bluuebunny/crossref_metadata_2025 huggingface dataset:
Ohmic nanocontacts to GaAs using undoped and p-doped layers of low-temperature-grown GaAs
Authors: Lee Takhee , Chen Nien-Po , Liu Jia , Andres R. P. , Janes D. B. , Chen E. H. , Melloch M. R. , Woodall J. M. , Reifenberger R.
Published: July 2002

The development and characterization of high-performance nanocontacts to n-GaAs are reported. The nanocontacts can be made to both undoped and p-doped low-temperature-grown GaAs (LTG:GaAs) cap layers. The geometry of the nanocontact is well characterized and requires the deposition of a 4 nm single-crystalline Au cluster onto an ohmic contact structure which features a chemically stable LTG:GaAs surface layer prepared using an ex situ chemical self-assembly technique. A self-assembled monolayer of xylyl dithiol (HS–CH2–C6H4–CH2–SH) is required to provide mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. For the case of an undoped LTG:GaAs cap layer, a specific contact resistance of 1×10−6 Ω cm2 and a current density of 1×106 A/cm2 have been measured from scanning tunneling microscopy. When a p-doped LTG:GaAs cap layer is used, the corresponding values are 1×10−7 Ω cm2 and 1×107 A/cm2, respectively. Improved surface stability as evidenced by a lower oxidation rate for p-doped LTG:GaAs provides a natural explanation for the higher-quality ohmic contact properties of the nanocontact to the p-doped LTG:GaAs cap layer.
Multiple gratings simultaneously formed in holographic polymer-dispersed liquid-crystal displays
Authors: Bowley Chris C. , Fontecchio Adam K. , Crawford Gregory P. , Lin Jau-Jeng , Li Le , Faris Sadeg
Published: July 2002

We report single-layer holographic polymer-dispersed liquid-crystal materials with multiple reflection and/or diffraction gratings. Three switchable gratings are observed in a single film formed in a three-beam interference pattern. Two reflection gratings and one transmission grating are formed, corresponding to each of the three possible two-beam interference fringes. We present scanning electron microscopy and electro-optic results which are in good agreement with our model based on matrix methods and coupled-wave theory.
Integration of Pb(Zr0.52Ti0.48)O3 epilayers with Si by domain epitaxy
Authors: Sharma A. K. , Narayan J. , Jin C. , Kvit A. , Chattopadhyay S. , Lee C.
Published: July 2002

High-quality lead zirconate titanate films (PZT) have been grown on yttrium barium copper oxide (YBCO) bottom electrode by domain epitaxy where integral multiples of lattice constants match across the interface. The YBCO films were epitaxially fabricated on Si (100) by introducing epilayer geometry of strontium titanate/magnesium oxide/titanium nitride. Pulsed-laser ablation was used to evaporate these five stoichiometric targets in a high vacuum chamber. X-ray diffraction and high-resolution transmission electron microscopy techniques were employed to gain understanding of the structure, crystallinity, and interfaces in these epilayers. The electrical characterization of the PZT films with evaporated silver contacts resulted in superior values of spontaneous polarization, remnant polarization, and coercive fields. This heterostructure opens a way for integration of epitaxial single-crystal PZT-based capacitors with silicon-based devices.
Fabrication of nanometric zinc pattern with photodissociated gas-phase diethylzinc by optical near field
Authors: Yamamoto Y. , Kourogi M. , Ohtsu M. , Polonski V. , Lee G. H.
Published: July 2002

In situ patterning of zinc on a nanometric scale has been accomplished by photodissociation of gas-phase diethylzinc by optical near field. By using an ultraviolet optical fiber probe with an aperture diameter of 60 nm, dots with full width at half maxima of 60 and 70 nm were deposited, and were separated by 100 nm. The aspect ratio of the dots increased at the rate of 0.03/μJ, as the optical near field energy increased. A T-shape pattern was also fabricated by scanning the probe in an optical near field microscope system.
Infrared continuous-wave laser gain in neodymium aluminum borate: A promising candidate for microchip diode-pumped solid state lasers
Authors: Jaque D. , Enguita O. , Solé J. Garcı́a , Jiang A. D. , Luo Z. D.
Published: July 2002

In this letter, continuous-wave laser gain at 1.06 and 1.3 μm is demonstrated in the neodymium aluminum borate [NdAl3(BO3)4] system. End pumping is achieved by means of a tunable argon-pumped Ti:sapphire and a diode laser. We report on low absorbed pump power at threshold and high pump-to-laser conversion efficiencies (up to 43% and 25% for laser oscillations at 1.06 and 1.3 μm, respectively) in a 0.3 mm long crystal. These good laser properties together with some other spectroscopic parameters described in this work (such as a 100% absorbance, a broad absorption spectrum around 800 nm and high emission cross section at 1.3 μm) make NdAl3(BO3)4 a promising material for diode-pumped microchip lasers.
Propagation of guided optical waves in thick GaN layers grown on (0001) sapphire
Authors: Ciplys D. , Gaska R. , Shur M. S. , Rimeika R. , Yang J. W. , Khan M. Asif
Published: July 2002

