bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocation Multiplication in the Early Stage of Deformation in MO Single Crystals
Authors: Hsiung L. M. , Lassila D. H.
Published: April 2011

Initial dislocation structure in annealed high-purity Mo single crystals and deformation substructure in a crystal subjected to 1% compression have been examined and studied using transmission electron microscopy (TEM) techniques in order to investigate dislocation multiplication mechanisms in the early stage of plastic deformation. The initial dislocation density is in a range of 106 σ 107 cm−2, and the dislocation structure is found to contain many grown-in superjogs along dislocation lines. The dislocation density increases to a range of 108 σ 109 cm−2, and the average jog height is also found to increase after compressing for a total strain of 1%. It is proposed that the preexisting jogged screw dislocations can act as (multiple) dislocation multiplication sources when deformed under quasi-static conditions. The jog height can increase by stress-induced jog coalescence, which takes place via the lateral migration (drift) of superjogs driven by unbalanced line-tension partials acting on link segments of unequal lengths. The coalescence of superjogs results in an increase of both link length and jog height. Applied shear stress begins to push each link segment to precede dislocation multiplication when link length and jog height are greater than critical lengths. This “dynamic” dislocation multiplication source is suggested to be crucial for the dislocation multiplication in the early stage of plastic deformation in Mo.
Properties of the Deformation Microstructure in Al-Rich γ-TiAl Deformed by Ordinary Dislocations
Authors: Grjégori Fabienne , VeyssiÈRe Patrick
Published: April 2011

In the near vicinity of the [021] load orientation, γ-TiAl deforms via ordinary dislocations (Burgers vector b = 1/2<110]). As for deformation by <110] dislocations, the flow stress shows a peak at about 600°C. Results of an extensive microstructural investigation aimed at identifying the origin of this mechanical anomaly are presented. The analysis was conducted on single crystals oriented for single slip. It confirmed that ordinary dislocations tend to align themselves along the screw direction. This preferential line direction becomes gradually accentuated as the deformation temperature is raised up to the peak temperature. This effect is accompanied by a strong tendency towards forming cusps, but there is indication that the immobilisation along the screw direction takes place prior to dislocation pinning. In the vicinity of the peak temperature, screw dislocations gather in the form of bundles. No clear correlation is found between the temperature dependence of the flow stress and that of the density of pinning points. The relationship between these microstructural findings and the occurrence of a flow stress anomaly is discussed.
Deformation of Al-Rich γ-TiAl by <110] Dislocations Over the Domain of Flow Stress Anomaly
Authors: Grégori Fabienne , Veysslère Patrick
Published: April 2011

The flow stress of Al-rich γ-TiAl peaks at a temperature located between about 600°C and 1000°C. We present mechanical data on samples oriented so as to deform on a <011]{111} system in single slip. These data are complemented by TEM observations in samples strained at selected temperatures (RT, 400°C, 600°C and 800°C). The present paper addresses (i) properties of dislocation organisation, (ii) the dissociation mode of <110] dislocations, (iii) locking properties of <110] dislocations and (iv) the formation of faulted dipoles.
A Systematic Investigation of Strain Relaxation, Surface Morphology and Defects in Tensile and Compressive InGaAs/InP Layers
Authors: Ferrari C. , Lazzarini L. , Salviati G. , Natali M. , Berti M. , De Salvador D. , Drigo A.V. , Rossetto G. , Torzo G.
Published: April 2011

The results of a systematic investigation by transmission electron microscopy (TEM), cathodoluminescence (CL), Rutherford backscattering (RBS), X-ray diffraction and topography and scanning force microscopy (SFM) techniques on several InGaAs/InP compressive and tensile strained layers covering the misfit range from −2.3 to 1.5×10−2 and grown by the metal organic vapor phase epitaxy (MOVPE) technique are reported. In compressively strained films the same dependence for the residual strain vs the film thickness as for the InGaAs/GaAs is found whereas a different strain release rate and different extended defects are found in tensile stressed InGaAs alloy. In particular in tensile stressed samples, grooves, planar defects and cracks are present in addition to the interfacial network of misfit dislocations. The correlation between the observed planar defects and the mechanisms of strain relaxation in the case of tensile strained layers is discussed.
X-Ray Microbeam Diffraction Mapping of Different Texture Scales Important in Fatigue Cracking or in Large Deformations
Authors: Stock S.R. , Rek Z.U.
Published: April 2011

Polychromatic synchrotron x-ray microbeam diffraction allows one to record Laue patterns simultaneously from all of the grains within the column of material irradiated. This paper describes 3-D texture mapping in polycrystalline Al and Cu samples. Transmission methods for determining the depths of diffracting volumes are outlined. The focus is on how the texture scale between micro- (i.e., individual grain orientations) and macro-texture defines fatigue crack paths in Al-Li 2090 T8E41: very large volumes of near single crystal material lead to large asperity formation, and at least forty volume percent of the material consists of these volumes. Results on grain subdivision in Cu after large deformations are also briefly reviewed.
Microstructurally Induced Ductile Deformation Mechanisms and Grain-Boundary Effects in Polycrystalline Aggregates
Authors: Ashmawi W. M. , Zikry M. A.
Published: April 2011

