bluuebunny/crossref_metadata_2025 huggingface dataset:
Gap States in Hydrogenated Amorphous Silicon—Carbon Alloys
Authors: Fiorini P. , Evangelisti F. , Frova A.
Published: March 2011

Tail and defect states in the gap of a-SixCl-x:H alloys have been studied by measurements of spectral photoconductivity. The variation of defect—state density versus x is found to be negligible. By comparison with PDS results the ητ product has been determined and found to be almost independent of photonenergy and to strongly decrease with inclusion of carbon. This effect is attributed to changes in the transport mechanism combined with an increased recombination rate associated with the widening of the gap.
The First Tetrahedrally Bonded Diamond–Like Compounds with Metallic Conductivity
Authors: Sawan Y.
Published: March 2011

In this paper we report for the first time the discovery of metallic conductivity in some tetrahedrally bonded diamond–like ternary oompounds 1 v vl with the general formula A3B C4 · Such compounds are characterized by a Valence Electron Concentration (VEC) of four valence electrons per atom. The preparation and characterization of one of the investigated compounds Cu3AsSe4 will be presented.

The transition metal atoms in such a compound are connected by non-transition metal atoms (Se) via coordinate locallized bonds in a tetrahedral environment. Such an environment should lead to a splitting in the 3d filled orbitals of the transition metal because of lattice field. A delocalization of the 3d electrons is expected to take place via back donation, through an overlap with the empty antibonding orbitals on the non-metal atoms to minimize the negative formal charge on the transition metal atoms. If such an overlap leads to continuous electron path through the whole material, metallic conductivity could be visualized. The detailed bonding model will be discussed.
Critical Materials Parameters for the Development of Amorphous Silicon Alloys
Authors: Ovshinsky Stanford R. , Adler David
Published: March 2011

The desired properties of solar cells are discussed, and a relative figure of merit for the comparison of cells fabricated using different technologies is described. The advantages of utilizing amorphous silicon alloys as the active material in solar cells are enumerated. Selected materials properties of these alloys are described and the physics of their electronic structure is discussed in detail. The necessary steps for achieving commercially viable cells based on amorphous silicon alloys are listed, and it is demonstrated how each of them has been achieved using a technology that incorporates fluorine throughout the entire process. The chemical and physical basis for the superiority of fluorinated material is presented in detail. Continuous web large-area high-efficiency multijunction solar cells are in production. Dual band gap multijunction cells have been tested under continuous air mass 1 exposure for over 2000 hours and show essentially no degradation. Some recent results are presented.
Thin Film Silicon Power Modules: Challenges and Opportunities for Materials Science
Authors: Morel Don L.
Published: March 2011

A survey of the materials issues involved in the development of thin film Si:H power modules is presented. Though there are significant opportunities for advancement of Si:H itself, especially in terms of new alloys, the range of opportunities is extended dramatically when all aspects of the product -- from gaseous fuels through encapsulants -- are included. Examples from each of these areas are presented to Indicate the current state of development as well as the needs for the future. Viable products exist today, but the extent of greater success depends critically on materials innovation.
Stability of P-I-N Amorphous Silicon Solar Cells with Boron-Doped and Undoped I-Layers
Authors: Catalano Anthony , Arya Rajeewa R. , Kerns Ralph C.
Published: March 2011

Boron-doping the i-layer in p-i-n amorphous silicon solar cells improves the device performance when the density of impurities in the undoped i-layer material is high (< 1020 cm-3). While this technique can boost the initial device efficiencies for poor quality i-layer material, our devices degrade faster than devices made with undoped, low impurity i-layer material. We have measured the degradation of photovoltaic parameters as a function of continuous AM1 exposure time for devices with and without B-doped i-layers. For single junction p-i-n solar cells with comparable initial conversion efficiencies (< 7%, area < 1cm2) we find that our devices containing i-layers deposited from gas mixtures containing 2–3 ppm diborane degrade faster than devices containing undoped i-layers. Similar effects are observed when two-junction stacked cells with B-doped i-layers are compared to two-junction stacked cells with undoped i-layers.
Experimental and Theoretical Analysis of the above Threshold Characteristics of Amorphous Silicon Alloy Field Effect Transistors
Authors: Hack M. , Shur M. , Hyun C. , Yaniv Z. , Cannella V. , Yang M.
Published: March 2011

Experimental studies on the above threshold characteristics of amorphous silicon alloy thin films transistors show that the field-effect mobility has a weak temperature dependence and that the current-voltage characteristics are described by a power law dependence on gate voltage, with an exponent greater than two. These results are in good agreement with our theoretical model, which demonstrates that in the above threshold regime, the Fermi level at the semiconductor-insulator interface lies in the tail states which control device performance. Analysis of the above threshold characteristics shows the characteristic energy variation of the exponential tail state distribution to be ∼22meV and that the on current changes by a factor of only two over the temperature range lO−90°C.
Characteristics of the Blocking Layers in the a-Si:H Photoreceptor
Authors: Kakinuma H. , Nishikawa S. , Watanabe T. , Nihei K.
Published: March 2011

