bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructure Development in Copper Welded by the FSW-Process
Authors: Källgren Therese , Sandström Rolf
Published: April 2011

To ensure safe storage of nuclear fuel waste, copper canisters are proposed as corrosion barrier. One alternative for sealing the copper canisters is Friction Stir Welding (FSW). During the joining process friction heat and mechanical deformation appear between the rotating tool and the material being welded. Liquid metal will not form, since this is a solid state welding process. Three distinct microstructural zones are developed namely the nugget, the thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ). The nugget is in the centre of the weld, where the pin is located and where severe plastic deformation occurs that leads to recrystallisation. Surrounding the nugget, the TMAZ is only partially recrystallised, due to lower temperature increase and deformation compared to the nugget. The third zone, HAZ, surrounds the TMAZ. The initial nugget can have a classic round aluminium nugget image, when the welding conditions are cold, but the steady state nugget, is wider near the shoulder and shorter in the weld root.
On Pressure Pulse Techniques for Testing Low Permeability Geological Materials and Formations
Authors: Zhang M. , Takeda M.
Published: April 2011

The accurate hydraulic characterization of low permeability geological materials and formations has important practical implications, such as the geological disposal of radioactive nuclear waste. In this paper, we discuss pressure pulse techniques, which are methods that are commonly adopted both in the laboratory and in situ for testing low permeability geological materials and formations. The results obtained in this study illustrate that: 1) the use of nominal values for water compressibility will lead to significant underestimation of permeability from the pressure pulse tests; 2) specific storage is also an important parameter that should be evaluated simultaneously with permeability; 3) to shorten the time required to measure low permeability in the laboratory, it is advisable to use disk-shaped specimens; 4) when a geological formation is anisotropic, the value of permeability calculated using an isotropic model is between the maximum and minimum permeabilities of the geological formation; 5) the values of permeability and specific storage derived from an in situ pressure pulse test may only reflect the hydraulic properties of the domain closely surrounding the test well. Caution should be exercised when interpreting data derived from the pressure pulse tests for the purpose of safety assessment.
Harmonisation of Site Characterisation and Performance Assessment Modelling - The Relative Importance of Surface Sorption and Matrix Interaction Phenomena
Authors: Crawford James , Moreno Luis , Neretnieks Ivars
Published: April 2011

In the context of geological repositories for nuclear waste disposal, the goal of site characterisation (SC) is to obtain input data that can later be used as a basis for performance assessment calculations (PA). Performance assessment is required to give an indication as to whether the repository will behave as intended over the geological timescales relevant for risk analysis.

Processes that may be important in a PA setting for constraining radionuclide transport may not necessarily be dominating, or indeed may not even be observable during SC investigations.

In this paper it is shown that the migration of sorbing tracers is governed largely by surface sorption phenomena in typical SC-type field experiments whereas in a PA scenario, matrix interaction instead can be expected to play an overwhelmingly important role. This study uses data and settings that are representative for fractured rocks in Sweden, but the method used and the conclusions may have more general applicability.
The Feasibility of USD Method in Tracking Past Hydrogeochemical Changes in the Fennoscandian Shield
Authors: Rasilainen Kari , Suksi Juhani , Ruskeeniemi Timo , Pitkänen Petteri
Published: April 2011

Uranium-series disequilibrium (USD) method is the only paleohydrogeological method that for the time scale relevant for performance assessments can detect and directly date past hydrogeochemical changes. Such changes cause U mobility which is seen as uranium-series disequilibrium. We have reviewed the method taking into account experimental and interpretation aspects and identified pros seem to outweigh identified cons. Reported USD studies in Finland and Sweden have earlier been mostly qualitative but a quantitative methodology has been developed for future studies.
Study of the Uranium Heterogeneous Diffusion through Crystalline Rocks and Effects of the “Clay-Mediated” Transport
Authors: Alonso U. , Missana T. , García-Gutiérrez M. , Patelli A. , Ravagnan J. , Rigato V.
Published: April 2011

Rock matrix diffusion is one of the possible mechanisms for radionuclide retardation in a deep geological high-level radioactive waste repository, and it is usually considered that radionuclides diffuse as solutes through the rock. Nonetheless, the potential effects that clay, from the bentonite barrier, may induce on the radionuclides migration should be taken into account. Furthermore, transport models generally assume that the whole mineral surface is accessible to transport, whereas transport is highly conditioned by the heterogeneous mineral distribution, since different minerals may act as preferential pathways, while others may present higher sorption capability. It is therefore necessary to determine the actual surface area accessible to transport.

