bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation Spectra in the Time-Dependent Density-Functional Theory with Gradient Correction
Authors: Uimezawa Naoto , Saito Susumu
Published: April 2011

We study tile optical absorption spectra of Na clusters using the time-dependent density-functional theory with gradient correction. A jellium-sphere background model, which is free from basis-set incompleteness error and is suitable for the comparison of various theoretical methods, is adopted. For energies of surface-plasinon excitations governing profiles of photoabsorption spectra with huge oscillator strengths., the gradient correction by van Leeiiwen and Baerends with correct asymptotic behavior of the effective potential is found to show considerable improvement over the time-dependent local-density approximation.
Optical Absorption and Electronic Excitations in Hydrogenated Silicon Clusters
Authors: Vasiliev Igor , Öĝüt Serdar , Chelikowsky James R.
Published: April 2011

The study of electronic excitations in hydrogen terminated silicon clusters is important for understanding the optical properties of confined systems such as quantum dots and porous silicon. Here we calculate the excitation energies and absorption spectra for SinHm clusters using linear response theory within the time-dependent local density approximation (TDLDA). We find the computed excitation energies and photoabsorption gaps agree with available experimental data. The TDLDA optical absorption spectra exhibit a substantial blue shift with respect to the spectra calculated within the time-independent local density approximation. This trend is consistent with other theoretical calculations for excited state properties, such as the Bethe-Salpeter technique.
Phase Coarsening as a Relaxation Phenomenon
Authors: Marsh S. P.
Published: April 2011

Phase coarsening, also known as Ostwald ripening, is generally treated as a growth phenomenon in which the average particle size increases during isothermal aging. However, coarsening is a relaxation process driven by a reduction in the excess interfacial energy of a two-phase structure. This viewpoint introduces a single temporal offset parameter that relates the experimental clock to the asymptotic time scale. Analysis of ripening as a relaxation phenomenon permits accurate determination of the kinetics that is less sensitive to particle morphology. Implications of this approach on measurement and interpretation of ripening kinetics will be discussed.
Optimizing The Metalloid Content in Bulk Metallic Glasses
Authors: Jin O. , Schwarz R. B. , Alamgir F. M. , Jain H.
Published: April 2011

We have studied the effect of phosphorus content on the formation and properties of bulk Pd-Cu-P and Pd-Ni-P metallic glasses. It was found that the molar volumes of the Pd-Ni-P bulk glasses plotted as a function of P content follow a straight line whereas those for the Pd-Cu-P glasses follow two straight lines of different slopes, intersecting at a critical P concentration. The transition in molar volume behavior is attributed to a change in the alloy's short-range order with increasing P content. The origin of this structural change is discussed based on x-ray photoelectron spectroscopy (XPS) measurements.
A Self-Similar Solution for the Growth Rate of a Compound Layer in Thin-Film Binary Diffusion Couples
Authors: Zhang Huifang , Wong Harris
Published: April 2011

The diffusion controlled growth of a compound phase AnB between two thin films of material A and B is studied with the nonlinear Kirkendall effect included. Previous models of the growth rate do not solve the diffusion equation, and thus do not fully utilize the predictive capability. This paper describes a self-similar transformation that reduces the nonlinear, time-dependent diffusion equation with two free boundaries into a nonlinear ordinary differential equation, which is solved numerically by a shooting method. It is found that the intrinsic diffusion coefficients of A and B in AnB can be determined from the positions of the interfaces without using the concentration profile. This provides a simpler method for measuring intrinsic diffusion coefficients.
Characterization of the Early Stages of Phase Separation by Atom probe Tomography
Authors: Miller M. K.
Published: April 2011

An atom probe tomography characterization of the early stages of phase separation in the FeCu-Ni system has been performed. The development of the copper-enriched regions into copper precipitates has been investigated during isothermal ageing at 573 K and 673 K for times up to 10,000 h. The formation and growth of the secondary precipitates in the nickel based superalloy, Alloy 718, have been determined during isothermal ageing at 873 K. These secondary precipitates were found to be a mixture of the L12-ordered γ' phase and the DO22-ordered γ” phase.
Solid Solutions (PbI2)x – (BiI3)1-x
Authors: Dmitriev Yu.N. , Bennett P.R. , Cirignano L.J. , Gupta T.K. , Klugerman M. , Shah K.S.
Published: April 2011

System (PbI2)x – (BiI3)1-x of two layered semiconductors PbI2 and BiI3 with various composition were synthesized and grown both as bulk single crystals (by vertical Bridgman method) and films (by thermal vacuum deposition). Grown composites were tested by different techniques (X-ray difflaction analysis, optical transmission, I – V measurements, response to X-ray pulse excitation). Property versus composition showed nonlinear behavior of measured parameters. The structural model of ordered structures formation in this system, which was based upon local electrical neutrality of unit cells, was worked out. It has been experimentally demonstrated that a formation of ordered structures is possible within (PbI2)x-(BiI3)1-x system, which is consistent with the suggested model. The possibility of crystal structures formation as ordered sets of BiI3 and PbI2 layers was also checked.
Investigation of Electronic Surface States and its Correlation to Surface Modifications in Femtosecond UV-Laser Treated n(100) GaAs
Authors: Railkar T. A. , Malshe A. P. , Brown W. D. , Hullavarad S. S. , Bhoraskar S. V.
Published: April 2011

