bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of feature broadening in direct-write optical lithography
Authors: Bradley R. Mark , Eschmann Andrea , Au Lee Siu
Published: July 2002

In direct-write optical lithography (DWOL), an intense, nearly-resonant standing light wave of wavelength λ is used to focus a collimated atomic beam. In this way, a grating structure is deposited on the substrate. We argue that during DWOL adatoms move primarily as a result of collisions with incident atoms. The resulting equation of motion for the film surface describes the way in which the features smooth and broaden as the film thickness increases. We find that as the film thickness grows large, the topography tends to a steady state form with amplitude proportional to λ4. No matter how well focused the incident atomic flux is, in the steady state the surface corrugations have a full-width-at-half-maximum that is greater than 45% of the peak-to-peak distance.
Potential energy curves and dipole transition moments to the ground state of the system Ar*(3p54s, 3P, 1P)+Ne
Authors: Petsalakis Ioannis D. , Buenker Robert J. , Lieberman Heinz-Peter , Alekseyev Aleksey B. , Devdariani Alexander Z. , Theodorakopoulos Giannoula
Published: July 2002

Relativistic core-potential calculations have been carried out on Ω states resulting from the interaction of Ar*(3p54s, 3P, 1P) with ground state Ne atoms. The results yield the correct asymptotic limits for the atomic states of Ar while shallow minima (700–800 cm−1) at large internuclear distances, 7–8 bohr, are obtained for the excited states. Dipole transition moments between pairs of states have been calculated and strong radiative transitions are predicted from excited states to the ground state. The 1(I) state, correlating with the metastable P23 state of Ar is found to have a small dipole transition moment at short and intermediate nuclear distances leading to a radiative lifetime for this state of 8.3 μs.
Flexible transition state theory for a variable reaction coordinate: Derivation of canonical and microcanonical forms
Authors: Robertson Struan , Wagner Albert F. , Wardlaw David M.
Published: July 2002

A completely general canonical and microcanonical (energy-resolved) flexible transition state theory (FTST) expression for the rate constant is derived for an arbitrary choice of reaction coordinate. The derivation is thorough and rigorous within the framework of FTST and replaces our previous treatments [Robertson et al., J. Chem. Phys. 103, 2917 (1995); Robertson et al., Faraday Discuss. Chem. Soc. 102, 65 (1995)] which implicitly involved some significant assumptions. The canonical rate expressions obtained here agree with our earlier results. The corresponding microcanonical results are new. The rate expressions apply to any definition of the separation distance between fragments in a barrierless recombination (or dissociation) that is held fixed during hindered rotations at the transition state, and to any combination of fragment structure (atom, linear top, nonlinear top). The minimization of the rate constant with respect to this definition can be regarded as optimizing the reaction coordinate within a canonical or microcanonical framework. The expression is analytic except for a configuration integral whose evaluation generally requires numerical integration over internal angles (from one to five depending on the fragment structures). The form of the integrand in this integral has important conceptual and computational implications. The primary component of the integrand is the determinant of the inverse G-matrix associated with the external rotations and the relative internal motion of the fragments.
Angular distribution of ionic fragments in the dissociation of SO22+
Authors: Masuoka Toshio , Kobayashi Ataru
Published: July 2002

The photoion–photoion coincidence technique has been used to measure the angular distributions of the O++S+ and O++SO+ fragments in the dissociative double photoionization of SO2 in the 37–120 eV range with linearly polarized synchrotron radiation. The present work shows that the obtained values of the asymmetry parameter β are close to zero (−0.2⩽β⩽0.2) over the whole energy range of photoexcitation, indicating that the fragmentation is almost isotropic in both cases of O++S+ and O++SO+ channels. It is found that the symmetry analysis which has been partially successful for explaining the behavior of β values observed for linear molecules or for molecules with cylindrical symmetry is unsatisfactory to account for the observed energy dependence of β values in the dissociation processes of SO22+. For more quantitative understanding of β values, the dynamical effects such as asymmetric bending or stretching motion of the SO2 molecule prior to the ionic fragmentation, rather than the static effects such as electron configuration of parent ions, may provide a satisfactory model for the observed β values.
Theoretical evidence for a bound doubly-excited B21(C 1s,n→π*2) state in H2CO below the C 1s ionization threshold
Authors: Trofimov A. B. , Gromov E. V. , Moskovskaya T. E. , Schirmer J.
Published: July 2002

The group of three lowest singlet C 1s-excited states of formaldehyde H2CO is studied theoretically. The equilibrium geometries are determined at the restricted open-shell Hartree–Fock (ROHF) level and refined total energies are obtained using the multireference configuration interaction (MRCI) approach. In agreement with an earlier prediction [Chem. Phys. 122, 9 (1988)] the second lowest singlet state, B21, is characterized by a doubly excited, “two particle–two hole” (2p–2h), configuration C 1s,n→π*2. Our calculations predict that H2CO in the B21(2p–2h) state has a stable pyramidal equilibrium structure with a barrier to inversion of 0.28 eV, the valence angle being close to 107°. The calculated length of the CO bond is 1.390 Å. The B21(2p–2h) state is shown to be also bound with respect to all possible dissociation and rearrangement processes. The lowest predicted dissociation energy for the B21 state (H2CO*→H2+CO* reaction) is 0.29 eV (6.69 kcal/mol). The rationalization of the great stability of the B21(2p–2h) state is the similarity of its electronic structure to that of the first singly-excited state A″2(n) of nitroxyl radical H2NO⋅. The neighboring states B11(C 1s→π*) and A11(C 1s→3s) are characterized within the same framework. Spectroscopic implications and possibilities for the experimental identification of the B12(2p–2h) state are discussed.
Detection of terahertz radiation from longitudinal optical phonon–plasmon coupling modes in InSb film using an ultrabroadband photoconductive antenna
Authors: Gu Ping , Tani Masahiko , Sakai Kiyomi , Yang T.-R.
Published: July 2002

