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Researchers at North Carolina State University recently discovered the impact of single and double helix screw dislocations on the crystal growth and mechanical properties of solid state materials. Screw dislocations [...]

The atomic structure of dislocation cores is decisive for the understanding of plasticity in crystalline solids. The core structure of dislocations in sapphire introduced by high-temperature plastic deformation has been investigated with the use of the negative spherical-aberration imaging technique. The ability of this technique to discriminate oxygen columns from aluminum (Al) columns, combined with reproduction of subtle contrast features by image simulation, leads to a markedly detailed atomic model of the dislocation cores. The partial dislocations are Al-terminated, with electrical neutrality being achieved because half of the Al columns are missing. These partials also undergo core spreading, which results in random occupancy of both tetrahedrally and octahedrally coordinated sites, though Al in tetrahedral coordination never occurs in a perfect crystal Unusual dislocation core structures may be present in other technologically important nonmetallic solids. 10.1126/science.1192319

As the laser spot size in microscale laser shock peening is in the order of magnitude of several microns, the anisotropic response of grains will have a dominant influence on its mechanical behavior of the target material. Furthermore, conventional plasticity theory employed in previous studies needs to be re-examined due to the length scale effect. In the present work, the length scale effects in microscale laser shock peening have been investigated. The crystal lattice rotation underneath the shocked surface was determined via electron backscatter diffraction. From these measurements, the geometrically necessary dislocation (GND) density that the material contains has been estimated. The yield strength increment was then calculated from the GND distribution by using the Taylor model and integrated into each material point of the finite element method (FEM) simulation. Finite element simulations, based on single crystal plasticity, were performed for the process both with and without considering the GND hardening, and the comparison has been conducted. [DOI: 10.1115/1.3160370]

The periodic makeup of carbon nanotubes suggests that their formation should obey the principles established for crystals. Nevertheless, this important connection remained elusive for decades and no theoretical regularities in the rates and product type distribution have been found. Here we contend that any nanotube can be viewed as having a screw dislocation along the axis. Consequently, its growth rate is shown to be proportional to the Burgers vector of such dislocation and therefore to the chiral angle of the tube. This is corroborated by the ab initio energy calculations, and agrees surprisingly well with diverse experimental measurements, which shows that the revealed kinetic mechanism and the deduced predictions are remarkably robust across the broad base of factual data. carbon | catalysis | kinetics | nucleation | synthesis

Various experiments are conducted to examine the equilibrium morphology of misfit twins in an epilayer with different elastic constraints from its substrate. The misfit twin morphology is represented by the equilibrium twin width and equilibrium twin mean spacing of the periodic array, which is highly affected by the difference in the elastic constants of the epilayer and its substrate.

The article presents a newly discovered analytical solution, based on the dislocation theory of twinning. It is which is commonly used to determine the critical thickness of the misfit twin formation in an epilayer with different elastic constraints from its substrate.

The morphology of basal plane dislocations (BPDs) in 4H-SiC homoepitaxial layers was investigated using plane-view transmission x-ray topography and molten KOH etching. Based on the morphologies three types of BPDs are distinguished which include interfacial dislocations, curved dislocations and circular loop dislocations around micropipes.

The effect of dislocations, grain boundaries and bulk recombination on minority carrier lifetime in multicrystalline silicon were studied by developing a new model. Minority carrier lifetime was measured using the carrier density imaging (CDI) technique, grain boundaries were then mapped and characterized by electron backscatter diffraction (EBSD) and sub grain boundaries were found to be recombination active and dominating the number of active grain boundaries.

The strain relief and dislocation motion in III-nitride films that are grown on step-free 4H-SiC mesas are studied. The results show that the nucleation of threading dislocations in the absence of interfacial steps is an extremely difficult process, which proves that the strain relief for such III-nitride films is not dominated by the greater excess stress driving dislocation tilt.

The effects of inserting compressively or tensely strained layers into the buffers of compositionally graded SiGe on its strain relaxation and threading dislocation density (TDD) are studied. The analysis reveals that samples with compressive strained layers exhibit lower TDD and more enhanced relaxation, as compared to the tensely strained ones.

