bluuebunny/crossref_metadata_2025 huggingface dataset:
Alteration of Natural Uranyl Oxide Hydrates in Si-Rich Groundwaters: Implications For Uranium Solubility
Authors: Finch R.J. , Ewing R.C.
Published: March 2011

The uranyl oxide hydrates are common initial corrosion products of uraninite (nominally U02+x) during weathering. In the presence of dissolved silica these early-formed phases alter to uranyl silicates (most commonly soddyite, U2SiO8-2H2O, and uranophane, CaU2Si2O11·6H2O). Uraninite, however, usually contains radiogenic Pb, and the earlyformed Pb-poor uranyl oxide hydrates alter incongruously to uranyl silicates plus Pb-enriched uranyl oxide hydrates such as curite. Similar to dissolved silica, radiogenic Pb may also serve to limit the mobility of U in nature by fixing U in solid phases. Curite may also play an important role in the formation of uranyl phosphates, which are significantly less soluble than the uranyl silicates, and control U solubility in many groundwaters associated with altered U ore.
Uranium Redistribution and Fixation During Chlorite Weathering at Koongarra, Australia
Authors: Murakami Takashi , Isobe Hiroshi , Nagano Tetsushi , Nakashima Satoru
Published: March 2011

The chemical forms of iron minerals and uranyl species have been examined non-destructively by visible microspectroscopy and electron microprobe analysis in order to obtain a better understanding of uranium redistribution and fixation mechanisms. The uranyl species are associated with sub-micron sized iron minerals, such as hematite (Fe2O3), goethite (FeOOH), and ferrihydrite (Fe4-5(OH,O)12). Additionally, some uranium is fixed to goethite in weathered chlorite grains. Sub-micron sized saleeite, Mg(UO2)2(PO4)2. 1OH2O, is the most probable uranyl phase associated with the iron minerals. This suggests that the uranium fixation in the vicinity of the Koongarra uranium ore deposits is the result of the coprecipitation and sorption of microcrystals of saleeite on to the iron minerals released during weathering of chlorite.
The Long-Term Dissolution Behaviour of the Pamela Borosilicate Glass SM527 - Application of SA/V as Accelerating Parameter
Authors: Lemmens K. , Iseghem P. Van
Published: March 2011

The Al2O3 rich borosilicate glass SM527 was submitted to corrosion tests with glass surface area to solution volume ratios ranging from 10 to 10000 m-1. This latter condition would correspond with a 1000 fold acceleration relative to the reference MCCI condition. Powdered glass was used to reach SANV ratios of 500 m-1 and more. The leaching solutions were either distilled water or referred to Boom clay disposal conditions. The results based on the boron concentration in solution revealed a relatively linear dependence on SA/V.t0.5 in the pure solutions, on the longer term (DW and clay water). Diffusion is suggested to be the process governing the glass dissolution, although other processes should not be excluded. In a clay / clay water mixture (slurry), long term dissolution seems to be limited by saturation. Short term data for boron are largest in the clay slurry, but with time the boron concentrations converge to similar values in the three media considered. The use of SA/V as an accelerating factor is promising, but certainly requires additional research.
Coffinitization - A Mechanism for the Alteration of UO2 under Reducing Conditions
Authors: Janeczek J. , Ewing R.C.
Published: March 2011

A structural and chemical analogue of UO2 in spent fuel, uraninite, UO2+x, is unstable in the presence of dissolved silica (>10-3.6 mg/L) under reducing conditions and transforms into coffinite, USiO4 nH20. Coffinite may incorporate numerous elements into its structure including actinides and fission products. Conditions favorable for coffinitization of UO2 in spent fuel may occur in repositories in granitic, basaltic, and tuffaceous rocks.
Source-Term Constraints for the Proposed Repository at Yucca Mountain, Nevaida, Derived from the Natural Analog at PeÑa Blanca, Mexico
Authors: Murphy William M. , Pearcy English C.
Published: March 2011

The source term for nuclear waste repository performance assessments can be constrained by the solubilities of radioelement-bearing solids and/or the rates of release of radioelements from nuclear waste forms. Both solubility and rate limits for the proposed repository at Yucca Mountain, Nevada, can be assessed using information from the natural analog at PeÑa Blanca, Mexico. Petrographic and field relations indicate that uraninite oxidation and transformation to secondary uranyl silicate minerals have been rapid relative to mass transport of uranium out of the PeÑa Blanca system. The rate limiting process for uranium removal is likely to be advective transport in groundwaters with uranium contents controlled by interactions with uranyl silicate minerals such as uranophane. A maximum limit on the rate of uraninite oxidation at PeÑa Blanca is calculated to be 0.032 tons of UO2per year using geologic constraints on the amount of oxidation and the available time.
Interaction of Water and Compacted Sodium-Bentonite in Simulated Nuclear Waste Disposal Conditions
Authors: Melamed A. , Pitkänen P. , Olin M. , Muurinen A. , Snellman M.
Published: March 2011

Alteration of compacted sodium-bentonite (Volclay MX-80) caused by groundwater in simulated repository conditions for high level radioactive waste, was studied in an experiment where bentonite (wrapped by a copper cylinder) was let to react with two types (A,B) of synthetic granitic groundwater that are distinguished by their initial concentration of potassium and chloride. The reaction took place in ambient conditions at a temperature of 75 °C and proceeded during several time intervals up to 36 months.

