bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic Mesoscale Modeling of Elastic-Plastic Deformation
Authors: Staroselsky A. , Bulatov V.V.
Published: April 2011

Plastic response of a solid under stress depends on its crystallographic structure and morphology. Two of the major mechanisms of plasticity in metals are crystallographic slip and twinning. The purpose of this work is to analyze the influence of local stress distribution on slip and twin nucleation and propagation and to examine how this behavior depends on the interaction among slips, twins, and grain boundaries. We formulate a simple model in which slip and twin systems are defined at appropriate angles to each other. Plastic flow is treated as a Markovian stochastic process consisting of a series of local inelastic transformations (LITs) in the representative volume elements (RVE). The probabilities of LITs per unit time are defined in the framework of transition-state theory. By varying the types of allowed LITs and/or the scale of RVE, plastic deformation is modeled at different structural levels, from a small volume of single crystal to the aggregate response of an isotropic polycrystalline solid. An important feature of this model is that evolution of the internal stress distribution is traced explicitly throughout the simulation run. This allows us to examine conditions of slip and twinning in considerable detail. In particular, we observe that twinning occurs through a nucleation-and-growth mechanism whose rate is controlled by the size of the critical nucleus of the new phase.
Strain Tensors at the Atomic Scale
Authors: Horstemeyer M.F. , Baskes M.I.
Published: April 2011

Almansi and Green strain tensors are developed for use in large deformation molecular dynamics/statics simulations that employ Embedded Atom Method (EAM) potentials for metals. The strain tensors are formulated with respect to the deformation gradient. A scalar potential function is used with a weighting function that is dependent upon a cutoff radius for the deformation gradient. For a homogeneous or inhomogeneous deformation, a cutoff distance of one lattice parameter can be used to approximate local strain level. Inhomogeneous deformation reveals different results for Almansi and Green strain tensors indicating that the small strain assumption cannot be used to determine large atomic strains.
Single Crystal Elastic Moduli of Disordered Cubic Alloys
Authors: Hartley Craig S.
Published: April 2011

A review of the relationship between elastic moduli and interatomic force constants precedes the description of a method for determining the composition dependence of single crystal elastic moduli of disordered alloys having the face-centered cubic structure. The method treats the alloy as a virtual crystal, characterized by an effective pair potential between atoms. Results of calculations are presented using experimental data on Cu-rich Cu-Al alloys.
The Structure of Defects Formed by Absorption of Crystal Dislocations in Interfaces in the HCP Metals
Authors: Bacon D.J. , Pond R.C. , Serra A.
Published: April 2011

Atomic-scale computer simulation has been used to investigate the interaction of crystal dislocations with two interfaces in hexagonal-close-packed (HCP) metals, namely the {1012} twin boundary and a <1210>/90° tilt boundary that is incommensurate in the direction perpendicular to the tilt axis. Crystal dislocations are absorbed in the tilt boundary with concomitant reconstruction of their cores. In the twin boundary, a broader range of interactions is observed, including defect transmission from matrix to twin and decomposition in the interface into discrete defects. The role of crystallographic features and interfacial structure is elucidated by comparing interaction processes in the two interfaces. The core structure of interfacial defects can be complex and contributes significantly to total defect energy.
Twinning Deformation in Martensite Microstructure
Authors: Ackland G.J. , Pinsook U.
Published: April 2011

Molecular dynamics is used to study twinning deformation in a martensite microstructure obtained from rapid cooling β zirconium through the bcc-hcp transition. The microstructure is composed of (1011) twin boundaries and boundary dislocations which sometimes spread across the twins to form stacking faults. A series of such equilibrium microstructures subjected to discrete, increasing < 1012 > (1011) shear strain. The stress-strain curve has stick-slip behaviour with yield stress of ≈ 5.OKbar and yield strain of ≈ 3.8%. Deformation occurs through movement of twin boundaries in segments between boundary dislocations. Straight perfect twin boundaries do not move.
Influence of Grain Size on Recrystallisation During hot Working of Austenitic Stainless Steels
Authors: Whiteman J. A. , Choi Y. , Sellars C.M.
Published: April 2011

During the hot rolling of austenitic stainless steels, complete static recrystallisation is expected between passes unless finishing temperatures are low. Typically progressive refinement takes place to grain sizes in the range 20–50μm. However, most experimental studies of the effect of strain, strain rate, temperature and initial grain size on recrystallisation kinetics and recrystallised grain size under hot working conditions have been carried out on initial grain sizes greater than 50μm. Empirical relationships from these data and from more limited results of CMn steels have been extrapolated to smaller grain sizes for use in models of microstructural evolution during rolling.

