bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of hydrogen implantation temperature on ion-cut of silicon
Authors: Lee J. K. , Nastasi M. , Theodore N. David , Smalley A. , Alford T. L. , Mayer J. W. , Cai M. , Lau S. S.
Published: June 2004

We have studied the effect of ion implantation temperature on the nature of cleavage and layer transfer, and the electrical properties in hydrogen implanted p-Si. The lattice damage and the hydrogen concentration in the as-implanted Si and transferred Si films were analyzed with elastic recoil detection, respectively. Implantations performed at −140 °C [low temperature (LT)] and room temperature (RT) resulted in a variation in the thickness and surface morphology of the transferred layers. The transferred layer from room temperature hydrogen ion implantation was both thicker and atomically smoother than the transferred layer produced by −140 °C hydrogen implantation. The as-transferred layer obtained from RT-implanted p-Si wafer was n-type, but converted to p-type after annealing at 650 °C or higher. The transferred layer obtained from LT-implanted Si wafer was highly resistive even after high temperature annealing. These variations were observed to be correlated with the damage profiles measured by ion channeling; channeling data showed that the room temperature implantation provided a deeper and narrower damage distribution than that obtained from the −140 °C implantation. The nature of the implantation damage was evaluated with the aid of IR spectroscopy and was found to consist of Si–H defects. The type and population of these defects were observed to be dependent on the ion implantation temperature. In both room temperature and −140 °C implantations, the presence of the implantation damage facilitated the nucleation of Si–H defects that developed into H platelets, which were the precursor defects for the cleavage and the layer transfer.
Time-resolved nonlinear luminescence of excitonic transitions in GaN
Authors: Yamada Yoichi , Yoshida Yohei , Taguchi Tsunemasa , Miyake Hideto , Hiramatsu Kazumasa , Iyechika Yasushi , Maeda Takayoshi
Published: June 2004

Excitonic optical properties of GaN have been studied by means of time-resolved nonlinear luminescence spectroscopy, which was based on an excitation correlation technique under excitation at which a transition of the dominant radiative recombination process occurred from donor-bound excitons to biexcitons. Nonlinear luminescence observed at a donor-bound-exciton line resulted from the superposition of three components: a faster-decay component of a superlinear signal, a slower-decay component of a sublinear signal, and a positive base; that is, a much slower-decay component of a superlinear signal as compared with the time range employed in the present measurement (∼600 ps). The sublinear signal was attributed to the state-filling effect of donor-bound excitons, and the positive base was attributed to the saturation of nonradiative recombination centers. In addition, the superlinear signal was attributed to the stimulation of biexciton luminescence, which appeared close to the donor-bound-exciton luminescence. This type of time-resolved nonlinear spectroscopy enabled us to observe separately the contributions of bound excitons and biexcitons even at the same excitation level, through the difference in optical nonlinearity.
Reversible crystallization of a-Si1−xGex alloys under the combined effect of light and temperature
Authors: Martin P. , Torres A. , Jiménez J. , Rodrı́guez A. , Sangrador J. , Rodrı́guez T.
Published: June 2004

A reversible ordered state induced by the combined action of above band-gap light and temperature in a-Si1−xGex layers with 0⩽x<0.38 is reported. The experimental conditions for the observation of this effect are described, as well as the main phenomenological aspects. The illumination of these layers with above band-gap light from an Ar+ laser in the temperature range from 110 to 190 °C induces changes in the Raman spectra that reveal the formation of small ordered clusters (only a few lattice parameters large). The ordered state is not metastable in the sense reported for other structural transformations in these materials, once it was formed it disappears by both cooling down and heating up beyond the above-mentioned lower and upper temperature thresholds. The effect reappears when the sample is settled at a temperature within the above range under illumination. The analysis of the Raman spectra allows to estimate the crystallite size, the amount of internal stress and the fraction of ordered material. The possible mechanisms governing this effect are discussed.
Carrier relaxation in closely stacked InAs quantum dots
Authors: Nakaoka T. , Tatebayashi J. , Arakawa Y. , Saito T.
Published: June 2004

We have investigated the carrier relaxation in closely vertically stacked InAs quantum dots by time-resolved photoluminescence (PL) and micro-PL measurements. The PL decay and the excitation spectrum in the closely stacked dots are much different with those in single layer dots. The PL decay in the stacked dots strongly depends on the PL energy. The decay time in the lower energy side of the PL increases with the number of stacked dot layers. These suggest the existence of a cascadelike relaxation channel via nonresonant tunneling between the stacked dots. The nonresonant tunneling is consistent with the results of micro-PL measurement which allows us to access single columns of the stacked dots. A broad near-resonant absorption in a single column of the dots is explained on the basis of the nonresonant tunneling.
Dynamic drag of solute atmosphere on moving edge dislocations—Phase-field simulation
Authors: Hu S. Y. , Choi J. , Li Y. L. , Chen L. Q.
Published: June 2004

