bluuebunny/crossref_metadata_2025 huggingface dataset:
Band gap of amorphous and well-ordered Al2O3 on Ni3Al(100)
Authors: Costina I. , Franchy R.
Published: July 2002

The vibrational and electronic properties of amorphous and well-ordered alumina formed on Ni3Al(100) were investigated using high-resolution electron energy loss spectroscopy. The structure of well-ordered alumina was analyzed by low-energy electron diffraction. The amorphous Al2O3 films are prepared by adsorption of O2 at room temperature, while the well-ordered Al2O3 are obtained by direct oxidation of Ni3Al at 1150 K. The band gap energy is ∼3.2 and ∼4.3 eV for amorphous alumina and well-ordered alumina thin films respectively. The lowering of the band gap with respect to the bulk value of Al2O3 is associated with defect-induced states located in the band gap.
Uncrossability constraints in mesoscopic polymer melt simulations: Non-Rouse behavior of C120H242
Authors: Padding J. T. , Briels W. J.
Published: July 2002

An important feature of a melt of long polymers is that the bonds of the chains cannot cross each other. This seemingly simple fact has a great impact on the long time dynamics and rheology of the material. In this paper an algorithm is described that explicitly detects and prevents bond crossings in mesoscopic simulations of polymers. The central idea is to view the bonds as slippery elastic bands which can become entangled. The method is applied to a simulation of a coarse-grained melt of C120H242, in which each chain is represented by six blobs. The long time dynamics and zero-shear rate rheology are investigated and the relative importance of uncrossability and chain stiffness is established. As a result of the uncrossability of the chains, we observe a subdiffusive exponent in the mean square displacement of the chains, a stretching of the exponential decay of the Rouse mode relaxations, an increase of relaxation times associated with large scales, and a slowing down of the relaxation of the dynamic structure factor. These results are in agreement with results from previous microscopic molecular dynamics simulations. Finally, an increased viscosity as compared to the Rouse model is observed, which is attributed to slowly decaying interchain stress components.
Thermal expansion and elastic properties of InN
Authors: Wang Kai , Reeber Robert R.
Published: July 2002

The thermal expansion coefficients of wurtzite structure InN are evaluated within the constraints of a basic model and predicted for an extended temperature range. Together with the elastic constants provided earlier, this information gives a basis for optimizing thin-film growth conditions and thereby reducing the residual stresses in group-III-nitride thin-film devices.
Tailoring the temperature coefficient of capacitance in ferroelectric varactors
Authors: Gevorgian S. , Petrov P. K. , Ivanov Z. , Wikborg E.
Published: July 2002

Two BaxSr1−xO3, films with different contents of Ba (Ba0.25Sr0.75O3 and Ba0.75Sr0.5O3), separated by a MgO film, are used to fabricate planar varactors with low temperature coefficient of capacitance (TCC), high tunability and low losses. The TCC of the varactors are small in the temperature interval between the dielectric permittivity peaks of the Ba0.25Sr0.75O3 and Ba0.75Sr0.5O3 films.
Quantum physical systems as classical systems
Authors: Cassa Antonio
Published: July 2002

A physical system showing a classical (deterministic) behavior to an observer can appear to be a quantum system to another observer unable to distinguish between some distinct states.
Magnetically switched quantum waveguide qubit
Authors: Harris J. , Akis R. , Ferry D. K.
Published: July 2002

Recently, quantum computing has gained notoriety as a computational method that has the possibility to greatly increase the speed of calculations compared with traditional, binary computing. To this end, simulations of a quantum controlled-not gate implemented by a quantum waveguide are examined. The waveguide is composed of two quantum wires and a tuned coupling region and the simulations consist of a solution to Schrödinger’s equation via the Usuki method of mode matching. The use of Gaussian wave packets in previous work on this topic is questioned and various methods of tuning the coupling region in order to guide the quantum wave are investigated. Simulations are shown for a tuned waveguide in which the wave can be switched by the application of an external magnetic field.
On the structure of Picard–Fuchs type equations for Liouville–Arnold integrable Hamiltonian system on cotangent phase spaces
Authors: Samoilenko Anatoliy M. , Prykarpatsky Anatoliy K. , Taneri Ufuk
Published: July 2002

There are studied in detail the structure properties of integral submanifold imbedding mapping for a class of algebraically Liouville integrable Hamiltonian systems on cotangent phase spaces and related with it so called Picard–Fuchs type equations. It is shown that these equations can be in general regularly constructed making use of a given a priori system of involutive invariants and proved that their solutions in the Hamolton–Jacobi separable variable case give rise exactly to the integral submanifold imbedding mapping being as known a main ingredient for Liouville–Arnold integrability by quadratures of the Hamiltonian system under regard.
Nano-dewetting: Interplay between van der Waals- and short-ranged interactions
Authors: Müller M. , MacDowell L. G. , Müller-Buschbaum P. , Wunnike O. , Stamm M.
Published: July 2002

