bluuebunny/crossref_metadata_2025 huggingface dataset:
Strong Piezoelectric Effects in Unstrained GaN Quantum Wells
Authors: Langer R. , Simon J. , Konovalov O. , Pelekanos N.T. , André R. , Barski A. , Godlewski M.
Published: April 2011

Optical properties of GaN/AIGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary A1GaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.
Pendeo-Epitaxy of Gallium Nitride and Aluminum Nitride Films and Heterostructures on Silicon Carbide Substrate
Authors: Gehrke T. , Linthicum K. J. , Thomson D. B. , Rajagopal P. , Batchelor A. D. , Davis R. F.
Published: April 2011

Pendeo-epitaxy of individual GaN and AlxGa,1-xN films and single- and multi-layer heterostructures of these materials have been achieved on a columnar GaN seed layer using metallorganic vapor phase epitaxy. These structures have been characterized using scanning electron microscopy and atomic force microscopy. The RMS roughness value of the grown side wall plane (1120) of these structures was 0.099 nm.
Comparative Growth of AIN on Singular and Off-Axis 6H and 4H-SiC by MOCVD
Authors: Wilson S. , Dickens C. S. , Griffin J. , Spencer M. G.
Published: April 2011

A comparison study of the growth of aluminum nitride (AIN) single crystal epitaxy on 6H-SiC and 4H-SiC substrates has been performed. The material has been characterized using atomic force microscopy (AFM) and reflective high energy electron diffraction (RHEED). AIN crystals were deposited on the following 6H-SiC substrates: singular with and without an initial SiC epilayer, and 3.5° off-axis with and without an initial SIC epilayer. AIN crystals were deposited on 8.0° off-axis 4H-SiC with and without initial SIC epilayers. AFM shows that the deposition of AIN on 6H-SiC and 4H-SIC with an initial SiC epilayer displays high quality quasi-two dimensional growth as atomically flat or step flow epitaxy.
Optical Properties of Si-DOPED AlxGal-xN/AlyGal-yN (x=0.24-0.53, y=0.11) Multi-Quantum-Well Structures
Authors: Hirayama H. , Aoyagi Y.
Published: April 2011

We demonstrate strong ultraviolet (UV) (280-330nm) photoluminescence (PL) emission from multi-quantum-well (MQW) structures consisting of AlGaN active layers fabricated by metal-organic chemical-vapor-deposition (MOCVD). Si-doping is shown to be very effective in order to enhance the PL emission of AlGaN QWs. We found that the optimum values of well thickness and Si-doping concentration of AlxGal-xN/AlyGal-yN (x=0.24-0.53, y=0.11) MQW structure for efficient emission were approximately 3nm and 2×1019cm-3, respectively. In addition, the PL intensities of AlGaN, GaN and InGaN quantum well structures are compared. We have found that the PL emission at 77K from a Al0.53Ga0.47N/Al0.11Ga0.89N MQW is as strong as that of InGaN QWs.
Fast Lateral Epitaxial Overgrowth of Gallium Nitride by Metalorganic Chemical Vapor Deposition Using A Two-Step Process
Authors: Marchand H. , Ibbetson J.P. , Fini P.T. , Wu X.H. , Keller S. , DenBaars S.P. , Speck J.S. , Mishra U.K.
Published: April 2011

We demonstrate a two-step process wherein the lateral epitaxial growth (LEO) of GaN from <1010>-oriented stripes is initiated at a low V/II1 ratio to produce smooth, vertical {1120} sidewalls, and where the V/III ratio is subsequently raised in order to increase the lateral growth rate. We find that the formation of the {1101} facets is inhibited using this two-step process, and that it is possible to maintain the {1120} sidewalls while achieving a large lateral growth rate. The ratio of lateral to vertical growth rate has been increased by up to factor of 2.6 using this approach relative to identical growth conditions without the initiation at low V/III ratio. The effect of lateral growth rate on the structural properties of the stripes is discussed.
The Behavior of Ion-Implanted Hydrogen in Gallium Nitride
Authors: Myers S. M. , Headley T.J. , Hills C.R. , Han J. , Petersen G.A. , Seager C.H. , Wampler W.R.
Published: April 2011

Hydrogen was ion-implanted into wurtzite-phase GaN, and its transport, bound states, and microstructural effects during annealing up to 980°C were investigated by nuclear-reaction profiling, ion-channeling analysis, transmission electron microscopy, and infrared (IR) vibrational spectroscopy. At implanted concentrations vl at.%, faceted H 2 bubbles formed, enabling identification of energetically preferred surfaces, examination of passivating N-H states on these surfaces, and determination of the diffusivity-solubility product of the H. Additionally, the formation and evolution of point and extended defects arising from implantation and bubble formation were characterized. At implanted H concentrations ^0.1 at.%, bubble formation was not observed, and ion-channeling analysis indicated a defect-related H site located within the [0001] channel.
Electrical Characterization of Sputter Deposition Induced Defects in n-GaN
Authors: Auret F. D. , Goodman S. A. , Koschnick F. K. , Spaeth J.-M. , Beaumont B. , Gibart P.
Published: April 2011

