bluuebunny/crossref_metadata_2025 huggingface dataset:
The Influence of Fumaric Anhydride on Bioadhesive Polymer Compositions
Authors: Santos C.A. , Freedman B.D. , Ghosn S. , Mathiowitz E.
Published: April 2011

Bioadhesive polymers are useful as drug delivery systems designed to adhere to the gastrointestinal lumen. The interaction between polymer and mucosal tissue influences residence time of the polymeric device and greatly affects the bioavailability of encapsulated drug. Poly(fumaric-co-sebacic anhydride) [P(FA:SA)] demonstrated impressive results in a variety of in vitro and in vivo experiments designed to test bioadhesion of microspheres. Among different molar ratios of P(FA:SA), adhesive measurements increased with increasing fumaric acid (FA) content. Using a modified microbalance technique, P(FA:SA)10:90 yielded a tensile work measurement of 82.99±12.76 nJ (mean±SEM) while that for P(FA:SA)70:30 was 453.23±47.73 nJ. A low molecular weight substance, fumaric anhydride prepolymer (FAPP), the oligomer form of fumaric acid, was incorporated into the microspheres and greatly increased the bioadhesive properties of P(FA:SA) as well as those of the relatively non-bioadhesive polymer poly(caprolactone) (PCL). Tensile work of P(FA:SA)20:80 was 32.95±5.42 nJ, and P(FA:SA)20:80 with 25% FAPP yielded a tensile work measurement of 556.28±113.12 nJ. Adhesion testing with PCL yielded a tensile work measurement of 7.93±1.84 nJ, while that for PCL with 25% FAPP was 1629.54±307.55 nJ. The effect of FA on drug delivery was evaluated in vitro using the P(FA:SA):FAPP blend with the everted intestinal sac technique. A low molecular weight drug (sodium salicylate) encapsulated in P(FA:SA)20:80 passed through the everted intestinal lumen to the interior of the sac to yield a concentration of 11.17±0.93 mg/dL, and drug encapsulated in P(FA:SA)20:80 with 10% FAPP yielded a concentration of 16.25±1.68 mg/dL (P<0.05). Unencapsulated drug passed through the intestinal lumen to yield a concentration of 8.09±-0.36 mg/dL. These experiments demonstrate that fumaric anhydride could be a very important component in bioadhesive polymer systems.
Investigations on the Electrochemical Behavior of Zr-Al-Cu-Ni Bulk Metallic Glass
Authors: Buchholz K. , Gebert A. , Mummert K. , Eckert J. , Schultz L.
Published: April 2011

The passivation behavior of bulk glassy Zr55Al10Cu30Ni5 alloy samples in weakly alkaline sulphate solution (pH = 8) is investigated in comparison to the arc-melted crystalline alloy, to the main alloying component zirconium and to aluminium. Results of potentiodynamic and potentiostatic polarization measurements reveal the formation of a stable passivating surface film on the glassy alloy grown by a high-field mechanism. Auger electron spectroscopic investigations of anodized sample surfaces show that the films formed on the glassy Zr-Al-Cu-Ni alloys exhibit a composition gradient in cross-sectional direction. In 0.001M NaCl electrolytes bulk glassy Zr-Al- Cu-Ni samples are susceptible to pitting corrosion, which is due to the existence of crystalline inclusions.
Glass Formation and Phase Separation In The Vitreloy Type Bulk Metallic Glasses
Authors: Hays Charles C. , Kim Paul , Johnson William L.
Published: April 2011

Results of calorimetric, differential thermal analysis, and structural measurements are presented for a series of bulk metallic glass forming compositions in the Zr-Ti-Cu-Ni-Be alloy system. The structural data identify the competing crystalline phases, formed on cooling from the liquid state, for various compositions in the Zr-Ti-X quasi-ternary phase diagram; with the Cu-Ni-Be ratio given by X = Be9Cu5Ni4. For this region of the ndimensional phase space (n=5), the bulk glass forming range is extensive and the calorimetric data exhibit thermal features associated with the occurrence of phase separation in the undercooled liquid state prior to primary crystallization. The topology of the composition manifold is complex; manifest by dramatic changes in crystallization behavior for small changes in the Zr-Ti-X ratio. Alloys with large supercooled liquid regions, Δ, are observed; ΔT≈ 135 K. Zr-Ti-Cu-Ni-Be alloy compositions with eutectic temperatures less than that of pure Al (Tm = 933 K) were also synthesized.
A Relation Between Fragility, Specific Heat, and Shear Modulus Temperature Dependence for Simple Supercooled Liquids
Authors: Granato A. V.
Published: April 2011

It is often said that the fragility and specific heat discontinuity at the glass transition are related characteristic features of glasses. We give a relation between these quantities for simple liquids, and also find that the temperature dependence of the shear modulus should be closely related to these quantities. According to the Interstitialcy Theory, the liquid state shear modulus G = Go, exp (-βc) = Go exp {-γ[(T/To)-l ]}, where Gois the shear modulus at a reference temperature To, which can be taken to be the glass temperature. In these relations, βis the diaelastic shear susceptibility, c is the interstitialcy concentration, T is the temperature, and dc/dT = γ/βTo. It has been proposed by Dyre, Olsen and Christensen that U in the viscosity η = ηo exp (U/kT) be given as the work done in shoving aside particles during a diffusion step, and therefore be proportional to the (liquid state) shear modulus. If so, then combining the above relations, the fragility F = [d log η/d (Tg/T]T=Tg becomes F=(1+γ) log [η (Tg)/ηo], where γ = - (Tg/G)(dG/dT). Then, since δCv, = UF(dc/dT), where UF is the interstitialcy formation energy, one obtains further with reasonable simplifying approximations δCv/Cv∼γG(T)/Goo, where Goo is the zero temperature crystalline shear modulus. For a typical fragile glass γ ∼ 2 (F ∼ 50) with G(Tg)/Goo ∼ 1/2, then δCv/Cv, ⊼ 1, ut close to zero for strong glasses, in fair agreement with available experimental results. The perspective given by the Interstititialcy Theory is then that the fragility is given phenomenologically by the temperature dependence of the shear modulus, or microscopically by the rate of increase of the equilibrium interstitialcy concentration with temperature.
Decomposition in Pd40Ni40P20 Bulk Metallic Glass
Authors: Miller M. K. , Schwarz R. B. , He Yi
Published: April 2011

