bluuebunny/crossref_metadata_2025 huggingface dataset:
Intense blue-light emission from carbon-plasma-implanted porous silicon
Authors: Liu Weili , Zhang Miao , Lin Chenglu , Zeng Zhaoming , Wang Lianwei , Chu Paul K.
Published: July 2002

We have investigated the room-temperature photoluminescence (PL) characteristics of porous-silicon plasma implanted with carbon. Before implantation, the porous silicon made by anodizing emits intense orange light. After carbon-plasma-immersion ion implantation, the orange light disappears and blue light appears. Furthermore, intense blue light is obtained after annealing at 400 °C for 30 min. Analytical results show that the quenching of orange light and appearance of blue light are due to the reduction of the size of nanocrystallites caused by implantation. The effects of different annealing temperature on the light-emission properties of the implanted porous silicon are also studied. The intensity decreases with increased temperature from 600 to 1000 °C, but the PL intensity increases drastically again after annealing at 1250 °C due to the formation of a substance.
Instantaneous normal mode analysis of hydrated electron solvation dynamics
Authors: Yang Chao-Yie , Wong Kim F. , Skaf Munir S. , Rossky Peter J.
Published: July 2002

The instantaneous normal mode (INM) method is implemented in the context of mixed quantum-classical molecular dynamics (MD) simulations and applied to the analysis of the short-time solvation dynamics of the hydrated electron. Numerically suitable equations for computing the solvent dynamical matrix (Hessian) for both ground and excited adiabatic electronic states are derived using analytical derivative methods of quantum chemistry. Standard diagonalization of the Hessian leads to the sets of eigenfrequencies and eigenvectors that underlie the INM theory. Comparison of the hydrated electron and pure water INM spectra and the corresponding mode participation ratios shows that the quantum solute enhances the participation of collective low-frequency unstable modes (imaginary frequencies) at the expenses of stable ones. Distinct differential INM spectra, involving distinct solvent configurational averages, are introduced to describe the changes experienced by the solvent INMs upon the vertical excitation of the electron. The overall picture is that the INMs associated with lower frequency translational and rotational motions, as well as fast librational reorientations are markedly affected by the photoexcitation, as opposed to the localized internal vibrations of the individual water molecules. The INM solvation response for the upward transition calculated from the real modes agrees with the response obtained directly from the energy gap time correlation up to approximately 100 fs. The agreement extends over much longer times for downward transitions. The INM analysis of the solvation responses following vertical upward and downward transitions reveals that diffusive translational and librational motions are both important mechanisms for the early stages (≲50 fs) of the solvent response, with the latter dominating the first half of this time period. It is also shown that the short-time solvent relaxation involves the combined motion of molecules from the first and second hydration shells. In addition, the linearized INM solvation response calculated for D2O indicates a significant (∼36%) solvent isotope effect in the first 25 fs of the response, where the decay is Gaussian. These results are compared with previous studies of the hydrated electron solvation dynamics.
Practical scheme for three-dimensional simulation of electron cyclotron resonance plasma reactors
Authors: Yasaka Yasuyoshi , Uda Nobuki
Published: July 2002

A three-dimensional simulation code which calculates wave propagation, plasma transport, and gas phase chemical reactions self-consistently in an electron cyclotron resonance plasma reactor has been developed. The code is designed to treat the three-dimensional inhomogeneity with a reasonable accuracy in a realistic configuration of the reactor within an acceptable computational time using common computer resources. The profiles of electromagnetic wave fields and the temporal evolution of plasma parameters and radical densities in a bounded, inhomogeneous, cylindrical system have been calculated. The code can resolve azimuthal asymmetry of the plasma associated with a rectangular waveguide coupling or an asymmetric injection of reactive gases as well as the radial and axial variations.
Nucleation of Ga2O3 nanocrystals in the K2O–Ga2O3–SiO2 glass system
Authors: Ceccato R. , Maschio R. Dal , Gialanella S. , Mariotto G. , Montagna M. , Rossi F. , Ferrari M. , Lipinska-Kalita K. E. , Ohki Y.
Published: July 2002

A multitechnique approach, consisting of x-ray diffraction, differential thermal analysis, low frequency Raman scattering from the acoustic vibrations of nanoclusters, and transmission electron microscopy associated with selected area diffraction, has been used to study the nucleation and crystallization processes in SiO2–Ga2O3–K2O glasses. The specific aim was to determine the structure and the size distribution of nanoparticles embedded in the glass matrix. It has been found that nearly spherical nanocrystals of β-Ga2O3, with a size of ∼2–3 nm, nucleate during thermal treatments at 900 °C. Crystallization was observed after annealing at higher temperature. The amount of the crystalline phase and the mean size of the nanocrystals increased with heat treatment, time and temperature. β-Ga2O3 was the only crystalline phase to appear in all glass samples.
Inductively coupled plasma etching of InP using N2/H2
Authors: Chen Hsin-Yi , Ruda Harry E. , Navarro Alvaro Zapata
Published: July 2002

Inductively coupled plasma etching of InP in N2/H2 is demonstrated. The dependence of etch rates on N2/H2 composition, radio frequency power and etching pressure is presented. An optimized process is developed and shown to be suitable for the slow, well-controlled, etching of InP-based nanostructures, while yielding excellent surface morphology.
Determination of oscillator strength of C–F vibrations in fluorinated amorphous-carbon films by infrared spectroscopy
Authors: Wang X. , Harris H. , Temkin H. , Gangopadhyay S. , Strathman M. D. , West M.
Published: July 2002