We have used current-voltage (I-V) measurements to assess and compare the electrical characteristics of resistively evaporated and sputter deposited Au Schottky contacts on epitaxially grown GaN. These I-V measurements revealed that resistively deposited Au contacts exhibited excellent rectification properties: high barrier height, low reverse current and good ideality factor (n = 1.04). In contrast, sputter deposited contacts had poor characteristics: low barrier height, high reverse current and non-linear forward I-V characteristics. The cause of this is thought to be defects introduced at and near the surface during sputter deposition. Deep level transient spectroscopy (DLTS) showed that at least four defects, with energy levels at 0.22±0.02 eV, 0.30±0.01 eV, 0.40±0.01 eV and 0.45±0.10 eV below the conduction band, were introduced in the GaN during sputter deposition. The first of these defects has similar electronic properties as a radiation induced defect in GaN, speculated to be the nitrogen vacancy, while the second appears to be the same as a defect in the as-grown material. The latter two defects have not previously been observed in as-grown or processed epitaxial GaN.
Electroluminescence Properties of InGaN/AlGaN/GaN Light Emitting Diodes with Quantum Wells
Authors: Yunovich A.E. , Kudryashov V.E. , Turkin A..N. , Kovalev A.N. , Manyakhin F.I.
Published: April 2011

Electroluminescence spectra of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures with single and multiple quantum wells (QWs) are analyzed by models of radiative recombination in 2D-structures with band tails. Equations of the model fit spectra quite good in a wide range of currents. Parameters of the fit are discussed and compared for single and multiple QWs. Tunnel effects play a sufficient role in blue LEDs with single QWs at low currents; they can be neglected in LEDs with multiple QWs. A new spectral band was detected at the high energy side of the spectra of green LEDs with multiple QWs; it is attributed with large scale inhomogenities of In distribution in InGaN QWs.)
Characterization of Hot-Electron Effects on Flicker Noise in III-V Nitride Based Heterojunctions
Authors: Ho W.Y. , Fong W.K. , Surya Charles , Tong K.Y. , Lu L.W. , Ge W.K.
Published: April 2011

We report experiments on hot-electron stressing in commercial III-V nitride based heterojunction light-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the I-V characteristics, electroluminescence, Deep-Level Transient Fourier Spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics. The room temperature electroluminescence of the devices exhibited 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by measuring the Deep-Level Transient Fourier Spectroscopy, which indicated an increase in the density of deep-traps from 2.7 × 1013 cm−3 to 4.21 × 1013 cm−3 at El = Ec – 1.1eV. The result is consistent with our study of I/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. Our experiments show large increase in both the interface traps and deep-levels resulted from hot-carrier stressing.
Influence of Si-Doping on Carrier Localization of Mocvd-Grown InGaN/GaN Multiple Quantum Wells
Authors: Cho Yong-Hoon , Schmidt T. J. , Bidnyk S. , Song J.J. , Keller S. , Mishra U.K. , DenBaars S.P.
Published: April 2011

We have systematically studied the influence of Si doping on the optical characteristics of InGaN/GaN multiple quantum wells (MQWs) using photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy combined with studies of optically pumped stimulated emission and structural properties from these materials. The MQWs were grown on 1.8-μm-thick GaN layers on c-plane sapphire films by metalorganic chemical vapor deposition. The structures consisted of 12 MQWs with 3-nm-thick InGaN wells, 4.5-nm-thick GaN barriers, and a 0.1 -μm-thick Al0.07Ga0.93N capping layer. The Si doping level in the GaN barriers was varied from 1 × 1017 to 3 × 1019 cm3. PL and PLE measurements show a decrease in the Stokes shift with increasing Si doping concentration. The 10 K radiative recombination lifetime was observed to decrease with increasing Si doping concentration (n), from ∼ 30 ns (for n < 1 × 1017 cm-3) to ∼4 ns (for n = 3 x 1019 cm-3). To elucidate whether non-radiative recombination processes affect the measured lifetime, the temperature-dependence of the measured lifetime was investigated. The reduced Stokes shift, the decrease in radiative recombination lifetime, and the increase in structural and interface quality with increasing Si doping indicate that the incorporation of Si in the GaN barriers results in a decrease in carrier localization at potential fluctuations in the InGaN active regions and the interfaces.
Theory of the Gain Characteristics of InGaN/AlGaN QD Lasers
Authors: Andreev A.D. , O'Reilly E.P.
Published: April 2011

We present a theoretical analysis of the gain characteristics of InGaN/AlGaN quantum dot (QD) lasers. We calculate the elastic strain distribution caused by the lattice mismatch between the QD and the barrier using an original method which takes into account the hexagonal symmetry of the structure's elastic properties. The method is based on an analytical derivation of the Fourier transform of the strain tensor. The proposed approach is combined with a plane-wave expansion method to calculate the carrier spectrum and wave functions. The many-body gain of a laser containing a periodic array of QDs is calculated using the Padé approximation. We show that band gap reduction and the Coulomb enhancement of the interband transition probability can significantly modify the gain spectrum in InGaN/AlGaN QD lasers.
Patterning III-N Semiconductors by Low Energy Electron Enhanced Etching (LE4)
Authors: Gillis H.P. , Christopher M.B. , Martin K.P. , Choutov D.A.
Published: April 2011

Fabricating device structures from the III-N wide ba-ndgap semiconductors requires anisotropoic dry etching processes that leave smooth surfaces with stoichiometric composition after transferring high-resolution patterns with vertical sidewalls. The purpose of this article is to describe results obtained by a new low-damage dry etching technique that provides an alternative to the standard ion-enhanced dry etching methods in meeting these demands for processing the HI-N materials.
Effect of Transition Temperature, Structure and Anisotropy on the Deformation and Failure of Polymers
Authors: Samuels Robert J.
Published: April 2011

