bluuebunny/crossref_metadata_2025 huggingface dataset:
The contribution of the N- and C- terminal domains to the stretching properties of intermediate filaments
Authors: Kreplak Laurent
Published: April 2011

The animal cell cytoskeleton consists of three interconnected filament systems: actin containing microfilaments (MFs), microtubules (MTs), and intermediate filaments (IFs). Among these three filaments systems, IFs are the only one that show high extensibility at both the single filament and network levels. In this work, I am presenting a simple model of IFs extensibility based on the current structural knowledge of the filaments. The only extra information added to this model compared to previous ones is the fact that the unfolded N- and C-termini of IF proteins are sandwiched between adjacent coiled-coil rod domains within the filaments. Since we know the contour length and typical persistence length of these unfolded termini, it is possible to predict the persistence length of a single filament, its maximal extensibility and the onset of coiled-coil unfolding. The predictions of the model are in good agreement with experiments on single desmin IFs stretched on a surface by AFM and on vimentin and desmin networks probed by rheology.
Mechanics of Collagen in the Human Bone: Role of Collagen-Hydroxyapatite Interactions
Authors: Katti Kalpana S , Pradhan Shashindra Man , Katti Dinesh R
Published: April 2011

Here, we report results of our simulations studies on modeling the collagen-hydroxyapatite (HAP) interface in bone and influence of these interactions on mechanical behavior of collagen through molecular dynamics and steered molecular dynamics (SMD). Models of hexagonal HAP (10-10) and (0001) surface, and collagen with and without telopeptides were built to investigate the mechanical response of collagen in the proximity of mineral. The collagen molecule was pulled normal and parallel to the (0001) surface of hydroxyapatite. Water molecules were found have an important impact on deformation behavior of collagen in the proximity of HAP due to their large interaction energy with both collagen and HAP. Collagen appears stiffer at small displacement when pulled normal to HAP surface. At large displacement, collagen pulled parallel to HAP surface is stiffer. This difference in mechanical response of collagen pulled in parallel and perpendicular direction results from a difference in deformation mechanism of collagen. Further, the collagen molecule pulled in the proximity of HAP, parallel to surface, showed marked improvement in stiffness compared to absence of HAP. Furthermore, the deformation behavior of collagen not only depends on the presence or absence of HAP and direction of pulling, but also on the type of mineral surface in the proximity. The collagen pulled parallel to (10-10) and (0001) surfaces showed characteristically different type of load-displacement response. In addition, here we also report simulations on 300 nm length of collagen molecule indicating the role of length of model on the observed response in terms of both the magnitude of modulus obtained as well as the mechanisms of response of collagen to loading.
Humidity induced softening leads to apparent capillary effect in gecko adhesion
Authors: Chen Bin , Gao Huajian
Published: April 2011

Humidity can lower the stiffness of beta keratin, which is the main component of spatula pads terminated at the bottom of a gecko's toe. To see if this softening can influence gecko adhesion, numerical simulation of the vertical peeling of a spatula pad adhered on a rough surface is performed, which shows that the reduction in material stiffness indeed leads to substantial increases in the pull-off force ″(Chen, B. and Gao, H. International Journal of Applied Mechanics, 2010). This work provides an alternative explanation of experimental observations on the capillary effect in gecko adhesion.
Mechanism of Grain Growth During Annealing of Si-Al Electrical Steel Strips Deformed in Tension
Authors: B J. Salinas , R A. Salinas
Published: April 2011

The mechanism of recrystallization as a result of annealing during 600–7200 seconds at 700 °C of a Si-Al, low C electrical steel strip is investigated in samples deformed in tension. The evolution of grain size during annealing is evaluated by optical microscopy and electron backscatter diffraction in the scanning electron microscope. It is found that grain growth starts after an incubation time of 600 s with no apparent evidence of primary recrystallization. After that, the grain size-time relationship exhibits two different stages. Initially, the grain size increases linearly with time up to about 3600 s. During this time, some selected grains grow until they consume the deformed microstructure. In the second stage, the rate of growth decreases significantly and a final grain size of about 150 m is reached after 7200 seconds of annealing. Grain orientation spread maps obtained from EBSD data of deformed and partially recrystallized samples during the stage of linear growth reveals that the growing grains exhibit lower misorientation and therefore smaller stored energy than the non-recrystallized matrix grains. Analysis of image quality maps reveal that the IQ values for {100}orientations are higher than those observed for {111}orientations thus suggesting that the {100}orientations grow at the expense of {111}orientations by a mechanism of strain-induced boundary migration.
Performance of Cement Systems with Nano-SiO2 Particles Produced Using Sol-gel Method
Authors: Sobolev Konstantin , Flores Ismael , Torres Leticia M. , Cuellar Enrique L. , Valdez Pedro L. , Zarazua Elvira
Published: April 2011

The reported research examines the effect of 5–70 nm SiO2 nanoparticles on the mechani-cal properties of nanocement materials. The strength development of portland cement with nano-SiO2 and superplasticizing admixture is investigated. Experimental results demonstrate an in-crease in the compressive strength of mortars with SiO2nanoparticles. The distribution of nano-SiO2 particles within the cement paste plays an essential role and governs the overall perfor-mance of these products. Therefore, the addition of a superplasticizer is proposed to facilitate the distribution of nano-SiO2 particles. The application of effective superplasticizer and high-speed dispergation are found to be very effective dispersion techniques that improve the strength of su-perplasticized portland cement mortars, reaching up to 63.9 MPa and 95.9 MPa after aging dur-ing 1 and 28 days, respectively. These values compare favorably with the observed compressive strengths of reference portland cement mortars of 53.3 MPa and 86.1 MPa. It is concluded that the effective dispersion of nanoparticles is essential to obtain the composite materials with im-proved performance.
Development of an Algorithm for Random Packing of Multi-Sized Spherical Particles
Authors: de la Garza-Gutiérrez H. , Plascencia-Barrera G. , de la Torre S. D.
Published: April 2011