At the end of each time interval the water was chemically analysed for determination of possible changes in composition. Chemical and mineralogical changes in the bentonite were investigated by using NH4Cl extractions, XRD and microprobe (SEM, EDS) analyses and were studied as a function of the reaction time (months) as well as of the distance (mm) from the contact front with water.

Sodium ions were found to migrate out from the bentonite while being replaced by other cations such as calcium, magnesium and to some extent, particularly during the reaction of the bentonite with water B, by potassium. No clear evidence was found for the fixation of potassium ions in the interlayer position of montmorillonite clay and the transformation to illite. The main mineralogical change in the bentonite was from Na- to Ca-rich montmorillonite. Secondary processes were the dissolution-precipitation of sulphur compounds, dissolution of gypsum and carbonates and the dissolution-precipitation of copper compounds.
Characterization of Lanthanoid Phases Formed Upon Glass Dissolution in Salt Solutions
Authors: Rother Anette , Lutze Werner , Schubert-Bischoff Peter
Published: March 2011

This communication gives a detailed characterization of some molybdate solid solutions and cerianite-type material which formed on the French borosilicate nuclear waste glass R7T7 upon corrosion in various saturated salt solutions at 110°C, 150 °C and 190 °C. The glass contained lanthanoid elements, such as neodymium, lanthanum, praseodymium, cerium and yttrium, but did not contain actinoid elements, except uranium and thorium. Various solid solutions containing lanthanoids (Ln) were found on the glass surface after corrosion, including powellite solid solutions and cerianite-type material. The secondary solid phases are characterized based on quantitative microchemical and structural analyses. These phases are expected to incorporate actinoids such as americium and curium in acid magnesiumcontaining salt solutions. The phases then constitute an additional barrier against migration of these radionuclides, which would otherwise be in the aqueous phase.
The Current Status of the Radionuclide Migration Experiment at the Grimsel Underground Rock Laboratory
Authors: Alexander W.R. , Bradbury M.H. , Mckinley I.G. , Heer W. , Eikenberg J. , Frick U.
Published: March 2011

This paper reviews the current status of the Grimsel radionuclide migration experiment, with especial reference to the advances of the last four years. Details are provided on the unusually extensive support programme and the general philosophy of undertaking such a large and costly field experiment.
Aem Analyses of SRL 131 Glass Altered as a Function of SA/V
Authors: Mazer James J. , Bates John K. , Biwer Bruce M. , Bradley C. R.
Published: March 2011

Experimental studies of silicate glass/water reactions at low temperatures have previously identified the glass surface area-to-solution volume ratio (SA/V) as a significant rate determining parameter [1-4]. The value produced when SA/V is multiplied by reaction time, hereafter referred to as SVT, has been proposed as a scaling factor for comparing experimental results collected under different test conditions and for extrapolating short-term results to longer periods of time. Developing an understanding of the effect of SAN is needed for modeling experimental results where SA/V ranges in value or may vary during experiments. It is also useful to understand the effect of SA/V for modeling natural systems where this value almost certainly varies, such as during the hydrothermal diagenesis of natural glasses or projecting the long-term reaction of water and borosilicate nuclear waste glass in a geologic repository.
The Structure of Alteration Layers on Cast Glass Surfaces
Authors: Oversby V. M. , Phinney D. L.
Published: March 2011

Alteration layers developed on SRL-165 simulated waste glasses in dilute sodium silicate/bicarbonate leaching solutions have been examined by Secondary Ionization Mass Spectrometry (SIMS) using fine-scale, multiple-element depth profiling. Selected samples were examined with an imaging detector system, which demonstrated the horizontal homogeneity of the layer development at all depths within the layer. After 1 day of reaction at 90°C the reaction layer shows depletion of glass elements to a depth of 0.2 μm. The surface of the layer in contact with the solution shows enrichment of Si, Al, and alkali elements even at this short reaction time, suggesting the early stages of development of secondary aluminosilicate phases. With increased reaction time, the layer thickens to about 1.3 μm at 91 days, while the evidence for aluminosilicate development at the surface of the layer becomes more prominent. Penetration of hydrogen into the “unreacted” glass proceeds to a depth of about 0.5 μm deeper than the alkali depletion zone. This suggests the mechanism of initial reaction of the glass is by attack of the silicate structure by molecular water or hydroxide ion rather than by alkali-hydrogen ion exchange. The simple structure of the layers developed in the silicate solution is in contrast to the complexity of layer structure found when glasses are reacted in deionized water. Since the conditions for geologic disposal will be closer to those used in the silicate leaching experiments, these results hold promise for the ability to model the system to predict long-term performance after disposal in a repository.
Instability in a-Si:H Thin-Film Transistor: A New Method to Discriminate Between Charge Injection and Defect Creation
Authors: Foglietti P. , Fortunato G. , Mariucci L. , Parisi V.
Published: March 2011

In the present work, in order to discriminate the main source of instability in a-Si:H TFTs, the determination of both threshold voltage and flat-band voltage has been performed after bias-stressing the devices with different gate voltages and at different temperatures. Flat-band voltage was determined by the space-charge photomodulation technique. From the close correlation observed between the two quantities, we conclude that the predominant instability mechanism is represented by change in the gate insulator charge at and near the semiconductor/insulator interface.
Electron Range and Electron Generation Function in a-SLH
Authors: Najar S. , Equer B. , Chévrier J-B.
Published: March 2011

