bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature-dependent morphology of three-dimensional InAs islands grown on silicon
Authors: Sharma P. C. , Alt K. W. , Yeh D. Y. , Wang K. L.
Published: July 2002

The temperature dependence of morphology of InAs islands on Si grown through Stranski–Krastanow mode is investigated by atomic force microscopy. Formation of islands in the range of 15–50 nm is observed for depositions at various temperatures for the same monolayer coverage. Growth temperatures between 400 and 425 °C are found to yield dense ensembles of islands with uniform dimensional distributions. Found to exhibit long-term stability, these islands undergo morphological transformation when annealed at temperatures above 700 °C. Ostwald ripening occurs in these islands through an enhanced surface diffusion mechanism at high annealing temperatures. The results of annealing experiments indicate surface diffusion being the dominant mechanism responsible for morphological changes in these island structures rather than the heterointerface diffusion.
Annealing behavior of p-type Ga0.892In0.108NxAs1−x (0⩽X⩽0.024) grown by gas-source molecular beam epitaxy
Authors: Xin H. P. , Tu C. W. , Geva M.
Published: July 2002

P-type, Be-doped GaInNAs layers (1100 Å thick) are grown on GaAs substrates by gas-source molecular beam epitaxy with a nitrogen radical beam source. High-resolution x-ray rocking curves show that the Ga0.892In0.108NxAs1−x peak shifts closer to the GaAs substrate peak with increasing N concentration, indicating reduced strain. After rapid thermal annealing (RTA) at 700 °C for 10 s, the Ga0.892In0.108As sample suffers strain relaxation, but the N-containing samples remain pseudomorphically strained, suggesting better thermal stability of GaInNAs. The wavelength of room-temperature photoluminescence redshifts from 0.988 to 1.276 μm, due to large band gap bowing, with N concentration increased from 0 to 0.024. Secondary ion mass spectrometry results show no Be diffusion, but hydrogen incorporation alongside N. The free carrier concentration is decreased by one order of magnitude mainly due to H passivation, but after RTA at 700 °C, it is increased to half that of GaInAs due to the reduced H concentration. The product of carrier concentration and Hall mobility is increased from one-tenth to about half that of the GaInAs sample.
Laser-induced direct formation of C54 TiSi2 films with fine grains on c-Si substrates
Authors: Chen S. Y. , Shen Z. X. , Chen Z. D. , Chan L. H. , See A. K.
Published: July 2002

In this letter, we report on the direct synthesis of C54 TiSi2 films with fine grains by pulsed-laser irradiation from Ti deposited on Si substrates, using a Q-switched Nd:YAG laser. The films were characterized using micro-Raman spectroscopy, high-resolution transmission electron microscopy, and atomic force microscopy. In comparison with the C54 TiSi2 using the conventional rapid thermal annealing (RTA) of 35 nm thick Ti/Si, which has an average grain size of about 110 nm and film thickness of 50 nm, the laser-induced C54 TiSi2 films vary from 13 to about 42 nm in thickness with different laser scanning speed and the grain size is 85 nm on average. The TiSi2/substrate Si interface is smooth on the atomic scale. Our results demonstrate the unique advantages of the laser-induced formation technique and its potential in deep submicron semiconductor technology. We propose that the C54 phase is formed by solid-state diffusion, rather than melting.
Observation of inter-sub-level transitions in modulation-doped Ge quantum dots
Authors: Liu J. L. , Wu W. G. , Balandin A. , Jin G. , Luo Y. H. , Thomas S. G. , Lu Y. , Wang K. L.
Published: July 2002

The inter-sub-level transitions in modulation-doped Ge quantum dots are observed. The dot structure is grown by molecular-beam epitaxy, and consists of 30 periods of Ge quantum dots sandwiched by two 6 nm boron-doped Si layers. An absorption peak in the midinfrared range is observed at room temperature by Fourier transform infrared spectroscopy, which is attributed to the transitions between the first two heavy-hole states of the Ge quantum dots. This study suggests the possible use of modulation-doped Ge quantum dots for improved infrared detector applications.
Switching properties of Pb(Nb, Zr, Ti)O3 capacitors using SrRuO3 electrodes
Authors: Aggarwal S. , Jenkins I. G. , Nagaraj B. , Kerr C. J. , Canedy C. , Ramesh R. , Velasquez G. , Boyer L. , Evans J. T.
Published: July 2002

We report on ferroelectric properties of polycrystalline sol-gel derived Pb(Nb, Zr, Ti)O3 (PNZT) thin films with SrRuO3 (SRO) electrodes. The processing temperature of the bottom electrode was varied between 550 and 850 °C. The polarization of the ferroelectric capacitors was dependent on the processing temperature of the bottom electrode. The capacitors exhibit low switching fields (40 kV/cm), high resistivity (1011 Ω cm at 3 V) and high remanent polarization values (19 μC/cm2 at 3 V), desirable properties for high-density ferroelectric memories. The activation field for these capacitors was measured to be ∼350 kV/cm and the polarization values exhibited a shallow dependence on the pulse width from 1 s to 1 μs. Fatigue, logic state retention, and dynamic imprint tests indicate robust capacitors from a memory viewpoint.
Asymmetry in fatigue and recovery in ferroelectric Pb(Zr,Ti)O3 thin-film capacitors
Authors: Chae B. G. , Park C. H. , Yang Y. S. , Jang M. S.
Published: July 2002

