Ab Initio Quantum Chemical Studies of Hydrogen and Halogen Migration on the Diamond (110) Surface
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INTRODUCTION Mobile surface adsorbates are known to play a role in growth processes for semiconductors such as silicon and gallium arsenide. However, it has not been established what role surface mobility plays in diamond growth. It is expected to be less important for diamond, since under chemical-vapor deposition (CVD) conditions diamond surfaces are generally terminated by hydrogen, which blocks migration pathways. In CVD environments containing halogens [1] or oxygen, a fraction of the surface sites may also be terminated by F, Cl, 0, or OH. Under optimal conditions for CVD growth, some fraction of the surface terminator is missing, exposing open surface radical sites. The surface coverage is determined by a balance of abstraction by gas-phase radicals, and recombination onto radical sites. Under hydrogen-based CVD conditions, modeling studies suggest that between several percent and 40% of the surface sites are open [2]. If the surface-terminating species are mobile on the diamond surface, then the open radical sites may be regarded as mobile. This may clear pathways for hydrocarbon adsorbate diffusion. Barriers to migration of hydrocarbon adsorbates on the (100) (2xl):H surface have been estimated to be at least 1.9 eV [3]. Therefore, at least on this particular surface, long-range diffusion of adsorbates across terraces to steps would be unlikely at typical diamond growth temperatures (kT - 0.1 eV). However, mobility of surface terminators may have more subtle implications for diamond CVD growth mechanisms. To date, no theoretical study of growth mechanisms has allowed for mobility of either hydrocarbon adsorbates or surface terminators, usually H. These studies often find that there are pathways which are blocked due to steric hindrance from neighboring terminators, or that the reaction must proceed through a highly strained 349 Mat. Res. Soc. Symp. Proc. Vol. 317. @1994 Materials Research Society
C-~x
(a) (1,2) migration
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(b) (1,3) migration
(c) (1,4) migration
Figure 1: Reactant cluster geometries, X = H, F, or Cl. (110) lies in the plane of the paper. intermediate. Both of these conclusions could be radically altered by even a small degree of mobility of surface terminators. Herein we report ab initio quantum chemical calculations of the barriers to migration of H, F, and Cl on a diamond surface. We restrict attention to the (110) surface, which has a higher growth rate than either the (100) or (111) surfaces [4]. (110)-type sites also appear at steps on the (100) and (111) surfaces.
METHOD As a model for the (110) surface, we consider constrained clusters in which all surface C-C bonds not involved in the reaction are replaced by C-H bonds. This leads to the clusters shown in Figure 1. The internuclear C-C distances were fixed at 1.54 A, the value for bulk diamond [5]. All C-H bond lengths on the peripheral hydrogens were fixed at 1.10 A, a value typical for hydrocarbons. The only atom whose coordinates were optimized during the calculation was the migrating X (H, F, or Cl). We carried out sel
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