Fluorinated amorphous-carbon (a-CFx) films deposited by plasma-enhanced chemical-vapor deposition were investigated by Fourier transform infrared transmission spectroscopy and Rutherford backscattering. The proportionality constant between the fluorine concentration and the integrated absorption of C–F vibration modes is 3.52±0.3×1019 cm−2, and is constant within experimental uncertainty over a wide range of processing conditions. It is shown that the fluorine content can be accurately determined from the infrared absorption spectrum of a-CFx films.
Density functional studies on the adsorption and decomposition of SO2 on Cu(100)
Authors: Rodriguez José A. , Ricart Josep M. , Clotet Anna , Illas Francesc
Published: July 2002

Density functional calculations in combination with cluster and slab models (periodic super-cell approach) were used to investigate the bonding and chemistry SO2 on Cu(100). For small coverages of SO2, the stability of the different bonding modes increases according to the η1-S<η2-S,O<η2-O,O<η3-S,O,O sequence. Large coverages make impossible η3-S,O,O bonding, and under such conditions the most stable conformations involve η2-O,O or η2-S,O bonding. These adsorption geometries can be expected when SO2 is coadsorbed with substantial amounts of O. The bonding mechanism of SO2 on copper involves a Cu(3d,4s)→SO2(LUMO) electron transfer that leads to a weakening and elongation of the S–O bonds. The η3-S,O,O conformations exhibit the biggest adsorption energies, the largest charge transfers, and the weakest S–O bonds. These conformations are ideal precursors for the dissociation of the SO2 molecule. From a thermochemical viewpoint, it is much easier to generate SO3(3SO2→2SO3+S,ΔE=∼+1 kcal/mol) than to form SO(SO2→SO+O,ΔE=∼+20 kcal/mol) as an intermediate during the decomposition of sulfur dioxide on Cu(100). SO and SO3 behave as net electron acceptors when bonded to copper, with the electron density on their S atoms increasing in the following order: SO3<SO2<SO<S. At small coverages, SO3 prefers an adsorption geometry in which its C3v axis is perpendicular to the surface and the molecule is bonded to copper through the oxygen atoms (η3-O,O,O bonding). In a crowded surface, the SO3 can be forced into a η2-S,O bonding conformation to minimize lateral adsorbate↔adsorbate repulsions. The multidentate nature of SO2 and SO3 opens the possibility for a complex DeSOx chemistry on metal surfaces.
Differentiation of the ground vibrational and global minimum structures in the Ar:HBr intermolecular complex
Authors: Castillo-Chará Jairo , Lucchese Robert R. , Bevan John W.
Published: July 2002

A fully three-dimensional morphed potential energy surface is reported for Ar:HBr. The morphed potential was obtained from parametrized scaling and shifting transformations of an ab initio potential. The optimum parameters of the morphed potential were determined by a regularized nonlinear least-squares fit to available experimental data. The rovibrational dynamics of the complex were computed using an adiabatic separation of the H–Br intramolecular stretching mode from the intermolecular modes of the system. The ground rovibrational state of the morphed potential was found to have the hydrogen-bound structure Ar–HBr. This state was 10.99 cm−1 more stable than the corresponding state having the van der Waals structure, Ar–BrH, in agreement with experimental data. However, the global minimum of the morphed potential was found to have the van der Waals structure, Ar–BrH. This structure was 20.9 cm−1 lower in energy than the local minimum having the hydrogen-bound structure, Ar–HBr.
Sphere to cylinder transition in a single phase microemulsion system: A theoretical investigation
Authors: Blokhuis Edgar M. , Sager Wiebke F. C.
Published: July 2002

The sphere to cylinder transition in a one-phase droplet microemulsion system is studied theoretically. Within the framework of the curvature energy model by Helfrich, it was already shown by Safran et al. [J. Phys. (France) Lett. 45, L-69 (1984)] that for a certain range of the curvature parameters (rigidity constants and spontaneous curvature), a transition occurs from spherical droplets to infinitely long cylinders through a region where both spheres and cylinders are present. Our aim is to further investigate this region in a quantitative way by including—in addition to curvature energy—translation entropy, cylinder length polydispersity, and radial polydispersity. In this way we are able to obtain structural information on the spheres and cylinders formed, their respective volume fractions, and polydispersity, and provide a more detailed comparison with experimental results.
Raman study of phonon modes in ErVO4 single crystals
Authors: Guedes I. , Hirano Y. , Grimsditch M. , Wakabayashi N. , Loong C.-K. , Boatner L. A.
Published: July 2002

The phonon modes of a pure ErVO4 crystal were determined at room temperature using Raman scattering methods, and the observed frequencies were assigned according to group theory in terms of the internal modes of the VO43− ions and the external modes of the Er(VO4) lattice. The assignments of the phonon modes match well with the overall phonon systematics of the rare-earth orthovanadate series, and the results presented here reinforce the general trend of bonding strength in the zircon series of RVO4, RAsO4, and RPO4 materials.
Assessment of exchange-correlation functionals for the calculation of dynamical properties of small clusters in time-dependent density functional theory
Authors: Marques M. A. L. , Castro Alberto , Rubio Angel
Published: July 2002

