bluuebunny/crossref_metadata_2025 huggingface dataset:
Photo-oxidation of germanium nanostructures deposited by the cluster-beam evaporation technique
Authors: Sato S. , Nozaki S. , Morisaki H.
Published: July 2002

The photo-oxidation of germanium (Ge) nanostructures deposited by the cluster-beam evaporation technique is systematically studied. The significant photo-oxidation is found in the Ge-liquid nitrogen temperature (LNT) film, which was deposited on substrates whose temperature was kept at liquid nitrogen temperature during the deposition but not in the Ge-RT film, whose substrate temperature was room temperature. The difference in photo-oxidation of both types of Ge films is explained by denseness of Ge nanostructures. The threshold photon energy for photo-oxidation is close to the optical band gap estimated from the absorption spectrum of the as-deposited Ge-LNT film. The modified electron-active oxidation model is proposed to explain the photo-oxidation of Ge nanostructures. It is also hypothesized that the final size of Ge cores is determined by the incident photon energy and that the size of Ge cores becomes more uniform for prolonged photo-oxidation.
The contribution of surface tension driven flows in flux line erosion
Authors: Tsotridis George
Published: July 2002

An interesting manifestation of surface tension driven flows is the phenomenon of “flux line erosion.’’ It consists of the scouring or erosion of the refractory crucible by a melt in the vicinity of a gas-melt interface. A mathematical model has been developed on the assumption that the wall erosion profile can be explained by the disruption of the dissolution concentration gradients by buoyancy and surface tension driven convective flows. The problem is formulated by considering the Navier–Stokes equations coupled with the energy and diffusion equations. The velocity field is determined by the equations of motion and energy, whereas the concentration distribution is determined by the convective diffusion equation. All these equations are mutually dependent since the surface tension is a concentration function. The concentration distribution depends on the velocity distribution, and the driving forces of the flow are the buoyancy and surface tension forces. The relative contributions of these forces in determining the shape and rate of the erosion profile have been studied. It was found that surface tension flows play a significant role in determining the erosion patterns of the refractory walls.
Effects of film thickness on the structural and magnetic properties of CoCrTa/Co3O4/Pt perpendicular thin-film media
Authors: Song Xing , Sivertsen John , Judy Jack
Published: July 2002

This paper reports the effects of film thickness of room temperature RF sputtered trilayer CoCrTa/Co3O4/Pt films on the structural and perpendicular magnetic properties of CoCrTa magnetic layer. Using a 100 Å Pt film with a weak (111) texture as the buffer layer improves the nucleation and initial growth of the (111) oriented Co3O4 seed layer, which in turn induces a significantly strong (00.2) texture in CoCrTa films. If the Pt is too thin, there are not enough (111) oriented sites created for the nucleation of Co3O4. If the Pt is too thick, the large grain size with significant size variation in the magnetic layer (originating from the thick buffer layer) leads to poor perpendicular magnetic properties. A 50 Å Co3O4 seed layer sputtered on 100 Å Pt is most effective in promoting a strong (00.2) texture in CoCrTa films by providing (111) oriented sites for (00.2) growth of CoCrTa without creating significant lattice mismatch. For CoCrTa films sputtered on this composite underlayer, the in-plane coercivity is not sensitive to the film thickness whereas the perpendicular coercivity is. The maximum Hc⊥ (perpendicular coercivity) ∼1650 Oe with an S∥ (in-plane remanant squareness) ∼13% is obtained when the film thickness is ∼2000 Å. The effect of the size of domain clusters on perpendicular coercivity Hc⊥ has also been studied by magnetic force microscopy.
Multilayers as spin-wave emitting diodes
Authors: Berger L.
Published: July 2002

Magnon-electron interaction is enhanced in the vicinity of an interface between ferromagnetic and normal layers, in metallic thin films. When a dc current crosses this interface, stimulated emission of spin-waves is predicted to take place. Beyond a certain critical current density ≃107 A/cm2, a spontaneous precession of the magnetization is predicted to arise. This SWASER (Spin Wave Amplification by Stimulated Emission of Radiation) is the magnetic equivalent of the injection laser. In the earlier theories, the thickness of the precessing magnetic layer was assumed to be larger than the electron mean free path. We now treat the case of general value of that thickness L2x. The current-induced amplification rate of spin-waves is proportional to L2x in the limit L2x≃0, and goes through a maximum for L2x≃π/|k↑−k↓|. Here, k↑ and k↓ are the spin-up and spin-down Fermi wave numbers. At larger L2x, the amplification rate of spin-waves decreases towards zero. On that general decrease are superposed damped periodic variations of wavelength ≃2π/|k↑−k↓|.
Investigation of short wavelength intersubband transitions in InGaAs/AlAs quantum wells on GaAs substrate
Authors: Asano Takashi , Noda Susumu , Abe Tomoki , Sasaki Akio
Published: July 2002

InGaAs/AlAs quantum wells on GaAs substrates were studied for near-infrared intersubband transitions. The relationships between the properties of the intersubband absorption and the structural/growth conditions of the quantum wells were quantitatively investigated by using theoretical calculations based on a k-p perturbation method. The total intersubband absorption magnitude is shown to be closely related to the leakage of free carriers from the Γ minimum of the well to the X minimum of the barrier. This work provides important information for design of devices utilizing short wavelength intersubband transitions.
Relationship between refractive indices and hydrogen concentration in proton exchanged LiNbO3 waveguides
Authors: Korkishko Yu. N. , Fedorov V. A.
Published: July 2002

