bluuebunny/crossref_metadata_2025 huggingface dataset:
A Model of Wafer Bonding by Elastic Accommodation
Authors: YU H. H. , SUO Z.
Published: April 2011

Two clean and flat wafers can adhere spontaneously. The technique has recently led to novel electronic and optoelectronic devices. The adhesion arises from short-range interatomic forces between wafer surfaces, which can be represented by a reduction in surface energy associated with the transformation of two surfaces into one interface. The wafers, however, are seldom perfectly flat; the misfit has to be accommodated by elastic distortion, plastic deformation, or mass transport. We model elastic accommodation in this paper. The distortion causes the wafers to gain elastic energy. If the surface energy reduction dominates over the elastic energy gain, the wafers will bond. We solve the three dimensional elastic field in the misfit wafers analytically. The conditions for bonding are established, and practical implications discussed.
Integration of Silicon and Diamond, Aluminum Nitride or Aluminum Oxide for Electronic Materials
Authors: Bengtsson Stefan , Bergh Mats , Söderbärg Anders , Edholm Bengt , Olsson Jörgen , Ericsson Per , Tiensuu Stefan
Published: April 2011

Material integration for the formation of advanced silicon-on-insulator materials by wafer bonding and etch-back will be discussed. Wafer bonding allows combining materials that may not be possible to grow on top of each other by any other technique. In our experiments, polycrystalline diamond, aluminum nitride or aluminum oxide films with thickness of 0.1-5 µm were deposited on silicon wafers. Bonding experiments were made with these films to bare silicon wafers with the goal of forming silicon-on-insulator structures with buried films of polycrystalline diamond, aluminum nitride or aluminum oxide. These silicon-on-insulator structures were aimed to address self-heating effects in conventional silicon-on-insulator materials with buried layers of silicon dioxide. The surfaces of the deposited diamond films were, by order of magnitude, too rough to allow direct bonding to a silicon wafer. In contrast the deposited aluminum nitride and aluminum oxide films did allow direct bonding to silicon. Bonding of the diamond surface to silicon was instead made through a deposited and polished layer of polycrystalline silicon on top of the diamond. In the case of the aluminum nitride electrostatic bonding was also demonstrated. Further, the compatibility of these insulators to silicon process technology was investigated.
Dynamics of Wet Oxidation of High-Al-Content III-V Materials
Authors: Ashby Carol I. H.
Published: April 2011

Oxidation of layers of high-Al-content III-V materials by water vapor has become the enabling process for high-efficiency vertical cavity surface emitting lasers (VCSELs) and has potential applications for reducing substrate current leakage in GaAs-on-insulator (GOI) MESFETs. Because of the established importance of wet oxidation in optoelectronic devices and its potential applications in electronic devices, it has become increasingly important to understand the mechanism of wet oxidation and how it might be expected to affect both the fabrication and subsequent operation of devices that have been made using this technique. The mechanism of wet oxidation and the consequence of this mechanism for heterostructure design and ultimate device operation are discussed here.
Fermi-Level effect and junction carrier concentration effect on p-Type dopant distribution in IlI-V Compound superlattices
Authors: Chen Chang-Ho , Gösele Ulrich M. , Tan Teh Y.
Published: April 2011

The pronounced segregation phenomenon in the distribution of p-type dopants Zn and Be in GaAs and related III-V compound heterostructures has been explained quantitatively by treating simultaneously the processes of dopant atom diffusion, segregation, and the effect of heterojunction carrier concentrations on these two aspects. Segregation of a dopant species between two semiconductor heterostructure layers is described by a model incorporating (i) a chemical effect on the neutral species; and (ii) in addition, a Fermi-level effect on the ionized species. The process of Zn and Be diffusion in GaAs and related compounds is governed by the doubly-positively-charged group III element self-interstitials whose thermal equilibrium concentration and hence also the Zn and Be diffusivities exhibit also a Fermi-level dependence, i.e., in proportion to p2.A heterojunction is consisting of a space charge region with an electric field, in which the hole concentration is different from those in the bulk layers. This influences the junction region concentrations of and of Zn− or Be−, which in turn influence the distribution of the ionized acceptor atoms. The overall process involves diffusion and segregation of holes, , Zn− or Be−, and an ionized interstitial acceptor species. The junction electric field also changes with time and position.
Peak Voltage Insensitivity to Quantum Well Width in Resonant Interband Tunneling Diodes
Authors: Kitabayashi H. , Waho T. , Yamamoto Masafumi
Published: April 2011

We have investigated the peak voltage (Vp) in InAs/A1Sb/GaSb/A1Sb/InAs double-barrier resonant interband tunneling (DBRIT) diodes with various GaSb well widths (Lw). After eliminating the voltage drop resulting from parasitic resistance included in the measured peak voltage (Vpm), we found that the intrinsic peak voltage (Vpint) was almost insensitive to Lw and had a small, and virtually constant value (∼ 60 mV) at Lw, ranging from 10 to 30 monolayers. This insensitivity contrasts with the peak voltage of the conventional resonant tunneling diodes with intraband tunneling, which is sensitive to changes in Lw. This insensitivity can be explained by the fact that Vpint in Sb-based BDRIT diodes is affected by the occupied states in the collector rather than by the resonance level itself in the interband tunneling process.
High Performance CuMetallized GaAs HEMTs Processing and Reliability
Authors: Feng T. , Dimoulas A. , Strifas N. , Christou A.
Published: April 2011