The characterization of a-Si:H structure, using an electron beam in the keV range, is an interesting alternative to the more common photon beam techniques. a-Si:H device characterization by the determination of collection efficiency and other electronic parameters can be performed by variable energy EBIC method. But this requires an accurate knowledge of electron generation function and electron range in a-Si:H. In this paper we present an experimental determination of electron generation function and electron range in a-Si:H. To do this, a series of identical n-i-p a-Si:H diodes with a thin aluminium top electrodes were fabricated and varying thicknesses of a-Si:H layer were deposited on it. In EBIC measurements, the n-i-p diode was reverse biased at maximum potential. The electron range of a-Si:H was determined directly by measuring the energy at which electron beam is completely stopped in the top layer and no carrier generation is possible in the n-i-p diode. The generation function is then deduced from EBIC contrast measurements between the aluminium electrode and the top a-Si:H layer.
Kinetics of a-Si:H Crystallization Induced by Gold at Low Temperatures
Authors: Pasa A.A. , Schubert M.B. , Abel C.-D. , Beyer W. , Losch W. , Bauer G.H.
Published: March 2011

The Au-induced crystallization of a-Si:H has been studied by evaporating Au films of different thicknesses onto intrinsic glow discharge deposited a-Si:H layers. The presence of a sharp peak in the Raman spectra (FWHM≈9 cm-1, ω516 cm-1) of samples with a Au thickness larger than 2 nm, which have been annealed in vacuum at 400K≤T≤600K, indicate that the crystallites have approximately the same size (6nm) regardless of the annealing conditions. An investigation of crystallization versus Au-film thickness revealed, that the total crystallized volume is increasing with Au thickness, and furthermore a saturation of the crystallized volume takes place, most probably due to an exhaustion of the Au reservoir. The increase of crystallization rate with temperature follows an Arrhenius-like dependence with an activation energy of 1.1 eV. Changes in hydrogen content as a consequence of the crystallization have been monitored by H-effusion measurements: Au-coated a-Si:H samples show a strong H2 evolution at temperatures substantially lower than uncoated ones.
Microstructure, Optoelectronic Properties and Saturated Defect Density of A-SL:H Prepared in VHF-Glow Discharge Using AR and XE Dilution
Authors: Kroll U. , Finger F. , Dutta J. , Keppner H. , Shah A. , Howling A. , Dorier J.-L- , Hollenstein Ch.
Published: March 2011

Films were prepared under argon and xenon dilution with silane concentrations ranging from 3 to 100% using the very high frequency glow discharge (VHF-GD) technique. The H-content and the microstructure were determined by IR-spectroscopy. A surface profiler was used to measure the stress and thickness of the films. The surface roughness of the films was evaluated by the UV-light reflectance loss. The samples were further characterized by dark- and photoconductivity, by CPM and by PDS, both in annealed and light-soaked state.

Down to silane concentrations of about 10–20% film properties change only little; however, for both dilution series the microstructure parameter shows a minimum and the internal stress a maximum near 20% silane concentration. At still higher rare gas dilution the film properties change drastically. Surprisingly the photoconductivity remains almost constant for all gas dilutions. It is shown how these changes in the film properties are linked with the light induced degradation.
VHF Plasma Deposition: A Comparative Overview
Authors: Shah A. , Dutta J. , Wyrsch N. , Prasad K. , Curtins H. , Finger F. , Howling A. , Hollenstein Ch.
Published: March 2011

The use of plasma excitation frequencies f in the VHF band (30–300 MHz), and particularly of f=70 MHz, for the high-rate deposition of amorphous hydrogenated silicon (a-Si:H) is described. Deposition rates, using monosilane (SiH4) as source gas, are thereby increased roughly five fold to over 10 Å/s as compared with the conventional case of RF plasma enhanced chemical vapour deposition with f=13.56 MHz. This may possibly be attributed to an enhancement in the high-energy tail of the electron energy distribution function (EEDF) of the plasma. Thereby, no noticeable deterioration in film properties is found.

Characteristics of VHF-deposited a-Si:H films are extensively reported, including properties like microstructure, hydrogen effusion behaviour and its low internal mechanical stress. High quality hydrogenated microcrystalline silicon (μc-Si:H) can be deposited at low substrate temperature and low plasma power densities thanks to VHF glow discharge. This can be linked to a reduction in sheath potential and to the energy of the ions arriving at the growing surface. Thereafter, use of VHF plasma in applications such as 100 μm thick a-Si:H layer for particle detectors and powder-free deposition of solar cells with efficiencies over 8% are reported.
Growth and Structure of Microcrystalline Silicon by Reactive DC Magnetron Sputtering
Authors: Feng G. F. , Katiyar M. , Yang Y. H. , Abelson J. R. , Maley N.
Published: March 2011

We have deposited microcrystalline silicon (μc-Si) at low temperature by reactive dc magnetron sputtering. The structure and crystallinity of the films are analyzed by in situ spectroscopie ellipsometry. μc-Si growth occurs at deposition conditions with substrate temperature between 150 – 300 °C and hydrogen partial pressure above 4 mtorr. We have also observed that the deposition rate strongly affects the interface structure. At a rate of 21 À/min, a 230 Å thick amorphous layer appears at the interface of film and glass substrate. The interface layer thickness decreases with deposition rate, and becomes undiscernible for growth rates ∼ 2 Å/min. The thickness of the interface layer is also found to depend on substrates.
Structural Studies of Amorphous Semiconductor Alloys Using X-Ray Absorption and Related Techniques
Authors: Boscherini F.
Published: March 2011

The use of X-ray absorption spectroscopy and related techniques in the investigation of the structure of amorphous group IV based semiconductor alloys is briefly reviewed, with emphasis on recent results.

