bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and Characterization of Magnetic Nanostructures on Carbon Nanotube Templates
Authors: Wu Yihong , Qiao Peiwen , Chong Towchong
Published: April 2011

ABSTRAT

We describe the growth of magnetic nanostructures on carbon nanotube templates. The nanotubes were grown by microwave plasma enhanced chemical vapour deposition. The as grown nanotubes were aligned reasonably well around the substrate normal directions. Although the nanotubes were quite straight, there were still some bent and tilt as revealed by the scanning electron microscope observations. Magnetic field has been used to re-align or re-assemble the nanotubes before they were used as the templates to grow magnetic nanostructures. Depending on whether there is a magnetic particle on the top tip of each nanotube and the density of the nanotubes, there are two different consequences of applying a magnetic field to the nanotubes. For nanotubes with magnetic particles attached to their top tips, the post-growth treatment by the magnetic field resulted in re-assembly of the nanotubes into micro-umbrella type of structures. For those without magnetic particles, however, the effect of magnetic field treatment is negligible; but after the deposition of thin magnetic layers, the field treatment made the nanotubes much straighter than what they originally were and aligned almost vertically to the substrates. The re-aligned or re-assembled nanotubes were used as the templates to grow magnetic nanostructures. It was found that most of the magnetic nanostructures exhibited characteristics similar to those of magnetic nanowires.
Pair-state formation in a nanocrystal: a theoretical perspective
Authors: Suyver J. F. , Meester R. , Meijerink A. , Kelly J. J.
Published: April 2011

Simulations of dopant pair-state distributions are presented for zincblende nanocrystals with different radii and for different dopant fractions. The probability of finding at least one pair-state and the concentration of pair-states were calculated on the basis of a statistical average of 105simulations per crystal size and dopant concentration. The distribution of nanocrystal lattice positions over the surface and the bulk of the crystal is computed. A mathematical description of the distributions, valid in any crystal lattice, is discussed. This removes the need for further simulations.
Depth Selective Mössbauer Spectroscopy as a Bridge between Surface and Small Particle Phenomena in Magnetic Materials
Authors: Stahl B. , Ghafari M. , Hahn H. , Kamzin A. , Hanzel D.
Published: April 2011

A most feasible tool in characterizing magnetic properties is Mössbauer spectroscopy. By a combination with electron spectroscopy, the depth profile of the Mössbauer excitation is experimentally accessible [Liljequist01, Stahl97]. Depth selective Mössbauer studies in the vicinity of the Néel temperatures of an antiferromagnetic FeBO3 single crystal and a α-FeOOH thin film and of the Morin transition in an β-Fe2O3 single crystal are presented. Especially in FeBO3, the divergence of both, the spatial correlation between atomic moments and the autocorrelation of the local moment, could be observed in the critical region.
A molecular dynamics investigation on grain disappearance at a triple junction in polycrystalline silicon
Authors: Satta Alessandra , Colombo Luciano , Cleri Fabrizio
Published: April 2011

Topological changes in microstructure are strictly related to the microscopic evolution of triple junctions (TJ). The three-sided grain disappearance, usually called T2 process, is here investigated via 3D-atomistic modeling. In particular the stability of a three-sided grain insertion in a triple junction in silicon is studied within the framework of Molecular Dynamics simulations. The Stillinger-Weber interatomic potential is adopted and constant-traction border conditions are considered to ensure a proper embedding of the atomistic region in a virtually infinite bulk continuum. Dealing with the T2-event, the critical radius below which the three- sided inner grain become unstable is evaluated to be three to four times the lattice constant of silicon. Moreover, we show that the instability sets in through the amorphization of the central shrinking grain.
Pulse frequency effect on neutron damage in α-Iron:A KMC analysis
Authors: Perlado J.M. , Lodi D. , Dominguez E. , Ogando F. , Prieto J. , de la Rubia T. Diaz , Caturla M. J.
Published: April 2011

The pulsed nature of the irradiation and the high neutron dose are the critical factors in an Inertial Fusion Energy reactor (IFE). The damage that structural materials suffer under these extremes conditions require a careful study and assessment. The goal of our work is to simulate, trough the multiscale modelling approach, the damage accumulation in μ-Fe under conditions relevant to a IFE Reactor. We discuss how the pulse frequency, 1 Hz, 10 Hz, and the dose rate of 10 ⊏2 and 10 ⊏1 dpa/s affect the damage production and accumulation. Results of the damage that this demanding environment can produce on a protected first structural exposed to 150 keV average recoil ion will be presented. A further comparison it has been made with the damage produced by a continuous irradiation at similar average dose.
Computer Simulation of Hydride Precipitation in Bi-crystalline Zirconium
Authors: Ma X.Q. , Shi S.Q. , Woo C.H. , Chen L.Q.
Published: April 2011

γ-hydride precipitation and growth in a zirconium bi-crystal were simulated using a phase field kinetic model. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations for the structural variables and the Cahn-Hilliard diffusion equation for the concentration variable. The morphology evolution of γ-hydride with and without external load was simulated. It is demonstrated that nucleation density of the hydride at the grain boundary increases as compared to the bulk and favorable hydride precipitation at the grain boundary become weaker when an external load is applied.
In-plane Thermal and Electronic Transport in Quantum Dot Superlattice
Authors: Khitun A. , Liu J.L. , Wang K.L. , Chen G.
Published: April 2011

