*3.2. Adsorption of SiHCl<sup>3</sup> Molecules on the Surface of Polysilicon Si(100)*

The molecular structure of SiHCl<sup>3</sup> is tetrahedral, and we discuss here the adsorption process of optimized SiHCl<sup>3</sup> on the Si(100) surface. Figure 4 shows six possible adsorption positions on the surface, which can be described as the hydrogen top-one-positive position, hydrogen bottom-one-front position, hydrogen top-two-front position, hydrogen side-onefront position, hydrogen side-two-front position, and hydrogen bottom-two-front position. After the optimization calculation, the distance between the molecule and the surface was found to have been shortened, indicating that SiHCl<sup>3</sup> is prone to react with the surface of the Si(100) unit cell. This conclusion can also be drawn from the changes in the distance before and after adsorption at the six adsorption sites listed in Table 3. When SiHCl<sup>3</sup> is adsorbed at the second bottom-front position of the hydrogen, its distance changes from 3.000 Å to 2.340 Å. The Si atom of SiHCl<sup>3</sup> forms a covalent bond with the Si atom at the bridging position, indicating that the SiHCl<sup>3</sup> molecule has been adsorbed on the Si(100) unit cell. Simultaneously, it is observed that the Si–Cl bond of the SiHCl<sup>3</sup> molecule is distinctly elongated, which indicates that the Cl atom is attracted by the Si atom adjacent to the bridge site.

**Figure 4.** Diagram of molecular structures and distances of SiHCl<sup>3</sup> before and after adsorption on the Si(100) surface. Diagrams (**a**–**f**) show the adsorption structures before reaction, (**a'**–**f'**) those after reaction ((**a**–**f**,**a'**–**f'**) show hydrogen in the top-one-positive, bottom-one-front, top-two-front, side-one-front, side-two-front, and bottom-two-front positions, respectively).


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

Table 4 shows the calculated adsorption energy of SiHCl<sup>3</sup> on the Si(100) surface. The adsorption energies obtained for the six adsorption structures are all negative, indicating that SiHCl3 reacts readily with the Si(100) surface. The order of the adsorption energies of the several positions is as follows: E(hydrogen side-two-front position) > E(hydrogen top-two-front position) > E(hydrogen top-one-positive position) > E(hydrogen bottomone-front position) > E(hydrogen side-one-front position) > E(hydrogen bottom-two-front position). The adsorption energy (E) of SiHCl<sup>3</sup> is greatest when the adsorption structure of the second hydrogen base is in the front position, indicating that adsorption is more likely to occur in this orientation, at the front position of the hydrogen base 2. From the calculated adsorption energies, it can be seen that the adsorption of SiHCl<sup>3</sup> molecules in the hydrogen bottom-two-front position corresponds to the most favorable orientation, and the calculated adsorption distances confirm this conclusion.

**Table 4.** Adsorption energy of SiHCl<sup>3</sup> after adsorption on the Si(100) surface.


To gain a deeper understanding of the charge distribution in the adsorption process of SiHCl3, we can use the electronic levels to characterize the reaction. The differential electron density and charge distribution of the SiHCl<sup>3</sup> molecules adsorbed on the surface of the Si(100) unit cell from the face center downward were calculated. The calculated results are shown in Figure 5a,b. From the results shown in the figure, it can be seen that the color representing the charge concentrated in the bond between the Si atoms on the Si(100) surface and the Si atom in SiHCl<sup>3</sup> is darker than that for other bonds. This not only shows that electrons have been transferred after adsorption, but also shows that a large amount of charge is transferred in this local area. Looking at the image of the adsorbed SiHCl<sup>3</sup> molecule, it is seen that the three Cl atoms and the bonding angles between the H and Si atoms in the molecule have changed, and both the Cl and H atoms show an upward posture. In addition, it can be seen that the silicon atoms on the surface and the two Si atoms at the right and back of the bridge silicon atoms adjacent to the bonded Si atoms and the corresponding Cl atoms form an electron cloud. This proves that the two have an attractive effect.

**Figure 5.** Differential charge density (**a**) and charge distribution (**b**) after SiHCl<sup>3</sup> is adsorbed on the Si(100) surface.
