bluuebunny/crossref_metadata_2025_split huggingface dataset:
Radiation Damage of a Glass-Bonded Zeolite Waste Form Using Ion Irradiation
Authors: Storey B. G. , Allen T. R. 
Published: April 2011

ABSTRACTGlass-bonded zeolite is being considered as a candidate ceramic waste form for storing radioactive isotopes separated from spent nuclear fuel in the electrorefming process. To determine the stability of glass-bonded zeolite under irradiation, transmission electron microscope samples were irradiated using high energy helium, lead, and krypton. The major crystalline phase of the waste form, which retains alkaline and alkaline earth fission products, loses its long range order under both helium and krypton irradiation. The dose at which the long range crystalline structure is lost is about 0.8 dpa for helium and 0.1 dpa for krypton. Because the damage from lead is localized in such a small region of the sample, damage could not be recognized even at a peak damage of 50 dpa. Because the crystalline phase loses its long range structure due to irradiation, the effect on retention capacity needs to be further evaluated.
Temperature Dependence and Annealing Effects of Absorption Edges for Selenium Quantum Dots Formed By Ion Implantation In Silica Glass
Authors: Ueda A. , Wu M. , Mu R. , Tung Y.-S. , Henderson D. O. , Meldrum A. , Zuhr R. A. , Budai J. D. , White C. W. , Keay J. C. , Feldman L. C. 
Published: April 2011

ABSTRACTWe have fabricated Se nanoparticles in silica substrates by ion implantation followed by thermal annealing up to 1000°C, and studied the Se nanoparticle formation by optical absorption spectroscopy, Rutherford backscattering spectrometry, X-ray diffraction, and transmission electron microscopy. The sample with the highest dose (1 × 1017 ions/cm2 ) showed the naonparticle formation during the ion implantation, while the lower dose samples (1 and 3 × 1016 ions/cm2) required thermal treatment to obtain nano-sized particles. The Se nanoparticles in silica were found to be amorphous. After thermal annealing, the particle sizes became larger than the exciton Bohr radius for bulk Se. Thus, the absorption edges for different doses approached the value of bulk after thermal annealing. The temperature dependent absorption spectra were also measured for this system in a temperature range from 15 to 300 K.
Initial Phase Formation During Interdiffusion
Authors: Perepezko J. H. , Park J. S. , Landry K. , Sieber H. , Bassani M. H. da Silva , Edelstein A. S. 
Published: April 2011

ABSTRACTIn multiphase materials systems involved in coatings, composites or multilayered structures, diffusion treatments often results in the development of intermediate phases at the reaction interfaces. While diffusional growth of phases has received much attention, the initial phase evolution involves a nucleation stage as well. The development of metastable phases during solid state interdiffusion demonstrates that the nucleation reaction can be controlling in some cases. For alloy systems with extensive solubility, intermediate phase nucleation is proceeded by interdiffusional mixing in order to achieve the required supersaturation. This leads to the identification of a critical concentration gradient for the onset of phase nucleation.The concentration gradient and the relative magnitudes of the component diffusivities provide a basis for a phase selection strategy and the application of a kinetic bias to modify the phase selection. For multicomponent alloy systems, the identification of the operative diffusion pathway is central to the control of phase formation. Experimental access to the nucleation stage of reaction is facilitated in thin film multilayer samples where the results from systems with both extensive and limited solubility offer new insight into the phase formation kinetics.
Interface Morphological Stability of Unidirectionally Solidified RE123 Superconductor
Authors: Sumida M. , Shiohara Y. , Umeda T. 
Published: April 2011

ABSTRACTA simple model is proposed to analyze the interface stability of the RE123 superconductor in accordance with the constitutional supercooling criterion. As the single crystal growth of the 123 phase is largely dependent on the growth interface stability, a quantitative analysis has been required. From the numerical analysis for the case of peritectically solidified Sm123, it was clarified that the constitutional supercooling must exist in the liquid when the 123 growth interface comes close to a 211 particle. It could also predict that larger 211 particle radius, smaller volume fraction of the 211 particles, larger growth rate, or smaller imposed temperature gradient cause easy occurrence of the constitutional supercooling. The growth rate and a 211 particle radius are determining parameters. Further consideration of the nucleation at the L/211 interface just ahead of the 123 growth front could describe the 123 growth morphological transition from with the planar interface to the equiaxed blocky.
Interface Diffusion in Polysynthetically-Twinned Tiai
Authors: Luzzi D. E. , Imamura D. , Inui H. , George E. P. , Heatherly L. , Yasuda H. , Mori H. , Yamaguchi M. 
Published: April 2011