We present a theoretical model in order to describe both thermal and electronic in-plane transports in quantum dot superlattice. The model takes into account the modifications of electron and phonon transport due to the space confinement caused by the mismatch in electronic and thermal properties between dot and host materials. The developed model provides the analysis of the in-plane superlattice electronic and thermal properties versus quantum dot size and their arrangement. Numerical calculations were carried out for a structure that consists of multiple layers of Si with regimented germanium quantum dots. The simulation results of the lattice thermal conductivity are in a good agreement with experimental data.
Recent Progress Toward an Integrated Multiscale-Multiphysics Model of Reactor Pressure Vessel Embrittlement
Authors: Wirth B. D. , Odette G. R. , Stoller R. E.
Published: April 2011

The continued safe operation of nuclear reactors and their potential for lifetime extension depends on ensuring reactor pressure vessel integrity. Reactor pressure vessels and structural materials used in nuclear energy applications are exposed to intense neutron fields that create atomic displacements and ultimately change material properties. The physical processes involved in radiation damage are inherently multiscale, spanning more than 15 orders of magnitude in length and 24 orders of magnitude in time. This paper reports our progress in developing an integrated, multiscale-multiphysics (MSMP) model of radiation damage for the prediction of reactor pressure vessel embrittlement. Key features of the fully integrated MSMP model include: i) combined molecular dynamics (MD) and kinetic lattice Monte Carlo (KMC) simulations of cascade defect production and cascade aging to produce cross-sections for vacancy, self- interstitial and vacancy-solute cluster size classes for times on the order of seconds; ii) an integrated reaction rate theory and thermodynamic code to predict the evolution of nanostructural and nanochemical features for times on the order of decades; iii) a micromechanics model to calculate the resulting mechanical property changes. This paper will focus on the combined use of MD and KMC to simulate the long-term rearrangement (aging) of defects in displacement cascades and thus, produce late-time production cross-sections for vacancy and vacancy cluster features.
Combination of Continuum and Atomistic Approaches for the Study of Dislocation Nucleation from Atomic Size Surface Defects
Authors: Brochard Sandrine , Beauchamp Pierre , Grilhé Jean
Published: April 2011

Atomistic simulations realized on an f.c.c. crystal containing atomic size surface defects (step and groove) show that the defects are privileged sites for dislocation nucleation. Before nucleation, an elastic shear, precursor of the dislocation, appears in the plane in zone with the step where the dislocation will be nucleated. In order to explain the strong localization of the localized elastic precursor shear, we have analyzed the stress concentration near the surface defects using the continuum point force approach. For the step case, the origin of the localized shear is related to an increase in the interplanar separation due to the stress concentration.
Multi-Scale Simulations of Silicon Etching by Halides: Effects of Surface Reaction Rates.
Authors: Kratzer Matthias , Steinhögl Werner , Kersch Alfred , Sachse Tanja , Höink Volker
Published: April 2011

We investigate the reactive ion etching of amorphous silicon by halides using a hierarchy of models on different time and length scales. The feature evolution is modeled using a two- dimensional cell based Monte-Carlo feature scale simulator. The fluxes, the energy distributions, and the angular distributions of the wafer-incident particles are provided by a hybrid plasma sheath simulator. The relevant surface reaction rates are calculated by a molecular dynamics simulator using a Stillinger-Weber representation of the interatomic potential. Our investigations show that the surface reaction rates are strongly determined by the particular surface morpho- logy, which, in turn, is strongly influenced by the kinetic properties of the impinging particles. Thus, we link the molecular dynamics simulator into the model as a whole.

As results, we present calculations for the etching of amorphous silicon by fluorine, chlorine, and bromine. A Stillinger-Weber representation of the bromine and the silicon-bromine potential which was not yet available in literature is additionally developed. We discuss the different morphologies of halogenated silicon surfaces as a consequence of the energy distri- bution and the angular distribution of the impinging particles. Comparisons of the sputter yield functions of bare amorphous silicon surfaces and corresponding halogenated surfaces exhibit considerable differences, qualitative as well as quantitative.
Dielectric Polarization of Materials: A Modern View
Authors: Resta Raffaele
Published: April 2011

The concept of macroscopic polarization is the basic one in the electrostatics of dielectric materials: but for many years this concept has evaded even a precise microscopic definition, and has severely challenged quantum-mechanical calculations. This concept has undergone a genuine revolution in recent years (1992 onwards). It is now pretty clear that—contrary to a widespread incorrect belief—macroscopic polarization has nothing to do with the periodic charge distribution of the polarized crystal: the former is essentially a property of the phase of the electronic wavefunction, while the latter is a property of its modulus. An outline of the modern viewpoint is presented. Experiments invariably address polarization derivatives (permittivity, piezoelectricity, pyroelectricity,…) or polarization differences (ferroelectricity), and these differences are measured as an integrated electrical current. The modern theory addresses this same current, which is dominated by the phase of the electronic wavefunctions. First-principle calculations based on this theory are in spectacular agreement with experiments, and provide thorough understanding of the behavior of dielectric materials.
AB-Initio Calculation of the optical Properties of BN(110) Surface.
Authors: Cappellini Giancarlo , Satta Guido , Palummo Maurizia , Onida Giovanni
Published: April 2011

