bluuebunny/crossref_metadata_2025 huggingface dataset:
Umklapp Scattering and Heat Conductivity of Superlattices
Authors: Simkin M.V. , Mahan G.D.
Published: June 2019

The mean free path of phonons in superlattices is estimated. It is shown to be strongly dependent on the superlattice period due to the Umklapp scattering in subbands. It first falls with increasing the superlattice period until it becomes comparable with the latter after what it rises back to the bulk value. Similar behavior is expected of heat conductivity, which is proportional to the mean free path.
Material Analysis by Ultrasonic Atomic Force Microscopy
Authors: Miyasaka Chiaki , Jia Lily , Tittmann Bernhard R.
Published: June 2019

Spray-dried ceramic powders (e.g., Al2O3) are composed of a plurality of granules, each of which, includes ceramic particles and organic binders. It is assumed that the binders become concentrated in the surface layer of the granule in accordance with its type or its volume mixed into a ceramic portion of the granule. However, evidence to prove the assumption was limited because conventional microscopes were not able to clearly visualize the segregation. This paper presents a technique for imaging detailed structure of the spray-dried ceramic powders with the ultrasonic-atomic force microscope (U-AFM). The distribution of binder vis-a-vis Al2O3 particles is highly resolved with good contrast. The distribution was confirmed by nano -indentation. Thus, the U-AFM is shown to be a useful diagnostic tool for the development of approaches to spray-dried process evaluation.
Nuclear Magnetic Resonance Studies of Granular Flows – Current Status
Authors: Altobelli Stephen A. , Caprihan Arvind , Fukushima Eiichi , Seymour Joseph D.
Published: June 2019

Nuclear magnetic resonance (NMR) is a non-intrusive method that can characterize not only the particulate density but also velocity and velocity fluctuation parameters. A survey of all the known NMR measurements of granular flow will be followed by a brief description of NMR as it applies to granular flow. Two new experiments, both involving flows in partially filled rotating horizontal cylinders, will be described. First, the effect of a stationary blade to suppress the azimuthal velocity of particles being brought up and deposited into the flowing layer on flow-velocity profiles will be studied. Suppressing the azimuthal velocity reduces the deviation of the velocity profile from a quadratic dependence on the height above the rigid layer. Second, a new NMR scheme will be presented that yields spatial distributions of collisional correlation times for macroscopic particles undergoing granular flow. It is based on Pulsed-Gradient Spin-Echo strategy that is commonly used to measure molecular diffusion in liquids. The scheme will be demonstrated with an example from shear flow in a partially-filled horizontal cylinder. Spatially resolved collisional correlation times and velocity fluctuation intensities are derived from the measurements and have values of ∼1 ms and ∼10-3 m2/s2 respectively, at the center of the free surface for 2 mm particles in a 70 mm diameter cylinder rotating at 2.36 rad/s.
Impulse and Low Frequency Acoustic Wave Propagation in Granular Beds
Authors: Sen Surajit , Manciu Marian , Tehan Victoria , Hurd Alan J.
Published: June 2019

The study of sound propagation in granular beds at frequencies exceeding a MHz has been a subject of study for many years. Much remains to be learnt about sound propagation at lower frequencies. We shall present our studies on the problem of impulse propagation and briefly comment on low frequency acoustic propagation in model granular beds using particle dynamical simulations. The following results will be briefly discussed. (i) Impulses propagating as solitary waves in 1-D granular chains with Hertz contacts in the absence of precompression. (ii) The effects of uniform precompression and gravitational loading on wave propagation. (iii) Impulse propagation in 3-D granular beds. The research presented shall highlight the intrinsically nonlinear nature of wave propagation in granular beds.
Friction and Flow in Granular Materials
Authors: Behringer R. P. , Kondic L. , Metcalfe G. , Schaeffer D. , Tennakoon Sarath G. K.
Published: June 2019

We probe the transitions between solid-like and fluid-like granular states in the presence of shaking in the horizontal and vertical directions. These transitions are fundamental to other aspects of granular flow such as avalanche flow, in which there is a free surface. Key control parameters include accelerations in the vertical and horizontal directions, Γi = Aiω2i/g, for shaking of the form si = Ai cos(ωit + φi), i = h, v. Here, g is the acceleration of gravity. Also important is the relative phase between the two modes of shaking. We focus on low to moderate dimensionless accelerations, 0 < Γv,h < 1.6. We consider first the case Γv = 0, i.e. pure horizontal shaking. In this case, there is a hysteretic transition between solid and fluid states, where the fluid state consists of a sloshing layer of material of height H plus additional transverse flow. The hysteresis is lifted in the presence of a modest amount of fluidization by gas flow, or if a slight overburden is provided. We also identify a time scale, τ, for the transition between the phases that diverges inversely as the distance ε = (Γh–Γhc)/Γhc, from the appropriate transition points, i.e. as τ α ε-1. We identify a new convective mechanism, associated with horizontal shearing at the walls, as the mechanism that drives the transverse convective flow. For combined horizontal and vertical shaking, there exist a related set of novel dynamics and stability properties. These include the spontaneous formation of a static heap and a transition to flow, similar to the flow state under horizontal shaking, when the vertical acceleration Γv < 1. A simple friction model provides a good description of the steady states and a reasonably good description of the transition to flow. Horizontal and vertical shaking frequencies that differ by a small amount can lead to a novel switching state, as the relative phase, φh—φv, shifts over time.
Persistence of Granular Structure during Die Compaction of Ceramic Powders
Authors: Walker William J.
Published: June 2019

