bluuebunny/crossref_metadata_2025 huggingface dataset:
Design of Holographic Recording Conditions with Optical Densities of Thick Azopolymer Films
Authors: Yasuda Shin , Kawano Katsunori , Minabe Jiro , Maruyama Tatsuya , Hayashi Kazuhiro , Ogasawara Yasuhiro
Published: April 2011

We evaluated the relation between the degradation of the Bragg selectivity and the optical densities of the films by use of the shift-multiplexing method with a spherical reference wave. Our recording media were prepared by blending methylazobenzene-containing polymer (MAP) and cyanobiphenyl-containing polymer (CBP). We fabricated four kinds of 250μm-thick films whose optical densities were increased with MAP concentrations that ranged from 10wt% to 70wt%. As a result of the evaluation, the film with a higher optical density degraded the Bragg selectivity more significantly. The degradation of the Bragg selectivity affected the bit error rate (BER). In addition, we obtained a guide to holographic recording conditions for achieving a low BER as to the films with non-negligible optical densities.
Nuclear Glass Alteration in Clay: Assessment of the Effect of Direct Contact between the Materials through Experimental and Modeling Approach
Authors: Gin S. , Thierry F. , Minet Y.
Published: April 2011

A new approach is proposed for discussing the reliability and predictability of the models intended to evaluate the performance of glass packages under geological disposal conditions. The r(t) model developed by the CEA is used in this study to simulate original laboratory test results, and the validity of the predictions is then verified experimentally. This approach allows us to check that the key mechanisms are correctly simulated, even if they are simplified. A review of the experimental results (glass alteration kinetics and morphology of the alteration film) suggests that physically separating the glass and clay under geological disposal conditions could considerably diminish the silica pump effect attributed to reactive clays such as Boom clay or FoCa7 clay.
An Investigation into the Oxidation State of Molybdenum in Simplifies High Level Nuclear Waste Glass Compositions
Authors: Short R. J. , Hand R. J. , Hyatt N. C.
Published: April 2011

The redox state of Mo in glasses containing simplified simulated high level nuclear waste (HLW) streams has been investigated using Electron Spin Resonance (ESR) and X-ray Photoelectron Spectroscopy (XPS). Melts performed in highly oxidising or neutral (air) atmospheres contained Mo6+, but our study indicates that the Mo can be at least partially reduced to Mo4+ or Mo3+ by melting in a reducing atmosphere. The implications for glasses containing a full simulated HLW stream are that the formation of detrimental crystalline phases containing Mo6+ upon heat treatment may be avoided by reducing the oxidation state of Mo during melting.
Incorporation of AlCl3 and As2O3 in Composite Cement Systems
Authors: Hill J , Sharp J H
Published: April 2011

As part of an investigation into the consequences of adding inorganic metal salts to composite cements, based on blastfurnace slag (BFS) and pulverised fuel ash (PFA), the effect on the hydration behaviour of adding AlCl3 or As2O3 to the mix water of a number of cementitious systems was investigated using isothermal conduction calorimetry (ICC) and x-ray diffraction (XRD). Four cement systems were investigated; ordinary Portland cement (OPC), 3:1 BFS: OPC, 9:1 BFS:OPC and 3:1 PFA:OPC. AlCl3·7H2O and As2O3 were added to the mix water at 0.1% and 1% concentrations. Results from ICC indicated that the addition of As2O3 had very little effect on the setting and early hydration of any of the cements. AlCl3, however, inhibited the setting of the composite cements at the 1% addition level but accelerated the OPC at 0.1%. The expected hydration products were observed, with the additional observation of Friedel's salt in the presence of aluminium chloride additions and vaterite in both the aluminium and arsenic-containing systems after 180 days hydration.
Corrosion of Copper in 1 M NaCl under Strictly Anoxic Conditions
Authors: Bojinov Martin , Laitinen Timo , Mäkelä Kari , Snellman Margit , Werme Lars
Published: April 2011

The corrosion of copper in 1 M NaCl has been investigated at room temperature and at 80°C by an on-line resistance probe, solution analysis of dissolved copper and weight loss measurements. At room temperature, corrosion of the copper as indicated by an increase of the probe resistance has been detected during the first 60–80 h of exposure. After the corrosion potential has reached the immunity region of Cu, a decrease in the resistance of the sensor is observed, probably due to redeposition of Cu from the solution. It can be concluded that the corrosion of copper at room temperature virtually stops after 60–80 h due to the anoxic conditions established in the experiments. The similarity in the trends of the corrosion and redox potential during exposure seems to indicate that Cu(II) contained in the native oxide on Cu and/or formed by homogeneous oxidation of Cu(I) during the initial oxic period of exposure acts as a redox-agent determining the chemical conditions in the closed system.
The Corrosion Rate of Copper in a Test Parcel at the Äspö Hard Rock Laboratory
Authors: Rosborg Bo , Eden David , Karnland Ola , Pan Jinshan , Werme Lars
Published: April 2011

Cylindrical copper electrodes have been installed in a test parcel at the Äspö Hard Rock Laboratory and real-time corrosion monitoring was initiated in May 2001. The test parcel was emplaced on October 29, 1999, and will be retrieved in 2004. The three electrodes, each of about 100 cm2 surface area, are installed in bentonite block 36, where the temperature is about 24°C.