The wetting behavior of a polymer liquid on a coated substrate is investigated via numerical self-consistent field calculations and experiments. The polymer does not wet the coated substrate, but the substrate might stabilize a mesoscopic film. Experiments observe a stable mesoscopic film of nanometer thickness in coexistence with macroscopic drops at high temperatures. Upon cooling, this mesoscopic polymer film breaks up into droplets (nano-dewetting). Our self-consistent field calculations suggest that the stability of the mesoscopic film is determined by a subtle interplay between van der Waals forces and short-ranged forces due to the distortion of the profile in the vicinity of the substrate.
Transverse field dependence of the planar Hall effect sensitivity in Permalloy films
Authors: Jen S. U. , Lee J. Y. , Yao Y. D. , Chen W. L.
Published: July 2002

A series of Permalloy films, of various shapes such as a square, a rectangle, a circle, and a rhombus, was made. The sample length to width ratio L/w varied from 1 (i.e., square) to 29 (i.e., rectangle). We studied how the planar Hall effect (PHE) signal was affected by adding a transverse field Hy along the easy-axis direction of the sample. It was found that in a certain range of Hy, the PHE sensitivity S might become inoperative, i.e., S changed linearly as a function of Hy from −Smax to +Smax (or vice versa), where Smax was the maximum sensitivity. This phenomenon is explained by considering the Zeeman-energy and the single-domain-rotation effects. In particular, for the square sample, the following data exist: (1) Smax is as high as 310 Ω/T at the film thickness t=500 Å, and (2) the inoperative range for Hy is the narrowest among all the samples. From this study, we conclude that the field of the Earth He—as long as its horizontal component is larger than 0.25 Oe—can be employed to stabilize the magnetic structure of a Permalloy element and to achieve the best PHE performance with Smax.
Bulk electronic structure of SrTiO3: Experiment and theory
Authors: van Benthem K. , Elsässer C. , French R. H.
Published: July 2002

Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3. Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states. The experimentally determined indirect band gap energy is 3.25 eV, while the direct band gap energy is 3.75 eV. The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV. The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV. The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 3.25 eV up to 15 eV. The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV. The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV. Interband transitions from the O 2s band to the conduction bands appear at 26 eV. A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons.
Does a magnetic barrier or a magnetic-electric barrier structure possess any spin polarization and spin filtering under zero bias?
Authors: Xu H. Z. , Okada Y.
Published: July 2002

In this letter, we have clarified that there is no spin polarization and spin filtering in a magnetic barrier structure as well as in a magnetic–electric barrier structure using our explicit expressions for electron transmission probability. Our results are found to be contradictory to those of A. Majumdar [Phys. Rev. B 54, 11911 (1996)] and G. Papp and F. M. Peeters [Appl. Phys. Lett. 78, 2184 (2001)]. We have shown the significant spin polarization and spin filtering observed by these authors were caused by a mistake in their transmission probability.
Variation with the intermolecular distance of properties dependent on the electron density in hydrogen bond dimers
Authors: Gálvez O. , Gómez P. C. , Pacios L. F.
Published: July 2002

The variation with the intermolecular distance of features in hydrogen bond (HB) dimers dependent on the electron density ρ(r) are studied in four complexes representative of weak/medium HB interactions. Topological properties, energy densities and integrated atomic properties are obtained with ρ(r) of dimers at B3LYP/6-311++G(d,p) optimized structures obtained upon fully relaxing the geometry of monomers. The dependence of A–H⋯B bond properties on intermolecular R(H⋯B) distances allows to characterize the nature of the interaction as monomers move nearer from infinite separation. At long distances the interaction is only electrostatic while for separations about 1 Å larger than the equilibrium distance Req, quantum effects arising from ρ(r) begin to dominate. In the immediate neighborhood of Req the interaction is mainly led by the stabilization of the H-donor due in turn to energy lowerings in A and B atoms associated to polarization effects. The mutual penetration of electron densities of donor and acceptor monomers provokes a considerable reduction of atomic volumes for H and B atoms which reveals in the form of redistribution rather than transfer of charge. This range of distances exhibits noncovalent bond features but shortly after, when monomers approximate a few tenths of Å below Req, characteristics typical of covalent interactions begin to appear while the rate of change of all the ρ(r)-dependent properties increases rapidly.
Rotation in collisional strongly coupled dusty plasmas in a magnetic field
Authors: Kaw Predhiman K. , Nishikawa Kyoji , Sato Noriyoshi
Published: July 2002

Recent experiments on strongly coupled plasmas in a magnetic field and a background of neutrals have revealed that the dust component exhibits a rigid rotation of order a fraction of a radian/s. The rotation arises spontaneously and seems to be in the direction of ion rotation (which in turn is determined by a competition between E⃗×B⃗ and diamagnetic rotations of the ions in the collisional magnetized plasma). In this Letter a possible interpretation of these results, based on the use of fluid equations, is offered.
Band alignments in metal–oxide–silicon structures with atomic-layer deposited Al2O3 and ZrO2
Authors: Afanas’ev V. V. , Houssa M. , Stesmans A. , Heyns M. M.
Published: July 2002

