bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructures of AlN Buffer Layers for the Growth of GaN on (0001) Al2O3
Authors: Yeadon M. , Kim W. , Botchkarev A. E. , Mohammad S. N. , Morkoc H. , Gibson J. M.
Published: April 2011

Ill-nitride semiconductors are emerging as highly promising candidates for the fabrication of wide band-gap electronic and opto-electronic devices. Sapphire ((α-A12O3) is currently one of the primary substrates of choice for the growth of GaN despite a large lattice mismatch. Significant improvements in the quality of III-nitride layers have been demonstrated by exposure of the substrate to reactive nitrogen species followed by deposition of a low temperature AIN or GaN buffer layer. In this paper we present a study of the evolution of the surface topography and defect microstructure of nitrided α-A12O3 substrates and AIN buffer layers deposited by reactive molecular beam epitaxy (RMBE). Their influence on the morphology and properties of GaN layers is also discussed. Both nitridation time and AIN deposit thickness were varied systematically, at different temperatures and buffer growth rates. The microstructures were characterized using the atomic force microscope (AFM) and transmission electron microscope (TEM). Initial growth studies are ideally suited to in-situ experiments, and further investigations are also in progress using a unique UHV TEM with the facility for in-situ RMBE.
Morphology and Dielectric Properties of Reactively Sputtered Aluminum Nitride Thin Films
Authors: Randolph A. G. , Kurinec S.K.
Published: April 2011

Aluminum nitride thin films (∼ 100 mn) have been deposited on silicon substrate by reactive sputtering using Al target in 1:1 Ar:N2 environment. The atomic force microscopy examination revealed continuous microcrystalline film structure. The Auger electron spectroscopic analysis show the presence of oxygen in the films. The annealing at 850 C in nitrogen is found to cause recrystallization and further oxidation of the films. The films can be characterized as lossy dielectrics with relative permittivity ∼ 10, higher than the bulk value of 8.9. Annealing the films is found to reduce anion vacancies and improve the dielectric strength within a range of a few MV/cm in these thin films.
Growth of Hexagonal Gallium Nitride Films On The (111) Surfaces of Silicon with Zinc Oxide Buffer Layers
Authors: Kim Y. , Kim C. G. , Lee K-W. , Yu K-S. , Park J. T. , Kim Y.
Published: April 2011

The growth of gallium nitride films on sapphire substrates has not been straightforward because of the large lattice mismatch between gallium nitride and sapphire. Zinc oxide is structurally the closest material to gallium nitride and therefore is finding use as the substrate for gallium nitride. Single crystal wafers of zinc oxide are hard to obtain and very expensive. However, a thin layer of zinc oxide on a suitable substrate might solve this problem. In this work, highly c-axis oriented zinc oxide buffer layers were grown on Si(lll) substrates at temperatures 410–540 °C by chemical vapor deposition of bis(2,2,6,6-tetramethyl–3,5-heptanedionato)zinc, Zn(tmhd)2, and the hexagonal GaN films were subsequently deposited on them at 500 °C using the single precursor tris(diethyl -μ-amido-gallium), [(C2H5)2 GaNH2]3. The compound Zn(tmhd)2 was found to require oxygen for the deposition of zinc oxide. In the case of gallium nitride, low pressure chemical vapor deposition of tris(diethyl-μ-amido-gallium) worked reasonably well with or without a carrier gas. The buffer layers and the GaN films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and reflection high energy elctron diffraction (RHEED).
Observation of Midgap States in GaN with Optical-Isothermal Capacitance Transient Spectroscopy
Authors: Hacke P. , Miyoshi H. , Hiramatsu K. , Okumura H. , Yoshida S. , Okushi H.
Published: April 2011

Optical-isothermal capacitance transient spectroscopy (O-ICTS) was used to distinguish the deep levels which occur in unintentionally doped n-type GaN by means of their characteristic optical cross section. GaN grown by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were compared. Correspondence between optical and thermal emission characteristics of previously discovered levels, E2 (∼Ec-0.55 eV) and E4 (∼EC-1.0 eV), were clearly determined by observing their sequential appearance in the ICTS spectra. Whether by thermal or optical stimulation, the emission from E4 was found to be broad in nature; it is consequently believed to involve a defect. The total measured concentration of deep levels, including a prominent level which photoionizes in the range 2.5 to 3.0 eV below the conduction band, is greater in the GaN grown by MOVPE than by HVPE that was tested.
Photoluminescence, Reflectance, and Magnetospectroscopy of Shallow Excitons in GaN
Authors: Skromme B. J. , Zhao H. , Goldenberg B. , Kong H. S. , Leonard M. T. , Bulman G. E. , Abernathy C. R. , Pearton S. J.
Published: April 2011

We report several new aspects of the excitonic properties of heteroepitaxial GaN grown on sapphire or 6H-SiC. In particular, we observed the n = 2 free exciton associated with both A and B excitons (which are distinct from the n = 1 C exciton) using reflectance and 1.7 K photoluminescence. We also studied the behavior of the n = 2 A-exciton using magnetoluminescence in fields up to 12 T. The large diamagnetic shift and splitting positively confirm the identification, yielding an exciton binding energy of about 26.4 meV. Several previous identifications of the n = 2 free exciton yielding a smaller exciton binding energy are probably in error, based on our results. We have also detected the two-electron replica of the neutral donor-bound exciton for the first time in GaN and observed its splitting pattern in magnetic fields up to 12 T. This feature is 22 meV below the principal neutral donor-bound exciton peak, independently of strain shifts in the overall spectrum. It yields a precise donor binding energy of 29 meV for the shallow residual donor in material grown by metalorganic chemical vapor deposition and gas-source molecular beam epitaxy, considerably smaller than that of the residual donor reported earlier in hydride vapor phase epitaxial material (about 35.5 meV).
Variation of GaN Valence Bands with Biaxial Stress: Quantification of Residual Stress and Impact on Fundamental Band Parameters
Authors: Edwards N. V. , Yoo S. D. , Bremser M. D. , Horton M. N. , Perkins N. R. , Weeks T. W. , Liu H. , Stall R. A. , Kuech T. F. , Davis R. F. , Aspnes D. E.
Published: April 2011