Experiments were conducted to investigate the locking dislocations by oxygen in Czochralski silicon (Cz-Si) with different concentrations of shallow dopants. In the 450-550 degree Celsius temperature range, the gradient of the rise in dislocation unlocking stress was found to be approximately a factor of 3 higher in highly antimony doped n-type Cz-Si than in low boron doped p-type Cz-Si.

Depth and in-plane profiling of strain, Ge composition, and defects in strained-Si/[Si.sub.1-x][Ge.sub.x]/Si heterostructures using micro-Raman imaging is reported. For the top strained-Si layer, two-dimensional UV-Raman images of the frequency and bandwidth of the Si band reveal that film crystallinity is correlated with the magnitude of in-plane strain.

An investigation on the plastically deformed Cu-Sn-5Zn ternary alloys with Sn concentrations 1, 25, and 5 wt % is conducted. Results reveal that the change in stacking fault probability with Sn concentration for ternary Cu-Sn-5Zn alloys is similar to Cu-based binary alloy (Cu-Sn) system but behaves in a different manner from Cu-1Sn-Zn ternary alloy systems.

An alternative formulation for extended defect kinetics was presented. These infinite nonlinear sums in previous model can be represented without loss of generality.

Research is presented concerning the use of theoretical and experimental investigations to determine the locking of dislocations in atoms of oxygen within Czochralski-silicon at temperatures ranging from 350- to 700-degrees centigrade.

Dislocation array energy in a strained epitaxial layer deposited on a finite substrate, is studied.

The dislocation mechanism for melting of elements is analyzed.

Research describing gallium arsenide based strained-layer semiconductor lasers is presented. In particular pure edge-type misfit dislocations are investigated and a model of their formation is developed.

Dislocations in gallium arsenide crystals are examined.

A new study investigates the relative stability and formation energies of faulted and perfect dislocation loops in silicon.

A new study investigates how the dislocation paths in crystals with paramagnetic obstacles are influenced by a magnetic field.

The effect of low-temperature growth in reducing threading dislocation (TD)densities in large mismatch heteroepitaxy gallium arsenide (GaAs) substrates has been studied. Results indicate that higher dislocation mobility from nonstoichiometric defects does not influence the diminution of TD density in low-temperature GaAs. High-temperature TD was also observed to have an effective annihilation radius between 500 to 5,000 angstroms.

HTREM images of naturally deformed, mylonitized biotite crystals were examined to determine where deformation-related defects occur in the structure. Dislocations were found to be associated with the plane of oxygen atoms that constitutes the base of the octahedral sheet and the apical plane of the tetrahedral sheet, rather than in the interlayer as has been generally assumed. Consistent with previous investigations, this region was also confirmed as the position at which stacking faults occur. The location of these defects was determined by comparing sub-unit-cell scale fringes in experimental images to simulated images with dislocation cores and stacking faults. Previous investigations have generally relied on indirect techniques and have yielded ambiguous results.

Research was conducted to examine the reconstruction effects on dislocation mobility of 90 degrees and 30 degrees partials in Si using two computationally efficient interatomic potentials. The interatomic potentials and numerical methods used to determine the relaxed atomic configurations and corresponding energies are discussed. It is suggested that increasing bond ionicity across the series of zinc-blende semiconductors will effect a reduction in reconstruction bonding and increase dislocation mobility.

Research was conducted to examine the influence of threading dislocations, Mg doping and etching on the photoconductivity spectra in GaN. A significantly enhanced spectral cutoff was demonstrated in nominally undoped GaN compared to heavily Mg-doped GaN. Results illustrate that etching/polishing causes increased surface recombination and reduces the expected quantum efficiency at photon energies above the band-gap energy of GaN.

Research was conducted to examine the hydrostatic compression of the perfect solid cyclotrimethylene trinitramine (C3H6N6O6), or RDX, and the crystal with a molecular vacancy. A combination of the ab initio Hartree-Fock method and the many-body perturbation theory was used for the study. Theoretical results were shown to be in good agreement with experimental results obtained for the behavior of RDX samples with different densities such as solid and porous materials.