We present a detailed study of different exchange-correlation (xc) functionals in describing the dynamical properties of finite systems. For that purpose, we calculated the static polarizabilities, ionization potentials, and optical absorption spectrum of four small clusters, Na2, Na4, SiH4, and Si2H6, using a real-space, real-time technique. The computed static polarizabilities and ionization potentials seem to be in rather good agreement with the available experimental data, once the proper asymptotics of the potential are taken into account. The same conclusion holds for the absorption spectra, although the xc kernels in use do not provide a sufficiently strong attractive interaction between electrons and holes, leading to spectra slightly shifted towards higher energies. This deficiency is traced back to the insufficient description of dynamical effects in the correlation functional. Furthermore, it is shown that the xc potential used to obtain the ground state is the key factor to get reasonable spectra, whereas the choice of the xc kernel just amounts to small, although important, quantitative changes.
Representations of the q-deformed algebra Uq′(so4)
Authors: Havlı́ček M. , Klimyk A. U. , Pošta S.
Published: July 2002

We study the nonstandard q-deformation Uq′(so4) of the universal enveloping algebra U(so4) obtained by deforming the defining relations for skew-symmetric generators of U(so4). This algebra is used in quantum gravity and algebraic topology. We construct a homomorphism φ of Uq′(so4) to the certain nontrivial extension of the Drinfeld–Jimbo quantum algebra Uq(sl2)⊗2 and show that this homomorphism is an isomorphism. By using this homomorphism we construct irreducible finite-dimensional representations of the classical type and of the nonclassical type for the algebra Uq′(so4). It is proved that for q not a root of unity each irreducible finite-dimensional representation of Uq′(so4) is equivalent to one of these representations. We prove that every finite-dimensional representation of Uq′(so4) for q not a root of unity is completely reducible. It is shown how to construct (by using the homomorphism φ) tensor products of irreducible representations of Uq′(so4). [Note that no Hopf algebra structure is known for Uq′(so4).] These tensor products are decomposed into irreducible constituents.
Light responsive polymer field-effect transistor
Authors: Narayan K. S. , Kumar N.
Published: July 2002

We report the effect of light incident on a polymer-based field-effect transistor and demonstrate the utility of light as an additional controlling parameter of the transistor state. The transistor exhibits large photosensitivity indicated by the sizable changes in the drain–source current at low levels of light. The response here is considerably higher than that from existing organic/polymeric planar, two-terminal photodetectors due to an additional process contributing to the enhancement. The light-responsive polymer transistor opens up a device-architecture concept for polymer-based electronics.
Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics
Authors: Namkoong Gon , Doolittle W. Alan , Brown April S. , Losurdo Maria , Capezzuto Pio , Bruno Giovanni
Published: July 2002

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resulting in improved structural and optical properties of GaN epitaxial films. On the other hand, the annealed buffer grown on sapphire nitrided at temperatures lower or higher than 200 °C, has islands of GaN nuclei revealing the sapphire substrate, and ultimately, resulting in degraded GaN epitaxial film quality as demonstrated by photoluminescence and x-ray diffraction measurements. The results can be traced back to the chemistry of the nitridation process.
Coherent x-ray pulse generation in the sub-Ångström regime
Authors: Keitel C. H. , Hu S. X.
Published: July 2002

A mechanism is proposed to generate femtosecond coherent light pulses with a wavelength shorter than the dimension of a single atom. X rays of such short wavelength and high energy are shown to arise from the relativistic interaction of multiply charged ions with short and intense near-optical laser pulses. The generated x-ray pulses have thus potential to be useful in investigating the dynamics of chemical and biological systems with a subatomic resolution.
IrO
2
-based thick-film resistors
Authors: Tankiewicz S. , Morten B. , Prudenziati M. , Golonka L. J.
Published: July 2002

IrO 2 -based thick-film resistors were prepared and their microstructure/electrical property relationship was studied as a function of the firing temperature Tf and composition. The resistor microstructure exhibits a notable change, from a strongly segregated structure for Tf=750 °C, to a quasihomogeneous one when the firing temperature increases to Tf=850 °C. Likewise, a substantial change is observed in the sheet resistance (Rs) dependence on the volume fraction (v) of the conductive phase. For resistors prepared at Tf=750 °C the percolation power law Rs≈(v−vc)−t (where vc and t are the critical volume fraction and exponent, respectively) is observed, while samples fired at Tf⩾850 °C follow the relation ln Rs≈a(v−v0) where a and v0 are proper constants. In addition, the slope of plot a is affected by the firing temperature Tf as well as by the measuring temperature at T<50 K. Very high temperature coefficients of resistance are measured at cryogenic temperatures. The longitudinal gauge factor values are in the range from 3 to 12.4 according to the resistor composition and firing temperature, with the highest values in samples fired at 950 °C with sheet resistance in the range from 104 to 107 Ω/□.
Complex impedance of a spin injecting junction
Authors: Rashba Emmanuel I.
Published: July 2002