Proton-exchanged LiNbO3 optical waveguides (HxLi1−xNbO3) exhibit very complex structural behavior which is different from that known for their powders. Seven crystallographic phases have been identified in the HxLi1−xNbO3 layers. A method to determine the proton concentration in different phases has been proposed. Correlation between the crystal structure, hydrogen concentration, and the ordinary and extraordinary refractive indices has been experimentally determined allowing one to explain some optical phenomena observed and to predict the characteristics of a great variety of proton exchanged waveguides.
In situ high-temperature x-ray diffraction study on domain evolution in ferroelectric (Pb,La)TiO3 epitaxial thin films
Authors: Kang Young Min , Baik Sunggi
Published: July 2002

The cooling process encountered during fabrication of epitaxial Pb1−xLaxTiO3 (PLT, x=0.00–0.12) thin films prepared on MgO(001) single-crystal substrates is simulated using an x-ray high-temperature attachment, and x-ray diffraction measurements are performed at various temperatures, during which a ferroelectric 90° domain structure has been evolved. The lattice constants and the degree of c-axis orientation (α) of the epitaxial PLT films are evaluated as a function of temperature below the deposition temperature (650 °C). The 0.00 La-PLT film shows nucleation of c domains at TC, growth of them at the expense of a domains below TC, and c-domain dominant structure at room temperature. By contrast, the 0.12 La-PLT film shows an abrupt evolution of the c-domain dominant structure at TC, and a higher degree of c-axis orientation at room temperature. The full width at half-maximum of the 003 perovskite peak is used to quantify the crystal quality of the films as a function of temperature. Considerable change in crystal quality occurs after the cubic-to-tetragonal phase transformation. Significant improvement of the crystal quality of the PLT films with high La concentration have been realized predominantly at the Curie temperature when the films transformed to the tetragonal phase.
Characterization of sputtered barium strontium titanate and strontium titanate-thin films
Authors: Baumert B. A. , Chang L.-H. , Matsuda A. T. , Tsai T.-L. , Tracy C. J. , Gregory R. B. , Fejes P. L. , Cave N. G. , Chen W. , Taylor D. J. , Otsuki T. , Fujii E. , Hayashi S. , Suu K.
Published: July 2002

Sputtered Ba1−xSrxTiO3 (BST) and SrTiO3 (STO) films and capacitors made with these dielectrics have been characterized with respect to physical and electrical properties. Specific capacitance values included a high of 96 fF/μm2 for BST films deposited of 600 °C and a high of 26 fF/μm2 for STO films deposited at 400 °C. Leakage current densities at 3.3 V for the most part varied from mid 10−8 to mid 10−6 A/cm2. All of the dielectrics are polycrystalline, although the lowest temperature STO films have a nearly amorphous layer which impacts their capacitance. Grain size increases with deposition temperature, which correlates to higher dielectric constants. The lattice parameter of the BST films is larger than that of bulk samples. Capacitance, leakage, breakdown, and lifetime results are reported.
Out-of-plane two-dimensional photonic band structure effects observed in the visible spectrum
Authors: Rosenberg A. , Tonucci R. J. , Shirley Eric L.
Published: July 2002

We have investigated the visible-region transmission spectra of light propagating in directions which lie outside the array plane of a two-dimensional photonic crystal. These photonic crystals consist of arrays of glass rods inside a glass matrix, with periodicities comparable to the wavelengths of interest. When the propagation direction is within the array plane, attenuations corresponding to the Brillouin-zone boundaries are observed, while for out-of-plane propagation all attenuation features shift to shorter wavelengths. Interesting polarization-dependent effects appear for out-of-plane propagation.
X-ray interference effect as a tool for the structural investigation of GaInAs/InP multiple quantum wells
Authors: Marschner T. , Brübach J. , Verschuren C. A. , Leys M. R. , Wolter J. H.
Published: July 2002

We present x-ray diffraction (XRD) investigations of the structure of nominally lattice-matched GaInAs/InP multiple quantum well (MQW) structures grown by chemical beam epitaxy (CBE). To obtain information about the individual MQW layers and the interface structure we make use of the x-ray interference effect between two layers of equal lattice constant but different layer thickness separated by ultrathin strained (interfacial) layers. This effect predicted by the dynamical diffraction theory provides a powerful tool to quantitatively investigate ultrathin single quantum well structures and monolayer thin interfaces as well as MQW structures. For a given switching sequence during CBE growth, we determine the interface structure of GaInAs/InP MQW structures within the limits given by XRD theory. Additionally we found that an As gradient from the GaInAs quantum well layers into the InP barrier layers is present. The influence of the substrate off-orientation, the growth rate, and the group V flux in the InP barriers on the total amount of strain incorporated into the InP layers is shown. The obtained results indicate that the mechanism of As incorporation into InP layers is similar to the mechanism observed for the As incorporation into (qua)ternary GaxIn1−xAsyP1−y layers.
Mechanisms of ion beam mixing in metals and semiconductors
Authors: Nordlund K. , Ghaly M. , Averback R. S.
Published: July 2002

