Si thin films and micro/nano structures have been widely used for various applications such as semiconductor and MEMS devices, solar cells, battery electrodes, etc. While dry processes such as chemical vapor deposition have been conventionally applied for their fabrication, electrochemical approaches attract attention for their capability to form uniform and precise structures to wide and non-flat surfaces, with high productivity. In this paper we describe the results of our investigation on Si electrodeposition, mainly focusing upon the process development as well as mechanistic understanding of the deposition process through experimental and theoretical approaches.

Since the redox potential of Si species is very negative, nonaqueous systems such as organic solvents, molten salts or ionic liquids have been applied for the electrodeposition of Si [1-3]. We have mainly used low temperature (up to 40°C) ionic liquid (IL), trimethylhexyl ammonium (TMHA⁺) bis-trifluorosulfonyl imide (TFSI^-), as electrolyte [4]. The TMHA-TFSI is hydrophobic IL in which SiCl₄, the precursor of Si, could be sufficiently dissolved. The electrodeposition was carried out potentiostatically using a three electrode sell. Au/n-Si or FTO substrate, Pt wire, and Ag/Ag⁺ were used as working, counter, and reference electrodes, respectively.

By optimizing the deposition parameters, uniform Si films were obtained. The results of structural and compositional analyses indicated that the as-deposited films were amorphous and contained impurities such as C, while their amount could be reduced by annealing. It was also confirmed that the electric properties of the deposited films could be controlled by adding dopant species such as AlCl₃ in the bath for the electrodeposition [5]. In order to investigate the mechanism of the deposition process, we applied electrochemical quartz crystal microbalance (EQCM) and the results indicated that the reduction of SiCl₄ to Si proceeded via intermediate species such as Si (III) and Si (II) [4, 6]. In situ X-ray reflection measurements were also employed using synchrotron radiation lightsource in combination with density functional theory (DFT) calculation for nanoscopic analysis of the structure of IL-electrode interface during the deposition [7]. It was indicated that layered structure consisting of Si multimer such as Si₂Cl₆ dimer was formed at the surface of electrode prior to the deposition of elemental Si, suggesting formation of the intermediate states during the initial deposition stage of Si.

The authors would like to thank Dr. J. Komadina, Dr. A. Mehta, Mr. M. Nishida, and Mr. H. Takai for their experimental help and valuable discussion. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program.

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[3] F. Endres, A.P. Abbott, D.R. MacFarlane, Electrodeposition from Ionic Liquids, Wiley-VCH, 2008.

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

[5] Y. Tsuyuki, H. Takai, Y. Fukunaka, T. Homma, Jpn. J. Appl. Phys., in press.

[6] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183, 49 (2015).

[7] Y. Tsuyuki, T. Fujimura, M. Kunimoto, Y. Fukunaka, P. Pianetta, T. Homma, J. Electrochem. Soc., 164, D994 (2017).