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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.

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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Nanoselective area growth and characterization of dislocation-free InGaN nanopyramids on AlN buffered Si(111) templates

We report the metal organic chemical vapor deposition growth of dislocation-free 100 nm thick hexagonal InGaN nanopyramid arrays with up to 33% of indium content by nano-selective area growth on patterned AlN/Si (111) substrates. InGaN grown on SiO{sub 2} patterned templates exhibit high selectivity. Their single crystal structure is confirmed by scanning transmission electron microscope combined with an energy dispersive X-ray analysis, which also reveals the absence of threading dislocations in the InGaN nanopyramids due to elastic strain relaxation mechanisms. Cathodoluminescence measurements on a single InGaN nanopyramid clearly show an improvement of the optical properties when compared to planar InGaN grown under the same conditions. The good structural, morphological, and optical quality of the InGaN nanostructures grown on AlN/Si indicates that the nano-selective area growth technology is attractive for the realization of site-controlled indium-rich InGaN nanostructure-based devices and can also be transferred to other highly mismatched substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electron mobility of self-assembled and dislocation free InN nanorods grown on GaN nano wall network template

A kinetically controlled two-step growth process for the formation of an array of dislocation free high mobility InN nanorods (NRs) on GaN nanowall network (NWN) by Molecular Beam Epitaxy is demonstrated here. The epitaxial GaN NWN is formed on c-sapphire under nitrogen rich conditions, and then changing the source from Ga to In at appropriate substrate temperature yields the nucleation of a self assembled spontaneous m-plane side faceted-InN NR. By HRTEM, the NRs are shown to be dislocation-free and have a low band gap value of 0.65 eV. Hall measurements are carried out on a single InN NR along with J-V measurements that yield mobility values as high as ≈4453 cm{sup 2}/V s and the carrier concentration of ≈1.1 × 10{sup 17} cm{sup −3}, which are unprecedented in the literature for comparable InN NR diameters.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

GaAsP solar cells on GaP/Si with low threading dislocation density

GaAsP on Si tandem cells represent a promising path towards achieving high efficiency while leveraging the Si solar knowledge base and low-cost infrastructure. However, dislocation densities exceeding 10{sup 8} cm{sup −2} in GaAsP cells on Si have historically hampered the efficiency of such approaches. Here, we report the achievement of low threading dislocation density values of 4.0–4.6 × 10{sup 6} cm{sup −2} in GaAsP solar cells on GaP/Si, comparable with more established metamorphic solar cells on GaAs. Our GaAsP solar cells on GaP/Si exhibit high open-circuit voltage and quantum efficiency, allowing them to significantly surpass the power conversion efficiency of previous devices. The results in this work show a realistic path towards dual-junction GaAsP on Si cells with efficiencies exceeding 30%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Advanced time integration algorithms for dislocation dynamics simulations of work hardening

Efficient time integration is a necessity for dislocation dynamics simulations of work hardening to achieve experimentally relevant strains. In this work, an efficient time integration scheme using a high order explicit method with time step subcycling and a newly-developed collision detection algorithm are evaluated. First, time integrator performance is examined for an annihilating Frank–Read source, showing the effects of dislocation line collision. The integrator with subcycling is found to significantly out-perform other integration schemes. The performance of the time integration and collision detection algorithms is then tested in a work hardening simulation. The new algorithms show a 100-fold speed-up relative to traditional schemes. As a result, subcycling is shown to improve efficiency significantly while maintaining an accurate solution, and the new collision algorithm allows an arbitrarily large time step size without missing collisions.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2023

Comparing theoretical predictions of radiation-free velocities of edge dislocations to molecular dynamics simulations

Transonic defect motion is of interest for high strain-rate plastic deformation as well as for crack propagation. Ever since Eshelby's 1949 prediction [J. D. Eshelby, Proc. Phys. Soc. A 62, 307 (1949)] in the isotropic limit of a “radiation-free” transonic velocity $v$ RF = $\sqrt{2}$c T , where shock waves are absent, there has been speculation about the significance of radiation-free velocities (if they truly exist) for defect mobility. Here, in this work, we argue that they do not play any significant role in dislocation dynamics in metals, based on comparing theoretical predictions of radiation-free velocities for transonic edge dislocations with molecular dynamics simulations for two face-centered cubic metals: Ag, where theory predicts radiation-free states, and Cu, where it does not.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Linear Complexions: Metastable Phase Formation and Coexistence at Dislocations