AlGaAs/GaAs based high electron mobility transistors (HEMTs) with Cu/Ti metallized gates have been fabricated. Copper gates were used to achieve low gate resistance and to minimize the hydrogen induced device degradation. The DC measurement of the processed AlGaAs/GaAs HEMTs with Cu/Ti gates shows comparable performance to similar Au based GaAs HEMTs. The Cu-based HEMTs were also subjected to elevated temperature testing under 5% H2 –N2 forming gas up to 250°C and 8 hours and no degradation due to hydrogen effects was found.
Application of SiO2 Films deposited by TICS/O2 PECVD to InSb MISFET
Authors: Sugiura O. , Akiba T. , Idris I.
Published: April 2011

We have fabricated n-channel InSb MISFETs using SiO2 films as gate insulators. The insulator was deposited by PECVD using tetra-isocyanate-silane (TICS) and oxygen (O2) as source gases. Threshold voltage and carrier mobility at 77K were 0.5V and 4200cm2/Vs, respectively.
Optimization of Microstructure and Dislocation Dynamics in InxGa1-xP Graded Buffers Grown on GaP by Movpe
Authors: Kim A.Y. , Fitzgerald E.A.
Published: April 2011

To engineer high-quality InxGal-xP graded buffers on GaP substrates (InxGa1-xP/GaP), we have explored the evolution of microstructure and dislocation dynamics in these materials. We show that the primarily limiting factor in obtaining high-quality InxGa1-xP/GaP is a new defect microstructure that we call branch defects. Branch defects pin dislocations and result in dislocation pileups that cause an escalation in threading dislocation density with continued grading. The morphology of branch defects is dominated by growth temperature, which can be used to suppress the strength or density of branch defects. In the absence of branch defects, we observe nearly ideal dislocation dynamics that are controlled by the kinetics of dislocation glide. This new understanding results in two primary design rules for achieving high-quality materials: 1) control branch defects, and 2) maximize dislocation glide kinetics. Combining these design rules into optimization strategies, we develop and demonstrate processes based on single and multiple growth temperatures. With optimization, threading dislocation densities below 5 × 106 cm−2 are achieved out to x = 0.39 and a nearly steady-state relaxation process is recovered.
Deep Level Characterization of Interface-Engineered ZnSe Layers Grown by Molecular Beam Epitaxy on GaAs
Authors: Hierro A. , Kwon D. , Ringel S. A. , Brillson L. J. , Young A. P. , Franciosi A.
Published: April 2011

Deep level defects have been detected and analyzed in epitaxial ZnSe layers grown by molecular beam epitaxy (MBE) on GaAs and on In0.04Ga0.96As using deep level optical spectroscopy (DLOS). A series of samples, which differ only in the initial Zn:Se beam pressure ratio (BPR = 1:1, 1:10, 10:1) during the growth nucleation step, were characterized by DLOS in order to assess the dependence of bulk deep level formation on interface nucleation conditions. The transient and steady state photocapacitance measurements were performed using 100 W Quartz Halogen and 450 W Xe lamps as light sources, in the spectral range of 0.9 to 2.9 eV with a resolution better than 0.02 eV. The capacitance transients were recorded for time windows of 10 msec to 5 sec after light excitation of the sample, which was kept at a temperature of 100 K. Using semi-transparent Au Schottky contacts, several deep levels in the ZnSe layer were detected for all BPR's, with optical threshold energies of 1.1, 1.46 and 1.9 eV below the conduction band. These energies were obtained from the slope of the capacitance transient at different time intervals and were confirmed by steady state photocapacitance. The concentration of the levels was in the range 1012 to 1014 cm−3. Both the 1.1 and 1.46 eV trap concentrations were found to depend strongly on lattice mismatch conditions, whereas the latter was shown to largely depend on BPR. The optical threshold of the 1.9 eV trap correlates well with a ˜1.9 eV cathodoluminescence (CL) peak, which has been previously associated with either Zn vacancies or Gazn substitutional defects in Zn-deficient material.
Growth and Characterization of GaN and ALxGA1−xN Thin Films Achieved Via Lateral- and/or Pendeo-Epitaxial Overgrowth on 6H-SIC(0001) Substrates
Authors: Davis Robert F. , Nam O-H. , Zheleva T. S. , Bremser M. D. , Linthicum K. J. , Gehrke T. , Rajagopal P. , Thomson D. B. , Carlson E. P.
Published: April 2011

Discrete and coalesced monocrystalline layers of lateral- and pendeo-epitaxially grown GaN and AlxGal−xN layers originating from GaN stripes deposited within windows contained in SiO2 masks or from side walls of GaN seed structures containing SiNx top masks have been grown via organometallic vapor phase deposition on GaN/AlN/6H-SiC(0001) substrates. Multilayer heterostructures of GaN and AlxGal−N were also achieved. Scanning and transmission electron microscopies and atomic force microscopy were used to evaluate the microstructures, the type and distribution of dislocations and the surface roughness of the resulting films. The extent and microstructural characteristics of the laterally overgrown GaN regions were a strong function of stripe orientation and temperature. These regions contained a low density of dislocations. The RMS roughness of the (1120) sidewall plane of the pendeoepitaxial structures was approximately 0.100 nm.
Evolution of Fractal Particles in Systems with Conserved Order Parameter
Authors: Kalinin S.V. , Gorbachev D.L. , Tomashevitch K.V. , Borisevich A.Yu. , Vertegel A.A. , Markworth A.J. , Tretyakov Yu. D.
Published: April 2011