The corrosion monitoring is performed with linear polarization resistance (LPR), harmonic distortion analysis (HDA) and electrochemical noise techniques. A value on the Stern-Geary coefficient is required to calculate the corrosion rate from the measured LPR data. A default value of 10.3 mV has been used, but an actual value can in fact be obtained from the HDA. The corrosion rate will be overestimated if the frequency of the voltage perturbation for the LPR measurements is not low enough. Electrode impedance measurements have been performed to verify this.

Two and a half year after emplacement the recorded corrosion rate of copper in the above bentonite block amounts to about 2.2 μm per year (using a default value of 10.3 mV for the Stern-Geary coefficient and a 0.01 Hz voltage perturbation frequency). The actual corrosion rate is estimated to less than 0.7 μm per year (considering a measured Stern-Geary coefficient of 6.5 mV and the findings from the electrode impedance measurements).
Excavation Damage Zone in the Experimental Deposition Holes at Äspö and Comparison to Existing Data
Authors: Autio Jorma , Malmlund Hanna , Hjerpe Thomas , Kelokaski Maarit , Siitari-Kauppi Marja
Published: April 2011

Disposal in deep, stable bedrock is currently one concept for isolating high-level wastes from the environment. Repository for high-level waste in rock excavated using different drilling techniques is surrounded by an excavation damaged zone (EDZ) which properties have been changed. The micro fracturing of samples taken from the experimental deposition holes in the underground Hard Rock Laboratory at Äspö were investigated by the 14C polymethylmetha-crylate (14C-PMMA) method and scanning electron microscopy (SEM) to evaluate the impact of EDZ on migration. The porosity of the damaged rock zone is clearly higher than the porosity of undisturbed rock. The thickness of the crushed zone with significantly higher porosity is a few millimetres and the average depth of the damaged zone (i.e. a clear increase in porosity found) is from 5 to 20 mm from the hole wall. The apertures of the inter- and intragranular fractures in the crushed zone varied from 5 to 30 μm according to SEM examination. Earlier results of porosity, diffusivity and permeability measurements in granites were compiled and the results of the porosity values of Äspö diorite were compared to the porosity values measured in other types of granites. The results were compiled in permeability-diffusivity-porosity space and were found to form a plane that could be used to estimate the range of diffusivity and permeability of the Äspö diorite.
Migration Behavior of Plutonium in Compacted Bentonite under Reducing Condition by Using Electromigration
Authors: Idemitsu Kazuya , Xia Xiaobin , Kikuchi Yoshiro , Inagaki Yaohiro , Arima Tatsumi , Mitsugashira T. , Hara M. , Suzuki Y.
Published: April 2011

Carbon steel is one of the candidate overpack materials for high-level waste disposal and is expected to assure complete containment of vitrified waste glass during an initial period of 1000 years in Japan. Carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The reducing condition will be expected to retard the migration of redox-sensitive radionuclides by lowering their solubilities. Therefore, the presence of corrosion products of iron in buffer material is important to discuss the migration behavior of redox-sensitive radionuclides. Plutonium electromigration experiments in bentonite have been carried out with source of iron ions supplied by anode corrosion of iron coupon. Plutonium migrated from the iron anode toward cathode as deep as 1 mm of the interior of bentonite within 24 h. Thus plutonium chemical species would have positive charge and were estimated as PuOH2+or PuCl2+by the thermodynamic calculation.
Modelling of Solute Transport under Flow Conditions Varying in Time, Using the Channel Network Model
Authors: Moreno Luis , Crawford James , Neretnieks Ivars
Published: April 2011

Transport of radionuclides from a repository to the biosphere is calculated in the presence of varying boundary conditions. This may occur in Sweden, for example, during post-glacial rebound. To calculate the transport of radionuclides, the flow field has to be calculated at each time step. An alternative approach is to determine the flow field at certain time intervals; i.e., modelling the dynamic flow system as a sequence of punctuated steady states. This can be done in a rather simple way by using the Channel Network model (CNM), in which radionuclide transport is calculated by particle tracking. Intuitively one would expect such a simple procedure to be permissible and lead to small errors if the time intervals are so short that only small changes in flow rates and flow directions take place. In this paper, the technique is applied to a study case. This describes a repository located at a sub sea level location outside the coastline; where, owing to the land lift that is taking place in Scandinavia the coastline is advancing towards the repository. An important issue to be determined is the minimum number of time intervals that need to be used in order to obtain a reliable solution to radionuclide transport. It is found that the number of time intervals needed is strongly dependent on the variation in the boundary conditions. For the changing conditions used in the example tested, a number on the order of 5–9 time intervals allows a sufficiently good representation of the transport.
Corrosion of Archaeological Artefacts from the Olviya Site in Ukraine
Authors: Demchenko Larisa V. , Zlobenko Borys P. , Manichev Vyacheslav I. , Kadoshnikov Vadim V. , Spasova Ludmila V.
Published: April 2011