Terahertz radiation from longitudinal optical (LO) phonon–plasmon coupling modes in InSb films is observed using an ultrafast photoconductive antenna detector. We demonstrate a response frequency of up to 7 THz for a low-temperature-grown GaAs-based photoconductive antenna gated with 25 fs laser pulses. It is found that the emission frequencies of the coupling modes are dependent only on the residual carrier density, not on the excitation carrier density. It is also found that the LO phonon–plasmon oscillations in semiconductors can serve as an efficient THz source.
Shape transition of calcium islands formed by electron-stimulated desorption of fluorine from a CaF2(111) surface
Authors: Batzill M. , Snowdon K. J.
Published: July 2002

Extremely low-energy electrons emitted from a heated filament have been used to deplete fluorine from a cleaved CaF2(111) surface. Calcium left behind on the surface reorganizes itself in islands. A shape transition for the islands from a compact shape to an elongated shape has been observed for islands larger than ∼20 nm in diameter. Some cleavage steps show preferential fluorine erosion, which leads to the formation of very long calcium nanowires of uniform width ∼15 nm parallel to the step edge. The observed island and wire morphology is explained by energy minimization of strained commensurate islands.
Evaluating free energy, enthalpy, and entropy of protonated water clusters by a grand canonical Monte Carlo simulation
Authors: Kusaka Isamu , Oxtoby David W.
Published: July 2002

We report the results of a Monte Carlo simulation of ion clusters using the polarizable model potentials for hydronium ion and a water molecule proposed by Kozack and Jordan [J. Chem. Phys. 96, 3120 (1992); 96, 3131 (1992)]. The grand canonical Monte Carlo simulation aided by the umbrella sampling technique allows us to evaluate very efficiently the equilibrium distribution of ion clusters of various sizes. Thermochemical data of interest, such as the standard free energy, enthalpy, and entropy of protonated water clusters follow immediately from this distribution.
Photostability enhancement of an azobenzene photonic polymer
Authors: Galvan-Gonzalez A. , Belfield K. D. , Stegeman G. I. , Canva M. , Chan K.-P. , Park K. , Sukhomlinova L. , Twieg R. J.
Published: July 2002

We have found that the presence of a methacrylate group on an azobenzene electro-optic chromophore can provide an enhancement in photostability of up to one or two orders of magnitude. Systematic studies involving the change in chromophore structure and atmospheric environment indicate that the photostabilization is due to antioxidant action by the unsaturated functional group.
Dynamics of optical gain in InxGa1−xN multi-quantum-well-based laser diodes
Authors: Kawakami Yoichi , Narukawa Yukio , Omae Kunimichi , Fujita Shigeo , Nakamura Shuji
Published: July 2002

Dynamical behavior of optical gain formation has been assessed at room temperature in the InxGa1−xN multi-quantum-well (MQW) based laser diodes (LDs) by employing pump and probe spectroscopy with a pulse width of 150 fs. The LDs are composed of (a) In0.1Ga0.9N–In0.02Ga0.98N MQW and (b) In0.3Ga0.7N–In0.05Ga0.95N MQW, whose stimulated emissions correspond to near ultraviolet (390 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to the LQL by 500 meV for the sample (b). It was found that the photogenerated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the time scale of about 5 ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photoexcitation.
Comment on “Colliding null shells of matter” [J. Math. Phys. 33, 1065 (1992)]
Authors: Halilsoy M.
Published: July 2002

The space–time created by the colliding null shells is combined with the metric of colliding waves. By this method evolution and collision of waves with exact backreaction can be obtained in the de Sitter and Kasner space–times.
Response to “Comment on ‘Quantum-confinement effects on the optical and dielectric properties for mesocrystals of BaTiO3 and SrBi2Ta2O9’ ” [J. Appl. Phys. 88, 6092 (2000)]
Authors: Kohiki Shigemi , Takada Syozo , Shimizu Akihiko , Yamada Kenichi , Higashijima Hirotaka , Mitome Masanori
Published: July 2002

In this reply, the authors show that the argument by Scott regarding the band gap of bulk SrBi2Ta2O9 (SBT) is not based on concrete evidence. The authors will also show additional data from a Raman study of a powdered SBT sample to prove that the surface of the specimen was not covered by Bi2O3.
Hot electron transport in AlN
Authors: Collazo R. , Schlesser R. , Roskowski A. , Davis R. F. , Sitar Z.
Published: July 2002

The energy distribution of electrons that were transported through a thin intrinsic AlN film was directly measured as a function of the applied field. The measurements were realized by extracting the electrons into vacuum through a semitransparent Au contact and measuring their energies using an electron spectrometer. At moderate applied fields (100 kV/cm), the energy distribution was found to follow a Maxwellian model corresponding to a temperature of 2700 K and a drift component below the spectrometer resolution. At higher fields, intervalley scattering was evidenced by the presence of a second peak at 0.7 eV. This coincides well with the energy position of the LM valleys in AlN.
An experimental and computational study of the methyl formate radical ultraviolet spectrum
Authors: Hansen Jaron C. , Li Yumin , Francisco Joseph S. , Szente Joseph J. , Maricq M. Matti
Published: July 2002