Ion beam mixing was investigated in crystalline and amorphous semiconductors and metals using molecular dynamics simulations. The magnitude of mixing in an amorphous element compared to its crystalline counterpart was found to be larger by a factor of 2 or more. Mixing in semiconductors was found to be significantly larger than in a face-centered-cubic (fcc) metal of corresponding mass and atomic density. The difference in mixing between amorphous and crystalline materials is attributed to local relaxation mechanisms occurring during the cooling down phase of the cascade. Comparison of mixing in semiconductors and metals shows that short range structural order also has a significant influence on mixing. The mixing results in fcc metals indicate that the role of the electron–phonon coupling in the evolution of collision cascades may be less significant than previously thought.
Influence of temperature and interface charge on the grain-boundary conductivity in acceptor-doped SrTiO3 ceramics
Authors: Hagenbeck Rainer , Waser Rainer
Published: July 2002

The temperature dependence of both the grain-boundary potential barrier height and the conductivity across the grain-boundary space-charge depletion layer in acceptor-doped SrTiO3 ceramics has been investigated by a numerical simulation technique. The underlying model is that of a back-to-back double Schottky barrier at the grain boundary. The influence of the amount of positively charged donorlike grain-boundary interface states on the charge transport behavior of the grain-boundary region is also discussed. An interpretation in terms of a defect chemistry model of the bulk and the space-charge depletion layer, which on both sides surrounds the grain-boundary core is presented. The temperature behavior of the potential barrier at the grain boundary can be divided into three different regimes: a linear regime, and subsequently, saturation and decreasing regimes. The theoretical explanation for this behavior is given. Two different spatial conductivity profiles at the grain boundaries have to be considered, which clearly can be identified by two different characteristic thermal activation energies of the effective grain-boundary conductivity. The barrier height itself is not equal to the the thermal activation energy of the effective grain-boundary conductivity. The electrical characteristics of the grain boundaries can be influenced deliberately by decorating the grain boundaries with suitable dopants.
Electron spin resonance features of interface defects in thermal (100)Si/SiO2
Authors: Stesmans A. , Afanas’ev V. V.
Published: July 2002

Electron spin resonance (ESR) on thermal (100)Si/SiO2 predominantly exhibiting either the Pb0 or Pb1 interface defect confirms the Pb1 point symmetry as monoclinic-I with g1=2.0058, g2=2.00 735±0.00 010, and g3=2.0022, where the g2 direction is at 3°±1° (towards the interface) with a 〈111〉 direction at 35.3° with the interface plane. Its line width is found weakly dependent on magnet angle, exhibiting a strain induced spread σg⊥∼0.00 035 in g⊥ about 2–3 times less than typical for Pb in (111)Si/SiO2. For Pb0, an axially symmetric g matrix is observed, with g∥=2.0018 and g⊥=2.0081, and σg⊥∼0.0009. From comparison of salient ESR data, it is concluded that Pb and Pb0 are chemically identical; however, systematic fabrication-induced variations in defect environment will lead to second order systematic shifts in average properties. The Pb1 defect is provisionally pictured as an unpaired Si bond on a defect Si atom at slightly subinterface plane position in the Si substrate, possibly facing an oxygen atom.
Role of grain-boundary sliding in diffusional creep of polycrystals
Authors: Mori T. , Onaka S. , Wakashima K.
Published: July 2002

The kinetics of deformation caused by grain-boundary sliding and diffusion is discussed for a two-dimensional model polycrystal. It is shown that boundary sliding is a prerequisite to induce the bulk diffusion. The sliding must also occur concurrently with the diffusion for a polycrystal to continue diffusional creep. Rate equations for deformation of the polycrystal are also formulated when both of the bulk and boundary diffusions are operative in addition to the boundary sliding. It is shown that the two diffusional processes are exclusive in general. Only when boundaries slide extremely fast, the strain rate of the polycrystal in a steady state becomes the simple sum of the strain rate achieved by the bulk diffusion and that by the boundary diffusion.
Analysis of lattice strains measured under nonhydrostatic pressure
Authors: Singh Anil K. , Balasingh C. , Mao Ho-kwang , Hemley Russell J. , Shu Jinfu
Published: July 2002

The equations for the lattice strains produced by nonhydrostatic compression are presented for all seven crystal systems in a form convenient for analyzing x-ray diffraction data obtained by newly developed methods. These equations have been used to analyze the data on cubic (bcc α-Fe) and hexagonal (hcp ε-Fe) systems. The analysis gives information on the strain produced by the hydrostatic stress component. A new method of estimating the uniaxial stress component from diffraction data is presented. Most importantly, the present analysis provides a general method of determining single crystal elastic constants to ultrahigh pressures.
Modulation response of tunneling injection laser: Analytical approach
Authors: Rusek P. , Wartak M. S. , Weetman P.
Published: July 2002

The modulation response of a tunneling injection laser has been studied by an analytic solution of the rate equations taking into account carrier transport effects. The formula for K factor has been derived. We have found that the modulation bandwidth increases when coupling between quantum well and separate confinement increases and that the value of K factor is underestimated in a tunneling injection laser. It indicates that the observed high modulation bandwidth in tunneling injection laser is mostly due to reduction of hot carrier effects.
Urbach–Martienssen’s tail in the absorption spectra of the ordered vacancy compound CuIn3Se5
Authors: Wasim S. M. , Marı́n G. , Rincón C. , Sánchez Pérez G.
Published: July 2002