Gallium arsenide (GaAs) is an important III-V semiconductor, due to its applicability to several high-end and consumable electronic devices. However, GaAs, unlike silicon (Si), does not have a stable native oxide that can stabilize and protect the surface from further oxidation and degradation. In this paper, we report the use of femtosecond laser-based modification of the GaAs surface, leading to its stabilization against oxidation, the use of ultrafast laser-based processing and its effectiveness compared to conventional laser treatment. One of the prominent advantages of ultrafast lasers include an almost non-existent heat affected zone (HAZ). The surface stabilizing and passivating effects were confirmed by depth-profiling x-ray photoelectron spectroscopic (XPS) measurements, atomic force microscopy (AFM) measurements, and a surface sensitive spectroscopic technique called thermally stimulated exoelectron emission (TSEE) measurements.
Nonlinear Ripple Formation in Sputter Erosion
Authors: Barabási A.-L. , Kahng B. , Jeong H. , Park S.
Published: April 2011

We investigate the morphological features of sputter eroded surfaces, demonstrating that while at short times ripple formation is described by the linear theory, after a characteristic time the nonlinear terms determine the surface morphology. We also show that the morphological transitions induced by the nonlinear effects can be detected by monitoring the surface width and the erosion velocity.
Environmental Scanning Electron Microscopy as a Tool to Study Shrinkage Microcracks in Cement-Based Materials
Authors: Bisschop J. , Mier J.G.M. Van
Published: April 2011

In this paper a method is described to observe shrinkage microcracks on ‘wet’ specimen cross-sections of cement-based materials with Environmental Scanning Electron Microscopy (ESEM). A sample cooling device which can be used in the ESEM chamber was built to control the relative humidity above a microscope sample. The accuracy of measuring relative humidity is determined to be 5% at a sample temperature of 3°C. A microscope sample preparation method and a pump-down sequence of the ESEM-chamber, both without any drying of the sample, are described. Preliminary results show that in the studied mortar the visibility of shrinkage microcracks on a ‘wet’ specimen cross-section is low due to closure of microcracks by swelling of the cement paste.
Epitaxy and Atomic Structure Determination of Au/TiO2Interfaces by Combined EBSD and HRTEM
Authors: Cosandey F. , Stadelmann P.
Published: April 2011

We have studied the effects of deposition conditions on the epitaxial orientation of Au or TiO2(110) and on the atomic structure of Au/TiO2interfaces by combined EBSD and HRTEM. Two experimental conditions were explored consisting of deposition of a 12 nim Au film at 300K followed by annealing at 770K and direct deposition of a 12 nm Au film at 770K. Deposition at 300K followed by annealing at 770K give rise to a (111)Au//(110)TiO2epitaxial orientation relationship, while direct deposition at 700K temperature give rise to an epitaxial orientation relationship given by (112)Au//(110)TiO2. For both orientations, two epitaxial variants are observed which are twin related. The (112)Au//(110)TiO2orientation has been found to minimize the interfacial lattice misfit while maximizing the number of Au-Ti bonds across the interface.
STEM Analysis of the Segregation of Bi to Σ19a Grain Boundaries in Cu
Authors: Sigle W. , Chang L.-S. , Gust W. , Rühle M.
Published: April 2011

Cu bicrystals were doped with different amounts of Bi and annealed at various temperatures. The segregation at the grain boundary (GB) was analyzed by energy dispersive X-ray analysis (EDX) in the scanning transmission electron microscope (STEM). It is found that with both increasing temperature and increasing Bi content the amount of faceting of the GB increases, finally reaching complete faceting. This final state is distinguished by brittle behavior. By analyzing the shape of pores at the GB we found that the Cu surface energy anisotropy is probably increased by Bi surface segregation.
Atomic and Electronic Structure Analysis of Σ=3, 9 and 27 Boundary, and Multiple Junction in β-SiC
Authors: Tanaka K. , Kohyama M.
Published: April 2011

The atomic structures of σ=3, 9 and 27 boundaries, and multiple junctions in β-SiC were studied by high-resolution electron microscopy (HREM). Especially, the existence of the variety of structures of σ=3 incoherent twin boundaries and σ=27 boundary was shown by HREM. The structures of σ=3, 9 and 27 boundary were explained by structural unit models. Electron energy-loss spectroscopy (EELS) was used to investigate the electronic structure of grain boundaries. The spectra recorded from bulk, {111}σ=3 coherent twin boundary (CTB) and {1211}σ=3 incoherent twin boundary (ITB) did not show significant differences. Especially, the energy-loss corresponding to carbon 1s-to Φ* transition was not found. It indicates that C atoms exist at grain boundary on the similar condition of bulk
The Effect of Different Oxidizing Atmospheres on the Initial Kinetics of Copper Oxidation as Studied In Situ UHV-TEM
Authors: Bharadwaj Mridula D. , Gupta Anu , Gibson J. Murray , Yang Judith C.
Published: April 2011

Effect of moisture on the oxidation of copper was studied using in situ UHV-TEM. The ultra high vacuum condition is required for minimum contamination effects. The initial observations show that the water vapor reduces the oxide as well as reduces the rate of oxidation if both oxygen gas and water vapor are simultaneously used. Based on these observations, we have speculated on the role of moisture in the solid state reactions involved in copper oxidation
Multi-and Single-Walled Boron Nitride Nanotubes Produced From Carbon Nanotubes by a Substitution Reaction
Authors: Golberg D. , Bando Y. , Han W. , Bourgeois L. , Kurashima K. , Sato T.
Published: April 2011

