bluuebunny/crossref_metadata_2025 huggingface dataset:
Doping of GaN by ion implantation
Authors: Alves Eduardo J. , Liu C. , Silva Maria F. da , Soares José C. , Correia Rosário , Monteiro Teresa
Published: April 2011

In this work we report the structural and optical properties of ion implanted GaN. Potential acceptors such as Ca and Er were used as dopants. Ion implantation was carried out with the substrate at room temperature and 550 °C, respectively. The lattice site location of the dopants was studied by Rutherford backscattering/channeling combined with particle induced X-ray emission. Angular scans along both [0001] and [1011] directions show that 50% of the Er ions implanted at 550 °C occupy substitutional or near substitutional Ga sites after annealing. For Ca we found only a fraction of 30% located in displaced Ga sites along the [0001] direction. The optical properties of the ion implanted GaN films have been studied by photoluminescence measurements. Er- related luminescence near 1.54 µm is observed under below band gap excitation at liquid helium temperature. The spectra of the annealed samples consist of multiline structures with the sharpest lines found in GaN until now. The green and red emissions were also observed in the Er doped samples after annealing.
Determination of the Distribution of Ion Implantation Boron in Silicon
Authors: Wang Te-Sheng , Cullis A.G. , Collart E.J.H. , Murrell A.J. , Foad M.A.
Published: April 2011

Boron is the most important p-type dopant in Si and it is essential that, especially for low energy implantation, both as-implanted B distributions and those produced by annealing should be characterized in very great detail to obtain the required process control for advanced device applications. While secondary ion mass spectrometry (SIMS) is ordinarily employed for this purpose, in the present studies implant concentration profiles have been determined by direct B imaging with approximately nanometer depth and lateral resolution using energy-filtered imaging in the transmission electron microscopy. The as-implanted B impurity profile is correlated with theoretical expectations: differences with respect to the results of SIMS measurements are discussed. Changes in the B distribution and clustering that occur after annealing of the implanted layers are also described.
Optical and Structural Changes of Fe Implanted Sapphire
Authors: Marques Carlos P. , Alves Eduardo J. , McHargue Carl J. , Silva Maria F. da , Soares José C. , Correia Rosário , Soares Manuel J. , Monteiro Teresa
Published: April 2011

Single crystalline colorless α-Al2O3 samples were implanted with several fluences of Fe+ ions in the range of 1×1016 to 5×1017 Fe+ cm-2 at room temperature. Optical absorption and luminescence measurements were carried out before and after annealing in reducing and oxidizing atmospheres. The structural changes were studied with RBS/channeling and x-ray diffraction. After implantation, the damage induces a brownish coloration in the samples for fluences below 2×1017 Fe+ cm-2. The optical spectra are characterized by an absorption band centered at 200 nm. This band is strongly reduced after annealing at 1100 °C in reducing atmosphere and a new well-defined band develops around 350 nm. This new band shifts to lower values with the implanted fluence, which is an indication of its correlation with the dimensions of the iron precipitates formed in the implanted region. The existence of these precipitates was confirmed by x-ray diffraction and TEM. The samples implanted with fluences above 1×1016 Fe+ cm-2 annealed in oxidizing atmosphere display different optical absorption spectra, with respect to those annealed in reducing atmosphere, characterized by an increase in the intensity of the peak at 200 nm. Luminescence measurements show the presence of F and F+ centers in the samples. The existence of these defects can be explained by the need of charge compensation and strain release due to the formation of mixed iron oxide or metallic precipitates.
Modification of the electronic properties of a-Si1-xCx:H by Fe+ ion implantation
Authors: Tsvetkova T. , Balabanov S. , Amov B. , Angelov Ch. , Zuk J. , Maczka D. , Adriaenssens G.J. , Iakoubovskii K.
Published: April 2011

The electrical and optical properties of hydrogenated amorphous silicon-carbide alloy films (a-Si1-xCx:H), modified by Fe+ ion implantation (D = 1×1016 - 1×1017 cm-2), have been investigated. Optical transmission spectra have been measured in the visible range (400-900 nm) and considerable absorption edge shift to the longer wavelength region has been registered, together with well-defined decrease of transmittance in the whole measured range. These effects are increased with the dose and are similar for samples with different carbon content (x1=0.08 and x2=0.15). Room temperature (RT) surface electrical conductivity is also increased by Fe+ implantation and the effect is most pronounced for the highest doses (D ~ 1017cm-2). The temperature dependence of the conductivity was measured in the temperature range from RT to 250 oC. The activation energy is considerably reduced and the effect is again strongest for the highest doses. From the above results we conclude that high doses Fe+ implantation in a-Si1-xCx:H affects both the localised states deep in the gap and the shallow states in the band tails.
Atomistic simulations of the Ostwald ripening of Si nanoparticles ion beam synthesized in SiO2
Authors: Bonafos C. , Carrada M. , Colombeau B. , Altibelli A. , Assayag G. Ben , Garrido B. , Lopez M. , Perez-Rodriguez A. , Morante J. R. , Claverie A.
Published: April 2011