The energy barrier height Φ for electrons at the interfaces of various metals (Mg,Al,Ni,Cu,Au) with nanometer-thin Al2O3 and ZrO2 layers grown on (100)Si by atomic layer deposition has been directly measured using internal photoemission of electrons into the insulator. The behavior of the metal/Al2O3 contacts with increasing metal electronegativity XM resembles that of the metal/SiO2 interfaces with ideality factor dΦ/dXM≈1. The metal/ZrO2 contacts exhibit a less ideal behavior with dΦ/dXM≈0.75. The metal–silicon work function differences in structures with Al2O3 and ZrO2 insulators appear to be considerably larger than in the structures with thermally grown SiO2, suggesting the presence of a negative dipole layer at the metal/deposited oxide interface.
Strong acceptor density and temperature dependences of thermal activation energy of acceptors in a Mg-doped GaN epilayer grown by metalorganic chemical-vapor deposition
Authors: Cheong M. G. , Kim K. S. , Kim C. S. , Choi R. J. , Yoon H. S. , Namgung N. W. , Suh E.-K. , Lee H. J.
Published: July 2002

P-type GaN layers were grown on sapphire by metalorganic chemical-vapor deposition and then rapid thermal annealing (RTA) was performed to electrically activate Mg impurities. Varied acceptor densities were obtained by RTA temperature and Mg concentration. Temperature-dependent Hall effects show that the thermal activation energy of the acceptor (EA) is strongly dependent on the acceptor density (NA), approximated by EA(0)=372−1.16×10−18 NA meV at 0 K. A strong temperature dependence of EA was also obtained in this study.
Effect of Ni substitution on the microstructure and coercivity of Sm(CobalFe0.1NiyZr0.04)8.5 magnets
Authors: Tang W. , Zhang Y. , Hadjipanayis G. C.
Published: July 2002

A systematic study has been undertaken to understand the microstructure and coercivity in Ni substituted Sm(CobalFe0.1NiyZr0.04)8.5 magnets using magnetometry and transmission electron microscopy. These samples exhibit a cellular and lamellar microstructure which is typical of Cu substituted Sm(Co,Fe,Cu,Zr)z magnets. However, the coercivity obtained by Ni substitution is very low. With increasing Ni content y from 0.04 to 0.24, the coercivity increases slightly from 0.4 to 1.5 kOe. Thermomagnetic analysis shows that Ni goes into both the Sm2Co17 and SmCo5 phases. This makes it impossible to obtain a large domain wall energy gradient at the interface of the 2:17 and 1:5 phases, leading to low room temperature coercivity. The abnormal temperature dependence of coercivity Hc(T), which is found in R2Co17 magnets consisting of cellular/lamellar microstructure, is also observed in these magnets. A maximum coercivity of 3.6 kOe was obtained at 480 °C in Sm(CobalFe0.1Ni0.18Zr0.04)8.5. The abnormal behavior in Hc(T) can be explained by both the domain wall pinning and nucleation models.
Anion solvation at the microscopic level: Photoelectron spectroscopy of the solvated anion clusters, NO−(Y)n, where Y=Ar, Kr, Xe, N2O, H2S, NH3, H2O, and C2H4(OH)2
Authors: Hendricks Jay H. , de Clercq Helen L. , Freidhoff Carl B. , Arnold Susan T. , Eaton Joseph G. , Fancher Chuck , Lyapustina Svetlana A. , Snodgrass Joseph T. , Bowen Kit H.
Published: July 2002

The negative ion photoelectron spectra of the gas-phase, ion-neutral complexes; NO−(Ar)n=1–14, NO−(Kr)1, NO−(Xe)n=1–4, NO−(N2O)n=3–5, NO−(H2S)1, NO−(NH3)1, and NO−(EG)1 [EG=ethylene glycol] are reported herein, building on our previous photoelectron studies of NO−(N2O)1,2 and NO−(H2O)1,2. Anion solvation energetic and structural implications are explored as a function of cluster size in several of these and as a result of varying the nature of the solvent in others. Analysis of these spectra yields adiabatic electron affinities, total stabilization (solvation) energies, and stepwise stabilization (solvation) energies for each of the species studied. An examination of NO−(Ar)n=1–14 energetics as a function of cluster size reveals that its first solvation shell closes at n=12, with an icosahedral structure there strongly implied. This result is analogous to that previously found in our study of O−(Ar)n. Inspection of stepwise stabilization energy size dependencies, however, suggests drastically different structures for NO−(Ar)2 and O−(Ar)2, the former being “Y” shaped, and the latter being linear. While stepwise stabilization energies usually provide good estimates of ion–single solvent dissociation energies, in the cases of NO−(Ar)1, NO−(Kr)1, and NO−(Xe)1, it is possible to determine more precise values. A plot of these anion–solvent dissociation energies shows them to vary linearly with rare gas atom polarizability, confirming the dominance of an ion-induced dipole interaction in these complexes. Extrapolation of this trend permits the estimation of NO−⋯ (rare gas atom) interaction energies for helium, neon, and radon, as well. The relative strengths of the molecular solvents, N2O, H2S, NH3, H2O, and EG are reflected in their stepwise stabilization energies and in the degree of broadening observed in their photoelectron spectra.
Spontaneous acoustic emission from strong shocks in diatomic gases
Authors: Mond Michael , Rutkevich Igor
Published: July 2002

