bluuebunny/crossref_metadata_2025 huggingface dataset:
Empirical Dissolution Rate Law for the Glass R7T7 Contacting Halite- and Silica-Saturated Brines
Authors: Grambow B. , Lutze W. , Müller R.
Published: March 2011

We report on the time dependence of release of glass constituents during static dissolution experiments with the COGEMA glass R7T7 in saline MgCl2 and NaCl dominated solutions at temperatures between 110 and 190°C. The experiments were performed at high S/V values to ensure silica saturation almost from the start of the tests. The results show a square root of time dependence indicating diffusion as rate-controlling with apparent diffusion coefficients similar to that of water diffusion in alkali silicate, borosilicate glasses or long-term weathered obsidians.
Effect of Platinoids on French LWR Reference Glass Properties
Authors: Pacaud F. , Fillet C. , Jacquet-Francillon N.
Published: March 2011

Nine samples of the “R7T7” glass composition selected to vitrify fission product solutions in France were prepared with added platinoid elements (ruthenium, rhoditun and palladium) in soluble form and as insoluble metal particles in solution, and their major properties were measured. Regardless of the initial form when added to the glass the platinoids always formed the same heterogeneous inclusions in the final glass: RuO2 precipitates which were often found as aggregates, and polymetallic (Pd, Rh & Te) inclusions.

The particles tended to settle in the molten glass. The viscosity increased by about 20% at 1100°C. The mechanical properties and short-term leach rates were not significantly affected. Crystallization increased by a factor of 2 or 3 in heat-treated glass specimens but did not exceed a few volume percent. However, as the short-term leach rate did not significantly increase, the glass properties were very satisfactory.
Radiological Protection Criteria for Solid Waste Disposal: Historical Developments and New Ideas
Authors: Cooper J R , Mobbs S F , Barraclough I
Published: March 2011

The historical development of international advice on radiological protection criteria for solid waste disposal is described, and problems associated with the interpretation of existing criteria are discussed. These problems arise largely because of the long timescales over which radionuclides may be released from solid waste repositories. In an attempt to aid in the interpretation of existing international advice, the UK National Radiological Protection Board is issuing new guidance on radiological protection criteria for solid waste disposal and its application. Aspects of this new guidance are described.
Mechanisms for the Retardation of Uranium (VI) Migration
Authors: Silva Robert J.
Published: March 2011

This report covers the investigation of mechanisms for the retardation of U(VI) migration. It describes (1) the measurement of the solubility of UO22+as afunction of pH and equilibration time, (2) the calculation of solubility using solution thermochemical data, (3) the measurement of the sorption of UO22+by silica as a function of pH and supporting electrolyte concentration, and (4) attempts to simulate the uranium sorption isotherms in terms of the U solution thermochemical data, measured properties of the silica and proposed sorption reaction mechanisms.
Dissolution Kinetics of UO2. I. Flow-Through tests on UO2.00 Pellets and Polycrystalline Schoepite Samples in Oxygenated, Carbonate/Bicarbonate Buffer Solutions at 25°C
Authors: Nguyen Son N. , Weed Homer C. , Leider Herman R. , Stout Ray B.
Published: March 2011

The modelling of radionuclide release from waste forms is an important part of the performance assessment of a potential, high-level radioactive waste repository. Since spent fuel consists of UO2 containing actinide elements and other fission products, it is necessary to determine the principal parameters affecting UO2 dissolution and quantify their effects on the dissolution rate before any prediction of long term release rates of radionuclides from the spent fuel can be made.
Leaching of Actinides from Nuclear Waste Glass: French Experience
Authors: Vernaz E.Y. , Godon N.
Published: March 2011

Results are presented for a large number of aqueous leach tests on glass specimens doped with 237Np, 238Pu, 239pu, 241Am or 244Cm. The effects of the leaching mode (static or dynamic), temperature and environmental materials are discussed. The amount of actinides contained in the alteration film is characterized by the retention factor RF = NL(B)/NL(act). The neptunium retention factor ranged from 1 to 10 depending on the test conditions; for plutonium and americium, the retention factor varied from 6 to 1300.

In general, the retention factor increased with the temperature. The very important influence of the environmental materials shows that there is competition for actinide retention between the alteration film and the surrounding materials, but in every case the amount of ca activity released into solution is very low.
X-RAY Photoelectron Spectroscopic Study of Cigar Lake Uranium Ore: A Natural Analog for Used Fuel
Authors: Sunder S. , Cramer J.J. , Miller N.H.
Published: March 2011

X-ray photoelectron spectroscopy (XPS) was used to study samples from the Cigar Lake uranium deposit in northern Saskatchewan. Two uranium-rich sections, CS-615 and CS-620 were studied. Peaks due to U, 0, C, Pb, Si, Ca, Al, S, Cu and Th were seen in the XPS spectra. Concentrations of Pb of up to 14 wt% were measured by XPS and correspond to the Pb ingrowth by radioactive decay during the 1.3 billion years since the ore was formed. High-resolution spectra were recorded for the U, Pb, 0, and C bands. Lead was in the +2 oxidation state in all samples. The carbon signal indicated the presence of organic carbon, while the oxygen bands indicated the presence of significant amounts of water in the samples. The samples from the CS-615 section had U6+/U4+ratios between 0.16 and 0.29. However, the U6+/U4+ratios for section CS-620 were about 0.7 but this is attributed to oxidation of the ore by water during the drilling and cutting processes. The significance of the results for disposal concepts in nuclear fuel waste management programs is discussed.
Immobilization of Actinides by Hydroxylapatite
Authors: Gauglitz R. , Holterdorf M. , Franke W. , Marx G.
Published: March 2011

In cooperation with the Ministry of Research and Technology we investigated the influence of hydroxylapatite (HAP) added to the backfill material of a radioactive waste disposal located in a salt dome. The time dependence of the sorption and precipitation of radionuclides has been studied in three different praxisrelevant salt brines under variation of temperature and pH-value.