Research was conducted to examine the chemical composition and structure of Si3N4/thermal (native and wet) SiO2 interface in oxide-nitride-oxide structures using secondary ion mass spectroscopy, Auger electron spectroscopy and electron energy loss spectroscopy calculations. Si-Si defects were observed through numerical simulation of the Si-Si bond's electronic structure to act as either hole or electron traps. Results suggest the abnormally large electron and hole capturing at the interface.

Research was conducted to examine the origin of the S-pit defects in almost dislocation-free GaP crystal grown by the liquid encapsulated vertical Bridgman method. It was proposed that the vacancy-type Frank dislocation loop surrounding a stacking fault is formed by the condensation of excess gallium vacancies during cooling of the GaP crystal. The phosphorus precipitates arise from the condensation of emitted phosphorous interstitials accompanied by the growth of the loop.

Research was conducted to examine the electroplastic effect under the simultaneous superposition of electric and magnetic fields. The free surface was perturbed by an electromagnetic field. The deformation of a bismuth crystal to the application of mechanical stress and concurrent constant magnetic field and an impulse electric current was investigated. A special device was designed as a supplement to the standard microhardness measuring equipment to illustrate the deformation in single crystals of bismuth.

Research was conducted to examine mosaic structures in partially relaxed Si0.7Ge0.3 epilayers grown on Si(001) substrates via x-ray double- and triple-axis diffractometry. Results demonstrate that the films contain mosaic regions laterally separated by perfect regions at the early stages of strain relaxation. Findings also suggest that the conventional method of estimating dislocation density from the x-ray rocking curve width fails in the experiment.

Research was conducted to create a semicontinuum model that combines certain elements of the atomistic and continuum picture of the dislocation kink-pair problem. The model is essentially an extension of the Pierls-Nabarro model where the dislocation core is seen as composed of linear elastic regions that are linked to each other along surfaces over which a nonlinear misfit force law acts. The general form of the orientation dependence of Peierls stress at zero temperature is predicted by the model.

Research was conducted to examine the dose dependence of as-implanted damage and the density of threading dislocations formed after MeV implants into Si. An analysis of the role of the amorphization and damage in the evolution of dislocation microstructure was also carried out viva Rutherford backscattering spectroscopy and channeling. Results are compared to and correlated with the dose dependence of the density of threading dislocations formed i the samples after furnace annealing.

A new flexible boundary condition that is capable of applying the simulations for a moving dislocation and is an effective method to use for dynamic and static problems is presented. The properties of a screw dislocation such as stable core structure, Peierls stress and glide property are measured in a square lattice under the flexible boundary condition described. Results suggest that the boundary responds to the changes of the atomic region associated with glide motion but is not responsive to lattice vibrations with high frequency and large amplitude emitted from the dislocation line.

Tunnelling electron microscopy was used in studying the intra-grain structure and variations in high-angle grain boundaries misorientations in copper oxide superconductors. YBCO samples were prepared through the Liquid Phase Removal Method. Results show that misorientation angles for the platelet and low-angle grain boundaries are between O.5 to 5 degrees. The misorientation is due to the alternation of the misorientation direction and does not go beyond 6 degrees. The misorientation of high-angle boundaries is influenced by the presence of platelet boundaries and low-angle grain boundaries.

Research was conducted to examine the configuration of dislocations in lateral overgrowth GaN films. Transmission electron microscopy and cathodoluminescence were used to investigate the microstructure of sublimation lateral overgrowth GaN. Results demonstrate that the dislocations also affect the yellow emission and not just the band edge emission. The dislocations were also shown to propagate laterally in the overgrowth region.

Research was conducted to examine the surfaces of GaN films deposited by metalorganic chemical vapor deposition and molecular beam epitaxy using atomic force microscopy. The objective was to explain how the surface structures form and how they can be controlled. Results demonstrate that the spiral hillock formation was dependent on the critical radius of curvature of the step-line and the miscut terrace width.

Research was conducted to examine a thermodynamical theory of elastoplasticity by introducing the concept of dislocation density tensor based on two fundamental principles of thermodynamics, namely, maximal entropy production rate and the principle of increase of entropy. A set of thermodynamically consistent governing equations of plastic spin and back stress was proposed by associating them with the dislocation density tensor. Results indicate that to achieve a thermodynamically consistent theory for kinematic hardening, the free energy density should have the dislocation density tensor as one of its arguments.

