bluuebunny/crossref_metadata_2025 huggingface dataset:
Volume Effects in Bulk Metallic Glass Formation
Authors: Yavari A. R. , Inoue A.
Published: April 2011

Atomic volume effects in amorphous alloys obtained by rapid solidification and in bulk glasses are briefly reviewed. It is recalled that at high undercoolings, the release at the growth fronts, of the volume corresponding to the reduction of the molar volume upon solidification can sharply accelerate crystal growth in the melt through viscosity reduction. A method is then proposed for estimating the volume of mixing ΔVmix which is negative for elemental additions to glass-forming liquid alloy. It is argued that negative ΔVmix reduces atomic mobility in easy glass-forming alloys thus allowing the suppression during cooling, of nucleation and growth of crystallites. The ZrCuNi system is used as an example for applying this reasoning. It is shown that Al or Ti addition to ZrNiCu alloys lead to strongly negative ΔVmix and expected sharp drops in diffusion-controlled crystal growth kinetics in the melt.
Distribution Coefficients and Apparent Diffusion Coefficients of Cesium in Compacted Bentonites
Authors: Okamoto Akiko , Idemitsu Kazuya , Furuya Hirotaka , Inagaki Yaohiro , Arima Tatsumi
Published: April 2011

Distribution coefficients and apparent diffusion coefficients of cesium in some compacted bentonites were determined by the penetration profile method. Cylindrical compacted bentonites with the dry density of 0.8 to 1.6 Mg/m3 were contacted with tracer solutions containing 1000, 100 or 10 ppm of cesium. The apparent diffusion coefficients were obtained from the concentration profiles of cesium in compacted bentonites. The distribution coefficients were obtained concurrently by dividing the intercepts of the profiles by the concentration of the tracer solution. The apparent diffusion coefficients of cesium in compacted bentonite were obtained in the range of 0.42 to 9.6· 10−12 m2/s. The apparent diffusion coefficients in the compacted bentonite contacted with three different concentrations of cesium tended to decrease with increasing dry density of the specimen; but, they had no dependence on cesium concentration within a factor of 3 at the same dry density. The distribution coefficient of cesium for the specimens contacted with three different concentrations of cesium were obtained in the range of 0.3 to 90 L/kg and had little dependence on dry density. The distribution coefficients obtained in the compacted bentonites were dependent on pH of the solution rather than concentration of cesium. These distribution coefficients obtained in the compacted bentonites were 10 to 1000 times smaller than those obtained by batch experiments. The data suggest that not all sorption sites for cesium are available in highly compacted bentonite. It is necessary to consider surface diffusion as a significant migration mechanism of cesium through the compacted bentonites at very high pH condition such as 12.
Modelling the Activity of Sulphate-Reducing Bacteria and the Effects on Container Corrosion in an Underground Nuclear Waste Disposal Vault
Authors: King Fraser , Kolar Miroslav , Stroes-Gascoyne Simcha , Bellingham Peter , Chu James , Dawe Paula V.
Published: April 2011

A mathematical model has been developed to predict the extent of sulphate reduction by sulphate-reducing bacteria (SRB) in an underground nuclear waste disposal vault and the consequences for corrosion of the waste package. The model is based on a series of massbalance equations that describe the kinetics of sulphate reduction by two types of SRB (one organotrophic and one chemoheterotrophic), the growth and death of SRB, the supply and consumption of nutrients (acetate and hydrogen) and reactants (SO42−) and the consumption of sulphide by precipitation with aqueous Fe(II) or by container corrosion. The disposal vault is described by a series of 1-D mass-transport barriers representing the buffer and backfill materials and fractured rock. The model can also be used to simulate the effects of γ-radiation and desiccation of the buffer and backfill materials on the extent of microbial activity.
A Quantitative Assessment of Microbiological Contributions to Corrosion of Candidate Nuclear Waste Package Materials
Authors: Lian T. , Martin S. , Horn J. , Jones D.
Published: April 2011

The U.S. Department of Energy is contributing to the design of a potential nuclear waste repository at Yucca Mountain, Nevada. A system to predict the contribution of Yucca Mountain (YM) bacteria to overall corrosion rates of candidate waste package (WP) materials was designed and implemented. DC linear polarization resistance techniques were applied to candidate material coupons that had been inoculated with a mixture of YM-derived bacteria with potentially corrosive activities, or left sterile. Inoculated bacteria caused a 5- to 6-fold increase in corrosion rate of carbon steel C 1020 (to approximately 7-8μm/yr), and an almost 100-fold increase in corrosion rate of Alloy 400 (to approximately μm/yr) was observed due to microbiological activities. Microbiologically Influenced Corrosion (MIC) rates on more resistant materials (CRMs: Alloy 625, Type 304 Stainless Steel, and Alloy C22) were on the order of hundredths of micrometers per year (μm/yr). Bulk chemical and surfacial endpoint analyses of spent media and coupon surfaces showed preferential dissolution of nickel from Alloy 400 coupons and depletion of chromium from CRMs after incubation with YM bacteria. Scanning electron microscopy also showed greater damage to the Alloy 400 surface than that indicated by electrochemical detection methods.
Ion Exchange in High-Level Nuclear Waste Encapsulation: Atomistic Modeling of Equilibrium Coefficients and Isotherms
Authors: Kletskova T. , Czerwinski K. , Gelbard E. , Yip S.
Published: April 2011