Ab initio molecular orbital theory is coupled with laser flash photolysis experiments to study the UV spectrum and absorption cross section of the methyl formate radical. The vertical excitation energies for four of methyl formate radical's excited states are calculated at the MRCI level of theory. In the region between 220 and 340 nm, two electronic transitions are identified to explain the experimental UV spectrum.
Variable-dose (1017–1020 cm−3) phosphorus ion implantation into 4H–SiC
Authors: Handy Evan M. , Rao Mulpuri V. , Holland O. W. , Jones K. A. , Derenge M. A. , Papanicolaou N.
Published: July 2002

Multiple-energy box profile elevated-temperature (700 °C) phosphorus ion implantations were performed into 4H–SiC in the doping range of 1×1017–1×1020 cm−3. The implanted material was annealed at 1500, 1600, or 1650 °C with an AIN encapsulant to prevent degradation of the SiC surface. Within this temperature range the sheet resistance does not change significantly for a given dose. The percentage of electrical activation of the P donors initially decreased with increasing implant dose for P-implant concentration up to 3×1019 cm−3 and then increased again at higher doses. For 1×1020 cm−3 P implant, a carrier concentration of 4×1019 cm−3 was measured at room temperature. In the 1017 cm−3 P doping concentration range substitutional activation greater than 85% was measured. Despite performing the implants at 700 °C, a significant amount of as-implanted damage was observed in the Rutherford backscattering (RBS) spectrum, even for 1018 cm−3 range P implantations. The RBS yield after annealing is near the virgin level for P concentrations up to 1×1019 cm−3, but above this concentration the RBS yield is above the virgin level, indicating a significant amount of residual lattice damage in the crystal.
Influence of electronic states on precipitation of metallic As clusters in LT-GaAs
Authors: Otsuka N. , Tasaki Y. , Yamada T. , Suda A. , Melloch M. R.
Published: July 2002

The influence of electronic states of the semiconductor matrix on the precipitation of metallic As clusters in GaAs epilayers grown by molecular beam epitaxy at low temperatures were studied. From x-ray diffraction and Hall effect measurements, the presence of free carriers was found to occur in a certain time after the start of the coarsening stage of the precipitation. Transmission electron microscope observations indicate that redistributions of As clusters between the doped and undoped regions starts at the same time with the appearance of free carriers. Redistributions of As clusters occurs in pn junction structures in such a way that depletion zones of the pn junctions become free from As clusters, leading to close correlation of widths of precipitate free zones with those of the depletion zones of the pn junctions for various dopant concentrations. These observations are explained with a tendency of As metal clusters of reducing free carriers in the surrounding semiconductor matrix and hence keeping it as an intrinsic semiconductor.
Cation-vacancy ordering in dehydrated Na6[AlSiO4]6
Authors: Shannon Scott R. , Campbell Branton J. , Metiu Horia , Blake Nick P.
Published: July 2002

The low-temperature cation-ordered superstructure of anhydrous sodium sodalite, a zeolite with composition Na6[AlSiO4]6, has been determined through the use of both density functional theory (DFT) and classical force-field lattice energy minimizations. The charge-balancing Na+ cations are assumed to occupy their characteristic locations within the cubic alumino–silicate framework near the centers of the 6-ring windows. Within the constraints of the volume-doubled pseudotetragonal supercell reported in a previous x-ray diffraction study [B. Campbell, S. R. Shannon, H. Metiu, and N. P. Blake (submitted)], all possible arrangements of cations and vacancies amongst the 6-ring window sites were considered. Force-field calculations employing the ab initio based potential energy function derived by Blake, Weakliem, and Metiu [J. Phys. Chem. B 102, 67 (1998)] and the empirical shell-model potential of Catlow et al. [J. Chem. Soc. Commun. 1984, 1271; Mol. Simul. 1, 207 (1988)], were used to perform full lattice-energy minimizations of each configuration, and to assess their relative stabilities both before and after minimization. The most stable configurations were then examined in more detail via ab initio density functional calculations in the generalized gradient approximation. The lowest-energy supercell ordering proved more stable than the lowest-energy parent cell ordering, and also yielded a pseudotetragonal distortion (space group Pnc2) and a calculated diffraction pattern that qualitatively match experimental results. The structural influences that contribute to the low energy of the correct vacancy ordering are described in detail.
Possible production of C3N4 in the microwave-discharge plasma of C2H2/N2
Authors: Fujii Toshihiro , Muraki Junichi , Arulmozhiraja Sundaram , Kareev Michael
Published: July 2002

C 3 N 4 species were unexpectedly detected in an analysis of the products in a C2H2/N2 microwave discharge using Li+ ion-attachment mass spectrometry. The mass spectra exhibited a Li+ adduct peak at a m/z of 99, which was tentatively assigned to C3N4. The possibility of the production of C3N4 was investigated. Discharge conditions were varied with a view toward increasing the production rate and confirming the presence of this compound in the gas phase. The production rate was found to increase with the nitrogen concentration. Both ab initio and density functional theory calculations were also performed.
Resonance Raman scattering from Cd1−xZnxS nanoparticles dispersed in oxide glass
Authors: Arora Akhilesh K. , Rajalakshmi M.
Published: July 2002