We report on the propagation of guided optical modes in insulating and Si-doped n-GaN layers grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. The effective refractive indices of the guided modes were measured using the prism-coupling technique for layers with thicknesses from 1 to 6 μm. Ordinary and extraordinary refractive indices of GaN layers at wavelength of 632.8 nm were evaluated from the comparison of the measured index values and the index values calculated by solving the characteristic equation for a planar optical waveguide. The obtained results are in a good agreement with the step-like index profile model, indicating a high optical homogeneity of the investigated layers.
Low-energy electron-beam patterning of amine-functionalized self-assembled monolayers
Authors: Harnett C. K. , Satyalakshmi K. M. , Craighead H. G.
Published: July 2002

Patterned amine-functionalized self-assembled monolayers have potential as a template for the deposition and patterning of a wide variety of materials on silicon surfaces, including biomolecules. Results are presented here for low-energy electron-beam patterning of 2-aminopropyltriethoxysilane and (aminoethylaminomethyl)phenethyltrimethoxysilane self-assembled monolayers on silicon substrates. On these ultrathin (1–2 nm) monolayers, lower electron beam energies (<5 keV) produce higher resolution patterns than high-energy beams. Auger electron spectroscopy indicates that low-energy electron exposure primarily damages the amine groups. At 1 keV, a dose of 40 μC/cm2 is required to make the patterns observable by lateral force microscopy. Features as small as 80 nm were exposed at 2 keV on these monolayers. After exposure, palladium colloids and aldehyde- and protein-coated polystyrene fluorescent spheres adhered only to unexposed areas of the monolayers.
On the strong influence of the photoelastic effect in the collinear mirage deflection
Authors: Salazar A. , Gateshki M. , Sánchez-Lavega A.
Published: July 2002

We report in this letter on the strong influence of the polarization state of the probe beam in the amplitude and phase of the collinear mirage deflection. A model of the collinear mirage deflection that takes into account the photoelastic effect in the sample has been developed. The agreement between the model predictions and the experimental results is excellent.
Mechanism for the anomalous degradation of silicon space solar cells
Authors: Karazhanov S. Zh.
Published: July 2002

We propose a mechanism to explain the anomalous degradation of n+-p-p+ silicon space solar cells. Distinct from previously known mechanisms, it has been shown that the anomalous increase and abrupt decrease of short-circuit current are caused by corresponding changes of the minority carrier lifetime and a conversion of conductivity type. The majority carrier density decreases abruptly due to trapping by the radiation-induced deep donors, which results in an increase of carrier lifetime and resistance, conversion of conductivity type, and anomalous change of solar cell performance. Peak values of the carrier lifetime and short-circuit current decrease with increasing illumination intensity and are sensitive to variations of the weak optical illumination.
Trajectory overlaps and local magnification in three-dimensional atom probe
Authors: Vurpillot F. , Bostel A. , Blavette D.
Published: July 2002

Local magnification effects related to the presence of a second phase in three-dimensional atom probe have been investigated using a simulation of ion trajectories from the analyzed sample surface. Spherical precipitates containing only B atoms embedded in pure A solid solution were considered. The magnification was found to vary drastically from 0.5 to 2.0 times when the evaporation field of B (EB) was varied from 1.15 EA to 0.85 EA. The trajectories were found to overlap over distances close to 1 nm only when the reduced evaporation field (εB=EB/EA) is outside of a gap ranging from 0.9 to 1.1. Simulations indicate that the “measured” composition in the inner core of precipitates is not biased in this gap. This is also the case for particles which have a diameter larger than a critical value of 2 nm.
Plasma drift and nonuniformity effects in plasma immersion ion implantation
Authors: Keidar M. , Monteiro O. R. , Brown I. G.
Published: July 2002

Measurements of the ion current collected by a substrate biased to high voltage have been carried out in plasma immersion ion implantation with a filtered vacuum arc plasma source. We have found that the ion saturation current increases with applied voltage and that this effect depends upon the angle of the normal to the substrate with respect to the plasma stream and on the distance of the substrate from the plasma duct exit. We also found that the ion current increases with increasing angle of the normal to the substrate with respect to the plasma stream. A model was developed for the sheath expansion in a nonuniform plasma with substantial ion drift velocity. We find that nonuniformity and high drift velocity lead to a decrease in sheath thickness. In a nonuniform plasma, the ion saturation current increases with applied voltage. The predictions of the model were found to be in good agreement with experiment.
Magnetic entropy change in Ni51.5Mn22.7Ga25.8 alloy
Authors: Hu Feng-xia , Shen Bao-gen , Sun Ji-rong
Published: July 2002

A considerable magnetic entropy change has been observed in Ni51.5Mn22.7Ga25.8 alloys under a field of 0.9 T. This change originates from a sharp magnetization jump which is associated with a martensitic to austenitic structure transition. The large low field entropy change and the adjustable martensic–austensic transition temperature indicate a great potential of Ni–Mn–Ga as working materials for magnetic refrigerants in a wide temperature range.
High-dielectric-constant ceramic-powder polymer composites
Authors: Bai Y. , Cheng Z.-Y. , Bharti V. , Xu H. S. , Zhang Q. M.
Published: July 2002

