bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Defect-Related Photoluminescence in GaN
Authors: Chen L. , Skromme B. J. , Mikhov M. K. , Yamane H. , Aoki M. , DiSalvo F. J. , Wagner B. , Davis R. F. , Grudowski P. A. , Dupuis R. D.
Published: April 2011

Broad, low temperature photoluminescence (PL) peaks near 3.4–3.42 eV in GaN have previously been associated with basal-plane stacking faults. Recently, we observed unusually sharp and highly structured PL peaks in this region in high quality bulk GaN grown from a Na/Ga flux, some of which display characteristic shifts and narrowing as a function of excitation power. Here, we study these peaks as a function of excitation intensity and crystal polarity, and compare them to those observed in GaN grown on off-axis SiC or sapphire (0001) substrates by metalorganic chemical vapor deposition (MOCVD). In the off-axis material on either substrate, similar peaks are observed to those in the bulk samples. In addition, a low energy peak near 3.21 eV is observed, which does not occur in the bulk material.
Optical Study of Localized and Delocalized States in GaAsN/GaAs
Authors: Xu Z. Y. , Luo X. D. , Yang X. D. , Tan P. H. , Yang C. L. , Ge W. K. , Zhang Y. , Mascarenhas A. , Xin H. P. , Tu C. W.
Published: April 2011

Taking advantages of short pulse excitation and time-resolved photoluminescence (PL), we have studied the exciton localization effect in a number of GaAsN alloys and GaAsN/GaAs quantum wells (QWs). In the PL spectra, an extra transition located at the higher energy side of the commonly reported N-related emissions is observed. By measuring PL dependence on temperature and excitation power along with PL dynamics study, the new PL peak has been identified as a transition of the band edge-related recombination in dilute GaAsN alloy and delocalized transition in QWs. Using selective excitation PL we further attribute the localized emission in QWs to the excitons localized at the GaAsN/GaAs interfaces. This interface-related exciton localization could be greatly reduced by a rapid thermal annealing.
Reduction of Threading Dislocation Density in AlGaN by Indium Incorporation
Authors: Kang H. , Feng Z. C. , Ferguson I. , Guo S. P. , Pophristic M.
Published: April 2011

The addition of indium, even to small concentrations, to AlGaN has resulted in improved optical and doping properties for these materials. This paper is the first report of improved structural properties for indium containing AlGaN layers. A systematic series of the AlGaN layers with nominal concentration of 20% aluminum were grown by metal-organic chemical vapor deposition with traces amounts of indium incorporated into the layers (up to 0.15% indium). X-ray diffraction analysis of the layers was completed using Williamson Hall plots and reciprocal space mapping to investigate any change in the columnar structure of the initial AlGaN layers. It was found that the threading dislocation densities and lateral coherence length showed a systematic variation with indium incorporation. The threading dislocation density is lowered as indium composition increased with a corresponding increase in lateral coherence length. This indicates that even the incorporation of trace amounts of indium improves the structural properties of these epilayers.
Experimental Analysis and a New Theoretical Model for Anomalously High Ideality Factors (n ≫ 2.0) in GaN-based p-n Junction Diodes
Authors: Shah Jay M. , Li Yunli , Gessmann Thomas , Fred Schubert E.
Published: April 2011

Diode ideality factors of 2.0–8.0 have been reported in GaN-based p-n junctions. These values are much higher than the expected values of 1.0–2.0 as per the Sah-Noyce-Shockley theory. We propose a fundamentally new model for the high ideality factors obtained in GaN-based diodes. This model is based on the effect of moderately doped unipolar heterojunctions as well as metal–semiconductor junctions in series with the p-n junction. A relation for the effective ideality factor of a system of junctions is developed. A detailed experimental study is performed on diodes fabricated from two different structures, a bulk GaN p-n junction structure and a p-n junction structure incorporating a p-type AlGaN/GaN superlattice. Bulk GaN p-n junction diode displays an ideality factor of 6.9, whereas the one with the superlattice structure displays an ideality factor of 4.0. In addition, device simulation results further strengthen the model by showing that moderately doped unipolar heterojunctions are rectifying and increase the effective ideality factor of a p-n junction structure.
Symposium Y: Gan and Related Alloys MOCVD AlGaN/GaN HFETs on Si: Challenges and Issues
Authors: Rajagopal Pradeep , Roberts John C. , Cook J. W. , Brown J. D. , Piner Edwin L. , Linthicum Kevin J.
Published: April 2011

AlGaN/GaN based high power, high frequency high electron mobility transistors (HEMTs) have been in development for over a decade. Although much progress has been made, AlGaN/GaN HEMT technology has yet to be commercialized. The choice of silicon as the substrate for the growth of GaN-based epi layers will enable commercialization of AlGaN/GaN based HEMTs, because of its maturity, scalability, reproducibility and economy. One of the epitaxial issues pertaining to the growth of AlGaN/GaN HEMTs on Si is the understanding of parasitic losses that can adversely impact the RF device performance. The effect of the III-N MOCVD process on the resistivity of the Si substrate, and correlations between the Si substrate resistivity and AlGaN/GaN HEMT RF characteristics are presented. Optimization of the MOCVD growth process led to a reduction in parasitic doping of the Si substrate. This resulted in the following improvements: (a) small signal gain increased from 17 to 21dB, (b) the cut-off frequency increased from 7 to 11GHz and (c) the maximum frequency of oscillation improved from 12 to 20GHz. This optimized process will enhance performance of AlGaN/GaN HEMTs at higher frequencies.
The Structure of Dislocations in GaN Grown by MBE as a Function of the Gallium to Nitrogen Ratio
Authors: Baines Marcus Q. , Cherns David , Novikov Sergei V. , Manfra Michael J. , Thomas Foxon C.
Published: April 2011

