bluuebunny/crossref_metadata_2025 huggingface dataset:
New Ferroelectric Liquid Crystal Polymers for Nonlinear Optics Applications
Authors: Thurmes William N. , More Kundalika M. , Vohra Rohini T. , Wand Michael D. , Walba David M. , Keller Patrick
Published: April 2011

Initial results of a project directed towards the design and synthesis of ferroelectric liquid crystal polymers (FLCPs) for second order nonlinear optics (NLO) applications are described. FLCP glasses represent a novel type of solid, a truly noncrystalline solid with thermodynamically stable polar order. FLCPs with useful magnitude of the second order susceptibility χ(2) and processability not possible with poled polymers or crystals have been obtained.
Enhanced Photoluminescence from Erbium-Doped Gap Microdisk Resonator
Authors: Chu D. Y. , Wang X. Z. , Bi W. G. , Espindola R. P. , Wu S. L. , Wessels B. W. , Tu C. W. , Ho S. T.
Published: April 2011

The fabrication and optical properties of an erbium-doped gallium phosphide microdisk resonator pumped by a Ti-sapphire laser at 980 nm were investigated. Enhanced Er3+ intra-4f-shell photoluminescence was observed in the microdisk resonator compared to a thin film, and is attributed to a microcavity effect. At low pumping power intensity, the photoluminescence from erbium-doped gallium phosphide microdisks is an order of magnitude more intense than that from a thin film sample.
Aspects of Polymer Erosion
Authors: Göpferich Achim , Shieh Lisa , Langer Robert
Published: April 2011

The importance of non-enzymatic chemical erosion for the release of drugs from degradable polymers is discussed. In order to have purely erosion controlled systems, polymer erosion has to be faster than polymer swelling or drug diffusivity inside degradable polymers. Therefore, fast hydrolyzing polymers are especially suited for the manufacture of erosion controlled drug delivery systems. Poly(anhydrides) are one such polymer class and are presented in more detail. It is shown that it is possible to predict drug release from such systems using discrete Monte Carlo Models. Such models are useful for the design of new implant type drug delivery systems.
Poly(α-Hydroxy Ester)/Short Fiber Hydroxyapatite Composite Foams for Orthopedic Application
Authors: Thomson Robert C. , Yaszemski Michael J. , Powers John M. , Harrigan Timothy P. , Mikos Antonios G.
Published: April 2011

A process has been developed to manufacture biodegradable composite foams of poly(DL-lactic- co-glycolic acid) (PLGA) and hydroxyapatite short fibers for use in bone regeneration. The processing technique allows the manufacture of three-dimensional foam scaffolds and involves the formation of a composite material consisting of a porogen material (either gelatin microspheres or salt particles) and hydroxyapatite short fibers embedded in a PLGA matrix. After the porogen is leached out, an open-cell composite foam remains which has a pore size and morphology defined by the porogen. The foam porosity can be controlled by altering the volume fraction of porogen used to make the composite material. Foams made using NaCl particles as a porogen were manufactured with porosities as high as 0.84±0.01 (n=3). The short hydroxyapatite fibers served to reinforce the PLGA. The compressive yield strength of foams manufactured using gelatin microspheres as a porogen was found to increase with fiber content. Foams with compressive yield strengths up to 2.82±0.63 MPa (n=3) with porosities of 0.47±0.01 (n=3) were manufactured using 30% by weight hydroxyapatite fibers in the initial composite prior to leaching. These composite foams with improved mechanical properties may also be expected to have enhanced osteoconductivity and hence provide a novel material which may prove useful in the field of bone regeneration.
Growth and Doping of AlxGa1−xN Deposited Directly on α(6H)-SiC(0001) Substrates via Organometallic Vapor Phase Epitaxy
Authors: Bremser M.D. , Perry W.G. , Edwards N.V. , Zheleva T. , Parikh N. , Aspines D.E. , Davis R.F.
Published: April 2011

Monocrystalline AlxGa1−xN(0001) (0.05 ≤ x ≤ 0.70) thin films, void of oriented domain structures and associated low-angle grain boundaries, have been grown at high temperatures via OMVPE directly on vicinal and on-axis α(6H)-SiC(0001) wafers using TEG, TEA and ammonia in a cold-wall, vertical, pancake-style reactor. The surface morphologies were smooth and die densities and distributions of dislocations were comparable to that observed in GaN(0001) films grown on high temperature A1N buffer layers. Double-crystal XRC measurements showed a FWHM value as low as 186 arc sec for the (0002) reflection. Spectra obtained via CL showed strong near band-edge emissions with FWHM values as low as 31 meV. The compositions of the AlxGa1−xN films were determined using EDX, AES and RBS and compared to the values of the bandgap as measured by spectral ellipsometry and CL emissions. A negative bowing parameter was found. Controlled n-type, Si-doping of AlxGa1−xN for x ≤ 0.4 has been achieved with net carrier concentrations ranging from ≈ 2 × 1017 cm−3 to 2 × 1019 cm−3. Acceptor doping with Mg for x < 0.13 was also successful.
A Modeling Study of GaN Growth by MOVPE
Authors: Safvi S.A. , Redwing J.M. , Tischler M.A. , Kuech T.F.
Published: April 2011

A model for the growth of gallium nitride in a vertical metalorganic vapor phase epitaxy reactor is presented. For a mixture of non-dilute gases, the flow temperature and concentration profiles are predicted. The results show that the growth of GaN epilayers is through an intermediate adduct of TMG and ammonia. Growth rates are predicted based on simple reaction mechanisms and compared with those obtained experimentally. Loss of adduct species due to polymerization leads to lowering in growth rate. An attempt to quantify loss of reacting species is made based on experimentally observed growth rates.
Photoluminescence and Sims Studies of Hydrogen Passivation of Mg-Doped P-Type Gallium Nitride
Authors: Li Y. , Lu Y. , Shen H. , Wraback M. , Hwang C.-Y. , Schurman M. , Mayo W. , Salagaj T. , Stall R. A.
Published: April 2011

