bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical characterization of bulk ZnSeTe solid solutions
Authors: Su Ching-Hua , Feth S. , Zhu Shen , Lehoczky S. L. , Wang Ling Jun
Published: July 2002

Optical characterization was performed on wafers sliced from crystals of ZnSe, ZnTe, and ZnSe1−xTex(0<x<0.4) grown by physical vapor transport. Energy band gaps at room temperature were determined from optical transmission measurements on 11 wafers. A best fit curve to the band gap versus composition x data gives a bowing parameter of 1.45. This number lies between the value of 1.23 determined previously on ZnSeTe bulk crystals and the value of 1.621 reported on ZnSeTe epilayers. Low-temperature photoluminescence (PL) spectra were measured on six samples. The spectra of ZnSe and ZnTe were dominated by near band edge emissions and no deep donor-acceptor pairs were observed. The PL spectrum exhibited a broad emission for each of the ZnSe1−xTex samples, 0.09<x<0.39. For x=0.09, this emission energy is about 0.2 eV lower than the band gap energy measured at low temperature. As x increases the energy discrepancy gradually decreases and reduces to almost zero at x=0.4. The single broad PL emission spectra and the spectra measured as a function of temperature were interpreted as being associated with the exciton bound to Te clusters because of the high Te content in these samples.
Magnetic pair-making double exchange coupling in Ru substituted orthomanganites, La0.7A0.3Mn0.9Ru0.1O3
Authors: Sahu Ranjan K. , Manoharan S. Sundar
Published: July 2002

A detailed study of Ru substitution at the Mn site in La0.7A0.3Mn0.9Ru0.1O3 (A=Ca, Sr, Pb, Ba) polycrystalline samples shows an unusual magnetic pair between Ru+4/Ru+5 and Mn+4/Mn+3 redox couple. As a result, in the entire 10 at. % Ru-substituted compositions, the TC varies only by 10–30 K. The similarity of Mn4+/Ru4+:eg parentage facilitates a redox interaction between Mn and Ru ions, Ru+4/+5/Mn+3/+4. Ruthenium (IV) low spin state has a magnetic moment of 2.7–2.9μB, and with an extended 4d orbital, it enhances exchange coupling between Mn and Ru sites.
Effect of overgrowth temperature on the photoluminescence of Ge/Si islands
Authors: Schmidt O. G. , Denker U. , Eberl K. , Kienzle O. , Ernst F.
Published: July 2002

Ge/Si islands grown with molecular-beam epitaxy at 630 °C are overgrown with Si at different temperatures Tcap, and their photoluminescene spectra are recorded. Both the island-related and wetting-layer-related energy transitions redshift with lowered Tcap, which is explained by reduced material intermixing. The mandatory growth interruption, which is introduced during the temperature drop, causes island ripening and shifts the island (wetting layer) photoluminescence peaks slightly to lower (higher) energies. The growth interruption quenches the quantum efficiency of the wetting layer by more than an order of magnitude, whereas the island-related photoluminescence intensity even slightly increases. The island’s superior resistance against growth interruptions, and hence interface contamination, is explained by effective carrier localization in the Ge nanostructures. Room-temperature photoluminescence is reported for Ge islands overgrown at 460 °C.
Application of a new reverse Monte Carlo algorithm to polyatomic molecular systems. I. Liquid water
Authors: da Silva Fernando Luı́s B. , Olivares-Rivas Wilmer , Degrève Léo , Åkesson Torbjörn
Published: July 2002

Using a new reverse Monte Carlo algorithm, we present simulations that reproduce very well several structural and thermodynamic properties of liquid water. Both Monte Carlo, molecular dynamics simulations and experimental radial distribution functions used as input are accurately reproduced using a small number of molecules and no external constraints. Ad hoc energy and hydrogen bond analysis show the physical consistency and limitations of the generated RMC configurations.
Tensile stress evolution during deposition of Volmer–Weber thin films
Authors: Seel Steven C. , Thompson Carl V. , Hearne Sean J. , Floro Jerrold A.
Published: July 2002

A simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer–Weber mechanism. The generation of a tensile stress was attributed to the impingement and coalescence of growing islands, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism. To model the process of island coalescence, finite element methods were employed and yielded average tensile stresses more consistent with experimental observations than those predicted using previously reported analytical models. A computer simulation was developed that models the process of film growth as the continuous nucleation of isolated islands, which grow at a constant rate to impinge and coalesce to form a continuous polycrystalline film. By incorporating the finite element results for stress generation and a microstructure-dependent stress relaxation model, the simulation qualitatively reproduced the complex temperature-dependent trends observed from in situ measurements of stress evolution during the deposition of Ag thin films. The agreement includes simulation of the decreasing stress relaxation rate observed during deposition at increasing temperatures.
Microstructure and properties of giant magnetoresistive granular Au80Co20 alloys
Authors: Vrenken H. , Kooi B. J. , De Hosson J. Th. M.
Published: July 2002