In the present research, the evolution of fractal aggregates in systems with conserved order parameter is studied. The aggregates are generated by diffusion limited aggregation (DLA). This model describes such important processes as annealing of dendrite inclusions in solids, healing of cracks in ceramics, temperature-induced transformations in composites, relaxation of rough surfaces, aging of colloid particles, etc. It is shown that the evolution in fractal media differs significantly from the evolution from the initially homogeneous state and leads to the different values of the scaling exponent.
Multiscale Modeling of Recrystallization
Authors: Miodownik M.A. , Holm E.A. , Godfrey A.W. , Hughes D.A. , Lesar R.
Published: April 2011

We propose a multi length scale approach to modeling recrystallization which links a dislocation model, a cell growth model and a macroscopic model. Although this methodology and linking framework will be applied to recrystallization, it is also applicable to other types of phase transformations in bulk and layered materials. Critical processes such as the dislocation structure evolution, nucleation, the evolution of crystal orientations into a preferred texture, and grain size evolution all operate at different length scales. In this paper we focus on incorporating experimental measurements of dislocation substructures, misorientation measurements of dislocation boundaries, and dislocation simulations into a mesoscopic model of cell growth. In particular, we show how feeding information from the dislocation model into the cell growth model can create realistic initial microstructures.
Kinetic Monte Carlo Simulations of Fecu Alloys
Authors: Domain C. , Becquart C.S. , Duysen J.C. Van
Published: April 2011

The steel vessels of pressurized water reactors are embrittled by neutron irradiation. It is well known that copper atoms play an important role in the embrittlement and that different Cu-containing defects such as Cu-rich clusters (sometimes called atmospheres), Cu precipitates and Cu-vacancy complexes have been identified experimentally. It is still difficult to link the formation of these defects to the primary damage resulting from the neutron inducing displacement cascades. Therefore, we investigate the evolution of the primary damage in FeCu alloys using kinetic Monte Carlo simulations based on a vacancy diffusion mechanism. The calculations rely on adapted, phenomenological, n-body potentials that satisfactorily reproduce properties of FeCu. At room temperature, experimentally identified defects such as Cu-vacancy complexes (one Cu atom bound to three or four vacancies) form in the course of our simulations. Furthermore, it appears that complex defects such as a Cu atom linked to two vacancies are very mobile and are responsible for the formation of small Cu clusters.
Self-Consistent Kinetic Lattice Monte Carlo
Authors: Horsfield Andrew , Dunham Scott , Fujitani Hideaki
Published: April 2011

We present a brief description of a formalism for modeling point defect diffusion in crystalline systems using a Monte Carlo technique. The main approximations required to construct a practical scheme are briefly discussed, with special emphasis on the proper treatment of charged dopants and defects. This is followed by tight binding calculations of the diffusion barrier heights for charged vacancies. Finally, an application of the kinetic lattice Monte Carlo method to vacancy diffusion is presented.
AB-Initio Based Calculations of Vacancy Formation and Clustering Energies Including Lattice Relaxation in Fe3Al
Authors: Muratov Leonid S. , Cooper Bernard R.
Published: April 2011

Vacancy formation and clustering significantly affect structural properties of transition-metal aluminides. Ab-initio quantum mechanical total-energy calculations using a full-potential linear combination of muffin-tin orbitals (LMTO) technique provide a convenient method of studying relevant characteristics such as changes in density of states, and charge redistribution around defects. Augmented with Hellmann-Feymann forces, LMTO allows calculations of relaxation geometries and relaxation energies. We have performed such calculations for vacancies and antisite substitutional point defects in Fe3Al with DO3 crystallographic structure. There are two limiting factors complicating calculations of defect formation energies directly from ab-inito calculations. The first is that a single defect, due to the lattice periodicity necessitated by the use of ab-inito total energy techniques, cannot be considered as an isolated defect, even in the maximum computable simulation cell. Unlike previous calculations [ I ], which did not find a dependency on the size of the simulation cell, our calculations have shown a significant difference in results for 32- and 16- atom cells. This difference provides information about vacancy clustering since it can be explained by a relatively small attractive interaction energy ~0.2 eV between two vacancies located in adjacent simulation cells and separated by the lattice constant distance (5.52Å) [2]. By comparing the internal energies for two configurations of 30 atom cells (32 atom - 2 vacancies) we were able to estimate that the attractive interaction between two vacancies could reach 1.2 eV. The second complication is the fact that chemical potentials of elements cannot be directly extracted from the total energy calculations for the compound. To deal with this problem, we considered two possible approximations and compared results, which were found to be quite similar for iron vacancies.
An Effect of Hydrostatic Compression on Defects in Energetic Materials: AB Initio Modeling Maija
Authors: Kuklja M. , Kunz A. Barry
Published: April 2011

First-principle theoretical investigation of the basic defects such as a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack in organic explosive molecular crystals is presented. As an example we considered solid RDX (C3H6N6O6) which is well studied unstable solid. It was established that external hydrostatic pressure changes optical properties of defect-free RDX as well as of the crystal with defects narrowing the band gap. The lattice defects (especially dislocations) are identified with the so-called “hot spots.” The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbitals of N-NO2 group is analyzed. Favorable conditions for molecular dissociation due to electronic excitation are shown.
Understanding Structure and Electronic Properties of Extended Self-Interstitial Defects in Silicon
Authors: Alippi P. , Colombo L.
Published: April 2011

