bluuebunny/crossref_metadata_2025 huggingface dataset:
The mechanism of sputter-induced orientation change in YBCO films on MgO (001)
Authors: Huang Y. , Vuchic B. V. , Baldo P. , Buchholz D. B. , Mahajan S. , Lei J. S. , Markworth P. R. , Merkle K. L. , Chang R. P. H.
Published: April 2011

The mechanisms of the sputter-induced orientation change in YBa2Cu3O7-x(YBCO) films grown on MgO (001) substrates by pulsed organometallic beam epitaxy (POMBE) are investigated by x-ray diffraction, Rutherford backscatter spectroscopy (RBS), cross-section TEM (XTEM) and microanalysis. It is found that the W atom implantation concurring with the ion sputtering plays an important role in effecting the orientation change. This implantation changes the surface structure of the substrate and induces an intermediate layer in the initial growth of the YBCO film, which in turn acts as a template that induces the orientation change. It seems that the surface morphology change caused by ion sputtering has only a minor effect on the orientation change.
Tailoring the Intergranular Phases in Silicon Nitride for Improved Toughness
Authors: Sun E. Y. , Becher P. F. , Plucknett K. P. , Waters S. B. , Hirao K. , Brito M. E.
Published: April 2011

Intergranular glass phases can have a significant influence on the fracture resistance (R-curve behavior) of silicon nitride ceramics and appears to be related to the debonding of the β-Si3N4/oxynitride-glass interfaces. Applying the results from β-Si3N4-whisker/oxynitride-glass model systems, self-reinforced silicon nitrides with different sintering additive ratios were investigated. Silicon nitrides sintered with a lower Al2O3:Y2O3 additive ratio exhibited higher steady-state fracture toughness together with a steeply-rising R-curve. Analytical electron microscopy studies suggested that the different fracture behavior is related to the Al content in the SiAlON growth band on the elongated grains, which could result in differences in interfacial bonding structures between the grains and the intergranular glass.
Interfacial Characterization with Spectrum-Lines and Multivariate Statistical Analysis
Authors: Anderson I. M. , Bentley J.
Published: April 2011

Changes in the electron energy-loss near-edge fine structure (ELNES) of the O-K and Co-L edges have been characterized in cobalt oxide in the vicinity of a boundary between periclase-(CoO) and spinel- (Co3O4) structured phases. The data were acquired with a conventional transmission electron microscope (CTEM) equipped with an imaging filter and operated in spectrum-line mode. Independent spectral features were identified by multivariate statistical analysis (MSA) of the raw data. The MSA of the spectrum-image acquired at the Co-L edge indicated a transition over ∼12 nm in the spectral features at the phase boundary. The variation of the ELNES at the O-K edge varied appreciably only in the interface region, whereas that at the Co-L edge also indicated subtle gradients in both phases adjacent to the interface area. These gradients are consistent with a change in the average oxidation state of the cobalt cations in the vicinity of the interface. A second independent component of the ELNES was also identified at the Co-L edge. Possible interpretations of this second component are discussed.
Self-Monitoring of Strain and Damage by Carbon Fiber Polymer-Matrix Composite
Authors: Wang Xiaojun , Chung D.D.L.
Published: April 2011

Self-monitoring of damage and dynamic strain in a continuous crossply (0°/90°) carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0° direction, R in this direction and R perpendicular to the fiber layers were measured. Upon tension to failure, R in the 0° direction first decreased (due to increase of degree of 0° fiber alignment) and then increased (due to 0° fiber breakage), while R perpendicular to the fiber layers increased monotonically (due to increase of degree of 0° fiber alignment and delamination). Upon cyclic tension, R (0°) decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. For a 90° unidirectional composite, R (0°) increased reversibly upon tension and decreased reversibly upon compression in the 0° direction, due to piezoresistivity.
Photo-Induced Polarization Recovery in PZT Thin Film Capacitors
Authors: Mansour S. A. , Rao A. V. , Bement A. L. , Liedl G.
Published: April 2011

Lead Zirconate Titanate (PZT) ferroelectric thin film capacitors were fabricated with metallic platinum and conducting Indium Tin Oxide (ITO) contacts. PZT thin films were fabricated using metallorganic decomposition (MOD) while a combination of MOD and RF-sputtering was used in fabricating the ITO-PZT-ITO capacitors. Photo-induced changes, manifested by an increase in switchable polarization, were studied before and after 108 switching cycles fatigue using white and monochromatic light. An increase in photo-induced changes was observed at 3.65eV light energies using monochromatic light using both capacitors. The increase was attributed to the excitation of electrons from PZT valence band into the conduction band causing an increase in film conductivity. However, polarization increase in Pt-PZT-Pt capacitor was more pronounced than ITO-PZT-ITO when white light was used. Some of the response in fatigued Pt-PZT-Pt capacitors was attributed to the excitation of electrons from the platinum Fermi level to oxygen vacancy sites trapped at the Pt-PZT interface by absorption of infrared radiation of white light. The latter observation implied a relationship between PZT fatigue and photo-induced effects.
Combination of Different Methods to Characterize Micromechanical Sensors
Authors: Buchhold R. , Büttner U. , Nakladal A. , Sager K. , Hack E. , Brönnimann R. , Sennhauser U. , Schroth A.
Published: April 2011

In the last years many micromachined sensors for measuring quite different quantities have been developed (e.g. [1][2]). The successful design of silicon micromachined sensors and their conversion into commercial products is still limited by the lack of full understanding of complex fault mechanisms. In order to detect those mechanisms it seems to be advantageous to investigate interrelated phenomena by different analysis methods. By combining results of several means the true nature of disturbing effects can be determined much more easily.