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) have been used to analyse the core structure of dislocations in GaN grown by molecular beam epitaxy (MBE) as a function of the gallium to nitrogen ratio. Ga-rich samples had a much smoother morphology; TEM observations showed that amorphous deposits decorated some dislocations and occasional surface pits, but weak beam and end-on imaging suggested that, away from the growth surface, dislocations of all types had closed core structures, in contrast to previous observations (Hsu et al, Appl. Phys. Lett. 78, 3980 (2001), Baines et al, Mat. Res. Soc. Symp. Proc. 743, L2.5 (2003)). Ga-poor samples were found to have much rougher surfaces; dislocations were often at the centers of deep surface pits but were observed to be undecorated and to have closed core structures. It is concluded that in growth under Ga-rich conditions, decoration of dislocation cores depends on the accumulation of Ga at surface pits, rather than being a fundamental property of dislocation formation.
Growth of Rhombohedral B12P2 Thin Films on 6H-SiC(0001) By Chemical Vapor Deposition
Authors: Lu Peng , Edgar J. H. , Pomeroy J. , Kuball M. , Meyer H. M. , Aselage T.
Published: April 2011

The parameters necessary to deposit oriented rhombohedral boron phosphide (B12P2) thin films on on-axis Si-face 6H-SiC(0001) substrates by chemical vapor deposition are reported. Ultra high purity BBr3 and PBr3 were used as reactants, with hydrogen as the carrier gas. The BBr3 to PBr3 flow rate ratio was adjusted to obtain good surface morphology of the B12P2 films. BBr3 to PBr3 ratios in the range of 1 to 1.5 produced smooth surfaces and moderate growth rates of 10μm/hr. Higher growth rates were obtained by increasing the BBr3 flow rate, but the surfaces became very rough. The c-axis of the B12P2 film was aligned with the c -axis of the substrate at temperatures between 1650°C-1700°C. The surface morphologies were investigated by SEM and the crystalline properties of the films were characterized by XRD and Raman spectroscopy.
Submicrometer scale growth morphology control for the making of photonic crystal structures
Authors: Gil-Lafon E. , Trassoudaine A. , Castelluci D. , Pimpinelli A. , Saoudi R. , Parriaux O. , Muravaud A. , Darraud C.
Published: April 2011

The feasibility of micrometer scale dielectric periodic structures by using a single selective hydride vapour phase epitaxy (HVPE) step was assessed. HVPE is a near-equilibrium growth process which offers perfect selectivity whatever the pattern design, thus giving rise to a great flexibility. The HVPE growth is also mainly governed by the intrinsic anisotropy of the surface kinetics of the crystal. We demonstrate here that micrometer scale dielectric periodic structures, constituted of perfectly defined 1μm wide GaAs beams alternately stacked with air, can be grown by selective HVPE by controlling the hierarchy of the growth rates of the low index faces of the III-V crystal via the growth temperature and the composition of the vapour phase. Potential of the HVPE growth technic for the making of submicrometer scale structures is finally discussed.
Growth and Properties of AlGaInP Resonant Cavity Light Emitting Diodes (RCLEDs)on Ge/SiGe/Si Substrates
Authors: Kwon O. , Boeckl J. , Lee M. L. , Pitera A. J. , Fitzgerald E. A. , Ringel S. A.
Published: April 2011

AlGaInP visible resonant cavity light emitting diodes (RCLEDs) were grown and fabricated on low-dislocation density, SiGe/Si metamorphic substrates by molecular beam epitaxy. A comparison with devices grown on GaAs and Ge substrates showed that not only did the RCLED device structure successfully transfer to the SiGe substrate, but also a higher optical output power was obtained. This is a result of a high thermal conductivity and an enhanced lateral current spreading. In addition, the growth of an AlGaAs current spreading layer and a modified top metal contact were incorporated to the SiGe RCLED to improve the device performance. With these improvements, a 410μm × 410μm device was fabricated with an optical power of 166μW at 665nm peak wavelength under 500mA current injection and an extremely narrow full width half maximum (FWHM) value of 3.63nm for electroluminescent emission under 50mA injection current.
Feature Size and Density Effects in Wet Selective Etching of GaAs/AlAs p-HEMT Structures with Organic Acid - Peroxide Solutions.
Authors: Kulkarni Vinay S. , Prasad Kanti , Quinn William , Spooner Frank , Sung Changmo
Published: April 2011