Dislocation-density based multiple-slip constitutive formulations and specialized computational schemes are introduced to account for large-strain ductile deformation modes in polycrystalline aggregates. Furthermore, new kinematically based interfacial grain-boundary regions and formulations are introduced to account for dislocation-density transmission, absorption, and pile-ups that may occur due to grain-boundary misorientations and properties.
Self-Affine Measurements on the Fracture Surface of Plastic Materials by AFM
Authors: Reyes E. , Guerrero C. , González V. , Hinojosa M.
Published: April 2011

The self-aff'me behavior of fracture surfaces of polymeric materials was qualitatively and quantitatively studied. SEM images of fracture surfaces of both polypropylene and polystyrene show Chevron marks at several magnifications. In addition, for polystyrene the mirror and Hackle zones were also observed. For quantitative analysis, the average roughness exponent, ζ, of height profiles generated by AFM images, was estimated by applying the variable bandwidth method. Values of ζ=0.788 and ζ=0.810 were obtained for polypropylene and polystyrene, respectively. These results are in very good agreement with the claimed universal exponent of 0.8 reported in the literature for other non-polymeric materials. By choosing the AFM appropriate operating conditions, measurements of roughness on plastic material surfaces could be performed.
Atomistic Simulations of Dislocation-Interface Interactions in the Cu-Ni Multilayer System
Authors: Rao Satish I. , Hazzledine Peter M.
Published: April 2011

Multilayered Cu-Ni has a peak yield strength four orders of magnitude higher than either Cu or Ni because the multitude of interfaces obstruct glissile dislocations. The barrier strengths of the interfaces may be traced to four mismatches across an interface: modulus, lattice parameter, chemical and slip geometry. This paper describes sample embedded atom method (EAM) simulations of dislocations crossing interfaces, designed to separate the effects of the four mismatches. The results confirm some classical calculations and emphasize the importance of three new effects (i) an interface-chemical effect in which dislocations are trapped by core spreading in the interface, (ii) a coherency-chemical effect caused by coherency strains changing effective stacking fault energies and (iii) a coherency-modulus effect in which coherency strains change elastic moduli (and hence the Koehler stress) significantly.
Structure and Stability of Grain Boundaries in Molybdenum with Segregated Carbon Impurities
Authors: Janisch R. , Ochs T. , Merkle A. , Elsässer C.
Published: April 2011

The segregation of interstitial impurities to symmetrical tilt grain boundaries (STGB) in bodycentered cubic transition metals is studied by means of ab-initio electronic-structure calculations based on the local density functional theory (LDFT). Segregation energies as well as changes in atomic and electronic structures at the ΣE5 (310) [001] STGB in Mo caused by segregated interstitial C atoms are investigated. The results are compared to LDFT data obtained previously for the pure Σ5 (310) [001] STGB in Mo. Energetic stabilities and structural parameters calculated ab initio for several crystalline Molybdenum Carbide phases with cubic, tetragonal or hexagonal symmetries and different compositions, MoCx, are reported and compared to recent high-resolution transmission electron microscopy (HRTEM) observations of MoCx, intergranular films and precipitates formed by C segregation to a Σ5 (310) [001] STGB in a Mo bicrystal.
Crystallographic Texture and Yield Behavior of Al-Cu-Li (2195) Plate
Authors: Crosby K.E. , Mirshams R.A. , Pang S.S.
Published: April 2011

Existing experimental texture analysis capabilities allow testing of theories on plasticity using polycrystal models. Aluminum-lithium alloys, which are particularly suited for aerospace applications due to excellent strength to weight ratio, typically possess pronounced texture. Al- Cu-Li (2195) thick plates were deformed by cold rolling to various thickness reductions. The plates exhibit a texture gradient through the plate thickness accompanied by a variation in yield strength values. The difference in yield strength values is related to the texture variation in terms of the texture components (ideal crystallographic orientations) identified from experimental measurements. A modified Taylor-based polycrystal plasticity model developed by Kocks and his colleagues is used to predict yield surfaces by incorporating texture data. Results of this investigation show that the texture intensities measured at certain ideal orientations increase or decrease with increasing deformation. These textural changes influence the value and anisotropy of the yield strength of the alloy.
Dislocations and Internal Stresses in Thin Films: A Discrete-Continuum Simulation
Authors: Lemarchand C. , Devincre B. , Kubin L.P. , Chaboche J.L.
Published: April 2011

The plasticity of thin films and layers is of considerable technological interest. For instance, the relaxation of internal stresses in semiconducting epitaxial layers has been the object of many studies [1, 2]. This relaxation is usually treated via the concept of critical thickness, the latter being defined as the maximum layer thickness below which dislocations cannot spontaneously move and relax the internal stresses. The various internal stresses present in epitaxial layers (e.g. the misfit and elastic incompatibility stresses at the film/substrate interface and the image force in a free-standing film) can be computed within a continuum frame. However, the way they influence the motion of a dislocation has not yet been computed, even in a approximate manner. An useful approximation that allows treating the boundary condition at the surface of a free-standing film consists of making use of the concept of image dislocation. Then, the critical stress for moving a dislocation in a free-standing film is the same as that of a capped layer of thickness twice that of the film. To date, models and dislocation dynamics (DD) simulations are available that involve several levels of approximation for the treatment of the dislocation/interface and dislocation/surface interactions [3–7]. For reasons that are not clearly understood, however, these models predict critical thicknesses that are systematically larger than the expected ones. The comparison with experiment is, in addition, made difficult because stresses have to be artificially introduced to replace the internal stresses and approximations have to be done to treat the image stresses. In the present work it is shown that it is now possible to fully account for the contribution of the various sources of internal stresses to the critical stress for the motion of a threading dislocation. This is performed numerically with the help of a hybrid code that combines a DD code for the treatment of the dislocation dynamics and a Finite Element (FE) code for the treatment of the boundary conditions. In what follows, several applications of this discrete-continuum model (DCM) to the study of dislocation motion in epitaxial layers are presented. The motion of a dislocation in a thin film is considered, including the image force and successively adding a misfit stress and an elastic incompatibility stress at the film/substrate interface.
Atomic-Scale Design for Enhanced Low Temperature Twinning in ZrCr2-Based Laves Phases
Authors: Kim W.-Y. , Luzzi D.E.
Published: April 2011