This paper describes a comprehensive approach for investigating failure mechanisms in piezoresistive pressure sensors. Sophisticated methods of signal analysis were combined with special semiconductor techniques, determination of thermo-mechanical properties, Finite-Element- (FE-) simulation and Michelson interferometry. By that it was possible to separate disturbing mechanical as well as electrical effects and their relation to sensor output and sensor accuracy, respectively. In particular, we discovered complex influences of passivation layer systems (differing in geometrical and technological parameters) on sensor accuracy.
Constitutive Equation of NiTi Superelastic Wire
Authors: Yang F. , Pu Z. J. , Wu K. H.
Published: April 2011

During the superelastic deformation process, because of the involvement of the austenite-martensite phase transformation, a superelastic wire will experience a self-heating or self-cooling process due primarily to the latent heat of the material. As the strain rate increases, and conditions become more adiabatic, the self-heating and self-cooling will cause a temperature rise upon loading and will drop upon unloading. As a consequence, an apparent effect of the strain rate on the superelastic behavior in the shape-memory alloys with a large diameter or more adiabatic conditions will be noticed. In the present paper, a constitutive stress-strain-strain rate equation is proposed to describe the self-heating behavior. In order to verify the model, a series of experiments have been conducted to study the effect of the strain rate, wire diameter, and adiabatic condition on the superelastic behavior of the shape-memory alloy wire. As will be shown later, the proposed equation can predict the behavior of the superelastic wire accurately, and the prediction is in good agreement with the experimental data.
The Two-Way Memory Effect by Pre-Straining in a 45Ni50Ti5Cu Alloy
Authors: Airoldi G. , Ranucci T.
Published: April 2011

The 45Ni50Ti5Cu shape memory alloy exhibits one single thermoelastic martensitic transformation from the parent B2 phase to monoclinic B19’and looks as an appealing system to be used for the two way memory effect (TWME). First results, appeared in literature, on the TWME in 45Ni50Ti5Cu indicate that in this alloy high figures for the two way strain can be obtained. The prestrain method is here investigated to imprint the TWME in a 45Ni50Ti5Cu alloy: a pseudoplastic deformation higher than the reorientation strain is applied in martensite, followed, after unloading, by a recovery of the one-way strain by heating to 110°C. The two way memory strain is detected in the following thermal cycle. Results show the two way memory strain can be imprinted in just one cycle, but it can even considerably be increased by repeating the procedure: the highest figure for the two way memory strain is however obtained whenever a plastic unrecoverable strain is accumulated in the specimen in some prestraining cycles. Often the maximum two way strain is of the same order as the plastic strain and the former decreases with the increase of the latter.
Actuator Applications with Single-Crystals of Cu-Zn-Al Shape Memory Alloy
Authors: Yawny A. A. , Sade M. , Lovey F. C.
Published: April 2011

In the present work the use of single-crystals of Cu-Zn-Al Shape Memory Alloys (SMA) in actuator applications is analyzed. The actuator considered here, a device capable of doing work in response to temperature changes, is based on a single-crystal nucleus of a Cu-Zn-Al SMA coupled to a conventional spring that represents the load to be displaced. A special experimental stage was designed for performing controlled thermal cycles under load. In this way the effects of different parameters (cycle number, friction, temperature range, load level) on the actuator behavior can be studied. From the results obtained, the use of a single-crystal of an SMA in a thermostatic device is analyzed and compared with the commercial wax actuator performance.
Stress Evolution During Crystallization and Isothermal Annealing of Titanium-Nickel on (100) Si
Authors: Zhang Jinping , Grammon D. S.
Published: April 2011

Thin films of near-equiatomic titanium-nickel are capable of thermally induced shape-strains and have recently been applied to silicon-based micromachined reversible actuators in force-production devices requiring energy-density levels substantially exceeding those available from piezoelectric, electromagnetic, electrostatic, or bimetal systems. Reversible actuation requires the presence of a resetting agent, or ‘bias’ force, capable of deforming the martensite phase during the exothermic transformation on cooling. Here, we show that both the initial martensite ‘programming’ force, and the bias force for subsequent reversible actuation, can readily be provided by manipulating intrinsic and extrinsic stresses developed during low-pressure sputter deposition of TiNi, and subsequent cooldown from either the deposition process temperature or from the crystallization annealing temperature. The stability of both intrinsic and extrinsic stresses at temperatures below the deposition temperature allows considerable flexibility in the design of the force-producing system, and reversible stress-production to levels beyond 0.7 GPa are shown to be readily achieved, corresponding to energy-density levels approaching 10 MJ-m−2. The present paper will review recent results from our group on the development of residual stresses in TiNi sputtered on (100) Si, using the wafer-curvature method applied during isochronal and isothermal vacuum annealing of both amorphous and crystalline thin films, with an emphasis on the combined influence of deposition temperature and annealing temperature on stress-relaxation rates, and on the response of the system to temperature cycling in the displacive transformation range.
The Electrical Resistance Properties of Shape Memory Alloys
Authors: Airoldi G. , Pozzi M. , Riva G.
Published: April 2011

Shape memory alloys are known for their ability to build up large deformations under an applied stress, either in martensitic phase or in the pseudoelastic region.