A new computational algorithm is introduced for packing simulation of spherical elements/particles into an imaginary container with three main possible geometries, cubic, cylindrical and spherical. The performance of the algorithm depends directly on the strategy or logic considered to solve the problem and the quality of its computational implementation. The combination of these two factors let the packing algorithm here presented and named as Octant Packing Random Algorithm (OPRA) to reduce the computation time between 2 and 127 times, when compared with the simplest or classical Packing Algorithm. OPRA is designed to reduce the number of comparisons needed to accept or reject a new position for an element/particle to be allocated into the virtual container. OPRA considers the container as if it were divided into 8 equal cells or octants limiting the overlap detection for a new position.
Stress Ratio Effect on Fatigue Behavior of Aircraft Aluminum Alloy 2024 T351
Authors: Benachour M. , Hadjoui A. , Benguediab M. , Benachour N.
Published: April 2011

Aluminum alloy series 2xxx, 6xxx, 7xxxx and 8xxx enjoy the widest use in aircraft structural applications. Among these materials, aluminum alloy 2024 remains the most commonly used and especially in T351 temper situation. The fatigue crack propagation behaviour of aluminum alloy 2024 T351 has been investigated using V-notch specimen in four bending test. A series of stress ratios from 0.10 to 0.50 was investigated in order to observe the influence of stress ratio on the fatigue life and fatigue crack growth rate (FCGR). The increase in FCGR, which occurs as the stress ratio is increased from 0.10 to 0.50, is generally attributed to an extrinsic crack opening effect. In T-S orientation and at low stress intensity factor, the increasing of stress ratio increase the FCG. Experimental results are presented by Paris law when coefficients C and m are affected by stress ratio. Contrary, at high stress intensity factor, the effect of stress ratio is reversed. We notice a decreasing of fatigue crack growth rate with an increasing of stress ratio. This effect may be explained by microstructure effect in (T-S) crack growth. The analysis of stress ratio effect by Elber model, shown that this model gives bad interpolation in this situation and the parameter characterized the crack closure factor will be adjusted.
Effect of Hot Band Annealing on the Microstructure and Mechanical Properties of Low Carbon Electrical Steels
Authors: Gutiérrez-Castañeda E. , Salinas-Rodríguez A.
Published: April 2011

Magnetic properties of grain non-oriented low-C electrical steels are improved when the hot rolled strip is annealed (HBA) prior to cold rolling and final annealing treatments. This improvement results from development of a {100} texture in the large grained ferrite microstructure produced during final annealing. HBA at 800–850 °C results in rapid decarburization and elimination of carbide particles which have caused concerns about the suitability of the mechanical properties in the final product. In this work, samples taken from a hot rolled electrical steel coil are subjected to HBA during 150 minutes at 850 °C, cold rolled and finally annealed three minutes at temperatures between 700 and 1000 °C. The resulting tensile properties are compared with those of samples subjected to a similar processing route but without the HBA treatment and samples of industrially semi-processed grain non-oriented electrical steel decarburized 16 hours at 750 °C. It is shown that the yield strength of samples with and without HBA depends on the final grain size according to the Hall-Petch relationship; the final grain size depends strongly on annealing temperature. However, the HBA treatment causes the strength to decrease by a factor of about 2.5 and the ductility to increase by a factor of about 1.5. It is observed that the microstructure and tensile properties of the semi-processed electrical steel subjected to a final decarburization annealing are identical to those observed in material subjected to HBA in the present work. These results indicate that the HBA treatment not only improves the magnetic properties but also leads to a significant reduction of production time for grain non-oriented electrical steels.
Bacterial Adhesion on Polyelectrolyte Modified Microstructured Titanium Surfaces
Authors: Almaguer-Flores Argelia , Sánchez-Cruz Yolloxóchilt R. , Park Jung Hwa , Olivares-Navarrete René , Dard Michel , Tannenbaum Rinna , Schwartz Zvi , Boyan Barbara D.
Published: April 2011

Micron-scale and submicron-scale surface roughness enhance osteoblast differentiation on titanium (Ti) substrates and increases bone-to-implant contact in vivo. However, bacterial adhesion is also strongly influenced by roughness and surface chemistry. The aim of this study was to investigate if chemical surface modifications alter initial bacterial attachment. To achieve this, two polyelectrolyte layers [chitosan (Ch) and poly(L-lysine) (PLL)] were used to coat Ti surfaces with different roughness (PT [Ra<0.3μm], SLA [Ra≥3.0μm]). Bacterial attachment was evaluated using Aggregatibacter actinomycetemcomitans, Actinomyces israelii, Campylobacter rectus, Eikenella corrodens, Fusobacterium nucleatum, Parvimonas micra, Porphyromonas gingivalis, Prevotella intermedia and Streptococcus sanguinis. After 24h incubation, bacteria were detached from the samples with sonication and the counting plate technique was performed to determine the number of colony forming units (CFU's). Additionally, surfaces were observed by scanning electron microscopy to determine bacteria surface coverage. Statistical significance was determined using ANOVA followed by Bonferroni's modification of Student's t-test. The results showed that polyelectrolyte coatings did not affect surface roughness. Modified surfaces were more hydrophilic than the controls. PT surfaces covered by Chi exhibited lower CFUs than the same surface covered by PLL or the control PT (140 × 105/mL, 343 × 105/mL and 283 × 105/mL, respectively). The opposite effect was observed on the SLA surfaces, PLL coated samples shown lower CFUʼs than Chi or uncoated SLA (199 × 105/mL, 229 × 105/mL and 227 × 105/mL, respectively). The Chi layer appeared to reduce bacterial adhesion only on the smooth surfaces. In contrast, PLL coatings reduced bacterial attachment on rougher surfaces. These results suggest that chemical modification of Ti without alteration of surface roughness affects oral bacterial attachment, and could be useful to prevent peri-implantitis related diseases.
A Correlation Between Soluble and Insoluble Fiber With the Elastic Modulus in Four Varieties of Bamboo
Authors: Sanchez-Echeverri L. A. , Contreras-Padilla M. , Rodriguez-García M. E.
Published: April 2011