Theory of the ac spin injection from a ferromagnetic electrode into a normal conductor through a tunnel or Schottky contact is developed. Diffusion and relaxation of nonequilibrium spins results in a frequency-dependent complex impedance Z(ω) controlled by the spin relaxation rates and the resistances involved. Explicit expression for Z(ω) is presented. Experimental investigation of the frequency dependence of 𝒵 should allow measuring spin relaxation times in both conductors, their effective resistances, and also the parameters of the contact controlling the spin injection.
Simultaneous calculation of the helical pitch and the twist elastic constant in chiral liquid crystals from intermolecular torques
Authors: Germano Guido , Allen Michael P. , Masters Andrew J.
Published: July 2002

We present a molecular simulation method that yields simultaneously the equilibrium pitch wave number q and the twist elastic constant K2 of a chiral nematic liquid crystal by sampling the torque density. A simulation of an untwisted system in periodic boundary conditions gives the product K2q; a further simulation with a uniform twist applied provides enough information to separately determine the two factors. We test our new method for a model potential, comparing the results with K2q from a thermodynamic integration route, and with K2 from an order fluctuation analysis. We also present a thermodynamic perturbation theory analysis valid in the limit of weak chirality.
A theoretical study of the azide (N3) doublet states. A new route to tetraazatetrahedrane (N4): N+N3→N4
Authors: Bittererová Martina , Östmark Henric , Brinck Tore
Published: July 2002

The potential energy surfaces for the low-lying doublet states of the azide radical (N3) have been computed at the complete active space self-consistent field (CASSCF) level with the CAS(15,12) active space. The cc-pVTZ and aug-cc-pVTZ basis sets have been employed throughout the present work. Energies, geometries and harmonic frequencies were determined for the N3 linear ground electronic state (2Πg), a stable C2v ring structure (2B1), and a Cs transition state (2A″) connecting the ring and linear structures. Other N3 (C2v) stationary points (2A2, B12, and A12) have been characterized, as well. The vertical excitation energies for the doublet excited states of the N3 linear (2Πg) and stable ring (2B1) isomers were calculated using CASSCF and multireference configuration interaction [MRCI-SD(Q)] methods. A new route to tetraazatetrahedrane [N4(Td)] has been proposed on the N4 singlet potential energy surface within Cs symmetry. MRCI-SD(Q) calculations predict that N4 (Td) can be formed from atomic nitrogen in the D2 state and N3 (C2v, B12) in a barrierless exothermic reaction. The energy difference (D0) is 135.4 kcal/mol at the MRCI-SD(Q) level.
Adsorption and dissociation of O2 on Ir(111)
Authors: Xu Ye , Mavrikakis Manos
Published: July 2002

The adsorption and dissociation of dioxygen on the Ir(111) surface have been studied using periodic self-consistent density functional theory (DFT-GGA) calculations. Three di-σ-type chemisorbed molecular precursors are identified: One is located over bridge sites and has a binding energy of −1.3 eV and a bond length of 1.4 Å; the other two are located over threefold hollows and have similar binding energies of ca. −1.2 eV and bond lengths of ca. 1.5 Å. None are magnetic, suggesting a peroxo (O22−) nature for these precursors. The minimum energy path for O2 dissociation is determined using the nudged elastic band method, and a very small activation energy is found, ca. 0.06 eV.
Anti-domain-free GaP, grown in atomically flat (001) Si sub-μm-sized openings
Authors: Ohlsson B. J. , Malm J.-O. , Gustafsson A. , Samuelson L.
Published: July 2002

We demonstrate a method for growth of GaP nanocrystals on Si(001), developed to avoid defects related to antiphase domain boundaries in the proximity of the GaP/Si interface. The technique is based on sub-μm-sized selective-area epitaxy of GaP on atomically flat Si in masked openings. We have used field-emission scanning electron microscopy together with transmission electron microscopy to illustrate the method with examples of monocrystalline GaP nanocrystals. The optical properties of the nanocrystals were investigated by low-temperature cathodoluminescence.
Torus knots and polynomial invariants for a class of soliton equations
Authors: Ricca Renzo L.
Published: July 2002

In this paper is shown how to interpret the nonlinear dynamics of a class of one-dimensional physical systems exhibiting soliton behavior in terms of Killing fields for the associated dynamical laws acting as generators of torus knots. Soliton equations are related to dynamical laws associated with the intrinsic kinematics of space curves and torus knots are obtained as traveling wave solutions to the soliton equations. For the sake of illustration a full calculation is carried out by considering the Killing field that is associated with the nonlinear Schrödinger equation. Torus knot solutions are obtained explicitly in cylindrical polar coordinates via perturbation techniques from the circular solution. Using the Hasimoto map, the soliton conserved quantities are interpreted in terms of global geometric quantities and it is shown how to express these quantities as polynomial invariants for torus knots. The techniques here employed are of general interest and lead us to make some conjectures on natural links between the nonlinear dynamics of one-dimensional extended objects and the topological classification of knots.
Escape orbits for non-compact flat billiards
Authors: Lenci Marco
Published: July 2002

It is proven that, under some conditions on f, the non-compact flat billiard Ω={(x,y)∈R0+×R0+; 0≤y≤f(x)} has no orbits going directly to +∞. The relevance of such sufficient conditions is discussed.
Minireview of stochastic resonance
Authors: Wiesenfeld Kurt , Jaramillo Fernan
Published: July 2002