In these materials the single scattering contribution to the x-ray absorption signal contains information mainly on first shell coordination; this is illustrated by showing trends in the local structure of amorphous semiconductor alloys. This type of analysis can be complemented by using other methods. For example, X-ray scattering yields information on second shell structure which reflects the influence of bond angle disorder.

Recent results on a-Si-1-xCx:H are then reported. Presence of both carbidic and graphitic carbon, interatomic distances which are constant with composition and a clear tendency towards chemical order are found. These results are compared to some recent structural models.
Realistic Models of Hydrogenated Amorphous Si
Authors: Drabold D. A. , Fedders P. A.
Published: March 2011

We have recently produced experimentally plausible structural models of 12a-Si:H. We discuss the structural, vibrational and electronic properties of these models, and indicate how such structures can be obtained.
Contributions to Silicon-Hydrogen Bond-Stretching Frequency in Amorphous Si Alloys
Authors: Jing Z. , Whitten J. L. , Lucovsky G.
Published: March 2011

We have performed ab initio calculations and determined the bond-energies and vibrational frequencies of Si-H groups that are: i) attached to Si-atoms as their immediate, and also more distant neighbors; and ii) attached to three O-atoms as their immediate neighbors, but are connected to an all Si-atom matrix. These arrangements simulate bonding geometries on Si surfaces, and the calculated frequency for i) is in good agreement with that of an Si-H group on an Si surface. To compare these results with a-Si:H alloys it is necessary to take into account an additional factor: the effective dielectric constant of the host. We show how to do this, demonstrating the way results of the ab initio calculations should then be compared with experimental data.
Interpretation of Stretched-Exponential Defect Kinetics in a-Si:H
Authors: Redfield David
Published: March 2011

A physical interpretation is given for the widespread observation of stretched-exponential time dependences for both light-induced generation and thermal anneal behavior of a-Si:H. This interpretation is based on a distribution of simultaneous rate processes, each of which represents a subset of normal metastable defects having a simple exponential behavior; it is thus an adaptation of one used for relaxation in glasses. The distribution can be evaluated from an observed stretched exponential, and for the case of thermal anneal with activated time constants, the stretch parameter should increase with temperature quite generally. The shape and width of the distributions of time constants and activation energies are inferred from these relations and properties of observed stretched exponentials.
Potential Role for Charged Dangling Bonds in Transient-Lesr of Light-Soaked a-Si:H
Authors: Saleh Z. M. , Tarui H. , Tsuda S. , Nakano S. , Kuwano Y.
Published: March 2011

Transient light-induced electron spin resonance (LESR) at 120 K, has been used to investigate deep defects in a-Si:H through changes in the lineshape. When the lineshape is deconvoluted into narrow and broad components, the narrow component is found to decrease, relative to the broad component, with increasing light-soaking time. Similar changes are not observed, however, in as-deposited or annealed films regardless of deposition and annealing conditions. An important role for charged dangling bonds is proposed to explain these changes and we suggest that intrinsic (stable) and light-induced (metastable) defects play different roles in transient-LESR and may occupy different energy distributions in the gap.
Anomalous Diffusion of Hydrogen and the Dependence of the Diffusion Constants on Hydrogen Content in Hydrogenated Amorphous Silicon
Authors: Shinar R. , Jia H. , Wu X.-L. , Shinar J.
Published: March 2011

The diffusion constant of hydrogen DH(t) in hydrogenated amorphous silicon (a-Si:H) is strongly dependent on the Si-bonded H content CH of the films. It increases by over four orders of magnitude for CH ranging from 1 to 19 at. %. In an rf sputter-deposited film of CH ∼5 at. % it increases with time at 300 ≤ T ≤ 362°C. The dispersion parameter α in DH(t) = D∞ (ωt)-αis thus negative. This observation and the increase of α with T above a sample-dependent temperature Tτ are discussed in relation to low temperature structural relaxation processes in the amorphous network.
Comparison of Electron Beam-Induced and Light-Induced Defect Saturation in a-Si:H
Authors: Grimbergen M. , Lopez-Otero A. , Fahrenbruch A. , Benatar L. , Redfield D. , Bube R. , McConville R.
Published: March 2011

Generation, saturation, and annealing characteristics of metastable defects formed by electron beam irradiation at 20 keV and photon irradiation at 1.9 eV have been compared. Saturation density reached by electron irradiation is temperature independent over the range 225 K to 300 K, although a small activation energy of the generation rate may be present. This differs from observed temperature dependent light-induced saturation from 330 K to 470 K, although differences are expected because of the separate temperature ranges and dissimilar carrier excitation rates. The electron beam-induced saturated defect density is about 5 times larger than for light-induced saturation at 350 K and high light intensity (generation rate ≈ 1022cm-3s-1). Defects formed by electron irradiation anneal at 300 K with a stretched exponential time constant three orders of magnitude smaller than for light-induced defects. After electron irradiation, dark conductivity relaxes faster than photoconductivity. Once the dark Fermi level becomes constant during defect density relaxation, photoconductivity is inversely proportional to the defect density.
Photodegradataion and Stability of a-Si Prepared at High Deposition Rates
Authors: Vepřek Stan , Ambacher Oliver , Vaněček Milan
Published: March 2011