Pseudomorphic HEMT (p-HEMT) devices are used in a number of wireless communication applications including power amplifiers in the 17–50 GHz range, low noise amplifiers and switches. Selective wet etching is often used to form the gate regions of these devices to avoid plasma damage associated with dry etching. We have investigated the wet etching of small (8μm to 0.5μm) features with organic acid - hydrogen peroxide solutions. Two acid solutions were used as a selective etchant for GaAs using AlAs etch stop layers in a p-HEMT structure grown by MBE. The etched features were characterized by AFM, SEM, and TEM techniques. The etch depth uniformity and reproducibility were found to depend on a number of factors including feature size, feature density, etching chemistry, agitation and surface tension. When features with a range of size and density were placed in close proximity in a layout we found that the etch rate of the different features was a function of density, size and most importantly the etch chemistry. One etchant solution exhibited a 12% difference in etch rate from the smallest feature to the largest, while another solution exhibited uniform etching of all features regardless of size or density. Both solutions produced specular etched surfaces in GaAs and AlGaAs. However, the AlAs etch stop showed a non-uniform surface morphology after etching. The surface morphology of the AlAs etch stop is one factor that limits the over etch which can be designed into the process. The most important factors to be considered in designing a selective etch process will be presented.
Acceptors in undoped gallium antimonide
Authors: Lui M. K. , Ling C. C. , Chen X. D. , Cheah K. W. , Li K. F.
Published: April 2011

Undoped GaSb materials were studied by temperature dependent Hall (TDH) measurements and photoluminescence (PL). The TDH data reveals four acceptor levels (having ionization energies of 7meV, 32meV, 89meV and 123meV) in the as-grown undoped GaSb samples. The 32meV and the 89meV levels were attributed to the GaSb defect and the VGa-related defect. The GaSb defect was found to be the important acceptor responsible for the p-type nature of the present undoped GaSb samples because of its abundance and its low ionization energy. This defect was thermally stable after the 500°C annealing. Similar to the non-irradiated samples, the 777meV and the 800meV PL signals were also observed in the electron irradiated undoped GaSb samples. The decrease of the two peaks' intensities with respect to the electron irradiation dosage reveals the introduction of a non-radiative defect during the electron irradiation process, which competes with the transition responsible for the 777meV and the 800meV PL peaks.
The Influence of GaSb Layer Thickness on the Band Gap of InAs/GaSb Type-II Superlattices for Mid-Infrared Detection
Authors: Haugan Heather J. , Szmulowicz Frank , Brown Gail J.
Published: April 2011

The effect of small changes in GaSb layer thickness on the photoresponse spectrum of InAs/GaSb superlattices (SLs) designed for mid-infrared detection was systematically investigated. The samples were grown by molecular beam epitaxy with precisely calibrated growth rates. The basic SL used for this study consisted of 40 periods of InAs (20.5 Å)/GaSb (X Å), where the nominal value for X was adjusted from 18 to 27 Å in four different samples. An InSb-like interface (IF) was inserted between the layers to balance the SL strain. By decreasing the GaSb width, the photoresponse cut-off wavelength (λc) was adjusted from 4.03 μm to 4.55 μm, i.e., the SL energy band gap is being decreased. This decrease in the energy separation between the first heavy hole band (HH1) and the first conduction band (C1) as the GaSb layer is narrowed is counter intuitive. However, this experimental trend can be explained by a modified envelope function approximation (EFA) calculation that includes the effect of in-plane asymmetry at InAs/GaSb interfaces. As expected, the HH band is pushed away from the top of the GaSb valence band as the GaSb layer width becomes narrower. However, at the same time the C1 band is significantly broadened by the increased wave function overlap of the electron states in the InAs layer. The trend to smaller band gap with narrower GaSb layers and other effects of the design changes on the photoresponse spectrum are discussed.
Interface Properties of Al/Ni Multilayers as Deposited and Following Volume Combustion Synthesis
Authors: Holtz M. , Aurongzeb D. , Daugherty M. , Chandolu A. , Yun J. , Berg J. M. , Temkin H.
Published: April 2011

We report a volume combustion synthesis study of Al/Ni multilayers. The alternating layers of pure Al and pure Ni were grown on silicon and glass substrates using electron beam evaporation. Layers of equal thickness and with bilayer period of 50 nm were grown with total thickness to 1 μm. We focus in this study on the interface properties of as deposited and following anneals up to 660°C with 100°C intervals. Anneal duration was generally 10 min with longer anneals to verify consistency with previously published results. Interface structure was probed using X-ray reflectivity (XRR). Based on interface roughness, we identify three temperature ranges. In the as-deposited to 260°C range the interface properties remain intact, as seen from the XRR interference fringes beyond the critical angle. From 360 to 460°C fringes disappear, indicating the loss of smooth interface morphology. The third temperature range is 500 to 560°C in which the XRR interference fringes are clearly seen, signifying the presence of layering. We describe these results based on a model in which alloy domains form at the interface and grow with temperature and time. Differences in lateral and vertical growth rates of alloy domains are naturally taken into account. At 660°C, the melting temperature of Al, and above the fringes completely vanish. X-ray diffraction is used to identify the nickel aluminide compounds formed upon anneal.
Performance and Characterization of Nanoenergetic Materials at Los Alamos
Authors: Son Steven F.
Published: April 2011

Novel properties associated with nanostructured materials, including nanoenergetic materials, have attracted a great deal of interest recently. Metastable Intermolecular Composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. They can have high energy densities and exhibit combustion velocities above 1 km/s. These properties make them very attractive in a number of applications, including lead-free primers and igniters. However characterizing these materials is challenging, and the mechanism responsible for the propagation of reaction in loose compacts is not well understood. In this paper I will present an overview of efforts at Los Alamos to characterize and better understand the reaction mechanisms of these advanced energetic materials. We also have continuing efforts to make new MIC materials. Two applications are illustrated for MIC materials.
Application and Characterization of Nanomaterials in Energetic Compositions
Authors: van der Heijden A. E. D. M. , Bouma R. H. B. , van der Steen A. C. , Fischer H. R.
Published: April 2011

