Simulations of Low-Energy Ion Bombardment and Epitaxial Growth
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SIMULATIONS OF LOW-ENERGY ION BOMBARDMENT AND EPITAXIAL GROWTH E. CHASON, P. BEDROSSIAN, J.Y. TSAO, B.W. DODSON AND S.T. PICRAUX Sandia National Laboratories, Albuquerque, NM 87185
ABSTRACT We have performed computer simulations of epitaxial growth and low-energy ion bombardment for comparison with reflection high-energy electron diffraction (RHEED) mesurements. The simulations are based on a hybrid Monte Carlo/rate equation approach which includes the processes of defect creation (adatom and surface vacancy), surface diffusion, and attachment and detachment from steps and islands. In this work, we focus on simulating the experimental observations of ion-induced RHEED oscillations and cancellation of RHEED oscillations during simultaneous ion bombardment and growth. For the interaction of the low-energy ion with the surface, we consider two mechanisms: preferential sputtering (where the sputtering cross section depends on the atomic coordination) and mobile vacancies. Our results indicate that the primary interaction of the ion beam with the surface is probably through the creation of mobile vacancies, and that the degree of preferential sputtering is not large.
INTRODUCTION Recently, we have discovered oscillations in the reflection high-energy electron diffraction (RHEED) intensity during low-energy ion bombardment of Si (001) surfaces [I]. In analogy with similar oscillations observed during epitaxial growth [2], we identify them as the consequence of ion-induced layer-by-layer removal of Si atoms from the surface. Subsequent scannning tunneling microscope (STM) studies confirmed the existence of monolayer-deep clusters formed during ion bombardment [3]. Further RHEED studies combining ion bombardment and epitaxial growth indicate that the ion beam can effectively remove surface roughness created by prior growth. Although these experiments clearly indicate that the effects of the low-energy ion bombardment are limited to the surface, they are not sufficient to determine the atomistic mechanism controlling the ion-surface interaction. The observed surface morphologies and RHEED dynamics are the result of a complex interaction among multiple surface processes, including surface defect creation (adatoms and vacancies), defect annihilation, surface diffusion, and attachment and detachment from steps and clusters. In order to model this complexity, we have conducted computer simulations of ion bombardment and epitaxial growth which allow us to determine the effects of various mechanisms on the evolution of the surface morphology and compare these results with the experimental observations. The two mechanisms we consider are preferential sputtering (where the sputtering cross section depends on the atomic coordination) and mobile vacancies.
SIMULATION The simulation is based on a hybrid Monte Carlo/rate equation technique that has also been presented in previous publications [4,5]. The simulation is performed on a cubic lattice, with solid-on-solid restrictions. We associate a transition rate with all the surface p
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