A-Si films with a constant optical band gap were prepared by plasma CVD from silane at deposition rates up to 17 Å/s under controllably varied conditons and characterized by a number of techniques. Photodegradation under AM1 simulator has been done for 50 hours and for 4–17 hours under 2 Watt/cm2 dye laser illumination at a wavelength corresponding to an absorption coefficient of about 104 cm-1 up to saturation. Samples with different initial value of σ(d) and σ(ph) show different rates of photodegra-dation, but they reach very similar saturation values. The fast photode-gradation with the dye laser is equivalent to a long term (several thousand hours) photodegradation under AM1. The AMI photodegradation for 50 to 100 hours does not yield any reliable measure of the stability of the films. The best films, which were prepared at a high deposition temperature show a low saturation defect density of about 5·1016 cm-3 (CPM-data).
Defect Kinetics and Saturation in Amorphous Silicon-Germanium Alloys
Authors: Schumm G. , Abel C.-D. , Bauer G.H.
Published: March 2011

A quantitative comparison of defect kinetics under high-intensity light-soaking was carried out on a-Si:H and a-SiGe:H. A simple technique that ensures identical experimental conditions (generation rates and generation profiles) in samples of different bandgaps is demonstrated. It is shown that the apparent dependence on bandgap of the susceptibility to light-induced degradation is mainly due to changes in intrinsic carrier concentrations without the need to adhere to any structural changes.
Subgap Absorption and Electrical Properties of Compositional Multilayers of a-Si:H and Alloys
Authors: Bernhard Norbert , Bauer Gottfried H.
Published: March 2011

Urbach tails and midgap absorption, as determined by PDS, and temperature dependent coplanar and sandwich conductivity were systematically investigated on compositional multilayers of a-Si:H and its alloys with Ge and C, under the main aspect of a critical evaluation with respect to a possible existence of quantum size effects. Both electronic well and barrier widths were varied, with and without keeping the mean composition of the multilayers constant. Neither the overall dependences of the Urbach slopes, nor of the conductivity activation energies and room temperature αdark values are consistent with a quantization picture. Other effects, as imposing on each other the own sublayer's degree of structural disorder, and adjustment of the Fermi level by charge transfer, play a dominant role and may feign quantum size effects, if in a multilayer series only variation of the well widths is considered.
Optimization of the Deposition Conditions for High-Gap a-Si,Ge:H,F Alloys
Authors: Morin P.A. , Wang N.W. , Wagner S.
Published: March 2011

We report the deposition parameters for optimized a-Si,Ge:H,F alloys in the range of optical (Taue) gap 1.5 leV to 1.62eV. These deposition parameters were optimized using the saturated defect density as a figure of merit. We report initial defect densities at or below 2.5×1016 cm-3, saturated defect densities below 9×1016 cm-3, photoconductivities (at G = 1021 cm-3s-1) between 8.7×10-6 Scm-1 and 7×10-5 Scm-land photosensitivities between 104 and 105 for alloys in this range of optical gap.
Optoelectronic Properties of Amorphous SiGec Alloys
Authors: Kolodzey J. , Schwarz R. , Wang F. , Muschik T. , Krajewski J. , Shekhar R. , Barteau M. , Plättner R. , Günzel E.
Published: March 2011

We describe the optoelectronic characteristics of hydrogenated amorphous silicon germanium carbon (a.Si1-x-yGexCy:H) alloys prepared by plasma deposition from SiH4/GeH4/CH4/H2 gas mixtures. a-Si1-x-yGexCy:H is a homogeneous random alloy having a variable optical gap depending on composition, with properties similar to those of amorphous Si-Ge alloys of the same optical gap but with improved thermal stability. Calculations show that if the ratio of Ge/C atomic fractions is 8.2, the average bond length matches that of unalloyed amorphous a-Si:H with the possibility of reduced defect densities at heterointerfaces. After light-soaking with high intensity white light, a sample having a 1.3 eV optical gap exhibited no Staebler-Wronski change in its properties.
Non-Gaussian 1 / f Noise and Conductance Fluctuations in Hydrogenated Amorphous Silicon
Authors: Parman C. E. , Israeloff N. E. , Kakalios J.
Published: March 2011

Statistical analysis of the 1/f noise power spectrum of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) find that the noise arises from a small number of correlated fluctuators. The noise power displays a complicated time dependence with the noise power changing in both magnitude and as a function of frequency. Spectral analysis of these noise power fluctuations display an approximate 1/f frequency dependence. These results are surprising given the effective volume (∼10-7 cm3) of the sample, and indicate that cooperative dynamics govern conductance fluctuations in a-Si:H.
Quasi-Fermi Energy and Steady-State Recombination Demarcation Level in a-Si:H
Authors: Liu J. Z.
Published: March 2011

A specially designed photoconductivity experiment directly shows that, above room temperature, the steady-state recombination lifetime of the free electrons in undoped amorphous silicon can be described by the free electron quasi-Fermi energy alone. The apparent discrepancy on recombination demarcation levels between the Rose model and the Simmons-Taylor model is resolved by including the effect of the occupation function. The significance of the free electron quasi-Fermi level is discussed.
Mobility-Lifetime Products in a-Si:H and the Dangling Bond Recombination Model
Authors: Morgado E.
Published: March 2011

