bluuebunny/crossref_metadata_2025 huggingface dataset:
Cooling Rate Dependence of Fatigue Life Time (Nf) of Cu-Al-Ni Alloy Prepared by Liquid-Quenching
Authors: Mori K. , Tanaka Y. , Furuya S. , Okada T. , Komatsuzaki K. , Nishi Y.
Published: April 2011

Since the fatigue life time is a serious problem of the Cu-Al-Ni alloy, it is difficult to apply for practical use. The authors have investigated the influence of the cooling condition of liquid-quenching on fatigue property. The liquid-quenching is performed by a piston-anvil apparatus. It is easy to control the solidification condition. The fatigue life time increases with increasing the cooling rate. The long life time is due to the small grain size, the small volume of γ1' phase, the low density of lattice defects and the low transformation temperature.
Enhanced Mechanical Properties of Smart Polymer-Matrix Hybrid Composite by Shape Memory Effect
Authors: Izui H. , Hamada K. , Ogawa K. , Taya M. , Inoue K.
Published: April 2011

New smart hybrid composite with TiNi shape memory alloy (SMA) fiber as reinforcement is designed to be composed of TiNi fiber and carbon/epoxy (CFRP) composite. One-dimensional composite modeling is utilized to evaluate residual stresses of the smart composite generated by shape memory effects. The modeling indicated that the tensile strength of TiNi/CFRP composite is improved due to the shape memory effect of TiNi fiber for both parallel and perpendicular configurations of TiNi fiber with respect to the direction of carbon fiber. Modeling also shows the tensile strength is increased with increasing both volume fraction of TiNi fiber and amount of prestrain.
Computational Models for Multicomponent Diffusion in Polymeric Materials
Authors: Rossi G. , Samus M. A.
Published: April 2011

Situations where a polymeric material is exposed to a solvent mixture so that the different components within the mixture can diffuse into the polymer are common both in industrial applications and in biological processes. Often one of the components is taken up preferentially and its presence affects the diffusion properties of the remaining components. The problem of accounting for processes of this type has not been dealt with in a systematic way. This may in part be due to the difficulty of characterizing experimentally the separate diffusion behavior of the various components: data of this kind are now becoming available for simple binary mixtures. In order to model this class of problems, a lattice model involving a polymer matrix (M) and two diffusing components (A and B) has been introduced. The Monte Carlo evolution of the system has been examined for different values of the local A–M, B–M and A–B interactions. These results shed light on the microscopic origin of selective uptake.
Scaling for the Coalescence of Microfractures before Breakdown
Authors: Zapperi S. , Ray P. , Stanley H. E. , Vespignani A.
Published: April 2011

We study the behavior of fracture in disordered systems close to the breakdown point. We simulate numerically both scalar (resistor network) and vectorial (spring network) models with threshold disorder, driven at constant current and stress rate respectively. We analyze the scaling of the susceptibility and the cluster size close to the breakdown. We observe avalanche behavior and clustering of the cracks. We find that the scaling exponents are consistent with those found close to a mean-field spinodal and present analogies between the coalescence of microfractures and the coalescence of droplets in a metastable magnetic system. Finally, we discuss different experimental conditions and some possible theoretical interpretations of the results.
Ac Response of Heterogeneous Materials: A Numerical Study
Authors: Rodriguez A. A. , Valbuena J.
Published: April 2011

The ac and dc electrical properties of composite materials are studied using hierarchical lattices. First we show that the hierarchical model can correctly account for the main scaling properties of critical percolative structures. Then we study the effect of potential disorder by assuming that the microscopic conductances are distributed according to a power law distribution function. We find that in the limit of strong disorder, the predictions are in qualitative agreement with reported experimental measurements.
Influence of sheet electron beam irradiation (SEBI) on water wettability of hydroxy apatites for artificial bone
Authors: Kawano J. , Izumi H. , Oguri K. , Tonegawa A. , Kawai T. , Ochi M. , Nishi Y.
Published: April 2011

An influence of sheet electron beam irradiation (SEBI) on the wettability is investigated of the hydroxy apatite (HAP) [Ca10(PO4)6(OH)2]. The wettability is one of the important factors to control bio-compatibility. The SEBI is homogeneously performed by an electrocurtain processor. The temperature of the sample is below 323 K just after the irradiation. The wettability is evaluated by measuring the wet angle θ in a drop of water. The SEBI increases the wettability. Based on rate process, the influence of SEBI on wettability is discussed. Using the SEBI, we can precisely control the surface condition of HAP.
Adhesion of Soft Biological Shells controlled by Bending Elasticity and Macromolecular Networks
Authors: Simson R. , Albersdörfer A. , Sackmann E.
Published: April 2011

We present an interferometrie technique allowing reliable measurements of bending modulus κ, membrane tension Σ and adhesion energy W of cells crawling on substrates. All three parameters are important for cell locomotion and reflect the local balance of attractive and repulsive forces between cell and substratum as well as the internal coupling of cell membrane and the underlying cytoskeleton. Mutants of the Dictyostelium ameba lacking an important cytoskeletal protein, Cortexillin, exhibited a markedly reduced bending modulus and adhesion energy as compared to wild type Dictyostelium. In addition, experiments with model membrane systems suggest that the combination of attractive and repulsive forces results in a local clustering of receptors mediating cell adhesion.
Driven Granular Media and Dissipative Gases: Correlations and Liquid-Gas Phase Transitions
Authors: Williams D.R.M.
Published: April 2011