We present an introductory overview of the subject of stochastic resonance. As researchers’ interest in the phenomenon has spread from physics to biology, new questions both fundamental and practical have emerged. After reviewing some key aspects of the subject, we describe a promising candidate for exploring the possible beneficial effects of random noise in sensory transduction.
Equipotential surfaces of the model reaction H+H2: Struggling with three dimensions
Authors: Kristyan Sandor
Published: July 2002

The solution of the complete Schrödinger equation for scattering and electronic structure calculations is the objective of this work, focusing on the triatomic systems. The mathematical difficulties of solving the Schrödinger equation are analyzed, what can be done easily and what needs large scale computational work. An overview of basic atomic and molecular quantum mechanics is given, including how adiabatic and diabatic approximations are used in calculating scattering cross sections. Effective interaction potentials are discussed in detail, how to compute them and what they look like. Besides describing how computers have opened the door to real computation of the full spatial behavior of these multicomponent systems, some simple graphical images significantly aid the development of intuition of how the system actually behaves. The graphical representation focuses on the potential energy surfaces of the H3 electronic structure.
Bubble, bubble, boil, and trouble
Authors: Rothman Daniel H. , Kadanoff Leo P.
Published: July 2002

Multiphase fluid mixtures present not only fascinating examples of nonequilibrium pattern formation, but also illustrate rather general questions concerning new emergent levels of organization. To illustrate these issues, we use lattice-gas cellular automata to study a buoyant mixture of hot and cold bubbles. We find that, depending on the volume fraction of bubbles, the model can exhibit either a coarsening instability familiar from studies of sedimentation, or a convective instability similar to the large-scale flows of Rayleigh–Bénard convection.
Parallelization of three-dimensional spectral laser–plasma interaction code using High Performance Fortran
Authors: Elisseev V. V.
Published: July 2002

We present the results from a parallelization of sequential code used for investigating parametric instabilities in large-scale laser-produced plasmas. The physical model consists of a system of two nonstationary, nonlinear, second-order partial differential equations. The solver is semi-implicit in time and employs Fourier and Chebyshev discretization in a three-dimensional space domain. High Performance Fortran is used for the parallelization of the solver. A detailed performance analysis of the parallel code (speedup, scalability, and load balancing) has been carried out for implementation on an IBM RS/6000 SP system using the XL HPF compiler. © 1998 American Institute of Physics.
Dependence of coercivity on the anisotropy field in the Nd2Fe14B-type sintered magnets
Authors: Sagawa M. , Hirosawa S. , Tokuhara K. , Yamamoto H. , Fujimura S. , Tsubokawa Y. , Shimizu R.
Published: July 2002

The temperature dependence of the coercivity (HcI)of Nd15(Fe1−xCox)77B8 and (Nd1−xDyx)15Fe77B8 sintered magnets and that of the saturation magnetization (Ms) and the anisotropy field (HA) of Nd2(Fe1−xCox)14B and (Nd1−xDyx)2Fe14B single crystals have been observed in the temperature range between 295 and 800 K. The dependence of the coercivity on the major magnetic properties of the matrix phase in the Nd-Fe-B based magnets are investigated using the μ0HA vs μ0HcI+Ms plot. It is demonstrated that this method of analysis is useful in studying the coercivity mechanism of the Nd-Fe-B based sintered magnets.
Thin-film interactions in Si/SiO2/W-Ti/Al-1% Si system
Authors: Chang Peng-Heng , Liu Hung-Yu , Keenan J. A. , Anthony J. M. , Bohlman J. G.
Published: July 2002

The thin-film metal-metal interactions in the Si/SiO2/W-Ti/Al-1% Si system have been studied by transmission electron microscopy, glancing angle x-ray diffraction, Rutherford backscattering spectrometry, and secondary ion mass spectrometry techniques. The reaction is predominantly controlled by the diffusion of tungsten into the Al–1% Si layer and the sequence of the compound formation follows the prediction of the binary Al-W phase diagram. It was shown that Al12W forms first after sintering for 30 min at 450 °C. The Al12W phase then grows from the W-Ti/Al-1% Si interface into the Al layer. At a temperature somewhere between 500 and 520 °C the entire Al layer is transformed to Al12W and then a more tungsten-rich phase, Al5W, starts to form at the W/Al12W interface. As the sintering temperature is increased, the Al5W phase grows into the Al12W layer. The transformation of Al to Al12W is accompanied by a significant deterioration of the top surface quality. The internal morphology of the Al-W phase is highly twinned. Its formation is accompanied by a drastic increase in the sheet resistance. The Ti in the W-Ti layer also out-diffuses into the top Al layer. Titanium forms Al3Ti particles which are dispersed inside as well as at the grain boundaries of the Al12W grains. The differences in the compound formation between the present system and other systems, such as Si/W-Ti/Al and Si/PtAl/W-Ti/Al, are discussed.
Luminescence properties of (GaAs)l(AlAs)m superlattices with (l,m) ranging from 1 to 73
Authors: Jiang D. S. , Kelting K. , Isu T. , Queisser H. J. , Ploog K.
Published: July 2002