Copper and bronze artefacts of Olviya archaeological collection dated the first century B.C. were the objects of this study. Susceptibility to soil corrosion of archaeological artefacts from pure copper and bronze was investigated. Detailed mineralogical and metallographic investigations were performed on specially prepared samples of metallic copper, such as cut and polished sections of a cylindrical body, etc. They reveal a complex picture of metal structure and mineralogical features that can be attributed to both original technological process of manufacturing and to alteration during the burial and weathering history. Corrosion products were researched and the thickness of the corrosion layer formed for the long period of time was determined. It is shown, that the corrosion rate of artefacts produced by casting depends on composition alloys and change of metal structure after the next mechanical-thermal processing. The soil corrosion rate of copper alloys also depends on the redox conditions in the soil of Olviya site.
A Comparison of Site Characterisation Data and Modelling Results from a Radiological Assessment of the Drigg Low Level Radioactive Waste Disposal Site
Authors: Small J. S. , Abraitis P. K. , Beadle I. R. , Johnstone T. L. , Kelly P. , Pettit C. L. , Stevens G. A.
Published: April 2011

The biogeochemical processes controlling release of radionuclides from solid low level radioactive waste (LLW) disposed at the Drigg site have been represented in a computational model in order to determine the effects of varying major element chemical parameters and solubility limits of radionuclides. The model is used to provide a source term for radiological safety assessment calculations for the groundwater pathway. Site data has been compared to the results for the initial stages of the model in order to build confidence in the conceptual and mathematical model used to derive the source term for 234U, the key dose contributor, and other radionuclides which are influenced by the biogeochemical processes. Measured aqueous concentrations of radionuclides whose release from LLW is sorption controlled are compared to the model results as a basis to discuss the uncertainty in sorption input parameters used.
Accelerated Stress Testing of a-Si:H TFTs for Amoled Displays
Authors: Sakariya Kapil , Ng Clement K.M. , Huang I-Heng , Sultana Afrin , Tao Sheng , Nathan Arokia
Published: April 2011

In this work, we have developed a method for accelerated stress testing of TFT driver circuits for AMOLED display backplane applications. Based on high current and temperature stress measurements, acceleration factors have been retrieved, which can be used to significantly reduce the testing time required to guarantee a 20000-hour display backplane lifespan.
Statistical Modeling of Grain-Enhanced Polysilicon Thin-Film Transistor in Consideration of Grain Boundary Distribution
Authors: Cheng C. F. , Poon M. C. , Kok C. W. , Chan M.
Published: April 2011

A probabilistic model to predict the statistical distribution of grain boundaries in the thin-film transistor (TFT) with an arbitrary transistor-to-grain size ratio is proposed in this paper. Performance of the TFTs, such as carrier mobility, can be estimated and the corresponding performance variation of the fabricated devices becomes predictable when the statistical distribution of grain boundaries is known. The proposed model is still applicable even when the transistor size becomes comparable to the grain size. Reliability and accuracy of the modeling results have been extensively verified by experimental data. It is believed that the model can provide design and optimization guidelines of device variation for grain-enhanced polysilicon TFT technology.
Increase of temperature and crystallinity during electrical switching in microcrystalline silicon
Authors: Hu Jian , Stradins Paul , Branz Howard M. , Wang Qi , Weinberg-Wolf J.R. , Harley E.C.T. , Lawyer Chris , Huie Brittany , Han Daxing
Published: April 2011

We investigate electrical stressing and switching in hydrogenated microcrystalline silicon (mc-Si:H) by thermal, and optical and electrical measurements of Cr/mc-Si:H/metal thin-film structures. Boron-doped microcrystalline Si films of 30-50 nm thick are deposited by hot-wire chemical vapor deposition (HWCVD) on Cr-coated glass at 160°C and contacted with Ag or Al. Switching in devices of size 5 to 30 mm is stimulated by a current-ramp from 10 nA to 50 mA. We find that the voltage across the mc-Si:H devices initially increases logarithmically with current, then saturates at 2∼3 V, and finally drops to a low value of 1 to 1.5 V. This drop indicates a permanent decrease of device resistance to below 1 kW. During current stressing, the surface temperature increases with the bias current, and the surface reflectivity changes. After switching, a small increase in crystalline fraction can be observed by micro-Raman scattering measurements. The observations suggest electrothermal processes which cause changes in microstructure of the mc-Si bulk during current stress.
Si Wire Light Emission Changes During Si/SiOx Interface Formation
Authors: Becerril-Espinoza F.G. , Torchynska T. V. , Rodríguez M. Morales , Khomenkova L. , Scherbina L.V.
Published: April 2011

Photoluminescence (PL), PL excitation, Raman scattering, IR absorption spectra as well as PSi surface morphology, have been studied as a function of Si/SiOx interface formation during PSi ageing in ambient air with the aim to reveal a PL mechanism. It is shown that fresh- prepared PSi layers created at low values of the anodization current Ia is characterized by “red” emission band centered at 1.72 eV, while the samples prepared at higher values of Ia have “orange” PL band centered at 2.00 eV. During oxidation in ambient air two processes take place at the PSi ageing: the oxidation of small size Si nano-crystallites up to their disappearing and the change of silicon oxide composition at the Si/SiOx crystallite surface. These processes initiate changes in PL and PL excitation spectra. The peak position of “red” PL band shifts to the high-energy up to 1.80-1.85 eV while the “orange” band does not change essentially.
Reversible Ordering of a-Si1-xGex by the Combined Effect of Light and Temperature
Authors: Martín P. , Torres A. , Jiménez J. , Rodríguez A. , Sangrador J. , Rodríguez T.
Published: April 2011