The relationship between microstructure and giant magnetoresistance (GMR) of granular Au80Co20 was investigated. Two different processing routes were explored. With the melt spinning technique the microstructure appeared to be so coarse that it was not expected to exhibit any substantial GMR effect. On the other hand, with the procedure of solid-solution annealing and water quenching afterwards, a suitable nanostructure was prepared that showed a GMR of 29% at 10 K and 50 kOe. Subsequent annealing causes coarsening of Co particles. In additional spinodal decomposition occurred for a certain temperature range and a loss of coherency of the Co particles with respect to the Au was observed with high-resolution transmission energy microscopy. At magnetic fields above ∼20 kOe, all annealed alloys showed a saturating magnetization, whereas the resistance is still steadily decreasing, challenging the presumed mathematical relationship between GMR and overall magnetization.
Driving force for adatom electromigration within mixed Cu/Al overlayers on Al(111)
Authors: Rous P. J.
Published: July 2002

A multiple scattering theory is used to study the effect of the addition of Cu upon the driving force for the electromigration of Al adatoms on an Al(111) surface. The theory employs a computational framework that combines a layer-Korringa Kohn–Rostoker calculation with the coherent potential approximation to describe the carrier scattering by a compositionally disordered overlayer. We investigate how the substitution of Cu for Al adatoms in the disordered overlayer alters the average effective wind valence of the Al and Cu adatoms within the overlayer. For adatom coverages smaller than approximately 0.2, we find that the effect of Cu substitution is to reduce the magnitude average effective wind valence of the Al adatoms by approximately +0.4%(Cu)−1 at 273 K.
Evolution with light soaking of the conduction band tail of amorphous-silicon-like materials
Authors: Longeaud C. , Roy D. , Hangouan Z. Teukam
Published: July 2002

The influences of light soaking and annealing processes on the density of states above the Fermi level for different types of intrinsic amorphous-silicon-like materials are presented. After only 2 mn of light soaking, an increase of the dangling bonds (DB) density if noticeable. After saturation, an increase of both the DB and the conduction band tail states is observed. It is also shown that the influence of the annealing process is material dependent. These data are supported by some other recent experimental results.
Relativistic calculations of the rotational
g
factor of the hydrogen halides and noble gas hydride cations
Authors: Enevoldsen Thomas , Rasmussen Torben , Sauer Stephan P. A.
Published: July 2002

The rotational g factors of the hydrogen halides, HX (X=F,Cl,Br,I), and noble gas hydride cations, XH+ (X=Ne,Ar,Kr,Xe), have been calculated at the level of the random phase approximation (RPA) as relativistic four-component linear response functions as well as nonrelativistic linear response functions. In addition, using perturbation theory with the mass-velocity and Darwin operators as perturbations, the relativistic corrections have been estimated as quadratic response functions. It was found that the four-component relativistic calculations give in general a more negative electronic contribution to the rotational g factor than the nonrelativistic calculations with relativistic corrections ranging from 0.2% for HF and NeH+ to 2.9% for XeH+ and 3.5% for HI. The estimates of the relativistic corrections obtained by perturbation theory with the mass-velocity and Darwin operators are in good agreement with the four-component results for HF, HCl, NeH+, and ArH+, whereas for HI, KrH+, and XeH+ they have the wrong sign.
Unique method to electrically characterize a single stacking fault in silicon-on-insulator metal–oxide–semiconductor field-effect transistors
Authors: Yang J. , Neudeck G. W. , Denton J. P.
Published: July 2002

A unique and simple method is demonstrated for characterizing the electrical behavior of a single stacking fault in thin-film fully depleted silicon-on-insulator (SOI) metal–oxide–semiconductor field-effect transistors (MOSFETs). SOI islands were created using selective epitaxial growth/epitaxial lateral overgrowth technology. P-channel MOSFETs, with the presence of a single stacking fault entirely in the channel region, were measured. The influence of a single stacking fault on device current–voltage characteristics was determined and compared to that of nearby identical devices without stacking faults. It was found that the threshold voltage increased and saturation current decreased, but had low subthreshold leakages. P-channel MOSFETs, with a single stacking fault crossing the gate and penetrating into the source and drain, had high subthreshold leakage currents.
Characterization of ultrathin silicon oxide films with mirror-enhanced polarized reflectance Fourier transform infrared spectroscopy
Authors: Cui Zhenjiang , Takoudis Christos G.
Published: July 2002

Ultrathin silicon oxide films thermally grown on Si (100) are characterized with mirror-enhanced polarized reflectance Fourier transform infrared spectroscopy (MEPR-FTIR). MEPR-FTIR is proposed to effectively probe properties of ultrathin films. Using a mirror and a polarizer, MEPR-FTIR overcomes the difficulty of weak IR intensities normally encountered in ultrathin gate dielectrics such as SiO2; the intensity of the silicon oxide longitudinal optical (LO) mode is found to increase by a factor of about 20. Therefore, FTIR spectrometers with sensitivity down to 0.01% may allow even submonolayer probing of silicon oxide on Si substrates. The relationship between film thickness and IR intensity of the LO mode at 1252 cm−1 is presented for silicon oxide films as thin as a few Å thick. Independent measurements with ellipsometry, cross-sectional transmission electron microscopy, and x-ray photoelectron spectroscopy as well as theoretical predictions using the general Fresnel function are utilized. Although nonlinear overall, the theoretically predicted relationship between the MEPR-FTIR intensity and film thickness is almost linear within the range 0–50 Å. Experimental data are shown to agree very well with the theoretical calculations for two different kinds of oxides: thermal oxides grown at 1050 °C and native oxides. Such results suggest that the MEPR-FTIR can be useful as an ex situ or in situ technique for thickness measurements and, in fact, for other properties of ultrathin gate dielectrics.
Ordered domains and polar clusters in lead magnesium niobate Pb(Mg1/3Nb2/3)O3
Authors: Jin H. Z. , Zhu Jing , Miao Shu , Zhang X. W. , Cheng Z. Y.
Published: July 2002