A solute atmosphere forms around a dislocation at temperatures where the diffusion of solute atoms is comparable to or faster than the dislocation motion. The dynamic interactions between diffusive solutes and moving dislocations were studied using a phase-field model which takes into account both the chemical interactions in the solid solution and the elastic interactions among solutes and dislocations. The average velocity of dislocations as a function of solute concentration, solute mobility, dislocation mobility, and applied stresses was investigated. The dragging stress of a solute atmosphere and the solute flow around a moving dislocation are presented and analyzed. Some features of dynamic interactions between a single dislocation and its solute atmosphere are observed.
Lattice density functional theory of molecular diffusion
Authors: Matuszak Daniel , Aranovich Gregory L. , Donohue Marc D.
Published: June 2004

A density functional theory of diffusion is developed for lattice fluids with molecular flux as a functional of the density distribution. The formalism coincides exactly with the generalized Ono–Kondo density functional theory when there is no gradient of chemical potential, i.e., at equilibrium. Away from equilibrium, it gives Fick’s first law in the absence of a potential energy gradient, and it departs from Fickian behavior consistently with the Maxwell–Stefan formulation. The theory is applied to model a nanopore, predicting nonequilibrium phase transitions and the role of surface diffusion in the transport of capillary condensate.
Electronic behaviors of high-dose phosphorus-ion implanted 4H–SiC (0001)
Authors: Negoro Y. , Katsumoto K. , Kimoto T. , Matsunami H.
Published: June 2004

High-dose phosphorus-ion (P+) implantation into 4H–SiC (0001) followed by high-temperature annealing has been investigated. Annealing with a graphite cap largely suppressed the surface roughening of implanted SiC. The surface stoichiometry of implanted SiC was examined by x-ray photoelectron spectroscopy. Electronic behaviors of P+-implanted SiC are discussed based on Hall effect measurements. There is no significant difference in the sheet resistance between SiC annealed with a graphite cap and without a graphite cap. The sheet resistance (resistivity) takes a minimum value of 45 Ω/□ (0.9 mΩcm) at an implant dose of 6.0×1016 cm−2. The sheet resistance shows a weak temperature dependence.
Measurement of low Schottky barrier heights applied to metallic source/drain metal–oxide–semiconductor field effect transistors
Authors: Dubois Emmanuel , Larrieu Guilhem
Published: June 2004

This article investigates the extraction of low Schottky barrier heights in the perspective of integration of metal–oxide–semiconductor field effect transistors (MOSFET) with a metallic source/drain. A test structure composed of two back-to-back junctions is proposed to characterize materials with a low Schottky barrier. To complete the proposed measurement setup, particular attention is placed on a Schottky transport model that continuously combines thermionic emission, field emission, and barrier lowering due to image charge. In the case of platinum silicide (PtSi) contact, it is shown that Arrhenius plots can be accurately reproduced over a wide range of temperature and applied bias. A consolidation of the measurement strategy and of the associated transport model is also performed through measurements and simulations on a long channel p-type Schottky barrier silicon-on-insulator MOSFET with PtSi source/drain. A excellent agreement between simulated and experimental current-voltage characteristics is obtained for a zero-field barrier height of 0.14 eV consistent with the value (0.145 eV) that best fits the Arrhenius plot measured on test structures. The corresponding bias-dependent effective barrier height in the 0.11–0.12 eV range is therefore confirmed at the device level.
Controlled synthesis of carbon nanoparticles by arc in water method with forced convective jet
Authors: Sano Noriaki , Charinpanitkul Tawatchai , Kanki Tatsuo , Tanthapanichakoon Wiwut
Published: June 2004

Forced convective jet applied onto carbon arc plasma has significant effects on the formation of carbon nanoparticles (CNPs) containing multiwalled carbon nanotubes mixed with multishelled nanoparticles produced by the “arc in water” method. There is an optimized flow rate of the convective jet that leads to the highest production yield, largest hydrodynamic diameter of the products, and the reduced crystalline defects. With the convective flow rate of 1.36 dm3/min, the highest yield and production rate of CNPs could be obtained at 48.5% and 9.32 g/h, respectively. These effects of the forced convective jet originate from the enhanced quenching of carbon clusters vaporized from graphite anode. Analysis of convective flow field subjected to the arc plasma zone is conducted to indicate that synthesis of CNPs by the arc in water method can be elaborately controlled.
Quantum confinement contribution to porous silicon photoluminescence spectra
Authors: Cooke D. W. , Muenchausen R. E. , Bennett B. L. , Jacobsohn L. G. , Nastasi M.
Published: June 2004