We investigate the fatigue and refreshment by dc-electrical field of the electrical properties of Pt/Pb(Ti,Zr)O3/Pt ferroelectric capacitors. We find an asymmetry in the refreshment, that is, the fatigued state can be refreshed by application of negative high dc voltage to the top electrode, but no refreshment is measured by positive dc-voltage application. We also find that the fatigue can be prevented by driving the capacitor asymmetrically.
A two-component phosphor approach for engineering electroluminescence phosphors
Authors: Jones T. C. , Park W. , Summers C. J.
Published: July 2002

The electroluminescence (EL) properties of SrS:Cu, SrS:Ag, and SrS:Ag,Cu thin-film phosphors are reported. SrS:Cu and SrS:Ag are shown to exhibit very different EL properties with SrS:Cu possessing a luminance and efficiency of 30.8 cd/m2 and 0.2 lm/W, compared to SrS:Ag whose luminance and efficiency are 0.7 cd/m2 and 0.001 lm/W, respectively. In contrast, SrS:Ag,Cu exhibits the high luminance and efficiency of SrS:Cu, but the saturated chromaticity of SrS:Ag, which exhibits an excellent blue chromaticity of (0.17,0.15). The improved electroluminescent luminance, efficiency, and color purity of SrS:Ag,Cu are explained by the energy transfer from the Cu+ to Ag+ ions. These results suggest a concept to improve EL performance through the use of a class of two-component phosphors, which exhibit superior color, brightness, and excitation efficiency.
Carbon nanotube-modified cantilevers for improved spatial resolution in electrostatic force microscopy
Authors: Arnason S. B. , Rinzler A. G. , Hudspeth Q. , Hebard A. F.
Published: July 2002

The resolution of electrostatic force microscopy (EFM) is enhanced when multiwalled carbon nanotubes are used as extensions on conventional silicon cantilevers. Multiwalled nanotubes provide robust, high aspect ratio, conducting tips that minimize topographic dependence of gradients in the capacitance between the tip/cantilever and the substrate. Comparison of simultaneously acquired topographical and EFM images taken at the intersection of overlapping electrodes of electrically biased Al–Al2O3–Al tunnel junctions confirm the improved performance. This enhancement enables us to determine the surface contact potential differences between individual nanotubes within a bundle with resolutions of 5 mV and 10 nm.
Room-temperature photochemical hole burning in Eu3+-doped Al2O3–SiO2 glass
Authors: Nogami Masayuki , Hayakawa Tomokatsu , Ishikawa Tomotaka
Published: July 2002

Persistent spectral hole burning was observed at room temperature in Eu3+-doped Al2O3–SiO2 glass. The glass was obtained by heating in hydrogen gas to reduce some Eu3+ ions to Eu2+ and the spectral hole was burned in the excitation spectrum of the F70→I50 transition of Eu3+. The depth of the hole burned at 200 K was ∼7% of the total excitation intensity and independent of temperature up to 350 K. A possible mechanism for hole burning is the electron transfer between the Eu3+ ions and the Al-related defect centers.
Asymmetric luminescence line shape and exciton energy relaxation in Zn1−x−yMgxCdySe epilayers
Authors: Shen J. X. , Pittini R. , Oka Y. , Guo S. P. , Tamargo M. C.
Published: July 2002

The asymmetric line shape of the transient photoluminescence (PL) spectra of high-quality Zn1−x−yMgxCdySe epilayers is deconvoluted to a narrow and a broad Gaussian peak at each time delay. The fitted energy difference between the two peaks corresponds to one longitudinal optical (LO) phonon energy. We assign the narrow peak to the recombination of free excitons (FXs) and the broad peak to the recombination of excitons localized in the broad band tail characteristic for this quaternary compound. The broad PL peak is found to arise from the energy relaxation of the FX at the exciton mobility edge to the localized states by LO phonon emission. Therefore, the population peak of the localized excitons is formed at one LO phonon energy below the FX for all times after the laser excitation.
History-dependent orientational order of rubbed polyimide for liquid-crystal alignment
Authors: Mahajan Milind P. , Rosenblatt Charles
Published: July 2002

A polyimide film that was spin coated onto a glass substrate was multiply rubbed along different directions and studied using ellipsometry and atomic force microscopy. The data show a minimum required rubbing strength for the onset of orientational order in the polyimide. When over rubbed along an axis perpendicular to the first rubbing direction, a smaller rubbing strength was required for the onset of order along this direction. This behavior indicates that the polyimide had been partially disentangled by the initial rubbing, needing only weaker rubbing to be reoriented by the second rubbing.
Carbon-related defects in proton-irradiated,n-type epitaxial Si1−xGex
Authors: Leervad Pedersen T. P. , Larsen A. Nylandsted , Mesli A.
Published: July 2002

C i and CiCs defects, created by proton irradiation of n-type, strain-relaxed, epitaxial Si1−xGex of 0.005⩽x⩽0.5, have been studied using deep-level transient spectroscopy (DLTS). The ionization enthalpies of the two defects relative to the conduction band edge, ΔH, are found to increase linearly with increasing Ge content. It is shown that the corresponding levels are not pinned to any of the band edges. Furthermore, it is shown that, for both defects, the slopes, δΔH/δx, as well as the full width at half maximum (FWHM) of the corresponding DLTS peaks, are similar. These observations are in agreement with conclusions deduced from previous electron-paramagnetic resonance (EPR) measurements in pure silicon, stating that, for both defects, the trapped electron is preferentially located at the Ci atom because of its larger electronegativity as compared to those of silicon and germanium. The anneal temperature of the Ci defect, and correspondingly the in-growth temperature of the CiCs complex, increase with increasing Ge content. This is equivalent to an increasing retardation of the diffusion of Ci in Si1−xGex with increasing Ge content.
Importance of the indentation depth in tapping-mode atomic force microscopy study of compliant materials
Authors: Bar G. , Delineau L. , Brandsch R. , Bruch M. , Whangbo M.-H.
Published: July 2002