ABSTRACTThe utility of polysynthetically-twinned (PST) TiAI, which contains a high density of parallel, atomically-flat interfaces within a set of identical crystallographic orientations, as a potential model system for a detailed investigation of interface diffusion is explored. Macroscopic PST crystals were grown in an optical float zone furnace. Thin films were cut from oriented crystals and polished with <112> directions normal to the film. After sputter cleaning, Ag was deposited on one side of the TiAI thin films. Auger spectra were obtained from these films over a wide range of sputter/anneal conditions. The Al and Ti concentrations were analyzed as well as the important impurity elements, S, Ar, C, N and O. Using the present data and existing knowledge of the microstructure and crystallography of PST TiAI, the potential of this material for providing a detailed understanding of the atomistic mechanisms of interface diffusion is analyzed.
Derivation of the Kinetics of Phase Transformations with Nucleation and Growth Mechanism
Authors: Yu G. , Lee S. T. , Lai J. K. L. 
Published: April 2011

ABSTRACTThe kinetics of phase transformation which follows a nucleation-and-growth mechanism was studied by using probability theory. From the calculation of the survival probability for each individual site, the general equation for describing the transformation kinetics, in which the nucleation and growth rates are considered as a function of time and space, is derived. In comparison to the classical derivation by Avrami, the new derivation is logical and transparent. The extension of the treatment by using the definition of the multiple-survival probability leads to the exact solutions of the time dependent grain size distribution functions during transformation. A new understanding of fundamental relationships for the microstructural analysis can be achieved by comparing different kinds of size distribution functions. By applying the principles of the analytical treatment to the simulation, model systems of vast size can be handled for very complicated transformation process.
Lattice Location and Luminescence Behavior of Rare Earth Elements Implanted in GaN
Authors: Dalmer M. , Restle M. , Stötzler A. , Vetter U. , Hofsäss H. , Bremser M. D. , Ronning C. , Davis R. F. ,   Isolde
Published: April 2011

AbstractSingle crystalline GaN-layers were implanted with radioactive 167Tm and 169yb ions, and their lattice sites were determined using the emission channeling technique. After the decay of 167Tm to 167Er, photoluminescence studies were performed. Upon room temperature implantation, rare earth atoms immediately occupy relaxed substitutional sites with an average relaxation of about 0.025 nm. Isochronal annealing treatments up to 800 °C and co-implantation of oxygen to a dose an order of magnitude greater than that of the Tm or Yb do not influence the rare earth lattice sites. A variety of different rare earth related luminescence lines are observed, and co-implantation of oxygen strongly changes the line intensities.
Characterizations of Mg Implanted GaN
Authors: Chi Gou-Chung , Pong B. J. , Pan C. J. , Teng Y. C. , Lee C. H. 
Published: April 2011

AbstractWith 150KeV Mg+ ion implantation, the optical and structural characteristics of GaN films were studied. Post-implant annealing up to 1000°C was performed in N2 ambient with a rapid thermal annealing (RTA) system, without an encapsulation layer. We observed a green band photoluminescence from Mg-implanted GaN. This green band photoluminescence should be associated with Mg induced defect-clustering in GaN. We also use the x-ray diffraction method to study the correlation between structure defects and implantation. We observed an extra shoulder peak at the small angle side of the GaN[0004] diffraction peak. The origin of this shoulder may be attributed to implanted magnesium induced GaN lattice strain.
Aging of InGaN/AlGaN/GaN Light-Emitting Diodes
Authors: Yunovich A. E. , Kovalev A. N. , Kudryashov V. E. , Imanyakin F. , Turkin A. N. 
Published: April 2011

AbstractChanges in luminescent spectra, current-voltage, and capacitance-voltage characteristics were studied versus time of working at forward currents for light-emitting diodes based on InGaN/AlGaN/GaN heterostructures. The samples of blue and green LEDs with a single quantum well InGaN active layers were studied during 102-2.103 hours at currents 30–80 mA. An increase in efficiency at the first stage and a decrease one at the second stage were detected. The main changes were detected at low currents in blue diodes for tunnel components of current and radiation. Activation of acceptors Mg because of residual H atoms are going out of complexes Mg-H and creation of donor type defects are proposed as models for two stages. A model of injection stimulated (non-thermal) underthreshold creation of defects can explain the results.
InGaN Double-Heterostructures and Dh-Leds on Hvpe Gan-on-Sapphire Substrates
Authors: Boutros K. S. , Flynn J. S. , Phanse V. , Vaudo R. P. , Smith G. M. , Redwing J. M. , Tolliver T. R. , Anderson N. G. 
Published: April 2011

