bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the adsorption of norbornadiene on the Si(001) surface: The predominance of non-[2+2]-cycloaddition products
Authors: Bilić Ante , Reimers Jeffrey R. , Hush Noel S.
Published: June 2003

Norbornadiene (NBE) chemisorbs to a Si(001) surface in a flagpolelike structure that has potential as an anchor point for nanoscale molecular devices to the surface. Its bindings to the reconstructed Si(001)-(2×1) surface and a partially depassivated Si(001)-(2×1)-H surfaces are modeled by slab-based density functional theory using the PW91 density functional. This method is shown to quantitatively and qualitatively reproduce many known properties of bulk silicon, the silicon surface reconstruction, and the gas-phase NBE molecule. Four strongly bound adsorbate configurations are found, with the C–C bonds located either above a Si–Si dimer row or trough, oriented either parallel or perpendicular to each other. The calculated binding energies are 96, 85, 81, and 72 kcal mol−1 for the perpendicular row, perpendicular trough, parallel row, and parallel trough configurations, respectively, evaluated at quarter-monolayer coverage on the bare surface, with hydrogen passivation of the surrounding sites having little influence. These results indicate that the observed structural disorder for NBE adsorption on the bare surface at very high coverage results from kinetic rather than thermodynamic control of the reaction products. Such kinetic control is shown to be associated with large barriers in excess of 40 kcal mol−1 for possible adsorbate annealing processes, with desorption into a (partially or fully) physisorbed precursor state being required. Enhanced disorder is also predicted arising from the strong partial binding of NBE through one alkene linkage only, with the analogous four structural motifs being calculated to be very similar in energy. The lowest-energy single-alkene-bonded structure is predicted to be of the parallel–above-row type, consistent with the observed structures for most monoalkene adducts. Preference for the uncommon perpendicular binding of NBE is predicted to arise from unfavorable interactions within the silicon lattice when parallel binding occurs on adjacent rows, a binding motif that is observed for only the simplest monoalkene, ethylene, and only at high coverage. The primary reaction products of NBE are not those predicted by a [2+2] cycloaddition reaction between C=C and Si=Si double bonds, suggesting that, in general, this is not the mechanism for chemisorption of alkenes on Si(001). Rather, the reaction products are those expected on the basis that the silicon dimer bond is biradical in nature. Careful structural, polarization, and band-structure analyses of the reconstructed surface are also shown to provide no evidence for the existence for a doubly bonded silicon dimer.
Atomistic Monte Carlo simulations of three-dimensional polycrystalline thin films
Authors: Rubio J. Emiliano , Jaraiz Martin , Martin-Bragado Ignacio , Hernandez-Mangas Jesus M. , Barbolla Juan , Gilmer George H.
Published: June 2003

An atomistic Monte Carlo code to simulate the deposition and annealing of three-dimensional polycrystalline thin films is presented. Atoms impinge on the substrate with selected angular distributions, and grains are nucleated with different crystalline orientations, defined by the tilt and rotation angles. Grain boundaries appear naturally when the islands coalesce, and can migrate during both deposition and annealing simulations. In this work we present simulations of aluminum films. We examine the influence of the temperature, deposition rate, and adhesion to the substrate on the morphology of polycrystalline thin films. The simulations provide insight into the dominant microscopic mechanisms that drive the structure evolution during thin film processing.
Relativistic and correlated calculations on the ground and excited states of ThO
Authors: Paulovič Jozef , Nakajima Takahito , Hirao Kimihiko , Lindh Roland , Malmqvist Per Åke
Published: June 2003

We report on the performance of the third-order Douglas–Kroll ab initio model potential (DK3-AIMP) method-based electron-correlated spin–orbit calculations. Our treatment assumes that the problem can be separated into a spin–free correlation treatment and a spin–orbit calculation. The correlation effects were calculated using the multistate complete active space second-order perturbation method, and the spin–orbit effects were treated by means of the restricted active space state interaction spin–orbit method, where the spin–orbit effects were approximated by the Douglas–Kroll type of atomic mean-field spin–orbit method. We used our method for illustrative calculations on the ground and low-lying electronic states of thorium monoxide. For a proper description of the inner core region in the spin–orbit calculations, an auxiliary spin–orbit basis set was introduced. The DK3-AIMP-based electron-correlated spin–orbit calculations on ThO yield good agreement with corresponding all-electron results and with the available experimental data. This confirms that the DK3-AIMP method can be easily combined with highly accurate correlation treatments and relativistic effects, both of which are vital for studying the actinides. To our knowledge, the literature contains no references to AIMP calculations on the low-lying states of ThO.
A new instrument to measure the surface resistance of superconducting samples at 400 MHz
Authors: Mahner E. , Calatroni S. , Chiaveri E. , Haebel E. , Tessier J. M.
Published: June 2003