The presence of Urbach–Martienssen’s tail in the absorption coefficient spectra just below the fundamental absorption edge is observed in several samples of the ordered vacancy compound CuIn3Se5 at different temperatures below 300 K. An empirical relation that takes into account the effect of structural disorder and modification in the contribution of the thermal term, caused due to the deviation from the ideal stoichiometry 1:3:5, is proposed to explain the change of Urbach’s energy KT/σ with temperature. The phonon energy involved in the electron–phonon interaction, in the absence of structural disorder, is estimated to be around 23 meV.
Observations of and model for insular grains and grain clusters formed during anomalous grain growth in N18 superalloy
Authors: Messina René , Soucail Michèle , Baudin Thierry , Kubin Ladislas Pierre
Published: July 2002

Anomalous grain growth has been studied in the solid state in an N18 superalloy. An unexpected observation was the presence of small grains, either isolated or clustered, embedded in the large grains. The misorientations of the grains and clusters with respect to the large grain have been measured by electron backscattering diffraction. It is found that a majority of the isolated grains exhibit boundaries of low interfacial energy, while in the clusters the misorientations mainly correspond to high energy random boundaries. A simple wetting model in terms of interfacial energies is proposed and discussed, which explains the occurrence of the low energy insular grains. A specific mechanism for the occurrence of the large energy grains is proposed, which qualitatively explains the experimental distributions of misorientations.
Maximizing loop squareness by minimizing gradients in the microstructure
Authors: Branagan D. J. , Kramer M. J. , Tang Y. L. , McCallum R. W.
Published: July 2002

Enhanced energy product and higher operating fields can be achieved by improving the squareness of the demagnetization loop. In isotropic materials, increasing loop squareness can be achieved by minimizing microstructural gradients and maximizing homogeneity. Achieving homogeneity by melt spinning can be difficult due to localized changes in cooling conditions from extrinsic factors such as gas entrapment between the melt and the chill surface and intrinsic effects resulting from the stochastic nature of nucleation on a surface. These localized variations are reduced by altering the solidification kinetics of the alloy, thus reducing its sensitivity to localized changes in cooling conditions. This was achieved through development of a nine-element modified Nd–Fe–B alloy which solidified with a uniform glass structure at much lower cooling rates. After crystallization, enhanced microstructural homogeneity was verified by electron microscopy and described by parameters based on the loop shape. The nine-element alloy had squarer loops and uniformly high hard magnetic properties (>12 MG Oe) for cooling rates over 10 m/s.
An investigation of magnetic reversal in submicron-scale Co dots using first order reversal curve diagrams
Authors: Pike Chris , Fernandez Andres
Published: July 2002

First order reversal curve (FORC) diagrams are a powerful method of investigating the physical mechanisms giving rise to hysteresis in magnetic systems. We have acquired FORC diagrams for an array of submicron-scale Co dots fabricated by interference lithography. These dots reverse magnetization through the nucleation and annihilation of a single-vortex state. Using FORC diagrams, we are able to obtain precise values for the nucleation and annihilation fields involved in magnetic reversal. Our results indicate, however, that there are actually two distinct paths for vortex annihilation: When a complete magnetic reversal takes place, a vortex enters on one side of a dot and exits out the opposite side. But if the magnetization is returned to its original orientation before a complete reversal has occurred, then the vortex will exit on the same side from which it has entered. We are unable to obtain a precise field value for this later path of annihilation; however, it is shown that, statistically, the vortex annihilates with greater ease when it exits out the same side from which it has entered.
Electron-irradiation-induced deep levels in n-type 6H–SiC
Authors: Gong M. , Fung S. , Beling C. D. , You Zhipu
Published: July 2002

The fluence-dependent properties and the annealing behavior of electron-irradiation-induced deep levels in n-type 6H–SiC have been studied using deep-level transient spectroscopy (DLTS). Sample annealing reveals that the dominant DLTS signal at EC−0.36 eV (labeled as E1 by others) consists of two overlapping deep levels (labeled as ED3L and ED3H). The breakup temperature of the defect ED3L is about 700 °C. The ED3H center together with another deep level located at EC−0.44 eV (so-called E2) can withstand high-temperature annealing up to 1600 °C. It is argued that the involvement of the defect ED3L is the reason that various concentration ratios of E1/E2 were observed in the previous work. The revised value of the capture cross section of the deep-level ED3H has been measured after removing ED3L by annealing. A deep level found at EC−0.50 eV is identified as a vacancy–impurity complex since it was found to have a lower saturated concentration and weak thermal stability. Two other deep levels, EC−0.27 eV and EC−0.32 eV, which were not observed by others because of the carrier freeze-out effect, are also reported.
Origin of the deep center photoluminescence in CuGaSe2 and CuInS2 crystals
Authors: Krustok J. , Schön J. H. , Collan H. , Yakushev M. , Mädasson J. , Bucher E.
Published: July 2002