AbstractWe report on the growth of InGaN films, and the fabrication and characterization of GaN homojunction LEDs and InGaN double heterostructure (DH) LEDs on HVPE GaNon- sapphire substrates. The use of these substrates facilitates the III-nitrides growth process, as it avoids the use of complicated buffer layers. We have achieved InGaN films with strong and sharp band-to-band photoluminescence (PL) from 370 to 540 nm. Typical In 0.o9Ga0. g9N/GaN DH films had double-crystal XRD FWHM ∼ 300 arcsec, and 400 nm peak PL emission with FWHM ∼ 100 meV. DH-LEDs were fabricated with InGaN layers with various compositions, and produced strong electroluminescence (EL) in the blue and blue/green regions.
Dc and Microwave Characteristics of High Transconductance AlGaN/GaN Heterostructure Field Effect Transistors on Sic Substrates
Authors: Chen Q. , Yang J. W. , Khan M. A. , Ping A. T. , Adesida I. 
Published: April 2011

AbstractHigh quality AJGaN/GaN heterostructures have been successfully deposited on both nand p-type SiC substrates. Heterostructure field effect transistors fabricated using these layers exhibited high channel current density (1.71 A/mm), well behaved pinch-off characteristics, and excellent extrinsic transconductance (Gm = 229 mS/mm). There is negligible channel current degradation up to a source to drain bias of 20 V as opposed to devices grown on sapphire substrates. The 0.25 μm gate-length devices fabricated on the heterostructures grown on p-type SiC has allowed us to extract a cutoff frequency of 53 GHz. The cutoff frequency showed little deterioration with increasing drain bias voltage. These results demonstrate for the first time the high frequency and high power operation potential of the heterostructure field effect transistors based on AlGaN grown on SiC.
Experimental Study of Sputter Deposited Contacts to Gallium Nitride
Authors: Piquette E. C. , Bandić Z. Z. , McGill T. C. 
Published: April 2011

AbstractA variety of metal contacts to n-GaN is investigated, including Al, Au, Ce, Ti, Cr, Mg, Sb, W, and Mo, which were deposited using DC magnetron plasma sputtering onto MBE grown GaN. The contacts were characterized by current-voltage analysis and contact resistances were measured by the circular geometry transmission line method. The effects on contact resistance and Schottky barrier properties by surface sputtering treatments, GaN doping, and post-deposition annealing are reported.
Characteristics of GaN Schottky Diode Grown on Sapphire Substrate by Mocvd
Authors: Egawa T. , Ishikawa H. , Yamamoto K. , Jimbo T. , Umeno M. 
Published: April 2011

AbstractWe report here the characterization of metalorganic chemical vapor deposition grown GaN layer on sapphire substrate with (0001) orientation (c-face). The 2.4-μm-thick GaN layer on sapphire substrate showed the specular surface morphology. The full width at half-maximum of X-ray double crystal rocking curve was 214 arcsec. Photoluminescence measurement at 4.2 K showed the emissions from the two free excitons (FEA and FEB), acceptor and donor bound excitons (ABE and I2) and the first excited state of FEA. The etch pit density (EPD) was measured using molten KOH etching was 2∼4×108 cm-2 . The electron mobilities from Hall effect measurement were 585 cm2/V·s for the electron concentration 1.1×1017 cm-3 at 300 K, and 1217 cm2/V·s for 2.4×1016 cm-3 at 77 K, respectively. The measured mobility values are much higher than the previously reported values. The fabricated Pt/Ti/Au Schottky diode on GaN layer revealed the ideality factor 1.04 and the barrier height 0.9 eV which was calculated from forward current-voltage characteristics. The reverse current was as low as 1 mA for the reverse voltage 100 V. The barrier height was also determined to be 1.4 eV from the capacitance-voltage measurement. The electron-beam induced current observations were performed on the fabricated GaN Schottky diode. The dark spot was observed and the dark spot density was estimated to be 2×108 cm-2, which was comparable to the EPD measured using molten KOH etching. GaN metal semiconductor field effect transistors (MESFETs) with 3-μm gate-length and 15-μm gate-width have also been fabricated on high-quality GaN single crystalline layer. Maximum transconductance 25 mS/mm and the drain-source current 210 mA/mm have been achieved for the GaN MESFET.
Selection, Growth, and Characterization of Gate Insulators on Mocvd Gallium Nitride for the Use in High Power Field Effect Devices
Authors: Therrien R. J. , Nam O. H. , Bremser M. D. , Lithicum K. , Nimii H. , Carlson E. P. , Lucovsky G. , Davis R. F. 
Published: April 2011