Recent development of a physically based model for the effects of initial grain size, assuming that site saturated nucleation occurs at grain corners, grain edges, grain faces and at intragranular sites leads to interdependence of the effects of strain and grain sizeon nucleation density and hence on recrystallised grain size and recrystallisation rate. Experimental evidence available in the literature and some new results on finer grained Type 316 stainless steel are reviewed and compared with the expectations from the model.
The Determination of Evolving Microstructure Using Constitutive Relationships
Authors: Diak B.J. , Saimoto S.
Published: April 2011

The evolution of the constitutive parameter, the mean slip distance, λ, is monitored during tensile deformation of 3.2 μm grain size aluminum at 200 K. Transmission electron microscopy (TEM) confirms that the grain size, D, sets an upper limit to λ.
Thixotropy of Semisolid Metals
Authors: Alexandrou Andreas N. , Burgos Gilmer R. , Entov Vladimir M.
Published: April 2011

Understanding the time-dependent flow behavior of metal alloys in semisolid state is essential for the further development of the process. In the present investigation, the thixotropic behavior of semisolid slurries is modeled using conservation equations and the Herschel-Bulkley fluid model. The rheological parameters are assumed to be functions of the solid volume fraction, and of a structural parameter that changes with processing history. The evolution of the structural parameter is described by a first order kinetic differential equation that relates the rate of build-up and break-down of the solid skeleton. The model is implemented into a computer code to predict die filling.
Experimental Analysis and Crystallographic Model of Plastic Deformation after a Change of Loading Path in Mild Steel Polycrystals
Authors: Hoc T. , Rey C.
Published: April 2011

Strain localization in mild steel submitted to a sequential loading paths is investigated at macroscopic, mesoscopic and microscopic scales. The experimental results demonstrate that the morphology of the localization and the nominal load-displacement curves depend on the microstructural anisotropy. A crystalline model using a finite element code is proposed. The anisotropy is described by a hardening matrix whose terms correspond to dislocation-dislocation interactions and depend on the evolution of the dislocation densities on the activated slip systems during the sequential tests. The strain localization predicted by this model fits with the experimental observation and allows us to assume that localization is correlated to the saturation on the activated slip systems.
Complementary Experimental Techniques for Multi-Scale Modeling of Plasticity
Authors: Levine L. E. , Long G. G. , Black D. R.
Published: April 2011

Some recently-developed experimental techniques, such asin situultra-small-angle Xray scattering (USAXS), have demonstrated a capability for measuring aspects of dislocation structure evolution that are inaccessible to other experimental methods. However, no single technique can provide the entire range of information required by theoretical and computational researchers. It is only through the synergy of several experimental techniques (such as USAXS, transmission electron microscopy, and X-ray diffraction imaging) that much of the required quantitative information can be obtained. Ultimately, the development of additional new experimental techniques will also be required.
Nano-Sized Semiconducting Oxide Powders for Thick Film Gas Sensors: From Powder Processing to Environmental Monitoring Devices
Authors: Traversa Enrico , Carotta Maria Cristina , Martinelli Giuliano
Published: April 2011

This paper reports the study of semiconducting oxides to develop gas sensors in thick-film form for use in atmospheric pollutant monitoring devices. The investigation was achieved with the following steps: selection of the suitable oxides and of their most appropriate processing method to obtain nano-sized powders, fabrication using screen-printing technology of thick-film sensors from these powders, and electrical measurements in laboratory and in the field. Chemical routes such as sol-gel techniques and thermal decomposition of heteronuclear complexes have been used to prepare nano-sized powders of n-type (TiO2) and p-type (LaFeO3 and SmFeO3) semiconducting oxides. Thick-film gas sensors have been produced by screen-printing technology. Pastes have been prepared and printed on laser precut 96% alumina substrates, each 2×2 mm element being provided with a heater, comb-type Au contacts and a Pt-100 resistor for controlling the operating temperature. The firing of the films has been performed in conditions able to keep grain size at nanometer level. Electrical responses to some major polluting gases (CO, NO, NO2 and O3) have been tested in laboratory and in the field, and compared with results of the analytical techniques approved by the international standards.
Synthesis and Characterization of Mn Doped CdS Quantum Dots from a Single Source Precursor
Authors: Malik M. Azad , O'Brien Paul , Revaprasadu N.
Published: April 2011

CdS and Mn-doped CdS capped with TOPO (tri-n-octylphosphineoxide) have been prepared by a single source route using bis(methylhexyldithiocarbamato)cadmium(II) and manganese dichloride as precursors. The nanoparticles obtained show quantum size effects in their optical spectra with the CdS nanoparticles exhibiting near band-edge luminescence. The PL spectrum of the doped CdS nanoparticles have an emission maximum at 585 nm attributed to the 4T1-6A1 electronic transition of Mn in a tetrahedral site. However the PL spectrum changes over time (weeks) and gave a deep trap emission. The Selected Area Electron Diffraction (SAED) and X-ray diffraction (XRD) pattern show both CdS and the Mn doped CdS particles to be of the hexagonal phase. Transmission Electron Microscopy (TEM) and High Resolution TEM show well-defined images of nanosize particles with clear lattice fringes. ESR spectra and ICP results confirm the presence of Mn in the CdS nanoparticles.
Synthesis and Characterization of Strongly Fluorescent CdTe Nanocrystal Colloids
Authors: Mikulec Frederic V. , Bawendi Moungi G.
Published: April 2011

We present a synthesis of colloidal CdTe nanocrystals whose absolute room temperature quantum yields are routinely above 60%. The preparation is based on the trioctylphosphine oxide (TOPO) method reported by Murray, with a more stalbe tellurium precursor now used as the chalcogenide source. The photoluminescence is continuously tunable over the range 590-760 nm and is as narrow as 135 meV (45 nm) FWHM. No deep trap luminescence is detected for the diameter range 4-11 nm. CdTe nanocrystals are characterized by UV/vis absorption, photoluminescence emission, transmission electron microscopy, and powder X-ray diffraction.
Size Effect in Germanium Nanostructures Fabricated by Pulsed Laser Deposition
Authors: Hassan K.M. , Sharma A.K. , Narayan J. , Muth J.F. , Teng C.W. , Kolbas R.M.
Published: April 2011