Most studies of Si nanocrystals ion beam synthesized in SiO2have shown that a link exists between the observed physical properties and the characteristics of the « populations » of nanoparticles (size-distribution, density, volume fraction). Nevertheless, the direct measurement of these parameters by Transmission Electron Microscopy (TEM) is very difficult and predictive simulations are essential to make the kinetic study complete. This paper presents atomistic simulations aimed at predicting the kinetic evolution of the Si nanoparticles during annealing, taking into account eventual interaction effects. The theoretical results are compared to TEM measurements of the size-distributions observed after high temperature annealing. Experimentally, the growth of these nanoparticles is very slow but their size significantly increases when increasing the initial Si excess. Simulations are in agreement with these experimental results only when taking into account possible interactions between neighboring particles for initial supersaturations larger than 10 at. %.
Effects of Ga-Irradiation On Properties of Materials Processed by A Focused Ion Beam (FIB)
Authors: Wanzenboeck H. D. , Langfischer H. , Lugstein A. , Bertagnolli E. , Grabner U. , Pongratz P. , Basnar B. , Smoliner J. , Gornik E.
Published: April 2011

Focused Ion Beam (FIB) technology allows to process various materials within a lateral range below 100 nm. The feasibility to mechanically sputter as well as to direct-write nanostructures and the fact that Ga-ions are utilized is unique for this method. The focused Ga-ions are used to locally induce a chemical vapor deposition of volatile precursor molecules adsorbed on a surface. Local deposition of metals and dielectrics has been achieved on a sub-µm scale utilizing a focused ion beam. This method is highly suitable for advanced microelectronic semiconductor fabrication. However, material specifications are narrow for these tailor-made applications. The effect of the Ga-ions implanted into the material both during sputtering and deposition has been realized as a key parameter for the function of FIB processed microelectronic devices. For Si-based semiconductors Ga can be used as dopant intentionally implanted into a Si substrate to locally modify the conductivity of Si. The results of locally confined ion irradiation on the surface roughness of a Si surface have been exploited by atomic force microscopy (AFM). Both local sputter depletion of the sample surface as well as sub-µm deposition of selected metals or dielectrics by ion-induced chemical vapor deposition (CVD) has been examined. The penetration depth and the distribution of Ga ions during the deposition process have been studied by simulation and experimentally by profiling with secondary ion mass spectroscopy (SIMS). Transmission Electron Microscopy (TEM) of cross-sections of the ion processed materials has revealed amorphisation of the crystalline substrate. For focused ion beam assisted deposition the effects of ion irradiation on the interface to the substrate and the local efficiency of the deposition are illustrated and discussed. The prospects of focused ion beam processing for modification of microelectronic devices in the sub-µm range and the limitations are demonstrated by the examples shown.
Spontaneous formation of nanometer-scale self-organized structures in Ag-Cu alloys under irradiation
Authors: Enrique Raúl A. , Bellon Pascal
Published: April 2011

Ion-beam irradiation can be used as a processing tool to synthesize metastable materials. A particular case is the preparation of solid solutions from immiscible alloys, which have been achieved for a whole range of systems. In this process, enhanced solute concentration is obtained through the local mixing induced by each irradiation event, which if occurring at a high enough frequency, can outweigh demixing by thermal diffusion. The resulting microstructure forms in far from equilibrium conditions, and theoretical results for these kind of driven alloys have shown that novel microstructures exhibiting self-organization can develop. To test these predictions, we prepare Ag-Cu multilayered thin films that we subject to 1 MeV Kr+-ion irradiation at temperatures ranging from room temperature to 225 °C, and characterize the specimens by x-ray diffraction, TEM and STEM. We observe two different phenomena occurring at different length scales: On the one hand, regardless of the irradiation temperature, grains grow under irradiation until reaching a size limited by film thickness (~200 nm). On the other hand, the distribution of species inside the grains is greatly affected by the irradiation temperature. At intermediate temperatures, a semi-coherent decomposition is observed at a nanometer scale. This nanometer-scale decomposition phenomenon appears as an evidence of patterning, and thus confirms on the possibility of using ion-beam irradiation as a route to synthesize nanostructured materials with novel magnetic and optical properties.
A Finite Element Study on the Nanoindentation of Thin Films
Authors: Chen Xi , Vlassak Joost J.
Published: April 2011

Nanoindentation is a technique commonly used for measuring thin film mechanical properties such as hardness and stiffness. Typically, shallow indentations with contact depths less than 10-20% of the film thickness are used to ensure that measurements are not affected by the presence of the substrate. In this study, we have used the finite element method to investigate the effect of substrate and pile-up on hardness and stiffness measurements of thin film systems. We find that: i) for soft films on hard substrates, the hardness is independent of the substrate as long as the indentation depth is less than 50% of the film thickness; ii) as soon as the hardness exceeds that of the substrate, the substrate effect becomes significant, even for indentations as shallow as 5% of the film thickness; iii) if the film is at least 40 times harder than the substrate, the plastic zone is mostly confined to the substrate while the film conforms to the deformed substrate by bending. We define a substrate effect factor and construct a map that may be useful in the interpretation of indentation measurements on thin films. It is found that the yield stress mismatch is a key factor characterizing the hardness of thin film system, and the elastic mismatch is important when making stiffness measurements. The results obtained in this study are very useful when it is difficult to avoid the influence of the substrate on the measurements.
Epoxy/Aluminum Adhesion as Measured by Contact Mechanics (JKR) in the Presence of an Organic Contaminant
Authors: Woerdeman Dara L. , Emerson John A. , Giunta Rachel K.
Published: April 2011