An atom probe field ion microscope and 3-dimensional atom probe characterization of the solute distribution in a bulk Pd40Ni40P20 metallic glass in the as-cast state and after annealing has been performed. Statistical analysis of the atom probe atom-by-atom data detected the presence of short range ordering in the as-cast alloy. Phase separation at the nanometer level is observed in glassy samples after annealing above the glass-transition temperature. Crystallization proceeds by phase separation into three distinct crystalline phases. Atom probe analysis of the alloy annealed for 1 h at 410°C revealed that the primary nickel phosphide phase contained significant levels of palladium, the palladium-rich Pd3P phosphide phase contained low levels of nickel and there was a small amount of a palladium-nickel solid solution.
Thermochemistry and Kinetics of Gas-Phase Reactions Relevant to the CVD of Coatings: New Data for Process Models
Authors: Allendorf M. D. , Melius C. F. , McDaniel A. H.
Published: April 2011

Understanding the role of gas-phase reactions is an important step in the development of useful CVD process models. In this article, we review the general types of gas-phase reactions that can occur and discuss quantum-chemistry techniques for predicting their thermochemistry and kinetics. We also describe the use of high-temperature flow reactors to measure gas-phase reaction kinetics. Coupling these theoretical and experimental methods is a powerful approach to the characterization of CVD precursor chemistry. We illustrate this in a discussion of the reaction between BC13and NH3, which is important in the deposition of hexagonal boron nitride coatings.
Temperature Dependence of Wafer Backside Deposition in Single Wafer CVD Reactor
Authors: Wang Q. , Liu Dennis D. , Yeh Jim
Published: April 2011

A quantitative experiment was designed to characterize in detail the backside silicon nodule formation (BSNF) on a single wafer reactor. It is found that there are two BSNF related vapor processes. One is the silicon mass transfer between wafer and wafer susceptor, which is controlled by the temperature difference between wafer and the susceptor. The other is the chemical vapor deposition process due to the presence of precursor (trichlorosilane) at the vicinity of the wafer backside. The contributions of these two processes to BSNF, however, have opposite dependencies on wafer temperatures. The silicon mass transfer from susceptor to wafer backside decreased with increasing wafer temperature. The trichlorosilane deposition increases with increasing wafer temperature. The effects of these two processes are also strongly dependent on susceptor configuration. It is found that having a wafer in direct contact with the susceptor could minimize the trichlorosilane deposition process, but silicon mass transfer is still present at the edge of the wafers. This study demonstrates that careful process control and susceptor design are necessary to eliminate wafer backside deposition.
Nitrogen Defect Concentration in Chemical Vapor Deposited Homoepitaxial Diamond at High TemperatureS
Authors: Yan Chih-Shiue , Vohra Yogesh K.
Published: April 2011

Homoepitaxial diamond films were grown on polished single crystal {100} oriented natural type Ila diamond circular substrates using high density microwave plasma chemical vapor deposition (MPCVD). Twelve homoepitaxial diamond films were grown under a variety of substrate temperatures (1000 C-2000 C), methane concentration (1% - 3% in hydrogen gas) and processing pressure(60 - 150 torr). Electron paramagnetic resonance (EPR) studies demonstrate that nitrogen is largely incorporated as singly substitutional impurity (P1-center) and nitrogen's concentration is in the range of 10 -100 parts per million. Nitrogen's concentration also shows a systematic decrease with increasing substrate temperatures. The temperature variation of nitrogen at high temperatures is discussed within a model where single substitutional nitrogen aggregates to form nitrogen defects which are not EPR active.
Deposition and Properties of Diamond-Like Carbons
Authors: Robertson J
Published: April 2011

We review for diamond-like carbon the various deposition methods, the deposition mechanisms of subplantation and ion-assisted addition, the characterisation methods such as Raman and electron energy loss spectroscopy, its mechanical properties and some applications as a coating material.
Surface Phenomena of the Thin Diamond-Like Carbon Films
Authors: Polyakov V. I. , Rukovishnikov A. I. , Khomich A. V. , Druz B. L. , Kania D. , Hayes A. , Prelas M. A. , Tompson R. V. , Ghosh T. K. , Loyalka S. K.
Published: April 2011

Diamond-like carbon films (DLC) 10–40 nm thick were deposited on quartz substrates on an interdigitated planar array of 20 μm Ni electrodes already prepared by lithographic techniques. The influence of the adsorbed molecules onte electrical properties of the thin DLC films was investigated. Current and capacitance-voltage characteristics were examined. Charge-based deep level transient spectroscopy (Q-DLTS) was used for study of adsorption and desorption processes. The strong sensitivity of Q-DLTS spectra to the presence of the vapor water and isopropyl alcohol was found. For example, the Q-DLTS signal for some deposited DLC film was changed more then in order in presence of the water vapor. Such strong surface phenomena of the thin DLC films may be exploited in novel gas sensor devices.
Effect of Processing Parameters on the Chemistry of Magnetron Sputtered Ti-Al Thin Films
Authors: Kale A. S. , Seal J S. , Beaulieu K. , Sundaram K. B.
Published: April 2011