Amorphous Si and a-Si1-xGex (0 ≤= x ≤= 0.38) films illuminated by above bandgap light, and heated between 110 and 180°C, undergoes an ordering transition, which is reversed to the amorphous phase by either heating up above 180°C or cooling down below 110°C. This structural change is investigated by Raman spectroscopy. The changes observed in the Raman spectra reveal the formation of small ordered clusters (only a few lattice parameters in diameter). The ordered state was not observed by the only effect of either light or temperature, suggesting that specific temperature and light excitation conditions are necessary to generate and support the ordered state, that appears as a nano-crystalline structure.
Performance analysis of a-Si:H detectors deposited on CMOS chips
Authors: Miazza C. , Dunand S. , Wyrsch N. , Shah A. , Blanc N. , Kaufmann R. , Cavalier L.
Published: April 2011

Image and particle sensors based on thin-film on CMOS technology are currently being developed at our laboratory. In this technology, amorphous silicon detectors are vertically integrated on top of dedicated CMOS chips. For both, vision and particle detection, this approach is expected to enhance the performances. In fact very high fill factors, increased sensitivity, and integration level, coupled with extremely low dark current density values can potentially be attained.

A first optimization of the a-Si:H diodes (>1mm2) on glass substrates, with the primary focus on reducing dark current densities, gave Jdark values as low as 1 pA/cm2 (at -1 V for 1 m thick detectors). These detectors were then deposited on CMOS readout chips, but so far this step was unfortunately accompanied by an increase in Jdark to values over 10 nA/cm2.

Here, the possible cause for such an increase in Jdark as well as possible “remedies” against this effect will be discussed; the principle cause is supposed to be the influence of chip topology. Possible solutions include surface treatments as well as the use of metal-i-p diode configuration. Results obtained so far with these methods are given.
Femtosecond Far-Infrared Studies of Photoconductivity in a-Si:H and a-SiGe:H
Authors: Nampoothiri A. V. V. , Dexheimer S. L.
Published: April 2011

We present femtosecond time-resolved studies of the frequency-dependent photoconductivity in the far-infrared spectral range (∼ 1 – 10 meV) in PECVD a-SiGe:H and a-Si:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers into the extended states and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties, which are directly related to the ac photoconductivity, as the carrier distribution evolves in time. We find that the frequency-dependent conductivity measured on picosecond time scales shows a strongly non-Drude behavior, with components of the response fitting to a power-law frequency dependence, reflecting processes associated with localized states.
The Status of and Challenges in CdTe Thin-Film Solar-Cell Technology
Authors: Compaan Alvin D.
Published: April 2011

Polycrystalline CdTe thin-film solar cells have shown high potential for low cost, large-area module fabrication. But successful large-scale commercial production has been elusive. Fabrication of the basic n-CdS/p-CdTe heterojunction is possible by a wide variety of methods, including close-spaced sublimation, vapor-transport deposition, electrodeposition, chemical bath deposition, and magnetron sputtering. An overview of these methods is presented as well as the role of the postdeposition “activation” treatment using CdCl2and issues related to the difficulty of obtaining low resistance back contacts to CdTe. We present some of our recent fabrication results using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses the challenges of fabricating tandem cells with efficiencies over 25%.
High-Rate Plasma Process for Microcrystalline Silicon: Over 9% Efficiency Single Junction Solar Cells
Authors: Matsui Takuya , Matsuda Akihisa , Kondo Michio
Published: April 2011

This paper presents microcrystalline silicon (μ c-Si:H) p-i-n (superstrate-type) solar cells fabricated by 100 MHz plasma-enhanced chemical vapor deposition (PECVD) at i-layer deposition rates of >2 nm/s. Under high-rate conditions, in particular, the deposition pressure is found to play a dominant role in determining short circuit current (Jsc) of solar cell. With anincrease in deposition pressure from 3 to 7-9 Torr, Jsc increases by more than 50% due to a significant improvement in the long wavelength (>600 nm) responses, which essentially leads to high efficiency (∼8%) solar cells in the 2-3 nm/s deposition rate range. Further progress in solar cell efficiency has been made by the improvement of TCO/p and p/i interfaces. As a result, efficiency reaches 9.13% (Jsc=23.7 mA/cm2,Voc=0.528 V,FF=0.73) with a 2.3μm-thick i-layer grown at 2.3 nm/s. Transmission electron microscopy and secondary-ion mass spectroscopy studies reveal that samples prepared at lower pressure (∼4 Torr) comprise many grain boundaries due to disordered grain growth, which induces an anomalous incorporation of atmospheric impurities (predominantly oxygen) after exposing sample to air. In contrast, the high-pressure process (<7 Torr) provides denser grain columns coalesced with [110]-oriented crystallites, which in turn inhibits impurities from penetrating deeper in the film. Based on above results, we propose that the less post-oxidation behavior associated with the denser microstructure of high-pressure-grown μc-Si:H is responsible for the excellent charge collection in p-i-n solar cells.
Materials and Interface Optimization of Heterojunction Silicon (HIT) Solar Cells Using in-situ Real-Time Spectroscopic Ellipsometry
Authors: Levi D.H. , Teplin C.W. , Iwaniczko E. , Ahrenkiel R.K. , Branz H.M. , Page M.R. , Yan Y. , Wang Q. , Wang T.H.
Published: April 2011