The compositional fluctuation and correlation between the ordered domain and the polar cluster in lead magnesium niobate Pb(Mg1/3Nb2/3)O3 have been studied by analytical electron microscopy and the image processing technique. It indicates that there is a strong compositional fluctuation of Mg/Nb ratio in both the ordered and the disordered regions, and that the degree of fluctuation of the Mg/Nb ratio is stronger in the boundary area than in either the ordered or disordered regions. Antiphase domain boundaries and dislocations were found, with the boundary facets of ordered domains being primarily {111} with a lesser {001} orientation. The fringe image of Nb and (Nb+Mg) is distorted randomly, which is thought to be caused by compositional fluctuation of the Mg/Nb ratio and local layer displacement. The boundary and dislocation area were found to be a region of severe distortion. The random interdomain-boundary area, with strong compositional fluctuation and lattice strain, may play an important role in forming a center for polar clusters.
Band gaps and localization in acoustic propagation in water with air cylinders
Authors: Ye Zhen , Hoskinson Emile
Published: July 2002

Multiple scattering of waves leads to many peculiar phenomena such as complete band gaps in periodic structures and wave localization in disordered media. Within a band gap, excitations are evanescent; when localized, they remain confined in space until dissipated. Here, we report acoustic band gap and localization in a two-dimensional system of air cylinders in water. Exact numerical calculations reveal the unexpected result that localization is independent of the precise location or organization of the scatterers. Localization occurs within a finite region of frequencies, coincident with the complete band gap for a periodic lattice of scatterers. Inside the gap or localization regime, a previously uninvestigated collective behavior of the cylinders appears.
Temperature-independent hole mobility in discotic liquid crystals
Authors: Kreouzis T. , Donovan K. J. , Boden N. , Bushby R. J. , Lozman O. R. , Liu Q.
Published: July 2002

Experimental measurements are presented of the hole mobilities of four conjugated discotic systems, forming columnar liquid crystals, as a function of temperature. The measurements cover the crystalline/glassy phase and mesophase of these materials. It is a remarkable fact that the mobility is almost independent of temperature in the range 30 °C–170 °C. Various explanations of a weak temperature dependence exist and these are explored. They include the small polaron of Holstein in the nonadiabatic limit and the effect of the dynamic disorder present in the system.
Gas-phase nuclear magnetic relaxation in Xe129 revisited
Authors: Moudrakovski I. L. , Breeze S. R. , Simard B. , Ratcliffe C. I. , Ripmeester J. A. , Seideman T. , Tse J. S. , Santyr G.
Published: July 2002

In this contribution gas-phase Xe129 spin-lattice relaxation time measurements are extended to conditions (pressure, temperature, magnetic-field strength, isotope composition) not previously used. It is shown that wall effects become apparent at densities below ∼20 amagat, and that these become dominant below ∼3 amagat. A significant new discovery from field-dependent studies is that, in addition to the previously identified field-independent spin–rotation relaxation operative in the bulk gas, there is a contribution from the modulation of the chemical shift that depends on the square of the applied magnetic-field strength. The weak temperature dependence of the relaxation times can be understood in terms of the opposite temperature coefficients of the field-independent and field-dependent contributions to the relaxation. The spin–rotation contribution and its temperature dependence are calculated and found to be in good agreement with the experimental data. The low field, low density limit for the relaxation time is 56.3±2.6 hr⋅amagat at 295 K in the absence of wall effects.
Nucleation in a slit pore
Authors: Talanquer V. , Oxtoby D. W.
Published: July 2002

We have employed density functional theory in statistical mechanics to study the nucleation of the gas–liquid transition inside a slit pore. This is the simplest kind of pore, in which a fluid is confined between two infinite and identical planar surfaces. Equilibrium capillary condensation in such systems has been studied in the past. Here, we explore the kinetics of nucleation of the new phase and show that the critical nucleus can either be attached to one of the planes or can bridge the two planes, depending on the conditions of the experiment. We show that the macroscopic capillarity approximation is not quantitatively accurate, but can provide a useful qualitative picture provided that the line tension is incorporated. Comparisons are made with recent simulations of phase transitions in slit pores.
Families of exact solutions of a two-dimensional gravity model minimally coupled to electrodynamics
Authors: Moayedi S. K. , Darabi F.
Published: July 2002

Three families of exact solutions for two-dimensional gravity minimally coupled to electrodynamics are obtained in the context of R=T theory. It is shown, by supersymmetric formalism of quantum mechanics, that the quantum dynamics of a neutral bosonic particle on static backgrounds with both varying curvature and electric field is exactly solvable.
Fabrication of CaS:Pb blue phosphor by incorporating dimeric Pb2+ luminescent centers
Authors: Yun Sun Jin , Kim Yong Shin , Park Sang-Hee Ko
Published: July 2002