We study the optical properties of the nonpolar (110) surface of cubic Boron Nitride calculated within the first-pinciple DFT-LDA scheme. The reflectance anisotropy (RA) spectrum is analyzed in relation to the better known spectrum of the GaAs(110) surface. The influence of the ionicity on the surface relaxation, the surface states character and the surface optical spectra, is studied. Comparisons with existing data for the surface under study and results to the GaN(110) surface are also given.
Molecular-Dynamics Simulations of Magnetic Structures in Metallic Systems
Authors: Meyer Ralf , Lewis Laurent J.
Published: April 2011

Recently a method has been proposed which allows the calculation of complex magnetic structures from a simple d-band tight-binding Hamiltonian including Coulomb and exchange interactions with the help of a molecular-dynamics simulations. In this article an improved version of this ap- proach is suggested which retains the rotational symmetry of the simulated system. The improved algorithm is applied to systems with electron numbers in the range 7 ≤nd ≤ 8.
Efficient Reconstruction of Multi-Phase Morphologies from Correlation Functions
Authors: Rozman Michael G. , Utz Marcel
Published: April 2011

A highly efficient algorithm for the reconstruction of microstructures of heterogeneous media from spatial correlation functions is presented. Similar to previously proposed algorithms, the new method relies on Monte Carlo optimization, representing the microstructure on a discrete grid. An efficient way to update the correlation functions after local changes to the structure is introduced. In addition, the rate of convergence is substantially enhanced by selective Monte Carlo moves at interfaces. Speedups over prior methods of more than two orders of magnitude are thus achieved. The algorithm is ideally suited for parallel computers. The increase in efficiency brings new classes of problems within the realm of the tractable, notably those involving several different structural length scales and/or components.
Coupled atomistic-mesoscopic model of polycrystalline plasticity
Authors: Cleri Fabrizio , D'Agostino Gregorio , Satta Alessandra , Colombo Luciano
Published: April 2011

We discuss a microstructure evolution framework which couples atomic-level information about extended-defect interactions into a mesoscopic model; the latter, in turn, describes the dy-namic evolution of a statistical population of grain boundaries and dislocations. Atomistic simulations are carried out by means of molecular dynamics simulations on both isolated and interacting dislocations, grain boundaries, triple junctions, microcracks; the reference material for such studies is, at present, Silicon with the Stillinger-Weber potential. The mesoscale model describes the motion of discrete triple junctions (and, consequently, of the continuous network of adjoining grain boundaries) embedded in a continuous medium containing a homogenous, evolving distribution of dislocations.
A Simulation Study of Tracer Diffusion Concentration Profiles Resulting from the Transition from Dislocation Pipes to a Grain Boundary Slab
Authors: Belova Irina V , Murch Graeme E
Published: April 2011

In the present study we examine the well-known analysis in which the dislocation pipe diffusivity is determined by means of a grain boundary type analysis of the tail of a tracer concentration depth profile. We use a Monte Carlo grid method for testing the analysis. The results show that the analysis is really only satisfactory when the spacing between the dislocations is roughly twice the diffusion length (Dlt)½ where Dland t are the lattice diffusivity and time respectively.
A Kinetic Study of Hydrocarbons Reactivity on Palladium Catalysts through a DFT Approach
Authors: Bertani Valeria , Cavallotti Carlo , Masi Maurizio , Carrá Sergio
Published: April 2011

Palladium clusters have been chosen to represent a typical supported heterogeneous catalyst and their interaction with hydrocarbons has been investigated theoretically. The calculations were performed through density functional theory and the Becke-Lee-Yang-Parr hybrid (B3LYP) functional was adopted to calculate exchange and correlation energy. An effective core potential basis set (ECP on core electrons and Dunning/Huzinaga on outer electrons) was found sufficiently accurate to reproduce experimental data. Clusters containing up to seven Pd atoms were considered and their interaction with hydrogen, methane and ethane and their fragments was analyzed and a kinetic study of the system was performed. Transition states structures and energies were calculated through quantum mechanics and kinetic constants were derived from a statistic thermodynamic approach. On the basis of such information, a kinetic model that accounts for ethane transformations. Finally the kinetic scheme was embedded in a plug flow reactor model and simulations were performed to test the validity of the developed mechanism. In this way information obtained at the atomic scale were adopted to study phenomena occurring on the much higher reactor scale.
Residual Stresses in Silicon Nitride Based Composites Using Synchrotron Radiation
Authors: Lee Myungae , Xiao Yanan , Wittmer Dale E. , Mini Susan M. , Graber Timothy J.
Published: April 2011

In order to measure the residual strain (stress) in the silicon nitride based composites, synchrotron based x-ray diffraction was employed on the BESSRC beamline at the APS. The residual stress for both the baseline Si3N4 and the Si3N4-TiN composites were determined from the (441) and (531) reflections, obtained by applying the three-dimensional 2θ-sin2ψ method. In both the baseline Si3N4 and the Si3N4-TiN composites, after thermal shocking, compressive residual stresses were developed in both directions parallel and perpendicular to the specimen's surface. The average residual stresses in the direction parallel to the sample surface were much higher than in the perpendicular direction.