Photoluminescence (PL) spectra of freshly etched and ambient-aged porous silicon specimens have been measured and analyzed to extract spectral contributions due to quantum confinement (QC) and non-QC effects. It is shown that all spectra can be deconvolved into five Gaussian bands with typical peak energies 1.59, 1.76, 1.84, 1.94, and 2.07 eV. Ambient aging induces ∼0.1 eV blueshift in each of the three highest energy peaks, which is attributed to QC effects. In contrast, the two lowest energy peaks remain unshifted as expected for non-QC effects. Because size of the porous silicon nanoparticles is deduced from the magnitude of blueshift, it is imperative to correctly identify the spectral components associated solely with QC. The three closely spaced Gaussian bands are summed and the resulting single Gaussian band is analyzed with the model of Islam and Kumar [J. Appl. Phys. 93, 1753 (2003)]. Results show that peak energy of the freshly etched sample is 1.86 eV with average nanoparticle size L0=3.90 nm and dispersion σ=0.065 nm. Ambient aging for 40 days causes this peak to blueshift to 2.07 eV and reduces the nanoparticle size to L0=3.37 nm with dispersion σ=0.088 nm. The reduction in particle size (ΔL=0.53 nm) is attributed to oxidation of the nanocrystalline surface, and corresponds to approximately 2 monolayers of oxide growth.
Damage effects produced in the near-surface region of x-cut LiNbO3 by low dose, high energy implantation of nitrogen, oxygen, and fluorine ions
Authors: Bentini G. G. , Bianconi M. , Correra L. , Chiarini M. , Mazzoldi P. , Sada C. , Argiolas N. , Bazzan M. , Guzzi R.
Published: June 2004

The damage effects produced in the near-surface region of x-cut LiNbO3 by low dose, high energy implantation of carbon, nitrogen, oxygen, and fluorine ions are investigated as a function of the dose and substrate temperature during the implant process. The damage profiles were obtained by the Rutherford backscattering RBS-channeling technique, whereas the compositional profiles were performed by secondary ion mass spectrometry. The experimental results showed that the mechanisms governing the damage formation at the surface are strongly connected to the interaction of defects produced when the electronic energy loss exceeds a given threshold close to 220 eV/Å. In particular, we observed a damage pileup compatible with a growth of three-dimensional defect clusters.
Raman scattering and photoluminescence of As ion-implanted ZnO single crystal
Authors: Jeong T. S. , Han M. S. , Youn C. J. , Park Y. S.
Published: June 2004

We have converted the surface of undoped ZnO bulk into the As-doped p-ZnO layer by means of the As ion implantation method. After postimplantation annealing, the As-related properties were investigated by using Raman scattering and photoluminescence (PL) experiments. The Raman spectrum shows that the E2high peak obtained from the As-doped p-ZnO shifted toward the higher energy side of 0.55 cm−1 in comparison with that of the undoped ZnO bulk. This result is related to the stress increment of the sample surface due to the As implantation. After the As-implanted p-ZnO annealed at 800 °C, the PL spectroscopy reveals the neutral acceptor bound exciton emission (A∘,X) at 3.3589 eV. This emission shows a tendency to quench the intensity and extend the emission linewidth with increasing temperatures. Also, two As-related peaks associating with recombination emissions between free electrons and acceptor holes were observed at 3.3159 and 3.2364 eV. In addition, the I–V characteristic curves of the p–n homojunction fabricated with the As ion implantation method clearly have shown the behavior of the diode.
Atomic level scanning transmission electron microscopy characterization of GaN/AlN quantum wells
Authors: Mkhoyan K. A. , Kirkland E. J. , Silcox J. , Alldredge E. S.
Published: June 2004

GaN quantum wells in an AlN matrix are characterized using scanning transmission electron microscopy. The width of the quantum wells and sharpness of the interfaces are measured with composition sensitive annular dark field imaging and electron energy-loss spectroscopy. The effects of beam broadening inside the specimen are discussed and mechanisms to minimize it are suggested. The quantitatively measured intensity of the N K-edge versus position is compared with the propagating beam intensity obtained from multislice calculations. Possible effects of strain in the structure on its electronic states and energy-loss spectra are also discussed.
Influence of solvent quality on polymer solutions: A Monte Carlo study of bulk and interfacial properties
Authors: Addison C. I. , Louis A. A. , Hansen J. P.
Published: June 2004

The effect of solvent quality on dilute and semidilute regimes of polymers in solution is studied by means of Monte Carlo simulations. The equation of state, adsorption near a hard wall, wall–polymer surface tension, and effective depletion potential are all calculated as a function of concentration and solvent quality. We find important differences between polymers in good and θ solvents. In the dilute regime, the physical properties for polymers in a θ solvent closely resemble those of ideal polymers. In the semidilute regime, however, significant differences are found.
NMR measurement of self-diffusion coefficients by slice selection
Authors: Leclerc S. , Trausch G. , Escanyé J.-M. , Canet D.
Published: June 2004

Most of the time, so-called inversion–recovery experiments concern longitudinal nuclear magnetization of the whole sample, the region of interest being limited by the transmitting–receiving coil. Here we address the question of what occurs if the region of interest is purposely limited to a thin slice selected by means of procedures employing magnetic field gradients. Gradients of both magnetic fields (B0, the static magnetic field, and B1, the radio-frequency magnetic field) can be used. In this study we resorted essentially to B1 gradients and novel procedures, based on the natural inhomogeneity of the B1 field delivered by a saddle coil, are described. It is obvious that molecules leaving and entering the slice during the evolution (recovery) period should influence the magnetization recovery. Molecular self-diffusion is responsible for such effects, experimentally visible and accounted for by an appropriate theory which has been approximated for by permitting an easy physical assessment. This approach should lead to alternative methods for measuring self-diffusion coefficients.
Theoretical analysis and computer simulation of fluorescence lifetime measurements. I. Kinetic regimes and experimental time scales
Authors: Yang Shilong , Cao Jianshu
Published: June 2004

