Anisotropic atom diffusion on Si(553)-Au surface

Using density functional theory, we study atomic diffusion on the Si(553)-Au surface. We have calculated the potential energy surfaces for atoms of different chemical elements (Ag, Au, Mg, In, and Si). By tracing the adatom positions and the energetics of a fully relaxed surface, we have calculated energy barriers, diffusion paths, and hopping rates along and across the steps. The results show that in most cases, the surface diffusion is strongly anisotropic, while the energy barriers and hopping rates have a strong dependency on the chemical species of the adatom. In the case of Au, Ag, and Mg atoms, the diffusion can be realized in two separate one-dimensional channels, featuring different mechanisms of the diffusion.

Figure 1

Side view (top) and top view (bottom) of the atomic structure of the Si(553)-Au surface, according to the model of Ref. 48. Silicon and gold atoms are marked by light green and orange balls, respectively. The main building blocks of the surface structure, i.e., a honeycomb chain (HC) and double Au chain together with the surface unit cell are also indicated. Note that atoms forming the Au chain are dimerized.

Figure 2

The projected density of states (PDOS) of the step-edge Si atoms (Si${}_1$ and Si${}_2$) and part of the double-Au chain (Au${}_1$ and Au${}_2$), the HC Si atoms (Si${}_3$ and Si${}_4$), and the other part of the Au chain (Au${}_3$ and Au${}_4$), and isolated Pb, In, Mg, Ag, Au, Si atoms.

Figure 3

Potential energy surface of an In atom on the Si(553)-Au surface (left) and the energy barriers (right) along the diffusion trajectories highlighted on the PES landscape. Labels Au and Si on the left side of the left panel indicate the positions of the double-Au chain and the step-edge Si atoms, respectively. The diffusion path marked by A passes through the lowest energy adsorption sites, shown as deep minima of PES. The parallelogram shows the unit cell of the surface with its vertices indicating positions of the step-edge Si atoms (compare Fig. 1). Note the differences in the abscissas of the left and right panels. The label X in the left panel describes the position in the direction parallel to the steps, while the length shown in the right panel is measured along the diffusion paths.

Figure 4

The same as in Fig. 3 for Ag atom.

Figure 5

The lowest energy structural configurations of In atoms in the step-edge diffusion channel [(a)–(d)]. (e) Distances of the Pb atom to the Si${}_1$, Si${}_2$, Au${}_1$, and Au${}_2$, indicated in Fig. 1, together with the energy barriers (curve A in Fig. 3) vs the position of the In atom in the $\left[\overline{1}10\right]$ direction (not along the diffusion path). Insets show the positions of the In atom on this energy barrier landscape.

Figure 6

The lowest energy structural configurations of Ag atoms in the middle-terrace diffusion channel [(a)–(f)]. (g) Distances of the Ag atom to the Si${}_3$, Si${}_4$, Au${}_3$, and Au${}_4$, indicated in Fig. 1, together with the energy barriers vs the position of Ag atom in the $\left[\overline{1}10\right]$ direction. Insets (a)–(f) show the positions of the Ag atoms on this energy barrier landscape.