A 400-MHz niobium quadrupole resonator has been manufactured to study the rf properties of superconducting bulk and thin film samples at low temperatures. We describe the apparatus, i.e., the construction of the resonator, field calculations with MAFIA, and the experimental procedure. In first validation tests the surface resistance Rs of a reactor-grade bulk niobium sample as a function of temperature and applied rf field has been investigated by using a calorimetric “rf-dc-compensation” method. A critical temperature Tc=9.15±0.02 K, a thermal conductivity λ(4.2 K)=6.9±0.7 W/mK, a residual resistance Rres=19.0±0.3 nΩ, and a superconducting energy gap of Δ/kBTc=1.82±0.01 have been measured. At 4.2 K we achieved a calorimetric detection limit for Rs of 0.16 nΩ at a peak field of 25 mT.
White luminescence from Si+ and C+ ion-implanted SiO2 films
Authors: Pérez-Rodrı́guez A. , González-Varona O. , Garrido B. , Pellegrino P. , Morante J. R. , Bonafos C. , Carrada M. , Claverie A.
Published: June 2003

The microstructural and optical analysis of SiO2 layers emitting white luminescence is reported. These structures have been synthesized by sequential Si+ and C+ ion implantation and high-temperature annealing. Their white emission results from the presence of up to three bands in the photoluminescence (PL) spectra, covering the whole visible spectral range. The microstructural characterization reveals the presence of a complex multilayer structure: Si nanocrystals are only observed outside the main C-implanted peak region, with a lower density closer to the surface, being also smaller in size. This lack of uniformity in their density has been related to the inhibiting role of C in their growth dynamics. These nanocrystals are responsible for the band appearing in the red region of the PL spectrum. The analysis of the thermal evolution of the red PL band and its behavior after hydrogenation shows that carbon implantation also prevents the formation of well passivated Si/SiO2 interfaces. On the other hand, the PL bands appearing at higher energies show the existence of two different characteristics as a function of the implanted dose. For excess atomic concentrations below or equal to 10%, the spectra show a PL band in the blue region. At higher doses, two bands dominate the green–blue spectral region. The evolution of these bands with the implanted dose and annealing time suggests that they are related to the formation of carbon-rich precipitates in the implanted region. Moreover, PL versus depth measurements provide a direct correlation of the green band with the carbon-implanted profile. These PL bands have been assigned to two distinct amorphous phases, with a composition close to elemental graphitic carbon or stoichiometric SiC.
Internal elemental microanalysis combining x-ray fluorescence, Compton and transmission tomography
Authors: Golosio Bruno , Simionovici Alexandre , Somogyi Andrea , Lemelle Laurence , Chukalina Marina , Brunetti Antonio
Published: June 2003

Conventional x-ray transmission tomography provides the spatial distribution of the absorption coefficient inside a sample. Other tomographic techniques, based on the detection of photons coming from fluorescent emission, Compton and Rayleigh scattering, are used for obtaining information on the internal elemental composition of the sample. However, the reconstruction problem for these techniques is generally much more difficult than that of transmission tomography, mainly due to self-absorption effects in the sample. In this article an approach to the reconstruction problem is presented, which integrates the information from the three types of signals. This method provides the quantitative spatial distribution of all elements that emit detectable fluorescent lines (Z⩾15 in usual experimental conditions), even when the absorption effects are strong, and the spatial distribution of the global density of the lighter elements. The use of this technique is demonstrated on the reconstruction of a grain of the martian meteorite NWA817, mainly composed of low Z elements not measured in fluorescence and for which this method provides a unique insight. The measurement was done at the ID22 beamline of the European Synchrotron Radiation Facility.
Gas phase electronic spectra of the linear carbon chains HC2n+1H (n=3–6,9)
Authors: Ding H. , Schmidt T. W. , Pino T. , Boguslavskiy A. E. , Güthe F. , Maier J. P.
Published: June 2003

The B 3Σu−←X 3Σg− transitions of HC13H and HC19H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A 3Σu−←X 3Σg− spectra have been observed for HC2n+1H (n=3–6) than before with many new vibronic bands identified. The spectra were obtained by means of a mass selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The electronic structures of this series of molecules (n=2–9) in both the ground and excited states have been investigated using DFT, MP2, and state-averaged CASSCF theories. The three lowest dipole allowed electronic transition systems are A 3Σu−←X 3Σg−, B 3Σu−←X 3Σg−, and C 3Πu←X 3Σg−, located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A 3Σu−←X 3Σg− system is found to be of medium intensity and the B 3Σu−←X 3Σg− transition is predicted to be very strong. This is a result of configuration mixing in the excited states. The oscillator strength of the lowest energy transition is not strongly dependent on the length of the chain, but that of the B 3Σu−←X 3Σg− system increases monotonically with size. The C 3Πu state is Rydberg in character. The astrophysical implications are considered and an upper limit of the column densities of these carbon chains in diffuse clouds has been estimated as 1013 cm−2, based on calculated oscillator strengths.
Combined effects of diffusion, nonuniform-gradient magnetic fields, and restriction on an arbitrary coherence pathway
Authors: Zielinski Lukasz J. , Sen Pabitra N.
Published: June 2003