The configuration-controlled regime and the diffusion-controlled regime of conformation-modulated fluorescence emission are systematically studied for Markovian and non-Markovian dynamics of the reaction coordinate. A path integral simulation is used to model fluorescence quenching processes on a semiflexible chain. First-order inhomogeneous cumulant expansion in the configuration-controlled regime defines a lower bound for the survival probability, while the Wilemski–Fixman approximation in the diffusion-controlled regime defines an upper bound. Inclusion of the experimental time window of the fluorescence measurement adds another dimension to the two kinetic regimes and provides a unified perspective for theoretical analysis and experimental investigation. We derive a rigorous generalization of the Wilemski–Fixman approximation [G. Wilemski and M. Fixman, J. Chem. Phys. 60, 866 (1974)] and recover the 1/D expansion of the average lifetime derived by Weiss [G. H. Weiss, J. Chem. Phys. 80, 2880 (1984)].
Rydberg-valence interactions of CO, and spectroscopic evidence characterizing the C′ 1Σ+ valence state
Authors: Eidelsberg Michèle , Launay Françoise , Ito Kenji , Matsui Takashi , Hinnen Paul C. , Reinhold Elmar , Ubachs Wim , Huber Klaus P.
Published: June 2004

Rotationally cold absorption and two-photon ionization spectra of CO in the 90–100 nm region have been recorded at a resolution of 0.3–1.0 cm−1. The analyses of up to four isotopomers seek to clarify the observations in regions where the Rydberg levels built on the ground state X 2Σ+ of the ion interact with valence states of 1Σ+ and 1Π symmetry. Previous observations of the 3sσ, B 1Σ+ Rydberg state, reviewed by Tchang-Brillet et al. [J. Chem. Phys. 96, 6735 (1992)], have been extended to energies above its avoided crossing with the repulsive part of the D′ 1Σ+ valence state where resonances of varying intensities and widths have been attributed to the fully coupled 3sσ or 4sσ and D′ potentials, and where the B state approaches a second avoided crossing with the C′ 1Σ+ valence state [Cooper and Kirby, J. Chem. Phys. 87, 424 (1987); 90, 4895 (1989); Chem. Phys. Lett. 152, 393 (1988)]. Fragments of a progression of weak and mostly diffuse bands, observed for all four isotopomers, have been assigned to the C′←X transition. The least-squares modeling of the 4p and 5p complexes reveals the 3pπ, E 1Π Rydberg state to be one of the perturbers, violating the Δv=0 selection rule for Rydberg–Rydberg interactions on account of its rapid transition with increasing v from Rydberg to valence state. A second 1Π perturber, very loosely bound and clearly of valence type, contributes to the confusion in the published literature surrounding the 5p, v=0 complex.
All-forward semiclassical simulations of nonlinear response functions
Authors: Mukamel Shaul , Maddox Jeremy B.
Published: June 2004

We propose a quantum trajectory algorithm for computing nonlinear response functions of condensed phase molecular systems based on a time-ordered expansion of the density matrix. The nth-order response function is expressed as a sum of 2n impulsive response pathways representing trajectories involving zero, one, and up to n interactions with short external pulses. These are evaluated using a forward propagation algorithm based upon a Liouville space extension of the Bohmian propagation method.
Using point defect engineering to reduce the effects of energy nonmonochromaticity of B ion beams on shallow junction formation
Authors: Shao Lin , Chen John , Zhang Jianming , Tang D. , Patel Sanjay , Liu Jiarui , Wang Xuemei , Chu Wei-Kan
Published: June 2004

We have shown that energy contamination introduced by deceleration technology, for increasing the beam currents available for low energy boron implants, can affect fabricated junctions adversely. Energy contamination at a level of 0.1% can extend the profile of 0.5 keV B implants 10 nm deeper after a 1050 °C spike annealing. A highly monoenergetic beam with energy contamination less than 0.1% is required for submicron devices. Furthermore, we have used MeV implantation as a technique of point defect engineering (PDE) to control boron diffusion. PDE can reduce boron clustering and enhance boron activation. Diffusion of B in the tail region has been reduced significantly and the boron profile is much sharper. PDE lowers the critical requirement for beam purity. We conclude that shallower and sharper box-like boron junctions can be achieved by PDE with sub-keV B implants with highly monoenergetic beams.
Bias-dependent carrier and phonon dynamics in semiconductor–metal heterointerfaces
Authors: Chang Y.-M.
Published: June 2004

Coherent longitudinal optical phonons in a GaAsP/Au heterointerface are impulsively generated and detected with time-resolved second-harmonic generation. It is found that the photoexcited electrons, ballistically transporting from the metal layer into the semiconductor region, can rapidly screen the near surface depletion field. This transient screening process launches coherent longitudinal optical (LO) phonons in the heterointerface. The relaxation time of photoexcited nonthermal equilibrium electrons and the dephasing times of coherent LO phonons decrease significantly as a forward bias is applied to the heterointerface. This phenomenon is attributed to the strong carrier–carrier and carrier–phonon scatterings induced by a large direct current flowing across the heterointerface.
Microstructure and magnetic properties of FePt and Fe/FePt polycrystalline films with high coercivity
Authors: Takahashi Y. K. , Seki T. O. , Hono K. , Shima T. , Takanashi K.
Published: June 2004