Boron nitride (BN) multi-walled (MWNT) or single-walled (SWNT) nanotubes were synthesized from carbon MWNT and SWNT templates, respectively, under heating of C nanotubes together with boron trioxide in a flowing nitrogen atmosphere. B and N atom substitution for C atoms in the nanotubular shells during C oxidation by the B2O3 vapor in the N2 flow is thought to underlie the formation mechanism. Structural and chemical BN MWNT/SWNT analyses were performed by means of high-resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS), respectively. In particular, BN MWNT shell structure and defects, and electron irradiation stability were studied. Finally, the prospects of using the substitution technique for the synthesis of other advanced nanostructures made of BN (nanorods, nanoplates and nanocones) are discussed.
Twisting of Single-Walled Carbon Nanotube Bundles
Authors: Qin Lu-Chang , Iijima Sumio
Published: April 2011

Bundles of single-walled carbon nanotubes produced by laser ablation are often seen twisted in morphology, forming nanoropes which would better suit the function for improvement of structural stability in mechanical applications. The rope-like configuration is explained with a model that relates the twisting morphology to the helicity difference between the neighboring nanotubes. This selfassembly phenomenon is attributed to the fact that a twisted structure has lower total energy compared with the raft-like parallel bundles when the helicities of neighboring nanotubes are different.
Nanomechanical Properties of Amorphous Carbon and Carbon Nitride Thin Films Prepared by Shielded Arc Ion Plating
Authors: Tajima N. , Saze H. , Sugimura H. , Takai O.
Published: April 2011

Hydrogen free amorphous carbon (a-C) and carbon nitride (a-C:N) were synthesized by means of shielded arc ion plating in which a shielding plate was inserted between a target and a substrate in order to reduce macroparticle deposition onto the substrate. Using a graphite target as a cathode, thin films of a-C and a-C:N were prepared in an arc discharge plasma of argon or nitrogen gas, respectively, at a pressure of 1 Pa. Based on nanoindentation, mechanical properties of these films were studied in relation to substrate bias voltage (Vs). The a-C films prepared at Vs ranging from -50 to -200 V consisted of diamond-like phase and showed hardness higher than 20 GPa with its maximum of 35 GPa at Vs = -100 V. However, hardness of the films deposited at Vs < 300 V was less than 7 GPa indicating that the films were converted to graphite-like phase due to excessive ion impact in Ar plasma. Wear resistance of the a-C films was closely related to their hardness. Namely, a harder a-C film was more wear resistant. On the contrary, hardness of the a-C:N films was less dependent on Vs. It remained in the range of 10 to 15 GPa and was much lower than the maximum hardness of the a-C films. Nevertheless, the wear resistance of the a-C:N films was comparable to or much better than the a-C films. In particular, the a-C:N film prepared at Vs = -300 V was so wear resistant that the film showed no apparent wear when rubbed with a diamond tip less than 100 nm in tip-diameter at a contact force of 20μN. The presence of β-C3N4like phase characterized by a N1 s XPS peak at 400.5 eV has found to be crucial for high wear resistance of the a-C:N films
Poly(SNS) Electrochemical Synthesis and Processability
Authors: Otero T. F. , Villanueva S. , Brillas E. , Carrasco J.
Published: April 2011

The flow of an anodic current density of 0.5 mA. cm−2 through a 5 mmol/l solution of 2,5-di-(2-thienyl)pyrrole (SNS), in acetonitrile, gives dark-violet and uniform polymer films coating the platinum electrode in presence of different electrolytes. The process is named either electropolymerization or electrogeneration.
Electrochemical Investigation of 2, 2′-Dlaminobenzyloxydisulfide
Authors: Su Y. Z. , Gong K. C.
Published: April 2011

A new conducting polymer, poly(2, 2′-diaminobenzyloxydisulfide), has been proposed as a positive material suitable for secondary lithium batteries. With the aim of better understanding the process of polymerization and depolymerization of 2,2′-diaminobenzyloxydisulfide(DABO). The redox behavior, kinetic reversibility and adsorption of DABO have been investigated at platinum electrode in acetonitrile/tetrahydrofuran solution by using linear sweep voltammetry, the cyclic sweep voltammetry and the rotating disk electrode technique. These experiments clearly showed the reaction is chemically reversible but kinetically slow at ambient temperature and charge transfer is the rate-determining step, but chemical dimerizaton is at equilibrium. The results are common to many organic disulfides. Furthermore, the striking observation from cyclic voltammograms is the smaller separation of the anodic and cathodic peak owing to the specific structure of DABO, compared with other organic disulfides. This results indicates the redox reaction of DABO is higher kinetically reversible and poly (DABO) positive material is expected to deliver higher power output or energy efficiency.
Polymer-Gel Phase-Transition as the Mechanism of Muscle Contraction
Authors: Pollack Gerald H.
Published: April 2011

The thesis offered here is that the muscle-contraction mechanism is similar to the mechanism of contraction in many artificial muscles. Artificial muscles typically contract by a phasetransition. Muscle is thought to contract by a sliding-filament mechanism in which one set of filaments is driven past another by the action of cyclically rotating cross-bridges—much like the mechanism of rowing. However, the evidence is equally consistent with a mechanism in which the filaments themselves contract, much like the condensation of polymers during a phasetransition. Muscle contains three principal polymer types organized neatly within a framework. All three can shorten. The contributions of each filament may be designed to confer maximum strength, speed and versatility on this biological machine. The principles of natural contraction may be useful in establishing optimal design principles for artificial muscles.
Giant Electrostrictive Response in Poly(vinylidene-fluoride hexafluoropropylene) copolymer
Authors: Lu Xiaoyan , Schirokauer Adriana , Scheinbeim Jerry
Published: April 2011

