bluuebunny/crossref_metadata_2025 huggingface dataset:
The Adsorption of Gallium on the Cleaved Surface of InP,InAs and InSb
Authors: Rodrigues W.N. , Mönch W.
Published: March 2011

Studies of chemical trends may provide additional insights in complicated reactions between adsorbates and substrates. Here, the interaction of gallium with InP, InAs and InSb cleaved surfaces was investigated by using photoemission spectroscopy (UPS).

Ga was evaporated from a Knudsen-type cell which was placed at a distance of approximately 15 cm in front of the surfaces cleaved in situ. After the evaporation rate, which was monitored by using a quartz oscillator, had stabilized the amount of Ga deposited was determined from the exposure time. The film thicknesses given in A are nominal values since Ga was found to exhibit island growth. The energy distribution curves (EDC-s) of the photoemitted electrons, which were excited by HeI and HeII radiation provided by a home-built discharge lamp, were recorded by using a single or a double-pass cylindrical mirror analyzer.
Correlations Between Electrical Properties and Solid-State Reactions in Co/N-GaAs Contacts: A Bulk and Thin-Film Study
Authors: Shiau F.-Y. , Chang Y. A.
Published: March 2011

A fundamental and comprehensive approach has been taken to study Co//GaAs interfacial reactions, using phase diagram determination, bulk and thin-film diffusion couple studies, and electrical characterization. Phase formation sequences and interfacial morphologies are found to be similar in bulk and thin-film couples. Thermodynamic and kinetic analyses are used to rationalize the contact formations. The electrical properties of the contacts are correlated to the phase formation sequences and phase diagram information.
Experimental and Theoretical Analysis of Strain Relaxation in GexSi1-x/Si(100) Heteroepitaxy
Authors: Hull R. , Bean J.C.
Published: March 2011

By analyzing in-situ strain relaxation measurements of GexSi1-x/Si(100) epitaxy in a Transmission Electron Microscope, we are able to quantify the fundamental parameters which describe strain energy relaxation via misfit dislocation introduction. Quantitative descriptions of misfit dislocation nucleation, propagation and interaction processes are derived. The numerical parameters obtained from these experiments are then incorporated into a predictive theoretical model of strain relaxation whichrelies only upon experimentally measured quantities. Good agreement between experiment and theory is obtained over a wide range of data.
Electron-Beam-Induced-Current (EBIC) Imaging of Defects in Si1−xGeX Multilayer Structures
Authors: Sturm J.C. , Xiao X. , Garone P.M. , Schwartz P.V.
Published: March 2011

The electron-beam-induced-current (EBIC) technique has been used to image dislocations and other defects at strained Si: Sil−xGex epitaxial interfaces and in overlying epitaxial layers grown by Limited Reaction Processing. Depending upon the bias conditions and test structure, one can distinguish between interface defects and those in overlying films. We have found that for a low density of misfit dislocations, a high quality (defect-free) overlying epitaxial layer can be grown, but for a high density of dislocations certain line defects propagate upwards in the overlying layers.
Studies of Interface Mixing in a Symmetrically Strained Ge/Si Superlattice
Authors: Bowman R. C. , Adams P. M. , Chang S. J. , Arbet V. , Wang K. L.
Published: March 2011

Raman scattering and x ray diffraction were used to study the effects of annealing on a Ge/Si superlattice that had been grown upon a Ge0.4Si0.6 alloy buffer layer which distributes the strain between the layers. Anneals above 910K caused substantial mixing at the Ge-Si interfaces. The interdiffusion coefficients obtained from the x-ray data were found to obey an Arrhenius relation with an activation energy of 3.1±0.2eV. Initial intermixing seems to be dominated by the diffusion of Si atoms into the Ge layers via a vacancy mechanism.
Defects at the Interface of GaAsxP1−x/Gap Grown by Vapor Phase Epitaxy
Authors: Takeda Seiji , Hirata M. , Fujita H. , Sato T. , Fujii K.
Published: March 2011

Defects and microstructures in a ternary GaAsxP−x compound have been studied by transmission electron microscopy. The compound was grown on a (100) GaP substrate by vapor phase epitaxial. Crystal growth striation contrast was detected in a TEM image. This contrast was explained by local compositional variation of As and P. The distribution of misfit dislocations in the interface region was also studied.
Effects of Charge Transfer on the Microscopic Theory of Strain-Layer Epitaxy
Authors: Tsu Raphael , Zypman Fredy , Greene Richard F.
Published: March 2011

Whenever an atom of As is substitutionally replaced by a P-atom in a compound such as GaAs, due to the difference in ionicity, local puckering is effected by charge transfer. This paper deals with a calculation of the total energy stored, Eex = E(strain) + E(interface) = Es + Eint. For thick epilayers of GaP on GaAs, Eex is dominated by Es. However, for severalatomic layers of GaP on GaAs, the dominant stored energy is actually represented by the electrical dipole-dipole energy at the interface. The creation of dislocations may release the strain energy, it cannot affect the interface dipoleterm. Only inter-diffusion can lower the interface energy.
Formation of Insulating Layers in GaAs-Aigaas Heterostructures
Authors: Hobson W. S. , Pearton S. J. , Abernathy C. R. , Neida A. E. Von
Published: March 2011