The fraction of soluble and insoluble fiber was determinate by AOAC International official method for dietary fiber in four varieties of Bamboo called Guadua Angustifolia Kunth. The elastic modulus of the four bamboo varieties was determined using an universal machine INSTROM 4401 based on the ASTM standard method D-143. Finally, the fiber content and the mechanical behavior were correlated using the Pearson coefficient calculated by statistical package STATGRAPHICS 5.1 The fiber content is the main factor responsible for the bamboo mechanical properties. The results show that the all fiber content into the bamboo culms is related with the mechanical behavior. These results confirm that the Guadua Bamboo is a natural composite material with aligned fibers embedded in a matrix formed by soluble fiber.
Grinding Admixture for Portland Cement Obtained from The Acetic Fermentation of Molasses
Authors: Jiménez-Alvarez Francisco J. , Téllez-Jurado Lucía
Published: April 2011

The acetic acid is used for the manufacture of grinding admixtures in the cement industry. The most admixtures used by commercial milling are mixtures of amine acetates, glycols and sugars. The commercial formulations contain 10% acetic acid as amine acetate. In this work the amine acetate was synthesized using the product from fermentation of molasses. In this paper amine acetate was synthesized using the product from acetic fermentation of molasses and it was followed by fermentation of molasses through FT-IR and pH measurement. The fermented product was neutralized with tertiary amine, and finally were adding glycols into the grinding admixture. This admixture was compared with a commercial admixture, having a similar size distribution in both cases.
Ultrasound Influence on Kinetics of Phenol Sulphoxidation in Water Solutions in the Presence of Cobalt Complexes Immobilized on Polyacrylic Acid
Authors: T. V. C.c.s. Shakieva
Published: April 2011

The influence of hypersonic on phenol sulfoxidation kinetics in the presence of cobalt complexes fixed on polyacrylic acid (PAA) is investigated in the soft conditions (313-333K). Products arep- ando-phenolsulfonic acids. Phenol conversion under the investigated conditions reaches 98-100 %. The ultrasonic sound in the interval from 0 to 60 kHz leads to aggrandizement of trivalent cobalt concentration in a solution, as a result, increasing the reaction activity. The enlargement of ultrasound influence (USI) modulation frequency and the exposure time results in destruction of a binuclear complex, which may be responsible for enhanced catalytic activity. KEDWORDS: ultrasound influence; polyacrylic acid; immobilized cobalt complexes; kinetics
NO Reduction by CO Catalyzed with Au/TiO2 in Oxygen-rich Condition
Authors: Wang J. A. , Toledo J. A. , Angeles C. , Moran-Pineda M. , García-Ruiz A.
Published: April 2011

Gold nanoparticles supported on titania catalysts with different Au loadings were prepared and evaluated in the reaction of NO reduction by CO in an oxygen rich condition. The crystalline structures of the Au/TiO2 materials were refined with Rietveld method. TiO2 support chiefly contains anatase phase, having a crystalline size ranged from 5 to 15 nm. Au particles have an average crystal size approximately 2-5 nm as Au concentration less 3 wt %. In the reaction of NO + CO + O2, the Au/TiO2catalysts show a selectivity to 100 % N2, neither NO2 nor N2O was yielded in the reaction temperature between 25 and 400 °C, which strongly indicates that Au/TiO2catalysts are much superior to the other catalysts like Pt/TiO2 catalysts on which N2O was usually produced in the reaction temperature below 200 °C and NO2 was produced in the reaction temperature above 300 °C under a similar reaction condition.
Hydrogen Production via Methane Decomposition Using Ni and Ni-Cu Catalysts Supported on MgO, Al2O3and MgAl2O4
Authors: Pola José F. , Valenzuela Miguel A. , Córdova Iván A. , Wang J. A.
Published: April 2011

Ni (10%) and Ni-Cu (50 and 25%, respectively) catalysts supported on alumina, magnesia and magnesium aluminate were synthesized. The characterization was carried out by X-ray diffraction, nitrogen physisorption, temperature programmed-reduction, Raman spectroscopy and SEM. The catalysts were tested in the methane decomposition reaction using a tubular fixed bed reactor operated in the range of 500-580°C under atmospheric pressure. A higher activity was observed with the bimetallic catalysts supported on alumina and magnesium aluminate. These results were explained in terms of Ni-Cu alloy formation and weak metal-support interaction. In the case of monometallic catalysts, a strong metal-support interaction was detected, which revealed the lowest activity and stability compared with the bimetallic catalysts. The formed carbon was a combination of amorphous and graphite.
Production of Di-isopropyl ether (DIPE) over 12-Molybdophosphoric Acid Supported on ZrO2. An Alternative Octane Enhancer for Lead-free Petrol
Authors: Hernández-Cortez J. G. , López-Salinas E. , Manríquez Ma. , Picquart M.
Published: April 2011