We develop a general framework for analyzing the effects of restricted geometries and inhomogeneous (nonuniform-gradient) magnetic fields on the relaxation of nuclear magnetization. The formalism naturally separates the effects of radio-frequency pulses by introducing the field scattering kernel F(t)≡〈[B(t)−B(0)]2〉 which captures all the interactions of the diffusing spins with the inhomogeneous field and with the confining walls. F(t) is the fundamental building block in the computation of the magnetization in any sequence of pulses. We use it to derive explicit formulas for the attenuation of the echoes of a general coherence pathway and thus arbitrary pulse trains. The short-time and long-time results, proved rigorously, are model-independent and hold for arbitrary geometries, both closed, such as a single cell or pore, and open, such as a connected porous medium. In open geometries, we compute the magnetization for all times, using a model form of the time-dependent diffusion coefficient. We apply our formalism to a few common sequences and study in detail the stimulated-echo (STE) and the Carr–Purcell–Meiboom–Gill (CPMG). We find that the STE is much more sensitive to the effects of restriction than the CPMG and that its long-time attenuation will be less than that of the CPMG, in sharp contrast to the free-diffusion behavior.
Theoretical study of the molecular motion of liquid water under high pressure
Authors: Yamaguchi T. , Chong S.-H. , Hirata F.
Published: June 2003

The pressure effects on the molecular dynamics of liquid water are investigated using the site–site generalized Langevin modified mode-coupling theory. The calculations are performed for temperatures from 273 to 373 K and densities from 0.9 to 1.2 g/cm3. The static structure factor required as input is obtained from the reference interaction-site model hypernetted chain integral equation. The shear viscosity, the dielectric relaxation time, the translational diffusion coefficient, and the first-rank reorientational relaxation times are evaluated. All these quantities show unusual pressure dependence in the low-density, low-temperature region in that the molecular mobility is enhanced by applying the pressure. The magnitude of the enhancement is larger on the reorientational motions than on the translational ones. These tendencies are consistent with experimental observations, although the quantitative agreement is not so good. An analysis of the theory indicates that the decrease in the dielectric friction on the collective polarization at small wave numbers upon increasing pressure is the principal reason for the pressure-induced enhancement of the dielectric relaxation and the decrease in the dielectric relaxation time affects other motions. The decrease in the dielectric friction is caused by the decrease in the number-density fluctuation around the low-wave-number edge of the first peak of the structure factor by compression. The comparison between the results for water and acetonitrile extracts two characteristic features of water that are important for the anomalous pressure effect on its molecular motion. The first one is the small collisional friction on the reorientation due to the spherical repulsive core, and the second one is the strong short-range Coulombic interaction caused by the formation of the hydrogen bonding. A theoretical calculation on a model diatomic liquid consisting of oxygen and hydrogen atoms proposes that the above two characteristic properties of water are sufficient for the emergence of the anomalous pressure dependence. This conclusion is also supported by the molecular dynamics simulation performed on the same model diatomic liquid.
Variational mean spherical scaling approximation for nonspherical ions: The case of asymmetrical dimers
Authors: Santana Samuel , Velázquez Esov S.
Published: June 2003

The primitive model for electrolytes constituted by asymmetric dimers (two charged spheres of arbitrary radii in contact) in a continuous solvent is treated in the variational mean spherical scaling approximation, a generalization of the mean spherical approximation. These theories are extensions of the linearized Poisson–Boltzmann (or Debye–Hückel) approximation that take into account the excluded volume effect of all the ions in the solution. The variational mean spherical scaling approximation is derived from a variational principle in which the energy is obtained from simple electrostatic considerations, and where the entropy is a universal function. We show that this approximation yields the correct limiting thermodynamics for both low and high concentration for ions of arbitrary shape. This work is an extension of our previously presented treatment for symmetric dimers.
Effect of composition on vacancy mediated diffusion in random binary alloys: First principles study of the Si1−xGex system
Authors: Ramanarayanan Panchapakesan , Cho Kyeongjae , Clemens Bruce M.
Published: June 2003

We present the results of a systematic study using the density functional theory (within the local density approximation) of the effect of composition on the self-diffusion of silicon and germanium in silicon–germanium alloys diffusing by a vacancy mechanism. The composition dependence of the vacancy formation energy was calculated. A database of ab initio migration energy barriers for vacancy migration in different local environments was systematically developed by considering the effect of the first nearest neighbor sites explicitly and the effect of the other sites by a mean field approximation. A kinetic Monte Carlo simulation based on the migration energy barrier database was performed to determine the dependence (on the composition) of the activation energy for the diffusion of Si and Ge in Si1−xGex. A detailed study of the variation of the correlation factor with composition and temperature in Si1−xGex was performed using the results of the kinetic Monte Carlo simulation. These analyses constitute essential building blocks to understand the mechanism of vacancy mediated diffusion processes at the microscopic level.
Solitonic mechanism of structural transition in polymer–clay nanocomposites
Authors: Gendelman O. V. , Manevitch L. I. , Manevitch O. L.
Published: June 2003