A computational modeling approach, based on molecular dynamics and related atomistic simulation techniques, is formulated to analyze the confinement of radionuclides in prepared nuclear wasteform for long-term disposal. While the intent is to exploit the ability of simulation to provide a unified treatment of chemical as well as kinetic processes at the molecular level, it is recognized that the problem is too complex to be studied in its entirety at the outset. To demonstrate the feasibility and effectiveness of the atomistic modeling approach, a first step is to determine an equilibrium isotherm for cation exchange, on the premise that a predominant release mechanism is likely to involve ion exchange. As a prototypical application, Cs-Na exchange on sodalite is studied using explicit interatomic potential models. Initial results for the equilibrium Cs concentration in dehydrated sodalite suggest that the effects of frame relaxation need to be taken into account. The present study will be extended to treat an aqueous environment for the loaded wasteform, as well as the effects of radiation damage to the zeolitic framework.
Oxidative alteration of Ce-rich pyrochlore: HRTEM/EELS investigation
Authors: Xu Huifang , Wang Yifeng
Published: April 2011

Transmission electron microscopy (TEM) and associated electron energy-loss spectroscopy (EELS) study show intergrowth of Ce4+-rich pyrochlore (metamict) and Ce3+-rich pyrochlore (partially metamict) in a Ce-rich pyrochlore from a rare earth element (REE) ore deposit of Inner Mongolia, Northern China. The partially metamict material is Ba-free and dominated by Ce3+. However, the metamict material is Ba-bearing and dominated by Ce3+,. The Ce4+-rich pyrochlore may result from radiation damage by alpha decay that also causes oxidation of Fe 2+ in titanite, and the interaction with a Ba-bearing oxidizing fluid. The oxidation of Ce3+ in the primary pyrochlore is accompanied by in the loss of REE, Ca, and Pb, a daughter product of U via alpha decay, during the alteration. However, most REE were incorporated in the alteration product, the Ce4+-rich pyrochlore. Based on EDS and EELS analyses, the chemical formulae of the partially metamict Ce3+-rich pyrochlore and metamict Ce4+-rich pyroeblore can be written as: (Ca, Ce3+, U, Pb) 2(Ti, Nb)2O7−x(OH)x, and (Ba, Ca, Ce4+, U)2 (Ti, Nb)2O7−y(OH)y, respectively. Ce is the most abundant element among all REE. It is proposed that the alteration takes place in solid-state with oxidizing fluid as a catalyst. The alteration kinetics is controlled by diffusion processes of aqueous species in metamict pyrochlore.
Molecular Dynamics Study of a Nuclear Waste Glass Matrix With Plutonium
Authors: Meis C. , Delaye J. M , Ghaleb D.
Published: April 2011

Molecular dynamics simulation techniques were applied to model the incorporation of plutonium in the French nuclear waste glass matrix. Born-Mayer-Huggins analytical potentials were established to characterize short-range interactions between Pu–O and Pu–Pu pairs; the potentials were fitted to the structural properties of plutonium dioxide in the light of a recent experimental study showing that plutonium is found as Pu(IV) in the glass. The transferability of the established potentials to the glass structure is discussed, and the potential parameters are further refined by molecular dynamics simulations in an aluminoborosilicate glass to obtain mean Pu–O interatomic distances and firstneighbor coordination numbers matching the experimental values as closely as possible. Previously published Born-Mayer-Huggins potentials supplemented by Stillinger-Weber three-body terms were used for oxygen-cation and cation-cation interactions. The difficulties encountered in establishing a Pu–O potential that provides satisfactory results in both oxides and glasses are also discussed.
Testing the Grambow Glass Dissolution Model by Comparing it With Monte Carlo Simulation Results
Authors: Aertsens Marc
Published: April 2011

We have developed a Monte Carlo (simulation) model to describe glass dissolution. This model uses similar parameters to those used in the Grambow model, which has been the main glass dissolution model during the last decade. While the Grambow model is macroscopic, ours is microscopic. This assures that the value of our parameters is time independent and allows changes in the gel structure (not just concentrations) to be described.

We relate our parameters to those of the Grambow model and test whether the basic assumptions of the Grambow model are consistent with our simulation results. In these simulations, as the solution evolves towards silica saturation, the silica concentration in solution can become higher than the (final) silica saturation concentration. A possible explanation for this behavior, which is observed experimentally as well, is that the silica saturation concentration is not constant as a function of time. Initially, the silica saturation concentration is the glass saturation concentration. The glass saturation concentration is higher than the final silica saturation concentration, which corresponds to saturation of the gel. In the Grambow model, the initial dissolution rate (at the gel/water interface) and the diffusion coefficient of silica in the gel are not constant either. The existence of a final dissolution rate depends on the silica content of the glass and on the protection provided by the gel layer. In the simulations, a protective gel is mainly formed by the adsorption (precipitation) of dissolved silica particles from the solution in contact with the gel. This makes us expect that (1) the gel would be non protective for dynamic tests (with a low silica content in solution) and (2) the gel would be very protective in static tests with high surface to volume ratios.
Grain Size and Burnup Dependence of Spent Fuel Oxidation: Geological Repository Impact
Authors: Kansa E. J. , Hanson B. D. , Stout R. B.
Published: April 2011

Further refinements to the oxidation model of Stout et al. have been made. The present model incorporates the burnup dependence of the oxidation rate and an allowance for a distribution of grain sizes. The model was tested by comparing the model results with the oxidation histories of spent-fuel samples oxidized in thermogravimetric analysis (TGA) or oven dry-bath (ODB) experiments. The experimental and model results are remarkably close and confirm the assumption that grain-size distributions and activation energies are the important parameters to predicting oxidation behavior. The burnup dependence of the activation energy was shown to have a greater effect than decreasing the effective grain size in suppressing the rate of the reaction U4O9→U3O8. Model results predict that U3O8 formation of spent fuels exposed to oxygen will be suppressed even for high burnup fuels that have undergone restructuring in the rim region, provided the repository temperature is kept sufficiently low.
Migration Experiments in Compacted Ca-Bentonite
Authors: García-Gutiérrez M. , Missana T. , Molinero J. , Yllera A. , Mingarro M.
Published: April 2011

Migration experiments were carried out in compacted Ca-bentonite as a complement of the FEBEX project large scale tests.