In the present study, the strain response of a new class of copolymers of PVF2 is investigated. Electrostrictive strains were measured in poly(vinylidene-fluoride hexafluoropropylene), P(VF2-HFP), using a capacitance method (air-gap capacitor), as a function of electric field. Three different thermal treatments (ice water quenched, air quenched and slow cooled) were given to samples of composition 5% and 15% HFP. Strains greater than 4 % were observed in the 5% HFP ice water quenched P(VF2-HFP) copolymer. Values of elastic modulus were lower for the quenched 5 % films than for the slow cooled ones, and in both cases they were higher than previously studied polyurethane elastomers and poly(vinylidene-fluoride trifluoroethylene) copolymers. Key words: Electrostrictive Polymer, Poly(vinylidene-fluoride hexafluoropropylene) copolymer, Ferroelectic, X-ray, DSC.
Superplasticity in Nb3Al/Nb In Situ Composites
Authors: Hanada S. , Fang W.
Published: April 2011

Microstructures of a binary Nb-15.8at%Al alloy ingot were controlled by isothermal forging and heat treatment to produce equiaxed, fine grains of Nb3Al and Nb solid solution (Nb33). Nb3Al/Nb33 two phase alloy (in-situ composite) is found to exhibit superplasticity especially when one of the constituent phases, Nb33, is supersaturated. During superplastic deformation Nb33 transforms to Nb3Al, and Al content in Nb33 decreases. After superplastic deformation the microstructure consisting of equiaxed grains is left unchanged, although a slight grain growth is observed. It is suggested that stress induced by grain boundary sliding is effectively accommodated through dislocation glide and climb in the soft Nb33
A Superplasticity Theory based on Dynamic Grain Growth
Authors: Mayo M.J. , Seidensticker J.R.
Published: April 2011

The linear relation between dynamic grain growth rate and strain rate appears to be constant across all superplastic materials—regardless of whether the system examined is metallic or ceramic, and regardless of the stress exponent exhibited during deformation. The simplicity and universality of the dynamic grain growth law suggest it might be useful as a foundation for a theory for superplasticity. One attempt at such a theory is presented. It is argued that stress leads to the development of anisotropic grain shapes that then require recovery through directionally biased grain growth events. Once this mathematical relationship between stress and grain growth rate is developed, it is inserted into the existing dynamic grain growth - strain rate law to arrive at a phenomenological law for superplasticity.
Cavity Nucleation in AL 5083 Alloys
Authors: Chandra N. , Chen Z.
Published: April 2011

In this paper we address the controversial issue of nucleation of cavities in Al 5083 alloys. We focus on the origin of cavities during the manufacture of these alloys into SPF (superplastic forming) sheet form. Experimental observations on the pre-existing cavities in this alloy are made using optical and electron microscopy. The effects of rolling direction and state of stress during superplastic deformations on the formation of cavities are also discussed. Numerical simulations of the sheet manufacturing process are carried out to understand the effect of hard phase/matrix, mechanical properties and interfacial strength on the origin of cavities. Based on the numerical results, a simplified model relating the process, material parameters and the cavity nucleation is presented.
Processing by Equal-Channel Angular Pressing: Potential for Achieving Superplasticity
Authors: Furukawa Minoru , Nemoto Minoru , Horita Zenji , Langdon Terence G.
Published: April 2011

Equal-channel angular (ECA) pressing is a processing procedure whereby a very severe plastic strain is imposed on a sample without any change in the cross-sectional dimensions of the material. This processing method leads to a substantial grain refinement, producing grains which are within the submicrometer or even the nanometer scale. This paper discusses the potential for using this method to prepare materials for superplasticity. The results demonstrate that it is possible to achieve superplastic deformation in selected materials subjected to ECA pressing and, in addition, there is the possibility of extending the superplastic region so that it occurs at very rapid strain rates.
A Model for Asymmetric Hysteresis in Piezoceramic Materials
Authors: Smith R.C. , Ounaies Z.
Published: April 2011

This paper focuses on the characterization of hysteresis exhibited by piezoelectric materials at moderate to high field levels. For soft materials in which dipoles are easily reconfigured, the hysteresis loop is observed to be rotationally symmetric about the zero field, zero polarization point and symmetric models can be employed. In harder materials, however, the loops are no longer rotationally symmetric which necessitates the development of commensurate characterization techniques. The model considered here is based upon the quantification of reversible and irreversible changes in polarization due to bending and translation of domain walls pinned at inclusions inherent to the materials. The performance of the model is illustrated through comparison with PZT4 data
Sr(Ti,Fe)O3: Materials for a Temperature Independent Resistive Oxygen Sensor
Authors: Menesklou W. , Schreiner H.-J. , Moos R. , Härdtl K.H. , Ivers-Tiffée E.
Published: April 2011

The metal oxide strontium titanate is a promising material for resistive high temperature oxygen sensors (T > 1000 K), that can be applied to control combustion engines. However, a disadvantage of this and of many other metal oxides is the strong temperature dependence of conductivity. In this work, we show that the temperature dependence of Sr(Ti1-x, Fex)O3-δ can be adjusted by the iron content. For x = 0.35 the thermal activation of the conductivity is near zero for a distinct temperature and oxygen partial pressure range (T = 1000 K - 1200 K, PO2 = 10 to 105 Pa). Short response times in the range of 10 ms can be realized by using sensors in thick film technology
Processing and Characterization of Cofired Capacitor and Varistor Ceramics
Authors: Malič B. , Kosec M. , Razinger J. , Živič Z.
Published: April 2011