AbstractMetal-insulator-semiconductor (MIS) capacitors have been fabricated on n-type GaN (0001) films using thermally grown Ga2O3, remote plasma enhanced chemical vapor deposited (RPECVD) SiO2, and molecular beam epitaxy (MBE) AIN as the gate insulator and Al as the gate electrode. Each GaN epitaxial layer was grown by organometallic chemical vapor deposition (OMCVD) on a 6H-SiC(0001) substrate on which was previously deposited a 1000Å buffer layer of AIN. Nitrogen-free polycrystalline films of Ga2O3 were grown on the GaN. Capacitancevoltage measurements of capacitors fabricated from this oxide showed distinct depletion and accumulation regions with significant leakage. The AIN and SiO2 capacitors demonstrated better electrical characteristics than the Ga2O3 because of lower leakage. The RPECVD SiO2/GaN heterostructures, in particular, showed good agreement with the curves calculated for an ideal oxide and a small amount of hysteresis.
Analysis of Optical Gain of Strained Wurtzite InxGa1-xN/GaN Quantum Well Lasers
Authors: Chong T. C. , Yeo Y. C. , Li M. F. , Fan W. J. 
Published: April 2011

AbstractThe valence subband structures, density-of-states, and optical gain of (0001) wurtzite (WZ) InxGa1-xN/GaN quantum wells (QWs) are studied using a numerical approach without analytical approximations. We used the effective-mass parameters of GaN and InN derived using the Empirical Pseudopotential Method. By varying the well width and mole fraction of In in the well material, the effects of quantum confinement and compressive strain are studied. A narrower well width and a higher In mole fraction in the well lead to TE enhancement and TM suppression of the optical gain. From the relationship between the optical gain and the radiative current density, we obtain the transparent current density for a single QW to be 200 A/cm2. Further, we analyze the InxGa1-xN/GaN/AlGaN separate confinement heterostructure multiple-QW laser structure. It is shown that a suitable combination of well width and number of QWs should be selected in optimizing the threshold current density in such MQW lasers
Optical Gain in Bgan Lattice-Matched to (0001) 6H-SiC
Authors: Honda T. , Tsubamoto M. , Kuga Y. , Kawanishi H. 
Published: April 2011

AbstractThe optical gain of a BGaN ternary system lattice-matched to 6H-SiC was estimated. The parameters used in this estimation, such as effective mass and a bandgap energy, were estimated according to Harrison's theory. The optical gain was estimated using the density matrix theory with intraband relaxation broadening. The transparency carrier density of BGaN is slightly only smaller than that of GaN. It may be possible to fabricate a BGaN-based semiconductor laser operating in the UV spectral region.
Epitaxial Growth and Properties of Mg-Doped Gan Film Produced by Atmospheric Mocvd System With Three Layered Lammar Flow Gas Injection
Authors: Akutsu N. , Tokunaga H. , Waki I. , Yamaguchi A. , Matsumoto K. 
Published: April 2011

AbstractMg-doped GaN films with a variety of Mg concentrations were grown on sapphire (0001) by horizontal atmospheric metalorganic chemical vapor deposition (MOCVD) system with three layered laminar flow gas injection in an attempt to study the Mg doping effects on film quality. The increase of Mg concentration induced an increase of x-ray rocking curve full width at half maximum (FWHM) and degradation of surface morphology. Secondary ion mass spectroscopy (SIMS) analysis shows increase of Si and O, associated with Mg-doping concentration. Si and O concentrations of Mg-doped film are up to 5×1016cm−3 and 5×1017cm−3 at Mg concentration of 4.5×1019cm−3, respectively. Strong 380nm emission and weak 430nm emission were observed by photoluminescence (PL) measurement at room temperature for as-grown Mg-doped GaN films which shows p-type conductivity after thermal annealing. While, in highliy Mg-doped GaN films which do not show the p-type conduction after thermal annealing, 430nm and/or 450nm emission were dominating. The highest room temperature free hole concentration achieved was p=2.5× 1018cm−3 with mobility μp=l.9cm2/V s.
Gallium Nitride Multioperate Optoelectronic Devices
Authors: Sidorov V. G. , Drizhuk A. G. , Sidorov D. V. 
Published: April 2011