Titanium-aluminum alloys are proved to be a viable candidate material in the field of structural coatings due to their high temperature creep and oxidation resistance in addition to being a lightweight material. In the present paper, we study the effect of applied power on the mechanical properties and surface chemistry of dc-magnetron sputtered Ti3AI thin films. While SEM and XRD investigate the structure and morphology of the deposited films, XPS and AES proved to be an essential tool for studying the surface chemistry and detailed chemical bonding information of the Ti3AI thin films. Glancing angle XPS and Auger depth profiling were carried out to study the chemistry at a few layers beneath the surface to monitor the changes in the stoichiometry of Ti3AI films. Microhardness measurements indicate an increase in the thin film hardness with increased sputtering power. These data were further compared to thin films deposited under liquid N2 temperatures.
Optical Properties and Corrosion Resistance of Durable Silver Coatings
Authors: Fuqua P. D. , Barrie J. D.
Published: April 2011

There is a need in the optics community for highly reflective mirrors that do not degrade in harsh environments. While silver can be a highly efficient reflector of visible light, it suffers from poor resistance to corrosive atmospheric gases. Here, we investigate a durable silver coating scheme, in which silver is sputter deposited between very thin layers of nichrome nitride. Excellent corrosion resistance is achieved with a silicon nitride overcoat. Samples were exposed to a variety of atmospheres and it was found that the nichrome nitride dramatically increased corrosion resistance when compared to bare silver or silicon nitride coated silver. Similar results were found when electrochemical impedance spectroscopy was used to evaluate samples. The enhanced corrosion resistance is achieved even when the thickness of the nichrome nilTide adhesion layer is too thin to be considered continuous.
Cesium Sorption on Tonalite and Mica Gneiss
Authors: Siitari-Kauppi M. , Hölttä P. , Pinnioja S. , Lindberg A.
Published: April 2011

Different approaches for measuring the interaction between radionuclides and rock matrix are needed to test the compatibility of transport models and retardation experiments. In this work sorption of cesium (134Cs) was studied on unaltered mica gneiss, and on unaltered, moderately altered and strongly altered tonalite. The crushed rock samples were sieved into seven fractions from 0. 1 mm to 3.15 mm. The mass distribution ratio values for each fraction was determined using the static batch method. Cesium sorption onto different minerals was demonstrated using digital image analysis in order to interpret thin section autoradiographs. The autoradiographic method based on irradiation-induced luminescence properties in feldspars was applied in order to estimate the alteration of tonalites.

Cesium sorption on moderately altered and strongly altered tonalite was stronger than on unaltered rocks owing to larger specific surface areas and the composition of alteration minerals. Strongest sorption was found on biotite. Mass distribution ratio, Rd, values of 0.3–1.1 m3·kg−1 for unaltered rocks and 0.9–3.4 m3·kg−1 for altered rocks were determined. Moderate dependence on surface area was found for mica gneiss after I and 3 days sorption time. For strongly altered tonalite only slight dependence on surface area was found. Research-articled -values increased as a function of sorption time due to diffusion into the particles. Higher increase in values for larger diameter particles indicated the availability of internal specific surface areas.
Sorption of Radionuclides Onto Cement Materials Altered by Hydrothermal Reaction
Authors: Sugiyama D. , Fujita T.
Published: April 2011

The sorption of radionuclides onto cement materials is a very important parameter when considering the release of radionuclides from radioactive wastes. Once the composition or crystallinity of the constituent minerals of a cement material is changed by alteration in the disposal environment, its sorption ability might be affected. In this study, the effect of hydrothermal alteration on the sorption properties of two cement grouts and Calcium Silicate Hydrogels (CSH-gels) is investigated by using the batch sorption technique. In the case of strontium (a model cation) sorption, the distribution ratio for Ordinary Portland Cement (OPC) and OPC/Blast Furnace Slag blended cement (OPC/BFS) decreased as the alteration temperature increased. This is mainly caused by the decrease of sorption onto CSH-gel which is a constituent of cement materials. In the case of selenium (selenite, a model anion) sorption, the distributionratio decreased as the alteration temperature increased for OPC treated in both distilled water and groundwater, and for OPC/BFS in groundwater. This is attributed to the decomposition of ettringite which sorbs anions. The distribution ratio for OPC/BFS in distilled water increased as the alteration temperature increased, although ettringite decomposed. This is attributed to the formation of monosulphate which also sorbs anions. These results show that the hydrothermal alteration of cement mineral phases in a disposal environment may cause changes which could increase or decrease the sorption of radionuclides onto cements depending on the cement composition and radionuclide speciation.
Purification of Alkaline Waste Solutions from Np and Pu
Authors: Guelis A. V. , Shilov V. P. , Krot N. N.
Published: April 2011