The electrical resistance of shape memory alloys, traditionally used to define the transformation temperatures, shows interesting features when considering its modification as a function of strain. The electrical resistance in NiTiCu ribbons, obtained by melt spinning, is here examined, either in martensite or in the pseudoelastic range or under a constant applied stress across the transformation range. In each examined case, the results obtained show that the electrical resistance follows a linear relationship as a function of the imprinted strain.
Shape Memory Thin Films of the System Ti- (Ni-Pd-Cu)
Authors: Quandt E. , Holleck H.
Published: April 2011

Free-standing shape memory thin films of the system Ti-Ni-Pd-Cu exhibiting the two-way-effect have been fabricated by d.c. magnetron sputtering onto unheated substrates followed by annealing and training processes. Their transformation temperatures were investigated by differential scanning calorimetry and electrical resistivity measurements. By the Ni-Pd substitution the transformation temperatures (austenite/martinsite finish temperature; Af/Mf) could be varied between 32°C/-38°C for the binary TiNi films to 570°C/498°C for the binary TiPd films. By the Ni-Cu substitution the transformation hysteresis (Af/Mf) could be reduced from 70°C for the binary film to 20°C for films with 10 at % Cu. A similar behavior was observed for TiNiPdCu films.
Radial, Planar and Helical Anisotropies Induced in Cylindrical Amorphous CoP Multilayers
Authors: Favieres C. , Caroca C. , López E. , Sánchez M. C. , Sánchez P. , Madurga V.
Published: April 2011

Electrolytic cylindrical amorphous CoP multilayers, grown on copper wires, with controlled continuos variation of the magnetic anisotropy, in both, direction and magnitude, have been produced. The inner zone of the samples has been grown to exhibit radial anisotropy and the overlayer has been obtained with planar longitudinal anisotropy. Our studies have revealed that the magnetization at the surface of the samples is strongly coupled with the bulk anisotropy for thickness of the overlayer of ≈ 1.5 μm. Furthermore, it has been found that an angular deformation applied to the samples, when the copper wire is subjected to a torsion, modifies the direction of the longitudinal anisotropy to the helical direction. Also, a large spontaneous Matteucci effect is induced by means of this angular deformation.
Magnetostrictive Composite Material Systems Analytical/Experimental
Authors: Duenas T. A. , Hsu L. , Cakman G. P.
Published: April 2011

Experimental and theoretical results are presented for a composite magnetostrictive material system. This material system contains Terfenol-D particles blended with a binder resin and cured in the presence of a magnetic field to form a 1–3 composite. Test data indicates that the magnetostrictive material can be preloaded in-situ with the binder matrix resulting in orientation of domains that facilitate strain responses comparable to monolithic Terfenol-D. Two constitutive equations for the monolithic material are described and a concentric cylinders model is used to predict the response of the composite structure. Experimental data obtained from the composite systems coincide with the analytical models within 10%. Particle size, resin system, and volume fraction are shown to significantly influence the response of the fabricated composite system.
Polarization and Piezoelectric Properties of a Nitrile Substituted Polyimide
Authors: Simpson Joycelyn , Ounaies Zoubeida , Fay Catharine
Published: April 2011

This research focuses on the synthesis and characterization of a piezoelectric (β-CN)-APB/ODPA polyimide. The remanent polarization and piezoelectric d31 and g33 coefficients are reported to assess the effect of synthesis variations. Each of the materials exhibits a level of piezoelectricity which increases with temperature. The remanent polarization is retained at temperatures close to the glass transition temperature of the polyimide.
Parameter Optimization of a Microfabricated Surface Acoustic Wave Sensor for Inert Gas Detection
Authors: Ahuja S. , DiVenere A. , Ross C. , Chien H. T. , Raptis A. C.
Published: April 2011

This work is related to designing, fabricating, and testing a surface acoustic wave sensor to be used for detecting metastable inert gases, particularly helium. The assembly consists of two microsensor configurations: (a) a reference device with no deposition at the delay line and (b) a sensing device with an Au-activated TiO2 e-beam-deposited thin film on the delay line. The interdigitated transducers and delay lines are fabricated by photolithography techniques on a single Y-cut LiNbo3 substrate oriented for Z-propagation of the acoustic waves. Variation in electrical conductivity of the Au-activated TiO2 film due to exposure to metastable He is translated as a frequency change in the assembly. Various characteristics of the surface acoustic microsensor have been studied to better understand and optimize the variation of acoustic wave velocity and the operating frequency of the microdevice. Methods for the TiO2 thin-film deposition are discussed.
Synthesis and Spectroscopie Analysis of Smart Photochromic Materials
Authors: Mo Yeon-Gon , Dillon R. O. , Snyder P. G.
Published: April 2011

Sol-gel processing was used to dope photochromic materials into metal alkoxide-polymer and pure polymer materials. The films on silicon and quartz substrates were examined, with and without UV irradiation, by UV/VIS spectroscopy, ellipsometry and FTIR.

The UV/VIS spectroscopy showed that the doped matrices were photochromic in the visible and near infrared.