We have fabricated Ge nanostructures buried in AlN and Al2O3 matrices grown on Si(111) and sapphire substrates by pulsed laser deposition. Our approach involved three-dimensional island growth of low band-gap material followed by a layer of wide band-gap material. The nanodots were uniformly distributed in between alternating layers of AlN or Al2O3. It was observed that these nanodots exhibit crystalline structure when grown at 300-500 °C. The average size of Ge islands was determined to be ∼5-15 nm, which could be varied by controlling laser deposition and substrate parameters. The Raman spectrum showed a peak of the Ge-Ge vibrational mode downward shifted upto 295 cm− which is caused by quantum confinement of phonons in the Ge-dots. The photoluminescence of the Ge dots (size ∼15nm) was blue shifted by ∼0.266 eV from the bulk Ge value of 0.73 eV at 77 K, resulting in a distinct peak at ∼1.0 eV. The spectral positions of both E1 and E2 transitions in the absorption spectra at room temperature and 77K shift toward higher energy as the Ge dot size decreases. The interpretation of these behaviors in terms of quantum confinement is discussed in this work, and the importance of pulsed laser deposition in fabricating novel nanostructures is emphasized
Functionalisation of CdSe and GaAs Quantum Dots
Authors: Nørager Sebastian J. , Lazell Michael , O'Brien Paul
Published: April 2011

In this communication we report the preliminary results of in-situ functionalisation, of Q-CdSe and Q-GaAs using thiols, (HSC12H25), dithiols, (HSC9H18SH), pyridine derivatives (4-ethylpyridine, 4-butylpyridine) and carboxylic acids (mercaptopropionic acid).

The functionalisation of the capping group provides a potential route to the assembly of 2D arrays and supperlattices. Anchorage of the nanocrystals to surfaces for use in devices, such as solar cells and Grätzel type cells is also discussed.
Sputtering Effects and Two Dimensional Arrangement of Nanoparticles in Insulators Under High Flux Cu Implantation
Authors: Kishimoto N. , Lee C.G. , Umeda N. , Takeda Y. , Gritsyna V.T.
Published: April 2011

Application of negative heavy ions, alleviating surface charging on insulators, enables us to conduct low-energy and high-flux implantation, and leads to a well-defined tool to fabricate near-surface nanostructures. Negative Cu ions of 60 keV, at high doses, have generated nanocrystals in amorphous(a-)SiO2 with a size (∼10 nm) suitable for nonlinear optical devices. The kinetic processes, inside the solid and at the surface, are studied by cross-sectional TEM and tapping AFM, respectively. In a-SiO2, nanoparticles spontaneously grow with dose rate, being controlled by the surface tension and radiation-induced diffusion. Furthermore, the nanospheres give rise to a two-dimensional (2D) arrangement around a given dose rate. The 2D-distribution occurs in coincidence with enhanced sputtering where a considerable Cu fraction sublimates from the surface. The dose-rate dependence of nanoparticles indicates that the surface-sputtering process influences the intra-solid process and contributes to the 2D-distribution. A self-assembling mechanism for 2D-arrangement of nanospheres is discussed taking into account contribution of the surface sputtering.
Preparationand Properties of Colloidal Particles. Silica on Yttrium Iron Garnet
Authors: Godoi R.H.M. , Jafelicci M. , Portillo Joaquim
Published: April 2011

Various properties of particles can be altered by coating them with a layer of different chemical composition. Yttrium iron garnet (YIG) particles has been coated with silica for control of their sintering, corrosion resistance, and stabilization of magnetic properties. This silica cover was obtained by hydrolysis of tetraethylorthosilicate (TEOS) in 2-propanol. This material was characterized by transmission (TEM) electron microscopy, (XEDS) X-ray energy-dispersive spectrometry, (XPS) X-ray photoemission spectroscopy and (VSM) vibrating sample magnetometry. YIG was heterocoagulated by silica as indicated by TEM micrographies. XPS measurements indicated that only binding energy for silicon and oxygen was found on the silica shell, which confirms that the YIG was covered. The values of the saturation magnetization differ from the heterocoagulated system to well-crystallized YIG.
Composite Nanowires from Ion Beam Modification of Si Nanowires
Authors: Zhou X. T. , Peng H. Y. , Shang N. G. , Wang N. , Bello I. , Lee C. S. , Lee S. T.
Published: April 2011

Composite nanowires with typical diameters of 30-100nm, which consisted of Si, β-SiC, amorphous carbon were converted from Si nanowires by ion beam deposition. The Si nanorods were exposed to broad low energy ion beams. The low energy hydrocarbon, argon and hydrogen ions, generated in a Kaufman ion source, reacted with Si nanowires and formed the composite nanowires. It has been assumed that the reaction pathway to form the composite nanowires were driven by both thermal diffusion and kinetic energic of interacting particles.
Supersonic Cluster Beam Synthesis of Nanophase Materials
Authors: Milanil P. , Iannotta S. , Biasioli F. , Piseri P. , Barborini E.
Published: April 2011

