bluuebunny/crossref_metadata_2025 huggingface dataset:
Photomking Determination of Mobility and Lifetime in Intrinsic a-Si:H
Authors: Tang Yi , Braunstein R. , Roedern Bolko Von
Published: March 2011

A photomixing technique was employed to determine the lifetime and the mobility in intrinsic a-Si:H, by measuring the power of the beat frequency in conjunction with the dc. photocurrent. Both the mobility and lifetime were found to decrease continuously following the light-soaking process, which indicates the existence of long-range potential fluctuations and a change in the recombination processes due to light-soaking. For the first time, evidence is provided that degradation is accompanied by loss of mobility, a fact that is neglected in all prevailing models for the Staebler-Wronski effect. Intrinsic a-Si:H samples produced by glow-discharge deposition and the hot wire technique were employed in this work.
Experimental Determination of the Dark Fermi Level in Hydrogenated Amorphous Silicon.
Authors: Dawson R.M. , Nag S. , Gunes M. , Wronski C.R. , Bennett M. , Li Y.M.
Published: March 2011

The position of the dark fermi level in hydrogenated amorphous silicon (a-Si:H) is important in determining its electrical properties and is a key parameter in the detailed modelling of materials and devices. The activation energies of conductivities have been investigated on intrinsic a-Si:H films from various laboratories where the slopes of the Arrhenius plots have ranged from 0.27 eV to 0.87 eV. In many cases, marked differences are found between the results obtained from two and four probe measurements, highlighting the importance of the four probe configuration. Results are presented which help to explain the scatter in the measured activation energies between intrinsic films of a-Si:H. Differences in the activation energies are discussed in terms of current limiting processes which act in series with the material bulk resistance. It will be shown that the conductivity of the film and the contact are key factors in assessing whether the position of the fermi level can be accurately determined from a two contact measurement.
Transient Versus Steady State Measurements on the Transport Properties of a-SI:H Films
Authors: Vieira M. , Martins R. , Fortunato E. , Soares F. , Guimarães L.
Published: March 2011

Transient and steady-state measurements of the ambipolar diffusion length (L*) in undoped a-Si:H films have been carried out through Flying Spot Technique (FST) and Spectral Photovoltage Technique (SPT) using either Schottky or pin structures. The FST is based on the photovoltage response of a monochromatic light that moves with a constant velocity in the interface direction, while in SPT the optical excitation is achieved by changing the absorption coefficient of the incident light and so, the light depth penetration. In FST the additional photo-effect due to the light spot movement allows to infer separately L* and the effective lifetime, (τ*) while in SPT the curve shape of the spectral response allows to estimate the interface behavior and L*. The obtained results reveal that the transient measurement is useful for determining the transport properties of the bulk while the steady technique seems to be more useful to infer the role of the interface on device performances. Limitations of the above techniques will also be reported.
Subband Recombination in a-Si:H
Authors: Gu S.Q. , Taylor P.C.
Published: March 2011

Photoluminescence (PL) in a-Si:H has been studied as a function of excitation energy from above the gap down to well below the gap. Although the width of the PL peak shows no strong dependence on the excitation energy at low temperatures, the low energy tail of bandtail PL exhibits an approximately exponential drop with decreasing energy, I ≈ exp(hν/δE), where δE increases with decreasing excitation energy for excitation energy below about 1.65 eV. This behavior is attributed to the slow downward hopping of holes in the valence band tail that are generated below a thermalization threshold at low temperatures.
Free Carrier Lifetime in a-Si,Ge:H Alloys
Authors: Young D.A. , Fauchek P.M. , Liu Y.M. , Nighan W.L. , Fortmann C.M.
Published: March 2011

The lifetime of carriers injected optically in the extended states of amorphous silicon-germanium alloys has been measured by time-resolved pump and probe optical techniques using either a femtosecond dye laser or a picosecond free electron laser. When Ninj > 1018 cm-3, the lifetime of the carriers is in the picosecond time domain. Our results are comparable to what we have observed previously in a-Si:H and very recently in a-Si,C:H. There are two lifetime regimes: at high densities, the recombination is bimolecular and nonradiative, whereas at lower densities, the recombination tends to be monomolecular but still nonradiative. The origin of these lifetimes is discussed.
Characterization of Arsenic-Implanted Amorphous Silicon
Authors: Knoch Lynnita , Theodore N. David , Tam Gordon , Pennell Ron
Published: March 2011

In this study, amorphous silicon and polycrystalline silicon films were implanted with arsenic and subjected to varied low temperature (<900°C) anneal conditions and characterized using TEM. The microstructure is of interest for later correlation with electrical measurements. The amorphous deposition produces larger, more irregular grains with more strain than does the polysilicon deposition for a single-step rapid thermal anneal (RTA) cycle. This can be explained by the number of critical nucleii and the rate of grain growth. The sheet resistivity, as measured by four-point probe, correlates to the deposition conditions. A two-stage anneal makes the grains less irregular by reducing the roughness of the grains and decreasing the strain in the grains. For a given deposition condition, the final microstructure is most strongly influenced by the first anneal. The second anneal produces no clear change in grain size. 800°C anneals result in larger grains than 900°C anneals. This is explained by the presence of less critical nucleii for 800°C anneals. In comparing short and long durations of RTA, the short duration produced slightly larger grains than the long duration RTA due to greater nucleation in the longer RTA wafers. In the case of RTA versus furnace anneals, RTA produces larger, more irregular grains, with more strain in the grains. A model in terms of the size of critical nucleii is used to explain the difference.
Tunneling Spectroscopy of Amorphous Semiconductors
Authors: Wen J. , Kakalios J.
Published: March 2011