The JKR contact mechanics approach is employed to analyze the effects of surface contaminants on adhesive bonding, as well as quantify the level of contamination at which adhesive strength decreases. The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to join the two surfaces, subsequently applying a tensile load to assess the energy dissipation mechanisms involved in the debonding process. In the present work, we monitor the interactions between a diglycidyl epoxy elastomer and an aluminum oxide substrate in the presence of an organic contaminant. Furthermore, we present a method by which surface contamination can be quantified using a single number, referred to as the adhesion hysteresis parameter, H.
Adhesion of a Rigid Cylinder to an Incompressible Film
Authors: Yang Fuqian , Zhang Xinzhong , Li J.C.M.
Published: April 2011

The adhesion between a rigid cylindrical particle with a flat end of radius a and an incompressible elastic film of thickness h deposited on a rigid substrate was studied. The contact surfaces between the particle and the film and between the film and the substrate are either frictionless (slip) or perfectly bonded (stick). Using integral equations, the stress distribution in the contact area was solved and used to obtain the load required to press the particle onto the thin film. Using a thermodynamic method, the pull-off force to separate the particle from the film was obtained numerically and analytically. For a>>h, the pull-off force is proportional to a2/h1/2 if it is frictionless on both contact interfaces and is proportional to a3/h3/2 if it is frictionless between the particle and thin film and bonded between the thin film and the substrate. For a<h/2 the pull-off force is proportional to a3/2 independent of h and the boundary conditions. To verify this the self adhesion of PDMS [poly(dimethylsiloxane)] was determined experimentally and the a3/2 relation was confirmed. The self-adhesion of PDMS was found to increase with the square root of contact time suggesting molecular diffusion as the dominant mechanism.
Correlation of Nanoindentation and Conventional Mechanical Property Measurements
Authors: Rice Philip M. , Stoller Roger E.
Published: April 2011

A series of model ferritic alloys and two commercial steels were used to develop a correlation between tensile yield strength and nano-indentation hardness measurements. The NanoIndenterII® was used with loads as low as 0.05 gf (0.490 mN) and the results were compared with conventional Vickers microhardness measurements using 200 and 500 gf (1.96 and 4.90 N) loads. Two methods were used to obtain the nanohardness data: (1) constant displacement depth and (2) constant load. When the nanohardness data were corrected to account for the difference between projected and actual indenter contact area, good correlation between the Vickers and nanohardness measurements was obtained for hardness values between 0.7 and 3 GPa. The correlation based on constant nanoindentation load was slightly better than that based on constant nanoindentation displacement. Tensile property measurements were made on these same alloys, and the expected linear relationship between Vickers hardness and yield strength was found, leading to a correlation between measured changes in nanohardness and yield strength changes.
Plastic Zone Development Around Nanoindentations
Authors: Woodcock C. L. , Bahr D. F. , Moody N. R.
Published: April 2011

Johnson's cavity model relating indenter geometry and deformation resulting from elastic-plastic indentations is appropriate for a wide variety of materials. In the case of nanoindentations in single crystal BCC metals, limitations are reached when creep is not fully accounted for. Both the standard Berkovich and cube corner geometries show that the ratio of plastic zone radius to contact radius increases with the duration of time at the peak load. Indenter tip geometry is shown to play an important role in this phenomenon. Length scale phenomena, such as the indentation size effect, are also subject to various interpretations. The traditional definition of hardness does not produce similar trends with indentation length scale between the blunt Berkovich geometry and the sharper cube corner tip. However, the ratio of plastic zone radius to contact radius proves to be a tip geometry independent method of assessing the plasticity of these metals.
Nanoscratch Deformation Response of Carbon Nitride Thin Films
Authors: Charitidis C. , Patsalas P. , Logothetidis S.
Published: April 2011

The mechanical performance of thin carbon nitride (CNx) films deposited onto Si(100) substrate by sputtering, was investigated in the load range of 2-20 mN using nanoindentation and nanoscratch techniques. The microstructure of these films was characterized by the inhomogeneous embedding of crystalline grains into a N-poor amorphous matrix. A load dependent transition was found in both the scratch, as well as the friction responses. Below 5 mN, nanoscratching showed completely elastic behavior of the film, while above 10 mN a mixed elastic-plastic behavior was identified. Testing under a normal load of 20 mN resulted in local grooving at the film surface; however, in-situ profiling of the scratch trace showed no evidence of film failure. Both the coefficient of friction (μ), as well as the plastic deformation of the films were found increasing with the normal load applied following an exponential low.
Effect of Film Thickness on the Nanoindentation Measurements of Hard Diamondlike Carbon Films Prepared by Pulsed Laser Deposition
Authors: Wei Q. , Sankar J. , Sharma A. K. , Narayan J.
Published: April 2011