We study a simple model of a granular material or powder where the particles are excited by an external noise source and dissipate energy by inelastic collisions. Due to the inelastic collisions between particles there is an effective interaction between them. In one dimension this leads to long-range correlations between the particles in a gas phase despite the absence of long-range forces between the particles. In two dimensions the dissipative effects cause a very sharp liquid-gas phase transition at which the susceptibility has a pronounced peak.
Separation of DNA Using Ferrofluid Array Electrophoresis
Authors: Mayer Pascal , Bibette Jerôme , Viovy J. L.
Published: April 2011

We present a new method for electrophoretic separation of DNA, Ferro fluid Array Electrophoresis (FAE). The method uses a stabilized suspension of an hydrophobic ferrofluid in aqueous buffer as the separating medium. When this suspension is placed in a slab cell and submitted to a magnetic field perpendicular to the slab plane, it organizes into a regular array of columns with micron-sized spacing. DNA migrating in this maze leads to size-fractionation. Resolution of lambda phage (48.5 kbp) and T4 (140 kbp) DNA molecules in 30 mn is achieved. The motion of individual DNA molecules during FAE is observed using fluorescence videomicroscopy, and the molecular mechanisms responsible for separation are discussed in the light of recent computer simulations. During migration, large DNA molecules temporarily wrap around the impenetrable ferrofluid columns. They disengage by slippage, like a rope on a pulley, and the dependence of the disengagement time upon DNA size is responsible for the size-fractionation.
DNA End-to-End Distance Change Due to Divalent Counterion Condensation Studffid by Pulse Gel Electrophoresis
Authors: Li Anzhi Z. , Huang Haiyan , Marx Kenneth A.
Published: April 2011

The conformation change of DNA fragments due to divalent counterion condensation onto DNA was investigated by pulse gel electrophoresis, and interpreted by gel models (Reptation and Henry model) and Manning's counterion condensation theory. The measured mobility reductions μ/μ0 of λ-DNA-Hind III fragments, ranging from 23.13 to 2.027 kilobase pairs, due to interaction with divalent cation Mg2+ (1–400 μμ), and Ca2+(0–40 μM) in tris-borate buffer were well fit by Manning's Counterion Condensation (CC) theory. We observed the normalized mobility reduction to be shifted by a small amount Δ(μ/μ0) relative to the CC prediction value. Δ(μ/μ0) is a function of DNA length, and the ion environment (divalent concentration C2 and ionic strength). The ‘shift’ phenomena only occurred close to where C2 began dominating the counterion binding, a condition described by the monovalent/divalent cation isocompetition point. Combining our observation with theoretical considerations, we conclude that the divalent counterion condensation changes the DNA fragments' conformation, resulting in an end-to-end distance decrease which is molecular weight dependent. The effect was enhanced by an increase of divalent ion concentration and a decrease of the ionic strength.
A Study of Impacting Droplets of an Emulsion or Surfactant Solution on Solid Substrates
Authors: Vignes-Adler M. , Prunet-Foch B. , Legay F. , Mourougou N.
Published: April 2011

Coating of solid surfaces with uniform, thin liquid films occurs in many industrial processes. A common process consists of spraying some liquids or emulsions on a freshly created solid surface. In this context, we have investigated the impact of a single droplet of various emulsions and surface-active solutions on a solid substrate using a high frequency fluorescent visualization technique (1 picture every 0.25 ms). Whatever the materials in presence, the drop spreads and then retracts under the action of inertia and capillarity respectively. Inertia induces spreading and generates a peripheral rim which is unstable to fingering. Then contact line instabilities appear under the form of festoons with pure liquids which are damped with surface-active solutions and amplified with emulsions. When the adsorption kinetics of the surfactant is slow, the equilibrium surface tension is not restored during the duration of the experiment and the drop can bounce back.
Hysteresis Phenomena in Nanoconfinements: Density Functional Theory and Experiments
Authors: Neimark A.V. , Ravikovitch P.I.
Published: April 2011

Hysteresis phenomena are observed in many nanoporous environments during adsorption/desorption isothermal quasiequilibrium cycles. A non-local DFT model has been developed for predicting adsorption/desorption isotherms in nanopores of different geometries in the wide range of pore sizes (0.5–10 nm) based on given intermolecular fluid/fluid and fluid/solid potentials. Depending on the confining geometry and the intermolecular potentials two types of hysteresis phenomena are occurred: capillary condensation/evaporation that implies volume filling of pores with a liquid-like matter, and layering adsorption/desorption that implies sequential step-wise formation of adsorption layers. It is shown that the DFT model qualitatively describes these phenomena and is in a reasonable quantitative agreement with some of the experiments.
Structure and Phase Transitions in Partially Confined Smectic Liquid Crystals
Authors: Martinez-Miranda L. J. , Shi Yushan , Kumar Satyendra
Published: April 2011