Recently, we have designed and produced several transition metal Laves phases with low-temperature compressive ductility. These improved alloys demonstrate that manipulation of atomic-scale structure can have a drastic effect on meso-scale deformation behavior. To gain a basic understanding of the role of atomic-scale substitutions on the room temperature mechanical properties, a systematic investigation of the Laves ZrCr2-based alloy system alloyed with Hf, Nb, Ta and Ti was conducted and is reported here. Extensive room temperature ductility was obtained in the Hf-alloyed ternary Laves phase alloy system. Mechanical twinning is found to be the predominant deformation mode at room temperature in this alloy system. It is emphasized that Hf substitution in the Zr sublattice of ZrCr2 may play the most prominent role in changing the local electronic structure resulting in easier twinning.
Electrochemical Atomic-Layer Epitaxy: Electrodeposition of III-V and II-VI Compound Semiconductors
Authors: Wade Travis L. , Flowers Billy H. , Vaidyanathan Raman , Mathe Kenneth , Maddox Clinton B. , Happek Uwe , Stickney John L.
Published: April 2011

Electrochemical atomic-layer epitaxy (EC-ALE) is an approach to electrodepositing thin-films of compound semiconductors. It takes advantage of underpotential deposition (UPD), deposition of a surface limited amount (a monolayer or less) of an element at a potential less negative than bulk deposition, to form a thin-film of a compound--one atomic layer at a time. Ideally, the 2-D growth mode should promote epitaxial deposition.

Many II-VI and a few III-V compounds have been formed by EC-ALE. TI-VI films such as CdSe, CdS, and CdTe have been successfully formed. In addition, deposition of III-V compounds of InAs and InSb are being explored, along with initial studies of GaAs deposition. Depositions of the I-VI systems are better understood so this report will focus on the III-V's, particularly InAs and InSb.

Building compounds an atomic layer at a time lends electrochemical-ALE to nanoscale technology. Deposited thickness ranged from a few nanometers to a few hundred. The films are typically characterized by atomic-force microscopy (AFM), X-ray diffraction (XRD), electron microprobe analysis (EPMA) and ellipsometry. InAs deposits are also characterized by infrared reflection absorption.
Electrical Properties of Nanocrystalline Tungsten Trioxide
Authors: Hoel A. , Kish L.B. , Vajtai R. , Niklasson G.A. , Granqvist C.G. , Olsson E.
Published: April 2011

Tungsten oxide is a material of significant interest for applications in several areas. It can, for example, be used as the electrochromic film in smart windows. Tungsten trioxide nanoparticles were produced using an advanced gas evaporation unit in which the tungsten was oxidized in low pressure ambient air. The tungsten trioxide particles were formed via vapor condensation and were deposited by gas deposition technique to avoid coagulation effects. The average size of the primary particles was around 5 nm, depending on the heating power and the pressure. The particles exhibited a body centred cubic structure. The impedance spectrum of particle deposits showed resonance and negative capacitance effects. The correlation between fabrication conditions, structure and impedance spectrum is discussed.
Solar Cell Contacts Using Nano-Sized Dispersions
Authors: Schulz Doug , Ribelin R. , Wu X. , Jones K.M. , Matson R.J. , Curtis C.J. , Gessert T. , Ginley D.S.
Published: April 2011

Nano-sized dispersions have been employed as precursor inks for the spray deposition of contacts to both Si and CdTe materials. In the case of Si, nano-sized Al particles (nano-Al) were dispersed and spray deposited onto p-type Si. Annealing above the eutectic temperature causes alloy formation yielding a p+ layer with p ∼ 10−4 Ω•cm. For CdTe, nano-sized Te particles (nano-Te) were dispersed and sprayed onto CdTe/CdS/SnO2/glass heterostructures. Contact to the CdTe layer occurred during a 30 min anneal in He (T = 215 to 255 °C). These solar cells were finished by spin-coating the Te layer with Ag paint and subsequently annealing in air (100 °C / 1 h). This approach produces solar cells with open circuit voltages (Voc) from 720 to 800 mV, short circuit current densities (Jsc) from 18 to 20 mA/cm2 and efficiencies up to 10.3%. The performance of these cells was similar to those produced using the standard NREL contact.
Structure of Polycrystalline Silicon Films by Glow-Discharge Decomposition using SiH4/H2/SiF4 at Low Temperature
Authors: Tsuchida R. , Syed M. , Inokuma T. , Kurata Y. , Hasegawa S.
Published: April 2011