We present the characterization of supersonic cluster beam deposition as a viable technique for the synthesis of nanostructured materials. Stable and intense cluster beams can be obtained with a pulsed microplasma cluster source. This technique has been applied to produce TiNi nanostructured thin films on various substrates at room temperature. The morphology and the structure of the film are strongly influenced by the precursor clusters. Films characterized by crystallite sizes of a few tens of nanometers can be grown without recrystallization by thermal annealing. The stoichiometry of the original TiNi alloy is maintained.
Magnetic Entropy Change of Nanocomposites Composed of a Silver Matrix and Grains of Iron-Oxide Or -Nitride
Authors: Yamamoto T. A. , Tanaka M. , Shiomi K. , Nakayama T. , Nishimaki K. , Nakagawa T. , Numazawa T. , Katsura M. , Niihara K.
Published: April 2011

Magnetocaloric effect of nanocomposites composed of iron-oxide or iron-nitride grains dispersed in a silver matrix was studied by calculating magnetic entropy change ΔS induced by a change in applied magnetic field H. These nanocomposites were synthesized by the inert gas condensation technique and nitridation by heat treatment in an ammonia gas stream. Average sizes of the iron-containing grains were 10-35 nm. Magnetic phases in the materials were Fe3O4 or γ -Fe2O3 for the oxide-composites and γ-Fe4N or ε -Fe3N for the nitride-composites. Values of the ΔS were obtained by applying a thermodynamic Maxwell's relation, (∂S / ∂H)T = (∂M / ∂T)H, to data set of magnetization M measured at various temperatures T. They clearly indicated significant enhancement due to the nanostructure as predicted.
Nanocomposites nc-TiN/a-Si3N4/a- and nc-TiSi2 with Hardness Exceeding 100 GPa and High Fracture Toughness
Authors: Vepřiek S. , Niederhofer A. , Moto K. , Nesládek P. , Männling H. , Bolom T.
Published: April 2011

In course of further development of our generic concept for the design of novel superhard nanocomposites [1,2] we have recently developed new multi-phase ultrahard nano-composite coatings with Vickers mirohardness of 80 to 105 GPa which is in the range of natural diamond. The coatings show a high elastic recovery of up to 90% upon a relatively large indentation deformation. The hardness measured by the depth sensing technique agree with those calculated from the area of the remaining pseudoplastic deformation. The very high apparent fracture toughness is illustrated by the absence of any radial cracks upon in-dentation with a large load of I N into 10.3 p.m thick film [3]. The unusual combination of high hardness, elastic recovery and apparent fracture toughness was attributed to the ample possibility of cracks deflection, meandering and termination during loading and cracks closure upon unloading [1,2,4]. The suggested nanostructure of the coatings has been elucidated on a basis of a complex analysis by means XRD, EDX, ERD, XPS and HR TEM.
Vanadium Oxide Nanotubes with Diamine Templates
Authors: Krumeich F. , Muhr H.-J. , Niederberger M. , Bieri F. , Reinoso M. , Nesper R.
Published: April 2011

Diamine vanadium oxide nanotubes (DA-VOx-NTs) have been prepared in high yields applying two different synthesis routes, namely either a sol-gel reaction followed by a hydrothermal treatment or an exchange reaction. The first route is analogous to the well-known preparation of VOx-NTs containing monoamines: a vanadium(V) alkoxide is mixed with an α, ω- diamine (H2N[CH2]nNH2with 12≤n≤20), the resulting adduct is hydrolyzed, and, after aging, a hydrothermal treatment leads to the nanotubes in pure form. The exchange of the monoamine template in VOx-NTs against diamines represents an alternative access to DA-VOx-NTs. This reaction occurs readily and is also applicable for diamines with short CH 2 chains like, e.g., in ethylene diamine. The TEM characterization of the DA-VOx-NTs confirms the tubular morphology which is widely preserved in the products of the exchange reactions. The characteristic feature of as-synthesized DA-VOx-NTs is a thick tube wall with many VO, layers while the exchange products are tubes comprising thin walls or bent packs of layers.
Phase Transition and Control of Self-Assembled Mesostructured Vanadia/Surfactant Composites
Authors: Honma I. , Yagi Y. , Zhou H. S. , Endo A. , Miyayama M. , Kudo T.
Published: April 2011

The mesostructured vanadia/surfactant composites are synthesized by self assembled processes and phase transition has been investigated. The lamellar mesostructure of the vanadia/surfactant has been shifted to monoclinic and hexagonal mesophase by low temperature annealing, possibly due to the change of the inorganic/surfactant ratio.
Optical Characterization of a Spheroidal Nanoparticle on a Substrate
Authors: Román-Velázquez Carlos E. , Noguez Cecilia , Barrera Rubén G.
Published: April 2011

With the help of a spectral formalism recently formulated, we study the effects in the optical response of the material properties of a nanoparticle lying over a substrate. A spectral representation was formulated to calculate the optical response of spheroidal nanoparticles including multipolar effects. We present our results in terms of Differential Reflectance spectra that can be compared directly with measurements. We have found that multipolar contributions depend in the shape of the particle and type of substrate.
Hysteresis Loop Shifts in Magnetic Field Cooled FeOOH Nanoparticles
Authors: Seehra M.S. , Roy Paromita , Manivannan A.
Published: April 2011