We describe two methods for producing thermally stable high resistivity layers in GaAs-AlGaAs heterostructures. These rely on the interaction of implanted ions with dopant impurities already present in a buried layer in the heterostructure. In the first case, oxygen implanted at a concentration above that of the acceptors in p-type GaAs is shown to create thermally stable, highresistivity material only in the case of Be-doping in the GaAs. The effect is not seen for Mg-, Znor Cd-doping. Similarly there is no apparent interaction of 0 with n-type dopants (S or Si). The Be-O complex in p-type GaAs is a deep donor, creating material whose sheet resistivity shows a thermal activation energy of 0.59 eV. In the second case oxygen implantation into n+ AlGaAs, followed by annealing above 600°C, creates a deep acceptor level that compensates the shallow donors in the material. Temperature dependent Hall measurements show the resistivity of the compensated AlGaAs has a thermal activation energy of 0.49 eV, in contrast to a value of 0.79 eV for non-induced damage compensation.
Defect Heterojunction Model for Anomalous Photoresponse of P/N GaAs Grown on N-Ge Substrates
Authors: Partain Larry , Grounner Marc
Published: March 2011

A GaAs p/n junction grown on a n-Ge substrate has an anomalous photoresponse that includes a quantum yield response starting at the 0.7-eV band edge of Ge. In terrestrial sunlight, its I-V characteristic has an anomalous “notched” shape that is progressively removed by increasing the infrared light content. The I-V is well modeled by a space-charge-limited-current diode theory with a GaAs/Ge interface defect density of 1.92 (1012) cm−2 eV−1 at 0.4 eV below the conduction band edge. Similar defect densities have been reported for GaAs/oxide interfaces and for a GaAs p/n junction interface specially selected for high defect densities.
Crystallinity of Isolated Silicon Epitaxy (ISE) Silicon-on-Insulator Layers
Authors: Allen L.T.P. , Zavracky P.M. , Vu D.P. , Batty M.W. , Henderson W.R. , Boden T.J. , Bowen D.K. , Gorden-Smith D. , Thomas C.R. , Tjahjadi T.
Published: March 2011

Isolated silicon epitaxy (ISE) is a proven method of producing single crystalline silicon-on-insulator (SOI) material with excellent electrical properties. The presence of the remaining isolated dislocation trails in the epitaxial silicon has led to this investigation of the crystallinity throughout the ISE SOI layer and across the isolated dislocations. The structural perfection of these layers has been examined by defect etching, Nomarski optical microscopy, electron channeling patterns, and with more sensitivity using double crystal synchrotron X-ray diffraction and topography. Defect etching reveals the dislocation density within the layers of production ISE SOI material to be ~5×l0 5 /cm2. Electron channeling pattern techniques have reached the resolution limit of angular orientation resolution for the isolated silicon layer. Finally, synchrotron studies have shown that orientation homogeneity across 5" wafers are preserved to 0.006° and the variation in orientation across the defect trails to be, in general, less than 10 arcsec (0.003°), indicating single crystalline ISE SOI production material.
EPR Studies of DX Center Related Paramagnetic States in Ga0.69 Al0.31 As:Sn
Authors: Von Bardeleben H.J. , Bourgoin J.C. , Basmaji P. , Gibart P.
Published: March 2011

The electron paramagnetic resonance study of the DX center in Sn doped direct gap Ga0.69Al0.31 As shows the existence of a shallow effective mass like excited configuration of this defect. The photoexcitation spectrum for this transformation has a threshold at 0.8 eV; the photoionization of the DX center is not a transition to the lowest r conduction band as previously assumed. After photoexcitation additional paramagnetic defects are observed.
Electron-Radiation-Induced Epitaxial Growth of CoSi2 on Si(111)
Authors: Nieh C. W. , Lin T. L. , Fathauer R. W.
Published: March 2011

We report electron-radiation-induced epitaxial growth of CoSi2 on Si(111) from an amorphous Co/Si 1:2 mixture with epitaxial CoSi2 nuclei. Under the electron beam of a Philips EM430 electron microscope, the epitaxial nuclei grow parallel to the surface with a growth rate orders of magnitude higher than that for thermally activated growth. The substrate temperature during irradiation and the electron energy dependence were studied. Electron-radiation-induced growth shows very weak temperature dependence in the temperatures between 100°K and 300°K and the activation energy is 0.03 eV. The growth rate increases significantly as the electron energy increased to 200 KeV which is about the threshold energy for displacing Si atoms.
Electric Field Dependence of the Electron Drift Velocity in Hydrogenated Amorphous Silicon
Authors: Devlen R. I. , Schiff E. A. , Tauc J. , Guha S.
Published: March 2011

We present room temperature measurements on the electron drift velocity in hydrogenated amorphous silicon (a-Si:H) for large electric fields (1.3 × 105 to 3 × 105 cm/s). The experimental time resolution was ˜ 1ps and velocities as high as 8 × 105 cm/s were observed. The electron transport was non-dispersive, and the velocity was found to be a superlinear function of the applied field.
Study of the Density of Bulk Gap States in Undoped a-Si:H by Depletion Width Modulation Electron Spin Resonance
Authors: Essick J. M. , Cohen J. D.
Published: March 2011

