bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Coordination Polyhedra Symmetry in Pyrochlore and Zirconolite Structures
Authors: Troole A. Y. , Stefanovsky S. V.
Published: April 2011

Zirconolite and pyrochlore are considered as promising host phases for high level waste (HLW). However, correct information on substitution mechanisms, forms of dopants incorporation in their structures and distortions in coordination polyhedra is presently unavailable. To clarify these points we use the electron paramagnetic resonance (EPR). Pyrochlore and three of zirconolite polytypes: zirconolite-2M, zirconolite-3T, and zirconolite-30 are considered. Pyrochlore is the “parent” structure for zirconolite since any zirconolite variety is produced by means of distortion of the initial pyrochlore structure. Space groups of pyrochlore and basic polymorphous zirconolite varieties found from XRD and TEM data, as well as interatomic distances and angles, were taken from reference data. This allows the determination of the most probable sites for impurities, substitution mechanisms, and local symmetry of coordination polyhedra (initial). Ions chosen for EPR were Gd (III) as an analog of trivalent rare earth and actinide elements which are also occurred in HLW and Fe (III) as a typical corrosion product which occurs in all HLW. For Gd (III) a strong ligand field approximation is suggested, theoretical computation using perturbation theory in this approximation has been carried out. All the non-diagonal members plus magnetic field were chosen as perturbation and formulae for transition frequencies, estimations of fine structure and g-factors parameters in the given approximation have been obtained.
Americium-Curium Vitrification Process Development (U)
Authors: Fellinger A. P. , Baich M. A. , Hardy B. J. , Jannik G. T. , Jones T. M. , Marra J. E. , Miller C. B. , Miller D. H. , Peeler D. K. , Snyder T. K. , Stone M. E. , Witt D. C.
Published: April 2011

The successful demonstration of sequentially drying, calcining and vitrifying an oxalate slurry in the Drain Tube Test Stand (DTTS) vessel provided the process basis for testing on a larger scale in a cylindrical induction heated melter. A single processing issue, that of batch volume expansion, was encountered during the initial stages of testing. The increase in batch volume centered on a sintered frit cap and high temperature bubble formation. The formation of a sintered frit cap expansion was eliminated with the use of cullet. Volume expansions due to high temperature bubble formation (oxygen liberation from cerium reduction) were mitigated in the DTTS melter vessel through a vessel temperature profile that effectively separated the softening point of the glass cullet and the evolving oxygen from cerium reduction. An increased processing temperature of 1470°C and a two hour hold time to fine any remaining bubbles successfully reduced bubbles in the poured glass to an acceptable level. The success of the preliminary process demonstrations provided a workable process basis that was directly applicable to the newly installed Cylindrical Induction Melter (CIM) system, making the batch flowsheet the preferred option for vitrification of the americium-curium surrogate feed stream.
Corrosion of Aluminum-Uranium Alloys in Water Vapor at 200 °C
Authors: Lam Poh-Sang , Sindelar Robert L. , Barrett Kelli Y.
Published: April 2011

Specimens of aluminum-uranium alloys at 10 and 18 weight percent uranium were exposed to a saturated water vapor condition at 200 °C up to about 12 weeks and compared to previous results for aluminum 1100. The aluminum-uranium materials exhibited a range of initial corrosion rates and approached similar rates with the formation of a passive film of boehmite (Al2O3·H2O). The initial corrosion rates of the aluminum-uranium materials were one to four times higher than that for aluminum 1100. It is postulated that a micro-galvanic coupling between the large UA14 particles and the aluminum matrix has caused this difference. Sectioning the exposed specimens shows different characteristics of the oxide layers. In the oxide on the aluminum-10% uranium alloy (Al-10%U), small uranium aluminide particles can be seen in a boehmite matrix and do not seem to be corroded. The oxide film on the aluminum- 18% uranium alloy (Al-18%U) appears to have two distinct oxide layers. The outer layer has mass aggregates in a boehmite matrix, while the inner layer contains UA14 particles as in the case of Al-10%U.
Stochastic Modeling of Long-Term Waste Package Degradation Incorporating Expert Elicitation on Corrosion Processes
Authors: Lee J. H. , Mori K. G. , Longsine D. E. , Bullard B. E.
Published: April 2011

The Viability Assessment (VA) of the potential repository for high-level nuclear waste at Yucca Mountain, Nevada was completed [1]. In the VA design concept, a two-layer waste container provides the primary component of the engineered barrier system (EBS). The VA reference design specifies a 100-mm thick carbon steel as the outer barrier and a 20-mm thick Alloy 22 as the inner barrier. A stochastic simulation model was developed to analyze long-term performance of the waste package in the potential repository. The model was developed by incorporating the latest corrosion data and models for the candidate waste package materials and the information developed from the Waste Package Degradation Expert Elicitation (WPDEE) [2].