The yield strength, the failure strength and strain behavior of Isotactic Polypropylene are all a function of the molecular orientation in the polymer. Further, although Isotactic Polypropylene and PMDA-ODA polyimide are very different polymers, they behave similarly when the strain rate is slow enough for the polymer to be at the low strain rate limit of its failure envelope. A master True Stress-Total True Strain curve is obtained for both polymers.
The Mechanism of Ductile Fracture in Constrained Thin Films
Authors: Kassner M.E. , Kennedy T.C , Schrems K.K.
Published: April 2011

The mechanism of low macroscopic-plastic-strain, ductile fractures under various high triaxial stresses in constrained thin silver films was investigated. Particular emphasis was placed on investigating ductile fracture by unstable cavity growth. The various multi-axial loads to failure were experimentally measured. FEA analysis was used to determine the corresponding high triaxial stress-states within the interlayer, also considering cavity-cavity interaction, residual stresses and elastic incompatibility stresses across interfaces. These were compared to the stresses required for cavity instability. Ductile fracture under high triaxial stresses, associated with low macroscopic strains, appears to be explained by unstable cavity growth, where cavity growth may occur without large macroscopic (e.g., < 0.05) plastic strains.
Atomic-Scale Structure and Properties of Thin Epitaxial SrRuO3 Films Grown on SrTiO3 (100) by Pulsed Laser Deposition
Authors: Satyalakshmi K.M. , Zakharov N. D. , Hesse D. , Koren G.
Published: April 2011

Transition metal oxides with perovskite structure exhibit a wide range of electrical properties. SrRuO3 (SRO), a perovskite oxide with high conductivity, finds applications as an electrode layer in ferroelectric devices and as a barrier layer in superconductor-normal metal-superconductor (SNS) Josephson junctions involving high Tc superconductors. Here we report on the structure and properties of thin SRO films on SrTiO3 (100) substrates grown by pulsed laser deposition. AFM investigations revealed that the SRO films exhibit island growth. The island size increases with thickness, and at a typical thickness of about 20 nm a connected network of islands is formed. The resistivity of the SRO films grown at different substrate temperatures (700 °C - 850 °C) exhibits metallic behavior with a ferromagnetic transition at 150 K. The films with typical thicknesses of 20 nm, grown at a substrate temperature of 775 °C exhibited the lowest resistivity of 200 μΩ-cm at 300 K, whereas the films grown at lower (700 °C) and higher (850 °C) temperatures showed a much higher resistivity at 300 K. By AFM and high resolution TEM investigations, this behavior is correlated with the atomicscale structure of the films. The film with the lowest room temperature resistivity showed an almost defect-free orthorhombic structure, whereas the film grown at 850 °C contained structural defects in high density, such as SrO-rich antiphase boundaries and twins located at the boundaries between the islands. The films grown at 700 °C showed a cubic perovskite structure, which is obviously the reason for their high resistivity.
Influence of Ti additions on the Texture and Dielectric Properties of LSBN-BST Complex Ceramics
Authors: Portelles J. , Amorín H. , Siqueiros J. M. , Castilión F.F. , Martínez E. , Aguilera S.
Published: April 2011

The stoichiometric La-Ti doped strontium-barium niobate ceramic system was obtained for different concentrations of Titanium ions (Ti=1,3,5,10,30,50 %), where the Nb5− was substituted by Ti4+ according to the following expression Sr0.255 La0.03Bao0.7Nb2−yTiyO6−y/2 (LSBN-BST). The oxygen vacancies thus created which, together with the vacancies in the A sites of the tetragonal tungsten-bronze (TTB) material structure, have an important influence on their texture and dielectric properties, due to the crucial role of the occupancy of the crystallographic sites. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and thermoelectric analyses are reported. We report the first results for thin films of LSBNT- 7 % obtained by the PLD technique. The diffraction pattern reveals the presence of the LSBNT structure without annealing.
Ferroelectric Properties of SBT (Sr/Bi/Ta = 0.8/2.3/2) Thin Films Using a Novel Chemical Solution Deposition
Authors: Kim Seung-Hyun , Kim D.J. , Streiffer S.K. , Maria J-P. , Kingon Angus I.
Published: April 2011

A new chemical route using an alkanolamine chelating agent for synthesis of ferroelectric Sro0.8Bi2.3Ta2O9 (SBT) thin films is investigated. The addition of alkanolamine provided stability to the SBT solution by retarding the hydrolysis and condensation rates. This solution could be stored for 30 days without any appreciable change of the properties. The crystallinity and the microstructure of the SBT thin films improved with increasing annealing temperature, and strongly influenced the ferroelectric properties. This alkanolamine modified films annealed at 800 °C exhibited low voltage saturation, high remanent polarization and resistance to fatigue after 1010 switching cycles. These electrical properties are favorable for non-volatile memory applications.
Control of Grain Size of Pb(Zr,Ti)O3 Thin Films by MOCVD and the Effect of Size on the Electrical Properties
Authors: Fujisawa H. , Nakashima S. , Shimizu M. , Niu H.
Published: April 2011