We have investigated the effect of film thickness on the nanoindentation measurements of hard diamondlike carbon (DLC) films. The DLC films were deposited on Si (100) substrates by pulsed excimer laser deposition (KrF, λ=248nm, duration=25 ns, energy density about 3.0 J/cm2, repetition rate 10 Hz) in high vacuum (∼5×10−7torr) at room temperature for various periods of time (from 5 minutes to 20 minutes). The thickness of the films was measured by optical profilometry, and ranges from 200 to 50 nm. The nanohardness and elastic modulus of these films were measured by a depth-sensing nanoindentation technique with NanoindenterXP. Preliminary experimental observations show that the nanoindentation results are a function of film thickness. Both the hardness and Young's modulus versus displacement curves of the 200nm thick film exhibit a peak at around tenth of the film thickness. The rest of the samples with smaller film thickness show plateaus, which are higher than the hardness and Young's modulus values of the substrate.
An Assessment of the Mechanical Strengths of Aluminide-based Thin Coatings
Authors: Li S.Y. , Ng H.P. , Ngan Alfonso H.W.
Published: April 2011

Titanium aluminide and nickel aluminide-based thin coatings were synthesized by magnetron sputtering from intermetallic TiAl and Ni3Al alloy targets on nickel substrates. Both types of aluminide coatings exhibited high surface hardness values that varied with the degree of heat treatment. The hardness of the coatings was investigated using micro- and nano- indentation techniques. In order to estimate the intrinsic strength of the films, the indentation size effects of the apparent hardness were analyzed by the Jönsson-Hogmark model and a model recently proposed by the authors. The analysis indicated that the strengths of the aluminide coatings may considerably exceed their strengths in bulk.
Mechanical Response of Diamond at Nanometer Scaes: Diamond Polishing and Atomic Force Microscopy
Authors: Pérez Ruben , Jarvis Murray R. , Payne Michael C.
Published: April 2011

Total energy pseudopotential methods are used to study two different processes involving the mechanical interaction of diamond nanoasperities and diamond surfaces: the wear processes reponsible for diamond polishing, an the mechanical deformation of tip and surface during the operation of the Atomic Force Microscope in contact Mode (CM-AFM). The strong asymmetry in the rate of polishing between different dirctions onthe diamond (110) surface is explained in terms of on atomistic mechanism for nano-groove formation. The pst–polishing surface morphology and the nature of the polishing residue epredicted by this mechanism are consistent with experimental evidence. In the case of CM-FAM, our calculations show that a tip terminated in a single atom is able to sustain forces in excess of 30 nN. The magnitude of the normal force was unexpectedlyfound to be verye similar for th approach on top of an atom or on a hollow position on the surface. This behaviour is due to tip relaxations induced by the interaction with the surface. These forces are also rather insensitive to the chemical nature of the tip apex.
Effect of Surface Steps on Dislocation Structure During Nanoindentation
Authors: Zimmerman Jonathan A. , Klein Patrick A. , Foiles Stephen M.
Published: April 2011

Nucleation of dislocations during nanoindentation is a prime example of how nanoscopic details play an important role in the evolution of macroscopic mechanical behavior. Experimental studies of nanoindentation show that surface irregularities, such as steps, alter the mechanical response during indentation. Atomistic simulations using the embedded atom method are used to examine the indentation of a Au(111) crystal with a surface step. The amount of force needed to nucleate dislocations decreases significantly for indentation close to the step. A recently formulated atomistic deformation metric, the slip vector, is used to quantify dislocation content. This metric is used with the local atomic stress to determine the critical resolved shear stress necessary for dislocation nucleation. Atomic stress fields are also compared with continuum calculations performed using non-linear elasticity with a constitutive relation based on the same interatomic potential. This constitutive model shows good agreement of indentation forces and stresses with the atomistic simulations.
Computational Nanotribology: SAMs for MEMS
Authors: Berry Rajiv J. , Wintrich Nicole L. , Bharadwaj Rishikesh K. , Schwartz Martin
Published: April 2011

Self-assembled monolayers (SAMs) consisting of hydrocarbon chains attached to silica walls were evaluated computationally for their high temperature stability, life cycle and performance in microelectromechanical systems (MEMS). Ab initio calculations at sufficiently high level of theory were conducted on model compounds to predict the bond strengths holding the monolayer tethered to the MEMS device and relate them to its thermal stability. Non-equilibrium molecular dynamics (NEMD) simulations under sliding periodic boundary conditions were employed to compute the frictional force as a function of applied load. The NEMD trajectories were analyzed for the structure and chain dynamics of the SAMs and compared with NEMD and equilibrium MD results for the fluid. The significance of monolayer penetration depth, monolayer gauche fraction, wall thermostat characteristics and the size of the simulation box on the computed results were investigated.
Microstructural Changes to Xenon Naonoclusters in Aluminum under 1 MeV Electron Irradiation.
Authors: Donnelly S. E. , Birtcher R. C. , Allen C. W. , Furuya K. , Song M. , Mitsuishi K.
Published: April 2011