Photoluminescence (PL) of CuGaSe2 and CuInS2 single crystals, either as grown or Cu annealed, reveals a broad and clear deep emission band at hν≈Eg−0.6 eV. In both of these as-grown materials this band has a similar doublet structure with the two D1,D2 subbands separated by about 100 meV. After the Cu annealing all samples became highly compensated and an additional deep PL band (W band) appeared on the high energy side of these D bands. This suggests a closely similar origin of the emission for the both materials. By a straightforward model calculation we show that the changes in the shape and intensity of these emission bands—due to variation of temperature, excitation intensity or due to the Cu annealing—are well explained if we assume that the D1 and D2 PL subbands originate in the recombination between the closest and the second closest donor–acceptor pairs, with the essential ingredient of the emission center being an interstitial donor defect, i.e., either Cui or Gai in CuGaSe2 and Cui or Ini in CuInS2. The W band in both compounds appears to be due to the recombination of an electron from this deep donor level with a hole in a deep localized state of the valence band tail.
Study of magnetization and crystallization in sputter deposited LiZn ferrite thin films
Authors: Dash J. , Prasad Shiva , Venkataramani N. , Krishnan R. , Kishan Pran , Kumar Nitendar , Kulkarni S. D. , Date S. K.
Published: July 2002

LiZn ferrite films of composition Li00.5−x/2Mn0.1ZnxFe2.35−x/2O4 with x=0.32 were rf sputter deposited on fused quartz substrates at ambient temperature. The as-deposited films were found by x-ray diffraction to be amorphous but magnetic, and showed large high field susceptibility. The films were studied after they were annealed at various temperatures up to 850 °C. It was observed that the films crystallize upon annealing and the value of the saturation magnetization increases with annealing temperature. The high field susceptibility, on the other hand, decreases with increasing anneal temperature. The measured ferromagnetic resonance spectra of these films indicated that the films consist of at least two different magnetic materials. A significant portion in the film crystallizes and the value of saturation magnetization of this portion tends to the bulk value as annealing temperature is increased. However, a small portion of the film remains in a highly defective state all through, even up to annealing temperatures of 850 °C. The high field susceptibility data indicates that point defects could play an important role in determining the magnetic properties of these films.
Built-in electric-field effects in wurtzite AlGaN/GaN quantum wells
Authors: Grandjean N. , Damilano B. , Dalmasso S. , Leroux M. , Laügt M. , Massies J.
Published: July 2002

AlGaN/GaN quantum well (QW) structures are grown on c-plane sapphire substrates by molecular beam epitaxy. Control at the monolayer scale of the well thickness is achieved and sharp QW interfaces are demonstrated by the low photoluminescence linewidth. The QW transition energy as a function of the well width evidences a quantum-confined Stark effect due to the presence of a strong built-in electric field. Its origin is discussed in terms of piezoelectricity and spontaneous polarization. Its magnitude versus the Al mole fraction is determined. The role of the sample structure geometry on the electric field is exemplified by changing the thickness of the AlGaN barriers in multiple-QW structures. Straightforward electrostatic arguments well account for the overall trends of the electric-field variations.
Optimizing well doping density for GaAs/AlGaAs p-type quantum well infrared photodetectors
Authors: Shen A. , Liu H. C. , Szmulowicz F. , Buchanan M. , Gao M. , Brown G. J. , Ehret J.
Published: July 2002

Effects of well doping density on the performance of GaAs/AlGaAs p-type quantum well infrared photodetectors are systematically studied. We find that for devices covering the 3–5 μm wavelength region and operating at about 100 K, the optimum two-dimensional doping density is in the range 1–2×1012 cm−2, which maximizes the background limited infrared performance temperature and dark current limited detectivity. Increasing the doping density not only enhances the peak absorption but also broadens the linewidth pronouncedly.
Carbon-coated Fe–Co(C) nanocapsules prepared by arc discharge in methane
Authors: Dong X. L. , Zhang Z. D. , Jin S. R. , Kim B. K.
Published: July 2002

Characterization and magnetic properties of Fe–Co(C) nanocapsules were investigated systematically by means of x-ray diffraction, Mössbauer spectroscopy, x-ray photoelectron spectroscopy, transmission electron microscopy, energy dispersive spectroscopy analysis, chemical analysis, oxygen determination, and magnetization measurement. The effects of elemental carbon, decomposed from a methane atmosphere in carbon arc process, on the phase structures, magnetic states and surface characterization were studied. Carbon atoms favor forming a new core/shell type structure, consisting of a carbon coating and a core containing a carbon solution. The mechanism of formation of the nanocapsules as well as origin of ferromagnetism and paramagnetism are discussed.
Localized stress near and the thermal expansion of Al2Cu precipitates in an Al thin film matrix
Authors: Lokker J. P. , van der Pers N. M. , Verbruggen A. H. , Janssen G. C. A. M. , Jongste J. F. , Radelaar S.
Published: July 2002

The thermal expansion coefficient of Al2Cu precipitates embedded in an Al–Cu thin film matrix is determined by in situ x-ray diffraction measurements on free films. The phases present and the thermal expansion or contraction are simultaneously observed during a temperature ramp. The Al2Cu precipitate has a tetragonal structure. The thermal expansion coefficient of the Al2Cu phase in the a direction is significantly smaller than that of the surrounding Al matrix while the thermal expansion in the c direction is comparable to that of the Al matrix. Upon cooling from 500 °C, the mismatch of the thermal expansion between the precipitate and the Al matrix results in compressive and tensile stresses in the aluminum matrix near the precipitates and in compressive stresses in the precipitates. Those stresses are estimated to be on the order of 100 MPa.
Segregation effects of Li, K, and F in Si during depth profiling by oxygen ions
Authors: Deenapanray Prakash N. K. , Petravic Mladen
Published: July 2002