It has been shown in recent years that the ground state of polymer–clay nanocomposites corresponds to phase-separated, intercalated, or exfoliated state dependent on external conditions. That is why the mechanism of structural transitions between such states is a subject of great scientific and practical importance. A simple “kink” model of melt intercalation in conditions of a shear flow proposed earlier [V. V. Ginzburg, O. V. Gendelman, and L. I. Manevitch, Phys. Rev. Lett. 86, 5073 (2001)] deals with the degenerate case when the energies of phase-separated and intercalated states are equal. Here, we consider a general, nondegenerate case, taking into account the nonequivalence of the aforementioned energies, and develop the model for the case of more general external stress conditions. The potential energy per unit area taking into account the enthalpic and entropic terms in the free energy of the confined polymer, as well as van der Waals and electrostatic interaction between the clays platelets themselves, is approximated by two parabolas. The analytic solution of the appropriate nonlinear dynamical problem has been found in strongly damped limit. Such a solution is manifested as loss of mechanical stability of the aggregated state. It is followed by formation of solitonic excitation, whose propagation leads to structural transition. As a result, we are able to compute the threshold compression force depending on external stress or shear flow intensity that provides the possibility of intercalation and to outline some kinetic peculiarities of the process.
Pure rotational spectrum and model calculations of indole–water
Authors: Blanco Susana , Lopez Juan C. , Alonso Josè L. , Ottaviani Paolo , Camináti Walther
Published: June 2003

The molecular beam Fourier transform microwave spectra of two isotopomers of the 1:1 complex between indole and water have been measured. The water molecule has been reliably located in the complex from these experimental data. The complex has a Cs symmetry with an N–H⋯O hydrogen bond and the plane of the H2O molecule perpendicular to the indole plane. The two-dimensional potential energy surface of the internal rotation and inversion of water in the complex, evaluated with B3LYP/6-31G** or MP2/6-31G** quantum chemical calculations, suggests the tunneling motion of water to take place with the contribute of both motions. The experimental evidence combined with flexible model calculations, indicate, however, that the tunneling motion is mainly an internal rotation of water around its C2 symmetry axis.
ac losses in circular disks of thin YBa2Cu3O7 films in perpendicular magnetic fields
Authors: Suenaga M. , Solovyov V. F. , Li Q. , Ye Z. , Wiesmann H. J. , Iwakuma M. , Fukui M. , Toyota K. , Funaki F. , Johansen T. H. , Shantsev D. V. , Clem J. R.
Published: June 2003

The ac losses at 20 and 30 Hz were measured for two disk-shaped YBa2Cu3O7 films in perpendicular peak applied magnetic fields up to ∼0.2 T in liquid nitrogen. One of the films had a significantly higher critical-current density than the other as determined from the loss measurements. Also, it exhibited a more-uniform flux penetration around the circumference of the disk than the other as observed by magneto-optical images of these films in perpendicular dc fields. The results from this film were compared with theoretical predictions of ac losses for disks of thin superconducting films in perpendicular magnetic fields using the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 55, 9355 (1994)], and the Kim critical-current model [D. V. Shantsev et al., Phys. Rev. B 61, 9699 (2000)]. The asymptotic Bean model predictions for low and high fields were in reasonably good agreement with the data. The numerical calculation of the losses following Shantsev et al. was found to give extremely good agreement with the loss data throughout the entire field range of the measurement when the Kim model for the critical-current density was used.
Bond-length-alternation and the hyperpolarizabilities of a charged soliton in polyenic chains
Authors: An Z. , Wong K. Y.
Published: June 2003

Nonlinear optical responses of a charged soliton were studied using a model charged polyenic chain. It was found that simple derivative relations exist between the spatial profile of the bond-length-alternation and the profiles of the real-space description of the linear polarizability and the first and second hyperpolarizabilities of the chain. These relations can be understood if the soliton is assumed to undergo a sliding translational motion under the influence of an external electric field.
Modulation of charge transport in diamond-based layers
Authors: Serra A. , Manno D. , Siciliano T. , Micocci G. , Tepore A. , Rossi M. , Terranova M. L. , Sessa V. , Piccirillo S. , Orlanducci S.
Published: June 2003

Doping of diamond by substitutional insertion of metallic species or production of diamond/metals nanocomposite layers has been obtained by a hybrid chemical vapor deposition based technique. The potential of such an approach makes it possible to obtain a wide class of purposely designed diamond-based structures characterized by specific properties of charge transport. Reflection high-energy electron diffraction, scanning electron microscopy and x-ray dispersive spectrometry have been used to study the structural and compositional characteristics of some Nd-, W- and Ti-containing diamond films. The peculiar electrical properties conferred to the host diamond layers by the insertion of various metals have been investigated in the range of 25–500 K by performing Hall effect and conductivity measurements. The mechanism of charge transport and the electrical properties of these materials are found to be mainly governed by organization of the metallic species, which can be in different forms, such as dispersion at the atomic scale and the distribution of isolated clusters or aggregates localized at grain boundaries. Depending on the microstructure, the resulting materials can behave as p-type semiconductors, characterized by resistivity values as low as 3.3×10−3 Ω cm and high values of Hall mobility, or show metal-like conduction, with resistivity as low as 2.2×10−1 Ω cm. The insertion of metallic species does not perturb the crystalline quality of the host diamond matrix and, consequently, the layers produced combine the outstanding properties of diamond with electrical behavior that can be modulated for specific applications.
Physical and electrical characterization of HfO2 metal–insulator–metal capacitors for Si analog circuit applications
Authors: Hu Hang , Zhu Chunxiang , Lu Y. F. , Wu Y. H. , Liew T. , Li M. F. , Cho B. J. , Choi W. K. , Yakovlev N.
Published: June 2003