Solid acid catalysts of 12-Molybdophosphoric acid (MoP) supported on hydrous zirconia have been synthesized using impregnation method. Structural transformations due to thermal effects have been studied by different techniques including X-Ray diffraction, thermogravimetric analysis (TGA), infrared absorption (DRIFT), Raman spectroscopy and31P MAS NMR. These techniques showed that the heteropolyacid present undegradedKegginstructure. The transformation of heteropoly anion of pure HPMo to MoO3begins at a temperature lower than that for its ZrO2-supported counterpart. The activity of ZrO2-supported (MoP-Z) catalysts in the 2-propanol decomposition presents a high selectivity to the formation of DIPE and propene, this selectivity is related with acid sites determined by thermodesorption (TPD) of NH3.
The Study of Charge Transport in Nanoscale DNA Structures
Authors: Bart G. , Singh M. R. , Zinke-Allamang M.
Published: April 2011

We have studied the variable range hopping (VRH) mechanism for polarons in DNA structures using an exponential density of states. Due to the electron-phonon interaction localized polarons are formed in the DNA helix. The unwinding of DNA increases molecular orbital overlap between bases while decreasing the base-to-base distance. These types of vibrations create phonons. We consider that DNA has a band tail which has an exponential density of states and we have calculated the temperature- and the electric field dependence of the conductivity. We compare our model with the experiments of the electrical conductivity of samples of double-stranded H5N1 genes of avian Influenza virus DNA. Our theory is able to explain their data.
Low Resistance Ohmic Contacts to n-Hg1−xCdxTe Using a HgTe Cap Layer
Authors: Leech Patrick W. , Reeves Geoffrey K.
Published: April 2011

Non-alloyed ohmic contacts of HgTe on Hg1−xCdxTe (x = 0.60) with metallisations of Ti, In and Au have been investigated. Layers of HgTe with thickness in the range from 0.1 μm to 1.0 μm were grown by organometallic epitaxy either as an abrubt or a graded junction, depending on the in-situ annealing conditions. The layer thickness and the extent of interdiffusion with the Hg1−CdxTe were determined using Rutherford backscattering spectrometry (RBS). The results have shown that an abrupt rather than a graded structure was essential in order to achieve the minimum value of specific contact resistance, ρc, of ≈5 × 10−5 Ωcm2. In addition, a critical thickness of HgTe (≥0.2 μm) was required in order to obtain a substantial reduction in ρc. For these metal/HgTe/Hg1−xCdxTe contacts, the metal Ti has produced the lowest values of ρc and greatest adhesion to the HgTe. Both of these properties have been attributed to the strong interfacial reaction of the overlayer of Ti with HgTe.
Quality of Bulk CdTe Substrates and its Relation to Intrinsic Defects
Authors: Meyer B. K. , Hofmann D. M. , Stadler W. , Emanuelsson P. , Omling P. , Weigel E. , Müller-Vogt G. , Wienecke F. , Schenk M.
Published: April 2011

Both the cadmium vacancy (Vcd) and the tellurium vacancy (VTe) in CdTe are identified by means of electron paramagnetic resonance (EPR). The Vcd is a double acceptor and the EPR spectrum is observed in its single negative charge state. The symmetry is found to be trigonal, which can be explained in a model in which the hole occupies a dangling bond t2 orbital and the orbital degeneracy is removed by a static Jahn-Teller distortion. The hyperfine interaction shows that the hole is localised on one of the four Te neighbours. The EPR spectrum of VTe+ reveals cubic (unperturbed) symmetry and the hyperfine structure shows that the unpaired electron is equally spread over the four Cd neighbours

Photo-EPR measurements locate the 0/+ state of VTe at Ev + 0.2 eV and the 2-/- acceptor level of VCd to be situated less than 0.47 eV above the valence band.
Defect Reduction in Cd1-xZnxTe Epilayers on GaAs Substrates
Authors: Chadda Saket , Malloy Kevin , Reno John
Published: April 2011

Cd0.91Zn0.09Te/CdTe multilayers of various period thicknesses were inserted into Cd0.955Zn0.045Te bulk alloys grown on (001) GaAs. The net strain of the multilayer on the underlying Cd0.955Zn0.045Te was designed to be zero. X-ray diffraction full width at half maximum (FWHM) was used as a means to optimize the period thickness of the multilayer. Transmission electron microscopy of the optimum period thickness samples demonstrated four orders of magnitude decrease in the threading dislocation density. Mechanism of bending by equi-strained multilayers is discussed.
An Array of CdTe Detectors for Inaging Applications
Authors: Eisen Y. , Polak E.
Published: April 2011

The performance characteristics of a linear array composed of several hundred miniature spectroscopy grade CdTe detectors are described. The linear array is utilized in a X-ray security imaging system. It operates either in an energy dispersive mode to highlight certain atomic elements in the luggage or in a regular mode for obtaining only the transmission image of the luggage. The array functions in the photon counting mode and tolerates count rates as high as 106 counts/sec. The array has high quantum efficiency and good spatial resolution which result in excellent image quality. The dynamic range, the electronic noise, the energy resolution, the homogeneity of response among detectors and the image quality are presented.
Performance Characteristics of N-Type SI-SI/GE Quantum well Infrared Photodetectors
Authors: Shadrin V. D. , Coon V. T. , Serzhenko F. L.
Published: April 2011

