Low-cost and simple fabrication methods of Si thin films have been required to meet increasing demands for solar cells, semiconductor materials, and so on. Si electrodeposition is one of the useful techniques, for it is low-cost and suitable for large scale production and nano-structure fabrication. We have proposed the Si electrodeposition process using SiCl₄ as Si source and ionic liquids Trimethyl-n-hexylammmonium bis(trifluoromethylsulfonyl)imide (TMHA-TFSI) as solvents [1,2], and have attempted to optimize the deposition process, for which understanding of the molecular level mechanisms of Si electrodeposition reaction is significant. In this study we carried out theoretical analysis on the reaction mechanism of SiCl₄ in ionic liquids by density functional theory (DFT), mainly focusing on Si-Si bond formation just after electron transfer from Si electrode surface. Two reaction schemes about the Si-Si bond formation as an initial stage of Si electrodeposition process are hypothesized; (i) SiCl₄ forms Si-Si bond with Si surface directly, (ii) SiCl₄ forms Si-Si bond with another SiCl₄nearby. More favorable scheme was identified by comparing theoretically obtained energy profiles of scheme (i) and (ii). All DFT calculations were performed by Gaussian 09, with B3LYP as an exchange-correlation functional, 6-31+G** as basis sets for C and H, LANL2DZdp ECP as a basis set for Si. To model ionic liquids as solvent surrounding reactants, ONIOM method was applied; main reactants were expressed by quantum mechanics and solvent molecules were expressed by molecular mechanics with universal force field (UFF) parameters. Since ionic liquids generally exhibit complicated behaviors in dielectric constant, polarized continuum model (PCM) that usually provides sufficiently appropriate solvent model in the case of water solvent is not capable to work well in this situation.

Energy profiles obtained by the DFT calculation showed that the scheme (ii) was more favorable than the scheme (i), suggesting that just after accepting electrons, SiCl₄ forms Si-Si bond with another SiCl₄ species, rather than with Si surface. Mulliken charge analyses showed that the electron population of covalent Si-Si bonds in Si surface decreased after the bond formation with SiCl₄, which was caused by highly electronegative Cl atoms of SiCl₄. This charge distribution destabilized the covalent bond within products of scheme (i), which made scheme (ii) more favorable. Solvation structure of products with ionic liquids showed that products of scheme (ii) interacted with both cation TMHA⁺ and anion TFSI^-, whereas the product of scheme (i) interacted with only TMHA⁺. This result implies that the ionic liquid molecules significantly influence the stability of the products, promoting Si-Si bond formation among SiCl₄species as shown in scheme (ii).

From these results, we hypothesized the scheme of Si electrodeposition at initial stage as follows; a SiCl₄ molecule near electrode surface receives electrons to react with another SiCl₄molecule nearby to form Si-Si bond, generating relatively smaller size intermediates, which are stabilized by the solvation of ionic liquids. These smaller size intermediates continue to receive electrons near electrode surface to organize larger size intermediates or to deposit on the Si surface, forming Si film.

This study was financially supported by JST Core Research for Evolutionary Science and Technology (CREST). T. F. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University from MEXT, Japan.

References

[1] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim, Acta., 100, 236, 2013

[2] Y. Tsuyuki, T. A. Pham Huynh, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta., 183, 49, 2015.