The aim of the present work was to prepare a monolithic varistor-capacitor element from a commercial ZnO-based varistor and a relaxor based on lead-magnesium niobate. The materials were chosen on the basis of similar sintering temperatures, in the range from 900 to 1000°C. “Sandwich”-structured composite-pellets were uniaxially dry-pressed and sintered at the conditions that lead to shrinkages observed on the pellets of separately sintered materials. The microstructural analysis of the ‘sandwich’ interface revealed a good contact between the two parts without a noticeable interaction layer. Selected functional properties of both varistor and relaxor parts were adequate
Aluminum Nitride Thin Films for Microwave Filter and Microsystem Applications
Authors: Dubois M.-A. , Muralt P. , Sagalowicz L.
Published: April 2011

Results on growth, characterization and resonator applications of AIN thin films grown by pulsed dc reactive magnetron sputtering are presented. In order to allow deposition on electrode films, moderate deposition temperatures of 300 to 400 °C were applied. On (001) sapphire, monocrystalline films were obtained. On (111)-textured platinum films, a columnar and (001) textured microstructure was established which exhibited quasi single-crystal properties in relevant materials constants, such as e33, e31, and c33. The films have been successfully applied in bulk wave resonators operating at GHz frequencies, allowing for the derivation of acoustic parameters. A zero thermal frequency drift was obtained with a membrane resonator due to a SiO2 layer as compensating element.
Wafer-Level Strength and Fracture Toughness Testing of Surface-Micromachined MEMS Devices
Authors: Kahn H. , Tayebi N. , Ballarini R. , Mullen R.L. , Heuer A.H.
Published: April 2011

Determination of the mechanical properties of MEMS (microelectromechanical systems) materials is necessary for accurate device design and reliability prediction. This is most unambiguously performed using MEMS-fabricated test specimens and MEMS loading devices. We describe here a wafer-level technique for measuring the bend strength, fracture toughness, and tensile strength of MEMS materials. The bend strengths of surface-micromachined polysilicon, amorphous silicon, and polycrystalline 3C SiC are 5.1±1.0, 10.1±2.0, and 9.0±1.0 GPa, respectively. The fracture toughness of undoped and P-doped polysilicon is 1.2±0.2 MPa√m, and the tensile strength of polycrystalline 3C SiC is 3.2±1.2 GPa. These results include the first report of the mechanical strength of micromachined polycrystalline 3C SiC.
Ferroelectric Composite of Ba1−xSrxTiO3 With Al2O3 and MgO Synthesized by Sol-Gel Method
Authors: Sharma Pramod K. , Jose K. A. , Varadan V. V. , Varadan V. K.
Published: April 2011

Barium strontium titanate with low dielectric constant, low loss tangent and high tunability, has unique application as phase shifter in radar and antenna. This work presents an alternative fabrication of the composite of barium strontium titanate with metal oxides such as Al2O3 and MgO. Ba1−xSrxTiO3 (BST), xergels of various compositions (x = 0.2, 0.4, 0.5 and 0.6) were prepared by sol-gel method. Xerogels were calcined at 800 °C. The structure of the final powders was investigated by x-ray diffraction (XRD). The resultant BST was then mixed with metal oxides in the desired weight percentage and ball milled in an organic solvent (ethyl alcohol) with a binder (acrylic polymer). The final samples were sintered at 1250 °C in air. The dielectric properties of the composites were determined by using impedance analyzer at the frequency of 1MHz. This work has reported the effect of Al2O3 and MgO, on the electronic behavior of the final BST-metal oxide composites.
Deposition of Crack-Free BaTiO3 and Pb(Zr,Ti)O3 Films Over 1 μm Thick Via Single-Step Dip-Coating
Authors: Kozuka H. , Kajimura M. , Katayama K. , Isota Y. , Hirano T.
Published: April 2011

BaTiO3 and PZT films were prepared by single-step dip-coating from alkoxide-acetate solutions containing polyvinylpyrrolidone (PVP). Crack-free BaTiO3 and PZT films over 1 μm in thickness were obtained via single-step deposition. Stepwise heating of the gel films was found to improve densification of BaTiO3 films, reducing the thickness and increasing the optical transmittance, which was not, however, the case with PZT films, where the stepwise heating rather induced crack formation, leading to degraded transmittance. Residual stress was evaluated on spin-coating BaTiO3 films by measuring the substrate curvature, where a significant reduction in tensile stress was found to be caused by PVP.
Tuned Infrared Emission From Lithographically-Defined Silicon Surface Structures
Authors: Daly James T. , Johnson Edward A. , Stevenson William A. , Greenwald Anton C. , A. Wollam John , George Thomas , Jones Eric W.
Published: April 2011

Photonic bandgap structures have received much attention as optical and infrared filters with controllable narrow-band absorbance. There is a need, however, for the same kind of control of the thermal emittance of surfaces for applications ranging from control of radiative heat transfer to gas absorption spectroscopy. We report on the fabrication of photonic bandgap structures on silicon surfaces using standard lithographic techniques. Substrate resistivity varied from n− to n+ and in some cases background surface emissivity was suppressed with a high reflectivity coating such as aluminum. We have measured the infrared reflectance and emittance of these patterned surfaces. Peak absorption wavelength and spectral purity (linewidth) correlate with photonic bandgap feature size and spacing as well as surface conductivity. We demonstrate band emission with a sharp short wavelength cut-off from these structures when heated.
Temperature Dependence of Photoresponse in p-Type GaAs/AlGaAs Multiple Quantum Wells: Theory and Experiment
Authors: Szmulowicz F. , Shen A. , Liu H. C. , Brown G. J. , Wasilewski Z. R. , Buchanan M.
Published: April 2011