Performing batch experiments uranium and thorium phosphates were precipitated fromUO2(NO3)2andTh(NO3)4solutions at 75°C, the amounts of the relevant actinide compounds also exceeding that of HAP. The isolated precipitates were investigated by use of X-ray-powder-diffractometry and SEM. For the various uranium systems two mineral phases were obtained. From solution II and also from Q-brine the mineral saléeiteMg(UO2)2(PO4)2·9H2Owas formed. From solution III meta-autuniteCa(UO2)2(PO4)2·6H2O, a dehydration product of autunite, was precipitated.
Diffusivity of Uranium(VI) in Water-Saturated Inada Granite.
Authors: Idemitsu Kazuya , Furuya Hirotaka , Murayama Kenji , Inagaki Yaohiro
Published: March 2011

In the safety assessment of high level radioactive waste disposal, a significant retardation of nuclides can be expected. When radionuclides released from an underground repository are transported with the moving groundwater along cracks in the rock, the radionuclides will be retarded by not only adsorption on the surface of the cracks but also diffusion into the micropores of rock matrix.

Diffusivities of U(VI) were measured in water-saturated granite that was mined in the middle of Japan. The measured penetration profile of U(VI) was composed of two parts. Those were a steep slope near the surface and a gradual slope in the deeper part. This profile was successfully explained by considering two diffusion paths in granite. One diffusion path was possibly a fissure with a width of a few microns and another was a network of submicron pores. The volume of submicron pores was approximately 80% of the total pore volume in the granite. The orders of magnitude of apparent diffusivities for U(VI) were 10−13-10−12 m2/s through the fissure and 10−15 m2/s through the network of pores. The ratio of geometrical factors in the two paths was also calculated from each apparent diffusivity and was almost unity. This ratio calculated suggests that the pore network in the granite matrix has a similar shape to the fissure, thus the microstructure of the granite seems to be fractal.
Radioelement Behaviour in a Cementitious Environment
Authors: Bayliss S. , Haworth A. , McCrohon R. , Moreton A.D. , Oliver P. , Pilkington N.J. , Smith A.J. , Smith-Briggs J.L.
Published: March 2011

The Nirex Safety Assessment Research Programme (NSARP) comprises scientific research to support the safety assessment of a deep geological repository for low- and intermediate-level radioactive wastes. The NSARP addresses several areas of research, one of which is a continuing investigation of the behaviour of radioelements in a cementitious environment. This paper draws together recent experimental measurements of solubility and sorption under such conditions, along with the results of interpretive thermochemical and sorption modelling. Under conditions similar to those expected in the repository, the predicted solubility of selenium is about 3xl0−8M. Experimental measurements confirm that at low redox potential the solubility of selenium is substantially less than that previously observed under an air atmosphere, and is closer to the predictions of the thermochemical modelling. The solubility of sodium stannate has been studied after heating solutions at 50, 80 and 200°C. Crystalline tin solids were formed at all temperatures. For solutions heated at 200°C the solids were identified as cassiterite. The solubilities of all of the crystalline solids were 1x10−6M to 1x10−9M which are at least five orders of magnitude lower than that of the original sodium stannate. Distribution ratios (Rd) between 1x103 and 5x103 ml g-1 have been measured for the sorption of tetravalent technetium on to a cementitious material. Preliminary results from an in-diffusion experiment involving americium and an intact cement sample are in good agreement (at 1sx104 ml g-1) with Rd values determined from previously reported batch sorption experiments.
Diffusivity and Porosity in Rock Matrix Related to the Ionic Strength in the Solution
Authors: Valkiainen M. , Uusheimo K. , Olin M. , Muurinen A.
Published: March 2011

The nature of diffusivity and porosity in rock was studied as a function of various parameters. The phenomena of main interest were dead-end porosity, ion-exclusion and sorption. The rock types studied were rapakivi granite, granite and gneiss, and tracer techniques with 36Cl, 22Na+ and 3H (HTO) were used as a research method. A mathematical solution for outdiffusion from a porous cylinder was developed by applying a corrected form of Fick's second law for a case where part of the pores are so-called dead-end pores. With this model the theoretical curve could be closely fitted to the measured values. It was found that the rock-capacity factor is an increasing function of the ionic concentration of the solution in the case of Cl indicating ion-exclusion, while the opposite is true in the case of Na+ indicating ion-exchange type sorption. The effective diffusion coefficient was also found to vary as a function of the salinity in the case of 36Cl. In the case of 22Na, the effect was opposite and weaker. The diffusion of tritium through the rock samples was clearly higher than the diffusion of 36Cl. Part of the difference is explained by the smaller effective porosity for 36Cl. The rest can probably be explained by the steric effects on the chloride ion caused by the negatively charged pore surfaces in the narrow pores.
Cationic Corrosion Inhibitors for Alkaliborosilicate Glass
Authors: Zwicky H.U. , Graber TH. , Grauer R. , Restani R.
Published: March 2011

The dissolution behaviour of two alkali borosilicate glasses has been investigated at 90°C in the presence of potential corrosion inhibitors. The added materials were selected on the basis of surface chemistry considerations: Since the dissolution rate of silicate solid phases in alkaline solutions increases with growing negative surface charge, it should be expected that sorbing cations reduce the reaction rates. Due to the formation of insoluble hydroxides and silicates and to the dominance of neutral or negatively charged hydroxo complexes in the pH range of interest, the selection of potential inhibitors is very restricted.

