*3.3. Co-Adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> on the Surface of Polysilicon Si(100)*

A BCl<sup>3</sup> molecule and a SiHCl<sup>3</sup> molecule were placed together on the surface of the Si(100) supercell to investigate the adsorption reaction. Figure 6 shows the structural changes before and after the co-adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> on the Si(100) surface. Compared to the structure before adsorption in Figure 6a, it can be concluded from the structure after adsorption (Figure 6b) that the Si atom in the SiHCl<sup>3</sup> molecule forms a covalent bond with the B atom in BCl3. A Cl atom in each of the BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules undergoes a dissociation reaction and is adsorbed on the Si atom at the nearest bridge site on the Si(100) surface. The bond lengths of the other Cl–B bonds in the BCl<sup>3</sup> molecule are significantly elongated, and the B atom is adsorbed on the Si(100) surface at the same time. These results indicate that BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules are placed on the Si(100) surface simultaneously. After the adsorption reaction, it is evident that BCl<sup>3</sup> is more easily adsorbed on the Si(100) surface than is SiHCl3. When BCl<sup>3</sup> and SiHCl<sup>3</sup> coexist on the Si(100) surface, they are more likely to have an effect than SiHCl<sup>3</sup> alone on the surface. Moreover, their simultaneous presence there will be accompanied by a dissociation process; in which each dissociates a Cl atom, and the Cl atom in the BCl<sup>3</sup> molecule is attracted by the Si(100) surface.

**Figure 6.** Structure changes before (**a**) and after (**b**) the co-adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> .

A microscopic electronic analysis was also performed of the charge distribution and differential electron density when BCl<sup>3</sup> and SiHCl<sup>3</sup> are present together on the surface of polysilicon. The darker blue color between BCl<sup>3</sup> and SiHCl<sup>3</sup> in Figure 7b indicates that there is a large amount of electron transfer, and it can be seen that the B–Cl bond in the BCl<sup>3</sup> molecule is distinctly elongated, and a dissociated Cl atom is adsorbed on the Si(100) surface, indicating that it charge transfer has occurred during the adsorption process. From the charge density results in Figure 7a, indicating charge transfer between the surface and the molecules, we can also see where the electrons accumulate, which proves the occurrence of the adsorption reaction.

**Figure 7.** Differential charge density (**a**) and charge distribution (**b**) before and after the co-adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> on the surface.

#### *3.4. Electronic Structure Analysis*

To further understand the bonding mechanism of the adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> on the Si(100) surface, the adsorption density of states, partial density of states, and Mulliken charge distribution were calculated, and the results are shown in Figure 8 and Table 5. It can be observed that the Si3s orbital lies far away in energy from the Fermi level on the surface of the Si(100) unit cell. After the adsorption reaction, the electrons of the Si3s orbital are not easily transferred. The valence band where electron transfer occurs is Si3p, which constitutes the valence band on the left side of the Fermi level on the surface of the Si(100) unit cell. Although it is theoretically impossible for the Si3s orbital to transfer electrons, a very small amount of electron transfer does occur from the Si3s orbital after the reaction. The valence state on the right side of the Fermi level is composed of Si3d orbitals, and the bandgap between the top of the valence band and the bottom of the conduction band is 1.502 eV. After the reaction, the bandgap is reduced to 0.087 eV. The narrowing of the bandgap indicates that the Si is more likely to react, and its chemical properties become more active, which in turn indicates that it is more likely to interact with BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules.

**Figure 8.** State density changes of BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules after the adsorption on the Si(100) cell surface. (**a**) is the density of states of Si(100); (**b**) is the density of states for the BCl<sup>3</sup> adsorption model; (**c**) is the density of states for the SiHCl<sup>3</sup> adsorption model; (**d**) is the density of states for the co-adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> .


**Table 5.** Mulliken charge co-adsorption of BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules on the surface of Si(100) unit cell.

By analyzing the electronic density of states of BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules adsorbed on the surface of the Si(100) unit cell in Figure 8, it is found that after the two molecules are adsorbed on the surface of the unit cell, their band gaps are narrowed and the electrons inside the molecules are transferred to the outermost electron layer and to the conduction band, which is more reactive. Based on molecular chemical structure theory, we know that the B atom in the BCl<sup>3</sup> molecule has three coordination sites, and the electron configuration of B is 1s22s22p<sup>1</sup> , showing that B is an electron deficient atom. Six electrons are provided

by three Cl atoms, and each of the three Cl atoms also provides a filled 2p orbital that is side-by-side with an empty p orbital of the B atom and is thus able to form a strong π bond. The formation of bonds by sp<sup>2</sup> hybridization can explain the three B–Cl bonds in BCl3. Similarly, for the SiHCl<sup>3</sup> molecule, electrons are transferred to the outermost electron layer. The Si atom in the SiHCl<sup>3</sup> molecule has four coordination sites and the outermost electron configuration is 3s23p<sup>2</sup> , which is eventually completed by the four covalent bonds formed with Si. This can explain the origin of the four covalent bonds, and it can be concluded that the spatial configuration of SiHCl<sup>3</sup> is a that of a regular tetrahedron.