The effects of characteristics of the blocking layers in multilayered a-Si:H photoreceptors on electrophotographic properties are reported. The structure of the photoreceptor is Al/a-Si:H(p)/a-Si:H(i)/a-Si1−xCx. The thickness and boron doping ratio of the bottom p-type layer and the thickness and compositional fraction x of the surface a-Si1−xCx layer are varied. It is shown that the dark decay time is particularly sensitive to the thickness and doping ratio of the p-type layer. These dependences are discussed assuming a space charge layer in the p-type layer. It is also shown that the thickness and x of the a-Si1−xCx layer greatly influence the residual voltage and effective quantum efficiency of photocarrier generation.
High Performance Polycrystalline Silicon Thin Film Devices
Authors: Hawkins William G.
Published: March 2011

The goal of this work was to produce a fabrication process for high performance polycrystalline silicon thin film MOS devices. We have fabricated p-channel devices with mobilities of 35 cm2/V-sec and n-channel devices with mobilities of 50 cm2/V-sec by tailoring the process for depositon of the channel layer, by gate oxidation of the channel at high temperature, and by use of plasma hydrogenation. Under optimal conditions deduced from the study, device threshold voltages are close to zero. Leakage currents in the off-state are less than 0.1 pA/µm of channel length. Fabrication of the devices requires four mask levels and employs standard process steps. Therefore, polycrystalline silicon devices are attractive candidates for a variety of electronics applications, including thin film logic over large area.
Highly Conductive and Wide Band GaP Microcrystalline Silicon Films Prepared by Photochemical Vapor Deposition and Applications to Devices
Authors: Nishida S. , Tasaki H. , Konagai M. , Takahashi K.
Published: March 2011

Doped hydrogenated microcrystalline silicon (μc-Si:H) and fluorinated hydrogenated microcrystalline (μc-Si:F:H) films were prepared by the mercury photosensitized decomposition of a disilane-hydrogen or a difluorosilane-hydrogen gas mixture, respectively. The maximum dark conductivity and optical band gap of μc-Si:H films were respectively 20 S•cm−1 and ∼2.0 eV for n-type and 1 S•cm−1 and 2.3 eV for p-type. A higher dark conductivity as much as 50 S•cm−1 and a wide gap of 2.0 eV were obtained for n-type μc-Si:F:H. It is most significant that the gaseous ratio of hydrogen to disilane should be enhanced to obtain such a highly conductive and wide gap film. The crystallinity of the photo-deposited μc-Si:H films appeared to be improved in comparison with that of films by the conventional plasma glow discharge technique.
Selected Area Epitaxial Regrowth of Amorphous Si/(100) Si Structures by Laser Annealing
Authors: Christou A. , Varmazis C. , Efthimiopoulos T. , Fotakis C.
Published: March 2011

Excimer laser KrF (248 nm) annealing at 93 mj/cm2 and 175 mJ/cm2 has been found to recrystallize amorphous silicon on (100)Si. The major impurities introduced by excimer laser annealing are carbon, while surface roughness remains as a major problem. Channel mobilities measured on MOSFETs processed on epitaxially regrown silicon were 98-115 cm2/v.s. Leakage currents between recrystallized silicon regions were 1-2 uA/cm2.
Determination of Density of Localized States in Amorphous Silicon Alloys From the Low Field Conductance of Thin N-I-N Diodes
Authors: Shur Michael , Hack Michael
Published: March 2011

We describe a new technique to determine the bulk density of localized states in the energy gap of amorphous silicon alloys from the temperature dependence of the low field conductance of n-i-n diodes. This new technique allows us to determine the bulk density of states in the centre of a device, and is very straightforward, involving fewer assumptions than other established techniques. Varying the intrinsic layer thickness allows us to measure the,density of states within approximately 400 meV of midgap.

We measured the temperature dependence of the low field conductance of an amorphous silicon alloy n-i-n diode with an intrinsic layer thjckness of 0.45 microns and deduced the density of localised states to be 3xlO16cm−3 eV−1 at approximately 0.5 eV below the bottom of the conduction band. We have also considered the high bias region (the space charge limited current regime) and proposed an interpolation formula which describes the current-voltage characteristics of these structures at all biases and agrees well with our computer simulation based on the solution of the complete system of transport equations.
Light Effects on Grain Boundary Properties in Silicon
Authors: Cheng L. J. , Shyu C. M.
Published: March 2011

We have studied the photoconductivity of grain boundaries in p–type silicon. The result demonstrates the applicability of the technique for the measurement of minority carrier recombination velocity at the grain boundary. The experimental data are consistent with the thought that the recombination velocity increases with the boundary state density and light intensity.
Effects of Grain Boundaries in GaAs Solar Cells
Authors: Benner John P. , Eugene Blakeslee A.
Published: March 2011

Many problems exist in current attempts to develop polycrystalline GaAs as a basis for thin-film solar cells. Some of these problems arise from the direct interaction of carriers, both dark and photo-generated, with grain boundaries. Others are more indirect; e.g., shunting currents due to the grain boundary-enhanced diffusion of contaminating impurities. This paper describes several of these effects, including the influence of system chemistry on grain properties, the correlation of device parameters with grain size, and grain boundary passivation experiments. A review of various approaches to solving the problems confronting the field is given, and an attempt is made to interpret reported observations in terms of existing theoretical models.
High-Spatial-Resolution Diffusion Length Measurements in Polycrystalline GaAs
Authors: Fletcher Robert M. , Ken Wagner D. , Wicks G. W. , Ballantyne J. M.
Published: March 2011