The grain size of MOCVD-Pb(Zr,Ti)O3 (PZT) thin films was successfully controlled by changing the grain size of Ir bottom electrodes and by changing the growth rate of PZT films. In Ir/PZT/Ir/SiO2/Si capacitors, the grain size of PZT thin films increased from 120 to 240nm as the grain size of bottom Ir electrodes increased from 50 to 200nm. The dielectric constants of PZT thin films increased from 760 to 1440 as the grain size increased from 120 to 240nm. Remanent polarization increased and coercive field decreased as the grain size increased. This dependence of electrical properties on the grain size coincided with that of ceramics.
The Effect of Atmospheric Water Vapour on the Temperature Dependence of Capacitance in BaxSr1−xTiO3 Thin Films
Authors: O'Neill D. , Gregg J. M.
Published: April 2011

Thin film ceramic Au/BaxSr(1−x)TiO3/YBCO/MgO planar capacitors with x ranging from 1 to 0.4 were fabricated using pulsed laser deposition (PLD). Structural and chemical characterisation showed the ferroelectric layers to be of perovskite crystallography, with cation stoichiometry close to that of the target. Auger depth profiling showed the interface between ferroelectric and lower electrode to be reasonably chemically sharp.

Capacitance versus temperature measurements performed in air showed a pronounced difference in behaviour between bulk and thin film devices. Bulk samples exhibited the expected capacitance anomaly at the Curie temperature. In contrast, thin film capacitors showed a prominent peak in capacitance at temperatures between 7 and 25°C on heating, irrespective of composition and did not show any evidence of expected Curie anomalies.

Further characterisation under vacuum and with deliberate additions of water vapour to the vacuum chamber indicated that the peaks observed in thin film devices described above could be attributed to the presence of atmospheric water vapour. This demonstrates that water vapour in the air can have a significant effect on the dielectric response of thin film capacitors around room temperature.

Studies on higher quality films showed a reduced response to water vapour.
The Role of Precursor Chemistry in the Ferroelectric Properties of Donor Doped Pb(Zr,Ti)O3 Thin Films
Authors: Fè Laura , Malič B. , Norga G. , Kosec M. , Wouters D. J. , Bartic T. A. , Maes H. E.
Published: April 2011

Modified sol-gel processes have been developed for the preparation of precursor solutions of undoped, La and Ta doped PZT (25/75). These processes use different solvents (methoxyethanol, butoxyethanol) and different lead sources (lead acetate, lead oxide). Due to variations in the structure and in the composition of the solutions, significantly different thermal decomposition behaviors were found. These inevitably affected texture and microstructure of the sol-gel derived thin films, demonstrating the important role of precursor chemistry in the improvement of film properties. In the case of tantalum doped Pt/PZT/Pt ferroelectric capacitors a rectangular hysteresis loop, featuring high Pr, was obtained for one specific precursor chemistry.
Domain Structure of Poled Ferroelectric (111) PZT (PbZr0.25Ti0.75O3) Films
Authors: Zybill C.E. , Boubekeur H. , Radojkovic P. , Schwartzkopff M. , Hartmann E. , Koch F. , Groos G. , Rezek B. , Bruchhaus R. , Wersing W.
Published: April 2011

Films of (111) oriented self-polarized, tetragonal ferroelectric PZT crystallites on (100)Si/SiO2(250 nm)/(111) Pt (50 nm) have been investigated by STM, AFM and SAXS. After metallization of the PZT surface with a Cr-Ni film (5.2 nm thickness) or a Ti film (5.0 nm thickness), single domains were visible on the metal surface by STM measurements as parallel stripes. The lamellar stripes had a width of 10.5 – 25.2 nm and a vertical corrugation of 0.9 – 3.0 nm at the intersection line of the domain walls with the crystallite surface.

High resolution AFM with EBD supertips on unmetallized samples revealed areas of typically several µm in diameter showing crystallites with perfectly parallel aligned (90°) domains of 10 - 15 nm width with their boundaries along {110} planes. Single domain walls were visible as a trace on the surface by a negative corrugation effect of 1.0 – 1.5 nm. This corrugation is assumed to be a reflection of the strain distribution normal to the surface. Furthermore, coherency (oxygen) defects are accumulated at the interface between 90 ° twin domains.

SAXS investigations allowed to estimate a mean value of domain thickness of 17.5 nm. Exertion of stress (5.1 104Nm−2) to the film resulted in an increase of domain width by ∼1%.
High Dielectric Constant Hf-Ti-Sn-O Off-Axis Cosputtered Films
Authors: Schneemeyer L. F. , Dover R. B. van , Fleming R. M.
Published: April 2011

New Hf-Sn-Ti-O thin-film dielectric materials were explored using a compositional-spread approach. Thin films of composition Hf0.2Sn0.05Ti0.75O2 prepared at 250°C have excellent dielectric properties: 40-50 nm thick films with a dielectric constant of 40-60 were obtained, depending on the processing conditions, yielding a specific capacitance of 9-17 fF/µm2. Breakdown fields were measured to be about 4 MV/cm, yielding a Figure of Merit εε0Ebr=19 µC/cm2. Leakage currents, measured at 1 MV/cm, were in the range 10−7- 10−6 A/cm2. Film properties are strongly dependent on substrate temperature during deposition. Like their recently reported zirconium analogs, these materials are of interest for use in Si-IC device technology, for example as storage capacitors in DRAM.
Piezoelectric Films and Coatings for Device Purposes
Authors: Sayer Michael , Lukacs Marc , Olding Tim , Pang Guofeng , Zou Lichun , Chen Yan
Published: April 2011