The CaS:Pb phosphor emitting blue electroluminescence (EL) induced by dimeric Pb2+ luminescent centers was deposited using the controlled atomic layer deposition technique. The effect of the incorporation method and concentration of Pb2+ ions on the color of high luminance CaS:Pb phosphor was investigated at the deposition temperatures of 350 and 400 °C. The blue EL emission at 440–445 nm could be maintained with the Pb concentration ranging from 0.6 to 2.5 mol % contrary to the earlier works that had demonstrated very sensitive dependence of color on Pb2+ concentration. The selective incorporation of dimeric Pb2+ ions was attributed to the reactivity of tetraethyl lead as Pb precursor as well as the controlled deposition technique.
A new united atom force field for α-olefins
Authors: Nath Shyamal K. , Banaszak Brian J. , de Pablo Juan J.
Published: July 2002

A new united-atom force field is proposed for α-olefins. The proposed force field is used to study vapor–liquid equilibria for pure components and binary mixtures. Results of phase equilibria simulations are found to be in good agreement with available experimental data.
The fabrication of reproducible superconducting scanning tunneling microscope tips
Authors: Naaman O. , Teizer W. , Dynes R. C.
Published: July 2002

Superconducting scanning tunneling microscope tips have been fabricated with a high degree of reproducibility. The fabrication process relies on sequential deposition of superconducting Pb and a proximity-coupled Ag capping layer onto a Pt/Ir tip. The tips were characterized by tunneling into both normal-metal and superconducting films. The simplicity of the fabrication process, along with the stability and reproducibility of the tips, clear the way for tunneling studies with a well-characterized, scannable superconducting electrode.
Photoluminescence and photo-modulated reflectance spectra of ion-implanted GaAs/AlGaAs coupled quantum wells
Authors: Fu Y. , Willander M. , Miao Z. L. , Lu W.
Published: July 2002

Ion-implantation effects on a series of comparable asymmetrically-coupled GaAs/AlGaAs quantum wells processed by implantation method have been studied by photoluminescence (PL) and photo-modulated reflectance (PR) spectroscopy measurements. The peaks in the PL and PR spectra originate from the intersubband transitions between the ground states of electron state and hole states. Corresponding peak positions in the PL and PR spectra shift to higher energies following the increase of the ion-implantation dose due to the enhanced Al diffusion across the GaAs/AlGaAs and GaAs/AlAs heterointerfaces in the system. The observation of the reduction and finally the quench of the PL and PR peaks due to the increased implantation dose is explained by the reduced lifetime of carriers in the system due to the increased scattering processes.
Defect recognition via longitudinal mode analysis of high power fundamental mode and broad area edge emitting laser diodes
Authors: Klehr A. , Beister G. , Erbert G. , Klein A. , Maege J. , Rechenberg I. , Sebastian J. , Wenzel H. , Tränkle G.
Published: July 2002

A nondestructive method is presented which allows a precise detection of defects and their positions inside the cavity of semiconductor lasers. The defect recognition is based on the measurement of the longitudinal mode spectrum below threshold and the inspection of its Fourier transformation. Using a theoretical model, it is shown that a small distortion inside the cavity leads to a peak in the Fourier transformed spectrum from which the position of the distortion relative to the facets can be determined. For a ridge waveguide laser we find a direct correlation between defects identified by the analysis of the longitudinal mode spectrum and cathodoluminescence imaging. The applicability of this method for nondestructive defect recognition will also be demonstrated for broad area laser diodes with lateral multimode emission. The investigations reveal that the presented method can be used to assess the crystal quality of manufactured laser diodes.
Spectroscopic properties of Nd-doped glass for 944 nm laser emission
Authors: Lu K. , Dutta N. K.
Published: July 2002

A tunable laser operating near 944 nm is important for the detection of water vapor in the atmosphere. In this work, we have carried out a systematic study of Nd doped fiber glass materials which can be used for this purpose. The emission cross sections of these materials at 944 nm have been calculated. The wavelength dependence of emission of these materials has been studied. Among the silica-based materials Nd doped silica fiber glass has the largest cross section for 944 nm laser emission. The codopants reduce this cross section. Some Nd doped nonsilica materials such as Ba0.25Mg0.75Y2Ge2O12 and chloride glass have higher stimulated emission cross section than silica-based materials.
Photons confined in hollow microspheres
Authors: Artemyev M. V. , Woggon U. , Wannemacher R.
Published: July 2002

Incorporation of CdSe quantum dots into a thin (<1 μm) surface shell of polymer microspheres (R∼2–4 μm) is achieved. The room-temperature emission spectra of single, hollow microcavities show several, spectrally well-separated cavity modes in the red-orange spectral range which have been assigned to high-Q whispering gallery modes (WGM) with radial quantum number n=1 and high angular quantum number l. An enhancement of the cavity finesse Q by a factor of about 10 with respect to CdSe-doped bulk polymer microspheres is found.
Depth-resolved cathodoluminescence microanalysis of near-edge emission in III-nitride thin films
Authors: Gelhausen O. , Phillips M. R. , Toth M.
Published: July 2002