A technique for making rapidly-scanned, high-spatial resolution measurements of minority-carrier diffusion length is applied to the characterization of polycrystalline GaAs. Measurements on a large-grained n-type epitaxial layer show gradual variations of hole diffusion length from 0.1µm to l.lµm across the wafer as well as occasional small step changes at grain boundaries. By trading resolution for speed, the technique would be well suited to the nondestructive evaluation of large-area epitaxial layers.
Advantages of MIS Processing on Passivated Polycrystalline Silicon
Authors: Rajeswaran G. , Thayer M. , Rao V. J. , Anderson W. A.
Published: March 2011

Wacker polycrystalline silicon shows enhanced grain boundary activity after a high temperature (950° C) anneal. It is possible to passivate this effect in a hydrogen plasma. The low temperature (600° C) processing of MIS technology does not activate grain boundaries or deteriorate a passivated specimen. Activated grain boundaries with MIS structures can be used to assess the character of recombination currents. It is concluded that MIS processing is advantageous for passivated polycrystalline silicon.
Potential Applications of Poly-Silicon as an Electronic-Device Material
Authors: Bartelink Dirk J.
Published: March 2011

Poly-crystalline silicon can be regarded as a true electronic material if good p-n junctions can be made in it or if its state of depletion can be altered by reasonable gate voltages. The degree of perfection with which it must exhibit these electronic-material properties depends on whether the application is as a technology in VLSI (or other bulk-Si substrate use) where devices with bulk-crystalline properties are available or as the principal active material in Large Area Integration (LAI), such as flat-panel display addressing circuits, where the competition is much less demanding. The three main detrimental effects of grain boundaries on electronic-device function are the action of grain boundary traps as (1) extra charge centers, (2) lifetime killers, and (3) mobility-reducing scattering centers. The current trend in reducing or almost eliminating grain boundaries by laser recrystallization or lateral epitaxy has the effect of reducing the average number of traps. In terms of potential applications of the material, the improvement derived from these procedures must be balanced against other contraints placed on the overall structure. For example, the thickness and quality of the lower oxide (and interface) in these processes will determine whether an electronically active lower gate function is practical. Some currently envisioned applications include load devices in inverters either as resistors or as depletion transistors, common-gate staked CMOS structures, dual-channel MOSFET's, and other “vertical VLSI” applications. The systems-level topological advantages of stacked structures and the circuit-performance improvement provided by the ground plane in dielectrically isolated devices must also be balanced against the extra cost and yield loss of additional masking level complexity and design complication.
Theory Of ZnO Varistors
Authors: Mahan G.D.
Published: March 2011

The theory of ZnO varistors has evolved along with the increasingly detailed experimental description [1-2]. The extreme nonlinearity in the currentvoltage curves naturally led the early investigators to suggest the well-known nonlinear mechanisms, such as space-charge limited currents or electron tunneling. Later experiments made these first proposals untenable [3-9]. Recently a second generation of theories has been proposed, with many different mechanisms suggested for the extreme nonlinearity [10-15]. Here we wish to review these different theories, and also the experiments which bear upon their validity.
Electrical Properties of ZnO-Bi2O3 Metal Oxide Heterojunction — A Clue of a Role of Intergranular Layers in ZnO Varistors
Authors: Eda Kazuo
Published: March 2011

Zinc Oxide (ZnO) Ceramics-Bismuth Oxide (Bi2O3) Metal Oxide thin film heterojunction made by sputtering technique showed a highly non-Ohmic property. The voltage-current characteristics and the dielectric properties showed dependence on Bi2O3 metal oxide thin film thickness.

In this paper after reviewing and discussing the electrical properties of ZnO varistors, the role of intergranular layers in the ZnO varistor is discussed based on experimental results with the heterojunction.
Thermal Runaway in Metal Oxide Varistors
Authors: Seitz Martin A. , Hirthe Richard W. , Potter Mike E.
Published: March 2011

The conditions leading to thermal runaway in metal oxide varistors are discussed in terms of its power dissipation. An equivalent circuit is used to rationalize the similarities and differences between the DC and 60 Hz ac behavior.
Preparation of Oriented GaAs Bicrystal Layers by Vapor-Phase Epitaxy Using Lateral Overgrowth
Authors: Salerno Jack P. , Mcclelland R. W. , Vohl P. , Fan John C. C. , Macropoulos W. , Bozler C. O. , Witt A. F.
Published: March 2011

This paper describes a novel technique that utilizes vapor-phase epitaxy to grow bicrystal semiconductor layers with predetermined rotation axis, misorientation angle, and grain boundary plane. The geometrical structure of the grain boundary in each layer is therefore completely specified. The technique has been demonstrated by using the AsCl3-GaAs-H2 method to grow a series of GaAs bicrystals, each containing a [110] tilt boundary formed by a grain with a (111)B boundary plane and a grain rotated from (111)B by a selected misorientation angle. The results of initial electrical measurements indicate that the height of the potential barrier associated with each grain boundary varies with the misorientation angle.
The Electronic Properties Of Semiconductor Grain Boundaries
Authors: Seager C. H.
Published: March 2011