The reference case results showed that only a small fraction of waste packages fail by localized corrosion (i.e., pit penetrations). The analysis also indicated the degradation mode that is most important to long-term waste package degradation is general corrosion (or passive dissolution) of the inner barrier under dripping conditions. However, the general corrosion rates for dripping conditions used in the analysis have a range over three to five orders of magnitude. This is due mainly to a lack of information on local chemical and electrochemical conditions on the inner barrier after the outer barrier breach. This paper discusses further the areas and issues that need improvement to reduce uncertainty in the waste package degradation analysis. It also discusses additional waste package degradation modes and associated processes that need to be included in the future analysis for the potential repository to enhance the confidence of the analysis.
Scientific Bases for Cladding Credit as a Barrier to Radionuclide Release at the Proposed Yucca Mountain Repository+
Authors: Ahn T. M. , Cragnolino G. A. , Chan K. S. , Sridhar N.
Published: April 2011

The performance of Zircaloy nuclear fuel cladding in the environment of the proposed Yucca Mountain (YM) high-level waste (HLW) repository is evaluated. Because the uniform aqueous corrosion/oxidation rate is extremely slow, this evaluation focuses on mechanical failure, localized corrosion, stress corrosion cracking (SCC), hydrogen embrittlement, and initial failure. Mechanical failure is expected to result from (1) disruptive events such as rockfalls from seismicity, faulting, and igneous activities, (2) creep, and (3) splitting by oxidation of the spent fuel (SF) matrix. Effects of chloride ions and radiolysis are evaluated in localized corrosion and SCC. Embrittlement can be caused by delayed-hydride cracking (DHC) and hydride reorientation. Among these cladding failure modes, rockfalls, other disruptive events and initial defects can be important to performance prior to the container failure by corrosion. Confirmatory tests are required to evaluate the susceptibility to splitting by secondary mineral formation, localized corrosion, SCC, and hydride reorientation. Reliable temperature calculations with backfilling are also required. After breach, cladding may still assure slow release of radionuclides through perforations because localized failures may limit the exposure of the SF matrix and may provide high mass-transfer resistance. Failures prior to the repository emplacement arising from reactor operation, pool storage, dry storage, and transportation are also considered.
Alternative Assessments of the Performance of the Yucca Mountain Candidate HLW Repository
Authors: Kessler J. H. , MCGuire R. K. , Vlasity J. A. , Klos R. A.
Published: April 2011

This paper summarizes some of the most recent total system performance assessment (TSPA) work supported by EPRI for the proposed repository at Yucca Mountain, Nevada. In Part I of the paper, (standard) TSPA analyses are presented. In Part II examples of two types of analyses that augment standard TSPAs are provided that suggest: (1) many components of the Yucca Mountain system contribute to overall hazard reduction; and (2) the biosphere dose conversions used, based on reasonably maximizing assumptions about future human behavior, provide a reasonable upper bound. These additional analyses should provide further confidence when considering more (standard) TSPA analyses.
The Modeler's Influence on Calculated Solubilities for Performance Assessments at the Äspö Hard-Rock Laboratory
Authors: Emrén A. T. , Arthur Randy , Glynn Pierre D. , McMurry Jude
Published: April 2011

Four researchers were asked to provide independent modeled estimates of the solubility of a radionuclide solid phase, specifically Pu(OH)4, under five specified sets of conditions. The objectives of the study were to assess the variability in the results obtained and to determine the primary causes for this variability.

In the exercise, modelers were supplied with the composition, pH and redox properties of the water and with a description of the mineralogy of the surrounding fracture system A standard thermodynamic data base was provided to all modelers. Each modeler was encouraged to use other data bases in addition to the standard data base and to try different approaches to solving the problem.

In all, about fifty approaches were used, some of which included a large number of solubility calculations. For each of the five test cases, the calculated solubilities from different approaches covered several orders of magnitude. The variability resulting firom the use of different thermodynamic data bases was in most cases, far smaller than that resulting from the use of different approaches to solving the problem.
Effective Diffusivities of Iodine, Chlorine, and Carbon in Bentonite Buffer Material
Authors: Kato H. , Nakazawa T. , Ueta S. , Muroi M. , Yasutomi I. , Fujihara H.
Published: April 2011

Effective diffusivities of iodine, chlorine, and carbon in mixtures of bentonite and sand were determined by measuring the effective diffusivities of common chemical compounds labeled with radioactive isotopes of these elements. For carbon, both inorganic and organic carbon compounds were used in order to consider the variety of chemical forms of carbon possible in a radioactive waste repository. The bentonite content and dry density of the bentonite–sand mixture were varied. Two chemically different aqueous solutions, representing concrete pore water and bentonite pore water, were used to represent different conditions that could affect diffusivity in bentonite buffer material in a hypothetical radioactive waste disposal situation.

The effective diffusivities of iodine, chlorine, and carbon tended to decrease with increasing bentonite content and dry density of the mixture. In the presence of simulated concrete pore water, the effective diffusivities for iodine, chlorine, and carbon in the bentonite mixtures were not higher than those obtained when simulated bentonite pore water was used. Except for some organic compounds, the measured effective diffusivities were lower than that of tritiated water under the same experimental conditions. This was attributed primarily to exclusion of anions from the bentonite pores. The effective diffusivity of carbon depended on its chemical form. The effective diffusivity of the anionic forms of organic carbon tested (carboxylic acids ) was as low as that of inorganic anionic carbon.