We describe an approach to assess the quality of III-nitride thin films using depth-resolved cathodoluminescence (CL) microanalysis. In this procedure, the depth-resolved peak shift due to self-absorption of the near-edge CL emission is calculated using Monte Carlo simulation techniques and compared with measured peak shift values. A discrepancy between the experimental and modeled data indicates the presence of an exciton peak shift due to strain, near-edge defects, and alloy fluctuation. Depth-resolved peak shift analysis of the near-edge CL from an undoped 700 nm thick Al0.057Ga0.943N film grown on a (0001) Al2O3 substrate is presented to demonstrate the utility of the method.
Nonequilibrium photoresponse of YBa2Cu3O7−x granular films to 8 mm microwave radiation
Authors: Xu Ke-xi , Zhou Shi-ping , Bao Jia-Shan
Published: July 2002

Nonequilibrium photoresponse behavior has been investigated for YBa2Cu3O7−x (YBCO) granular films to 8 mm microwave radiation under various bias currents and magnetic fields. The measurements reveal that the nonequilibrium photoresponse mode occurs only in the tail region of the resistance transition curve R(T) from the normal to the superconducting state, where transportation behavior of the granular superconducting film is found to be characterized by the Kosterlitz–Thouless (KT) phase transition model. Based on the KT model, the photoresponse mechanism has been interpreted in terms of the depinning process of the unbinding vortices, which are generated from the decoupling process of the vortex–antivortex pairs by current, and are held at the intrinsic pinning sites of the granular high-Tc superconducting films at low temperature. Under the co-action of the bias current and the incident microwave photons, these unbinding vortices will be driven out of the pinning center, creating viscous motion in the Josephson junction array system. An analytical result of the unbinding vortices density n(T,I) induced by applied current has been worked out based on the model of two-dimensional Josephson junction arrays that is employed as a model system for the YBCO granular films. The distribution of the n(T,I) is found to be analogous to that of the photoresponse measured in the temperature region of 2/3TKT<T<TKT. Additionally, the measurements reveal that the magnitude of the photoresponse is linearly increased with an increase of the incident microwave power. These results imply that the nonequilibrium photoresponse induced by microwave irradiation may be intrinsically related to the decoupling process of the vortex–antivortex pairs, as well as to the depinning dynamics of the unbinding vortices in the granular high-Tc superconducting films.
Electrostatic potential and quantum transport in a one-dimensional channel of an induced two-dimensional electron gas
Authors: Tkachenko O. A. , Tkachenko V. A. , Baksheyev D. G. , Pyshkin K. S. , Harrell R. H. , Linfield E. H. , Ritchie D. A. , Ford C. J. B.
Published: July 2002

We discuss quantization of the conductance in short, ultraclean one-dimensional quantum wires of a design where an electron gas is induced electrostatically. Two-level sets of gates allow independent control of electron density in the constriction and in the reservoirs, thus varying the conductance G as a function of the bias on the gates G(Vtg,Vsg). Up to 12 clean well-resolved conductance G(Vsg) plateaus confirm the high quality of the constriction. The experimental curves are modeled using three-dimensional self-consistent calculations in the Thomas–Fermi approximation of the electrostatic potential of the constriction and solution of the two-dimensional problem of electron transport in the calculated potential. Our calculations are in qualitative agreement with the experiment.
Quantum dynamics on new potential energy surfaces for the H2+OH→H2O+H reaction
Authors: Yang Minghui , Zhang Dong H. , Collins Michael A. , Lee Soo-Y.
Published: July 2002

We report dynamics on two interpolated global potential energy surfaces (PES) for the H2+OH reaction. The first PES is based on both QCISD(T)/6-311++G(3df,2pd) and MRCI/aug-cc-pVTZ ab initio calculations. In the second version, the energies at the interpolation data points are improved to the UCCSD(T)/aug-cc-pVQZ level. Fully converged integral cross sections were calculated on these two PESs for the title reaction for the ground rovibrational and two rotationally excited initial states. Good agreement between theory and experiment for rate coefficients for temperatures up to 1050 K are only achieved on the second PES.
Two-state approximation for aggregate states of clusters
Authors: Berry R. Stephen , Smirnov Boris M.
Published: July 2002

The concepts of the phase and change of state for clusters are analyzed. The connection between a phase or state change and the thermodynamic parameters of a cluster is analyzed within the framework of a two-state model. This model aggregates the excited configurational states of each state of the cluster. The model allows one to estimate parameters of clusters and bulk in the range of the phase or state change. The approach has sufficient generality to be applicable not only to order–disorder changes and evaporation but also to such processes as ionization and vibronic (electronic-to-thermal) energy conversion. The last of these raises a question regarding one step in the interpretation of some recent experimental studies of cluster melting.
Statistical mechanics and molecular simulation of adsorption of ternary gas mixtures in nanoporous materials
Authors: Xu Lifang , Tsotsis Theodore T. , Sahimi Muhammad
Published: July 2002

We report the results of extensive equilibrium molecular dynamics simulation of adsorption of ternary gas mixtures in a class of model nanoporous materials, namely, carbon molecular sieve membranes (CMSMs), which are being utilized for separation of gas mixtures. We use a novel three-dimensional molecular pore network model for CMSMs based on Voronoi tessellation of the simulation cell. The simulations with such a model allow us to investigate the effect of the morphology of the pore space, i.e., its pore size distribution and pore connectivity, on the adsorption isotherms. We also extend a statistical mechanical theory of adsorption of single gases and their binary mixtures, recently developed by us, to ternary mixtures and use the results of the molecular simulations to test its accuracy. The theory is shown to provide very accurate predictions for the simulation results over wide ranges of the temperature and porosity of the membrane. The performance of the theory is also compared with that of another theory of adsorption based on a virial expansion.
X-ray reflectivity of self-assembled structures in SiGe multilayers and comparison with atomic force microscopy
Authors: Meduňa M. , Holý V. , Roch T. , Stangl J. , Bauer G. , Zhu J. , Brunner K. , Abstreiter G.
Published: July 2002

