bluuebunny/crossref_metadata_2025 huggingface dataset:
Hall measurements of bilayer structures
Authors: Jain S. H. , Griffin P. B. , Plummer J. D.
Published: January 2003

Hall measurements performed on a p-type layer on an n-type substrate showed n-type characteristics at room temperature, p-type behavior at low temperature, and a singularity at intermediate temperatures. The results can be explained by considering the contribution to the Hall readings from the n-type substrate, which is made possible by leakage through the p–n junction. A theoretical description has been developed to understand Hall coefficients in bilayer structures with opposite carrier types.
Effect of inductively coupled plasma oxidation on properties of magnetic tunnel junctions
Authors: Song Ohsung , Lee Y. M. , Yoon C. S. , Kim C. K.
Published: January 2003

Magnetic tunnel junctions consisting of Ta(50 Å)/NiFe(50 Å)/IrMn(150 Å)/CoFe(50 Å)/Al(13 Å) –O/CoFe(40 Å)/NiFe(400 Å)/Ta(50 Å) with a 100×100 μm2 junction area were prepared. The AlOx tunnel barrier was produced by oxidizing the 13 Å thick Al metal using inductively coupled plasma (ICP) for 30–360 s and the ensuing junction properties were characterized as a function of oxidation time. It was found that a junction oxidized for 80 sec exhibited the highest magnetoresistance ratio, 30.3%, at room temperature. It was also shown that the junctions with an ICP oxidized tunnel barrier maintained the tunneling magnetoresistance ratio over 15% even when the insulator layer was oxidized for a prolonged period, well beyond the optimal oxidation time. The large processing window for the insulator oxidation was attributed to the dense amorphous AlOx structure formed by the ICP oxidation.
Efficient thermal rate constant calculation for rare event systems
Authors: Corcelli S. A. , Rahman J. A. , Tully J. C.
Published: January 2003

We present an efficient method for computing thermal reaction rate constants that can be applied to systems in which transitions from reactant to product are infrequent. The method can be applied to high-dimensional, disordered systems which exhibit too many transition states to be identified, and for which useful reaction coordinates cannot be easily defined. The focus of our method is the time correlation function C(t), the normalized partition function for trajectories that begin in the reactant region and end in the product region after a time t; the time derivative of C(t) is the reaction rate constant, k(t). We use an umbrella potential to select initial positions from improbable regions of the reactant configuration space. We then compute C(t) directly by choosing random thermal momenta and asking if the resulting dynamical trajectory reaches the product region in time t. Since dynamical trajectories are run on the true potential energy surface, without the umbrella, re-crossing effects are included correctly. The initial condition bias introduced by the umbrella is removed by a weighting factor. We test the method on a simple two dimensional model potential and on a model for the isomerization of a diatomic in a Weeks–Chandler–Andersen fluid, and show that it gives accurate and precise rates with substantial reduction in computer time.
Characteristics of single defect laser modes in a two-dimensional square lattice photonic crystal slab
Authors: Ryu Han-Youl , Kim Se-Heon , Park Hong-Gyu , Lee Yong-Hee
Published: January 2003

Square lattice photonic crystal single defect nanocavity structures are formed in free-standing slab InGaAsP materials emitting near 1.55 μm. Laser operations are achieved from two types of resonant modes, a degenerate dipole mode and a nondegenerate whispering-gallery-like mode. Each laser mode is identified by the measurement of mode shapes, quality factors, and polarization. The threshold absorbed pump power is estimated to be less than 0.2 mW for the whispering gallery mode. It is found, by rate equation analyses, that nonradiative surface recombination, which takes >60% of carrier recombination near threshold, is one of the main limiting factors for the low-threshold operation of photonic crystal single defect lasers.
Development of microstructure in Cr and Cr/CoCrPt films made by pulsed laser deposition
Authors: Shima M. , Ross C. A.
Published: January 2003

Cr films and Cr/CoCrPt bilayer films have been grown using ion-beam-assisted pulsed laser deposition (PLD). High mobility conditions such as a substrate temperature above 350 °C, a low deposition rate, and a high laser energy promote the formation of a {100} bcc crystallographic preferred orientation in the Cr layer, while a {110}-oriented film is formed under other conditions. The {100} orientation can be formed at lower temperatures if the film is bombarded by energetic Ar ions during growth. CoCrPt grows with the hcp-{112̄0} orientation on bcc-Cr {100} underlayers, which is the same epitaxial relationship that occurs in sputtered Cr/Co-alloy films used in hard disk recording media. PLD CoCrPt films also have magnetic properties broadly similar to those of sputtered films. The PLD film microstructure development is interpreted in terms of the preferential nucleation of {100}-oriented Cr crystals during the early stages of film growth.
Stability analysis of conducting jets under ac radial electric fields for arbitrary viscosity
Authors: González Heliodoro , García F. Javier , Castellanos Antonio
Published: January 2003