We have investigated the microstructure and the magnetic properties of FePt and Fe/FePt polycrystalline thin films with high coercivity. The L10 FePt particulate film deposited on a heated amorphous SiO2 substrate showed a large coercivity (Hc) as high as 23 kOe. Contrary to an epitaxially grown single crystal FePt film, the Hc did not show a drastic decrease when the film morphology changed from particulate to continuous. The polycrystalline film with a thickness of 100 nm exhibited a coercivity of 13 kOe in spite of its simple processing route. This high coercivity is attributed to the magnetic domain pinning at the grain boundaries. By depositing Fe layers on the particulate FePt films, an increase of remanence and energy product was observed as a result of the exchange coupling of the Fe and FePt layers.
Physical model of quantum-well infrared photodetectors
Authors: Jovanović V. D. , Harrison P. , Ikonić Z. , Indjin D.
Published: June 2004

A fully quantum-mechanical model for electron transport in quantum-well infrared photodetectors (QWIPs) is presented, based on a self-consistent solution of the coupled rate equations. The important macroscopic parameters like current density, responsivity, and capture probability can be estimated directly from this first-principles calculation. The applicability of the model was tested by comparison with experimental measurements from a GaAs/AlGaAs device, and good agreement was found. The model is general and can be applied to any other material system or QWIP design.
Spin-polarized transport in diluted GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions
Authors: Tao Y. C. , Hu J. G. , Liu H.
Published: June 2004

Taking into account the basic physics of diluted ferromagnetic semiconductors (DMS), in which the variation of the splitting energy with temperature is included, we apply a quantum-mechanical approach to studying the spin-polarized transport in GaMnAs/AlAs/GaMnAs DMS tunnel junctions. It is shown that tunneling magnetoresistance first rapidly increases and then decreases with increasing barrier thickness, exhibiting a peak at an optimum value of barrier thickness. We also find that the normalized conductance difference decreases with the enhancement of temperature. The theoretical results can reproduce the main feature of the experiments.
Molecular reorientation dynamics due to direct current voltage-induced ion redistribution in undoped nematic planar cell
Authors: Pagliusi P. , Zappone B. , Cipparrone G. , Barbero G.
Published: June 2004

We discuss the influence of mobile ions in liquid crystalline materials on the dc voltage-induced molecular reorientation dynamics. We investigate the conventional case of an undoped planar nematic sample aligned by thin polymeric films deposited on the electrodes. A simple model for the ion drift towards the electrodes has been developed in the limit of negligible diffusion current in order to obtain the time dependence of the internal electric field and, subsequently, the dynamics of the director distribution. This model describes the buildup of two electric charge double layers near each electrode reducing the effective voltage drop across the nematic sample. We report the measurement of the dynamics of the phase retardation in a conventional 5CB planar cell when a stepwise dc voltage above the Freedericksz threshold is applied to the electrodes. We demonstrate that our model reproduces the experimentally determined curve, thereby providing reliable values for the physical parameters used in the fitting procedure.
Nanostructuring of silicon (100) using electron beam rapid thermal annealing
Authors: Johnson S. , Markwitz A. , Rudolphi M. , Baumann H.
Published: June 2004

A technique for the rapid, uncomplicated and lithography free fabrication of silicon nanostructures on both n-type and p-type Si(100) substrates is presented. The nanofabrication method employs electron beam rapid thermal annealing of Si(100) substrates which have undergone no prior processing and thus still contain the native oxide. The resulting nanostructures are distributed across the entire Si surface and are square based and aligned to the [110] direction. Nanostructure growth was only observed in the temperature range 800–1200 °C and has been shown to occur following annealing durations as short as 3 s. Nanopillars over 20 nm high have been fabricated following annealing for 120 s. The initial stage of nanostructure growth involves thermal decomposition of the native oxide resulting in atomic scale disorder of the Si surface. Following complete oxide desorption, diffusive Si species migrate across the surface in response to diffusion barriers established on the strained potential-energy surface, nucleating islands at kinetically favored sites. With continued annealing the island number and size evolves according to crystal ripening processes. Enhancement of the oxide desorption and crystal growth rates due to electron irradiation are discussed.
Study of oxygen influences on carbon doped silicon oxide low k thin films deposited by plasma enhanced chemical vapor deposition
Authors: Wang M. R. , Rusli  , Xie J. L. , Babu N. , Li C. Y. , Rakesh K.
Published: June 2004