AbstractOpportunity of creation of several optoelectronic devices such as optron, photoreceiver, switch device, IR-to-visible signal transformer and others based on i-n-GaN light emitting diodes (LED) is shown. Technology and properties of GaN LED in relation to the desired device properties are discussed. A discussion of issues relating to electrical and optical positive feed back between the device elements is also included.
Nitride Laser Diodes With InGaN Based Mqw Structures
Authors: Sugiura Lisa , Nishio Jobji , Onomura Masaaki , Nunoue Shin-Ya , Itaya Kazuhiko , Ishikawa Masayuki 
Published: April 2011

AbstractAdvantages of applying III-V nitride materials for short wavelength light-emitting devices despite their extremely high dislocation density are discussed from the viewpoint of dislocation motion. There are also difficulties proper to these materials, which make it difficult to fabricate laser diodes. We present recent works to realize high performance laser diodes. We introduce the nitrogen ambient metalorganic chemical vapor deposition (MOCVD) growth which realizes the highly p-typed GaN films without any post-treatments. Some of our results respecting the room temperature pulsed operation of the conventional laser diode and the advanced inner stripe (IS) laser diode with InGaN based multi-quantum-well (MQW) grown by MOCVD are reported.
Properties of Free-Standing GaN Bulk Crystals Grown by HVPE
Authors: Melnik YU. , Nikolaev A. , Nikitina I. , Vassilevski K. , Dmitriev V. 
Published: April 2011

AbstractGaN wafers 200 μm thick and 30 mm diameter were fabricated. GaN was grown by hydride vapor phase epitaxy on SiC substrates and removed from the substrate by reactive ion etching. Lateral size of the GaN wafers was equal to the size of the initial SiC substrates. GaN wafers were cleaved in pieces and these pieces were characterised. It was found that after the fabrication, GaN crystals were slightly deformed and strained. An anneal at 830°C in nitrogen ambient eliminated the residual strains. The FWHM of ω-scan (0002) x-ray rocking curve for annealed crystals was less than 140 arcsec for both sides of the best GaN crystals. The lattice constants measured from both sides of the crystals were c =5.1853±0.0003 Å and a = 3.1889±0.0001 Å. The Nd– Naconcentration determined by a mercury probe was about 2×1017cm−3for as-grown GaN surface and about 2×1019cm−3for former interface surface. Photoluminescence spectrum taken at 17 K revealed an edge peak at 3.472 eV with the FWHM value of 2.3 meV. A ratio of the edge peak intensity to the intensity of yellow band was higher than 1000. Initial TEM experiments were performed.
Ultra-Smooth ZnO Buffer Layers on (001) Sapphire
Authors: Drehman A. J. , Wang S.-Q. , Yip P. W. 
Published: April 2011

AbstractUsing off-axis reactive rf sputtering, we have grown extremely smooth, nearly epitaxial, (001) oriented ZnO films on c-axis sapphire substrates. Atomic Force Microscopy was used to determine that these films are extremely smooth, having an rms roughness of only a few tenths of a nanometer. Based on high resolution x-ray diffraction (HXRD), the ZnO is highly oriented, with a rocking curve width of less than 400 arc seconds for the (006) diffraction peak, and only somewhat larger for the (112) reflection. HXRD Phi scans show that the ZnO (112) reflection is rotated in the a-b plane by 30 degrees from the sapphire (113) direction. These two measurements indicate excellent in-plane orientation. We are investigating the use of these buffer layers for subsequent GaN growth. Electrical resistivities of the films exceeded 100 kΩ-cm making ZnO a potential candidate as an insulating buffer layer.
Organometallic Vapor Phase Lateral Epitaxy of Low Defect Density GaN Layers
Authors: Nam O. H. , Zheleva T. S. , Bremser M. D. , Thomson D. B. , Davis R. F. 
Published: April 2011

AbstractLateral epitaxial overgrowth (LEO) of GaN layers has been achieved on 3 μm wide and 7 μm spaced stripe windows contained in SiO2 masks on GaN/AIN/6H-SiC(0001) substrates via organometallic vapor phase epitaxy (OMVPE). The extent and microstructural characteristics of lateral overgrowth were a complex function of stripe orientation, growth temperature and triethylgallium (TEG) flow rate. A high density of threading dislocations, originating from the interface of the underlying GaN with the AIN buffer layer, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The second lateral epitaxial overgrowth layers were obtained on the first laterally grown layers by the repetition of SiO2 deposition, lithography and lateral epitaxy.