A temporal linear modal stability analysis is presented for conducting viscous liquid jets flowing with nonzero velocity relative to an ambient gas and subjected to an ac radial electric field. Parametric resonance between natural dc frequencies and the frequency (or multiple) of the imposed ac field eventually leads to destabilization of the jet for perturbations with wave numbers in the stable domain. In this way, it is possible to obtain drops of smaller size. The main result is the extension of the stability analysis to liquids of arbitrary viscosity using a dynamical approach, instead of previous variational models valid for slightly viscous liquids. The effect of the outer gas in relative motion is taken into account in the framework of currently available semiempirical theories. A brief discussion of the dispersion relation for dc fields is included as the natural starting point for the discussion of the ac case. Use of the 1-D averaged model for axisymmetric perturbations, an alternative to the 3-D approach, allows a complete determination, in this particular case, of the distribution and nature of roots of the dispersion relation in the complex plane. The theoretical study presented here is ready to be compared to future experiments in the Rayleigh and first wind-induced regime, as no relevant instability mechanisms have been excluded; namely, capillary instability, viscous damping, quasi-electrostatic pressure effects, Kelvin–Helmholtz instability corrected to account for the gas viscosity, and finally, parametric resonance.
Application of time-dependent current-density-functional theory to nonlocal exchange-correlation effects in polymers
Authors: van Faassen M. , de Boeij P. L. , van Leeuwen R. , Berger J. A. , Snijders J. G.
Published: January 2003

We provide a successful approach towards the solution of the longstanding problem of the large overestimation of the static polarizability of conjugated oligomers obtained using the local density approximation within density-functional theory. The local approximation is unable to describe the highly nonlocal exchange and correlation effects found in these quasi-one-dimensional systems. Time-dependent current-density-functional theory enables us to describe ultranonlocal exchange-correlation effects within a local current description. Recently a brief account was given of the application of the Vignale–Kohn current-functional [G. Vignale and W. Kohn, Phys. Rev. Lett. 77, 2037 (1996)] to the axial polarizability of oligomer chains [M. van Faassen, P. L. de Boeij, R. van Leeuwen, J. A. Berger, and J. G. Snijders, Phys. Rev. Lett. 88, 186401 (2002)]. With the exception of the model hydrogen chain, our results were in excellent agreement with best available wavefunction methods. In the present work we further outline the underlying theory and describe how the Vignale–Kohn functional was implemented. We elaborate on earlier results and present new results for the oligomers of polyethylene, polysilane, polysilene, polymethineimine, and polybutatriene. The adiabatic local density approximation gave good results for polyethylene, which were slightly modified by the Vignale–Kohn functional. In all other cases the Vignale–Kohn functional gave large improvements upon the adiabatic local density approximation. The Vignale–Kohn results were in agreement with best available data from wave function methods. We further analyze the hydrogen chain model for different bond length alternations. In all these cases the Vignale–Kohn correction upon the adiabatic local density approximation was too small. Arguments are given that further improvements of the functional are needed.
The H2–H complex: Interaction-induced dipole surface and infrared absorption spectra
Authors: Gustafsson Magnus , Frommhold Lothar , Meyer Wilfried
Published: January 2003

A quantum chemical dipole surface of interacting H2–H pairs is obtained and collision-induced absorption spectra are computed for temperatures from 200 to 1000 K and frequencies from 0 to 6000 cm−1. The effect of the anisotropy of the potential energy is investigated and turns out to be almost negligible at the temperature for which a close-coupled quantum calculation was done. The smallness of the effect stems from the short range character of the anisotropic potential components for H2–H. Accordingly the isotropic potential approximation could be applied in most of the present calculations. The accuracy of the dipole surface as well as that of the ab initio potential energy surface that enters the calculations of the spectra are believed to permit prediction of absolute spectral intensities with an accuracy in the 5% range.
Optical and mechanical characteristics of nanocrystalline boron carbonitride films synthesized by plasma-assisted physical vapor deposition
Authors: Cao Z. X. , Oechsner H.
Published: January 2003

Nanocrystalline boron carbonitride thin films were prepared using the electron-cyclotron- wave-resonance plasma-assisted deposition, whereby the energy for precursor ions was adjusted between 70 and 180 eV. Fourier-transform infrared spectroscopy confirmed the presence of ternary sp3-bonded structure and high-resolution transmission electron microscopy revealed a punctured lattice. The deposits unfold grains of about 200 nm in dimension under atomic force microscope, yet they exhibit extremely flat surfaces with a root-mean-square roughness less than 3 nm. For a 1.2 μm thick film, the transmittance in the visible light range is as high as 80%. The Vicker’s hardness measures over 28 GPa. Therefore, this ternary material is competitive to nanocrystalline diamond in application as protective coatings for optical components. Remarkably, also very strong photoluminescence peaked at 430 nm was detected in the as-deposited films at room temperature.
Predictive model for CO2 generation and decay in building envelopes
Authors: Aglan Heshmat A.
Published: January 2003

Understanding carbon dioxide generation and decay patterns in buildings with high occupancy levels is useful to identify their indoor air quality, air change rates, percent fresh air makeup, occupancy pattern, and how a variable air volume system to off-set undesirable CO2 level can be modulated. A mathematical model governing the generation and decay of CO2 in building envelopes with forced ventilation due to high occupancy is developed. The model has been verified experimentally in a newly constructed energy efficient healthy house. It was shown that the model accurately predicts the CO2 concentration at any time during the generation and decay processes.
Exciton localization and quantum efficiency—A comparative cathodoluminescence study of (In,Ga)N/GaN and GaN/(Al,Ga)N quantum wells
Authors: Jahn U. , Dhar S. , Brandt O. , Grahn H. T. , Ploog K. H. , Watson I. M.
Published: January 2003