The precipitated hydroxides of Al, Zn, Cd and Pb were chosen as the solid phases. Compared with the experiments in pure water, glass dissolution is enhanced in presence of Al(OH)3 and Zn(OH)2. The concentration of the aqua ions is too low for an inhibiting effect. Sorption of silicic acid on the hydroxide accelerates the glass dissolution. Addition of PbO had a strong inhibiting effect in a 28 days experiment. At longer times an insoluble lead silicate is formed and thereby the glass dissolution rate is increased. Cd on the other hand is still effective as an inhibitor after 365 days.

Based on the present results, the possibility of using glass corrosion inhibitors in a repository is considered not to be worthwhile.
Hydrogenated Amorphous Silicon/Aluminum Interaction at Low Temperatures
Authors: Shahidul Haque M. , Naseem H. A. , Brown W. D. , Ang S. S.
Published: March 2011

Annealing effects on the Al/a-Si:H structure in the temperature range 100°C to 300°C have been studied. Aluminum was evaporated on device quality phosphorous doped n+ a-Si:H films deposited in a UHV-PECVD system. The transmission line model (TLM) technique was used to measure the sheet resistance and contact resistivity of the films. For samples where Al covered the entire a-Si:H surface during annealing, sheet resistance and contact resistivity were found to decrease monotonically with annealing temperature. On the other hand, samples annealed after patterning Al pads showed a minimum in sheet resistance and contact resistivity at temperatures between 170°C and 200°C with a steep increase beyond 250°C. Optical and scanning electron microscopy, as well as surface profilometry, were used to study the surface morphology. A simple model involving Al interaction with the a-Si:H at the surface is proposed to explain the electrical behavior.
X-Ray Detectors Based on “Thick” a-Si:H Layers Deposited by the VHF-GD Process
Authors: Chabloz P. , Keppner H. , Baertschi V. , Shah A. , Chatellard D. , Egger J.-P. , Denoréaz M. , Jeannet E. , Germond J.-F. , Vuilleumier R.
Published: March 2011

In spite of its low absorption coefficient for X-rays, amorphous Silicon can be an interesting alternative approach for X-ray detection because of its low cost, its potential for large-area deposition and the possibility to deposit on a curved surface. For this application, basically two approaches have been proposed up to now: either a thick solar cell type n-i-p structure (the i-layer as to be sufficiently thick i.e typically 50 μm or more), or a normal solar cell type n-i-p structure (with a relatively thin i-layer, i.e (typically 1 to 2 μm) together with a fluorescent layer emitting visible light composed e.g. of CsJ. In this paper, we present first results of a X-ray detectors with thick i-layers (15 to 100 μm) prepared by the high deposition rate VHF-GD technique introduced at our laboratory. Detectors with low leakage currents (<4nA/cm2) under high reverse bias voltages (about 100 V) could be fabricated at rates as high as 22 Å/s. As substrates, aluminium as well as TCO-coated glass substrates were used. The detectors have a n-i-p structure, where highly conductive (100 S/cm) n-doped μc-Si:H was first deposited. For the substrate preparation, a high energy Ar plasma was applied before the first deposition step; in this way excellent sticking conditions could be achieved, although in the thicker detectors considerable curvature due to the internal mechanical stress could be observed. A medical X-ray radiation source was used, where the detector was exposed to a continuous X-ray spectrum at acceleration voltages between 80 kV and 240 kV. The paper presents measurements on the linearity of the detector, as well as on the value of the reverse current in the dark, which must be as small as possible to have the best signal to noise ratio.
1Improvement of Thick a-Si Radiation Detectors by Field Profile Tailoring
Authors: Drewery J.S. , Cho G. , Jing T. , Kaplan S.N. , Mireshghi A. , Perez-Mendez V. , Wildermuth D.
Published: March 2011

Application of thick (∼50 μm) a-Si p-i-n diodes as a direct radiation detector for minimum ionizing particles is hampered by the need to apply large bias voltages in order fully to deplete the detecting intrinsic layer, which typically contains 5–10×1014 ionizable dangling bonds per cm3. By insertion of thin p-type layers at intervals within the intrinsic layer, the required depletion voltage can be reduced by a factor of at least 1/(n+1) where n is the number of layers inserted. This principle is demonstrated for devices approximately 12μm in thickness. It is shown that electron losses within the p type layer can be kept to minimum by choice of a low doping concentration for the introduced p layers.
Dynamic Thresholding with the Three-Terminal Optically Addressed Spatial Light Modulator
Authors: Rice Robert A. , Close Peter J. , Moddel Garret
Published: March 2011

Standard, non-patterned optically addressed spatial light modulators (OASLMs), comprising an a-Si:H photodiode and a ferroelectric liquid crystal (FLC), have a fixed write-light intensity threshold when driven at a particular frequency. We present a patterned OASLM in which the threshold may be varied. The structure and operation of this device is significantly different from the standard OASL.M. While the square-wave drive for the non-patterned OASLM provides a time-varying voltage for the device, the thresholding OASLM uses a dc drive, with spatially separated bias voltages. By varying the voltage applied to a patterned metal layer between the a-Si:H and FLC, we achieved dynamic thresholding over one decade of write-light intensity. Fabricated devices have response times as low as 200 μsec, and contrast ratios as high as 50:1.
Optimization of Storage Spatial Light Modulators Incorporating Amorphous Silicon as Photoconductor
Authors: Slobodin D. , Sugiarto D. , Bone M. , Kahn F.
Published: March 2011