This paper describes a study of the photoresponse of long-wavelength (LWIR) and mid-infrared (MWIR) p-type GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) as a function of temperature and QWIP parameters. Using an 8x8 envelope-function model (EFA), we designed and calculated the optical absorption of several bound-to-continuum (BC) structures, with the optimum designs corresponding to the second light hole level (LH2) coincident with the top of the well. For the temperature-dependent study, one non-optimized LWIR and one optimized MWIR samples were grown by MBE and their photoresponse and absorption characteristics measured to test the theory. The theory shows that the placement of the LH2 resonance at the top of the well for the optimized sample and the presence of light-hole-like quasi-bound states within the heavy-hole continuum for the nonoptimized sample account for their markedly different thermal and polarization characteristics. In particular, the theory predicts that, for the LWIR sample, the LH-like quasi-bound states should lead to an increased Ppolarized photoresponse as a function of temperature. Our temperature dependent photoresponse measurements corroborate most of the theoretical findings with respect to the long-wavelength threshold, shape, and polarization and temperature dependence of the spectra.
Novel Diffraction Pattern for Optical Coupling in QWIP
Authors: Masalkar Prafulla , Nishino Hironori , Matsukura Yusuke , Tanaka Hitoshi , Yamamoto Kousaku , Miyamoto Yoshihiro , Fujii Toshio
Published: April 2011

An optical coupling diffraction pattern for quantum well infrared photodetectors is developed using simulation. It provides high coupling efficiency and fabrication ease. Improved performance has been experimentally verified.
Investigation of New Geometries for High Duty Cycle Far-Infrared p-Type Germanium Lasers
Authors: Chamberlin Danielle R. , Bruendermannw Erik , Haller Eugene E.
Published: April 2011

We report on increasing the pulse length and repetition rate of p-type germanium lasers through miniaturization, increased electric field uniformity, and improved cooling. We have recently demonstrated that it is possible to improve the electric field uniformity necessary for an efficient laser and at the same time decrease the electrical input power by using a geometry with d/L>>1, where d is the distance between electrical contacts and L is the length in the direction of the Hall field. In order to achieve good heat sinking along with a large d/L ratio, we have developed a new, planar contact geometry. Attaching an undoped, high-resistivity, single-crystal Si heat sink to the base of the Ge planar contact laser increases the duty cycle by a factor of 5.5. In order to further decrease the input power by decreasing the volume of laser crystals in the planar contact geometry, we show as a proof-of-concept the use of polished strontium titanate single crystals as electrically insulating far-infrared mirrors based on restrahl band reflection. The physical phenomena underlying these improvements in this novel geometry will be discussed.
Electronic Structure Engineering of the Linewidth Enhancement Factor in Mid-Infrared Semiconductor Laser Active Regions
Authors: Flatté Michael E. , Olesberg J.T. , Boggess T.F.
Published: April 2011

The linewidth enhancement factor is a fundamental parameter that characterizes the limit of spectral purity and the tendency for filamentation in a semiconductor laser. For either narrow-line or high-power operation, it is generally desirable that the linewidth enhancement factor be minimized. This parameter depends primarily on the shape of the differential gain spectrum of the active region material. In mid-infrared laser materials, band structure engineering has been used routinely to minimize the effects of nonradiative recombination. This same approach can be used to tailor the differential gain spectrum of these materials to minimize the linewidth enhancement factor.

Favorable features of the electronic structure of the material include 1) band-edge dispersion in the conduction band which shifts the peak of the gain away from the band edgeCand 2) intersubband absorption resonances, which can be designed to lie above the peak gain in energy. Both of these strategies shift the peak of the differential gain closer to the peak gain, and thus reduce the linewidth enhancement factor. These electronic structure design strategies can be used to reduce the linewidth enhancement factor in type-II InAs/GaInSb systems to almost 1.0, which is significantly smaller than typical values of 2.5–5 in type-I strained quantum wells. We note that a linewidth enhancement factor close to 1 is much smaller even than a typical linewidth enhancement factor in visible or near-infrared semiconductor lasers. Furthermore in some materials the peak of the differential gain lies in a region of positive gain, indicating that a grating could be used to select a mode with negligible linewidth enhancement factor.
Positive and Negative Photoconductivity in Lead Telluride Doped With Gallium Epitaxial Films
Authors: Akimov B.A. , Bogoyavlenskiy V.A. , Ryabova L.I. , Vasil'kov V.N.
Published: April 2011

We report on the photoconductivity and transient in n-PbTe(Ga) epitaxial films prepared by the hot wall technique on the <11 I>-BaF2 substrates. Photoconductivity measurements performed under continuous and pulse illumination in the temperature range 4.2-300 K give the photoresponse to be composed from negative and positive parts. The experimental results are interpreted assuming the mixed valence of Ga in lead telluride.
Kinetics of Light Induced Changes in Protocrystalline Thin Film Materials and Solar Cells
Authors: Koval Randy , Niu Xinwei , Pearce Joshua , Jiao Lihong , Ganguly Gautam , Yang Jeffrey , Guha Subhendu , Collins Robert W. , Wronski Christopher R.
Published: April 2011