As part of a cooperation between several TNO institutes, including TNO Prins Maurits Laboratory, recently a new initiative on nanotechnology was started. The research subjects within this initiative can be roughly divided into two areas: (1) Instrumentation for analysis and manufacture at nano-scale and (2) Nanoscale engineering techniques to create materials and components (including their applications). Currently the research at TNO Prins Maurits Laboratory is focusing on the application of reactive nanomaterials to decontaminate surfaces from e.g. bacteria or toxic chemicals, the use of plasma's to generate nanomaterials like carbon nanotubes, and the application and characterization of nanomaterials in energetic formulations (e.g. explosives, propellants and pyrotechnic compositions). In this paper results on the latter subject will be presented in more detail. Also results will be included of other research projects involved with energetic/reactive nanomaterials.
Embedded Cluster Model: Application to Molecular Crystals
Authors: Kuklja Maija M. , Zerilli Frank J. , Sushko Peter
Published: April 2011

Multiscale modeling using an embedded cluster approach is presented and applied to study the structure and properties of molecular crystals. We discuss the results of hydrostatic compression modeling of 1,1-diamino-2,2-dinitroethylene obtained with the embedded cluster model and the Hartree-Fock method and compare these with the full periodic crystal structure calculations. Details of the electronic structure of the perfect, highly compressed material are discussed. The results demonstrate the applicability of the embedded cluster model. We show that the band gap of the perfect material is not sensitive to hydrostatic compression, but some changes induced by the pressure take place in the valence band.
Reactivity of Nanosize Aluminum with Metal Oxides and Water Vapor
Authors: Puszynski Jan A.
Published: April 2011

It has been well documented that the reactivity of nano-powders as well as bulk properties of materials derived from nano-particulates might be significantly different than those obtained from micron-sized grains. This paper addresses several aspects related to the characterization of coated and uncoated aluminum nanopowders, mixing of binary nanopowders, and the reactivity of nanosize aluminum with copper oxide under unconfined and confined conditions. It was found that the volumetric method is more versatile method than TGA or calorimetry for determination of reactive aluminum in coated aluminum nanopowders. It was shown that wet mixing in the presence of dispersants is essential to obtain mixtures with a high level of concentration uniformity. The combustion front velocity can be increased both by using an excess of aluminum in MIC systemand by coating of aluminum nanopowders or addition of dispersants during wet mixing. Protective organic coating with hydrophobic groups is essential to protect aluminum nanopowder from the reaction with moisture at higher relative humidity levels.
Nanofractography Of Composition B Fracture Surfaces with AFM
Authors: Lanzerotti Y. D. , Sharma J. , Armstrong R. W. , McKenney R. L. , Krawietz T. R.
Published: April 2011

The characteristics of TNT (trinitrotoluene) crystals in the fracture surface of Composition B (a melt-cast mixture of TNT and RDX) have been studied using atomic force microscopy (AFM). The size of TNT crystals has been examined by analyzing the surface structure that is exhibited after mechanical failure of the Composition B. The failure occurs when the material is subjected to high acceleration in an ultracentrifuge and the shear or tensile strength is exceeded. AFM examination of the topography of the Composition B fracture surface reveals fracture across columnar grains of the TNT. The width of the columnar TNT grains ranges in size from ∼ 1 μm to ∼ 2 μm. Their height ranges in size from ∼ 50 nm to ∼ 300 nm. Flat TNT columns alternate with TNT columns containing river patterns that identify the direction of crack growth. Steps in the river patterns are a few nanometers in depth. The TNT constitutent fracture surface morphology is shown to occur on such fine scale, beginning from adjacent columnar crystals only 1–2 μm in width, and including river marking step heights of only a few nanometers, that AFM-type resolution is required.
Cutting Reactive Foils Without Igniting Them (A Femtosecond Laser Machining Approach)
Authors: Picard Yoosuf N. , Liu Hsiao-Hua , Speys Stephen J. , McDonald Joel P. , Adams David P. , Weihs Timothy P. , Yalisove Steven M.
Published: April 2011

Ni/Al nanostructured multilayer foils were machined with femtosecond pulse-length laser irradiation at various fluences. Scanning electron microscopy, back-scattered electron detection, and atomic force microscopy were used to characterize the resulting laser modified regions. We show that material removal at the micron scale is possible with no ignition of a self-propagation reaction emanating from the irradiated areas, a danger minimized by the fact that the extremely short time duration of the pulse produces negligible heat dissipation into the multilayer system. Nevertheless, initial AFM and BSE results give indication that multilayers may be intermixing and reacting locally in areas smaller than the laser beam diameter, though the exact ignition mechanism is still to be determined.
Compound Formation in Ti-Doped Sodium Aluminum Hydrides
Authors: Majzoub E.H. , Stumpf R. , Spangler S. , Herberg J. , Maxwell R.
Published: April 2011