The theory of multiple quantum well n-type Si-SiGe IR detectors is presented. The coefficient of photoabsorption, quantum efficiency and responsivity of Si-SiGe quantum well detector are calculated taking in consideration the effects of depolarization and electron-electron exchange interaction. We show that the Si-SiGe quantum well detectors possess lower dark current and better performance characteristics compared to GaAs-AlGaAs photodetectors.
Photoconductive (PC) and Photovoltaic (PV) Dual-Mode Operation III-V Quantum well Infrared Photodetectors for 2–14 μm IR Detection
Authors: Li Sheng S. , Wang Y. H. , Chuang M. Y. , Ho P.
Published: April 2011

We present four new types of III-V quantum well infrared photodetectors (QWIPs) operating in photoconductive (PC) and photovoltaic (PV) modes for the wavelength range from 2 to 14 μm. These dual-mode (DM) operation QWIPs were grown by the MBE technique using GaAs/AlGaAs, AlAs/AlGaAs, and InGaAs/InAlAs material systems. Based on the bound-to-miniband (BTM) and the enhanced bound-to-continuum (BTC) intersubband transition schemes, these detectors provide the features of large absorption coefficient, low dark current, and high detectivity in the wavelength of interest, and show promising for use in large area IR focal plane array image sensor applications.
Process Optimization for the Fabrication of the Thin PtSi Schottky Barrier Diode IRCCD
Authors: Wang Wang-Nang , Ho Chia , Shiue Jee-Ming
Published: April 2011

High quality abrupt junction PtSi thin film prepared by the MBE system with in situ precleaning and annealing was obtained. Wet etching and low energy hydrogen plasma excitation to generate H-terminated Si surface and low temperature thermal desorption were used to clean the substrate. IRCCD thus fabricated achieved the quantum efficiency 0.8%. TEM, STM/AFM, AES/ESCA, and RBS were used to monitor the fabrication processes.
Iridium Silicide Formation by Rapid Thermal Annealing
Authors: Rodriguez T. , Wolters H. , Almendra A. , Sanz-Maudes J. , Da Silva M.F. , Soares J.C.
Published: April 2011

Iridium silicides formation by rapid thermal annealing (RTA) under vacuum at several temperatures in the range of 350 to 650°C has been investigated. The substrates and the silicide films were analyzed by Rutherford backscattering spectrometry (RBS) and Auger electron spectroscopy (AES). At 350°C, no distinguishable phase was detected for 240 seconds of annealing time. At 400°C, for processing time up to 45 seconds only Ir1Si1 was formed, for longer processing time Ir1Si1.75 was formed too. At higher temperatures even for very short processing time, Ir1Si1.75 was formed. Ir, Ir1Si1 and Ir1Si1.75 were present simultaneously if the iridium film was thick enough and the processing time was long enough too. For thin iridium layers, the Ir1Si1 formed was totally converted to Ir1Si1.75, if the annealing time was long enough. Formation rates were observed to be three to five orders of magnitude faster than the reported for furnace annealing.
A Shubnikov-De Haas Study of Tilted Magnetic Field in the HgTe/CdTe Superlattice
Authors: Lo Ikai , Mitchel W. C. , Boeringer D. , Harris K.A. , Yanka R.W. , Mohnkern L.M. , Reisinger A.R. , Myers T.H.
Published: April 2011

We have measured the Shubnikov-de Haas (SdH) effect in a HgTe/CdTe superlattice (SL) with a tilted magnetic field in the absence of an external electric field. We found that the peaks of the SdH oscillation changed with Bcosθ, indicating the two-dimensional character of the electron gas. The carrier concentration of the two-dimensional electron gas (2DEG) is equal to 2.95×1011 cm−2. The 2DEG shows the existence of a Stark ladder, which is caused by the internal electrostatic field, near the interface between the substrate and the SL. From the temperature dependence of the SdH measurements, we also show that the 3DEG in the SL miniband contributes to the non-oscillatory magnetoresistance and that the mobility of 3DEG in the miniband is lower than that of 2DEG in the Stark ladder.
The Photoemission from Superlattices of III-V Semiconductors with Graded Interfaces Under Quantizing Magnetic Field
Authors: Ghatak Kamakhya P. , De Badal
Published: April 2011

In this paper we have studied the photoemission from super-lattices of III-V semiconductors under magnetic quantization by formulating a new dispersion law. It is found, taking InAs/GaSb super-lattice with graded interfaces as an example that the photoemission, increases with increasing electron concentration in an oscillatory way and increases with decreasing magnetic field in the magnetic quantum limit. Besides, the photoemission in superlattices is much greater than that of the constituent materials and the well-known results for wide-gap materials have also been obtained from our generalized analysis. In addition, the theoretical analysis is in agreement with the experimental datas as given elsewhere.
Physics of the Meyer-Neldel Rule in Amorphous Silicon
Authors: Branz Howard M. , Yelon Arthur , Movaghar Bijan
Published: April 2011

We show that the meyer-Neldel Rule (or compensation law) arises from the entropy effect of accumulating several excitations in order to surmount an activation barrier. This explanation applies both to annealing and to conductivity phenomena in a-Si:H. We consider three consequences of this theory for a-Si:H : 1) MN processes in a-Si:H involve coupling to a bath of “optical” phonons; 2) dc conductivity involves strongly electron-phonon coupled states rather than extended electrons and a fixed density of states; and 3) time-of-flight exponents depend upon the MN temperature.
Molecular Dynamics Studies of Defect Motion in a-Si:H
Authors: Fedders P. A.
Published: April 2011

