bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

ABSTRACTPseudomorphic 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

ABSTRACTUndoped 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

ABSTRACTThe 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

ABSTRACTWe 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

ABSTRACTNovel 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

ABSTRACTAs 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

ABSTRACTMultiscale 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

ABSTRACTIt 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

ABSTRACTThe 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

ABSTRACTNi/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

ABSTRACTRenewed 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

ABSTRACTThe 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

ABSTRACTA 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.
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

ABSTRACTOne 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

ABSTRACTOptical 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

AbstractHigh 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

ABSTRACTA 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

ABSTRACTDue 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

ABSTRACTUAsSe 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.