Glass spheres were used as a model system to investigate granule failure during die compaction. Stresses within an assembly of spheres follow a network of pathways. When the spheres are of uniform composition, the magnitude of the stresses within a pair of contacting granules is a function of the locally transmitted stress and the diameter of the two spheres. Results obtained using glass spheres demonstrated the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while others (∼40% by volume) persist at even the highest applied loads. Within a distribution of granule sizes, those granules with smaller diameter were seen to have a higher probability of failure at low pressure than were larger granules. These results are consistent with those observed during die compaction of granulated alumina powder.
MBE Growth of GaN using NH3 and Plasma Sources
Authors: Sampath A.V. , Bhattacharyya A. , Sandeep I. , Ng H.M. , Iliopoulos E. , Moustakas T.D.
Published: June 2019

We report on a comparative study of the growth of GaN in an arsenic free MBE system using either the method of plasma activation of molecular nitrogen or catalytic decomposition of ammonia on a heated substrate. We find that while growth with a plasma source leads to smooth films only under Ga- rich conditions, growth with ammonia leads to smooth films under ammonia-rich conditions. In both cases we find a 2×2 surface reconstruction when using an AlN buffer, which is evidence that material grown with this buffer layer has the Ga-polarity. In the case of plasma growth we also investigated the use of a GaNbuffer and found that at the growth temperature the surface is unreconstructed, however it undergoes 3×3 reconstruction upon cooling to 300 °C. This observation is evidence that material grown on a GaN buffer has the N-polarity.
Multiscale Simulations of Brittle Fracture and the Quantum-Mechanical Nature of Bonding in Silicon
Authors: Bernstein N. , Hess D.
Published: June 2019

We simulate the microscopic details of brittle fracture in silicon by dynamically coupling empirical-potential molecular dynamics of a strained sample to a quantum-mechanical description of interatomic bonding at the crack tip. Our simulations show brittle fracture at loads comparable to experiment, in contrast with empirical potential simulations that show only ductile crack propagation at much higher loading. While the ductility of the empirical potentials can be attributed to their short range, it is unclear whether the increased range of the tight-binding description is sufficient to explain its brittle behavior. Using the multiscale method we show that at a temperature of 1100 K, but not at 900 K, a dislocation is sometimes nucleated when the crack tip impinges on a vacancy. While this result is too limited in length and time scales to directly correspond to experimental observations, it is suggestive of the experimentally observed brittle to ductile transition.
Al Grain Boundary Embrittlement Promoted by Na Impurity: An ab initio Study
Authors: Lu Guang-Hong , Kohyama Masanori , Yamamoto Rayoichi
Published: June 2019

We calculate the electronic structure of AlΣ9 tilt grain boundary with substitutional Na impurity atoms by first principles pseudopotential method. Results show that by Na segregation Al grain boundary expands and the valence charge density decreases significantly along the boundary. There is no stronger bond than metallic bond in the boundary even with Na impurity. We therefore conclude that the mechanism of Na-promoted Al grain boundary embrittlement should be one kind of ‘decohesion model’.
Intrinsic and Extrinsic Size Effects in Plasticity by Dislocation Glide
Authors: Sevillano J. Gil
Published: June 2019

A classification of size effects (SE) in plasticity is attempted. ”Intrinsic” SE are perceived when any internal length scale directly influencing some process or property interferes with the size of the material region where the process is going on or when two internal length scales directly affecting the same process or property interfere. ”Extrinsic” SE arise from the external imposition of spatial gradients in the plastic process or by the building up of internal gradients by the (externally induced) process itself. In dislocation-mediated plasticity plastic strain gradients are resolved by the storage of geometrically necessary dislocations (GND) leading to prominent size effects. Of course, mixed effects with intrinsic and extrinsic contributions can be found as well as superposed effects involving more than two characteristic lengths (i.e., size effects on size effects).The inclusion of both types of SE in continuum or crystallographic theories is commented.
Micromechanical Modeling of Two-Phase Steels
Authors: Nygårds Mikael , Chandrasekaran Dilip , Gudmundson Peter
Published: June 2019

A two-dimensional micromechanical model based on the finite element method is presented to model two-phase ferritic/pearlitic steels, by aid of generalised plane strain elements. A periodic representative cell containing 100 ferrite grains, and the desired fraction pearlite is used. By applying periodic boundary conditions, loading by an average stress or strain state is possible.

Uniaxial tensile tests were performed on specimens containing the ferrite and pearlite microstructures, and on two-phase materials containing 25% and 58% pearlite respectively. The stress-strain data of the pearlite material is used to fit a laminar dependent Taylor relation to represent the pearlite workhardening. Thereafter, laminar spacings in the two-phase materials are measured, and the total stress-strain response of the materials is modelled. Comparisons between generated data and experiments show good agreement up to a strain of 2%.
Defect Accumulation and Recovery in Ion-Implanted 6H-SiC
Authors: Jiang W. , Weber W. J. , Wangxya C. M.
Published: June 2019

Single crystal wafers of <0001>-oriented 6H-SiC were irradiated at different temperatures using a variety of ion species. The disorder on both the Si and C sublattices has been studied in situ using a combination of ion beam analyses in multiaxial channeling geometry. The fraction of the irradiation-induced defects surviving simultaneous recovery processes decreases with decreasing ion mass and with increasing irradiation temperature. Some of the Si and C defects are well aligned with the <0001> axis and the rate of C disordering is higher than that of Si disordering. Three recovery stages in Au2+-irradiated 6H-SiC have been identified.
Possible shallowing of nitrogen donors in diamond
Authors: Miyazaki Takehide , Uda Tsuyoshi
Published: June 2019