We provide the widest estimate thus far of the range of tensile and compressive stress (−3.8 to 3.5 kbar) that GaN epitaxial material can withstand before relaxation occurs, and an unambiguous determination of the spin-orbit splitting Δso = 17.0 ± 1 meV for the material. These are achieved by analyzing 10K reflectance data for the energy separation of transitions between the uppermost valence bands and the lowest conduction band of wurtzitic GaN as a function of biaxial stress for a series of GaN films grown on both Al2O3 and 6H-SiC substrates. Our data explicitly show the nonlinear behavior of the excitonic energy splittings B-A and C-A vs. the energy position of the A exciton, which stands in contrast to the linear approximations used by previous workers analyzing material grown only on Al2O3 substrates. Further, the lineshape ambiguities present in GaN reflectance spectra that hindered the accurate determination of such excitonic energies have also been resolved by analyzing these data in reciprocal space, where critical point energies are determined by phase effects to an accuracy of ±0.5 meV.
Shallow Impurity States in Wurtzite and Zinc Blende Structure GaN
Authors: Wang R. , Ruden P. P. , Kolnik J. , Oguzman I. , Brennan K. F.
Published: April 2011

Calculations of shallow donor states for wurtzite and zincblende structure GaN, and acceptor states for zincblende GaN in an effective mass approximation are presented. Band parameters were taken from experiment or determined from band structure calculations. The effect of wavevector dependent screening is examined, based on dielectric functions calculated from empirical pseudopotential band structures. For donor states, the effects of electron-phonon coupling and free carrier screening in the Thomas-Fermi and Debye approximations are discussed briefly.
Blue Luminescence from SiOx Films Containing Ge Nanocrystals
Authors: Zacharias M. , Weigand R. , Bläsing J. , Christen J.
Published: April 2011

We present a systematic analysis of SiOx alloys films containing Ge nanocrystals prepared by dc magnetron sputtering. Increasing the sputtering power (50–175W) reduces the average Ge nanocrystal size exponentially down to ∼2nm. Broad band photoluminescence spectra are observed in the visible at room temperature centered at ∼3eV or/and ∼2eV. Neither the 3eV nor the 2eV luminescence can be correlated to the change in the size. Excluding the quantum-confined origin, the presence of a luminescence center located in the inhomogeneous strain field of the Ge nanocrystal surface is discussed.
Soft X-Ray Emission Studies of the Electronic Structure in Silicon Nanoclusters
Authors: Van Buuren T. , Dinh L. N. , Chase L. L. , Siekhaus W. J. , Jimenez I. , Terminello L. J. , Grush M. , Callcott T. A. , Carlisle J. A.
Published: April 2011

Density of states changes in the valence and conduction band of silicon nanoclusters were monitored using soft x-ray emission and absorption spectroscopy as a function of cluster size. A progressive increase in the valence band edge toward lower energy is found for clusters with decreasing diameters. A similar but smaller shift is observed in the near-edge x-ray absorption data of the silicon nanoclusters.
Raman Scattering Investigation of Superconductivity in Si46 Clathrates
Authors: Fang S. L. , Grigorian L. , Rao A. M. , Eklund P. C. , Dresselhaus G. , Dresselhaus M. S. , Kawaji H. , Yamanaka S.
Published: April 2011

Room temperature Raman scattering spectra are reported for the type II superconductors MxBaySi46 (M-Na, K) which were recently shown to exhibit Tc's ∼ 3.5 K. The spectra are compared to those of other Si46-clathrates which exhibit normal metallic behavior down to 2K. Thirteen of the twenty first-order Raman frequencies predicted by group theory have been detected, and the frequencies are found to be sensitive to the particular dopants. The Raman linewidths observed for the MxBaySi46 system are comparable to those observed for Na8Si46 and K7Si46. The data, taken collectively, suggest that the line broadening in the metallic Si-clathrates is due to important contributions from both the electron-phonon interaction as well as to a random filling of the Si cages.
Synthesis of Mn2+ Doped CdS Nanocrystals Embedded in a Sol-Gel Silica Matrix: Characterization of the Luminescence Activator
Authors: Counio G. , Esnouf S. , Gacoin T. , Barboux P. , Hofstaetter A. , Boilot J.-P.
Published: April 2011

Mn2+-doped CdS nanocrystals (1.2 to 2.4 nm in diameter) embedded in organic-inorganic silica xerogels have been synthesized. Extensive studies (EXAFS, ESR and ENDOR) allow us to localize the ions responsible for the bright luminescence observed in such materials (quantum yield of 7%). The average number of Mn2+ per nanocrystal is in the 0.2–0.8 range, and the emission arises from an energy transfer from surface trapped carriers to Mn2+ ions.
Depth Gradients in Porous Silicon: How to Measure Them and How to Avoid Them
Authors: Thönissen M. , Berger M. G. , Theiβ W. , Hilbrich S. , Krüger M. , Arens-Fischer R. , Billat S. , Lerondel G. , Lüth H.
Published: April 2011