We study the quantum efficiency (η) and transition energy (Et) as a function of excitation density and temperature in (In,Ga)N/GaN multiple quantum wells (MQWs) fabricated by molecular-beam epitaxy (MBE) and metal-organic chemical-vapor deposition (MOCVD), as well as in an MBE-grown GaN/(Al,Ga)N MQW. A method based on cathodoluminescence spectroscopy is proposed to be suitable for a reproducible measurement of the power dependence of η and Et. The experimental results are fit to a recently developed model allowing for a distinction of localization and electric-field effects for η and Et, as well as for the extraction of the localization energy, density of localization centers, and radiative recombination rate of localized excitons. In the (In,Ga)N/GaN MQWs grown by MBE and MOCVD, we found a value of the localization energy of 34 and 100 meV, respectively. In the MBE-grown GaN/(Al,Ga)N MQW, the exciton recombination is dominated by quasifree excitons even at low temperatures.
Internal waves generated from a turbulent mixed region
Authors: Dohan K. , Sutherland B. R.
Published: January 2003

Using two nonintrusive visualization methods, laboratory experiments are performed to examine the internal wave field underlying a turbulent region generated by a vertically oscillating grid. The first method uses dye-lines to mark the vertical motions of isopycnal layers and the second uses “synthetic schlieren” to visualize the entire wave field. In a range of experiments, the strength of the stratification is varied so that the buoyancy frequency N=0.33–1.40 s−1. In all cases, large tank-scale standing wave modes are established which last throughout the experiment. The amplitudes of the isopycnal lines, Aξ, follow a power law relation Aξ∼N−1.5. The synthetic schlieren technique allows us to visualize turbulent eddy-scale waves and to isolate the properties of the strongest downward propagating waves at the base of the turbulent layer. These waves have a surprisingly narrow range of frequencies and vertical wavenumbers. The angles of wave propagation from the vertical for the dominant waves lie in the range Θ=42°–55°. The amplitudes in the wave field follow a relation Aξ∼N−1.68. These waves are of large amplitude: their vertical displacement is from 2% to 4% of their horizontal wavelength, a significant fraction of the breaking amplitude.
Formation of Au0.6Ge0.4 alloy induced by Au-ion irradiation of Au/Ge bilayer
Authors: Som T. , Ayyub P. , Kabiraj D. , Kulkarni N. , Kulkarni V. N. , Avasthi D. K.
Published: January 2003

We report on the formation of a-axis oriented Au0.6Ge0.4 alloy on a Si(100) substrate on 120 MeV Au-ion irradiation of a Au/Ge bilayer and subsequent vacuum annealing at 360 °C. Irradiation-induced changes occurring across the Au/Ge interface were studied using Rutherford backscattering spectrometry. Phase identification was done by x-ray diffraction and the surface morphology of the samples was studied by scanning electron microscopy. Formation of oriented Au0.6Ge0.4 alloy was confirmed by transmission electron microscopy and discussed on the basis of swift heavy ion induced effects followed by thermal annealing.
High-current capillary discharge with prepulse ablative plasma
Authors: Levin M. , Marcus G. , Pukhov A. , Zigler A. , Sasorov P.
Published: January 2003

Generation of axisymmetric stable, long plasma channels with temperatures of 8 eV and electron densities ∼1019 cm−3 by a high-current evaporating-wall capillary discharge with prepulse ablative plasma is reported. Results of spectroscopic measurements of the temperature and electron density of plasma produced in a polyethylene capillary are presented. The discharge provides a convenient source of dense highly ionized plasmas for laser-plasma interaction studies.
Thermoelectric properties of icosahedral quasicrystals: A phenomenological approach
Authors: Maciá Enrique
Published: January 2003

In this work, we introduce a phenomenological model describing the thermoelectric power of icosahedral quasicrystals. On the basis of a realistic model for the spectral conductivity, obtained from ab initio band-structure calculations [C. Landauro and H. Solbrig, Physica B 301, 267 (2000)], we derive a closed analytical expression for the Seebeck coefficient, satisfactorily describing its temperature dependence S(T) over a wide temperature range. We introduce four phenomenological coefficients relating the electronic structure to characteristic features of the experimental S(T) curves. By comparing our analytical results with available experimental data we relate the sensitivity of the thermopower curve to minor variations in the chemical composition to a systematic shift of the Fermi-level position.
Contact angle measurements for adhesion energy evaluation of silver and copper films on parylene-n and SiO2 substrates
Authors: Gadre Kaustubh S. , Alford T. L.
Published: January 2003