Grain boundaries play an important role in determining the operating characteristics of devices such as varistors and thin film solar cells and transistors. In the last several years detailed one-electron expressions have been developed for the transport coefficients of majority and minority carriers at grain boundaries. These theories have been successful in predicting the measured electrical properties of bicrystals in lightly doped silicon. The status of our understanding in this area will be reviewed in some detail. Recent extensions of these calculations which have been necessary to explain measurements on grain boundaries in degenerately doped GaAs will also be reviewed.
Investigations of Fracture Zones in Crystalline Rock by Borehole Radar
Authors: Olsson Olle , Falk Lars , Forslund Olof , Lundmark Lars , Sandberg Eric
Published: March 2011

A borehole pulse radar system has been developed as part of the International Stripa Project with the objective to identify and characterize fracture zones at a considerable distance from boreholes. The radar uses very short pulses, which are transmitted and received by dipole antennas inserted into the boreholes. The pulses are extremely broadband with center frequencies of 25–60 MHz corresponding to wavelengths of a few meters in the rock. At 25 MHz the attenuation in the Stripa granite is 28 dB/100 m and the pulse velocity is approximately 128 000 km/s. Reflection measurements have been used to identify fracture zones and determine their position and orientation. The zones often cause strong and well-defined reflections. Improvements in the pulse form and numerical filtering of the data have consequently made the radar a very efficient instrument for locating fracture zones. During measurements in Stripa reflections from fracture zones have been observed more than 100 m from the borehole.
Glass/Water Reaction with and without Bentonite Present - Experiment and Model
Authors: Grambow B. , Hermansson H. P. , Björner I. K. , Werme L.
Published: March 2011

In nuclear waste repository design bentonite has been included as part of the backfill for its sorbtive capacity and low water permeability. Nevertheless, it cannot keep the waste form dry once intrusion of groundwater has occured [1]. Leach experiments [2], [3] with the radioactive nuclear waste form borosilicate glass JSS-A have been performed with and without bentonite present to provide a database which allows the long term stability of the glass in aqueous environment to be forecasted and the chemical effects of bentonite to be studied.
The Role of Groundwater Oxidation Potential and Radiolysis on Waste Glass Performance in Crystalline Repository Environments
Authors: Jantzen Carol M. , Bibler Ned E.
Published: March 2011

Laboratory experiments have shown that groundwater conditions in a granite repository will be as reducing as those in a basalt repository. Chemical analysis of the reduced groundwaters confirmed that the Fe2+/Fe3+ couple controls the oxidation potential (Eh). The reducing groundwater conditions were found to decrease the time-dependent release of soluble elements (Li and B) from the waste glass. However, due to the lower solubility of multivalent elements released from the glass when the groundwaters are reducing, these elements have significantly lower concentrations in the leachates.

Gamma radiolysis reduced the oxidation potential of both granitic and basaltic groundwater in the absence of both waste glass and oxygen. This occurred in tests at atmospheric pressure where H2 could have escaped from the solution. The mechanism for this decrease in Eh is under investigation but appears related to the reactive amorphous precipitate in both groundwaters. The results of these tests suggest that radiolysis may not cause the groundwaters to become oxidizing in a crystalline repository when abundant Fe2+ species are present.
Characterization of Lead-Iron Phosphate Nuclear Waste Glasses
Authors: Schiewer Edwin , Lutze Werner , Boatner Lynn A. , Sales Brian C.
Published: March 2011

Lead-iron phosphate glasses have recently been proposed as a new primary disposal medium for both commercial nuclear reactor wastes and some types of high-level radioactive U.S. defense wastes [1–41. The initial work on the characterization of lead-iron phosphate nuclear waste glasses, hereafter named phosphate glasses, concentrated on the preparation, thermal properties, and devitrification behavior (both during and after preparation) of these glasses. Aqueous corrosion rates were measured at 90°C in distilled water, acidic and basic solutions, low eH distilled water, and in the “reference’ natural ground water appropriate to disposal in a tuffaceous formation (i.e., J-13 well water [5]). The results of these initial investigations were extremely encouraging and indicated that homogeneous, highly leach resistant (at 90° C) phosphate glasses loaded with either commercial or high-level U.S. defense wastes could be prepared at relatively low temperatures (process temp.:≤ 1050°C). The chemical stability of the phosphate glasses was found to be primarily due to the stabilizing effect of iron on the structure of lead phosphate glass. This effect is illustrated in Fig. 1 where the leachate conductivity at 90°C is plotted versus time for phosphate glasses with varying iron concentrations. The addition of 9 wt.% iron oxide to lead metaphosphate glass [Pb(PO3)2] increases the chemical durability of the glass in water by a factor of about 104. The tendency of pure lead phosphate glasses to crystallize is also greatly suppressed by the addition of iron oxide [3,4]. Lead metaphosphate glass can be completely crystallized by heating the glass in air at 30O°C for several hours. Lead-iron phosphate glasses, however, can be heated in air for several hundred hours at a temperature as high as 500°C without exhibiting any evidence of crystallization.
Intergranular Films and Pore Surfaces in Synroc C
Authors: Myhra S. , Segall R. L. , Smart R. ST. C. , Turner P. S , White T. J.
Published: March 2011