The results of an atomistic investigation on the coalescence mechanisms of self-interstitial {311} defects are presented. Formation energies and equilibrium configurations of defect structures are determined by tight-binding molecular dynamics simulation. We focus on the characterization of the lattice strain field around the defect complex: By means of the determination of the atomic stress distribution, we discuss how it may influence the formation mechanisms of the planar {311} structures. We also attempt a correlation between structural features and electronic properties through the analysis of defect-related orbitals occupations and inverse participation ratios.
First-Principles Study of π-Bonded (100) Planar Defects in Diamond
Authors: Zapol Peter , Curtiss Larry A. , Gruen Dieter M.
Published: April 2011

A periodic density functional study of the high-energy π-bonded (100) stacking fault in diamond that can serve as a prototype of a twist grain boundary has been carried out. Information on formation energies, geometries and the electronic structure has been obtained. A single point electronic structure calculation of a ∑5 twist grain boundary based on the geometry taken from a molecular dynamics simulation has also been performed.
Atomistic Studies of Generic Tilt Grain Boundary Structures
Authors: Pawaskar D. N. , Miller R. , Bai R. , Schwartzman A. , Phillips R. , Briant C. L.
Published: April 2011

We investigate the atomic-scale structures of high-sigma (∑) tilt grain boundaries in aluminum using lattice statics calculations. In particular, we examine the efficacy of the structural unit model (SUM) in the context of long-period boundaries. Our investigation of both the equilibrium and metastable structures for certain high ∑ boundaries may necessitate a revision of the SUM formalism. We also consider further departures from high symmetry boundaries by considering the structural rearrangements induced by steps on boundaries.
Point Defect Interactions with Extended Defects in Silicon
Authors: Justo J. F. , Antonelli A. , Fazzio A.
Published: April 2011

We investigated the interaction of point defects (vacancy and self-interstitials) with an intrinsic stacking fault in silicon. The calculations were carried out using ab initio total energy methods. The results show that point defects at an intrinsic stacking fault display a different behavior as compared to the same defect in the crystalline environment. This is evidenced by differences in formation energies and electronic structures. These results suggest that there is migration of point defects from the bulk to a stacking fault. This could affect the dislocation mobility in the crystal.
Bias Potentials for Ttyperdynamics Simulations
Authors: Rudd W. G. , Voter A. F.
Published: April 2011

We describe our studies of two methods for generating bias potentials for use in hyperdynamics simulations. In the first method, first reported by Steiner, et al, the bias potential is the additional energy needed to keep the total potential energy above a fixed level. This potential exerts a negligible computational load and is very easy to code. The second technique involves computing the iterative Hessian-based bias potential as usual, but including in those calculations only the atoms in a small “active region” that surrounds the area in which a state transition is expected to occur. This “subspace hyperdynamics” method is less costly than full HD. The extra computational effort required for HD scales as the size of the active region(s), instead of with the size of the entire simulation domain.

We have carried out extensive tests of both methods on two problems, the diffusion of adatoms on silver surfaces and the migration of vacancies in bulk silver. Using the first method, to obtain large boosts the bias potential must be so high that many transitions are prevented from occurring. Our results for subspace hyperdynamics are promising; we obtain the same results as with full hyperdynamics, but with considerably less computational effort.
Thermal Conductivity of Solid Argon by Classical Molecular Dynamics
Authors: Kaburaki Hideo , Li Ju , Yip Sidney
Published: April 2011

Following the Green-Kubo formalism in linear response theory, the lattice thermal conductivity of solid argon is determined by using classical molecular dynamics simulation to calculate the heat current correlation function. Comparing the absolute conductivities obtained using the Lennard-Jones potential with experiments, we find the predicted results to uniformly underestimate the measurements in magnitude, whereas the calculated temperature dependence corresponds well with the data. The temporal behavior of the heat current autocorrelation function shows that while a single exponential decay description is appropriate at elevated temperatures, below the half of the Debye temperature, the heat current relaxation clearly consists of two stages, an initial rapid decay associated with local dynamics followed by a slower component associated with the dynamics of lattice vibrations (phonons).
Bond-Order Potentials for Molybdenum and Niobium: An Assessment of Their Quality
Authors: Mrovec M. , Vitek V. , Nguyen-Manh D. , Pettifor D. G. , Wang L. G. , Sob M.
Published: April 2011

The bond-order potentials (BOP) have been constructed for Mo and Nb. These potentials are based on the real-space parametrized tight-binding method in which diagonalization of the Hamiltonian is avoided by direct calculation of the bond-order. In this scheme the energy consists of three parts: The bond part that comprises contributions of d electrons and introduces into the scheme the covalent character of bonding, the central-force many-body part that reflects the environmental dependence of sp overlap repulsion and a pair-wise contribution. The potentials were tested by calculation of energy differences between the bcc and several alternate structures and by investigating the trigonal deformation path. These calculations have been made in parallel using BOP and the full-potential linearized augmented plane-wave method. The central-force many-body Finnis-Sinclair type potentials have also been included into the study of the deformation path. This evaluation of BOP reveals that the potentials reproduce very closely the ab initio results and are, therefore, very suitable for atomistic studies of extended defects in the transition metals.
Hydrogenated Amorphous Silicon Nitride: Structural and Electronic Properties
Authors: Justo J. F. , Mota F. De Brito , Fazziom A.
Published: April 2011