For poly-Si films prepared by a plasma-enhanced chemical vapor deposition, we examined the changes in the local structure caused by adding H2 and/or SiF4 in the SiH4 feed gases and by changing supplied rf power values. The conditions of low rf power supply, low H2 addition, and SiF4 addition allow formation of films with microcrystalline or nanocrystalline structures. In addition, the H2 or SiF4 addition was found to be effective in promotive growth of <111> or <110> grains, respectively. In such low crystallized films, it was suggested that high-angle boundary would be formed, leading to a decrease in the density of SiH2 and Si dangling bonds, and to an increase in g values.
Magnetic Hardening of Mechanichally Alloyed SmFe11−xCoxTi
Authors: Bessais L. , Djega-Mariadassou C.
Published: April 2011

Mechanical alloying of ternary SmFe11−xCoxTi (x = 0, 0.5, 1, 1.5, 2) alloys was carried out under an Ar atmosphere. Milled samples were annealed for 30 min in a vacuum at different temperatures Ta from 650 °C to 1150 °C. The effects of heat treatment, on structure and magnetic property changes, have been investigated by means of x-ray diffraction using the Rietveld method, Mössbauer spectroscopy and differential sample magnetometer. Tetragonal ThMn12-type structure is observed for samples annealed at Ta > 900 °C. For 650<Ta<800 °C the TbCu7 type phase was identified as the major phase. Between these two regions a mixture of TbCu7 and ThMn12-type nanocrystalline phases is obtained with a maximum of the coercive field Hc (Hc > 5kOe). The Mössbauer spectra relative to the hexagonal phase show sextuplets broadened by the statistical occupancies of the iron sites. An enhancement of the magnetic properties results from the Co substitution.
Nanocomposite Material, Liquid Crystal-Aerosil Particles: Dielectric and Photon Correlation Spectroscopy Investigations
Authors: Aliev F.M. , Sinha G.P.
Published: April 2011

Nematic liquid crystal filled with Aerosil particles prospective inorganic-organic nanocom-posite material for optoelectronic application has been investigated by broadband dielectric spectroscopy (BDS) and photon correlation spectroscopy (PCS). The aerosil particles of diameter ≈ 10 nm in filled nematic liquid crystals form a network structure with linear size of LC domains about 250 nm and with random distribution of the director orientation of each domain. This material has a very developed liquid crystal-solid particle interface that makes the role of the surface layers of LC important in the determination of the properties of the material. BDS provides information on reorientational motion of polar molecules of liquid crystal while PCS probes dynamics of collective modes associated with director fluctuations. We found that the properties of 5CB are considerably affected by the network. Two bulk-like dielectric modes due to the rotation of molecules around short axes and the tumbling motion were observed in filled 5CB. Additionally, a low frequency relaxation process and dispersion of dielectric permittivity due to conductivity were observed. The treatment of the surface of filling particles has strongest influence on the properties of the slow process and it is less important for molecular modes. PCS experiment shows that two new relaxation processes appear in filled 5CB in addition to the director fluctuations process in bulk.
Fe-Al2O3 Nanocomposite: Synthesis and Magnetic Properties
Authors: Santos A. , Macedo W. A. A. , Ardisson J. D. , Viegas A. D. C. , Schmidt J. E.
Published: April 2011

We have investigated the synthesis and the structural and magnetic properties of Fe nanoparticles embedded in Al2O3 matrix, a granular nanocomposite prepared by sol-gel processing. Samples with volumetric Fe content ranging from 20 to 62% were obtained starting from Al nitrate and Fe sulfate as precursors and the preparation method results initially in a mixture of Fe- and Al oxides. The conversion of Fe oxides into metallic Fe was done by calcination at 800°C followed of reduction at 600°C for 2 hours, in H2 atmosphere. Our results indicated the following phases in the amorphous alumina matrix, after reduction: α-Fe, the main phase, α- and γ-Fe2O3, Fe3O4 and some interstitial Fe2+ and substitutional Fe3+ atoms in the Al2O3 lattice. For the investigated system, Fe reduction rate was very sensitive to the sample porosity and, with the applied method, we have obtained reductions of the Fe atoms into metallic Fe ranging from 45 to 68%, preserving the mean diameter of the oa-Fe nanoparticles between 55 and 80 nm. VSM measurements at room temperature resulted in coercivity between 450 and 630 Oe and saturation magnetization between 40 and 110 emu/g. Magneto-transport measurements in samples with 25% metallic Fe (and 51% total Fe) reveal ΔR/R close to 2% at room temperature.
Thin Polytetrafluoroethylene Organosilane Nanocomposite Films Used as Ultra Low Dielectric Constant Materials in Microelectronics
Authors: Qu Shichun , Rosenmayer Tom , Xu Ping , Spevack Perry
Published: April 2011

Replacing the current on-chip dielectric materials such as silicon dioxide, which has a dielectric constant of approximately 4.0, with low dielectric constant materials can greatly improve the performance of high density VLSI device by reducing crosstalk and capacitive delay. Polytetrafluoroethylene (PTFE) has the lowest dielectric constant (k < 2.0) of any full density material, which makes it a promising candidate for this IC application. Pure PTFE thin films cast from PTFE nanoemulsion containing sub-20nm PTFE particles, though thermally stable, have some inherent sub-optimal properties. These include adhesion to other inorganic materials and mechanical strength at high processing temperatures. In order to improve these properties, we have developed a PTFE / silicon compound nanocomposite material. Initial tests have shown that this nanocomposite material has significantly improved high temperature mechanical properties and interfacial properties between the composite and inorganic materials such as silicon, silicon oxide, silicon nitride and some metals. The surface roughness of the thin film coatings is also reduced compared to pure PTFE thin film coatings. The coatings require no separate adhesion promoter to be applied to the substrate prior to deposition. Characterization work has been carried out with different techniques such as DMA, ToF-SIMS, XPS and AFM with hydrofluoric acid (HF) selective etching, in order to understand this novel nanocomposite and its surface and interfacial properties.
A Nano-Composite Poly(Methyl-Methacrylate) Bone Cement
Authors: Gomoll Andreas H. , Bellare Anuj , Fitz Wolfgang , Thornhill Thomas S. , Scott Richard D. , Jemian Peter R. , Long Gabrielle G.
Published: April 2011