The stability properties of strong shock waves in diatomic ideal gases are investigated. Shock instabilities in diatomic gases may play a role in the dynamics of astrophysical molecular clouds, hypersonic flight applications, and inertial confinement schemes. It is shown that the dissociation processes alone do not give rise to shock instabilities. It is further demonstrated that the shock’s front becomes unstable under spontaneous acoustic emission due to the ionization processes and only for those perturbations that are characterized by thermal nonequilibrium between the electrons and the heavy particles (atoms, ions, and molecules). To show that, the classical Dyakov–Kontorovich stability criterion is modified in order to take into account the effects of the perturbed electronic temperature. Numerical calculations for diatomic nitrogen indicate that the spontaneous acoustic instability occurs on the descending portion of the Hugoniot curve in the density–pressure plane. In addition, it is found that the threshold for spontaneous acoustic instability decreases with the upstream density.
Controlling doping and carrier injection in carbon nanotube transistors
Authors: Derycke V. , Martel R. , Appenzeller J. , Avouris Ph.
Published: July 2002

Carbon nanotube field-effect transistors (CNTFETs) fabricated out of as-grown nanotubes are unipolar p-type devices. Two methods for their conversion from p- to n-type devices are presented. The first method involves conventional doping with an electron donor, while the second consists of annealing the contacts in vacuum to remove adsorbed oxygen. A comparison of these methods shows fundamental differences in the mechanism of the transformation. The key finding is that the main effect of oxygen adsorption is not to dope the bulk of the tube, but to modify the barriers at the metal–semiconductor contacts. The oxygen concentration and the level of doping of the nanotube are therefore complementary in controlling the CNTFET characteristics. Finally, a method of controlling individually the contact barriers by local heating is demonstrated.
Dynamical instabilities in liquid nuclear magnetic resonance experiments with large nuclear magnetization, with and without pulsed field gradients
Authors: Jeener J.
Published: July 2002

In this article we extend previous studies of the dynamical instabilities to incorporate the roles of the tipping angle, the overall shape of the sample, (optional) pulsed field gradients in various directions, and radiation damping. The linear stability for small perturbations is discussed analytically in detail, and numerical simulations are used to have a glimpse at the turbulent spin motion for large perturbations.
Prediction of electromigration failure in passivated polycrystalline line
Authors: Sasagawa Kazuhiko , Hasegawa Masataka , Saka Masumi , Abé Hiroyuki
Published: July 2002

Mechanical stress or atomic density distribution in passivated metal lines is induced by electromigration, and plays an important role in the mechanism of electromigration damage. Recently, a governing parameter for electromigration damage in passivated polycrystalline lines, atomic flux divergence (AFDgen*), was formulated by adding the effect of the atomic density gradient to the governing parameter for electromigration damage in unpassivated polycrystalline lines, AFDgen. The latter has already been utilized to construct a prediction method for electromigration failure in unpassivated lines, as the first step toward the development of a practical method. In this article, a prediction method for electromigration failure in a passivated polycrystalline line is proposed using AFDgen*. It is shown that the proposed method is able to predict the failure location as well as the lifetime of the passivated polycrystalline lines. Both the lifetime and the failure location in a passivated polycrystalline line are predicted by means of numerical simulation of the failure process covering the building up of atomic density distribution, void initiation, void growth, and ultimately—line failure. The usefulness of this method is verified by an experiment where two passivated straight lines with different lengths are treated.
Organic oscillator and adaptive amplifier circuits for chemical vapor sensing
Authors: Crone B. K. , Dodabalapur A. , Sarpeshkar R. , Gelperin A. , Katz H. E. , Bao Z.
Published: July 2002

Organic transistor based circuits that can be employed for chemical vapor sensing, are described. Such circuits have improved sensing characteristics in comparison with discrete transistor based sensors. Complementary ring oscillator based sensors have a stronger response to analytes such as octanol and allyl propionate compared to a single transistor. A fabrication process that combines organic semiconductor circuitry with Si is described. The design and advantages of adaptive differential amplifiers with high gain and feedback are described. Voltage gains of ∼20 allow the detection of weak odorant inputs and the adaptive feedback allows for improved background elimination.
Use of forecasting signatures to help distinguish periodicity, randomness, and chaos in ripples and other spatial patterns
Authors: Rubin David M.
Published: July 2002

Forecasting of one-dimensional time series previously has been used to help distinguish periodicity, chaos, and noise. This paper presents two-dimensional generalizations for making such distinctions for spatial patterns. The techniques are evaluated using synthetic spatial patterns and then are applied to a natural example: ripples formed in sand by blowing wind. Tests with the synthetic patterns demonstrate that the forecasting techniques can be applied to two-dimensional spatial patterns, with the same utility and limitations as when applied to one-dimensional time series. One limitation is that some combinations of periodicity and randomness exhibit forecasting signatures that mimic those of chaos. For example, sine waves distorted with correlated phase noise have forecasting errors that increase with forecasting distance, errors that are minimized using nonlinear models at moderate embedding dimensions, and forecasting properties that differ significantly between the original and surrogates. Ripples formed in sand by flowing air or water typically vary in geometry from one to another, even when formed in a flow that is uniform on a large scale; each ripple modifies the local flow or sand-transport field, thereby influencing the geometry of the next ripple downcurrent. Spatial forecasting was used to evaluate the hypothesis that such a deterministic process—rather than randomness or quasiperiodicity—is responsible for the variation between successive ripples. This hypothesis is supported by a forecasting error that increases with forecasting distance, a greater accuracy of nonlinear relative to linear models, and significant differences between forecasts made with the original ripples and those made with surrogate patterns. Forecasting signatures cannot be used to distinguish ripple geometry from sine waves with correlated phase noise, but this kind of structure can be ruled out by two geometric properties of the ripples: Successive ripples are highly correlated in wavelength, and ripple crests display dislocations such as branchings and mergers.
Self-similarity, renormalization, and phase space nonuniformity of Hamiltonian chaotic dynamics
Authors: Zaslavsky G. M. , Edelman M. , Niyazov B. A.
Published: July 2002