A ceramic-powder polymer composite, making use of a relaxor ferroelectric polymer that has a high room-temperature dielectric constant as the matrix, is developed. The experimental data show that the dielectric constant of the composites with Pb(Mg1/3Nb2/3)O3–PbTiO3 powders can reach more than 250 with weak temperature dependence. In addition, the composites under a proper preparation procedure exhibit a high breakdown field strength (>120 MV/m), leading to a maximum energy storage density of more than 15 J/cm3. Experimental results also indicate that the high electron irradiation does not have much effect on the dielectric behavior of Pb(Mg1/3Nb2/3)O3–PbTiO3 powders, possibly due to the relaxor nature of the ceramic.
Surface photovoltage spectroscopy, photoreflectance, and reflectivity characterization of an InGaAs/GaAs/GaAlAs vertical-cavity surface-emitting laser including temperature dependence
Authors: Huang Y. S. , Malikova L. , Pollak Fred H. , Shen H. , Pamulapati J. , Newman P.
Published: July 2002

We have investigated an InGaAs/GaAs/GaAlAs vertical-cavity surface-emitting laser using surface photovoltage spectroscopy (SPS) in the temperature range of 15 K<T<400 K. For comparison purposes, we have also (a) measured temperature-dependent photoreflectance (PR) and normal-incidence reflectivity (NIR); and (b) calculated the number of photoexcited carriers (PEC), which is related to the SPS signal. The SPS spectra exhibit both the fundamental conduction to heavy-hole (1C–1H) excitonic transition and cavity mode plus a rich interference pattern related to the properties of the mirror stack. The PR data show only the 1C–1H excitonic transition (plus Franz–Keldysh oscillations) while the cavity mode is detected by NIR. The temperature variation of 1C–1H/cavity mode is the same for SPS and PR/SPS and NIR. The SPS interference pattern from the DBRs is in good agreement with the PEC calculation. This experiment demonstrates the considerable potential of SPS for the characterization of these devices.
Space-charge controlled fast electron beam pulsing
Authors: Ruan Chong-Yu , Fink Manfred
Published: July 2002

The generation of fast electron beam pulses by voltages in the range of 1 V is presented. Two emission mechanisms are described, where the one which utilizes thermal electron confinement in the proximity of a sharp filament proves to be more advantageous. We have observed pulsewidth of 5 ns, limited only by our recording instruments. Due to the superb optical quality and tunability, this electron beam modulation mechanism can find applications in time-resolved diffraction/imaging, microwave system, time-of-flight spectrometer, and electron beam lithography.
Multidimensional hierarchies of (1+1)-dimensional integrable partial differential equations. Nonsymmetric ∂̄-dressing
Authors: Zenchuk A. I.
Published: July 2002

In this paper the ∂̄-problem has been constructed for a class of multidimensional integrable partial differential equations (PDE) which can be classified as the multidimensional hierarchies of the (1+1)-dimensional systems of integrable PDE. We introduce the nonsymmetric dressing procedure for this purpose. Among the examples we consider the (n+1)-dimensional (n>1) hierarchies of nonlinear Schrödinger, modified Korteveg–de Vries, and Camassa–Holm equations.
The role of infrared divergence for decoherence
Authors: Kupsch Joachim
Published: July 2002

Continuous and discrete superselection rules induced by the interaction with the environment are investigated for a class of exactly soluble Hamiltonian models. The environment is given by a boson field. Stable superselection sectors can only emerge if the low frequences dominate and the ground state of the boson field disappears due to infrared divergence. The models allow uniform estimates of all transition matrix elements between different superselection sectors.
Classification of certain integrable coupled potential KdV and modified KdV-type equations
Authors: Foursov Mikhail V.
Published: July 2002

In this article we describe the classification of integrable symmetrically coupled potential KdV and modified KdV-type equations that possess higher symmetries. Restricting our attention to the systems that cannot be decoupled by a change of dependent variables, we obtain 11 previously unknown classes of integrable equations. In some cases we present Hamiltonian or bi-Hamiltonian formulations.
Nonlinear thermal instability in optically thin plasmas
Authors: Steele C. D. C. , Ibáñez S. M. H. , Sira E.
Published: July 2002

This paper contains a study of the stability of a slablike thermal structure constituted by an optically thin plasma with solar abundances in which there is generation and loss of energy, and a thermal diffusion of heat dependent on both density and temperature. The analysis is carried out by means of a second order approximation. A general expression for the Landau constant is obtained. Close to the first order marginal state, this constant is completely determined by the power of the heat diffusion law and the first and second order derivatives with respect to density and temperature of the heat/loss function. Normally, near the first-order marginal state, the second order is such that cooling perturbations are enhanced. However, it is possible to find circumstances where a finite perturbation may lead to the enhancement of heating. First and second order bifurcation points are determined when this function has a power dependence on density and temperature. The regions with a different kind of instability or stability have been determined on the plane defined by the ratios between the dynamical and the relaxation time scale (α) and between the dynamical and the heating time scale (ε). Additionally, the time evolution at the onset of the nonlinear regime is analyzed.
State-selected photodissociation dynamics of CS2+ in the C̃  2Σg+ state
Authors: Hwang Wan Goo , Kim Hong Lae , Kim Myung Soo
Published: July 2002