The ellipsometric data were obtained with a variable angle of incidence spectroscopie ellipsometer (VASE). The Cauchy and a combined Cauchy-Lorentz model were used to fit the unirradiated and irradiated films, respectively. The optical constants of the films showed significant changes upon irradiation. This means that the absorption coefficient and hence the emissivity of the films is being modulated with UV irradiation. The VASE-fitted thicknesses of the films were in the range of 1 to 6 microns.

In the FTIR spectra, the spiropyran doped samples have shown IR transmission changes in the two spectral regions (6–7 μm and 7.5–8.5 μm) where changes are expected due to band opening. The transmission ratio for UV irradiated to unirradiated samples decreased by as much as about 24% at aparticular IR wavelength.
Local Viscoelasticity of Biopolymer Solutions
Authors: Schnurr B. , Gittes F. , Olmsted P. D. , Schmidt C. F. , Mackintosh F. C.
Published: April 2011

We describe a new, high-resolution technique for determining the local viscoelastic response of polymer gels on a micrometer scale. This is done by monitoring thermal fluctuations of embedded probe particles. We derive the relationship between the amplitude of fluctuations and the low-frequency storage modulusG′, as well as the relationship between the fluctuation power spectrum, measured between 0.1 Hz and 25kHz, and the complex shear modulusG((ω). For both, semiflexible F-actin solutions and flexible polyacrylamide (PAAm) gels we observe high-frequency power-law dependence in the spectra, which reflects the behavior of the shear modulus. However, we observe distinctly different scaling exponents forG((ω)in F-actin and PAAm gels—presumably due to the semiflexible nature of the actin filaments.
Front propagation in laser-tweezed lipid bilayer tubules
Authors: Olmsted Peter D. , Mackintosh Fred C.
Published: April 2011

We study the mechanism of the ‘pearling’ instability seen recently in experiments on lipid tubules under a local applied laser intensity. We argue that the correct boundary conditions are fixed chemical potentials, or surface tensions Σ, at the laser spot and the reservoir in contact with the tubule. While most qualitative conclusions of previous studies remain the same, the ‘ramped’ control parameter (surface tension) implies several new features. We also explore some consequences of front propagation into a noisy unstable medium.
Conformal Map on Rough Boundaries Application to Fracture
Authors: Roux Stéphane , Vandembroucq Damien
Published: April 2011

A conformai mapping technique is presented which allows to solve efficiently harmonic and bi-harmonic problems in semi-infinite domains limited by a rough boundary. This technique is applied to obtain the statistical distribution of flux and elastic stress in the vicinity of a self-affine boundary. This computation justifies the occurence of Weibull statistics for the strength of glass fibers, with Weibull modulus depending on the roughness amplitude. Another application concerns the determination of the local mode III stress intensity factor ahead of a rough crack. Extension of the method to surface stress-corrosion, and interfacial crack propagation are discussed.
A Model of Crack Propagation in a 2D Heterogeneous Material
Authors: Daguier P. , Bouchaud E. , Lapasset G.
Published: April 2011

A continuous numerical model of pure mode I crack propagation in a bidimensional heterogeneous material is presented. This model describes the propagation of a macrocrack into a two-phases brittle material containing a finite density of second phase precipitates. The morphology of cracks produced for various mechanical and microstructural conditions is analysed. It is shown that the simulated cracks are self-affine with a roughness index ≃ 0.6 independent of the microstructure. Relevant length scales, on the contrary, strongly depend on the microstructural parameters, and indicate an optimum density leading to a maximum fracture toughness.
Anomalous Relaxation in Disordered Media
Authors: Araujo Mariela , Gonzalez Orlando
Published: April 2011

Several random walk-algorithms are used to model relaxation processes on disordered structures. Substrates are regular lattices in d = 1,2 with disorder variables attached to each lattice site. For the simulations, we consider diffusing particles obeying different rules according to the transport process being studied. Relaxation description is done in the time domain by calculating the characteristic function F(k,t), and in the frequency domain through the study of the complex susceptibility χ(k,ω). Two types of relaxation mechanisms are seen both in simulations and in Nuclear Magnetic Resonance (NMR) experiments done on sandstones. These are expressed as stretched exponential forms of the relaxation function, and the Cole-Cole form of the imaginary part of the complex susceptibility. Longitudinal and transversal NMR measurements were done on fully saturated samples at 90 MHz. NMR relaxation data can be qualitatively understood using the random walk models proposed here.
Kinetics of Periodic Pattern Formation in Diffusion-Reaction in a Gel
Authors: Kuo C-S. , Bansil R. , Cabarcos E. L.
Published: April 2011

A video photographic study of the kinetics of the precipitation reaction Na2HPO4 + CaCl2 → CaHPO4 in agarose gels is presented. Periodic precipitation, known as Liesegang bands, are observed when one of the reactants is incorporated in the gel and the other allowed to diffuse in. The time evolution of the first Liesegang band's profile provides a direct confirmation of the supersaturation model, with a decrease in intensity as the monomer concentration is depleted and the formation of a broad plateau behind the sharp band from large particles left after the incorporation of the smaller mobile particles into the rapidly growing aggregate front. The time dependence of leading edge of the precipitate front agrees with the predictions of the reaction-diffusion-supersaturation model of Liesegang band formation.
Diffusion Limited Wetting
Authors: Steiner U. , Klein J.
Published: April 2011