Through diffusion and permeation experiments with tritiated water were used to perform a deep characterisation of the clay diffusion parameters (diffusion coefficient, effective porosity and permeability ). In through diffusion experiments, a variation of tracer concentration in both reservoirs was allowed. Since in these experimental conditions an analytical solution of the diffusion problem does not exist, an analytical approximation was used and the results compared with a numerical model which takes into account the presence of the sintered steel filters confining the clay plug.

The migration behaviour of Se in this compacted bentonite has been studied and the retardatioci factor obtained. The calculated Kd is at least one order of magnitude lower than the one obtained by means of batch sorption experiments due to a lower availability of sorption sites when increasing the compaction degree.
New Data on the HyrkkÖlÄ Native Copper Mineralization: A Natural Analogue for the Long-Term Corrosion Of Copper Canisters
Authors: Marcos N. , Ahonen L. , Bros R. , Roos P. , Suksip J. , Oversby V.
Published: April 2011

The Hyrkkölä U-Cu mineralization located in south-western Finland is reassessed with reference to the corrosion mechanisms affecting the stability of native copper and the time-scales of corrosion processes. The mineral assemblage native copper – copper sulfide occurs in open fractures at several depth intervals within granite pegmatites (GP). The surfaces of these open fractures have accumulations of uranophane crystals and other unidentified uranyl compounds. The secondary uranium minerals are mainly distributed around copper sulfide grains. Microscopic intergrowths of copper sulfides and uranyl compounds also have been observed. Groundwater samples were collected from the vicinity of the Cu samples. The hydrogeochemical features of these samples indicate that the present conditions are oxidising. The minimum age of U(VI) transport and deposition is about 200 000 years. This age is indicated by 234U/238U and 230Th/234U activity ratios of uranophane. The age of the hexavalent uranium precipitation may be somewhat later than the last influxes and/or demobilisation of sulfur.

The mineral assemblage native copper – copper oxide (cuprite) occurs only at one depth interval within altered granite pegmatite. The fracture surface was coated by smectite. The content of uranium in smectite was 69–75 ppm U. The 234U/238U and 230Th/234U activity ratios of smectite showed that it has been exposed to recent groundwaters (e.g., during the last million years). The pH of the groundwater at this interval was near neutral (6.9). The copper grains present at this fracture surface were as large as 1 mm in diameter and had rims of cuprite of 0.01 to 0.1 mm thick. The smallest grains were totally oxidised.
Passive Films and Blistering of Titanium
Authors: Bedrossian Peter J. , Farmer Joseph C. , McCright R. Daniel , Phinney Douglas L. , Estill John C.
Published: April 2011

Coupons of titanium alloys under consideration as components of the Engineered Barrier System in the proposed repository at Yucca Mountain have been evaluatedfor their passive film composition and stability. Oxide depths and compositions on specimens exposed in long-term corrosion testing for one year were determined with x-ray photoemission spectroscopy. The specimens removed from long-term testing, as well as separate coupons polarized cathodically in an electrochemical cell, exhibited blistering associated with hydride formation in both scanning electron microscopy and atomic force microscopy.
Low Temperature Kinetics for the Growth and Decay of Band-Tallcarriers and Dangling Bonds in Hydrogenated Amorphous Silicon
Authors: Schultz Niko , Taylor P.C.
Published: April 2011

In hydrogenated amorphous silicon (a-Si:H), the kinetics of the light induced production of silicon dangling bonds and long-lived band-tail electrons and holes has been measured at temperatures between 65 and 340 K using light induced electron spin resonance (LESR). Below about 150 K the measurement of Si dangling bonds is masked by the accumulation of long-lived band-tail carriers. The kinetics of the growth and decay of these long-lived, trapped band-tail carriers consists of very fast components (τ < ms) and very long components (τ > h). Optical quenching of these long-lived carriers is not efficient at quenching energies of 0.6 eV. Afler removal of these long-lived band tail carriers by annealing at about 250 K we find that the total production of silicon dangling bonds at 65 K after 10 h of illumination is about a factor of five less than at 340 K. The dangling bond production resulting from 10 h of illumination is well fit to an underlying mechanism that, if thermally activated, exhibits an activation energy of approximately 10 meV.
Structural Changes and Hydrogen Motion in A-SI:H Observed by Proton Nmr
Authors: Baugh Jonathan , Han Daxing , Wang Qi , Wu Yue
Published: April 2011

Proton nuclear magnetic resonance (NMR) is applied to investigate hydrogen dynamics and microstructures in a-Si:H. In addition to the generic broad and narrow lines observed in all aSi:H, an additional narrow line (about 1 kHz wide) is observed as the temperature is raised above RT. This narrow line is shifted to the up-field by about 4 ppm with respect to the generic narrow line of a few kHz in width. Below 150°C, the change of the proton spectrum with temperature is reversible; the hydrogen associated with this additional narrow line is shown to originate from hydrogen originally associated with the broad line. The spin-lattice relaxation time T1 of this up-field shifted narrow line is about 0.4 s. This short T1, along with its small linewidth, suggests that this line is associated with molecular hydrogen, possibly trapped in sites of atomic dimensions.
Current Noise Measurements of Surface Defect States in Amorphous Silicon
Authors: West P. W. , Kakalios J.
Published: April 2011