We have applied real-time spectroscopic ellipsometry (RTSE) as both an in-situ diagnostic and post-growth analysis tool for hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction with intrinsic thin-layer (HIT) solar cells grown by hot-wire chemical vapor deposition. RTSE enables precise thickness control of the 5 to 25 nm layers used in these devices, as well as monitoring crystallinity and surface roughness in real time. Utilizing RTSE feedback, but without extensive optimization, we have achieved a photovoltaic energy conversion efficiency of 14.1% on an Al-backed p-type Czochralski c-Si wafer coated with thin i and n layers on the front. Open-circuit voltages above 620 mV indicate effective passivation of the c-Si surface by the a-Si:H intrinsic layer. Lifetime measurements using resonant coupled photoconductive decay indicate that surface recombination velocities can approach 1 cm/s. RTSE and transmission electron microscopy show that the intrinsic a-Si:H i-layers grow as a mixture of amorphous and nano-crystalline silicon.
Unexpected Mode of Plastic Deformation in Cu Damascene Lines Undergoing Electromigration
Authors: Budiman Arief S. , Tamura N. , Valek B. C. , Gadre K. , Maiz J. , Spolenak R. , Caldwell W. A. , Nix W. D. , Patel J. R.
Published: April 2011

An unexpected mode of plastic deformation was observed in damascene Cu interconnect test structure during an in-situ electromigration experiment and before the onset of visible microstructural damages (void, hillock formation). We show here, using a synchrotron technique of white beam X-ray microdiffraction, that the extent of this electromigration-induced plasticity is dependent on the line width. The grain texture of the line might also play an important role. In wide lines, plastic deformation manifests itself as grain bending and the formation of subgrain structures, while only grain rotation is observed in the narrower lines. This early stage behavior can have a direct bearing on the final failure stage of electromigration.
Organic Thin-Film Transistors Fabricated with Alignment-Free Printing Technique
Authors: Ando Masahiko , Kawasaki Masahiro , Imazeki Shuji , Sasaki Hiroshi , Kamata Toshihide
Published: April 2011

A novel process for fabricating alignment-free, printable, organic thin-film transistors is presented. This process exploits a self-assembly phenomenon in which soluble nanomaterials such as metal nanoparticles and organic molecules are self-assembled into a device structure. To demonstrate this process, solution-processed source and drain electrodes were self-aligned to a gate electrode by using a hydrophobic self-assembled monolayer (SAM) optically patterned onto the gate electrode with a back-substrate exposure technique. An organic semiconductor film deposited on the patterned SAM was selectively ordered and substantially self-aligned to the gate electrode. A field-effect mobility of 0.15 cm2/Vs and on/off current ratio of 105 were experimentally demonstrated when pentacene molecules were used as the semiconductor and silver nanoparticles were used as electrode materials.
Slip Transfer across Hetero-Interfaces in two-phase Titanium Aluminum Intermetallics
Authors: Wiezorek Jörg M.K. , Zhang Xiao-Dong , Fraser Hamish L.
Published: April 2011

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations.
Grain Boundary Mechanics – Influence of Mechanical Stress Fields on Grain Boundaries
Authors: Winning Myrjam
Published: April 2011

The reaction of grain boundaries to mechanical stresses is reviewed. Results of in-situ experiments on planar, symmetric tilt grain boundaries with different tilt axes (<112>, <111> and <100>) as well as twist grain boundaries with <100> rotation axis will be presented. It was found that the motion of planar grain boundaries can be induced by an imposed external stress irrespective of the angle of misorientation i.e. irrespective whether the grain boundary was a low or high angle grain boundary. The observed activation enthalpies of the stress induced grain boundary motion allow conclusions on the migration mechanism. The motion of planar low and high angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary. A sharp transition between low and high angle grain boundaries was observed for different tilt axes. The fact that boundaries can also be moved by mechanical forces sheds new light on microstructure evolution during elevated temperature deformation.
Nickel Silicide Work Function Tuning Study In Metal-Gate CMOS Applications
Authors: Yuan Jun , Pan Grant Z. , Chao Yu-Lin , Woo Jason C.S.
Published: April 2011

Mid-gap work function (∼4.7eV) for mono-nickel-silicide (NiSi) was obtained by extrapolating flat band voltages of metal-oxide-semiconductor (MOS) capacitors with different gate oxide thickness. Both silicidation temperature and time can affect the nickel silicide work function as a result of different Ni:Si ratio close to the gate oxide interface. Arsenic implantation into the polysilicon before silicidation can shift the NiSi work function towards the silicon conduction band, which makes it suitable for high performance NMOS applications. The physical mechanism responsible for this work function shift is arsenic pile-up at the oxide interface during the nickel silicidation process. Therefore, dual work function metal gate can be obtained by using a single gate full silicidation process. Silicidation temperature and time also affect the work function shift from arsenic dopant, and the incomplete gate silicidation can have the maximum work function modification effect. Arsenic activation temperature before silicidation was found to have a significant effect on the work function shift. Un-annealed samples exhibit a minimum shift in work function due to the low dopant pile-up concentration at the oxide interface.
Composition Dependence of the Judd-Ofelt Intensity Parameters in TeO2-PbF2: Tm3+ Glasses
Authors: Kabalci Idris , Ozen Gonul , Kurt Adnan , Sennaroglu Alphan
Published: April 2011