Among many doping issues of diamond semiconductors, establishment of a shallow n-type impurity is a very important and yet challenging subject. Although both nitrogen (N) and hydrogen (H) are omnipresent impurities in diamond, they create only deep half-filled states in the energy gap. In this study, we present a theoretical proposal of a mechanism that makes N donors in diamond as shallow as possible. A complex of two adjacent substitutional N atoms with H sitting between the N atoms, an N-H-N defect, has a donor level approximately 1 eV shallower than of an isolated substitutional N defect.
Defect Engineering in CCD Image Sensors
Authors: McColgin William C. , Shantharama Lingadahalli G. , Lavine James P.
Published: June 2019

Defect engineering principles are integral to the design and manufacture of high-quality CCD image sensors. As examples, we describe the use of epitaxial silicon for defect control, hydrogen passivation of interface defects, and several forms of impurity gettering. The high sensitivity of image sensors to contaminants reveals that boron segregation gettering of iron dominates gettering by oxygen precipitates for both fast and slow cooling cycles. We estimate that the gettering efficiency for iron is 99.95%.
Radiation hardness of AlGaN/GaN based HEMTs
Authors: Vitusevich S.A. , Klein N. , Belyaev A.E. , Danylyuk S.V. , Petrychuk M.V. , Konakova R.V. , Kurakin A.M. , Rengevich A.E. , Avksentyev A.Yu. , Danilchenko B.A. , Tilak V. , Smart J. , Vertiatchikh A. , Eastman L.F.
Published: June 2019

In this work we present steady-state characteristics and low-frequency noise spectra of AlGaN/GaN based high electron mobility transistors (HEMTs) exposed to gamma ray radiation. The devices with a variety of gate length (150-350 nm) and width (100-400 νm) were irradiated by 60Co gamma rays with doses in the range of 104-109 Rad and flux of 102 Rad/s. Dose dependencies of basic operating parameters of the transistors, such as saturation current (Isat), transconductance (gm), channel conductance (gc), and threshold voltage (VT) are analysed. Our study show that visible changes of above mentioned parameters are observed under relatively small doses (105 Rad) and strongly depend on the HEMT's topology. The transconductance decreases and threshold voltage becomes more negative for all devices while deviation of these parameters from its initial values does not exceed 20% at highest irradiation dose. At the same time variation of the channel conductance as well as saturation current depends to a high extent on the gate voltage (Vg). At |Vg| < |Vcr|, both Isat and gc show a reversal proportional to the cumulative dosage of radiation. However, at |Vg| > |Vcr|, drain saturation current and channel conductance increase with the cumulative dosage of radiation. The effect is more pronounced in short-length-gate devices.
Nitrogen-Stabilized H2* Defects in GaP:N
Authors: Janotti A. , Zhang S. B. , Wei Su-Huai
Published: June 2019

The presence of N in III-V compounds such as GaAs and GaP drastically changes the behavior of H. We studied atomic structures of N-related hydrogen complexes in GaP:N using first-principles method. We show that the formation of the strong N-H bond is responsible for the stabilization of H2* complex that is otherwise unstable against the formation of H2 molecule. This provides the first theoretical proof that H2* can be stable in III-V semiconductor. The previously proposed H-N-H dihydride model is found to be unstable against spontaneously transforming into H2*, which involves only monohydrides, H-N and H-Ga. The calculated local vibration frequencies and isotope shifts are in good agreement with experiment.
Application of X-ray synchrotron techniques to the characterization of the chemical nature and recombination activity of grown-in and process-induced defects and impurities in solar cells
Authors: Buonassisi T. , Vyvenko O.F. , Istratov A.A. , Weber E.R. , Schindler R.
Published: June 2019

Results of the application of a combination of synchrotron radiation based analytical techniques, X-ray Beam Induced Current (XBIC) and microprobe X-ray Fluorescence (ν-XRF) to the analysis of shunts and lifetime limiting defects in solar cells are reported. XBIC, a new lifetime measurement technique similar to the Laser Beam Induced Current (LBIC) technique, uses a focused X-ray beam to generate minority charge carriers, which are then collected by the p-n junction of the solar cell. The X-ray beam is focused down to a spot size varying from approximately 1×1 νm to 5×5 νm, depending on the settings of focusing mirrors and slits. The sample stage is moved by computer-controlled step motors with sub-micron accuracy. Since the X-ray Beam Induced Current, which characterizes the minority carrier diffusion length in the spot where the X-ray beam hits the sample, and the X-ray Fluorescence signal, which characterizes the chemical nature of the precipitates under the beam, are measured at the same time, the chemical nature of the defects and impurities and their recombination activity can be studied simultaneously, in situ, and with a micron-scale resolution.