Measurements of conductance fluctuations in coplanar hydrogenated amorphous silicon (a-Si:H) are reported as a function of surface etching treatments. The noise power spectrum displays a broadened Lorentzian peak, associated with surface damage by CF4 reactive ion etching (RIE), whereas surface etches using ion milling or wet chemicals remove the Lorentzian spectral feature and only a 1/f spectral form for frequency f is observed. The Lorentzian spectral feature can be explained by trapping-detrapping from surface states induced by the RIE etch, which cause fluctuations in the depletion width of the space charge region near the film surface. The thermally activated Lorentzian comer frequency is a measure of the degree of band bending and the Fermi energy at the thin film surface.
An Experimental Evaluation of Modulated Photocurrent Spectroscopy as A Density of States Probe
Authors: Reynolds S. , Main C. , Webb D.P. , Rose M.J.
Published: April 2011

Modulated and Fourier-transformed transient photocurrent (MPC and TPC-FT) spectroscopies have been evaluated through a study of the density and capture properties of localised states in as-prepared and light-soaked PECVD a-Si:H samples over a range of temperatures and optical excitations. Both techniques return a conduction band tail state characteristic energy of approximately 22 meV. However, defect state spectra differ in detail and are strongly influenced by dc optical excitation. A feature correlating with the quasi-Fermi level position is observed, but the capture coefficient implied (of order 10-6 cm3 s-1) is some two orders greater than that calculated from thermal activation of emission frequencies. Such a value would suggest an implausibly low absolute density of defects. Possible explanations are briefly discussed and additional investigations proposed.
Capacitance Spectroscopy of Defects in a-SI:H/c-SI Heterostructures
Authors: Rösch M. , Unold T. , Pointmayer R. , Bauer G.H.
Published: April 2011

We investigate defects at the interface in heterodiodes of hydrogenated amorphous silicon and monocrystalline silicon by frequency and temperature dependent capacitance measurements. The interpretation of the experimental results is supported by numerical simulations of capacitance experiments via transient calculations of defect charging and decharging in the diodes. A defined variation of waver surface treatments prior to amorphous silicon deposition shows a clear correlation of interface defects determined by capacitance measurements with current-voltage characteristics.
Hydrogen Dilution Effect on the Crystallinity of Silicon Films Grow by Hot Wire Cell Method
Authors: Ichikawa M. , Takeshita J. , Yamada A. , Konagai M.
Published: April 2011

A new process, the Hot Wire Cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low temperature and high growth rate. In the Hot Wire Cell method, reactant gases are decomposed as a result of reacting with a heated tungsten filament placed near to a substrate and polycrystalline silicon films can be deposited at a growth rate of 1.2nm/s without hydrogen dilution and 0.9nm/s with the use hydrogen dilution. The film crystallinity changed from amorphous to polycrystalline due to the addition of hydrogen, thus hydrogen dilution was effective for improving film crystallinity. Furthermore, we obtained (220) oriented polycrystalline silicon thin films with a 90% crystal fraction by the use of hydrogen dilution. These results showed that the Hot Wire Cell method is promising for the deposition of device-grade polycrystalline silicon films for photovoltaic applications.
Growth and Properties of Microcrystalline Germanium-Carbide Alloys
Authors: Herrold Jason , Dalal Vlkram L.
Published: April 2011

We report on the preparation and properties of microcrystalline (Ge,C) alloys grown using a remote, reactive H plasma beam deposition technique. The plasma beam was generated using an ECR reactor. The films were grown at low temperatures (300 - 400°C) on glass, stainless steel and c-Si substrates. The optical properties of the films were measured using spectrophotometer and two-beam photoconductivity techniques. The C content was measured using XPS techniques. We find up to 3% C incorporation in the lattice. The degree of crystallinity determined using Raman spectroscopy was very good. X-ray diffraction measurements indicated grain size in the range of a few tens of nm. The best crystallinity was obtained on conducting substrates, indicating the importance of H ion bombardment in promoting crystallinity. We find that the absorption curves for increasing C content remain similar to the curves for c-Ge, but are shifted to higher energies. Thus, the absorption curve is sharper than for c-Si in the same energy range. The defect densities remain low for the range of C content measured. Because of its sharper absorption curve compared with c-Si, the material may be attractive for photovoltaic energy conversion.
Resistless Patterning of Hydrogenated Amorphous Silicon Films
Authors: Hollingsworth Russell E. , Hemdon Mary K. , Collins Reuben T. , Benson J.D. , Dinan J.H. , Johnson J.N.
Published: April 2011