Aluminum films containing solid Xe precipitates have been subjected to 1 MeV electron irradiation in a high-voltage electron microscope. High-resolution images have been recorded on videotape in order to monitor the changes to the system resulting from the passage of electrons through the film. Inspection of the video recordings reveals that complex, rapid processes occur under the electron beam. These include shape changes, the creation and movement of extended defects within the Xe lattice, movement of small clusters, coalescence of neighboring clusters and the apparent melting and resolidification of the Xe. An interpretation of many of the observations is presented in terms of the interaction of the nanoclusters with defects created in the aluminum by the high-energy electrons.
Inhomogeneous Swelling Near Grain Boundaries in Irradiated Materials: Cavity Nucleation and Growth Saturation Effects
Authors: Dudarev S. L.
Published: April 2011

The effect of inhomogeneous nucleation and growth of cavities near grain boundaries illustrates the failure of the standard rate theory to describe the kinetics of phase transformations in irradiated materials under cascade damage conditions. The enhanced swelling observed near grain boundaries is believed to result from the competition between the diffusional growth of cavities and their shrinkage due to the interaction with mobile interstitial clusters. Swelling rates associated with the two processes behave in a radically different way as a function of the size of growing cavities. For a spatially homogeneous distribution of cavities this gives rise to the saturation of swelling in the limit of large irradiation doses.

We investigate the evolution of the population of cavities nucleating and growing near a planar grain boundary. We show that a cavity growing near the boundary is able to reach a size that is substantially larger than the size of a cavity growing in the interior region of the grain. For a planar grain boundary the magnitude of swelling at maximum is found to be up to eight times higher than the magnitude of swelling in the grain interior.
The Effect of Bulk Composition on Swelling and Radiation-Induced Segregation in Austenitic Alloys
Authors: Allen T. R. , Cole J. I. , Dietz N. L. , Wang Y. , Was G. S. , Kenik E. A.
Published: April 2011

Changes in bulk composition are known to affect both radiation-induced segregation and microstructural development, including void swelling in austenitic stainless steel. In this work, three alloys (designations corresponding to wt%) have been studied: Fe-18Cr-8Ni alloy (bulk composition corresponding to 304 stainless steel), Fe-18Cr-40Ni (bulk composition corresponding to 330 stainless steel), and Fe-16Cr-13Ni (bulk composition corresponding to 316 stainless steel). Following irradiation with high-energy protons, the change in hardness and microstructure (void size distribution and grain boundary composition) due to irradiation was investigated. Increasing the bulk nickel concentration decreases void swelling, increases matrix hardening, and increases grain boundary chromium depletion and nickel enrichment. The analysis shows that decreases in lattice parameter and shear modulus due to radiation- induced segregation (RIS) correlate with decreased void swelling and a decreased susceptibility to irradiation assisted stress corrosion cracking (IASCC). Traditional thinking on IASCC assumed RIS was a contributing factor to cracking. It may, however, be that properly controlled RIS can be used to mitigating cracking.
Atomistic Mechanism of Nucleation and Growth of Voids in Cu Studied by Computer Simulation
Authors: Shimomura Y. , Mukouda I.
Published: April 2011

A TEM observation of fission neutron-irradiated copper at 300°C shows that the maximum size of stacking fault tetrahedra (sft) observed is 6 nm of edge length which corresponds to a cluster of 280 vacancies and the minimum size of voids is 2.2 nm in diameter which corresponds to a cluster of 470 vacancies. The result suggests that a vacancy cluster whose size is smaller than 300 vacancies grows to sft while a cluster whose size is larger than 500 vacancies relaxes to a void in 300°C-irradiated copper. A computer simulation of molecular dynamics (MD) with an isotropic EAM potential examined this model. It is found that a vacancy cluster that is smaller than 300 vacancy segregates to a (111) platelet and relaxes to an sft. Small vacancy clusters which are generated at damage cascade cores aggregate to spherically distributed vacancies for the size of more than 500 vacancies, and relax to several (111) platelets, which finally form a vacancy (111) polyhedron. Inside a polyhedral vacancy platelet, vacancies are confined and grow to a void at high temperature.
Mobility of Interstitial Clusters in HCP Zirconium
Authors: de Diego Nieves , Osetsky Yuri N. , Bacon David J.
Published: April 2011

Clusters of self-interstitial atoms (SIAs) formed in displacement cascades in metals irradiated with energetic particles play an important role in microstructure evolution under irradiation. They have been studied in the fcc and bcc metals by atomic scale computer simulation and in this paper we present the first results of a similar study of SIA clusters in an hcp crystal. Clusters of 4 to 30 SIAs were modelled over a wide temperature range using molecular dynamics and a many-body Finnis-Sinclair type interatomic potential for Zr. The results show a qualitative similarity of the dynamic properties of clusters to those for cubic metals. In particular, all clusters larger than 4 SIAs exhibit fast thermally-activated one-dimensional glide, which is in a <1120> direction in the hcp case. Due to the crystallographic features of the hcp lattice, this mechanism leads to two-dimensional mass transport in basal planes. Smaller clusters (≤ 4 SIAs) exhibit behaviour peculiar to the hcp structure, however, for they can migrate two-dimensionally in the basal plane. The jump frequency, activation energy and correlation factors of clusters have been estimated and comparison drawn between the behaviour of SIA clusters in different structures.
Effect of Annealing and Microwave Hydrogen Plasma Treatment on Structural, Chemical, and Electronic Properties of Ion Irradiated Diamond Films
Authors: Laikhtman Alexander , Hoffman Alon
Published: April 2011