We present the results of the applications of these techniques to low lifetime regions in fully processed solar cells. The solar cells were pre-characterized by LBIC and thermography, and regions of interest (containing shunts) were selected. An ν-XRF scan in this area of low lifetime revealed the presence of silver and titanium far from the contact strip, suggesting a process-induced defect.
Raman and optical absorption studies of silicon carbide structure damage by ion implantation
Authors: Muntele Claudiu I. , Muntele Iulia C. , Ila D. , Poker David B. , Hensley Dale K.
Published: June 2019

The work reported here deals with studying the defects induced by heavy ion implantation as well as the degree of crystalline lattice recovery after annealing in a high purity argon environment between 600 and 1600 °C. We implanted 6H, n-type silicon carbide with Pd and Au ions at 1015 ions/cm2, and used Micro-Raman (MR) and optical absorption (OA) spectroscopy techniques for investigating the lattice properties and damage evolution at various stages during the fabrication process.
The Effect of Defects and Dopants on Thermal Conduction in GaN Films
Authors: Zou J. , Kotchetkov D. , Balandin A.A. , Florescu D.I. , Pollak F.H.
Published: June 2019

We present a theoretical investigation of the effects of dislocations, impurities and dopants on the thermal conductivity of GaN layers. It is shown that the experimentally observed decrease of the room-temperature thermal conductivity with increasing doping density is a result of enhanced phonon relaxation on silicon dopant atoms. Scattering of acoustic phonons on free carriers plays a relatively minor role in GaN. The functional dependence of the thermal conductivity on doping density is in good agreement with experiment. A developed model can be used for thermal budget calculation in high-power density GaN devices.
Comparative Investigation of Photoluminescence of In- and Si- doped GaN/AlGaN Multi-Quantum Wells
Authors: Sun W. H. , Wang L. S. , Chua S. J.
Published: June 2019

The GaN/AlGaN multi-quantum-wells (MQWs) have been grown via metalorganic chemical vapor deposition (MOCVD). Micro-Photoluminescence (PL) measurement has been performed on non-, In- and Si- doped GaN/AlGaN multi-quantum-well samples in the temperature ranges of 90-300 K. In the non-doped GaN/AlGaN multi-quantum-wells we observed the free exciton peak at 3.4587 eV at 90 K. Other exciton related peaks are located at 3.4346, 3.4177, 3.394, 3.3129eV, which are probably associated with the strongly localized excitons involving the defects. In In-doped GaN/AlGaN MQWs, the free exciton peaks have a slight red-shift from 3.4712 to 3.4629 eV, but the PL intensities become stronger with increasing trimethylindium (TMIn) flow from 10.6 to 42.6-νmol/min. With Si-doping in the well layers, photoluminescence exhibits an envelope of exciton bands ranged from 3.4796eV (free exciton) to 3.43915eV. The excitonic peaks in the bands vary in intensity and position with sample temperature. In addition, we have also observed the LO phonon replica of AlGaN interacted by the laser line due to the resonance effect.
The Strain relaxation and dislocation density of SiGe films in micron size window with different mask material grown by MBE
Authors: Zhang Xiang-jiu , Xiong Hui , Hu Jihuang , Jiang Zuiming , Fan Yongliang , Lin Jun , Chen Yuerui
Published: June 2019

The strain relaxation and the misfit dislocation density in micron size windows with different mask materials were studied. Experiments showed that the misfit dislocation density and the strain in the SiGe films would be affected not only by the size of the windows but the mask material. Experiments also showed that after annealing the SiGe films grown in the micron size windows would be much more stable than films grown in the large area on the same wafer.
Thermal Evolution of Deuterium in 4H-Sic
Authors: Delamare R. , Ntsoenzok E. , Sauvage T. , Shiryaev A. , Veen A. van , Dubois Ch.
Published: June 2019

4H-SiC samples were implanted at room temperature with 30 keV D+ ions at a dose of 5×1016 D+/cm2. Nuclear reaction analysis (NRA) and secondary ion mass spectroscopy (SIMS) measurements were performed to study the deuterium profiles after subsequent annealing at 1000-1250°C for 10min.Also, analytical techniques: RBS/C and thermal desorption spectroscopy (TDS) were carried out to characterize the evolution of implantation induced defects upon annealing. According to the NRA measurements, no deuterium release was found in the sample annealed at 1000°C. However, increasing the temperature to 1150°C led to a 40% decrease of deuterium content. Similar results about the evolution of D profiles upon annealing have also been obtained by SIMS measurements. In addition, SIMS measurements show that the maximum of the deuterium concentration shifts to the surface. Deuterium desorption at annealing temperatures higher than 1000°C was further confirmed by TDS experiments. Results from RBS/C indicated that during the desorption of deuterium, the implantation induced damage was annealed. These results are discussed.
Preparation and Characterization of Tin/Carbon Composites for Lithium-Ion Cells
Authors: Guidotti Ronald A. , Irvin David J. , Even William R. , Gross Karl
Published: June 2019

A number of Sn/C composites were prepared for evaluation as anode materials for Li-ion cells. In one case, samples were prepared by incorporation of Sn species into organic precursors that were then pyrolyzed under an Ar/H2 cover gas to prepare the Sn/C composites. They were also prepared by decoration of various types of carbon with nanoparticles of Sn by electroless deposition using hydrazine. The carbons examined included a disordered carbon prepared in house from poly(methacrylonitrile), a mesocarbon microbead (MCMB) carbon, and a platelet graphite. The Sn/C composites were examined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) and were also analyzed for Sn content. They were then tested as anodes in three-electrode cells against Li metal using 1M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) solution. The best overall electrochemical performance was obtained with a Sn/C composite made by electroless deposition of 10% Sn onto platelet graphite.
Synthesis and Processing of High Capacity, High Cycle life and High Discharge Rate Defective Manganospinel films for Rechargeable batteries
Authors: Singh Deepika , Hofmann Heinrich , Kim Won-Seok , Craciun Valentin , Singh Rajiv K.
Published: June 2019