Piezoelectric devices generally require film thicknesses in the range 2-200µm and have a response which is perturbed by the substrate. Sol gel and sol gel composite technology provide a manufacturable process for many purposes with a piezoelectric coupling coefficient for 40ptm thick PZT approaching kt= 0.34. Techniques such as laser machining or micromolding are required for patterning thick devices. Measurement and applications of piezoelectric coatings in medical imaging, high temperature ultrasonics and flexure devices are discussed.
Epitaxial KNbo3 and its Nonlinear Optical Properties
Authors: Nichols Barbara M. , Wessels Bruce W. , Belot John A. , Marks Tobin J.
Published: April 2011

The nonlinear optical properties of epitaxial KNbO3 were studied for films grown by metalorganic chemical vapor deposition on different substrates. X-ray diffraction revealed epitaxial KNbO3 with a (110) orientation for layers deposited on single crystal strontium titanate. Whereas, films grown on (100) spinel exhibited a multi-variant structure, consisting of the (110) and (001) orientations. The volume fraction of each crystallographic variant in the film was calculated. Second harmonic generation measurements at 532 nm were performed to determine the ferroelectric domain variants present. The Maker-fringe technique was employed to obtain the effective nonlinear optical susceptibility of the films, which ranged from 0.1 - 1.8 pm/V. The SHG intensity profiles reveal a strong dependence upon substrate type.
Control of Ferroelectric Properties of PbZrxTi1−xO3 Thin Film for Electron Emission Device Driven by Low Voltage
Authors: Yamagata Y. , Yamazato M. , Ikegami T. , Ebihara K. , Narayan J. , Grishin A.M.
Published: April 2011

Epitaxial PbZr0.52TiO0.48O3/YBa2Cu3O7−x heterostructures on Nd:YAlO3 and MgO substrates were fabricated by KrF pulsed laser deposition. The coercive electric field of the PZT films increased with decrease of the film thickness from 1.2 μm to 0.04 μm, while the magnitude of spontaneous polarization was almost constant in this thickness range. It was found that the dependence of the film thickness d on the coercive electric field Ec was Ecæ d−2/3. This results from that the PZT/YBCO heterostructure has the one dimensional ferroelectric domain growth without non-ferroelectric phase. The polarization of AuIPZT/YBCO/(MgO or YAlO) capacitors can be changed by the applied voltage below 5 V.
Effect of Solvent Condition on the Dynamic Response of Polymer Brushes
Authors: Kilbey S. M. , Schorr P. , Tirrell M.
Published: April 2011

The frictional response of two opposing polymer brushes subjected to steady shear was studied as a function of solvent condition and degree of compression using the Surface Forces Apparatus (SFA). The brushes were made by preferential adsorption of polyvinylpryridinepolystyrene (PVP-PS) block copolymers adsorbed onto atomically smooth surfaces from a dilute solution of toluene. Extremely parallel lateral motion was imparted to one surface and the response was detected at the opposing brush-covered surface. When the brushes were bathed in a good solvent, it was necessary to strongly compress the layers in order to observe frictional forces transmitted between the surfaces. However, when the solvent was changed to a near-theta solvent, large frictional forces were measured at weaker compressions. The onset of these frictional forces occurred at distances comparable to the distance at which the opposing layers contact one another and rapidly increase as the brushes are compressed. Arguments are advanced that this behavior is attributable to frictional interactions between the polymer chains and the solvent.
Wetting Of Ion Containg Polymers: Micelles Versus Molecular Absorption
Authors: Hill T. A. , Jiao X. , Martin C. W. , Demarteau D. D. , Perahia D.
Published: April 2011

The wetting behavior of perfluorinated ionomers has been investigated as a function of polymer-surface interaction, the nature of the counter-ion and the structure of the polymer in the solvent from which it has been cast onto the surface. Two different ion-containing polymers, with a per-fluoro-carbon backbone were studied. They differ in the chemical composition and the polarity of their side chains. AFM results together with neutron reflectivity, light scattering and surface tension measurements had shown that the structure of the polymer in the parent solution has a profound effect on the wetting properties as well as the surface structure of the polymers.
Crossover Time Behavior in a+b⃗c And a+2b⃗c Reaction-Diffusion Front Systems in a Capillary
Authors: Park Sung Hyun , Yen Andrew , Shi Zhong-You , Kopelman Raoul
Published: April 2011

The crossover time from the early-time regime to the asymptotic regime for the A+B⃗+C reaction-diffusion system with initially separated reactants has been derived analytically by matching the rate expressions for the two regimes. The crossover time expression thus obtained shows an explicit dependence on the rate constant (k) and the initial reactant concentrations (ao, bo). For the A+2B⃗C system we performed computer simulations using two different methods. Crossover behaviors for the global reaction rate and reaction front width have been observed for both methods. The crossover time depends on the mechanism of the reaction.
Monte Carlo Study of Etching at Silica-Water Interface
Authors: Yen Andrew , Zhao Xiaolin , Kopelman Raoul
Published: April 2011

