[Analysis Example] Surface Reconstruction

Calculation of the surface structure of Si

A solid surface is energetically less favorable than the bulk state because the atoms that make up the surface lose their bonds with surrounding atoms. The magnitude of the energy loss is known as the surface energy, which has different values depending on the surface orientation (ref：[Case Studies] Surface Energy of Pigments).The electronic state near the surface is different from that in the bulk, and the face spacing near the surface of metals and other materials is known to deviate from that inside the crystal. In covalent solids, it is known that not only one-dimensional structural relaxation but also two-dimensional relaxation occurs in the plane.

Here, the relaxation of the surface structure of Si(001) is investigated by ab initio MD calculations.MD calculations for the Si(001) surface were performed using the MD calculation function of the SIESTA modeler. We used PBE as the functional and SZP as the basis function. The time step was 2fs and the total calculation was 1ps. In this calculation, the atoms in three layers near the surface were excluded and constrained. Figure 1 plots the potential energy during the MD calculation, and the snapshot at 0.8 ps shows the formation of the dimer structure. From the change in potential energy, we can see that it goes beyond some energy valley structures, and it is possible that the dimer structure is not reached by optimization calculations.

Fig.1　ab initio MD calculation of surface reconstruction of Si(001)