Significant effort to develop a robust rechargeable battery has been put in the past two decades. The efforts were mainly focused on developing rechargeable battery systems which exhibit high capacity, long cycle life and high discharge rate capabilities. LiMn2O4 based cathodes have been researched extensively as they are not only economical but also environmentally desirable. Research includes composition and doping variation, formation of novel phases and microstructural tailoring, but none of the material modifications have successfully satisfied all the above mentioned performance criteria. In this paper we show a correlation between processing parameters, microstructure and electrochemical performance of Li-Mn-O cathode films. In addition we discuss the formation of metastable oxygen-rich lithium manganospinels, using a unique ultraviolet assisted deposition process. These defective films exhibit high capacity (> 230 mAh/gm), long cycle life (less than 0.05 % capacity loss per cycle for the first 700 cycles), and high discharge rates (> 25 C for 25 % capacity loss). The long cycle life and high capacity was attributed to the ability to cycle the Mn+ valence to less than 3.5 without onset of Jahn-Teller structural transformation, while the high discharge rate was attributed to the extremely high diffusivity of Li+ in the defective Li1-δMn2-2δO4 phase.
Texture Evolution in Zr Grain-refined by Equal Channel Angular Pressing
Authors: Yu S.H. , Ryoo H.S. , Shin D.H. , Hwang S.K.
Published: June 2019

To explore a possibility of obtaining an ultra fine grain size in hcp materials, equal channel angular pressing (ECAP) was applied to commercially pure Zr with the initial grain size of 20 μm. To ensure the crack-free specimen during the severe deformation it was necessary to adopt different die designs at the two processing temperatures, room temperature and 350°C. Submicrometer scale grains, in the order of 200 nm, were obtained by employing the die design of 90°/20° and the deformation temperature of 350°C. With the proper choice of the processing parameters, the refined grains were surrounded by the high angle boundaries. During the severe plastic deformation, the crystal texture underwent a significant change, from a fiber texture to that of a strong (0002) component mixed with a medium {1010} components, suggesting a dual  slip mode of deformation on the plane of the maximum shear stress.
Carbon Nanotube-Perovskite-Composites as New Electrode Material
Authors: Weidenkaff Anke , Ebbinghaus Stefan G. , Lippert Thomas , Montenegro Macarena J. , Reller Armin
Published: June 2019

In this paper we describe the synthesis and characterisation of La1-xAxCoO3 (A=Ca, Sr) (0 < x < 0,5) of different morphologies using pulsed laser deposition, ceramic methods and alternative soft-chemistry techniques. Furthermore the potential use of a La1-xCaxCoO3/carbon nanotube composite material for oxygen electrodes is discussed.
HVPE-GROWN AlGaN/GaN HEMTs
Authors: Luo B. , Ren F. , Mastro M. A. , Tsvetkov D. , Pechnikov A. , Soukhoveev V. , Dmitriev V. , Baik K. H. , Pearton S. J.
Published: June 2019

High quality undoped AlGaN/GaN high electron mobility transistors(HEMTs) structures have been gorwn by Hydride Vapor Phase Epitaxy (HVPE). The morphology of the films grown on Al2O3 substrates is excellent with root-mean-square roughness of ∼0.2nm over 10×10μm2 measurement area. Capacitance-voltage measurements show formation of dense sheet of charge at the AlGaN/GaN interface. HEMTs with 1μm gate length fabricated on these structures show transconductances in excess of 110 mS/mm and drain-source current above 0.6A/mm. Gate lag measurements show similar current collapse characteristics to HEMTs fabricated in MBE- or MOCVD grown material.
Mocvd Zinc Oxide Films for Wide Bandgap Applications
Authors: Rice C.E. , Tompa G.S. , Provost L.G. , Sbrockey N. , Cuchiaro J.
Published: June 2019

ZnO is a wide bandgap (3.2 eV) semiconductor with potential application in LEDs, lasers, and transparent transistors, among other uses. These applications require uniform thickness, high quality materials (amorphous, poly- or single crystal), pinhole- and defect-free-single-and multilayer-conformal coatings. These attributes are generally best achievable by MOCVD. We have mounted a significant effort to develop automated MOCVD systems and process technologies for single and multicomponent oxides. The reactors use high speed rotation and are of a vertical orientation built to all metal UHV standards. We have demonstrated reactor scaled performance from 3” to 12” diameter depositions planes with modeling scales through 24” diameter. Metalorganics are used for zinc and dopant sources as well as dopant gases to optimize performance at low pressures. In this paper we will discuss our most recent results with epitaxial ZnO films, achievements in p-type doping, multilayer structures, and polycrystalline doped ZnO films.
Cathodoluminescence, Electroluminescence, and Degradation of ZnCdSe Quantum Well Light Emitting Diodes
Authors: Nikiforov A. Yu. , Cargill G. S. , Guo S. P. , Tamargo M. C. , Chen Y.-C.
Published: June 2019

We have studied luminescence and degradation of a ZnCdMgSe/ZnCdSe/ZnCdMgSe QW LED grown by MBE and lattice matched to its InP substrate. Results have been obtained from time-resolved and bias-dependent cathodoluminescence spectroscopy and from electroluminescence spectroscopy. Reversible and irreversible variations in luminescence intensity were observed during bias cycling and electron bombardment. Cathodoluminescence from the QW LED was modeled by calculating current generation and transport, bias-dependent energy levels, envelope wave functions, overlap integrals, and carrier escape rates. Results of these calculations agree qualitatively, but not quantitatively, with bias-dependent intensity and wavelength changes observed in the experiments.
High-Performance AlGaN-Based Visible-Blind Resonant Cavity Enhanced Schottky Photodiodes
Authors: Kimukin Ibrahim , Biyikli Necmi , Kartaloglu Tolga , Aytür Orhan , Ozbay Ekmel
Published: June 2019