For the formation of single layers and layer systems the depth homogeneity of porous silicon (PS) plays a key role. We have measured qualitatively and quantitatively structural gradients in p-PS layers with increasing layer thicknesses by fitting reflectance spectra in the infrared and by measuring thickness oscillations in the reflectance during the formation of PS layers. These results allow us to give recipes for the formation of PS layers with a homogenous optical thickness in depth.
Roles of Surface Termination in Photoluminescence Mechanisms of Porous Si
Authors: Suda Y. , Obata K. , Kumagai A. , Koshida N.
Published: April 2011

The relationship between the oxidation states and PL properties and the effects of H/O termination exchange on the PL properties of PS have been investigated using synchrotron radiation photoemission spectroscopy (SR-PES), Auger electron spectroscopy (AES), Fourier transform infrared (FTIR), and photoluminescence (PL) techniques. The energy band gap, the peak energy and FWHM of the PL spectrum are almost unchanged by the oxidation process and by the H/O termination exchange. After the oxidation, the PL peak intensity decreased, suggesting the creation of nonradiative centers. In the H/O termination exchange experiment, the PL peak intensity decreased by more than 65% upon annealing. However, it recovered the initial PL intensity by oxygen exposure. These results suggest that the surface termination itself functions to eliminate the nonradiative centers without depending on the termination species of hydrogen or oxygen, and that the skeletal structure of PS crystallites is important in the PL mechanisms.
Fabrication and Characterization of Light Emitting Porous Silicon and Polymer Nanocomposites
Authors: Duttagupta S. P. , Fauchet P. M. , Chen X. L. , Jenekhe S. A.
Published: April 2011

We report the fabrication of nanocomposites by the infiltration of polymers into porous silicon. Polymers such as polyamide, polystyrene, PMMA, and PVC were chosen because they are commonly available and have been extensively studied. The pore-filling was accomplished by either diffusion of the polymer molecules into porous silicon or in-situ polymerization of the monomer. The Vickers hardness arid the thermal conductivity of the samples were measured. There was a difference in the nanocomposite characteristics depending on whether the samples were as-anodized or had been annealed in oxygen. By infiltrating polyamide into an as-anodized sample, a 42% increase in hardness and a 24% increase in thermal conductivity were observed at room temperature, without any degradation of luminescence.
Localisation of Carriers in Porous Silicon
Authors: Mihalcescu I. , Vial J. C. , Romestain R.
Published: April 2011

We analyze the intensity and decay time evolution of the porous silicon luminescence upon anodic oxidation, aging, chemiral thinning and temperature variation. Strong analogies are pointed out for the photoluminescence intensity as well as for the photoluminescence decay shape evolution. They are interpreted by the variation of the extension of the carrier wavefunction induced by the modification of potential barrier efficiencies. No additional assumption such as hopping of carriers was necessary to explain the decay shapes well fitted by stretched exponential. On the contrary our observations and our simple model are in favor of a strong localization of carriers. Some experimental results are revisited within the frame of this model.
Room Temperature Band-Edge Luminescence from Silicon Grains Prepared by the Recrystallization of Mesoporous Silicon
Authors: Moore Karen L. , Tsybeskov Leonid , Fauchet Philippe M. , Hall Dennis G.
Published: April 2011

Room-temperature photoluminescence (PL) peaking at 1.1 eV has been found in electrochemically etched mesoporous silicon annealed at 950°C. Low-temperature PL spectra clearly show a fine structure related to phonon-assisted transitions in pure crystalline silicon (c-Si) and the absence of defect-related (e.g.P-line) and impurity-related (e.g.oxygen, boron) transitions. The maximum PL external quantum efficiency (EQE) is found to be better than 0.1% with a weak temperature dependence in the region from 12K to 400K. The PL intensity is a linear function of excitation intensity up to 100 W/cm2. The PL can be suppressed by an external electric field ≥ 105 V/cm. Room temperature electroluminescence (EL) related to the c-Si band-edge is also demonstrated under an applied bias ≤ 1.2 V and with a current density ≈ 20 mA/cm2. A model is proposed in which the radiative recombination originates from recrystallized Si grains within a non-stoichiometric Si-rich silicon oxide (SRSO) matrix.
Light Emission from Intrinsic and Doped Silicon-Rich Silicon Oxide: from the Visible to 1.6 ΜM
Authors: Tsybeskov L. , Moore K. L. , Fauchet P. M. , Hall D. G.
Published: April 2011

Silicon-rich silicon oxide (SRSO) films were prepared by thermal oxidation (700°C-950°C) of electrochemically etched crystalline silicon (c-Si). The annealing-oxidation conditions are responsible for the chemical and structural modification of SRSO as well as for the intrinsic light-emission in the visible and near infra-red spectral regions (2.0–1.8 eV, 1.6 eV and 1.1 eV). The extrinsic photoluminescence (PL) is produced by doping (via electroplating or ion implantation) with rare-earth (R-E) ions (Nd at 1.06 μm, Er at 1.5 μm) and chalcogens (S at ∼1.6 μm). The impurities can be localized within the Si grains (S), in the SiO matrix (Nd, Er) or at the Si-SiO interface (Er). The Er-related PL in SRSO was studied in detail: the maximum PL external quantum efficiency (EQE) of 0.01–0.1% was found in samples annealed at 900°C in diluted oxygen (∼ 10% in N2). The integrated PL temperature dependence is weak from 12K to 300K. Light emitting diodes (LEDs) with an active layer made of an intrinsic and doped SRSO are manufactured and studied: room temperature electroluminescence (EL) from the visible to 1.6 μmhas been demonstrated.
Improved Interference Filter Structures Made of Porous Silicon
Authors: Thönissen M. , Berger M. G. , Krüger M. , Theiβ W. , Hilbrich S. , Arens-Fischer R. , Billar S. , Lüth H.
Published: April 2011