State-selected photodissociation dynamics of CS2+ molecular ion has been investigated by an optical–optical double resonance technique. The CS2+ molecular ions were prepared by [2+1] REMPI and the predissociative C̃ 2Σg+ state was populated by the B̃ 2Σu+←X̃ 2Πg,3/2 followed by C̃ 2Σg+←B̃ 2Σu+ transition. The product branching ratios, CS+/S+, for selected vibrational levels in the C̃ state and average kinetic energy releases in the CS+ and S+ production channels were measured from the time-of-flight mass spectra. It has been found that excitation of the bending vibration enhances the CS+ production channel more than the S+ channel. In addition, an isotropic fragment distribution for different polarizations of the dissociating laser light has been observed, from which the dissociation time was estimated to be longer than 20 ps. Dynamical aspect of the reaction revealed by the present investigation is discussed together with the previous spectroscopic results.
Investigation of molybdenum-carbon films (Mo–C:H) deposited using an electron cyclotron resonance chemical vapor deposition system
Authors: Rusli  , Yoon S. F. , Huang Q. F. , Yang H. , Yu M. B. , Ahn J. , Zhang Q. , Teo E. J. , Osipowicz T. , Watt F.
Published: July 2002

We have recently proposed a technique for depositing metal incorporated carbon films (Me–C:H) based on an electron cyclotron resonance chemical vapor deposition (ECR) process. This technique employs an ECR plasma derived from the excitation of source gases CH4 and Ar, together with two grids embedded within the chamber that serve as the source of the metal. It has been successfully applied for the deposition of tungsten–carbon films (W–C:H) which have been shown to exhibit a wide range of electrical, optical, and microstructural properties. These properties can be controlled through varying the deposition conditions such as the bias voltages at the grids and the substrate holder, and the flow ratio of CH4/Ar. In this work, we report on the growth and characterization of molybdenum–carbon (Mo–C:H) films deposited using the above technique incorporating two pure Mo grids. The effect of radio-frequency induced direct-current (dc) bias at the substrates was investigated. It was found that the resistivity of the films decreased by 9 orders of magnitude, and the optical gap decreased by more than 2 eV with increasing bias voltage from −38 to −130 V. The results suggest that the substrate dc bias has a crucial effect on the incorporation of Mo into the a-C:H films and the resulting microstructures, with larger bias voltages leading to an increase in the Mo fractions in the films. Concurrently, the hardness of the films was found to deteriorate from 22 to 10 GPa. The structures of these Mo–C:H films were characterized using x-ray diffraction and Raman scattering. Mo was found to exist in the forms of Mo and MoC and Mo2C. The experimental results are interpreted in terms of the effects of ion energy on the structure of the films having Mo clusters embedded within an amorphous carbon matrix.
The Goldberg–Sachs theorem in linearized gravity
Authors: Dain Sergio , Moreschi Osvaldo M.
Published: July 2002

The Goldberg–Sachs theorem has been very useful in constructing algebraically special exact solutions of the Einstein vacuum equation. Most of the physically meaningful vacuum exact solutions are algebraically special. We show that the Goldberg–Sachs theorem is not true in linearized gravity. This is a remarkable result, which sheds light on the understanding of the physical meaning of the linearized solutions.
Density-functional studies of amphiphilic binary mixtures. I. Phase behavior
Authors: Napari Ismo , Laaksonen Ari , Strey Reinhard
Published: July 2002

Density-functional theory with interaction site model is applied to study binary mixtures consisting of Lennard-Jones atoms with hard-spherical cores (monomers) and bonded Lennard-Jones atoms of two hard-sphere sites (dimers). Strong amphiphilic interaction is shown to result in lamellae in the bulk condensed phase. A phase diagram showing disorder-lamellar phase boundaries and tricritical points is presented for a system with reduced interaction between the molecular species. Density distributions in the lamellar phase indicate that the mole fraction in the lamellae is determined by the concentration of the monomers.
Electrical measurements on p+–p−–p+ homoepitaxial diamond capacitors
Authors: Inushima Takashi , Matsushita Takahiro , Mamin Rinat F. , Ohya Seishirou , Shiomi Hiromu
Published: July 2002

Conductance versus voltage and capacitance versus voltage (C–V) characteristics are investigated for p+–p−–p+ capacitors over a temperature range of 40–300 K, where the p+ layer is heavily doped homoepitaxial diamond and has impurity-band conduction and the p− layer is slightly doped with valence-band conduction. Above 200 K, the capacitors behave like a semiconductor–insulator–semiconductor diode with interface barrier height of about 0.07 eV. The C–V curve agrees closely with the standard theory of semiconductor–insulator–semiconductor structure and shows formation of the deletion layer at the p+ layer on the interface. The Cole–Cole plot of conductance versus susceptance reveals that there is a virtual trap level in the p− layer which is located about 0.06 eV above the valence band.
Local analysis of the morphological properties of single-wall carbon nanotubes by Fresnel projection microscopy
Authors: Binh Vu Thien , Vincent P. , Feschet F. , Bonard J.-M.
Published: July 2002