The effects of hydrogen passivation in MOCVD grown Mg doped p-type GaN were studied using low temperature (5K) photoluminescence (PL) and secondary-ion-mass spectroscopy (SIMS). GaN films with different Mg doping level were annealed at 700° C in N2 ambient with different annealing times. The SIMS results indicate that the hydrogen concentration increases with increasing Mg doping level in the as-grown Mg:GaN film. After 20 minutes of annealing, most of the hydrogen escapes from the film. The 3.455 eV PL peak before annealing and the 3.446 eV peak after annealing found in the Mg doped samples were attributed to the exciton bound to the Mg-H complex and to the Mg acceptor, respectively. The shift of the bound exciton peak to higher energy (3.465 eV) in the lightly doped sample is due to an effective n-type compensation associated with an annealing-induced increase in the nitrogen vacancies. In heavily doped Mg:GaN, the decreases in the integrated PL intensity after 700° C annealing may be associated with the hydrogen depassivation of nonradiative recombination centers in the film. The increase of PL intensity in the lightly doped sample after annealing is attributed to the reduction of defects by the annealing process.
The Microstructural Study of Aluminum Nitride Thin Films: Epitaxy on the Two Orientations of Sapphire and Texturing on Si
Authors: DOVIDENKO K. , OKTYABRSKY S. , NARAYAN J. , RAZEGHI M.
Published: April 2011

The microstructural study of wide-band gap semiconductor AlN thin films grown on (0001) and (1012) sapphire and (111), (100) Si was carried out using plan-view and cross-sectional high-resolution electron microscopy and x-ray diffraction. The films were grown by MOCVD from TMAl + NH3 + N2 gas mixture. Epitaxial relationship for AIN grown on (0001)α 01-Al2O3 was determined to be the following: (0001)AIN ║ (0001)sap with the 30° in-plane rotation - [0110]AIN ║ [1210]sap. We report also TEM observation of the following epitaxial relationship of the AlN/(1012)α-Al2O3 heterostructure: (1120)AIN ║ (1012)sap; [0001]AIN ║ [1011]sap and [1100] AIN ║ [1210]sap. These epitaxial relationships were determined to be controlled by the bonding of Al and O ions at the interface. The study of interfaces and the defects present in the film was also carried out. Main type of defects were established to be inverted domain boundaries, misfit and threading dislocations - in the films on (0001) sapphire, and stacking faults of high density in the films on (1012) sapphire. The epitaxial AIN films on (0001) sapphire contained dislocation density about 1010 cm−2 and exhibited device quality electrical characteristics. The films on both orientations of Si were found to be highly <0001> textured polycrystalline.
Identification of Transition Metals in GaN
Authors: Pressel K. , Heitz R. , Eckey L. , Loa I. , Thurian P. , Hoffmann A. , Meyer B.K. , Fischer S. , Wetzel C. , Haller E.E.
Published: April 2011

We present a photoluminescence study of residual transition metal contaminants in hexagonal GaN layers. We observe three no-phonon lines peaking at 1.3 eV, 1.19 eV and 1.047 eV. The no-phonon line at 1.3 eV is caused by the internal electronic transition 4T 1→ 6A1 of Fe3+. The 1.19 eV emission, which was first attributed to Cr4+, is caused by Ti2+. GaN layers intentionally doped with Cr, V, or Ti during growth have been investigated. Only the Ti doped samples snow an intense sienal of the 1.19 eV emission. The experimental data of the luminescence center at 1.047 ev, which appears as natural contaminant only in GaN layers grown by the sandwich technique, fit best to the 4T2(F) → 4A2(F) transition of Co2+. The three no-phonon lines show characteristic phonon sidebands. Most of them correspond to phonon modes observed in Raman spectroscopy.
Optical Properties of Mg-GaN, GaN/AlGaN SCH structures, and GaN on ZnO Substrates
Authors: Morkoç H. , Kim W. , Aktas Ö. , Salvador A. , Botchkarev A. , Reynolds D. C. , Look D. C. , Smith M. , Chen G. D. , Lin J. Y. , Jiang H. X. , Schmidt T.J. , Yang X.H. , Shan W. , Song J.J. , Goldenberg B. , Litton C. W. , Evans K.
Published: April 2011

GaN films and GaN/AlGaN heterostructures have been gro wn by MBE. GaN films doped with varying levels of Mg indicate effective mass acceptor at low doping concentrations, as determined from strong photoluminescence emission at about 380 nm. As the Mg concentration is increased the photoluminescence emission line red shifts considerably, indicating the formation of Mg-related or induced complexes whose lifetimes are relatively short. GaN/AlGaN separate confinement heterostructures grown on sapphire show strong near ultraviolet stimulated emission at room temperature in a side-pumping configuration. The pumping threshold for stimulated emission at room temperature was found to be ∼90 kW/cm2. Initial GaN films grown on ZnO substrates show the A exciton in low temperature photoluminescence. ZnO is being considered for nitride growth because of its stacking order and close lattice match.
Temperature Dependence of the Absorption Band Gap Edge of GaN
Authors: Manasreh M. O. , Sharma A. K.
Published: April 2011