Recently passive optical devices like filter structures or waveguides based on porous silicon have attracted high interest due to their easy and cheap fabrication. We have formed interference filters using porous silicon by changing the current density during formation. For the specific design of these filter structures a calibration of the etch rate and the refractive indices is required. Therefore we have determined the effective dielectric function for different substrate doping levels and anodization current densities by fitting reflectance spectra. Based on these results different kinds of reflectance filters consisting of discrete layers (Bragg reflectors, Fabry-Perot filters) as well as filters with a continuous change of the refractive indices with depth (rugate-filters) can be realised. Furthermore we present applications of these filter structures such as anti-reflectance coatings and high quality mirrors.
A Microscopic Model for the Dielectric Function of Porous Silicon
Authors: Cruz M. , Beltran M. R. , Wang C. , Tagüeña-Martinez J.
Published: April 2011

Micro and nano-structures have opened a new area in materials research since they present interesting phenomena such as efficient luminescence and localization of carriers. An important example of these new materials is porous silicon (PS). It is considered that the quantum confinement is an essential cause of the opto-electronic properties of PS [1], thus microscopic analysis should be performed. We have developed a supercell model to study PS with a tight-binding Hamiltonian, where an sp3s* basis set is used. In an otherwise perfect silicon structure empty columns of atoms are produced and passivated with hydrogen atoms [2]. In this work we calculate the dielectric function and compare it against experimental data for bulk c-Si, ultrathin c-Si films and PS. We discuss the importance of considering the relaxation of the electron wavevector (k) conservation in order to include disorder effects in PS.
Post-Deposition Annealing and Hydrogenation of Hot-Wire Amorphous and Microcrystalline Silicon Films
Authors: Conde J. P. , Brogueira P. , Chu V.
Published: April 2011

Amorphous and microcrystalline silicon films deposited by hot-wire chemical vapor deposition were submitted to thermal annealing and to RF and electron-cyclotron resonance (ECR) hydrogen plasmas. Although the transport properties of the films did not change after these post-deposition treatments, the power density of a Ar+ laser required to crystallize the amorphous silicon films was significantly lowered by the exposure of the films to a hydrogen plasma. This decrease was dependent on the type of hydrogen plasma used, being the strongest for an ECR plasma with the substrate held at a negative bias, followed by an ECR hydrogen plasma with the substrate electrode grounded, and finally by an RF hydrogen plasma.
Fracture of Nanophase Ceramics: A Molecular-Dynamics Study
Authors: Nakano Aiichiro , Kalia Rajiv K. , Omeltchenko Andrey , Tsuruta Kenji , Vashishta Priya
Published: April 2011

New multiscale algorithms and a load-balancing scheme are combined for molecular-dynamics simulations of nanocluster-assembled ceramics on parallel computers. Million-atom simulations of the dynamic fracture in nanophase silicon nitride reveal anisotropie self-affine structures and crossover phenomena associated with fracture surfaces.
Structure, Mechanical Properties, and Dynamic Fracture in Nanophase Silicon Nitride via Parallel Molecular Dynamics
Authors: Tsuruta Kenji , Omeltchenko Andrey , Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya
Published: April 2011

Million-atom molecular-dynamics (MD) simulations are performed to study the structure, mechanical properties, and dynamic fracture in nanophase Si3N4. We find that intercluster regions are highly disordered: 50% of Si atoms in intercluster regions are three-fold coordinated. Elastic moduli of nanophase Si3N4 as a function of grain size and porosity are well described by a multiphase model for heterogeneous materials. The study of fracture in the nanophase Si3N4 reveals that the system can sustain an order-of-magnitude larger external load than crystalline Si3N4. This is due to branching and pinning of the crack front by nanoscale microstructures.
Fluorophlogopite and Taeniolite: Synthesis and Nanocomposite Formation
Authors: Moore Gregory J. , Zavalij Peter Y. , Whittingham M. Stanley
Published: April 2011

Sodium fluorophlogopite and lithium taeniolite have been synthesized by new routes for application in lithium batteries. The fluorophlogopite synthesized by a high temperature solid state reaction, was found to be non-water-swellable and unreactive towards several mono- and divalent ions. However it was found to readily undergo ion-exchange with both copper and iron ions, with concomitant swelling to a bilayer water state. This swelled material reacted readily with long chain amines and other molecules and ions behaving like a regular swellable silicate. A taeniolite precursor was synthesized by mild hydrothermal reactions, and annealed into a well crystalline layer solid, that reacted readily with organics to form ordered composites that have potential use as battery electrolytes and cathodes.
Synthesis and Lcst Behavior of Thermally Responsive Poly(N-Isopropylacrylamide)/Layered Silicate Nanocomposites
Authors: Messersmith Phillip B. , Znidarsich F.
Published: April 2011