The aim of the present work is the identification of the uranium preferential pathways to the granite, both in presence or absence of bentonite clay. Results showed that uranium as solute diffused in specific mineral areas, indicating that the actual surface area accessible to matrix diffusion, and/or sorption on the surface, is significantly lower than the whole mineral surface. By the other hand, the uranium in presence of the clay was randomly distributed on the surface, and penetrated into the granite mainly through “defects” (as fractures or grain boundaries); its migration being enhanced on specially fractured or disturbed areas.
A Conceivable Technique of Measuring Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as the Driving Force for Migration
Authors: Löfgren Martin , Neretnieks Ivars
Published: April 2011

Sorption coefficients are traditionally obtained in batch experiments if the sorbent is strongly or intermediately sorbing. In a batch experiment the rock is crushed and this could increase the surface area as well as induce new and fresh surfaces. Therefore there is some concern whether sorption coefficients obtained in batch experiments represents those of intact rock. Performing sorption experiment by diffusion in intact rock with intermediately and strongly sorbing species seems impossible in practice due to extremely long experimental times. In this paper the possibility of increasing the migration rate in the rock by two of three orders, thus enabling KD measurements of intermediately sorbing species in intact rock, is discussed. The increase in migration ratehas already been achieved successfully in so called through electromigration experiments using non-sorbing species. Here a potential gradient acts as the main driving force. In our experiments the migration rate was increased 320 times by using a potential drop of only 9 volts.
Modification of the Tournemire Argilite in Contact with Simplified Concrete Fluid
Authors: Devol-Brown Isabelle , Stammose Denise , Ahamdach Noureddine , Geiss Olivier
Published: April 2011

Argillaceous rocks are considered as potential host rocks for radioactive waste repositories. The design of an underground disposal often includes large amounts of concrete which are proposed as waste matrix, backfill material and structural components. The leaching of concrete by groundwater produces alkaline solutions that will react with the host rock inducing modifications of the geological barrier performances. In order to examine the effects of an alkaline plume on the properties of claystone, an experimental program is being developed in our institute involving batch, diffusion and percolation experiments. This paper presents the results obtained with first batch studies performed at 25°C and 70°C over 3 months. The effect of high pH solutions (corresponding to the early period of leaching) on the composition of a claystone (originating from Tournemire - France) was examined. The changes in solutions composition were monitored as a function of temperature and V/m ratio. In spite of the short contact time between solution and solid, some modifications in these two phases were noticed. The interactions between solids and solutions were more pronounced at 70°C than at 25°C, resulting in pH and potassium concentration decrease, while the sodium concentration remained constant. Moreover, the dissolution of mineral phases such as dolomite and kaolinite was proposed.
iCONNECTClub: Integrated CONtinuum and NEtwork Approach to Groundwater Flow and Contaminant Transport
Authors: Holton David , Hartley Lee J , Hautojärvi Aimo , Marschall Paul , Selroos Jan-Olof
Published: April 2011

Models of groundwater flow and transport are routinely used as part of the safety assessment of a site, and increasingly to aid the site investigation strategy and design of a proposed repository. This emphasis puts greater demands on those models to reflect a higher level of realism to incorporate additional information in the vicinity of the repository. This paper describes a successful approach to incorporate these different scales, from canister- to regional-scale, within one model. Such an approach is able to resolve the details of groundwater flow around the waste packages in the overall hydrological setting of the site.
Hyperalkaline Cement Leachate-Rock Interaction and Radionuclide Transport in a Fractured Host Rock (HPF Project)
Authors: Mäder Urs , Frieg Bernd , Puigdomenech Ignasi , Decombarieu Michel , Yui Mikazu
Published: April 2011

The HPF project (Hyperalkaline Plume in Fractured rock) at the Grimsel Test Site comprises an underground long-term field experiment in a shear zone, in-situ radionuclide transport experiments, two laboratory core infiltration experiments, sophisticated reactive transport modeling exercises, studies on radionuclide stability and solubility, innovative on-line measurement techniques and development of equipment for high-pH conditions (K-Na-Ca-OH, pH = 13.4 at 15 °C). Results to date indicate a decrease in the overall transmissivity of the tested shear zone over a duration of 2 years accompanied by channeling of flow as evidenced by repeat dipole tracer testing with Na-fluorescein, 82Br, 131I, 24Na, and 85Sr. The associated evolution in fluid chemistry indicates the in situ formation of Ca-Si-hydrates. Tracer transport modeling of dipole tests are based either on a heterogeneous porous medium approach or on discrete fracture models. Reactive transport modeling is achieving reasonable agreement with a laboratory core infiltration experiment. Integral to the project are supporting sorption / stability studies, colloid measurements, and development of analytical and measurement techniques.
Location Control of Crystal Grains in Excimer Laser Crystallization of Silicon Thin Films for Single-Grain TFTs
Authors: Kumomi Hideya , Wakiyama Hiroaki , Nakagawa Gou , Makihira Kenji , Asano Tanemasa
Published: April 2011

Location of crystal grains is controlled in excimer laser crystallization (ELC) of amorphous Si (a-Si) thin films, aiming at a high-performance single-grain thin film transistor (TFT) whose channel is inside a single crystal grain with no grain boundary in the channel. The location control is achieved by manipulating seed-crystal forming sites in the starting thin film. The sites are small portions of the a-Si thin film, typically 1 μm in diameter, only in which nanometer-sized crystallites are embedded in the amorphous matrix. During the ELC, at least one crystallite survives the melting duration and serves as a seed crystal for the resolidification of the surrounding molten silicon. As a result, large crystal grains are formed at the predetermined sites. The TFTs whose channels are fabricated at the location-controlled crystal grains exhibit higher performance than the random polycrystalline Si (poly-Si) TFTs.
Are the Current Models Helpful to Understanding Staebler-Wronski Degradation?
Authors: Roedern Bolko von
Published: April 2011