Renewed interest in hydrogen storage materials has resulted in the development of Ti-doped NaAlH4. Different doping methods such as mechanical milling with powdered TiCl3, or wet doping in solvents such as tetrahydrofuran (THF), yield enhanced kinetics. Still, the location and action of the Ti dopant is an open question. In order to address titanium substitution in the bulk, we present lattice parameter measurements of crushed single crystals which were exposed to Ti during growth. Rietveld refinements suggest that the titanium does not appear to enter the bulk by this method of exposure. Therefore, reaction products are investigated by x-ray diffraction of completely reacted samples of solvent-mixed versus mechanically milled 3 NaAlH4+TiCl3. Formation of TiAl3 is observed in mechanically milled materials, but not solution mixed samples, where bonding to THF likely stabilizes Ti-based nano-clusters. The Ti in these clusters is activated by mechanical milling.
Enhancing Hydrogen Adsorption by Metal Incorporation in Carbon Multi-Wall Nanotubes Produced by Continuous Hot Wire Chemical Vapor Deposition
Authors: Dillon A. C. , Parilla P. A. , Alleman J. L. , Mahan A. H. , Gilbert K. E. H. , Jones K. M. , Heben M. J.
Published: April 2011

The hydrogen storage properties of carbon single-wall and multi-wall nanotubes (SWNTs and MWNTs), graphitic nanofibers, and other nanostructured carbons have recently become the subject of considerable debate. Reported capacities range from ∼ 0–60 wt%. Hot wire chemical vapor deposition (HWCVD) has recently been adapted for a continuous growth process for high-density carbon MWNTs. Multi-wall nanotube growth is optimized in 1:5 CH4:Ar at 150 Torr with reactor temperatures of 400 and 550°C for static and flowing gases, respectively. Ferrocene is employed to provide a gas-phase catalyst. Highly graphitic nanotubes can be continuously deposited with iron content as low as 15 wt% and carbon impurities below thermal gravimetric analysis detection limits. The MWNTs are simply purified to ∼99.5 wt% with minimal structural damage and with a 75 wt% yield. Hydrogen adsorption is observed for low pressures at near ambient temperatures on the as-synthesized MWNTs containing iron nanoparticles. However, no hydrogen adsorption, is observed at near ambient temperatures for the purified MWNTs or for purified MWNTs that were subsequently combined with iron micro/nano-particles via sonication. These results indicate that an intimate metal/graphitic carbon interaction is required for unanticipated hydrogen adsorption at near ambient conditions.
A Molecular Dynamics Simulation Study Towards Understanding the Effects of Diameter and Chirality on Hydrogen Adsorption in Singlewalled Carbon Nanotubes
Authors: Cheng Hansong , Cooper Alan C. , Pez Guido P. , Kostov Milen K. , Cole Milton W. , Stuart Steven J.
Published: April 2011

A force field methodology has been developed for the description of carbon-carbon and carbon-molecular hydrogen interactions that is ideally suited to modeling hydrogen adsorption on single-walled carbon nanotubes (SWNT). The method makes use of existing parameters of potential functions developed for sp2 and sp3 hybridized carbon atoms and allows accurate representation of molecular forces on curved carbon surfaces. This approach has been used in molecular dynamics (MD) simulations for hydrogen adsorption in SWNT. The results reveal significant nanotube deformations, consistent with ab initio MD simulations, and the calculated energies of adsorption at room temperature are comparable to the reported experimental heats of adsorption for H2 in SWNT. The efficiency of this new method has permitted the MD simulation of hydrogen adsorption on a wide range of SWNT types, varying such parameters as nanotube diameter and chirality. The results show that these SWNT physical parameters have a substantial effect on the energies of adsorption and hydrogen capacities.
Novel Pd/SiO2 Membrane Supported on Porous Stainless Steel for High Temperature Hydrogen Separation
Authors: Su Caili , Jin Tetsuro , Kuraok Koji , Matsumura Yasuyuki , Yazawa Tetsuo
Published: April 2011

Thin Palladiun/SiO2 composite membrane supported on porous stainless steel has been fabricated by a novel preparation procedure. SiO2 colloid suspensions with different particle sizes were applied to modify the pore size of the substrate and also form an intermediate SiO2 layer to support the palladium layer. Palladium nuclei were seeded by a chemical vapor deposition process using Pd(F6acac)2 as metal precursor, and then a palladium layer thinner than 10 μm was prepared by electroless plating. The membrane had a hydrogen permeance of 2.7×10−6 mol/m2.s.Pa and a permselectivity of PH2 / PN2 in the range of 300–400 at 773K. The preparation process showed that SiO2 layer had a unique property for palladium adhesion compared with other studied metal oxides. The seeding process by chemical vapor deposition showed this step is significant for the preparation of a defect-free membrane. A novel electroless plating process was also applied using a weak acidic both, which is different from the usually used basic ones containing N2H4. The surface morphology and component of the membrane have been studied by scanning electron micrograph (SEM) and energy-dispersive X-ray analysis (EDX). The permeance and permselectivity of the membrane to hydrogen have bean measured at different temperatures.
Investigations of Actinide Metals and Compounds under Pressure Provide Important Insights into Bonding and Chemistry
Authors: Haire R. G. , Heathman S. , Le Bihan T. , Lindbaum A. , Iridi M.
Published: April 2011

One effect of pressure on elements and compounds is to decease their interatomic distances, which can bring about dramatic perturbations in their electronic nature and bonding, which can be reflected in changes in physical and/or chemical properties. One important issue in the actinide series of elements is the effect of pressure on the 5f-electrons. We have probed changes in electronic behavior with pressure by monitoring structure by X-ray diffraction, and have studied several actinide metals and compounds from thorium through einsteinium. These studies have employed angle dispersive diffraction using synchrotron radiation, and energy dispersive techniques via conventional X-ray sources. The 5f-electrons of actinide metals and their alloys are often affected significantly by pressure, while with compounds, the structural changes are often not linked to the involvement of 5 f-electron. We shall present some of our more recent findings from studies of selected actinide metals, alloys and compounds under pressure. A discussion of the results in terms of the changes in electronic configurations and bonding with regard to the element's position in the series is also addressed.
VUV and Soft X-ray Spectroscopy of Actinides
Authors: Olson Clifford G. , Joyce John J. , Durakiewicz Tomasz , Guziewicz Elzbieta , Butterfield Martin
Published: April 2011