Fermi level and light intensity dependences of electron and hole lifetimes have been calculated using a recombination model which considers positively correlated dangling bonds as the only localized states in the gap. The model equations have been solved numerically taking into account the non-equilibrium statistics of correlated electrons and the Fermi level dependence of the defect density. The results are in agreement with the anticorrelated behavior of the majority' and minority carrier μτ products observed in a-Si:H. The majority carrier lifetime is found to be more sensitive to the photogeneration rate than the minority carrier lifetime. The position of the Fermi level with respect to the energies of the D° and D- centers in the gap is a determinant factor of the (°τ)e/(μτ)h ratio.
Assessment of Free Electron to Free Hole Ratios in a-Si:H By Ambipolar Diffusion and Photoconductivity Measurements
Authors: Pipoz Patrice , Sauvain Evelyne , Hubin Jacques , Shah Arvind
Published: March 2011

The authors report on systematic photoconductivity σph and ambipolar diffusion length Lamb measurements carried out on a series of lightly p- and n-doped samples and monitored throughout the entire light-induced degradation process. The results are interpreted based on a transport model involving free carriers (nf and pf), localized charge on bandtails, as well as dangling bond and dopant charges.
Loss Peaks in the AC Conductivity of a-Si:H
Authors: Schirone L. , Guseinov Ya. Yu. , De Cesare G. , Ferrari A. , Califano F.P.
Published: March 2011

The electrical conductivity of a-Si:H films was investigated in the temperature range 300K - 450K under an alternative electric field whose frequency was in the range 100 Hz - 6×106 Hz. Loss peaks were detected, located at a temperature dependent frequency and superimposed to a nearly linear dependence on frequency. Their amplitude has found to be sensitive to Staebler-Wronski Effect. The observed photodegradation-sensitive loss peaks have been described in terms of a Simple Pair Hopping (SPH) mechanism, involving the electronic states associated to dangling bonds, superimposed to a broadband process described by Correlated Barrier Hopping (CBH) model.
Toward 20% Efficiency With a-Si // poly-Si Tandem Solar Cell
Authors: Yoshimi M. , Ma W. , Horiuchi T. , Lim C. C. , De S. C. , Hattori K. , Okamoto H. , Hamakawa Y.
Published: March 2011

A series of experimental investigations has been made on the a-Si // poly-Si tandem solar cell which is one of the most promised candidate of high cost-performance photovoltaic cell, e.g., high efficiency, low cost with almost no light induced degradation. Employing high conductivity with wide optical band gap p type microcrystalline SiC (μ-SiC) as a window material together with a-SiC as an interface buffer layer and also n type μc-Si as a back ohmic contact layer in the poly-Si based bottom cell, the conversion efficiency of 17.2 % has been obtained. Combining an optically transparent a-Si p-i-n cell as a top cell with an optical coupler between the top and the poly-Si bottom cell, a total efficiency of 20.3 % has been obtained so far on the four-terminal stacked mode structure. A systematic technical data for the optimization of cell structure variation on the developed tandem solar cells are presented and further possibility to improving the performance are discussed.
Hydrogenation of B and P Implanted LPCVD Amorphous Silicon
Authors: Pietruszko Stanislaw M.
Published: March 2011

The results of the investigation of doping by B and P ion implantation into LPCVD amorphous silicon films in the range from 2*1015 to 2*1021 atoms/cm3 are presented. The room temperature conductivity increases to 10-2 Ω-1 cm-1 and to 10-2 Ω-1 cm-1 for the highest B and P doping, respectively. The subsequent hydrogenation (2.5 and 5 at%) by ion implantation increases the doping efficiency for P doping. For B doping efficiency increases at the low and decreases for the high doping range. The results of conductivity measurements vs temperature of doped and hydrogenated films are presented.
High Quality a-SiO:H Films and Their Application to a-Si Solar Cells
Authors: Fujikake S. , Ohta H. , Asano A. , Ichikawa Y. , Sakai H.
Published: March 2011

Hydrogenated amorphous silicon oxide (a-SiO:H) films were prepared by rf glow discharge decomposition of SiH4, CO2 and H2 gas mixture. These films showed better properties than a-SiC:H films. By applying the a-SiO:H to the p-layer, we attained an efficiency of 12.5% for 1cm2 single junction solar cells and a total-area efficiency of 10.1% for 30cm × 40cm tandem submodules.
Effects of Low Level Graded I-Layer Doping on the Stability of A-SI:H Solar Cells
Authors: Fischer D. , Pellaton N. , Keppner H. , Shah A. , Fortmann C. M.
Published: March 2011

This work reports on attempts to tailor the electric field of a-Si:H solar cells by the graded low-level doping of the intrinsic layer to optimize conversion efficiency in the degraded state. Based on wavelength dependent collection measurements and numerical modeling, the degradation behavior of doped and undoped cells is explained in terms of the interaction of dopants and the light-induced space-charge. Low level doping is shown to shift the electric field away from the p/i interface towards the bulk of the i-layer. This results in a better carrier collection from the back part of the solar cell, and solar cells with improved stabilized red light conversion efficiency can be realized. These cells can be readily applied as bottom cells of stacked solar cells.
Effect of Plasma Treatment of the Tco on a-Si Solar Cell Performance
Authors: Demichelis F. , Galloni R. , Madan A. , Pirri C.F. , Rava P. , Ruth M. , Schropp R.E.I. , Summonte C. , Tresso E.
Published: March 2011

