Electrodeposition of semiconductor materials such as Si with very negative reduction potentials has been attempted in various non-aqueous solvents [1-3]. In particular, since ionic liquids exhibit relatively large electrochemical windows and are applicable under low temperature condition, they are expected to be one of the promising solvents for Si electrodeposition. Several studies have suggested the capability of ionic liquids to electrodeposit Si [4-6]. We also applied trimethylhexylammonium bis(trifluoromethylsulfonyl) imide (TMHATFSI) ionic liquid for the electrodeposition of Si [7-9]. To control this deposition process more precisely, its reaction mechanism should be elucidated further in detail. In the present study, we analyzed the reaction mechanism of SiCl₄ during Si electrodeposition in ionic liquids using three approaches; 1) Electrochemical quartz crystal microbalance (EQCM), 2) X-ray reflectivity (XRR), and 3) Density functional theory (DFT) calculation. Electrodeposition of Si was conducted in TMHATFSI at 40 ^oC, controlled by thermo block. 3 electrodes cell (W.E. : Au, R.E. : Ag/Ag⁺, C.E. : Pt) was employed for the Si film electrodeposition. During XRR measurement, 2 electrodes cell (W.E. : Au, C.E. : Au) was used at 30 ^oC, controlled by thin thermo plate. EQCM measurement was carried out in Ar atmosphere, while the mixture of Ar and N₂ was introduced into the electrolytic cell during the XRR measurement. The XRR measurement data were analyzed by GenX software. DFT calculation was performed by Gaussian09 package to support the XRR measurements.

Results of the EQCM measurements during the cyclic voltammetry indicated a preliminary stage prior to the electrodeposition, in which no mass increase was detected in spite of substantial amount of charge transfer, suggesting the sequential reduction of SiCl₄ through Si (III) and Si (II) species over the multiple stages. These were thought to be intermediate state in Si electrodeposition. Such a state was then investigated by using XRR measurement. The intensity of the reflectivity gradually decreased with the progress of the Si electrodeposition. Analyzed results of the XRR measurement indicated that a layer, which is thought to be polymer-like structure of Si containing dimer such as Si₂Cl₆, would be generated prior to the formation of metallic Si (0) films. This polymer-like structure was also supported by the results of DFT calculation; energy profiles showed that Si-Si bond formation between two SiCl₄ molecules was more favorable than that between SiCl₄ and Si surface, after SiCl₄ received electrons from the surface.

These results suggest that the cathodic reaction of SiCl₄ proceeds through the formation of intermediate states containing polymer-like structure of Si.

We would like to thank Dr. Apurva Mehta (SLAC National Accelerator Laboratory) and Mr. Trevor Petach (Department of Physics, Stanford University) for their kind advices and helps with the experiment. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Y. T. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University, from MEXT, Japan.

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