We studied the response of a cantilever tapping on polydimethylsiloxane (PDMS) samples of different crosslink density. It is shown experimentally that the tip deeply penetrates into the compliant PDMS samples. A more compliant material leads to a larger indentation such that at a given set-point ratio the indentation force is nearly constant on samples of different elastic moduli. This confirms the simulations by J. Tamayo and R. Garcia [Appl. Phys. Lett. 71, 2394 (1997)] that phase contrast acquired at constant set point does not depend on the sample’s modulus if other contrast relevant parameters remain identical. PDMS samples of different crosslink density are distinguished in terms of amplitude and phase versus distance measurements if the tip-sample interaction is made substantially large and indentation is taken into account.
Nanoheteroepitaxial growth of GaN on Si by organometallic vapor phase epitaxy
Authors: Zubia D. , Zaidi S. H. , Brueck S. R. J. , Hersee S. D.
Published: July 2002

Nanoheteroepitaxy has recently been proposed as a technique for significantly extending the thickness of pseudomorphic growth in mismatched heterostructures. This letter reports the experimental application of nanoheteroepitaxy for the growth of GaN on patterned 〈111〉 oriented silicon-on-insulator substrates by organometallic vapor phase epitaxy. Transmission electron microscopy reveals that the defect concentration decays rapidly away from the heterointerface as predicted by nanoheteroepitaxy theory. The melting point of the nanoscale islands is found to be significantly reduced, enhancing substrate compliance and further reducing the strain energy in the GaN epitaxial layer.
Electrical fracture toughness for conductive cracks driven by electric fields in piezoelectric materials
Authors: Fu Ran , Qian Cai-Fu , Zhang Tong-Yi
Published: July 2002

Fracture of piezoelectric materials can be caused by either mechanical loads or by electric fields. It is shown in lead zirconate titanate ceramics that conductive cracks driven by electric fields can be described by the same fracture mechanics concepts used for mechanical loads, but with a much higher fracture toughness. The electrical fracture toughness GICE=223.7±17.0 N/m (95% confidence) is 25 times higher than the mechanical fracture toughness GICM=8.7±0.4 N/m (95% confidence). This is due to the greater energy dissipation that is possible under electric loading but not possible under mechanical loading in the brittle ceramics of lead zirconate titanate.
Control of Coulomb blockade oscillations in silicon single electron transistors using silicon nanocrystal floating gates
Authors: Takahashi Nobuyoshi , Ishikuro Hiroki , Hiramoto Toshiro
Published: July 2002

We have fabricated single-electron transistors (SETs) with Si nanocrystal floating gates, and experimentally demonstrated the control of the peak positions of Coulomb blockade oscillations. The positive voltage applied to the gate makes channel electrons tunnel into the floating dots, and the injected electrons raise the potential of quantum dots in SET, resulting in a shift of peak positions of Coulomb blockade oscillations. In addition, from the temperature dependence of device characteristics, it is confirmed that the potential fluctuations caused by random distribution of the Si nanocrystals have a slight influence on the shape of the Ids-Vg curves at practical high temperatures.
Explicit and closed-form expressions for describing magnetic behaviors of uniaxial anisotropy materials
Authors: Hur Jeen , Paek Mun Cheol , Cho Kyoung-Ik , Shin Sung-Chul
Published: July 2002

We present explicit, analytic, and closed-form expressions for describing magnetic behaviors of materials having the magnetic anisotropy energy of K sin2 θ. The functional forms of the expressions depend on the ratio of an applied field to an anisotropy field and the ratio of the lowest critical field, for a domain wall to nucleate or move, to the anisotropy field. The present expressions for functions of experimentally measurable quantities gave an elegant method of fitting the quantities to the torque measurement. Furthermore, they may be useful to analyze the ferromagnetic resonance experiment and to measure the distribution of magnetic properties of magnetic aggregates.
The formation of 360° domain walls in magnetic tunnel junction elements
Authors: Portier X. , Petford-Long A. K.
Published: July 2002

The formation of 360° domain walls has been observed in exchange-biased magnetic tunnel junction elements using Lorentz transmission electron microscopy. These domain walls occur under certain circumstances and remain stable up to a high external field (∼150 Oe) compared to the value observed to achieve the antiparallel state of the free- and pinned-layer magnetizations (∼20 Oe). They have been found to play an important role during reversal to the parallel magnetization state, inducing a much lower switching field and a very different reversal mechanism.
A femtosecond, polarization-sensitive optically addressed modulator based on virtual exciton effects in an anisotropically strained multiple quantum well
Authors: Wraback M. , Shen H.
Published: July 2002

High contrast and femtosecond speed have been achieved in an optically addressed anisotropically strained multiple quantum well modulator by exploiting polarization-sensitive optical nonlinearities associated with the interaction of resonantly excited excitons with a population of nonresonantly created excitons that adiabatically follows the temporal intensity profile of the excitation pulse. The resultant modulation is characterized by a contrast ratio of 380:1, corresponding to a dynamic polarization rotation of 18°, and a pulse-width-limited full width at half maximum of 165 fs.
Effect of Sb composition on lateral oxidation rates in AlAs1−xSbx
Authors: Chavarkar P. , Mishra U. K. , Mathis S. K. , Speck J. S.
Published: July 2002