We have measured the growth with time t of a wetting layer (of thickness l(t)) at the surface of a thin film of a binary liquid (polymer) mixture. Over a wide range of experimental parameters, our data is well described by a model of diffusion limited wetting which takes into account the finite film thickness. In this model, l(t) is a function of time which sensitively depends on the nature of the interfacial potential: a detailed comparison shows that long range van-der-Waals forces provide the main driving force for the build-up of the wetting layer.
Adsorption Monolayer Formation in Confined Geometry: Apparent Fractal Dimension
Authors: Qkunev A.G. , Aristov Yu.I.
Published: April 2011

Here we present experimental results on adsorption of fatty acids, aliphatic alcohols, and acetone on the mesoporous silica gels with various porosity. Conclusion has been drawn that fractal resolution analysis of the silica surface may lead to apparent values of the surface dimension. These data are suggested to be analyzed by a simple model taking into account adsorbed layer thickness and steric difficulties of monolayer formation in narrow pores.
Phase Transitions of Organic Fluids Confined in Porous Silicon
Authors: Faivre C. , Dolino G. , Bellet D.
Published: April 2011

This paper reports experimental data on both the solid-liquid and liquid-vapour phase transitions of an organic fluid confined in the pore network of porous silicon, by using respectively differential scanning calorimetry and X-ray diffraction. Due to the nanometric pore sizes of this material, the surface effects have a strong influence, shifting the transition parameters (lower melting temperature and lower condensation vapour pressure respectively). In particular, the effect of chemical dissolution on the pore size distribution of porous silicon layers has been investigated.
Electron Escape from a Quantum Well
Authors: Lavine James P. , Picker Harvey S.
Published: April 2011

The quantum mechanical escape rate is calculated for an electron in a one-dimensional potential well. First-order time-dependent perturbation theory is used for the bound-to-bound and the bound-to-free transitions. The bound-to-free transition probability decays exponentially with bound energy. The fraction of one-electron systems in a bound state decays exponentially with time. The characteristic time constant grows exponentially with an increasein the depth of the potential well.
Photophysics of High Concentration Systems at a Solution / Solid Interface: An Evanescentwave Study
Authors: Elliott J.A. , Rumbles G. , De Mello A.J. , Anderson H.L.
Published: April 2011

For many species the study of photophysical behaviour by fluorescence spectroscopy has been confined to low concentration as a result of the influence of the inner filter effect. In this paper we report how the total internal reflection technique of evanescent wavefluorescence spectroscopy (EWIFS) allows both the collection of distortion free steady-state fluorescence emission spectra and quantitative analysis of such species, in ‘bulk’ solution environment, to significantly higher concentration than can be achieved with a conventional ( right-angle ) geometry. A study of two species, the laser dye rhodamine 101 and anovel meso-substituted zinc prophyrin is presented.
Glass-Forming Liquids in Mesopores Probed by Solvation Dynamic and Dielectric Techniques
Authors: Yan X. , Streck C. , Richert R.
Published: April 2011

The orientational dynamics of organic supercooled liquids of low molecular weight confined to the geometry of porous glasses are studied by two highly related techniques, the optical method of probing the dynamics of solvation regarding a chromophoric host molecule and dielectric relaxation spectroscopy. The dielectric results display marked effects of the confinement to mesopores in terms of altered structural dynamics which appear to separate into a raster and slower responses relative to the bulk liquid. We also demonstrate that there is no trivial relation between the ε*(ω) data and the liquid dynamics in these heterogeneous samples. These effects are partially paralleled by the solvation dynamics results, but with the spatial range inherent in the optical technique being inconsistent with associating the fast and slow dynamical components to spatially distinct regimes. We conclude on the slow component being a signature of non-ergodicity which arises from the competition between the length scale of cooperativity and the pore size.
Scanning Probe Microscope Study of Mixed Chain-Length Phase-Segregated Langmuir-Blodgett Monolayers
Authors: Koleske D.D. , Barger W.R. , Lee G.U. , Colton R.J.
Published: April 2011

Using a scanning probe microscope, the influence of adhesion on measured film height of mixed-chain-length fatty acid Langmuir-Blodgett monolayers was investigated. A 1:1 mixture of CH3[CH2]22COOH and CH3[CH2]14COOH, was deposited in which the long chain-length acid, C24, was segregated from the short chain-length acid, C16. Two experiments were performed, contact scanning and force curve mapping. From both experiments, the film height difference measured between the C16 and C24 regions was larger than expected. Adhesion of the tip was 20% larger over the C16 acid regions. Using a compressional modulus derived from film pressure vs. area isotherms of the pure components, the adhesion difference and apparent filmheight difference over the two regions can be understood.
Polymer Brushes in Strong Shear Flow
Authors: Grest Gary S.
Published: April 2011