A surface adsorption-diffusion-reaction model is proposed and tested by Monte Carlo simulations. The solvent shell effect is considered as a geometric constraint and the Langmuir adsorption mechanism is also incorporated. An anomalous power law of t1/2 is obtained for the amount of material removed from the surface vs. time. The result agrees well with etching experiments of SAM on a silica surface. We conclude that the unusual power law results from an interplay of vacancy growth and fusion at the surface.
Material Selection and Layered Assembly for Shock Wave Attenuation
Authors: Segars R. A. , Piscitelle L.
Published: April 2011

Theoretical models and experimental data taken primarily from the literature provide predictive capabilities of the sound speed in porous materials and hence their acoustic impedance can be determined. Applying the acoustic approximation (weak discontinuity) at the interfaces of the porous material along with an attenuation model describing conditions within the porous system good fits to shock tube data on an elastomeric foam and on layers of kevlar were obtained.
Localized Optical Excitations and Two-Exciton Spectroscopy of Phenylacetylene Dendrimers
Authors: Chernyak V. , Poliakov E. Y. , Tretiak S. , Mukamel S.
Published: April 2011

The one- and two-exciton manifolds of conjugated dendrimers possessing fractal geometries are studied using the Frenkel exciton model. Two-photon spectra can be used to determine both the magnitude and the sign of short-range coupling among segments. Self-similarity and the high degree of symmetry make it possible to compute the one-exciton states and the optical response with reduced numerical effort that scales linearly rather than exponentially with the number of generations. The third-order optical response and exciton scattering matrix are expressed in compact forms using irreducible representation of optical excitations, totally avoiding the expensive explicit calculation of two-exciton eigenstates.
Selective Grafting of Amine Groups on Polyethylene by Means of Modulated RF NH3 Plasmas
Authors: Creatore M. , Cicala G. , Favia P. , Lamendola R. , d'Agostno R.
Published: April 2011

Plasma treatments in power modulated NH3 RF glow discharges have been performed for modifying the surface of polyethylene (PE). Time Resolved Optical Emission Spectroscopy (TR-OES) has been utilized for monitoring the emission of active species during the modulation period of the discharge, Electron Spectroscopy for Chemical Analysis (ESCA) has been used for studying the surface composition of PE before and after the derivatization of plasmagrafted -NH2 groups. We present our preliminary results here, which show how modulated NH3 plasmas can effectively improve the grafting selectivity of -NH2 respect to all other N-containing groups.
Deposition of Transparent Conductive Tin Oxide thin Films by Pacvd for Sensor Applications
Authors: Arefi-Khonsari F. , Hellegouarc'h F. , Planade R. , Amouroux J.
Published: April 2011

Non stoichiometric thin tin oxide films have been deposited using the PACVD technique in a RF glow discharge diode or triode reactor at low pressure (15 Pa) and at low temperature (25–100°C). The films deposited in the diode configuration were amorphous and presented a high transmittance (95%) in the visible region. The conductivity of the films deposited in the triode configuration can be increased by 4 orders of magnitude with respect to the ones deposited in the diode configuration (from 0.01 to 100 ω−1 cm−1) with a decrease of the gap energy (from 3.5 to 2.5 eV). This increase of the conductivity was accompanied by a slight decrease of the grain size (95 to 45 nm). The very small grain size of the deposited films makes them excellent candidates for gas sensors due to their good sensitivities to different molecules such as ethanol and propane.
Ultra High Rate, Wide Area, Plasma Polymerized Films from High Molecular Weight/Low Vapor Pressure Liquid or Liquid/Solid Suspension Monomer Precursors
Authors: Affinito J. D. , Gross M. E. , Mounier P. A. , Shi M-K , Graff G. L.
Published: April 2011

A new process has been developed for the high rate vacuum deposition of solid films from high molecular weight/low vapor pressure liquid, or even liquid/solid suspension, monomer precursors. The gas resulting from the flash evaporation of a liquid monomer mixture, or from a suspension of insoluble solid particles in liquid monomer, is used as the support medium for a glow discharge in a Plasma Enhanced Chemical Vapor Deposition-like (PECVD) process. Due to the high molecular weight/low vapor pressure nature of the precursors, the plasma of the flash evaporated gas cryocondenses at extremely high rate on substrates at ambient, and higher, temperatures. Upon condensation the liquified plasma immediately begins to polymerize to form a solid film due to the high concentration of radicals and ions contained in the liquid film. The process has been successfully implemented in a vacuum roll coating system in a roll-to-roll deposition process. Polymer films, Molecularly Doped Polymer (MDP) composite films of polymer containing light emitting organic molecules, and MDP composite films of polymer containing organic dye molecules have been deposited at thicknesses ranging from 0.1 microns to 24 microns at webs speeds as high as 96 linear meters per minute. This new deposition process will be discussed along with some properties of the films fabricated with this new process.
Transition from Surface Treatment in a Batch Process to a Continuous Process
Authors: Holländer A , Royer E. , Behnisch J.
Published: April 2011

For the improvement of the adhesion to ethylene vinylacetate copolymer (EVA), poly (ethylene terephthalate) (PET) films were treated with low pressure plasmas in a laboratory batch reactor and in a pilot scale semi-continuous treatment system. The effects of the treatments were investigated by mechanical testing of the PET-EVA laminates, by XPS, by chemical derivatization for XPS, and by contact angle measurements including a data processing for the determination of Lewis acid/ base properties of the surfaces. It was found that the adhesion between the polymer surfaces seems to be based on rather specific interactions which ask for certain conditions for the surface treatment. The differences between the batch treatments and the semi-continuous treatments are discussed.
Thin Film Plasma Deposition of Allylamine; Effects of Solvent Treatment
Authors: van Os M. T. , Menges B. , Förch R. , Knoll W. , Timmons R. B. , Vancso G. J.
Published: April 2011