Coprecipitation of Np and Pu with Co(OH)3 and Fe(OH)3, obtained by the Method of Appearing Reagents from Co(NH3)6 Cl3, K2FeO4, and Na2[Fe(CN)5NO]2H2O from alkaline solutions and from simulants of Hanford Site tank wastes has been studied. Removal of Np and Pu from pH 10.5 to 0.4 M NaOH is semiquantitative with decontamination factors of 80 to 400. The presence of EDTA and HEDTA interferes with the precipitation of Co(OH)3 and Fe(OH)3. The deleterious effect of EDTA was eliminated by Ca2 addition at a 1:1 molar ratio of Ca2:EDTA. The deleterious effect of HEDTA was eliminated by the addition of 0.11 M excess precursor Co(NH3)6Cl3 to increase the content of carrier Co(OH)3.
Formulation, Testing, and Structural Characterization of High-Zirconium High-Level Waste Glasses
Authors: McKeown David A. , Muller Isabelle S. , Buechele Andrew C. , Pegg Ian L. , Kendziora Christopher A. , Scales Charles R.
Published: April 2011

A range of compositions of high-zirconia borosilicate glasses were formulated and their structures investigated by a combination of techniques. These compositions have potential applications for high-level nuclear waste storage in combination with advanced reprocessing methods. Raman and Zr EXAFS data were collected for a series of glasses spanning a range of zirconia concentrations. The Raman spectra indicate that Zr acts as a silicate network modifier, where the silicate tetrahedral network depolymerizes as the zirconia content increases. Zr EXAFS analysis indicates that Zr is found in octahedral sites, and to a minor extent, sevencoordinated sites. As the zirconia content increases, the fraction of seven-coordinated Zr-sites increases; this may be the cause of ZrO2 baddeleyite crystallization that was observed in some Zr-rich glasses investigated.
Inventories of Iodine-129 and Cesium-137 in the Gaps and Grain Boundaries of LWR Spent Fuels
Authors: Gray W. J.
Published: April 2011

Performance assessment calculations that support geologic disposal of spent nuclear fuel in a potential repository at Yucca Mountain, Nevada, are based in part on the assumption that 2% of the total inventories of 135Cs, 129I, and 99Tc are located in the gap and grain-boundary regions where they could dissolve rapidly if the spent fuel were to be contacted by groundwater. Actual measured values reported here for a few light-water reactor (LWR) spent fuels show that the combined gap and grain-boundary inventories of 129I approximately equaled the fission-gas release fractions. For 137Cs, the combined gap and grain-boundary inventories were approximately one third of the fission-gas release fractions. These measured values can be used to replace the 2% estimate and thus reduce the uncertainties in the calculations.
Multi-Scale Near-Field Thermohydrologic Analysis of Alternative Designs for the Potential Repository at Yucca Mountain
Authors: Buscheck T. A. , Gansemer J. , Nitao J. J. , Delorenzo T. H.
Published: April 2011

A multi-scale, thermohydrologic (TH) modeling methodology has been developed that integrates the results from 1-, 2-, and 3-D drift-scale models and a 3-D mountain-scale model to calculate the near-field TH variables affecting the performance of the engineered barrier system (EBS) of the potential repository at Yucca Mountain. This information was used by Total System Performance Assessment—Viability Assessment (TSPA-VA) and is being used by the ongoing TSPA, supporting the License Application Design Selection, to assess waste-package (WP) corrosion, waste-form dissolution, and radionuclide transport in the EBS. Line-load WP spacing, which places WPs nearly end to end in widely spaced drifts, results in more locally intensive and uniform heating along drifts, causing hotter, drier, and more uniform conditions on WPs than point-load spacing, which is used in the VA design. Backfilling drifts with a granular material with coarse, well-sorted, nonporous grains (e.g., a coarse quartz sand) results in a large, persistent reduction in RH on WPs; point-load spacing allows only the medium-to-high-heat-output WPs to benefit from RH reduction, but line-load spacing enables all WPs to benefit.
Radionuclide Migration Analysis Using A Discrete Fracture Network Model
Authors: IJIRI Y. , SAWADA A. , WEBB E.K. , WATARI S. , HATANAKA K. , UCHIDA M. , ISHIGURO K. , UMEKI H. , DERSHOWITZ W.S.
Published: April 2011

This paper describes an approach for assessing the geosphere performance of nuclear waste disposal in fractured rock. In this approach, a three-dimensional heterogeneous channelnetwork model is constructed using a stochastic discrete fracture network (DFN) code. Radionuclide migration in the channel-network model is solved using the Laplace transform Galerkin finite element method, taking into account advection-dispersion in a fracture network, matrix diffusion, sorption in the rock matrix as well as radioactive chain decay. Preliminary radionuclide migration analysis was performed for fifty realizations of a synthetic block-scale DFN model. The total radionuclide release from all packages in the repository was estimated from the statistics of the results of fifty realizations under the hypothesis of ergodicity. The interpretation of the result of the three-dimensional network model by a combination of simpler one-dimensional parallel plate models is also discussed.
Impact of Ionic Strength on Colloid Mobility in Saturated And Unsaturated Porous Media
Authors: Gamerdinger A. P. , Kaplan D. I. , H. J. H.
Published: April 2011