Stimuli responsive polymeric hydrogel composites were synthesized by room temperature copolymerization of N-isopropyl acrylamide and methylene bisacrylamide (crosslinking monomer) in an aqueous suspension of Na-montmorillonite. Hydrogels containing 3.5 weight % of montmorillonite exhibited a lower critical solution temperature (LCST) similar to unmodified PNIPAM hydrogel (approximately 32°C), and underwent a reversible 60–70% volume shrinkage when heated from ambient temperature to above the LCST. However, hydrogels containing 10 weight% montmorillonite did not exhibit a measurable LCST, and underwent considerably less shrinkage when heated. A solvent exchange reaction was used to replace the water with an acrylic monomer, which was polymerized in-situ to create a delaminated montmorillonite/polymer nanocomposite.
Production of Nanostructured Iron Oxide Particles via Aerosol Decomposition
Authors: Joutsensaari J. , Kauppinen E. I.
Published: April 2011

Nanostructured iron oxide particles with average size below 100 nm were produced by aerosol decomposition method starting from an aqueous iron nitrate solution. Air, nitrogen, or mixture of hydrogen (7 %) and nitrogen were used as the carrier gas. Gas-phase particle number size distributions were determined with a differential mobility analyzer. Particle morphology and crystallinity were studied with scanning (SEM) and transmission (TEM) electron microscopes. Crystalline phase composition of the particles was studied with X-ray diffraction (XRD). The average gas-phase diameter of particles produced in air or N2 reduced from 80 to 47 nm when temperature was increased from 500 to 1100 °C. In H2 rich environment, the reduction of average size was larger, from 80 nm at 500 °C to 45 nm already at 900 °C. SEM results showed that very small crystallites (5–10 nm) were formed on the surface of the particles produced in N2 at 500 °C. When the processing temperature was increased to 700, 900 and 1100 °C, the crystallites on the particle surfaces were grown to 15–30, 30–60 and 60–180 nm, respectively. TEM results show that very small particles (<50 nm) were single crystals and larger particles were polycrystalline with crystallite size of about 50 nm at 700 °C in H2/N2. Magnetite particles were produced from aqueous iron nitrate solution at 500 °C in H2/N2 and at 900 °C in N2 according to XRD results.
Microstructure Analysis of Melt-Spun Al3Ti Intermetallics by XRD and EXAFS
Authors: Chen Jinmin , Frazier W. E. , Barrera E. V.
Published: April 2011

In an effort to expand the composition range over which Al3Ti is stable, various amounts of niobium were substituted for titanium and processed by melt-spinning. Several samples were annealed both at 600°C and 1000°C for 24 hours. The effects of processing parameters such as wheel speed, the amount of niobium, and annealing temperatures on the structure were investigated by XRD and EXAFS. XRD showed that for all the samples the only structure present was DO22-The DO22 structure was stable even after the high temperature heat treatments. By means of EXAFS, niobium atoms were observed to occupy titanium sites in the DO22 structure. Furthermore, in the unannealed samples, increasing wheel speed of the melt spinning process or the niobium concentration tended to distort the crystal structure. It was observed that Ti EXAFS had different results from the Nb EXAFS beyond their occupying similar sites, which suggested there may exist some composition zones, i.e. rich Nb zone or rich Ti zones, although the structures present were still DO22. The samples were found to experience different distortions as a function of annealing temperatures.
Texture Studies of Gamma Titanium Aluminide Sheets Produced by Melt Overflow Rapid Solidification
Authors: Weaver M. L. , Garmestani H. , Das G.
Published: April 2011

Texture analyses have been conducted on gamma titanium aluminide (γ-TiAI) strips produced using the recently developed plasma melt overflow process. The results indicated that <101] deformation textures persisted in the γ-phase while <0002>, <1010> and <1020> texture components were all observed in the α2-phase. After annealing at 1065°C/48 hrs., the γ-phase textures did not change while the α2-phase changed from a basal to a <1010> texture. It is suggested that the texture development in direct cast γ strips produced using this technique are a direct result of lattice rotations and residual stresses caused by nonuniform cooling.
Microstructural Changes During Long-Term Tension Creep of Two-Phase γ-Titanium Aluminide Alloys
Authors: Oehring M. , Ennis P. J. , Appel F. , Wagner R.
Published: April 2011

Long-term tension creep tests were performed on a Ti-48 at.% Al-2 at.% Cr alloy in order to assess the material behaviour under the intended service conditions for structural parts in turbine engines. Deformation processes and microstructural changes were investigated by TEM on a specimen loaded to 140 MPa for 5988 h at 700 °C. At lamellar boundaries the emission of interfacial dislocations was observed and is thought to contribute significantly to the high primary creep rate of the material. Under the creep conditions gliding dislocations apparently become locked by the heterogeneous formation of precipitates along their cores. Lamellar interfaces revealed ledges which indicates that they migrate during creep.
Deformation Mechanisms Responsible for the Creep Resistance of Ti-Al Alloys
Authors: Morris M. A. , Lipe T.
Published: April 2011

Two γ-based Ti-Al alloys with similar grain sizes and, respectively, lamellar and duplex microstructures have been creep tested at 700°C and constant stresses ranging between 280 and 430 MPa. TEM observations have confirmed that the duplex alloy deforms by extensive mechanical twinning whose density increases with applied stress and increasing strain. The new twin interfaces subdivide the γ grains throughout the primary stage of creep. At the onset of the minimum creep rate, the twin interfaces in the duplex alloy behave in the same way as the γ/γ or the α2/γ interfaces in the lamellar alloy. However, single dislocations were also present and it appears that in both alloys the deformation process is controlled by the accumulation and emission of dislocations from the different interfaces.
Creep Deformation of a Fully Lamellar Gamma Based Titanium Aluminide Alloy
Authors: Herrouin F. , Bowen P. , Jones I. P.
Published: April 2011