The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the strength and toughness of each composite. The results suggested that there should be a maximum thermal shock temperature, within the range of 1000 °C to 1100 °C, for improving the fracture toughness for both the baseline Si3N4 and the Si3N4-TiN composites. Also, the addition of the TiN appeared to improve both the strength and toughness of the baseline composition.
Microcracks in Carbon/Carbon Composites: A Microtomography Investigation using Synchrotron Radiation
Authors: Paris Oskar , Peterlik Herwig , Loidl Dieter , Rau Christoph , Weitkamp Timm
Published: April 2011

The mechanical properties of brittle matrix composites such as carbon/carbon (C/C) are closely related to the generation and propagation of microcracks. A better understanding of the role of microcracking requires a quantification of the three-dimensional morphology of the crack network. In this study we demonstrate that phase contrast microtomography using synchrotron radiation is a unique tool to get 3D information about cracks in C/C. This is shown for three different C/C specimens subjected to different final heat treatment temperatures (HTT). The results are discussed qualitatively with respect to the influence of HTT on the distribution of microcracks and their relevance for the mechanical properties of C/C.
Structure and Bonding of Metallic Nanowires Prepared in Nanoporous Alumina Membranes Studied by EXAFS, XANES and HED
Authors: Kröll M. , Benfield R. E. , Dore J. C. , Grandjean D. , Schmid G.
Published: April 2011

The structural properties of metallic nanowires prepared within the pores of nanoporous alumina membranes are investigated using EXAFS, XANES and High Energy X-Ray Diffraction (HED). Gold, silver, copper and iron nanowires show the same structure and atom-atom distances as the corresponding bulk metals. The structural properties of these metals do not change if the wire diameter is varied. Cobalt nanowires, however, show a size dependent mixture of fcc and hcp phases. The amount of fcc-Co, which in the bulk is only known as a high temperature modification, increases with decreasing wire diameter. This leads to interesting changes in the magnetic properties of Co nanowires.
X-RAY Magneto-Optics in Lanthanide Materials
Authors: Starke K. , Heigl F. , Vollmer A. , Kaindl G.
Published: April 2011

We demonstrate the feasibility of element-specific magnetization reversal studies applying x-ray magneto-optical Kerr effect (XMOKE) to lanthanide systems. In particular, we will first describe XMOKE spectra at 4d-4f excitation thresholds of Gd and Tb in some detail, before showing element specificity at the example of a Gd/Y/Tb-trilayer metal film. On the basis of experimental Tb M4,5absorption spectra, we may anticipate substantial MO signals in specular reectivity at M4;5edges of all lanthanide elements, provided they carry a non-vanishing net 4f-magnetization.
Society Photoemission and Photoabsorption Study of YNi2−xCoxB2C Superconductors
Authors: Hsu L.-S. , Guo G. Y. , Chen C.-J. , Lan M.-D. , Lee J.-F.
Published: April 2011

The electronic structures of five polycrystalline YNi2−xCoxB2C (x=0, 0.05, 0.1, 0.15, and 0.2) borocarbide superconductors were studied by photoemission and photoabsorption spectroscopies and theoretical calculations. The valence-band (VB) photoemission spectrum is compared with the theoretical total and partial density-of-states (DOS) curves. The VB satellite is peaked at a binding energy (EB) of 6 eV. The Ni K-edge x-ray absorption near edge spectra (XANES) are compared with the calculated XANES spectra for these intermetallic compounds. Extended x-ray absorption fine structure (EXAFS) spectra at the Ni and Co K edges are analyzed to yield the structural parameters. The decrease of the superconducting transition temperatures (Tc) with addition of Co dopant in these compounds is due to a decrease of the total DOS at the Fermi level (EF).
Proton-gated Molecular Devices Based on Rod-shaped Metal Complexes Immobilized on Solid Surface
Authors: Haga Masa-aki , Shiozawa Youichi , Suzuki Shinichiro , Inoue Miyao
Published: April 2011

Rod-shaped dinuclear Ru/Os complexes, [Os(bimpy-X)(BL)M(bimpyH2)]n+( M = Os or Ru; bimpyH2 = 2,6-bis(benzimidazol-2-yl)pyridine, X = anchoring group such as disulfide and phosphonic acid, BL =bis tridentate ligand), as a potential candidate for molecular devices have been successfully self-assembled on gold or indium-tin oxide (ITO) electrode by using anchoring ligands. The Ru/Os assembled complexes on the solid surface were well characterized by means of MALDI-TOF mass spectrometry, XPS, and electrochemical measurements. Novel proton-coupled electron transfer reaction of the self-assembled dinuclear complex on M(bimpyH2) moiety (M = Ru or Os) was confirmed by analyzing the plots of oxidation potential, E1/2, vs pH. In the case of M = Os and BL = 1,4-bis(terpyridyl)benzene (btpb), two stepwise one-electron oxidation waves were observed at 0.67 V for the Os(III/II) process and 0.88 V vs Ag/AgCl for the Ru(III/II) process in CH3CN/H2O at pH 1.0. The E1/2-pH diagram reveals the intercrossing of Ru(II/III) and Os(II/III) processes at around pH 5.5. Therefore, the intramolecular electron transfer can be controlled by pH. Proton-gated electron rectifying effect was observed in this Ru-Os dinuclear complex.
Components for AlGaN/GaN Power Amplifiers
Authors: Jacobs B. , Wingelaar P. , Kramer M. , Falcone S. , Karouta F. , Hek P. De , Suijker E. , Dijk R. Van
Published: April 2011