Measured effective diffusivities were compared with those calculated using a model based on electrical double layer theory. The theory was applied to calculate distributions of electrolyte ions and diffusing ions in the bentonite pores. The calculated effective diffusivities showed good agreement with the measured values.
The Channel Network Model and Field Applications
Authors: Khademi B. , Moreno L. , Neretnieks I.
Published: April 2011

The Channel Network model describes the fluid flow and solute transport in fractured media. The model is based on field observations, which indicate that flow and transport take place in a threedimensional network of connected channels. The channels are generated in the model from observed stochastic distributions and solute transport is modelled taking into account advection and rock interactions, such as matrix diffusion and sorption within the rock. The most important site-specific data for the Channel Network model are the conductance distribution of the channels and the flow-wetted surface. The latter is the surface area of the rock in contact with the flowing water. These parameters may be estimated from hydraulic measurements. For the Äspö site, several borehole data sets are available, where a packer distance of 3 metres was used. Numerical experiments were performed in order to study the uncertainties in the determination of the flowwetted surface and conductance distribution. Synthetic data were generated along a borehole and hydraulic tests with different packer distances were simulated.

The model has previously been used to study the Long-term Pumping and Tracer Test (LPT2) carried out in the Äspö Hard Rock Laboratory (HRL) in Sweden, where the distance travelled by the tracers was of the order hundreds of metres. Recently, the model has been used to simulate the tracer tests performed in the TRUE experiment at HRL, with travel distance of the order of tens of metres. Several tracer tests with non-sorbing and sorbing species have been performed.
A Study of Rock Matrix Diffusion Properties by Electrical Conductivity Measurements
Authors: Ohlsson Y. , Neretnieks I.
Published: April 2011

Traditional rock matrix diffusion experiments on crystalline rock are very time consuming due to the low porosity and extensive analysis requirements. Electrical conductivity measurements are, on the other hand, very fast and larger samples can be used than are practical in ordinary diffusion experiments. The effective diffusivity of a non-charged molecule is readily evaluated from the measurements, and influences from surface conductivity on diffusion of cations can be studied. A large number of samples of varying thickness can be measured within a short period, and the changes in transport properties with position in a rock core can be examined.

In this study the formation factor of a large number of Äspö diorite samples is determined by electrical conductivity measurements. The formation factor is a geometric factor defined as the ratio between the effective diffusivity of a non-charged molecule, to that of the same molecule in free liquid. The variation of this factor with position along a borecore and with sample length, and its coupling to the porosity of the sample is studied. Also the surface conductivity is studied. This was determined as the residual conductivity after leaching of the pore solution ions. The formation factor of most of the samples is in the range 1E-5 to 1E-4, with a mean value of about 5E-5. Even large samples (4-13 cm) give such values. The formation factor increases with increasing porosity and the change in both formation factor and porosity with position in the borecore can be large, even for samples close to each other.

The surface conductivity increases with increasing formation factor for the various samples but the influence on the pore diffusion seems to be higher for samples of lower formation factor. This suggests that the relation between the pore surface area and the pore volume is larger for samples of low formation factor.
Bedrock Characterisation of Four Candidate Repository Sites in Finland As Determined by He-Gas Methods
Authors: Hartikainen J. , Hartikainen K. , Timonen J. , Hautojärvi A.
Published: April 2011

At the end of the year 2000, one of four sites will be chosen as the final repository site in Finland. Therefore accurate and comprehensive statistics of the bedrock characteristics such as porosity [% ] and effective diffusion coefficient [m2/s ] of these sites are of importance. Altogether 115 rock samples from the four sites were measured by different He-gas methods to achieve this goal.

The results obtained indicate that the average bedrock properties at these sites are quite similar. Variations among individual samples and different rock types within one repository site were larger than variations among the averaged values of the four sites. Some indication of increased microfracturing was also found in samples of small size.
Investigation of the Long -Term Performance of Betafite and Zirconolite in Hydrothermal Veins From Adamello, Italy
Authors: Lumpkin G. R. , Day R. A. , McGlinn P. J. , Payne T. E. , GierÉ R. , Williams C. T.
Published: April 2011

Betafite and zirconolite occur in Ti-rich hydrothermal veins emplaced within dolomite marble in the contact aureole of the Adamello batholith, northern Italy. Zirconolite contains up to 18 wt% ThO2 and 24 wt% UO2, and exhibits strong compositional zoning. Some zirconolite grains were corroded by the hydrothermal fluid. Betafite, the Ti-rich member of the pyrochlore group, often occurs as overgrowths on zirconolite. The betafite is weakly zoned and contains 29-34 wt% UO2. In terms of end -members, betafite contains approximately 50 mole percent CaUTi2O7 and is the closest known natural composition to the pyrochlore phase proposed for use in titanate waste forms. Amorphization and volume expansion of the betafite caused cracks to form in the enclosing silicate mineral grains. Backscattered electron images reveal that betafite was subsequently altered along crystal rims, particularly near the cracks. EPMA data reveal little difference in composition between altered and unaltered areas, except for lower totals, suggesting that alteration is primarily due to hydration. The available evidence demonstrates that both betafite and zirconolite retained actinides for approximately 40 million years after the final stage of vein formation. During this time, betafite and zirconolite accumulated a total alpha -decay dose of 3-4 x 1016 and 0.2-2 x 1016 α/mg, respectively.
Nuclear Waste Package Corrosion Behavior in the Proposed Yucca Mountain Repository
Authors: Sagüés A. A.
Published: April 2011