The tunneling transmission coefficient as calculated by the WKB and the multi-potential approximation (MPA) is compared for tunneling through an a-Si:H/a-SiNx:H double barrier structures. The MPA method leads to shifts in the location of the quantum well states, which complicates the interpretation of quantum size effects in amorphous silicon multilayers. Experimental studies of tunneling currents through a-Si:H/a-SiNx:H single barrier structures are presented. At low temperatures the conductance data resembles tunneling data for metal/insulator/metal structures with an asymmetric barrier, while at room temperature a pronounced peak is observed in the conductance-volatge characteristics.
Improvement in Wide-Gap A-SI:H For High-Efficiency Solar Cells
Authors: Tsuge Sadaji , Hishikawa Yoshihiro , Okamoto Shingo , Sasaki Manabu , Tsuda Shinya , Nakano Shoichi , Kuwano Yukinori
Published: March 2011

A hydrogen-plasma treatment has been used for the first time to fabricate wide-gap, high-quality a-Si:H films. The hydrogen content (CH) of a-Si:H films substantially increases by the hydrogen-plasma treatment after deposition, without deteriorating the opto-electric properties of the films. The photoconductivity (σph) of ≥ 10-5 ο-1 cm-1, photosensitivity ( σ ph/σ d) of > 106 and SiH2/SiH of <0.2 are achieved for a film with CH of ∼25 atomic >%. The optical gap of the film is > 1.70 eV by the (α h ν )1/3 plot, and is >2 eV by the Tauc's plot. The open circuit voltage of a-Si solar cells exceeds 1 V conserving the fill factor of > 0.7 when the wide-gap a∼Si:H films are used as the i-layer, which proves the wide band gap and low defect density.
Electron Transport in a-Si1-xGex:H Solar Cells
Authors: Wang Qi , Antoniadis Homer , Schiff E. A. , Guha S.
Published: March 2011

We report a study of the correlation between solar cell parameters and electron transport parameters in a series of a-Si1-xGex:H alloys with bandgaps spanning the range 1.42 – 1.72 eV. Fill factors and open circuit voltages were measured in p-i-n solar cells with 0.32 μm thick i-layers. The electron drift mobility and deep-trapping mobility-lifetime product were measured using transient photocurrent techniques on p-i-n cells with thicker i-layers. The open circuit voltage tracked the bandgap accurately. Both the electron drift mobility and the deep-trapping mobility lifetime product correlated reasonably well with the fill factor measured with illumination absorbed near the p-i interface. We present a discussion of the relationship of the fill factor to these transport parameters.
Kinetics of Light-Induced Degradation of a-Si:H Solar Cells Compared to I-Layer Films
Authors: Li X.R. , Wagner S. , Bennett M. , Fonash S.
Published: March 2011

We report the CPM defect density of a-Si:H material and the performance characteristics of pin solar cells during high-intensity light-soaking. In one group of experiments we compared the effects of soaking with monochromatic light from a Kr+ laser to white light from a Xe arc lamp. The effects are identical for the same electron-hole pair generation rate. In a second group of experiments we light-soaked at the two different temperatures of 50°C and 90°C. At 90°C the defect density saturates at a lower value than at 50°C, and correspondingly the cell performance parameters saturate at higher values.
Effect of Interface and Surface on the Performance of a-Si:H TFTs
Authors: Jang Jin , Jung Moon Youn , Yoo Sun Sung , Song Hyon Kyun , Jun Jung Mok
Published: March 2011

We have studied the effects of interface and surface on the performance of hydrogenated amorphous silicon(a-Si:H) thin film transistors. The effects of rf power, the buffer layer between the gate insulator and a-Si:H, and the surface oxidation on the performance on the a-Si:H TFTs have been investigated. By introducing suitable buffer layer, we can increase the mobility up to 2.1 cm2/Vs. The surface oxidation gives rise to the electron accumulation near the surface.
Work Function Difference Between N-Type μc-Si Gate Electrodes Deposited by Remote Pecvd and P-Type c-Si Substrates in Mos Capacitors
Authors: Lee D.R. , Lucovsky G.
Published: March 2011

We have used remote PECVD to deposit highly-doped μc-Si films for use as gate electrodes in MOS capacitors. The flatband voltages of capacitors with different oxide thicknesses have been measured by high-frequency capacitance-voltage and have been used to determine the work function difference between an n-type μc-Si electrode and a p-type c-Si substrate. This value for ϕμs is discussed in terms of a band diagram for the stacked-gate MOS device, and is compared to the expected value.
An Investigation of a-Si:H Thin Film Transistors by A Comparison of Transient Measurements and Numerical Simulations.
Authors: Willums M.F. , Hack M. , Lecomber P.G. , Shaw J.
Published: March 2011

Transient measurements of the source-drain current ISD of amorphous silicon (a-Si:H) thin film transistors are compared with the results of two dimensional simulations. In particular, we have investigated the effect of different amorphous silicon layer thicknesses on the transient response. It is found that the dynamic response of a transistor with 0.4 μm a-Si:H is significantly slower than that of a device with only 0.06 μm of a-Si:H.
Cleaning Procedures for UHV Cluster-tool MOS Fabrication
Authors: Sofield C J , Murrell M P , Sugden S , Heyns M , Verhaverbeke S , Welland M E , Golan B , Barnes J
Published: March 2011

We have investigated the effect on a silicon surface of both wet chemical and cluster-tool UV/ozone cleaning, prior to UHV processing to fabricate MOS test structures. The physical and chemical condition of the Si surface has been examined by Scanning Tunnelling and Atomic Force Microscopy (STM, AFM) and Medium Energy Ion Scattering (MEIS). After MOS fabrication some of the structures were examined by Cross-sectional Transmission Electron Microscopy (TEM). The electrical performance of the MOS test sets were characterized by breakdown voltage measurements.