The behavior of a-Si.H photoconductors under conditions used in storage photoaddressed spatial light modulators (PASLM) is described. The differences observed in high and lov voltage operation are explained in terms of charge collection efficiency. A voltage dependent resistance term is proposed for PASLM modelling. The effects of photocapacitance are assessed and found to be of significance only under conditions of low drive voltage and higher illumination levels.
Study of a-Si:H/μc-Si:H Heterojunction as Photosensor for Large Screen Projection Display
Authors: Han Weiqiang , Han Gaorong , Qiao Jianmin , Du Pija , Zhao Danmei , Ding Zishang
Published: March 2011

Liquid crystal light valve(LCLV) using an a-Si:H/μc-Si:H heterostructure as the photosensor and the nematic liquid crystal as the modulator has been firstly presented for large screen projection display. The a-Si:H photoconductor and μc-Si:H light blocking layer were prepared by a modified glow discharge CVD method. The optoelectric and structure properties of the μc-Si:H films deposited at different deposition conditions have been studied. The a-Si:H film and the continuously deposited pc-Si:H film possibly form an a-Si:H/μc-Si:H heterojunction. The electrical and optoelectric properties of the heterojunction has been studied.

LCLV using a-Si:H/μc-Si:H heterostructure has shown that the most aspects of the device performance can be improved.
Amorphous Silicon Image Sensor Arrays
Authors: Powell M J , French I D , Hughes J R , Bird N C , Davies O S , Glasse C , Curran J E
Published: March 2011

We have developed a technology for 2D matrix-addressed image sensors using amorphous silicon photodiodes and thin film transistors. We have built a small prototype, having 192×192 pixels with a 20μm pixel pitch, and assessed its performance. The nip photodiodes can have dark current densities of less than 1011 A.cm-2 (up to 5V reverse bias) and peak quantum efficiencies of 88% (at 580nm). We operated the sensor in real time mode at high speed (50 Hz frame rate and 64μS line time). The image sensor has a low noise performance giving a dynamic range in excess of 104. The maximum crosstalk is about 2%, which allows at least 50 grey levels. The bottom contact of the photodiode acts as a light shield from light through the substrate, which enables the sensor to be operated as an intimate contact image sensor to image a document placed directly on top of the array. In this mode, the CTF was 75% at 2 lp.mm1. Good quality images are demonstrated in both front projection and intimate contact imaging modes.
Dependence of 1.00 MeV Proton Radiation Resistance of a-Si:H Alloy Solar Cells on Cell Thickness
Authors: Woodyard James R.
Published: March 2011

Rutherford bacJcscattering spectrometry has been used to determine the role of thickness and composition on the radiation resistance of a-Si:H based alloy solar cells. Single and dual-junction cells with thicknesses between about 0.23 and 1.1 microns were studied. Cells were irradiated with 1.00 MeV protons using fluences between 1E13 and 1E15 cm-2; the radiation resistance was determined using J-V measurements. The fluences produced significant changes in Jsc. Cells with thicknesses in the 0.23 to 0.69 micron range degraded about the same when irradiated with the same fluences; thicker single-junction cells degraded with both thickness and fluence. Thirty percent Ge in a-Six,Ge1-x:H cells results in poorer radiation resistance. It appears that Ge is more effective than Si in stabilizing 1.00 MeV proton-induced defects.
Pulsed Laser Crystallization of Amorphous Silicon Films: Effects of Substrate Temperature and Laser Shot Density
Authors: Johnson R. I. , Anderson G. B. , Ready S. E. , Fork D. K. , Boyce J. B.
Published: March 2011

A recent report on pulsed laser crystallization of a-Si thin films concluded that substrate bias temperatures up to 400°C in combination with laser fluences below 500 mJ/cm2 had little effect on grain size and transport properties. The current report describes the effects of substrate bias temperature up to 500°C and laser fluence up to 540 mJ/cm2 on grain size, mobility and Si (111) x-ray peak intensities. Results indicate that substrate bias temperatures above 400°C, in combination with high laser shot densities and large laser beam spot energies (> 500 mJ/cm2), are a factor in Improving these film properties.
Analysis of VHF Glow Discharge of A-SI:H Over a Wide Frequency Range
Authors: Zedlitz Ralf , Heintze Moritz , Bauer Gottfried H.
Published: March 2011

a-Si: H was deposited in a VHF-plasma in a wide frequency range up to 250MHz. The deposition rate was found to increase with excitation frequency up to 180MHz, where values of 35Å are achieved maintaining good film properties

The onset of this increase occurs near 60MHz, when the excitation frequency exceeds the inelastic electron-neutral collision rate. From optical emission of hydrogen and a He/Ar line pair the high energy tail of EEDF was probed. It is found, that the rise in deposition rate is solely due to the increased plasma density and not to an enhancement of the high energy tail of the EEDF, as postulated previously.
Microstructure and Dangling Bond Defects in Amorphous Hydrogenated Silicon Deposited Near Room Temperature
Authors: Cheung Man Ken , Petrich Mark A.
Published: March 2011