We present the results of an X-ray scattering study on partially confined smectic-A (layered) LC films. This partial confinement is achieved by placing the LC material inside the open grooves of a glass grating. Samples prepared in this manner are confined in the direction perpendicular to the gratings. Samples in which the LC is contained entirely inside the grooves develop an induced molecular tilt, which results in a compression of the smectic layers. The molecular tilt varies as the amount of the LC material increases, eventually forming a thin overlayer film above the gratings. As the thickness varies, a second region develops in the films. The layer spacing in this region is close to the bulk layer spacing. This structural evolution is coupled to a variation in the nematic-to-smectic-A phase transition temperature of the samples from the bulk. In addition, the nature of the phase transition is driven first order. The effects of partial confinement on a sm-C* LC film is discussed briefly.
Translational Diffusion of Liquids Confined in Nanopores
Authors: Korb J.-P. , Whaley M. , Bryant R. G.
Published: April 2011

We present a theory of nuclear spin relaxation and experiments to probe the microdynamics of polar solvents at the surface of calibrated microporous glass beads. The theory is tested using the magnetic field dependence of the proton spin-lattice relaxation rate recorded with a field-switched magnetic relaxation dispersion spectrometer.
Broad-Band Dielectric Spectroscopy of Liquid Crystals Confined in Random and Cylindrical Pores
Authors: Sinha G.P. , Aliev F.M.
Published: April 2011

Using dielectric spectroscopy in the frequency range 0.1 Hz-1.5 GHz, we investigated the influence of confinement on the dynamic properties of polar nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores as well as in parallel cylindrical pores with different, pores sizes. The confinement has a strong influence on the dielectric properties of LC which resulted in the appearance of a low frequency relaxational process (f≤ 10 KHz) absent in bulk and a strong modification of modes due to the molecular rotation around short axis and librational motion. The differences between bulk and confined behavior are: (a) - the dielectrically active modes in confined LC are not frozen even at temperatures about 20 degrees below the bulk crystallization temperature; (b) - in the temperature range corresponding to the anisotropie phase in pores,lnτ, where τ is the relaxation time corresponding to the molecular rotation around short axis, is not a linear function of 1/T and there is an evidence for smectic type order formation at sufficiently low T; (c) - the retardation factorg= τ/τisis ≃ 1.5, where as the typical value ofgin bulk nematic liquid crystals is ≃ 4; (d) - smooth and small changes inTat phase transition in pores suggest that the “isotropie” phase of LC in pores is not bulk like isotropie phase with complete disorder in molecular orientations, and some degree of orientation order still persists.
Relaxation Phenomena and Thermodynamics of Liquids at Very High Pressures
Authors: Oliver W. F.
Published: April 2011

Complex liquid glass-forming systems ranging from those composed of simple molecules to polymer melts and amorphous polymers have been studied extensively as a function of temperature resulting in a basic understanding of liquid-state dynamics and glass transition phenomenology as these systems are supercooled to the vitreous state. An important aspect of this problem that remains largely unexplored, and that is relevant to the topic of this symposium, involves liquid-state dynamics and vitrification (as well as crystallization) in the regime of high pressure and high density. We describe work on “fragile” to “intermediate strength” simple organic glass-forming liquids where both temperature (T) and pressure (P) are varied. Diamond anvil cells are used to achieve pressures exceeding 10 GPa. Several optical and light scattering techniques are used to explore both static and dynamic properties of these systems. High-pressure Brillouin scattering enables us to model the longitudinal relaxation time in these systems as well as their equations of state. These can now be refined by direct measurements of the pressure dependence of the glass transition, Tg(P). Finally, we summarize depolarized light scattering studies which allow us to compare both the isobaric and isothermal evolution of structural (α) and fast (β) relaxation processes.
Temperature-Dependent Dynamics of Microconfined Cs2
Authors: Loughnane B. J. , Farrer R. A. , Deschenes L. A. , Fourkas J. T.
Published: April 2011

Femtosecond optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) has been used to study the reorientational dynamics of CS2 in microporous glasses over a wide range of temperatures. Microconfinement is shown to affect the behavior of the liquid at all the temperature studied. The results are interpreted in terms of a two-state model of the confined liquid.
Instability and Frontal Breakup in Super-Meniscus Films
Authors: Kataoka Dawn E. , Troian Sandra M.
Published: April 2011

Surface tension gradients created by a nonuniform temperature distribution in athin liquid film can force vertical spreading beyond the equilibrium meniscus [1]. Experiments designed to probe the flow behavior of super-meniscus films have shown that the leading edge can either spread uniformly with complete surface coverage or become corrugated and breakupinto long slender rivulets. We show that within linear stability analysis, both the conditions for unstable flow and the most unstable wavelength compare favorably with recent experiments reported in the literature.
NMR Experiments on Molecular Dynamics in Nanoporous Media: Evidence for Lévy Walk Statistics
Authors: Kimmich Rainer , Zavada Tatiana , Stapf Siegfried
Published: April 2011