We have found correlations between the electrical performance of the MOS devices, the structure of the Si surface prior to oxidation, and the details of the UHV fabrication technique. In particular any MOS device fabricated on a Si surface thermally cleaned in UHV prior to oxidation has a poor breakdown strength. We have found that this is the result of the formation of silicon carbide on the Si surface at high temperature and the subsequent local disruption of the oxidation step of MOS fabrication by the SiC. A UHV cleaning procedure has been developed to avoid this C contamination problem.
Chemical Structure of Native Oxide Grown on Hydrogenterminated Silicon Surfaces
Authors: Takakura M. , Yasaka T. , Miyazaki S. , Hirose M.
Published: March 2011

Chemical bonding features and suboxide compositions in native oxide grown on chemically-cleaned hydrogen-terminated Si(100) surfaces stored in pure water have been studied by using surface sensitive infrared spectroscopy and x-ray photoelectron spectroscopy. The LO phonon peak for the native oxide is located at 1210cm−1, which is shifted to a significantly lower wavenumber side than the ultrathin thermal oxide peak at 1250cm−1. This is because an appreciable amount of SiHx bonds are incorporated in the native oxide/Si interface and such hydrogen termination in the network dramatically reduces strained bonds in the interface. Very weak Si2+ suboxide signal from the oxide grown in pure water is also explained by the incorporated SiHx bonds which interrupt the Si2+ suboxide formation in the interface.
Pre-oxidation Silicon Cleaning and its Relation to MOS Reliability and Process-Induced Damage
Authors: Kasi S. R. , Rubloff G. W. , Cohen S. A. , Hsia L. C.
Published: March 2011

Material and device reliability issues play a critical role in the choice of materials and processing options for ULSI technology and manufacturing. Recently, chemical surface treatments (e.g., fluorine incorporation, nitridation) have been shown to benefit MOSFET gate dielectric reliability. This paper describes the impact of different pre-oxidation chemical surface treatments on the reliability characteristics of MOS structures by using a combination of x-ray irradiation and electrical stress-induced charge injection techniques. X-ray irradiation at 1-2 keV increases oxide-trapped charge and the density of interface states substantially (80-800X), but these changes depend significantly on the chemistry of pre-oxidation cleaning. Furthermore, the specific surface treatment strongly affects subsequent device reliability; MOS degradation is up to 5X lower when fluorine-based surface pre-cleaning is used.
Spectroscopic Ellipsometric Analysis of Surface and Subsurface Damage in Chemical-Mechanical Polished Semiconductors
Authors: Xiong Yi-Ming , Snyder Paul G.
Published: March 2011

Spectroscopic ellipsometry (SE), with the added capability of variable angle of incidence, is a sensitive technique for nondestructive surface and interface analysis. In this work, we demonstrate the application of this technique to the quantitative characterization of surface and subsurface damage in Si and GaAs, induced by conventional chemical-mechanical polishing. We show that the optimum selection of angle of incidence and spectral range is of crucial importance for detecting and evaluating surface and subsurface quality. To adequately describe the measured SE data, a two-layer model (i.e. oxide overlayer/subsurface damage layer/substrate) was found to be necessary. In this model, the oxide overlayer and subsurface damage layer were each modeled as a simple physical mixture of the chosen constituents, using the Bruggeman effective medium approximation. SE analysis clearly indicated the presence of surface microscopic roughness and subsurface damage for all the samples investigated. Typical results for thickness were 2 to 3 nm of surface overlayer, and 15 to 40 nm of damage beneath it, containing about 1 ∼ 2% volume fraction of either oxide or voids. As verification, atomic force microscopy (for surface characterization) and wet chemical oxide stripping (for subsurface analysis) were used to substantiate the SE results.
In Situ Control of Native Oxide Growth for Semiconductor Processes
Authors: Morita Mizuho , Ohmi Tadahiro
Published: March 2011

In situ control methods of native oxide growth on Si surfaces at room temperature and during the temperature ramp-up are proposed for metal/Si contact formation, lowtemperature Si epitaxy, and very-thin thermal oxide film formation, based on analyses of factors dominating the oxide growth. Low-resistance W/Si contacts are formed by N2 gas sealed processing of HF cleaning right before W chemical vapor deposition(CVD). Highquality epitaxial Si films are grown at a low temperature of 550°C using Si2H6 molecular flow pre-showering to suppress the oxide growth caused by water or oxygen in a CVD reactor. Very thin gate oxide films with high insulating performance are realized by the preoxidation step at 300°C to form one-molecular-layer oxide for passivation and by the subsequent temperature ramp-up step in ultraclean Ar gas to prevent oxide growth and an increase of surface microroughness before the thermal oxidation step.
Characteristics and Recovery of SI Surfaces Plasma Etching in CHF3 / C2F6
Authors: Park H.-H. , Kwon K.-H. , Koak B.-H. , Lee S.-M. , Kwon O.-J. , Kim B.-W. , Lee J.-W. , Yoo J.-B. , Sung Y.-K.
Published: March 2011