We have studied the interface morphology of SiGe/Si multilayers by means of specular and nonspecular x-ray reflectivity under grazing incidence. The samples were grown by molecular beam epitaxy on silicon substrates with (001) surface orientation and with different directions of the surface misorientation. X-ray reflectivity measurements in different azimuths are compared to data from atomic force microscopy, which are used to simulate the x-ray experiments. With this combination of experimental techniques we have determined the structural properties, in particular the ordering of different features present at the sample surface and inside the multilayer at the SiGe/Si layer interfaces.
Comparative study on the effects of ion density and ion energy on diamond-like carbon deposited by electron cyclotron resonance chemical vapor deposition
Authors: Yoon S. F. , Tan K. H. , Rusli  , Ahn J. , Huang Q. F.
Published: July 2002

Diamond-like carbon films were deposited using electron cyclotron resonance (ECR) chemical vapor deposition incorporated with a screen grid under different dc bias voltages to compare the effect of ion density and ion energy on the film properties. Langmuir probe measurements and optical emission spectroscopy were used to characterize the ECR plasma, while the films were characterized using Raman and infrared (IR) spectroscopies, hardness, and optical gap measurements. The plasma measurements showed that the ion density, hydrogen atom density, and CH density decreased monotonously following increase in the dc bias voltage. Raman spectra and optical gap measurements indicate the films became more graphitic with lower content of sp3-hybridized carbon atoms as the dc bias voltage was increased. An increase in hydrogen content was found in films prepared at relatively high dc bias voltage, as indicated by IR measurements. Films deposited at −150 V exhibit maximum hardness. The results show the ion density has a stronger effect on the film deposition rate and hydrogen content, while the ion energy affects the film properties more predominantly by changing the bonding structure.
Direct observation of the field-stimulated exoemission sites at tungsten surfaces using field ion microscopy
Authors: Shiota T. , Umeno M. , Dohkuni K. , Tagawa M. , Ohmae N.
Published: July 2002

The spatial distribution of the field-stimulated exoemission (FSEE) from the W tip surface annealed at 800 K for 600 s and the atomic arrangement of the emitting surface were correlated using field ion microscopy (FIM) and field emission microscopy. The FSEE was observed at around the (111) plane of the annealed W tip surface. FIM observation of the annealed W tip revealed the existence of a pyramid-like protrusion at the W(111) surface. From these experimental results, a new emission model of the FSEE was proposed relating to the field-assisted surface structural change. This model deals with the buildup/collapse of the pyramid-like protrusion at the W(111) surface under the effect of negative high electric field. The temperature dependence of the FSEE reported previously [Shiota et al., J. Appl. Phys. 85, 6811 (1999)] was qualitatively explained by this emission model.
Ultrashort pulse ultraviolet laser treatment of n(100) GaAs: Microstructural modifications and passivation effects
Authors: Railkar T. A. , Malshe A. P. , Brown W. D. , Hullavarad Shiva S. , Bhoraskar S. V.
Published: July 2002

Gallium arsenide (GaAs) is one of the most important materials among the III–V family, especially in view of its applicability to optoelectronic devices. However, it is known that GaAs, unlike silicon (Si), does not possess a stable native oxide that can passivate and protect the surface. This article reports the use of femtosecond (fs) laser-based modification and passivation of the GaAs surface, where femtosecond laser-based processing was shown to be particularly useful, effective, and more convenient compared to conventional laser treatment. The fs laser treatment involves an almost nonexistent heat affected zone, which implies that there is virtually no thermal damage to the volume of material surrounding a processed region. The surface passivating effects were confirmed by depth-profiling x-ray photoelectron spectroscopic measurements. In addition, scanning electron microscopy and atomic force microscopy measurements lead to a possible explanation of the passivation mechanism. Further, a relatively novel technique called thermally stimulated exoelectron emission was used to verify the existence of surface passivation. This measurement technique detects “cold electron emission” from trapping centers at the surface of material under scrutiny.
Multiwalled carbon nanotubes as ultrasensitive electrometers
Authors: Roschier L. , Tarkiainen R. , Ahlskog M. , Paalanen M. , Hakonen P.
Published: July 2002

We show that it is possible to construct low-noise single-electron transistors (SETs) using free-standing multiwalled carbon nanotubes. The 1/fα-noise of our devices, 6×10−6e/Hz at 45 Hz, is close in the performance to the best metallic SETs of today.
Simulation of the initial transient of the Si+ and O+ signals from oxygen sputtered silicon by means of independent models on sputtering and secondary ionization
Authors: Serrano J. J. , De Witte H. , Vandervorst W. , Guzmán B. , Blanco J. M.
Published: July 2002