Direct observations of free-standing single-wall carbon nanotubes (SWNTs), either as individual tubes or bundled in ropes, are performed using a Fresnel projection microscope (FPM). The analysis is done with the support of systematic numerical simulations of Fresnel diffraction images. The images of individual SWNTs are characterized by bright lines, while semitransparency wire fringe patterns are observed for thin ropes. Ropes with a higher number of individual tubes present the conventional Fresnel fringe patterns of an opaque wire. This differentiation in the FPM images between fringe patterns and bright line patterns allows for an easy detection of individual SWNTs. FPM also allows us to detect the presence of localized defects along SWNTs, and, in particular, the characterization of nanotube junctions. Although the exact structure of the junctions cannot be deduced presently from the images they, at least, indicate the presence of strong electric fields associated with the structural connections between the SWNTs.
Generating folded protein structures with a lattice chain growth algorithm
Authors: Gan Hin Hark , Tropsha Alexander , Schlick Tamar
Published: July 2002

We present a new application of the chain growth algorithm to lattice generation of protein structure and thermodynamics. Given the difficulty of ab initio protein structure prediction, this approach provides an alternative to current folding algorithms. The chain growth algorithm, unlike Metropolis folding algorithms, generates independent protein structures to achieve rapid and efficient exploration of configurational space. It is a modified version of the Rosenbluth algorithm where the chain growth transition probability is a normalized Boltzmann factor; it was previously applied only to simple polymers and protein models with two residue types. The independent protein configurations, generated segment-by-segment on a refined cubic lattice, are based on a single interaction site for each amino acid and a statistical interaction energy derived by Miyazawa and Jernigan. We examine for several proteins the algorithm’s ability to produce nativelike folds and its effectiveness for calculating protein thermodynamics. Thermal transition profiles associated with the internal energy, entropy, and radius of gyration show characteristic folding/unfolding transitions and provide evidence for unfolding via partially unfolded (molten-globule) states. From the configurational ensembles, the protein structures with the lowest distance root-mean-square deviations (dRMSD) vary between 2.2 to 3.8 Å, a range comparable to results of an exhaustive enumeration search. Though the ensemble-averaged dRMSD values are about 1.5 to 2 Å larger, the lowest dRMSD structures have similar overall folds to the native proteins. These results demonstrate that the chain growth algorithm is a viable alternative to protein simulations using the whole chain.
Eigenvalues in spectral gaps of the two-dimensional Pauli operator
Authors: Besch Alexander
Published: July 2002

We consider purely magnetic two-dimensional Pauli operators H with a spectral gap, perturbed by a magnetic field λBs=λda⃗s, λ⩾0. Assuming that Bs and a⃗s vanish at infinity, we ask whether eigenvalues will cross the gap as λ→∞. Furthermore, we give an example of a two-dimensional Pauli operator H with periodic magnetic field of zero flux which has at least one spectral gap.
Structure of liquid GeSe: A first principle study
Authors: van Roon F. H. M. , Massobrio C. , de Wolff E. , de Leeuw S. W.
Published: July 2002

We have investigated liquid GeSe by first-principle molecular dynamics simulations performed in the framework of density functional theory. Our partial structure factors are in good agreement with recent experimental data. In particular, we do not find any evidence of intermediate range order, as indicated by the absence of a first sharp diffraction peak for low magnitudes of the momentum transfer in the partial structure factors. Comparison between experimental and calculated partial pair correlation functions gαβ(r) yields a less satisfactory agreement, non-negligible differences being found particularly in the shape and peaks position of the gGeGe(r) and gSeSe(r) for short distances (<4 Å). The origin of these discrepancies are discussed in the light of recent experimental and theoretical findings on other GexSe1−x disordered systems. Analysis of the atomic configurations reveals that this liquid GeSe does not exhibit a regular network structure. A variety of bonding configurations are found and, in particular, the subunits GeSe3 and SeGe3 are frequently observed. A conspicuous amount of Ge atoms form homopolar bonds, thereby favoring the occurrence of Ge–Ge–Se3 groups in which the Ge–Ge bonds have a very short lifetime (∼0.5 ps). However, neither these groups nor the GeSe3 and SeGe3 subunits are predominant and play the role of the GeSe4 tetrahedra in the liquids GeSe4 and GeSe2, where structural order extended well beyond the first shell of neighbors. The lack of a prevailing structural feature in liquid GeSe correlates well with the absence of intermediate range order.
Formation and dissolution of microcrystalline graphite in carbon-implanted GaN
Authors: Sun W. H. , Wang S. T. , Zhang J. C. , Chen K. M. , Qin G. G. , Tong Y. Z. , Yang Z. J. , Zhang G. Y. , Pu Y. M. , Zhang Q. L. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

Two sharp bands at ∼1350 and ∼1600 cm−1 were observed in the Raman spectra of carbon-implanted GaN after postimplantation annealing treatments. The intensities of these two bands increased while their full widths at half maximum decreased with increasing annealing temperature. The observation of these two bands indicates the formation of microcrystalline graphite in C-implanted GaN. Hall measurements demonstrated that some dispersed C in GaN acted as acceptors and played a role in reducing electron concentration and Hall mobility. The facts that in 1100 °C furnace annealing the intensities of these two Raman peaks decreased rapidly to zero and the resistivity increased by 3 orders of magnitude indicate the dissolution of microcrystalline graphite at this temperature.
The ν1 and ν2 bands of Ar⋯HN2+: A joint theoretical/experimental study
Authors: Botschwina Peter , Oswald Rainer , Linnartz Harold , Verdes Dorinel
Published: July 2002