The density of gap states in the bulk of intrinsic a-SiH has been probed using depletion width modulation electron spin resonance spectroscopy (DWM ESR) as well as complementary capacitance techniques. Our sample consists of a 2.85 micron thick layer of undoped a-Si:H sandwiched between a large area Pd Schottky barrier contact and a p+ c-Si substrate, allowing charge-counting capacitance and spin-counting ESR measurements to be performed on the same sample. DWM ESR measures the spin change that occurs when electrons are removed from gap states by increasing reverse bias on a Schottky barrier. The phase of the signal indicates whether unpaired spins are created or destroyed with increasing bias, thus revealing the defect charge state at the Fermi level The DWM ESR signal from our sample indicates that removing electrons from gap states leads to a reduction of the spin signal. This is qualitatively consistent with a band of Do states near EFo. However, for all metastable states and at all temperatures studied, the magnitude of the DWM ESR signal account for only about 14% of the responding charge as determined by junction capacitance. This would occur only if the majority of the filled bulk gap states were doubly occupied with a small correlation energy Ueff. In that case the majority of transitions caused by depletion width modulation would be D−/D+ which would produce no spin change. Model calculations show that the small observed Do/D+ DWM ESR signal would indeed result from the occupation statistics for a near zero value of Ueff at the moderate temperatures of the experiment. Comparison with our data sets the overall limits If −20meV < Ueff < 50 meV. Implication for recent models of the density of states in intrinsic a-Si:H are discussed.
Emission Energy Dependence of Transient Photoluminescence in Hydrogenated Amorphous Silicon
Authors: Longeaud C. , Merk E.
Published: March 2011

The dependence upon emission energy of the transient photoresponse of photoluminescence in amorphous silicon in the time range 100 ns to 20 μs, and for temperatures between 100 K and 160 K has been the subject of a detailed study. Analysis of this dependence, as a function of temperature, is carried out in terms of band tail carrier thermalisation and recombination. By using the simple multiple trapping treatment for the thermalisation of photoexcited carriers in their respective band tails, and by allowing for a fast time- and temperaturedependent recombination through defects deep in the gap, we are able to reproduce the correct energy and time dependence, as well as the temperature behaviour. On the basis of this analysis, the significance of the dispersive parameters deduced from transient photoluminescence decays is discussed. Comparison with the results of transient experimental techniques such as time of flight will be given.
Substrate Interactions in the Formation of Amorphous Silicon/Dielectric Interfaces
Authors: Parsons G. N. , Kim S. S. , Lucovsky G.
Published: March 2011

Using Remote PECVD we have deposited layers of “device quality” a-Si:H and Si-based dielectrics. We find that the formation of depletion or accumulation layers at a-Si:H/dielectric interface depends on the specific dielectric (SiO2 or Si3N4) and on the film deposition sequence. In addition, we have studied process-gas/substrate interactions by in-situ Auger Electron Spectroscopy (AES) and determined that Si-O or Si-N bonds are produced at a-Si and c-Si surfaces exposed to plasma excited He/O2, or NH3 mixtures, respectively. These process-gas/substrate interactions occur in parallel with film deposition and are correlated with the electronic properties of the interfaces, and the layer deposition sequence.
Raman Studies of Boron Induced Structural Changes in Amorphous Silicon-Boron Alloys
Authors: Yang G. , Bai P. , Tong B. Y. , Wong S. K. , Hill I.
Published: March 2011

A new amorphous Si1-x.Bx. alloy with composition of boron x ranging from 0.01 to 0.5 was produced in a low pressure chemical vapor deposition (LPCVD) system. We performed Raman scattering experiments on the a-Si1−xBx alloys and specifically monitored the Si-Si TO-like mode at around 480cm−1 and the TA-like mode at around 130cm−1. A pronounced broadening of the TO-like band as well as a decrease of the intensity ratio ITo/ITA are observed with an increase of boron concentration in the a-Si1−x Bx alloys. Based on the Raman spectra, the structural changes induced by boron incorporation in the a-Si network are discussed in the conceptual framework of the continuum random network (CRN) model. We conclude that boron incorporation enhances the structural disorder and induces topological changes in the amorphous network.
The Determination of the Quality of a-Si:H Films for Application in Solar Cells
Authors: Grunow P. , Herm D. , Könenkamp R. , Küppers U. , Kunst M. , Neitzert H. C. , Werner A.
Published: March 2011

Several stationary and transient techniques are used to characterize a large number of intrinsic a-Si:H films prepared in a narrow range of production conditions. Correlation between stationary photoconductivity, the activation energy of the dark conductivity and the long-time range decay of the transient photoconductivity and photoinduced absorption is observed. The applicability of a-Si:H films for solar cells in view of these properties is discussed.
Trapping of Atomic Hydrogen in a-Si:H from G – D Silane Plasma
Authors: Cruz-Jimenez S. A. , Muhl S. , Salcedo R.
Published: March 2011

Plasma Enhanced Chemical Vapor Deposition CPECVD) is used extensively for the preparation of amorphous materials. However, to date we do not have a full description of the deposition process. By this we refer to the following steps ; the source gas decomposition [1], the gas phase reactions, diffusion within the plasma [a], adsorption of the various species, solid-gas reactions, nucleation and subsequent film growth [3–7]. To a large extent the diversity of the processes which are involved in film formation explain the observed variation in the characteristics of supposedly identical material made in different laboratories. Even with such variations certain trends relating the properties of the materials with the growth processes are apparent. In particular it is well established that hydrogen saturation of the dangling bonds is essential. Although how much hydrogen is optimum, and how it is incorporated in the growing film are questions of considerable importance.
Thin Film Photovoltaic Options – an Overview
Authors: Stone Jack L.
Published: March 2011