This contribution reviews the compatibility of Staebler-Wronski models with experimental data and observations. The review will show that neither the “bond-breaking models” (originally proposed by Dersch and Stutzmann) nor the “defect conversion” models (originally proposed by Adler) can explain all observations on films and/or solar cells. It has been well accepted for some time that experimental stress and recovery phenomena, both on films and devices, always identify both “slow” and “fast” degradation and recovery mechanisms. It is argued that the quintessential understanding of the Staebler-Wronski mechanisms will come from identifying a fundamental physical process that provides a quantitative understanding of the “coupling” between the slow and fast mechanisms.
Two terminal large area single and double p-i-n devices for image and color recognition
Authors: Louro P. , Vieira M. , Fantoni A. , Fernandes M. , Carvalho C. Nunes de , Lavareda G.
Published: April 2011

Large area hydrogenated amorphous silicon single and stacked p-i-n structures with low conductivity doped layers are proposed as monochrome and color image sensors. All have the same intrinsic layer and in the doped layers the resistivity and optical gap was controlled through the addition of methane to the doping gas. The current-voltage characteristics and the spectral sensitivity under different illumination conditions are analyzed. The sensor output characteristics are evaluated under different read-out voltages and scanner wavelengths. To extract information on image range, intensity and color, a modulated light beam scans the sensor active area at three appropriate bias voltages and the photoresponse in each scanning position (“sub-pixel”) is recorded. The investigation of the sensor output under different scanner wavelengths and varying electrical bias reveals that the response can be tuned, thus enabling color separation. The operation of the sensor is exemplified and supported by a numerical simulation.
Monolithically Integrated p- & n- Channel Thin Film Transistors of Nanocrystalline Silicon on Plastic Substrates
Authors: Cheng I-Chun , Wagner Sigurd
Published: April 2011

Inverters made of monolithically integrated p- and n-channel thin film transistors of nanocrystalline silicon were demonstrated on both Corning 1737 glass and Kapton E polyimide substrates. The TFT's geometry is staggered top-gate, bottom-source/rain. A nc-Si:H seed layer promotes the structural evolution of the nc-Si:H channel. Electron field-effect mobilities of 15 - 30 cm2V−1s-1 and hole mobilities of 0.15 - 0.35 cm2V−1s−1 were obtained. Slightly lower carrier mobilities were observed in the TFTs made on polyimide than on glass substrates. High gate leakage currents and offsets between the supply HIGH voltages and the output voltages in the inverters indicate that the low-temperature gate dielectric needs improvement.
In-situ Observation of Silicon Epitaxy Breakdown with Real-Time Spectroscopic Ellipsometry
Authors: Teplin Charles W. , Levi Dean H. , Wang Qi , Iwaniczko Eugene , Jones Kim M. , Branz Howard M.
Published: April 2011

We use in-situ real-time spectroscopic ellipsometry to observe the breakdown of silicon epitaxy during growth by hot-wire chemical vapor deposition (HWCVD) on Si (100) substrates. Representative data is presented for the two types of epitaxy breakdown that we have observed: 1) an immediate transition to hydrogenated amorphous silicon (a-Si:H), and 2) a slower transition where a-Si:H cones nucleate and grow until they eclipse further epitaxial growth. Simple models, consistent with transmission-electron and atomic-force micrographs, describe the evolution of both types of breakdown showing that real-time spectroscopic ellipsometry is a useful tool for monitoring the growth of epitaxial silicon.
Amorphous and Microcrystalline Silicon-based Photovoltaic
Authors: Guha Subhendu
Published: April 2011

The last two decades have witnessed tremendous progress in the science and technology of amorphous and microcrystalline silicon-based photovoltaic. Advances in the understanding of materials and devices have led manufacturers to expand their production capacity; the production of solar panels based on this technology exceeded 25 MW in 2003. Hydrogen dilution in the active gas mixture during deposition has played a key role in improving the quality of the materials and the performance of the devices. In this paper I shall review the properties of the optimum material for device application, and discuss the production status. I shall also report on the new opportunities that are opening up for these products for space and stratospheric applications.
Structural and Electronic Properties of SiCl4-based Microcrystalline Silicon Films
Authors: Beyer Wolfhard , Carius Reinhard , Lejeune Michael , Zastrow Uwe
Published: April 2011

Structural and electronic properties of SiCl4-based microcrystalline silicon films were studied. A rather dense (non-porous) material structure is obtained near the transition to amorphous material, in particular at substrate temperatures of 250°C and above. Boron doping results in very high conductivity values while for phosphorus doping only lower values are reached. This latter effect is attributed to a different microstructure with lower crystalline fraction, higher hydrogen and chlorine content and increased porosity in highly phosphorus- doped material.
Process Modeling for Advanced Devices
Authors: Law Mark E. , Jones Kevin S. , Radic Ljubo , Crosby Robert , Clark Mark , Gable Kevin , Ross Carrie
Published: April 2011

As device lots become more and more expensive, process modeling is increasingly important. Process simulation and modeling is increasingly sophisticated but the accuracy remains a problem. There is generally a time lag between the introduction of a particular process and its accurate modeling – the problem of “yesterday's technology modeled tomorrow”. For many problems, absolute accuracy isn't required. Relative trends provide excellent information about the process in question. For this reason, process simulation is still a useful technique for guiding process development.
Growth of epitaxial CoSi2 from Cobalt Carbonyl on Si(100) Substrate
Authors: Singanamalla R. , Greve D.W. , Barmak K.
Published: April 2011