High resolution electron microscopy and scanning electron microscopy were used to determine the distribution of intergranular films and microvoids in Synroc C. Diffraction contrast derived from these films, which were 1–3 nm thick, showed then to be ill-defined crystallographically, and they may be described as glassy. Pores were usually several micrometers in extent and occurred principally in rutile-rich areas. The chemical composition of these structural features was obtained using analytical transmission electron microscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and scanning Auger microscopy. Within intergranular films, elemental enhancement of cesium, sodium, potassium and aluminium, and possibly silicon and molybdenum was observed. Enhancement of cesium, sodium, phosphorous, aluminium and silicon was found at triple points. Fracture faces preferentially expose grain boundaries, and dissolution of these surfaces proceeds rapidly at ambient temperatures. During the fabrication of Synroc C, microvoids trap cesium vapour, and after cooling this condenses onto pore surfaces. Recognition of the (simulated) waste species which segregate at grain boundaries and pores, permitted the reinterpretation of published leach data for monolithic and crushed Synroc C.
Nuclear Energy and Nuclear Waste Management
Authors: Forsberg Professor Hans G.
Published: March 2011

I can honestly say that I am very proud to get this opportunity to open this symposium and provide a few opening remarks on nuclear energy and nuclear waste management. I was once a nuclear chemist myself. Moreover, the Royal Swedish Academy of Engineering Sciences, IVA, has since her start in 1919 been concerned with all aspects of energy questions and energy politics but particularly in recent decades with the nuclear energy issues.
The Corrosion of Spent UO2 Fuel in Synthetic Groundwater
Authors: Forsyth R. S. , Werme L.O.
Published: March 2011

Leaching of high burnup BWR fuel for up to 3 years showed that both U and Pu attain saturation rapidly at pH 8.1, giving values of 1–2 mg/l and 1 μg/l respectively. The leaching rate for Sr-90 decreased from about 10−5/d to 10−7/d but was always higher than the rates for U, Pu, Cm, Ce, Eu and Ru. Congruent dissolution was only attained at pH values of about 4.

The hypothesis that alpha radiolytic decomposition of water is a driving force for UO2 corrosion even under reducing conditions has been examined in leaching tests on low burnup (low alpha dose-rate) fuel. No significant effect of alpha radiolysis under the experimental conditions was detected.
Derivation of a Waste Package Source Term for NNWSI from the Results of Laboratory Experiments
Authors: Oversby Virginia M. , Wilson Charles N.
Published: March 2011

Results are presented for the dissolution of Turkey Point pressurized water reactor (PWR) spent fuel in J-13 well water at ambient hot cell temperatures. These results are compared with those previously obtained on Turkey Point fuel in deionized water, on H. B. Robinson PWR fuel in J-13 water, and by other workers using various fuels in dilute bicarbonate groundwaters. A model is presented that represents the conditions under which maximum dissolution of spent fuel could occur in a repository sited at Yucca Mountain, Nevada. Using an experimentally determined upper limit of 5 mg/l for uranium solubility in J-13 water, a fractional release rate of 6.4 × 10−8 per year is obtained by assuming that all water entering the repository carries away the maximum amount of uranium.
Radiation Damage in Natural Titanite by Crystal Structure Analysis
Authors: Fleet M. E. , Henderson G. S.
Published: March 2011

Study of radiation damage in natural titanite (CaTiSiO5) permits estimation of the long-term performance of titanite-bearing glass ceramics in the immobilization of radioactive waste. Single-crystal X-ray structure analyses have been made on two geologically old, U, Th-bearing titanites (sphenes), on an undamaged gem-quality titanite and on their annealed equivalents. The alpha-damaged titanite crystal structure is characterized by a marked increase in magnitude of the apparent thermal parameters of all structural positions, reflecting a general increase in positional disorder. The ratio of observed structure factors (F0-damaged/F0-annealed) may be used to estimate the overall structural damage.
Alpha-Radiolysis of Aqueous Solutions
Authors: Christensen Hilbert , Bjergbakke Erling
Published: March 2011

The effects of the radiolysis of water exposed to mixed alpha- and beta-radiation originating from spent fuel have been calculated. The water is assumed to have penetrated the copper canister and fuel cladding, and then to exist as a 30 μm thin surface film on the fuel pellets.

The combined effects of alpha and beta-radiation, and the presence of iron in the water have been found to be important parameters. The beta-radiation will lower the yield of hydrogen, as will the presence of low concentrations of iron ions. The most likely conditions for water exposed fuel give rise to a total production of 1 mol H2per m2fuel surface after 1 million years. A stoichiometric amount of oxygen is also formed.

Calculations have also been carried out in connection with experiments on alpha-radiolysis from Am-241 or from spent fuel pellets. In the case of alpha-radiolysis from Am-241 it was calculated that a 20 cm2thin layer of 4 mCi Am-241 produced 1.4 × 10−3cm3hydrogen per day.