Copper and silver films are being considered for future multilevel interconnect systems. The reduction of feature size has also demanded the use of different low-dielectric materials (e.g., parylenes) in place of conventional silicon dioxide based layers. Adhesion of these materials with each other is a major hurdle in the reliable and durable performance of the devices. Contact angle measurements are used to measure adhesion energies of Cu and Ag layers on substrates of either SiO2 and Pa–n. Qualitative tape-test analysis indicates improved adhesion of these films with anneal and plasma treatment. Surface energy increase of parylene–n using oxygen plasma treatment is demonstrated using sessile water-drop method. The increase in adhesion for the Ag/Pa–n system is attributed to increased roughness and presence of carbonyl groups on the surface. The contact angle measurements are corrected to compensate for the effect of roughness. The adhesion energy for Ag/Pa–n system increases from 0.33 to 1.28 N/m with plasma treatment. Higher-surface energies of copper at room temperature attribute to higher-copper adhesion energy when compared to that of silver.
Characterization of CdTe quantum dots grown on Si(111) by hot wall epitaxy
Authors: Ferreira S. O. , Paiva E. C. , Fontes G. N. , Neves B. R. A.
Published: January 2003

We report on the growth and characterization of CdTe quantum dots on Si(111) by direct island nucleation. The samples were grown by hot wall epitaxy on Si(111) substrates treated with diluted HF solution only and the resulting hydrogen-terminated surface permitted the growth of good quality CdTe layers using substrate temperatures below 300 °C. The samples, grown at very low growth rates, were investigated by atomic force microscopy. Our measurements show that this system follows the Volmer-Weber growth mode, with nucleation of isolated CdTe islands on the Si substrate surface even for just 0.6 monolayers of evaporated material. As the growth proceeds, the density and size of quantum dots increase until the point at which they start to coalesce to form a uniform layer. We describe the size and density distribution of these islands as a function of growth time and substrate temperature. The results show that the Volmer-Weber growth mode can be successfully used to obtain self-assembled quantum dots of CdTe on Si, with reasonable size dispersion, using an inexpensive growth technique.
Size-dependent elastic moduli of platelike nanomaterials
Authors: Sun C. T. , Zhang Haitao
Published: January 2003

A semicontinuum model is presented for nanostructured materials that possess a platelike geometry, such as ultra-thin films. In contrast to the classical continuum theory, the current model accounts for the discrete nature in the thickness direction. In-plane Young’s modulus, and in-plane and out-plane Poisson’s ratios are investigated with this model. It is found that the values of the Young’s modulus and Poisson’s ratios depend on the number of atom layers in the thickness direction and approach the respective bulk values as the number of atom layers increases.
m-line spectroscopy for optical analysis of thick LiNbO3 layers grown on sapphire substrates by radio-frequency multistep sputtering
Authors: Dogheche E. , Lansiaux X. , Remiens D.
Published: January 2003

Thick films of lithium niobate (LiNbO3) have been epitaxially grown on sapphire substrates by multistep radio-frequency magnetron sputtering. We have investigated the optical properties of the deposited layers by the m-line spectroscopy technique. The relationship between the film-substrate interface, the surface morphology of the LiNbO3 layer, and the optical measurements has been carefully established. Since this optical technique is sensitive to material defects, a detectable degradation of the optical data is observed when the microstructure of the LiNbO3 film changes. In particular, the angular width of the excited optical guided modes (Δθ) varies with the film thickness. This agrees perfectly with the evolution of the surface roughness as a function of the film thickness. We report a surface roughness rms of 2 nm corresponding to an angular width Δθ of 0.1° for the TE0 mode. The accuracy and the sensitivity of this technique can be advantageously compared to the classical tool of atomic force microscopy.
Magnetic properties enhancement of Nd2Fe14B/α–Fe nanocomposites with a combined addition of Cu and Ti
Authors: Yang S. , Song X. P. , Liu X. S. , Li S. D. , Qin W. , Gu B. X. , Du Y. W.
Published: January 2003

The effect of combined Cu and Ti additions on the magnetic properties and phase transformation behavior of Nd8Fe86B6 nanocomposites prepared by melt spinning have been investigated. For pure ternary Nd8Fe86B6 ribbons, the α–Fe phase precipitates directly from amorphous phase and its grains grow quickly during crystallization. But for Nd8Fe86B6 ribbons with a combined addition of Cu and Ti, the crystallization temperature is increased by about 30 °C and a metastable hexagonal phase with a TbCu7-type structure is formed prior to the α–Fe and Nd2Fe14B phase. When the annealing temperature is increased, a metastable 1:7 phase is decomposed into the α–Fe and Nd2Fe14B phase. At the same time, the growth of α–Fe grains is suppressed and the microstructure becomes more uniform. When the content of Cu reaches 1.0 at. % and that of Ti reaches 0.5 at. %, the optimal magnetic properties Hci=4.8 kOe, σr=110 emu/g, (BH)max=15 MGOe were obtained after annealing at the temperature of 650 °C for 8 min. So, a combined addition of Cu and Ti is very effective in improving the magnetic properties and reducing grain size and, in particular, the size of α–Fe grains.
Semiflexible random A–B block copolymers under tension
Authors: Debnath Pallavi , Cherayil Binny J.
Published: January 2003