The microstructure of high-density amorphous hydrogenated silicon (a-S.i:H) films deposited at 50°C substrate temperature was revealed by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies to be similar to that of “device-quality” a-Si:H films deposited at standard “optimum” conditions. However, optical absorption measurements of these low microstructure 50°C films with photothermal deflection spectroscopy indicate that they have higher densities of gap state defects and localized band tail states than “device-quality” films deposited at standard substrate temperatures. The correlation between the amount of microstructure and electronic properties is not unique. A low amount of microstructure is a necessary, but not sufficient, requirement for high electronic quality a-Si:H films.
Small-Angle X-Ray Scattering Studies of Glow-Discharge-Produced a-SiGe:H Alloys
Authors: Jones Scott J. , Chen Yan , Williamson D. L. , Mooney G.D.
Published: March 2011

Electron density fluctuations associated with microstructural features on a scale from about 1 to 25 nm in glow-discharge-deposited a-Si1-xGex:H films were studied by the technique of small-angle x-ray scattering (SAXS). Films prepared in four different deposition systems (in different laboratories) have been characterized and a general increase in the SAXS signal with increasing x is observed. Density deficiencies determined from film flotation measurements lead to the correlation of the increased scattering intensities with increases in the volume fractions of micro voids. Modeling of the data yields void size distributions that demonstrate significantly more of the larger voids (2 to 6 nm) than found at x=0 (around 1 nm). For the alloys with x>0.4, the scattering at the smallest angles was observed to decrease substantially upon tilting of the sample relative to the x-ray beam. This result contrasts with the small or no changes in SAXS upon tilting device-quality x=0 films. This anisotropie scattering associated with the tilting experiments has been modeled with distributions of ellipsoidal microvoids that are preferentially oriented with their major axes normal to the film plane. This latter result is consistent with a columnar-like microstructure. However, one film with x=0.37 shows no evidence for such microstructure.
Bond-Angle Variation and Microstructure in Hydrogenated Amorphous Silicon
Authors: Berntsen A. J. M. , Van Den Boogaard M. J. , Van Sark W. G. J. H. M. , Van Der Weg W. F.
Published: March 2011

A series of hydrogenated amorphous silicon (a-Si:H) films was deposited by rf glow-discharge deposition using various processing conditions. We have studied microstructure in the films by means of infrared absorption spectroscopy. Small-angle X-ray scattering measurements were used to determine the microvoid fractions of a few selected samples. Our results show that both the void fraction and the amount of microstructure can be varied either by changing the substrate temperature or by H2 dilution. Bond-angle variation in the films was probed by Raman scattering measurements. The Raman data indicate that the substrate temperature is the main variable that determines the bond-angle variation. We conclude that the presence of microvoids in a-Si:H does not influence the structural disorder of the amorphous matrix surrounding the voids. Our results are in agreement with experimental work on microvoids in a-Si1-xCx:H, and model calculations on voids in a-Si.
Small-Angle X-Ray Scattering from a-Si:H and a-SiC:H Alloys Prepared by Reactive DC Magnetron Sputtering
Authors: Chen Yan , Jones S.J. , Williamson D.L. , Yang S. , Maley N. , Abelson J.R.
Published: March 2011

The microstructure of amorphous silicon-based alloys prepared by reactive magnetron sputter (RMS) deposition has been examined on a scale from 1 to 25 nm using small-angle x-ray scattering (SAXS) and compared to that from films prepared by standard glow-discharge (GD) technology. Device-quality RMS a-Si:H material is found to have a larger microvoid fraction than device-quality GD a-Si:H and to have a significantly different size distribution. Addition of by the RMS technique using CH produces enhanced SAXS similar to the introduction of C via CH4 in GD material. A significant difference in the SAXS from RMS a-Si:H and a-SiC:H films compared to GD films is the observation of some oriented microstructure, most likely columnar in nature. Flotation density measurements of the same films examined by SAXS support the assumption that the SAXS originates primarily from microvoids.
Evidence for Hydrogen-Hydrogen Interactions During Diffusion in a-Si:H
Authors: Jackson W. B. , Tsai C.C. , Santos P. V.
Published: March 2011

The effects of H concentration on H transport has been measured by deuteration of H depleted samples which were re-hydrogenated at various concentrations and on samples subjected to various annealing protocols. A remarkable observation is that the trap density appears to increase with H concentration. Extended thermal annealing appears to increase the trap depth but does not markedly alter the number of traps. The results suggest that H introduced from the plasma is less strongly bound than H residing in the film during annealing. Nucleation and growth of H clusters is introduced as a possible model accounting for these transport results.
Defect Equilibration and Stabilization in Two Types of Low-Spin-Density a-Si:H
Authors: McMahon T.J.
Published: March 2011

Relaxation data provide evidence that thermally generated defects with spin observed in thick, undoped hydrogenated amorphous silicon (a-Si:H) films can be stabilized with alternate structural configurations. Some quenched-in metastable spins, equilibrating further at a lower temperature, relax and become more resistant to annealing. Some of these defects take longer to anneal than any of the defects equilibrated only at the higher temperature. An obvious crossing of annealing data curves occurs on one of two differently prepared films. Crossover has important physical implications in deciding between defect models.