Beyond its applications in silicon MOSFETs, cobalt disilicide is potentially useful for the formation of epitaxially overgrown quantum dots and quantum wires. We report the formation of cobalt silicide on Si (100) and its overgrowth by silicon. Cobalt films approximately 3.5 nm in thickness have been deposited on Si (100) using a cobalt carbonyl organometallic source. We observe by reflection high-energy electron diffraction (RHEED) that the deposited cobalt film is converted to epitaxial cobalt disilicide upon annealing at 850°C, considerably higher than the silicidation temperature typically observed for sputtered cobalt. We explain this behavior with the aid of secondary ion mass spectrometry (SIMS) profiling of the carbon and oxygen impurity concentrations. The cobalt disilicide layers have been overgrown with silicon at 625°C. We contrast the surface topology measured by atomic force microscopy (AFM) just after cobalt growth, after silicidation, and after overgrowth with silicon. The surface has been observed to be rough after growth of silicon by silane exposure for 15 min at 625°C at 0.55 mTorr of chamber pressure. The surface roughness prevails even after overgrowing with silicon for 60 min at same chamber temperature and pressure conditions.
BIC Formation and Boron Diffusion in Relaxed Si0.8Ge0.2
Authors: Crosby R. T. , Radic L. , Jones K. S. , Law M. E. , Thompson P.E. , Liu J. , Klimov M. , Craciun V.
Published: April 2011

The relationships between Boron Interstitial Cluster (BIC) evolution and boron diffusion in relaxed Si0.8Ge0.2 have been investigated. Structures were grown by Molecular Beam Epitaxy (MBE) with surface boron wells of variant composition extending 0.25 [.proportional]m into the substrate, as well as boron marker layers positioned 0.50 [.proportional]m below the surface. The boron well concentrations are as follows: 0, 7.5×1018, 1.5×1019, and 5.0×1019 atoms/cm3. The boron marker layers are approximately 3 nm wide and have a peak concentration of 5×1018 atoms/cm3. Samples were ion implanted with 60 keV Si+ at a dose of 1×1014 atoms/cm2 and subsequently annealed at 675°C and 750°C for various times. Plan-view Transmission Electron Microscopy (PTEM) was used to monitor the agglomeration of injected silicon interstitials and the evolution of extended defects in the near surface region. Secondary Ion Mass Spectroscopy (SIMS) concentration profiles facilitated the characterization of boron diffusion behaviors during annealing. Interstitial supersaturation conditions and the resultant defect structures of ion implanted relaxed Si0.8Ge0.2 in both the presence and absence of boron have been characterized.
Enhancement of Boron Activation in Shallow Junctions by Hydrogen
Authors: Vengurlekar A. , Ashok S. , Kalnas C. E. , Theodore N. D.
Published: April 2011

The ability to activate greater amounts of dopants at lower temperatures is a persistent contingency in the continual drive for device scaling in Si microelectronics. We report on the effect of incorporating atomic hydrogen on the activation of implanted boron in shallow junctions. Hydrogen incorporation into the sample was carried out by exposure to an electron cyclotron resonance (ECR) hydrogen plasma. Enhanced activation was observed in hydrogenated samples for post-implantation annealing temperatures of 450°C and below, as measured by spreading resistance profilometry, and confirmed by identical boron atomic profile in hydrogenated and unhydrogenated samples. The enhancement in boron activation at lower temperature is attributed to the creation of vacancies in the boron-implanted region, the lattice-relaxation effect by the presence of atomic hydrogen, and the effect of atomic hydrogen on boron-interstitial cluster formation.
Textural Evolution of Cu Damascene Interconnects after Annealing
Authors: Cho Jae-Young , Lee Hyo-Jong , Kim Hyoungbae , Szpunar Jerzy A.
Published: April 2011

Textural evolution of Cu interconnects having a different line width was investigated after annealing. Texture was measured on the surface of Cu interconnects using EBSD (electron backscattered diffraction) techniques including GBCD (grain boundary character distribution). To analyze a relationship between the stress distribution and textural evolution in the samples investigated, the micro stresses were calculated for the different line width at 200°C using FEM (finite element modeling). In this investigation, it was found that the inhomogeneity of stress distribution in Cu interconnects is an important factor is necessary for understanding textural transformation after annealing. A new interpretation of textural evolution in damascene interconnects lines after annealing is suggested, based on the state of stress and the growth mechanisms of Cu electrodeposits.
Reel to Real: Prospects for Flexible Displays
Authors: Allen Kimberly
Published: April 2011

Although it is tempting to begin with a standard opening statement like “Flexible displays have been generating much recent interest,” the fact is that flexible displays have fascinated people for about 50 years and have been discussed seriously for at least 20 years. The difference now is that display and backplane technology have evolved to the point where reasonably attractive-looking demonstrations of flexible screens are possible.
Patterned growth of organic small-molecule layers
Authors: Steudel Soeren , Janssen Dimitri , Verlaak Stijn , Heremans Paul
Published: April 2011