We combined empirical and ab initio methods to study structural and electronic properties of amorphous silicon nitride. For such study, we developed an interatomic potential to describe the interactions between silicon, nitrogen, and hydrogen atoms. Using this potential, we performed Monte Carlo simulations in a simulated annealing scheme to study structural properties of amorphous silicon nitride. Then this potential was used to generate relevant structures of a-SiNx:Hy which were input configurations to ab initio calculations. We investigated the electronic and structural role played by hydrogen incorporation in amorphous silicon nitride.
Molecular Orbital Calculations on Atomic Structures of Si-Based Covalent Amorphous Ceramics
Authors: Matsunaga K. , Matsubara H.
Published: April 2011

We have performed ab-initio Hartree-Fock molecular orbital calculations of local atomic structures and chemical bonding states in Si-N covalent amorphous ceramics. Solute elements such as boron, carbon and oxygen were considered in the Si-N network, and the bonding characteristics around the solute elements were analyzed. When a nitrogen atom is substituted by a carbon atom, it was found that Si-C bonds reinforce the Si-N network due to strong covalency.
Nanocrystal Growth in Crystalline Insulators Irradiated with High-Current Cu IONs
Authors: Kishimoto N. , Umeda N. , Takeda Y. , Lee C.G. , Gritsyna V.T.
Published: April 2011

Negative Cu ions of 60 keV have been applied to generate metal nanocrystals embedded in insulators. Crystalline (c-), amorphous (a-)SiO2 and a spinel oxide, MgO·2(Al2O3), were irradiated at various dose rates up to about 100 µA/cm2, at a total dose of 3.O×1016 ions/cm2. In a-Sio2, morphology of nanoparticles and the resultant optical property changed with dose rate and, at a critical condition, showed in-plane arrangement of nanocrystals. The optical property of c-Si0 2 (a-quartz) was qualitatively similar to that of a-SiO2, but the implanted region of c-SiO2 showed different irradiation responses. The c-SiO2 was susceptible to radiation-induced amorphization but the nanoparticle morphology was still different from a-SiO2, suggesting a stronger depth-oriented driving force. Unlike SiO2, the spinel oxide showed good structural stability against the implantation and no tendency of the long-range atomic rearrangement. The results indicate that the crystallinity and the relevant interactions may play an important role in the nanoparticle growth during the implantation.
Dose Rate Dependence of the Amorphization of Silicon Carbide
Authors: Snead L.L. , Zinkle S.J. , Eatherly W.S. , Hensley D.K. , Vaughn N.L. , Jones J. W.
Published: April 2011

Single crystal silicon carbide (SiC) has been 2 MeV silicon ion irradiated in various irradiation temperature and ion flux ranges to measure the effect of these parameters on the critical dose for amorphization. The temperature and flux range for which amorphization was observed ranged from 80 to 400 K and 0.066 to 3 × 104 dpa/s, respectively. The critical dose, Dcrit was found by locating the depth of the boundary between partially crystalline and fully amorphous material using dark field TEM from samples prepared in cross section. This depth was compared to the damage profile as calculated using the TRIM-96 code. The temperature dependence of Dcrit is found to agree well with previously reported values, though new evidence suggests a defect species becoming mobile in the 250-300 K range. Also of significance is that Dcrit was dependent on flux at 340 K, ranging from 0.79 displacements per atom at the lowest ion flux to ∼0.6 dpa at the highest flux level. The dose rate dependence of Dcrit, is compared with a chemical rate theory model previously described by the authors. It is seen that the dose rate dependence is substantially weaker than theorized. An extrapolation of the measured dose rate dependence is also compared with recent data on fast neutron amorphized SiC.
Formation of ZnAl2O4 and MgAl2O4 Spinel in Al2O3 by Ion Implantation
Authors: White C.W. , Meldrum A. , Sonder E. , Budai J.D. , Zuhr R.A. , Withrow S.P. , Henderson D.O.
Published: April 2011

ZnAl2O4 spinel has been formed in Al2O3 by ion implantation. Sequential implantation of Zn and 0 in overlapping profiles followed by annealing in Ar + H2 gives rise to a nearly continuous epitaxial layer of ZnAl2O4 oriented with (111) planes parallel to (0001) planes of Al2O3. If only Zn is implanted, then discrete bands of ZnAl2O4 oriented with (422) planes parallel to (0001) planes of Al2O3 are produced. By similar methods, oriented MgAl2O4 spinel also has been produced in Al2O3 by sequential Mg + O implantation.
Interphase Integrity of Neutron Irradiated SIC Composites
Authors: Snead L. L. , Lara-Curzio E.
Published: April 2011

SiC/SiC composites were fabricated from Hi-Nicalon™ fibers with carbon, porous SiC and multilayer SiC interphases. These materials were then irradiated in the High Flux Beam Reactor with fast neutrons at 260 and 900–1060°C to a dose of l.1×1025 n/m2 corresponding to 1.1 displacements per atom (dpa). Results are presented for bend strength of both non-irradiated and irradiated materials. Within the interphases studied the multilayer SiC interphase material showed the least degradation (8-20%) in ultimate bend stress, while porous SiC underwent the greatest degradation (∼35%). The fiber matrix interphases are studied with TEM for both non- irradiated and irradiated materials. While no irradiation induced microstructural evolution of the interphase was observed, debonding of the interphase from the fiber was observed for all cases. This debonding is attributed to tensile stresses developed at the interface due to densification of the Hi-Nicalon™ fiber. Residual stress analysis of the fiber matrix interface indicates that the irradiation-induced densification of Hi-Nicalon™ and the volumetric expansion of the CVD SiC matrix cause tensile stresses well in excess of those which can be withstood by these, or any other viable SiC composite interphase.
Microstructure of Swift Heavyion Irradiated MgAl204 Spinel
Authors: Zinkle S.J. , Matzke Hj. , Skuratov V.A.
Published: April 2011