First principles molecular dynamics methods are used to study the motion of defects including dangling bonds and H. We study motion among H passivating dangling bonds, bond centered H, and tetrahedral H. We find that relaxation effects can reduce activation energies by up to one eV. Our studies also include the actual simulation of the time evolution of an a-Si:H system for several picosec. Here we observe the motion of a dangling bond over several hops and the motion of an H atom by means dangling bond exchange.
Light Induced Defects in a-Si:H, Temperature Dependence of their Creation and Anneal and their effect on Photocarrier Lifetime
Authors: Stradins Paul , Fritzsche Hellmut , Tran Minh Q.
Published: April 2011

We compared the effect of light soaking on the photoresponse and defect concentration ND of samples prepared by normal glow discharge, by remote plasma discharge, by the heated mesh and by the hot wire deposition methods. After exposure to 4×1027 cm−3 absorbed photons all samples have the nearly the same Np and photoresponse. At low temperatures additional defects with small anneal energies are created. Defects created at low temperatures were found to relax between 100K and 300K before they anneal. These new results cannot be explained by present models of defect creation. The kinetics of defect creation at low temperatures is discussed.
A Low Temperature Plasma-Assisted Deposition Process for Microcrystalline Thin Film Transistors, TFTS
Authors: He S.S. , Lucovsky G.
Published: April 2011

The drive-current of low-temperature (∼300°C) deposited TFTs has been increased by replacing the a-Si:H channel, and source and drain materials with μc-Si. Lightly B2H6 doped, near-intrinsic μc-Si has been used as the channel layer of the TFTs, and n+ μc-Si was used for the source and drain contacts. The compensation of intrinsic defects in the undoped μc-Si by boron doping increases the dark conductivity activation energy from -0.35 eV to 0.8 eV. TFTs were fabricated in a bottom gate structure, and required an H2 plasma treatment to produce devices with effective channel mobilities of -6.8 cm2/V-s and threshold voltages of -3.7 V in the saturation region.
The Role of the Band GAP in the Light-Induced Degradation of Amorphous Silicon Alloys
Authors: Unold Thomas
Published: April 2011

The dependence of the light-induced degradation on the band gap and hydrogen content in Amorphous silicon and related alloys is discussed. A simple model is presented which predicts that the light-induced degradation should be strongly correlated with the optical gap. In particular, for materials with Egap smaller than approximately 1.2eV no degradation is expected. The implications and consequences of the model for the task of reducing metastability in Amorphous silicon alloys are discussed.
Hydrogen Bonding Configurations Determined From H Evolution
Authors: Jackson W. B. , Nickel N.H. , Johnson N.M. , Pardo F. , Santos P. V.
Published: April 2011

Hevolution spectra were measured for hydrogenated Amorphous silicon (a-Si:H) films of varying thickness. A low temperature evolution peak at about 380 °C was determined to arise from a surface monolayer. The bulk limited main peak of the evolution spectrum arises from a very narrow range of binding energies (<0.2 eV). The evolution spectra of a-Si:H, hydrogenated polycrystalline Si, and c-Si containing platelet structures are virtually identical in both size and temperature of the various peaks. Less than 5% of the H is released in an evolution peak occurring at about 800 °C, a peak possibly due to H bound at isolated dangling bonds.
Moving Photocarrier Grating Technique for Mobility and Lifetime Measurements in Amorphous Semiconductors
Authors: Hundhausen M. , Haken U. , Ley L.
Published: April 2011

In this paper we describe a new method for the determination of the carrier lifetime (r) and the carrier Mobilities (μn, μp) in semiconductors. This technique utilizes a moving intensity grating that is generated by superposition of frequency shifted laser beams for the illumination of the sample. The Material parameters are extracted from the short circuit current in the sample induced by the moving grating as a function of it's velocity (vgr). We solve the continuity equations and Poisson's equation in the small signal approach for the modulated electron and hole densities and show how these densities result in an electric field that in turn acts on the electrons and holes in order to yield a short circuit current density jsc. From a fit of this expression for jsc to experimental data we determine μn, μp and τ.
Amorphous to Polysilicon growth and “Sunflower Effect” Observed in Catalytic-CVD
Authors: Matsumura Hideki , Hosoda Youichi , Furukawa Seijiro
Published: April 2011

Polysilicon films with a large mobility can be deposited at tempertures lower than about 400 °C by the catalytic chemical vapor deposition (cat-CVD) Method using a silane and hydrogen gas mixture. The mechanism of polysilicon growth and the structure of polysilicon itself are studied by TEM and X-ray diffraction. It is found that the polysilicon grows from an initial step of the deposition but that the grains start to orient preferentially along a (220) direction as the film grows. It is also found that such preferential axis of the grains is likely to orient toward a heated catalyzer as a sunflower always likes to face the sun.
Improved Electrical and Transport Characteristics of Amorphous Silicon by Enriching with Microcrystalline Silicon
Authors: Mireshghi A. , Hong W.S. , Drewery J. , Jing T. , Kaplan S.N. , Lee H.K. , Perez-Mendez V.
Published: April 2011