We demonstrate the effect of antimony (Sb) composition on the oxidation mechanism of AlAs1−xSbx (x<0.21) layers on GaAs substrates. It has been demonstrated that addition of a group-III element like Ga to AlAs slows the rate of oxidation. In contrast, addition of a group-V element like Sb to AlAs changes the oxidation mechanism in more than one way. The oxidation rate increases with Sb addition, and the oxidation reaction changes from a diffusion-limited mechanism to a reaction-rate-limited mechanism at higher oxidation temperatures. This is attributed to the increase in the permeability of the oxide. Nonuniform segregation of Sb is observed upon oxidation. The activation energy of the oxidation reaction-rate constant initially decreases with the Sb composition upto 10%, further Sb addition increases the activation energy.
Microwave surface resistance of HgBa2CaCu2O6+δ thin films
Authors: Aga Roberto S. , Yan Shao-lin , Xie Yiyuan , Han Siyuan , Wu Judy Z. , Jia Quan-Xi , Kwon Chuhee
Published: July 2002

Microwave surface resistance (Rs) has been measured on c-axis-oriented superconducting HgBa2CaCu2O6+δ (Hg-1212) films. A cavity perturbation method was employed using a high-Q Nb cavity cooled at 4.2 K. For the best film, an Rs as low as ∼0.3 mΩ was observed at 10 GHz up to ∼120 K on Hg-1212 films that have smooth surface morphology and high critical current density near 2 MA/cm2 at 100 K and self-field. This result suggests that Hg-1212 films are very promising for microwave applications above 100 K.
High-resolution scanning capacitance microscopy of silicon devices by surface beveling
Authors: Giannazzo Filippo , Priolo Francesco , Raineri Vito , Privitera Vittorio
Published: July 2002

High-resolution scanning capacitance measurements were carried out magnifying the sample dimensions by a double beveling method. A magnification of ten times has been reached, but in principle even higher magnifications can be obtained. For depth magnifications the reverse junction carrier spilling has to be considered. The measurements indicate that the amount of the spilling effect is in agreement with the models developed to date. The method was successfully applied directly to silicon devices and it demonstrates that accuracy well below tip dimensions can be easily reached. Junction depths as well as channel lengths can be determined with a high resolution.
Electronic transport mechanisms of nonalloyed Pt Ohmic contacts to p-GaN
Authors: Jang Ja-Soon , Seong Tae-Yeon
Published: July 2002

We report on the electronic transport mechanisms for nonalloyed Pt Ohmic contacts to p-GaN which were surface treated using a buffered oxide etch solution and (NH4)2Sx. Measurements show that the value of the effective Richardson constant (A**) is 12 A cm−2 K−2, which is considerably smaller than the theoretical value of 103.8 A cm−2 K−2. Based on Hall-effect results, the two-step surface-treated contact is modeled to consist of a Pt/p+-/p-GaN structure, and the conventionally treated contact consists of a Pt/p-GaN structure. The theoretical results obtained using these models are compared with the experimental data. It is shown that for the conventionally treated contact thermionic emission dominates the current flow, whereas for the two-step surface-treated contact, field emission is dominant.
Microscopic model of ferroelectricity in stress-free PbTiO3 ultrathin films
Authors: Ghosez Ph. , Rabe K. M.
Published: July 2002

The ground-state polarization of PbTiO3 thin films is studied using a microscopic effective Hamiltonian with parameters obtained from first-principles calculations. Under short-circuit electrical boundary conditions, (001) films with thickness as low as three unit cells are found to have a perpendicularly polarized ferroelectric ground state with significant enhancement of the polarization at the surface.
Electrically induced changes in polymer dispersed liquid crystals
Authors: Cupelli D. , Macchione M. , Nicoletta F. P. , De Filpo G. , Chidichimo G.
Published: July 2002

This letter reports the changes induced in polymer dispersed liquid crystal films by applying high intensity electric fields (charge process). It has been observed that lower reorientation fields and sharper OFF–ON transitions characterize samples after the charge process. Scanning electron microscopy and dielectric investigation show that the charge process induces both a morphology and a dielectric variation in films. Nevertheless a reduction in the surface anchoring energy of the liquid crystal at droplet interface has been postulated as the changes in the dielectric properties and droplet size cannot entirely justify the enhancement of electro-optical properties. The role played by the temperature at which the charge process is performed is also discussed.
Field emission study of diamond-like carbon films with scanned-probe field-emission force microscopy
Authors: Inoue Takahito , Ogletree D. Frank , Salmeron Miquel
Published: July 2002

Using a tip as an anode, a scanning force microscope (SFM) with an electrically conducting tip allows simultaneous measurement of both field-emitted currents and surface electronic properties with high lateral resolution. The principle of the method and its application to field emission from chemical vapor deposition diamond-like carbon films are presented. By simultaneously imaging the topography and field-emission current distribution with a 100 nm tip-surface separation, we correlated emission, topography, and dielectric properties. Subsequent contact SFM images of the same regions correlated topography and conductivity on the nanometer scale. The electrostatic force between tip and surface showed fluctuations on a millisecond time scale during field emission. This is probably due to charging and discharging of deep traps in the diamond-like carbon film.
The role of interface states in hydrogen-annealing-induced mobile proton generation at the Si–SiO2 interface
Authors: Vanheusden K. , Devine R. A. B.
Published: July 2002