Ohmic contacts and Schottky contacts were made on an undoped AlGaN/GaN FET structure. Despite the high Al content (33%), we were still able to obtain a contact resistance of 0.3 ωmm. Pulsed measurements showed the large effect of self-heating even for circular contacts with a radius of 50 μm. The behavior ofthe Ni/Au Schottky contacts is according to the charge control model; the reverse current and capacitanceonly scale with the area of the diode. Tests with polygon type diodes showed no dependence of the reverse current on the number of polygon corners. The reverse current decreased when the devices were aged at 400°C for 30 hrs. Coplanar Waveguide discontinuities were realized on AlN substrates. A scalable lumped element model was derived from measurements for T-junctions, transmission lines, bends and crosses.
Analysis of Electron Transport in a High-Mobility Freestanding GaN Substrate Grown by Hydride Vapor-Phase Epitaxy
Authors: Yun F. , Morkoc H. , Rode D. L. , Tsen K. T. , Farina L. , Kurdak C. , Park S. S. , Lee K. Y.
Published: April 2011

Semiconductor nitrides grown on substrates with a large lattice mismatch typically contain extended and point defects that prevent the full potential of this material system from being attained. Among allthe substrate options explored so far, freestanding GaN templates appear ideal for homoepitaxial growth of GaN films. To this end, hydride vapor-phase epitaxial (HVPE) grown GaN templates with a thickness of more than 200 μm were thermally lifted off from the sapphire substrate and mechanically polished. The defect densityof such a template is expected to be non-uniform in the growth direction, especially near the back surface which was in close vicinity of the sapphire substrate. We, therefore, studied the transport properties of this template before and after the removal of a 30 μm region from the back-side. For as-prepared GaN, Hall mobilities of 1100 cm2/V-s and 6800 cm2/V-s were obtained at 295 K and 50 K, respectively. A simultaneous fitting of mobility and carrier concentration was used to quantify the contribution ofdifferent scattering mechanisms. When the backside was etched by ∼30 μm, Hall mobilities improved to 1200 cm2/V-s at 295 K and 7385 cmsup2/V-s at 48 K, respectively. A numerical solution of the Boltzmann transport equation (BTE) that deals with the inelastic nature of electron scattering by polar optical mode was employed to determine the acceptor concentration. Raman spectroscopy was employed to obtain LO and TO phonon energies, which were then used in the above-mentioned calculations. The best fittings of the mobility and carrier concentration data yield an average acceptor concentration of 4.9×1015 cm-3 and a donor concentration of 2.1×1016 cm-3 for the as-prepared GaN. The average acceptor concentration decreased to 2.4×1015 cm-3 after etching of the backside, which confirms that the etched-away region contained higher density of defects. The donor activation energy is derived to be 25.2 meV. Our analysis demonstrated high quality of the freestanding GaN substrate with the highest reported electron mobility for wurtzite GaN.
Growth of 3C-SiC Layers on Silicon Substrates with a Novel Stress Relaxation Structure
Authors: Irokawa Yoshihiro , Yamada Noboru , Kodama Masahito , Kachi Tetsu
Published: April 2011

Silicon (Si) substrates having cavities just beneath the surface layer (multi-cavity Si substrates) were examined whether they worked as the stress relaxation structure in 3C-SiC heteroepitaxial growth on Si. Single crystalline 3C-SiC layers were grown on the multi-cavity Si substrates by means of low pressure chemical vapor deposition (LPCVD). The layers' quality was characterized by the cross-sectional TEM observations and the Micro-Raman spectroscopy. The TEM results showed that this structure reduced the defect density in the 3C-SiC layers. The averaged full width at half-maximum (FWHM) of LO Raman mode in the 3C-SiC layerson the multi-cavity Si substrates became narrower than that on the conventional Si substrates. Furthermore, Schottky barrier structures showed that the reverse leakage current of the diodes using the multi-cavity Si substrates is smaller than that using the conventional Si substrates. These results indicate that the multi-cavity Si substrates are effective for stress relaxation in the 3C-SiC layers.
Electrical properties of MBE grown Si-doped AlxGa1−xN as a function of nominal Al mole fraction up to 0.5
Authors: Ahoujja M. , Yeo Y. K. , Hengehold R. L. , Nostrand J. E. Van
Published: April 2011

Hall-effect measurements were conducted on Si-doped AlxGa1−xN films grown on sapphire substrate by gas source molecular beam epitaxy. The Al mole fraction in the 1 [.proportional]m thick AlxGa1−xN was 0.0, 0.3, and 0.5, and the Si doping concentration was kept at a nominal value of 1018 cm−3. Variable temperature Hall-effect measurements reveal a presence of a highly degenerate n-type region at the AlxGa1−xN /sapphire interface. This degenerate interfacial layer dominates the electrical properties below 30 K and significantly affects the properties of the AlxGa1−xN layer. Thus, by using a two-layer conducting model, the carrier concentration and mobility of the AlxGa1−xN layer alone are obtained.
Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire.
Authors: Visconti P. , Reshchikov M. A. , Yun F. , Jones K. M. , Morkoç H. , Passaseo A. , Piscopiello E. , Pomarico A. , Cingolani R. , Lomascolo M. , Catalano M.
Published: April 2011