We have deposited n-i-p diodes with microcrystalline intrinsic layers for radiation detection applications. The diodes show interesting electrical characteristics which have not been reported before. From TOF Measurement for our best samples we obtained μe values which are about 3 times larger than our standard a-Si:H. for μτ values approximately a factor of 2 improvement was observed. The N*D values derived from hole-onset measurements show lower ionized dangling bond density than normal a-Si:H Material. We have proposed a simple model which can very well explain the experimental results.
Interpretation of High Field Transport in A-Si:H Based on the Effective Temperature Definition
Authors: Nebel Christoph E. , Street R. A.
Published: April 2011

The effective temperature definition which combines electric field and lattice temperature is critically discussed based on dark conductivity and drift mobility data measured over a wide range of temperature (10 K ≤ T ≤ 300 K) and electric field (102 V/cm ≤ F ≤ 6×105 V/cm). The reasonable values for the localisation lengths of electrons (αe ≡ 7–8 A) and holes (αh, ≡ 4 A) as well as the overall good agreement between predicted and deduced features support the effective temperature concept. A statistical transit time equation for transport at low temperature and high field is given and discussed based on time-of-flight data measured at T = 40 K.
Modeling of Reverse Bias Differential Charge Experiments in Amorphous Silicon
Authors: Shapiro Finley R.
Published: April 2011

An analytic model is developed for differential charge measurements which avoids the use of the depletion approximation. In this experiment the incremental charge in the device is calculated as the difference between the total steady-state junction charge at two nearly equal voltages, similar to a very low frequency capacitance Measurement. All of the calculations, including an iteration to find the bulk Fermi level, can easily be performed using a spreadsheet program on a personal computer. It is shown that the results of these calculations agree extremely well with simulations performed with a complete numerical device simulator. Thus, the model useful for the analysis of experimental data
Charged Defect State Distributions Obtained from the Analysis of Photoconductivities in Intrinsic a-Si:H Films
Authors: Güneş Mehmet , Collins R. W. , Wronski C. R.
Published: April 2011

Steady-state photoconductivity, sub-bandgap absorption and electron spin resonance (ESR) Measurements were carried out on annealed and light soaked intrinsic hydrogenated Amorphous silicon (a-Si:H) films. The experimental results were modeled using detailed numerical Model. The defect densities derived from the sub-bandgap absorption in the light soaked films were correlated with the ESR spin densities. Selfconsistent fitting of the data was obtained using a gap state distribution which consists of positively charged defect states above, negatively charged defect states below and neutral defect states at about Midgap. Both the annealed and the light degraded states are modeled using the same distribution of gap states whose densities increase upon light soaking with a slight increase in the ratio of the neutral to charged defect densities. These results on intrinsic a-Si:H are consistent with those of charged defect Models.
Silicon Carbon Alloys Produced by VHF and Conventional PECVD. A Comparison of their Properties.
Authors: Crovini G. , Demichelis F. , Pirri C.F. , Tresso E. , Meier J. , Dub Ail S. , Shah A.
Published: April 2011

The Very High Frequency (70 MHz) PECVD has recently proven its ability to produce Amorphous silicon with high deposition rates (10 Å/s) without affecting the quality of the Material. A comparative study of the optoelectronic properties of undoped silicon carbon alloys produced by Very High Frequency and by conventional RF (13.56 MHz) is carried out. Conductivity, infrared absorption, optical transmission and deep defect densities via PDS and ESR have been Measured. Deposition parameters under study aie methane fraction and hydrogen dilution. In contrast to conventional PECVD, we observe for VHF depositions an increase of the deposition rate with the addition of Methane. Larger energy gap and smaller Urbach energy values seem to indicate a better incorporation of carbon in the VHF case. A study of the degradation induced by light is also presented.
Nanoscale Study of the Hydrogenated Amorphous Silicon Surface
Authors: Tanenbaum D.M. , Laracuente A. , Gallagher A.C.
Published: April 2011

A scanning tunneling microscope has been used to study the topology of the surface of device-quality, hydrogenated Amorphous silicon deposited by rf discharge from silane or “hot wire” CVD. The substrates were oxide-free single-crystal silicon or GaAs. Films studied were either grown in our laboratory and observed with no air exposure, or grown at other laboratories producing device-quality photovoltaic cells and viewed after air exposures of less than 30 Minutes. Thin films (10 nm) representing early growth stages appear significantly smoother than the thicker films. The topology of thick films (> 50 nm) has large variations over individual samples. While many regions can be characterized as “rolling hills”, atomically flat areas are sometimes observed nearby in our films. In most regions the observed slopes were 10% or less from the horizontal, but some steep-sided valleys, indicating incipient voids, are seen. Overall surface roughness measured on sub-Micron areas of our films is very inhomogeneous. Uniformity of the films grown off site was much better, although no atomically flat regions were observed, surface roughness can be estimated.
Boron Doped A-Si,C:H Grown by Reactive Magnetron Sputtering from Doped Targets
Authors: Liang Y.H. , Yang S.-Y. , Nuruddin A. , Abelson J.R.
Published: April 2011

We have deposited p+ a-Si,C:H films by reactive magnetron sputtering from boron doped target with 1 at.% B. We have investigated the influence of H2 pressure on the electrical and microstructural properties of the doped a-Si,C:H films. The boron concentration in the film is ∼2–4×1020cm−3. The incorporation of B atoms decreases by a factor of 2 at the highest H2 pressure. We have obtained films of Taue bandgap ∼1.8–1.84eV with dark conductivity 2–8×10− 6Ω−1cm−1, thermal activation energy ∼0.28–0.33eV and νμτe ∼1–3×10−8cm2/V. This result is comparable with glow discharge B doped a-Si,C:H film. We discuss the film Microstructure, as reflected in infra-red & thermal H evolution spectra, vs. the absolute composition of sputtered boron doped a-Si,C:H films.
Excimer Laser Annealing of Amorphous Silicon Films
Authors: Viatella J. , Singh R.K. , Thakur R.P.S.
Published: April 2011