The optical absorption near the fundamental absorption edge in GaN thin films grown on sapphire substrates is studied as a function of temperature. The absorption band edge was determined from either the energy position of the exciton line in samples grown by metalorganic chemical vapor deposition technique, or from the first derivative of the absorption spectra in samples grown by molecular beam epitaxy technique. The band edge energies determined in the temperature range of 13 – 300 K were fitted with Varshni empirical relationship: Eg(K) = Eg(0) – α T2/(T + θD) and with the expression: Eg(K) = Eg(0) – κ/[exp(θE/T) – 1]. It is found that Eg(0), α, θD, and θE to be sample-dependent, which suggests that defects and dislocations significantly affect the optical band edge in GaN.
Fine Structure of the 3.42 eV Emission Band in GaN
Authors: Fischer S. , Wetzel C. , Walukiewicz W. , Haller E.E.
Published: April 2011

A luminescence band centered around 3.42 eV is commonly observed in GaN. Its appearance has been tentatively assigned to an oxygen donor level. Stimulated laser activity has also been reported at this energy. We present a study of this band in GaN grown by high temperature vapor phase epitaxy (HTVPE). The high quality of this material, with an excitonic line width as narrow as 3 meV, allows us to distingish four different peak positions of this luminescence band. They appear at 3.4066, 3.4121, 3.4186, and 3.4238 eV (T = 6 K). Within the experimental error the lines exhibit an equidistant spacing of 6 meV. They show a pressure behavior similar to shallow levels described by effective mass theory. We discuss our results in the context of incorporation of oxygen and structural defects in GaN.
Light Generating Carrier Recombination and Impurities in Wurtzite GaN/Al2O3Grown by MOCVD
Authors: Kaufmann U. , Kunzer M. , Merz C. , Akasaki I. , Amano H.
Published: April 2011

We have studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR) un-doped, n-doped and p-doped thin wurtzite GaN layers grown by metal-organic chemical vapor deposition on sapphire substrates. From the PL data for free excitons an accurate value of the free A-type exciton binding energy and a more accurate estimate for the hole effective mass is deduced. The localization energies of the Mg and the Zn neutral acceptor bound excitons are found to be in good agreement with Haynes’ rule. A sharp emission line, assigned to free electron recombination at a 116 meV shallow acceptor, together with three additional weak zero-phonon-lines (ZPLs), assigned to distant donor-acceptor (DA) pairs, are reported for the first time. The chemical nature of this acceptor and that of three residual donors, inferred from the DA pair ZPLs, is discussed. The effects of strain in thin GaN layers on a dissimilar substrate like sapphire are emphasized with respect to the energetic position of narrow PL lines. The ODMR data obtained for undoped, Mg-doped and Zn-doped GaN layers provide insight into the recombination mechanisms responsible for the broad yellow (2.25 eV), the violet (3.15 eV) and the blue (2.8 eV) PL bands, respectively. The ODMR results for Mg and Zn also show that these acceptors donotbehave effective mass like and indicate that the acceptor hole is mainly localized in the nearest neighbor shell surrounding the acceptor core.
Exciton Lifetimes in GaN
Authors: Bergman J.P. , Harris C. , Monemar B. , Amano H. , Akasaki I.
Published: April 2011

We have performed time resolved photoluminescence measurements of the exciton recombination in different GaN samples at low temperatures. In epitaxial layers the decay time of the free exciton is typically faster than 100 ps. This is due to a dominating non-radiative recombination process. In thick bulk samples we have resolved and measured the decay time of the free exciton with a value of about 200 ps. We believe that this value is close to the radiative lifetime for free excitons in GaN. We have also shown that excitation transfer occurs between free and bound exciton states. We have furthermore measured the decay of the donor and acceptor bound excitons, and obtained values of the decay time of 250 ps and 1200 ps, respectively.
Laser Ablation Process and Growth Control of Oxide Thin Films in Laser Molecular Beam Epitaxy
Authors: Kawai T. , Nismkawa H. , Koguchi K. , Kanai M.
Published: April 2011

In order to study mechanism of laser ablation, the time-of-flight spectra of monovalent ions generated by ablation of alkaline earth metals are measured. The obtained spectra show that the ion generation is caused by 5-, 4- and 3-photon processes for Ca, Sr and Ba, respectively, when ArF excimer laser is used as a light source. This behavior suggests that laser ablation of alkaline earth metals is triggered by excitation of electrons at the highest inner-core level. Control of layer-by-layer growth processes by monitoring emission from the ablation plume is also reported. The emission measurement is utilized for estimation of each layer thickness in layer-by-layer growth. This conventional technique is very effective for automatic control of layer-by-layer growth.
Pulsed Laser Deposition and Crystallization of Transparent Conducting Thin Films
Authors: Phillips H. M. , Yunjun Li , Zhaoqi Bi , Binglin Zhang
Published: April 2011

Transparent conducting thin films of approximately 1000-2000 Å were deposited on glass, quartz and silicon substrates using standard pulsed laser deposition techniques with two different targets (Sn and SnO2) and with three different laser wavelengths (1.06, 0.532 and 0.266 urn) from a Q-switched Nd:YAG laser. Composite films of SnO2 and Sn with high optical transmission were obtained using a Sn target and a background oxygen pressure of 20 Pa with optical transmission over most of the visible spectrum exceeding 80%. Electrical resistivities of approximately 10-2 Ω-cm were obtained. Using SnO2 targets, predominantly amorphous phase SnO2 films were deposited on Si substrates and then transformed into polycrystalline Sn3O4 by laser induced crystallization (λ = 1.06 μm). The electrical resistivity of these films was also permanently reduced by a factor greater than 1000.
Pulsed Laser Deposition of SrBi2Ta2O9 Thin Films for Nonvolatile Memory Applications
Authors: Werner S. , Thomas D. , Streiffer S.K. , Auciello O. , Kingon Angus I.
Published: April 2011