Resonance Raman scattering from 1 and 2 LO phonons is investigated from Cd1−xZnxS nanoparticles dispersed in oxide glass before and after annealing. The resonance profiles are found to exhibit asymmetry which is attributed to the interference effects arising from a nonresonant contribution to the polarizability. The energies of the resonances are found to be consistent with the direct interband transition energies estimated from the reported optical data.
High temperature mobility of CdTe
Authors: Franc J. , Grill R. , Turjanska L. , Höschl P. , Belas E. , Moravec P.
Published: July 2002

The Hall mobility of electrons μH is measured in CdTe in the temperature interval 450–1050 °C and defined Cd overpressure in near-intrinsic conditions. The strong decrease of μH above 600 °C is reported. The effect is explained within a model of multivalley conduction where both electrons in Γ1c minimum and in L1c minima participate. The theoretical description is based on the solution of the Boltzmann transport equation within the relaxation time approximation including the polar and acoustic phonon intravalley and intervalley scatterings. The Γ1c to L1c separation ΔE=0.29−10−4T (eV) for the effective mass in the L valley mL=0.35m0 is found to best fit the experimental data. Such ΔE is about four times smaller than it is predicted by first-principle calculations.
Inhomogeneous speed effects on H2 vibrational line profiles in ternary mixtures
Authors: Joubert P. , Bruet X. , Bonamy J. , Robert D. , Chaussard F. , Saint-Loup R. , Berger H.
Published: July 2002

A study of speed inhomogeneous broadening of the hydrogen vibrational line profiles in the collisional regime for ternary mixtures is reported. The Q(1) line of H2 in H2–Ar–N2, H2–He–Ar, and H2–He–N2 mixtures is investigated by high resolution stimulated Raman spectroscopy for various concentrations and temperatures. A model, successfully used for binary mixtures, is extended to ternary mixtures. An excellent agreement is obtained between theory and experiment for H2–Ar–N2, by using the collisional parameters previously obtained from binary mixtures study. For H2–He–Ar and H2–He–N2, H2–He collisions play a “hardening” effect in the H2 soft speed memory mechanism for H2–Ar or H2–N2. The present experimental results allow us, via the frame of our model, to determine the “hardness” parameter values for H2–He (inaccessible from binary mixtures data) and to get an accurate description of the spectral line shape. This study should be useful for hydrogen coherent anti-Stokes Raman spectroscopy thermometry in H2/air flames at high pressure through its possible extension to the H2–N2–H2O system.
Densities of states in gases, liquids, and solids
Authors: Poland Douglas
Published: July 2002

From heat capacity data one can calculate a set of energy moments for any system. Using the maximum-entropy method, these energy moments can be used to construct an approximate energy probability distribution which, in turn, can be used to calculate the density of states as a function of energy. The density of states is a temperature independent function from which the free energy, internal energy, entropy, and heat capacity for the system can be obtained plus the energy probability distribution and its potential at any temperature. We compare the behavior of the density of states for various substances in a single phase and as one goes through different types of phase transition.
Localization of random copolymers at a sharp interface: Effect of excluded volume
Authors: Dwyer Steven , Trott Michael , Chen Zheng Yu
Published: July 2002

We investigate the localization of random copolymers using the model of Garel et al. [Europhys. Lett. 8, 9 (1989)], with the incorporation of excluded volume effects, based on a trial potential approach. Analytic solutions are produced for the case of evenly distributed randomness. Scaling behavior is predicted from numerical solutions for the case of uneven randomness. We find a critical temperature associated with monomer size, and a logarithmic increase in localization dimension near this transition.
Nucleation and coarsening during epitaxy on a substrate subject to periodic strain: Spatial ordering and size uniformity
Authors: Mattsson Thomas R. , Metiu Horia
Published: July 2002

The diffusion constant of an adsorbed atom changes if the surface is under strain. Because of this, it is reasonable to expect that all diffusion-dependent phenomena occur differently on a strained surface. Here we study how homogeneous, metal-on-metal epitaxy is modified when it takes place on a (100) surface on which we have imposed a periodic inhomogeneous strain field. The periodic field studied here has square symmetry and a period of a hundred lattice sites. The strain is such that an atom diffusing on the surface has a tendency to drift toward the center of the unit cell of the strain field. We deposit atoms on this surface, up to a coverage of 0.15 atoms/site; then we stop deposition and allow the ensemble of islands formed during deposition to coarsen. If the deposition flux and the temperature of the surface are chosen properly, the ensemble of islands generated by this procedure is nearly periodic in space and has a greatly improved island-size uniformity.
Wetting of glass surfaces by ionic magnetic fluids: Effect of the concentration and the pH of the solution and of the size of the magnetic grains on a surface stabilized birefringent layer
Authors: Magalhães M. , Figueiredo Neto A. M. , Bee A. , Bourdon A.
Published: July 2002

The wetting of glass surfaces by ionic magnetic fluids (ferrofluids) is studied, using an improved birefringence technique, as a function of the concentration and size of the colloidal grains, and the pH of the solution. The problem is considered as a phase transition induced by the surface field on a thin ferrofluid layer which is in contact with the glass walls. The induced optical phase shift increases with the grain concentration. The penetration length in the bulk, induced by the ordered surface ferrofluid layer, calculated assuming that the order parameter at the surface is independent of grain concentration, is of the order of 2–4 grain diameters. The grain size, in the range from 2.5 to 9.5 nm, and the solution pH do not seem to be relevant parameters for the induced phase shift.
Simplified embedding schemes for the quantum-chemical description of neutral and charged point defects in SiO2 and related dielectrics
Authors: Erbetta Davide , Ricci Davide , Pacchioni Gianfranco
Published: July 2002