Plan view and cross-section transmission electron microscopy was used to investigate the microstructure of magnesium aluminate spinel (MgAl2O4) following room temperature irradiation with either 430 MeV Kr, 614 MeV Xe, or 72 MeV I ions. The fluences ranged from 1×1016/m2 (single track regime) to 1×1020/m2. Destruction of the ordered spinel crystal structure on both the anion and cation sublattices was observed in the ion tracks at low fluences. At intermediate fluences, the overlapping ion tracks induced the formation of a new metastable crystalline phase. Amorphization with a volumetric expansion of ∼35% was observed in spinel irradiated with swift heavy ions (electronic stopping powers >7 keV/nm) at fluences above 1×1019/m2. These results demonstrate that swift heavy ion radiation can induce microstructural changes not achievable with conventional elastic collision irradiation at comparable temperatures.
Damage Evolution in Xe-Ion Irradiated Rutile (TiO2) Single Crystals
Authors: Li Fuxin , Lu Ping , Sickafus Kurt E. , Evans Caleb R. , Nastasi Michael
Published: April 2011

Rutile (TiO2) single crystals with (110) orientation were irradiated with 360 keV Xe2+ ions at 300K to fluences ranging from 2×1019 to 1×1020 Xe/m2. Irradiated samples were analyzed using: (1) Rutherford backscattering spectroscopy combined with ion channeling analysis (RBS/C); and (2) cross-sectional transmission electron microscopy (XTEM). Upon irradiation to a fluence of 2×1O19 Xe/m2, the sample thickness penetrated by the implanted ions was observed to consist of three distinct layers: (1) a defect-free layer at the surface (thickness about 12 nm) exhibiting good crystallinity; (2) a second layer with a low density of relatively large- sized defects; and (3) a third layer consisting of a high concentration of small defects. After the fluence was increased to 7×1019 Xe/m2, a buried amorphous layer was visible by XTEM. The thickness of the amorphous layer was found to increase with increasing Xe ion fluence. The location of this buried amorphous layer was found to coincide with the measured peak in the Xe concentration (measured by RBS/C), rather than with the theoretical maXimum in the displacement damage profile. This observation suggests the implanted Xe ions may serve as nucleation sites for the amorphization transformation. The total thickness of the damaged microstructure due to ion irradiation was always found to be much greater than the projected range of the Xe ions. This is likely due to point defect migration under the high stresses induced by ion implantation.
Effect of Stress and Impurities on Preferential Amorphization on Grain Boundaries in Polycrystalline Silicon
Authors: Takeda Mitsuhiro , Ohnuki Somei , Watanabe Seiichi , Abe Hiroaki , Naramoto Hiroshi , Okamoto Paul R. , Lam Nigh Q.
Published: April 2011

Clarifying the local amorphization on the grain boundaries, the in-situ observation during ion-irradiation was carried out for poly-crystalline Si film. The critical dose of amorphous formation increased exponentially with increasing temperature, where the local amorphization was developed at middle temperature. The critical dose was affected by the doped impurity and the grain size. The preferential amorphization on and near grain boundaries had two processes; first stage with rapid growth rate and second stage with almost constant growth rate. The importance of stress was demonstrated from the acceleration due to the stress on the first stage of amorphization.
Exploring the Excited States of Vacancy Defects in Silica
Authors: Song J. , Corrales L. R. , Jònsson H.
Published: April 2011

A study of vacancy defect formation is carried out using a density functional theory molecular dynamics simulation methodology. The objective is to study the effects of electronic excitations in crystalline and non-crystalline silica polymorphs. We present an initial study focusing on the formation energetics of distinct oxygen vacancy defect states in α-quartz.
Radiation Effects in Zircon, Hafnon, and Thorite: Implications for Pu Disposal
Authors: Meldrum A. , Zinkle S.J. , Boatner L.A. , Wu M. , Mu R. , Ueda A. , Henderson D.O. , Ewing R.C.
Published: April 2011

Synthetic ZrSiO4, HfSiO4, and ThSiO4 single-crystal specimens were irradiated by 800 keV Kr+ ions, and the microstructural evolution was observed in-situ in a transmission electron microscope. All three compounds were found to become amorphous up to temperatures in excess of 600°C. Using a new model, the activation energies for annealing were found to be in the range of 3.1 to 3.6 eV for these compounds. At temperatures above 600°C, the orthosilicates were observed to decompose into the component oxides (e.g., tetragonal ZrO2 + amorphous SiO2 in the case of zircon). A single-crystal zircon specimen was also irradiated with a pulsed picosecond Nd:YAG laser operated at 355 nm, and the resulting microstructure was investigated by optical absorption, SEM, AFM, and TEM techniques.
Anomalous Small-Angle X-Ray Scattering Characterization of Model Pressure Vessel Alloy Microstructures
Authors: Alexander Dale E. , Kestel B. J. , Jemian P. R. , Odette G. R. , Klingensmith D. , GRAGG D.
Published: April 2011