The migration behavior of Li, K, and F during secondary ion mass spectrometry (SIMS) depth profiling was investigated in both n- and p-type Si using different oxygen bombardment conditions. The presence of an electric field across the surface oxide is shown to be the major driving force for both the segregation of Li and K at the SiO2/Si interface and the antisegregation of F into the oxide. Room temperature SIMS measurements revealed that K segregates at the oxide side of the SiO2/Si interface, whereas Li segregates at the silicon side of the interface. We have also found that the field-induced segregation of Li and antisegregation of F are less pronounced in high resistivity (11 000–16 000 Ω cm) p-type Si than in low resistivity (∼0.011 Ω cm) n-type Si. Although Li segregates at the Si side of the interface in both types of Si, some Li, however, remains at the oxide side in the high resistivity p-type Si. The high solid solubility of Li in amorphous Si is also considered as a driving force for its segregation.
Screening technique measurement of ac imaginary susceptibility and critical current density of YBa2Cu3O7−δ films
Authors: Acosta C. R. , Acosta M. , Sosa Victor , Ares O.
Published: July 2002

The screening technique was used to study systematically the ac susceptibility of superconducting YBa2Cu3O7−δ thin films in the temperature range of 32–83 K. We used the Fourier transform of the pick-up signal, coming from the secondary coil, to estimate the critical current density. From the experimental data we observed regularities in the imaginary part of ac susceptibility, and confirmed that the flux creep mechanism is dominant.
Evolution of the microstructure in microcrystalline silicon prepared by very high frequency glow-discharge using hydrogen dilution
Authors: Vallat-Sauvain E. , Kroll U. , Meier J. , Shah A. , Pohl J.
Published: July 2002

A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH4/(SiH4+H2) varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy (TEM) and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a (220) crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherrer’s equation illustrates the known limitations of the latter method for grain size determination in complex microstructures.
Cobalt-ferrite nanoparticles: Structure, cation distributions, and magnetic properties
Authors: Li Sichu , John Vijay T. , O’Connor Charles , Harris Vincent , Carpenter Everett
Published: July 2002

Cobalt–ferrite nanoparticles have been synthesized in water-in-oil microemulsions (reversed micelles) with varying cation composition. The microenvironment provides a template effect that controls the size and particle shape. Transmission electron microscopy reveals that the particles are nanospheres with particle size ranging from 12 to 18 nm. X-ray diffraction results indicate that at low Co2+:Fe2+ ratio (1:10 and 1:5) in the precursor, the particles retain an essentially ferrite structure (γ-Fe2O3). However, the cobalt–ferrite phase (CoFe2O4) forms upon further increase of the Co2+ content. The materials are found to exhibit superparamagnetism. The blocking temperatures and coercivities are dependent on the Co2+:Fe2+ ratio in the system.
Temperature dependence of magnetic hysteresis of RCox:Co nanocomposites (R=Pr and Sm)
Authors: Liu J. P. , Skomski R. , Liu Y. , Sellmyer D. J.
Published: July 2002

The temperature dependence of magnetic hysteresis of RCox:Co (R=Pr and Sm) nanocomposite films is reported. These films are prepared by dc and rf sputtering and subsequent thermal processing. It is found that the squareness of the hysteresis loops deteriorates with decreasing temperature. This is attributed to the enhanced magnetic anisotropy of the hard phases at low temperatures. The analysis of the magnetic reversal process shows that the anisotropy enhancement leads to a transition from cooperated to independent behavior. This analysis agrees well with the experimental results.
Nanoscale electrodeposition: A new route to magnetic nanostructures?
Authors: Schindler W. , Hofmann D. , Kirschner J.
Published: July 2002

Localized electrochemical deposition of small Co clusters on Au (111) has been achieved by using the tip of a scanning tunneling microscope (STM) as an electrochemical nanoelectrode. In contrast to most of the reported techniques to create nanostructures, this approach avoids irreversible modifications of both substrate and deposit during the preparation process. A special polarization routine of the STM tip allows a purely electrochemical growth of Co clusters with diameters as small as 15 nm.
The refractive index of AlxGa1−xAs below the band gap: Accurate determination and empirical modeling
Authors: Gehrsitz S. , Reinhart F. K. , Gourgon C. , Herres N. , Vonlanthen A. , Sigg H.
Published: July 2002

The refractive indices of AlxGa1−xAs epitaxial layers (0.176⩽x⩽1) are accurately determined below the band gap to wavelengths, λ<3 μm. The layers are grown on GaAs substrates by molecular beam epitaxy metal organic and chemical vapor deposition with thicknesses ranging from 4 to 10 μm. They form improper waveguide structures with the GaAs substrate. The measurements are based on the excitation of the improper waveguide modes with grating couplers at 23 °C. The refractive indices of the layers are derived from the modal propagation constants in the range of 730 nm<λ<830 nm with an estimated uncertainty of Δn=5×10−4. The temperature coefficient of the refractive index is investigated in the same spectral range. From the effective indices of the TE and TM modes, we derive the strain-induced birefringence and the elasto-optic coefficients. High-resolution x-ray diffraction is used to determine the strain of the layers. The layer compositions are obtained with inductively coupled plasma atomic emission spectroscopy. The measurement range of the refractive index is extended from the direct gap to λ<3 μm by observing the Fabry-Pérot interference fringes of the transmission spectra of isolated layers. The measured values of the refractive index and the elasto-optic coefficient are compared to calculated data based on semiempirical models described in the literature. Published data of the index of refraction on GaAs, AlAs and GaP are analyzed to permit the development of a modified Sellmeier approximation. The experimental data on AlxGa1−xAs can be fitted over the entire composition range 0⩽x⩽1 to provide an accurate analytical description as a function of composition, wavelength, and temperature.
Geometrical considerations in model systems with periodic boundaries
Authors: Theodorou Doros N. , Suter Ulrich W.
Published: July 2002

