Pattern Selection of Surface-based Nanostructures
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Pattern Selection ofSurface-based Nanostructures
E. G. Wang1,2, B. G. Liu1, J. Wu1, W. G. Zhu1 and Z. Zhang3,2 1 Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, P. R. China 2 International Center for Quantum Structures, CAS, Beijing, P. R. China 3 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
ABSTRACT A rich variety of two-dimensional patterns can be formed in the early stages of film growth. In this paper, we will show that, when a surfactant layer is used to mediate the growth, a counter-intuitive fractal-to-compact island shape transition can be induced by increasing deposition flux or decreasing growth temperature. Specifically, we introduce a reaction limited aggregation (RLA) theory, where the physical process controlling the island shape transition is the shielding effect of adatoms stuck to stable islands on incoming adatoms. Also discussed is the origin of a transition of compact islands from triangular to hexagonal then to inverted triangular in Pt (111) homoepitaxy with the presence of CO adsorbates. We will provide a coherent and unified picture for the interpretation of these intriguing observations based on kinetic Monte Carlo simulations, with energy barriers from first-principles calculations. INTRODUCTION Nanoscale structures on surfaces are at the forefront of exploratory work for next generation devices in the fields of electronics and optics, which rely on materials of ever increasing complexity and decreasing size. In most cases these nanostructures must be fabricated through either homoepitaxial or heteroepitaxial growth. The understanding of the kinetics involved in the formation and stability of nanostructures on surfaces is of importance for the fast-growing area of nanotechnology. So far, substantial experimental and theoretical studies have been focused on the evolution of morphological features on the surface and the growth modes, and also the decay of three dimensional islands. In this paper we will present a recent study of the novel formation mechanisms of nanostructures on surfaces. REACTION LIMITED AGGREGATION IN SURFACTANT-MEDIATED EPITAXY
In heteroepitaxial growth, the presence of strain often leads to three-dimensional (3D) growth mode since the equilibrium structure involves strain-relieving effects. A breakthrough was made in 1989 when Copel et al [1] demonstrated that the use of a single layer of As can improve the heteroepitaxial growth of Ge on Si. This remarkable behavior was termed the “surfactant P2.1.1
effect”. Since then, a large number of experimental and theoretical efforts have been devoted to understand the surfactant effect [2]. Now surfactants have been successfully used to modify growth mode in various systems. However, most of these works concentrate on the mechanism of how surfactants change the growth mode from 3D island to layer-by-layer (LBL) growth [3-6], while little effort has been made to the understanding of the pattern formation of 2D islands in the early stage of
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