Calculations simulating the conditions of experiments on the leaching of spent fuel have shown that both alpha- and gamma-radiation accelerate the dissolution. The rate is strongly dependent on the dose rate and on the area of the water film covering the fuel surface and fuel fissures.
Corrosion Rate of Unalloyed Steels and Cast Irons in Reducing Granitic Groundwaters and Chloride Solutions
Authors: Simpson J. P. , Schenk R. , Knecht B.
Published: March 2011

Measurements of the corrosion rate of unalloyed steel have been made under conditions representative for repositories in the granitic bedrock of Northern Switzerland using two independent methods: (1) Immersion tests in two representative groundwaters, with and without bentonite; (2) Hydrogen evolution measurements in these groundwaters. The immersion tests were carried out at 80°C and 140°C. In both cases the corrosion rate was higher than 50 μm/a as determined at the end of the first observation period of about 500 h. Corrosion rates of under 10 μm/a were estimated after the first 500 h. The corrosion rates were similar in both groundwaters with mineralisations of approx. 3 g/11 and 14 g/11 respectively, and were generally higher at 80°C than at 140°C. The hydrogen evolution measurements allow an hourly determination of the corrosion rate, with a sensitivity expressed as a corrosion rate of better than 0.1 μm/a. The results generally confirm the observation made in the immersion testing; high corrosion rates were observed over the first few days but then decreased to values well below 10 μm/a. The steady state corrosion rates measured were 1.1 μm/a, 6.5 μm/a, and 2.5 μm/a at 25°C, 50°C, and 80°C respectively in the water with the higher mineralisation. The inverse temperature effect on corrosion rate above 50°C is attributed to a change in the nature of the passive film at higher temperatures.
Geochemical Modelling of the Alteration of a Granitic Rock Around a Repository for Spent Nuclear Fuel
Authors: Fritz Bertrand , Kam Marie , Tardy Yves
Published: March 2011

The alteration of a granitic rock around a repository for spent nuclear fuel has been simulated considering the geothermal effect through an increase of temperature. The results of the simulation were considered in terms of mass transfers and their volumic consequences. The dissolution of primary minerals by alteration tends to increase the porous volume due to fissures and cracks, while the precipitation of secondary minerals and particularly here calcite and clay minerals will tend to decrease the porosity by sealing the cracks. The results of the simulations show that an increase of temperature from 10 to 100°C will favour the alteration of the granitic rock by the solutions circulating today in the rock. This rock is characterized by a very low fissure porosity and the water velocity is consequently particularly low. These conditions favour intense water-rock interactions and the water-rock system will probably reach the stability field of secondary clay minerals predicted by the calculations: the porosity will decrease. A combination of these thermodynamic mass balance calculations and hydrodynamic considerations, with a kinetic approach of mineral dissolutions gives a first attempt to calibrate the modelling in the time scale.
Diffusion Mechanisms of Strontium, Cesium and Cobalt in Compacted Sodium Bentonite
Authors: Muurinen Arto , Rantanen Juha , Penttilä-Hiltunen Pirkko
Published: March 2011

For a porous water-saturated material where diffusion in the pore water, sorption on the solid material and diffusion of the sorbed ions (surface diffusion) occur, a diffusion equation can be derived where the apparent diffusivity includes two terms. One represents diffusion in the pore-water, the other surface diffusion.

In this research diffusion mechanisms were studied. The apparent diffusivities of strontium, cesium and cobalt in compacted sodium bentonite were measured by a non-steady state method. The sorption factors were adjusted using different sodium chloride solutions, groundwater and addition of EDTA for saturation of the bentonite samples. The corresponding sorption factors were measured by a batch method.

The results suggest that cations diffuse also while being sorbed. A combined pore diffusion-surface diffusion model has been used to explain the transport and the corresponding diffusivities have been evaluated. The surface diffusivities (Ds) of Sr and Cs were 8-9·10-12 m2/s and 4-7·10-13 m2 /s respectively. The pore diffusivity eD of Cs was 3.5.10-11 m2 /s which has been used also for Sr.

The sorption mechanism of Co seems to be different from that of Sr or Cs and the results allow no specific conclusions of the diffusion mechanism of Co. The apparent diffusivity of Co ranged from 2·10-14 to 7·10-14 m2/s.

The anionic Co-EDTA seems to follow some other diffusion mechanism than the cations.
The Solubility of Actinides in the Near-Field
Authors: Ewart F. T. , Howse R. M. , Thomason H. P. , Williams S. J. , Cross J. E.
Published: March 2011

The solubility of some of the radiologically important actinide elements have been determined in a water whose chemistry was representative of that believed to exist in potential repositories in the U.K. The solubilities of the actinides have been determined as a function of the pH of the water. The results of these experiments have been compared with the results of predictions made using the PHREEQE geochemical modelling code in order to test and validate the data base used in the model. In the light of these comparisons, suggestions are made for alternative values for the thermodynamic data and for further studies.
Mechanical Behaviour of Host Rock Close to H.L.W. Disposal Cavities in a Deep Granitic Formation
Authors: Hoorelbeke J. M. , Dourthe M.
Published: March 2011

The construction of a H.L.W. repository in a deep granitic formation creates mechanical disturbances in the rock on the scale of the massif and in the nearfield. Amongst all the disturbances noted in the nearfield, this study is concerned with examining the evolution of stresses linked with the excavation of the rock and the rise in temperature in the proximity of the waste packages. Several linear elasticity calculations were made using on the one hand finite element models and on the other simple analytical models. These calculations concern two different storage concepts -“in room” concept and “in floor” concept- whose differences in mechanical behaviour are analysed. A study of sensitivity with regard to the characteristics of the rock and to the initial geostatic stresses is presented. The comparison of the calculated stresses with three-dimensional failure criteria gives a clear indication of the satisfactory behaviour of granite for final storage. However, the need for experimental study and complementary calculation must be emphasised.
Numerical Calculation of the Time and Space Dependent Temperature Distribution in a HLW Repository of Finite Extension and Temporal Step-by-Step Disposal Procedure
Authors: Hahne K. , Schlich M. , Korthaus E.
Published: March 2011

With the program system FAST-STEP the temporal step-by-step disposal procedure of a HLW repository and its finite geometry can be taken into consideration in the calculation of the time and space dependent temperature distribution. This will be obtained through the application of a suitable coupling of calculation models treating both far field and near field phenomena.