A combined theoretical and experimental study of the ν1 and ν2 stretching vibrations of Ar⋯HN2+ is presented. These correlate asymptotically with the almost local NH and NN stretching vibrations of free HN2+, but undergo mode mixing and are strongly influenced by vibrational anharmonicity in the complex. The first observation of the strong ν2 band by means of diode laser absorption spectroscopy in a supersonic planar plasma is reported. Its band origin is found at 2041.1802(3) cm−1. New results are presented for the ν1 band with origin at 2505.5000(2) cm−1. The ground-state rotational constant is determined from both band systems as 0.080 868(6) cm−1.
Microchannel plate for high-efficiency field emission display
Authors: Yi Whikun , Jin Sunghwan , Jeong Taewon , Lee Jeonghee , Yu SeGi , Choi Yongsoo , Kim J. M.
Published: July 2002

The efficiency of a field emission display was improved significantly with a newly developed microchannel plate. The key features of this unit and its fabrication are summarized as follows: (a) bulk alumina is used as a substrate material, (b) channel location is defined by a programed-hole puncher, and (c) thin film deposition is conducted by electroless plating followed by a sol–gel process. With the microchannel plate between the cathode and the anode of a field emission display, the brightness of luminescent light increases three- to fourfold by electron multiplication through an array of pores in the device. In addition, the fabricated microchannel plate prevents spreading of electrons emitted from the cathode tips, thus improving both display resolution and picture quality.
Integrity of thin, laser-driven flyer plates
Authors: Watson S. , Field J. E.
Published: July 2002

Laser-driven flyers were launched from substrate backed aluminum films. They were produced by single pulses from a Q-switched Nd:yttrium–aluminum–garnet laser, of 10 ns duration and typically up to a few hundred mJ energy, with a focused spot size of approximately 1 mm diameter. The aluminum films were between 2 and 6 μm thick and the flyers typically achieved average velocities of a few km s−1. The integrity of the flyers was studied using streak photography and microscopy of the impacted flyers. Threshold times and distances prior to plasma penetration were used as a measure of integrity and were calculated for films of different thickness, launched with various laser pulse energies.
Representation of quantum mechanical resonances in the Lax–Phillips Hilbert space
Authors: Strauss Y. , Horwitz L. P. , Eisenberg E.
Published: July 2002

We discuss the quantum Lax–Phillips theory of scattering and unstable systems. In this framework, the decay of an unstable system is described by a semigroup. The spectrum of the generator of the semigroup corresponds to the singularities of the Lax–Phillips S-matrix. In the case of discrete (complex) spectrum of the generator of the semigroup associated with resonances, the decay law is exactly exponential. We explain how this profound difference between the quantum Lax–Phillips theory and the description of unstable systems in the framework of the standard quantum theory emerges. The states corresponding to these resonances (eigenfunctions of the generator of the semigroup) lie in the Lax–Phillips Hilbert space, and therefore all physical properties of the resonant states can be computed. In the special case of a time-independent potential problem lifted trivially to the quantum Lax–Phillips theory, we show that the Lax–Phillips S-matrix is unitarily related to the S-matrix of standard scattering theory by a unitary transformation parametrized by the spectral variable σ of the Lax–Phillips theory. Analytic continuation in σ has some of the properties of a method developed some time ago for application to dilation analytic potentials. We work out an illustrative example of the theory using a Lee–Friedrichs model, which is generalized to a rank one potential in the Lax–Phillips Hilbert space.
Photodissociation of polarized diatomic molecules in the axial recoil limit: Control of atomic polarization
Authors: Underwood Jonathan G. , Powis Ivan
Published: July 2002

A quantum mechanical treatment of the photofragment angular momentum polarization following photodissociation of diatomic molecules is presented. This treatment extends that of Siebbeles et al. [J. Chem. Phys. 100, 3610 (1994)] by considering photodissociation of a molecule whose angular momentum is polarized in the laboratory frame, and also treats properly the angular momentum coupling between the two photofragments. The formalism treats coherent excitation of dissociative surfaces and the consequences of nonadiabatic coupling between surfaces. The possibility of exploiting the parent molecule polarization in order to control the photofragment polarization when both parallel- and perpendicular-type dissociations are active is discussed. An example is given in which significant control over the molecular frame polarization of the fragments is achieved following photolysis of a parent molecule prepared by an electric dipole transition, which may be of use in the study of photoinitiated bimolecular reactions.
Micro-optomechanical sensor for optical connection in the near field
Authors: Belier B. , Santoso A. , Bonnafe J. , Nicu L. , Temple-Boyer P. , Bergaud C.
Published: July 2002

A micro-optomechanical sensor has been used to collect and carry optical information at the nanometer scale. This simple device consists of a SiNx microfabricated cantilever used simultaneously as an atomic-force microscope (AFM) probe and optical waveguide. When approaching the micro-optical cantilever close to the surface, the tip converts evanescent waves into radiative waves that propagate within the cantilever over long distances. Since the device is also employed as an AFM probe, mechanical contact is fully controlled as the tip approaches the surface, thus avoiding any crash. This sensor has been used to collect optical information from an optical fiber in the near field.
The resolution of distributed photorefractive multiple quantum well devices
Authors: Abeeluck A. K. , Garmire E. , Canoglu E.
Published: July 2002

