The energetics involved in the dimer diffusion between row and trough sites are shown in Fig. 2. While it is reasonable to assume that the dimer would pass through the metastable B-state before passing off of the row (thereby minimizing its profile in the direction of its diffusion), the energy values shown in Fig.1 are relatively independent of this assumption. The energy of state B relative to state A, 59+9 meV, and the barrier between these states, 0.70+0.08 eV, were obtained by Swartzentruber et al. by tracking the dimer through A-B rotations at room temperature [1]. The relative energy of state C, 61+16 meV, was found by appealing to the Boltzmann relation for the equilibrium distribution of equally accessible states. The energy barrier from the B to the C state, 1.36+0.06 eV, was found by applying an Arrhenius relation. In spite of this being a three state system, for temperatures well above the energy difference between states A and B, the row to trough transition rate can be described by an Arrhenius relation with the energy barrier being that from B to C and the prefactor v/2 where v is assumed to be the same as for on the row diffusion 1013.2+0.6 . Therefor, in this temperature range, this can be considered a system of only two states, in the trough and on the row.
The diffusion of dimers across dimer rows may have important implications for the homoepitaxial growth of Si on Si(001). For growth regimes where there are large numbers of dimers present, we expect that the quality of the epitaxial growth process depends on the area accessible to diffusing dimers prior to being incorporated into the crystal. We have shown that at a critical threshold near 450K, dimer diffusion converts from one-dimensional to two-dimensional, with the onset of diffusion across dimer rows. Below this temperature, dimers are confined to move along single dimer rows between defects and step edges. Above this temperature, dimers are able to escape from these one-dimensional traps and interact with features over a much larger area. In doing so, we may expect that dimers can not only find favorable epitaxial locations more easily, but also more effectively convert metastable structures into epitaxial structures. Such conversion processes have recently been proposed to be important in low temperature epitaxial growth [3]. Furthermore, it has been proposed that it is the C configuration, and not A or B, which binds additional monomers, forming a cross configuration, and promotes one-dimensional growth at low temperatures [1,3], making conversion to the C configuration itself important in understanding epitaxial growth at low temperatures. The interactions of dimers with various metastable and epitaxial structures, dimer-dimer interactions, and the lifetime of dimers on the Si(001) surface are currently being studied in our lab to help clarify the role of dimers in epitaxial growth.
[1] T. Yamasaki and T. Uda. Phys. Rev. Lett. 76, 2949-2952 (1996).
[2] B. S. Swartzentruber, A. P. Smith, and H. Jonsson, Phys. Rev. Lett. 77, 2518 (1996).
[3] G. Brocks and P. J. Kelly, Phys. Rev. Lett. 76, 2362 (1996).