Poly(methyl methacrylate) (PMMA) based orthopaedic bone cements contain 1-3 Pim size radiopacifier particles. Incomplete dispersion of these particles leads to the presence of 50-200 μm size agglomerates. These large defects are sites of high-stress concentration that reduce the fracture toughness of PMMA. In this study, the micrometer-sized radiopacifying particles of a commercial bone cement were replaced by nanosized fillers. Both, commercial and nanocomposite PMMA bone cements were characterized using ultra-small angle x-ray scattering and low voltage scanning electron microscopy; mechanical properties were evaluated using ASTM standard tensile testing. The results showed a substantial reduction of particle agglomerate size and a significant increase in tensile properties of the nanocomposite over that of the standard microcomposite bone cement.
Nanocomposites Based Upon Alumina and Zirconia Pillared Clays Loaded with Transition Metal Cations and Clusters of Precious Metals: Synthesis, Properties and Catalysis of NOx Selective Reduction by Hydrocarbons
Authors: Sadykov Vladislav A. , Bunina R. V. , Alikina G. M. , Doronin V. P. , Sorokina T. P. , Kochubei D. I. , Novgorodov B. N. , Paukshtis E. A. , Fenelonov V. B. , Derevyankin A. Yu. , Ivanova A. S. , Zaikovskii V. I. , Kuznetsova T. G. , Beloshapkin B. A. , Kolomiichuk V. N. , Plasova L. M. , Matyshak V. A. , Konin G A. , Rozovskii A. Ya. , Tretyakov V. F. , Burdeynaya T. N. , Davydova M. N. , Ross J. R. H. , Breen J. P. , Meunier F. C.
Published: April 2011

Thermally stable alumina and zirconia pillared clays loaded with copper and cobalt cations and silver nanoparticles were synthesized. The structural and surface features of these nanosystems were studied and compared with those of bulk analogs -partially stabilized zirconias and γ-alumina loaded with the same active components. Specificity of the catalytic properties of nanocomposites in the reactions of nitrogen oxides reduction by propane, propylene and decane in the excess of oxygen appears to be determined both by the degree of interaction between pillars and active components and the type of reducing agent.
Characterization of Voids in Rutile Nanoparticles by Transmission Electron Microscopy
Authors: Turner S.
Published: April 2011

Rutile nanoparticles containing voids or cavities have been characterized using transmission electron microscopy. The general morphology of the voids has been determined from images of nanoparticles in different orientations. In general, the longest dimension is along the c axis of rutile. Many of the voids show a prismatic morphology with dipyramid terminations. The prism consists of primarily four {110} faces with rounded or faceted comers between the primary faces. The pyramidal terminations can appear ovoid or faceted. A major facet plane of the pyramids is (101). A model consistent with the morphology of many voids in rutile nanoparticles is proposed.
Power of Mass Spectrometry for Analyzing Organometallic Molecules as Precursor of Nanomaterials
Authors: Strouse G.F. , Gaumet J.J.
Published: April 2011

A mass spectrometry method (Electrospray Mass Spectrometry [ESMS]) for analyzing organometallic precursors of nano-materials is described. We show that application of low-cone voltage ESMS is a useful technique for the rapid analysis of intact organometallic precursor molecules when both positive and negative ionic modes are analyzed. This method shows promise for extrapolation to analysis of the dynamics of growth in nano-scale materials.
Micro-Raman Study of Self-Assembled Nanostructures: (1−x)PZN:xPT Solid Solution
Authors: Gupta S. , Katiyar R. S. , Guo R. , Bhalla A. S.
Published: April 2011

Relaxor ferroelectrics are one of the important classes of self-assembled nanostructure composite materials. Interesting features associated with the nanoregions give rise to the most interesting device related characteristics and unusual properties in these materials. Besides, they possess the largest property coefficients by themselves or when modified with lead titanate (PT). In this report, a detailed temperature dependent study has been carried out on (1-x)PZN-xPT relaxors with compositions x = 0.05 and 0.085 using polarized Raman scattering under optical and E-field variables and inferred the structure-property relations in order to obtain information to characterize the material for matching the application criteria. In addition, phase transitions associated with the relaxors have also been investigated to understand the polarization mechanism(s) for the unpoled and poled specimens.
Thermoelectric Properties of Bi2Te3-Sb2Te3 Compounds Prepared by Ma-Pulse Discharge Sintering Process
Authors: Park R.E. , Park Y.H. , Abe T.
Published: April 2011

The Bi2Te3-Sb2Te3 compounds with the composition of useful thermoelectric cooling materials were prepared by mechanical alloying-pulse discharge sintering process. Effects of the process on the Seebeck coefficient, electrical resistivity and thermal conductivity were investigated. Temperature dependence of the Hall coefficient was also observed in the temperature range 80 - 325 K.