Carbon doped silicon oxide SiO(C, H) low k thin films (k∼2.9) deposited by the plasma enhanced chemical vapor deposition technique from trimethylsilane (3MS) and oxygen (O2) have been studied. Two types of chemical vapor deposition process recipes, namely CVD1 and CVD2 were applied for film deposition. CVD1 is an initial recipe that resulted in films with a uniform deposition rate irrespective of film thickness and a low dielectric constant of about 2.9. However, it suffers from extraordinarily high post particle counts. CVD2 is a modified recipe adopted to address this issue. The main difference between the two recipes is that a pump down step immediately before the film deposition has been omitted in recipe CVD2, and this has successfully reduced the particle counts to a satisfactory level. However when using recipe CVD2, the deposition rate is nonuniform and the dielectric constant is slightly above 3.0, attributed to the residual oxygen in the process step prior to film deposition. In this study we investigate the effects of oxygen incorporation on the properties of SiO(C, H) films. Surface defects and their element composition, film thickness, refractive index, dielectric constant, and chemical bonding analysis of the films have been carried out. The chemical composition and structure of films deposited by recipes CVD1 and CVD2 show very slight differences, and are also slightly nonuniform along the film depth. Although the nonuniformity does not have much effect on the dielectric constants of the SiO(C, H) films, it may pose a potential challenge for these low k films in terms of advanced integration.
A finite-size dynamic-scaling approach for the diffusion front of particles
Authors: Chappa Verónica C. , Albano Ezequiel V.
Published: June 2004

The scaling behavior of the diffusion front, originated by the random motion of particles under a concentration gradient, is studied by means of the Monte Carlo method and solving the diffusion equation. Simulations are performed on the square lattice using confined geometries of size M×L, where the gradient is established along the M direction while periodic boundary conditions are set along the L direction. A dynamic scaling Ansatz is proposed such as the width of the front [w(M,t)] scales as w(M,t)∼Mαf(t/Mα/β), where α and β are the roughness and growing exponents, respectively. This proposal is based on the fact that the development of w is constrained by the gradient, that decays as M−1, in contrast to the standard Family–Vicsek scaling Ansatz where correlations are constrained by the lateral dimension of the sample. It is found that the roughness exponent exhibits a systematic dependence on the sample size that can be rationalized in terms of a finite-size correction. Extrapolation to the thermodynamic limit gives α=4/7, in excellent agreement with theoretical predictions linking the diffusion system to the percolation problem. The evaluation of the growing exponent gives β=0.30±0.02, leading us to the conjecture β=2/7 for the exact value.
An intrinsic reaction coordinate calculation of the torsion-internal rotation potential of hydrogen peroxide and its isotopomers
Authors: Halpern Arthur M. , Glendening Eric D.
Published: June 2004

Intrinsic reaction coordinate (IRC) calculations of the internal rotation (torsional) potentials for H2O2 and its isotopomers HDO2 and D2O2 were carried out at the CCSD(T)/CBS//aug-cc-pVDZ level. Two extrapolation methods were used to obtain energies in the complete basis set (CBS) limit. The full IRC potential was constructed from scans from the C2v (cis) and C2h (trans) transition states to the equilibrium C2 (gauche) structure. The IRC potential for H2O2 was fit to a five-term Fourier function; coefficients were compared with values obtained from spectroscopic data. The twofold IRC torsional potentials were used to obtain torsional eigenvalues, which yielded values of the transitions between various nτ states. These results compare favorably with Raman and near-infrared data. Our calculations provide values of the cis and trans barriers of 2495 and 364 cm−1, respectively, which are in good agreement with both previously calculated and experimentally derived values. It appears that coupling between torsional motion and other degrees of freedom is not significant in these molecules.
Extracting dominant pair correlations from many-body wave functions
Authors: Beran Gregory J. O. , Head-Gordon Martin
Published: June 2004

The singular value decomposition of the n-particle excitation operator as determined by coupled cluster or perturbation theory is used to extract the dominant and interesting electron–electron correlations from complex molecular wave functions. As an example of the very general formalism, the decomposition of the T̂2 operator obtained from coupled cluster doubles calculations is used to analyze the strength and character of pair correlations in a variety of molecules with interesting electronic structure. The magnitude of the largest singular value(s) determines the strength of the correlation(s), and the corresponding right- and left-hand singular vectors characterize the physical and spatial nature of the correlations. The primary advantage of this tool over natural orbital analysis is that it provides direct associations between the occupied and virtual geminals involved in the correlations.
The permanent dipole moment of gas-phase para-amino benzoic acid revisited
Authors: Rubio-Pons Òscar , Luo Yi
Published: June 2004

The permanent dipole moment of para-amino benzoic acid has been calculated at various theoretical levels, including Hartree–Fock, second-order Møller–Plesset perturbation (MP2), coupled cluster singles and doubles (CCSD), and triples corrections CCSD(T), and hybrid density functional theory at B3LYP level. It is found that the B3LYP method fails to provide correct results for the geometry and the permanent dipole moment. These results are significantly improved by MP2 calculations. Our best estimated dipole moments obtained at CCSD and CCSD(T) levels are in good agreement with experiment.
Excitation energies from time-dependent density-functional theory beyond the adiabatic approximation
Authors: Ullrich C. A. , Burke Kieron
Published: June 2004