We have designed, fabricated and tested resonant cavity enhanced visible-blind AlGaN-based Schottky photodiodes. The bottom mirror of the resonant cavity was formed with a 20 pair AlN/AlGaN Bragg mirror. The devices were fabricated using a microwave compatible fabrication process. Au and indium-tin-oxide (ITO) thin films were used for Schottky contact formation. ITO and Au-Schottky devices exhibited resonant peaks with 0.153 A/W and 0.046 A/W responsivity values at 337 nm and 350 nm respectively. Temporal high-speed measurements at 357 nm resulted in fast pulse responses with pulse widths as short as 77 ps. The fastest UV detector had a 3-dB bandwidth of 780 MHz.
Investigation of Thin Film Growth of B12As2 by Chemical Vapor Deposition
Authors: Nagarajan R. , Edgar J.H. , Pomeroy J. , Kuball M. , Aselage T.
Published: June 2019

The chemical vapor deposition of icosahedral boron arsenide, B12As2, on 6H-SiC (0001) (on and off-axis) substrates was studied using hydrides as the reactants. The effects of temperature and reactant flow rates on the phases deposited and the crystal quality were determined. The growth rate increased with temperature from 1.5μm/h at 1100°C to 5 μm/h at 1400°C and decreased thereafter. X-ray diffraction revealed that the deposits were amorphous when the deposition temperature is below 1150° C. Above 1150°C, smooth B12As2 films were formed on 6H-SiC substrates with an orientation of (0001) B12As2 parallel to 6H-SiC (0001). Raman spectroscopy confirmed the strongly c-axis oriented nature of B12As2 film on 6H-SiC.
Solar Blind (Al,Ga)N Metal-Semiconductor-Metal Devices for High Performance Flame Detection
Authors: Mosca Mauro , Reverchon Jean-Luc , Grandjean Nicolas , Omnès Franck , Duboz Jean-Yves , Ribet Isabelle , Tauvy Michel
Published: June 2019

In this work we report on solar blind (Al,Ga)N photovoltaic metal-semiconductor-metal (MSM) detectors with a cutoff wavelength as short as ∼270 nm. (Al,Ga)N heterostructures, that allow backside illumination, were grown on sapphire substrates by both metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). We observed that both the interdigitated electrodes and the contact areas contribute to the overall photocurrent. In order to avoid the parasitic current due to the contact pads, we developed a new process where the electrodes are deposited on the (Al,Ga)N surface whereas the contact pads are deposited on an insulator (dielectric) layer besides the electrodes. Some layers develop microcracks related to excess stress. In that case, we showed that both the dark current and the responsivity strongly depend on the crack density. By using our two-level process, we could reduce the parasitic effects of cracks on the dark current. Several dielectrics were tested and our results are reported; values of dark current < 10 fA have been measured at 10 V bias voltage. An extremely high performance can then be reached in these ultraviolet solar blind detectors.
High-Speed Solar-Blind AlGaN Schottky Photodiodes
Authors: Biyikli Necmi , Kimukin Ibrahim , Kartaloglu Tolga , Aytür Orhan , Ozbay Ekmel
Published: June 2019

We report high-speed solar-blind AlGaN-based Schottky photodiodes. AlGaN/GaN heterostructure device layers were grown on sapphire substrate. The devices were fabricated on AlGaN/GaN heterostructuresusing a microwave compatible fabrication process. Schottky photodiodes with Au and indium-tin-oxide (ITO) Schottky contacts were fabricated. Current-voltage, spectral responsivity, and high-speed measurements were performed. Both Schottky samples exhibited very low sub-pA dark currents at high reverse bias. A bias dependent spectral responsivity was observed with a peak responsivity of 89 mA/W at 267 nm, and 44 mA/W at 263 nm for Au and ITO-Schottky devices respectively. Time-based high-frequency measurements at 267 nm resulted in pulse responses with rise times and pulse-widths as short as 13 ps and 74 ps respectively. The fastest solar-blind detector had a record 3-dB bandwidth of 1.10 GHz.
GaN Epitaxial Growth by Molecular Beam Epitaxy utilizing AlGaN Buffer Layer with Nanopipes
Authors: Yun F. , He L. , Reshchikov M. A. , Morkoç H. , Jasinski J. , Liliental-Weber Z.
Published: June 2019

GaN layers were grown on AlGaN with nanopipes by molecular beam epitaxy (MBE) and analyzed. AlGaN films were grown by MBE using rf-plasma nitrogen source under metal-rich condition. Within the Al composition range of 0.5-0.6, open-end nanopipes were formed at the surface of AlGaN films with a density of ∼6×109 cm-2 and a size ranging from 10 to 20 nm. These nanopipes, observed within ∼300 nm of the surface, served as a nanoporous AlGaN template for re-growth of GaN epilayers. GaN epilayers grown to different thickness by MBE were studied for their microstructural and optical properties. For an AlGaN buffer layer with dislocation density of 3×1010 cm-2 near its surface, the overlaying GaN layers with thickness ranging from 0.1 μm to ∼2μm were grown and analyzed by transmission electron microscopy for dislocation density. The GaN layer started with hexagonal islands on the nanopiped AlGaN and began to coalesce at about 0.1μm thickness. At a thickness of 2.0 μm, the dislocation density reduced to ∼1×109 cm-2. Low temperature photoluminescence data demonstrate the improved optical quality of GaN epilayer grown on the porous AlGaN buffer layer.
Pulsed Laser Deposition in a Zinc Alloy Corrosion Study
Authors: Guerrero-Penalva R. , Farías M.H. , Cota-Araiza L.
Published: June 2019