Tm3+ -doped glasses with the composition of (1-x)TeO2-xPbF2, where x=10, 15, 20, 25 mol.% were synthesized and, their thermal and absorption measurements were investigated. All the glasses were transparent. The absorption bands corresponding to the absorption of the 1G4, 3F2, 3F3, 3F4, 3H5, and 3H4 levels from the 3H6 ground level of the Tm3+ ion were observed in the optical UV/VIS/NIR absorption measurements. We calculated the integrated absorption cross sections of each band except that of 3H5 level was found to vary with composition of the PbF2. The absorption measurements were first made to determine the spontaneous emission probabilities of the 4f-4f transitions of the Tm3+ ions. The calculations were made by using the Judd - Ofelt theory. The Ω2 parameter shows the strongest dependence on the host composition and it increases with the increasing PbF2 amount. The values Ω4 increases rather slowly while the value of Ω6 is practically independent of the composition. The strong dependence of the parameter Ω2 indicates that this parameter is related to the structural change and symmetry of the local environment of the Tm3+ ions in this glass.
Preparation of Carbon Nano-Materials Using Arc Discharge in Liquid
Authors: Suda Y. , Kawasaki H. , Ohshima T. , Nakashima S. , Kawazoe S. , Toma T.
Published: April 2011

Carbon nanomaterials have been prepared using a simple production method of pulsed arc discharge between pure graphite electrodes in liquid. High-resolution transmission electron microscopy observation revealed that two kinds of nanomaterials can be produced in liquid. One of them is powdery products floating on the water surface, and another one is bulky deposits sank down in the water bottom. Some of them have multi-walled carbon nanofibers. Mean diameter of the nanofibers was approximately 30 nm. Optical emission of C+(426.7nm), Hα(656.3nm), Hβ(486.1nm), Hγ(434.0nm), C2 swan bands and CO(457.1nm) can be observed in the arc discharge by using photonic multichannel spectral analyzer. These results indicate that the carbon nanomaterials have been produced by pulsed arc discharge in liquid.
Ab-Initio Calculations for the Electronic Spectra of Cubic and Hexagonal Boron Nitride
Authors: Satta Guido , Cappellini Giancarlo , Olevano Valerio , Reining Lucia
Published: April 2011

We present state of the art fist-principles calculations for the optical spectra and the loss functions of bulk boron nitride in the cubic (c-BN) and in the hexagonal (h-BN) phases. We start from a DFT-LDA density functional Khon-Sham bandstructure to investigate the influence of many-body effects beyond the Random Phase Approximation (RPA) on the optical spectra through the inclusion of self-energy and excitonic effects by a GW calculation and the solution of the Bethe-Salpeter equation. For the loss function we only perform RPA calculations. We show to which extent the description of many-body effects is important for a meaningiful comparison with experiment, and when they can be neglected.
Site-specific formation of nanoporous silicon on micro-fabricated silicon surfaces
Authors: Ou Fung Suong , Saraf Laxmikant V. , Baer Donald R.
Published: April 2011

We demonstrated an ability to control the location of the porous silicon formation during electro-chemical etching by the introduction of micro- fabricated defects. This ability may open new doors for chemical and biological applications of porous silicon. In this paper, we have focused on the experimental procedure used to form this site-specific porous silicon area on micro- fabricated silicon surface. We also compared our results with work carried out by Saraf et al[1] during the early stage of porous silicon formation in conventional electrochemical etching.
Optical properties of CdSe and CdTe nanoparticles embedded in SiO2 films
Authors: Babu Dayal P. , Mehta B. R. , Paulson P. D.
Published: April 2011

CdSe and CdTe nanoparticles dispersed in silicon dioxide films have been grown by magnetron sputtering technique followed by thermal annealing. Effect of thermal annealing conditions on the structural and optical properties of CdTe and CdSe nanoparticles has been studied. Glancing angle x-ray diffraction (GAXRD), optical absorption (OA), transmission electron microscope (TEM) and spectroscopic ellipsometry (SE) techniques have been used to study the crystal structure, particle size and dielectric function spectra of nanoparticles. ε2 spectra of CdTe nanoparticles show a decrease in dielectric constant values in comparison to bulk CdTe. ε2″ spectrum of CdTe nanoparticles show four critical points in the electronic band structure of CdTe. Optical absorption studies in the case of CdTe and CdSe nanoparticles clearly show the blue shift of fundamental absorption edge due to quantum confinement effect.
Self-oriented Growth of GaN Films on Molten Gallium
Authors: Li Hongwei , Chandrasekaran Hari , Sunkara Mahendra K , Collazo Ramon , Sitar Zlatko , Stukowski Michael , Rajan Krishna
Published: April 2011