Thin films of HfO2 high-k dielectric have been prepared by pulsed-laser deposition at various substrate temperatures and pressures. X-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy and ellipsometry were used to characterize the deposited films. Experimental results show that substrate temperature has little effect on the stoichiometry, while deposition pressure plays an important role in determining the ratio of Hf and O. It is also found that the optical properties of the HfO2 thin films have strong dependence on both the deposition temperature and pressure. The electrical properties of HfO2 metal–insulator–metal (MIM) capacitors were investigated at various deposition temperatures. It is shown that the HfO2 (56 nm) MIM capacitor fabricated at 200 °C shows an overall high performance, such as a high capacitance density of ∼3.0 fF/μm2, a low leakage current of 2×10−9 A/cm2 at 3 V, low-voltage coefficients of capacitance, and good-frequency dispersion properties. All of these indicate that the HfO2 MIM capacitors are very suitable for use in Si analog circuit applications.
Solvent effects on the electronic structure of a newly synthesized two-photon polymerization initiator
Authors: Wang Chuan-Kui , Zhao Ke , Su Yan , Ren Yan , Zhao Xian , Luo Yi
Published: June 2003

Time-dependent hybrid density functional theory in combination with polarized continuum model has been applied to study the solvent effects on the geometrical and electronic structures, as well as one- and two-photon absorption processes, of a newly synthesized asymmetrical charge-transfer (CT) two-photon absorption (TPA) organic molecule. The TPA cross section calculated from a generalized two-state model and solvatochromic shift of the CT state are found to be solvent dependent, for which a nonmonotonic behavior with respect to the polarity of the solvents has been observed. The calculated properties are in good agreement with the experimental data available. The character of the CT state is visualized by plotting its charge density difference from ground state, in which an excess of electron density on the donor side of the molecule is found. This implies that the excited molecule is ready to donate its electron to the surroundings. The energetic aspect of the electron donation is discussed by examining the solvent dependence of the molecular ground state oxidation potential. The importance of the electron correlation for describing the two-photon absorption is also demonstrated.
Deformation in the heat affected zone during spot welding of a nickel-based single crystal
Authors: Barabash O. M. , Babu S. S. , David S. A. , Vitek J. M. , Barabash R. I.
Published: June 2003

Constrained melting and solidification and associated thermal cycling during welding produces extensive microstructural changes. Such changes are observed during spot welding of single-crystal Ni-based superalloy TSM-75. Analysis of Laue diffraction patterns attained from the heat-affected-zone indicates the presence of a high density of dislocations. Dislocation rearrangement results in local rotations around the [100] direction, which is perpendicular to the temperature gradient. These dislocations exist over a large fraction of the heat-affected zone. The analyses also showed that the dislocation density was maximal near the fusion line and gradually decreases with an increase in the distance from the fusion line. The dislocation density increased with the cooling rate.
Relativistically corrected nuclear magnetic resonance chemical shifts calculated with the normalized elimination of the small component using an effective potential-NMR chemical shifts of molybdenum and tungsten
Authors: Filatov Michael , Cremer Dieter
Published: June 2003

A new method for relativistically corrected nuclear magnetic resonance (NMR) chemical shifts is developed by combining the individual gauge for the localized orbital approach for density functional theory with the normalized elimination of a small component using an effective potential. The new method is used for the calculation of the NMR chemical shifts of Mo95 and W183 in various molybdenum and tungsten compounds. It is shown that quasirelativistic corrections lead to an average improvement of calculated NMR chemical shift values by 300 and 120 ppm in the case of Mo95 and W183, respectively, which is mainly due to improvements in the paramagnetic contributions. The relationship between electronic structure of a molecule and the relativistic paramagnetic corrections is discussed. Relativistic effects for the diamagnetic part of the magnetic shielding caused by a relativistic contraction of the s,p orbitals in the core region concern only the shielding values, however, have little consequence for the shift values because of the large independence from electronic structure and a cancellation of these effects in the shift values. It is shown that the relativistic corrections can be improved by level shift operators and a B3LYP hybrid functional, for which Hartree–Fock exchange is reduced to 15%.
Molecular nitrogen (N2−) acceptors and isolated nitrogen (N−) acceptors in ZnO crystals
Authors: Garces N. Y. , Wang Lijun , Giles N. C. , Halliburton L. E. , Cantwell G. , Eason D. B.
Published: June 2003