Research was conducted to examine a macroscale hardening law formulation of geometrically necessary dislocation boundaries (GNB) strengthening mechanisms that are mapped to the orientation distribution of crystallites in to assign slip system shear flow resistance in a polycrystal plasticity model. A mapping procedure of the macroscale hardening surface to the length scale of grains determined crystallographic shear flow resistance. Results indicate that the hardening surface model of GNB strengthening mechanisms can afford improved predictive capability of nonproportional loading behavior of copper.

Research was conducted to examine the stability of short-range reactions between two dislocations of parallel line vectors which glide on two parallel slip planes in BCC crystals. The elastic interaction between the two dislocations based on their stress fields at finite speed and the kinetic energy of moving dislocations resulting in inertial forces were two features that were highlighted in DD simulations. Results suggest that the dynamic formation of dislocation dipoles or tilt wall embryos takes place only over a small range of the phase space.

Research was conducted to examine the elastic field and the interaction energy associated with surface steps and adatoms. A harmonic model is proposed that presents a number of considerable advantages when compared to continuum models of Embedded Atom Method (EAM) simulations. Results are in good agreement with atomic EAM simulation results as well as the far-field predictions of continuum elastic theory.

Research was conducted to examine a new method for accurate computations of mutual and self interactions between dislocation loops. The approach involved dislocation loops that are segmented with appropriate parametric equations representing the dislocation line vector. Results provide evidence of the efficiency and the potential advantages of the proposed method for calculations of the elastic field of arbitrary-shape, 3-D dislocation loops.

Research was conducted to illustrate the usefulness of the Embedded Atom Method in the study of atomistic finite deformation simulations. Three items of interest to the field of continuum mechanics were observed, namely, a spatial size scale effect, oscillating global shear stress under simple shear conditions and a length scale bridging idea that is proposed to demonstrate the bridging of spatial length scales related to plastic energy.

A study was conducted to analyze photoinduced dislocation lines on the (111) face of C60 singles crystals using atomic force microscopy. The crystals were prepared by the improved descending-temperature vapor-phase technique. The crystal surfaces were then imaged in the dark and under illumination. Experimental results indicated that different types of dynamic features correlated with the different growth rates of the substructures.

Nonlinear ultrasonic experiments were conducted on martensitic steels which varied in carbon content. Nonlinear parameter beta increased monotonically with carbon content and hardness but the ultrasonic longitudinal phase velocity, upsilon(sub L), remained virtually the same for all samples. Therefore, it is concluded that upsilon(sub L) is not sensitive to microstructural variations between samples while beta is sensitive to them. Results also provide evidence that the observed increase in beta can be caused by dislocation-related effects in the specimen.

A study was conducted on the Ostwald ripening of end-of-range defects in Si. End-of-range defects refer to the dislocation loops that nucleate beneath the ion implantation-induced crystalline/amorphous interface in Si. Results show that the density of dislocation loops decreases with time while the square of the mean radius of the loop population increases. Diffusion was also identified as the limiting mechanism for dislocation loop growth.

A novel scheme of plan-view high resolution electron microscopy, which allows electronically enhanced kink motion on 30 degrees partial dislocations in Ge at atomic resolution, is presented. An analysis of sequential images taken from the same dislocation lines indicated that generation of kink pairs occurred during the observation period and geometrical kinks of different signs were mobile even at room temperature.

The velocity of misfit dislocation threading segments in real time during ultrahigh vacuum (UHV) chemical vapor deposition heteroepitaxial growth of thin SiGe epilayers on Si (001) was measured using ultrahigh vacuum transmission electron microscopy. No measurable difference in dislocation velocities during growth and post-growth annealing of samples with an atomically clean surface was observed unlike previous observations in the InGaAs/GaAs (001) system. Results suggest an unappreciated interaction between the surface of the system and moving dislocations.

A study was conducted on the magnetic hardening mechanism in nanocrystalline Nd2Fe14B with 0.1 at% additions of Cu, Cr or Zr. Small additions of Cr or Cu were found to promote pinning-type behavior retention and marginal coercivity increase while Zr addition reduced coercivity and changed the mechanism to nucleation type. Results indicate that coercivity mechanism regulation requires both the magnetic nature and location of impurity.