A detailed description of fractional kinetics is given in connection to islands’ topology in the phase space of a system. The method of renormalization group is applied to the fractional kinetic equation in order to obtain characteristic exponents of the fractional space and time derivatives, and an analytic expression for the transport exponents. Numerous simulations for the web-map and standard map demonstrate different results of the theory. Special attention is applied to study the singular zone, a domain near the island boundary with a self-similar hierarchy of subislands. The birth and collapse of islands of different types are considered.
On a simple recursive control algorithm automated and applied to an electrochemical experiment
Authors: Rhode M. A. , Rollins R. W. , Dewald H. D.
Published: July 2002

We review a simple recursive proportional feedback (RPF) control strategy for stabilizing unstable periodic orbits found in chaotic attractors. The method is generally applicable to high-dimensional systems and stabilizes periodic orbits even if they are completely unstable, i.e., have no stable manifolds. The goal of the control scheme is the fixed point itself rather than a stable manifold and the controlled system reaches the fixed point in d+1 steps, where d is the dimension of the state space of the Poincaré map. We provide a geometrical interpretation of the control method based on an extended phase space. Controllability conditions or special symmetries that limit the possibility of using a single control parameter to control multiply unstable periodic orbits are discussed. An automated adaptive learning algorithm is described for the application of the control method to an experimental system with no previous knowledge about its dynamics. The automated control system is used to stabilize a period-one orbit in an experimental system involving electrodissolution of copper.
Diffusion of aluminum in ion-implanted α-Ti
Authors: Räisänen J. , Anttila A. , Keinonen J.
Published: July 2002

The diffusion of Al in polycrystalline ion-implanted α-Ti has been studied in the temperature range 600–850 °C using ion-beam techniques. Diffusion couples were created by ion implantation. The time-dependent concentration profiles were monitored by the use of the nuclear resonance broadening technique through the 27Al(p,γ) 28Si reaction. The effect of the implantation energy and implanted dose on the diffusivity of Al has been investigated. The value of 1.62±0.11 eV for the activation energy and (7.4±9.8)×10−7 cm2/s for the frequency factor was obtained. The present result is discussed in the framework of the Ti diffusion barrier used in semiconductors.
Magnetization process in devitrified glassy alloy
Authors: O’Handley R. C. , Megusar J. , Sun S-W. , Hara Y. , Grant N. J.
Published: July 2002

Co84Nb10B6 metallic glasses have been devitrified so as to produce a series of specimens with increasingly coarser microstructure. Samples annealed for 1 h at Ta≤700 °C remain largely glassy with a distribution of fine cobalt precipitates. Their coercivities increase with mean precipitate diameter. Samples annealed for 1 h at Ta≥700 °C are fully crystallized (fcc Co plus a Co-Nb boride). Their coercivities decrease with increasing mean grain size. An abrupt increase in coercivity occurs between Ta=650 and 700 °C.
Laser ablation of organic polymers: Microscopic models for photochemical and thermal processes
Authors: Garrison Barbara J. , Srinivasan R.
Published: July 2002

Irradiation of organic polymers by short pulses of far-UV (e.g., 193 nm) laser light causes ablative photodecomposition (APD) of the material. This etching process occurs cleanly leaving behind a well-defined pit. Longer wavelength (e.g., 532 nm) laser light also ablates material from a polymeric solid. However, this process is distinct from APD in that the sample near the pit is distorted and melted. Microscopic models are presented here for both the photochemical and thermal processes. The photochemical model predicts that well-defined pits will be formed, that narrow angular distributions of the ablated material should be observed, and that the average perpendicular ejection velocity will be 1000–2000 m/s. The thermal model predicts melting or distortion of the solid and a broad angular distribution of the ejected material.
Time-dependent heat flow calculation of cw laser-induced melting of silicon
Authors: Schvan P. , Thomas R. E.
Published: July 2002

The time evolution of the temperature profile during cw laser processing is calculated including the effects of melting using computer simulation. Results, presented for single-crystal silicon irradiated by an argon ion cw laser, show that high scanning speed can result in a non-steady-state situation for the temperature rise. In this case the maximum temperature and melt depth become functions of the dwell time and also less sensitive to the incident laser power, as a result of which scanning speed can be effectively used to control melting process induced by cw laser. Experimental results presented support the calculated dependence of the maximum melt depth on the laser power and scanning speed.
Flexoelectric properties and optical activity of a nematic liquid crystal
Authors: Barbero G. , Strigazzi A.
Published: July 2002

The rotation of the polarization plane of a polarized light beam through a hybrid aligned liquid-crystal cell is described, by taking into account the joint effect of the weak anchoring and of the flexoelectricity in the presence of an electric field normal to the initial deformation plane and smaller than Fréedericksz’ threshold [H. Kelker and R. Hatz, Handbook of Liquid Crystals (Chemie, Weinheim, 1980), pp. 160–188]. Such a rotation is shown to be proportional to the square root of the difference between the actual and the critical thickness of the cell in the case of small thicknesses (which are interesting in the display technology) whereas in the case of larger thicknesses the same relation deduced by Durand’s group for strong anchoring is obtained.
Diffusion of manganese in silicon studied by deep-level transient spectroscopy and tracer measurements
Authors: Gilles D. , Bergholz W. , Schröter W.
Published: July 2002