Field-cycling 1H and 2H NMR relaxometry and field-gradient 1H NMR diffusometry were applied to polar and nonpolar liquids filled into porous glasses and fineparticle agglomerates (SiO2, ZnO, TiO2, globular proteins). The orders of magnitude of the length scales of the pore spaces ranged from 100 to several 102 nm. Pronounced differences of the spin-lattice relaxation dispersion for “weak” (nonpolar) and “strong” (polar) adsorption were found. In the latter case, the correlation times of the adsorbate orientation are up to eight orders of magnitude longer than in bulk. Trans-lational diffusion in liquid surface layers was directly studied with the aid of field-gradient NMR diffusometry in systems where the free liquid was frozen. The spin-lattice relaxation dispersion can be explained on the basis of reorientations mediated by translational displacements (RMTD) of adsorbate molecules on the surfaces. Thisprocess appears to be enhanced by a Levy walk mechanism so that the propagator adopts the form of a Cauchy distribution. The evaluated surface correlation functions are characterized by surface correlation lengths in the same order as the pore diameters, that is, up to three orders ofmagnitude larger than the length scale of dipolar interaction.
Gas Transport in Sol-Gel Derived Porous Carbon Aerogels
Authors: Reichenauer G. , Fricke J.
Published: April 2011

Due to their high electrical conductivity, their large specific surface area and their high porosity sol-gel derived nanoporous carbons are promising materials for electrodes, e.g. in water desalination systems or fuel cells. In order to optimize their properties with respect to these applications information is needed about transient and steady state transport through the interconnected pores.

Dynamic gas expansion and time resolved permeation measurements allow to determine the relevantquantities, i.e. the permeability, the ratio of gas phase to surface diffusion and the volume of dead end pores along with the tortuosity.

Experimental data on nanoporous carbons of different density are presented. All samples investigated were prepared via pyrolysis of resorcinol formaldehyde aerogels. The measurements were performed with different gases below 0.1 MPa.
Spectroscopie Studies of Confined Molecules Under Shear
Authors: Soga Iwao , Dhinojwala Ali , Cho Yoon-Kyoung , Granick Steve
Published: April 2011

We introduce two new techniques with the sensitivity to measure spectroscopie properties of confined molecules under shear. One concerns rheo-optics of fluids in micron-sized gaps. The experimental approach consists in applying periodic shear motion to fluid films confined between parallel plates separated by controllable film thickness from 0.1 to 100 micrometers. A key aspect is the concurrent ability to perform spectroscopie experiments (infrared or dielectric spectroscopy, or x-ray or neutron spectroscopy) of these fluids during flow.

The second technique consists in flow field experiments of polymer adsorption and orientation. The experimental approach is to produce one-directional shear, at rates up to 104 sec−1, past adsorbed polymer layers of thickness less than 0.01 micrometers. The selective augmentation of vibrational modes oriented preferentially along the chain backbone is then used to probe chain orientation in the direction of flow in a laminar flow field.
Search for Explaining the Staebler-Wronski Effect
Authors: Fritzsche H. , Az Tucson
Published: April 2011

For twenty years we searched to understand the Staebler-Wronski effect (SWE). New results continue to emerge which invalidate prior interpretations. Recent evidence shows that the SWE is not associated with impurities. Long-range hydrogen diffusion is ruled out because the SWE occurs with comparable efficiency between 400K and the lowest temperatures. Nonradiative geminate recombinations might be important since high fields reduce the SWE significantly. It disappears when the bandgap or the photon energy falls below a critical value. The creation of a metastable density of dangling bond defects has been considered to be its sole manifestation. However, there is mounting evidence for light-induced structural changes which extend throughout the material. The weak bond breaking model emerges as the only viable explanation of the SWE if the expected spatial correlation between defects and hydrogen is destroyed by subsequent recombination events. The SWE is reduced by a favorable microstructure and low hydrogen content. It is suggested that defect pairs have larger recombination coefficients than isolated defects.
Identification of the Dominant Electron Deep Trap in a-Si:H: A Critical Test of the Defect Pool vs. Defect Relaxation Pictures
Authors: Kwon Daewon , David Cohen J.
Published: April 2011

Modulated photocurrent (MPC) measurements in intrinsic a-Si:H reveal a prominent band of electron deep traps with a thermal emission energy near 0.6eV. We have identified this defect band by directly comparing MPC and ESR spectra for both an intrinsic and a lightly n-type doped sample for a various metastable states such that the Fermi level, EF, ranges from less than 0.5eV to more than 0.7eV below Ec. This comparsion unambiguously demonstrates that the MPC band arises from the Do charge state of the defects (specifically, the D−&Do transition). This identification is also confirmed when the quasi-Fermi level is varied by the application of light bias even though the peak emission rate from the MPC defect band is changed by more than a factor of 100. These observations specifically rule out the possibility of large populations of charged defects in intrinsic samples predicted by proponents of the defect pool model. Instead, observed behaviors have a natural explanation in terms of a defect relaxation process.
Selfconsistent Analysis of Mobility-Lifetime Products and Subgap Absorption on Different PECVD A-SI:H Films
Authors: Jiao Lihong , Semoushikina S. , Lee Yeeheng , Wronski C. R.
Published: April 2011

The photoconductivity and subband gap absorption measurements over a wide range of generation rate(G) have been carried out ondiluted and undiluted a-Si:H films. It is found that in these high quality films there are significant differences in the functional dependence of mobility-lifetime(μτ) products on G. In additionto the different values of subgap absorption(a) there are also distinct differences in the dependence of a on photon energy (E) as well as G. It is difficult to selfconsistently analyze the results on theundiluted film with the previously used three gaussian distribution, particularly at high generation rates. Selfconsistent analysis isobtained when the (+/0) transitions of negative charged defects andthe (0/-) transitions of positive charged defects are introducedrespectively closer to the valence and conduction bands. This new gapstate distribution is a better representation for the defectpool model and potential fluctuation model.
Photoconductivity and Density of States in “Microstructural” Amorphous Silicon
Authors: Budaguan B. G. , Aivazov A. A. , Radosel'sky A. G. , Popov A. A.
Published: April 2011