Measurements of the magnetization M as a function of temperature (5K - 300K) and applied magnetic field H (up to 50 kOe) in 30 Å particles of FeOOH are reported. M increases with decreasing T, peaking at TB = 65 K below which the ZFC (zero-field-cooled) and the FC (field-cooled) data separate. Hysteresis loop measured at 10 K for ZFC shows an open loop up to 40 kOe with coercivity = 2 kOe. For the FC case, the loop shifts and the loop-shift increases with the cooling field ItL, approaching saturation above Hc = 20 kOe. From the variation of M vs H above TB, a magnetic moment/particle μp = 300 μB is determined. These results suggest that the FeOOH nanoparticles have an antiferromagnetically ordered core with uncompensated surface spins yielding μp and the surface spins order in a spin-glass-like state below TB, possibly due to interparticle interactions.
Preparation of Nanocrystalline Bismuth-Containing Powders through Solution-Phase Reductions
Authors: Foos Edward E. , Berry Alan D. , Snow Arthur W. , Armistead J. Paul
Published: April 2011

In an attempt to both prepare nanocrystalline bismuth and understand the fundamental chemistry behind the formation of this potentially interesting material, we have examined the reduction of BiCl3 in the presence of strongly coordinating solvents and ligands. These studies have resulted in the formation of bismuth powders with approximate average particle sizes of between 20 and 40 nm which exhibit large size distributions. The simultaneous reduction of both gold and bismuth precursors, done in an attempt to better control the final particle size, instead produces Au2Bi of comparable dimensions. There is no evidence that the ligands utilized in either of these systems remain bound to the final product. These nanocrystalline powders have been characterized through XRD and TEM, and full details of the synthesis are presented.
Temperature Dependence of the Electrical Resistivity in Nanocrystalline Gold Films Made by Advanced GaS Deposition
Authors: Ederth J. , Kiss L. B. , Niklasson G. A. , Granqvist C. G. , Olsson E.
Published: April 2011

Nanocrystalline thin Au films with grain size 10 - 76 nm have been analyzed regarding the temperature dependence of the electrical resistivity. A sudden change in the power function, ρ α Tn, was found at ∼10 K, where n = 1.7 in the range 5 - 10 K and n = 3.3 in the range 10 - 15 K. This effect disappears after annealing at 773 K for 0.5 h in air at atmospheric pressure. After the annealing the grain size was ∼ 100 nm. This is an indication of interference between electron-phonon scattering and electron-grain boundary scattering in nanocrystalline materials at low temperatures.

The temperature coefficient of resistivity, TCR, increased with increasing grain size at any temperature and the position of the maximum TCR was shifted towards lower temperatures with increasing grain size.
Microstructure and Mechanical Properties of Oxide Eutectic Fibers
Authors: Durbin S. D. , Yoshikawa A. , Hasegawa K. , Lee J.-H. , Epelbaum B. M. , Fukuda T. , Waku Y.
Published: April 2011

Some oxide eutectic materials, such as sapphire/YAG, display an intricate “Chinese script” microstructure which strongly influences properties such as mechanical strength. We used the micro pulling-down technique to grow fibers of a series of such materials at rates over 20 mm/min, giving a uniform microstructure with a characteristic size as low as 150 nm. Bulk and surface characteristics of this microstructure were investigated by electron and atomic force microscopy. Smaller microstructure size gave higher tensile strength at 1500°C. Annealing of the fibers produced some coarsening of the microstructure, but increased the strength at lower temperatures.
Structure and Stability of Nano-Scale Melt Spun Mg-Ca-Zn Alloys
Authors: Jardim P.M. , Solórzano I.G. , Sande J.B. Vander , You B.S. , Park W.W.
Published: April 2011

Alloys, with six different compositions in the system Mg-Ca-Zn were produced by melt spinning. The aging behavior of alloys was investigated by measuring the changes in microhardness after isochronal aging and the microstructure was analyzed by means of Transmission Electron Microscopy(TEM), Energy Dispersive X-ray Spectrometry (EDS), Scanning Transmission Electron Microscopy (STEM) and Scanning Electron Microscopy (SEM). All six compositions in the as-solidified condition show a difference in microstructure between the wheel contact side (zone A) and the free surface side (zone B) as a result of the differences in the solidification rate across the ribbons. One of the alloys was chosen to be more deeply investigated in this work as it exhibited grain boundary films and presented the highest peak hardness among the low Ca alloys. The comparison between the two microstructural zones in this alloy can aid in understanding of the phase transformation steps during cooling with a model which is proposed here.
Nano-Photoluminescence Studies of Self-Assembled Quantum Dots
Authors: Htoon H. , Yu Hongbin , Kulik D. , Keto J. W. , Baklenov O. , Holmes A. L. , Shih C. K.
Published: April 2011

Two simple and effective far-field-optics-based methods capable of isolating photoluminescence peaks of different individual self assembled quantum dots (SAQD's) with nanometer scale precision are presented. By using these methods, we performed the temperature and electric field dependent studies on the optical properties of SAQD's. We found temperature induced inter-dot carrier transfer among neighboring quantum dots (QD's) and observed the quantum confined stark effect (QCSE).
Kinetic Aspects of Island Nucleation Derived from Near Equilibrium Growth Experiments
Authors: Christiansen S. H. , Becker M. , Wawra H. , Albrecht M. , Strunk H. P.
Published: April 2011