Such an observation necessitates the use of defect models with more complexity than variations based on two-level energy-configuration coordinate diagrams. Long annealing times may not be controlled simply by high energy barriers; the annealing of a spin may require many dispersive steps with smaller energy barriers. Models that stablize a spin by either hierarchically constrained defect configurations or the trapping of a mobile hydrogen in a distribution of sites can be used to explain this result. The fact that crossover is more obvious on the sample that has an oriented microstructure implicates film structure.
Macro-Trench Studies of Surface Reaction Probability of a-Si:H Film Growth
Authors: Nuruddin A. , Doyle J. R. , Abelson J. R.
Published: March 2011

Using a “macro-trench” technique, the surface reaction probabilities β of the a-Si:H growth precursors for remote hollow cathode silane discharge (HC) and reactive magnetron sputter deposition (RMS) are measured. Both deposition methods produce state of the art photo-electronic quality a-Si:H. For the HC case, β= 0.28 ± 0.05, whereas for RMS deposition β ≈ 0.97 ± 0.05. We conclude that β is not universally correlated with film quality, and discuss mitigating factors present in RMS deposition that permit high quality film to be deposited despite the high film precursor reactivity.
A Defect Density of ∼ 1014 cm-3 in Hydrogenated Amorphous Silicon Deposited at High Substrate Temperatures
Authors: Ganguly Gautam , Matsuda Akihisa
Published: March 2011

The idea of surface mobility of growth precursors determined material quality has been exploited by raising the substrate temperature above the conventional 250°C and the ensuing thermal depletion of the surface hydrogen coverage compensated by increasing the precursor flux (deposition rate) to prepare ultra low defect density hydrogenated amorphous silicon.
The Influence of the Void Structure on Deuterium Diffusion in a-Si:H
Authors: Van Den Boogaard M. J. , Jones S. J. , Chen Y. , Williamson D. L. , Hakvoort R. A. , Van Veen A. , Van Der Steege A. C. , Bik W. M. Arnold , Van Sark W. G. J. H. M. , Van Der Weg W. F.
Published: March 2011

We have used small-angle X-ray scattering (SAXS) and Doppler-broadening measurements of positron-annihilation radiation to study changes in the microvoid distribution in PECVD a-Si:H films during annealing. From a comparison of data on deuterium diffusion with information obtained from SAXS we conclude that changes, during annealing, in the dispersive character of deuterium diffusion are likely to be caused by void formation through clustering of smaller structural defects.
New Results on Enhanced Deuterium Diffusion Under Illumination in Amorphous Silicon
Authors: Branz Howard M. , Asher Sally E. , Nelson Brent P.
Published: March 2011

We measure the light-enhancement of D diffusion in hydrogenated amorphous silicon and determine that the mechanism for the effect is an increase of the rate of Si-D bond breaking under illumination. We exclude light-induced heating of the sample and light-induced excitation of D between dissimilar materials as sources of the light-enhancement. It is a bulk effect, most likely caused by excess carriers. We are able to observe the light-induced effect with 380 mW-cm-2 of red light, an intensity only slightly larger than the intensity normally used to induce the Staebler-Wronski effect. At room temperature, the effect is unobservable and we derive an upper bound of 2 × 10-8 photon-1 for the efficiency of light-induced Si-D bond breaking. Implications for the Staebler-Wronski effect are discussed.
Lithium Stability and Diffusion In a-Si:H
Authors: Beyer Wolfhard , Herion Jörg , Hagner Heribert , Zastrow Uwe
Published: March 2011

SIMS measurements of implanted Li in (doped and undoped) a-Si:H, a-Si and a-Si:H alloys are discussed. The results suggest basically the same Li diffusion process in p- and n-type a-Si:H with negatively charged acceptors or dangling bonds acting as traps for the positively charged diffusing Li ions. Li is less stable in a-Si than in a-Si:H; Li stability in a-Si:H is enhanced at high concentrations of implanted Li. Both diffusion and stability are modified by doping gradient-related electric fields.
The Role of Charged Defects in Photo-Degradation of Hydrogenated Amorphous Silicon
Authors: Dalal Vikram L. , Chopra Sanjiv , Knox Ralph
Published: March 2011

We examine the role of charged defects in inducing degradation of electronic properties of a-Si:H upon exposure to light. We measure the kinetics of decay of photo-conductivity of a-Si:H films at different light intensities, and the corresponding changes in mid-gap optical absorption. We find that the initial, rapid decay of photo-conductivity can be modeled guite well by invoking Adler's model of conversion of charged defects to neutral dangling bonds(D- to D° conversion). A consequence of this conversion is a decrease in sub-gap absorption upon photo-induced degradation, which we observe. Therefore, we conclude that charged defects coexist with neutral defects in a-Si:H, and they play a major role in early stages of photo-degradation.
Isolation of Temperature Effects on the Kinetics of Light Induced Defect Generation in a-Si:H
Authors: Benatar L. , Grimbergen M. , Fahrenbruch A. , Lopez-Otero A. , Redfield D. , Bube R.
Published: March 2011

Data are presented here that show the effects of temperature on the kinetics of metastable defect formation in undoped a-Si:H over the range 45°-110°C. CPM (Constant Photocurrent Method), photoconductivity, and dark conductivity measurements were made and provide independent checks of the defect generation behavior. A stretched exponential description of defect formation as a function of time was used to fit the CPM defect density data. The stretched exponential time constant, τSE, is thermally activated with an apparent activation energy of 1 eV, a value that agrees well with data for defect anneal and solar cell degradation. The data indicate that thermal terms are not negligible for temperatures as low as 45°C, and therefore should be included in any model of the kinetics of defect formation. The role of adistribution of anneal energies and the regimes of dominance of thermal and optical rate terms are discussed in the context of the model.
Current Induced Degradation of a-Si:H Pin and Schottky Switches
Authors: Nieuwesteeg K.J.B.M. , Boogaard J. , Oversluizen G.
Published: March 2011

Forward-bias current stress experiments were performed on α-Si:H p-i-n and Schottky switches at several temperatures and at current densities up to 6 A/cm2. In Schottky diodes, current stressing results in a lowering of the forward-bias SCLC current together with an increase of its thermal activation energy. The reverse current is unaffected. The rate of degradation of the forward current increases with increasing temperature. From a comparison of the degradation behaviour of Schottky's with different barrier height we find that the rate of degradation is correlated to the minority-carrier injection ratio of the Schottky contact. The effects are interpreted as being due to metastable state creation in the bulk α-Si:H. The rectifying properties of the metal-to-semiconductor contact are relatively stable to current stress.