Optical and photoelectron spectroscopies using VUV and Soft X-ray photons are powerful tools for studies of elemental and compound actinides. Large changes in the relative atomic cross sections of the 5f, 6d and sp electrons allow decomposition of the character of the valence bands using photoemission. Resonant enhancement of photoelectrons and Auger electrons at the 5d core threshold further aids the decomposition and gives a measure of elemental specificity. Angle-resolved photoemission can be used to map the momentum dependence of the electronic states. The large changes in relative cross section with photon energy yields further details when the mapping is done at equivalent points in multiple zones. Spectra for well understood rare earth materials will be presented to establish spectral characteristics for known atomic character initial states. These signatures will be applied to the case of USb to investigate f-d hybridization near the Fermi level.
Electronic Structure and Surface Science of delta Plutonium
Authors: Butterfield M. , Durakiewicz T. , Guziewicz E. , Joyce J. J. , Moore D. P. , Arko A. J. , Morales L. A.
Published: April 2011

High resolution photoelectron spectroscopy (PES) studies were conducted on a δ-phase Plutonium sample cleaned by laser ablation and gas dosed with O2 and H2. The measurements were made with an instrument resolution of 60 meV and with the sample at 77 K. The PES data strongly support a model with Pu2O3 growth on the metal and then PuO2 growth on the Pu2O3 layer at this temperature. In vacuum, the PuO2 reduces to Pu2O3 at room temperature with a pressure of 6×10−11 Torr. In the case of H2 dosing the hydrogen appears to penetrate the surface and disrupt the valence band as evidenced by a drop in intensity of the peak at EF which is not accompanied by a drop in the main 5f manifold at ∼2eV.
X-ray Absorption Fine Structure Studies of Speciation of Technetium in Borosilicate Glasses
Authors: Lukens Wayne W. , Shuh David K. , Muller Isabelle S. , McKeown David A.
Published: April 2011

A series of glass samples were prepared analogously to high level waste glass using either glass frit or glass precursors combined with a high level waste surrogate containing NaTcO4. Three different technetium species were observed in these glasses depending upon the synthesis conditions. If the glasses were prepared by reducing NaTcO4 to TcO2•2H2O with hydrazine or if a large amount of organic material was present, inclusions of TcO2 were observed. If no organic material was present, technetium was incorporated as TcO4−. If only a small amount of organic material was present, isolated Tc(IV) sites were observed in the glass. The relative technetium retention of these glasses was estimated from the Tc K-edge height, and had no correlation with the oxidation state of the technetium. Pertechnetate was well retained in these glasses.
Elaboration and characterization of britholites loaded with tetravalent actinides
Authors: Terra Olivier , Audubert Fabienne , Dacheux Nicolas , Guy Christophe , Podor Renaud
Published: April 2011

Due to a natural occurrence, a good resistance to radiation damage and a low solubility in neutral and basic conditions, Nd-britholite of formula Ca9Nd(PO4)5(SiO4)F2 was already considered as a potential host matrix for the specific immobilization of trivalent actinides. The incorporation of tetravalent actinides like Th, U or Ce in the structure was examined through the elaboration of Ca9Nd1−xAnIVx(PO4)5−x(SiO4)1+xF2 samples. This study was the early beginning of the incorporation of 239Pu and/or 238Pu in order to evaluate the effects of α-decay on the britholite structure. The preparation of the samples was realized through dry chemistry methods using mechanical grinding in order to optimize the conditions of synthesis. The results showed that the incorporation of thorium in britholite structure was successful up to 20 wt.% leading to the formation of solid solutions. For U(IV), the incorporation was incomplete: only 5–8 wt.% of uranium was incorporated instead of 10 wt.% expected. The secondary phase, which was uranium enriched, was identified as the calcium uranate CaU2O5+y. The incorporation of Pu(IV) was studied using Ce(IV) as a surrogate. The results were quite good: major part of cerium was introduced in the structure but was partly reduced in Ce(III) during the heating treatment.
Electronic structure of UAsSe and USb2 compounds: the 5f photoemission
Authors: Guziewicz E. , Durakiewicz T. , Butterfield M. T. , Olson C. G. , Joyce J. J. , Arko A. J. , Sarrao J. L. , Wojakowski A. , Cichorek T.
Published: April 2011

UAsSe and USb2 single crystals were studied at 15K by angle-resolved photoemission spectroscopy (ARPES) in the photon energy range between 20 eV and 110 eV. The high energy and momentum resolution (24 meV and 0.09Å−1, respectively) allows observation of very narrow and dispersive photoemission features within 100 meV of the Fermi level. The natural linewidth of the near EF feature of USb2 was found to be less than 10 meV with the dispersion of 10 meV observed in normal emission spectra. The near EF feature in UAsSe is slightly broader, situated closer to the Fermi edge, and exhibits 20 meV dispersion in the normal emission spectra. It gives evidence that neither UAsSe nor USb2 have purely 2D electronic structure. Both these compounds should be treated as quasi-2D materials.
Composition of uranium oxide surface layers analyzed by μ-Raman Spectroscopy
Authors: Siekhaus Wigbert J.
Published: April 2011