Model systems with periodic boundaries in the shape of an arbitrary parallelepiped are discussed. The locus of minimum images with respect to a given atom is determined, and algorithms for computing minimum image distances, as well as locating significantly interacting atom pairs are presented. It is also shown that, by exhausting the structural information provided by the model system, pair distribution functions can be computed for distances up to the semidiagonal of the minimum image locus. Comparison of the algorithms proposed with existing ones on a typical model system clearly shows that the new algorithms are correct and superior in computational efficiency, for cubic as well as noncubic systems.
Rate coefficients for S(1D) removal at 300 K
Authors: Black G. , Jusinski L. E.
Published: July 2002

S(1D) atoms have been produced by photodissociation of OCS at 248 nm and resonance-enhanced photoionization has been used to follow their decay. Rate coefficients for removing S(1D) have been determined for C2H4, C2H6, H2, CH4, N2O, OCS, CO2, N2, O2, and Ar at 300 K and upper limits determined for SF6, Ne, and He. (45±10)% of the collisions of S(1D) with OCS produce S(3P) atoms.
Theory of energy transfer between molecules near solid state particles
Authors: Hua X. M. , Gersten J. I. , Nitzan A.
Published: July 2002

The theory of energy transfer between a donor molecule and an acceptor molecule near a solid state particle is developed. The particle is modeled as a spheroidal shape and the molecules are allowed to be at arbitrary positions in space. It is found that there exist zones of activity in which the molecules display significantly enhanced energy transfer. We develop a formalism which allows us to describe nonradiative and radiative decay and energy transfer in a unified manner.
Theory of electronic transport in molecular crystals. III. Diffusion coefficient incorporating nonlocal linear electron–phonon coupling
Authors: Munn R. W. , Silbey R.
Published: July 2002

Electronic transport in molecular crystals is studied for simultaneous local and nonlocal linear electron–phonon coupling using a generalized polaron Hamiltonian derived previously. Nonlocal coupling increases the scattering, giving lower band contributions and higher hopping contributions. It also gives a phonon-assisted term which dominates at high temperature, leading eventually to a constant diffusion coefficient whose magnitude depends on the ratio of the nonlocal to local coupling and is independent of transfer integral. Incorporating nonlocal coupling thus mainly increases the magnitude of the difffusion coefficient and decreases its temperature dependence.
Adiabatic wave functions beyond the Born–Oppenheimer approximation: Phase linking between electrons and nuclei
Authors: Marechal Y.
Published: July 2002

In order to improve the Born–Oppenheimer treatment of adiabaticity we impose to the electronic and nuclear wave functions to have a well defined phase difference. It enables us to correctly evaluate matrix elements of such quantities as momenta which are poorly evaluated in the Born–Oppenheimer description. The phase factor obeys an equation which we solve by successive approximations and leads to simple expressions for this phase factor. Taking the coupling of two harmonic oscillators as an exactly soluble example which may also be treated within this adiabatic approximation we show that the proposed formalism is rather easy to use and leads to significant improvements with respect to Born–Oppenheimer treatments. We suggest that this framework might be useful when treating systems which are weakly coupled but are not so different in their time evolutions as are electrons and nuclei.
Density functional calculations of molecular bond energies
Authors: Becke A. D.
Published: July 2002

The calculation of molecular bond energies is a sensitive test of exchange-correlation approximations in density functional theory. The well known local density approximation (LDA) gives excellent bond lengths and vibrational frequencies, but seriously overestimates dissociation energies. Therefore, we have investigated the effect on bond energies of nonlocal corrections to the LDA exchange-correlation functional. We consider the nonlocal correction term of Langreth and Mehl, and also a new semiempirical exchange energy correction. Significant improvements over the LDA dissociation energies are obtained in calculations on first-, second-, and third-row homonuclear diatomic systems.
The Eu(III) ion as luminescent probe: Laser-spectroscopic investigation of the metal ion sites in an 18-crown-6 complex
Authors: Bünzli Jean-Claude G. , Pradervand Georges-Olivier
Published: July 2002