Using liquid phase epitaxy from Bi solution, a by its nature a near equilibrium growth process, we study the kinetics of island formation in the heteroepitaxial system SiGe/Si(001) as dependent on growth temperature, growth rate and composition (which also determines the lattice misfit between layer and substrate). As a main result island formation can be described by classical nucleation theory, moreover, it can be described as any other crystallization process such as solid state crystallization of amorphous silicon or crystallization from a melt, provided that the limited size the islands can grow into is correctly considered. In consequence, after an incubation time period that depends on the growth temperature, islands nucleate and cover the substrate surface with time. The activation energy of island nucleation is 0.84±0.13eV. The coverage with islands depends only on the undercooling and is independent of the cooling rate in case near equilibrium growth conditions are maintained. In these cases the islands have the shape of truncated pyramids with four {111}– side facets and a base width λ that only depends on the misfit f (λ ∝ 1/f2). Deviations from the equilibrium growth stage at high growth rates (thus higher growth driving forces) result in the formation of a higher density of smaller islands with smaller facet angles. At higher growth rates, some kinetic influences begin to appear indicated by the additional appearance of shallower pyramids with four {115}– facet side faces.
Reciprocal Space X-Ray Mapping and Transmission Electron Microscopic Studies of Coincided δ-Inas and As-Cluster Superlattices in Gaas Films Grown by Molecular-Beam Epitaxy at Low Temperature
Authors: Chaldyshev V. V. , Bert N. A. , Faleev N. N. , Musikhin Yu. G. , Kunitsyn A. E. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R. , Werner P. , Takeda Y.
Published: April 2011

InAs/GaAs superlattices with thin (0.5–1 monolayer) δ-InAs insertions were grown by molecular beam epitaxy at low (150–200°C) temperature. The as-grown samples contained up to 1020 cm−3 arsenic antisite defects. Transmission electron microscopic study revealed no extended defect and showed that the real thickness of δ-InAs insertions is 3–4 monolayers. This thickness seems to be due to short-range roughness of the growth surface. Low diffuse scattering and extended interference picture were observed for such superlattices by x-ray diffraction study. Superlattices of two-dimensional cluster sheets were produced by annealing of the δ-InAs/GaAs superlattices at 500–600°C. Precipitation of excess arsenic at InAs δ-layers was found to be accompanied by enhanced In-Ga intermixing, roughening the InAs δ-layers, and smoothing the x-ray interference picture. No evidence for any self-ordering in the system of nanoscale As clusters was revealed using x-ray mapping in reciprocal space.
Growth Phenomena of Quantum Dot Structures in the Ingaas System Investigated by Tem Techniques
Authors: Werner P.
Published: April 2011

The contribution is mainly focused on the formation and properties of InGaAs islands (quantum dots, QD) in a GaAs matrix, viz. a system showing properties similar to those of other materials systems. Depending on the growth techniques applied (MBE or MOCVD), the islands/dots differ in size, shape, chemical composition, and lattice strain. These parameters influence strongly the optical properties of QD structures, which might consist of single layers or complex 3-dimensional arrays. To reveal the correlation between the morphology/structure and the optical behavior of such systems different analytical methods are available, more effective in some combined applications. The present paper is mainly focused on the possibilities and limitations of transmission electron microscopy (TEM) to analyze the lattice structure of QDs down to the nm-range. Such TEM investigations imply a general problem: to deconvolute separately information on the QD size, shape (e.g., pyramidal or spherical islands), strain and composition. TEM imaging techniques used for such structural analyses will be described, including conventional diffraction contrast and high-resolution electron microscopy, respective examples of which will be presented. For the chemical analysis energy sensitive techniques (e.g., energy filtered images) are appropriate methods. Applications and limitations of such investigations will be discussed.
X-Ray and Tem Studies of Short-Range Order in Al1−xInxAs Thin Films
Authors: Kulik Joseph , Forrest Rebecca , Li Jianhua , Golding Terry , Moss Simon C. , Bai Jianming
Published: April 2011

X-ray scattering and transmission electron microscopy (TEM) have been used to study short-range order in Al1−xlnxAs thin films grown by molecular beam epitaxy. Two samples grown on (001) InP at temperatures of 370°C and 400°C are characterized. The first exhibits simultaneous triple-period-A and CuPt-A short-range order with a rather short correlation range of about 2.4 nm normal to the (111) planes. Within these (individual) planes the concentration, however, is uniform over a considerably greater distance – about 6 to 9 nm – leading to a highly anisotropic scattering. This observation of triple-period short-range ordering in a sample that exhibits 2×1 surface reconstruction during growth is unexpected. We also report on the first observation of the coexistence of triple-period-A and CuPt-B short-range order. The diffuse scattering exhibits significant intensity anomalies that we attribute to atomic displacements associated with the short-range order. The second sample exhibits CuPt-B short-range ordering with scattering that is significantly streaked, suggestive of lamellar-shaped ordered domains. This sample also exhibits intensity anomalies that must be associated with atomic displacements. The first sample contains a high density of twin lamellae, while both samples contain high densities of stacking faults leading to additional narrow streaking along symmetry-allowed 〈111〉 directions. These growth faults most likely arise from the relatively low growth temperatures.
Epitaxial Growth of CuAu–Ordered CuInSe2 Structural Polytypes by Migration Enhanced Epitaxy
Authors: Stanbery B. J. , Chang C.-H. , Kim S. , Kincal S. , Lippold G. , Ahrenkiel S. P. , Li L. , Anderson T. J. , Al-Jassim M. M.
Published: April 2011