A significant improvement in corrosion resistance of the protecting oxide of alloys has been observed when adding small amounts of reactive elements, such as yttrium, this effect has been called reactive element effect (REE). The general mechanism of the REE has not been determined yet. In this work, we study a growing of a yttrium oxide film and its interaction with the phases η and α that constitutes the alloy Zn-22Al-2Cu named ZinalcoTM The alloy's surface was coated by a pulsed laser deposition technique. The deposit is controlled and characterized by x-ray photoelectron spectroscopy. The mechanism by which the reactive element produce its effects in this alloy is explained by the preferential interaction among the active sites related to the zinc rich phase and enhancing aluminum movement toward the surface where it is oxidized and the protection film formed.
Studies on Ultra-short Laser Micro Structuring
Authors: Zergioti I. , Papazoglou D. G. , Gamaly E. , Rode A. , Fotakis C.
Published: June 2019

A comparative study of the effect of ultrashort (0.5 ps) and short (30 ns) pulses on Laser transfer of Cr is presented in this paper. The dynamics of the process was investigated by stroboscopic schlieren imaging for time delays up to 3 νsec following the laser irradiation pulse. In contrast to the ns laser the directionality of the ejected material is very high in the case of the sub-ps laser process. The narrow angular divergence (3°) of the sub-ps pulses permits the direct dynamic transfer of the material and opens up new application possibilities for the fabrication of high spatial resolution microstructures.
Novel Growth of Biodegradable Thin Films via Matrix Assisted Laser Processing
Authors: Mercado A.L. , Fitz-Gerald J.M. , Johnson R. , Talton J.D.
Published: June 2019

The ability to controllably deposit polymers onto flat or curved surfaces in a quasi-dry environment while retaining native-like structure is of extreme importance to the medical and microelectronics communities. Current applications range from protective and conformal coatings for microelectronics to sustained drug delivery platforms in the pharmaceutical industry. In this research, biodegradable thin films of poly(DL-lactide-co-glycolide) (PLGA), were deposited onto flat substrates of Si and NaCl using a pulsed excimer laser, (λ= 248 nm) with fluences ranging from 0.1 – 1.0 J/cm2 via matrix assisted pulsed laser evaporation (MAPLE). Results are shown from scanning electron microscopy (SEM) to study morphological features and Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR) to measure chemical structure compared to original PLGA.
Direct Photo-Imprinting in High Photosensitive Organically Modified Germanosilicate (ORMOGSIL) Glasses
Authors: Jang Jae Hyeok , Kang Dong Jun , Bae Byeong-Soo
Published: June 2019

Direct photo-imprinting of both surface relief pattern and refractive index modulation upon the organically modified germanosilicate (ORMOGSIL) glass using its large volume change was performed by ultraviolet exposure. A large refractive index increase up to 10-2 is induced by ultraviolet-induced densification in the ORMOGSIL glasses. The photosensitivity in the ORMOGSIL glass was enhanced by introducing a photo-polymerizable methcrylate group in the glass structure. Also, a surface AFM scans and optical microscope images of unetched sample show that the volume compaction in the ultraviolet illuminated region is associated with periodic pattern inscription.
Development, Application and Validation Of Models for Waste Package Long-Term Performance; Current Trends
Authors: Werme Lars O. , Grambow Bernd
Published: June 2019

Current trends in modelling waste package performance are reviewed mainly from the perspective of the Swedish SKB studies. Examples are given, which illustrate the approaches for modelling different waste forms, i.e. HLW glass and spent nuclear fuel, and candidate canister materials, such as copper and steel. The relative importance of thermodynamics, reaction kinetics and near-field transport are discussed.
Corrosion Behavior of Container Materials in Grande Ronde Basalt Groundwater
Authors: Anantatmula R. P. , Fish R. L.
Published: June 2019

Three candidate waste package container materials were tested for 5 mo at 200 °C in Hanford Site Grande Ronde Basalt groundwater (9.75 pH) under anoxic conditions (<0.1 mg/L dissolved oxygen in water). The materials were cast carbon steel (American Society for Testing and Materials (ASTM) A27), wrought carbon steel (American Iron and Steel Institute (AISI) 1020), cupro- nickel 90-10, and Fe9CrlMo steel. Testing was performed in 1-L titanium autoclaves at a pressure of 6.9 MPa and a flow rate of ∼O.02 mL/min. Anoxic conditions were achieved by constantly sparging the synthetic groundwater in the reservoir with argon. In addition, the groundwater was conditioned by placing a 2.54-cm layer of crushed basalt (∼O.635 cm average size) at the bottom of each autoclave. The average corrosion rates at 200 °C were 0.9 μm/yr for cupronickel 90-10 and Fe9CrlMo steel, 1.1 μm/yr for cast carbon steel, and 1.4 μm/yr for wrought carbon steel. Pitting was not detected in any of the specimens. Posttest analysis of the corrosion speci- mens indicated the formation of a thin film of iron smectite clay on the surface of all specimens. In addition, magnetite formation immediately adjacent to the specimen surface was observed in the iron-base alloys, consistent with previous investigations. Based on the present investiga- tions, all the materials exhibited significantly lower corrosion rates than were used in the Environmental Assessment (∼5 μm/yr) [1] for calculating acceptable container lifetimes. However, longer term general corrosion tests and tests to study the container materials susceptibility to other degradation modes are necessary prior to making a final evaluation of the suitability of these materials for use in fabricating high-level nuclear waste containers.
Leaching Studies of Cement-Glass Package Containing Sodium Borate
Authors: Okamoto Masahiro , Chino Koichi , Baba Tsutomu , Izumida Tatsuo , Kawamura Fumio , Kikuchi Makoto
Published: June 2019