We will demonstrate a novel approach to the problem of patterning organic small molecule layers, which is compatible with printing techniques. By locally changing the chemistry of the dielectric surface by means of Self-Assembly Monolayers (SAM), we predefine active and passive areas of the circuit. The subsequent growth of the organic film by sublimation takes place under growth conditions which cause the formation of a high-mobility, well- connected two-dimensional (2D) grains in the predefined active area contrasting with ill- connected, low-mobility 3D-grains in the passive area. We will explain this patterned growth in terms of a microscopic theory of nucleation of organic semiconductor on inert substrates and present experimental results.
Reel-to- Reel Cassette Cluster Tool System for Thin Film Transistor and Four Terminal Solar Cell Fabrication
Authors: Madan Arun
Published: April 2011

We present a new type of system architecture to fabricate thin film silicon devices on flexible substrates, which uses the inherent advantages of a cluster tool normally used in the production of amorphous silicon thin film transistors, solar cells, etc. In this, the flexible substrate material is contained within a cassette which includes a reel to reel operation. As in the current cluster tools used for planar substrates, the cassette is transported to a process chamber using a robotic arm. When the entire roll in the cassette has been processed, it is transported into other chambers for further processing. We also show that a four terminal device structure (e.g. amorphous Si and stable low band gap nano-crystalline Si cells) potentially can lead to high efficiency (>15%), stable, low cost solar cells on a plastic substrate. Lastly we discuss the use of a pulsed PECVD deposition technique, which allows that the structure of the nano-crystalline Si films can be altered from 111 to 220 in a controllable way at a low temperature of <170C.
Crack Trapping Effect of Recalcitrant Grain Boundary Islands in Fe-3wt%Si Alloy
Authors: Qiao Yu , Kong Xinguo
Published: April 2011

In the polycrystalline Fe-Si alloy when a cleavage front transmits from one grain to another, it first penetrates stably through the grain boundary at a number of break-through points that distribute along the front quasi-periodically. As the critical energy release rate is reached, the unstable crack jump occurs and the recalcitrant grain boundary islands (RGBI) between the break-through points are left behind the verge of propagating, bridging across the crack flanks. Eventually, the RGBI are sheared apart, which leads to a 10%-30% increase in fracture resistance. In this paper, this process is investigated through an energy analysis. The influence of the size/spacing ratio of RGBI on the grain boundary toughness is discussed in detail.
Roughness Scaling of Fracture Surfaces in Polycrystalline Materials
Authors: Seppäaläa Eira T. , Reed Bryan W. , Kumar Mukul , Minich Roger W. , Rudd Robert E.
Published: April 2011

The roughness scaling of fracture surfaces in two-dimensional grain boundary networks is stud- ied numerically. Grain boundary networks are created using a Metropolis method in order to mimic the triple junction distributions from experiments. Fracture surfaces through these grain boundary networks are predicted using a combinatorial optimization method of maximum flow — minimum cut type. We have preliminary results from system sizes up to N = 22500 grains suggesting that the roughness scaling of these surfaces follows a random elastic manifold scaling exponent ζ = 2/3. We propose a strong dependence between the energy needed to create a crack and the special boundary fraction. Also the special boundaries at the crack and elsewhere in the system can be tracked.
Direct Measurement of Impurity Segregation Concentration at Grain Boundaries for Polycrystalline Materials using EBSD and 3D Reconstruction of SEM images of Etch Grooves
Authors: Cornen Marilyne , Le Gall René
Published: April 2011

Impurity segregation at grain boundaries in polycrystalline alloys is known to have a tremendous impact on the material properties such as mobility, cohesion.. But, direct measurement of grain boundary chemistry is quite complex and there are quite few results concerning polycrystals. In this paper we present an indirect method to measure segregation- misorientation dependence on polycrystalline Ni-S alloys using both EBSD and 3D reconstruction of etch grooves. Samples of Ni-S alloy (7.2 ppm at) have been annealed at different temperatures to get equilibrium segregation at grain boundaries. Then they have been etched near the transpassive potential to form etch grooves, whose geometry depends on the sulfur segregation level. Grain boundaries misorientation statistics and first results about the segregation-misorientation function are given here.
Preparation of SrS:Ce/ZnO Core-Shell Nanoparticles Using Reverse Micelle Method
Authors: Kusakari Yusuke , Ishizaki Shinji , Kobayashi Masakazu
Published: April 2011

SrS:Ce nanoparticles were prepared using a reverse micelle method and coated by ZnO. The mono-dispersed nanoparticles whose size was as small as 3nm were observed by transmission electron microscope (TEM) and dynamic light scattering (DLS) method. Optical properties of the core-shell SrS:Ce/ZnO nanoparticle was studied. UV-visible absorption and photoluminescence (PL) properties were measured and compared with those of SrS:Ce and SrS nanoparticles. Bright blue photoluminescence (around 440 nm) was observed from SrS:Ce at a room temperature when nanoparticles were excited using a Hg lamp. The PL peak intensity enhancement was observed after forming the ZnO shell. The optical absorption profile of nanoparticles was associated with the band edge feature of SrS. The coating of the SrS:Ce surface by ZnO has affected the electronic structure of SrS:Ce nanoparticles.
Near band-edge and excitonic behavior of GaAsN epilayers grown by Chemical Beam Epitaxy
Authors: Coaquira J. A. H , Bhusal L. , Zhu W. , Fotkatzikis A. , Pinault M.-A. , Litvinchuk A. P. , Freundlich A.
Published: April 2011