The effects of SiO2 reactive ion etching (RIE) in CHF3 / C2F6 on the surface properties of the underlying Si substrate have been studied by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) techniques. The observed two major modifications are (i) a ∼50nm thick silicon layer which contains carbon and fluorine and (ii) 2∼3nm thick residue layer composed entirely of carbon, fluorine, oxygen and hydrogen on the silicon surface. The thermal behaviors of attributed peaks for C 1s, Si 2p, O 1s and F 1s of residue film have been analyzed after in-situ resistive anneal under ultra high vacuum (UHV) condition. C-F1, C-F2 and C-F3 bonds decompose and form C-CFx (x≤3) bonds above 200°C. Above 400°C, C-CFx bonds also decompose to C-C/H bonds. For recovery of the modified silicon surface, reactive ion etched specimens have been exposed to an oxygen plasma. By XPS analysis, the effect of an O2 plasma treatment has been revealed to be completed within 20min. With an O2 plasma pre-treated, a rapid thermal anneal (RTA) treatment as low as 500°2 is found to be effective for removal of impurities in the silicon.
Single Step Low Temperature In-Situ Substrate Cleaning for Silicon Processing
Authors: Ramm Juergen , Beck Eugen , Steiner Franz-Peter , Pixley Ralph E. , Eisele Ignaz
Published: March 2011

Silicon wafers as obtained from the manufacturer are immersed in a large area argon/hydrogen plasma for surface cleaning. The plasma discharge is maintained between a heated cathode and the grounded process chamber at discharge voltages of about 30 V for which discharge currents up to 100 A can be chosen. In this regime, neither the chamber walls nor the substrates are sputtered. Chemical reactions at the wafer surface are assumed to be mainly stimulated by low energy electron bombardment. The etch rates for diamond-like carbon (DLC) on silicon wafers were determined for selected discharge parameters and compared with the previously obtained results for SiO2. It was found that 5 minutes in-situ cleaning prepares the silicon wafers for homoepitaxy at 500 °C and higher substrate temperatures, whereas a short anneal at 500 °C was necessary to obtain low temperature homoepitaxy at 300 °C on (100) and 400 °C on (111) silicon. This seems to be due to hydrogen passivation of silicon during the cleaning procedure.
Comparison between Air and UV/Ozone Surfaces Passivation Methods of GaAs (100) Substrates
Authors: Ingo G.M. , Padeletti G. , Mattogno G. , Scandurra A.
Published: March 2011

Air and UV/ozone exposure have been used for growing sacrificial thin oxide layers on HCl etched GaAs (100) surfaces. Passive films have been then desorbed by vacuum thermal cleaning in order to prepare suitable surfaces for molecular beam epitaxy (MBE) growth. Structure, thickness and chemical composition of passive films and desorbed surfaces have been studied by ellipsometry and angle dependent X-ray photoelectron spectroscopy (ADXPS). The results have indicated an increased chemical reactivity of both arsenic and gallium during UV/ozone exposure, compared to that of air exposure, that produces Ga (III) oxide enriched films. Furthermore, XPS results have also shown that the thermal desorption behavior are different. In particular, GaAs (100) after short term UV/ozone exposure and oxide film desorption, has an As/Ga surface atomic ratio close to unity and a level of carbon contamination below the XPS detectability. On the contrary, air exposed surfaces never have a stoichiometric composition and carbon is not completely removed by a vacuum heating.
Structure of Sulphur-Passivated InP(100)-(1×1) Surface
Authors: Lu Z.H. , Tao Y. , Yang B.X. , Feng X.H. , Mitchell D.F. , Yelon A. , Graham M.J. , Sacher E.
Published: March 2011

X-ray photoelectron spectroscopy, X-ray photoelectron diffraction, and S K edge X-ray absorption near-edge structure were used to characterize the air-stable S-passivated InP(100)-(lxl) surface prepared in (NH4)2S solution. The results show that one monolayer of sulphur which is bonded only to In is adsorbed on the surface. The S is found to occupy bridge-bonded sites, and the orientation of the In-S bond is determined to be in the [011] azimuth, with an In-S-In bridge-bond angle of about 100°.
tudy of the Surface Cleaning Effects to the InGaAs Quantum Well Wires by Atomic Force Microscopy and Photoluminescence Spectroscopy
Authors: Tan I-Hsing , Stone Song Shi , Prater Craig , Mirin Richard , Hu Evelyn , Bowers John , Merz James , Hansma Paul
Published: March 2011

The efficacy of residual photoresist removal on the top surface of the InGaAs QWW grating and the effects of surface oxides on the optical property of quantum well wires (QWWs) were examined through atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. Different resist removal treatments, including acetone, ozone and diluted HC1 were evaluated. Both AFM and PL measurements reveal that with the surface cleaning processing we have developed, high luminescence efficiency from the QWWs is conserved after removal of the residual photoresist.
Cleaning and Oxidation of Heavily Doped Si Surfaces
Authors: Yasaka T. , Uenaga S. , Yasutake H. , Takakura M. , Miyazaki S. , Hirose M.
Published: March 2011