It has been reported in previous works that using of RF 55 kHz PECVD method allows to deposit microstructural inhomogeneous a-Si:H films at high deposition rate (10–20Å/c) and with high photoconductivity The structural analysis with using of IR spectroscopy and atomic force microscopy (AFM) performed in this work have shown that these films possess a relatively regular microstructure consisting of “grains” with characteristic size of∼300–500Å. The regular microstructure of investigated films differs them from inhomogeneous a-Si:H with deteriorate electronic properties. At the same time the diffraction analysis didn't reveal the presence of microcrystals. Therefore, we denoted our films as “microstructural” a-Si:H films In this work we performed the modeling of the photoconductivity of “microstructural” a-Si:H films to analyze the density-of-states (DOS) responsible for recombination kinetics For this purpose different approaches to photoconductivity modeling have been used to simulate the experimentally measured temperature dependence of photoconductivity The comparative analysis of results of these simulations and ESR measurements have shown that recombination in high photoconductive ‘microstructural’ films is controlled by deep neutral states
Defect Luminescence in Films Containing Ge and GeO2 Nanocrystals
Authors: Zacharias M. , Atherton S. J. , Fauchet P. M.
Published: April 2011

Amorphous SiOx alloys containing Ge or GeO2 nanocrystals are produced by dc-magnetron sputtering and controlled crystallization. The samples are investigated by Raman scattering, transmission electron microscopy, photo luminescence and excitation spectroscopy. Under UV excitation, both types of films luminesce around 3.1 eV, with identical PL line shapes and subnanosecond PL dynamics. The strongest PL intensity is found for the films containing GeO2 crystals and for the largest nanocrystals. These results are a clear indication that although the blue luminescence is without a doubt correlated with the formation of Ge (or GeO2) nanocrystals, it is not produced by the radiative recombination of excitons confined in the nanocrystals. Possible mechanisms for the luminescence are discussed, including defects at the nanocrystal/matrix interface or in the matrix itself.
Initial Growth of Microcrystalline Silicon on Atomically Flat Hetero-Substrate
Authors: Saitoh K. , Kondo M. , Matsuda A.
Published: April 2011

Initial growth of microcrystalline silicon μ-Si:H) deposited on an atomically flat GaAs (001) wafer using a RF glow-discharge decomposition of hydrogen diluted monosilane gas mixture has been studied by means of atomic force microscope (AFM), Auger electron spectroscopy (AES), and cross-sectional transmitssion electron microscopy (XTEM).

It is shown that the initial growth of μc-Si:H deposited at a substrate temperature of 50∼250°C consists of four successive stages, i.e., (1) a layer-by-layer growth of a-Si:H up to d ∼5 Å, (2) island formation of a-Si:H, (3) the coalescence of the islands and the nucleation of microcrystallite at d∼ 10–40 Å depending on the growth temperature, and (4) a rapid roughening with microcrystalline growth.
Novel Plasma Control Method in PECVD for Preparing Microcrystalline Silicon
Authors: Nishimiya T. , Kondo M. , Matsuda A.
Published: April 2011

A novel plasma enhanced vapor deposition (PECVD) technique employing biased wall (BW) method has been developed for the enhanced growth rate of the hydrogenated microcrystalline silicon (μc-Si:H) films. Using this method, we have achieved a growth rate of more than 6Å/sec for the formation of μc-Si-H having an average grain size of 200Å; at 350°C.
Growth of polycrystalline silicon at low temperature on hydrogenated macrocrystalline silicon(μ c-Si:H) seed layer
Authors: Park W. S. , Jang Y. H. , Takeya M. , Jong G. S. , Ohmi T.
Published: April 2011

High quality polycrystalline silicon (poly-Si) films were deposited on SiO2 substrate using hydrogenated microcrystalline silicon (μ c-Si:H) seed layer by very high frequency (182.5MHz) sputtering system and Chemical Vapor Deposition (CVD) system in pure Ar ambient at atemperature of 300°C. In this study, we investigated the correlation between the initial seed layer conditions and the grown layer, and studied the nucleation process. Electrical characteristics and surface roughness of the grown layer as a function of film thickness and deposition rate will be shown. Grown layers were deposited on several kinds of hydrogenated seed layers in pure Ar ambient. The resistivity and Hall mobility of the grown layer are 0.33Ω.cm and 7.5cm2/V-sec respectively for a film with thickness of 3000Å. In this measurement, we found that the resistivity is lower as deposition rate is slower, and when deposition rate is relatively high, the resistivity increases with thickness. On the contrary, in the case of low deposition rate, resistivity decreases as film thickness increases. For the deposition rate of 0.32Å/sec, the surface roughness ranges over 1.39Å-8.52Å. This is a very smooth surface compared to hydrogenated polycrystalline silicon films. The resistivity of grown layer dramatically decreases in proportion to that of the seed layer. From Reflection High Energy Electron Diffraction (RHEED) patterns, it is clearly shown that the film is crystallization of occurred in the earlier stage than that of conventional deposited film. Our results shows that the electrical characteristics of the films are affected by the conditions of the seed layer and the deposition rate. Consequently, we conclude that the initial conditions are very important for the crystal growth of the as-deposition poly-Si.
An Absorption Study of Microcrystalline Silicon Deposited by Hot-Wire CVD
Authors: Diehl F. , Herbst W. , Schröder B. , Oechsner H.
Published: April 2011