Photoluminescence and absorption spectroscopy experiments were performed on as grown and thermally annealed GaAs1-xNx with nitrogen content in the range of 0.75–7.1%. At low temperature, the photoluminescence spectra exhibits two set of features: (i) a relatively broad peak at low energy and near to the vicinity of the predicted band gaps and (ii) a sharp excitonic feature at higher energy (about 100 meV for x>4%). Post growth thermal annealing processes systematically favor stronger excitonic emissions, and a notable intensity reduction of the deeper (defect related) luminescence. The low temperature binding energy of the higher energy excitonic peak is found to be consistent with the increase of the electronic effective masses. A careful examination of the data obtained in this work suggests that for higher nitrogen content (x>4%), the fundamental band gap of GaAsN is located at significantly higher energies than those commonly accepted for these alloys.
Good Temperature Performances of 870nm Resonant Cavity Light Emitting Diode (RCLED)
Authors: Laih Lih-Wen , Wu Yi-Hao , Laih Li-Hong , Hong Rong-Moo , Guo Hao-Chung , Yu Jung-Lung , Huang Yu-Hsiang , Chang Yi-An , Chang Ren-Jiun , Yang Chun-Hui , Wu I-Tsung
Published: April 2011

A high performance of wavelength 870nm resonant cavity LED (RCLED) was fabricated. The high performance of InGaAs/GaAs multiple quantum wells (MQWs) and distributed Bragg reflector (DBR) were employed to achieve the high transmission rate. Two devices A and B were fabricated in this paper. Device A has an offset of 10nm between active region gain and resonant gain, and device B without it. Due to the wavelength shift of active region gain is faster than that of DBR's resonant gain at higher temperature. Device A shows the better temperature performances than device B. A cutoff frequency of 60MHz, a low forward voltage of 1.6V, a output power of 1mW at 10mA and a output power temperature variation (ΔP/ΔT) of –0.02dBm/ °C with chip dimensions of 220um × 220um and 85um diameter emitting window are obtained.
Fabrication of ZnO Coated ZnS:Mn2+ Nanoparticles
Authors: Ishizaki Shinji , Kusakari Yusuke , Kobayashi Masakazu
Published: April 2011

The fabrication of ZnO coated ZnS:Mn2+ nanoparticles were achieved using simple methods. ZnS:Mn2+ nanoparticles were prepared by a mechanical milling method. Coating of ZnO was then performed using a simple chemical method. Structural properties were evaluated by the X-ray powder diffraction (XRD) and the transmission electron microscope (TEM). Optical properties of the ZnO coated ZnS:Mn2+ nanoparticle were characterized by exciting the particle included solvent using an UV-LED (400 nm). Bright orange color fluorescence was observed, and the fluorescence intensity of ZnO coated ZnS:Mn2+ nanoparticles was enhanced as compared to uncoated ZnS:Mn2+ nanoparticles. The capping of the core surface has probably terminated surface defects of ZnS:Mn2+ nanoparticles, and resulted in the improved fluorescence intensity.
Structural investigation of InAs/InGaAs/InP nanostructures: origin and stability of nanowires.
Authors: Nieto L. , Gutiérrez H. R. , Bortoleto J. R. R. , Magalhães-Paniago R. , Cotta M. A.
Published: April 2011

In this letter we present results on the growth of InAs nanowires (NW's) on InGaAs lattice-matched to (100) InP substrates by Chemical Beam Epitaxy. We observed that the nanostructure stability depends on the thickness of the InGaAs layer. This effect may result from two different conditions: the nanostructure strain field depth and/or compositional modulation in the buffer layer. Our investigation shows that anisotropic strain relaxation for nanowires grown on InGaAs is faster than for those grown on InP but the elastic energy in the nanostructures is no different from the InAs/InP case. These results suggest that the InAs strain relaxation does not depend significantly on the InGaAs buffer layer thickness. Nevertheless, transmission electron microscopy images show an additional stress field superimposed on that usually observed for the InAs nanostructures, which is attributed to compositional modulation in the ternary layer.
InAs Quantum Dots for Optoelectronic Device Applications
Authors: Stewart K. , Barik S. , Buda M. , Tan H. H. , Jagadish C.
Published: April 2011

In this paper we discuss the growth of self-assembled InAs quantum dots (QDs) on both GaAs and InP substrates by low pressure Metal Organic Chemical Vapor Deposition. The influence of various growth parameters, such as the deposition time, the QD overlayer growth temperature, the V/III ratio and the group III and/or group V interdiffusion on QD formation are discussed and compared for the two systems. Stacking issues and preliminary results for an InAs/GaAs QD laser are also presented.
Superfluorescence of Ion Beam Synthesized Dense-Packed Embedded CdSe Nanoclusters
Authors: Karl H. , Groβhans I. , Huber P. , Stritzker B.
Published: April 2011