Properties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on c-plane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≍ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9×108 - 2 ×109 cm−2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≍ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors.
Persistent photocurrent effects in GaN/AlGaN multiquantum wells
Authors: Bonfiglio A. , Traetta G. , Lomascolo M. , Passaseo A. , Cingolani R.
Published: April 2011

PhotoCurrent (PC) experiments on GaN/AlGaN multiple quantum wells, both spectrally and temporally resolved, have shown that a photopersistence effect exists in our samples even at room temperature. A peak in the rise and decay times of PC vs. time appears in correspondence of wavelengths typical of Yellow-Band features while this is not seen in PL. This fact seems to indicate that YB defects give rise here to carrier trapping rather than recombination. Furthermore, the observed persistence times seem to be peculiar of MultiQuantum Wells and this possibly calls into play the role of built-in fields in the defects dynamics.
Stress Effects in p-Type AlGaN/GaN Heterostructures
Authors: Sutandi Agustinus , Ruden P. Paul , Brennan Kevin F.
Published: April 2011

The physics of bulk wurtzite-structure III-nitride materials and of III-nitride heterostructures includes many phenomena that can be modulated by the application of stress. In particular, p- type material is expected to display a rich variety of piezo-resistive and piezo-optic effects that originate from the stress-induced modulation of lattice polarization charges, of valence band energies, and of bulk, surface, and interface defect states in the band gap. Here we focus on the expected effects of in-plane uniaxial on p-channel AlGaN/GaN heterostructures grown along the hexagonal axis on sapphire substrates. The valence band structure in the channel region is calculated self-consistently in the framework of a six-band Rashba-Sheka-Pikus (RSP) Hamiltonian. Stress-effects are included (in linear elastic theory) through deformation potentials and through the modulation of interfacial polarization charges associated with the piezoelectric nature of the constituent materials.
Characterization of Inversion Domains in GaN by Electric Force Microscopy in Conjunction with Transmission Electron Microscopy and Wet Chemical Etching
Authors: Yun F. , Visconti P. , Jones K. M. , Baski A. A. , Morkoç H. , Passaseo A. , Piscopiello E. , Catalano M. , Cingolani R.
Published: April 2011

Inversion domains (IDs) in III-nitride semiconductors degrade the performance of such devices, and so their identification and elimination is critical.An inversion domain on a Ga- polarity samples appears as an N-polarity domain, which has a polarization reversed with respect to the rest of the surface and therefore has a different surface potential. Surface-contact-potential electric force microscopy (SCP-EFM) is an extension of atomic force microscopy (AFM) that allows imaging of the surface electrostatic potential. Previously, we established the particular mode of operation necessary to identify inversion domains on III-nitrides using a control sample. We have now studied inversion domains in GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The existence of inversion domains was also verified by transmission electron microscopy (TEM) using multiple dark field imaging. In MOCVD grown GaN, we found predominant Ga-polarity with very low density of IDs, while in the MBE GaN, a mix polarity feature was identified.
Surface Evaluation of 6H-SiC after Doping by Diffusion
Authors: Gao Ying , Soloviev S. I. , Sudarshan T. S.
Published: April 2011

Doping by diffusion of aluminum into 6H-SiC has been carried out in the temperature range of 1800-2100°C. Aluminum carbide (Al4C3) is thought to be one of the best candidates for a p-type diffusion source material. A thin layer graphite mask was developed to protect the wafer surface from deteriorating by sublimation/epigrowth during high temperature diffusion. High-resolution optical microscope (HROM) and atomic force microscopy (AFM) were employed to evaluate the surface morphology of the diffused samples. The protective mask significantly decreased the surface roughness. In addition, secondary ion mass spectroscopy (SIMS) was used to identify the influence of the thin graphite mask on the diffusion properties in SiC. There were no significant differences in doping profiles in the samples with and without the graphite mask.
The Influence of Contact Composition, Pretreatment, and Annealing Gas on the Ohmic Behavior of Ti/Al-Based Ohmic Contacts to n-Al0.4Ga0.6N
Authors: Schweitz K. O. , Pribicko T. G. , Mohney S. E. , Isaacs-Smith T. F. , Williams J. , Pophristic M. , Gotthold D. , Birkhahn R. , Ferguson I.
Published: April 2011

As the group III nitride semiconductor technology matures, an increasing number of devices are being fabricated with high Al fraction AlGaN. In this study, ohmic behavior is achieved using Ti/Al/Pt/Au contacts to n-Al0.4Ga0.6N, which is the highest Al fraction for which ohmic contact formation has been reported. The effect of contact composition, pretreatment, and annealing conditions is studied by 30 s isochronal annealing experiments between 500°C and 1000°C. A specific contact resistance ρC of (5±3) × 10−5 ωcm2 is obtained using Ti(26 nm)/Al(74 nm)/Pt(50 nm)/Au(50 nm) contacts to n-Al0.4Ga0.6N annealed in N2 at 800°C; however, this value is shown to be artificially high because the metal sheet resistance RM is 4 ω/⊏ causing an artifact in the data analysis. All contacts with ρC < 10−3 ωcm2 exhibit a local minimum in ρC after annealing at 800°C. The observed increase in ρC upon annealing at 850°C and 900°C, however, is not an artifact originating from a change in RM. The top Au layer is found to play an active role in forming ohmic contacts with low ρC, since omitting the Au layer yields an increase in ρC of two orders of magnitude after annealing at 800°C. Furthermore, leaving out the Au layer requires an annealing temperature of 700°C to result in linear I-V curves for currents up to 100 µA, as opposed to 500°C when the Au layer is present. The role of Au is further studied in Ti(26 nm)/Al(74 nm)/Ni(50 nm)/Au(50 nm) contacts, where Rutherford backscattering spectroscopy reveals Ga in the metal layer and/or Au buried deeper than the original semiconductor-metal interface, and x-ray diffraction indicates the formation of new phases to happen concurrently with a decrease in ρC of three orders of magnitude.
Structural and Electrical Characterisation of Nickel Silicides Contacts on Silicon Carbide
Authors: Via F. La , Roccaforte F. , Raineri V. , Musumeci P. , Calcagno L.
Published: April 2011