A model system consisting of well-characterized quartz sand, negatively charged carboxyl-modified latex microspheres, and synthetic J- 13 groundwater was used as a first-step in defining limiting conditions for colloid transport at the proposed Yucca Mountain Repository for radioactive waste. Colloid (280-nm diameter) and tracer (bromide) transport was determined using column methods under steady-state flow conditions for both saturated and unsaturated moisture conditions. Experiments were conducted at three water contents (13%, 70%, and 100% saturated) and four ionic strengths (deionized water, and 0.1 x, 1x, and 10x the ionic strength of J-13 groundwater, which has an ionic strength of 0.0116). Colloid mobility in the 13% saturated columns was appreciably less than in the 70% and 100% saturated columns, whereas colloid mobility in the 70% and 100% saturated columns were quite similar. Ionic strength had a significant impact on colloid retention, becoming more pronounced in the drier systems. Colloids were mobile in deionized water and the 0.1x J-13 groundwater. Retention was approximately 20% in lx J-13 groundwater and 100% in the 10x J-13 groundwater. Compared with the 70% and 100% saturated conditions, colloid mobility decreased for the 10% saturated condition, with greater than 50% retention for the 1x J-13 groundwater system. As observed for the 70% and 100% saturated conditions, colloids in the 10x J-13 solution were essentially immobile. Colloid mobility is greatly affected by the ionic strength of the soil solution and this effect is moisture saturation dependent.
Study of Th4(PO4)4 P2O7 and Solid Solutions With U(IV), Np(IV) and Pu(IV): Synthesis, Characterization, Sintering and Leaching Tests
Authors: Dacheux N. , Thomas A. C. , Chassigneux B. , PichotI E. , Brandel V. , Genet M.
Published: April 2011

In the framework of radwaste storage, the thorium phosphate-diphosphate Th4(PO 4)4P 2O7 (TPD) has been synthesized in the ThO2 – P2O5 system. This material is very resistant to aqueous corrosion. In this paper we report the conditions of synthesis of TPD doped with small amounts of trivalent actinides and of solid solutions with uranium (IV), neptunium (IV) and plutonium (IV). Sintered samples of TPD and Th4_xUx(PO4)4P2O7 (TUPD) have been prepared after pressing at 100-800 MPa at room temperature then heating at 1250°C for 10 hours. The corresponding density varies from 87 to 93% of the calculated value from crystal data. Leaching tests of Th4 (PO4)4P2O7 doped with Am3+ or Cm3+ and solid solutions Th4_xUx(PO4)4P2O7 (TUPD) or Th4-xPux(PO4)4P2O7 (TPPD) have been performed in water and in several acidic media at room temperature and at 90'C. The dependence of the leaching rate on the pH has been determined: the partial order related to [H3O+] is in the 0.35 – 0.44 range while the apparent leaching rate is equal to about 1 x 10–5x–4 x 10–5 g/(m2d).
Methodologies for Predicting the Performance of Ni-Cr-Mo Alloys Proposed for High Level Nuclear Waste Containers
Authors: Dunn D. S. , Cragnolino G. A. , Sridhar N.
Published: April 2011

For the geologic disposal of the high level nuclear waste (HLW), aqueous corrosion is considered to be the most important factor in the long-term performance of containers, which are the main components of the engineered barrier subsystem. Container life, in turn, is important to the overall performance of the repository system. The proposed container designs and materials have evolved to include multiple barriers and highly corrosion resistant Ni-Cr-Mo alloys, such as Alloys 625 and C-22. Calculations of container life require knowledge of the initiation time and growth rate of localized corrosion. In the absence of localized corrosion, the rate of general or uniform dissolution, given by the passive current density of these materials, is needed. The onset of localized corrosion may be predicted by using the repassivation and corrosion potentials of the candidate container materials in the range of expected repository environments. In initial corrosion tests, chloride was identified as the most detrimental anionic species to the performance of Ni-Cr-Mo alloys. Repassivation potential measurements for Alloys 825, 625, and C-22, conducted over a wide range of chloride concentrations and temperatures, are reported. In addition, steady state passive current density, which will determine the container lifetime in the absence of localized corrosion, was measured for Alloy C-22 under various environmental conditions.
A Stem Study of P and Ge Segregation to Grain Boundaries in Si1-xGex Thin Films
Authors: Qin W. , Ast D. G. , Kamins T. I.
Published: April 2011

The segregation of phosphorus to grain boundaries in phosphorus implanted Si0.87Ge0.13 films, deposited by chemical vapor deposition (CVD), was directly observed by scanning transmission electron microscopy (STEM) with energy dispersive x-ray (EDX) microanalysis. The segregation was determined to be a thermal equilibrium process by measuring and comparing the average phosphorus concentrations at the grain boundaries in Si0.87Ge0.13 films subjected to 700, 750 or 800°C annealing, following the implantation and 1000°C annealing processes. The measured segregation energy was 0.28 eV/atom. No Ge segregation was found at grain boundaries in phosphorus implanted Si0.87Ge0.13 films by STEM x-ray microanalysis. Neither was evidence shown by STEM microanalysis that Ge segregated to grain boundaries in intrinsic Si1-xGex films with x = 0.02, 0.13 and 0.31. Secondary ion mass spectrometry (SIMS) analysis showed that these intrinsic Si1-xGex films contained 1019 to 4 × 1019/cm-3H, depending on the deposition temperature.
Experimental Verification of a Random Medium Model for the Optical Behaviour of Ultrathin Crystalline Silicon Layers Grown on Porous Silicon
Authors: Stalmans L. , Abouelsaood A.A. , Ghannam M.Y. , Poortmans J. , Bender H. , Caymax M. , Nijs J.
Published: April 2011

The random medium model for porous silicon (PS) is applied to a porous silicon layer to be used as a back reflector in a thin film silicon solar cell realized in an ultrathin epitaxially grown Si layer on PS. Three-layer-structures (epi/PS/Si) have been fabricated by RPCVD of 150-1000 nm epitaxial silicon layers on the porous surface of silicon wafers. The light reflection has been measured in the 300-1000 nm wavelength range. An excellent agreement is found between the experimentally measured reflectance curves and those calculated using the random medium model. The analysis shows that the epitaxial growth has led to an appreciable reduction in the porosity in the intermediate PS layer from about 60% to 20-30%.
Amorphous Silicon Alloy Materials and Solar Cells Near the Threshold of Microcrystallinity
Authors: Yang J. , Guha S.
Published: April 2011