A complex two phase γ-TiAl alloy, Ti-47Al-lCr-1Mn-2Ta-0.2Si (at.%) in a fully lamellar condition, has been creep tested at a stress of 200MPa and a temperature of 700°C. This simulates the in-service operating conditions for several potential gas turbine aero engine applications where creep resistance is a design limiting material property. The results have indicate that reduction in lamellae thickness and avoidance of feathery type microstructures contribute to improved creep resistance.
Creep in Single Crystal Ni3AI
Authors: Zhu W. , Jones I. P. , Fort D. , Smallman R. E.
Published: April 2011

Single crystals of Ni3AI (1 at%Ta) with a compression axis of [123] were subject to creep at a stress of 550 MPa and a temperature of 520 °C. Slip trace and TEM microstructural observations reveal that primary octahedral slip is responsible for the primary creep. In the secondary stage, cube cross slip (010) is operative. There is no obvious sign of inverse creep.
Site Occupancies in Ternary C15 Ordered Laves Phases
Authors: Kotula P. G. , Anderson I. M. , Chu F. , Thoma D. J. , Bentley J. , Mitchell T. E.
Published: April 2011

Site occupancies in three C15-structured AB2(X) Laves phases have been determined with Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI). In NbCr2(V), the results are consistent with exclusive site occupancies of Nb for the A sublattice and Cr and V for the A sublattice. The B-site occupancy of V can be interpreted in terms of electronic structure. In NbCr2(Ti), the results are consistent with Ti partitioning mostly to the A sites with some anti-site defects likely. In HfV2(Nb), the results are consistent with Nb partitioning between the A and A sites. The results of the ALCHEMI analyses of these ternary C15 Laves phase materials are discussed with respect to previously determined phase diagrams and first-principles total energy and electronic structure calculations.
Creep of Anomalous Ni3Ga
Authors: Lunt M J , Sun Y Q
Published: April 2011

Ni3Ga is among a number of LI2ordered intermetallic alloys whose yield stress increases with temperature. In this work we have examined the creep strength of [123] and [001] oriented Ni3Ga specimens in the temperature regime of the yield stress anomaly and confirmed that the creep strength shows the normal rapid decrease with temperature. Inverse creep occurs in the [001] specimens where slip is on the {111} planes only. [123] specimens exhibit steady-state creep and slip line and TEM observations have shown slip on the cube plane and dislocations of both <110> and <100> Burgers vectors are present. We have carried out creep tests of prestrained [123] samples and demonstrated that the storage of primary <110> {111} screw dislocations, locked in the Kear-Wilsdorf configuration, has no effect on creep.
Characterization of thin Polymeric Nanofoam films by Transmission Electron Microscopy and Small Angle Neutron Scattering
Authors: Briber R. M. , Fodor J. S. , Russell T. P. , Miller R. D. , Carter K. R. , Hedrick J. L.
Published: April 2011

Thin film polymer nanofoams are produced from triblock copolymers of a fluorinated polyimide, 3F/PMDA (derived from pyromelletic dianhydride (PMDA) and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (3F)) as the center block and polypropylene oxide (PO) as the end blocks. The nanofoam is produced using a three step process: 1.) spin casting the triblock copolymer onto a silicon substrate, 2.) thermal treatment in an Argon atmosphere to imidize the center block and 3.) thermal treatment in air to degrade the PO domains and form nanoscale voids. This process was characterized using both transmission electron microscopy (TEM) and small angle neutron scattering (SANS). For the TEM studies, Ruthenium tetroxide staining was used to enhance the contrast between the polyimide (PI) matrix and the PO microdomains or voids, which permitted a more detailed view of the microstructure of both the foamed and unfoamed materials. From the two dimensional Fourier transform of the micrographs the spatial correlation between the PO microdomains in the unfoamed material and between the voids in the foam were found. An interdomain separation distance of -37 nm was observed. SANS was performed to follow the imidization and foaming processes both in-situ and on a Si substrate. The SANS results indicated that the films are homogeneous when spun from solution and that the microphase separation of the PO domains occurs during the imidization step. The subsequent foaming step leaves the morphology generally intact with the PO domains converting to voids. A peak was found in the SANS curve with a spacing of about 26 nm, which is qualitative agreement with the TEM data.
Block Copolymer Microdomain Morphologies by Phase Detection Imaging
Authors: Leclère Ph. , Yu J. M. , Lazzaroni R. , Dubois Ph. , JéRôme R. , Brédas J. L.
Published: April 2011

Atomic Force Microscopy with Phase Detection Imaging is used to study the surface microdomain morphology of thick (i.e., ca. 2 mm) films of triblock copolymers, such as polymethylmethacrylate - block - polybutadiene - block - polymethylmethacrylate copolymers prepared by a well-taylored two-step sequential copolymerization promoted by a 1,3-diisopropenylbenzene based difunctional anionie initiator. By means of this new scanning probe microscopy technique, it is shown that the surface exhibits a segregated microphase structure, corresponding to the different types of components predicted theoretically by thermodynamic processes. We investigate the relationships between the size and characteristics of the microdomain structure as a function of the molecular parameters of the constituent polymers. Our data illustrate the interest of Phase Detection Imaging in the elucidation of surface phase separation in block copolymers.
Phase Behavior of Triblock Copolymers upon Incorporation of Non-Parent, Midblock-Associating Additives
Authors: Laurer J. H. , Mulling J. F. , Bukovnik R. , Spontak R. J.
Published: April 2011