The unique three-phase coexistence of metastable B2–FeNi with stable L1 0 –FeNi and L1 2 –FeNi 3 is discovered near edge dislocations in body-centered cubic Fe-Ni alloys using atomistic simulations. Stable nanoscale precipitate arrays, formed along the compression side of dislocation lines and defined as linear complexions, were observed for a wide range of compositions and temperatures. Here, by analyzing the thermodynamics associated with these phase transitions, we are able to explain the metastable phase formation and coexistence, in the process defining new research avenues for theoretical and experimental investigations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Coalescence dynamics of prismatic dislocation loops due to vacancy supersaturation

Recovery processes in metals with a high defect concentration are at the heart of a structural alloy's usability in engineering applications. However, such processes take place via nonconservative mechanisms mediated by point defect diffusion and are governed by their interactions with the underlying microstructure. Indeed, one of the main challenges in studying recovery processes is their intrinsic multiscale complexity, which often negates the direct use of experimental techniques. Here, in this Research Letter, we study the mechanism of dislocation loop coalescence in metals by simulating the interaction dynamics between vacancies and prismatic loops using a stochastic framework that seamlessly bridges point defect kinetics with dislocation mechanics. Our simulations reveal two fundamental discoveries: first, that climb processes are governed by local fluctuations that are not captured by continuum elastodiffusion models, and, second, that prismatic loops create internal traps for vacancies, leading to the formation of rings around the perimeter of the loops that affect how coalescence takes place. Our results show an excellent agreement with in situ experimental observations of loop coalescence in Fe under irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Designing an Epitaxially-Integrated DBR for Dislocation Mitigation in Monolithic GaAsP/Si Tandem Solar Cells

This work explores epitaxially-integrated distributed Bragg reflectors (DBR) as a strategy to mitigate the impact of threading dislocations on the performance of monolithic GaAs 0.75 P 0.25 /Si tandem solar cells. The constraints present due to materials availability and the optical transmission profile of the GaAs 0.75 P 0.25 top cell require the use of an enhanced bandwidth DBR design achieved by combining two narrow-band DBR’s with different central wavelengths. Here, the impact of this DBR structure on JSC, along with the competing effects of threading dislocation density (TDD), are investigated using a robust, experimentally informed analytical model. Implementing this DBR within the GaAs 0.75 P 0.25 /Si tandem cell structure is predicted to yield a short circuit current enhancement equivalent to an ~1.8× reduction in TDD, providing a clear demonstration of its promise as a design methodology for mitigating the impact of non-negligible defect populations.

14 SOLAR ENERGY↗
DOE OSTI2012

Hydrogenation of Dislocation-Limited Heteroepitaxial Silicon Solar Cells: Preprint

Post-deposition hydrogenation by remote plasma significantly improves performance of heteroepitaxial silicon solar cells. Heteroepitaxial deposition of thin crystal silicon on sapphire for photovoltaics (PV) is an excellent model system for the study and improvement of deposition on inexpensive Al2O3-coated (100) biaxially-textured metal foils. Without hydrogenation, PV conversion efficienciesare less than 1% on our model system. Performance is limited by carrier recombination at electrically active dislocations that result from lattice mismatch, and other defects. We find that low-temperature hydrogenation at 350 degrees C is more effective than hydrogenation at 610 degrees C. In this work, we use measurements such as spectral quantum efficiency, secondary ion mass spectrometry(SIMS), and vibrational Si-H spectroscopies to understand the effects of hydrogenation on the materials and devices. Quantum efficiency increases most at red and green wavelengths, indicating hydrogenation is affecting the bulk more than the surface of the cells. SIMS shows there are 100X more hydrogen atoms in our cells than dangling bonds along dislocations. Yet, Raman spectroscopy indicatesthat only low temperature hydrogenation creates Si-H bonds; trapped hydrogen does not stably passivate dangling-bond recombination sites at high temperatures.

