Atomistic Simulation of Low-Energy Beam Deposition
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ATOMISTIC SIMULATION OF LOW-ENERGY BEAM DEPOSITION BRIAN W. DODSON Sandia National Laboratories,
Albuquerque,
NM 87185
ABSTRACT Low-energy (50 eV) homoepitaxial beam deposition of silicon is simulated using many-body silicon potentials and molecular dynamics techniques. Results are presented for the case of a 50 eV neutral silicon beam incident on the (2xl) dimer reconstructed Si(100) surface. The beam is aligned along (110) symmetry directions, which are the most natural channeling directions in the silicon lattice. Roughly 10% of the incident beam atoms are scattered from the surface with a small fraction of their initial energy. About half of the incident atoms penetrate the lattice, but scatter strongly and come to rest within 10-15A of the surface. The remainder are steered accurately into the bulk (110) channels, where they penetrate some 30-100 A into the lattice. Those atoms which do not undergo bulk channeling cause considerable lattice damage to the near-surface (depth : 10A) region.
INTRODUCTION Low-energy beam deposition is currently under investigation as a possibility for growth of modulated semiconductor structures which cannot be grown using conventional thermal deposition techniques. However, because of the short timescale involved (typically picoseconds), experimental diagnostics are not capable of studying the initial interaction and thermalization of the incident beam atoms with the substrate. As a result, computer simulation of the early stages of the deposition process is potentially useful to guide the 1
experimental efforts . SIMULATION OF BEAM DEPOSITION The simulation of covalently bound materials requires explicit treatment of many-body interactions. The case of silicon has recently attracted considerable attention, and several reasonably successful empirical potentials have been developed. In the present work, the many-body silicon potential of Dodson2 is used. This potential takes the form of a Morse potential where the strength of the attractive term is a function of the locations of the atoms in the surrounding neighborhood. As a result, information concerning high-order many body interactions is included in this potential. This approach provides an accurate description of both bulk and surface properties of silicon. In this paper, beam deposition on the Si(100) surface is considered. This surface has a (2xl) reconstruction, in which the surface atoms attract to form rows of surface dimers. This reconstruction is also exhibited when the Si(100) surface is simulated using the Dodson potential 3 . The details of the reconstruction, however, are slightly different than the experimental observations. The amount of energy released in the reconstruction is roughly correct, as is the dimer separation, but the predicted dimer structure is symmetric, whereas LEED observations of the actual structure reveals a small asymmetry. This difference is not expected to affect the results of the current simulations. The silicon substrate is roughly a cube 20A on a side (totaling 576 atoms) having (
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