Time-dependent density-functional theory in the adiabatic approximation has been very successful for calculating excitation energies in molecular systems. This paper studies nonadiabatic effects for excitation energies, using the current–density functional of Vignale and Kohn [Phys. Rev. Lett. 77, 2037 (1996)]. We derive a general analytic expression for nonadiabatic corrections to excitation energies of finite systems and calculate singlet s→s and s→p excitations of closed-shell atoms. The approach works well for s→s excitations, giving a small improvement over the adiabatic local-density approximation, but tends to overcorrect s→p excitations. We find that the observed problems with the nonadiabatic correction have two main sources: (1) the currents associated with the s→p excitations are highly nonuniform and, in particular, change direction between atomic shells, (2) the so-called exchange-correlation kernels of the homogeneous electron gas, fxcL and fxcT, are incompletely known, in particular in the high-density atomic core regions.
Vibrationally inelastic collisions in H++CO system: Comparing quantum calculations with experiments
Authors: Dhilip Kumar T. J. , Kumar Sanjay
Published: June 2004

State-resolved cross beam experiments [H. Udseth et al., J. Chem. Phys. 60, 3051 (1974); J. Krutein and F. Linder, J. Chem. Phys. 71, 599 (1979); G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)], coupled with proton energy loss spectroscopy for the inelastic scattering of H+ from CO in the collision range of 10–30 eV show very low vibrational excitation of the target molecule. Stimulated by the experimentally observed low vibrational inelasticity in the system the ground and the first two low-lying excited electronic potential-energy surfaces have been computed using the ab initio multireference configuration interaction method. Quantum dynamics has been performed on the ground potential energy surface in the framework of vibrational close-coupling rotational infinite-order sudden approximation. The various computed dynamical attributes such as differential and integral cross sections, and average vibrational energy transfer are analyzed in detail, and compared successfully with the available experimental results.
Effect of internal gradients in the nuclear magnetic resonance measurement of the surface-to-volume ratio
Authors: Zielinski Lukasz J.
Published: June 2004

We consider a system of spins diffusing in a static inhomogeneous (nonuniform-gradient) magnetic field B in a restricted geometry and in the presence of surface relaxation. We show that the short-time diffusional decay of nuclear magnetization is controlled by the field scattering kernel F(t)≡〈[B(t)−B(0)]2〉, which is a measure of the average field inhomogeneity sampled by the spins in time t and does not depend on the particular sequence of radio-frequency pulses used. Magnetization in arbitrary sequences can be straightforwardly computed by evaluating elementary integrals of F(t). Diffusion takes place while the field is on, so that the spins precess as they diffuse, in contrast to the simpler problem of purely classical diffusion considered in [P. P. Mitra, P. N. Sen, and L. M. Schwartz, Phys. Rev. B 47, 8565 (1993)] which is applicable only to the ideal pulsed-field gradient experiment. We compute the short-time asymptotic form of F(t) and find that it depends on the surface-to-volume ratio (S/V) of the pore space as well as on the average of the gradients over the bounding surface. In a system with nonuniform gradients that vary faster near the surface than in the bulk, as for internal susceptibility fields, this gradient surface average may be much larger than the gradients in the bulk, significantly enhancing the apparent S/V. We discuss the application of our results to the widely used Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence as well as proposing a modification of it, which we term “padded” CPMG, that may be preferable in systems with significant surface relaxation. We indicate how each sequence can be used to probe the internal fields.
Elucidating the initial dynamics of electron photodetachment from atoms in liquids using variably-time-delayed resonant multiphoton ionization
Authors: Martini Ignacio B. , Schwartz Benjamin J.
Published: June 2004

We study the photodetachment of electrons from sodium anions in room temperature liquid tetrahydrofuran (THF) using a new type of three-pulse pump–probe spectroscopy. Our experiments use two variably-time-delayed pulses for excitation in what is essentially a resonant 1+1 two-photon ionization: By varying the arrival time of the second excitation pulse, we can directly observe how solvent motions stabilize and trap the excited electron prior to electron detachment. Moreover, by varying the arrival times of the ionization (excitation) and probe pulses, we also can determine the fate of the photoionized electrons and the distance they are ejected from their parent Na atoms. We find that as solvent reorganization proceeds, the second excitation pulse becomes less effective at achieving photoionization, and that the solvent motions that stabilize the excited electron following the first excitation pulse occur over a time of ∼450 fs. We also find that there is no spectroscopic evidence for significant solvent relaxation after detachment of the electron is complete. In combination with the results of previous experiments and molecular dynamics simulations, the data provide new insight into the role of the solvent in solution-phase electron detachment and charge-transfer-to-solvent reactions.
A technique for temperature mapping in fluorocarbon plasmas using planar laser-induced fluorescence of CF
Authors: Steffens Kristen L. , Sobolewski Mark A.
Published: June 2004

Planar laser-induced fluorescence measurements of CF A 2Σ+–X 2Π(1,0) were used to determine two-dimensional maps of rotational temperature in CF4 plasmas. Measured rotational temperatures are expected to be in equilibrium with the gas temperature due to the long chemical lifetime of CF relative to the collision rate. Experiments were performed in the capacitively coupled Gaseous Electronics Conference rf reference cell at pressures from 26.7 Pa (200 mTorr) to 107 Pa (800 mTorr) and powers of 10 to 30 W deposited in the plasma. Temperatures, which ranged from 273±15 K to 480±15 K, were fairly axially symmetric and increased with pressure and power. All plasmas were coolest near the electrodes, which provided a substantial sink for heat in the plasma. Highest temperatures were found at a radial position near the edge of the electrodes. The strong temperature gradients observed in the plasmas can have serious effects on density measurements that probe a single rotational level, as well as on reaction rate constants and interpretation of density gradients. The effects of water-cooling the electrodes and the presence of a silicon wafer on temperature were also measured.
Tree network channels as fluid distributors constructing double-staircase polymer electrolyte fuel cells
Authors: Senn S. M. , Poulikakos D.
Published: June 2004