The forward-bias I-V curves of p-i-n diodes degrade much faster than those of the Schottky switches. At the same time, the reverse-bias current increases due to the stress. The lower stability to current-stress of p-i-n diodes is ascribed to the much higher hole injection in the mesa. After a short time, the reverse-bias current becomes dominated by e-h generation from the created deep states in the i-layer and then gives a direct indication of its time dependence.
Growth of Ge/Si Amorphous Superlattices by Dual-Target DC Magnetron Sputtering
Authors: Järrendahl K. , Birch J. , Hultman L. , Wallenberg L.R. , Radnoczi G. , Arwin H. , Sundgren J-E.
Published: March 2011

Amorphous Ge/Si superlattices have been grown by dual-target dc magnetron sputtering at ambient temperature under different negative substrate bias voltages Vs. The films were studied with cross-sectional transmission electron microscopy (XTEM). All films were amorphous, but for Vs ≤ 110 V the superlattices exhibited a columnar structure. Within each column, the compositional modulation was clearly resolved except in the vicinity of the column boundaries. For 110<VS≤250V, dense films with homogeneous and well-defined layers were obtained. High-resolution XTEM studies showed that superlattices with individual layer thicknesses less than 1.5 nm and smooth interfaces could be fabricated. Increasing Vs > 250 V resulted in ion induced intermixing such that at Vs ≥ 450 no compositional modulation was observed. The obtained structural results are discussed in terms of available models for ion-irradiation effects during low-temperature ballistic growth.
Energy Differences Between the Si and the Ge Dangling Bond Defects in a-Si1-xGex Alloys
Authors: Cho S.M. , Davidson B.N. , Lucovsky G.
Published: March 2011

We have investigated the difference in the electronic energies of neutral Si and Ge dangling bond states in undoped a-Si1-xGex alloys as a function of the alloy composition, x, and local bond-angle distortions. The local density of states, LDOS, in a-Si1-xGex alloys has been calculated using nearest-neighbor interactions, and employing the Cluster Bethe Lattice method. We conclude that for ideal, tetrahedrally bonded amorphous semiconductors alloys, the Ge dangling bond energy is lower than that of Si dangling bonds by ∼ 0.13 eV, independent of the specific nearest neighbors to the dangling bond (3 Si-atoms, 2 Si-atoms and 1 Ge-atom, etc.), but that the spread in dangling bond energies associated bond-angle variations of the order of 6–8 degrees can be larger than this energy difference (∼0.3 eV or greater). This means that structural disorder, rather than chemical disorder causes Si and Ge-atom dangling bond states to overlap in their energy distributions.
Defect Saturation in a-SiGe:H(F) Alloys
Authors: Wang N.W. , Morin P.A. , Chu V. , Wagner S.
Published: March 2011

It is a question as yet unresolved whether the density of light-induced defects in a-Si:H reaches a saturated value that cannot fundamentally be exceeded, or whether the defect density is in all conditions a steady-state value that reflects carrier concentration and temperature. In our experiments on a-Si:H we have observed defect saturation at low temperature and high light intensity; on the other hand, data exhibiting no saturation have also been published. To learn more about this question we have carried out saturation experiments on a-SiGe:H(F) alloys. These alloys have lower defect freeze-in temperatures than a-Si:H and, presumably, lower annealing energies. Therefore, saturation should be more difficult to achieve in the alloys than in a-Si:H.

We have studied saturation for a-SiGe:H(F) samples to temperatures above the onset of thermal annealing and have observed that its behavior is similar to that seen in a-Si:H.
Optical Properties of Hydrogenated Amorphous Silicon, Silicon-Germanium and Silicon-Carbon Thin Films
Authors: Dawson R.M. , Li Y.M. , Gunes M. , Heller D. , Nag S. , Collins R.W. , Wronski C.R. , Bennett M. , Li Y.M.
Published: March 2011

The optical properties of solar cell grade hydrogenated amorphous silicon (a-Si:H), silicon germanium (a-SiGe:H) and silicon carbon (a-SiC:H) alloy thin films have been investigated over a wide photon energy range (0.8–4.8 eV) using a combination of subgap photoconductivity, reflection and transmission, and spectroscopie ellipsometry techniques in order to obtain accurate optical functions for solar cell modelling. Studies on films with thicknesses ranging from a few hundred Å to a few microns show that the optical spectra obtained by the different techniques agree closely over the energy ranges of overlap and display no thickness dependence from the Urbach tail energies and above. Thus, the results appear to be free of measurement and sample related artifacts. Three different methods provide a common value for the optical gap within ±0.02 eV and the result for a-Si:H is ∼0.1 eV below the mobility gap.
High-Bandgap a-SI:H Deposited by Concentric-Electrode rf Glow Discharge
Authors: Conde J.P. , Lau F. , Chu V. , Chan K. K. , Blum J.M. , Arienzo M.
Published: March 2011