The figure of merit, Z, was found to be about 4.0 × 10−1K−1 at room temperature in the 25%Bi2Te3-75%Sb2Te3 composition sintered at 618K using grain refined mechanically alloyed powders which had the size of under 32 μm. The value of Z was remarkably improved with a decrease of the thermal conductivity shown in the fine grain compacts fabricated by mechanical alloying-pulse discharge sintering process.
Thermally Driven Shape Instability of Multilayer Structures
Authors: Troche P. , Hoffmann J. , Herweg C. , Lang CH. , Freyhardt H.C. , Rudolph D.
Published: April 2011

The thermally induced shape instability of Fe/Au, Fe/Ag, and Nb/Cu multilayer systems during heat treatments was investigated. The disintegration temperature of these systems decreases with decreasing single-layer thickness. Up to a critical thickness, the disintegration temperature is proportional to the reciprocal layer thickness. The driving force of this process is related to the interfacial stress and the local variation of the interface curvature. After heat treatment, spherically shaped Fe and Nb nanoparticles, located in chains perpendicular to the substrate, were observed and depicted by means of transmission electron microscopy (TEM) and X-ray microscopy (XRM).
Tight-Binding Molecular Dynamics of Ceramic Nanocrystals Using Pc-Based Parallel Machines
Authors: Tsuruta Kenji , Totsuji Hiroo , Totsuji Chieko
Published: April 2011

Evolution of atomic and electronic structures of silicon-carbide (SiC) nanocrystals during sintering is investigated by a tight-binding molecular dynamics (TBMD) method. An O(N) algorithm (the Fermi-operator expansion method) is employed for calculating electronic contributions in the energy and forces. Simulations are performed on our eight-node parallel PC cluster. In a sintering simulation of aligned (no tilt or twist) SiC nanocrystals at T = 1000K, we find that a neck is formed promptly without formation of defects. Analyses of local electronic density-of-states (DOS) and effective charges reveal that unsaturated bonds exist only in grain surfaces accompanying the gap states. In the case of tilted (<122>) nanocrystals, surface structures formed before sintering affect significantly the grainboundary formation.
Electrical Properties of Nanoscale Tisi2 Islands on Si
Authors: Oh Jaehwan , Ham Hoon , Laloli Peter , Nemanich R. J.
Published: April 2011

Nanoscale TiSi2 islands are formed by electon beam deposition of a few monolayers of titanium followed by in situ annealing at high temperatures (800–1000°C). The typical island sizes were ˜10 nm. Electrical characteristics of these islands were probed using UHV-STM. I-V spectroscopies on these islands show single electron tunneling effects such as Coulomb blockade and Coulomb staircase at room temperature.
Novel Strain-Relief Mechanisms For Ge/Si(100) Coherent Islands
Authors: Drucker Jeff , Chaparro Sergio , Zhang Yangting , Chandrasekhar D. , Mccartney M. R. , Smith David J.
Published: April 2011

Novel strain-relief mechanisms for Ge/Si(100) coherent islands were identified. Ge/Si(100) islands were grown at temperatures, T, between 450 and 650°C for effective Ge coverages between 3.5 and 14.0 monolayers. The mean dome size increased and island dislocation was delayed as T increased. For T > 600°C, very large hut clusters populated a peak in island size distributions distinct from and between those of huts and domes. For T > 550°C, large dome clusters may form trenches at their base which extend well into the Si substrate. Increasing T reduced the island size for trench formation. Cross-sectional energy dispersive xray nanoanalysis confirmed that Si diffuses into the Ge clusters at T ≥ 550°C. Si interdiffusion was responsible for the increase in dome size and delay of dislocation with increasing T and the existence of very large hut clusters. AFM cross-sections indicate that there is a linear trench depth dependence on island size. A simple atomistic elastic model suggests that this experimentally observed self-limiting trench depth is kinetic rather than energetic in origin.
Atomic-Resolution Z-Contrast Imaging and its Application to Compositional Ordering and Segregation
Authors: Pennycook S. J. , Yan Y. , Norman A. , Zhang Y. , Al-Jassim M. , Mascarenhas A. , Ahrenkiel S. P. , Chisholm M. F. , Duscher G. , Pantelides S. T.
Published: April 2011

In the last ten years, the scanning transmission electron microscope (STEM) has become capable of forming electron probes of atomic dimensions making possible a new approach to high-resolution electron microscopy, Z-contrast imaging. Formed by mapping the intensity of high-angle scattered electrons as the probe is scanned across the specimen, the Z-contrast image represents a direct map of the specimen scattering power at atomic resolution. It is an incoherent image, and can be directly interpreted in terms of atomic columns. High angle scattering comes predominantly from the atomic nuclei, so the scattering cross section depends on atomic number (Z) squared. Z-contrast microscopy can therefore be used to study compositional ordering and segregation at the atomic scale. Here we present three examples of ordering: first, ferroelectric materials, second, III-V semiconductor alloys, and finally, cooperative segregation at a semiconductor grain boundary, where a combination of Z-contrast imaging with first principles theory provides a complete atomic-scale view of the sites and configurations of the segregant atoms.
X-ray Diffraction Study of Chalcopyrite ZnSnP2 Epitaxial Layers
Authors: Francoeur S. , Seryogin G. A. , Nikishin S. A. , Temkin H.
Published: April 2011

We apply the technique of x-ray diffraction to the determination of the crystallographic structure and the quantitative measurement of the order parameter of ZnSnP2 epitaxial layers. In bulk, ZnSnP2 it is possible to obtain highly ordered distribution of Zn and Sn atoms in the cation sublattice, but epitaxial growth often produces partially ordered layers. The ordered and disordered phases correspond to the chalcopyrite and sphalerite structures and their respective band gaps are 1.66 and 1.24 eV. Since ZnSnP2 is almost lattice-matched to GaAs. it is interesting candidate for optoelectronic applications.