It is demonstrated that a 450 °C anneal in hydrogen, known to passivate traps at the Si–SiO2 interface, can impede the generation of mobile protons during a subsequent hydrogen anneal at 600 °C. We further present a detailed reaction and diffusion model for the mobile proton buildup in the buried oxide of Si–SiO2–Si capacitors during a hydrogen anneal at 600 °C. In this model, unpassivated interface traps, located at and near the physical edges of the capacitor area, play a key role during the initial stages of the proton generation process. The mobile protons are generated near these edges and diffuse laterally along the Si–SiO2 interface.
Enhanced effect of polarization on electron transport properties in AlGaN/GaN double-heterostructure field-effect transistors
Authors: Maeda Narihiko , Saitoh Tadashi , Tsubaki Kotaro , Nishida Toshio , Kobayashi Naoki
Published: July 2002

In order to understand the effect of polarization on the electron transport properties in AlGaN/GaN heterostructures, the dependence of the two-dimensional electron gas mobility on the density has been examined both in the GaN single- and 200-Å GaN double-heterostructure field-effect transistors, by means of the Hall effect measurement under the gate-voltage application at 4.2 K. In the single heterostructure, a maximum mobility of 5700 cm2/V s has been obtained at 9.0×1012 cm−2; whereas in the double heterostructure, a higher maximum mobility of 8100 cm2/V s has been obtained at a lower electron density of 6.8×1012 cm−2. The results have been analyzed in terms of potential profiles and electron distributions in both structures where both piezoelectric and spontaneous polarization effects are taken into account. The analysis has shown that the observed mobility enhancement is mainly due to the enhanced polarization-induced electron confinement in the double heterostructure and, additionally, to the improvement of the interface roughness in the structure.
Atomic-level simulation of ferroelectricity in perovskite solid solutions
Authors: Sepliarsky M. , Phillpot S. R. , Wolf D. , Stachiotti M. G. , Migoni R. L.
Published: July 2002

Building on the insights gained from electronic-structure calculations and from experience obtained with an earlier atomic-level method, we developed an atomic-level simulation approach based on the traditional Buckingham potential with shell model which correctly reproduces the ferroelectric phase behavior and dielectric and piezoelectric properties of KNbO3. This approach now enables the simulation of solid solutions and defected systems; we illustrate this capability by elucidating the ferroelectric properties of a KTa0.5Nb0.5O3 random solid solution.
Polarized luminescence in CdS/ZnSe quantum-well structures
Authors: Schmidt M. , Grün M. , Petillon S. , Kurtz E. , Klingshirn C.
Published: July 2002

The photoluminescence from type II CdS/ZnSe quantum-well structures is found to be polarized with respect to the 〈110〉 directions with polarization degrees up to 20%. The absolute polarization direction is related to the interface bond directions in samples with differently prepared interfaces. The observations are explained by the detailed analysis of the epitaxial growth process and polarization sensitive luminescence experiments.
Scheme for the fabrication of ultrashort channel metal-oxide-semiconductor field-effect transistors
Authors: Appenzeller J. , Martel R. , Solomon P. , Chan K. , Avouris Ph. , Knoch J. , Benedict J. , Tanner M. , Thomas S. , Wang K. L. , del Alamo J. A.
Published: July 2002

We present a scheme for the fabrication of ultrashort channel length metal-oxide-semiconductor field-effect transistors (MOSFETs) involving nanolithography and molecular-beam epitaxy. The active channel is undoped and is defined by a combination of nanometer-scale patterning and anisotropic etching of an n++ layer grown on a silicon on insulator wafer. The method is self-limiting and can produce MOSFET devices with channel lengths of less than 10 nm. Measurements on the first batch of n-MOSFET devices fabricated with this approach show very good output characteristics and good control of short-channel effects.
Near-room-temperature tunneling magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.85Sr0.15MnO3/La0.67Sr0.33MnO3 device
Authors: Yin H. Q. , Zhou J.-S. , Goodenough J. B.
Published: July 2002

A trilayer La0.67Sr0.33MnO3/La0.85Sr0.15MnO3/La0.67Sr0.33MnO3 tunneling magnetoresistance (TMR) device has been epitaxially produced on a SrTiO3 substrate. The current–voltage curves show that tunneling process dominates the carrier transfer across the junction near room temperature. However, the tunneling effect becomes weak with decreasing temperature. Magnetoresistance of about 4%–6% has been observed below room temperature. The temperature dependence of TMR appears correlated to both the tunneling effect cross the junction and the spin polarization in La0.67Sr0.33MnO3 layers. The temperature dependence of the TMR effect in this device is different from that of other devices based on colossal magnetoresistance materials.
Total energy density as an interpretative tool
Authors: Cohen Morrel H. , Frydel Derek , Burke Kieron , Engel Eberhard
Published: July 2002

We present a formulation for the total-energy density within density-functional theory which is physically transparent and computationally feasible. We propose that it be used as a tool for the interpretation of computed energy and electronic structure changes during structural transformations and chemical reactions, augmenting the present use of changes in the electron density, in the Kohn–Sham local density of states, and in the Kohn–Sham energy density.
Response to “Comment on ‘A potential dependent polarizability?’ ” [J. Chem. Phys. 113, 3477 (2000)]
Authors: Shanker J. , Kushwah S. S.
Published: July 2002

In our reply we have emphasized that the polarizabilities of ions depend on the environment in which the ions exist. The polarizabilities are intimately related to the potential energies in molecules. A ‘single polarizable particle model’ considered by Donald et al. in their comment does not appear to be adequate for molecules and crystals.
Measurement of preheat due to fast electrons in laser implosions
Authors: Yaakobi B. , Stoeckl C. , Boehly T. , Meyerhofer D. D. , Seka W.
Published: July 2002

