Nanocluster Epitaxy by Annealing: Ag on H-terminated Si (111) Surfaces
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Nanocluster Epitaxy by Annealing: Ag on H-terminated Si (111) Surfaces B.Q. Li, Y.F. Shi, J. Bording and J.M. Zuo Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 ABSTRACT We report an experimental investigation on the morphology and orientation of Ag nanoclusters by RT deposition and subsequent annealing. We show that epitaxial Ag clusters of 2 ~ 6 nm in diameter can be synthesized in this way. The RT self-assembled Ag clusters grow as mostly single-crystal crystallites with Ag(111)//Si(111), but the in-plane orientation has a dispersion of ~ 9° centering at Si[110] direction. Upon annealing, the Ag clusters drastically rotated to the epitaxial configuration with the in-plane orientation aligned to the Ag[110]//Si[110] direction. The rotation and epitaxy of the Ag nanoclusters are explained based on a coincident site lattice model and interface energy minimization.
INTRODUCTION One of the most fundamental issues in self-assembly nanostructures is the orientation and epitaxy on surfaces. Fundamentals involved include the surface and interface energies, and the equilibrium shape of the nanostructures [1]. Epitaxy comes with low interface energy, which leads to better structural stability and lower defect density. While the epitaxy of continuous thinfilms is well studied, little is known about epitaxy of nanostructures. In general, epitaxy of nanostructures, together with the shape, is determined by the minimization of the sum of the surface, interface and elastic energies [2]. The specifics depend on the properties of the system. Here we report our study on a model system: Ag on H-terminated Si (111) surfaces with focus on the Ag nanocluster orientation or epitaxy and the interface structure. The growth of Ag on clean Si (111) has been extensively studied [3,4]. Ag interacts with clean reconstructed Si (111) surfaces with a number of intermediate phases, which complicates the Ag/Si(111) interface structure, e.g. Ag deposited onto Si (111)-(7x7) induces the Si (111)-( 3 x 3 )R30° surface reconstruction at temperatures above 200°C. In comparison, hydrogen terminating the dangling bonds of Si (111) surfaces avoids this complexity [5]. The H-terminated Si(111) surface has an ideal 1x1 terminated surface and a much reduced surface energy. Previous studies show that Ag grows in the Volmer-Weber mode on H-Si(111) [5]. Our recent work shows that Ag nanoclusters on H-Si (111) surfaces evolve in a step-like manner, originating from the cluster coalescence and shape transformation during growth due to the reduced substrate surface energy and enhanced surface diffusion [6]. Another attraction of this surface is in its capacity for nanoscale modification. It has been demonstrated that the hydrogen atoms on the surface can be extracted with a scanning tunneling microscope (STM) tip, by applying high positive bias voltages or large tunneling currents. The Si dangling bonds, formed in this way, act as preferential adsorption site
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