Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “LINE DEFECTS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

First-principles study of dislocations in hcp metals through the investigation of the (1121) twin boundary

Herein, we use first principles calculations to study the energy of the (1121) twin boundary in Zr, Zn, Mg, Ti, and Be. This boundary is important for understanding the microyielding and damping of hexagonal close-packed metals. The (1121) twin boundary is unique in that it is composed of--and can form by the glide of--basal dislocations nucleating at every c lattice parameter. The effect of the number of atoms between boundaries on the boundary energy, and the resulting lattice strains of the relaxed structures are quantified. It is shown that the energies obtained converge within 32-64 atoms/supercell. The structures with a higher second-order elastic constant term, c{sub 44}, also have higher boundary energies. It is further shown that the critical resolved shear stresses of the basal dislocations at 0 K, which make up the (1121) twin, are so low as to be below the threshold of the first principles calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Spontaneous formation of highly regular superlattice structure in InGaN epilayers grown by molecular beam epitaxy

In this letter, we have investigated the structural properties of thick InGaN layers grown on GaN by plasma-assisted molecular beam epitaxy, using two growth rates of 1.0 and 3.6 A/s. A highly regular superlattice (SL) structure is found to be spontaneously formed in the film grown at 3.6 A/s but not in the film grown at 1.0 A/s. The faster grown film also exhibits superior structural quality, which could be due to the surface roughness suppression caused by kinetic limitation, and the inhibition of the Frank-Read dislocation generation mechanism within the spontaneously formed SL structure.

36 MATERIALS SCIENCE↗
DOE OSTI2011

In-Situ RBS Channelling Studies Of Ion Implanted Semiconductors And Insulators

The experimental set-up at the ion beam facility in Jena allows the performance of Rutherford backscattering spectrometry (RBS) in channeling configuration at any temperature between 15 K and room temperature without changing the environment or the temperature of the sample. Doing RBS channeling studies at 15 K increases the sensitivity to defects, because the influence of lattice vibrations is reduced. Thus, the very early processes of ion induced damage formation can be studied and the cross section of damage formation per ion in virgin material, P, can be determined. At 15 K ion-beam induced damage formation itself can be investigated, because the occurrence of thermal effects can be widely excluded. In AlAs, GaN, and ZnO the cross section P measured at 15 K can be used to estimate the displacement energy for the heavier component, which is in reasonable agreement with other experiments or theoretical calculations. For a given ion species (here Ar ions) the measured cross section P exhibits a quadratic dependence P{proportional_to}P{sub SRIM}{sup 2} with P{sub SRIM} being the value calculated with SRIM using established displacement energies from other sources. From these results the displacement energy of AlN can be estimated to about 40 eV. Applying the computer code DICADA to calculate the depth distribution of displaced lattice atoms from the channeling spectra, indirect information about the type of defects produced during ion implantation at 15 K can be obtained. In some materials like GaN or ZnO the results indicate the formation of extended defects most probably dislocation loops and thus suggest an athermal mobility of defect at 15 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

In situ strain relaxation comparison between GaAsBi and GaInAs grown by molecular-beam epitaxy

The strain relaxation of GaAsBi is studied in order to determine both the maximum thickness before dislocations form for various misfits and the potential of GaAsBi for usage in the compositionally graded buffer of lattice-mismatched devices. Low-misfit GaAsBi epilayers are grown and compared with GaInAs, a well-studied material currently used in compositional grades. Relaxation behavior and dislocation energetics are compared using in situ wafer curvature. Samples are grown using molecular-beam epitaxy under similar conditions with misfit up to -0.81%. GaAsBi begins to relax at a lower thickness than GaInAs of the same mismatch. This leads to the majority of GaAsBi strain relaxation occurring with less material growth than comparable GaInAs. However, GaAsBi has greater residual strain than GaInAs after 2 {mu}m of growth. These results indicate that GaAsBi requires less elastic energy to form dislocations and more elastic energy to either encourage glide or multiply dislocations than GaInAs. GaAsBi shows less surface roughness than GaInAs for all samples, ruling out roughness as a source of dislocations and hindered glide in these alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Free Carrier Absorption due to Dislocation Scattering in GaN Quantum Wells

Free carrier absorption (FCA) is studied in quantum well structures assuming electrons to be scattered by dislocations via strain field. Expression for FCA coefficient, {alpha} is obtained assuming radiation field to be polarized along the plane of quantum well. Numerical results of {alpha}, as function of photon frequency, {Omega} and well width, d are presented. Calculations show, FCA to decrease with increase in {Omega} with a kink observed at {Omega} = 7.79x10{sup 13} s{sup -1} indicating onset of inter subband transitions. {alpha} is found to be proportional to d{sup -3} and to increase with increase in dislocation density.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

High-resolution x-ray diffraction investigation of relaxation and dislocations in SiGe layers grown on (001), (011), and (111) Si substrates