Samples used in this work were grown by gas source molecular beam epitaxy on GaAs substrates. Slight variations in growth conditions could be induced to produce partially ordered and disordered structures. Chalcopyrite ordering is determined by the observation of several characteristic reflections identifying the lower symmetry of this structure. For example, reflections from (101), (217) and (611) planes, strictly forbidden for sphalerite, were measured. The quantitative determination of the order parameter could be made by comparing intensities of a carefully chosen set of measured and calculated reflections. We show that while kinematic approximation can be used to model weak superstructure reflections, in the calculation of the strong, low-angle, fundamental reflections used for intensity normalization it is necessary to take into account extinction effects. Order parameters varying from 0 to 30% were obtained.
Reciprocal-Space and Real-Space Analyses of Compositional Modulation in InAs/AlAs Short-Period Superlattices
Authors: Follstaedt D. M. , Lee S. R. , Reno J. L. , Jones E. D. , Twesten R. D. , Norman A. G. , Ahrenkiel S. P. , Moutinho H. R. , Mascarenhas A. , Millunchick J. Mirecki
Published: April 2011

The microstructure of lateral composition modulation in InAs/AlAs superlattices grown by MBE on InP is examined. The use of x-ray diffraction, TEM, AFM, and STEM to characterize the modulations is discussed. Combining the information from these techniques gives increased insight into the phenomenon and how to manipulate it. Diffraction measures the intensity of modulation and its wavelength, and is used to identify growth conditions giving strong modulation. The TEM and STEM analyses indicate that local compositions are modulated by as much as 0.38 InAs mole fraction. Plan-view images show that modulated structures consists of short (≳0.2 μm) In-rich wires with a 2D organization in a (001) growth plane. However, growth on miscut substrates can produce a single modulation along the miscut direction with much longer wires (≲0.4 μm), as desired for potential applications. Photoluminescence studies demonstrate that the modulation has large effects on the bandgap energy of the superlattice.
Computer Simulations of the Self-Organized 3-D Islanding in Epitaxial Thin Film Systems
Authors: Zhang Y. W.
Published: April 2011

3-D computer simulations were carried out to investigate the effects of surface energy anisotropy and initial surface roughness on the self-organization of 3-D islands in epitaxial thin film systems. Under the present scheme, the driving force for surface diffusion is the gradient of surface chemical potential, which includes strain energy and surface energy. The simulations demonstrate that the typical sequence of surface evolution is from an initial random surface to a rippled surface, then to irregular islands and then to self-organized islands. Whether the final islands are regular or not depends on both the form of surface energy anisotropy and initial surface roughness. However, the stability and shape transition of the epitaxially strained islands are associated with the form of surface energy anisotropy. The comparisons between the simulations and various experiments were also discussed.
Diffusion and Distribution of Photoacid Generators in thin Polymer Films
Authors: Lin Qinghuang , Angelopoulos Marie , Babich Katherina , Medeiros David , Sundararajan Narayan , Weibel Gina , Ober Christopher
Published: April 2011

Distribution and diffusion of two fluorinated ionic photoacid generators (PAGs) in thin polymer films have been investigated by depth profiling of the intrinsic label elements of both the PAGs and a silicon containing carrier polymer with Rutherford Backscattering Spectrometry (RBS) and dynamic Secondary Ion Mass Spectroscopy (SIMS). Distribution and diffusion of the PAGs in a bilayer film stack, which consists of a thin silicon containing polymer film on top of a thick thermally cross-linked Novolak film on a silicon substrate, have been studied as a function of the Novolak cross-linking temperature. Deposition of the PAG containing polymer films on top of the cross-linked Novolak films by spin coating results in an interphase with enriched PAG. Subsequent annealing of the film stack causes expansion of the interphase and diffusion of the PAG into the underlying Novolak film when Novolak is cross-linked at lower temperatures. On the other hand, there is a uniform PAG distribution and no detectable diffusion of the PAG into Novolak when it is cross-linked at high temperatures. The variations in the PAG distribution and diffusion are attributed to the changes in the chemical and physical properties of Novolak during cross-linking.
Second Derivative Ballistic Electron Emission Spectroscopy in Au/(AlGa)As
Authors: Kozhevnikov M. , Narayanamurti V. , Smith D. L. , Chiu Yi-Jen
Published: April 2011

Our current study focuses on an analysis of the ballistic electron emission microscopy (BEEM) spectra of Au/(AIGa)As heterostructures to estimate quantitatively the effect of the carrier scattering in the metal, at the metal-semiconductor (m-s) interface and in the semiconductor on the multivalley carrier transport. The second derivative (SD)-BEEM spectra, representing the heterostructure transmission coefficient, show explicit partitioning of the contribution of different transport channels (Γ, L and X conduction valleys). Our analysis of SD-BEEM spectra by the developed theoretical model indicates that about 85–92% of the BEEM electrons are scattered at the nonepitaxial Au/GaAs interface. We also show that initial electron distribution among the conduction bands of the semiconductor, specified by the m-s interface scattering, is modified by the further hot-electron transport inside the semiconductor.
Modeling of Structural and Elastic Properties of InxGa1–xN Alloys
Authors: Grosse Frank , Neugebauer Jörg
Published: April 2011