In this article, constructal tree-like channel networks are introduced as a fuel cell fluid distribution concept, which also optimizes the shape of polymer electrolyte fuel cells. This concept is the main contribution of the article. To perform quantitative calculations based on this concept, a one-dimensional model, accounting for oxygen consumption in the feed channel, oxygen mass transfer between the channel and the backing layer, and oxygen mass transfer through the backing layer to the catalyst layer, is used to predict the polarization curve of a so-constructed polymer electrolyte fuel cell. Pressure drop and pumping power required for the fluid circulation is estimated. Multiobjective genetic search is performed to maximize the net power density with respect to constructal parameters and operating conditions, leading to the optimized tree network. It is found that the use of tree networks instead of the traditional, nonbifurcating serpentine channels in rectangular systems can provide substantially improved cell performance due their intrinsic advantage with respect to both mass transfer and pressure drop. The resulting “double-staircase” shape of the fuel cells differs from the traditional rectangular shape while maintaining simplicity, and it is determined based on the functionality of the flow distribution system.
Dynamical process of switch-off in a supertwisted nematic cell
Authors: Yang Fuzi , Dong Youmei , Ruan L. Z. , Sambles J. R.
Published: June 2004

The switch-off dynamics of a 180° supertwisted nematic (STN) are explored in detail by using a convergent beam system and the fully leaky guided-mode technique. From the dynamical guided-mode data and using the Ericksen–Leslie theory, the director structure in the cell at different times during switch-off is obtained. We have studied the dynamic switch-off from different applied voltages and have found that the relaxation time is independent of the voltage. From fitting the optical data, the effective viscosities are obtained. It appears that two viscosity coefficients, γ and η1, largely control the dynamics of switch-off. From the analysis of the director tilt profile in the cell, it is found that there is no backflow-induced increased tilt at the cell center in the STN cell during the switch-off. From liquid crystal hydrodynamics, this no-backflow dynamic process in the STN cell is well explained.
Electro-optical response of the ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer
Authors: Jeong Dae-Yong , Wang Y. K. , Huang M. , Zhang Q. M. , Kavarnos G. J. , Bauer F.
Published: June 2004

The electro-optical (EO) properties of the ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer were investigated in the wavelength region from 3 to 5 μm. A large Kerr effect was observed in which a refractive index change of −2.6% can be induced under an electric field of 80 V/μm. When combined with the electrostrictive strain, the terpolymer film exhibits a total −5.6% optical path-length change under a field of 80 V/μm. Calculations based on density functional theory suggest that such a large EO effect was caused mainly by the reorientation of the C–F dipoles under external field in the crystalline region. Furthermore, the results support the notion that in the ferroelectric relaxor terpolymer, there exist nanopolar regions that undergo reorientation under external field and generate the observed electrostrictive strain and EO responses.
Pure spin current gratings in semiconductors generated by quantum interference
Authors: Kerachian Y. , Nemec P. , van Driel H. M. , Smirl Arthur L.
Published: June 2004

We demonstrate that the quantum mechanical interference between the probability amplitudes for the two-photon absorption of a fundamental (1.55 μm)∼150 fs pulse and for the one-photon absorption of a noncollinearly propagating second-harmonic (775 nm) pulse can create transient, ballistic, purely spin-polarized current gratings in bulk GaAs at room temperature. For fundamental and second-harmonic pulses having orthogonal linear polarizations, two periodically modulated ballistic spin-polarized current gratings are injected that have opposite spins and opposite propagation directions at each point along the grating. Consequently, there is no initial modulation of the charge current, carrier population, or net spin. Before the carrier momentum relaxes, the transport associated with these spin currents forms two oppositely spin-polarized population gratings that are exactly out of phase spatially and that decay by electronic spin diffusion in a time of 3.2 ps. In addition, charge density gratings are directly produced by the quantum interference process, and they decay by ambipolar diffusion and recombination (∼17.6 ps). The polarization selection rules and sample orientation are used to separate the contributions of the current and density gratings.
Temperature effect on liquid crystal refractive indices
Authors: Li Jun , Gauza Sebastian , Wu Shin-Tson
Published: June 2004

A model describing the temperature effect of liquid crystal (LC) refractive indices is derived and confirmed by experiment. Two single LC compounds (5CB, 5PCH) and two mixtures (MLC-6241-000 and UCF-35) with different birefringence values were used to validate the model. This model fits all the experimental data well. For a low-birefringence LC mixture, if the operating temperature is far below its clearing point, the temperature-dependent refractive indices can be approximated as a parabolic form. This prediction is also experimentally validated. In addition, a linear relationship between 〈n2〉 and temperature is found.