Embedding methods specifically designed to treat large molecules with bulky ligands or in polar solvents are used to describe the electronic structure of point defects in the covalently bonded solids SiO2, Si3N4, and Si2N2O. The mechanical relaxation of the lattice around a given defect, in particular an anion vacancy or interstitial, is described using the ONIOM approach where the system is partitioned in two regions, the local defect treated at the gradient corrected DFT level, and the surrounding matrix treated with a semiempirical Hamiltonian. In this way clusters of 100 atoms and more are used to describe a portion of the solid of 10–15 Å of diameter. The long-range lattice polarization induced by a charged defect, a charged oxygen vacancy or a proton bound to O or N atoms, is estimated by means of the isodensity polarized continuum model, IPCM, and compared with the approximate Born’s formula. The two simplified embedding schemes provide a simple way to improve cluster models of neutral and charged defects in covalent materials.
New picosecond laser system for easy tunability over the whole ultraviolet/visible/near infrared wavelength range based on flexible harmonic generation and optical parametric oscillation
Authors: Maus Michael , Rousseau Els , Cotlet Mircea , Schweitzer Gerd , Hofkens Johan , Van der Auweraer Mark , De Schryver Frans C. , Krueger Arnd
Published: July 2002

A new laser-based and time-correlated single photon counting (TCSPC) detection system which allows easy and fast tuning of excitation wavelengths over a broad range from 240 to 1300 nm, with small gaps from 335 to 360 nm and 660 to 720 nm, has been built. The unique combination of a mode-locked Ti:sapphire laser, an optical parametric oscillator, pulse selectors, and harmonic generators delivers ultrafast laser pulses (1–2 ps) with variable repetition rates and excitation wavelengths. Performance characteristics of the laser system at different excitation wavelengths are reported and the TCSPC setup, which is characterized by a total instrument response function of 25 ps full width at half maximum, is described. Typical TCSPC measurements demonstrate the capability of the system of deriving decay or species associated excitation spectra.
Photoinduced current generated by subband gap optical pumping for the investigation of deep levels in semiconductor lasers
Authors: Zhao Z. , Su C. B.
Published: July 2002

It is shown that the measurement of photocurrents induced by optical pumping at subband gap wavelengths may be a useful technique for studying deep-level traps in semiconductor lasers. In this experiment a 1.3 μm wavelength laser is used to probe a 0.78 μm wavelength GaAs laser device. The trap density of a predegraded and postdegraded device was measured. The measurement indicates a significant increase in the trap density for the degraded device over the nondegraded device. Rate equations describing this excitation process are given and are shown to describe the experiment very well.
Explanation of the limiting thickness observed in low-temperature silicon epitaxy
Authors: Thiesen Jack , Branz Howard M. , Crandall Richard S.
Published: July 2002

Solution of the partial differential equation for diffusion of mobile atoms during solid film growth demonstrates that the observed phase transition in low-temperature silicon epitaxy is triggered by supersaturation of the growing layer with hydrogen. The limiting thickness of the epitaxial layer, hepi, is completely determined by measurable quantities: the flux of hydrogen, the hydrogen diffusion coefficient, and the layer growth rate. Our model accounts for the observed Arrhenius and growth rate dependence of hepi.
Negative differential-resistance device involving two C60 molecules
Authors: Zeng Changgan , Wang Haiqian , Wang Bing , Yang Jinlong , Hou J. G.
Published: July 2002

Negative differential-resistance (NDR) molecular device is realized involving two C60 molecules, one is adsorbed on the tip of a scanning tunneling microscope and the other is on the surface of the hexanethiol self-assembled monolayer. The narrow local density of states features near the Fermi energy of the C60 molecules lead to the obvious NDR effect. Such controllable tunneling structure and the associated known electronic states ensure the stability and reproducibility of the NDR device.
Analyses and proofs of multiexponential process of bacteriorhodopsin photoelectric response
Authors: Yao Baoli , Xu Dalun , Hou Xun , Hu Kunsheng , Wang Aojin
Published: July 2002

A bacteriorhodopsin (bR)-based photodetector is prepared with the structure of stainless steel slide/bR film/aqueous electrolyte gel/copper electrode. The photoelectric response signal to a 10 ns laser pulse is measured. A theory on the bR photoelectric kinetics is developed based on the concept of charge displacement current and the bR photocycle rate equations. Comparison between the theoretical and experimental results proves the multiexponential process of the bR photoelectric response to a short laser pulse. The decay time constants and amplitudes of each exponential component are obtained by data fitting.
Crystallization kinetics of amorphous Sm8Fe85Si2C5 alloy
Authors: Zhang X. Y. , Zhang K. Q. , Liu J. H. , Guan Y. , Zhang J. W.
Published: July 2002