The corrosion performance of spent nuclear fuel waste packages is becoming increasingly important in establishing the viability of the proposed Yucca Mountain repository system. Current package concepts propose the use of a 2 cm thick nickel-base superalloy (Alloy 22) shell as the main barrier to prevent corrosion penetration over many thousands of years. The expected package service conditions, as well as their variability and uncertainty, are discussed. The electrochemical conditions known to be responsible for passive behavior and its breakdown in Alloy 22 and similar alloys are examined in the light of the predicted repository environment. Durability prediction approaches and their conclusions are considered. Efforts to determine the relative impact of localized modes of failure and uniform passive dissolution on package durability are reviewed, along with open issues in need of resolution and alternative package designs. The basic question of the validity of extrapolating corrosion behavior over many times the duration of the present base of experience is addressed.
Open Site Tests on Corrosion of Carbon Steel:Containers for Radioactive Waste Forms
Authors: Barlnov A. S. , Ojovan M. I. , Ojovan N. V. , Startceva I. V. , Chujkova G. N.
Published: April 2011

Testing of waste containers under open field conditions is a component part of the research program that is being carried out at SIA “Radon”for more than 20 years to understand the long-term behaviour of radioactive waste forms and waste packages. This paper presents the preliminary results of these ongoing studies. We used a typical NPP operational waste, containing 137Cs, 134Cs, and 60Co as the dominant radioactive constituents. Bituminized and vitrified waste samples with 30–50 wt.% waste loading were prepared. Combined effects of climatic factors on corrosion behaviour of carbon steel containers were estimated using gravimetric and chemical analyses. The observations suggest that uniform corrosion of containers prevails under open field conditions. The upper limits for the lifetime of containers were derived from calculations based on the model of atmospheric steel corrosion. Estimated lifetime values range from 300 to 600 years for carbon steel containers with the wall thickness of 2 mm containing vitrified waste, and from 450 to 500 years for containers with the wall thickness of 2.5 mm that were used for bituminized waste. However, following the most conservative method, pitting corrosion may cause container integrity failure after 60 to 90 years of exposure.
Microstructure and Leaching Characteristics of a Technetium Containing Metal Waste Form
Authors: Johnson S. G. , Keiser D. D. , Noy M. , O'Holleran T. , Frank S. M.
Published: April 2011

Argonne National Laboratory is developing an electrometallurgical treatment for spent fuel from the experimental breeder reactor II. A product of this treatment process is a metal waste form that incorporates the stainless steel cladding hulls, zirconium from the fuel and the fission products that are noble to the process, i.e., Tc, Ru, Pd, Rh, Ag. The nominal composition of this waste form is stainless steel/15 wt% zirconium/ 1–4 wt% noble metal fission products. The behavior of technetium is of particular importance from a disposal point of view for this waste form due to its long half life, 2.14E5 years, and its mobility in groundwater. To address these concerns a limited number of spiked metal waste forms were produced containing Tc. These surrogate waste forms were then studied using scanning electron microscopy (SEM) and selected leaching tests.
Lead and Lead-Based Alloys as Waste Matrix Materials
Authors: Arustamov Arthur E. , Ojovan Michael I. , Kachalov Michael B.
Published: April 2011

Metals and alloys with relatively low melting temperatures such as lead and lead-based alloys are considered in Russia as prospective matrices for encapsulation of spent nuclear fuel in containers in preparation for final disposal in underground repositories. Now lead and leadbased alloys are being used for conditioning spent sealed radioactive sources at radioactive waste disposal facilities.
Epr of Fe3+ and Cr3+ Ions in NZP Ceramics
Authors: Troole A. Y. , Stefanovsky S. V. , Bogomolova L. D.
Published: April 2011

A new method to estimate the thermodynamic stability of crystalline host phases for radwaste is based on the determination of the degree of coordination polyhedra distortion. Analysis of the stability of sodium zirconium phosphate (NZP) structure containing Fe and Cr has been completed. The NZP structure can incorporate a limited amount of iron (∼2 mol. % Fe2o3). From the EPR study, Fe3+ and Cr3+ ions occupy two different structural positions substituting for Zr4+and Na+. Moreover, these ions can enter extra phases.
Fast Deposition of a-Si:H Layers and Solar Cells in a Large-Area (40×40 cm2) VHF-GD Reactor
Authors: Kroll U. , Fischer D. , Meier J. , Sansonnens L. , Howling A. , Shah A.
Published: April 2011

Large-area deposition of hydrogenated amorphous silicon has been investigated in a single-chamber industrial reactor with electrode dimensions of 40×40 cm2 in the plasma excitation frequency range of 60 to 120 MHz. The film thickness uniformity, analyzed by a light interferometry technique and a step profiler, has been compared with 2-dimensional interelectrode voltage measurements and calculations. The frequency of 80 MHz has been found to be a good compromise between the gain in deposition rate and the homogeneity requirements necessary for a-Si:H solar cells. Under these conditions and while using hydrogen dilution high deposition rates of 6-7 Å/s with a film uniformity of ±5% over a usable substrate size of 30×30 cm2 have been obtained.