We present a systematic investigation of (GaAs)l(AlAs)m (l≂m) superlattices by photoluminescence excitation at T=2 K in the energy range of 1.52–2.42 eV. The luminescence spectra of the superlattice samples differ strongly from the spectra of the ternary Al0.5Ga0.5As alloy. The luminescence peak energy is largest for l,m=3 and decreases when l,m<3. The superlattices with l,m≤15 show sidebands of low intensity. The excitation spectra for superlattices with l,m≤8 show threshold energies but no discrete states corresponding to light-hole related transitions. Superlattices with l,m>15 display a splitting of light- and heavy-hole states. The optical measurements confirm that this new material has properties which are very different from the ternary Al0.5Ga0.5As alloy.
Stress relaxation during thermal cycling in metal/polyimide layered films
Authors: Chen S. T. , Yang C. H. , Faupel F. , Ho P. S.
Published: July 2002

The stress relaxation behavior during thermal cycling of metal/quartz and metal/polyimide/quartz layered structures has been investigated using a cantilever bending beam technique. The metals chosen for this study include Cu and Cr, two materials with contrasting mechanical and interfacial bonding properties. A finite element analysis, as well as an analytical calculation, has been carried out to deduce the stress distribution in the layered structures. Results indicate that the extent of stress relaxation strongly depends on the intrinsic mechanical property of the metal film with significantly higher stresses residing in the Cr film than in the Cu film. The interfacial polyimide layer has been found to serve as an effective buffering layer to reduce the residual stress in metal films. Observations from transmission electron microscopy suggest that the stress is partially released through the deformation of the polyimide near the metal/polyimide interface.
Spin fluctuations in superconducting cuprates (invited)
Authors: Lyons K. B. , Fleury P. A.
Published: July 2002

Our recent reports of highly energetic spin fluctuations in cuprate insulators closely related to high-Tc superconductors have raised the possibility that the strong spin interactions thus manifested may mediate the superconducting pairing. In La2CuO4 the exchange measured by light scattering is 1100 cm−1, while in Ba2YCu3O6 it is 950 cm−1. In the case of Ba2YCu3O6+x it was also shown that the fluctuations persist at room temperature when carriers have been introduced to make the material superconducting. The fluctuations in this case exhibit a broad spectrum shifted to lower frequency, with no clearly defined peak in the spectrum. In this paper, we shall (1) review the theory of two-magnon light scattering and the selection rules applicable to the cuprates, (2) present the data described above, and (3) present data on other related systems, including Eu2CuO4, La2NiO4, and the superconducting phase of Ba2YCu3O6.7 (for T<Tc =60 K). Although the newest data show quite unequivocally that the spin fluctuations persist in the superconductor, there is as yet no detailed explanation of the observed line shape. Certainly, a simple first-order calculation for a spin-half Heisenberg square lattice gives a peak that is far narrower than that observed. Although a calculation based on a resonating valence bond model describes our line shape in insulating La2CuO4, other evidence contravenes this interpretation. It is thus not yet possible to say definitively whether the magnetic interaction is the root cause of the high-Tc phenomena. What is clear is that our data represent a clear and well-defined challenge for any detailed theoretical description of the cuprates.
Magnetic hardening of Sm-Fe-Ti alloys
Authors: Wecker J. , Katter M. , Schnitzke K. , Schultz L.
Published: July 2002

The coercivity Hci of melt-spun Sm-Fe-Ti alloys can be increased to above 4.0 kA/cm by the addition of Zr and Co. Co-substitutions also enhance the remanence Jr and suppress the formation of α-Fe, both of which result in a high-energy product, (BH)max, of up to 56 kJ/m3, the highest value reported so far for 1:12-type magnets. In Sm20Fe70Ti10 alloys prepared by mechanical alloying or by rapid quenching a new hard-magnetic phase is observed with room-temperature coercivities of 40 and 33 kA/cm, respectively.
Temperature dependence of the fundamental absorption edge of mercury cadmium telluride
Authors: Laurenti J. P. , Camassel J. , Bouhemadou A. , Toulouse B. , Legros R. , Lusson A.
Published: July 2002

We have investigated the temperature dependence of the fundamental absorption edge of a series of Hg1−xCdxTe alloys (with composition x ranging from 0.5 to 1). Analyzing our data in the light of the three-dimensional theory of direct-allowed excitons, we find precise values for the fundamental Γ8-Γ6 interband transition energy in a temperature range extending from 0 to 300 K. All experimental results, including previous data for HgTe and mercury-rich Hg1−xCdxTe alloys, are well accounted for using a simple empirical formula: Eg (eV)=−0.303(1−x)+1.606x−0.132x(1−x)+[6.3(1−x) −3.25x −5.92x(1−x)]10−4T2/[11(1−x)+78.7x+T]. This expression, which is valid for all compositions 0≤x≤1 and temperatures 0≤T≤500 K, predicts an alloy composition such that the band-gap energy is temperature independent: We find x=0.505. Finally, it can be used for technological application purpose (far-infrared detection as well as optical-fiber communications performed at realistic values of the temperature) and gives accurate values for the temperature and composition dependence of the effective mass in the semiconducting range of alloys.
Twin microstructure and effective particle size in (111) CdTe and ZnTe grown by metalorganic chemical vapor deposition
Authors: Raizman A. , Oron M. , Cinader G. , Shtrikman Hadas
Published: July 2002