A new solidification technique using cement-glass, which is a mixture of sodium silicate, cement, additives, and initiator of the solidification reaction, was developed for sodium borate liquid waste generated from pressurized water reactor (PWR) plants. The cement-glass could solidify eight times as much sodium borate as cement could, because the solidifying reaction of the cement-glass is not hindered by borate ions.

The reaction mechanism of sodium silicate and phosphoric silicate (initiator), the main components of cement-glass, was studied through X-ray diffraction and compressive strength measurements. It was found that three- dimensionally bonded silicon dioxide was produced by polymerization of the two silicates. The leaching ratio of cesium from the cement-glass package was one-tenth that of the cement one. This low value was attributed to a high cesium adsorption ability of the cement-glass and it could be theoretically predicted accordingly.
Integrated Testing of the Srl-165 Glass Waste Form
Authors: Phinney D.L. , Ryerson F.J. , Oversby V.M. , Lanford W.A. , Aines R.D. , Bates J.K.
Published: June 2019

Integrated testing of the important components of a glass waste form waste package has been performed in order to gain a better understanding of the processes of radionuclide release and transport in the near field environment. Based upon an interpretation of the depth of penetration of hydrogen in reacted SRL-165 glass we have modeled the radionuclide release from the glass as a combined process of (1) the diffusive exchange of alkalis and boron in the glass for hydrogen species in the solution (D=10−16 cm2/s) and (2) surface dissolution. Surface dissolution controls the release of components not exchanged by diffusion and takes place at a rate of 1.5-3.0 μm/yr. Subsequent to release the radionuclides may remain in the leach solution, diffuse into the tuff, or precipitate as secondary phases. Precipitation is particularly important for plutonium and americium. Diffusive transport of radionuclides through the tuff takes place at an extremely slow rate, D=10−16 cm2/s. As such, the mass of radionuclides incorporated in the tuff by diffusion during the tests is inconsequential relative to that in the leach solution (with the exception of plutonium) and can be ignored in mass balance calculations. Mass balance calculations based upon the release of radionuclides by surface dissolution of the glass waste form are in good agreement with observed solution chemistry when allowances are made for a pulse of dissolution early in the tests. This pulse may be due to either the rapid dissolution of high-energy surface features early in the inLegrated tests, or an initially high surface dissolution rate that decreases with time as silica saturation is approached [1], or a combination of the two.
On a Global Database Strategy for Public Engineering Decision Making
Authors: Fong Jeffrey T. , P.E.
Published: June 2019

A historical review of how engineers contributed to the solution of societal (public) and individual (private) problems is presented, with emphasis on the last thirty years when computers began to alter drastically the practice of engineering.

The concept of a 3-prong representation of the engineering decision making process, i.e., database, analysis, and judgment, is introduced. The importance of generating a suitable body of experimental and actual performance data and of coupling the database with a state-of-the-art analysis and extrapolation methodology for “public engineering” is illustrated in three case studies involving (a) the nuclear reactor safety study of 1975, (b) the Alaska pipeline weld defect assessment study of 1976, and (c) the Welding Research Council-Pressure Vessel Research Committee (PVRC) Round Robin Flaw Detection Program (1966-86) for thick-section steel welds.

For most of the public engineering projects involving long-term prediction of performance from short-term test data, the time-honored approach of supplementing experimental or service data with engineering judgment is frequently open to a public challenge. To remedy the situation, a new approach of using both the personal computers (PC) and the mainframes to (a) accelerate the peer review of existing data, (b) design a strategy for acquiring new data, and (c) facilitate a regional or global exchange of field data with standardized format, is outlined.

Application of the above approach to strengthen the judgment component of public engineering projects such as the management of high-level nuclear wastes is discussed through an example on the fatigue life prediction of stainless steel 304 piping and components.
Modeling of the Effect of Iron Corrosion Products on Nuclear Waste Glass Performance
Authors: Grambow B. , Zwicky H. U. , Bart G. , Bjorner I. K. , Werme L. O.
Published: June 2019

Experiments [1], performed in the frame of the JSS-project (joint Japanese [CRIEPI], Swiss [NAGRA], Swedish [SKB] project), demonstrated that the effect of iron corrosion products on the long-term stability of HLW-glass depends on the effective surface area of the corrosion products. The experimental results were described quantitatively by thermodynamic and kinetic modeling with the computer codes GLASSOL and PHREEQE. The model interprets the effect in terms of surface sorption of a monolayer of silica molecules from solution. Silica saturation occurs in solution only, when all surface sites are occupied. The effect of bentonite is also considered. The agreement between calculations and experiment is about as good as in the case of modeling the glass-water reaction only.