Addition of a block-selective homopolymer to a microphase-ordered block copolymer is known to result in preferential swelling of the chemically compatible microdomain. In this work, we examine the miscibility between a triblock copolymer and a relatively low-molecular-weight, chemically dissimilar, midblock-associating homopolymer and demonstrate that the homopolymer molecules residing in the swollen midblock matrix self-assemble to avoid repulsive interactions with neighboring microdomains. We extend this investigation to include systems composed of a very low-molecular-weight, midblock-associating additive (an oil). At high oil concentrations, the glassy copolymer endblocks micellize, resulting in the formation of a thermoplastic elastomer gel.
Interfacial Reaction and Electrochemical Activity of Thiokol Rubber/Conjugated Polymer Composites
Authors: Ke-Cheng Gong , Wen-Shi Ma
Published: April 2011

The highly electroactive thiokol rubber (TR)/ conjugated polymer (eg. polyaniline (PAn) or polypyrrole (PPy)) composite films were prepared by electropolymenzation deposition via one-step process in the electrolytic solutions containing aniline or pyrrole and TR oligomer. The electrocatalysis of PAn or PPy for the electrodepolymerization (reduction)- electropolymenzation (oxidation) reaction of TR in the interface between PAn or PPy and TR is determined by cyclic voltammograms. The differeme between the oxidation potential and the reduction potential is 0.05V and 0.36V or less for TR/PAn and TR/PPy composite films, respectively The chemical bands between the nitrogen atoms of PAn or PPy and the mercaptan groups of TR (oligomer) are formed in the electropolymenzation process that is indicated by XPS. The conductivities of TR/PAn and TR/PPy composite films and the stability of the cells consisting of those films are remarkably improved after electrochemical reduction with addition of a suitable conducting carbon black.
An Examination of Contemporary Iznik-Style Turkish Ceramics
Authors: Artal-Isbrand P. , Newman R.
Published: April 2011

Contemporary hand-made Iznik-style ceramics continue to be popular in Turkey. Made in some 50 workshops in Kiitahya and one workshop in Iznik, these wares generally show the same stratigraphy as the traditional Iznik wares produced during the Ottomon Empire from the 15th-17th centuries. While the materials utilized in the modern wares show some similarities to the traditional wares, the modern ceramicists have adopted many modern materials. For this paper, crossections of ceramics from four modern workshops were analyzed in an electron beam microprobe and the results compared with published information on traditional Iznik ceramics.
Mineralization in Archaeological Skin: An Interdisciplinary Approach to Conservation Processes and Contextual Relations
Authors: Cruz-Flores S. , Gama-Castro J.E.
Published: April 2011

Determining the composition and microdeposition of minerals, and their relation with the skin, are basic in order to understand the mechanisms, dynamics and environmental conditions that take place in the burial context. The present research has been developed with the objective of obtaining information that will contribute to increasing our knowledge of the modification processes in the preserved skin, verified inside an archaeological context.
Machine Vision as a Materials Conservation Technology
Authors: Samajdar S.
Published: April 2011

Conservation of archaeological monuments and artifacts is in need of precise and efficient techniques for materials condition evaluation in order to ensure accurate diagnosis followed by effective curative intervention. Machine Vision, already extensively used in the manufacturing industry, is introduced as a preservation technology to fulfill that need. Material degradation manifests itself as localized variations in the optical response (e.g., color & intensity) from the affected surface. Once captured through an appropriate recording device, the image of the surface is documented, enhanced, analyzed and displayed by use of Digital Image Processing (DIP) techniques. The resulting map of materials condition for the surface is demonstrably comprehensive, quantitative & accurate, and the process is fast and economical. Field work was done at the site of Mayan ruins in Tulum, Mexico, on the shore of the Caribbean sea. This paper focuses on a piece of structural rock having a large number of surface pores as the primary form of degradation. Employing DIP procedures, the material condition of the surface is quantitatively represented and characterized in terms of size as well as spatial distribution of the damage features. On a theoretical note, it is postulated that the sand from the beach, airborne by the sea breeze, had been the principal erosion agent. And a phenomenological model of the weathering mechanism has been developed centering on the fluid mechanical concept of terminal velocity. Close agreement between the field work data and the theoretical construct demonstrates the additional merit of the adapted technology as enabling a verifiable, and hence more meaningful, diagnosis of the weathering mechanism.
A Novel Stem-Based Mass Spectroscopic Technique: Applications to Catalytic Materials
Authors: Yang J. C. , Singhal A. , Bradley S. , Gibson J. M.
Published: April 2011

Very high angle (∼ 100mrad) annular dark-field (HAADF) images in a dedicated scanning transmission electron microscope (STEM) can be used to quantitatively measure the mass of a cluster on a support material. With knowledge of the annular dark field (ADF) detector efficiency, the absolute intensity of very HAADF images can be converted to elastic scattering cross-sections. By comparing the theoretical and experimental elastic scattering cross-sections, the number of atoms can be determined. Statistical measurement of absolute cross-sections from Re-6 clusters show good agreement with theoretical cross-sections. The experimental error corresponded to ±2 Re atoms. Our experiments demonstrate the exceptional stability of the Re-6 organometallic compound relative to Re-8 clusters. This technique is presently being applied to Pt clusters.
A Combined-Techniques Approach to Elucidating Crystalline Interface Atomic Structure
Authors: Dickey E. C. , Dra Vid V. P. , Pennycook S. J. , Nellist P. D. , Wallis D. J.
Published: April 2011