Significant deployment of the promising option of photovoltaics for energy will require cost-effective technologies that compete effectively with conventional sources. One such option utilizes thin films of a variety of photovoltaic materials. These thin films must be manufacturable in large quantities, and they must have high performance and acceptable reliability. Amorphous silicon (a-Si) was the first successfully demonstrated thin film to be widely adopted by industry. This material is already used to power a larger number of such consumer products as calculators, watches, and battery chargers. Recently, a-Si solar cells have been scaled up to large-area modules for power applications. Large fields of these modules have been deployed by utility companies for their evaluation. Polycrystalline thin films such as copper indium diselenide (CIS) and cadmium telluride (CdTe) have recently shown promise in following the path of a-Si. High-efficiency, large-area submodules have been successfully tested. By combining these materials in hybrid combinations, researchers have demonstrated much higher efficiencies. Even higher efficiencies have been demonstrated with more conventional materials such as silicon and gallium arsenide in thin-film form. Such devices have a high degree of acceptability because of their successful application for power uses in their non-thin-film form. Extensive examples are given to demonstrate the technical viability of these photovoltaic approaches for possible use in utility-scale power plants and in other near-term, highvalue markets.
Midgap Defects in a-SiGe:H Devices from Capacitance Measurements
Authors: Hegedus Steven S. , Zhou Theodore X.
Published: March 2011

We have measured the capacitance of a-SiGe:H p-i-n solar cells as functions of temperature, frequency and bias. The density of states at the Fermi level g(EF) has been determined from two methods. We find that g(EF) increases from 1–2×1016 to 5–10×1017/(cm3eV) as the Ge content increases from 0 to 63%.
Microvoids and Photoconductivities in a-SiC:H and a-Si:H
Authors: Mahan A. H. , Williamson D. L. , Nelson B. P.
Published: March 2011

Small microvoids having a radius of gyration between 3.3Å-4.6Å are found by the Small Angle X-Ray Scattering (SAXS) technique to exist in both glow discharge a-Si1−xCx:H (O<x<0.3) and a-Si:H (40°C<Ts<250°C) films. Included in these results are the first observation of microvoids, as detected by SAXS, in device quality a-Si:H. The volume fraction of microvoids (vf) is found to increase from 0.01 in device quality a-Si:H to greater than 0.20 for both the a-SiC:H(x=.30) and a-Si:H(Ts=40°C) films. Tilting experiments indicate that the microvoids are non-oriented (spherical) in a-SiC:H and are oriented (columnar) in non-device quality a-Si:H. The normalized coplanar photoconductivities of all samples decrease with increasing vf, but for a given vf the photoconductivity is larger in a-SiC:H than in a-Si:H. We discuss reasons for this behavior.
“Concentration Dependence of the DC Dark Conductivity in Silicon Alloys”
Authors: Castagno G. , Mezzetti E. Minetti
Published: March 2011

The behaviour of DC dark conductivity of amorphous silicon alloys versus modifier concentration is investigated. By increasing the modifier concentration, the conductivity decreases, reaches
Electron Trapping Properties of Silicon Nitride Determined by Avalanche Injection
Authors: Park Y. C. , Jackson W. B. , Johnson N. M. , Hagstrom S. B.
Published: March 2011

This paper presents the results of a comparison of the electron trapping in plasma-enhanced chemical vapor deposited nitrides (PECVD) to low pressure CVD (LPCVD) nitrides. The electron trapping was investigated using avalanche injection of electrons from the c-Si substrate over a thermal Si02 barrier into the various nitrides. The average trapping distance was 3–20nm in the various nitrides and the density of the traps was found to be around 2X1018 cm−3 for the PECVD nitrides while the density for LPCVD nitride was about 4X1018traps cm−3.
ESR in 13C Enriched a-Si1−xCx:H
Authors: Xu Xixiang , Kidoh Hideo , Morimoto Akiharu , Kumeda Minoru , Shimizu Tatsuo
Published: March 2011

The effect of using the 13C isotope on the ESR signal in a-Si1−x Cx:H is investigated. The ESR linewidth increases with increasing the C content for a-Si1−x Cx.:H with 12C, while it is almost unchanged for a-Si1−x Cx:H with 12C. However, a doublet line which is expected for a C dangling bond having the hyperfine interaction with the 13C nucleus was not detected. In the range of a small x, it is possible that Si dangling bonds with 13C at the back bonds are the origin of the wide ESR signal. In the range of a large x, 13C dangling bonds should exist, but they are in a clustered form, being observed as an exchange narrowed single line instead of the hyperfine split line.
IR and Sims Study of Hydrogen Diffusion in RF Sputter Deposited Boron Doped a-Si:H and Undoped a-Ge:H
Authors: Mitra S. , Wu X.-L. , Shinar R. , Shinar J.
Published: March 2011

Secondary ion mass spectrometry (SIMS) and IR measurements of long range deuterium motion in rf sputter deposited (rf sp) p-doped a-Si:H and undoped a-Ge:H are compared to recently published results on undoped rf sp a-Si:H, which exhibited strongly power-law time dependent diffusion constants (exponent α= 0.75±0.1) in films of as-deposited content of di-H and tri-H bonds (usually associated with microvoids) Ndo –4–5 at.%. In pdoped a-Si:H samples where Ndo-l.8–3.8at.%, the diffusion is much faster, but the exponent is similar. In undoped a-Ge:H exhibiting a stretch vibration band indicative of mono-H bonding only, the diffusion is about one order of magnitude faster than in undoped a-Si:H, and α = 0.23. The results are discussed in relation to both the multiple trapping (dispersive) and defect mediated diffusion models.
The Application of Thick Hydrogenated Amorphous Silicon Layers to Charged Particle and X-Ray Detection
Authors: Perez-Mendez V. , Cho G. , Fujieda I. , Kaplan S. N. , Qureshi S. , Street R. A.
Published: March 2011