Ferroelectric SrBi2Ta2U9 (SBT) thin films were synthesized by pulsed laser deposition (PLD) on platinized silicon substrates held at different substrate temperatures, from targets with different compositions. It was necessary to anneal films deposited at low temperature (525°C) at elevated temperatures in an oxygen atmosphere in order to achieve properties comparable to SBT thin films grown by the sol-gel technique. Polarization – electric field hysteresis loops showed saturation in the 2-5 V range with a remnant polarization 2Pr = 8-13 µC/cm2. Capacitors showed negligible fatigue up to 1010 switching cycles.
Investigation of Electronic Excitation Crystallization Enhancement in Amorphous Gesb Films Upon Ultrashort Laser Pulse Irradiation
Authors: Solis J. , Afonso C.N. , Hyde S.C.W. , Barry N.P. , French P.M.W.
Published: April 2011

Pulsed laser induced crystallization of amorphous GeSb films has been studied as a function of the laser pulse duration. The energy density crystallization threshold has been determined for pulses in the range from 400 fs to 8 ns. The threshold is observed to increase substantially for pulse durations in the ns range due to the existence of heat flow through the substrate while the pulse is still being absorbed. The energy density crystallization threshold remains constant within the experimental resolution as the pulse duration is decreased down to 800 fs. For the shortest pulse length used, (400 fs), a decrease in the threshold is observed, suggesting the possible existence of electronic excitation enhanced crystallization.
Laser Crystallization and Annealing of Ferroelectric Thin Films
Authors: Zhu J.S. , Lu X.M. , Liu X. , Tian W. , Yang Z. , Wang Y.N.
Published: April 2011

Ferroelectric PbZr0.44Ti0.56O3 film with pure ferroelectric phase was fabricated by Ar3+ and KrF laser crystallization technique from as-deposited amorphous films, with the substrate at room temperature. Laser annealing technique was also used to improve the quality of BaTiO3 (BT) films.
Excimer-Laser Crystallization of Patterned Si Films At High Temperatures Via Artificially Controlled Super-Lateral Growth
Authors: Song H. Jin , Im James S.
Published: April 2011

Based on the artificially controlled super-lateral growth approach, we have developed a novel excimer-laser-based high-substrate-temperature method for producing single-crystal Si islands on SiO2. By irradiating a photolithographically preconfigured sample, complete melting of an Si film is induced only at precisely predesignated locations within patterned and physically isolated islands. An intentionally incompletely melted section within each island initiates lateral growth of crystalline grains. A “bottleneck” portion of the island permits only one of the laterally growing grains to propagate into the main portion of the island. The low nucleation-to-growth-rate ratios that are attainable with high substrate temperatures (1000–1200 °C) can lead to nearly unlimited lateral growth distances; with a proper combination of the substrate temperature and the island dimension, the main area of an island—up to 50×50 μm2 in area—is readily converted into a large single-crystal region.
Laser-Controlled Etching of (Al,Ga)As Epitaxial Layers
Authors: Chong T.C. , Lu Y.F. , Lee A.
Published: April 2011

We report a systematic study of laser-controlled etching behaviors of AlxGa1−xAs epitaxial layers (x from 0 to 0.5) by varying parameters such as laser power, beam scanning speed, beam duty cycle, number of scans and different doping types and concentrations. The laser source was an argon ion laser emitting at 514.5 n-type layers were found to be significantly higher than those for p-type layers. Generally, higher etch rates were observed for layers with higher aluminium mole fraction. The etch rate was found to increase at an exponential rate with the laser power for CW beam, but this increase became linear when a chopped laser beam was used. The dependence of etch rates on different conditions can be largely accounted for by the differences in the built-in electric field at the surface as well as differences in the laser-induced local temperature rise in the layers.
Visible Luminescence from Laser-Induced Stain- and Dry Etched Silicon
Authors: Dimova-Malinovska D. , Tzolov M. , Malinowski N.
Published: April 2011

Light emitting silicon has been prepared by Ar laser (514.5 nm) induced stain etching and Nd:YAG impulse (532 nm) laser irradiation in air. Photoluminescence (PL), IR and XPS spectra have been studied. The intensity and position of the PL depend on the power or the energy and the duration of laser beam treatment during the etching. Correlation between the PL and chemical bonding is discussed.
Laser - Assisted Synthesis of Unique Graded Surface Composites
Authors: Singh R.K. , Fitz-Gerald J.
Published: April 2011

We have developed a novel laser assisted method to fabricate unique modified surface composites which have industrial applications ranging from improved adhesion of coatings to improved bio-compatibility. The surface consists of two materials with linear variation in composition, thus leading to linear variation in thermal expansion coefficients. The fabrication process is based on the use of a pulsed excimer laser with an energy fluence near the ablation threshold. Using this method we have been able to create adherent diamond coatings on cutting tool materials, high surface area bio-compatible hydroxyapatite ceramics and Mo/W surface composites. This process has also increased the adhesion of coatings in large thermal mismatched systems like diamond on steel substrates.
Laser Induced Nanofabrication on Titanium Thin Films
Authors: Huey B.D. , Bonnell D.A. , Akhsakhalian A.D. , Gorbunov A.A. , Sewing A. , Pompe W.
Published: April 2011

Illumination of titanium thin films with an argon-ion laser has been used to fabricate nanometer scale features by localized oxidation. The laser induces a temperature gradient in the metal film, within which oxidation may occur. Due to the non-linearity of the reaction with temperature, the reaction zone can be laterally confined to regions narrower than the diffraction limit of optical resolution. Scanning probe microscopy indicates widths ranging from 105 to 600 nm and heights of 0.8 to 30 nm. The possibility of forming novel structures is demonstrated.
Laser-induced Reactions in a Deep UV Resist System: Studied with Picosecond Infrared Spectroscopy
Authors: Lippert T. , Koskelo A. , Stoutland P. O.
Published: April 2011