This paper presents a concept of growing near single crystal quality GaN films over large areas through self-oriented growth of GaN platelets on molten gallium. The experiments were performed by nitridation of Ga films on amorphous quartz substrates using nitrogen plasma at low pressures of few mTorr. XRD texture analysis of the free standing GaN flakes with areas over 25 mm2 exhibited an overall c-axis tilt of 2.2°, while showing primary reflections from (0002) and (0004) planes. Further more, the cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small misorientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8° and 1°, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces with both twisted and perfect attachments.
Scanning Electron Microscopy Cathodoluminescence Studies of Piezoelectric Fields in an InGaN Multiple Quantum Well Light Emitting Diode
Authors: Bunker Kristin L. , Garcia Roberto , Russell Phillip E.
Published: April 2011

Scanning Electron Microscopy (SEM)-based Cathodoluminescence (CL) experiments were used to study the influence of piezoelectric fields on the optical and electrical properties of a commercial InGaN-based Multiple Quantum Well (MQW) Light Emitting Diode (LED). The existence and direction of a piezoelectric field in the InGaN-based LED was determined with voltage dependent SEM-CL experiments. The CL emission peak showed a blueshift followed by a redshift with increasing reverse bias due to the full compensation of the piezoelectric field. It was determined that the piezoelectric field points in the [000–1] direction and the magnitude was estimated to be approximately 1.0±0.2 MV/cm. SEM-CL carrier generation density variation and electroluminescence experiments were used to confirm the existence of a piezoelectric field in the InGaN-based MQW LED.
Optical Properties of II-IV-N2 Semiconductors
Authors: Muth John , Cai Ailing , Osinsky Andrei , Everitt Henry , Cook Ben , Avrutsky Ivan
Published: April 2011

Recently, wide band gap II-IV-N2 semiconductors such as ZnSiN2, and ZnGeN2 and ZnSiGeN2 have been synthesized, but very little is known about their band structure, optical properties, or electronic properties. Bulk crystals are hard to synthesize because high temperatures and pressures are required. The success in growing II-IV-N2 films epitaxially by MOCVD creates interesting opportunities. The crystal structure of II-IV-N2 compounds is orthorhombic, and when grown on r-plane sapphire can provide a suitable template for GaN growth. Optical transmission studies of the band edge of ZnSiN2 and ZnSiGeN2 with varying Si and Ge percentages were conducted. The indirect nature of the band gap was investigated, and prism coupling was used to obtain the refractive indices in the visible and NIR portion of the spectrum. Although the crystal symmetry was orthorhombic, the refractive indices indicated uniaxial optical properties. Optical loss measurements indicate that the films are suitable for waveguides and novel devices based on birefringent optical effects.
Characterization of the carrier confinement for InGaN/GaN light emitting diode with multiquantum barriers
Authors: Wang Jen-Cheng , Lin Chung-Han , Lin Ray-Ming , Nee Tzer-En , Fang Bor-Ren , Wang Ruey-Yu
Published: April 2011

Electroluminescence (EL) of InGaN/GaN multiquantum wells (MQW) in blue light-emitting diodes (LED) with multiquantum barriers (MQB) has been investigated. It was found that a device with an MQB structure exhibited better performance in carrier confinement, as well as higher temperature insensitivity, compared with the conventional MQW LEDs. The total cross sections ware obtained for those devices with and without MQBs, by fitting to the measurement of the spectral intensity. Not only the cross sections, but also the carrier lifetimes depending on temperature can therefore be mainly attributed to changes in Boltzmann population. All the detailed calculations are also agreement with the observations.
The composition dependence of the optical properties of InN-rich InGaN grown by MBE
Authors: Martin R.W. , Edwards P.R. , Hernandez S. , Wang K. , Fernandez-Torrente I. , Kurouchi M. , Nanishi Y. , O'Donnell K.P.
Published: April 2011

Study of the relationship between the composition and optical energies of InxGa1-xN has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from InxGa1-xN epilayers covering the full range of composition (0 < × < 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 < × < 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed.
AlGaN/GaN Field Effect Schottky Barrier Diode for a Low Loss Switching Device
Authors: Yoshida Seikoh , Ikeda Nariaki , Li Jiang , Hataya Kohji , Wada Takahiro , Takehara Hironari
Published: April 2011

ABSTRUCT:

We have proposed a novel field effect Schottky barrier diode (FESBD) with a dual Schottky structure combined with an AlGaN/GaN heterostructure in order to obtain a very low on-voltage. This diode has a dual Schottky structure of a very low Schottky barrier metal and a high Schottky barrier metal for obtaining a low on-voltage. The leakage current at a reverse bias was suppressed by the pinch-off based on field effect of a higher Schottky barrier metal, resulting in increasing the reverse breakdown voltage.