In the present study we correlate between the secondary electron emission (SEE) of variously treated Xe+ ion-damaged diamond films and their bonding structure in the near-surface region as identified by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy. The 50 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of near-surface diamond to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to microwave (MW) hydrogen plasma treatment for 30 min produces negative electron affinity diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of a positive electron affinity surfaces and drastically degrades their electron emission properties. Prolonged, up to three hours MW hydrogen plasma treatment of as-implanted diamond films gradually improves the crystal quality and results in further increase of SEE intensity. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. To fully recover electron emission properties it is necessary to both remove the defects and hydrogenate the diamond surfaces.
Surface and Bulk Microstructural Modifications in Amorphous Carbon Films after Post-Growth Low Energy Ion Beam Irradiation
Authors: Patsalas P. , Logothetidis S.
Published: April 2011

We present the crystallization effects occurring in sputtered amorphous Carbon (a-C) thin films deposited on Si induced by post-growth low energy (0.5-1.5 keV) Ar+ ion beam irradiation (IBI). The a-C films after IBI have the form of an amorphous matrix with embedded crystalline regions. X-ray diffraction and Electron Microscopy measurements identified the crystalline phases of carbon and SiC. We study in detail the effects of ion energy and fluence on the crystallization process. It was found that low fluence (∼2×1016 ions/cm2) of ions with an optimum ion energy (∼1.5 keV) promoted the diamond formation. X-Ray Reflectivity (XRR) and Spectroscopic Ellipsometry were used to study the amorphous matrix. XRR discriminated the IBI induced surface and bulk effects through the density and the a-C surface roughness, showing surface smoothing to be more prominent for low energy IBI.
Point Defects Observed in D-T neutron irradiated Copper, Silver and Gold at 288 K with a Rotating Target in FNS_JAERI
Authors: Shimomura Y. , Sugio K. , Ohkubo H. , Mukouda I. , Kutsukake C. , Takeuchi H.
Published: April 2011

A 14 MeV D-T(fusion) neutron irradiation was carried out at fusion neutron source facility (FNS) in Japan Atomic Energy Research Institute (JAERI). Specimen temperature was controlled to 288 K. Fluence was 6.1 × 1017to 1.1 × 1021 n/m2. Both TEM thin foil and bulk specimens were irradiated at the same position. At 1018 n/m2, defects observed were single isolated dot defects. With increasing fluence, dot defects changed to complicate structure and made groupings. In a dot group, interstitial clusters and vacancy clusters were observed together. The present result was explained by the modeling that point defects in a nascent damage cascade move in crystal at 288 K and form their defect groupings.
Ion-Irradiation-Induced Amorphization Of Cadmium Niobate Pyrochlore
Authors: Meldrum A. , Beaty K. , Boatner L. A. , White C. W.
Published: April 2011

Irradiation-induced amorphization of Cd2Nb2O7 pyrochlore was investigated by means of in-situ temperature-dependent ion-irradiation experiments in a transmission electron microscope, combined with ex-situ ion-implantation (at ambient temperature) and RBS/channeling analysis. The in-situ experiments were performed using Ne or Xe ions with energies of 280 and 1200 keV, respectively. For the bulk implantation experiments, the incident ion energies were 70 keV (Ne+) and 320 keV (Xe2+). The critical amorphization temperature for Cd2Nb2O7 is ∼480 K (280 keV Ne+) or ∼620 K (1200 keV Xe2+). The dose for in-situ amorphization at room temperature is 0.22 dpa for Xe2+, but is 0.65 dpa for Ne+ irradiation. Both types of experiments suggest a cascade overlap mechanism of amorphization. The results were analyzed in light of available models for the crystalline-to-amorphous transformation and were compared to previous ionirradiation experiments on other pyrochlore compositions.
Modelling of Copper Precipitation in Fe-Cu Alloys Under Irradiation
Authors: Barashev Alexander V. , Golubov Stanislav I. , Bacon David J.
Published: April 2011

Precipitation of copper-rich clusters is a major cause of in-service hardening of reactor pressure vessel steels and has attracted much attention. Experimental studies of microstructural changes in alloys under various conditions have revealed similarities and differences. It has been established that under ageing the precipitate ensemble experiences normal nucleation, growth and Ostwald ripening, a distinguishing feature of which is the bcc-9R-3R-fcc transformations the precipitates undergo during growth. The main effect of electron irradiation is believed to be enhancement of the diffusion of copper and hence acceleration of the kinetics. In the case of neutron irradiation, however, there are many aspects that are not clear. One is that at temperatures less than about 300°C the precipitate size is observed to be very small (∼1-3 nm), i.e. the coarsening rate is very low. In this paper we study this phenomenon by computer simulations based on the “mean-field” approach for describing microstructural evolution.
Doping of GaN by ion implantation
Authors: Alves Eduardo J. , Liu C. , da Silva Maria F. , Soares José C. , Correia Rosário , Monteiro Teresa
Published: April 2011