Photorefractive multiple quantum well (PRMQW) devices can achieve simultaneously higher resolution and greater sensitivity by distributing several highly trapping low temperature growth (LTG) layers at different depths in their intrinsic region. This article extends a previously derived analytical model of PRMQW devices to distributed photorefractive devices (DPDs) that consist of a cascade of smaller MQW regions sandwiched between LTG layers. This enables an understanding of how multiple trapping layers affect the resolution and sensitivity in these DPDs. In addition to an improvement in resolution, the model predicts an enhancement in diffraction efficiency at small grating spacings as the number of subdevices increases for a fixed total DPD length. This result is of significance in designing compact image processing systems that can operate at small grating periods but still be able to achieve a large signal-to-noise ratio for image processing and sensing applications.
Characteristic temperature study of GaAsP–AlGaAs tensile strained quantum well lasers
Authors: Tolliver Todd R. , Anderson Neal G. , Agahi Farid , Lau Kei May
Published: July 2002

Temperature characteristics of tensile strained GaAs1−yPy–Al0.35Ga0.65As single quantum well heterostructure laser diodes are examined. The dependence of the characteristic temperature (T0) on the quantum well composition is systematically studied in broad-area stripe lasers with identical quantum well widths (115 Å) and various cavity lengths. Laser diodes with 10 quantum well compositions ranging from y=0 (lattice matched) to 0.30 (∼1.1% tensile strain) and 5 cavity lengths ranging from 300 to 1500 μm are examined. Characteristic temperatures are found to be maximized for small (but generally nonzero) quantum well phosphorus compositions, with a maximum value of T0=159 K obtained for y=0.025, but decrease rapidly with increasing composition. Our results are analyzed via a theoretical model for the characteristic temperature of the transparency current in separate confinement quantum well lasers and comparison of our observations with other measured laser characteristics. These analyses suggest that the observed local maximum in T0(y) at small y is a tensile strain effect, whereas the decreased T0(y) at large y results primarily from loss of carrier confinement and barrier recombination.
Novel mechanism of fast relaxation of photo-induced anisotropy in a poly(malonic esters) containing p-cyanoazobenzene
Authors: Joo Won-Jae , Shin Hee-Deuk , Oh Cha-Hwan , Song Seok-Ho , Kim Pill-Soo , Ko Bong-Soo , Han Yang-Kyoo
Published: July 2002

The relaxation behavior of photo-induced anisotropy has been investigated in a side-chain liquid crystalline(LC) poly(malonic ester) (PCN). Abnormal phenomenon, increment of the induced birefringence, in decaying process was observed just after the pumping beam polarized perpendicularly to the recording beam was illuminated in the relaxation process. This is difficult to explain using the present models, the thermal cis-trans isomerization or the free volume effect for the relaxation process. We suggest a new model considering the elastic force between the side-chain and the backbone in the polymeric film. According to our model, the backbone can be reoriented together with the side-chain, and fast relaxation is associated with the elastic force.
Enhanced photoinduced birefringence in polymer microcavities
Authors: Piron Rozenn , Toussaere Eric , Josse Dominique , Zyss Joseph
Published: July 2002

We investigate photoinduced birefringence in functionalized polymer microcavities illuminated by a visible polarized cw laser beam. The cavity studied in this experiment consists of an assembly of two Bragg mirrors embedding a polymer film with tethered azobenzene dyes. Linearly polarized light induces orientationally selective excitation of the chromophore guests, resulting in optical anisotropy of the film. The mechanism of the phenomenon is based on random-to-axial statistical reorientation of the azochromophores following repetitive trans-cis-trans photoisomerization cycles. The resulting birefringence is inferred from analysis of the spectral shifts of the cavity Fabry–Pérot longitudinal modes as a result of illumination. A large value of the birefringence of the order of 10−2 is noted after currently unoptimized durations of the order of 30′ in the 1.3–1.5 μm range with possible applications in optical storage at WDM telecom-compatible wavelengths.
Interaction between a waveguide-fed narrow slot and a nearby conducting strip in millimeter-wave scanning microscopy
Authors: Hochman Amit , Paneah Peter , Leviatan Yehuda
Published: July 2002

Millimeter-wave microscopy is investigated with the aid of a simplified model. The model consists of a parallel plate waveguide opening into a ground plane and radiating in the presence of a perfectly conducting strip. The method of moments is used together with a quasi-static approximation to solve for the reflection coefficient. Numerical results included indicate that high resolutions are feasible with this apparatus.
Damage production in semiconductor materials by a focused Ga+ ion beam
Authors: Menzel R. , Gärtner K. , Wesch W. , Hobert H.
Published: July 2002

The semiconductor materials Si, SiC, GaP, InP, GaAs, and InAs were irradiated at normal incidence and room temperature with a focused Ga+ ion beam in order to investigate the damage production at high current densities on the order of some A cm−2. The samples were irradiated with ion fluences between 2×1013 and 2×1015 Ga+ cm−2 at an ion energy of 50 keV. The critical ion fluences for amorphization were determined by Rutherford backscattering spectrometry and by Raman spectroscopy. It was found that for SiC, GaP, and InP the number of displacements per atom necessary for amorphization is about the same one as that required for irradiation at low current densities, but in the cases of Si, GaAs, and InAs the high and low current density results differ remarkably. The reason for the different behavior of these materials is discussed.
Elastic energy approach to the strain relaxation mechanism by dislocation splitting and slip in twist-bonded substrates
Authors: Obayashi Y. , Shintani K.
Published: July 2002