Polymers end-grafted to a surface in the presence of a shear flow are studied by molecular dynamics simulations. The solvent velocity field is observed to penetrate only a short distance into the brush consistent with predictions based on self-consistent field theory. The deformation of the brush is small except when the shear rate γ is very large. In this limit, while some of the polymer chains are stretched in the direction of flow, the brush height actually decreases slightly, in contrast to several theoretical predictions. When two surfaces bearing end-grafted chains are brought into contact, the normal force increases rapidly with decreasing plate separation, while the shear force is significantly smaller. For low relative velocityvwof the two walls, the surfaces slide pass each other with almost no change in the chain's radius of gyration or the amount of interpenetration, while for very largevw, there is significant stretching and some disentanglement of the chains. The results are in qualitatively good agreement with recent experiments using the surface force apparatus.
Reactivity and Migration of Hydrogen in A-SI:H
Authors: Biswas R. , Li Qiming , Pan B. C. , Yoon Y.
Published: April 2011

Tight-binding molecular dynamics calculations reveal a new mechanism for hydrogen diffusion in hydrogenated amorphous silicon. Hydrogen diffuses through the network by successively bonding with nearby silicon and breaking their Si-Si bonds. The diffusing hydrogen carries with it a newly created dangling bond. These intermediate transporting states are densely populated in the network and have lower energies than H at the center of stretched Si-Si bonds.
Role of Material Structure on Molecular Diffusion of Hydrogen in a-Si:C:H Films
Authors: Ullersma E. H. C. , Inia D. K. , Habraken F.H.P.M. , Van Sark W.G.J.H.M. , Van Der Weg W. F. , Westerduin K. T. , Van Veen A.
Published: April 2011

We used Fourier Transform Infra-Red (FTIR) analysis of bi-layers of plasma-grown hy-drogenated amorphous silicon-carbide films to investigate the role of the material structure in the hydrogen diffusion process. In the bi-layers one layer was deposited using CH4/SiH4 and in the other layer CD4/SiD4 was applied. The carbon concentration was 20 at.%. In previous work we showed, using Elastic Recoil Detection (ERD) and Thermal Desorption Spectrometry (TDS), that the hydrogen moves molecular through these films in the temperature range 325 < T < 450 °C [1]. Using FTIR we obtained information about the number of Si-H and Si-D bonds and their change upon annealing. The FTIR data indicate a structural change during annealing. A comparison with the TDS spectra led us to the conclusion that at higher temperatures the out-diffusion of hydrogen stops because of the hindrance of the molecular transport.
Growth Mechanism of Hydrogen Clusters
Authors: Nickel N. H. , Anderson G. B. , Johnson N. M. , Walker J.
Published: April 2011

It is demonstrated that the exposure of polycrystalline silicon (poly-Si) to monatomic hydrogen results in the formation of H clusters. These H stabilized platelets appear in the near-surface region ( 100 nm) and are predominantly oriented along {111} crystallographic planes. Platelet concentrations of ≈5×1015, 1.5×1016-cm−3, and 2.4×1017 cm−3 were observed in nominally undoped poly-Si, phosphorous doped poly-Si (P=1017 cm−3), and phosphorous doped single crystal silicon (P>3×1018 cm−3), respectively. Results obtained on doped c-Si demonstrate that platelet generation occurs only at Fermi-level positions of Ec -EF < 0.4 eV.
An NMR Investigation of H Cluster Configurations in A-SI:H
Authors: Todd Stephen J. , Rutland Jonathan M. , Han Daxing , Wu Yue
Published: April 2011

In this work the characteristics of hydrogen clusters in hot filament assisted CVD and conventional glow discharge a-Si:H films are discussed. Computer simulations of the observed free-induction decays of the 1H NMR signals indicate that the distribution of the nearest-neighbor distances between H atoms in the H clusters is quite narrow in hot filament assisted CVD a-Si:H whereas the distribution is larger in glow discharge a-Si:H. This is clear evidence of improved structural order in hot filament assisted CVD a-Si:H. The relaxed hydrogenated divacancy and multi-vacancy models reproduce the main features of the observed free-induction decay in hotwire a-Si:H very well. Computer simulations of the multiple-quantum NMR spectra indicate that a relaxed hydrogenated divacancy configuration leads to good agreement with experimental observations in device quality glow discharge a-Si:H. Results of simulations based on other H cluster configurations are also discussed. These results provide restrictions on the possible models for H clusters in a-Si:H.
Influence of Defects on Excess Charge Carrier Kinetics Studied by Transient PC and Transient PA
Authors: Feist H. , Kunst M. , Swiatkowski C.
Published: April 2011

By comparison of transient photoconductivity (TPC) and transient photoinduced absorption (PA) the influence of the density of states in the bandgap on excess charge carrier kinetics is studied for a-Si:H films deposited at different temperatures and for state of the art a-Si:H films in two different states of light soaking. In both series the rising deep defect density leads to an enhancement of electron trapping rather than recombination via deep defects. The samples deposited at temperatures lower than 250°C additionally show a lower effective electron mobility, i.e. a broader conduction band tail.
Space Charge Memory Effect in a-Si:H at low Temperatures
Authors: Heck S. , Stradins P. , Fritzsche H.
Published: April 2011

Space charges build up near one or both electrodes of a photoconductor unless the thermoionic current balances the photocurrent. Space charges built up also when a voltage U is applied at low T to a semiconductor containing a non-equilibrium distribution of carriers. The presence of a space charge is observed as a relaxation-current transient when the sample is illuminated at zero bias. If one type of carrier is immobile the steady state photocurrent becomes zero.
Electronic Properties and Device Applications of Hot-Wire CVD Polycrystalline Silicon Films
Authors: Middya A. R. , Guillet J. , Brenot R. , Perrin J. , Bouree J. E. , Longeaud C. , Kleider J. P.
Published: April 2011