One of the most technologically important uses of organic photochemistry is in the imaging industry where radiation-sensitive organic monomers and polymers are used in photoresists. A widely-used class of compounds for imaging applications are diazoketones; these compounds undergo a photoinduced Wolff rearrangement to form a ketene intermediate which subsequently hydrolyses to a base-soluble, carboxylic acid. Another use of organic molecules in polymer matrices is for dopant induced ablation of polymers. As part of a program to develop diagnostics for laser-driven reactions in polymer matrices, we have investigated the photo-induced decomposition of 5-diazo-2,2-dimethyl-l,3-dioxane-4,6-dione (5-diazo Meldrum's acid, DM) in a PMMA matrix using picosecond infrared spectroscopy. In particular, irradiation of DM with a 60 ps 266 nm laser pulse results in immediate bleaching of the diazo infrared band (v = 2172 cm-1). Similarly, a new band appears within our instrument response at 2161 cm-1 (FWHM = 29 cm-1) and is stable to greater than 6 ns.; we assign this band to the ketene photoproduct of the Wolff rearrangement. Using deconvolution techniques we estimate a limit for its rate of formation of τ < 20 ps. The linear dependence of the absorbance change with the pump power (266 nm) even above the threshold of ablation suggest that material ejection take place after 6ns.
Influence of the Shape and Size of ahe Laser Spot on the Spatial Distribution of the Plasma Plume
Authors: Mele A. , Giardini Guidont A. , Kelly R. , Miotello A. , Orlando S.
Published: April 2011

Pulsed laser deposition may be accomplished by irradiating a target with a high intensity laser pulse. The flux of particles moving from the target to the substrate obeys gas-dynamic laws when the density of the emitted particles exceeds ∼1 monolayer/(10 ns). The angular distribution of the plume, which depends on various factors, will strongly affect the deposit characteristics. It has been found that an intensified CCD camera can be successfully used to analyze laterally in two orthogonal directions the profile of the expanding plume. A marked influence of laser spot dimension and shape on plume dynamics is observed for laser ablation of a number of materials such as metals, semiconductors and high Tc superconductors.
Study of Glass Transition Based on the Fragmentation Model
Authors: Masaki Y. , Kitagawa A. , Suzuki M.
Published: April 2011

Glass transition phenomena for a fictive V2-VI3 compound have been described on the basis of fragmentation model in which glass materials begin to fragmentize at a certain temperature and the fragment size becomes small with increasing temperature. By a numerical calculation, the heating rate dependence of glass transition temperature was clearly presented, and fragment sizes at glass transition and melting temperature were estimated as 1.4μm and 20Å respectively. Further, thermodynamic aspects were discussed using the model and it was concluded that the glass transition is a sort of the first order transition.
On the Shape Evolution of Coherent Precipitates: Discrete Atom Method
Authors: Lee Jong K. , Choy Jun H.
Published: April 2011

Morphological evolution of coherent precipitates with arbitrary transformation strain is studied via a discrete atom method. With a purely dilatational misfit strain, a soft precipitate tends to have a plate-like equilibrium shape, whereas a hard precipitate takes up a shape of high symmetry such as a circle. If the stiffness is comparable between the matrix and precipitate, however, the equilibrium shape depends on the degree of anisotropy, misfit strain, size, and interfacial energy. Shape bifurcation phenomena from circular to rectangular shapes are also revealed during clustering. With either a tetragonal misfit strain of mixed signs or a pure shear misfit strain, a precipitate takes up a plate-like shape whose major axis lies along the direction containing an invariant line in accordance with the continuum elasticity prediction. Both elastic anisotropy and elastic inhomo-geneity exert little influence on the preferred orientation relationship. With a misfit strain of combined shear and dilatation, a precipitate follows an orientation and shape dictated by both components of the misfit strain.
Simulation by Cellular Automata of the Crystallization of a Matrix Containing a Mobile Second Phase
Authors: Shelton R.K. , Dunand D.C.
Published: April 2011

Two-dimensional cellular automaton computer simulations were carried out to model the geometric interaction between growing matrix grains and mobile, equiaxed particles, thus simulating crystallization (respectively, recrystallization, phase transformation or solidification) of a material containing mobile particles. As a result of particle pushing by growing grains, particle accumulation at grain boundaries and triple-points and concomitant particle depletion within grains occurs. A range of grain nucleation rate, particle area fractions and particle settling rate was investigated under continuous grain nucleation conditions. While these parameters do not change the crystallization kinetics or grain size or shape, they strongly affect the particle spatial distribution and clustering during and after crystallization.
Observation of Crystallization, Precipitation, and Phase Transformation Phenomena in Si Rich Titanium Silicide Thin Films
Authors: Domenicucci A. , Gifford G. , Clevenger L.A.
Published: April 2011

Crystallization, precipitation, and phase transformation phenomena were observed in titanium suicide thin film samples during in situ heating experiments in a transmission electron microscope. The as-deposited TixSiy films were 110 nm in thickness with a composition of 1 Ti to 2.33 Si. Crystallization of the C49 phase was followed isothermally near the sputter deposition temperature. The movement of individual grain boundaries was recorded so that a “velocity of crystallization” could be calculated. The precipitation of excess silicon from the C49 phase was first observed in the 650°C to 750°C temperature range. The precipitates were predominantly of the incoherent type, with a smaller number existing at the grain boundaries. Ostwald ripening then occurred up to the C49 to C54 phase transformation which was accompanied by a dramatic increase in grain size. Grain boundary movement during the phase transformation was such that large precipitates, which were originally at C49-C49 boundaries, ended up within resulting C54 crystals. Many of these larger precipitates were found to exist as epitaxial “islands” at the TiSi2/Si substrate interface.
Competitive Phase Selection in Fe-Ni Alloy Droplets
Authors: Gärtner F. , Norman A. F. , Assadi H. , Greer A. L.
Published: April 2011