In this work we report the structural and optical properties of ion implanted GaN. Potential acceptors such as Ca and Er were used as dopants. Ion implantation was carried out with the substrate at room temperature and 550 °C, respectively. The lattice site location of the dopants was studied by Rutherford backscattering/channeling combined with particle induced X-ray emission. Angular scans along both [0001] and [1011] directions show that 50% of the Er ions implanted at 550 oC occupy substitutional or near substitutional Ga sites after annealing. For Ca we found only a fraction of 30% located in displaced Ga sites along the [0001] direction. The optical properties of the ion implanted GaN films have been studied by photoluminescence measurements. Er- related luminescence near 1.54 μm is observed under below band gap excitation at liquid helium temperature. The spectra of the annealed samples consist of multiline structures with the sharpest lines found in GaN until now. The green and red emissions were also observed in the Er doped samples after annealing.
The Effect of Interfacial Free Energies on the Stability of Microlaminates
Authors: Lewis A. C. , Mann A. B. , van Heerden D. , Josell D. , Weihs T. P.
Published: April 2011

Laminated composites with polycrystalline layers typically break down at high temperatures through grain boundary grooving and the pinch-off of individual layers. Such materials, when exposed to high temperatures, develop grooves where grain boundaries meet the interfaces between layers. The depths of the grooves are controlled by the ratios of grain boundary and interfacial free energies, γgb/γint. Depending on the dimensions of the grains, these grooves can extend through the entire layer, causing pinch-off at the grain boundary. This pinch-off destroys the layering and eventually leads to a gross coarsening of the microstructure. Because microstructural stability is critical to performance for most applications, the ability to understand and predict the stability of microlaminates is a necessary tool. An existing model of this capillarity-driven breakdown requires the interfacial free energies, γgb and γint, as input parameters. Both biaxial and uniaxial zero creep tests have been used in conjunction with transmission electron microscopy to measure these interfacial energies in Ag/Ni and Nb/Nb5Si3 microlaminates.
An Approach to the Mesoscale Simulation of Grain Growth
Authors: Kinderlehrer David , Livshits Irene , Manolache Florin , Rollett Anthony D. , Ta'asan Shlomo
Published: April 2011

The simulation of curvature driven growth in grain boundary systems is becoming an important tool in understanding the behavior of microstructure evolution and there is much distinguished work in this subject. Here we address the mesoscale simulation of large systems of grain boundaries subject to the Mullins equation of curvature driven growth with the Herring force balance equation imposed at triple junctions. We discuss several novel features of our approach which we anticipate will render it a flexible, scalable, and robust tool to aid in microstructural prediction. What is the result of the simulation? We discuss what such a simulation is capable of predicting, taking as a prototype the histogram of relative area population as it changes through the simulation. We do not use this data to seek the best distribution, like Hillert, Rayleigh, or lognormal. Instead we treat the set of distributions as the solution of an inverse problem for a time varying function and determine the equation they satisfy. This results in a coarse graining of the complex simulation to simpler system governed by a Fokker-Planck Equation. Even so, fundamental questions concerning the predictability of simulations of large metastable systems arise from these considerations.
Nanometer Scale Study of Segregation at Heterophase Interfaces of Molybdenum Nitride Precipitates in an α-Iron Matrix
Authors: Isheim Dieter , Csencsits Roseann , Seidman David N.
Published: April 2011

We are studying heterophase interfaces between molybdenum nitride precipitates and an α-iron matrix on a nanometer scale by electron and atom-probe field-ion microscopies. Fe-2 at.% Mo-0.5 at.% Sn and Fe-2 at.% Mo-0.4 at.% Sb are internally nitrided by annealing in an NH3/2 atmosphere. Nitridation at 550°C produces coherent thin platelet-shaped molybdenum nitride precipitates, while at 600°C semicoherent coarse plates and spheres also form. Segrega-tion of Sn and Sb is observed at the heterophase interfaces of only the semicoherent precipitates. The loss of coherency, that is the formation of misfit dislocations, is essential for substantial interfacial solute segregation to occur at heterophase interfaces.
Dislocation - Grain Boundary Interaction in Nickel Bicrystals Evolution of the Resulting Defects under Thermal Treatment
Authors: Priester Louisette , Poulat Sophie , Décamps Brigitte , Thibault Jany
Published: April 2011

The interactions between lattice dislocations and grain boundaries were studied in nickel bicrystals. Three types of grain boundaries, according to their energy, were investigated : singular σ3 {111}, vicinal near σ11 {311} and general near σ11 {332} grain boundaries. The experiments were performed by transmission electron microscopy using a set of techniques : conventional, weak beam, in situ and high resolution transmission electron microscopy. Dislocation transmission from one crystal to the other was only observed for σ3 {111} GB. It consists in a decomposition within the grain boundary of the trapped lattice dislocation followed by the emission of one partial in the neighbouring crystal. A high resolved shear stress is required to promote the emission process. Most often, the absorbed lattice dislocations or extrinsic grain boundary dislocations react with the intrinsic dislocation network giving rise to complex configurations. The evolutions with time and upon thermal treatment of these configurations were followed by in situ transmission electron microscopy. The evolution processes, which differ with the type of grain boundaries, were analyzed by comparison with the existing models for extrinsic grain boundary dislocation accommodation. They were tentatively interpretated on the basis of the grain boundary atomic structures and defects obtained by high resolution transmission electron microscopy studies.
Dislocation Dynamics in Intermetallic and Oxide Dispersion Strengthened (ODS) Alloys
Authors: Messerschmidta Ulrich , Gudera Susanne , Häusslera Dietrich , Bartscha Martin
Published: April 2011