We outline the characteristics of thick hydrogdenated amorphous silicon layers which are optimized for the detection of charged particles, x-rays and γ-rays. Signal amplitude as a function of the linear energy transfer of various particles are given. Noise sources generated by the detector material and by the thin film electronics - a-Si:H or polysilicon proposed for pixel position sensitive detectors readout are described, and their relative amplitudes are calculated. Temperature and neutron radiation effects on leakage currents and the corresponding noise changes are presented.
Electron Cyclotron Resonance Deposition of a-Si:H and a-C:H Films
Authors: Shing Y. H. , Yang C. L. , Allevato C. E. , Pool F. S.
Published: March 2011

Amorphous silicon (a-Si:H) and amorphous carbon (a-C:H) films have been deposited by electron cyclotron resonance (ECR) microwave plasma enhanced CVD. A high deposition rate of ˜ 25 Å/sec and a light-to-dark conductivity ratio of 1 × 105 for a- Si:H films have been achieved by the ECR process using a pure silane plasma. ECR microwave plasmas have been analyzed by in situ optical emission spectroscopy (OES) and have shown a strong H * emission at 434 nm indicating higher chemical reactivity than RF plasmas. The linear correlation between the film deposition rate and the SiH* emission intensity of ECR silane plasma suggests that SiH* species are related to the neutral radicals which are responsible for the a-Si:H film deposition. Hard and soft a-C:H films have been deposited by ECR with and without RF bias power, respectively. The RF bias to the substrate is found to play a critical role in determining the film structure and the carbon bonding configuration of ECR deposited a-C:H films. Raman spectra of these films indicate that ECR deposition conditions can be optimized to produce diamond films.
In-Situ Kelvin Probe Study of the SnO2 / p (a-Si:H) Interface. Effects of Hydrogen and Argon Plasma Treatment
Authors: Cabarrocas P. Roca i
Published: March 2011

We present an in-situ Kelvin probe study of the effects of hydrogen and argon plasma treatments on fluorine doped tin-oxide (SnO2) substrates, as well as the consequences of these treatments on the subsequent growth of a-Si:H. We found that the effect of both hydrogen and argon plasmas is to enhance the reactivity of SnO2 substrates in a silane plasma. Moreover, the, doping level (Fermi level) of boron doped a-Si:H layers deposited on SnO2 is reached more quickly when the substrate has been treated previously with an argon plasma. As a consequence, we expect a higher open circuit voltage (Voc) for SnO2/p-i-n solar cells in which the substrate has been treated by an argon plasma. The Kelvin probe results have been confirmed by Voc measurements on solar cells deposited on treated and untreated substrates.
Experimental and Theoretical Analysis of Resonant Tunneling Through a-Si:H/a-Si1−xCx:H Double-Barrier in p-i-n Structure
Authors: Jiang Yeu-Long , Hwang Huey-Liang
Published: March 2011

a-Si:H/a-Si1−x Cx:H double-barrier structure imbedded in the i layer were fabricated by glow discharge. By using the built-in field in the i layer and the external applied bias, the effects of photo-excited carriers driven by electric field in the i layer tunneling through the double-barrier quantum well were examined. Distinct current bumps are distinctly observed when the quantum well structure is imbedded in the middle section of the i layer. A simple theoretical analysis was developed to study the quantum transport behavior.
Band Offsets and Anomalous Deep Defect Distribution at the Hydrogenated Amorphous Silicon-Crystalline Silicon Interface via Junction Capacitance Techniques
Authors: Essick J. M. , Cohen J. D.
Published: March 2011

A novel a-Si:H/c-Si Schottky diode heterostructure device premits the study of the capacitive response to relatively low temperatures (25 K) and also allows fast pulse filling capture measurements of electrons from the c-Si substrate into a-Si:H defect states. These latter measurements, as well as capacitance vs. temperature measurements on these diodes, indicate a nearly zero conduction band offset (50±50 meV). We also have observed trapping of holes at the a-Si:H/c-Si valence band discontinuity ΔEv. A clear threshold for the subsequent optical release of these trapped holes by sub-bandgap light yields a value of ΔEv = 0.58±0.02 eV. Finally, photocapacitance spectra along with thermally stimulated capacitance (TSCAP) measurements indicate an anomalously large (l×1018/cm3) Gaussian-shaped defect band located a Ec - 0.88 eV with a FWHM of 0.46 eV. Model calculations of the high temperature capacitance-voltage dependence indicate these defects lie predominately within 350 Å of the a-Si:H/c-Si interface.
The Effect of Water Vapor and Oxygen in the Processing Environment on the Properties of Sputtered a-Si:H Films
Authors: Wang Cheng , Parsons G. N. , Lucovsky G.
Published: March 2011

A systematic study of the effect of the trace impurity of oxygen and water vapor in the processing environment on the infra-red (IR), optical and electrical properties, and defect density of sputtered a-Si:H films is reported. The results are that small contents of water introduced into the argon plasma extract H-atoms from the growth surface, thereby reducing total hydrogen content and polyhydride formation in films deposited at low substrate temperatures, <150°C. This leads to a decrease of the defect density, and hence, to improvements in the photoconductivity (Δσ) and the associated quantum efficiency-mobility-lifetime(ημτ) product.
Plasma Diagnostic of a TCDDC System Using a Quadropole Mass Spectrometer
Authors: Vieira M. , Maçarico A. , Martins R. , Ferreira I. , Guimarāes L.
Published: March 2011