In this paper we explore the conformational properties of random A–B block copolymers with semiflexible segments in a constant force field as a model of the behavior of biopolymers under tension. The degree of semiflexibility of individual blocks, which is characterized by a persistence length l, may range from values that correspond to complete flexibility of the block to values that correspond to nearly complete rigidity. The distribution of blocks is described by a two-state Markov process that generates the statistics governing the process of living polymerization in the steady state. Force-extension curves for this model of the polymer are calculated as an average over the chains conformations (which are described by a finitely extensible Gaussian model), as well as over both quenched and annealed distributions of the sequence of A and B blocks along the chain backbone. The results are sensitive to the nature of the sequence distribution: in the annealed case, the force-extension curves are sigmoidal for essentially all values of the parameters that characterize the conformational properties of the A and B blocks and their statistical correlations, while in the quenched case, the curves exhibit plateau regions that are reminiscent of various kinds of transitions in proteins and DNA.
Monte Carlo study of high-field carrier transport in 4H-SiC including band-to-band tunneling
Authors: Hjelm M. , Nilsson H-E. , Martinez A. , Brennan K. F. , Bellotti E.
Published: January 2003

A full-band ensemble Monte Carlo simulation has been used to study the high-field carrier transport properties of 4H-SiC. The complicated band structure of 4H-SiC requires the consideration of band-to-band tunneling at high electric fields. We have used two models for the band-to-band tunneling; one is based on the overlap test and the other on the solution of the multiband Schrödinger equations. The latter simulations have only been performed for holes in the c-axis direction, since the computer capacity requirement are exceedingly high. Impact-ionization transition rates and phonon scattering rates have been calculated numerically directly from the full band structure. Coupling constants for the phonon interaction have been deduced by fitting of the simulated low-field mobility as a function of lattice temperature to experimental data. Secondary hot electrons generated as a consequence of hole-initiated impact ionization are considered in the study for both models of band-to-band tunneling. When the multiband Schrödinger equation model is used for holes in the c-axis direction, a significant change in the electron energy distribution is found, since the hole impact-ionization rate is very much increased with this model. The secondary electrons increase the average energy of the electron distribution leading to a significant increase in the electron-initiated impact-ionization coefficients. Our simulation results clearly show that both electrons and holes have to be considered in order to understand electron-initiated impact ionization in 4H-SiC.
Heating-rate method for determination of kinetic parameters from thermally stimulated conductivity and luminescence
Authors: Opanowicz A. , Pietrucha P.
Published: January 2003

A method for determining the ratio of the recombination and trapping coefficients and the relative density of inactive deep traps on the basis of simultaneously measured thermally stimulated conductivity (TSC) and luminescence (TSL) is derived in the case of a temperature-dependent free electron lifetime. The method is based on the relation between the preexponential factor of the temperature dependence of the thermally stimulated free electron density (TSC) and the occupancy of active traps. Values of the preexponential factor and the trap depth corresponding to different values of the relative TSC intensity can be found from the TSC peak measured at different heating rates of the sample. Similar analysis of the TSL peak produces accurate values of the trap depth as well. The method for determination of the trap depth also yields accurate results from multipeak TSC and TSL curves. These results are more accurate than those found using the heating-rate method of Haering and Adams [Phys. Rev. 117, 451 (1960)], the initial-rise method of Garlick and Gibson [Proc. Phys. Soc. (London) 60, 574 (1948)], and the fractional TSL method of Gobrecht and Hofmann [J. Phys. Chem. Solids 27, 509 (1960)].
Ultrathin Ag films on H:Si(111)-1×1 surfaces deposited at low temperatures
Authors: Müller T. , Nienhaus H.
Published: January 2003

The growth, structure, and electronic properties of thin Ag films on H-terminated Si(111) surfaces were investigated with Auger electron and photoelectron spectroscopy (and atomic force and secondary electron microscopy). The films were either evaporated at room temperature (RT) or deposited at low temperature (LT) and subsequently annealed to RT in the thickness range between 1 and 50 monolayers (0.2–12 nm). The LT preparation leads to large Ag islands on a wetting monolayer which form a continuous Ag film above a critical thickness of 30 monolayers. Ultraviolet photoelectron spectra and work function measurements reveal a (111) surface orientation of the Ag islands. In constrast, RT deposition results in Stranski-Krastanov growth of smaller and irregularly shaped islands which do not form a continuous layer even up to film thicknesses of 45 monolayers.
Self-diffusion of boron in TiB2
Authors: Schmidt H. , Borchardt G. , Schmalzried C. , Telle R. , Weber S. , Scherrer H.
Published: January 2003

Self-diffusion studies of boron in polycrystalline TiB2 were carried out as a function of temperature, using a specially designed experiment with stable B10 tracers, B−enriched11 TiB2 samples, and secondary ion mass spectrometry for depth profiling. The diffusivities were extracted from the isotope depth profiles in the range between 950 and 1600 °C. They obey an Arrhenius behavior with an activation enthalpy of about ΔH=2.2 eV and a preexponential factor of D0=4×10−12 m2/s. Interpolation of the diffusivities to the melting point of 3225 °C reveals a very low value of about D(Tm)≈10−15 m2/s, which reflects the covalent bonds present in the material. A possible explanation for the low values obtained for D0 and ΔH is the assumption of a diffusion mechanism via vacancies, where in addition to thermal vacancies a substantial concentration of structural vacancies are present. The possible influence of grain boundaries and of the anisotropic crystal structure on the results is discussed together with crystallographic diffusion paths.
Ordinary refractive index of sapphire in uniaxial tension and compression along the c axis
Authors: Jones S. C. , Robinson M. C. , Gupta Y. M.
Published: January 2003