The diffusion of manganese in silicon was studied in the temperature range 900–1200 °C by deep-level transient spectroscopy and the tracer method, with particular emphasis on well-defined boundary conditions. The surface concentrations from the tracer method agree with solubility data and the concentration of electrically active interstitial manganese is found to be 60–70% of the total manganese concentration. Both methods yield identical diffusion coefficients which are described by an Arrhenius law, D(T)=(6.9±2.2)×10−4 cm2 s−1 exp [(−0.63±0.03)eV/kT].
Nitridation of Si (100) made by radio frequency plasma as studied by i
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u angular resolved x-ray photoelectron spectroscopy
Authors: Ermolieff Anne , Bernard Pascal , Marthon Sybil , Camargo da Costa José
Published: July 2002

X-ray photoelectron spectroscopy with variable takeoff angles has been used to study the nitridation of Si(100). The air-oxidized layer on the silicon film has been cleaned off by an Ar radio frequency (rf) plasma, and nitridation was obtained at temperatures around 400 °C in a rf plasma of a NH3 (5%)-N2(95%) mixture. N(1s) and Si(2p) photoelectrons lines have been analyzed as function of the angle θ between the surface of the sample and the detector slit at each step of nitridation to obtain depth profile. These lines have both been decomposed into two components due, on one hand, to the Si–N and the metallic bonds and, on the other hand, to the chemisorbed nitrogen and the N–Si bond. From the integrated intensities of these lines, concentration dependencies on depth and nitridation time have been studied; a nitridation kinetics has been deduced.
Photoablation of plasma polymerized polyacetylene films
Authors: Golombok M. , Gower M. C. , Kirby S. J. , Rumsby P. T.
Published: July 2002

Polyacetylenes are the simplest organic polymers which readily undergo photoablation. We describe and justify the use of a less well-known technique for synthesizing polyacetylenes and characterize the films so formed. This is of interest because polyacetylenes are organic semiconductors. Glow discharge polymerization yields layers around 1 μm thick. Efficient ablative decomposition of polyacetylene films requires multishot exposures. The role of conjugation in the chains and efficiency of the process as opposed to thermal decomposition is discussed. Because the polymer contains only carbon and hydrogen, ablation in vacuum yields readily identifiable products.
Vanadia reactions with yttria stabilized zirconia
Authors: Hertl W.
Published: July 2002

The reaction of vanadia films with yttria stabilized single-crystal cubic and polycrystalline cubic-tetragonal zirconias was studied using Rutherford backscattering (RBS) and x-ray diffraction, with some selected depth profiling studies. Vandium on zirconia readily oxidizes at 500 °C to form vanadia films. With increasing temperature, small amounts of vanadia diffuse into the zirconia. Near 650 °C the vanadia forms a eutectic solution at the surface and massive diffusion into the ZrO2 occurs. With rapid heating much of the vanadia volatilizes; with slow heating the vanadia diffuses large distances into the zirconia. Near 800 °C the vanadia principally reacts with yttria to form yttrium vanadate. At 980 °C and above, vanadia is completely reacted. Zirconia coated with materials which do not form a eutectic mixture (tantala, tantalum, vanadium) showed little reaction or diffusion at 900–1000 °C, but massive dissolution when a eutectic is formed (vanadia, chromia+vanadia). Yttria preferentially diffuses toward the surface when a reaction takes place to form YVO4 . When yttria is sequestered as the vanadate, the zirconia changes phase from cubic to monoclinic. Little difference in reactivity is observed between single-crystal and polycrystalline zirconia due to eutectic solution formation, which, with the phase separation, produces a very corroded surface.
Comparison of surface properties of sodium sulfide and ammonium sulfide passivation of GaAs
Authors: Besser R. S. , Helms C. R.
Published: July 2002

Sodium sulfide has been demonstrated to significantly improve the electrical properties of the ambient-exposed (100) GaAs surface. It has also shown usefulness in several practical device structures. Ammonium sulfide has been proposed to have similar properties and has already been demonstrated to improve InP and GaAs devices. We compare the photoluminescence behavior and band bending of GaAs samples treated with these two different sulfide species. We also compare the chemical nature of the surfaces treated using these two processes by Auger electron spectroscopy. The surface recombination velocity was found to be similarly affected by the two treatments and the band bending found to be essentially identical. Sodium sulfide produces a film and interfacial region rich in oxygen due to hydration. However, the ammonium sulfide was found to leave very little oxygen on the surface, even compared to conventional GaAs cleaning and etching procedures.
Homogeneous nucleation of diamond powder in the gas phase
Authors: Frenklach M. , Kematick R. , Huang D. , Howard W. , Spear K. E. , Phelps A. W. , Koba R.
Published: July 2002

