Atomic structure of carbon-induced Si(001)c(4X4) reconstruction as a Si-Sihomodimer and C-Si heterodimer network - art. no. 035306

A combination of low-energy electron diffraction, x-ray and ultraviolet pho toelectron spectroscopy, and scanning-tunneling microscopy studies, in conj unction with ab initio calculations leads us to suggest a model for the car bon (C)-induced Si(001)c(4x4) atomic structure. This surface superstructure is obtained in a defined range of C2H4 exposures at 600 degreesC. Experime ntal probes show that the c(4x4) superstructure involves C atoms in both su rface and subsurface sites. This is reflected in well-marked features in ph otoemission valence- and core-level spectra. Surface carbon atoms are stabi lized in Si-C heterodimers, with a surface density of about 0.25 monolayer (ML) [i.e., two C atoms per c(4x4) unit cell of eight atoms]. In the subsur face region, carbon atoms substitute for Si atoms in well-defined sites of the third or fourth layers of the Si substrate. The subsurface C density in creases with C2H4 exposure time up to a limit value of about 0.5 ML, within the c(4x4) surface structure. Further exposure disrupts the c(4x4) reconst ruction and leads to a (2x1) low-energy electron diffraction pattern. Inter action with atomic hydrogen shows that the surface contains a mixture of he terodimers (Si-C) and homodimers (Si-Si), with an 1:1 proportion. These ass ignments are supported by first-principle calculations, which yield valence band and core level states in fairly good agreement with the experiment. F urthermore, total energy calculations strongly favor C incorporation in sur face Si-C dimers and in third and fourth layer sites, and rule out C incorp oration in sites of the second Si layer. The most stable c(4x4) surface con figuration, suggested by our calculations, consists of alternate Si-C and S i-Si dimer lines. In such a configuration, surface carbon atoms in Si-C dim ers induce a surface stress that leads to charge redistribution and atomic relaxation of the adjacent Si-Si dimers, consistent with scanning-tunneling microscopy images. Additional C atoms (in excess of those accommodated in surface sites) are forced in selected compressive (alpha) sites of the thir d and fourth layers. This model is discussed with respect to the previous m odels published in the literature.