Oxide thickness and composition averaged over a few square millimeter have been measured with nm thickness resolution by diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. μ-Raman spectroscopy has been done on powders and bulk samples in the past, and can now be done on surfaces layers with μm lateral and depth resolution using con-focal microscopy. Here we apply con-focal-microscopy-based μ -Raman spectroscopy to a freshly polished/lightly oxidized and to heavily oxidized uranium to determine its sensitivity. The spectra show that μ-Raman spectroscopy does detect oxide thickness and oxide composition with high sensitivity.
High-Throughput Screening of Thermoelectric Materials; Application of Thermal Probe Method to Composition-Spread Samples
Authors: Yamamoto Atsushi , Kukuruznyak Dmitry , Ahmet Parhat , Chikyow Toyohiro , Ohuchi Fumio S.
Published: April 2011

In this paper, we demonstrate the use of thermal probe method that is capable of mapping Seebeck coefficient, thermal conductivity and contact resistance on a micrometer scale. We show the successful screening example on pseudo binary (Bi1−xSbx)2Te3 (0.5<x<1) bulk composition-spread sample prepared by conventional powder metallurgy process. Another demonstration is a novel attempt to combine the combinatorial PLD and the thermal probe method. A pseudo ternary diagram of nickel-copper-manganese oxides fabricated on Nb doped STO substrate was used for the screening. The mapping of electrical resistance over the ternary diagram yields a lot of information, which is essential for materials researches on complex, multi-composition systems.
Development of High-Throughput substrates for Generating Two-Dimensional Nanoparticles Assemblies and for Screening Protein Adsorption
Authors: Bhat Rajendra R. , Genzer Jan
Published: April 2011

We discuss methods leading to the fabrication of orthogonal substrates comprising surface-anchored polymer brushes, in which the polymer brush grafting density and molecular weight vary independently in two mutually perpendicular directions. We demonstrate that these orthogonal polymer substrates can be used as intelligent combinatorial platforms that facilitate the spatial distribution of nanoparticles and allow screening of protein adsorption on surfaces.
Ab Initio Structure Determination of Quasicrystals via Single Crystal X-Ray Diffraction
Authors: Takakura Hiroyuki , Yamamoto Akiji , Boissieu Marc de , Sato Taku J , Tsai An Pang
Published: April 2011

A standard approach for structure solution of ordinary crystals begins with solving the phase problem. We show that a similar procedure can be taken even in the case of quasicrystals using single crystal X-ray diffraction by applying anab initiostructure determination method called the low density elimination method. The first picture of the occupation domains, which must be specified in a higher-dimensional structure determination of quasicrystals, is obtained from a phase-reconstructed density. We present six-dimensional densities determined by this method and give their interpretation for several different types of icosahedral quasicrystals.
Low temperature plasticity of an AlPdMn quasicrystal
Authors: Mompiou Frédéric , Caillard Daniel
Published: April 2011

TEM observations have been performed in AlPdMn single-grain quasicrystal deformed at low temperature. They show that dislocation motion has occurred by climb associated with vacancy diffusion. At room temperature, deformation also occurs by crack followed by re-healing.
From Crystalline to Glassy: Crack Propagation Modes in Decagonal Quasicrystals
Authors: Rudhart Christoph , Gumbsch Peter , Trebin Hans-Rainer
Published: April 2011

The propagation of mode-I cracks in a two-dimensional decagonal model quasicrystal is studied by molecular dynamics simulations. The samples are endowed with an atomically sharp seed crack and a temperature gradient. Subsequently the crack is loaded by linear scaling of the displacement field. The response of the crack running into regions of increasing temperature is monitored.

For low temperatures below 30% of the melting temperature Tm the model-quasicrystal fails by brittle fracture. We observe that the crack follows the path of dislocations nucleated at its tip. The crack propagates along well defined planes and circumvents tightly bound clusters. In the medium temperature regime from 30% to 70% Tm the crack is blunting spontaneously by dislocation emission. In the range of 70%-80% Tm the quasicrystal fails by nucleation, growth and coalescence of micro-voids. This gradual, dislocation-free crack extension is caused by plastic deformation which is mediated by localized rearrangements comparable to so-called shear transformation zones in amorphous solids.
Medium-Range Order and Crystallization in Zr59Cu20Al10Ni8Ti3 and Zr57Cu20Al10Ni8Ti5 Metallic Glasses Investigated by NMR
Authors: Breitzke Hergen , Lüders Klaus , Scudino Sergio , Eckert Jürgen , Kühn Uta
Published: April 2011

Medium-range order and quasicrystal formation in Zr57Cu20Al10Ni8Ti5 and Zr59Cu20Al10Ni8Ti3 metallic glasses are investigated by means of nuclear magnetic resonance (NMR). The samples are prepared by rapid solidification and annealed to induce primary quasicrystal formation. 27Al and 63Cu NMR spectra and spin relaxation curves are recorded before and after the annealing process. The spectral width of 27Al and 63Cu NMR spectra is relatively small and can be taken as a proof for the presence of medium-range order in these metallic glasses. By simulations of 27Al and 63Cu NMR second moments in various binary Al and Cu compounds the medium-range orders L12 AlZr3 and C11b CuZr2 can be identified in both as-quenched metallic glasses. After heat treatment both metallic glasses show considerable changes in the 63Cu NMR second moments, whereas the 27Al NMR second moments show only relatively small changes. Furthermore, the 63Cu NMR second moment after heat treatment in Zr57Cu20Al10Ni8Ti5 is about half as large as in Zr59Cu20Al10Ni8Ti3 indicating different types of crystallization during heat treatment.
Amorphous Alloy Formation in Immiscible Cu-Ta and Cu-W Systems by Atomistic Modeling and Ion-Beam Mixing
Authors: Gong H. R. , Kong L. T. , Liu B. X.
Published: April 2011