In the same single-chamber deposition system at 80 MHz, 0.4 gtm thick single-junction a-Si:H solar cells with high performance were fabricated in a total process time of 16 minutes applying a continuous deposition process. Spectral response measurements indicate a minor boron contamination of the i-layer. Initial cell efficiencies of 7.1 % could be achieved for such a fast-grown a-Si:H solar cell.
Assessment of Intrinsic-Layer Growth Temperature to High-Deposition-Rate a-Si:H n-i-p Solar Cells Deposited by Hot-Wire CVD
Authors: Wang Qi , Iwaniczko Eugene , Xu Yueqin , Nelson Brent P. , Mahan A. H.
Published: April 2011

We report progress in hydrogenated amorphous silicon n-i-p solar cells with the i-layer grown by the hot-wire chemical vapor deposition technique. Early research showed that we grew device-quality materials with low saturated defect density (2 × 106/cm3), high initial ambipolar diffusion length (~2000 Å) and low hydrogen content (<1%). One of the major barriers to implementing this material into solar cells is the high substrate temperature required (>400°C). We re-assess the effects of low substrate temperature on the property of the films and the performance of the solar cells as an alternative avenue to solving this problem. We find that the material grown at 300°C can have similar values of saturated defect density and ambipolar diffusion length as the one grown greater than 400°C. We also study the effect of i-layer substrate temperature ranging from 280° to 440°C for n-i-p solar cells. We now consistently grow devices with Fill Factor (FF) greater than 0.66, with the best close to 0.70 at lower substrate temperature. A collaboration with United Solar System, in where they grew the p-layer and top contact, produced devices with initial efficiencies as high as 9.8%. We produce n-i-p solar cells with initial efficiencies as high as 8% when we grow all the hydrogenated amorphous silicon and top contact layers. All these i-layers are grown at deposition rates of 16 to 18 Å/sec. We need to further improve our p-layer and transparent conductor layer to equal the collaborative cell efficiency. We also report light-soaking results of these devices.
A Novel Excimer Laser Crystallization Method of Poly-Si Thin Film by Grid Line Electron Beam Irradiation
Authors: Park C-M , Lee M-C , Jeon J-H , Han M-K
Published: April 2011

Excimer laser annealing technique is proposed to increase the grain size and controlling the microstructure of polycrystalline silicon (poly-Si) thin film. Our method is based on the lateral grain growth during laser annealing. Our specific grid ion beam irradiation method was designed to maximize the lateral growth effect and arrange the location of grain boundaries. We observed well-arranged poly-Si grains up to micrometer order by transmission electron microscopy (TEM).
Structural Properties of Amorphous Silicon Produced by Electron Irradiation
Authors: Yamasaki J. , Takeda S.
Published: April 2011

The structural properties of the amorphous Si (a-Si), which was created from crystalline silicon by 2 MeV electron irradiation at low temperatures about 25 K, are examined in detail by means of transmission electron microscopy and transmission electron diffraction. The peak positions in the radial distribution function (RDF) of the a-Si correspond well to those of a-Si fabricated by other techniques. The electron-irradiation-induced a-Si returns to crystalline Si after annealing at 550°C.
Remote Silane Plasma Chemistry Effects and their Correlation with a-Si:H Film Properties
Authors: Kessels W.M.M. , Smets A.H.M. , Korevaar B.A. , Adriaenssens G.J. , Sanden M.C.M. Van de , Schram D.C.
Published: April 2011

A remote silane plasma, capable of depositing solar grade a-Si:H at a rate of 10 nm/s and with an up to ten times higher hole drift mobility than standard a-Si:H, has been investigated by means of several plasma diagnostics. The creation of the different reactive species in the plasma and their contribution to film growth has been analyzed and is correlated with the film properties obtained under various conditions. Furthermore, the first results on a n-i-p solar cell with the intrinsic a-Si:H film deposited by this remote plasma are presented.
Microstructure and Hydrogen Dynamics in a-Si1-xCx:H
Authors: Shinar R. , Shinar J. , Williamson D. L. , Mitra S. , Kavak H. , Dalal V. L.
Published: April 2011

Small angle x-ray scattering (SAXS), IR spectroscopy, and deuterium secondary ion mass spectrometry (DSIMS) were used to study the microstructure and hydrogen dynamics of undoped and boron-doped if-sputter-deposited (RFS) and electron cyclotron resonance (ECR)-deposited hydrogenated amorphous silicon carbides (a-Si1-xCx:H) with x ≤ 19 at.%. The SAXS measurements indicated residual columnar-like features and roughly spherical nanovoids of total content CnV ≤ 1.0 vol.%. The growth of CnV with annealing was due largely to an increase in the average nanovoid radius. It was noticeably smaller than in RFS a-Si:H films. The IR spectra demonstrated H transfer by annealing from mostly bulk-like Si-H groups to C-bonds. The H diffusion and its temperature dependence in undoped films resembled those of a-Si:H and were consistent with the SAXS and IR data. Suppression of long-range motion of most of the H atoms, consistent with increased CnV was observed in B-doped ECR films. However, a small fraction of the H atoms appeared to undergo fast diffusion, reminiscent of the fast diffusion in doped a-Si:H. The results are consistent with impeded relaxation processes of the Si network, caused by the presence of C atoms, and H trapping at C-H bonds.
Hydrogen Flip Model for Metastable Structural Changes in Amorphous Silicon
Authors: Biswas R. , Li Y.-P.
Published: April 2011

We propose a new metastable defect associated with hydrogen atoms in amorphous silicon. A higher energy metastable state is formed when H is flipped to the backside of the Si-H bond at monohydride sites. This defect is described by a double-well potential energy and occurs in addition to metastable dangling bonds. The dipole moment of this “H-flip” defect is larger and increases the infrared absorption. This defect accounts for large structural changes observed on light soaking including larger absorption and volume dilation.
Photoconductivity Transient Response from the Steady State in Amorphous Semiconductors
Authors: Main C. , Reynolds S. , Zollondz J.- H. , Bruggemann R.
Published: April 2011