In this paper, we carried out a planer-type FESBD for a large current operation. The AlGaN/GaN heterostructure was grown by a metalorganic chemical vapor deposition (MOCVD). A dual Schottky structure was fabricated using Ti/Al and Pt. An ohmic electrode was also Ti/Al. As a result, the on voltage of FESBD was below 0.1 V. The reverse breakdown voltage was also over 400 V by pinch-off effect. The switching time of the diode was shorter than 3 ns.
High pressure annealing of HVPE GaN free-standing films: redistribution of defects and stress
Authors: Paskova T. , Suski T. , Bockowski M. , Paskov P.P. , Darakchieva V. , Monemar B. , Tuomisto F. , Saarinen K. , Ashkenov N. , Schubert M. , Roder C. , Hommel D.
Published: April 2011

The effect of high temperature, high pressure annealing on morphology, optical and structural properties of free-standing GaN films grown by hydride vapor phase epitaxy is studied. The annealing is found to change the intensities of the photoluminescence peaks as a result of a redistribution of the impurities and native defects in the thick GaN films. A positron annihilation study shows a decrease of the Ga vacancy-related defects below the detection limit after the annealing. The defect redistribution is correlated with a flattening of the stress distribution across the thickness, as revealed by micro Raman study, and with a decrease of the curvature of the annealed free-standing films.
Synchrotron white beam x-ray topography (SWBXT) and high resolution triple axis diffraction studies on AlN layers grown on 4H- and 6H-SiC seeds
Authors: Raghothamachar Balaji , Dudley Michael , Dalmau Rafael , Schlesser Raoul , Sitar Zlatko
Published: April 2011

For nitride based devices such as LEDs, high power FETs and laser diodes, single crystal substrates of AlN are highly desirable. While the sublimation technique is suitable for growing bulk AlN crystals, appropriate seeds are also necessary for growing large diameter oriented boules. 4H- and 6H-SiC substrates which are readily available commercially can potentially be implemented as seeds for bulk AlN growth. However, issues regarding SiC decomposition at high temperatures, thermal expansion mismatch, single crystal growth, etc. need to be addressed. Towards this end, a series of growth experiments have been carried out in a resistively heated reactor using on and off-axis 4H- and 6H-SiC substrates as seeds for AlN growth from the vapor phase. Several hundred microns thick AlN layers have been grown under different growth conditions. Synchrotron white beam x-ray topography (SWBXT) has been used to map the defect distribution in the grown layers and high resolution triple axis x-ray diffraction (HRTXD) experiments were carried out to record reciprocal space maps from which tilt, mismatch and strain data can be obtained. These results are analyzed with respect to the growth conditions in order to gain a better understanding of this growth process.
Improvement of Operating Voltage and Luminescent Properties in Nanocrystalline Silicon Electroluminescent Device
Authors: Sato K. , Hirakuri K. , Izumi T.
Published: April 2011

We have investigated direct current (DC) operating voltage and luminescence properties of electroluminescent (EL) devices with and/or without a silicon dioxide (SiO2) layer in nanocrystalline Si (nc-Si) region/Si substrate interface. The device with the SiO2 layer showed red luminescence with a peak at 670 nm by applying the DC operating voltage above 4.0 V. When the SiO2 layer in the device was completely removed by the hydrofluoric acid (HF) treatment, the red luminescence from the device was observed at the DC operating voltage of 2.0 V. Moreover, the luminescent intensity was also increased more than one order of magnitude, because carriers were efficiently and easily injected into the nc-Si region by the removal of SiO2 layer. The red luminescence from the device could be clearly seen with the naked eye under the DC operating voltage above 3.0 V. These results indicate that the removal of SiO2 layer leads to the lowering of DC operating voltage and increase of luminance for the nc-Si based EL device.
Hydrogen Passivation of Er and Si Nanocrystallites in Er-doped SiO2- Increase in Photoluminescence-
Authors: Fukata N. , Li C. , Uematsu H. , Arai T. , Makimura T. , Murakami K.
Published: April 2011

Hydrogen passivation effect on the enhancement of photoluminescence (PL) of Er ions in SiO2films contained Si nanocrystallites (nc-Si) has been investigated. Er-doped SiO2films were fabricated by laser ablation of Er-deposited Si substrate in oxygen gas atmosphere. The PL intensity of Er ions and nc-Si were increased by hydrogen gas treatments, while ESR signal intensity of residual defects located at the interfaces between nc-Si and SiO2was decreased. These results indicate that hydrogen passivation of residual defects is useful for the enhancement of the Er PL.
Charge Transport in Silicon Nanocrystal Arrays
Authors: Krishnan R. , Xie Q. , Kulik J. , Wang X.D. , Krauss T.D. , Fauchet P.M.
Published: April 2011

Single layers of isolated, size-controlled silicon nanocrystals were prepared by thermal crystallization of a thin amorphous silicon layer sandwiched between silicon dioxide layers. A subsequent oxidation treatment ensured controlled increase in their lateral separation. The size of the nanocrystals, separation of the nanocrystals (from < 1 nm to ∼ 4 nm), stoichiometry of the resulting oxide and surface morphology were monitored with transmission electron microscopy, scanning transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. Mesoscopic charge transport studies performed with an electrostatic force microscope (EFM) revealed rapid lateral transport of charges when the nanocrystals were tightly packed (< 1 nm average separation) and interconnected. As the inter-nanocrystal separation was increased, lateral charge transport was rapidly suppressed. Nanocrystals separated by up to 3.6 nm retained the injected charges in a well-defined localized region (∼ 62 nm diameter region) for a time of the order of several days. The ability to switch from a very short to a very long retention time using the same structure by simply changing the post-growth processing conditions is attractive for various applications involving charge transport and localization.