In the solidification of Fe-Ni droplets (≤ 30 at.% Ni), the selection of different microstructures is dominated by the competition between the bcc and ccp phases. In drop-tube experiments ccp is the primary phase in some dilute (up to 7at% Ni) alloys although the bcc phase is favoured by a lower free energy and by a lower interfacial energy with the liquid. Competitive dendrite growth is a possible explanation for the formation of primary ccp. Comprehensive thermodynamic (CALPHAD) and kinetic modelling is undertaken to understand the growth competition. The origin of the observed primary phases is discussed.
Order-Disorder Transformations in the σ3 (111)/[110] Symmetrical Tilt Boundary in Tungsten
Authors: Wang Gui Jin , Vitck V.
Published: April 2011

The structure of the σ3 (111)/[110] symmetrical tilt boundary in tungsten was modeled by molecular statics using Finnis-Sinclair type many body potentials. Several multiple structures have been found which are composed of two types of structural units and the interaction energy between these units is negative. Hence, order-disorder structural transitions may occur in the boundary with structures being ordered and/or disordered mixtures of the two units. The uansilion temperature is found to be 1158K if only the internal energy and configurational entropy arc included when evaluating the free energy. However, the transition temperature is 782K if the vibrational entropy is also incorporated. This demonstrates that the vibrational contribution to the entropy may be as important as the configurational contribution when considering the inierfacial transformations.
Energetics of Manganese Oxides
Authors: Fritsch S. , Navrotsky A.
Published: April 2011

The energetics of some manganese oxides : pyrolusite ΜnO2, bixbyite Μn2O3, hausmannite Μn3O4 and manganosite MnO were studied by transposed temperature drop calorimetry and high temperature drop solution calorimetry in molten 2PbOB2O3 at 977 K. The enthalpies of oxidation at 298 K in the Mn-O system, determined by appropriate thermodynamic cycles, are (in kJ/mol O2) : −441.4 ± 5.8 for the reaction 6 MnO + O2 -> 2 Mn3O4, −201.8 ± 8.7 for 4 Mn3O4 + O2 -> 6 Mn2O3 and −162.1 ± 7.2 for 2 Mn2O3 + O2 -> 4 ΜnO2 These values agree very well with previous data obtained by equilibrium ρθ2 measurements and indicate that direct calorimetrie measurements are well suited to obtain reliable enthalpy of formation data for oxides containing manganese in the 2+, 3+ and 4+ states. The phase diagram of the Mn-O system was calculated using these new values of enthalpies.
In Situ Cross-Sectional Scanning Tunneling Microscopy Sample Preparation Technique
Authors: Kim Y.-C. , Nowakowski M. J. , Seidman D. N.
Published: April 2011

A novel in situ sample cleavage technique has been developed for fabricating specimens for cross-sectional scanning tunneling microscopy (XSTM) applications. This technique can be easily adapted to any ultrahigh vacuum (UHV) STM that has a coarse motion capability. A conducting diamond STM tip is used to create micron long scratches on Ge/GaAs or GaAs {001 }-type surfaces. These {001} surfaces are imaged with STM to observe scratch characteristics, and GaAs samples are cleaved to reveal {110}-type faces. Atomic resolution images of {110}-type GaAs surfaces are readily and reproducibly obtained.
Microstructural Evolution of Co2GaAs Thin Film on GaAs Substrate
Authors: Shin D.W. , Park C.G. , Kwak J.S. , Baik H.K.
Published: April 2011

Interfacial reactions and epitaxial growth of the microstructure between cobalt thin film and (100)-oriented GaAs substrate have been studied, using X-ray diffraction, transmission electron microscopy and EDS analysis. Cobalt thin films were deposited by e-beam evaporation, and were subsequently annealed at 400 °C.

The reaction layer evolved due to the annealing was identified as the ternary phase, Co2GaAs, that was grown epitaxially on the substrate. The new orientation relationship between Co2GaAs and GaAs substrate was found as <110>Co2GaAs // <100>GaAs and {001}Co2GaAs // {011}GaAs. These results can be well explained upon matching the unit cells of Co2GaAs and GaAs. Upon the annealing at 600°C, the Co2GaAs phase is usually decomposed to stable binary phases, CoGa and CoAs.
The Effect of Substrate Misorientation on the Evolution of Surface Morphology in Epitaxially Grown CaF2/Si(111) Heterostructures
Authors: Kim B. M. , Soss S. R. , Overney R. M. , Schowalter L. J.
Published: April 2011

We have studied the effect of substrate misorientation on the evolution of surface morphology in lattice-mismatched heterostructures for molecular beam epitaxial growth of a thin insulator (CaF2) on vicinal Si(111) surfaces with in-situ reflection high energy electron diffraction (RHEED) and ex-situ atomic force microscopy (AFM). At a substrate temperature of 770 °C, CaF2 growth is initiated by the formation of a reacted CaF layer followed by the complete overgrowth of an additional CaF2 monolayer (ml). However, CaF2 growth beyond these two ml depends on the degree of miscut of the Si substrate. On Si substrates tilted toward the [112] by a miscut angle ≥ 0.5°, the atomic step edges on the Si surface bunch together forming flat terraces that are ∼200 nm wide. In this case, the CaF2 growth beyond 2 ml proceeds by the nucleation and lateral propagation of thick CaF2 islands (whose height is determined by the step bunches) along the bunched step edges to eventually form a complete overlayer. For CaF2 films grown on substrates with a miscut angle < 0.5°, the CaF2 layer remains relatively uniform without the formation of thick islands. With further deposition, evidence for a dense network of misfit dislocations is observed on the surface of the CaF2 film.
Coarsening and Slope Selection During Crystal Growth and Etching of Ge(001)
Authors: Jay Chey S. , Van Nostrand Joseph E. , Cahill David G.
Published: April 2011