Homogeneous nucleation of diamond powder is reported. The experiments were performed in a low-pressure microwave-plasma reactor. The deposits were collected downstream of the reaction zone and subjected to wet oxidation to remove nondiamond carbons. The residues were analyzed by optical and electron microscopy, electron diffraction, and Raman spectroscopy. A variety of hydrocarbons diluted in argon, hydrogen, or oxygen gas mixtures were tested. In most cases only nondiamond materials, like graphite and carbyne, were obtained. Homogeneous nucleation of diamond was clearly observed in dichloromethane- and trichloroethylene-oxygen mixtures. The particles formed had crystalline shapes, mostly hexagonal. The largest particles were about 0.2 μm, although most of the particles were on the order of 50 nm in diameter. The powder was identified to be a mixture of polytypes of diamond.
Internal stress and internal friction in thin-layer microelectronic materials
Authors: Berry B. S. , Pritchet W. C.
Published: July 2002

Understanding of the static internal stress in supported thin layers can be aided by the use of dynamic mechanical analysis, with emphasis on internal friction measurements for the detection of defects susceptible to stress-induced ordering. In addition to the bending-bilayer and vibrating-reed methods used for separate measurements of internal stress and internal friction, a new vibrating-membrane method is described that enables both quantities to be measured concurrently on the same sample. Structural relaxation, solute sorption, orientational ordering, and surface reaction are identified as four general phenomena affecting the internal stress, and experimental results are given for each of these categories. The bending-bilayer method has been used to observe stress changes associated with structural relaxation in amorphous SiO films, and with the sorption and orientational ordering of oxygen in Nb films. The vibrating-membrane method has been used to study stress changes in boron-doped Si membranes, resulting from either hydrogen sorption or high-temperature oxidation. Hydrogen sorption leads to the appearance of a new internal friction peak located near 130 K, which is governed by an activation energy of 0.23 eV.
Improvements of optical and microwave properties of Bi-substituted magnetic garnets by O3 annealing
Authors: Tamada Hitoshi , Saitoh Masaki , Kaneko Masahiko
Published: July 2002

The effects of the heat treatment of Bi-substituted magnetic garnets in a flowing oxygen containing ozone with less than a few vol % (O3 annealing) on the optical absorption α in a near-infrared spectral region and on the ferromagnetic resonance linewidth ΔH have been investigated in detail. By O3 annealing for typically 3 h at a temperature of 700 °C and subsequent O2 annealing around 500 °C, the α is reduced to less than 1 dB/cm at a wavelength of 1.3 μm, and also the ΔH is reduced to 0.8 Oe at 9 GHz in a very reproducible manner. The reduction of both α and ΔH is thought to be caused by the decrease of Fe2+ ions originally formed mainly by oxygen vacancies. The only negative point of O3 annealing is that it inevitably accompanies some surface damage probably caused by the attack of oxygen atom radicals produced by the thermal dissociation of ozone which affects magnetostatic surface waves so as to increase the insertion loss. However, this problem has been successfully solved by protecting the surface with a thin oxide layer such as a Ta2O5 on. O3 annealing with a protective oxide layer on is extremely valuable because α is still reduced effectively and magnetostatic surface waves can propagate with a smaller insertion loss.
Thermal annealing effects of defect reduction in GaAs on Si substrates
Authors: Yamaguchi Masafumi , Tachikawa Masami , Itoh Yoshio , Sugo Mitsuru , Kondo Susumu
Published: July 2002

High-quality GaAs films with a dislocation density of 2×106 cm−2 on (100) Si substrates have been obtained by thermal cycle annealing using the metalorganic chemical vapor deposition method. Dislocation behavior in GaAs/Si has been considered. Significant reduction effects of dislocation density in the GaAs layers on Si due to thermal annealing have been analyzed by a simple model, in which annihilation such as coalescence of dislocations is assumed to be caused by dislocation movement under high thermal stress and temperature. Numerical analysis suggests that excellent quality GaAs/Si films with a dislocation density of less than 105 cm−2 will be realized if thermal cycle annealing is carried out more than 1000 times without thermal degradation of the GaAs/Si.
Heavily doped GaAs:Se. II. Electron mobility
Authors: Szmyd D. M. , Hanna M. C. , Majerfeld A.
Published: July 2002

A study of the mobility μ of Se-doped n+-GaAs grown by metalorganic chemical vapor deposition is presented. A significant decrease in μ is observed for n>1×1018 cm−3, which is a general characteristic of n+-GaAs. Previous explanations that the low values of μ are the result of autocompensation by the dopant are unsatisfactory in view of the universality of the decline in μ. A new formula is derived for the ionized impurity mobility μI for degenerately doped material which accurately predicts the experimental μ using no compensation and no adjustable parameters. The formula takes into account the increase of the effective mass m* due to nonparabolicity of the conduction band and due to distortion of the band by the donor atoms. For degenerate material, μI is inversely proportional to the square of m* at the Fermi energy EF. For uncompensated GaAs with n=1×1019 cm−3, m* at EF is 2.4 times m* for pure GaAs, and μ is only 1000 cm2/V s. Previous theories, which use the smaller optical effective mass m*opt in place of m*, predict higher mobilities and temperature variations of μ for n+-GaAs. The new formula correctly predicts that μ does not change with temperature when n>2×1018 cm−3.
Characterization of ZnO varistor degradation using lifetime positron-annihilation spectroscopy
Authors: Ramanachalam M. S. , Rohatgi A. , Schaffer J. P. , Gupta T. K.
Published: July 2002