Migration-Enhanced Epitaxy (MEE) has been successfully employed to grow epitaxial films of the ternary compound CuInSe2 on (001) GaAs that are ordered in a CuAu crystallographic structure rather than the compound's equilibrium chalcopyrite structure. Evidence for this structural polytype is provided by XRD, TEM–TED, and Raman scattering data. Film growth under the conditions employed for this study occurs in a Stranski-Krastanov mode. The effects of growth system parameters and overall stoichiometry on 3–D island formation are described, including our observation that ordering of those islands into quasiperiodic self-assembled arrays occurs only in the case of nonstoichiometric copper excess.
Morphological and Compositional Instabilities of Strained and Unstrained Alloy Layers
Authors: Glas F. , Patriarche G.
Published: April 2011

We study several aspects of the instability of epitaxial layers with respect to composition modulations and surface non-planarities. After recalling previous experimental and theoretical results, we first give a detailed calculation of the elastic energy of a lattice-matched layer where both these phenomena occur. As in the lattice-mismatched case, the elastic strain fields of the modulation and the undulation interact, but the critical temperature under which composition modulations become favorable remains the same as in the planar layers. However, when a composition modulation is present, the system is unstable to a specific class of surface undulations. We then discuss TEM experiments investigating systematically the morphological instability in superlattices containing either tensile or compressive layers. Growth on surfaces variously disoriented with respect to (001) allows us to study two debated points: Does the instability develop by bunching of steps already present on the substrate or through nucleation of new steps? What is the nature of the asymmetry between tensile and compressive layers? We show that in the compressive layers, the surface non-planarity develops by bunching of a large fraction of the original substrate steps, whereas for layers under tension facets appear.
Growth and Characterization of Inas Quantum Dots on Silicon
Authors: Hansen L. , Ankudinov A. , Bensing F. , Wagner J. , Ade G. , Hinze P. , Wagner V. , Geurts J. , Waag A.
Published: April 2011

Up to 1011 cm−2 InAs quantum dots (QD) can be grown on Silicon(001) by molecular beam epitaxy. This very new material system is on the one hand interesting with regard to the integration of optoelectronics with silicon technology on the other hand it offers new insight into the formation of QDs. We report on RHEED, TEM and Raman studies about (in-) coherence of the QDs and on an according to our knowledge so far unknown dewetting transition in this material system. The results are being discussed on the basis of a thermodynamic model, assuming a liquid-like behavior of a strained adlayer.
Profiling Composition Variations in Composition-Modulated GaP/InP Short-Period Superlattices Using Resonance Raman Scattering
Authors: Cheong H. M. , Zhang Yong , Norman A. G. , Perkins J. D. , Mascarenhas A. , Cheng K. Y. , Hsieh K. C.
Published: April 2011

We use resonance Raman scattering (RRS) and electroreflection (ER) measurements to profile the the composition and strain variations in laterally composition-modulated (CM) GaP/InP short-period superlattices (SPS's). The ER spectra of a GaP2.2/InP2.0 SPS give the fundamental band-gap energy at 1.69±0.05eV, which is about 210 meV lower than the band gap energy of a GaInP random alloy with the same overall composition. The RRS measurements reveal strong dependences of the phonon spectrum on the polarization and the excitation energy. In RRS spectra measured with the polarization of both excitation and scattered photons along the composition modulation direction, the GaP-like longitudinal optical (LO) phonon redshifts by 4.0±0.5 cm−1 near the resonance with the fundamental energy gap. On the other hand, when the polarizations are orthogonal to the composition modulation, the LO phonons redshift as much as 16 cm−1 at low excitation energies. A comparison of the experimental data with a model calculation gives the average In composition in the In-rich region as 0.70±0.02, and the average Ga composition in the Ga-rich region as 0.68±0.02. Our result also indicates that there are small volumes (less than 1% volume fraction) with very high In mole fraction.
Self-Organized Composition Modulation In Omvpe Ga1−xInxAsySb1−y/GaSb Epitaxial Heterostructures
Authors: Chen Y-C. , Bucklen V. , Rajan K. , Wang C. A. , Charache G. W. , Nichols G. , Freeman M. , Sander P.
Published: April 2011

Microstructures of lattice-matched Ga1−xInxAsySb1−y grown by organometallic vapor phase epitaxy (OMVPE) on (100) 6°→ (111)B GaSb substrates have been examined in detail by transmission electron microscopy. A three-dimensional self-organized composition modulation (SOCM) microstructure was found with an orientation inclined 10 degrees to the surface orientation when viewed in (011) cross-section. The periodicity of the SOCM increased from ˜13 nm to 20 nm, as x increased from 0.1 to 0.2 while the orientation of the SOCM remained the same. The fact that the orientation was not sensitive to the component composition indicated that substrate misorientation plays a major role in deciding this SOCM orientation. This may open fabrication opportunities for three-dimensional natural superlattices by engineering on the substrate misorientation.
Novel Stm Probe Assisted Site-Control of Quantum Dots With Nanometer Precision
Authors: Nakamura Hitoshi , Kohmoto Shigeru , Ishikawa Tomonori , Asakawa Kiyoshi
Published: April 2011