14 SOLAR ENERGY↗
DOE OSTI2018

Athermal Motion of Dislocations at the Front of a Loading Pulse

The character of dislocation propagation and arrest during activationless motion in alkali halide crystals under a pulsed load is considered. A model of structural transformation of impurity complexes caused by spatial reorientation of bound excitons under the action of external factors is proposed. In the framework of this model, it is possible to explain the well-known effects of photoplasticity, magnetoplasticity, and activationless motion of dislocations at the front of a loading pulse.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Wedge dislocations, three-dimensional gravity, and the Riemann-Hilbert problem

An expression for the free energy of an arbitrary static distribution of wedge dislocations in a solid is proposed. It represents a Euclidean version of (1+2)-dimensional gravity interacting with an arbitrary number of point particles. It is shown that the solution of the equilibrium equations leads to the Cauchy problem for effective equations determining the form of dislocations, while the problem of finding a metric leads to the Riemann-Hilbert problem for a frame with an O(3) monodromy representation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Effect of charged dislocation walls on mobility in GaN epitaxial layers

A theoretical model in the context of a conventional representation on traditional notion concerning Read cylinders for interpretation of mobility collapse as a function of the concentration of free carriers in GaN-based films is suggested. Along with phonon and impurity scattering mechanisms, electron scattering due to charged dislocations embedded into the walls is taken into account in the model. An expression is obtained for the height of the drift barrier depending on the concentration of free carriers. Based on the derived equations, the dependence of the location of the mobility minimum on the dislocation structure is interpreted.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of dislocations on the structural order parameter in a crystal upon torsional strain

The interaction of the structural subsystem with dislocations upon applying an elastic torsional strain has been studied. The consideration is performed in the terms of the Landau phenomenological theory with the refusal from the approximation that the irreducible vector magnitudes are constant. It is shown that additional spatial amplitude and frequency oscillations related to the change in the magnitudes of the structural order parameter and the dislocation density appear.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Intensity Distribution of the Three-Wave Diffraction from Dislocation Epitaxial Layers in the Reciprocal Space

The three-wave X-ray diffraction in strongly disordered epitaxial layers of GaN and ZnO is experimentally investigated. The charts of the intensity distribution in the reciprocal space are plotted in coordinates q{sub θ} and q{sub ϕ} for the most intensive three-wave combination (1010)/(1011) by means of subsequent θ- and ϕ-scanning. A nontrivial shape of the θ-sections of these contours at a distance from the ϕ center of reflection is revealed; it is different for different samples. For the θ-curves at the center of reflection, we observed a common peak that may be approximated by the Voigt function with a power-low decrease in the intensity at the wings; the decrease law (from–4.5 to–5.0) is found to be considerably greater than that for the similar curves of two-wave diffraction and not depending on the dislocation density and distribution in layers. In some films we observed a coarse-block structure; in addition, it follows from the distribution in the reciprocal space that these blocks are turned with respect to each other around a normal to the surface, which allows us to suggest the existence of low-angle boundaries between them, consisting exclusively of edge dislocations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Resonances of the Spatial Distribution of Dislocations upon Interaction with the Structural Order Parameter

The interaction of a structural subsystem with dislocations under the superposition of an elastic torsion strain and an additional uniaxial spatially nonuniform time-constant pressure is studied. The analysis is carried out within of Landau’s phenomenological theory with refusal of the approximation of constant magnitudes of irreducible vectors. The rise of spatial resonances of the dislocation density in these conditions is shown.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Forming an Edge Dislocation Image at Anomalous X-ray Transmission

The features of forming section and projection topography images of an edge dislocation normal to the crystal surface are considered for the case of X-ray anomalous transmission. Experimental images are analyzed using numerical simulation of the diffraction experiment. A new mechanism for image formation of defects arranged near the output crystal surface has been proposed. Differences between topography images of dislocations and rosettes of local misorientations of reflecting planes have been established.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Reciprocal-Space Maps of X-Ray Diffraction Intensity Distribution and Their Relation to the Dislocation Structure of Epitaxial Layers

X-ray diffraction (XRD) in asymmetric Bragg geometry was measured and XRD intensity distribution maps in the reciprocal space were constructed for GaN epitaxial layers with various degrees of structural perfection grown on c-sapphire substrates. It is established that equal-intensity lines (isolines) related to a regular system of perpendicular rectilinear threading dislocations are extended in a direction parallel to the surface. For a more chaotic distribution of dislocations with a large fraction of horizontal fragments, these isolines are rotated toward a direction perpendicular to the reciprocal lattice vector, although they still not attain a limiting position characteristic of the ideal mosaic crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

X-Ray Diffraction Analysis of Epitaxial Layers with the Properties of a Dislocation Filter

An approach to instant testing of epitaxial films with a sharp decrease in the threading dislocation density is proposed. High-resolution X-ray diffractometry, including reciprocal space mapping, has been used. The structure of GaAs/Si(001) heterosystems with low-temperature GaAs layers has been analyzed. A decrease in the density of threading dislocations in the GaAs film with the formation of a small-angle boundary has been detected.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