The Si+ and O+ signals, as obtained in secondary ion mass spectrometry (SIMS) analysis of silicon when using oxygen as the primary species, pass through an initial transient region before reaching the stationary state. We simulate this transient zone to check a phenomenological model for the secondary ionization of sputtered atoms. The simulation is split into two parts: the sputtering of neutrals obtained from implantation, sputtering, relocation/replacement, and diffusion, simulations and their subsequent ionization. The ionization phenomena are also described by some ad hoc fitting functions with which the SIMS measurements are better approached than with the model. The fitting functions and the model outputs are verified with experimental secondary ionization data.
An efficient method for the coordinate transformation problem of massively three-dimensional networks
Authors: Németh Károly , Coulaud Olivier , Monard Gérald , Ángyán János G.
Published: July 2002

A new and efficient algorithm is presented for the coordinate transformation problem of massively three-dimensional networks formed, e.g., by the atoms of crystal fragments or molecular clusters. The new algorithm is based on a divide-and-conquer technique to perform iterative coordinate transformation, applicable even for three-dimensional networks, with linear scaling memory and near linear scaling CPU time requirements. The new algorithm proved to be very fast in the coordinate transformation problems and geometry optimization of diamond fragments, water clusters, globular proteins, and proteins in solvent.
Photoluminescence properties of the Eu3+ activator ion in the TiO2 host matrix
Authors: Conde-Gallardo A. , Garcı́a-Rocha M. , Hernández-Calderón I. , Palomino-Merino R.
Published: July 2002

Titania (TiO2) is employed as a host for the Eu3+ activator ion. Thin films were produced by the sol-gel method on silicon and corning glass substrates and, depending on the nature of the substrate, they present different crystalline structure. The films show an intense red photoluminescence associated with the D05→7F2 transition of the electronic structure of Eu3+. The photoluminescence presents better characteristics for the films deposited on silicon wafers. For above band gap excitation the emission from the TiO2 matrix is obscured by the luminescence of the Eu3+ ions. The peak energy position, the spectral shape, and the width are insensitive to changes of temperature in the 12–300 K range, making the TiO2:Eu3+ a very attractive system for technological applications.
A long range spherical model and exact solutions of an energy enstrophy theory for two-dimensional turbulence
Authors: Lim Chjan C.
Published: July 2002

The equilibrium statistical mechanics of the energy–enstrophy theory for the two-dimensional (2D) Euler equations is solved exactly. A family of lattice vortex gas models for the Euler equations is derived and shown to have a well-defined nonextensive continuum limit. This family of continuous-spin lattice Hamiltonians is shown to be nondegenerate under different point vortex discretizations of the Euler equations. Under the assumptions that the energy, total circulation and the enstrophy (mean squared vorticity) are conserved, this lattice vortex gas model is equivalent to a long range version of Kac’s exactly solvable spherical model with logarithmic interaction. The spherical model formulation is based on the fundamental observation that the conservation of enstrophy is mathematically equivalent to Kac’s spherical constraint. This spherical model is shown to have a free energy that is analytic in the properly scaled inverse temperatures β̃ in the range 0=β̃*<β̃<β̃c=4π/3. Phase transitions occur at the positive value β̃c and β̃*=0. Spin–spin correlations are calculated giving two-point vorticity correlations that are important to the study of turbulence. There are exactly three distinct phases in the energy-enstrophy theory for 2D flows, namely (a) an uncorrelated high positive temperature phase, (b) an antiferromagnetic checkerboard vorticity pattern at low positive temperature, and (c) a highly correlated physical domain scale vorticity pattern (for instance, a large positive vorticity region surrounded by a sea of negative vorticity) at negative temperatures. The boundary β̃*=0 agrees with the known numerical and analytical results on the occurrence of coherent or ordered structures at negative temperatures. The critical temperature β̃c>0 is new, as is the corresponding checkerboard low positive temperature phase. Physical interpretations of the results in this paper are obtained.
Ultrastable laser array at 633 nm for real-time dimensional metrology
Authors: Lawall John , Pedulla J. Marc , Le Coq Yann
Published: July 2002

We describe a laser system for very-high-accuracy dimensional metrology. A sealed-cavity helium–neon laser is offset locked to an iodine-stabilized laser in order to realize a secondary standard with higher power and less phase noise. Synchronous averaging is employed to remove the effect of the frequency modulation present on the iodine-stabilized laser. Additional lasers are offset locked to the secondary standard for use in interferometry. All servo loops are implemented digitally. The offset-locked lasers have intrinsic linewidths of the order of 2.5 kHz and exhibit a rms deviation from the iodine-stabilized laser below 18 kHz. The amplitude noise is at the shot-noise limit for frequencies above 700 kHz. We describe and evaluate the system in detail, and include a discussion of the noise associated with various types of power supplies.
Lattice model for ferroelectric thin film materials including surface effects: Investigation on the “depolarizing” field properties
Authors: Baudry L. , Tournier J.
Published: July 2002

Ferroelectric properties of thin film materials like lead zirconate titanate differ from those observed for bulk material. It is well known that the surface can strongly influence phase transition phenomena. By the use of a thermodynamical approach, very similar to those generally used to describe phase transitions, we have proposed for studying ferroelectric states a two dimensional lattice model which allowed us to account for surface effects. These surface effects described by an inhomogeneous Landau–Devonshire theory were suggested to produce a finite size effect even if the surface charge is compensated at the metallic electrodes. The manifestations of the field induced by the bound charges in the polarization switching is discussed and is shown to depend strongly on the bulk correlation length and on the extrapolation length. Finally we have applied our model to investigate the influence of the surface effect and of the related bound charges on the thickness dependence of ferroelectric properties.
Modulation doped InGaAsP quantum well laser emitting at 1.55 μm
Authors: Choudhury N. , Dutta N. K.
Published: July 2002