Practical methods for directly patterning hydrogenated amorphous silicon (a-Si:H) films have been developed. Direct patterning involves selectively oxidizing the hydrogen passivated a-Si:H surface or laser crystallization of the bulk. The oxide or polycrystalline layer formed in this way then becomes a mask for subsequent hydrogen plasma etching. Methods for selective oxidation of the a-Si:H surface have been extensively studied. Examination of the pattern generation threshold dose for excitation wavelengths from 248 to 633nm provides indirect evidence for electron-hole recombination breaking of the silicon-hydrogen bond. An additional hydrogen removal mechanism was observed whereby simple proximity of a tapered fiber optic probe less than 30nm from the sample surface resulted in pattern generation. Patterns were generated in both intrinsic and doped a-Si:H films by several means, including contact printing with a mask aligner, in situ projection lithography with an excimer laser, and direct writing with a near-field scanning optical microscope (NSOM). Direct patterning of a-Si:H films has a wide range of potential applications. We have demonstrated a-Si:H as an in situ photoresist material for patterning HgCdTe infrared detector arrays with all process steps done in vacuum. We have also demonstrated 100nm line widths using NSOM writing with a photolithography goal. Direct patterning of a-Si:H could simplify the manufacturing of thin film transistors, or other devices that require patterned silicon films.
Near Infrared Response of Amorphous Silicon Detector Grown with Microcompensated Absorber Layer
Authors: Caputo D. , Cesare G. De , Nascetti A. , Palma F. , Tucci M.
Published: April 2011

In this work we demonstrate that radiation up to 2 μm induces photocurrent in a single junction amorphous silicon structure at room temperature. The absorber layer is a microcompensated film deposited using very low concentrations of dopant species. Device operation is based on optical excitation of thermal generated carriers from trap states toward valence and conduction band in the high electric field region of the structure. Transient and frequency response under different bias voltages and illuminations conditions are presented. The possibility to use the infrared sensor in low bit rate communication systems has been demostrated by including our detector in a front-end system and measuring its frequency responce.

Quantum efficiency measurement have been reproduced with a numerical model, able to take into account sub-band gap absorption into single films. Model results indicate the presence of a large valence band tail and a high number of dangling bonds and shallow defects ascribed to the presence of dopant atoms.
A High-Voltage Hydrogenated Amorphous Silicon Thin-Film Transistor for Reflective Active-Matrix Cholesteric LCD
Authors: Nahm J.Y. , Lan J.H. , Kanicki J.
Published: April 2011

A high-voltage hydrogenated amorphous silicon thin film transistor (H-V a-Si:H TFT) with thick double layer gate insulator (∼0.95 μm) has been developed for reflective active-matrix cholesteric liquid crystal displays. The double layer gate insulator consists of 0.85 and 0.10 μm thick benzocyclobutene and hydrogenated amorphous silicon nitride, respectively. This HV a-Si:H TFT operates at the gate-tosource and drain-to-source biases up to 100V without any serious leakage current degradation and device breakdown.
Melting and resolidification dynamics of a-Si and poly-Si thin films during excimer laser annealing
Authors: Hatano Mutsuko , Moon Seungjae , Lee Minghong , Suzuki Kenkichi , Grigoropoulos Costas P.
Published: April 2011

The liquid-solid interface motion and the temperature history of thin Si films during excimer laser annealing are observed by in-situ experiments combining time-resolved (∼Ins) electrical conductance, optical reflectance/transmittance at visible and near-IR wavelength, and thermal emission measurements. The existence of partial and complete melting regimes is elucidated. In the partial melting regime, the maximum temperature remains close to the melting point of aSi, since the laser energy is consumed on the latent heat of phase-change. In the complete melting regime, substantial supercooling, followed by homogeneous nucleation is observed. These phase transformations are consistent with the recrystallized poly-Si morphologies.
Physical Characteristics of Very Low Temperature Anodic Oxides of Polycrystalline Si Films
Authors: Plantier H. , Pelletier J. , Devine R. A. B. , Vincent G.
Published: April 2011

Controlled oxidation of polycrystalline and single crystal silicon has been carried out at temperatures < 100 °C using microwave excited, plasma assisted anodisation. Oxide thicknesses up to 30 nm have been obtained in times ∼ 15 minutes. The growth kinetics are similar for both types of Si. The results of infrared absorption measurements clearly indicate that the anodic oxides have a network structure significantly different to that of high temperature, thermally grown oxides and this is partly due to ultra-violet radiation present in the plasma during growth. Electrical measurements (CV, IV) are underway on simple MOS capacitors. The results indicate that the plasma oxides have acceptable levels of fixed oxide charge but that the breakdown electric fields are presently low. Preliminary data suggests that these very low temperature oxides are suitable for TFT applications though some technical problems need to be addressed. Given these reservations the processing of large area flat panel displays using this type of plasma assisted oxidation is perfectly feasible.
Exciton Dissociation and Mobility in Conducting Polymers and Oligomers
Authors: Jain S. C. , Geens W. , Poortmans J. , Mehra A. , Nijs J. , Mertens R.
Published: April 2011

A survey of the literature shows that reverse bias current IL of an illuminated conducting polymer Schottky diode increases with voltage. We suggest that this increase in IL with applied reverse bias is due to a combination of two factors: (1) increase of mobility, and (2) dissociation of excitons. The experimental results agree with the values of IL calculated using either of the two mechanisms. Therefore it is difficult to determine the relative importance of the two mechanisms. The relative importance can be determined only if reliable values of material parameters are available. We have fabricated Schottky diodes and FETs using 5-ring n-octyloxy-substituted oligo[p-phenylene vinylene](Ooct-OPV5) and C60. The mobility of the oligomer derived from the measured characteristics of the diode is 3.29×10−7cm2/Vs and from the FET data, 3.24 × 10−4 cm2/Vs. These results show that the mobility (and other material parameters) depend strongly on the structure of the device. Therefore for interpreting the IL data it is important to measure the material parameters on the same structure on which IL measurements are made.
Surface Treatment Effects on the Electron Emission Characteristics of Ultra Thin AIN Coated Molybdenum Tips
Authors: Kang D. H. , Zhirnov V. V. , Wojak G. J. , Sanwald R. C. , Park M. , Hren J. J. , Cuomo J. J.
Published: April 2011