Dense-packed embedded semiconductor quantum dot (QD) layers with a multimodal size distribution are representing new types of QD solids. Their optical and electronic properties are modified due to dipole-dipole interactions and tunneling effects. In this work sequential high dose ion implantation of Cd and Se and subsequent thermal treatment is used to synthesize QD assemblies with the required structural properties in the surface near region of 500 nm thick thermally grown SiO2 on Silicon. We used cw photoluminescence (PL) to study PL-yield as a function of pump laser power at low temperatures for different various stoichiometries and annealing conditions. In these embedded QD assemblies of mixed size distribution we detected a promising non-linear increase of the PL-intensity with laser excitation power. The exponents evaluated are maximal for implanted Cd:Se-dose ratios between 0.8 and 1.0. The power law dependence of the PL-yield on pump laser power will be discussed in context with electronic energy transfer between dense-packed QD's of different size, implanted dose ratios and postimplantation thermal treatment conditions.
Correlation between Photoreflectance Spectra and Electrical Characteristics of InP/GaAsSb Double Heterojunction Bipolar Transistors
Authors: Sugiyama Hiroki , Oda Yasuhiro , Yokoyama Haruki , Kobayashi Takashi , Uchida Masahiro , Watanabe Noriyuki
Published: April 2011

We report a photoreflectance (PR) characterization of InP/GaAsSb double-heterojunction bipolar transistor (DHBT) epitaxial wafers grown by metal-organic vapor-phase epitaxy (MOVPE). The origin of the Franz-Keldysh oscillations (FKOs) in the PR spectra was identified by step etching of the samples. FKOs from the InP emitter region were observed in the wafer with low recombination forward current at the emitter-base (E/B) heterojunction. In contrast, they did not appear when recombination current was dominant. The absence of the FKOs from the emitter indicates the high concentration of the recombination centers at the E/B heterojunction. We have also measured PR spectra from InAlP/GaAsSb/InP DHBT wafers. Pronounced FKOs from InAlP emitter reflect the suppression of recombination at E/B heterojunctions.
New Electroluminescence Spectrum from Co-doped ZnS:(Mn, Si) Films Prepared by Chemical Vapor Deposition with Laser Ablation
Authors: Ozawa Makoto , Satoh Tomomasa , Hirate Takashi
Published: April 2011

Co-doped ZnS:(Mn, Si) films were fabricated. The ZnS was synthesized by a low-pressure thermal chemical vapor deposition. Metal Zn vapor and H2S gas were used as the CVD-precursors. Mn and Si were doped using a laser ablation technique during the ZnS growth. A solid MnSi alloy (Mn:Si = 1:1) was used as the laser ablation target. The films were deposited at the range from 650°C to 750°C. At the deposition temperature of 650°C, only a EL emission peak at 585 nm same as conventional ZnS:Mn films appeared, i.e., the Si co-doping had no effects on the EL spectrum. At the deposition temperature of 700°C, the Si co-doping to ZnS:Mn film caused the shift of the EL emission peak at 585 nm to shorter wavelength by 15 nm and provided new EL emission at 760 nm. The film deposited at 750°C exhibited new UV and blue EL emissions at 390 nm and 450 nm, respectively, although the host material of the film differed from usual ZnS.
Growth and Characterization of bulk GaN by Ga Vapor Transport
Authors: Konkapaka Phanikumar , Wu Huaqiang , Makarov Yuri , Spencer Michael G.
Published: April 2011

Bulk GaN crystals of dimensions 8.5 mm × 8.5 mm were grown at growth rates greater than 200μm/hr using Gallium Vapor Transport technique. GaN powder and Ammonia were used as the precursors for growing bulk GaN. Nitrogen is used as the carrier gas to transport the Ga vapor that was obtained from the decomposition of GaN powder. During the process, the source GaN powder was kept at 1155°C and the seed at 1180°C. Using this process, it was possible to achieve growth rates of above 200 microns/hr. The GaN layers thus obtained were characterized using X-Ray diffraction [XRD], scanning electron microscopy [SEM], and atomic force microscopy [AFM]. X-ray diffraction patterns showed that the grown GaN layers are single crystals oriented along c direction. AFM studies indicated that the dominant growth mode was dislocation mediated spiral growth. Electrical and Optical characterization were also performed on these samples. Hall mobility measurements indicated a mobility of 550 cm2/V.s and a carrier concentration of 6.67 × 1018/cm3
Optical Characteristics of Amorphous III-V Nitride Thin Films
Authors: Khoshman Jebreel M. , Kordesch Martin E.
Published: April 2011

The optical constants and polarized optical properties of amorphous III-V nitride thin films, a-(Al, Ga, In) N, deposited by RF magnetron sputtering onto crystalline silicon, c-Si, (111) and glass substrates have been investigated over the wavelength range 300 – 1400 nm. The optical constants of a-AlN were obtained by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model while the optical constants of a-(In, Ga) N were determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200 – 1400 nm) and a-GaN (in the range 300 – 1400 nm) show the optical bandgap to be 5.9 ± 0.05 and 3.44 ± 0.05 eV. The absorption coefficient of a-InN (in the range 300 – 1400 nm) as a function of photon energy shows the absorption edge to be about 1.74 ± 0.05 eV. From the angle dependence of the p-polarized reflectivity we deduced Brewster and principal angles of these films. Measurement of the polarized optical properties revealed a high transmissivity (70 % – 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions.
On the dynamics of InGaN dot formation by RF-MBE growth
Authors: Yamaguchi Tomohiro , Einfeldt Sven , Figge Stephan , Kruse Carsten , Roder Claudia , Hommel Detlef
Published: April 2011