HF-treated Si surfaces and the oxidation kinetics in pure water or in clean room air have systematically been studied by x-ray photoelectron spectroscopy (XPS). The oxidation of heavily-doped n-type Si appears to proceed parallel to the surface, resulting in the layer-by-layer oxidation. The oxide growth rate in pure water for heavily-doped n-type Si is significantly higher than that of heavily-doped ptype Si. This is explained by the electron tunneling from the Si conduction band to adsorbed O2 molecules to form the O2 state. O2 ions easily decompose and induce a surface electric field, enhancing the oxidation rate. The growth rate of native oxide on heavily-doped n-type Si is less sensitive to the crystallographic orientations than the case of lightly doped Si where the steric hindrance against oxygen molecules significantly lowers the oxidation rate of the (110) and (111) surfaces. We suggest that the decomposed oxygen can penetrate into Si without steric hindrance. It is also found that the oxidation of heavily-doped n-type Si in pure water is effectively suppressed by adding a small amount (10 ∼ 3600 ppm) of HCI.
Low Temperature Epitaxy on H-Passivated Si(100) by Sputter Deposition
Authors: Chen Chau-Chen , Smith Donald L. , Anderson Greg , Hagstrom Stig B.
Published: March 2011

Conventional preparation of Si for epitaxy involves high-temperature heating to remove surface oxide or to remove H passivation left by HF cleaning. Attempted MBE on H-passivated Si(100) has resulted in amorphous films below 640 K. We show that the use of ion beam sputtered Si allows the growth of thick (300 nm) epitaxial layers on H-passivated Si(100) at high rate (0.65 nm/s) and at 483 K, well below the H desorption temperature. This is because the sputtered Si has translational kinetic energy of tens of eV, well above the Si-H bond strength of a few eV, so that the passivating H does not block the epitaxial bonding sites as it does in MBE. The ability to grow epitaxially on H-passivated Si not only reduces the maximum Si processing temperature required, but also keeps the substrate passivated against contamination until the onset of deposition.

During deposition, the LEED pattern changes gradually from the 1 × 1 of the H-passivated Si to the 2 × 1 reconstruction of bare or partially hydrogenated Si(1 00), yet SIMS shows that little of the disappearing H accumulates at the substrate interface or in the bulk of the film. Thermal desorption spectra show that its bonding remains similar to that on the substrate surface. Thus, it appears to float to the surface of the depositing film and become gradually knocked off of the surface over the course of a few hundred nm of deposition. Cross-sectional TEM lattice imaging shows near-perfect ordering of the epilayer and a barely distinguishable substrate interface.
In Situ Analysis of Surface Contaminant Desorption during Low-Temperature Silicon Substrate Cleaning using Reflection Electron Energy Loss Spectrometry
Authors: Wong Selmer S. , Nikzad Shouleh , Ahn Channing C. , Smith Aimee L. , Atwater Harry A.
Published: March 2011

We have employed reflection electron energy loss spectrometry (REELS), a surface chemical analysis technique, in order to analyze contaminant coverages at the submonolayer level during low-temperature in situ cleaning of hydrogen-terminated Si(100). The chemical composition of the surface was analyzed by measurements of the C K, O K and Si L2,3 core loss intensities at various stages of the cleaning. These results were quantified using SiC(100) and SiO2 as reference standards for C and O coverage. Room temperature REELS core loss intensity analysis after sample insertion reveals carbon at fractional monolayer coverage. We have established the REELS detection limit for carbon coverage to be 5±2% of a monolayer. A study of temperature-dependent hydrocarbon desorption from hydrogen-terminated Si(100) reveals the absence of carbon on the surface at temperatures greater than 200°C. This indicates the feasibility of epitaxial growth following an in situ low-temperature cleaning and also indicates the power of REELS as an in situ technique for assessment of surface cleanliness.
NH4F Pre-Cleaning of Silicon (100) for UHV-CVD Epitaxy
Authors: Cohen Susan L. , Blum Joseph , D'Emic Christopher , Gilbert Monica , Cardone Frank , Stanis Carol , Liehr Michael
Published: March 2011

NH4F etching of Si(100) surfaces has been evaluated as a pre-clean for UHV-CVD low temperature epitaxy of silicon. A combination of surface science and epitaxial growth experiments have found that while the NH4F treatment provides surfaces which have less carbon, oxygen and fluorine impurities than a standard dilute HF pre-clean, there is a significant density of crystallographic defects in the epitaxial films which limits the usefulness of this approach.
The Impact of Alpha-Radiolysis on the Release of Radionuclides from Spent Fuel in a Geologic Repository
Authors: Neretnieks Ivars
Published: March 2011

Spent nuclear fuel buried in deep geologic repositories may eventually be wetted by water. The alfa-radiation will radiolyse the water and produce hydrogen and oxidizing agents, mainly hydrogen peroxide and oxygen. The hydrogen will escape by diffusion and the oxidizing agents may attack the canister materials, oxidize the uranium oxide matrix or diffuse out and oxidize reducing agents in the surrounding rock.