Polycrystalline silicon films (5 to 30 μm thick) have been deposited on glass substrates at low temperatures (400–550 °C) with a rate of 15 Å/s by hot-wire chemical vapour deposition (HWCVD). The homogeneity of the deposited layer is ±5% on a 8 cm diameter substrate. The films have columnar microstructure and a textured surface. The undoped films (carrier concentration, 1011 cm−3) have a resistivity of 105-106 Ω-cm, activation energy of 0.50 ± 0.05 eV and Hall mobility of 14 ± 4 cm2 /V.s. By in situ gas phase doping, resistivity can be varied by six to seven orders of magnitude. Incorporation of dopant atoms such as boron into the film, strongly influences its morphological and crystallographic structure. The mobility lifetime product of undoped films is low (10−8 cm2/V), possibly due to the presence of a high density of dangling bonds defects and broad band-tails. This product can be improved by a factor of 5 to 10 by using in-situ hydrogen passivation in the same reactor at lower temperature (350–400 °C) within one hour. The results of many complementary experiments suggest that hydrogen treatment mainly improves carrier mobility by a factor of 3 to 4 by passivating extended defects. Preliminary results on application of these types of materials in unoptimized P-I-N solar cells on c-Si and glass substrates are presented.
Electronic Properties of Microcrystalline Silicon
Authors: Carius R. , Finger F. , Backhausen U. , Luysberg M. , Hapke P. , Houben L. , Otte M. , Overhof H.
Published: April 2011

The electronic and optical properties of microcrys tall ine silicon films prepared by plasma enhanced chemical vapour deposition are investigated with Hall-effect, electrical conductivity, photothermal deflection spectroscopy and photoluminescence measurements. In particular, the influence of the grain size and the crystalline volume fraction on the conductivity, the carrier density and the Hall mobility is investigated in highly doped films. A percolation model is proposed to describe the observed transport data. Photoluminescence properties were studied in un-doped films. It is proposed that the photoluminescence is due to recombination at structural defects similar to those observed in crystalline silicon.
Electroabsorption Spectra of Hydrogenated Amorphous and Microcrystalline Silicon
Authors: Jiang Lin , Schiff E. A. , Finger F. , Hapke P. , Koynov S. , Schwarz R. , Wyrsch N. , Shah A. , Yang J. , Guha S.
Published: April 2011

We report on electroabsorption spectra for plasma deposited thin films of hydrogenated silicon ranging from amorphous (a-Si:H) to microcrystalline (μc-Si-H) structures. The EA spectrum of a-Si:H deposited from silane with low hydrogen dilution were consistent with previous works; material prepared with high hydrogen dilution showed a 0.07 eV blue shift of the spectrum and somewhat stronger electroabsorption. μc-Si-H specimens have a sharp peak at 1.19eV; the spectrum is blue shifted by 0.03 eV and is significantly stronger than electroabsorption reported in single crystal silicon. Spectral features which have no correspondence to single crystal silicon were also observed in μc-Si-H. Specimens deposited using “cyclic” deposition and chemical annealing had electroabsorption spectra with both the 1.19 eV, crystalline feature and a band peaking at 2.02 eV which we attribute to strongly hydrogenated a-Si:H. We discuss applications of electroabsorption to determining the crystal fraction of microcrystalline material and to determining grain size distributions.
Injection Doping of Ultrathin Microcrystalline Silicon films Prepared by CC-CVD
Authors: Koynov S. , Grebner S. , Schwarz R. , Vassilev L. , Sieber I. , Schmidt M. , Fuhs W.
Published: April 2011

Recently, we have proposed a cyclic method, referred to as Closed Chamber CVD (CC-CVD), for the preparation of μc-Si films of high crystalline fraction at increased deposition rates. In this work we first report new process conditions of CC-CVD, which result in growth of highly crystalline films with a sharp interface on a foreign substrate. Then these conditions are further used together with a pulsed injection of B2H6 in an appropriate moment of each cycle, so that the disturbance of the crystallization process is prevented. A series of ultrathin μc-Si films, doped by this technique, is characterized by conductivity measurements, SEM, Raman Scattering, optical transmission and UV reflection. A strong reduction of the transient interface layer is achieved and conductivity as high as 2 S/cm
 with an activation energy of 27 meV is reached.
A Chemical Bonding Model for Photo-Induced Defects in Hydrogenated Amorphous Silicon (a-Si:H): Intrinsic and Extrinsic Reaction Pathways
Authors: Lucovsky G. , Yang H.
Published: April 2011

In device grade a-Si:H photo- or light-induced defect generation is an intrinsic effect for impurity concentrations of oxygen and nitrogen below about 1019 to 1020 cm−3; however, at higher concentrations it increases with increasing impurity content. Charged defect configurations are identified by empirical chemistry and are studied by ab initio calculations. This paper addresses: i) the chemical stability of charged defects, ii) the reaction pathways for defect metastability, and iii) the transition between extrinsic and intrinsic behavior.
Control of Enhanced Optical Absorption in μc-Si
Authors: Kaan Kalkan A. , Fonash Stephen J.
Published: April 2011