The use of x-ray diffraction techniques for studying twin microstructure and particle-size analysis of (111) CdTe and (111) ZnTe layers grown by metalorganic chemical vapor deposition on GaAs substrates is demonstrated. Two types of twins, lamella and double positioning, were identified, both composed of small particles (domains). A procedure is presented for the determination of the particle thickness and width as well as the lamella thickness. CdTe(111)B layers are typically characterized by a two-dimensional growth mechanism, smooth surface appearance, relatively high growth rate, and lamella twin structure. On the other hand, the (111)A-oriented layers (CdTe and ZnTe) exhibited a three-dimensional growth mechanism, rough surfaces, low growth rates, and double-positioning twins with a preferred twin orientation of the substrate.
Investigation of local piezoelectric properties of thin copolymer films
Authors: Güthner Peter , Glatz-Reichenbach Joachim , Dransfeld Klaus
Published: July 2002

We used the scanning nearfield acoustic microscope−a nontouching profilometer−to investigate the local piezoelectric activity of thin vinylidene fluoride trifluoroethylene copolymer films with a lateral resolution of 1 μm. For metallized as well as uncovered polymers we measured the surface topography. By stimulating the previously poled films with an ac electric field, the piezoelectric surface motion could be measured with high local resolution. Additionally we investigated in detail the change of the piezoactivity during the poling process.
Thermal modeling of zone-melting-recrystallization processing of silicon-on-insulator film structures
Authors: Miaoulis Ioannis N. , Wong Peter Y. , Lipman Joseph D. , Im James S.
Published: July 2002

A numerical simulation model that considers changes in thermal and radiation properties during zone-melting-recrystallization processing of silicon-on-insulator film structures with a graphite strip heater was developed. A series of numerical experiments were conducted to investigate the conductive, convective, and radiative parameters affecting the process. The variability of the thermal conductivity of silicon as a function of temperature at the premelting stage and the variation in thermal conductivity due to the multilayer nature of the structure were found not to affect of the temperature distribution during processing. The convective heat losses induced by the inert gas in the processing chamber does not affect significantly the temperature profiles in the film structure, but can reduce the melt zone width by 10%–25%. The study of radiative effects focused on the effects of varying the width and height of the graphite strip and the distance between the graphite strip and the film structure. These parameters significantly affect the temperature distribution and the width of the melt zone. The relationships between the convection and radiation parameters and the resultant melt zone width are presented in graphical form for use in design optimization.
Simulation of the gas-phase processes in remote-plasma-activated chemical-vapor deposition of silicon dielectrics using rare gas–silane-ammonia mixtures
Authors: Kushner Mark J.
Published: July 2002

Remote-plasma-activated chemical-vapor deposition (RPACVD) is a method whereby thin films are deposited with the substrate located out of the plasma zone. The lower rate of energetic ion and photon bombardment in RPACVD compared to conventional direct-plasma-enhanced chemical-vapor deposition (DPECVD) reduces damage to the substrate. The use of RPACVD also enables one to more carefully tailor the flux of radicals to the substrate compared to DPECVD. This selectivity results from both physically isolating the substrate from undesirable radicals and limiting the variety of chemical pathways that produce radicals. A model for RPACVD is described and results from the model are discussed in the context of comparing gas mixtures and geometries in which this selectivity may be achieved. The chemistries investigated are Rg/SiH4 (Rg=Ar, He) for deposition of Si and Rg/NH3/SiH4 (Rg=Ar, He) for deposition of Si3N4. It is found that the selectivity in producing radicals that can be obtained by excitation transfer from excited states of rare gases is easily compromised by reactor configurations that allow injected gases to penetrate into the plasma zone.
Modeling uphill diffusion of Mg implants in GaAs using suprem-i
v
Authors: Robinson Heyward G. , Deal Michael D. , Amaratunga G. , Griffin Peter B. , Stevenson David A. , Plummer James D.
Published: July 2002

Transient, uphill diffusion of implanted Mg in GaAs during a 900 °C anneal is simulated using suprem-iv. The diffusion is believed to occur via the substitutional-interstitial-diffusion (SID) mechanism, with excess interstitials and vacancies produced by the implantation process causing this abnormal diffusion behavior. The SID mechanism is shown to be equivalent, in terms of the governing equations, to the interstitial-dopant pair diffusion model used in suprem-iv. This allows one to use suprem-iv, a silicon process simulator that includes dopant–point-defect interactions, to model uphill diffusion once the appropriate diffusivity and defect parameters are included. The profiles of excess interstitials and vacancies produced by the implantation process are obtained from Boltzmann transport equation calculations. The transient uphill diffusion phenomenon can be well simulated using the diffusion model in suprem-iv, with the dopant diffusing from the region of excess interstitials toward the surface and the region of excess vacancies. Once the defect concentrations return to their steady-state levels, either by diffusion, recombination, or capture by sinks, the normal concentration-dependent diffusion into the substrate occurs.
Improvement of the thermal stability of W/C multilayers
Authors: Jiang Z. , Dupuis V. , Vidal B. , Ravet M. F. , Piecuch M.
Published: July 2002