This work presents a detailed characterization, using high-resolution x-ray diffraction, of multilayered Si{sub 1-x}Ge{sub x} heterostructures grown on (001), (011), and (111) Si substrates by reduced pressure chemical vapor deposition. Reciprocal space mapping has been used to determine both the strain and Ge concentration depth profiles within each layer of the heterostructures after initially determining the crystallographic tilt of all the layers. Both symmetric and asymmetric reciprocal space maps were measured on each sample, and the evaluation was performed simultaneously for the whole data set. The ratio of misfit to threading dislocation densities has been estimated for each individual layer based on an analysis of diffuse x-ray scattering from the defects.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The Growth of GaN on Si by the Beam Flux Modulation

AlGaN/GaN HEMT structure was grown on Si (111) substrate by plasma-assisted molecular beam epitaxy (PA-MBE) using a beam flux modulation methods. In this result, it was verified that the propagation of treading dislocation (TD) due to N-rich GaN layer was effectively suppressed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

ADVANCED-STEM-BASED DEEP LEARNING FOR SEMANTIC SEGMENTATION OF DEFECTS IN STEELS

In this work, we developed a novel deep convolutional neural network (DCNN) model, called DefectNet, for robust and automated semantic segmentation of three crystallographic defects including line dislocations, precipitates and voids commonly observed in structural metals and alloys. In previous work, we established an experimental protocol for a diffraction contrast imaging scanning transmission electron microscopy (DCI STEM) technique and tailored it specifically for imaging defects in popular iron-based structural alloy. Thus, the DefectNet was trained over a small but high-quality DCI STEM defect image sets of a HT-9 martensitic steel before and after neutron irradiation for 111.8 dpa at 412°C. For the defect quantification that typically takes at least half an hour even for an expert, with a good model the machine learning algorithm can produce more reproducible and reliable quantification in a few seconds. This work demonstrates the feasibility of using deep learning algorithms for fast and accurate defect quantitative analysis.

Machine Learning, Radiation effects, Image analysi↗
DOE OSTI2011

Low energy electron beam induced damage on InGaN/GaN quantum well structure

In this paper, low energy electron beam (5-20 keV, 0-500 {mu}As/cm{sup 2}) induced damage on a GaN/InGaN/GaN near-surface quantum well structure is studied. Exposure to low energy electron beam is shown to significantly reduce the optical quality of the structure. It is also observed that reducing the electron beam energy causes larger PL intensity reduction. This can be explained by considering the beam penetration depth, which is shown to be smaller with lower e-beam energies. The damage is believed to be attributed to enhanced dislocation mobility upon low energy electron beam irradiation. However, further studies are needed to confirm the mechanism. These results should be taken into consideration in low energy electron beam related sample characterization and preparation.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Effect of dislocation density on thermal boundary conductance across GaSb/GaAs interfaces

We report on the thermal boundary conductance across structurally-variant GaSb/GaAs interfaces characterized by different dislocations densities, as well as variably-rough Al/GaSb interfaces. The GaSb/GaAs structures are epitaxially grown using both interfacial misfit (IMF) and non-IMF techniques. We measure the thermal boundary conductance from 100 to 450 K with time-domain thermoreflectance. The thermal boundary conductance across the GaSb/GaAs interfaces decreases with increasing strain dislocation density. We develop a model for interfacial transport at structurally-variant interfaces in which phonon propagation and scattering parallels photon attenuation. We find that this model describes the measured thermal boundary conductances well.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effect of Accumulative Roll Bonding on Plastic Flow Properties of Commercially Pure Zirconium

Accumulative roll bonding (ARB) has been considered as one of the promising techniques for fabrication of ultra-fine grained (UFG) metallic materials. The ARB process consists of several cycles of cutting, stacking, and rolling of metal sheets, so very high strains can be induced in the material resulting in significant grain refinement and in the formation of UFG microstructures. The ARB technique has been applied to a wide range of metallic materials such as Al and Al alloys, Mg, Fe and steels, Zr, Cu, as well as composite materials. UFG metallic materials processed via ARB show increased strength. Despite a significant body of experimental research into the deformation behaviour of the ARB-processed materials, the fundamentals of their plastic deformation are not fully understood yet. This work focuses on the effect of grain refinement via ARB-processing on the mechanical behavior and on the strain-rate sensitivity of commercially pure Zr (99.8% purity). The mechanical properties of the as-received coarse-grained (CG) and UFG (as-ARB processed) samples were studied at room temperature at two different strain rates. Mechanical strain rate jump tests were performed during tensile deformation to estimate the instantaneous strain-rate sensitivity. The evolution of surface relief during plastic deformation of pure Zr in both CG and UFG conditions was studied. The features of plastic deformation of pure Zr in both CG and UFG conditions are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Improving optical performance of GaN nanowires grown by selective area growth homoepitaxy: Influence of substrate and nanowire dimensions