Positron-annihilation spectroscopy (PAS) was used in the lifetime mode to study changes in the grain-boundary defect equilibrium associated with dc-bias-induced degradation of a ZnO varistor. The PAS lifetime spectra were collected while the sample was under an applied bias ranging from 100 V ( ∼ 400 V/cm) to 500 V ( ∼ 2000 V/cm). The current through the sample was continuously monitored. The simple trapping model was used to interpret the lifetime PAS results and to obtain an estimate for the positron-capture rate. The experimental results show that an increase in the bias voltage results in a decrease in the positron-trap density and an increase in the positron lifetime associated with the dominant positron trap. These results are explained on the basis of a decrease in the concentration of negatively charged zinc vacancies at the grain boundary. The PAS results support the ion-migration model for degradation, which suggests that the bias-induced migration of positively charged zinc interstitials to the grain boundary reduces the concentration of negatively charged zinc vacancies at the boundary. This results in a reduction in the barrier height (degradation) and is consistent with the PAS data.
New III-V diluted magnetic semiconductors (invited)
Authors: Ohno H. , Munekata H. , von Molnár S. , Chang L. L.
Published: July 2002

A new class of diluted magnetic semiconductor (DMS) based on a III-V semiconductor is reviewed. The new DMS, (In,Mn)As, was made possible by low temperature molecular beam epitaxial growth. Magnetic measurements and x-ray diffraction showed homogeneous incorporation of Mn in the films under certain growth conditions, and inclusion of a MnAs-like phase if the conditions are not optimized. The films can be made either p- or n-type by choosing the growth conditions and/or doping. Homogeneous n-type (In,Mn)As layers were paramagnetic and showed negative magnetoresistance. On the other hand, remanent magnetization was observed in p-type samples at low temperature and an anomalous Hall effect associated with it. The presence of such effects was most readily explained in terms of formation of bound magnetic polarons. A first result of anomalous Hall effect in a heterojunction is also presented.
Low frequency and low temperature behavior of ZnO-based varistor by ac impedance measurements
Authors: Al Abdullah K. , Bui A. , Loubiere A.
Published: July 2002

This paper reports on an original work on the study of the behavior of ZnO-based varistor by ac impedance measurements. The measurements of complex impedance response to small ac signals applied to samples have been achieved in the frequency range 10−3–7×106 Hz and a temperature range −196–145 °C. In the temperature range below −120±10 °C, a complete semicircular relaxation phenomenon has been observed, which is in good agreement with the Debye model. In the temperature range above −120±10 °C, the appearance of the depression angle θ may arise from the heterogeneity of the barriers of the ceramic varistor. A Cole–Cole distribution function of the relaxation time constants was used to characterize the response of the ceramic device as a function of temperature. No observable influence in the response was introduced by either the space charge or the electrode. Using the leakage resistance measurements from the complex plane, two values of the activation energies were observed; 0.36±0.02 eV and 0.01±0.001 eV. Similarly, the boundary layer equivalent capacitance measurements detect two other energy levels: 0.06±0.005 eV and about 0 eV.
Determination of grain-boundary defect-state densities from transport measurements
Authors: Evans P. V. , Nelson S. F.
Published: July 2002

Transport of majority carriers across the potential barriers frequently found at grain boundaries in semiconductors is treated by considering sequential drift diffusion and thermionic emission processes. The regimes where the thermionic or drift-diffusion approximations can reasonably be made are readily determined from the analysis. Over a wide doping range, we find boundary transport to be under mixed control. Numerical calculations are made for a given barrier height with a range of bulk doping levels. A deconvolution scheme to determine defect-state densities is given and tested for a variety of densities of states.
In situ investigation of the amorphous silicon/silicon nitride interfaces by spectroellipsometry
Authors: Stchakovsky M. , Drévillon B. , Cabarrocas P. Roca i
Published: July 2002

We present an in situ study, by spectroellipsometry, of interfaces between amorphous silicon (a-Si:H) and silicon nitride (a-SiNx). The nature of the interface between amorphous silicon and silicon nitride depends on the order of deposition. A behavior compatible with an atomically abrupt interface is observed when a-SiNx is deposited on top of a-Si:H. On the contrary, a graded transition with a width estimated at ≊15 Å is observed when a-SiNx is deposited first. An NH3 plasma treatment has little influence on the interface properties. These different behaviors are attributed to the plasma process.
Mössbauer study on Fe16N2 films prepared by ion-implant nitrification of iron films
Authors: Nakajima Kensuke , Yamashita Tsutomu , Takata Masasuke , Okamoto Shoichi
Published: July 2002

Mössbauer spectroscopy was carried out to elucidate the magnetic structure of Fe16N2 with the giant magnetization. Samples were prepared by ion-implant nitrification of single-crystal iron films, followed by an annealing at 150 °C for 60 h. The nitrification developed three Fe atom environments, attributable to three different Fe sites in Fe16N2 structure. The magnetic moments of the first, second, and third nitrogen neighbor Fe atoms were determined to be 1.3, 2.5, and 3.8 μB, respectively. The quadrupole effect in Fe16N2 was interpreted consistently on the basis of the consideration that the magnetization is directed along [001] and nitrogen atoms had a negative charge.