Astract

Although plasma surface treatment has proven useful for both metal and diamond field emitters, there is at this time no study of the effect of plasma treatment on the field emission from a thin layer of AIN. In this study a 7 nm film of AIN deposited on a molybdenum emitter tip was treated with different plasmas (H2, O2, and N2). All surface treatments and measurements were carried out by using an in-situ technique inside the magnetron sputtering chamber. Changes in the measured I-V curves were not observed from the emitter after treatment with either hydrogen or nitrogen plasmas, but were observed after oxygen plasma treatment. In addition, it was found that a very thin coating of aluminum over the AIN further enhanced the emission characteristics. We propose that the lowering of the surface barrier is due to the electronegativity difference of the species forming the surface layer.
Mechanism of Efficient Field Emission from Carbon Nanotubes
Authors: Zhu W. , Bower C. , Zhou O. , Kochanski G. P. , Jin S.
Published: April 2011

We report observation of electron emission from individual carbon nanotubes. Two classes of emitters are observed, one emitting electrons with momentum nearly parallel to the nanotube axis, the other emitting electrons with nonzero momentum perpendicular to the tube axis. The emission pattern reflects the electronic structure of a particular tube and allows us to distinguish between structurally different nanotubes. These nanotube emitters exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm2. At electric fields as low as 4-7 V/μm, they emit technologically useful current densities, e.g. 10 mA/cm2. The emission characteristics and durability of the carbon nanotube cold cathodes offer promising applications for vacuum microelectronic devices.
Nickel-Related Optical Centers in Hpht Treated Synthetic Diamonds: Photoluminescence Study
Authors: Kupriyanov I.N. , Gusev V.A. , Pal'yanov Yu.N. , Borzdov Yu.M.
Published: April 2011

The results of a photoluminescence (PL) study of synthetic diamonds annealed over the temperature range 1500-2500°C are presented. For nickel- and nitrogen-containing diamonds a number of new, previously undocumented PL systems with zero-phonon lines (ZPLs) at 598.5, 622.6, 638.9, 752.1 and 877.2 nm has been found. Photoluminescence excitation (PLE) spectra for PL systems with ZPLs at 727.1 and 746.6 nm have been measured for the first time. Preliminary results on annealing behavior of the PL centers have been obtained. Of all the PL systems observed only that ones with ZPLs at 752.1 and 793.0 nm together with the S2 and S3 optical centers are present in diamonds annealed at temperatures higher than 2200°C.
First-Principles Calculation of Optical Spectra for Transition-Metal Impurities Doped in Zno and Zns
Authors: Ogasawara K. , Ishih T. , Oba F. , Tanaka I. , Adachi H.
Published: April 2011

The multiplet structures of Co2+ doped in ZnO, Co2+ doped in ZnS and Ni2+ doped in ZnS are calculated from first principles using the recently developed discrete variational-multielectron (DV-ME) method, in which the matrix elements of electron-electron repulsion are calculated numerically using the molecular orbitals obtained by cluster calculations. The transition probabilities between the multiplet states are also calculated from first principles using the many-electron wave functions obtained by the DV-ME calculations. The optical spectra of these materials are well reproduced, indicating that the effects of covalency and configuration interactions are properly taken into account in the present calculations.
A Sims Study of Electroluminescent Phosphors: SrS:Cu
Authors: Zhang Lily H. , Wang Larry , Tong Wusheng , Xin YongBao
Published: April 2011

This study has used secondary ion mass spectrometry (SIMS) as a technique for thin film EL material characterization. It has shown that the Cu dopant concentration in the SrS films directly correlates with the luminescent brightness of the EL devices. A series of SrS:Cu,Y were grown using MBE to study the Y co-doping effects. It has been found that Y peak concentration and areal density in the SrS increased as the Y evaporation cell temperature was increased. The maximum PL intensity was found in the sample grown in the middle of the Y cell temperature range used. The Y co-doping has shown to reduce the thermal quenching effects in SrS EL devices. Therefore, in this series of samples, a good correlation has been found between Y and Cu concentration and the EL device performance characteristics.
Self-Assembly Processes for Organic Led Transport Layers and Electrode Passivation
Authors: Malinsky J.E. , Li W. , Wang Q. , Cui J. , Chou H. , Marks T.J. , Jabbour G.E. , Shaheen S.E. , Anderson J.D. , Lee P. , Richter A.G. , Kippelen B. , Dutta P. , Peyghambarian N. , Armstrong N.
Published: April 2011

This contribution describes the use of self-limiting siloxane chemisoroption processes to self-assemble building blocks for the modification of vacuum-deposited organic LED (OLED) devices. One approach consists of the use of self-assembling OLED hole transport materials for application in hybrid self-assembled + vapor deposited two-layer devices. Another approach involves the application of self-limiting, chemisorptive self-assembly techniques to introduce thin dielectric films between the anode and hole transport layer of a vapor deposited two-layer OLED device.
Determination of the Coefficient of Piezoresistivity in Aluminum Alloy Interconnect Structures
Authors: Reilly Christopher J. , E John , Jr Sanchez
Published: April 2011