We present analysis, computer modelling and experimental measurements of the photoconductive decay which occurs on cessation of illumination, in amorphous semiconductors. We explore the processes of relaxation of the excess carrier distributions, and examine the relative rôles of re-trapping and recombination in a model case of an exponential trapping state profile, with monomolecular recombination. A variety of possible decay behaviour is revealed. We examine several plausible intuitive explanations of the decay process, including (a) the assumption that the rate limiting step in the decay process is the thermal release of trapped carriers from the vicinity of the quasi - Fermi level, and (b) multiple re-trapping at the quasi -Fermi level prior to recombination. Actual decay rates, however are often much faster than that predicted by these assumptions, and the generation rate dependencies do not follow the relation expected. These explanations are shown in detail to be largely erroneous. Results of experimental measurements of the decay from steady state and TPC in films of a-Si1-xCx:H are presented. While these appear initially to be at variance with the predictions of the present work, we demonstrate that the observations can be reconciled fully with theory.
Vibrational Properties of a-Si:H Films Containing Voids: Experiment and Modeling
Authors: Barriquand N. , Paillard V. , Cabarrocas P. Roca i , Landa G. , Djafari-Rouhani M.
Published: April 2011

In this paper, we present results about the vibrational properties of hydrogenated amorphous silicon films. We expect to explain the slight differences observed in the Raman spectra using atomic-scale modeling. In particular, we focuse on the correlation of our results to the density of samples. This should give quantitative structural information which could be correlated to both macroscopic properties and elaboration conditions.
Polymorphous Silicon: Transport Properties and Solar Cell Applications
Authors: Longeaud C. , Kleider J. P. , Gauthier M. , Brtiggemann R. , Poissant Y. , Cabarrocas P. Roca i
Published: April 2011

Transport properties of hydrogenated polymorphous silicon layers (pm-Si:H) deposited at 150°C under various pressures in the range 80-293 Pa in sandwich (Schottky and p-i-n diodes) and coplanar structures have been compared to those of hydrogenated amorphous silicon (a-Si:H) samples deposited at the same temperature in standard conditions. The layers have been studied as-deposited, annealed and after light-soaking. With increasing pressure up to 240 Pa: i) the density of states above the Fermi level decreases as determined by means of the modulated photocurrent technique, ii) the mobility-lifetime products of electrons and holes measured by means of steady-state photoconductivity and photocarrier grating techniques both increase. The highest values for the diffusion length of minority carriers exceed 200 nm. Capacitance measurements as a function of frequency and temperature show that the density of states at the Fermi level is lower in the pm-Si:H than in the a-Si:H films. After light-soaking the diffusion length of minority carriers in a-Si:H is reduced by a factor oftwo whereas it is less reduced or not affected in the pm-Si:H layers. Solar cells including this new material present an excellent stability.
Use of A GAS Jet Technique to Prepare Microcrystalline Silicon Based Solar Cells at High I-Layer Deposition Rates
Authors: Jones S.J. , Crucet R. , Deng X. , Doehler J. , Kopf R. , Myatt A. , Tsu D.V. , Izu M.
Published: April 2011

Using a Gas Jet thin film deposition technique, microcrystalline silicon (μc-Si) materials were prepared at rates as high as 15-20 Å/s. The technique involves the use of a gas jet flow that is subjected to a high intensity microwave source. The quality of the material has been optimized through the variation of a number of deposition conditions including the substrate temperature, the gas flows, and the applied microwave power. The best films were made using deposition rates near 16 Å/s. These materials have been used as i-layers for red light absorbing, nip single-junction solar cells. Using a 610nm cutoff filter which only allows red light to strike the device, pre-light soaked currents as high as 10 mA/cm2 and 2.2-2.3% red-light pre-light soaked peak power outputs have been obtained for cells with i-layer thicknesses near 1 micron. This compares with currents of 10-11 mA/cm2 and 4% initial red-light peak power outputs obtained for high efficiency amorphous silicon germanium alloy (a-SiGe:H) devices. The AM1.5 white light efficiencies for these microcrystalline cells are 5.9-6.0%. While the efficiencies for the a-SiGe:H cells degrade by 15-20% after long term light exposure, the efficiencies for the microcrystalline cells before and after prolonged light exposure are similar, within measurement error. Considering these results, the Gas Jet deposition method is a promising technique for the deposition of μc-Si solar cells due to the ability to achieve reasonable stable efficiencies for cells at i-layer deposition rates (16 Å/s) which make large-scale production economically feasible.
Microstructural Defects of Device Quality Hot-Wire Cvd Poly-Silicon Films
Authors: Rath J.K. , Tichelaar F.D. , Schropp R.E.I.
Published: April 2011