After the description of the single steps of the program system the influence of both aspects on the temperature distribution will be demonstrated with the aid of a test calculation. The possible fields of application of FAST-STFP will be pointed out at the end.
Swedish Nuclear High-Level Waste Program—Key Research Areas
Authors: Bjurström Sten , Papp Tönis
Published: March 2011

Nuclear power plays an important role in the Swedish power production. From this year on 45–50% of the Swedish electricity will be produced by nuclear power plants, the rest being hydro power.
Overheating In Silicon During Pulsed-Laser Irradiation?
Authors: Larson B. C. , Tischler J. Z. , Mills D. M.
Published: March 2011

Nanosecond resolution time-resolved x-ray diffraction measurements of thermal strain have been used to measure the interface temperatures in silicon during pulsed-laser irradiation. The pulsed-time-structure of the Cornell High Energy Synchrotron Source (CHESS) was used to measure the temperature of the liquid-solid interface of <111> silicon during melting with an interface velocity of 11 m/s, at a time of near zero velocity, and at a regrowth velocity of 6 m/s. The results of these measurements indicate 110 K difference between the temperature of the interface during melting and regrowth, and the measurement at zero velocity shows that most of the difference is associated with undercooling during the regrowth phase.
Time-Resolved Reflectivity Measurements of Silicon And Germanium Using A Pulsed Excimer Laser
Authors: Jellison G. E. , Lowndes D. H. , Mashburn D. N. , Wood R. F.
Published: March 2011

Time-resolved reflectivity measurements of silicon and germanium have been made during pulsed KrF excimer laser irradiation. The reflectivity was measured simultaneously at both 1152 and 632.8 nm wavelengths, and the energy density of each laser pulse was monitored. The melt duration and the time of the onset of melting were measured and compared with the results of melting model calculations. For energy densities just above the melting threshold, it was found that the melt duration was never less than 20 ns for Si and 25 ns for Ge, while the maximum reflectivity increased from the value of the hot solid to that of the liquid over a finite energy range. These results, along with a reinterpretation of earlier time-resolved ellipsometry measurements, indicate that, during the melt-in process, the near-surface region does not melt homogeneously, but rather consists of a mixture of solid and liquid phases. The reflectivity at the onset of melting and in the liquid phase have been measured at both 632.8 and 1152 nm, and are compared with the results found in the literature.
Raman Microprobe Analysis of Laser-Induced Microstructures
Authors: Fauchet P. M.
Published: March 2011

We study the composition, stress and structure variations across periodic surface undulations produced by pulsed laser illumination of semiconductors, by explosive crystallization of amorphous films, and by laser-assisted CVD. These variations are mapped out with a one micron spatial resolution using a Raman microprobe. Similarities and differences between the three cases are pointed out. These results are also compared to those obtained by deliberately exposing the sample to interfering beams.
Pulsed Co2 Laser Induced Melting and Nonlinear Optical Studies of GaAs
Authors: James R. B. , Christie W. H. , Mills B. E. , Burcham H. L.
Published: March 2011

We report new optical and structural properties of p-type GaAs that result from the absorption of high-intensity 10.6 μm radiation. Prior to the onset of surface melting, we find that the absorption coefficient decreases with increasing intensity in a manner predicted by an inhomogeneously broadened two-level model. As the energy density of the CO2 laser radiation is increased further, the surface topography shows signs of melting, formation of ripple patterns, and vaporization. Auger spectroscopy and electron-induced x-ray emission show that there is loss of As, compared to Ga, caused by the melting of the surface. Using plain-view TEM we find that Ga-rich islands are formed near the surface during the rapid solidification of the molten layer. Auger and SIMS measurements are used to study the incorporation of oxygen in the near-surface region, and the results show that oxygen incorporation can occur for GaAs samples that have been irradiated in air.
In Situ Time-Resolved Reflectivity Measurements of Growth Kinetics During Solid Phase Epitaxy: A Tool To Estimate Interface Non Planarity During Growth
Authors: Licoppe C. , Nissim Y.I.
Published: March 2011

The time resolved reflectivity technique is shown to give informations on the amorphous-crystalline interface evolution during solid phase epitaxial (SPE) regrowth in semiconductors. Two specific cases have been treated here. The first case is encountered in laser annealing when the growth front exhibits a curvature due to the combination of an inhomogeneous temperature distribution and a steep dependence of SPE growth rates with temperature. A computer simulation is carried out from an analytical determination of the laser induced temperature profiles to shape up the resultinq reflectivity signal. The second case is obtained when there is an evolution of interface roughness during regrowth. In order to simulate this effect a simple model is developed to treat the influence of diffusion of the reflected light at the interface, on the reflectivity modulation during SPE regrowth.
Formation of AlxGa1-xAs Alloy on the Semi-Insulating GaAs Substrate by Laser Bean Interaction
Authors: Joshi N. V. , Lehman J.
Published: March 2011