The ordinary refractive index of sapphire, at 532 nm, was measured in single crystals subjected to elastic, uniaxial tensile (and compressive) strain along the c axis in plate impact experiments. The refractive index under both tension and compression was determined from the difference between the uncorrected particle velocities from velocity interferometer measurements and the known particle velocities in symmetric impact experiments. Results to a peak stress of 100 kbar in tension and compression show that the refractive index has an overall monotonic, nonlinear dependence on density that is asymmetric about the ambient density. In compression, the refractive index decreases by an extremely small amount; in contrast, the change is relatively larger in tension. These refractive index results at large tensile stresses suggest that sapphire polarizability is quite different under compression and tension. Compression measurements by themselves can be approximated well by a linear refractive index–density relation, resulting in a window correction (at 532 nm) that is a constant fraction of the true particle velocity up to at least 115 kbar longitudinal stress. The present data also provide high-precision shock velocity results in compression and demonstrate the appropriateness of using second and third order elastic constants for calculating the elastic response of sapphire under uniaxial tensile strain loading.
InAs/GaAs quantum dot intermixing induced by proton implantation
Authors: Ji Yalin , Lu Wei , Chen Guibin , Chen Xiaoshuang , Wang Qing
Published: January 2003

We have investigated the intermixing effect of multilayer self-assembled InAs/GaAs quantum dots on photoluminescence (PL) spectra. Proton implantation combined with rapid thermal annealing is used to induce intermixing at the interface of InAs and GaAs. Intermixing results in a change of both the optical transition energy and the linewidth of the PL emission peaks. A blueshift up to 94.3 meV is obtained in the PL emission peaks. Our results show that proton implantation is an efficient method to tune the electronic states in self-assembled InAs/GaAs quantum dots.
Dynamic response of ceramic-metal composites: The TiC-Steel system
Authors: Klein B. , Frage N. , Dariel M. P. , Zaretsky E.
Published: January 2003

The dynamic response of a titanium carbide (TiC)–carbon steel, ceramic-metal composite, was studied in planar impact experiments, using a copper impactor with velocity in the 80–450 m/s range. The composites were prepared by pressureless infiltration of TiC ceramic preforms by molten steel. The metallic component had either a pearlitic or a martensitic microstructure, determined by an appropriate heat treatment. Fully dense composites, consisting of TiC and 1060 steel, in pearlitic and martensitic states, were used as reference samples. Values of the Hugoniot elastic limit and of the spall strength were derived from the velocity interferometer system for any refractor records of the free surface velocity profiles of the impacted samples. These properties are affected drastically by the confining stress that is induced in the TiC particles by the steel submatrix and is dependent on the microstructure of the latter. The results show unambiguously that the dynamic response of the cermets may be controlled by choosing an appropriate thermal treatment.
Arrays of (Zn,Mn)S quantum wires with well-defined diameters below 10 nm
Authors: Chen L. , Klar P. J. , Heimbrodt W. , Brieler F. J. , Fröba M. , Krug von Nidda H.-A. , Kurz T. , Loidl A.
Published: January 2003

Zn 1−x Mn x S with x=1% to 30% were formed inside the ordered pore systems of different mesoporous SiO2 matrices. Due to the highly ordered host structure, regular arrays of Zn1−xMnxS quantum wires with diameters of 3.1 nm and 5.6 nm, respectively, separated by 2 nm SiO2 barriers were obtained. The wires were characterized using photoluminescence (PL), PL excitation (PLE) spectroscopy, and electron paramagnetic resonance (EPR). The T14→A16 internal transition of the Mn2+(3d5) ions dominates the PL. The corresponding PLE spectra show higher internal Mn transitions and the band-to-band transition. EPR spectra and the energies of the internal Mn transitions are typical for Mn2+ on a cation site of (II,Mn)VI semiconductors. The crystal field parameters indicate that the wires are tensilely strained. Due to the comparable band gaps of the SiO2 and the Zn1−xMnxS and the small exciton Bohr radius in (Zn,Mn)S, the confinement effects in the wires are less than 150 meV.
Low dielectric constant a-SiOC:H films as copper diffusion barrier
Authors: Koh Yee Wee , Loh Kian Ping , Rong Liu , Wee A. T. S. , Huang Liu , Sudijono J.
Published: January 2003

A low-k dielectric barrier based on silicon oxycarbide for copper damascene processes has been developed in this work. The optimal process conditions that allow the deposition of silicon oxycarbide films with a dielectric constant of 3.74 and copper diffusion depth of 290 Å after thermal stress at 400 °C for 3 h has been identified. Copper diffusion depth is defined as the copper and dielectric interfacial region with three-order magnitude reduction in copper concentration. A multilayered structure consisting of black diamond/SiOC/Cu/TaN/Si is fabricated. 3-methyl silane and oxygen in varying concentration is used for the deposition of SiOC using plasma enhanced chemical vapor deposition. The composition of the films is studied by Fourier transform infrared spectroscopy. Dielectric constant and dielectric breakdown of the films are also evaluated. Secondary ion mass spectrometry is employed to investigate the copper diffusion property of the films. The electronic component of the dielectric constant has been found to be most significant in affecting the overall dielectric constant in SiOC films.
dc and microwave resistivities of SrRuO3 films deposited on SrTiO3
Authors: Rundqvist P. , Vorobiev A. , Gevorgian S. , Khamchane K. , Ivanov Z.
Published: January 2003