Studies have been carried out on the kinetics of light induced changes in thin films and solar cells fabricated in different laboratories. Unlike the many studies on undiluted a-Si:H films the kinetics reported here are for 1 sun illuminations carried out to the degraded steady state (DSS). The light induced changes at temperatures from 25 to 100°C were characterized with electron mobility lifetime products and subgap absorption in films and fill factors in solar cells. The significant changes in the degradation kinetics that occur in these materials in this t mperature range have further confirmed the importance of charged defects, not only in the annealed state, but also in the degraded steady states. Results are also presented on corresponding solar cell structures, which also indicate that defects other than neutral dangling bonds have to be taken into account if any meaningful evaluation is to be made of their solar cell properties.
Anisotropy In Hydrogenated Silicon Thin Films
Authors: Webb John D. , Nelson B. P. , Mahan A. H. , Theisen J. , Reedy R. , Perkins J. D. , Gedvilas L. M.
Published: April 2011

We investigated spatial non-uniformity and optical anisotropy in hydrogenated silicon thin films using infrared spectroscopy and other techniques. The films ranged in morphology from amorphous to micro-crystalline, as determined by Raman spectroscopy and X-ray diffraction (XRD). Trace element analysis of the films was carried out using secondary ion mass spectrometry (SIMS). We used polarized attenuated total reflection (ATR) measurements together with Fourier transform infrared (FTIR) spectroscopy to investigate anisotropy in the Si-H covalent bonding structure of the films, and also to detect oxide impurity phases present at certain sites in some samples. The FTIR-ATR measurements were performed using a germanium (Ge) contact microprobe with a 100-micron sampling area, as well as on films deposited on standard crystalline Si and Ge ATR substrates. The Ge contact ATR microprobe enabled relatively interference-free measurements of the IR spectra of a-Si:H and µc-Si:H films deposited on conductive substrates such as stainless steel and transparent conductive oxides, and is also sensitive to the spatial distribution of oxide precipitates in the films. This measurement is difficult or impossible using transmission or reflection IR spectroscopy.
Manipulation and Control of Nucleation and Growth Kinetics with Hydrogen Dilution in Hot-Wire CVD Growth of Poly-Si
Authors: Swiatek Maribeth , Holt Jason K. , Goodwin David G. , Atwater Harry A.
Published: April 2011

We systematically explore the relationship between gas-phase kinetics and film microstructure in the hot-wire CVD technique using diluted silane (1% in He) and additional hydrogen. Using a wire temperature of 2000°C, films were grown on Si (100) at 300°C using 1 mTorr SiH4 and 99 mTorr He at hydrogen pressures from 0-100 mTorr. Transmission electron microscopy and atomic force microscopy measurements indicated that continuous microcrystalline films had a columnar grain structure and that grain size increased from 40 nm using SiH4/He to 85 nm using SiH4/He/H2 with 20:1 H2:SiH4 ratio due to the etching of silicon by hydrogen. Etching rate measurements using a quartz deposition monitor show that, under the current deposition conditions, a transition from net film growth (0.17 nm/s using only SiH4) to net etching occurs at a H2:SiH4 ration of 80:1. The effect of atomic H on the nucleation density during the initial stages of growth has also been investigated, revealing a sublinear dependence of nucleation density with time and a decrease in nucleation density with increasing H2 dilution. High deposition rate growth with no H2 dilution has been achieved on a low-density array of seed nuclei produced using high H2 dilution.
A Case for Molecular Hydrogen Being the Mobile H Species in a-Si:H
Authors: Fedders P. A.
Published: April 2011

The mechanism (or mechanisms) of hydrogen transport in a-Si:H is of great intrinsic interest but is also important for the understanding of a number of phenomena such as light induced defects. We find that the literature includes many calculations for the activation energy for hydrogen diffusion that are at best misleading and often interpreted incorrectly. In this paper we summarize a number of calculations that apply to particles hopping in complex media and apply the results to a-Si:H. These calculations, coupled with existing estimates of the energy of various H configurations in a-Si:H, show that the standard picture is likely to be incorrect. However, previous work on the mobility of molecular hydrogen in crystalline Si coupled with some molecular dynamics simulations presented in this paper show that the mobile species of hydrogen in a-Si:H may well be molecular.
Photodegradation in a-Si:H Prepared by Hot-Wire CVD as a Function of Substrate and Filament Temperatures
Authors: Han Daxing , Yue Guozhen , Lin Jing , Habuchi Hitoe , Iwaniczko Eugene , Wang Qi
Published: April 2011

We have studied light-soaking effects, such as photoconductivity (PC) degradation kinetics, the changes of conductivity activation energy, Ea, and the defect density of states (DOS) in a-Si:H films deposited by hot-wire CVD. Films were deposited in a substrate temperature range from 280 to 440 °C for filament temperatures of 1900 and 2100 °C. We find that (a) the photodegradation kinetics does not follow the stretched exponential rule for all of the samples; (b) the Fermi level position moves up after light-soaking for most samples; and (c) the metastable defect DOS deduced from sub-band gap absorption is not consistent with that deduced from the electron mobility-lifetime product. The results are discussed according to the possible mechanism in which charged defects exist in hot-wire a-Si:H films.a
Device Quality Silicon Carbon Thin Films
Authors: Gemmer Christian , Schubert Markus B.
Published: April 2011