Species formed during the decomposition of silane-methane-hydrogen mixtures ([SiH4]x:[CH4]y:[H2]z), by spatial plasma separation technique using the TCDDC (Two Consecutive Decomposition and Deposition Chambre) systeml, are evaluated by mass spectrometric analysis and related with the structural and electro-optical properties for either amorphous (a-) or microcrystalline (μc-) thin films. Results obtained show that in the plasma region, the main reaction is the direct fragmentation of SiH4 by electron impact whilst near the growing surface, the main detected species are excited. The kind of species and their intensity depend strongly on the power density (dp), mixture gas ratio (g = CH4 /SiH4+CH4), static electromagnetic (ξ, B) fields and r.f. frequency (f), used. Since CH4 has a threshold decomposition higher than that one of SiH4, the species presented at the plasma region are, mainly, methyl, dimethyl and CH2-CH2 graphitic-like chains depending, mostly, on dp and on g. By diluting the mixture in H2, we observe the existence of active H2 species that, for high dp, may lead to a transition from the amorphous to microcrystalline phase, as well as a carbon incorporation in the amorphous tissue as graphitic-like bonds. This allow us to infer the merit of the TCDDC system in producing a-/μc-thin films that can be applied to photovoltaic and other applications.
A Single Phase ([Nzp]) Ceramic Radioactive Waste Form
Authors: Roy Rustum , Yang L.J. , Alamo J. , Vance E.R.
Published: March 2011

It has been shown that between 10 and 20% of a simulated PW–4b radwaste composition can be incorporated into a single nhase with the NZP (= ‘MaZr2 P3 o12’) structure. By changing the P/Na and Zr/Na molar ratios (i.e., adjusting the crystal chemical model of where each ion is located in the structure) it has been possible to outline a very ‘forgiving’ compositional regime both at the 10% and the 20% waste loading level within which one obtains one ([NZP]) or two ([NZP] and [monazite]) phases. We have also succeeded in substituting Tio2 for Zro2 in making a TiO2-rich [NTP] waste form analogous to the [NZ]] materials.

Thus we have succeeded in creating monophasic and diphasic ceramic waste forms which can be sintered below 1000° C. Only preliminary leach data have been obtained at 25° and 300°C, and they place this material with good ceramic forms.
The Effect of Solution Ph and Ion Concentrations on Leaching of Silicate Glass
Authors: Bunker Bruce C. , Arnold George W.
Published: March 2011

For complex glasses such as simulated nuclear wastes, glass dissolution is a complex process, involving selective leaching of cations, reprecipitation reactions, and protective film formation. In order to begin to understand this complex behavior, each of the above phenomena is being studied, one reaction at a time, on simple alkali silicate glasses under controlled environmental conditions. To date, two types of reactions have been investigated. The first reaction type is the selective leaching of cations from the glass, resulting in the formation of a hydrated “gel” layer on the glass surface. The second reaction type is the reaction of ionic or colloidal species in solution with this gel layer. Reactions in the second category include ion exchange reactions and sorption reactions which can result in protective film formation. Studies of these simple reactions have led to the development of new leaching models and to observations which begin to explain the behavior of complex glasses, and predict how glass dissolution should change as a function of the chemical environment.
Comparison of the Leachability of Three TRU Cement Waste Forms
Authors: Ross Wayne , Westsik Joseph , Roberts Frank , Harvey Clark
Published: March 2011

Cement waste forms prepared by three processes, casting, cold pressing, and FUETAP (Formed Under Elevated Temperatures and Pressure) have been compared for their leachability by using the MCC-1 leach test. The results indicate that releases of plutonium are not controlled by the waste form matrix and that there is no significant overall advantage to any of the three cement processes from a leachability viewpoint.
Preferential Dissolution Phenomena of Nuclear Waste Materials
Authors: Clarke D. R. , Flintoff J. F.
Published: March 2011

Preferential dissolution of polyphase nuclear waste materials in short term leach tests can exaggerate radionuclide release rates when extrapolated to the lifetime of the waste form. Possible preferential leach phenomena are associated with the presence of cracks, intergranular phases and readily soluble phases. The rate of dissolution and the microstructural connectivity of the most soluble phase determine the period over which perferential dissolution is observable. The connectivity of phases is amenable to control during processing by altering the starting green density of the precursor powders.
Evaporite Dissolution Relevant to the Wipp Site, Northern Delaware Basin, Southeastern New Mexico
Authors: Lambert Steven J.
Published: March 2011

Evaluation of the threat of natural dissolution of host evaporites to the integrity of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico has taken into consideration (1) the volume of “missing” rock salt, (2) the occurrence (or not) of characteristic dissolution brines, (3) geomorphic features, some of which are unrelated to dissolution, and (4) the time intervals over which dissolution may have been active. Even under the assumption that all “missing” halite was originally present and has been removed by dissolution, there is no evidence of active preferential removal of the lower Salado Formation halite by any geologically reasonable process. The geologic record contains evidence of dissolution in the Triassic and Jurassic; to constrain all removal of basinal halite to the late Cenozoic yields an unrealistically high rate of removal. Application to the lower Salado of a stratabound mechanism known to be active in Nash Draw, a near-surface feature within the Basin, allows a minimum survival time of 2,500,000 years to be predicted for the subsurface facility for storage of radioactive waste at WIPP. This calculation is based on an analysis of all known dissolution features in the Delaware Basin, and takes into account the wetter (pluvial) climate during the past 600,000 years.
Radionuclide Migration into Natural Fracture Surfaces of Granitic Rock
Authors: Torstenfelt B. , Eliasson T. , Allard B. , Andersson K. , Hoglund S. , Ittner T. , Olofsson U.
Published: March 2011