The resistance of Al thin film metallization interconnects has been measured during temperature cycling where thermal expansion mismatch between the Al and the substrate and passivation is used to induce interconnect strain. Both passivated and unpassivated interconnects were thermally cycled between 80K and 373K. The coefficient of piezoresistivity, defined as dp/dE, where ρ= resistivity and εv = volumetric strain, is measured to be 2.0 10−5Ω-cm in tension, consistent with previous results determined in bulk Al under hydrostatic pressure. We describe a model which incorporates the thermal, geometrical, and piezoresistance effects on the measured interconnect resistance during temperature changes. Thermal expansion mismatch strains and the degree of interconnect constraint by the rigid surrounding materials, which vary with interconnect aspect ratio, are considered. The application of piezoresistance analysis to measurement of local stresses and stress relaxation in advanced interconnects will be described.
Sidewall-Fence-Free Pt Etching with an Ar/O2 Mixed Gas Plasma
Authors: Chiang M.-C. , Pan F.-M. , Wei T.-C. , Chien H.-C. , Liou J.-S. , Chiu H.-K.
Published: April 2011

Platinum has been successfully patterned with a TiN mask in an Ar/O2 plasma.No fence residue was observed on the etched Pt pattern if the thickness of the TiN mask was less than 300 nm. During the etching process, titanium dioxide and platinum oxides were formed and redeposited on the mask and the Pt structure in the Ar/O2 plasma according to Auger and XPS analyses. The etch rates of Pt and TiN films decreased with increasing oxygen concentration in the Ar/O2 gas mixture. The addition of O2 inthe gas mixture could improve the sidewall slope of the etched Pt structure. This was ascribed to redeposited platinum oxides and TiO2 on the sidewall, which makes the sidewall more resistant to ion etching at a higher concentration of oxygen.
PECVD of Low Dielectric Constant Fluorine-Doped SiO2 Using TICS/O2/TFAA with In-Situ Annealing
Authors: Idris I. , Sugrura O.
Published: April 2011

A fluorine-doped silicon dioxide (SiO2:F) film with dielectric constant as low as 3.0 was deposited by introducing trifluoroacetic anhydride ((CF3CO) 2O TFAA) into our hydrogen-free SiO2 PECVD system using tetraisocyanatesilane (Si(NCO)4 TICS) and O2. The film was deposited at 100°C and 0.5 Torr. It was annealed by in-situ, 0.2 Torr O2 plasma treatment at 400°C for 1 hour, as well as post-metallization annealing in N2 ambient at 400°C for 1 hour. The refractive index and the ratio of infrared absorbance of Si-F/Si-O-Si peak intensity were 1.37 and 11.7%, respectively. No peak related to water absorption was clearly observed in its infrared spectrum even after contact-holes opened and dipped into boiling water for 2 hours.
Reliability of Electrodeposited Copper and Ecrcvd Siof Films for Multilevel Metallization
Authors: Lee Seoghyeong , Kim Yong-An , Lee Kyoung-Woo , Sohn Seil , Kim Young-Il , Yang Sung-Hoon , Oh Kyunghui , Park Jong-Wan
Published: April 2011

The effect of a post plasma treatment on the dielectric properties and reliability of fluorine doped silicon oxide (SiOF) films deposited by electron cyclotron resonance chemical vapor deposition (ECRCVD) was studied. Also, the thermal stability of an electrodeposited Cu / sputtered Ta interconnect system with SiOF intermetal dielectrics was examined by annealing in a vacuum furnace. The stability of the dielectric constant of SiOF films was improved by O2 post plasma treatment. Surface modification by the plasma treatment was effective in prevention of water absorption. The Cu/Ta/SiOF/Si system was thermally stable at least up to 500°C for 3h. For the Cu/Ta/SiOF/Si multilayer structure, the plasma treatment seemed to play a big role in suppressing the interdiffusion between SiOF and metal interconnects. By C-V measurement, the electrical stability of the Cu/Ta/SiOF/Si multilayer structure was found to be stable up to 500°C for 2 h.
Mask-Induced Strain in Gaas Layers Grown by Liquid Phase Epitaxial Lateral Overgrowth
Authors: Zytkiewicz Z.R. , Domagala J. , Dobosz D.
Published: April 2011

X-ray diffraction has been used to study the mask-induced strain in GaAs layers grown by the liquid phase epitaxial lateral overgrowth (ELO) on (100) GaAs substrates. SiO2 mask has been investigated to produce seeding areas for the ELO growth. It has been found that SiO2 attracts the ELO layers, which leads to their pronounced bending towards the oxide film and to the appearance of vertical strain in laterally grown parts of ELO. When SiO2 is removed by selective etching this strain disappears. We show evidence that the bending of ELO layers is reduced when the density of substrate dislocations is increased. This is explained as being due to enhancement of the initial vertical growth rate by dislocations supplying steps on the upper surface of ELO.
Epitaxial Growth of Atomically Flat Spin Dependent Tunneling Junctions
Authors: Li Y. , Wang S. X. , Mancoff F. B. , Clemens B. M.
Published: April 2011

Spin dependent tunneling junctions with epitaxially grown underlayers have been investigated to examine the possibility of achieving very flat and uniform barrier layers. Pt/Ni80Fe20 /Fe50Mn50/Ni80Fe20layers were deposited on sapphire (0001) substrates at different temperatures and monitored by in-situ reflection high energy electron diffraction (RHEED). The surface morphology has been found to depend strongly on the growth temperature. X-ray diffraction and magnetic hysteresis loop measurements were also performed to characterize the film structures
A Formation of SiO2/4H-SiC Interface by Oxidizing Deposited Poly-Si and High Temperature Hydrogen Annealing
Authors: Fukuda K. , Sakamoto K. , Nagai K. , Sekigawa T. , Yoshida S. , Arai K.
Published: April 2011