The crystallization kinetics of amorphous Sm8Fe85Si2C5 alloy has been investigated. As the alloy is heated to 900 °C, the crystalline phases of the alloy are composed of α-Fe phase and Sm2(Fe, Si)17Cx. At the beginning of crystallization the activation energy of the α-Fe phase is about 425 kJ/mol, it remains relatively constant as the crystallized fraction of the α-Fe phase is below 70%, and then it declines with increasing the crystallized fraction. When the crystallized fraction of the Sm2(Fe, Si)17Cx is below 50%, the activation energy of crystallization of the phase remains between 518 and 530 kJ/mol, and then decreases with the crystallized fraction. The crystallization behavior of the α-Fe phase and Sm2(Fe, Si)17Cx essentially results in the formation of an α-Fe/Sm2(Fe, Si)17Cx composite microstructure with a coarse grain size in annealed Sm8Fe85Si2C5 alloy, which is attributed to a difficult nucleation and an easy growth for both the α-Fe phase and Sm2(Fe, Si)17Cx in the alloy. Cu and Nb can be used to change the crystallization behavior of the α-Fe phase in the amorphous Sm8Fe85Si2C5 alloy, which is helpful to the formation of the α-Fe/Sm2(Fe, Si)17Cx nanocomposite microstructure with a fine grain size for the α-Fe phase in the alloy.
Anisotropic surface segregation of indium atoms in molecular beam epitaxy of InGaAs/GaAs quantum wells
Authors: Yamaguchi Koichi , Yasuda Yoshikatsu , Kovács András , Barna Peter B.
Published: July 2002

In order to investigate the dependence of indium surface segregation on step structures, InGaAs (InAs)/GaAs quantum wells (QWs) were grown by molecular beam epitaxy on vicinal GaAs(001) substrates, misoriented toward [011] and [01̄1] directions. High energy shift and narrow linewidth of photoluminescence spectra were observed for InAs QWs prepared on the [011]-stepped surface. By cross-sectional transmission electron microscope observation, it was found that broadening of InGaAs QWs was enhanced on the [011]-stepped surface. These phenomena were explained by the anisotropic segregation effect due to the difference in the step structure between the [011] step and the [01̄1] step.
Optimal design and quantum limit for second harmonic generation in semiconductor heterostructures
Authors: Goldoni Guido
Published: July 2002

The optimal design for infrared second harmonic generation (SHG) is determined for a GaAs-based quantum device using a recently developed genetic approach. Both the compositional parameters and electric field are simultaneously optimized, and the quantum limit for SHG, set by the trade-off between large dipole moments (favoring electron delocalization) and large overlaps (favoring electron localization), is determined. Optimal devices are generally obtained that have an asymmetric double quantum well shape with narrow barriers and a graded region sideways to the largest well. An electric field is not found to lead to improved SHG if the compositional parameters are optimized.
Impact ionization coefficients of Al0.8Ga0.2As
Authors: Ng B. K. , David J. P. R. , Plimmer S. A. , Hopkinson M. , Tozer R. C. , Rees G. J.
Published: July 2002

The impact ionization coefficients in bulk Al0.8Ga0.2As have been determined from photomultiplication measurements over the electric field range of 328–519 kV/cm. Unlike in AlxGa1−xAs (x⩽0.6), where the electron to hole ionization coefficients ratios (1/k) are less than 2, the 1/k value in Al0.8Ga0.2As was found to be greater than 10. Excess noise measurements corroborated the multiplication results, suggesting that this material may be a suitable multiplication medium for low noise avalanche photodiodes.
Crystallization of amorphous WNx films
Authors: Suh Bong-Seok , Cho Heung-Kuon , Lee Yoon-Jik , Lee Won-Jun , Park Chong-Ook
Published: July 2002

The microstructural changes of WNx films during annealing were analyzed using transmission electron microscopy and x-ray diffraction. Amorphous WNx films eventually crystallized to a two-phase mixture of W and W2N through the primary crystallization and eutectic crystallization. The resulting microstructure after annealing at 800 °C consisted of large primary crystals with a eutectic mixture of W and W2N microcrystallites in the intergranular boundary region. In case of the amorphous film with near-eutectic composition of W0.79N0.21, it transformed directly into a two-phase mixture of W and W2N through the eutectic crystallization at 600 °C without the primary crystallization. On the other hand, polycrystalline W2N films had a columnar structure and did not undergo any microstructural change during annealing up to 800 °C. Barrier properties of WNx film versus Cu diffusion are discussed in relation with the microstructural changes of the films.
The effect of radial dynamics on the stability of diffuse profile Z pinches
Authors: Zdravkovic D. , Coppins M. , Bell A. R.
Published: July 2002

The effect of radial dynamics on the ideal magnetohydrodynamic (MHD) linear m=0 instability in diffuse profile Z pinches is investigated with a two-dimensional (2D) MHD code. The radial dynamics impose a periodic behavior within which three distinct phases of instability development can be distinguished. The comparison with the Rayleigh–Taylor instability is presented. The behavior is best characterized as MHD instability enhanced by the radial dynamics.
A redundant internal coordinate algorithm for optimization of periodic systems
Authors: Kudin Konstantin N. , Scuseria Gustavo E. , Schlegel H. Bernhard
Published: July 2002

The algorithm for optimizing molecular geometries in redundant internal coordinates is extended to periodic systems. The lattice vectors are not explicitly included in the set of optimized coordinates, but are adjusted only implicitly via a combination of chemically meaningful inter- and intracell internal coordinates. The Wilson B matrix required for coordinate transformations is modified to include internal coordinate derivatives with respect to lattice vectors. The efficiency of the algorithm is demonstrated on a one-dimensional polymer, poly(p-phenylenevinylene), and a three-dimensional crystal of urea.
Decreasing the emission wavelength of GaAs–AlGaAs quantum cascade lasers by the incorporation of ultrathin InGaAs layers
Authors: Wilson L. R. , Cockburn J. W. , Steer M. J. , Carder D. A. , Skolnick M. S. , Hopkinson M. , Hill G.
Published: July 2002