Electron paramagnetic resonance (EPR) has been used to investigate molecular nitrogen and isolated nitrogen acceptors in single crystals of ZnO. These samples were grown by the seeded chemical vapor transport method with N2 added to the gas stream. A five-line EPR spectrum is observed at low temperature in the as-grown bulk crystals and is assigned to N2− molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei (14N, 99.63% abundant, I=1). The spin Hamiltonian parameters for the N2− center are g∥=2.0036, g⊥=1.9935, A∥=9.8 MHz, and A⊥=20.1 MHz, with the unique directions parallel to the c axis. Laser excitation at 9 K, with 325 or 442 nm light, eliminates the N2− spectrum (when the N2− convert to N20) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N− acceptors convert to N0). Removing the laser light and warming to approximately 100 K restores the crystal to its preilluminated state. In separate experiments, heating between 600 and 800 °C increases the number of N2− and N0 acceptors that can be observed. We suggest that the activation of these nitrogen acceptors occurs when complexes of hydrogen and nitrogen thermally dissociate. Further heating above 800 °C drives the two nitrogen acceptors to inactive forms.
Influence of the elastic strain on the band structure of ellipsoidal SiGe coherently embedded in the Si matrix
Authors: Usami Noritaka , Ichitsubo Tetsu , Ujihara Toru , Takahashi Tatsuya , Fujiwara Kozo , Sazaki Gen , Nakajima Kazuo
Published: June 2003

We report on a theoretical investigation of the elastic strain in an ellipsoidal SiGe inclusion coherently embedded in Si and its influence on the band structure of SiGe. The strain was calculated as a function of the Ge fraction in SiGe and the aspect ratio of the ellipsoid, and utilized to derive the shift of the band edge. When the principal axis of the ellipsoid was chosen to be parallel to [001], the band structure of SiGe was predicted to be Si like regardless of the aspect ratio. The band gap of strained SiGe was also calculated, and the deviation of the aspect ratio from unity was found to be effective to decrease the band gap due to the breaking of the crystal symmetry. These results suggest the importance of controlling strain, shape, and local Ge fraction in multicrystalline SiGe, which we propose as a promising material for solar cell applications.
Emission, thermocouple, and electrical measurements in SF6/Ar/O2 SiC etching discharges
Authors: Brown Michael S. , Scofield James D. , Ganguly Biswa N.
Published: June 2003

In SiC etching plasma devices, we have recorded plasma emission from Ar, F, and O atoms in SF6/Ar/O2 rf discharges as a function of pressure, input power, and mixture fraction. At fixed power, the emission intensities rise nearly linearly with increasing pressure between 100 and 300 mTorr; with pressure increases to 600 mTorr, the emission intensity rolls off due to the increase in collisional de-excitation. At fixed pressure, Ar and O atom emission shows a similar functional dependence on input power with a roll off at the higher powers due to decreasing reduced electric field strength (E/n, where n denotes the number density). In contrast, the F atom emission increase with increasing power is nearly linear. This reflects the fact that F atoms are produced by dissociative attachment of SF6 (for lower E/n conditions) in addition to direct electron impact dissociation. Electrical measurements, with a variable interelectrode gap discharge, indicate that the electric field to pressure (E/p) ratio does drop with increasing input power. Thermocouple measurements show that the ground electrode temperature increases with increasing power. The dissociative attachment of SF6 increases with increasing temperature as well. The SiC etch rate increases nearly linearly with input power up until the polymer buildup becomes the rate limiting process. At fixed pressure, the Ar emission from the 750 nm line decreases with increasing additions (up to 10%) of O2. This is due to resonantly enhanced quenching of the 4p (13.5 eV) manifold by oxygen atoms. In marked contrast, the F atom emission intensity increases suggesting F atom production by neutral species chemistry. Over the pressure (100–600 mTorr) and power (20–60 W) range studied, the Ar 750 nm emission line serves as a good actinometer for the 704 nm F line provided that there is not a high degree (or changing degree) of O2 dissociation. Resonant deactivation of the 750 nm line through collisional interaction with O atoms, can make the 750 nm line problematic. Under such conditions our previous work indicates that the Ar 641 nm line can provide an alternative actinometer. The excited state of the 641 nm transition lies above the O atom ionization limit making it immune from resonant quenching.
Optical characterization of CuIn1−xGaxSe2 alloy thin films by spectroscopic ellipsometry
Authors: Paulson P. D. , Birkmire R. W. , Shafarman W. N.
Published: June 2003

Optical constants of polycrystalline thin film CuIn1−xGaxSe2 alloys with Ga/(Ga+In) ratios from 0 to 1 have been determined by spectroscopic ellipsometry over an energy range of 0.75–4.6 eV. CuIn1−xGaxSe2 films were deposited by simultaneous thermal evaporation of elemental copper, indium, gallium and selenium. X-ray diffraction measurements show that the CuIn1−xGaxSe2 films are single phase. Due to their high surface roughness, the films are generally not suitable for ellipsometer measurements. A method is presented in which spectroscopic ellipsometer measurements were carried out on the reverse side of the CuIn1−xGaxSe2 films immediately after peeling them from Mo-coated soda lime glass substrates. A detailed description of multilayer optical modeling of ellipsometric data, generic to ternary chalcopyrite films, is presented. Accurate values of the refractive index and extinction coefficient were obtained and the effects of varying Ga concentrations on the electronic transitions are presented.
Linear stochastic magnetization dynamics and microscopic relaxation mechanisms
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: June 2003