A case study is presented in which HREM, Z-Contrast Imaging and EELS are used as complementary techniques for elucidating interface structure. The NiO-ZrO2(cubic) interface is investigated along two orthogonal directions by these electron imaging and spectroscopy techniques to reveal the three-dimensional interface structure. Based on findings from this study, a protocol is suggested for using all three experimental techniques to gain a thorough understanding of interface structures.
Chemical Imaging of InGaAs/InAiAs Quantum Wells
Authors: Mountjoy G. , Crozier P. A. , Fejes P. L. , Tsui R. K. , Kramer G. D.
Published: April 2011

We have applied high resolution chemical imaging in a transmission electron microscope to study compositional variations across InGaAs/InAIAs double quantum well structures. The structures of interest are grown on an InP substrate and consist of two 40 Å layers of InGaAs separated by 20 Å of InAlAs. For this (InGa)x(InAl)1-xAs system, we have been able to obtain compositional information with an accuracy of about 20 % and a maximum spatial resolution of 1/2 × 1/2 unit cell. The results clearly show irregularities on a monatomic scale.
Cross Sectional AFM of Oxidized Porous Silicon
Authors: Ye Weijun , Poler Jordan , Drozd B. , Hasan M. A.
Published: April 2011

Morphological studies of bulk porous-silicon (PS) are presented. Using the atomic force microscope, cleaved cross-sections of electrochemically processed porous-silicon were investigated. The porous-silicon/silicon interface was examined. Using a room temperature UV O3 generator, the porous silicon-samples were oxidized. Oxidation in air was also observed. The morphology of the oxidized porous-silicon is process dependent. AFM contrast was enhanced by selective wet chemical etching. Unusually fast oxide etching in BOE was observed. A possible oxide/PS model is proposed.
Kinetic Model of Thermal Donor Evolution
Authors: Kelton K. F. , Falster R.
Published: April 2011

Kinetic aspects of thermal donor (TD) formation in Czochralski silicon are shown to be consistent with the evolution of small oxygen clusters, as described within the classical theory of nucleation. Predictions for TD generation and interstitial oxygen loss are presented. Favorable agreement with experimental data requires that the rate constants describing cluster evolution be increased over those expected for a oliffusion-limited flux based on a normal diffusion coefficient for oxygen in silicon. This may signal an anomalously high diffusion rate for temperatures less than 500°C, as has been suggested by others. However, it may instead signal an enhanced concentration of free oxygen near clusters smaller than the critical size for nucleation. This is expected when the interfacial attachment rates become comparable with the rates at which oxygen atoms arrive in the vicinity of the sub-critical clusters. The link between thermal donor generation and oxygen precipitation processes demonstrated here provides a consistent framework for better understanding and controlling oxygen precipitation in silicon. Further, the kinetic TD generation and oxygen loss data provide a new window into the dynamical processes for small clusters, which underlie all nucleation phenomena.
Low Energy Implantation and Transient Enhanced Diffusion: Physical Mechanisms and Technology Implications
Authors: Cowern N. E. B. , Collart E. J. H. , Politiek J. , Bancken P. H. L. , Van Berkum J. G. M. , Kyllesbech Larsen K. , Stolk P.A , Huizing H. G. A. , Pichler P. , Burenkov A. , Gravesteijn D. J.
Published: April 2011

Low energy implantation is currently the most promising option for shallow junction formation in the next generations of silicon CMOS technology. Of the dopants that have to be implanted, boron is the most problematic because of its low stopping power (large penetration depth) and its tendency to undergo transient enhanced diffusion and clustering during thermal activation. This paper reports recent advances in our understanding of low energy B implants in crystalline silicon. In general, satisfactory source-drain junction depths and sheet resistances are achievable down to 0.18 micron CMOS technology without the need for implantation of molecular species such as BF2. With the help of defect engineering it may be possible to reach smaller device dimensions. However, there are some major surprises in the physical mechanisms involved in implant profile formation, transient enhanced diffusion and electrical activation of these implants, which may influence further progress with this technology. Some initial attempts to understand and model these effects will be described.
Experimental Investigation of Transient Enhanced Diffusion (TED) of Phosphorus Implants in Silicon in the MeV Range
Authors: Adam Lahir S. , Law Mark E.
Published: April 2011

Transient Enhanced Diffusion (TED) is one of the biggest modeling challenges present in predicting scaled technologies. Prior work has concentrated on TED resulting from sub 200 KeV implants. However, the results applicable to the low energy regime cannot be extrapolated per se into the high energy regime. Furnace anneals have been performed and through simulations it has been observed that there is a differential diffusivity enhancement between the part of the profile before the profile peak and that beyond the profile peak. Longer saturation times compared to the low energy implants have also been observed.
Formation of Counter Doped Shallow Junctions by Boron and Antimony Implantation and Codiffusion in Silicon
Authors: Solmi S. , Canteri R.
Published: April 2011

Very shallow p+/n junctions (lower than 60 nm) have been fabricated by implanting Sb and subsequently BF2, at a higher dose, in a n-type Si substrate. The preamorphisation with Sb avoids the B channeling and increases the n-type doping in the junction region, thus confining the depth of the p layer. Furthermore, both the transient enhanced diffusion, being the B implanted in a preamorphized layer, and the standard diffusion, due to the pairing between donors and acceptors, are strongly reduced. This procedure allows us to obtain very shallow junctions even after annealings with relatively high thermal budget, like 800 C/8h, or 900 C/lh, or 950 C/15min or 1000 C/60s. Dopant diffusion is strongly affected by the direct donor-acceptor interaction. Good agreement between experimental and simulation results can only be obtained using a simulation code which takes into account the formation of neutral, near immobile, Sb-B pairs.