One of the most effective techniques used to obtain high quality amorphous silicon alloys is the use of hydrogen dilution during film growth. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. As the hydrogen dilution increases, however, a threshold is reached, beyond which microcrystallites begin to form rapidly. In this paper, we review some of the interesting features associated with the thin film materials obtained from various hydrogen dilutions. They include the observation of linear-like objects in the TEM micrograph, a shift of the principal Si TO band in the Raman spectrum, a sharp, low temperature peak in the H2 evolution spectrum, a shift of the wagging mode in the IR spectrum, and a narrowing of the Si (111) peak in the X-ray diffraction pattern. These spectroscopic tools have allowed us to optimize deposition conditions to near the threshold of microcrystallinity and obtain desired high quality materials. Incorporation of the improved materials into device configuration has significantly enhanced the solar cell performance. Using a spectral-splitting, triple-junction configuration, the spectral response of a typical high efficiency device spans from below 350 nm to beyond 950 nm with a peak quantum efficiency exceeding 90%; the triple stack generates a photocurrent of 27 mA/cm2. This paper describes the effect of the improved materials on various solar cell structures, including a 13% active-area, stable triple-junction device.
Kinetics of Light-Induced Changes in P-I-N Cells with Protocrystalline Si:H
Authors: Koval R.J. , Koh J. , Lu Z. , Lee Y. , Jiao L. , Collins R.W. , Wronski C.R.
Published: April 2011

Studies have been carried out on the thickness dependent transition between the amorphous and microcrystalline phases in intrinsic Si:H materials (i-layers) and its effect on p-i-n solar cell performance [1]. P(a-SiC:H)-i(a-Si:H)-n(μcSi:H) cell structures were deposited with the intrinsic Si:H layer thickness and the flow ratio of hydrogen to silane, R=[H2]/[SiH4], guided by an evolutionary phase diagram obtained from real-time spectroscopic ellipsometry. The thickness range over which the fill factors are controlled by the bulk was established and their characteristics investigated with different protocrystalline i-layer materials (i.e., materials prepared near the amorphous to microcrystalline boundary but on the amorphous side). Insights into the properties of these materials and the effects of the transition to the microcrystalline phase were obtained from the systematic changes in the initial fill factors, their light-induced changes, and their degraded steady states for cells with i-layers of different thickness and H2 dilution.
Stability of Amorphous Silicon Thin Film Transistors
Authors: Wehrspohn R.B. , Deane S.C. , French I.D. , Hewett J. , Powell M.J.
Published: April 2011

Dangling bond defects are created during positive bias stress of amorphous silicon thin film transistors and there is an energy barrier between 0.9 and 1 eV for this process. We have studied how this energy barrier depends on the material parameters of the amorphous silicon, namely hydrogen content, hydrogen bonding, Urbach energy and intrinsic, deposition induced stress. We observe no dependence on the hydrogen content or hydrogen bonding type, but we do observe a clear dependence on the Urbach energy and the intrinsic stress. These measurements support a localized model for defect creation involving Si-Si bond breaking and the switching of a neighboring H atom to stabilize the broken bond. These results suggest that stable amorphous silicon TFTs can be obtained at low deposition temperatures by control of the deposition-induced, intrinsic stress.
Modelling of Drift Mobility Experiments on a-Si:H
Authors: Chen Wen Chao , Hamel Louis-André , Kemp Mathieu , Yelon Arthur
Published: April 2011

We present recent results of a study of the behavior of electronic carriers in a-Si:H, using the model of multiple trapping (MT) in an exponential density of states. In previous publications, using Monte Carlo simulations, we showed that the standard low field MT model gives reasonable agreement with experiment particularly if the Meyer-Neldel effect is included in the model. We report here on the results of including two other effects. First, we have included a simple model of field assisted detrapping, to take account of the effect of high fields. We obtain very good agreement with the results of measurements on both electrons and holes, from a number of laboratories. In addition, we show here that the validity of an effective temperature approach can be checked easily by comparison with experiment. Second, we have presented a simple model of rapid relaxation of trapped carriers. This model offers the possibility of removing the apparent inconsistency between these measurements, and other experiments.
Two Carrier Sensitization as a Spectroscopic Tool for a-Si:H
Authors: Fonseca L.F. , Weisz S.Z. , Rapaport R. , Balberg I.
Published: April 2011

In a recent letter we have reported the first observation of the phenomenon of minority carrier-lifetime sensitization in hydrogenated amorphous silicon (a-Si:H). We find now that combining the study of this phenomenon with the study of the well-known phenomenon of majority carrier lifetime sensitization, in this material, can provide direct information on its density of states (DOS) distribution. This finding is important in view of the limitations associated with other methods designed for the same purpose. We have carried out then an experimental study of the effect of light soaking on the phototransport in a-Si:H. We found that the increase of the dangling bond concentration with light soaking affects the sensitization and thermal quenching of the majority carriers lifetime. Using computer simulations, we further show that the details of the observations associated with the sensitization effect yield semiquantitative information on the concentration and character of the recombination centers in a-Si:H.
Collection Efficiencies Greater Than Unity by Electron Or Hole Gating in a-Si:H p-i-n Diodes
Authors: Zollondz J.-H. , Main C. , Reynolds S.
Published: April 2011