In situ straining experiments in a high-voltage electron microscope allow the observation of the dynamic behaviour of individual dislocations. Such experiments have been performed on the intermetallic alloys NiAl, NiAl containing 0.2 at% Ta, α-TiAl, and MoSi2, and the oxide dispersion strengthened (ODS) alloys INCOLOY MA956 and INCONEL MA754 in a wide range of temperatures. There are many similarities in the dynamic behaviour of dislocations in the different materials. In the intermetallic alloys, a transition occurs between low temperature mechanisms and a viscous motion in the temperature range of the flow stress anomaly. The viscous motion at high temperatures can be explained by diffusion processes in the dislocation cores, whichcan be described by the theory of the Cottrell effect. The diffusing species can be quite different, alloying components or intrinsic point defects like vacancies and antisite defects existing in the lattice or only in the dislocation cores. If the dislocations are straight and crystallographically oriented during their motion, they may be dissociated and move by a succession of glide and conservative climb between the partial dislocations. In the ODS alloys, the dislocations move between the oxide particles again in a viscous way. The relation between the dislocation dynamics and the strain rate sensitivity of the flow stress is discussed for thedifferent materials.
A New Continuum Scalar Model of Facets
Authors: Xin Tinghui , Wong Harris
Published: April 2011

Facets or planar surfaces appear often on crystalline solids, and need to be accurately modeled in studying surface evolution. Here we propose a model in which the radius of curvature of the equilibrium crystal surface is prescribed as a function of crystallographic orientation. In this approach, a facet is represented by the Dirac delta function with the weight of the delta function equal to the width of the facet plane. This model allows sharp corners on solid surfaces, but avoids the non-uniqueness of equilibrium surface profiles that plagues previous facet models. We demonstrate this approach by solving the equilibrium shape and surface energy of triangular crystals.
Amplitude Equation for a Dynamic Strain Aging Model: Beyond Linear Stability Analysis of Serrated Flow in Metallic Alloys
Authors: Rashkeev Sergey N. , Glazov Michael V. , Barlat Frédéric , Lege Daniel J.
Published: April 2011

A method for construction of “processing windows” to avoid negative strain rate sensitivity and associated serrated flow in some aluminum alloys is described. The method is based on the amplitude Ginzburg-Landau (GL) equations and analysis of bifurcation diagrams. The mathematical technique developed in the present work was applied to a specific aluminum alloy, Al-0.4%Mg-0.2%Si considered earlier in the literature [1-3], and yielded good results in terms of predicting the negative strain rate sensitivity regions in the “strain rate “temperature” parameter space. Using the GL-analysis it was demonstrated that even though the instability area is located in the region of intermediate strain rates, a qualitative difference exists between the areas of (relatively) fast and (relatively) slow strain rates. In the first case the dynamic behavior of the system is supercritical, in the second case it is subcritical. The second case is highly undesirable because it causes a sudden onset of stable stress serrations that are difficult to suppress, while in the first case the development of instability is gradual and, consequently, more easily controllable.
Growth of Cu-Ni-Sn Alloys in Pb Free CuSnAg Solder/Au-Ni Metallization Reactions
Authors: Zribi Anis , Borgesen Peter , Zavalij Lubov , Cotts Eric J.
Published: April 2011

Diffusion and phase formation were monitored in solder joints consisting of flip chips with Ni(V) under-bump metallizations bumped with Ag3.8Cu1.85Sn94.35 (atomic percentage) solder reflowed on printed circuit boards with Cu/Ni/Au metallizations. A ternary alloy, (CuNi)6Sn5, was observed to form during reflow at solder/Ni interfaces in these Ag3.8Cu1.85Sn94.35/Au/Ni solder joints. After extended thermal aging at 150 oC, a second ternary compound, (CuNi)3Sn4 forms at the Ni/(CuNi)6Sn5 interface. The growth of these alloys depletes the solder of essentially all Cu, changing its mechanical properties and melting temperature.
The Effects of a New Grain Boundary Phase in Ni-Al Alloy
Authors: Chiu Y.L. , Ngan A.H.W. , Jones I.P.
Published: April 2011

In a γ/γ' nickel-aluminum alloy, a new phase has been identified along grain boundaries by conventional transmission electron microscopy. This phase is identified to be of the C6Cr23 type structure with lattice parameter 10.48Å, which is approximately three times that of Ni 3Al or nickel. This grain boundary precipitate is found to be coherent with the γ' phase in one grain. The chemical composition of this grain boundary precipitate is determined by energy dispersive x-ray spectroscopy and parallel electron energy loss spectroscopy. The deformation mechanism of this grain boundary precipitate is investigated on post-mortem specimens which have been carefully deformed by nanoindentation after jet-polishing.