*3.1. Adsorption of BCl<sup>3</sup> on the Surface of Polysilicon Si(100)*

The molecular structure of BCl<sup>3</sup> is a plane regular triangle. The structure of BCl<sup>3</sup> adsorbed on the Si(100) surface was obtained by placing BCl<sup>3</sup> molecules on a super cell (3 × 3) surface and fully relaxing it. The surface optimization of BCl<sup>3</sup> with different structures was performed as follows. A BCl<sup>3</sup> molecule was used to establish stable adsorption models for three orientations: the side position, lateral position and positive position of the molecule. The changes in the distance before and after adsorption were compared and analyzed. The results are shown in Figure 2.

**Figure 2.** Diagrams of the molecular structure and distance of BCl<sup>3</sup> before and after adsorption on the Si(100) surface. Diagrams (**a**–**c**) show the adsorption structure before reaction, (**a'**–**c'**) the adsorption structure after the reaction, ((**a**,**a'**) show side position of the molecule, (**b**,**b'**), lateral position of the molecule, and (**c**,**c'**) positive position of the molecule).

Changes in the distance between the molecules and the surface before and after the reaction for the three adsorption structures were observed. It was found that when BCl<sup>3</sup> molecules are adsorbed on the Si(100) surface of the unit cell in the three possible structures, the distance before and after the adsorption has changed correspondingly, and the distance between the molecules and the surface of the unit cell in the three structural configurations is correspondingly shortened. This shows that an adsorption reaction between the BCl<sup>3</sup> molecule and the Si(100) surface occurs readily. The distances of the ortho-adsorbed BCl<sup>3</sup> molecule before and after optimization were markedly different, at 2.971 Å and 1.873 Å, respectively. The B atom in BCl<sup>3</sup> forms a covalent bond with the Si(100) surface and one of the B–Cl bonds is elongated, indicating that the Si atom has an attractive effect on Cl. Table 1 shows the change in distance before and after adsorption in each of the three positions.

**Table 1.** Distances before and after the adsorption of BCl<sup>3</sup> on the Si(100) surface.


The adsorption energies of the three adsorption structures were then calculated. The smaller the adsorption energy, the easier it is to adsorb, and vice versa. The calculated results for the adsorption energy of the side, lateral and positive positions of the molecule are shown in Table 2.

**Table 2.** Adsorption energy of BCl<sup>3</sup> after adsorption on Si(100) surface.


According to the data in Table 2, the adsorption energies of the three adsorption modes on the Si(100) surface are all negative, indicating that BCl<sup>3</sup> molecules are readily adsorbed by the Si(100) surface and that the adsorption reaction occurs easily on the surface. The relative adsorption energies of the three molecular orientations are as follows: E (lateral position) > E (side position) > E (positive position). It can be seen that the orientation in which BCl<sup>3</sup> molecules are most easily adsorbed on the Si(100) surface is the positive position. Thus, the change in the molecular adsorption distance after adsorption was verified, and the most favorable orientation for adsorption was the positive position.

Differential electron density analysis (Figure 3a) and calculation of the charge distribution (Figure 3b) of BCl<sup>3</sup> molecules adsorbed on the Si(100) surface were also carried out. The color of the Si atoms depicted at the bridge sites on the Si(100) surface is denser, indicating that a large amount of charge is transferred in this local area and that the charge has migrated after the adsorption reaction has occurred. The B–Cl bond on the right side is clearly elongated, and an electron cloud is formed between the Cl atom and the silicon atom at the adjacent bridge site on the right, indicating that the silicon atom at the adjacent bridge site on the right has an attractive effect on the Cl atom.

**Figure 3.** Differential charge density (**a**) and charge distribution (**b**) of BCl<sup>3</sup> adsorbed on the Si(100) surface of the unit cell.