Epitaxial SrRuO3 thin films on (001) SrTiO3 substrates grown by pulsed laser ablation at deposition temperatures ranging between 690° and 810 °C have been examined. The correlations between dc and microwave resistivity, microstructure, and surface morphology are obtained. According to the x-ray measurements the structure of all films is a mixture of highly oriented domains of strained SrRuO3 phases with different lattice parameters. The films grown at 780 °C consist mainly of orthorhombic phase I with a bulk lattice parameter close to the orthorhombic single crystal and reveal same resistivity values of 270 μΩ cm for both dc and microwave. Films obtained at other deposition temperatures contain a larger amount of orthorhombic phase II, with extended bulk lattice parameters and dc resistivity increases up to 1700 μΩ cm for the lowest deposition temperature. The corresponding microwave resistivity values are 50% higher than the dc ones. The dc resistivity of SrRuO3 films is in inverse proportion to the intensity of the (220) refection from orthorhombic phase I. The orthorhombic phase I domains reveal higher crystallinity (FWHM values are about 0.2°) and lower out-of-plane strain (0.4%) than corresponding values of the orthorhombic phase II domains (0.35° and 1.6%, respectively). According to AFM measurements the SrRuO3 films consist of a network of distinguishable islands separated by film areas with lower thickness. The islands consist mainly of orthorhombic phase II. At deposition temperatures more than 780 °C the film roughness is mainly due to holes formation caused, probably, by grain rotation and reevaporation of SrO chains. The difference between dc and microwave resistivity values is explained by influence of the film roughness.
Acoustically induced optical second harmonic generation in Pb4.7Ba0.3Ge3O11 crystals
Authors: Kityk I. V. , Zmija J. , Majchrowski A. , Ebothe J.
Published: January 2003

Acoustically induced optical second harmonic generation (AIOSHG) in ferroelectric Pb4.7Ba0.3Ge3O11 (PBG) crystal has been observed. With increasing acoustical power, the AIOSHG for Nd doped yttrium–aluminum garnet laser light (λ=1.06 μm) increases and achieves its maximum value at acoustical power density of about 1.75 W/cm2. The values of the AIOSHG for the PGB are considerably higher than for other photorefractive ferroelectrics. With decreasing temperature, the AIOSHG signal strongly increases below 29 K and correlates well with a ferroelectric phase transition detected by the differential scanning calorimetric method and spontaneous polarization measurements. The AIOSHG maxima were observed at acoustical frequencies lying within the ranges 12–17, 22 to 23, and above 26 kHz. Comparing the obtained results with the acoustically induced Raman spectra at different temperatures one can conclude that the effect is caused prevailingly by acoustically induced electron–phonon anharmonicity, which is larger in the vicinity of the ferroelectric phase transition.
Pulsed laser deposition of diamondlike carbon films on polycarbonate
Authors: Bonelli Marco , Miotello Antonio , Mosaner Paolo , Casiraghi Cinzia , Ossi Paolo M.
Published: January 2003

Diamondlike carbon films have been deposited on polycarbonate by pulsed laser deposition technique by irradiating highly oriented pyrolytic graphite with high-energy excimer laser pulses (248 nm wavelength, 20 ns duration, and up to 37 J/cm2 energy density). Irradiations were performed in different atmospheres: (1) moderate vacuum (10−2 Pa), (2) nitrogen atmosphere (1 Pa), and (3) argon atmosphere (1 Pa). The structure of the deposited films was analyzed with Raman spectroscopy. In vacuum-deposited films, a transition from mainly disordered graphitic carbon to up to 80% ta−C occurs above a laser energy density threshold of 7 J/cm2. No such transition was observed in films deposited in nitrogen up to energy densities as high as 33 J/cm2. In argon atmosphere the transition is only observed at high-laser energy density, ≈23 J/cm2. The results are discussed in terms of combined ballistic and chemical effects affecting both plume dynamics and bonding configuration of the growing film. The Fourier transform infrared spectroscopy showed that films deposited in nitrogen atmosphere contain nitrogen and hydrogen, thus becoming unstable when exposed to air. The hardness of ta−C coated polycarbonate, as measured by nanoindentation technique, is about 8 GPa, while when disordered graphitic carbon was deposited on the substrates hardness does not exceed 5 GPa. Measurement of the internal stress in the deposited films suggests that atomic relaxation occurs for laser-pulse energies exceeding 10 J/cm2, with residual stress values of the order of 1 GPa in films deposited in vacuum. In these conditions, given the strong interfacial chemical bonds, the adhesion strength is very high. In films deposited in nitrogen atmosphere the internal stress is almost constant for pulse laser energy up to 20 J/cm2 while it slowly increases at higher energies.
Design and evaluation of omnidirectional one-dimensional photonic crystals
Authors: Lee Hyun-Yong , Yao Takafumi
Published: January 2003

