Probing the Structure and Energetics of Dislocation Cores in SiGe Alloys through Monte Carlo Simulations

We present a methodology for the investigation of dislocation energetics in segregated alloys based on Monte Carlo simulations which equilibrate the topology and composition of the dislocation core and its surroundings. An environment-dependent partitioning of the system total energy into atomic contributions allows us to link the atomistic picture to continuum elasticity theory. The method is applied to extract core energies and radii of 60 degrees glide dislocations in segregated SiGe alloys which are inaccessible by other methods.Comment: 5 pages, to be published in Physical Review Letter

Simulation of mirror electron microscopy caustic images in three-dimensions

A full, three{dimensional (3D) ray tracing approach is developed to simulate the caustics visible in mirror electron microscopy (MEM). The method reproduces MEM image contrast resulting from 3D surface relief. To illustrate the potential of the simulation methods we study the evolution of crater contrast associated with a movie of GaAs structures generated by the droplet epitaxy technique. Speci�cally, we simulate the image contrast resulting from both a precursor stage and the �nal crater morphology which is consistent with an inverted pyramid consisting of (111) facet walls. The method therefore facilities the study of how self{assembled quantum structures evolve with time and, in particular, the development of anisotropic features including faceting

Thermally controlled widening of droplet etched nanoholes

We describe a method to control the shape of nanoholes in GaAs (001) which combines the technique of local droplet etching using Ga droplets with long-time thermal annealing. The cone-like shape of inverted nanoholes formed by droplet etching is transformed during long-time annealing into widened holes with flat bottoms and reduced depth. This is qualitatively understood using a simplified model of mass transport incorporating surface diffusion and evaporation. The hole diameter can be thermally controlled by varying the annealing time or annealing temperature which provides a method for tuning template morphology for subsequent nanostructure nucleation. We also demonstrate the integration of the combined droplet/thermal etching process with heteroepitaxy by the thermal control of hole depth in AlGaAs layers

Asymmetric coalescence of reactively wetting droplets

Coalescence of droplets during reactive wetting is investigated for the liquid Ga/GaAs(001) system. In situmirror electron microscopy reveals that coalescence predominantly involves the motion of one reactive droplet relative to the other. This behaviour differs significantly from coalescence in non-reactive systems and is associated with contact line pinning at a ridge/etch pit edge which is identified using atomic force microscopy and selective etching. A simple geometrical model is presented to describe the pinning

Dynamics of mass transport during nanohole drilling by local droplet etching

Local droplet etching (LDE) utilizes metal droplets during molecular beam epitaxy for the self-assembled drilling of nanoholes into III/V semiconductor surfaces. An essential process during LDE is the removal of the deposited droplet material from its initial position during post-growth annealing. This paper studies the droplet material removal experimentally and discusses the results in terms of a simple model. The first set of experiments demonstrates that the droplet material is removed by detachment of atoms and spreading over the substrate surface. Further experiments establish that droplet etching requires a small arsenic background pressure to inhibit re-attachment of the detached atoms. Surfaces processed under completely minimized As pressure show no hole formation but instead a conservation of the initial droplets. Under consideration of these results, a simple kinetic scaling model of the etching process is proposed that quantitatively reproduces experimental data on the hole depth as a function of the process temperature and deposited amount of droplet material. Furthermore, the depth dependence of the hole side-facet angle is analyzed

On the sensitivity of convergent beam low energy electron diffraction patterns to small atomic displacements

Multiple scattering simulations are developed and applied to assess the potential of convergent beam low-energy electron diffraction (CBLEED) to distinguish between various reconstructions of the Si(001) surface. This is found to be readily achievable through changes in pattern symmetry. A displacement R-factor approach is used to incorporate the angular content of CBLEED discs and identify optimal energy ranges for structure refinement. Defining a disc R-factor, optimal diffraction orders are identified which demonstrate an enhanced sensitivity to small atomic displacements. Using this approach, it was found that respective dimer height and length displacements as small as ±0.06 Å and ±0.20 Å could be detected

Modification of stress-strain behaviour in aromatic polybenzoxazines using core shell rubbers

2,2-Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a) is blended with a commercial core shell rubber (CSR), Genioperl P52, based on a siloxane core and an acrylic shell, at a range of loadings (1–32 wt.%). Scanning electron microscopy and energy-dispersive X-ray analysis reveals an even distribution with good cohesion between the resin and CSR particles. Measurements carried out by dynamic mechanical analysis and thermogravimetric analysis show modest improvements in glass transition temperature (6 °C) and significant enhancement of thermal stability (20%) when CSR (32 wt.%) was incorporated. Such improvements are linearly related to CSR content. Moderate reductions in modulus (30%) were observed with the highest (32 wt.%) loadings of CSR and were also linearly proportional to CSR content. Thermal analysis demonstrated a small inhibitory effect, with activation energy raised by 4% with the blend containing 32 wt.% CSR and 3% in the blend containing 8 wt.% CSR. It was found that mechanical stirring of the CSR particles into the molten BA-a monomer was the most practical solution for dispersion and effectively broke down CSR agglomerates in the bulk and produced void free samples upon curing, although some minor defects were apparent with higher loadings of core shell rubber. Four batches of dog bone specimens (containing 0, 8, 16 and 32 wt.% CSR) were manufactured and underwent tensile testing. An average increase in extension was observed from 0.82 mm for the pristine poly(BA-a), to 1.14 mm (32 wt.% CSR) was achieved. The introduction of CSR has a deleterious impact on tensile strength (24.67 MPa, pristine poly(BA-a) compared with 20.48 MPa containing 32 wt.% CSR; Young's modulus of 5.4 GPa for pristine poly(BA-a) compared with 3.1 GPa containing 32 wt.% CSR). Following tensile tests, scanning electron microscopy reveals rubber cavitation as the principal toughening mechanism