We attain good quality hydrogenated silicon carbon films grown by plasma-enhanced chemical vapor deposition. Similar to hydrogenated silicon, we observe a characteristic edge of crystallinity at medium hydrogen dilution ratios of the feedstock gases. In the transition regime between amorphous and nanocrystalline phase, our thin films exhibit a remarkable ratio of photocarrier mobility-lifetime product to dark conductivity of 105... 106 cm3A-1 and minimum light-induced degradation. The static index of refraction increases and the resonance energy decreases for films below the onset of crystallinity which points towards a higher compactness of the protocrystalline material. Hence, alloying of hydrogenated silicon with small amounts of carbon leads to the formation of SiC:H layers that feature an optical bandgap of 2.0 eV and simultaneously maintain the superior optoelectronic properties of protocrystalline silicon.
TEM and HREM study of silicon and platinum nanoscale ensembles in 3D dielectric opal matrix
Authors: Feoktistov N. A. , Golubev V. G. , Hutchison J.L. , Kurdyukov D. A. , Pevtsov A. B. , Schwarz R. , Sloan J. , Sorokin L.M.
Published: April 2011

In the present paper regular systems of silicon and platinum assemblies have been fabricated in a three-dimensional (3D) void sublattice of synthetic opal. The detailed TEM and HREM structure study of ‘opal-Si’ and ‘opal-Pt-Si’ composites was carried out. It was found that in regular composites ‘opal-Si’ the silica spheres were covered uniformly with a nanocrystalline silicon layer of up to 25-30 nm in thickness. To form the Pt-Si contact the silica spheres were coated with platinum layer before embedding silicon. The results obtained demonstrate a possibility of creating 3D multilayer semiconductor structure (p-n junctions, Schottky barriers etc.) on the inner surface of opal voids.
Poly-Si Thin Film Transistors Fabricated By Employing Selective Si Ion-Implantation And Excimer Laser Annealing
Authors: Lee Min-Cheol , Jeon Jae-Hong , Park Jin-Woo , Han Min-Koo
Published: April 2011

A new excimer laser annealing method is proposed in order to produce the poly-Si film with low defect density and large grain, by combining the selective Si ionimplantation and excimer laser annealing. Selective Si ion-implantation is employed to form artificial nucleation seeds in a-Si film prior to excimer laser annealing in order to increase the nucleation probability. The grain boundary location in poly-Si film has been controlled through implantation mask, and the grain size around micrometer order is obtained without any other process. TEM result shows that grain boundary is controlled according to mask pattern and the crystallinity of the poly-Si film is improved.
Light Intensity Exponents as Sensitive Tools for the Detection of Impurities in a-Si:H
Authors: Fonseca L. F. , Weisz S.Z. , Alpuim P. , Chu V. , Conde J.P. , Naides R. , Balberg I.
Published: April 2011

We have shown recently that the temperature dependence of the phototransport properties can yield information regarding the state distribution in the forbidden gap of semiconductors. Of these properties the light intensity exponents of both, the majority carriers, γe, and the minority carriers, γh, were found to be very sensitive to the details of this distribution. In particular, noting that sub 1/2 values of the exponents are very unusual we have studied their origin in some a-Si:H materials. Finding experimentally such sub 1/2 values of γh and running computer simulations of the recombination processes in a-Si:H led us to the conclusion that these low values are due to acceptor-like centers which have a relatively high capture coefficient for the holes. We attribute these centers to the unintentional oxygen doping in a-Si:H. We will show that the oxygen presence, usually ignored in the discussions of the phototransport properties of a-Si:H, appears to be, in many cases, the dominant factor in the properties of “intrinsic” a-Si:H.
Thin Film Transistors Made of Polysilicon Crystallized at 950°C on Steel Substrate
Authors: Wu Ming , Wagner Sigurd
Published: April 2011

We fabricated top-gate polycrystalline silicon thin film transistors on steel foil substrates with two processes: (1) Directly deposited n+ source/drain layers. (2) Ion-implanted n+ source/drains. The steel substrates enable reducing the crystallization time of amorphous silicon to polysilicon from 4 hours at 600°C to 20 seconds at 900°C. Thin film transistors were made from polysilicon channel material crystallized at 950°C for either 20 seconds or 20 minutes. The steel foil substrates were passivated with SiO2. The best thin film transistors to date have mobilities of above 30cm2/Vs in both the linear and saturated regions.
Thin Film Transistors with Electron Mobility of 40 cm2V−1s−1 Made from Directly Deposited Intrinsic Microcrystalline Silicon
Authors: Cheng I-Chun , Wagner Sigurd , Mulato Marcelo
Published: April 2011

We report top-gate n channel μc-Si:H TFTs with saturated electron mobilities up to 40 cm2V−1s−1 and ON/OFF ratios up to ~106. The μc-Si:H was grown from silane, dichlorosilane, and hydrogen. The glow discharge was excited at a frequency of 80 MHz to raise the growth rate to ∼1Å/sec, which is above that achievable with 13.56 MHz. Deposition temperatures were 230°C for the i-layer and 280°C (the highest temperature in the process) for the n+ source and drain layers. The TFTs were fabricated from 340-nm thick μc-Si:H films, and with a 300-nm thick gate insulator of plasma deposited SiO2.
Deposition of Heterogeneous Silicon Thin Films-Structure and Electric States-
Authors: Shimizu Isamu
Published: April 2011

Heterogeneous silicon thin films exhibited various microstructures were prepared by plasma enhanced(PE)-CVD from gaseous mixture of SiF4+H2 (SiH4) on glass substrate. Efficient parameters for controlling the microstructures were reviewed together with the way to grow crystalline seeds on glass by repeating the deposition and etching. Two step growth where polycrystalline silicon thin films were epitaxially grown on the seeds made on glass was proposed to accelerated the growth rate. The relationship between the microstructure and the carrier-transport properties are addressed, as well.