The influence of grain size on the enhanced optical absorption of μc-Si has been investigated using films of various grain sizes prepared by solid phase crystallization. We show that we can control this grain size and therefore the degree of absorption changes. For grain sizes below a threshold range significant absorption enhancement can be seen in the photon energy range of 1 to ∼3 eV and the absorption characteristics of these films show that the dominant mode of optical transitions is indirect. A correlation between first order Raman peak broadening and enhanced absorption was found suggesting both effects are related to confinement. A simple model was developed to see how confinement in the crystallites could influence indirect optical transitions.
Deposition and Characterization of Polycrystalline Silicon Films on Glass for thin Film Solar Cells
Authors: Bergmann R. B. , Krinke J. , Strunk H. P. , Werner J. H.
Published: April 2011

We deposit phosphorus-doped, amorphous Si by low pressure chemical vapor deposition and subsequently crystallize the films by furnace annealing at a temperature of 600°C. Optical in-situ monitoring allows one to control the crystallization process. Phosphorus doping leads to faster crystallization and a grain size enhancement with a maximum grain size of 15 μm. Using transmission electron microscopy we find a log-normal grain size distribution in our films. We demonstrate that this distribution not only arises from solid phase crystallization of amorphous Si but also from other crystallization processes based on random nucleation and growth. The log-normal grain size distribution seems to be a general feature of polycrystalline semiconductors.
Diffusion and Effusion of Hydrogen in Microcrystalline Silicon
Authors: Beyer W. , Hapke P. , Zastrow U.
Published: April 2011

The diffusion and effusion of hydrogen in hydrogenated microcrystalline silicon films deposited in an electron cyclotron resonance reactor were studied for various deposition temperatures Ts. For deposition temperatures below 250°C, hydrogen effusion is found to be dominated by desorption of hydrogen from internal surfaces followed by rapid out-diffusion of H2. Higher substrate temperatures result in an increased hydrogen stability suggesting the growth of a more compact material. For this latter type of samples, a hydrogen diffusion coefficient similar as in compact plasma-grown a-Si:H films is found despite a different predominant bonding of hydrogen according to infrared absorption.
Electrical and Optical Properties of Nano-Crystalline GaN and Nano-Crystalline GaN:H thin Films
Authors: Kobayashi S. , Nonomura S. , Abe K. , Gotoh T. , Hirata S. , Nitta S. , Kanemitsu Y.
Published: April 2011

Nano-crystalline GaN (nc-GaN) and hydrogenated nc-GaN (nc-GaN:H) thin films and thin film transistors (TFT) prepared by a reactive sputtering method have been studied. Hydrogen incorporation in nc-GaN film induces localized states at mid-gap energy. Thermal annealing at 400 °C and 600 °C creates mid-gap states which is detectable by electron spin resonance. Further thermal annealing treatment at 800 °C reduces the deep states in nc-GaN and nc-GaN:H. Photoluminescence spectra of the nc-GaN film have two broad peaks at 2.4 eV and 3.2 eV. The source-drain current voltage characteristics of the nc-GaN TFT is demonstrated for the first time. The obtained field effect mobility is 10−4 cm2/V-s. Thermal annealing at 800 °C improves the field effect mobility to 10−2 cm2/V-s.
Recrystallization of Amorphous Silicon Deposited on Ultra Thin Microcrystalline Silicon Layers
Authors: Wang F. , Wolfe D. , Lucovsky G.
Published: April 2011

This study reports on a method to reduce the thermal crystallization time and temperature of amorphous silicon films by initially depositing an ultra thin μc-Si:H seed layer. After rapid thermal annealing (RTA), films were characterized by means of Raman spectroscopy, x-ray diffraction, reflection high energy electron diffraction, atomic force microscopy, and dark and photocurrent. The results show that the microcrystalline particles in the seed layer act as nucleation centers, promoting crystallization of a-Si:H at lower temperatures and at shorter times, compared to a-Si:H films deposited without any seed layer. Additionally, it was found that the seed layer affects the orientation of the crystallized films. The dark current increases abruptly over 4 orders of magnitude in the first 15 second anneal, then decreases as the time increases, and tends to saturate. The photocurrent has an opposite behavior. These transport results can be understood in terms of a change in defect density and band gap shrinkage.
High Rate Deposition of a-SiNxH by VHF PECVD
Authors: Takagi T. , Nakagawa Y. , Watabe Y. , Takechi K. , Nishida S.
Published: April 2011

Very High Frequency (VHF) has been applied to the plasma enhanced chemical vapour deposition (PECVD) of hydrogenated amorphous silicon nitride films (a-SiNx:H) to fabricate amorphous silicon (a-Si:H) thin film transistors (TFTs). Especially, the effect of the excitation frequency on the deposition rate and the film quality of a-SiNx.H deposited in a SiH4/NH3/N2 plasma has been investigated. The films were prepared by VHF (40 MHz and 60 MHz) and HF (13.56 MHz) plasma enhanced CVD.

The optical bandgap, the hydrogen content, the Si-H/N-H ratio and TFT mobility for films deposited in VHF plasma did not change significantly with the increase in deposition rate up to 300 nm/min. Internal stress could be constrained to acceptable levels at very high deposition rates. In contrast, the film quality deteriorated with an increase of the deposition rate in HF plasma. There seems to be a parallel relation between the optical emission intensity and the deposition rate which depends on the excitation frequency.