Single junction a-Si p-i-n solar cells have been deposited by an Ultra High Vacuum (UHV) Multichamber PECVD system reaching an efficiency of 10.1% over 0.1 cm2 and 9.7% over 1 cm2. The effect of hydrogen treatments on the performance of the solar cells was studied on two different types of SnO2 coated substrates and was correlated with the I-V characteristics under AM1.5 (100 mW cm-2) illumination and the spectral response of the devices. The results show that modifications at the TCO/p-layer interface due to the hydrogen plasma treatments are strongly dependent on the initial characteristics of the TCO.
Characterization of the SnO2/a-SiC:H Interface in Amorphous Silicon Solar Cell by C-f-T Measurements
Authors: Caputo D. , De Cesare G. , Irrera F. , Palma F. , Salvatori S. , Vincenzoni R.
Published: March 2011

In this paper we present a new method for the characterization of the interface between the transparent conductive oxide (TCO) and the p doped amorphous silicon carbide layer in solar cells. The method is based on electrical capacitance measurements versus frequency in the temperature range: 20K-200K. We use Schottky diode structures (TCO-p-i-Ag) containing the p-i structure actually present in solar cells. Analysis of capacitance at different frequencies allows one to calculate an“activation energy”. Activation energy, temperature, position and shape of the capacitance step can be related to the density of states in the p type material by a model here presented. The effects of the p layer thickness and of the carbon content are investigated. A large increase of activation energy at short thickness is found, which can be related to interface damage, increasing in material with high carbon content. The technique shows an high spatial resolution and has the advantage to investigate the material as grown in the actual device.
Film Morphology, Excess Shunt Current and Stability in Triple-Junction Cells
Authors: McMahon T.J. , Bennett M.S.
Published: March 2011

When a textured conductive textured oxide (CTO)-glass substrate is used for either thin single p-i-n or thin multilayer p-i-n structures, a new mode of light-induced cell degradation occurs which has been associated with enhanced shunting. In this study, triple-junction cells were examined before and after light-soaking using dark current-voltage (I-V) analysis. Two degradation mechanisms are occurring: i) The classical bulk Staebler-Wronski defect generation mechanism causes an increase in diode quality factor, ii) In cells exhibiting more as-grown shunting behavior, light-soaking greatly enhances shunting. This enhanced shunting becomes the primary mode of degradation.

Reversible incremental current steps are also seen when measuring the (I-V) characteristics on diodes with excess shunt current due to light-soaking. This is a clear indication that the increase in shunting is caused by an increase in the number of shunt pathways and not by enhancing current through existing shunts. Optical-beam-induced current (OBIC) scans show some small defect areas of reduced current responsivity that are about 5–10 urn in diameter and may not be caused by holes in the back reflector metalization. Localized accelerated light-induced degradation is measured using the OBIC scanning laser beam on small areas (5 × 10-6 cm2), and there is some evidence that these areas degrade more than areas without defects.
Mobility Improvement Mechanism in a-Si:H TFTs With Smooth a-Si:H/SiNx Interface
Authors: Takechi K. , Uchida H. , Kaneko S.
Published: March 2011

The authors have investigated the mechanism for mobility improvement in a-Si:H TFTs with smooth a-Si:H/SiNx interface. SiNx surface roughness was evaluated by Atomic Force Microscope (AFM) measurement with nanometer resolution. The properties for a-Si:H initial growth layer near the a-Si:H/SiNx interface are affected by the SiNx surface roughness. By realizing a smooth SiNx surface, the properties for a-Si:H initial growth layer have been improved and high mobility TFT has been obtained. This high mobility TFT will have a great impact in application to high resolution liquid crystal displays.
Effects of Ultraviolet Radiation on the Characteristics of Hydrogenated Amorphous Silicon Thin Film Transistors
Authors: Lee Seong K. , Park Jin S. , Kim Yong S. , Hwang Jung R. , Oh Chang H. , Han Min K.
Published: March 2011

The experimental results regarding to the effects of ultraviolet (UV) light illumination on the characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) have been presented. The device parameters of a-Si:H TFT, such as threshold voltage, field-effect mobility, and subthreshold slope, have been degraded by electrical stress and visible light illumination, but substantially improved by UV radiation. This may be attributed to an annealing effect on the dangling-bond defects, involving a number of phonons generated by absorption of high energy UV photons in the a-Si:H TFT channel. It has been also observed that the off-current of a-Si:H TFT decreases remarkably while the on-current changes very little. From the experimental results, we report that the improved on/off current ratio of a-Si:H TFT may be achieved by UV radiation.
Si Surface Preparation: The Effect of Small Amounts of Carbon Contamination and Sputter Induced Surface Roughness
Authors: Baikie Iain D.
Published: March 2011

The sputter-anneal cleaning process is one of the most common methods of producing clean, defect free, crystalline Si surfaces. However the effects of small amounts of residual Carbon, probably the most insidious surface contaminant, and of Inhomogeneities in the sputter profile have not been investigated in any detail.

Using a very high resolution Kelvin probe coupled with Auger Electron Spectroscopy (AES), we have followed the changes in work function, together with surface contaminants, throughout the cleaning process. We show that very small amounts of surface carbon contamination, at or about the AES detection limit, lead to a significant increase in work function of the “Clean surface”. Further as little as 0.6% Carbon substantially influences the initial stages of oxidation of the Si(111) 7×7 surface by (i) hindering formation of the elementary dipole layer and (ii) enhancing oxygen penetration through the surface layer into deeper layers.

We have also measured work function profiles of a Si(111) 7×7 sample during sputtering. Inhomogeneities in the sputter profile, particularly around the periphery, are clearly evident and produce variations in work function of some 200 meV across the specimen. Repeated sputter-anneal cycles lead to a degree of disorder in these regions and this is reflected in a different oxidation behaviour.

