Modelagem da formação de estruturas tridimensionais em crescimento epitaxial

• Autor(a) principal: Leal, Fábio Fagundes
• Data de Publicação: 2011
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/955
• topic: Modelagem; Epitaxia; Morros; Monte Carlo; Pontos quânticos; Modelling; Epitaxy; Mound; Monte Carlo; Quantum dots; CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA::FISICA DA MATERIA CONDENSADA

In the present thesis we investigate the emergence of three- dimensional self-assembled structures in surfaces grown by molecular beam epitaxy (MBE). Kinetic Monte Carlo simulations were used to the modelling of two speci c cases of these morphologies: mound formation and nucleation of 3d nanometric islands called of quantum dots (QDs). The main process of the models proposed in this thesis is the thermally activated surface di usion. The main novelty of our model is in the implementation of the step barriers in terraces edges, usually called Ehrlich-Schwoebel (ES) barriers. We investigate two rules for the interlayer difusion: a bond dependent step barrier (BDSB) and height dependent step barrier (HSB). The work is divided in two parts. In the first one, we simulate the mound formation using BDSB and HSB rules and, in the second one, we simulate the nucleation of three-dimensional islands using HSB rule. For HSB, we have shown that a mounded morphology can be obtained even for a small step barrier while a self-a ne growth, consistent with the Villain-Lai-Das Sarma equation, is observed in the absence of an explicit step barrier in the HBS model. The surfaces exhibiting mounds are described by dynamical scaling laws associated to the super-roughness scaling regime, characterized by locally smooth surfaces (faceted) and a roughness exponent α > 1. The thin film phase is featured by surfaces with self-assembled three-dimensional structures with aspect ratio (height/width) that may increase or decrease with temperature, depending on the strength of step barrier. We have shown that the standard model with barrier only in descending steps losses stability and generate unstable morphologies with anomalously sharp and high columns for the investigated range of temperature. The BDSB rule, however, generates well-behaved mounded morphologies for the same ES barriers that lead to anomalous morphologies in the standard model. Moreover, mounds are also observed in BDSB rule when a same value is used for the step barrier felt by all particles independent if they are bonded or free particles or moving down- or upwardly in a step. The analysis of the three-dimensional island nucleation was motivated by the experimental results of Ferreira et al. [1] for the growth of CdTe QDs on Si(111) substrates. We modify the difusion rate of HSB model in order to study heteroepitaxial systems. The difusion rate depends now on the bonding energy between distinct chemical species. The main change of the model is the introduction of a step barrier that can vary with the growth temperature. Using a a barrier varying linearly with temperature, we were able to reproduce the experimental QDs distributions, as well as the dependence of the density and mean diameter of QDs as functions of the temperature. We have also captured details experimentally observed as the bimodal shape of the QDs size distributions without any additional parameter tuning.