The Theoretically Studies On The Mechanism For Small Molecule Adsorption On The Si Surface

Semiconductor silicon is the most important function material in microelectronic industry evolution. Following extension of application area, it has a higher requirement about its properties, such as light, heat, electricity, mechanics and so on. Chemical modification of silicon surface has been investigated to improve these properties. In this paper, it has been studied the mainly dissociative mechanisms of small molecules on the silicon surface. There have three systems to be studied:(1) H2S decomposition on the Si(111)-7x7 surface; (2) pyrazine and pyrimidine chemisorption on the Si(111)-7x7 surface (3) dimethylsilane and trimethylsilane dissociative adsorption on the Si(100)-2x1 surface.Firstly, density functional theory calculation has been used to investigate the dissociative adsorption of hydrogen sulfide molecules on the Si(111)-7x7 surface. The reaction pathways for four hydrogen sulfide molecules dissociation over the adatom and rest atom sites are explored theoretically. The calculated results demonstrate that the initial dissociation of the first H2S molecule on both different sites to form the adsorbed SH species is no energy barrier. The elevated temperature can result in further S-H bond dissociation, which is started by an SH insertion, and then followed by an H-transfer process. Subsequently, the second H2S attacks the adatom site (Sia), H atoms from the S-H of the second H2S and surface Sia-H are extracted to release the gas phase H2, and then the SH species from the second H2S occupies on the Sia site. Furthermore, the SH species inserts into Si-Si backbond of the adatom, followed by an H-transfer process. In the same way, the SH species from the third and the fourth H2S molecule are also inserted into remnant Si-Si backbonds of the adatom followed by an H-transfer process up to the formation of Si4+ state.Secondly, the chemisorption of pyrazine and pyrimidine on the reconstructed Si(111)-7x7 surface has been investigated by means of the density functional theory calculation in combination with a cluster model approach. Three chemisorption styles, i.e., dative-bond, [4+2]-like and [2+2]-like cycloadditions of pyrazine and pyrimidine onto the Si(111)-7x7 surface, have been considered comparatively. The calculations indicate that the formation of the [4+2]-like cycloadditions are thermodynamically and kinetically favored compared to the dative bonding and the [2+2]-like cycloaddition.Finally, the dissociative adsorption of dimethylsilane and trimethylsilane on the Si(100)-2x1 surfaces are calculated with DFT method combining Si15H16 double-dimer and Si23H24 V-trench clusters model. Three different reaction pathways via scission of Si—H, C—H, and Si—C are investigated theoretically. The calculated results indicate that Si—H dissociation pathways of dimethylsilane and trimethylsilane are the most kinetically favorable relative to the other two pathways.In this paper, there have investigated the mechanisms of chemisorption of hydrogen sulfide, pyrazine and pyrimidine on Si(111)-7x7 surface, and dimethylsilane and trimethylsilane on Si(100)-2x1 surface to expose the mechanisms of chemical modification of these small molecules on silicon surface, which provide the theoretical direction for experiment.