Series of GaN nanowires (NW) with controlled diameters (160–500 nm) and heights (420–1100 nm) were homoepitaxially grown on three different templates: GaN/Si(111), GaN/AlN/Si(111), and GaN/sapphire(0001). Transmission electron microscopy reveals a strong influence of the NW diameter on dislocation filtering effect, whereas photoluminescence measurements further relate this effect to the GaN NWs near-bandgap emission efficiency. Although the templates' quality has some effects on the GaN NWs optical and structural properties, the NW diameter reduction drives the dislocation filtering effect to the point where a poor GaN template quality becomes negligible. Thus, by a proper optimization of the homoepitaxial GaN NWs growth, the propagation of dislocations into the NWs can be greatly prevented, leading to an exceptional crystal quality and a total dominance of the near-bandgap emission over sub-bandgap, defect-related lines, such as basal stacking faults and so called unknown exciton (UX) emission. In addition, a correlation between the presence of polarity inversion domain boundaries and the UX emission lines around 3.45 eV is established.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Tracking defect type and strain relaxation in patterned Ge/Si(001) islands by x-ray forbidden reflection analysis

Plastic relaxation and formation of defects are crucial issues in the epitaxial growth of nanoparticles and thin films. Indeed, defects generate local stress in the crystalline lattice, which affects their surroundings and may lead to undesired effects such as reduced charge-carrier lifetime or nonradiative recombinations. Here, we use a nondestructive method based on x-ray diffuse scattering close to forbidden reflections to identify the defect types with a high sensitivity and quantify their average size and strain field. Combined with transmission electron microscopy, it offers opportunities to track both ensemble average and single defects inside three-dimensional structures. These techniques have been applied to partially embedded and high-Ge-content (x{sub Ge}=0.87{+-}0.06) dots selectively grown in 20-nm-sized pits on Si(001) surfaces through openings in a SiO{sub 2} template. The stress in the 20-nm-wide Ge islands is relaxed not only by interfacial dislocations but also by microtwins and/or stacking faults located at the interface, proving the importance of {l_brace}111{r_brace} planes and twinning in the relaxation process of nanometer-size Ge dots.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Dislocation engineering in SiGe heteroepitaxial films on patterned Si (001) substrates

We demonstrate dislocation engineering without oxide masks. By using finite element simulations we show how nanopatterning of Si substrates with (111) trenches provides anisotropic elastic relaxation in a SiGe film, generates preferential nucleation sites for dislocation loops, and allows for dislocation trapping, leaving wide areas free of threading dislocations. These predictions are confirmed by atomic force and transmission electron microscopy performed on overcritical Si{sub 0.7}Ge{sub 0.3} films. These were grown by molecular beam epitaxy on a Si(001) substrate patterned with periodic arrays of selectively etched (111)-terminated trenches.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Radiation-induced damage and evolution of defects in Mo

The formation of defects in bcc Mo lattice as a result of 50-keV Xe bombardment is studied via atomistic simulation with an interatomic potential developed using the force-matching ab initio based approach. The defect evolution in the cascade is described. Diffusion and interaction of interstitials and vacancies are analyzed. Only small interstitial atom clusters form directly in the cascade. Larger clusters grow only via aggregation at temperatures up to 2000 K. Stable forms of clusters demonstrate one-dimensional diffusion with a very high diffusion coefficient and escape quickly to the open surface. Point vacancies have much lower diffusivity and do not aggregate. The possibility of a large prismatic vacancy loop formation near the impact surface as a result of fast recrystallization is revealed. The mobility of the vacancy dislocation loop segments is high, however, the motion of the entire loops is strongly hindered by neighbor point defects. This paper explains the existence of the large prismatic vacancy loops and the absence of the interstitial loops in the recent experiments with ion irradiation of Mo foils.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Synergetic effects of dual-beam implantation on the microstructural development in silicon

We report a synergy effect on the microstructural development of silicon specimens as a result of dual-beam high temperature irradiation/implantation. In situ transmission electron microscopy experiments using two different experimental setups have been used, where the primary 50 keV Co{sup +} ion implantation beam was supplemented with either a 300 keV electron beam or a 500 keV Si{sup +} ion beam. In both cases, the secondary beam intensity was such that both beams created comparable overall primary damage. Completely different microstructural response has been found in these two cases. An intensive electron irradiation was found to sharply accelerate the evolution of dislocation structure, only weakly affecting the disilicide kinetics. On the contrary, the Si ion beam weakly affected the kinetics of either dislocation loops or coherent CoSi{sub 2} precipitates, but drastically increased the number density of thermodynamically unstable semicoherent precipitates. Possible microstructural reasons for the observed effects and the implications for both dislocation loop and cobalt disilicide nucleation mechanisms in high-temperature implanted TEM samples are discussed and supported by detailed molecular dynamics calculations of annealing of cascade remnants produced by the energetic silicon recoils.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Transport and strain relaxation in wurtzite InAs-GaAs core-shell heterowires

Indium-arsenide-gallium-arsenide (InAs-GaAs) core-shell, wurtzite nanowires have been grown on GaAs (001) substrates. The core-shell geometries (core radii 11 to 26 nm, shell thickness >2.5 nm) exceeded equilibrium critical values for strain relaxation via dislocations, apparent from transmission electron microscopy. Partial axial relaxation is detected in all nanowires increasing exponentially with size, while radial strain relaxation is >90%, but undetected in nanowires with both smaller core radii <16 nm and shell thicknesses <5 nm. Electrical measurements on individual core-shell nanowires show that the resulting dislocations are correlated with reduced electron field-effect mobility compared to bare InAs nanowires.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