A formation of SiO2/4H-SiC interfaces by oxidizing deposited poly-Si on a 4H-SiC substrate and high temperature hydrogen annealing at low pressure ( 8.5×102 Pa ) has been investigated. The oxidation rate of deposited poly-Si was approximately 100 times faster than that of a SiC. Hydrogen annealing more effectively reduced the flat band voltage shift ( ΔVfb ) of the 4H-SiC MOS structure than argon and vacuum annealing. Moreover, the good SiO2/4H-SiC interface was formed because ΔVfb decreased as the oxidation temperature increased.
Polytype Stability and Defect Reduction in 4H-SiC Crystals Grown via Sublimation Technique
Authors: Yakimova R. , Iakimov T. , Syvdijarvi M. , Jacobsson H. , Råback P. , Vehanen A. , Janzén E.
Published: April 2011

Reproducible growth of 4H-SiC with good crystalline quality has been obtained in a temperature interval around 2350°C and on 4H-SiC C-face seeds. It has been observed that morphological instability may appear at the initial stage of growth, causing formation of defects. Experimental evidence has been found that supersaturation and surface kinetics are responsible for the polytype stability, while growth front and growth mode address defect reduction. An explanation of the findings has been suggested. It has been shown that starting the growth with a relatively low growth rate ( ≈ 100 μm/h ) can be beneficial for the crystal quality.
AlGaN Microwave Power HFETs on Insulating SiC Substrates
Authors: Sullivan Gerry , Gertner Ed , Pittman Richard , Chen Mary , Pierson Richard , Higgins Aiden , Chen Qisheng , Yang Jin-Wu , Smith R. Peter , Perez Raul , Khan Abdur , Redwing Joan , McDermott Brian
Published: April 2011

AIGaN HFETs are attractive devices for high power microwave applications. Sapphire, which is the typical substrate for AlGaN epitaxy, has a very poor thermal conductivity, and the power performance of AlGaN HFETs fabricated on sapphire substrates is severely limited due to this large thermal impedance. We report on HFETs fabricated on high thermal conductivity electrically insulating SiC substrates. Excellent RF power performance for large area devices is demonstrated. On-wafer CW measurements at 10 GHz of a 320 micron wide FET resulted in an RF power density of 2.8 Watts/mm, and measurements of a 1280 micron wide FET resulted in a total power of 2.3 Watts. On-wafer pulsed measurements, at 8 GHz, of a 1280 micron wide FET resulted in a total power of 3.9 Watts. Design of a hybrid microwave amplifier will be discussed.
Thick Oxide Layers on N and P SiC Wafers by a Depo-Conversion Technique
Authors: Zhang Q. , Madangarli V. , Khlebnikov I. , Soloviev S. , Sudarshan T. S.
Published: April 2011

The electrical properties of thick oxide layers on n and p-type 6H-SiC obtained by a depoconversion technique are presented. High frequency capacitance-voltage measurements on MOS capacitors with a ∼ 3000 Å thick oxide indicates an effective charge density comparable to that of MOS capacitors with thermal oxide. The breakdown field of the depo-converted oxide obtained using a ramp response technique indicates a good quality oxide with average values in excess of 6 MV/cm on p-type SiC and 9 MV/cm on n-type SiC. The oxide breakdown field was observed to decrease with increase in MOS capacitor diameter.
Preparation of Conductive Tungsten Carbide Layers for SiC High Temperature Applications
Authors: Romanus H. , Cimalla V. , Ahmed S. I. , Schaefer J. A. , Ecke G. , Avci R. , Spiess L.
Published: April 2011

Thin tungsten carbide films of different compositions were prepared by DC magnetron sputtering of tungsten and carbon and subsequent annealing in different environments. The onset of carbide formation was around 800°C. Annealing in a pure hydrogen ambient generally results in carbon depletion in the layers with the formation of a dominant W2C phase. Adding propane enhances the carbon content in the layers and stimulates the formation of the WC phase. On silicon nitride substrates, variation of the propane concentration in an annealing environment allows a continuous alteration of the layer structure between polycrystalline single phase WC and a mixed layer with dominant W2C and with it, the adjustment of different values of the electrical resistance. In contrast, on thin (100)SiC layers a textured W2C phase was grown after annealing in propane/hydrogen at 900°C whereas at higher temperatures the formation of silicides was observed. In addition, the chemical composition and the temperature dependence of the electrical specific resistance were investigated and are also discussed.
In Situ Surface Passivation of GaAs by Thermal Nitridation using Metalorganic Vapor Phase Epitaxy
Authors: Sun Jingxi , Himpsel F. J. , Ellis A. B. , Kuech T. F.
Published: April 2011

An ammonia-based, in situ passivation of GaAs surfaces conducted within a metalorganic vapor phase epitaxy reactor is present. The shift of the GaAs surface Fermi level, and hence the surface charge density, resulting from this in situ passivation, has been studied using photoreflectance (PR) spectroscopy. Samples consisting of an undoped GaAs layer on highly doped n-GaAs (UN+) and p-GaAs (UP+) structures allow for the exact determination of the surface Fermi level position using PR These structures were grown by MOVPE and in situ thermal nitridation was performed after growth within the MOVPE system without exposure to the air. After nitridation, the surface Fermi level can be shifted by ∼ 0.23 eV towards the conduction band edge for UN+ structures and by ∼ 0.11 eV towards the valence band edge for UP+ structures from the normally mid-gap ‘pinned’ positions.