Linear magnetization dynamics with relaxation and noise from a thermal bath is analyzed for two general classes of microscopic damping mechanisms. The resulting stochastic differential equations are always in the form of a damped harmonic oscillator driven by a thermal field. Backtransformation from the oscillator coordinates to the magnetization variables always results in a stochastic Bloch–Bloembergen equation for the transverse magnetization components. Detailed evaluation of relaxation rates for the magnon-electron confluence process and the slow-relaxing impurity process is given. Using experimentally observed parameters, a combination of these two processes show good agreement for a set of ferromagnetic resonance linewidth measurements versus temperature, frequency, and film thickness in NiFe.
Maxwell's demon and quantum-dot cellular automata
Authors: Timler John , Lent Craig S.
Published: June 2003

Quantum-dot cellular automata (QCA) involves representing binary information with the charge configuration of closed cells comprised of several dots. Current does not flow between cells, but rather the Coulomb interaction between cells enables computation to occur. We use this system to explore, quantitatively and in a specific physical system, the relation between computation and energy dissipation. Our results support the connection made by Landauer between logical reversibility and physical reversibility. While computation always involves some energy dissipation, there is no fundamental lower limit on how much energy must be dissipated in performing a logically reversible computation. We explicitly calculate the amount of energy that is dissipated to the environment in both logically irreversible “erase” and logically reversible “copy-then-erase” operations carried out in finite time at nonzero temperature. The “copy” operation is performed by using a near-by QCA cell which plays the role of Maxwell's demon. The QCA shift register can then be viewed as a sequence of copy-then-erase operations where the role of the demon cell shifts down the line.
Correlated topographic and spectroscopic imaging beyond diffraction limit by atomic force microscopy metallic tip-enhanced near-field fluorescence lifetime microscopy
Authors: Hu Dehong , Micic Miodrag , Klymyshyn Nicholas , Suh Yung Doug , Lu H. Peter
Published: June 2003

A near-field optical imaging approach is demonstrated for simultaneous topographic and spectroscopic imaging with spatial resolution beyond the optical diffraction limit. The method combines metallic-tip-based tapping-mode atomic force microscopy (AFM) with fluorescence lifetime imaging microscopy (FLIM). The AFM metallic tip was formed by sputter coating a Si tapping mode tip with Au, in a way that forms a globular tip apex. Such tip apex generates high local electric field enhancement under laser illumination, which provides a strong electric-field interaction between the AFM tip and the fluorescent molecules under the tip. The tip perturbation of fluorescence gives the fluorescence lifetime changes that provide the AFM–FLIM imaging contrast. A finite element method simulation was used to further evaluate the electric near-field enhancement and electric field distribution originating from the metallic Au-coated AFM tapping-mode tip. We have demonstrated that spatially mapping the change in fluorescence lifetime and intensity is a promising approach to spectroscopic imaging at an AFM spatial resolution typically defined by the apex diameter of the AFM tips. The globular Au-coated AFM tip not only gives adequate spatial AFM tapping-mode imaging spatial resolution but also is “environmentally friendly” to soft samples, such as polymeric dye-labeled nanospheres and even biological specimens such as POPO-3 labeled DNA.
Swift heavy ion induced modification of C60 thin films
Authors: Bajwa Navdeep , Dharamvir K. , Jindal V. K. , Ingale Alka , Avasthi D. K. , Kumar Ravi , Tripathi A.
Published: June 2003

Modification of thin film samples of C60 on Si and quartz substrates, induced by irradiation of 110 MeV Ni ions at various fluences, was studied. The pristine and irradiated samples were investigated using Raman spectroscopy, electrical conductivity, and optical absorption spectroscopy. The Raman data and band gap measurements indicate that swift ions at low fluences result in aggregate formations involving multiple molecular units such as dimers or polymers. High fluence irradiation results in submolecular formations and amorphous semiconducting carbon, indicating overall damage (fragmentation) of the fullerene molecules. These submolecular units in the amorphous carbon network have been identified as nanocrystalline graphite.
Broadband ferromagnetic resonance spectrometer
Authors: Denysenkov V. P. , Grishin A. M.
Published: June 2003

The continuous wave ferromagnetic resonance (FMR) spectrometer operating in multioctave (0.05–40 GHz) frequency range has been built to investigate the magnetic properties of thin ferromagnetic films in the temperature range of 4–420 K. The spectrometer has two probeheads: one is the X-band microwave reflection cavity used to perform express room temperature measurements and the other is an in-cryostat microstrip line probe to carry out FMR experiments covering the entire frequency range offered by the microwave source. Very uniform and stable magnetic field up to 2.4 T, temperature 4 K–420 K, and continuous frequency scan performed by an HP8722D vector network analyzer provide various modes of operation. Both probe heads are equipped with two-circle high precision goniometers to ensure accurate characterization of magnetic anisotropy and magnetostatic waves spectra recording. Use of the phase sensitive detection, utilized by magnetic field modulation at audio frequency and computer triggering of the network analyzer, enables broadband spectrometer sensitivity to be as high as 1.3×1011 spins/Oe.
Slowness curves and characteristics of surface acoustic waves propagating obliquely in periodic finite-thickness electrode gratings
Authors: Laude Vincent , Ballandras Sylvain
Published: June 2003