The rate of radiolysis has been computed recently within the Swedish nuclear fuel safety projects KBS. It is strongly influenced by the amount of available water and by the presence of dissolved iron. The movement of the oxidizing agents out from the canister and their reaction with the reducing agents (mainly ferrous iron) in the Swedish crystalline rock has been modelled as well as the movement of the radionuclides within and past the redox front. Some substances such as uranium, neptunium and technetium will precipitate at the redox front and will be withdrawn from the water to a considerable extent.
Thermodynamic Coupling of Heat and Matter Flows in Near-Field Regions of Nuclear Waste Repositories
Authors: Carnahan C. L.
Published: March 2011

In near-field regions of nuclear waste repositories, thermodynamically coupled flows of heat and matter can occur in addition to the independent flows in the presence of gradients of temperature, hydraulic potential, and composition. The following coupled effects can occur: thermal osmosis, thermal diffusion, chemical osmosis, thermal filtration, diffusion thermal effect, ultrafiltration, and coupled diffusion. Flows of heat and matter associated with these effects can modify the flows predictable from the direct effects, which are expressed by Fourier's law, Darcy's law, and Fick's law. The coupled effects can be treated quantitatively together with the direct effects by the methods of the thermodynamics of irreversible processes. The extent of departure of fully coupled flows from predictions based only on consideration of direct effects depends on the strengths of the gradients driving flows, and may be significant at early times in backfills and in near-field geologic environments of repositories. Approximate calculations using data from the literature and reasonable assumptions of repository conditions indicate that thermal-osmotic and chemical-osmotic flows of water in semipermeable backfills may exceed Darcian flows by two to three orders of magnitude, while flows of solutes may be reduced greatly by ultrafiltration and chemical osmosis, relative to the flows predicted by advection and diffusion alone. In permeable materials, thermal diffusion may contribute to solute flows to a smaller, but still significant, extent.
Experimental Studies of Packing Material Stability
Authors: Allen C.C. , Lane D.L. , Palmer R.A. , Johnston R.G.
Published: March 2011

The Basalt Waste Isolation Project is conducting experiments to assess the stability of bentonite (sodium montmorillonite)and crushed basalt as waste package packing materials in a nuclear waste repository in basalt. The experiments are designed to identify changes in physical, chemical, and mineralogical properties that these materials could undergo in the repository environment. A series of bentonite dehy-dration experiments showed that after 1 year at 370°C the clay's structural and swelling properties were preserved and only reversible dehydration occurred. At 440°C, however, irreversible dehydration, collapse of the clay structure, and loss of swelling ability took place. Hydrothermal tests using bentonite, or an equal mixture of bentonite and basalt, along with synthetic groundwater, were also conducted. Abentonite + water experiment showed negligible structural alteration at 200°C, although some iron enrichment of the clay occurred. At 300°C, partial conversion of the mont-morillonite to an iron- and potassium-rich smectite occurred, along with the formation of secondary quartz and albite. An experiment in the basalt + bentonite + water system at 150°C resulted in partial etching of the basalt grains, but no detectable change in rock or clay mineralogy. At 300°C, the basalt was strongly etched. Furthermore, iron- and potassium-rich smectite apparently replaced montmorillonite as the dominant clay, and secondary silica, zeolites, and minor feldspar were formed. These mineral assemblages are thought to be metastable at 300°C, based on natural analogs. The kinetics and reaction paths of further transformations of packing materials are important areas for continued study.
Irradiation-Corrosion Evaluation of Metals For Nuclear Waste Package Applications in Grande Ronde Basalt Groundwater
Authors: Nelson J.L. , Westerman R.E. , Gerber F.S.
Published: March 2011

The corrosion behavior of several iron-base and titanium-base alloys was studied in synthetic Grande Ronde Basalt groundwater at temperatures of 150°C to 2500°C and under irradiation dose rates to 2 × 106 rad/hr. The objective of these ongoing studies is to help select one or more materials for waste-package canisters that will maintain their integrity for time periods up to 1,000 yr in a nuclear waste repository constructed in basalt. The corrosion rates of iron-base alloys under irradiated conditions were generally 2 to 3 times as high as those obtained on similar materials under nonirradiated conditions. The titanium alloys exhibited low corrosion rates but absorbed significant amounts of hydrogen under irradiated conditions.
Stochastic Analysis of Groundwater Traveltime for Long-Term Repository Performance Assessment
Authors: Clifton P.M. , Baca R.G. , Arnett R.C.
Published: March 2011

This paper describes a method of stochastically analyzing groundwater traveltime. The method uses a Monte Carlo technique to generate a suite of random spatial fields that are subsequently input to the groundwater flow and groundwater traveltime equations. Stochastic inputs to these equations can be (1) transmissivity (or hydraulic conductivity), (2) effective thickness (or effective porosity), or (3) boundary conditions. In a transient problem, storage coefficient (or specific storage) could also be stochastically treated. Spatial correlation in the random input fields is accounted for by means of a multivariate random-number generator, which requires the first two statistical moments of these fields to be specified. The output from the Monte Carlo analysis is a suite of groundwater traveltime realizations that can be used to derive exceedance probabilities. These probabilities provide a measure of the degree of confidence in meeting set criteria.