The effect of variation of the preparation parameters filament temperature Tfil, gas pressure p and hydrogen dilution (H2/SiH4-flow ratio) on the absorption spectra of microcrystalline silicon deposited by the hot-wire technique (hw-μc-Si:H) has been studied by means of Photothermal Deflection Spectroscopy (PDS). We find an enhanced absorption of the μc-Si:H compared to crystalline silicon in the band gap (defect absorption) as well as in the interband transition region. An increase of absorption has already been reported for μc-Si:H films prepared by different techniques. In the case of hw-pc-Si:H we observe a relation between the absorption enhancement and the crystallite size. Increasing the gas pressure from 35 to 400 mTorr (Tfil=1850°C) or the filament temperature from 1750°C to 1950°C (p=100mTorr) the crystallite sizes, deduced from X-ray diffraction measuements, range from 10 to 60 nm. An alteration of the hydrogen dilution by varying the flow ratio between 2.5 and 25 does not affect the crystallite size and the optical absorption remains constant. In our opinion the enhancement cannot be described by a simple superposition of an amorphous and a crystalline absorption coefficient weighted by the volume fractions of the amorphous and crystalline phase, respectively. The possible reasons for the enhanced absorption will be discussed. The variation of the crystallite size with deposition conditions offers the possibility to control the optical absorption of μc-Si:H which is important for incorporating the material either as window layers or intrinsic layers in solar cells.
Characterization of Monolayer-Level Composition and Optical Gap Profiles in Amorphous Silicon-Carbon Alloy Bandgap-Modulated Structures
Authors: Fujiwara H. , Koh Joohyun , Wronski C. R. , Collins R. W.
Published: April 2011

Over the past few years we have applied real time spectroscopie ellipsometry (RTSE) to characterize the structural, compositional, and optical gap profiles in continuously-graded amorphous silicon-carbon alloy films (a-Si1-xCx:H). Most recently, we have extended the RTSE methods to their monolayer sensitivity and resolution limits. In this study, continuous triangular variations in the carbon content × (0.02≤x≤0.24) within ∼25 to 130 Å thick graded layers were introduced at the i/p interfaces of the n-i-p solar cell structures using continuous variations in the flow ratio z=[CH4]/{[SiH4]+[CH4]} during rf plasma-enhanced chemical vapor deposition (PECVD). A virtual interface approximation has been applied to interpret the RTSE data collected during the growth of the graded interface layers. This analysis yields C-content depth-profiles with monolayer-level resolution and a compositional uncertainty of ±0.004. Even compositional gradients in which x changes by >0.2 within a few monolayers’ thickness are readily characterized. Lastly, a continuous increase in open circuit voltage with increasing graded interface layer thickness, saturating at ΔVoc=0.1 V after 100 Å, is observed in the n-i-p solar cells with graded layers. These results demonstrate the importance of the RTSE analysis in assessing bandgap engineered device designs.
Deposition Conditions for Large Area PECVD of Amorphous Silicon
Authors: Kuske J. , Stephan U. , Nowak W. , Röhlecke S. , Kottwitz A.
Published: April 2011

The production of amorphous silicon devices usually requires large area, high-deposition-rate plasma reactors. Non-uniformity of the film thickness at high power and deposition rate is found to be an important factor for large area deposition.

Increasing the radio frequency from the conventional 13.56 MHz up to VHF has demonstrated advantages for the deposition of a-Si:H films, including higher deposition rates and lower particle generation. The use of VHF for large area deposition leads to the generation of standing waves and evanescent waveguide modes at the electrode surface and on the power feeding lines. Thereby increasing the non-uniformity of the film thickness. The uniformity of the film thickness for an excitation frequency strongly depends on the deposition parameters e.g. pressure, input power, silane flow and the value of load impedances. With increasing exciting frequencies the range of deposition parameters for obtaining uniform films narrows.

Subsequently it is shown that for a large-area plasma-box reactor (500 × 600 mm2 plate size) with a double-sided RF electrode, the non-uniformity of the film decreases due to a homoge-neization of the electrode voltage distribution by using multiple power supplies and load impedances on the end of the RF electrode. The uniformity errors decrease from ±20% to ±2.4% (27.12MHz) and from ±40% to ±5.9% (54.24MHz). Experimental results of the film uniformity will be discussed in dependence on excitation frequencies and the deposition parameters.
Role of The Gas Flow Parameters on The Uniformity of Films Produced by PECVD Technique
Authors: Martins R. , Maçarico A. , Ferreira I. , Fortunato E.
Published: April 2011

The aim of this work is to present an analytical model able to interpret the experimental data of the dependence of film's uniformity on the discharge pressure, gas flow and temperature used during the production of thin films by the plasma enhancement chemical vapour deposition technique, under optimised electrode's geometry and electric field distribution. To do so, the gas flow is considered to be quasi-incompressible and inviscous leading to the establishment of the electro-fluid-mechanics equations able to interpret the film's uniformity over the substrate area, when the discharge process takes place in the low power regime.
a-Si:H and a-SiGe:H Alloys Fabricated Close to Powder Regime of RF PECVD
Authors: Middya A. R. , Suktihazra  , Ray Swati , Longeaud C. , Kleider J. P.
Published: April 2011