The evolution of surface morphology during low temperature crystal growth by molecular beam epitaxy and etching by low-energy ions is characterized by in-situ scanning tunneling microscopy. Epitaxial growth of Ge(001) at low temperatures in the vicinity of 155°C produces a pattern of growth mounds while etching at temperatures near 270°C produces a pattern of low aspect ratio pits. The characteristic in-plane length scale of the surface roughness d increase with a power law dependence on time but the exponent depends on temperature and process. Prior to the onset of amorphous growth, the amplitude of the surface roughness G1/2(d/2) increases more rapidly than d; i.e. the slope of the sides of the growth mounds increases with time. By contrast, the ratio of Gl/2(d/2) to d remains nearly constant during ion etching for a wide range of etching times.
On the Roughening of Ceramic Surfaces
Authors: Heffelfinger J. R. , Bench M. W. , Carter C. B.
Published: April 2011

The faceting of single-crystal ceramic surfaces has been investigated by atomic force microscopy, scanning electron microscopy and transmission electron microscopy. Single-crystals of α-Al2O3 (alumina) with different nominal surface orientations were annealed and used as a model system to investigate the faceting of a polished ceramic surfaces. The {1010} orientation of α-alumina, which facets into a hill-and-valley morphology, provides a dramatic representation of the different stages by which a surface facets. This surface starts faceting with the growth of an individual facet. The growth of this facet creates local surface disturbances that promote the nucleation of adjacent facets; thus, domains of faceted surface form on the otherwise smooth surface. Through time, these domains coalesce and the facets coarsen into a hill-and-valley morphology. Surfaces which facet into a terrace-and-step structure, such as (0001) and {1120} surfaces of alumina, are characterized for their progression of surface faceting and are compared with the stages of faceting observed for the {1010} surface.
Synchrotron White Beam Topography Studies of Residual Stress in SiC Single Crystal Wafers with Epitaxial Thin Films
Authors: Huang W. , Wang Q. , Dudley M. , Chiang F. P. , Parsons J. , Fazi C.
Published: April 2011

The residual stress in a 6H-SiC wafer with a 3C-SiC epitaxial overlayer is determined by the technique of Synchrotron white beam x-ray topography (SWBXT). The short wavelength and high energy attributes of synchrotron radiation are exploited to very accurately determine the wafer curvature. Different approaches including absorption edge contour (AEC) mapping, multiple diffraction line (MDL) analysis and diffracted x-ray beam divergence (DXBD) analysis in both transmission and reflection geometry are demonstrated. The residual stress distribution is calculated from the wafer curvature measurement.
Reciprocal Space Analysis of the Initial Stages of Strain Relaxation in SiGe Epilayers
Authors: Lee S. R. , Floro J. A.
Published: April 2011

Metastable SiGe films were grown by MBE on Si (001) substrates and annealed to promote varying degrees of partial relaxation. X-ray diffraction reciprocal-space analysis was then used to monitor the structural evolution of the displacement fields of the dislocation array with increasing misfit density. The diffuse-x-ray-scattering patterns of the dislocated heterolayers were compared with lineal-misfit densities determined by defect etching, leading us to develop a geometric model which provides a framework for understanding the early-stage evolution of the displacement fields of the dislocation array, and which also explicitly links diffuse x-ray intensity to misfit density. At low misfit density, the diffuse intensity arises from two-dimensional displacement fields associated with single-nonoverlapping dislocations. As misfit density increases, the displacement fields of individual dislocations increasingly overlap producing three-dimensional displacements. The evolving diffuse intensity reflects the transition from 2-D to 3-D displacement fields. Finally, it is demonstrated that the diffuse x-ray intensity of the strained epilayer can be used to accurately measure lineal misfit-dislocation densities from 400 to 20,000 lines/cm.
Defect Structure of Cu-Rich and In-Rich Chalcopyrite CuInSe2 Films Grown on GaAS
Authors: Hellman Olof , Tanaka Shun-Ichiro , Niki Shigeru , Fons Paul
Published: April 2011

Using Transmission Electron Microscopy, we examine the defect structure of Cu-rich and In-rich CuInSe2 films grown by Molecular Beam Epitaxy on GaAs (100) substrates. A surprisingly high density of cation sublattice stacking faults on (001) planes are observed in the Cu-rich films. Because these stacking faults are extremely flat and extend thousands of Ångstroms over the surface, and because they are not observed in other, non-Cu-rich films, we argue that they are a consequence of a surface structural change during growth, induced by the excess Cu. Two other types of defects are also observed: near the CuInSe2/GaAs interface, there is a high concentration of dislocations, stacking faults and domain boundaries. In the In-rich films, stacking faults and twin-type defects on {112} planes extend throughout the thickness of the grown film.
Technical Applications of Nanocrystalune Materials
Authors: Schulz R.
Published: April 2011