A preliminary application of this method using data from the deep basalts beneath the Hanford Site is also presented. The results illustrate how this method can be used to evaluate compliance with a technical criterion relating to groundwater traveltime.
The Corrosion of Spent Uo2-Fuel in Synthetic Groundwater
Authors: Forsyth P S , Svanberg K , Werme L
Published: March 2011

Segments of fuel and clad have been leached in deionized water and in groundwater. The leachants were centrifuged through membrane filters. Both centrifugate and the filters were analysed for U, Sr-90, α- and γ-emitters. The results are discussed in terms of preferential leaching, solubility limitations and adsorption effects. For U an apparent saturation at about 800 ppb was observed. Pu also appeared to attain saturation at a few ppb. For Sr the leach rate was 3·10-7/d after ca 400 days. Attempts to impose reducing conditions showed decreased leach rates.
Multicomponent Leach Tests in Standard Canadian Shield Saline Solution on Glasses Containing Simulated Nuclear Waste
Authors: Heimann Robert B. , Wood Donald D. , Hamon Richard F.
Published: March 2011

Leaching experiments on borosilicate glass frit and simulated nuclear waste glasses were performed as a preliminary to leaching experiments on glasses incorporating radioactive waste. The experimental design included (i) simulated waste glass, (ii) ASTM Grade-2 titanium container material, (iii) clay buffer material, (iv) Standard Canadian Shield Saline Solution, and (v) granitic rock.

Cumulative fractions of release for boron were determined, as well as the solution concentrations of silicon, iron, strontium and cesium. The leach rates for boron after 28 d were approximately 5x10-6 kg.m-2 .8-1 for experiments in stainless steel vessels, and 2xl0-7 kg.m-2 .s-1 in Hastelloy* vessels. There is an apparently strong relationship between the clay/groundwater ratio, the concentration of iron in the solution, and the concentrations of silicon, strontium, and cesium.
An Integrated Approach to the Description of Radionuclide Release and Transport in the Geosphere
Authors: Neretnieks Ivars , Rasmuson Anders
Published: March 2011

The basic models for predicting the transport of radionuclides in the near field and in the geosphere (far field), used in the Swedish KBS-3 work, are presented. The transport of corrosive substances to the canister is also accounted for.
Site Description and Selection Process
Authors: Vieth D. L.
Published: March 2011

Good afternoon Ladies and Gentlemen. In leading off this afternoon's session, I would like to establish a framework against which the papers that follow can be referenced. My discussion of the proposed tuff site at Yucca Mountain shall focus on:
Retardation of Radionuclides by Rock Units Along the Path to the Accessible Environment
Authors: Ogard A. E. , Wolfsberg K. , Daniels W. R. , Kerrisk J. , Rundberg R. S. , Thomas K. W.
Published: March 2011

The most likely mechanism for release of radionuclides from a repository is transport in water from the repository to the accessible environment. The processes that can retard the movement of radionuclides in water are sorption, precipitation, and diffusion. These processes can occur anywhere along flow paths from the repository to the accessible environment.
Geohydrology Surrounding a Potential High-Level Waste Repository in Richton Salt Dome, Mississippi
Authors: Teubner Michael D. , Murphy Peter J. , Swanson Owen E.
Published: March 2011

A site investigation study is currently being performed in the vicinity of Richton Salt Dome, Mississippi. Description of the geohydrology setting of the area is integral to assessment of the site's ability to isolate nuclear waste, and to the design and construction of repository shafts and seals. Two ground-water models are being developed to assist the evaluations and design efforts: a regional model to better define the regional groundwater flow system; and a near-dome model to refine the flow system around the dome. Initial results suggest that present conditions of ground-water flow away from the dome will meet the hydrologic requirements for a suitable repository site.
Far Field Flow Uncertainty Analysis for the Palo Duro Basin
Authors: Devary Joseph L. , Battelle Battelle , HARPER William V. , ONWI Onwi , SYKES Jon F. , WILSON John L.
Published: March 2011

In order to license an underground nuclear waste repository DOE must demonstrate its safety. Sensitivity and uncertainty analyses play a critical role in both performance assessment and licensing. This paper focuses on the application of a variety of techniques to arrive at a comprehensive sensitivity and uncertainty analysis for performance measures relating to ground-water travel in the Wolfcamp aquifer of the Palo Duro Basin (see Figure 1). Scenario analysis is not examined; rather, analysis is based on well understood models for flow in porous media. Both Monte Carlo and first order second moment techniques are utilized to obtain output uncertainties. The approach results in a logical, comprehensive analysis that blends both deterministic and statistical methods. It provides for an extensive screening of the parameters, accounts for parameter and spatial correlations, and quantifies uncertainties for the major performance measures in ground-water travel. The results are useful for both licensing and the identification of additional data needs.
The Composition and Stratigraphic Distribution of Materials in the Lower San Andres Salt Unit 4
Authors: Hubbard N. , Livingston D. , Fukui L.
Published: March 2011

The salt bed in depositional cycle 4 of the Permian Lower San Andres Formation, Palo Duro Basin, Deaf Smith County, Texas consists of massive salt interlayered with discrete bands and beds of anhydrite and claystone. The massive salt consists of about 90% halite, with 7% anhydrite and 3% clays disseminated in and among the halite crystals.

The halite in this salt bed contains fluid inclusions filled with a (Na, K, Mg)Cl brine, with an average Mg concentration of about 50,000 mg/liter. The anhydrite in the salt will saturate the brines in CaSO4, which in turn may coat the waste package with anhydrite because of the retrograde solubility of CaSO4. This may increase waste package lifetime to failure by corrosion.
The Geomicrobiology of Nuclear Waste Disposal
Authors: West Julia M. , McKinley Ian G.
Published: March 2011

The geomicrobiology of high-level nuclear waste disposal is a new field in the area of nuclear waste management. Until recently the likelihood of microbial contamination in a deep repository had not been considered, but possible biogeochemical effects of repository materials, radionuclide transport and groundwater chemistry must now be realistically assessed.