We have been observing that a-Si:H and a-SiGe:H films deposited under high chamber pressure or close to powder regime (500 to 2000 mT for a-Si:H and 200 to 1000 mT for a-SiGe:H) show many new features : the mobility lifetime product is 10 to 100 times higher and the density of states above Fermi level of a-Si:H and a-SiGe:H ([Ge] < 0.20) is lower than that of standard samples. This enhancement in photoconductivity can neither be attributed to autodoping nor to creation of sensitization centers in the samples. Hydrogen bonding of these films, is mostly monohydride and the density of microstructural (polyhydrides and microvoids) defects is low. Evidence for the presence of nanocrystals is noted. Though the films seems amorphous by all conventional techniques, they crystallize easily under low laser intensity. Kinetics of light induced degradation is very fast (less than 20 hrs), for a-Si:H and the value of photoconductivity in the light soaked state is comparable to that of the standard samples in the annealed state. The process gas utilization efficiency in this regime is also higher than in the low power and pressure regime.
Molecular Dynamics Investigations of Boron Doping in a-Si:H
Authors: Fedders Peter A. , Drabold D. A.
Published: April 2011

The rather low doping efficiency of B in a-Si:H is almost always explained by the argument that almost all of the B is incorporated into three-fold coordinated sites and that B is inert or non-doping in this configuration. Using ab initio molecular dynamics, we have studied the energetics and doping (electronic structure) consequences of B incorporation into a-Si:H both with and without H passivation. Our results suggest that the conventional view is in error and that the low doping efficiency is primarily due to H passivation. These results are consistent with the low doping efficiency of B as well as NMR studies on the large electric field gradients experienced by the B atoms and on NMR double resonance studies of B-H neighboring distances.
The Problem of the Top Cell for the Micromorph Tandem
Authors: Platz Rainer , Meier J. , Fischer D. , Dubail S. , Shah A.
Published: April 2011

A detailed study of the influence of different materials for the amorphous top cell on the stabilized efficiency of amorphous silicon/microcrystalline silicon (”micromorph”) tandem cells is presented. The authors investigate different amorphous i-layer materials which are applied in cells with varying thicknesses. It is shown that it is preferable to optimize the top cell in order to obtain a high current rather than a high voltage.

A simple optical and electrical model is presented which allows one to predict the optimum optical bandgap of the i-layer material of the top cell and to determine its optimum thickness for maximum stabilized efficiency. By means of this model it is shown that the optimum top cell for a total cell current of 26 mA/cm2 should contain an about 2500 Å thick i-layer with an optical gap Eo4 ≃ 1.8 eV. The model furthermore shows that it is desirable to obtain slightly bottom-limited conditions after degradation in order to maximize the output power.

A stabilized efficiency of 10.7 % for a micromorph tandem cell has been confirmed by an independent measurement (FhG-ISE). Another such cell yields a stabilized efficiency of 11.2 % as measured in our laboratory.
The Light Induced Changes in A-Si: H Materials and Solar Cells—Where We are Now
Authors: Wronski C. R.
Published: April 2011

The quest for understanding and especially controlling the reversible light induced changes in a-Si:H based materials has been ongoing for the last twenty years. This has been accompanied by a corresponding large effort in minimizing their effects on more efficient a-Si:H based solar cells. Despite the complexities in both the phenomena as well as the solar cells, progress has been made in both the scientific and technological arenas. This paper briefly reviews primarily studies on the characterization and reduction of the metastable changes in materials and the correlation of these changes with those in efficient solar cells. It will discuss the impact of studies on materials as well as the continuous advances made with “engineering” of solar cell structures on their improved stabilized performance.
Structural and Optical Properties of a-Si:H/μc-Si:H:B Junctions in the a-Si:H-Based n-i-P Solar Cell Configuration
Authors: Kohi Joohyun , Fujiwara H. , Wronski C. R. , Collins R. W.
Published: April 2011

We have extended previous real time spectroscopie ellipsometry (RTSE) capabilities in order to investigate the effects of H2-plasma treatment of i-type hydrogenated amorphous silicon (a-Si:H) on the deposition of the overlying p-type microcrystalline silicon (μc-Si:H:B)) in the formation of an n-i-p solar cell structure. In this study, we compare in detail the nucleation and growth of p-layers by plasma-enhanced chemical vapor deposition (PECVD) from SiH4 highly diluted in H2 on the surfaces of untreated and H2-plasma treated a-Si:H i-layers. We find that for intended single-phase μc-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap (∼2.0 eV Taue gap) amorphous layer nucleates and grows in the first ∼150 Å. This layer develops uniformly to a bulk thickness of ∼150 Å, but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 Å). In contrast, for p-layer PECVD under identical conditions on the H2-plasma treated i-layer, high-density crystalline nuclei form immediately. This conclusion is drawn on the basis of the unique optical properties of the bulk p-layer that develops on the surface of the H2-plasma treated i-layer. Specifically, an absorption onset near ∼2.5 eV is observed for a 48 Å fully-coalesced p-layer, as measured by RTSE at 200°C. For this μc-Si:H:B p-layer, the optical gap decreases by ∼0.15 eV with increasing thickness from 50 to 200 Å. This effect is attributed to a reduction in the quantum confinement energy with an increase in the average crystallite size in the film.
Internal Electric Field Profile in Thin Film Hydrogenated Amorphous Silicon Diodes Studied by the Transient-Null-Current Method
Authors: Han Daxing , Yeh Chenan , Wang Keda , Qiwang
Published: April 2011