AlxGal-xAs alloy was obtained on semi-insulating GaAs by laser beam interaction. For this purpose, thin layers (~150 A) of AlAs and GaAs were deposited by MICVD on the semi-insulating undoped GaAs substrate and irradiated with a high power laser beam. For a certain critical value of incident power (7.1 watts/mm2), a layer of AlxGal-xAs alloy was formed. The nature of the alloy was examined by Auger and Far infrared reflectance spectroscopy. The later technique reveals the characteristic mode of AlxGal-xAs confirming its formation. Disorder induced LA mode was also observed.
Time-Resolved Spectroscopy of Plasma Resonances In Highly Excited Silicon And Germanium
Authors: Malvezzi A. M. , Huang C. Y. , Kurz H. , Bloembergen N.
Published: March 2011

The dynamics of the electron-hole plasma in silicon and germanium samples irradiated by 20 ps, 532 nm laser pulses has been investigated in the near infrared by time-resolved picosecond optical spectroscopy. The experimental reflectivities and transmissions are compared with the redictions of the thermal model for degenerate carrier distributions through the Drue iformalism. Above a certain fluence, a significant deviation between measured and calculated values indicates a strong increase of the recombination rate as soon as the plasma resonances become comparable with the band gaps. These new plasmon-aided recombination channels are particularly pronounced in germanium.
Dynamics of Elementary Processes At Surfaces: Nitric Oxide Scattered From A Graphite Surface
Authors: Vach H. , Häger J. , Simon B. , Flytzanis C. , Walther H.
Published: March 2011

Molecular beam scattering from solid surfaces has long been recognized as a powerful means for investigation of gas-surface reaction dynamics. With the help of the recently developed laser-induced fluorescence and ionization techniques for state-selective detection, one can now measure the angular and velocity distributions of the scattered molecules together with their internal energy distributions. Such measurements fully describe the average energy and momentum exchanges between molecules and surfaces and give thus full information on the dynamics of the interaction. Recently, also the scattering of vibrationally excited NO molecules was investigated. The paper gives a review of new experiments with emphasis on the investigation of the scattering of NO molecules from a pyrographite surface. A simple model using transport properties of the solid is presented which accounts surprisingly well for the observed features.
Microstructural Studies of Pulsed-Laser Irradiated Graphite Surfaces
Authors: Speck J. S. , Steinbeck J. , Braunstein G. , Dresselhaus M. S. , Venkatesan T.
Published: March 2011

The surface structures of laser-irradiated samples of highly oriented pyrolytic graphite (HOPG) have been investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The samples were irradiated with a 30 ns ruby laser (λ = 6943 !A) pulse with energy fluences ranging from 0.1 to 3.0 J cm−2. Optically, the specimens show a damage region approximately 5 mm in diameter. The surface structure displays three characteristic regions: an outer boundary characterized by submicron carbon spheroids resting on the surface; an inner boundary characterized by both submicron spheroids and 1 to 5 [m ‘torn’ carbon layers which appear to have broken away from the graphite surface after irradiation; a central region characterized by a uniform density of spheroids and a pattern of surface upheavals which trace out a grain pattern similar to that of the pristine substrate. Electron diffraction patterns taken on the irradiated region indicate an ultra-fine grain 2-dimensionally ordered carbon. Qualitative trends in the areal density of different microstructural features are presented. In addition, a simple model explaining the observed features is given. All observations are consistent with the rapid solidification of liquid carbon.
Kinetics, Microstructure And Mechanisms of Ion Beam Induced Epitaxial Crystallization of Semiconductors.
Authors: Elliman R.G. , Williams J.S. , Maher D.M. , Brown W.L.
Published: March 2011

Ion-beam induced epitaxy is shown to be essentially athermal over the temperature range 200-400°C, and to exhibit no dependence on substrate orientation and little dependence on doping in this regime. On the other hand, the formation and propagation of defects during growth and the interaction of the advancing crystal-amorphous interface with implanted impurities is essentially identical for both thermally induced and ion-beam induced epitaxy. These observations lead to a simple model for ion-beam induced epitaxial crystallization in which epitaxial growth is nucleated by defects generated at, or near, the crystal-amorphous interface by the ion beam. Comparisons of ion-beam induced epitaxy and thermally induced epitaxy suggest that the 2.7 eV activation energy associated with the latter process is dominated by a 2.0 eV nucleation step.
Microstructure of Niobium Films Oriented By Non-Normal Incidence Ion Bombardment During Growth
Authors: Harper J.M.E. , Smith D.A. , Yu L.S. , Cuomo J.J.
Published: March 2011

We demonstrate that non-normal incidence ion bombardment applied during thin film growth has a pronounced alignment effect on crystallographic orientation. Restricted fiber texture is achieved in Nb films deposited at room temperature onto amorphous silica substrates with simultaneous 200 eV Ar+ ion bombardment at 20 degrees from glancing angle. Xray pole figure measurements and transmission electron diffraction show that the alignment direction is a channeling direction for the incident ions between (110) planes. The degree of alignment increases linearly with the fraction resputtered by the ion beam. Recommendations are given for optimizing this ion beam orientation effect.