The interaction of an adsorbate such as carbon with a Si(111) surface can affect significantly the local arrangement of atoms at a surface and in subsurface region. This can modify greatly the growth of structures at the nanoscale size. We investigated the adsorption of a carbon at Si(111):As-(1x1) and Si(111):H-(1x1) surfaces using Density Functional theory calculations within the local density approximation. These two systems are very important for the understanding of phenomena taking place at surfaces of semiconductors. To model the As- and H-terminated Si(111) surfaces, we replaced the topmost Si layer by As and saturated the Si dangling bonds with hydrogen respectively [1,2]. Upon relaxation of the slab, the As layer moves outwards with respect to the initial position of a Si atom in the unrelaxed Si(111) surface. This corresponds to an expansion of the As-Si2 interlayer spacing by 30% compared to the Si1-Si2 interlayer distance in the bulk terminated surface. In contrast, we found a contraction of the Si1-Si2 interlayer spacing by some 5% in the H-terminated Si(111) case. To determine the best adsorption site of carbon, different high symmetry configurations were taken into consideration. We placed a carbon atom successively at the fourfold (T4) and threefold coordinated (H3) sites or substituted for a Si atom in the S5 position. Our results showed that, at the clean surface, the most energetically favored position is the S5 adsorption site. Moreover, the substituted Si atom moves from the S5 position to the H3 site. Carbon occupies the H3 site at both passivated surfaces. This indicates that the adsorption mechanism of carbon is modified by the passivation of the surface by species like As and hydrogen.
References:
[1]M.A. Olmstead, R.D. Bringans, R.I.G. Uhrberg, R.Z. Bachrach, Phys. Rev. B, 34 (1986)6041
[2]G.S.Higashi, R.S.Becker, Y.J.Chabal, A.J.Becker, Appl.Phys.Lett.,,58 (1991)1656 |