Hydrogenated amorphous silicon (a-Si:H) with Tauc's optical bandgap (Eopt) between 1.7 and 2.1 eV was deposited at room-temperature by concentric-electrode plasma-enhanced chemical vapor deposition (CE-PECVD). The variation of Eopt was achieved by varying the flow of silane, without hydrogen dilution. The increase in Eopt results from an increase in the hydrogen concentration in the film from 7 to 15%. The photoconductivity shows a monotonie decrease with increasing Eopt and is 10'-7 Scnr-1 for the 2.1 eV sample. The Urbach energy (Eu) is observed to increase sharply with increasing Eopt, while the subgap defect (Ns) density remains approximately constant.
Comparative Study of the Optical and Vibrational Properties of a-SiNx:H Films Prepared from SiH4-N2 and SiH4-NH3 Gas Mixtures by rf Plasma
Authors: Campmany J. , Bertran E. , Andújar J.L. , Canillas A. , López-Villegas J. M. , Morante J.R.
Published: March 2011

The properties of amorphous silicon nitride films (a-SiNx:H) prepared by PECVD from SiH4-NH3 and SiH4-N2 gas mixtures have been determined by spectroscopie ellipsometry and FTIR spectroscopy as a function of the nitrogen concentration measured by XPS. The films are transparent for silane ratios [SiH4]/([SiH4] + [NH3]) < 20% and [SiH4]/([SiH4] + [N,]) < 1.5%. The refractive index shows a wide range of progressive variation from 3.2 for high silane concentrations to 1.8 for low silane concentrations. The hydrogen content of the low-absorbing films has much lower values for those obtained by SiH4 + N2 plasma than for those obtained by SiH4 + NH3 plasma. The results are discussed in terms of growth models of PECVD a-SiNx:H films from SiH4-NH3 and SiH4-N2 mixtures.
Investigation of Hydrogen and Nitrogen Thermal Stability in PECVD a-Sinx:H.
Authors: Fitzner M. , Abelson J.R. , Kanicki J.
Published: March 2011

We study the high temperature (≤1300°C) thermal evolution from hydrogenated amorphous silicon nitride (a-SiNx:H) films prepared by plasma enhanced chemical vapor deposition. Two principle peaks are found, one at 525–750°C associated with hydrogen release, and one at ≥950°C from hydrogen and nitrogen release. In nitrogen-rich films (x>4/3), the low temperature (525°C) peak intensity is smaller compared to silicon-rich films (x≤4/3), implying that hydrogen is more thermally stable in N-rich films. Helium dilution during film growth further reduces the low temperature peak intensity, producing the most thermally stable N-rich material, with the onset of hydrogen evolution occurring at ∼600°C. For a nitrogen-rich film, high temperature hydrogen evolution began at ∼900°C and was accompanied by nitrogen evolution starting at ∼950°C. UV-illumination of N-rich samples prior to thermal evolution produced no observable changes in the evolution spectra.
Percolation Behaviour in the Electrical Characteristics of Hydrogenated Amorphous Silicon Nitride Films.
Authors: López-Villegas J.M. , Garrido B. , Benrakkad M.S. , Samitier J. , Bertran E. , Canillas A. , Morante J.R.
Published: March 2011

The electro-optical properties of hydrogenated amorphous silicon nitride films (a-SiNx:H) prepared by rf glow discharge of SiH4 and N2 have been determined as a function of the silicon content in the alloy. The stoichiometry and structure of the layers have been studied by ellipsometry, infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Two different electrical behaviours have been found. The samples with x>0.8 show conductivity based on the Frenkel-Poole mechanism, while the samples with x<0.8 show quasi-ohmic conductivity. Both kinds of conduction and the transition between them are analyzed in the framework of the percolation theory. In this context, the correlation between the stoichiometry and structure of the layers with their electrical behaviour indicate that the transition from the Frenkel-Poole to the quasi-ohmic conduction is a consequence of the formation of conducting paths as the percolation threshold of Si-Si bonds is reached.
Photoluminescence in Nitrogen-Rich a-SiNx:H
Authors: Chen D. , Viner J.M. , Taylor P.C. , Kanicki J.
Published: March 2011

We report the measurement of photoluminescence (PL) from nitrogen-rich hydrogenated amorphous silicon nitride (a-SiN1.6:H) films. Excitation at 4.8 eV gives a PL spectrum of approximately Gaussian lineshape centered at 2.9 eV with a full width at half maximum of 1.4 eV. The PL intensity at 2.9 eV fatigues under continuous 4.8 eV excitation, but may be restored by thermal annealing or photobleaching with light of energy ≥ 2 eV. For emission at 2.9 eV, the PL excitation spectrum in the range 3.0 to 5.2 eV follows the a-SiN1.6:H film optical absorption edge at low energy and then decreases with a maximum at approximately 5.4 eV. The PL results are discussed within the framework of a negative effective electron-electron correlation energy (negative Ueff) for the dominant defect (silicon atom bonded to three nitrogen atoms).
Simulation of Steady State and Transient Phenomena in a-Si:H Pin Structures and Films
Authors: Brüggemann Rudi , Main Charles , Bauer Gottfried H.
Published: March 2011

In this paper we report on a range of simulation studies. The transient photocur-rent decay after switching off of steady illumination is investigated and we show that the protracted decay in undegraded pin-solar cells is a hole current. In degraded pin samples a change of sign in the photocurrent during the decay is observed by simulation (as is experimentally) which is attributed to an electron diffusion current in reverse direction. We study the distribution of defects according to the defect pool model in a pin structure. The varying dangling bond density with position of the Fermi level leads to large inhomogeneities in defect density across the i-layer in thermal equilibrium leading to increased band bending and giving increased recombination close to the interfaces under illumination. As a demonstration of another application of the simulation method we show how the quasi-steady state is established in the steady state photocarrier grating experiment in lock-in technique.