Role of bonding and coordination in the atomic structure and energy of diamond and silicon grain boundaries
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Role of bonding and coordination in the atomic structure and energy of diamond and silicon grain boundaries P. Keblinski Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, and Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
D. Wolf and S. R. Phillpot Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
H. Gleiter Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany (Received 27 March 1997; accepted 23 October 1997)
The high-temperature equilibrated atomic structures and energies of large-unit-cell grain boundaries (GB’s) in diamond and silicon are determined by means of Monte-Carlo simulations using Tersoff’s potentials for the two materials. Silicon provides a relatively simple basis for understanding GB structural disorder in a purely sp 3 bonded material against which the greater bond stiffness in diamond combined with its ability to change hybridization in a defected environment from sp 3 to sp 2 can be elucidated. We find that due to the purely sp 3 -type bonding in Si, even in highly disordered, high-energy GB’s at least 80% of the atoms are fourfold coordinated in a rather dense confined amorphous structure. By contrast, in diamond even relatively small bond distortions exact a considerable price in energy that favors a change to sp 2 -type local bonding; these competing effects translate into considerably more ordered diamond GB’s; however, at the price of as many as 80% of the atoms being only threefold coordinated. Structural disorder in the Si GB’s is therefore partially replaced by coordination disorder in the diamond GB’s. In spite of these large fractions of three-coordinated GB carbon atoms, however, the three-coordinated atoms are rather poorly connected amongst themselves, thus likely preventing any type of graphite-like electrical conduction through the GB’s.
I. INTRODUCTION
The low-pressure synthesis of rather pure nanocrystalline diamond films from fullerene precursors1 suggests that for a small enough grain size the diamond structure may be energetically preferred over graphite.2,3 Because of the small grain size of typically about 15 nm in these films,4 a non-negligible fraction of the carbon atoms is situated in the grain boundaries (GB’s). The surprisingly high wear resistance5 of these films even after the substrate is removed and their high corrosion resistance6 suggest that the grains are strongly bonded together. Also, grain-boundary carbon is thought to be responsible for the absorption and scattering of light in these films, their electrical conductivity, and electronemission properties.7,8 In spite of all these indications of a possibly critical role of GB carbon in the mechanical, electronic and optical properties of nanocrystalline diamond films, to date the atomic structures of the GB’s are essentially not known. That the electronic and optical properties of polycrystalline silicon films are significantly affected by the presence of GB’s is well known.9 For example, G
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