The Relationship Between Atomic Structure and Strain Distribution of Misfit Dislocation Cores at Cubic Heteroepitaxial Interfaces

The atomic reconstruction of a misfit dislocation (MD) core causes change in the strain distribution around the core. Several MD cores at the AlSb/GaAs (001) cubic zincblende interface, including a symmetrical glide set Lomer dislocation (LD), a left-displaced glide set LD, a glide set LD with an atomic step, a symmetrical shuffle set LD, and a 60° dislocation pair, were studied using simulated projected potential and aberration-corrected transmission electron microscope images. Image deconvolution was also used to restore structure images from nonoptimum-defocus images. The corresponding biaxial strain maps, εxx (in-plane) and εyy (out-of-plane), were obtained by geometric phase analysis using the GaAs substrate as the reference lattice. The results show that atomic structure characteristics of MD cores can be revealed by the strain maps. The strain maps should be measured from optimum-defocus images or restored structure images. Furthermore, the εxx strain map has been found more accurate than the εyy strain map for MD cores, and the specimen thickness should be below the critical thickness due to the influence of dynamical scattering.

Strain Fields around Dislocation Cores Studied by Analyzing Coordinates of Discrete Atoms

Dislocation core structures in Au and Cu crystals are investigated by means of quasicontinuum simulations combined with the embedded atom method potentials. A dislocation pair in a graphene sheet, which is observed by Warner et al. experimentally, is also analyzed in the present work. The strain fields around these dislocations in Au, Cu, and graphene crystals are calculated by analyzing the coordinates of discrete atoms, which is a strain tensor calculation method proposed by Zimmerman et al., and compared with theoretical predictions based on Foreman dislocation model. It is shown that the strain fields given by Zimmerman theory are completely suitable for describing the dislocation core structures of Au, Cu and graphene crystals. However, compared with the results of Au and Cu, the Zimmerman strain field in the vicinity of graphene dislocation core is a little less accurate, possibly due to the effect of lattice symmetry of graphene, which needs to be clarified in the future study.

Characterization of Threading Edge Dislocation in 4H-SiC by X-Ray Topography and Transmission Electron Microscopy

A threading dislocation (TD) in 4H-SiC, which was currently interpreted as a perfect threading edge dislocation (TED) by synchrotron monochromatic-beam X-ray topography (SMBXT) and molten KOH etching with Na2O2 additive, was performed comparative characterization using weak-beam dark-field (WBDF) and large-angle convergent-beam electron diffraction (LACBED) methods. The TD was suggested to be dissociated into a dislocation pair which can be observed in the WBDF image of g=-12-10. The TD, which was identified as b//[-12-10] by SMBXT observation, was unambiguously determined as b=1/3[-12-10] by LACBED analysis. In the case of perfect TED, it was found that the direction of Burgers vector derived from SMBXT observation corresponds to LACBED analysis.

Atomistic Simulation of the Energy Barrier for Dislocation Movement in Si

We examine the mobility of an edge dislocation pair on the shuffle plane in Si using action-derived molecular dynamics (ADMD). ADMD is one of the specially designed schemes for finding out the reaction pathways passing through transition states in the landscape of potential energy surfaces. Via ADMD calculations, the various structural changes of dislocation line with atomic resolution and their corresponding energy barriers are evaluated during the dislocation motion. The energy barrier for the movement of an edge dislocation pair on shuffle plane is about 0.24 eV. In this case, one bond between the atoms at the dislocation line is broken first, and then a new bond is formed with the neighboring atom. The movement of the dislocation line is achieved by a sequence of making new bond after bond-breaking of concerned atoms, which occur layer by layer. When the dislocation moves through this mechanism, energy barrier for the dislocation movement does not depend on the length of dislocation line. Thus the present result enables one to surmount the inherent limitation of Peierls-Nabarro’s two-dimensional continuum model, which may fail to describe successfully dislocation motion on the atomistic level.

STATISTIC MECHANICS FOR LINEAR DEFECT-MEDIATED MELTING

The melting of elemental solids is modelled as a dislocation pair-mediated transition on a lattice. Statistical mechanics of linear defects is used to obtain the relation between melting temperature and shear modulus. It is derived theoretically that the phase transition is the first order and the formula for latent heat is also derived.