We report measured electron and hole gating in thick a-Si:H (3.5 μm) p-i-n diodes under reverse bias conditions. Previous publications have shown very high collection efficiency values for electron gating (p-side bias, n-side probe) of up to 50 (i.e. 5000%) for measured and simulated data and predictions of up to 400 (i.e. 40000%) from simulations. Reversing the usual sides of illumination for (electron) gating a situation can be created where, by n-side bias and p-side probe illumination, holes can be gated to travel through the sample to be collected at the contact. Even though the holes have much lower mobility, by this process we can still obtain collection efficiencies greater than unity. This measurement is more difficult because of unwanted illumination by stray bias beam photons on the more sensitive p-side, caused by reflections within the apparatus. Simulation of this situation corroborates qualitatively the measured data. A wide ranging study of the gating phenomenon in relation to different incident wavelengths and photon fluxes for bias and probe beam is reported. We present comparisons of electron and hole gating by measurement and simulation and explain the phenomenon for both electron and hole gating in terms of field changes near to the incident bias interface.
Electronic Transitions in Mixed Phase Crystalline/Amorphous Silicon in the Low Crystalline Fraction Regime
Authors: Cohen J. David , Kwon Daewon , Chen Chih-Chiang , Jin Hyun-Chul , Hollar Eric , Robertson Ian , Abelson John R.
Published: April 2011

Amorphous silicon films were prepared by dc reactive magnetron sputtering under conditions approaching the phase transition to microcrystallinity. Using TEM imaging these films were found to contain clusters of 5 to 50 nm sized Si crystallites embedded in an amorphous silicon matrix. Photocapacitance and transient photocurrent sub-band-gap optical spectra of this material appear to consist of a superposition of a spectrum typical of amorphous silicon together with an optical transition, with a threshold near 1. 1eV, that exhibits a very large optical cross section. This transition arises from valence band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. Using modulated photocurrent methods we have determined that these states also dominate the electron deep trapping in this material. We argue that these states arise from defects at the crystalline-amorphous boundary.
Plasma Etching Conditioning of Textured Crystalline Silicon Surfaces for a-Si/c-Si Heterojunctions
Authors: Rosa R. De , Addonizio M.L. , Chiacchio E. , Roca F. , Tucci M.
Published: April 2011

The development of a hybrid heterojunction fabricated by growing ultrathin amorphous silicon by Plasma Enhanced Chemical Vapor Deposition using temperatures below 250°C offers the potential of obtaining high efficiency solar cells deposited on glassy substrates. The surface preparation represents one of the most critical steps. The first aim of etching is to remove the native oxide layer from the surface of the crystalline wafer, before amorphous layer deposition. The possibility of obtaining this goal with a dry procedure that reduces the exposure of the sample to the environment is not trivial.

We performed several dry etching processes but the best results were obtained using an etching process involving CF4/O2 gases. We have found evidence that plasma etching acts by removing the native oxide and the damaged surface of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis has confirmed that it is possible to find plasma process conditions where no appreciable damage and changes in surface morphology are induced. Detailed investigation was performed to find compatibility and optimization of amorphous layer deposition both on flat and textured cast silicon by changing the plasma process parameters. By using this process we achieved on cast silicon for solar applications photovoltaic conversion efficiencies of 12.9% on 51 cm2 and 9.2% on 45 cm2 active areas for amorphous crystalline heterostructure devices realized on monocrystalline and polycrystalline silicon respectively. We also investigated the compatibility of the process with industrial production of large area devices.
Deposition of a-Si:H Devices in a RTR System for Photovoltaic and Macroelectronic Applications
Authors: Scholz M. , Peros D. , Böhm M.
Published: April 2011

This work presents first results of potential manufacturing processes for integrated series connected hydrogenated amorphous silicon (a-Si:H) thin film solar modules and/or pindiode/TFT based macroelectronic circuits on flexible tapes. A RTR (Reel-To-Reel) deposition system on laboratory scale has been built, The system consists of seven metal sealed LIHV stinless steel chambers to obtain ultra high vacuum as a basis for high quality a-Si:H layers, in order to support continuous movement of the tape in the RTR process the chambers cannot be isolated from each other. The necessary pressure difference between the sputtering chambers and the PECVD (Plasma Enhanced Chemical Vapor Deposition) chambers is provided by pressure stages. They are optimized for high molecular flow resistance without any influence on the moving substrate tape. The back metal contacts and the semitransparent TCO (Transparent Conductive Oxide) contacts are deposited by rf magnetron sputtering, the a-Si:H film system is deposited by PECVD. Parallel to the film deposition a Nd:YAG laser patterning system is coupled into one chamber. This allows for instance a total manufacturing of integrated series connected solar modules in one system without breaking the vacuum. Our present investigations focus on the deposition of doped and intrinsic high quality a-Si:H based layers in neighboring chambers. The quality of semiconducting films deposited in adjacent chambers is studied with regard to potential contamination effects.
Thin-Film Transistors based on Hot-Wire Amorphous Silicon on Silicon Nitride
Authors: Stannowski B. , Meiling H. , Brockhoff A. M. , Schropp R. E. I.
Published: April 2011