The interfacial reaction and phase formation as a function of the annealing temperature (600÷1000°C) and times were investigated on nickel thin films evaporated on n type 6H-SiC (0001) substrate. The study was carried out employing a combination of Rutherford Backscattering Spectrometry, X-Ray Diffraction, Transmission Electron Microscopy and sheet resistance measurements. Also several TLM structures and Schottky diodes were fabricated with the same processes and a correlation has been found between the annealing process and the electrical measurements. The only nickel silicide phase that has been observed between 600 and 950 °C was the Ni2Si. The carbon of the consumed silicon carbide layer has been dissolved in the silicide film, during the reaction, forming carbon precipitates. The Ni2Si/SiC Schottky diodes show an almost ideal characteristics (n=1.07) and a barrier height of about 1.3 eV. From the electrical characterisation a non uniform Schottky barrier height seems to be formed.
Optical Characterization of Mg- and Si-Implanted GaN
Authors: Fellows James A. , Yeo Yung Kee , Hengehold Robert L. , Krasnobaev Leonid
Published: April 2011

The optical and electrical properties of Mg- and Si-implanted GaN were investigated using photoluminescence, cathodoluminescence, and Hall-effect measurements. Implantation of Mg, Si, Mg+Si, Mg+O, Mg+C, and Mg+P was made into undoped semi-insulating MBE-grown GaN at energies from 125 to 260 keV at room temperature and 800 oC with doses of 1x1014 to 5x1015 cm−2. The samples were capped with AlN and annealed at temperatures ranging from 1100 to 1300 oC for 9 s to 20 min. The dominant luminescence peak in all Mg-implanted and annealed GaN is a broad green luminescence (GL) band at 2.36 eV, which may be related to a deep donor-deep acceptor complex transition resulting from the Mg implant, residual implant damage, and/or native defects. The relative intensities of this GL band and secondary peaks from 2.75-3.28 eV vary as a function of implantation temperature, ion dose, species, and anneal temperature. All Mg single and dual implantation resulted in extremely resistive GaN layers, except Mg+Si, which resulted in weakly n-type GaN. However, the Si-implanted GaN produced an electrical activation efficiency as high as 73% after annealing at 1200 oC for 5 min.
Low Temperature Epitaxial Growth of ZnO Layer on Sapphire by Plasma-Assisted Epitaxy
Authors: Yamauchi Satoshi , Hariu Takashi
Published: April 2011

ZnO layer was epitaxially grown with a high growth rate around 800nm/hour on C-sapphire at relatively low temperature 400°C by plasma-assisted epitaxy in oxygen plasma which was excited by radio- frequency power at 13.56MHz. Photoluminescence spectra, which were dominated by strong donor-bound exciton emissions and free-exciton emissions at 10K, indicated deep levels native-defects were well prevented in the layers grown by the optimized growth condition including the power to discharge oxygen gas. Initial layers grown at very low temperature increased the probability of ZnO-crystal nucleation, compared with the direct growth on the sapphire, and were very effective in decreasing surface roughness and then improving the crystal quality of the successively grown ZnO layers. The full-width at half-maximum of donor-bound exciton emission was dependent on the initial layer growth condition and the narrowest value of the FWHM was obtained as 1.7meV on PAE-layers deposited at 400°C on the initial layer grown at 150°C.
Characterization of p-type 4H-SiC epitaxial layer grown on (11-20) substrate
Authors: Kojima Kazutoishi , Ohno Toshiyuki , Kushibe Mituhiro , Masahara Koh , Ishida Yuuki , Takahashi Tetuo , Suzuki Takaya , Tanaka Tomoyuki , Yoshida Sadafumi , Arai Kazuo
Published: April 2011

Growth and characterization of p-type 4H-SiC epitaxial layers grown on (11-20) substrates are reported. P-type 4H-SiC epilayers with smooth surface morphology have been grown on (11-20) substrates by low-pressure, hot-wall type CVD with SiH4–C3H8–H2–TMA system. The doping concentration can be controlled in the range from about 1×1016cm−3 to 1×1019cm−3. Anisotropy of the crystalline quality is observed by x-ray diffraction measurement. P-type epilayers, in which near band-gap emissions are dominated and D-A pair peak is not observed, are obtained. Hole mobility of (11-20) epilayers is smaller than that of (0001) epilayers probably due to the lack of crystalline quality compared to (0001) epilayers. The results of both low-temperature photoluminescence and the temperature dependence of Hall effect measurements indicate that the boron concentration as undoped impurity in (11-20) epilayer is lower than that of (0001) epilayer. This may be caused by the smaller incorporation efficiency of boron into (11-20) epilayer than that of (0001) epilayer.
Investigation of Mechanical Stresses in Underlying Silicon due to Lead-Tin Solder Bumps via Synchrotron X-Ray Topography and Finite Element Analysis
Authors: Kanatharana J. , Pérez-Camacho J.J. , Buckley T. , McNally P.J. , Tuomi T. , Danilewsky A.N. , O'Hare M. , Lowney D. , Chen W.
Published: April 2011