For the immiscible Cu-Ta and Cu-W systems, realistic n-body potentials are derived under an embedded-atom method through fitting the cross potentials to some physical properties obtained from ab initio calculations for a few possible metastable Cu-Ta and Cu-W crystalline phases, respectively. Based on the derived potentials, molecular dynamics simulations reveal that in the Cu-Ta system, 30 at. % of Ta in Cu is the critical composition for the crystal-to-amorphous transition in the Cu-rich Cu-Ta solid solutions, and that in the Cu-W system, amorphous alloys can be formed within the composition range of 20–65 at. % of W. Interestingly, amorphous alloys are indeed obtained by ion-beam mixing in properly designed Cu70Ta 30, Cu65Ta35, Cu60Ta 40, and Cu50Ta 50 multilayered films, while crystalline Cu and Ta remain in Cu75Ta25 multilayered sample, which matches well with the critical composition of 30 at. % of Ta predicted by simulation. Moreover, there have been experimental data, which are in support of the predicted composition range of the Cu-W system by simulations.
On the Fragility of Nb-Ni Based and Zr55-x Tix(CuNi)18.25+yBe26.25-y Bulk Metallic Glasses
Authors: Shadowspeaker Ludi A. , Busch Ralf
Published: April 2011

The heating rate dependencies of the glass transition temperature of the Ni65Nb35, Ni60Nb35Sn5, Ni59.35Nb34.45Sn6.2, Ni60(Nb40Ta60)34Sn6, and Ni57Fe3Nb35Sn5 metallic glass forming alloys were investigated with a differential scanning calorimeter (DSC). The relaxation time for each DSC experiment was plotted versus inverse temperature and a Vogel-Fulcher-Tamman (VFT) type relation was fitted to the data. The fragilities of the alloys were characterized with the fragility parameter, D*, and the VFT temperature, T0, which are the fit parameters from the VFT relation. It was found that for the binary alloy D* = 6.2, for the ternary alloys D* = 11.0, and that for the quaternary alloys D* was between 16.4 and 19.0. The D* increases monotonically as the number of components in the alloy is increased. This trend is also seen in Zr based alloys.
Molding of fine surface features into bulk metallic glass
Authors: McCracken Ivan , Busch Ralf
Published: April 2011

Molding of fine surface features into a bulk metallic glass (BMG) from the molten state was investigated. It was studied down to which a feature size in a mold could be reproduced in the BMG. A new casting apparatus was designed and built to carry out the experiments. The BMG used was Vitreloy 1™, Zr41.24Ti13.75Cu12.5Ni10Be22.5, and it was observed to replicate mold features down to several nanometers. This has been verified with SEM photographs of both the mold surfaces and the BMG parts. Processing variables, including the injection temperature of the molten BMG and the atmosphere, in which it is molded, appear to have a great effect on the ability to reproduce the fine features on the mold surface.
Medium-Range Order in High Al-content Amorphous Alloys Measured by Fluctuation Electron Microscopy
Authors: Stratton W. G. , Hamann J. , Perepezko J. H. , Voyles P. M.
Published: April 2011

We have used fluctuation electron microscopy (FEM) to measure nanoscale mediumrange order in amorphous Al92Sm8. Samples of this amorphous alloy formed by rapid quenching (melt-spinning) show a high density of pure Al nanocrystals (>1020 m-3) after low temperature (< 250 °C) devitrification. In samples amorphized by deformation (cold-rolling), primary Al-crystallization does not occur. This difference in devitrification behavior suggests an underlying structural difference in the amorphous state. FEM is a quantitative microscopy technique for determining nanoscale medium-range order in amorphous materials. Our measurements show that amorphous alloys formed by melt-spinning and cold-rolling have significant structural differences, and that annealing melt-spun alloy under conditions previously shown to modify the devitrification thermodynamics also changes the medium-range structure.
Ultra-thin strain relaxed SiGe buffer layers with 40% Ge
Authors: Lyutovich Klara , Kasper Erich , Oehme Michael
Published: April 2011

Virtual substrates with ultra-thin SiGe strain relaxed buffers have been grown on Si substrates by a method employing point defect supersaturation in the growing layers. A concept of the point defect influence on the strain relaxation and on defect interactions in layers has been proposed. A method is developed to increase the degree of relaxation in sub-100 nm SiGe buffer layers and to provide a smooth surface morphology. Layer growth has been realized by solid source molecular beam epitaxy in a chamber equipped with an in situ monitoring system. One of the growth stages, performed at a very low temperature, serves the generation of point defects. Strain relaxation tunable up to the high degree and a crosshatch-free surface morphology are demonstrated in 40nm thick SiGe buffers which contain 40-45% Ge.

Growth monitoring enables the control of the process window and the layer crystallization by a chosen mechanism.

Virtual substrates produced by the described method were successfully tested in nMODFET structures.