Fast electrons due to the two-plasmon decay instability have been shown to be generated in laser interaction experiments. The purpose of the present experiment is to determine the preheat level due to these electrons, using the Kα emission from a two-layer, Ti–V, planar target. The target is designed to separate the contributions of fast electrons and radiation to the observed Kα intensity. About 0.3% of the incident laser energy is found to be deposited as preheat due to fast electrons. It is shown that the preheat level can alternatively be determined directly from the hard x-ray continuum without the need to know the electron dynamics; the only required parameters are the total energy in hard x rays and the temperature of the fast electrons.
Study of the anisotropy of a passive scalar field at the level of dissipation
Authors: Gonzalez Michel
Published: July 2002

The anisotropy of a passive scalar field at the level of second-order moments of the scalar derivatives is studied starting from the exact equations for the components of the second-rank tensor E defined by Eij=2D(∂θ′/∂xi)(∂θ′/∂xj)¯ (D is the molecular diffusivity of scalar θ). Analysis requires also the equations for the components of a mixed tensor defined by the correlations between scalar and velocity gradients. After the examination of this set of equations, it is conjectured that, in the case of forcing by a mean scalar gradient in isotropic turbulence, the anisotropy of tensor E is produced by cliffs of temperature occurring in the direction of the mean gradient. A model for this mechanism is proposed including possible indirect influence of shear through the large-scale anisotropy of the scalar field. Predictions in the situation where a mean scalar gradient is combined with homogeneous shear agree tolerably well with experimental data. This suggests that the proposed picture describing production of small-scale anisotropy implied by mean gradient forcing has to be completed.
Experimental determination of the mechanical impedance of atomic force microscopy cantilevers in fluids up to 70 kHz
Authors: Scherer Marc P. , Frank Gerhard , Gummer Anthony W.
Published: July 2002

The high-frequency dynamical response of cantilevers used in atomic force microscopy (AFM) to an external force is important for applications such as dynamic AFM modes and dynamic mechanical measurements on biological tissues. In this paper we present a novel method for experimentally determining the mechanical driving-point impedance of an AFM cantilever up to frequencies of at least 70 kHz. A frequency-independent, point force was achieved by applying an electric field between the cantilever and a sharpened steel electrode positioned near (10 μm) the tip of the cantilever. The velocity of the cantilever in response to the force was measured with a laser interferometer. The method was tested in air and in tetrachloromethane (CCl4), a nonpolar, nondissociating fluid. The results agree with theoretical predictions. In addition, the mechanical impedance of a cantilever near a flat surface was measured.
Photodissociation of methyl nitrite on Ag(111): Nitric oxide ejection dynamics
Authors: Fieberg Jeffrey E. , White J. M.
Published: July 2002

The thermal and photochemical properties of CH3ONO adsorbed on Ag(111), with and without a thick spacer layer of hexane, C6H14, are described. Angle-resolved time-of-flight measurements of NO ejected during photolysis at 248 and 351 nm exhibit trimodal velocity distributions. Two of the components are wavelength dependent and nonthermal, and are modeled as NO ejection with and without partially thermalizing collisions with surrounding species. The third component of the distribution is wavelength independent and characterized by the bulk temperature of the adsorbate–substrate system. These three components are present for all coverages of CH3ONO, submonolayer to multilayer, and persist even when there is a spacer of C6H14. The photochemistry is dominated by direct excitation of CH3ONO; there is no evidence for NO ejection as the result of substrate excitation. Comparing NO time-of-flight distributions for 1 monolayer (ML) of CH3ONO on clean Ag(111) to those for 1 ML on a thick layer of C6H14, evidences substrate involvement in the dynamics of nascent NO.
Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. III. Isotropic and anisotropic Raman spectra of HCl in liquid SF6
Authors: Padilla A. , Pérez J. , Hernández A. Calvo
Published: July 2002

The isotropic and anisotropic Raman spectra of HCl diluted in liquid SF6 have been analyzed and compared with available experimental data. The theoretical profiles have been calculated with a non-Markovian spectral theory in which the statistical parameters of the isotropic and the anisotropic interaction were obtained from a microscopic extended cell model for the liquid. Memory and interference effects are qualitatively and quantitatively analyzed in terms of the time scales involved in the Raman spectra. A detailed study of intrabranch, interbranch, and cancellation mixing effects is carried out together with the analysis of the contribution of the different relaxation channels.
Simultaneous optimization of exponents, centers of Gaussian-type basis functions, and geometry with full-configuration interaction wave function: Application to the ground and excited states of hydrogen molecule
Authors: Tachikawa Masanori , Osamura Yoshihiro
Published: July 2002

We have extended the fully variational molecular orbital (FVMO) method to the full-configuration interaction (CI) wave function (full-CI FVMO). All variational parameters in the full-CI scheme, i.e., exponents and centers in Gaussian-type function (GTF) basis set, and nuclear positions, as well as the CI coefficients, are simultaneously optimized by using their analytical gradients. We have applied the full-CI FVMO method to the ground and electronic excited states of hydrogen molecule. In the ground state, the total energy (−1.174 015 hartree) and the internuclear distance (1.4016 bohr) obtained by the full-CI FVMO calculation with [8s4p2d] GTFs agree very well with the high-level calculation by the 249 term expansion in elliptic coordinates (−1.174 476 hartree and 1.4010 bohr, respectively). The excitation energies to the Σu+1, Πu1, Σg+3, and Πu3 Rydberg states calculated by the full-CI FVMO method with [8s4p2d] GTFs coincide with the experimental values within 52 cm−1. The present result can not be obtained with the conventional basis set approach because of the fact that our full-CI FVMO calculation gives an extremely accurate wave function with a relatively small number of basis functions owing to the extension of flexibility in the variational space.
Individual Shapiro steps observed in resistively shunted intrinsic Josephson junctions on Bi2Sr2CaCu2O8+x single crystals
Authors: Wang H. B. , Aruga Y. , Chen J. , Nakajima K. , Yamashita T. , Wu P. H.
Published: July 2002