Anomalous small-angle x-ray scattering (ASAXS) experiments were performed at energies near the Mn-K and Fe-K absorption edges on an Fe-0.9 Cu wt.%-1.0 Mn wt.% alloy subjected to either annealing at 450°C, 24 hrs or irradiation with 10 MeV electrons to 0.5 mdpa at 300°C. A Guinier analysis of the net scattered intensity revealed scatterers with radii of 16.5.16.8 Å and 16.4.17.3 Å for the irradiated and annealed foils, respectively. The variation in net scattered intensity with photon energy in the annealed sample was consistent with scattering contrast variations expected for Cu-rich, Cu-Mn precipitates. Minimal variation in net intensity (scattering contrast) with energy determined that Cu-Mn precipitates were not detected in the irradiated sample.
Microchemistry of Proton-Irradiated Austenitic Alloys Under Conditions Relevant to Lwr Core Components
Authors: Was G. S. , Allen T. R. , Busby J. T. , Gan J. , Damcott D. , Carter D. , Atzmon M. , Kenik E. A.
Published: April 2011

Over 1200 measurements of grain boundary composition and microstructure have been made on 14 different austenitic Fe-Cr-Ni alloys following proton irradiation in the temperature range 200-600°C and in the dose range 0.1-3.0 dpa. Grain boundary composition measurements revealed that Cr depletes at grain boundaries, Ni enriches and Fe can either enrich or deplete depending on alloy composition. Analysis of temperature and composition dependence of RIS revealed that the magnitude and direction of grain boundary segregation depends on alloy composition because the value of migration enthalpy differs among the alloy constituents, and diffusivities of the alloy constituents are composition-dependent. The dose dependence of segregation revealed ordering in Ni-base alloys and temperature dependence was used to show that RIS occurs by vacancy exchange rather than an interstitial binding mechanism. The dependence of segregation on composition is consistent with all known, relevant neutron data.
The Effect of Low Dose-Rate Irradiation on the Microstructure of 304 Stainless Steel
Authors: Cole J. I. , Allen T. R.
Published: April 2011

Changes in mechanical and corrosion properties caused by the development of radiation-induced microstructures have relevance to the aging and lifetime extension of light water reactors (LWR‘s). However, much of the current data related to microstructural development in irradiated metals are generated from studies carried out at much higher dose-rates than encountered in LWR‘s. An opportunity exists to study the influence of low dose-rate irradiation on microstructural development for a variety of structural and surveillance materials extracted from the experimental breeder reactor EBR-lI. In this study, irradiated 304 stainless steel hexagonal “hex” duct material is examined in order to compare microstructures in the dose-rate range of 10−7 - 10−9 dpa/sec. The samples, taken from the reflector locations in EBR-II, experienced a total dose between 10 and 12 dpa at a temperature of ∼375 °C. Transmission electron microscopy (TEM) analysis reveals that there is a moderate dose-rate effect on microstructural development for samples irradiated in the range of 2 × 10−8 to 4 × 10−8. dpa/sec, however a substantial dose-rate effect exists between dose-rates of 2 × 10−8 and 1 × 10−9 dpa/sec Results detail the development of the microstructure in terms of radiation-induced cavities, dislocations, and precipitates.
Origin and Influence of Pre-Existing Segregation on Radiation-Induced Segregation In Austenitic Stainless Steels
Authors: Kenik E. A. , Busby J. T. , Miller M. K. , Thuvander A. M. , Was G.
Published: April 2011

The pre-existing segregation at grain boundaries in two austenitic stainless steels has been investigated by atom probe field ion microscopy and analytical electron microscopy. In addition, the effect of radiation-induced segregation on the near-grain-boundary composition has been studied by analytical electron microscopy. Pre-existing enrichment of Cr, Mo, B, C and P and depletion of Fe and Ni near grain boundaries has been observed. Significant affinity between Mo and N in both alloys is indicated by the detection of MoN2+` molecular ions during field evaporation. The pre-existing segregation is modified by radiation-induced segregation resulting in Ni and Si enrichment near the boundary as well as depletion of chromium adjacent to the boundary resulting in a “W-shaped” Cr profile.
Dissolution of Copper Precipitates in An Fe−1·3Wt%Cu Alloy Under Fe+ Ion Irradiation
Authors: Nicol A. C. , Jenkins M. L. , Wanderka N. , Abromeit C.
Published: April 2011

The stability of Cu precipitates in an Fe-1.3wt%Cu alloy under 300 keV Fe+ion irradiation has been investigated using transmission electron microscopy and high-resolution electron microscopy. The irradiations were carried out between room temperature and 550°C at displacement rates of 103 to 10−2 dpa(s)−1 to fluences of up to 30 dpa. Copper precipitates were found to keep their shape but decrease in size under all irradiation conditions. The results are discussed within the framework of a competitive process between irradiation induced ballistic destruction of precipitates by cascades and irradiation-enhanced precipitation.
Modeling the Post-Yield Flow Behavior After Neutron and Electron Irradiation of Steels and Iron-Base Alloys
Authors: Dimelfi R.J. , Alexander D.E. , Rehn L.E.
Published: April 2011