After discussing various applications of nanocrystalline solids which are presently under development, this paper describes in more detail the potential use of nanocrystalline materials for the production of sodium chlorate, a paper bleaching agent. The technology and advantages of using nanocrystalline solids as cathode material for this electrochemical process are discussed. The paper concludes by presenting the world market and potential savings associated with the use of this new technology.
Ti-Zr-Ni Quasicrystals: Structure and Hydrogen Storage
Authors: Stroud R. M. , Viano A. M. , Majzoub E. H. , Gibbons P. C. , Kelton K. F.
Published: April 2011

Titanium-based icosahedral phases constitute the second largest class of quasicrystals. In contrast with other Ti-based icosahedral phases (i-phases), Ti-Zr-Ni i-phases are well ordered and their formation is inhibited by the presence of Si and O, elements that stabilize the Ti-3d transition metal quasicrystals. We present x-ray and DSC data that suggest that Ti-Zr-Ni i-phases form a different class of titanium-based quasicrystals that are closely related to the MgZn2 Laves phase. The DSC data also suggest that the i-phase may be stable in these alloys. The ability of Ti-Zr-Ni i-phases to absorb up to 62 atomic % of hydrogen is presented and discussed. This opens new avenues of investigation of the structure and dynamics of quasiperiodic phases using elastic and inelastic neutron scattering and nuclear magnetic resonance and may point to potential uses for quasicrystals in hydrogen storage applications.
Tem Investigation and Hardness Improvement of A N+ Implanted Al-Alloy
Authors: Serventi A.M. , Vittori Antisari M. , Guzman L. , Miotello A.
Published: April 2011

The surface modification induced by nitrogen ion implantation on the Al-alloy 7075 has been studied with the aim of understanding the microstructural evolution and the phase separation during the implantation process. 150 keV N2+ ions have been implanted at different temperatures from 373 K to 473 K, with a current density of 5–15 µA/cm2 on previously polished samples. The implanted dose was in the range 1 × 1017 N/cm2 - 5 × 1017 N/cm2. Vickers micro-hardness tests and friction coefficient measurements show a real improvement in the mechanical behaviour of the alloy after the treatment.

TEM observations of specimens treated at low temperature with different ion dose have been carried out at 200 kV on cross-sectional samples, prepared by ion beam milling. First results show the presence of small AlN hexagonal precipitates whose evolution is followed as a function of the implanted dose.
Evolution of the Nanostructure of Giant Magnetoresistance Heterogeneous Alloys CoxAg1-x Upon Annealing
Authors: Regnard J. R. , Revenant-Brizard C. , Dieny B. , Mevel B. , Mimault J.
Published: April 2011

Heterogeneous alloys CoxAg1-x with atomic concentrations x=0.20 and 0.35, presenting giant magnetoresistance properties, have been studied by Total Electron Yield X-ray absorption spectroscopy at liquid nitrogen temperature at the Co K edge after annealings at various temperatures. For both sample concentrations, the Co atoms are mainly in small filament agglomerates for the as-deposited and low temperature annealed samples (T<150°C). In this temperature range, the local Co atomic environment is very stable and disordered. A structural change has been detected by XANES and EXAFS analysis between the 250 and 400°C annealings. After a 400°C annealing during 10 min, the nanostructures evolve towards larger, more compact and well ordered granules. The Co-Co distance varies from 0.246 to 0.248 nm as the annealing temperature increases indicating that the small filament-shaped agglomerates are under strain in the Ag matrix. The Debye-Waller factor of the Co0.35Ag0.65 samples decreases continously as the annealing temperature increases, as this factor drops suddenly after a 250°C annealing for the Co0.20Ag0.80 sample. The Co agglomeration is steadily larger for the Co0.35Ag0.65 sample than for the Co0.20Ag0.80 one. The Co-Ag contribution is negligible for the Co0.35Ag0.65 sample, as it represents approximately one third of the nearest neighbors for the Co0.20Ag0.80 sample from the as-deposit up to a 250°C annealing stage.
Grain Growth Impediment of Fe-Based Nanocomposites During Heat Treatment at Elevated Temperature
Authors: Huang B. , Perez R.J. , Lavernia E.J.
Published: April 2011

Cryogenic ball milling of Fe-10 wt.%Al in liquid argon is shown to produce a nanocrystalline microstructure with high thermal stability against grain growth. The experimental evidence suggests that this stability may originate from the presence of fine γ-Al2O3 particles which pin the Fe grain boundaries. In contrast, Fe3O4 particles formed during cryomilling of Fe in liquid nitrogen were not able to impede grain growth during consolidation at 823 K. The interactions between the γ-Al2O3 particles and the grain boundaries during growth may be described by Gladman's theory. Additional stabilization against grain growth may have been provided by the presence of Al at the Fe grain boundaries. Following annealing at 1223 K, the grain size of the Fe-10 wt.%Al powders cryomilled in liquid argon was 13.3 ± 7.9 nm, but it exceeded 1700 nm following annealing at 1373 K. The loss of nanocrystalline structure may be attributed to the Ostwald ripening of γ-Al2O3 particles, resulting in abnormal grain growth.
In-situ Studies of a Novel Sodium Flame Process for Synthesis of Fine Particles
Authors: Steffens Kristen L. , Zachariah Michael R. , Dufaux Douglas P. , Axelbaum Richard L.
Published: April 2011

This study focuses on the optical characterization of a novel method of forming nanoscale titanium and boron particles, which can be used to form ceramic precursors such as TiB2. TiCl4 or BCl3 reacts with heated Na vapor in a counterflow diffusion flame reactor. After Na strips the Ti or B of its Cl atoms, nanosize Ti or B particles form and become encased in NaCl, which helps to prevent agglomeration and oxidation. The two-dimensional spatial distribution of the Na dimer has been optically interrogated using planar laser-induced fluorescence (PLIF) to clarify the influence of the concentration distributions and transport on particle formation rates.