The growth of InGaN on GaN (0001) by plasma-assisted molecular beam epitaxy was investigated with special focus on the dynamics of the formation of the dots. A metastable 2D growth regime, where the surface changes from smooth to rough by thermal treatment during growth interruption, existed previous to the 2D-3D transition. Both small regular-shaped dots and large irregular-shaped islands were observed. The large islands were suppressed by choosing correct growth conditions. The critical thickness for the transition from 2D to 3D growth also depended on the growth conditions. The growth of GaN capping layer to cover InGaN dot-structure was also attempted.
Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN
Authors: Reshchikov M. A. , Morkoç H.
Published: April 2011

We studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.
Efficient Luminescence from {11.2} InGaN/GaN Quantum Wells
Authors: Funato Mitsuru , Nishizuka Koji , Kawakami Yoichi , Narukawa Yukio , Mukai Takashi
Published: April 2011

InGaN/GaN multiple quantum wells (MQWs) with [0001], <11.2>, and <11.0> orientations have been fabricated by means of the re-growth technique on patterned GaN templates with striped geometry, normal planes of which are (0001) and {11.0}, on sapphire (0001) substrates. It was found that photoluminescence intensity of the {11.2} QW is the strongest among the three QWs, and its internal quantum efficiency was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {11.2} QW was about 0.39 ns at 14 K, which was 3.8 times shorter than that of conventional c-oriented QWs emitting at a similar wavelength. These findings are well explained by the high internal quantum efficiency in the {11.2} QW owing to the suppression of piezoelectric fields.
AlGaN/GaN HFETs and Insulated Gate HFETs DC and RF stability
Authors: Koudymov Alexei , Saygi Salih , Tipirneni Naveen , Simin Grigory , Adivarahan Vinod , Yang Jinwei , Asif Khan M.
Published: April 2011

The comparative study of the DC parameters and RF power stability of nitride-based conventional HFETs and Metal-Oxide-Semiconductor HFETs (MOSHFETs) is presented. The average lifetime under DC stress is estimated to be as high as 2.5 years at room temperature. Under the large-signal RF stress, the gate leakage current of conventional HFETs increases significantly with time showing faster degradation as compared to DC stressing. MOSHFETs demonstrate superior RF performance stability, which perfectly correlates with the DC stability data. It is shown that in conventional HFETs, the combination of the self-heating and positive dynamic gate bias leads to the defect accumulation in the AlGaN barrier under the gate. In MOSHFETs, the absence of the gate leakage is a key to stable RF performance.
SURFACE STABILIZATION FOR HIGHER PERFORMANCE ALGAN/GAN HEMT WITH IN-SITU MOVPE SIN
Authors: Germain M. , Leys M. , Derluyn J. , Boeykens S. , Degroote S. , Ruythooren W. , Das J. , Vandersmissen R. , Xiao D.P. , Wang W. , Borghs G.
Published: April 2011

Passivation of AlGaN/GaN HEMT by SiN has been shown by several authors to improve device performance. In this paper we demonstrate a new method of passivating the top AlGaN layer, that leads to a drastic improvement of the device characteristics. On top of the AlGaN/GaN HEMT layer structures, a thin SiN layer is grown at high temperature in-situ, i.e. in the MOCVD reactor prior to unloading the HEMT epiwafers. The quality of the SiN is shown to be very high. As the AlGaN surface is protected against any air contamination, ohmic contacts deposited directly on the SiN layer exhibit better properties than on uncapped AlGaN layers. Transistor performance increases impressively: the source-drain dc current, for a positive gate voltage of 2 V, increases from 0.5 A/mm to 1.2 A/mm for similar AlGaN/GaN HEMT structures by depositing SiN in-situ. RF characteristics also show some improvement. The positive impact of in-situ SiN layer has different origins, some being intrinsically linked: surface protection against oxide formation, better ohmic contact formation, passivation of the surface states, reduction of AlGaN relaxation. All this leads to an increase of the 2DEG carrier concentration.
Self-Organized GaN/AlN Superlattice Nanocolumn Crystals Grown by RF-MBE
Authors: Yamano Kouji , Kikuchi Akihiko , Kishino Katsumi
Published: April 2011

GaN nanocolumns including a GaN/AlN superlattice (SL) region were grown by rf-plasma assisted molecular beam epitaxy. The photoluminescence (PL) peak intensity of the GaN/AlN SL nanocolumns was 300∼500 times stronger than that of conventional GaN continuous films with a dislocation density of 3∼5×109 cm−2 and thickness of 3.75 μm grown by metalorganic chemical vapor deposition (MOCVD). The peak wavelengths of the GaN (10.2 ML)/AlN (15.2 ML) SL and GaN (7.7 ML)/AlN (12.4 ML) SL were 420 and 380 nm, respectively. The theoretically calculated transition wavelength agreed well with experimental values, suggesting that GaN/AlN SL nanocolumns involve a large built-in electrostatic field of about 5.8 MV/cm. The effect of the surface morphology of nanocolumns on the PL intensity was studied using GaN/AlN SL nanocolumns with different surface morphologies but with the same nanocolumn structure. Integrated PL intensity was increased by a factor of 2.2 upon changing the surface morphology from continuous to columnar.