This work was funded by the UK Department of the Environment and the Commission of the European Communities.
The Role of Colloids in Nuclear Waste Disposal
Authors: Avogadro A. , Marsily G.De
Published: March 2011

Aspects of formation and characterization of a radioactive colloidal fraction released by the waste form or produced by association with microcolloids naturally existing in ground water or produced either by corrosion of container material or by degradation of backfill material are discussed. A filtration model has been developed in order to describe colloidal transport under field conditions. Comparison between data obtained with laboratory column experiments and theoretical evaluations is presented.
Devitrification Behavior of SRL Defense Waste Glass
Authors: Bickford Dennis F. , Jantzen Carol M.
Published: March 2011

Simulated SRL waste was prepared with compositions varying in iron and aluminum content. Two batches with similar composition were produced with different amounts of reducing agent added. Samples were isothermally heat treated and used to derive time-temperature-transformation diagrams. Supplementary samples were cooled in a manner programmed to simulate the cooling curves of production canisters. Less than 10% total devitrification occurs during normal processing. However, when waste glass 165 was purposely devitrified, un to 30 volume percent total spinel and acmite formed. Formation of these species had minor effect on leachability in MCC-1 and accelerated leach tests.
Diffusion of Colloids and Other Waste Species in Brine–Saturated Backfill Materials
Authors: Nowak E. J.
Published: March 2011

The diffusion of plutonium (IV) and pertechnetate and the migration of colloidal gold in brine-saturated bentonite was measured. High ionic strength brine characterizes potentially intruding groundwater for radioactive waste repositories in salt. Plastic diffusion cells containing cylindrical diffusion columns were used for low density bentonite. Metal diffusion cells constructed entirely from Hastelloy C-276 were used for the diffusion of pertechnetate in highly compacted bentonite. Apparent distribution coefficients calculated from plutonium diffusion in two columns of low density bentonite are 2 m3/kg and 3 m3/kg. These values are in good agreement with the value of 3m3/kg that was calculated from previous batch sorption data. Pertechnetate anions migrated out of a brine-saturated high density bentonite diffusion column at rates that are too large to be rationalized with a simple diffusion theory. Additional measurements that take into account possible channeling of pertechnetate are required. Colloidal gold was excluded from low density brine-saturated bentonite, but colloidal gold may have channeled between the bentonite and the wall of the diffusion cell. These results support the effectiveness of brine-saturated bentonite as an engineered barrier to plutonium. The results also highlight the need for additional measurements of pertechnetate and gold transport in bentonite. Needs for transport measurements that take into account site-specific materials interactions are also described.
Leach Testing of Waste Glasses Under Near-Saturation Conditions
Authors: Grambow B. , Strachan D. M.
Published: March 2011

Two waste glasses, MCC 76-68 and C31-3, were leached in deionized water and 0.001 M MgCl2 for periods up to 158 days. At 57 days the gel layer was removed from some of the specimens and leaching continued for up to 100 days. Results from leaching in deionized water showed that the gel layer was not protective. Results from leaching in 0.001 M MgCl2 are in good agreement with the predicted results obtained from the use of the PHREEQE geochemical code and with sepiolite [Mg2Si306(OH)4] as the Mg-bearing precipitate. Both B and Si were predicted and observed to increase with increasing glass dissolution while maintaining sepiolite solubility. Both MCC 76-68 and C31-3 glasses showed increased leaching in 0.001 M MgCl2 upon removal of the layer. This suggests that the leaching mechanism is a function of the difference in the silica chemical potential between the glass and the gel layer.
Leach Behavior of SRL TDS-131 Defense Waste Glass in Water at High&Low Flow Rates
Authors: Barkatt Aaron , Sousanpour William , Barkatt Alisa , Boroomand Morad A. , Macedo Pedro B.
Published: March 2011

Leach tests carried out on SRL TDS-131 Defense Waste Class indicate that at high flow rates the controlling mechanism is simple corrosion. The matrix elements (Si, Al) are leached out at rates similar to those of the leaching of the alkalis and of boron, and the leaching process is nearly linear with time. At slow flow rates (below 1 m/yr) leaching becomes controlled by the build-up of a protective layer. Al and most of the Si remain in the leached surface layer. The leach rates decrease in the course of the test before leveling off at constant values which are almost inversely proportional to the contact time, indicating that leachate concentrations have become solubility-limited. The low concentrations observed at this stage indicate the formation of alteration products.
Effects of Metals and Metal Oxides on the Leaching of Nuclear Waste Glasses
Authors: Barkatt Aaron , Sousanpour William , Barkatt Alisa , Boroomand Morad A.
Published: March 2011

The effects of the presence of various metals and metal oxides on the leaching of nuclear waste glasses in water can be considerable even under rapid flow conditions. The systems studied here include SRL TDS-131 glass in the presence of lead and stainless steel, and PNL 76-68 glass in the presence of aluminum, alumina and their mixtures. Lead specifically suppresses dissolved silica, but does not inhibit dissolution of defense waste glass as a whole. Stainless steel specifically reduces the levels of dissolved Ca and Sr. Al and Al2 O3 exercise a general reducing effect on the rates of dissolution of commercial waste glass and the presence of alumina may have a beneficial role in enhancing waste package durability.