Irradiation hardening is an issue of practical importance as it relates to the remanent life and the nature of failure of reactor components exposed to displacement-producing radiation. Understanding these phenomena requires studies of fundamental microstructural mechanisms of hardening. In this paper, we analyze the tensile behavior of pressure vessel steels (A212B and A350) irradiated by neutrons and electrons. The results show that the post-yield true stress/true strain behavior can provide fingerprints of the different hardening effects that result from irradiation by the two particles, which suggests correspondingly different hardening microstructures for the two particles. Microstructurally-based models for irradiation-induced yield strength increases, combined with a model for strain hardening, are used to make predictions of the different effects of irradiation by the two particles on the entire flow curve that agree well with data.
Examination of Neutron-Irradiated Pressure-Vessel Steel Using Positron Annihilation Lifetime Spectrosopy
Authors: Cumblidge Stephen E. , Motta Arthur T. , Catchen Gary L.
Published: April 2011

On a variety of pressure-vessel (PV) steels, we have observed changes in the average positron lifetime with increasing (near end-of-life) neutron fluences. Samples were irradiated at reactor-temperature and subjected to post-irradiation annealing, and they were examined using positron annihilation lifetime spectroscopy (PALS). The measured average positron lifetimes in high-temperature (2900 C-300° C) irradiated PV steels decrease with increasing neutron damage up to fluences of 8.5×1018 cm−2 and increase again at higher fluences. Annealing of high-fluence, 300° C irradiated ASTM A508 PV steel samples produces an initial decrease in average positron lifetimes with increasing annealing temperatures of up to 400° C, followed by an increase in average positron lifetime with higher annealing temperatures, when samples were annealed in successive 24-hour steps. A sample of weld steel, irradiated to 2.2×1019 cm−2 at 290° C, shows similar behavior in which the minimum lifetime occurs at ≈ 450° C. These trends are similar to those seen in previous studies performed on VVER and other ferritic steels.
A Comparison of the microstructure and tensile behaviour of irradiated fcc and bcc metals
Authors: Baluc N. , Bailat C. , Dai Y. , Luppo M.I. , Schaublin R. , Victoria M.
Published: April 2011

As part of an on-going research program, findings are presented from a comparison of the microstructures and associated tensile properties of fcc and bcc materials after high energy proton irradiation, to fluences between 10−4 and 1 dpa, at 300-320 K. Results for this comparison between Cu, Pd, 304 and 316 stainless steel on one side and Fe and the F82H ferritic-martensitic low activation steel on the other are discussed, showing a strong difference in defect accumulation behaviour between the differing crystal structures. The overall deformation behaviour is similar, with an initial localised deformation taking place in all cases, even though the actual deformation mode itself might be different. Furthermore, a comparison is made with some of the materials that have also been irradiated with fission neutrons, showing no influence of the PKA spectra for these irradiation conditions.
From Molecular Dynamics to Kinetic Rate Theory: A Simple Example of Multiscale Modeling
Authors: Stoller Roger E. , Greenwood Lawrence R.
Published: April 2011

Radiation damage formation in iron has been investigated using the method of molecular dynamics simulation. The MD simulations have been used to determine primary defect production parameters for cascade energies up to 50 keV at temperatures from 100 to 900K. The energy dependence of these parameters has been used to determine appropriate neutron-energy- spectrum averaged damage production cross sections for various irradiation environments. Two applications of these effective cross sections are discussed. The first is an evaluation of neutron energy spectrum effects in commercial fission reactor pressure vessels. The second example deals with the use of these cross sections in the source term of a kinetic model used to predict void swelling and microstructural evolution. The simulation of the primary damage event by MD involves times less than 100 ps and a size scale of a few tens of nm, while the kinetic simulation encompasses several years and macroscopic sizes. This use of the MD results to develop an improved source term for rate theory modeling provides a simple example of multiscale modeling.
Exploring The Use of Pulsed Irradiations for Studying the Transient Sink Formed by Cascade Producing Radiation
Authors: Alexander Dale E.
Published: April 2011

An approach is suggested for using pulsed irradiation experiments to study attributes of the transient sink formed by cascade producing radiation. As an example, a simple model of radiation enhanced diffusion is proposed for a dual ion beam irradiation in which one of the beams is a pulsed, sink-producing heavy ion. It is demonstrated that by combining this model with diffusivity measurements evaluated as a function of the irradiation pulse wavelength, it is possible to quantitatively determine the transient sink strength and its characteristic lifetime.
On The Mechanism of Interstitial Cluster Migration in α -FE
Authors: Barashev A.V. , Osetsky Yu.N. , Bacon D.J.
Published: April 2011

Recent molecular dynamics (MD) computer simulations have shown that clusters consisting of up to a few tens of self-interstitial atoms (SIAs) are highly mobile along closed-packed crystallographic directions in pure copper and iron. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms of the cluster motion. In the present paper the results of MD modelling of the thermally-activated motion of clusters of 3, 9 and 17 SIAs in α-Fe in the temperature range from 90 to 1400 K are analyzed. The extensive MD data has enabled the migration of clusters, as well as that of individual SIAs in the clusters, to be treated with high statistical accuracy. The correlation between the motion of the centre of gravity of a cluster and the jumps of individual SIAs in the cluster is revealed. It is found that the SIAs in a cluster jump almost independently and their jump frequency depends on the number of SIAs in the cluster. This leads to a simple relationship between the jump frequency of a cluster and the number of SIAs in it. The cluster jump frequency exhibits a deviation from the Arrhenius relationship. The reason for this is discussed.