We have theoretically and experimentally studied a wide range of realistic fabrication parameters for the formation of omnidirectional photonic band gaps (PBGs) in one-dimensional photonic crystals (1D PCs). Photonic band structures in 1D PCs have been analyzed for the cases with and without a defect layer. It is shown that a defect state in the defect-mode PBG behaves like an electronic impurity level in the gap with a change in thickness (i.e., optical path) of the defect layer. Three types of 1D PC structures (Te/polystyrene, Si/SiO2, and ZnS/SiO2), which are designed for applications in the infrared region (especially for the wavelength of 1.55 μm) and in the visible region, are proposed and demonstrated their high reflectance (R) properties experimentally. For example, a six-pair Si/SiO2 1D PC fabricated with a structure parameter (period=388.5 nm, filling factor=0.406) exhibits a wide range omni-PBGs in the wavelength range of 1310–1742 nm. The measured R spectra are in very good agreement with calculated results. In particular, the R spectra of a ZnS/SiO2 1D PC in the wavelength range of 190–800 nm are almost the same as those calculated using complex refractive indices. We also propose conditions to obtain omni-PBGs and band structures for a high refractive-index ambient medium (n).
Effects of lateral quantum dot pitch on the formation of vertically aligned InAs site-controlled quantum dots
Authors: Yang Tao , Kohmoto Shigeru , Nakamura Hitoshi , Asakawa Kiyoshi
Published: January 2003

We discuss the effects of lateral quantum dot pitch on the formation of vertically aligned InAs site-controlled quantum dots (SCQDs) separated by thin spacers on GaAs substrates. Bilayers of InAs SCQD square arrays of various quantum dot-array pitches (100 nm, 75 nm, 50 nm, and 40 nm) are fabricated based on nanometer-scale site-control techniques and self-organizing epitaxy. In situ scanning tunneling microscope observations reveal that the vertical pairing probability between the two layers of SCQDs decreases with decreasing pitch, with a dramatic reduction when the pitch falls below 75 nm. Moreover, room-temperature μ-photoluminescence measurements show that the linewidth of the bilayers of vertically aligned SCQDs increases with declining pitch. Again, we found a remarkable increase in photoluminescence linewidth when the pitch falls below 75 nm. The observed results are attributed to lateral interactions between elastic strain fields induced by the first layer of buried InAs SCQDs.
Systematic analysis and control of low-temperature GaN buffer layers on sapphire substrates
Authors: Sumiya M. , Ogusu N. , Yotsuda Y. , Itoh M. , Fuke S. , Nakamura T. , Mochizuki S. , Sano T. , Kamiyama S. , Amano H. , Akasaki I.
Published: January 2003

The growth of low-temperature (LT) GaN buffer layers on sapphire substrates was systematically studied using x-ray photoelectron spectroscopy with regards to processes such as substrate treatment and deposition conditions, along with annealing treatments of the GaN buffer layer during two-step metalorganic chemical vapor deposition. Variations observed in the LT-buffer layer depended strongly on both the chemical state of the sapphire surface as a result of the substrate treatment and the subsequent annealing conditions. A 20 nm buffer layer on non-nitrided sapphire evaporated after the formation of islands during the conventional annealing process (N2, H2, and NH3 gas mixture). Adding H2 gas to the annealing ambient enhanced the evaporation and reduced the surface coverage. It was found that AlxGa1−xN was formed at the interface, which has a low evaporation coefficient. In contrast, a buffer layer deposited onto a nitrided sapphire substrate evaporated completely in a layer-by-layer mode. The buffer layer contained domains with N face (−c) polarity that were almost covered with a Ga face (+c) layer. It was found that using Ga-rich conditions (a lower V/III ratio) for the deposition suppressed the formation of the −c domains, even on the nitrided sapphire. High temperature (HT) GaN layers were deposited on these well-defined LT-buffer layers. The influence of the various conditions used in preparing the LT-buffer layer on the HT-GaN layers are discussed in terms of the crystalline quality and the polarity of the HT-GaN layers.
An ab initio study of the pathways for the reaction between CH3O2 and BrO radicals
Authors: Guha Sujata , Francisco Joseph S.
Published: January 2003

The pathways for the reaction between methylperoxy (CH3O2) and bromine monoxide (BrO) radicals have been examined by using the quadratic configuration interaction method. It is found that the most feasible pathway of the CH3O2+BrO reaction is the formation of CH3OOOBr as an intermediate during the reaction and its dissociation into CH2O and HOOBr. This study finds that besides CH3OOOBr, CH3OOBrO may also exist as an intermediate complex during the CH3O2+BrO reaction.
Multireference configuration interaction calculations of some low-lying states of positronium hydride
Authors: Saito Shiro L.
Published: January 2003

Multireference configuration interaction (MRCI) calculations of the ground state S2,1 and low-lying excited states P2,1, D2,1, and S2,3 of positronium hydride (PsH) were carried out, and their total energies and two-photon annihilation rates were presented. Our MRCI calculations gave more compact and accurate wave functions than recent large scale full CI ones, and the effectiveness of the MRCI method for positron-atom complexes was shown. Two extrapolation methods derived from an analysis of the convergence pattern of the total energies and two-photon annihilation rates with respect to angular momentum of orbitals were attempted in order to estimate full CI limit of those values. As a result, our reliable extrapolation method gave −0.788 642, −0.732 826, −0.724 642, and −0.739 829 a.u. for the total energies of the S2,1, P2,1, D2,1, and S2,3 states of PsH, respectively. For the two-photon annihilation rates of the respective states, 2.1078, 1.5702, 1.0980, and 1.0784 ns−1 were obtained, respectively.