , Grenet J. C. , Teissere M. , Neu G.
Published: July 2002

The possibility of growing strained GaAs layers on GaAs (100) substrates using a nonpseudomorphic GaAs1−xPx buffer layers is examined. It is demonstrated that by varying the phosphorus content in a thick buffer (significantly thicker than the critical thickness for strain relaxation), uniform biaxial stress magnitude can be monitored in GaAs, e.g., 8 kbar biaxial compression can be achieved by a P composition of 0.16 in the alloy. After an x-ray diffraction study of strain relaxation in the buffers, low temperature photoluminescence measurements are used to evaluate the effect of such a stress upon monitoring the near band gap properties of GaAs layers.