The uptake of cesium on natural fracture surfaces taken from granitic bedrock has been studied as well as the depth penetration into the underlying rock. The fracture minerals usually exhibit good sorptive properties with respect to cesium (with the exception of calcite). The surface coatings do not act as physical diffusion barriers and do not prevent a migration into the underlying rock. A considerable depth penetration into the rock (several mm over a period of three months) was observed. The dominating pathways into the rock matrix were the high-capacity minerals in grains or in microfissures. The calculated diffusivity in the rock matrix was of the order of 10−13 m2/s.
Salt Creep Design Consideration for Underground Nuclear Waste Storage
Authors: Li William T. , Wu Ching L. , Antonasc Nicholas J.
Published: March 2011

This paper summarizes the creep consideration in the design of nuclear waste storage facilities in salt, describes the non-linear analysis method for evaluating the design adequacy, and presents computational results for the current storage design. The application of rock mechanics instrumentation to assure the appropriateness of the design is discussed.

It also describes the design evolution of such a facility, starting from the conceptual design, through the preliminary design, to the detailed design stage The empirical design method, laboratory tests and numerical analyses, and the underground in situ tests have been incorporated in the design process to assure the stability of the underground openings, retrievability of waste during the operating phase and encapsulation of waste after decommissioning.
Studies on Speciation of Americium, Technetium and Neptunium in Simulated Vitrified-Waste Leachates
Authors: Bidoglio G. , De Plano A. , Chatt A.
Published: March 2011

Physico-chemical forms of several radionuclides are being investigated in order to characterize the migrating species in a deep geologic repository. Under ground water bicarbonate and carbonate concentration levels, Am(CO3)2 has been observed to be the most important complex while the formation of Am(OH)(CO3)22− has been indicated, stability constants of Am-complexes have been calculated, and a distribution diagram of Am(III) species as a function of pH is presented. Speciation of Tc has been studied using radiochemical and electromigration methods in oxic and anoxic environments; in effective diffusion coefficient of 1.37 × 10−5 cm2 S−1 for ToO4− has been obtained. Preliminary studies on Np(V) indicate that NpO2+ does not form strong complexes with carbonate and bicarbonate ions at ground water concentrations.
NWTS Waste Package Design and Materials Testing
Authors: Kircher J. F. , Bradley D. J.
Published: March 2011

It is generally accepted that waste to be disposed of in a deep geologic repository will be suitably packaged. The design of such packages depends on the criteria which they must meet and the geochemical environment in which they are to be placed. Both the criteria and the environment must be considered in determining the components required to make up the package, the functions those components are to perform, and the materials of which the components are to be made.
Mechanical Stability of a Cm-Doped Celsian Glass-Ceramic
Authors: Routbort J.L. , Offermann P. , Matzke Hj.
Published: March 2011

Radiation damage due to a α-decay of actinides will modify the material used to store nuclear wastes. For example, ionization-induced processes can fragment the oxide bonds of a glass thereby creating gas bubbles. Phase decomposition, swelling, and disordering of crystalline phases are other possibilities. The self-irradiation damage could also affect the fracture properties of the storage materials. Spontaneous failure and fragmentation of some brittle materials have been observed during storage. Yet the problem of fracture of irradiated waste materials has received very limited attention.
Waste Classification System for Low-Level Radioactive Wastes
Authors: Wickham L. E. , Rope S. K.
Published: March 2011

Some low-level radioactive waste represents both a chemical and radiological hazard, and it is desirable that management practices take this into account. This paper describes a waste classification system that could provide guidance on how mixed wastes could be separated for disposal, and the degree of disposal isolation each waste material or shipment needs. An example of the system's use is provided.
Leach Tests of Simulated Low-Level Transuranic Waste Forms Containing Transuranic Elements
Authors: Welch Jane M. , Sill Claude W. , Flinn John E.
Published: March 2011

Simulations of waste forms that might be produced by slagging pyrolysis incineration of low-level transuranic (TRU) wastes stored at the Idaho National Engineering Laboratory (INEL) have been fabricated containing the transuranic isotopes 237Np, 239pu, 24lAm, 244Cm at levels of approximately 1 μCi per gram of each.

Leach tests were performed using frit and vitrified monolithic specimens of average INEL TRU waste, portland cement monoliths made with frit as aggregate, and vitrified monoliths of INEL soil and simulated Rocky Flats sludge. Static leach tests were performed at 90, 70, 40, and 25°C in deionized water for up to 364 days. Leachates were analyzed for the TRU elements by alpha spectrometry. The following generalizations can be made:

1. Cemented frit and vitrified sludge waste forms produce leachates with the highest pHs (>11) and have the lowest TRU leach rates, 10−4 g/m2.d at 90°C.

2. Neptunium has a higher leach rate than the other three TRU elements by as much as two orders of magnitude for all waste forms tested except cemented frit.