Amine-functionalized thin films were prepared by plasma induced deposition of allylamine. Radio frequency (rf) plasma polymerization was carried out under both continuous wave (CW) and pulsed plasma conditions to control the film chemistry, all other process variables being held constant. Using plasma polymerized films for subsequent grafting reactions or for direct biomedical applications usually involves exposure of the films to a solvent environment. It was the major focus of this work to investigate both the chemical and physical effects of solvents on plasma polymerized allylamine films. Film properties were determined using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), Waveguide Mode Spectroscopy (WaMS) and Surface Plasmon Spectroscopy (SPS). WaMS provided an ideal opportunity to study the complex swelling and drying behaviour of these films by allowing a simultaneous study of the changes in film thickness and refractive index. Although the amine groups of the monomer were increasingly retained in the films as the RF duty cycle was lowered, a larger amount of oxygen was also found to be incorporated upon exposure to air. Extraction in ethanol led to a decrease in film thickness, especially for the films produced at low duty cycles, but appeared to have little effect on the film composition, as measured by XPS and FTIR.
Pulsed Plasma Polymerizations: Film Chemistry Control and Applications
Authors: WU Yuliang , Han Licheng M. , Thomes Brett E. , Qiu Haibo , Savage Charles R. , Lee Wei W. , Timmons Richard B.
Published: April 2011

Film chemistry control issues, as well as key properties of polymers synthesized under pulsed plasma conditions, are discussed. Distinctions between pulsed and continuous wave plasmas are examined, particularly as they relate to differences in the energy efficiency of film formation rates and to the range of available power inputs. Film stabilities, with special reference to polymers formed under very low power input conditions, are considered. Finally, selected applications involving use of the inherently high film chemistry controllability made available by the variable duty cycle pulsed plasma technique are described.
Highly Reflective and Surface Conductive Silver-Polyimide Films Fabricated Via Reduction of Silver(I) in a Thermally Curing Poly(Amic Acid) Matrix
Authors: Southward R. E. , Thompson D S. , Thompson D. W. , Scott J. L. , Broadwater S. T.
Published: April 2011

Highly reflective and surface conductive silvered polyimide films have been prepared by the incorporation of the (1,1,1-trifluoro-2,4-pentanedionato)silver(I) complex into a dimethylacetamide solution of the poly(amic acid) formed from 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) and 4,4'-oxydianiline (ODA). Thermal curing of the silver(I)- containing poly(amic acid) films leads to cycloimidization with concomitant silver(I) reduction yielding a reflective and conductive silvered film surface.
The Effects of Ground and Space Processing on the Properties of Organic, Polymeric, and Colloidal Materials
Authors: Frazier Donald O. , Paley Mark S. , Penn Benjamin G. , Abdeldayem Hossin A. , Smith David D. , Witherow William K. , Carswell William E. , Zugrav Maria I.
Published: April 2011

In recent years, a great deal of interest has been directed toward the use of organic materials in the development of high-efficiency optoelectronic and photonic devices. There is a myriad of possibilities among organic materials, which allows flexibility in the design of unique structures with a variety of functional groups. The use of nonlinear optical (NLO) organic materials as thin-film waveguides allows full exploitation of their desirable qualities by permitting long interaction lengths and large susceptibilities allowing modest power input. There are several methods in use to prepare thin films such as Langmuir-Blodgett (LB) and self-assembly techniques,2-4 vapor deposition.5-7 growth from sheared solution or melt,8,9 and melt growth between glass plates.10 Organic-based materials have many features that make them desirable for use in optical devices, such as high second- and third-order nonlinearity, flexibility of molecular design, and damage resistance to optical radiation. However, processing difficulties for crystals and thin films has hindered their use in devices.

We discuss the potential role of microgravity processing of a few organic and polymeric materials. It is of interest to note how materials with second- and third-order NLO behavior may be improved in a diffusion-limited environment and ways in which convection may be detrimental to these materials. We focus our discussion on third-order materials for all-optical switching, and second-order materials for frequency conversion and electro-optics. The goal of minimizing optical loss obviously depends on processing methods. For solution-based processes, such as solution crystal growth and solution photopolymerization, it is well known that thermal- and solutal-density gradients can initiate buoyancy-driven convection. Resultant fluid flows can affect transport of material to and from growth interfaces and become manifest in the morphology and homogeneity of the growing film or crystal. Likewise, buoyancy-driven convection can hinder production of defect-free, high-quality crystals or films during crystal and film growth by vapor deposition.
Magnetic Levitation of Liquid Droplets
Authors: Beaugnon E. , Tournwer R.
Published: April 2011

The magnetic levitation of diamagnetic materials in a high field superconducting magnet is presented. The magnetic force is determined from numerical calculations of the magnetic field components and the stability of levitation is studied using energy calculations. Amongst the numerous materials that can be levitated, experiments are shown for ethanol droplets.
Microgravity Solidification of Immiscible Alloys
Authors: Andrews J. B. , Hayes L. J. , Arikawa Y. , Coriell S. R.
Published: April 2011