From the Mulliken charge distribution of the two molecules shown in Table 5, it can be observed that when SiHCl<sup>3</sup> is adsorbed on the surface of the Si(100) unit cell, charge transfer occurs, and there are two charge transfer paths. The first is the transfer of 1.30 eV of charge from the Si atom of the SiHCl<sup>3</sup> molecule to the surface of the unit cell. The second is charge transfer from the surface of the unit cell to the three Cl atoms and the H atom of the SiHCl<sup>3</sup> molecule, a charge transfer of 1.06 eV. After fitting and offsetting, it is found that during the entire adsorption reaction of SiHCl<sup>3</sup> on the surface of the Si(100) unit cell, the Si atom in the SiHCl<sup>3</sup> has transferred a net 0.24 eV of charge to the surface of the Si(100) unit cell. When BCl<sup>3</sup> is adsorbed on the surface of the Si(100) unit cell, significant charge transfer also occurs, and there are two charge transfer paths. The first is a transfer of 1.52 eV from the B atom in BCl<sup>3</sup> to the surface of the unit cell. The second is a transfer of 1.23 eV from the surface of the unit cell to the three Cl atoms and one H atom in the BCl<sup>3</sup> molecule. After fitting and offsetting, during the entire adsorption reaction of BCl<sup>3</sup> on the surface of the Si(100) unit cell, the B atom in BCl<sup>3</sup> is seen to have transferred 0.29 eV of charge to the surface of the Si(100) unit cell.

According to the results for transfer of electrons between SiHCl3, BCl3, and the surface of polysilicon during the adsorption process, both molecules transfer charge to the surface of the unit cell during adsorption, with BCl<sup>3</sup> transferring 0.05 eV more charge to the surface of the Si(100) unit cell than SiHCl<sup>3</sup> does. It can be shown that the surface of the Si(100) unit cell has a definite adsorption effect on BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules, and that the B atoms in BCl<sup>3</sup> are more easily adsorbed on the Si(100) surface, which means that B and polysilicon will be deposited on the silicon rod together during the production process. The above results also explain the phenomenon that when BCl<sup>3</sup> and SiHCl<sup>3</sup> molecules are co-adsorbed on the surface of the Si(100) unit cell, the Si atoms in the SiHCl<sup>3</sup> molecules first form a covalent bond with the B atoms in BCl3.

The B impurities may deposit on the surface of the silicon rod competing with polysilicon and the trace B-compound impurities may be enriched by the influence of transport phenomena. The above two phenomena are the main reasons causing the high-impurity B in the silicon production. It is obvious that the adsorption of BCl<sup>3</sup> on silicon to cause the deposition of B on silicon surface. Therefore, transport phenomenon investigation may be effective method to reduce boron impurity inside poly-silicon, such as increasing mole fraction of H<sup>2</sup> and temperature.

#### **4. Conclusions**


(3) After the adsorption of SiHCl<sup>3</sup> and BCl3, 0.24 and 0.29 eV of charge, respectively, are found to have been transferred from the molecule to the surface of the unit cell. Both BCl<sup>3</sup> and SiHCl<sup>3</sup> are readily adsorbed on the surface of the Si(100) unit cell, but BCl<sup>3</sup> is more easily adsorbed. These results confirm that the B atom in BCl<sup>3</sup> in the adsorption model forms a covalent bond with the Si atom on the Si(100) unit cell surface, and the Si atom in the SiHCl<sup>3</sup> molecule forms a covalent bond with the B atom in BCl3.

**Author Contributions:** Conceptualization, L.H. and Y.H.; Data curation, F.C., Y.H. and N.Y.; Investigation, J.W., Y.H. and N.Y.; Methodology, N.Y. and G.X.; Software, L.T. and L.H.; Validation, Q.Y.; Writing—Original draft, Q.Y.; Writing—Review and editing, Q.Y., J.W. and Y.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China Project (No. 21566015 and No. 52074141).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Data sharing not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