The structure of [Eu(NO3)2(18-6)]3[Eu(NO3)6] has been probed by high-resolution excitation and emission spectra at 77 and 4.2 K. The compound contains centrosymmetric [Eu(NO3)6]3− ions with D2h, C2h, or Ci symmetry and three types of complex [Eu(NO3)2(18-6)]+ cations with D2 symmetry slightly distorted towards C2v. The luminescence spectra of Ln2Mg3(NO3)12 ⋅ 24H2O (Ln=Eu and La doped with 1% Eu) have also been recorded and are compared to the spectrum of the hexanitrato ion. The high-resolution study of the 5D0→7F1 transition at 4.2 K, both of polycrystalline samples and of monocrystals of the Eu-doped Gd complex, revealed that each metal site does not generate a single emission spectrum but, rather, a series of very similar spectra. These arise from molecules experiencing slightly different crystal field effects, that is, having slightly different conformations. The Eu(III) ion is therefore a quite sensitive luminescent probe responding to faint chemical differences often overseen in x-ray diffraction studies; it could be used to probe the conformational distribution of molecules in polycrystalline solids.
A calculation of the rotation–vibration states of He2H+
Authors: Lee J. S. , Secrest Don
Published: July 2002

A fit to a recent ab initio potential surface for HeHHe+ is presented and discussed. Calculations of the rotation–vibration spectra are made using the variation method with this potential surface. The calculations are carried out for both the 1H and 2H(D) isotope of H and comparisons of the spectra and vibrationally averaged bond length are discussed for the two species. A study of the dissociation to He+HeH+ is made and ‘‘vibrational bonding’’ is investigated. It is found that this system is not vibrationally bound. The deuterium substituted species is more strongly bound.
Density functional theory of nonuniform polyatomic systems. I. General formulation
Authors: Chandler David , McCoy John D. , Singer Sherwin J.
Published: July 2002

We extend the density functional theory of nonuniform fluids to the cases of systems composed of polyatomic species. By the method of Legendre transforms, one demonstrates the existence of a free energy density functional where the densities refer to the locations of interaction sites (not full molecular coordinates). A variational principle for the free energy is derived. The methodology retains nearly all the mathematical simplicity of the traditional theory of atomic fluids. Thus, it may provide a practical route to deriving mean field theories of assembly and phase transitions in complex systems. Certain nonlinearities intrinsic to polyatomic systems and absent in simple fluids become apparent in our analysis. These features are associated with the entropy density functional for systems with bonding constraints. They must be carefully assessed in accurate applications.
Structure, stability, and fragmentation of small carbon clusters
Authors: Raghavachari Krishnan , Binkley J. S.
Published: July 2002

Accurate ab initio calculations are performed to study the structures and energies of small carbon clusters (Cn, n=2–10). The effects of polarization functions and electron correlation are included in these calculations. Significant odd–even alternation is found in the nature of the cluster geometries with the odd-numbered clusters having linear structures and many of the even-numbered clusters preferring cyclic structures. Energetically, odd-numbered clusters (up to C7) are found to be more stable than the adjacent even-numbered clusters. Ionization potentials are calculated and used in conjunction with the cluster energies to explain the fragmentation behavior of small carbon cluster ions.
On the pressure equation for chain molecules
Authors: Honnell K. G. , Hall C. K. , Dickman R.
Published: July 2002

The pressure equation is derived for fluids containing chain molecules interacting with site–site potentials. Three models are considered: a flexible chain with harmonic potentials for bond-length and bond-angle variations, a flexible chain in the limit as the force constants for the vibrations become infinite, and a chain where bond angles and lengths are rigidly constrained to their equilibrium values. In the flexible model, the pressure is found to depend on only site–site correlations and contains both inter- and intramolecular contributions. Forces arising from angular potentials, however, do not appear in the equation. In the limit as the force constants for internal vibration become infinite, the intermolecular contribution to the pressure remains a function of pair correlations, but the intramolecular term becomes a function of chain orientation and depends on three-site correlations. The result is consistent with an expression used previously for dimers and trimers, but disagrees with a site–site equation previously used to study longer chains. The equation of state for the rigidly constrained model takes the same functional form as the equation for the infinitely stiff model. However, the two-molecule correlation function appearing in the rigid equation differs from its stiff counterpart as a result of the imposed constraints.
Laser double-resonance mesurements of rotational relaxation rates of HF( J=13) with rare gases, H2, and D2
Authors: Taatjes Craig A. , Leone Stephen R.
Published: July 2002

Rotational relaxation rates for HF(v=0, J=13) colliding with rare gases (He, Ne, Ar, Kr, Xe), H2, and D2 are measured using a transient-absorption double-resonance technique. The relaxation rate constants with rare gases (rotation-to-translation, R–T) decrease dramatically through the series He–Ne–Ar, then increase substantially through the series Ar–Kr–Xe, revealing the increased effectiveness of either highly impulsive or highly attractive collisions. Rate constants at 298 K are: (He), 1.3±0.2×10−11; (Ne), 2.2±0.4×10−12; (Ar), 9.3±0.9×10−13; (Kr), 4.4±0.5×10−12; (Xe), 6.2±0.6×10−12 cm3 molecule−1 s−1. The fact that the rate constants with hydrogen (1.1±0.2×10−10 cm3 molecule−1 s−1) and deuterium (1.2±0.2×10−10 cm3/molecule−1 s−1) are ten times faster than with helium underscores the importance of rotation-to-rotation (R–R) transfer in the rotational relaxation process. The decrease in rates from He to Ar can be understood by an impulsive-type model, analogous to the Schwartz, Slawsky, and Herzfeld (SSH) treatment of vibration-to-translation transfer. However, the increasing trend from Ar to Xe, while clearly due to the increasing magnitude of the attractive interaction, is not easily modeled.