This paper covers findings obtained from the microgravity directional solidification of immiscible aluminum-indium alloys during the Life and Microgravity Spacelab Mission in 1996. Three alloys, one of monotectic composition and two alloys containing an excess of indium above the monotectic (i.e., hypermonotectic compositions) were solidified using the Advanced Gradient Heating Facility (AGHF). Samples were processed in specialized ampoule assemblies containing pistons and a high temperature spring in a partially successful attempt to prevent void formation due to thermal contraction of the melt and solidification shrinkage. A comparison of compositional variations between microgravity processed and ground processed samples revealed compositional variations along the length of ground processed samples which were representative of results anticipated due to convective mixing in the melt. Flight samples showed an initial compositional variation indicative of minimal mixing in the melt. However, a discontinuity in the microstructure was observed which coincided with the presence of a void in the flight sample.
Creep of Fully Lamellar Near γ-TiAl Intermetallics
Authors: Beddoes J. , Zhao L. , Chen W. R. , Du X.
Published: April 2011

The influence of the fully lamellar morphology and third phase β on the creep properties of near γ-TiAl intermetallics is presented. Specifically, the effect of improved microstructural control obtainableby a stepped cool, involving furnace cooling and air cooling from the a single phase, on creep resistanceis demonstrated for three near γ-TiAl intermetallics: binary Ti-48AI, ternary Ti-48A1–2W and Ti-47A1–2Nb-IMn-0.5W-0.5Mo-0.2Si. The results indicate that appropriate stepped cooling can be used to reducethe lamellar interface spacing without the formation of Widmanstatten, feathery γ or γM structures, leadingto longer creep life and reduced creep strain rates. A second benefit of stepped cooling is preventionof βformation during cooling from the α phase, allowing controlled β precipitation during aging at 950 C. Creep tests on variously aged Ti-48A1–2W indicate that β precipitation along lamellar grain boundariesimproves creep resistance. Development of a uniform fully lamellar structure in Ti-47A1–2Nb-1Mn-0.5W-0.5Mo-0.2Si significantly improves creep resistance. Applying the stepped cool to this alloy allowsthe precipitation of β and silicides to be controlled during lower temperature aging.
Yield Stress Anomaly for 1/2〈112] {1111} Slip in y-Titanium Aluminide
Authors: Jiao S. , Bird N. , B. P. , Taylor G.
Published: April 2011

Transmission electron microscopy (TEM) studies of dislocation structures in single crystals of TiAl containing 54.5 or 54.7 at% Al deformed at different temperatures revealed the occurrence of slip on ½〈112]{ {111} at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existence of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for 30° dislocations and at high temperatures for edge dislocations. Both types of locks involve dissociation on two intersecting { 1111} type planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide.
Precipitation and Creep Strengthening Mechanisms in K5 Series γ-TiAl Alloys
Authors: Gouma P. I. , Subramanian K. , Kim Y-W. , Mills M. J.
Published: April 2011

A new generation of wrought γ-TiAl alloys having a good balance of properties at both low and high temperatures has recently been developed. Addition of light elements (C, Si) to the base alloy of this system (K5 material) has been shown to increase the creep strength of the material significantly. The microstructure of the materials with superior creep performance was found to consist of wellaligned rows of fine precipitates, delineating partially dissolved α2 laths. These precipitates were found to interact strongly with dislocation structures. This paper studies the mechanisms for α2 dissolution and fine particle precipitation in annealed samples of the K5 series alloys. Microstructural design issues that would allow for optimization of the particular strengthening mechanism will also be discussed.
Comparative Study of the Microstructure and Tensile Properties of Ni-Al Alloys with Fe and Cr Additions
Authors: Pekarskaya E. , Humphreys C. J. , Jones C. N.
Published: April 2011

The tensile behaviour of Ni-32at.% Al-5at.% Fe and Ni-32at.% Al-12 at.% Cr two-phase intermetallic compounds has been studied in the 20–750°C temperature range. TEM studies of the defect structures have been performed. Both alloys exhibit a brittle behaviour at room temperature, while at high temperature the Ni-Al-Cr alloy appears to be much more ductile that the Ni-Al-Fe alloy. Various reasons for the limited ductility and the brittle-to-ductile transition in these alloys are discussed.
Analysis of Weak-Beam Contrast from SESF/SISF Fault Pairs Associated with ½ <112] Superdislocations in TiAl
Authors: Kumar Mukul , Sriram S. , Schwartz Adam J. , Vasudevan Vijay K.
Published: April 2011

The diffraction contrast from dissociated ½<112] superdislocations in γ-TiAl intermetallic alloy cannot always be analyzed using conventional rules of diffraction contrast. In particular, the configuration involving three similar Shockley partials on adjacent planes has often been ruled out due to the absence of fringes indicating the presence of stacking faults. In order to determine the dissociated configuration, weak-beam transmission electron microscope observations of edge-oriented ½<112] superdislocations have been correlated with computer simulated images. Dissociation of these superdislocations into three similar ⅙<112] partial dislocations bounding a superlattice extrinsic and intrinsic stacking fault pair has been consequently determined from these analyses. It has been found that diffraction contrast alone cannot distinguish between the various configurations that lead to the formation of the fault pair, but the formation of an antiphase boundary or complex stacking fault linked dissociation or locking by stair rod dislocations can be ruled out.