We conclude that work function measurements are both simple to perform and can be utillsed to determine the presence of surface contamination and roughness. As the oxide layer thickness in MOS devices continues to diminish the detection and elimination of residual Carbon contamination will play an ever increasing role in determining device performance. The Kelvin probe can be used at elevated temperatures and pressures and has potential applications in alternative surface cleaning methods including flash annealing and chemical preparations.
Crystal Originated Singularities on Silicon Wafers After SC1 Cleaning
Authors: Morita E. , Ryuta J. , Tanaka T. , Shimanuki Y.
Published: March 2011

It is shown that a new type of singularity is formed on Si wafer surface by the Standard Cleaning 1 (SC1) of the RCA cleaning process″. Such singularities are perceived by laser particle counters as small particles on wafers. It is revealed that the singularities correspond to small shallow pits caused by the etching effect of the SCI cleaning solution. The origin of the pits seems to be some kind of defects in the melt–grown Si crystals. The authors named such “particles” as crystal originated “particles” (COPs).

The size–distribution of COPs after single SC1 cleaning cycle is estimated on the basis of variation in the number of COPs with the repeated cleaning cycles. It is revealed that the crystal pulling rate affects the size distribution of COPs. As the pulling rate becomes faster, COP becomes larger in number. From the total number of COPs after the first cleaning cycle and the etching depth, the volume density of origin of COPs can be estimated. The results show that, as the pulling rate becomes faster, the volume density of origin of COPs increases.
Surface Electronic States of Low Temperature H-plasma Cleaned Si(100) and Ge(100) Surfaces
Authors: Cho Jaewon , Schneider T.P. , Nemanich R.J.
Published: March 2011

The surfaces of Si(100) and Ge(100), cleaned at low temperatures by H-plasma, were studied by Angle Resolved UV-Photoemission Spectroscopy(ARUPS). In the case of Si(100), H-plasma exposure produced ordered H-terminated crystallographic structures with either a 2×1 or 1×1 LEED pattern while for Ge(100) a weak H-terminated 2x1 pattern was obtained. The hydride phases, found on the surfaces of the cleaned Si(100), were shown to depend on the temperature of the surface during H-plasma cleaning. The electronic states for the monohydride and dihydride phases were identified by ARUPS. When the plasma cleaned surface was annealed, the phase transition from the dihydride to monohydride was observed. For the Ge(100) surface, an ordered 2x1 monohydride phase was obtained from the surface cleaned at 180°C. After plasma exposure at ≤170°C a 1×1 surface was observed, but the ARUPS indicated that the surface was predominantly composed of disordered monohydride structures. After annealing above the H-dissociation temperatures, the dangling bond surface states were identified for both Si and Ge. The Si(100) and Ge(100) ARUPS spectra shifted after annealing, indicating that the H-terminated surfaces were unpinned.
Silicon Surface Cleaning by A Plasma in Afterglow
Authors: Charlet B. , Peccoud L.
Published: March 2011

Microwave plasma surface cleaning opens an interesting possibility for thin layer preparation. We have investigated the effect of hydrogen and NF3plasma induced by a microwave source. The sample is exposed in the downstream region achieved by a plenum. The cleaning process has been applied to a silicon (100) substrate. The cleaned surface has been analyzed by different methods of investigation: HRTEM, XPS, Therma-Wave and MOS capacitor (C-V) characteristics. The HRTEM observations show a defect-free interface, between the hydrogen cleaned silicon surface and the thermal oxide grown immediately after cleaning. In addition, the silicon crystalline network is not perturbed by induced defects. This was confirmed by Thermal Wave measurements which gave an excellent surface qualification.
Analysis of Plasma Oxidised Hg1-xCdxTe Surfaces
Authors: Leech P.W. , Moore R.L. , Crisman E.E. , Roberts C.B. , Stiles P.J.
Published: March 2011

We report for the first time on the application of Field Enhanced Plasma Oxidation (FEPO) to the passivation of Hg1-xCdxTe. Oxide layers of thickness 250-500Å were grown on Hg0.4Cd0.6Te at temperatures in the range from 30°C to 100°C. The index of refraction of the oxide layer, as determined by ellipsometry, was between 1.99 and 2.10. Auger depth profiles of the surfaces showed a compositional variation through the oxide with predominantly a mix of Cd and Te oxides in a film depleted in Hg. The oxides had dielectric constants of 3.6 to 4.5, and dielectric strength greater than 1 × 107 V/cm. The oxide/semiconductor interfaces had high densities of “slow” states but were capable of being depleted and accumulated.
MOCVD Growth of GaAs on Si with Low Temperature Preheating Process
Authors: Takagi N. , Eshita T. , Miyagaki S. , Kimura M. , Takasaki K.
Published: March 2011

A low temperature preheating process is developed for metalorganic chemical vapor deposition (MOCVD) growth of GaAs on wet chemical pretreated Si substrates. NH4 OH/H2 O2 is found to be most effective in decreasing the preheating temperature among the chemicals we tried: NH4 OH/H2 O2, H2SO4 /H2O2, or hot HNO3. By using NH4OH/H2 O2, the preheating temperature is reduced from 1000°C to 875°C. X-ray diffraction measurements and surface observations with an atomic force microscope (AFM) show that the GaAs film quality obtained with the 875 °C preheating process is better than that obtained with 1000°C preheating.