A number of parameters, such as gain, modulation response, linewidth enhancement factor and relative intensity noise, in a modulation-doped InGaAsP quantum well (QW) laser emitting at 1.55 μm have been theoretically investigated. The results indicate that the relaxation oscillation frequency for a p-type modulation doped QW laser is enhanced by a factor of more than 2 compared to that for undoped MQW lasers, the linewidth enhancement factor of a p-type modulation doped QW laser is reduced to 1/2 that of an undoped MQW laser and the relative intensity noise is reduced by a factor of >10 dB compared to that for undoped MQW lasers.
Supersonic Love waves in strong piezoelectrics of symmetry mm2
Authors: Darinskii A. N. , Weihnacht M.
Published: July 2002

A study has been made of the Love wave propagation on piezoelectric substrates of symmetry mm2. It has been shown that under certain conditions the velocity of the Love wave exceeds that of shear horizontal (SH) bulk waves in the substrate. This occurs when the slowness curve of SH bulk waves in the substrate either has a concavity or is convex with nearly zero curvature. For such “supersonic” Love waves to appear, it is also required that the substrate as well as the layer be specially oriented and that their material constants fulfill a number of inequalities. Numerical computations have been carried out for a number of structures. The results of numerical computations have been compared with approximate analytical estimations.
Propagation of coherent x rays in a multistep-index x-ray waveguide
Authors: Bongaerts J. H. H. , Zwanenburg M. J. , Zontone F. , van der Veen J. F.
Published: July 2002

We have developed and tested a multistep-index x-ray waveguide. The waveguide geometry allows for a controlled positioning of the waveguide gap down to the nanometer range. The waveguide consists of two closely spaced flat fused-silica substrates, coated with an aluminum layer. On top of the aluminum layer we deposited an optically transparent spacer layer. The aluminum layers are the mirrors of an optical interferometer, which is used to measure the separation and parallelism of the waveguide surfaces. The x rays are guided within the gap between the spacer layers. We have observed so-called cladding modes and demonstrate waveguiding in a multistep index structure having a gap width of 59 nm over a length of 5 mm.
Multiphonon excitonic absorption in semiconductors and quantum wells
Authors: Bardyszewski Witold , Prywata Mirosław , Yevick David
Published: July 2002

In this article, we present a straightforward procedure based on the cumulant expansion for computing the contribution of exciton–phonon coupling to the exciton line-shape function and thus absorption in bulk and quantum well semiconductors. Using the functional integral technique we derive the effective equation of motion for the two-particle Green’s function in the presence of interaction with phonons. We show that our many body formulation in the limit of linear coupling of carriers with polar longitudinal optical phonons in bulk semiconductors is equivalent to the model exciton–phonon Hamiltonian approach. Our calculations for the hydrogenic model of excitons in three- and two-dimensional systems reproduce not only the correct polaron induced shift (renormalization) of the exciton energy but also the observed asymmetric spectral line shape and exponential Urbach absorption tail below the fundamental absorption edge.
Photovoltaic properties of Schottky and p–n type solar cells based on polythiophene
Authors: Sicot Lionel , Geffroy Bernard , Lorin André , Raimond Paul , Sentein Carole , Nunzi Jean-Michel
Published: July 2002

The electrical and photovoltaic properties of organic Schottky and p–n type cells have been studied. Schottky devices are based on a spin-coated film of poly(3-butylthiophene) blend with the dye 1,3-bis-(dicyanomethylidene)-2-(4-dibutylaminobenzylidene)indane which is sandwiched between indium tin oxide (ITO) and gold electrodes. A depletion zone of 9.5 nm thick has been determined on a 220-nm-thick film. The conversion efficiency is equal to 0.01% under 100 mW cm−2 white light illumination. Effects of dye loading, film thickness, and light intensity are described. A model based on simple assumptions is used to fit the photoaction spectra. By evaporation under vacuum of a layer of N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic diimide between the ITO and the polymer film, we have fabricated hybrid polymer-molecular p–n type photovoltaic cells. The role of this molecule in the photogeneration has been studied. A constant power conversion efficiency of 0.15% has been measured under white light illumination between 0.2 and 200 mW cm−2.
Microoptical characterization of electroluminescent SrS:Cu,Ag thin films by photo- and cathodoluminescence observations
Authors: Poelman D. , Wauters D. , Versluys J. , Van Meirhaeghe R. L.
Published: July 2002

For SrS:Cu,Ag thin film phosphors, a postdeposition annealing step is imperative in order to obtain bright photo- or electroluminescence. However, it is not clear to date what is the most important effect of this treatment, a recrystallization of the host lattice or a redistribution of the activators. The present article discusses the microscopic emission characteristics of annealed photoluminescent layers SrS:Cu,Ag. Using cathodoluminescence measurements, it is shown that emission, originating from the grain boundaries, is very weak compared to the emission from the bulk of the grains.