Electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and Raman spectroscopy analysis of SiCl4 reduction in ionic liquids

• EQCM and XPS measurements were applied to understand the Si electrodeposition reaction mechanism.
• SiCl4 reduction apparently progressed by four-electron electroreduction.
• Si thin films electrodeposited from SiCl4 in trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl) imide (TMHATFSI) were characterized as amorphous Si with Si–Si bonds.
• An annealing treatment resulted in crystalline Si thin films with significantly lower C content than as-deposited material.

The reaction mechanism of Si electrodeposition in the ionic liquid trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl) imide (TMHATFSI) with SiCl4 was investigated using an electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The deposited films were further characterized by Raman spectroscopy. The EQCM method measured the frequency change in situ during electrodeposition, whereas XPS provided the Si mass concentration in the electrodeposited films. By combining these results, the mass change due to SiCl4 reduction was estimated. Then, the current efficiency for Si electrodeposition at various potentials was evaluated. The calculated current efficiency was almost 100% when it was assumed that SiCl4 reduction progressed by four-electron electroreduction. The XPS and Raman results showed that the deposited films were composed primarily of amorphous Si with Si–Si bonds. Crystalline Si thin films were obtained after annealing in an Ar gas stream.