Solder based flip-chip packaging has prompted interest in many integrated circuit (IC)packaging applications due to its many advantages in terms of cost, package size, electricalperformance, input/output density, etc. The ball grid array (BGA) is one of the most commonflip-chip packaging techniques used for microprocessor applications. However, mechanicalstresses induced by the flip-chip process can impact adversely on the reliability of production.

White beam synchrotron x-ray topography (SXRT), a non-destructive technique, has beenemployed to investigate the spatial extent of strain fields imposed on the underlying siliconsubstrate for Intelν®Pentiumν®III microprocessors due to the lead-tin solder bump process for BGApackaging. Large area and section back-reflection SXRT images were taken before and after asimulation of the reflow process at 350°C in atmosphere. The presence of induced strain fields inthe Si substrate due to the overlying bump structures has been observed via the extinction contrasteffect in these x-ray topographs. In addition, orientational contrast effects have also been foundafter the reflow process due to the severe stresses in the underlying silicon beneath the lead bumps.The estimated magnitudes of stress, ∣σ∣, imposed on the underlying silicon were calculated to be100 MPa. The spatial strains in the underlying silicon were relieved dramatically after the leadbumps were removed from the wafer, which confirms that the bumps are indeed a major source ofstrain in the underlying Si. Finite element analysis (FEA) has also been performed in 2-D planestrain mode. The magnitudes and spatial distribution of the stresses after the reflow process are ingood agreement with the SXRT results.
Dislocation Pattern Formation – Simulations of Annealing in Two Dimensions
Authors: Argaman Nathan , Levy Ohad , Makov Guy
Published: April 2011

A two-dimensional discrete dislocation dynamics code was developed and used to simulate dislocation patterning. For the present study of annealing, both glide and some climb were allowed in the simulations. In these circumstances patterning takes place even in the absence of external stresses and of dislocation sources. A triangular underlying lattice was assumed, with the three slip systems equally populated initially. Well-defined dislocation walls and cells are observed to form from random initial conditions. The structure coarsens with time, i.e. the typical size of the cells increases as annealing takes place (the smaller cells shrink and disappear from the structure). In the spirit of a bottom-up multiscale approach, it is suggested that a new simulation methodology should be developed, in which the discrete moving objects will be dislocation wall segments rather than individual dislocations.
The Interaction of a Circular Dislocation Pile-up with a Short Rigid Fiber: a 3-D Dislocation Dynamics Simulation
Authors: Khraishi Tariq A. , Zbib Hussein M.
Published: April 2011

This paper presents a dislocation dynamics simulation of the interaction of a circular dislocation pile-up with a short rigid fiber, say as in metal-matrix composites. The pile-up is composed of glide dislocation loops surrounding the fiber. This problem is treated here as a boundary value problem within the context of dislocation dynamics. The proper boundary condition to be enforced is that of no or zero elastic displacements at the fiber's surface. Such a condition is satisfied by a distribution of rectangular dislocation loops, acting as sources of elastic displacements, meshing the fiber's surface. Such treatment is similar to crack modeling using distributed dislocations and falls under the category of “generalized image stress analysis.” The unknown in this problem is the Burgers vectors of the surface loops. Once those are found, the Peach-Koehler force acting on the circular dislocation loops, and emulating the fiber's presence, can be determined and the dynamical arrangement of the circular pile-up evolves naturally from traditional dislocation dynamics analysis.
Teaching Undergraduate and Graduate Students Results From Recent Research As Part of a Class
Authors: Martínez-Miranda L. J. , Kidder J. , Lloyd I. , Briber R. J. , Wilson O. , Al-Sheikly M. , Salamanca-Riba L. G.
Published: April 2011

We have integrated recent research results in the curriculum of the University of Maryland, while leaving the basics intact. This allows us to teach the fundamental laws and to show the students how these help us to answer questions that are of importance today. This can be done as part of a laboratory and as part of a class. In the laboratory classes, we emphasize what is measured (the main topic), then measure samples recently done in the literature. If available, we measure two samples and have a discussion on what are the differences. These are then related to the manner the samples have been grown. In the classes, we show the basic equation or relation, and introduce how it can answer a question that is pertinent to today's research.
The Genesis Space Probe as a Platform for Materials Education
Authors: Hays Charles C.
Published: April 2011

NASA's Genesis mission provides an interesting opportunity for educational outreach in a wide variety of materials related disciplines. This mission is one the Discovery planetary exploration series, and its purpose is to return to earth quantitative data regarding the composition of the Sun's solar wind. In doing so, the mission requires a number of materials technology platforms that provide examples of how fundamental questions in science might be answered. Specifically, the Genesis mission provides an educational pathway showing how experiment design and the process of materials selection are combined to realize evidence that will increase our understanding of the cosmological transition from solar nebula to planetary bodies.