The computation of the diffraction of surface acoustic waves (SAWs) on piezoelectric materials is complicated by the anisotropy of their propagation, and requires their slowness curves to be known precisely. In addition, in actual devices these slowness curves are strongly affected by the massive electrodes used for SAW generation. We investigate the characteristics of SAWs propagating obliquely in periodic finite-thickness electrode gratings, with the mass-loading effect taken into account. The velocity of the surface waves, as well as the attenuation, piezoelectric coupling and beam steering are obtained as a function of the propagation angle with respect to the grating axis. The slowness curves for SAWs propagating under periodic electrode gratings are compared with the slowness curves for the same SAWs propagating under a thick homogeneous metallic layer. Numerical examples are presented for the SAWs of 42.75°YX quartz and X112.2°Y lithium tantalate, and the leaky SAWs of 36°YX lithium tantalate.
Optical nonlinear properties of InAs quantum dots by means of transient absorption measurements
Authors: Nakamura H. , Nishikawa S. , Kohmoto S. , Kanamoto K. , Asakawa K.
Published: June 2003

The optical nonlinear properties of self-assembled InAs/GaAs quantum dots (QDs) were experimentally verified by means of transient absorption measurements. A saturation pulse energy Ps of 13 fJ/μm2 and an absorption recovery time τr of 55 ps were obtained from transmission bleaching and pump/probe measurements for a waveguide sample with ten-layer-stacked QDs. An absorption saturation intensity Is of 2.5×104 W/cm2, calculated from Ps and τr, was found. The saturation pulse energy is up to an order of magnitude smaller than, or at least comparable with, the reported values for excitons in quantum wells of III–V compound semiconductors. The dipole length, as calculated from the absorption cross section, is of the same order as the lattice constant of the InAs QDs. The results are expected to experimentally verify that QDs show a delta-function-like density of states.
Incorporation of N into GaAsN under N overpressure and underpressure conditions
Authors: Zhongzhe Sun , Fatt Yoon Soon , Chuin Yew Kuok , Khai Loke Wan , Weijun Fan , Shanzhong Wang , Khee Ng Tien
Published: June 2003

GaAsN was grown by molecular beam epitaxy equipped with a radio frequency nitrogen plasma source. The N incorporation behaviors were investigated using a series of samples grown at different growth rates, As4/Ga ratios, and nitrogen fluxes within a growth temperature range from 420 to 560 °C. It was found that, for the GaAsN grown at higher growth rates (0.4–1.3 ML/s), the N concentration increased linearly following a decrease in growth rate, and kept independent of the arsenic pressure. For the GaAsN grown at lower growth rate (0.1–0.3 ML/s), the N concentration was significantly influenced by the arsenic pressure, and the increase in N concentration with decreasing growth rate follows a sublinear manner. We propose a model based on the incorporation competition of group V elements (N and As) under N underpressure and overpressure conditions, which can explain the different behaviors well.
Size effect on the ferroelectric phase transition in SrBi2Ta2O9 nanoparticles
Authors: Yu T. , Shen Z. X. , Toh W. S. , Xue J. M. , Wang J.
Published: June 2003

We report the size effect on the ferroelectric phase transition in SrBi2Ta2O9 nanoparticles. Samples with particle sizes between 11 and 71 nm were prepared by a room-temperature mechanical activation process followed by postannealing at different temperatures. The size of the particles was determined by x-ray diffraction with the aid of Scherrer’s equation. The ferroelectric phase transition in both SrBi2Ta2O9 nanoparticles and bulk sample were investigated by in situ Raman scattering. The results show that the transition temperature (Tc) decreases from its bulk value (605 K) as the size decreases. The size dependence of Tc can be described as Tc=605−1150/(D−2.1) (K), where D (nm) is the particle size. A critical size of 2.6 nm, below which ferroelectricity disappears, was obtained from this empirical expression.
Interface engineering of cBN films deposited on silicon substrates
Authors: Yang Hangsheng , Iwamoto Chihiro , Yoshida Toyonobu
Published: June 2003

We could determine the substrate pretreatment conditions required for the deposition of cubic boron nitride films without an interfacial amorphous layer by investigating nanostructures, chemical composition, and bonding states of the amorphous layer, which inevitably grew prior to the growth of cubic BN in ion-assisted chemical vapor deposition. The amorphous layer was composed of a native Si oxide layer and a complex oxide layer consisting of B, N, Si, and 10–20 at. % oxygen. However, by the substrate pretreatment, 1200 K heating in 20 mTorr H2 atmosphere for 120 min, the crystallinity of the Si substrate surface could be retained throughout the removal of the native oxide layer, and turbostratic BN was revealed to grow directly on the Si substrate. In addition, we could reduce the thickness of the turbostratic BN layer to less than 3 nm, and increase the adhesion strength markedly. The formation of the amorphous layer was found to be caused by two factors: the native oxide layer and the ion bombardment effects.