Kinetic Monte Carlo Simulation Of Surfactant-mediated Epitaxial Growth

The thin film growth mechanism is one of primary contents in the filed of thin film research. Experimentally, STM and AFM are used to observe the surface pattern and structure evolution in the process of thin film formation. Theoretically, computer simulation is an important method to study the atomistic processes of thin film growth. The meaning of computer simulation is that the details of atomistic processes hardly observed in experiment can be investigated, On the other hand, computer simulations can easily alter various parameters, such as deposition temperature, deposition rate, etc, in the process of thin film growth. In addition, computer simulation can be used to study the thin film growth in extreme growth conditions, such as high deposition rate, high deposition temperature, etc, which is difficult achieved in experiment. In the thin film epitaxy growth, the properties of the nucleation and growth of deposition atoms in the initial stage will directly affect the quality of whole thin film. In this thesis, we study the nucleation and growth behaviors of two-dimensional islands in the initial stage of thin film epitaxy growth by using kinetic Monte Carlo simulation. The main points of study are following:1. Expanding the RLA model. In the modified RLA model, the nucleation and growth of islands are controlled by the exchange process between adatoms and surfactant atoms. The exchange barrier in island edge depends on the number of the nearest atoms above and below the surfactant layer. The dependence of island shape and density on the deposition flux is investigated systematically.2. Nucleation and growth behavior of two dimensional islands in early stage of surfactant-mediated epitaxial growth have been studied by kinetic Monte Carlo simulation. Our model includes three main atomic processes: diffusion of adatoms on the surfactant terrace, exchange of adatoms with their underneath surfactant atoms, and reexchange of exchange adatoms with the lifted surfactant atoms for resurfacing to the top of the surfactant layer. The simulation results demonstrate that, at a giving deposition rate, with increasing temperature, the activated reexchange process gives rise to a transition of nucleation density N, and the corresponding transition temperature T divide the N～T curve into two different regions. In different temperature regions, the dependence of island density on deposition flux exhibits different characteristics. Multifractal spectra are used to describe quantitatively the distribution of island under different growth conditions. At the transition temperature T=T_x, the width of multifractal spectrum is smallest, and the top value is highest. As the temperature increased or decreased, the width of spectra increase, while the top value decrease.3. Simulations study the scaling behavior of island nucleus density and size distributions in the surfactant-mediated epitaxy. The results show that, the nucleus density and island size distributions of reexchange-controlled nucleation at high temperatures exhibit similar scaling behaviors to that of traditional diffusion-mediated nucleation at low temperatures. However, at intermediate temperatures, the exchanging-controlled nucleation leads to an increase of nucleus density with temperature, while the island size distribution scales to a monotonically decreasing scaling behavior.