Indium Arsenide/Gallium Arsenide Quantum Dots and Nanomesas: Multimillion-Atom Molecular Dynamics Solutions on Parallel Architectures.

Abstract

Multimillion-atom molecular dynamics (MD) simulations have been performed to study the flat InAs overlayers with self-limiting thickness on GaAs square nanomesas. The in-plane lattice constant of InAs layers parallel to the InAs/GaAs(001) interface starts to exceed the InAs bulk value at 12th monolayer (ML) and the hydrostatic stresses in InAs layers become tensile above ∼12 th ML. As a result, it is not favorable to have InAs overlayers thicker than 12 ML. This may explain the experimental findings of the growth of flat InAs overlayers with self-limiting thickness of ∼11 ML on GaAs nanomesas. We have also examined the lateral size effects on the stress distribution and morphology of InAs/GaAs square nanomesas using parallel molecular dynamics. Two mesas with the same vertical size but different lateral sizes are simulated. For the smaller mesa, a single stress domain is observed in the InAs overlayer, whereas two stress domains are found in the larger mesa. This indicates the existence of a critical lateral size for domain formation in accordance with recent experimental findings. The InAs overlayer in the larger mesa is laterally constrained to the GaAs bulk lattice constant but vertically relaxed to the InAs bulk lattice constant, consistent with the Poisson effect. Moreover, we have calculated surface energies of GaAs and InAs for the (100), (110), and (111) orientations. Both MD and the conjugate gradient method are used and the results are in excellent agreement. Surface reconstructions on GaAs(100) and InAs(100) are studied via the conjugate gradient method. We have developed a new model for GaAs(100) and InAs(100) surface atoms. Not only this model reproduces well surface energies for the (100) orientation, it also yields (1 x 2) dimer lengths in accordance with Ab initio calculations. Finally, a series of molecular dynamics simulations are performed to investigate the behavior under load of several 〈001〉 and 〈011〉 symmetrical tilt grain boundaries (GBs) in diamond. These MD simulations are based on the bond-order analytic potential. Crack propagation in polycrystalline diamond samples under an applied load is simulated, and found to be predominantly transgranular rather than intergranular.