We demonstrate that the emission wavelength of GaAs–Al0.33Ga0.67As quantum cascade lasers can be decreased significantly by incorporating InGaAs layers within the active regions. InAs monolayers are deposited during growth, with segregation effects resulting in the formation of thin InGaAs layers within the GaAs active region quantum wells of the laser. The InGaAs layers are positioned close to the antinodes of the lower laser level wave function, thus decreasing its confinement energy. The small spatial overlap of the InGaAs layers with the upper laser level minimizes the perturbation of the upper state. Consequently, the energy separation between the upper and lower laser levels increases, reducing the emission wavelength. The measured operating wavelength of 7.4 μm is the shortest reported for a GaAs–AlGaAs quantum cascade laser and is approximately 2 μm less than for an identical structure without InGaAs layers in the active regions.
Raman free-induction-decay measurements in low viscosity and supercooled toluene: Vibrational dephasing by shear fluctuations
Authors: Hubble Hugh W. , Lai Tianshu , Berg Mark A.
Published: July 2002

Total dephasing decay profiles of the ν12 (1002 cm−1) ring-stretching mode of toluene have been measured in the time domain in a range extending from the low viscosity, normal liquid (380 K, 0.26 cP) to the high viscosity, supercooled liquid (140 K, 4600 cP). In the region from 1 to 5 cP (250 to 200 K), the profile makes a transition from exponential to partially Gaussian, consistent with a loss of motional narrowing. In contrast to many interpretations of dephasing in terms of collisional dynamics, these data clearly indicate an important role for diffusive dynamics in vibrational dephasing. Above 10 cP, oscillations appear, and their period decreases with increasing viscosity. An analysis of the low to moderate viscosity region with a Kubo model shows that the frequency modulation time is dependent on the viscosity. The dephasing decay profiles are used to test a recent viscoelastic theory of dephasing [Chem. Phys. 233, 257 (1998)], which attributes the perturbation of the vibrational frequency to shear fluctuations in the liquid. A second dephasing process must be included to obtain reasonable fits in the viscosity range below 10 cP. The second process is consistent with a Gaussian frequency modulation whose modulation time is weakly viscosity dependent or has no viscosity dependence. We speculate that this process is due to rapid rotation about the toluene z axis and that the same process also causes the oscillations seen at higher viscosity. There is a good correspondence between the value of the infinite frequency shear modulus found from fitting the Raman free-induction-decay data and the value recently found by Larsen, Ohta, and Fleming by fitting photon-echo-peak-shift measurements in toluene [J. Chem. Phys. 111, 8970 (1999)]. This correspondence supports the idea that both vibrational dephasing and nonpolar electronic solvation are caused by the same physical mechanism.
Stochastic growth of localized plasma waves
Authors: Robinson P. A. , Cairns Iver H.
Published: July 2002

Localized bursty plasma waves are detected by spacecraft in many space plasmas. The large spatiotemporal scales involved imply that beam and other instabilities relax to marginal stability and that mean wave energies are low. Stochastic wave growth occurs when ambient fluctuations perturb the system, causing fluctuations about marginal stability. This yields regions where growth is enhanced and others where damping is increased; bursts are associated with enhanced growth and can occur even when the mean growth rate is negative. In stochastic growth, energy loss from the source is suppressed relative to secular growth, preserving it far longer than otherwise possible. Linear stochastic growth can operate at wave levels below thresholds of nonlinear wave-clumping mechanisms such as strong-turbulence modulational instability and is not subject to their coherence and wavelength limits. These mechanisms can be distinguished by statistics of the fields, whose strengths are lognormally distributed if stochastically growing and power-law distributed in strong turbulence. Recent applications of stochastic growth theory (SGT) are described, involving bursty plasma waves and unstable particle distributions in type III solar radio sources, the Earth’s foreshock, magnetosheath, and polar cap regions. It is shown that when combined with wave–wave processes, SGT also accounts for associated radio emissions.
Band-gap states and ferroelectric restoration in strontium bismuth tantalate
Authors: Li Biao , Koch F. , Chu L.
Published: July 2002

By means of photoluminescence (PL) and sub-band-gap (sub-Eg) optical illumination, the degradation and restoration of ferroelectric properties in strontium bismuth tantalate thin films have been investigated, and the existence of band-gap states is demonstrated. It is shown that the suppression and recovery of ferroelectricity are closely correlated with the change of PL intensity, since both switchable polarization and PL are related to Ta5+ ions in the TaO6 octahedron. Furthermore, the electric-field-induced restoration increases dramatically by the aid of sub-band-gap light (2.5 eV⩽hν⩽Eg) illumination absorbed in band-gap states.
Charge transfer interaction in the acetic acid–benzene cation complex
Authors: Kosugi Kentaroh , Inokuchi Yoshiya , Nishi Nobuyuki
Published: July 2002