Two types of poly-Si:H thin films made by Hot Wire CVD have been evaluated with respect to utilisation in solar cells. Poly-Si:H films made at high hydrogen dilution are highly porous and have large interconnected voids. The void density is 25000/μm-3 as determined by XTEM. On the other hand, poly-Si:H layers made at low hydrogen dilution have a compact structure and a much smaller density of voids. In these films, two types of voids exist: globular voids smaller than 15 nm, and elongated voids, often located between columns of large crystals of 150-250 nm wide at the top. The density for the 5 - 15 nm spherical voids is usually -50/μm3, but larger concentrations often occur locally, up to 1000/pm3, i.e., 0.05% volume fraction. High oxygen content in the poly-Si films made at high hydrogen dilution is largely due to post deposition intrusion of water vapour through the interconnected voids. Profiled layers are made by depositing device quality poly-Si:H layers (low hydrogen dilution) on top of a seed layer (high hydrogen dilution) of high nucleation density. Cells incorporating profiled poly-Si:H films as i-layers at a deposition rate of 0.5 nm/s were made on stainless steel substrates in the configuration SS/n-μc-Si:H(PECVD)/i-poly-Si:H(HWCVD)/p-μc-Si:H(PECVD)/ITO. For our n-i-p solar cell with poly-Si i-layer we obtained an efficiency of 4.41% and a FF of 0.607. Due to native surface texture a current density of 19.95 mA/cm2 is generated in only ~1.22 μm thick i-layer without back reflector.
Thin Film Transistors of Microcrystalline Silicon Deposited by Plasma Enhanced-CVD
Authors: Chen Y. , Wagner S.
Published: April 2011

We fabricated top gate TFTs of microcrystalline silicon (μc-Si) deposited at 360 °C. The TFTs have field-effect electron mobilities of up to 7.9 cm2/Vs in the saturation regime and 5.8 cm2/Vs in the linear regime. The highest ION/IOFF ratio is 105. Typical values for Vth is 6.5 V and for the subthreshold slope is 1.7 V/decade. The μc-Si is grown by PE-CVD from a source gas mixture of SiH4, SiF4 and H2, with a typical flow ratio of 1:20:200, at a pressure of 120 Pa and a power density of 160 mW/cm2. The TFT structure is built on un-passivated Coming 7059 glass, with 300 n+ μc-Si, 60 nm n+ μc-Si source and drain contact layers, 200 nm SiO2 or 300 nm SiNx gate insulator, and 100 nm Al gate, source and drain electrodes.
Optical Filter for Fabricating Self-Aligned Amorphous Si TFTS
Authors: Mei P. , Lu J. P , Chua C. , Ho J. , Wang Y. , Boyce J. B.
Published: April 2011

Self-aligned structures for bottom-gate amorphous Si TFTs provide a number of advantages, including reduced parasitic capacitance, smaller device dimensions, and improved uniformity in device performance for large-area electronics. A difficult challenge in making self-aligned TFT structures is the necessity of making source/drain contacts that exhibit low contact resistances and that are precisely aligned relative to the gate electrode. In this article, we describe a novel process for fabricating self-aligned amorphous Si TFTs. This process utilizes a pulsed excimer laser (308 nm) to dope or to activate dopants in a-Si to form the source/drain contacts. An important feature of the device design is an optical filter to protect the a-Si channel region from radiation damage during the 308 nm laser process. However, the optical filter allows the transmission of the uv light for lithography exposure from the backside of the substrate to align the channel region with the gate electrode. This new process enables the fabrication of high performance self-aligned a-Si TFTs with poly-Si source and drain contacts.
Real Time Optics of Amorphous Silicon Solar Cellfabrication on Textured Tin-Oxide-Coated Glass
Authors: Rovira P. I. , Ferlauto A. S. , An Ilsin , Fujiwara H. , Koh Joohyun , Koval R. J. , Wronski C. R. , Collins R. W.
Published: April 2011

A rotating-compensator multichannel ellipsometer has been used to measure the four spectra (1.4-4.0 eV) that describe the Stokes vector of the light beam reflected from the surface of an amorphous silicon (a-Si:H) p-i-n solar cell during fabrication on textured tin-oxide (SnO2) coated glass. The Stokes vector elements include the irradiance in the reflected beam (or the reflectance) and the parameters {(Q, χ), p} of the reflected beam, where Q and χ are the tilt and ellipticity angles of the polarization ellipse and p is the degree of polarization. The value of p deviates from unity in part due to the non-uniform nature of the textured SnO2 substrate film. An analysis of Q and χ that neglects the effects of the texture can provide the time evolution of the thicknesses, microscopic structure, and the optical properties of the component layers of the a-Si:H solar cell. Deviations of the measured reflectance spectra from those predicted on the basis of the (Q, χ) analysis provide the thickness dependence of the scattering and the evolution of the macroscopic structure of the solar cell. The measurement and analysis approach is important because of its potential application for real time monitoring of solar cell production. The analysis results also provide realistic inputs for optical modeling of the effects of texture in light trapping for solar cell efficiency enhancement.
Optical Modeling of a-Si Solar Cells
Authors: Sopori Bhushan , Madjdpour Jamal , Zhang Yi , Chen Wei , Guha Subhendu , Yang Jeff , Banerjee Arindam , Hegedus Steven
Published: April 2011

We describe applications of PV Optics to analyze the behavior of a metallic back-reflector on an a-Si solar cell. The calculated results from PV Optics agree well with the measured data on solar cells. Several unexpected results obtained from these calculations are qualitatively explained.
Amorphous Silicon Solar Cell Techniques for High Temperature and/or Reactive Deposition Conditions
Authors: Kanbe M. , Komaru T. , Fukutani K. , Kamiya T. , Fortmann C.M. , Shimizu I.
Published: April 2011