InxGa1-xN alloys are investigated by combining density-functional theory (DFT) calculations and all extended valence force field model (VFF). Based on a large nuniber of ordered InxGa1-xN supercells we determined the accuracy of various valence force field models with respect to ab initio DFT calculations. To get the correct energy difference between the stable wurtzite and the inetastable zincblende phase the inclusion of the ion-ion-interaction within the VFF’ model was necessary. As a direct consequence the inequivalent bonds of the wurtzite structure including the non-ideal c/a ratio and internal u parameter are described correctly. The wurtzite phase is found to be stable against the zincblende phase for all concentrations. The calculated elastic constants and their dependency on In alloy composition enables the determination of the local concentration by simultaneous measurement of the local lattice constants a and c.
Realization of Detailed Kinetic Models for the Growth of II-VI Compounds: Adopting DFT Calculations and Experimental Evidences
Authors: Cavallotti Carlo , Bertani Valeria , Masi Maurizio , Carrà Sergio
Published: April 2011

The kinetics of deposition of CdTe, ZnS and ZnSe films was studied. An overall microkinetically reversible kinetic scheme consisting of elementary reactions, comprehensive of gas phase and surface processes was developed. Kinetic constants of gas phase reactions were either found in the literature or determined through quantum chemistry methods. Kinetic constants of surface reactions were first guessed combining kinetic theory with quantum chemical calculations and then their values were refined to reproduce experimental data. The kinetics schemes so developed were tested through the simulation of deposition reactors. Measured growth rates and gas phase compositions were thus compared with those calculated. The major finding of this studies is that the growth of CdTe, ZnS and ZnSe can be represented adopting a surface kinetic scheme constituted essentially by the same fundamental steps.
Effect of Hydrogen Coverage on Silicon Thin Film Growth: Molecular Dynamics Investigation
Authors: Muramatsu Shinya , Shimada Masatoshi , Hirao Masahiko
Published: April 2011

Silicon thin films grown by CVD processes take various structures depending on the reaction conditions. In the case of low-temperature CVD, hydrogen coverage on the silicon substrate affects the growth as well. In order to analyze the effect of hydrogen coverage on elementary surface reactions, we performed molecular dynamics simulations in which gasphase radicals, H and SiH3, fell onto a hydrogen-terminated silicon surface. We prepared monohydride and dihydride Si(100) surfaces and modified their hydrogen coverage in a certain area in the range from 0.5 to 2.0 monolayers so as to describe the growing surface. As a result of our simulation, H radicals mainly caused H adsorption and H abstraction reactions and altered the hydrogen coverage of the surface. While reactions of SiH3 radicals with monohydride or dihydride surfaces rarely occurred, these radicals reacted more frequently with the modified surfaces. These results indicate that the change in the local hydrogen coverage caused by H radicals may induce subsequent surface reactions
Effects of the Electronic Structure on the Stability of Metallic Superlattices. Semi-Emipirical Calculations of the Total Energy
Authors: Mazzone A. M
Published: April 2011

In this work the total energy of metallic superlattices is evaluated at semi-empirical level. The superlattice has an heterogenous composition and it has two shapes, i.e.(i) two facingA/B wires and (ii) a hut-shaped island of the element A deposited on a rigid B substrate. The elements A and B are Ag, Cu, Pd, Pt and Fe. The calculations show the existence of competing bonding contributions. In fact, a strong bond arises from charge exchanges at the A/B interface and from the bulk-like bonds of interior atoms. On the contrary, surface atoms are undercoordinated and reduce the strength of bonding. These contributions act on opposite sense on the large structures and the geometry dependence of the binding energy suggests a self-limited growth, rather than Ostwald ripening.
Nanoscale Electron-Beam Processes and Its Application to Nanodevices
Authors: Komuro Masanori
Published: April 2011

In this paper, electron beam (EB) lithography and direct processes are summarized for application to nanometer-scale electron devices such as single electron transistor. As decreasing line width in lithography, the delineated patterns has fluctuation of line width, which is so-called line-edge-roughness. It is kwon that such a roughness is caused by aggregates of resist molecules. Therefore to avoid the rougphness to make smooth line edge, we have to use rather low molecular weight resist materials. As one of such candidates, EB exposure characteristics of thermally oxidized SiO2 film are described. Although the sensitivity of the resist is about a few C/cm2 which is lower than conventional resists, it is possible to delineate fine line and space pattern with 15 nm pitch and 5 nm width. This technique is extended to make miniature metal/insulator/metal junctions using SiO2/Si bilayer resist system and the following metal liftoff process. Metal layer is directly deposited on SiO2 substrates in WF6 gas ambient simultaneously with EB irradiation. The resistivity of the deposited film is about 6×10/4 Ωcm depending on hydrocarbon contamination of the substrate itself. We can make the deposited lines with line-width of about 10 nm using 3 nm diameter of the incident EB. From current-voltage characteristics for single tunnel junctions with various tunnel resistances, the barrier height is estimated to be about 0.2 eV. This result indicates clearly the junction properties can be controlled at least with the accuracy of minimum deflection increment of the EB system used here. It is successfully observed that single-electron-transistor produced by EB-induced deposition exhibits Coulomb oscillation at temperature of 230 K.