Geometrical and electronic structures of the acetic acid–benzene cation complex, (CH3COOH)⋅(C6H6)+, are studied experimentally and theoretically. Experimentally, a vibrational spectrum of (CH3COOH)⋅(C6H6)+ in the supersonic jet is measured in the 3000–3680 cm−1 region using an ion-trap photodissociation spectrometer. An electronic spectrum is also observed with this spectrometer in the 12 000–29 600 cm−1 region. Theoretically, ab initio molecular orbital calculations are performed for geometry optimization and evaluation of vibrational frequencies and electronic transition energies. The vibrational spectrum shows two distinct bands in the O–H stretching vibrational region. The frequency of the strong band (3577 cm−1) is close to that of the O–H stretching vibration of acetic acid and the weak one is located at 3617 cm−1. On the basis of geometry optimizations and frequency calculations, the strong band is assigned to the O–H stretching vibration of the cis-isomer of acetic acid in the hydrogen-bonded complex (horizontal cis-isomer). The weak one is assigned to the vertical trans-isomer where the trans-isomer of acetic acid interacts with the π-electron system of the benzene cation. The weakness of the high frequency band in the photodissociation spectrum is attributed to the binding energy larger than the photon energy injected. Only hot vertical trans-isomers can be dissociated by the IR excitation. The electronic spectrum exhibits two bands with intensity maxima at 17 500 cm−1 and 24 500 cm−1. The calculations of electronic excitation energies and oscillator strengths suggest that charge transfer bands of the vertical trans-isomer can be observed in this region in addition to a local excitation band of the horizontal cis-isomer. We assign the 17 500 cm−1 band to the charge transfer transition of the vertical trans-isomer and the 24 500 cm−1 band to the π–π transition of the horizontal cis-isomer. The calculations also suggest that the charge transfer is induced through the intermolecular C⋯O=C bond formed between a carbon atom of benzene and the carbonyl oxygen atom of acetic acid.
The H+n-C5H12/n-C6H14→H2(v′,j′)+C5H11/C6H13 reactions: State-to-state dynamics and models of energy disposal
Authors: Picconatto Carl A. , Srivastava Abneesh , Valentini James J.
Published: July 2002

The rovibrational state distributions for the H2 product of the H+n-C5H12/n-C6H14→H2+C5H11/C6H13 reactions at 1.6 eV collision energy are reported. The results are compared to measurements made on the kinematically and energetically similar H+RH→H2+R (RH=CH4, C2H6, and C3H8) reactions as well as the atom–diatom reactions H+HX→H2+X(HX=HCl, HBr). For the title reactions, as for all the comparison reactions, the product appears in few of the energetically accessible states. This is interpreted as the result of a kinematic constraint on the product translational energy. Characteristic of the H+RH reactions we have previously studied, the title reactions show increasing rotational excitation of the H2 product with increasing vibrational excitation of it, a correlation that gets stronger as the size of the alkane increases. Trends and variations in the product energy disposal are analyzed and explained by a localized reaction model. This model predicates a truncation of the opacity function due to competing reactive sites in the polyatomic alkane reactant, and a relaxation of the otherwise tight coupling of energy and angular momentum conservation, because the polyatomic alkyl radical product is a sink for angular momentum.
Single-electron effects in side-gated point contacts fabricated in low-temperature deposited nanocrystalline silicon films
Authors: Tan Y. T. , Kamiya T. , Durrani Z. A. K. , Ahmed H.
Published: July 2002

Single-electron effects have been observed up to 60 K in a side-gated point contact device fabricated in nanocrystalline silicon films. The films were phosphorus-doped and deposited at 300 °C by plasma enhanced chemical vapor deposition. Using transmission electron microscopy and Raman spectroscopy, the grain size, crystalline volume fraction, and grain boundary thickness are determined. The single-electron effects are associated with islands formed by crystalline silicon grains ∼4 nm in size, isolated by amorphous silicon regions ∼0.5 nm in thickness. The structural characteristics of the nc-Si film are correlated to the electrical behavior. The electrical transport mechanism at high temperatures is attributed to percolation conduction across a distribution of tunnel barriers with a maximum height of 40 meV.
Mixing of a passive scalar near a free surface
Authors: Shen Lian , Triantafyllou George S. , Yue Dick K. P.
Published: July 2002

We study numerically and analytically the mixing of a passive scalar in turbulent shear flow with a free surface. The Navier–Stokes and scalar mixing equations are solved by direct numerical simulation. We find that the mean concentration of the scalar exhibits a two-layer structure near the free surface: an outer layer, characterized by a fast reduction in the value of the turbulent diffusivity of the scalar, and a much thinner inner layer characterized by a fast reduction of the mean concentration gradient. We develop a similarity theory for the variation of the mean concentration, which agrees very well with the numerical results and provides us with scaling relations for the thickness of each layer.
High-performance metal–ferroelectric–insulator–semiconductor structures with a damage-free and hydrogen-free silicon–nitride buffer layer
Authors: Fujisaki Yoshihisa , Kijima Takeshi , Ishiwara Hiroshi
Published: July 2002

We developed high-performance metal–ferroelectric–insulator–semiconductor (MFIS) structures with a damage-free and hydrogen-free Si3N4 buffer layer as an insulator. We fabricated Si3N4 films by nitriding Si substrates with N2 and/or atomic N radicals generated by an rf radical cell. In contrast to conventional Si3N4 films, the radical-nitride Si3N4 films showed no hysteresis or flat-band shift in the capacitance–voltage (C–V) characteristics even after high-temperature treatments, such as crystallization annealing of ferroelectric thin films deposited on the buffer Si3N4. Using this radical nitride Si3N4 as a buffer layer, we fabricated MFIS diodes with a Pt/Bi3.25La0.75Ti3O12/Si3N4/Si structure. These diodes had good hysteresis in their C–V characteristics, resulting from remnant polarization of the ferroelectric films. However, no other parasitic effects originating in charge trapping and/or detrapping were observed.