The strain relaxation mechanism by the splitting and slip of a cross grid of dislocations in a heteroepitaxial film on a thin twist-bonded substrate is analytically investigated by considering the energy change due to the slip of the dislocation arrays. In this mechanism, the dislocation arrays resulting from the splitting of a cross grid of screw dislocations in the interface between the twist-bonded substrate and the supporting bulk substrate move toward the interface between the heteroepitaxial film and the twist-bonded substrate so that the mismatch strain is relieved. The energy change consisting of the interaction energy between the dislocation arrays and their images and the interaction energy between the two split dislocation arrays is obtained in the semi-infinite isotropic elastic solid. If the initial screw dislocation arrays dissociate into two partial dislocation arrays, the stacking fault energy must be added in the energy change. The thickness of the twist-bonded substrate at the zero of the energy change becomes the critical thickness at which the strain relaxation mechanism can be operative. The results show that the strain relaxation due to the slip of 60° perfect dislocation arrays does not tend to occur while the strain relaxation due to the slip of 30° partial dislocation arrays is possible if the twist angle is low and if the thickness of the twist-bonded substrate is small. Whether this mechanism was possible in the experiments in the literature is discussed.
Dielectric, thermal, and mechanical properties of the semiorganic nonlinear optical crystal sodium p-nitrophenolate dihydrate
Authors: Brahadeeswaran S. , Bhat H. L. , Kini N. S. , Umarji A. M. , Balaya P. , Goyal P. S.
Published: July 2002

Physical properties such as dielectric, specific heat, thermal expansion, and mechanical hardness have been measured for sodium p-nitrophenolate dihydrate single crystals. The dielectric measurements were made both as a function of frequency (in the range 15 kHz–10 MHz) and temperature (in the range 30–80 °C). For the range of frequencies measured the values were found to vary between 4.03 and 3.69 for ε11, 8.11 and 7.05 for ε22, and 5.71 and 5.36 for ε33 at room temperature, and piezoelectric resonances were observed at certain frequencies. The dielectric measurements made as a function of temperature did not exhibit any anomaly in the temperature range measured. The specific heat studies carried out for this compound showed ≈30% increase in Cp with the temperature in the measured range. The thermal expansion measured as a function of temperature in the range 30–80 °C exhibited linear behavior and the average thermal expansion coefficients were found to be α1=7.48×10−5 K−1, α2=4.30×10−6 K−1, and α3=1.99×10−5 K−1. The polar plots drawn for these coefficients were correlated with crystallographic features of the structure. The Vickers hardness measurements, carried out along the three crystallographic axes, also exhibited anisotropy.
The unperturbed state of dendrimers
Authors: Ganazzoli Fabio , La Ferla Roberto
Published: July 2002

We report a theoretical study of the unperturbed state of dendrimers, which is realized when the second virial coefficient becomes equal to zero. This condition is achieved through a vanishing of the intermolecular free energy, which is obtained by mutual compensation of the two- and three-body interactions between two molecules. This procedure, which permits us to determine the Θ temperature of dendrimers as a function of their generation, is coupled to the problem of the intramolecular conformation, determined by minimization of the intramolecular free energy. The latter accounts for the two- and three-body interactions within the molecule, and for the configurational entropy. We find that the Θ temperature is a decreasing function of the dendrimer generation g, the decrease becoming relatively fast at large g, but is almost independent of the number of segments (one or two in our case) between adjacent branch points. At the Θ temperature, the residual three-body interactions within the molecule not compensated by the two-body attractions induce a significant swelling over the random-walk conformation for g>2.
Imaging spectroscopy of quantum wells with interfacial fluctuations: A theoretical description
Authors: Di Stefano Omar , Savasta Salvatore , Martino Giovanna , Girlanda Raffaello
Published: July 2002

We present a theoretical approach for the simulation of scanning local optical spectroscopy in disordered quantum wells (QWs). After a single realization of the disorder potential, we calculate spectra on a mesh of points on the QW plane, thus obtaining a three-dimensional matrix of data from which we construct two-dimensional spectroscopic images of excitons laterally localized at interface fluctuations. Our simulations are in close agreement with the experimental findings, and contribute to the interpretation of spatially resolved spectra in QWs.
Interactions of atomic hydrogen with Cu(111), Pt(111), and Pd(111)
Authors: Nobuhara K. , Nakanishi H. , Kasai H. , Okiji A.
Published: July 2002

We calculate the potential energy curves for H adsorption on and absorption in Cu(111), Pt(111), and Pd(111) within the density functional theory. We show stronger dependence of the potential energies on the lateral position of the H atom at the position further from the surface in the case of Pt(111) than one in the case of Pd(111), and no lateral position dependence in the case of Cu(111). Contrary to this, when the H atom comes close to and adsorbs on the surface, the dependence of its adsorption energy (a depth of potential well outside the surface) on the lateral position in the case of Cu(111) is stronger than one in the case of Pd(111), and such dependencies are hardly seen, and the adsorption energies are nearly the same in the case of Pt(111). When the H atom penetrates the first layer of the surface, the activation barrier at a so-called face-centered-cubic hollow site in the case of Pd(111) is the lowest in three surfaces.