With a thin gold layer, we fabricated mesa-like shunted intrinsic Josephson junction stacks on Bi2Sr2CaCu2O8+x single crystals. Instead of the multibranch structure and large voltage jumps often observed in conventional intrinsic junctions, current–voltage characteristics typical of resistively shunted junctions were obtained. Individual Shapiro steps were clearly visible with irradiation at frequencies from a few to 20 GHz. The experiments demonstrated not only the ac Josephson effects directly but also the possibility of employing intrinsic Josephson junctions in high-frequency applications.
Microscopic structure of Cl− coordination shell in NiCl2 methanol solution: A neutron diffraction study
Authors: Adya Ashok K. , Kalugin Oleg N.
Published: July 2002

Neutron diffraction measurements were carried out on ∼1.4 molal solutions of NiCl2 in methanol under ambient conditions. First- and second-order difference methods with isotopic substitution on nickel, chlorine and hydroxyl hydrogen (Ho) were applied to derive the Cl−-dependent partial structure factors and radial distribution functions, RDFs. The nearest-neighbor Cl–(H/D)o distance and the solvation number of Cl− were determined to be 2.18(1) Å and ≅2, respectively, as compared to 2.21(3) Å and 3.6 in a 5.8 M solution of LiCl in methanol. The joint analysis of the pair RDFs, gNiCl(r) and gClCl(r), reveals that the electrolyte in its 1.4 molal methanolic solution mainly exists in the form of solvated contact ion pairs, [NiCl]s+, with very small content of solvated [NiCl2]so ion-triples.
Observation of resonant tunneling through a quantized state in InP quantum dots in a double-barrier heterostructure
Authors: Reddy C. V. , Narayanamurti V. , Ryou J. H. , Chowdhury U. , Dupuis R. D.
Published: July 2002

A direct spectroscopic signature associated with the quantized state of the charge carriers in three-dimensionally confined InP quantum dots (QDs) is reported using a ballistic electron emission microscopy (BEEM)/spectroscopy technique. The self-assembled InP QDs are sandwiched in an AlInP double-barrier heterostructure. The excellent nanometer-scale lateral resolution of the BEEM technique is used to investigate the current transport mechanism by the direct injection of electrons into a single quantum dot. The BEEM spectra taken on and off the dot revealed the presence of a localized state at around 0.1±0.02 eV above the ground state.
2.12 μm InGaAs–InGaAlAs–InP diode lasers grown in solid-source molecular-beam epitaxy
Authors: Kuang G. K. , Böhm G. , Grau M. , Rösel G. , Meyer R. , Amann M.-C.
Published: July 2002

We have fabricated InGaAs–InGaAlAs–InP strained quantum well lasers with wavelength as long as 2.12 μm in solid-source molecular-beam epitaxy. A continuous-wave threshold current density of 780 A/cm2 at room temperature and a characteristic temperature of 48 K have been achieved.
Thermodynamics of anisotropic surfactant micelles. I. The influence of curvature free energy on the micellar size and shape
Authors: Bergström Magnus
Published: July 2002

A novel theory for the structural behavior of surfactant micelles is expounded. The micelles are considered to be generally shaped as triaxial tablets with distinct thickness width and length, respectively, and may become spherical, spherocylindrical or disk-shaped in the special cases where two or more of the dimensions are equal. It is demonstrated that the average width and length of a tablet-shaped micelle with a fixed thickness is mainly determined by two constants, k1 and k2, related to the first and second order correction in curvature of the micellar end caps. The size of the micelles is found to be mainly determined by k1, whereas k2 influences the shape, i.e., the length-to-width ratio, of the micelles so that the micellar size increases with increasing k1 and the length-to-width ratio decreases with increasing k2. Hence, large positive values of k2 promote the formation of tablets rather than very long spherocylinders. An additional parameter related to the curvature of the straight cylindrical rims may influence the structure of the tablet-shaped micelles insofar k2 is close to or below zero.
Rotational spectroscopy of H3P⋯BrCl and the systematics of intermolecular electron transfer in the series B⋯BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, and PH3
Authors: Legon A. C. , Thumwood J. M. A. , Waclawik E. R.
Published: July 2002

Ground-state rotational spectra of the isotopomers H3P⋯79Br35Cl, H3P⋯81Br35Cl, H3P⋯79Br37Cl, H3P⋯81Br37Cl, D3P⋯79Br35Cl, and D3P⋯81Br35Cl, of the phosphine–bromine monochloride complex were observed by the pulsed-jet, Fourier-transform method, incorporating a mixing nozzle to preclude reaction among the component gases. Each isotopomer exhibited a symmetric-top-type spectrum which yielded accurate values of the spectroscopic constants B0, DJ, DJK, χaa (Br), χaa (Cl), Maa (Br), and Mbb (Br) on analysis. Interpretations of the changes in the B0 values with isotopomer showed that the intermolecular bond involves P and Br, with r(P⋯Br)=2.869(1) Å and that the BrCl bond increases in length by ∼0.04 Å on complex formation. Changes in the halogen nuclear quadrupole coupling constants when H3P⋯BrCl is formed lead, with the aid of the Townes–Dailey model, to the conclusion that a fraction δi=0.100(5) of an electron is transferred from P to Br on complex formation, while the polarization of BrCl by PH3 can be viewed as the transfer of 0.128(2)e from Br to Cl, leading to a net change of −0.028(5)e in the population of the 4pz orbital of Br. The complex is only of moderate strength, with an intermolecular stretching force constant kσ=11.5 Nm−1. Values of δi, similarly determined, for the series B⋯BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, or PH3, are presented. It is shown that the variation of δi with the ionization energy IB of the Lewis base B can be described by an expression δi=Aexp(−bIB). This behavior is compared with that for the corresponding series B⋯ICl.
Structural and optical properties of high-quality ZnTe homoepitaxial layers
Authors: Chang J. H. , Cho M. W. , Wang H. M. , Wenisch H. , Hanada T. , Yao T. , Sato K. , Oda O.
Published: July 2002