Low temperature processing is necessary for the fabrication of thin-film transistors for electronics-on-glass applications, including active matrix flat-panel displays. One method to achieve this involves the use of pulsed excimer-laser annealing of an Amorphous silicon layer on top of an SiO2 layer. The intense UV laser is absorbed in the Amorphous silicon region, Maintaining a low average temperature. The thickness of the underlying SiO2 layer affects the solidification velocity and hence the grain size of the annealed layer. Previous work has concluded that the resultant grain size is small (<100 nm) and further work is needed in finding ways of increasing grain size. This paper describes how grain size is affected by varying the thickness of the SiO2 layer. Correlations will be discussed.between the solidification velocities and grain size as affected by the varying thickness of the SiO2 layer. The paper includes a comparison between experimental and theoretical results, using equations based on energy balance considerations.
Efficient Visible Room Temperature Photoluminescence in Wide Gap Hydrogenated Amorphous Silicon-Carbon Alloys
Authors: Tessler Leandro R. , Solomon Ionel
Published: April 2011

We report a photoluminescence study on amorphous hydrogenated silicon carbon (a-Si1-xCx:H) alloys with carbon concentration in the range O < x < 0.5, prepared by PECVD in the “low-power” regime, that preserves the tetrahedral coordination of the carbon atoms. These samples have optical gaps higher than conventional “high power” alloys with the same carbon content. For carbon concentrations below x = 0.2 the photoluminescence behaves essentially as in pure a-Si:H with increased gap, Urbach energy and DOS. For higher carbon concentrations there is a change in the recombination process, that we attribute to a change in the dominating diffusion process of the photogenerated carriers. The integrated photoluminescence intensity for carbon-rich samples is very weakly dependent on the temperature, and at room temperature it approaches that of pure a-Si:H at 77K. For all samples, the photoluminescence bandwidth can be well described by a zero-phonon model.
Photobleaching of PL and Temperature Dependence of ESR in Nitrogen-Rich Amorphous Silicon Nitride Films
Authors: Chen D. , Viner J.M. , Taylor P.C. , Kanicki J.Z.
Published: April 2011

We report measurements in nitrogen-rich a-SiNx:H (x > 1.3) of visible light photobleaching of photoluminescence (PL) that has been fatigued with UV light. We also review the temperature dependence of electron spin resonance (ESR) from a-SiN1.6:H films. The samples were prepared by plasma-enhanced chemical vapor deposition (PECVD) at 400 and 250° C. The optical gap of a-SiN1.6:H is about 5 eV. In PL experiments, the PL intensity fatigues under UV excitation, finally reaching a saturated value. By illuminating with visible light, the UV effect is partially bleached, that is, the PL intensity is partially restored. The bleaching light ranges in energy from 2.0 to 4.8 eV. With increasing bleaching-light energy, the reinduced PL intensity increases, reaches a maximum at a bleaching energy of 3.8 eV, and then quickly decreases. In ESR experiments, there exists a small ESR signal in as-deposited films before UV excitation and the variation of the ESR signal with temperature is similar to that of a-As.
Modelling Heteroface of P.I.N Solar Cells for Improving Stability
Authors: Fantoni A. , Vieira M. , Martins R.
Published: April 2011

The introduction into a traditional p.i.n structure of two defective buffer layers near the p/i and i/n interfaces can improve the device stability and efficiency through an enhancement of the electric field profile at the interfaces and a reduction of the available recombination bulk centers. The defectous layer (“i'-layer”), grown at a higher power density, present a high density of defects and acts as “gettering centers” able to tailor light induced defects under degradation conditions. If the i-layer density of states remains below 1016 eV−1 cm−3 and assuming a Gaussian distribution of defect states, the gettering center distribution will not affect significantly the carrier population but only its spatial distribution.

We report here about a device numerical simulation that allows us to analyse the influence of the “i'-layer” position, thickness and density of states on the a-Si:H solar cells performances. Results of some systematic simulation from the ASCA program (AMorphous Solar Cell Analysis), and for different configurations will be presented.
A Study of a-Si:H/a-SiGe:H Tandem Solar Cells and Modules
Authors: Li Yuan-Min , Yang L. , Bennett M.S. , Chen L. , Jackson F. , Rajan K. , Arya R.R.
Published: April 2011

We report a study of a-Si:H/a-SiGe:H dual-bandgap p-i-n type tandem solar cells and Modules. Refined bandgap grading in the a-SiGe:H bottom cell results in higher Jsc (higher red response) without loss in FF or Voc. IMprovements in the tunnel junction consisting of doped widegap a-SiC:H and μC-Si lead to higher FF and lower optical loss. Undoped widegap a-SiC:H is used as interface layers in the top junction. Initial conversion efficiencies of 11.4% and 10.5% have been obtained for small-area cells and one-square-foot modules, respectively. The double junction cells and modules exhibit good stability and saturation of light-induced degradation, showing only ∼15% decrease in conversion efficiency after more than 1000 hours of simulated AM1.5 illumination. The stabilized efficiencies are 9.6% and 9.1%, respectively, for small area cell and large area Module. We also present some findings from the computer simulations of multijunction solar cell performance using a simple lump-circuit device Model.