Several promising new methods for amorphous silicon solar cell preparation involve high substrate temperatures and/or very reactive atmospheres. When incorporated into solar cells, the performance of these layers has often been less than expected due to enhanced diffusion and/or chemical reactions. This poor performance results from the harsh deposition environments. Deleterious effects include darken of TCO coated glass substrates due to hydrogen diffusion to and hydrogen reduction at the TCO interface when solar cells are prepared in the p-i-n deposition sequence. Alternatively, the deposition of TCO layers onto amorphous layers also involves rather harsh oxidizing conditions that have a deleterious effect on the top most amorphous silicon-based p-layers. Strategic use of blocking layers results in remarkably improved solar cell performance. A thin Cr layer (probably becoming Cr2O3) shows ability to improve the performance of both n-ip and p-i-n solar cells by inhibiting both O and H diffusion.
Buffer Layers for Narrow Bandgap A-SIGE Solar Cells
Authors: Liao X. B. , Walker J. , Deng X.
Published: April 2011

In high efficiency narrow bandgap (NBG) a-SiGe solar cells, thin buffer layers of unalloyed hydrogenated amorphous silicon (a-Si) are usually used at the interfaces between the a-SiGe intrinsic layer and the doped layers. We investigated the effect of inserting additional a-SiGe interface layers between these a-Si buffer layers and the a-SiGe absorber layer. We found that such additional interface layers increase solar cell VOC and FF sizably, most likely due to the reduction or elimination of the abrupt bandgap discontinuity between the a-SiGe absorber layer and the a-Si buffer layers. With these improved narrow bandgap solar cells incorporated into the fabrication of triple-junction a-Si based solar cells, we obtained triple cells with initial efficiency of 10.6%.
Characteristics of Different Thickness a-Si:H/Metal Schottky Barrier Cell Structures-Results and Analysis
Authors: Lu Zhou , Jiao Lihong , Koval Randy , Collins Robert W. , Wronski Christopher R.
Published: April 2011

Current-voltage, light I-V and internal quantum efficiency characteristics have been investigated in specular TCO/n+(a-Si:H)/i(a-Si:H)/Nickel Schottky barrier cell structures with protocrystalline intrinsic layers. The studies were carried out on structures with different thickness i layers after a degraded steady state had been reached with AMI.5 illumination at 27°C. These characteristics were modeled using the Analysis of Microelectronic and Photonic Structures(AMPS) and a gap state distribution, which includes charged defects, used in the analysis of results of detailed studies on thin films[1]. Fits are obtained to these characteristics for the different thickness cell structures using the same parameters as those used to fit the results on the corresponding intrinsic thin films. The results obtained from this study offer an approach to more reliable modeling of solar cells at their “end of life”.
High Resolution, High Fill Factor A-SI:H Sensor Arrays for Optical Imaging
Authors: Rahn J.T. , Lemmi F. , Mei P. , Lu J.P. , Boyce J.B. , Street R.A. , Apte R.B. , Ready S.E. , Schuylenbergh K.F. van , Nylen P. , Ho J. , Fulks R.T. , Lau R. , Weisfield R.L.
Published: April 2011

Amorphous silicon large area sensor arrays are in production for x-ray medical imaging. The most common pixel design works very well for many applications but is limited in spatial resolution because the available sensor area (the fill factor) vanishes in small pixels. One solution is a 3-dimensional structure in which the sensor is placed above the active matrix addressing. However, such high fill factor designs have previously introduce cross talk between pixels.

We present data for a design in which the a-Si:H p-i-n photodiode sensor layer has a continuous i-layer and top p+-layer, and a patterned n+-layer contact to the pixel. Arrays of 64 μm and 75μm pitch have been fabricated and are the highest resolution a-Si:H arrays reported to date. The resolution matches the pixel size, and sensitivity has been improved by the high fill factor. Comparison is made between arrays with standard TFTs and TFTs with self-aligned source and drain contacts. Data line capacitance is improved by use of the self-aligned contacts.

Measurements are included on the contact to bias capacitance. The high fill factor design greatly suppresses lateral leakage currents, while retaining ease of processing. Provided illumination levels remain below saturation, the resolution matches expectation for the pixel size.
More Insight into the Transient Photocurrent Response of Three-Color Detectors
Authors: Stiebig H. , Stannowski B. , Knipp D. , Wagner H.
Published: April 2011

Nipiin structures with controlled bandgap and mobility-life-time product exhibit excellent stationary properties. However, the photocurrent transients of these devices show reasonable delay times in the range of tens of milliseconds before reaching steady state. Hence, the transient behavior after light switching of three color detectors based on a nipiin structure was studied by a detailed analysis of experimentally determined and simulated data. The numerical simulation reproduces well the characteristic features. The delayed current onset and the sign reversal of the transient photocurrent response are explained by trapping of holes in the vicinity of the central p-layer.
Effects of Material Properties in Amorphous Silicon Color Detectors
Authors: Caputo D. , Colalongo L. , Irrera F. , Lemmi F. , Palma F. , Tucci M.
Published: April 2011

In this work we study the effects of material properties on the reading process of color detectors by using a two dimensional simulator for the transient regime. In particular, starting from the potential and charge distribution in the device, we describe effects of the density of defects on the self-bias process. Results show the possibility to engineer materials in order to optimize response speed of the device.