A series of W/C multilayers has been fabricated using a magnetron sputtering device. In order to establish comparison with other multilayers produced by several laboratories, their atomic structure and thermal evolution has been studied by small- and wide-angle x-ray scattering, respectively, and Raman spectroscopy. By plotting the nominal thickness determined by the sputtering speed versus the thickness obtained by simulation of small-angle x-ray scattering spectra the existence has been confirmed of a WC interface compound of about 0.7 nm formed during the multilayer deposition. The multilayers fabricated by the magnetron sputtering device showed higher thermal stability and less period expansion than those produced by different apparatus [Dupuis et al., J. Appl. Phys. 68, 5146 (1990), Jiang et al., J. Appl. Phys. 65, 196 (1989)]. Raman spectra indicated that the carbon in the W/C multilayers is more graphitized than in the previous W/C multilayer. The carbon is further graphitized after annealing at 1000 °C. The graphitization of carbon induced its density to decrease, that is, thickness to increase. Consequently, in the multilayers a smaller expansion of carbon is expected after annealing at 1000 °C since there is more graphitized carbon in the initial as-deposited state than in the previous W/C multilayer. This conclusion agrees very well with the thickness variation measured by x-ray diffraction.
Structural and electrical studies on rapid thermally processed ferroelectric Bi4Ti3O12 thin films by metallo-organic solution deposition
Authors: Joshi P. C. , Krupanidhi S. B.
Published: July 2002

Thin films of Bi4Ti3O12 that were pinhole free with uniform composition and thickness were prepared by the metallo-organic solution deposition technique on platinum coated silicon and bare silicon substrates. Crack free and crystalline films of 5000 Å thickness were fabricated by spinning and post deposition rapid thermal annealing treatment at 500 °C. The films exhibited good structural, dielectric, and ferroelectric properties. The effects of annealing temperature and time on the structural and electrical properties of the films were evaluated. The measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 184 and 0.039, respectively for a film rapid thermally annealed at 700 °C for a time of 10 s. Room temperature resistivity of 108 Ω cm and leakage current density of less than 10−7 A/cm2 were obtained for a 0.5-μm-thick film at an applied electric field of 100 kV/cm; establishing good insulating behavior. Ferroelectricity was confirmed by P–E hysteresis loops with remanent polarization and coercive field values of 4.4 μC/cm2 and 84 kV/cm, respectively. The high frequency C–V measurements on films in metal–ferroelectric–semiconductor configuration indicated good Si/Bi4Ti3O12 interface characteristics. The films showed good switching endurance under bipolar stressing at least up to 1010 switching cycles.
Dark current in quantum well infrared photodetectors
Authors: Liu H. C. , Steele A. G. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

Modeling the dark current in quantum well infrared photodetectors has been a topic of much recent research, but the implications of many of the underlying assumptions have not been clarified. We attempt to justify one such model and to provide physical insight for its success. We compare the dark current expression with experiments on several samples, and show that the model provides a good approximation for a wide range of device parameters including barrier thicknesses from 250 to 700 Å and number of wells from 4 to 32.
Emission life and surface analysis of barium-impregnated thermionic cathodes
Authors: Aida T. , Tanuma H. , Sasaki S. , Yaguchi T. , Taguchi S. , Koganezawa N. , Nonaka Y.
Published: July 2002

Emission life of small Ba-impregnated thermionic cathodes suitable for use in TV tubes is investigated systematically. Cathodes are covered with an Os layer to enhance the emissivity. They have an unusual emission life in that the emission current decreases gradually with time and then drops abruptly. The early gradual decrease in emission at high cathode temperatures is caused by the phase change of the Os layer to OsW2. A linear relationship is found to exist between the logarithm of the lifetime and the reciprocal of the cathode temperature. This life end is believed to happen after the end of the reduction reaction between BaO and the porous W body and after ceasing replenishment of the low work function Ba+-O− monolayer on the metal surface. A formula for the emission lifetime is derived from the assumption that the rate determining step is the Ba diffusion inside the pellet. Emission lifetime τ is shown to be given by τ=Ad2 exp(E/kT), where A is a constant, d is the pellet thickness, and E is the activation energy of Ba diffusion. This relation is confirmed experimentally.
Magnetic properties of reactively sputtered Fe1−xO and Fe3O4 thin films
Authors: Kim Y. K. , Oliveria M.
Published: July 2002

We have investigated the magnetic properties of Fe3O4 thin films with thicknesses below 1000 Å. Previous reports had indicated an anomalous decrease in the saturation magnetization of Fe3O4 films with thicknesses below 700 Å. The films were prepared by reactive rf magnetron sputtering and effects of film thickness, oxygen flow rate, and substrate temperature were examined. A region of acceptable magnetic properties, which do not vary with film thickness, for Fe3O4 films has been found at both RT and 400 °C. Within the processing region for single-phase Fe3O4, the magnetic properties did not vary significantly with oxygen partial pressure but did change with substrate temperature. The RT films are characterized by low saturation magnetization, 250 emu/cm3, and modest coercivity, 250 Oe. The films at 400 °C have a higher saturation magnetization, 330 emu/cm3, but also a higher coercivity, 450 Oe. The increased saturation magnetization at higher temperature results from the higher film density and lower degree of cation disorder; the higher coercivity at 400 °C is anomalous. In addition, at RT we have discovered a processing region where ferrimagnetic wüstite, Fe1−xO, is formed. The Fe1−xO films have good soft magnetic properties and warrant further study.