We demonstrate that the internal field of a thin a-Si:H pin solar cells can be measured using the transient-null-current method. This method was previously developed to measure the internal field profile in a-Si alloy Schottky barrier. The internal electric field profile was determined by measuring the forward-bias voltages that tune the transient photocurrents generated by a pulsed laser at a various wavelengths to zero. We adopt the same technique to a-Si:H p-i-n solar cells. In the case of p-i-n structure, we need to consider both space charge contributed by photogenerated carriers and carrier recombination which disturb the internal field. We use two critical conditions to minimize these effects. (1) To limit the contribution of photocarriers to space-charge distribution, the total charge collected is less than 10−10 C per pulse, and a repetition rate 1 Hz is used to ensure that the diode remains close to its equilibrium state. (2) The measuring time window is about 1 – 6 μs following the displacement current. Typically the RC constant of diode is < 1 μs and the rise time of the forward-bias recombination current is 6.0 × μs. We apply the signal average to process the forward-bias voltage. The error is within ± 0.05 V. With this technique we can study the effect of variety of structure design or processing on the device performance.
Conditions for Collection Efficiencies Greater than one Hundred Percent
Authors: Brüggemann R. , Zollondz J. H. , Main C. , Gao W.
Published: April 2011

An account is given for the conditions under which the collection efficiency in hydrogen ated amorphous silicon pin-diodes increases to values larger than 100 %. By specific bias illumination through the p-side bias generated photocarriers are collected under certain probe beam conditions of the collection efficiency measurement, leading to apparent large collection efficiencies. By numerical modelling we investigated the influence of the diode thickness, bias photon flux and probe absorption coefficient as well as applied voltage for possible sensor applications which may utilise this optical amplifying principle. The alternative with bias light through the n-side and probe light through the p-side is also explored. Collection efficiency values determined by the photogating of bias generated holes become only slightly larger than 100 % in contrast to the electron case where values in excess of 3000 % are presented.
Solar Cells from Mixed SiCi2H2 and SiH4 Plasmas
Authors: Payne A. M. , Crone B. K. , Wagner S.
Published: April 2011

We have deposited and analyzed solar cells from mixed SiCI2H2/SiH4 plasmas. Intrinsic and doped films were also grown to explain the cell results. The SiCI2H2 cells have lower fill factor and short circuit current density than the SiH4 cells due to the higher defect densities and lower photoconductivities of SiCI2H2/SiH4 material. Cell results showed low VQC if the n-layer was in contact with an i-layer deposited from SiCI2H2/SiH4. A pure silane buffer layer between the i- and n-layers was needed to attain the standard open circuit voltage. The reduced Voc results from a reduced conductivity of phosphorus-doped SiCI2H2/SiH4 material. However, comparison of doped films showed higher conductivities for boron-doped material grown from the SiCI2H2/SiH4 mixture than from pure silane.
Spin-Dependent Transport in Si Thin-Film Transistors
Authors: Kawachi G. , Graeff C F. O. , Brandt M. S. , Stutzmann M.
Published: April 2011

Defects and carrier transport processes in silicon based thin-film transistors (TFTs) are investigated by spin-dependent transport (SDT). The resonance signal arising from less than 106 defects in the hydrogenated amorphous silicon (a-Si:H) TFT is detected with a sufficient signal-to-noise ratio. The leakage current mechanism in a-Si:H under high source-drain fields is identified by SDT as electron hopping via defect states located at the interface between undoped a-Si:H and the passivation silicon nitride layer. At temperatures below 100K, spin-dependent hopping of electrons in conduction band tail states is observed. The change of the dominant transport path from extended states conduction to variable range hopping with decreasing temperature is confirmed. SDT measurements on polycrystalline silicon (poly-Si) TFTs having silicon nitride and silicon dioxide as the gate dielectric films reveal differences in the defect structure in these devices. The overall results demonstrate that SDT is a powerful method to probe paramagnetic defects and carrier transport in TFTs.
Stability of Amorphous Silicon Thin Film Transistors for Analog Circuit Applications
Authors: Hornsey R. I. , Mahnke T. , Madeira P. , Aflatooni K. , Nathan A.
Published: April 2011

Analog circuits using amorphous silicon thin film transistors offer significant advantages for in situ signal processing in large-area optical and x-ray imagers. However such circuits are susceptible to gate-bias-induced shifts in the threshold voltages of the constituent transistors. In this work, the change of threshold voltage for devices undergoing cycles of stress, relaxation and reverse bias is measured in order to determine the feasibility of resetting the threshold voltage electrically. It is concluded that, although the reverse bias does assist the recovery of the threshold voltage, the process is still not sufficiently rapid. An analog amplifier circuit is then described which uses negative feedback to achieve a gain that is stable to within 6% over a period of 8 hours.