The structural and optical properties of high-quality homoepitaxial ZnTe films are investigated. A substrate surface treatment using diluted HF solution plays a key role in growing device-quality ZnTe layers. X-ray diffraction analysis of ZnTe epilayers based on the crystal-truncation-rod method suggests that a homoepitaxial ZnTe film grown on a HF-treated substrate can be regarded as an ideal truncated crystal without an interfacial layer, while a ZnTe layer grown on a substrate without HF treatment suggests the presence of an interfacial layer which may lead to degraded crystallinity of ZnTe overlayers. The crystal quality of the homoepitaxial ZnTe layers with HF treatments are characterized by an extremely narrow x-ray diffraction linewidth of 15.6 arcsec and dominant very sharp excitonic emission lines with dramatically reduced deep-level emission intensity in the photoluminescence (PL) spectrum. Three bound excitonic emission lines at neutral acceptors are observed in the PL from the high-quality ZnTe homoepitaxial layers in addition to the free-exciton emission line, suggesting the presence of three different kinds of residual acceptor impurities.
Quantum-classical calculation of cross sections and rate constants for the H2+CN→HCN+H reaction
Authors: Coletti Cecilia , Billing Gert D.
Published: July 2002

Initially state-selected cross sections and thermal rate constants for the reaction CN+H2→HCN+H have been calculated using a mixed quantum-classical method: the vibrations of the two reagents are treated quantum mechanically by means of an exact solution of the time-dependent Schrödinger equation, while translational and rotational motions are treated classically. Coupling between quantum and classical degrees of freedom is realized within an effective Hamiltonian approach.
Carrier transfer and photoluminescence quenching in InAs/GaAs multilayer quantum dots
Authors: Sanguinetti S. , Padovani M. , Gurioli M. , Grilli E. , Guzzi M. , Vinattieri A. , Colocci M. , Frigeri P. , Franchi S.
Published: July 2002

A detailed study of the carrier transfer and photoluminescence (PL) quenching in stacked InAs/GaAs quantum dots (QDs) is presented. Vertically aligned QD structures, grown by atomic layer molecular beam epitaxy, with different numbers N of dot planes and different spacer thicknesses (d) were prepared and studied. The dependencies of carrier transfer from the GaAs barriers to the InAs QDs and of the PL quenching channels on the design parameters N and d have been identified by performing continuous wave and time-resolved PL measurements. We have found that both the radiative recombination and capture efficiency into the QDs are reduced by increasing N and by decreasing d, as a consequence of the deterioration of the interdot GaAs spacers induced by stacking.
Metamorphic InP/InGaAs double-heterojunction bipolar transistors on GaAs grown by molecular-beam epitaxy
Authors: Zheng H. Q. , Radhakrishnan K. , Wang H. , Yuan K. H. , Yoon S. F. , Ng G. I.
Published: July 2002

InP/InGaAs double-heterojunction bipolar transistor (HBT) structures were grown metamorphically on GaAs substrates by solid-source molecular-beam epitaxy. A linearly graded InxGa1−xP (x varying from 0.48 to 1) buffer layer was used to accommodate the strain relaxation. The crystallinity of the buffer layer and the HBT structure was examined by x-ray diffractometry. Devices with 5×5 μm2 emitter area showed a typical peak current gain of 40, a common-emitter breakdown voltage (BVCEO) higher than 9 V, a current gain cut-off frequency (fT) of 46 GHz, and a maximum oscillation frequency (fmax) of 40 GHz.
Positron and positronium chemistry by quantum Monte Carlo. V. The ground state potential energy curve of e+LiH
Authors: Mella Massimo , Morosi Gabriele , Bressanini Dario , Elli Stefano
Published: July 2002

The potential energy curve of e+LiH has been computed by means of diffusion Monte Carlo using explicitly correlated trial wave functions. This curve allows us to compute the adiabatic total and binding energies and the vibrational spectrum of e+LiH, and the adiabatic positron affinity of LiH. Using these results, we discuss the possibility to detect spectroscopically e+LiH in the gas phase, in order to have the first direct observation of a positron-containing system.
Monte Carlo approach to phase transitions in ferroelectromagnets
Authors: Gao X. S. , Liu J.-M. , Chen X. Y. , Liu Z. G.
Published: July 2002

A Monte Carlo algorithm based on the hybrid Ising-DIFFOUR model is proposed to investigate the phase transition in ferroelectromagnetic lattice in which the ferroelectric order and antiferromagnetic order coexist below a certain temperature. The Ising spin moment and ferroelectric displacement and their susceptibilities as well, as a function of temperature for systems of different magnetoelectric couplings, are simulated and compared with the mean-field approach. The typical antiferromagnetic transition at Néel point and the ferroelectric transition at Curie point are observed at zero coupling. It is demonstrated that a weak ferromagnetic order can be activated by introducing the magnetoelectric coupling or applying external magnetic field, while the external electric field has little effect on the ferromagnetic ordering behavior.