We present state-of-the-art thin-film transistors (TFTs) incorporating amorphous silicon i-layers deposited by hot-wire chemical vapor deposition. The TFTs are deposited on glow-discharge silicon nitride as well as on thermally-grown silicon dioxide. The devices on silicon nitride have a field-effect mobility above 0.7 cm2/Vs, a threshold voltage around 2 V and a sub-threshold slope as low as 0.5 V/dec. As commonly observed, the TFTs on silicon-dioxide have higher values for the threshold voltage and the sub-threshold slope. In the annealed state the hot-wire TFTs show almost the same properties as TFTs deposited by conventional plasma-enhanced chemical vapor deposition. Nevertheless, the stress-time dependent behavior under prolonged gate-voltage stress at elevated temperature is different from that of the glow-discharge devices. The hot-wire TFTs are clearly more stable than their glow-discharge counterparts. Furthermore, we found differences in the stress behavior of the hot-wire TFTs deposited on silicon nitride and silicon dioxide.
Temperature Dependent Transient Leakage Currents in Amorphous Silicon Thin Film Transistors
Authors: Lemmi F. , Street R. A.
Published: April 2011

The transient response of a-Si:H thin film transistors (TFT) provides information about gap states in the channel and about the TFT conduction mechanisms. We present results of a detailed parametric study of transient leakage currents in TFTs, extending from 10-3 sec to >100 sec and at temperatures up to 400K. The form of the transient decay depends most notably on the off-state gate voltage, which determines the leakage current mechanism and magnitude. The measurements use chains of many TFTs, such that we can measure dc on/off ratios of >1011.

In the sub-threshold region (e.g. Vg~-2V) some samples exhibit non-monotonic decay of the drain current, with a minimum followed by a slow increase to a steady state. Typically this effect is barely visible at room temperature due to the extremely low values of the dc current. Higher temperatures enhance the phenomenon, which has activation energy of about 1 eV for the current magnitude and time constant. We attribute the effect to back-channel electron conduction. This explanation is consistent with the form of the TFT transfer characteristics, and the weak drain-source voltage dependence excludes a contact injection mechanism.

At more negative gate off-state voltages, some samples exhibit increasing leakage currents, indicative of hole conduction. These samples feature a monotonically decreasing transient decay with a pronounced drain-source voltage dependence, consistent with contact injection being enhanced by a hole-enriched channel in the high field drain region.

Effects of on-state gate pulse width and amplitude are also discussed, as well as the effect of high temperature stress on the TFT transients.
High-Performance Damascene-Gate Thin Film Transistors
Authors: Ma Eugene , Wagner Sigurd
Published: April 2011

We report a novel TFT structure where the gate metal is embedded into a SiNx passivation layer. This allows the subsequent gate dielectric layer to be much thinner than in conventional bottom-gate structures. thereby reducing the threshold voltage and the sub-threshold slope. TFTs employing these damascene-gate structures were fabricated with SiNX gate dielectrics as thin as 50 nm. Such devices exhibit threshold voltages of 0.9 V, sub-threshold slopes of 0.1 V/dec, ION/IOFF current ratios of 106 and linear region field-effect mobilities of 0.6 cm2/Vs.
Flexible A-SI Based Solar Cells with Plastic Film Substrate
Authors: Ichikawa Yukimi , Fujikake Shinji , Tabuchi Katsuya , Sasaki Toshiaki , Hama Toshio , Yoshida Takashi , Sakai Hiroshi , Saga Misao
Published: April 2011

A flexible amorphous silicon (a-Si) based photovoltaic (PV) module has been developed. The a-Si solar cell fabricated on a heat-resistance plastic film with a thickness of 50μm has a new monolithic series-connected structure named SCAF to obtain a high output voltage required for practical use. Applying a-Si/a-SiGe double-junction structure to 40cm × 80cm SCAF solar cells, we obtained a stabilized efficiency of 9% with an excellent reproducibility. Flexible PV modules fabricated from the SCAF solar cells have been tested in several demonstration sites to study their durability under outdoor conditions.
Optimization of High Efficiency Amorphous Silicon Alloy based Triple-Junction Modules
Authors: Banerjee A. , Yang J. , Guha S.
Published: April 2011

A systematic approach has been used to scale up high efficiency 0.25cm2 active-area amorphous Si alloy based triple-junction devices to high-efficiency encapsulated modules of aperture area ~920cm2. In order to analyze the losses involved in the scale-up, intermediate aperture area, 40cm2 and 450cm2, modules have also been fabricated. The best stable active-area efficiency obtained on the small-area cells is 12.9%. The best initial efficiency of a ~920cm2 aperture area encapsulated module is 12.1%. National Renewable Energy Laboratory (NREL) has independently light soaked three of the ~920cm2 modules. They have measured a stable efficiency of 10.5% which represents a new world record. This paper presents various aspects of the large-area module work.
Amorphous Silicon Solar Cells Techniques for Reactive Conditions
Authors: Shimizu Satoshi , Okawa Kojiro , Kamiya Toshio , Fortmann C.M. , Shimizu Isamu
Published: April 2011

The preparation of amorphous silicon films and solar cells using SiH2Cl2 source gas and electron cyclotron resonance assisted chemical vapor deposition (ECR-CVD) was investigated. By using buffer layers to protect previously deposited layers improved a-Si:H(Cl) solar cells were prepared and studied. The high quality a-Si:H(Cl) films used in this study exhibited low defect densities (~1015cm-3) and high stability under illumination even when the deposition rate was increased to ~15A/s. The solar cells were deposited in the n-i-p sequence. These solar cells achieved VOC values of ~ 0.89V and ~ 3.9% efficiency on Ga doped ZnO (GZO) coated specular substrate. The a-Si:H(C1) electron and hole μτ products were ~10-8cm2/V.