We propose a novel site-control technique for strained quantum dots (QDs) based on nano-lithography using an STM integrated into a UHV STM/MBE multi-chamber system. A nano-scale deposit was formed on a GaAs surface by applying voltage between the GaAs surface and the tungsten tip of the STM. Since the deposit acted as a nano-mask, the subsequent GaAs growth formed a nano-hole just above the deposit. Subsequent InAs supply produced a QD on the hole site, and no QD was observed in any undesirable regions. We also observed the QD formation processes involved in the technique, based on step-by-step STM observations of the QD formation process. The observation directly revealed an InAs wetting layer formation with 1-ML thickness on the GaAs terraces followed by the QD formation in the Stranski-Krastanow growth mode.
Optical Characterization and Process Control of Top Surface Imaging
Authors: Luo Ying-Ying , Stauffer Craig , Ygartua Carlos , Chu Dinh , Hayzelden Clive
Published: April 2011

The use of selectively silylated resists to facilitate top surface imaging offers the potential for nanoscale lithography using both deep ultra violet (DUV) illumination and electron beam techniques. In this process, an exposure-generated crosslinking prevents silicon incorporation from silylating agents. In the non-crosslinked regions, a silylation agent reacts with OH groups in the resist to form silicon-oxygen bonds. During subsequent dry development in an oxygen plasma, the incorporated silicon attracts oxygen to form silicon dioxide, protecting the resist underneath. The adjacent un-silylated resist erodes (develops) anisotropically 25–100 times faster than the silylation-protected resist. The key to this process is the chemical formation of a silylated region at the top of the resist. A challenge associated with the silylation process, however, is lateral swelling of the silylated layer, which can lead to difficult dimensional control. The amount of swelling depends on the amount of incorporated silicon. Therefore, the uniformity and repeatability of the silylation process must be controlled. This paper will describe how spectroscopic ellipsometry has been used to characterize and monitor the resist silylation process in a non-destructive manner. The simultaneous characterization of the thicknesses and optical properties of a series of silylated resists will be presented. Optical metrology of the thickness of the silylated resists will be correlated with SEM cross-sectional analyses, and process uniformity will be quantified using 49-site wafer maps.
Extending the Possibilities of Near-Field Scanning Optical Microscopy for Simultaneous Topographical and Chemical Force Imaging
Authors: Nagy N. , Goh M. C.
Published: April 2011

The Near-field Scanning Optical Microscope (NSOM) is an innovative new form of surface microscopy, which can be used to obtain local spectroscopic information about surfaces, enabling the characterization of nanometer-sized regions. The most important component of this instrument is the scanning probe tip. In this paper, we discuss the production of a novel fiber optic probe that can be used in local spectroscopy with an NSOM, but also for
simultaneous imaging of topography and chemical forces. The probe consists of a bent, tapered silicon dioxide optical fiber. We have determined the rates of selective wet chemical etching of germanium dioxide doped pure silica optical fibers and used this information to optimize the probe etching process. A systematic approach for the development and testing of such probes is presented. The performance of the optical probes was characterized using surfaces prepared by the technique of microcontact printing. Phase and friction images of these surfaces were obtained using both standard atomic force microscopy tips and the optical fiber probe. The new optical probe was capable of distinguishing between different chemical regions on the patterned surface.
Effects of Interface Roughness and Embedded Nanostructures on Device Properties
Authors: Ting D. Z.-Y. , Mcgill T. C. , Chen N. Y. , Wang J. N. , Li R. G. , Wang Y. Q. , Ge W. K. , Schulman J. N.
Published: April 2011

We give an overview of the open-boundary planar supercell stack method (OPSSM), as a means for treating 3D quantum transport in mesoscopic tunnel structures. The flexibility of the method allows us to examine a variety of physical phenomena relevant to quantum transport. In this work we focus on the effects of interface roughness and embedded nanostructures in tunnel devices. Four representative applications of OPSSM are discussed: (1) interface roughness in double barrier resonant tunneling structures, (2) self-organized InAs quantum dot insertions in GaAs/AlAs double barrier structures, (3) tunneling characteristics of ultra-thin oxides with interface roughness, and, (4) embedded quantum wire model of dielectric breakdown. These examples demonstrate scattering and localization effects under different biasing conditions.
Cluster Model Study of the Incorporation Process of Excess Arsenic into Interstitial Positions of the GaAs Lattice
Authors: Marek T. , Kunsági-Máté S. , Strunk H. P.
Published: April 2011

We study an As2 molecule approaching a planar, non-reconstructed, As terminated GaAs(001) surface by using a suitable cluster and quantum mechanical ab-initio calculation methods. During our calculations the As2 molecule is always oriented perpendicularly to the surface and its bonding length may vary. We find a metastable position below the growing surface which facilitates the incorporation of the leading As atom into an interstitial position of the crystal during growth. We give a first model for this incorporation process.
Step Bunching during SiGe Growth on Vicinal Si(111) Surfaces
Authors: Hibino H. , Ogino T.
Published: April 2011

We investigate step bunching during SiGe growth on vicinal Si(111) surfaces. Step bunching occurs irrespective of the misorientation angle and direction of the vicinal surface, the growth temperature, and the Ge concentration. At 550°C, the average number of the steps in the bunch increases with the Ge concentration. After growth of 10-nm-thick SiGe layers, twodimensional islands are formed on the terraces, which indicates that the terrace width has already been saturated. Therefore, the terrace width is mainly determined by the diffusion length of the adatom. The average number of steps in the bunch increases with the Ge concentration because the diffusion length increases with the Ge concentration. The diffusion length also increases with the temperature. So the higher the temperature is, the larger the step bunch becomes.