3. Only the vitrified soil samples display a marked temperature dependence for leach rates of all four TRU elements.
Electric Arc Furnace Melting of Simulated Transuranic Wastes
Authors: Nafziger R. H. , Oden L. L.
Published: March 2011

As part of an interagency agreement between the Bureau of Mines, U.S. Department of the Interior, and the U.S. Department of Energy through its contractor, EG&G Idaho, Inc., the Bureau conducted six melting tests at its Albany Research Center to assess the feasibility of melting transuraniccontaining wastes. The tests were conducted with simulated wastes in a conventional refractory-lined electric arc furnace. Charge materials included concrete, soil, metal, wood, CaO- and Na2O-containing chemical waste sludges, cement, and polyethylene mixed in various proportions in both unburned and partially incinerated forms. The investigation showed that it is possible to melt these materials in a 1-metric-ton conventional electric arc furnace and separate the slag and metal provided that suitable fluxes are added to condition the silicious slags. However, the electric arc furnace cannot be considered an efficient incinerator. The molten slags were poured into 210-liter steel drums having a 0.64-cm-thick steel chill plate on the bottom. All slags were tapped from the furnace satisfactorily. The concrete and sludge materials required the most energy for melting (2.2 to 2.6 kwhr/kg). The highest electrode consumption occurred when the sludges were melted (0.04 kg/kg product). A high alumina-chrome refractory is satisfactory for use as a furnace lining in melting these wastes. Offgases and particulates from all of the tests were sampled and analyzed. The greatest amounts of particulate matter in the offgas streams were obtained from melting sludges and incinerated wastes.

It is recommended that if a conventional electric arc furnace is used to melt transuranic waste that it (1) be fed slowly to prevent excessive fumes and flames and (2) be operated continuously to minimize startup problems and improve efficiency.
Thermal Conductivity of Bentonite/Quartz High-Level Waste Package Backfill
Authors: Moss Marvin , Molecke Martin A.
Published: March 2011

A mixture of bentonite clay and quartz sand is being considered for use as a waste-package backfill, the material placed between a radioactive-waste canister and the repository host rock. Compacts of bentonite/quartz with weight-percent ratios of 100/0, 70/30, 50/50 and 30/70 were made at room temperature under a pressure of 100 MPa (15 ksi). Upon initial heating, the thermal conductivity of the 70/30 compact rose from 1.20 W/m·K at 298 K to 1.32 W/m·K at 373 K. After further heating to 473 K, it fell to 1.10 W/m·K, reflecting the loss of interlamellar water from the bentonite. The conductivity of the now-dehydrated compact was reproducible through several heating and cooling cycles, varying from 1.15 W/m·K at 573 K to 1.03 W/m·K at 298 K. The other mixtures were qualitatively similar to the 70/30: the 100/0, 50/50 and 30/70 dehydrated compacts displayed conductivities of 0.59, 1.06 and 0.83 W/m.K,respectively, at 298 K. Measured densities ranged from 1.98 to 2.12 g/cc. Combined geometric-mean and Maxwell models for thermal conduction in composite systems predict the measured results reasonably well. An analysis of the impact of backfills on high-level waste (HLW) package design indicates that no significant thermal penalty is imposed.
Inertial Effects in Transport of Radioactive Waste Through Engineered Barriers
Authors: Carnahan C. L.
Published: March 2011

Addition of an inertial term to the constitutive relation for mass flux leads to a mass transport equation which is hyperbolic and describes propagation of a distorted wave with a finite velocity. This approach eliminates the “instantaneous propagation paradox” inherent in parabolic transport equations based on Fick's law. Analytical solutions of the wave transport equation have been derived and have the properties that the leading edge of a solute front propagates at a finite, predictable velocity and is truncated by a step function which decreases in magnitude exponentially with time. The inertial effects on computed solute fronts are most evident near the leading edge, and have potential significance in the prediction of engineered barrier performance.
Corrosion of Ticode-12 in A Simulated Waste Isolation Pilot Project (Wipp) Brine
Authors: Ahn T. M. , Lee B. S. , Woodward J. , Sabatini R. L. , Soo P.
Published: March 2011

The corrosion behavior of TiCode-12 (Ti-0.3 Mo-0.8 Ni) high level nuclear waste container alloy has been studied for a simulated WIPP brine at a temperature of 150°C or below. Crevice corrosion was identified as a potentially important failure mode for this material. Within a mechanical crevice, a thick oxide film was found and shown to be the rutile form of TiO2, with a trace of lower oxide also present. Acidic conditions were found to cause a breakdown of the passive oxide layer. Solution aeration and increased acidity accelerate the corrosion rate. In hydrogen embrittlement studies, it was found that hydrogen causes a significant decrease in the apparent stress intensity level in fracture mechanics samples. Hydride formation is thought to be responsible for crack initiation. Stress corrosion cracking under static loads was not observed. Attention has also been given to methods for extrapolating short term uniform corrosion rate data to extended times.
Rheological and Physical Properties of Magesium Oxide and Silica Fume-Modified Cement Mortars Cured at High Temperature
Authors: Nakagawa Zenbe-e , White Elizabeth L. , Roy Della M.
Published: March 2011

Physical properties and placement characteristics of cementitious mortars have been studied for their potential as repository sealing materials. They contained various expansive agents and industrial by-products, and were investigated at curing temperatures up to 250° C, the upper limit of an emplacement site or generally of relevance in accelerated reaction studies. An expansive agent, magnesium oxide, and two industrial by-products, silica fume and granulated blast furnace slag have been used at different percentages in the mixtures. Excellent general performance, including very high strengths up to 240 MPa combined with very low intrinsic permeability <10−8 Darcy (μm2) were generated at 175°C on material having a viscosity of 5000 cP (mPa·s) at 38° C. One 1700 cP(mPa·s) material treated at 250°C had compressive strength >180 MPa and also <10−8 Darcy (μm2) permeability. MgO was found to accelerate formation of tobermorite and generally cause expansion; at 250° C expansion was also related to xonotlite formation.