Effects of Oxide Ions on the Electrodeposition Process of Silicon in Molten Fluorides

Abstract

We studied the effects of O2− ions on the coordination structure of Si ions and the electrodeposition process of Si films in molten fluorides using SiO2 powder (as a source of Si) and Li2O (as a source of O2−). High-temperature Raman spectroscopic data revealed that the dissolution of SiO2 in molten KF, LiF-KF, and LiF-NaF-KF without Li2O was proceeded by the formation of silicate ions, with the [Si2O5]2− ion acting as the dominant species. On the other hand, when 3.0 mol% Li2O was added to molten LiF-KF and LiF-NaF-KF, the intensity of Raman band due to [SiO3F]3− ion was increased compared to that without Li2O system, which indicated that the O2− ions could cause breakage of Si-O-Si bonds of SiO2 or [Si2O5]2− ions. Furthermore, the reduction currents, attributed to the reduction of Si ions, increased significantly by the addition of Li2O; moreover, the thickness and current efficiency of the electrodeposited polycrystalline Si layer prepared by potentiostatic electrolysis was improved. These results indicated that the O2− ions can change the coordination structure of Si ions in molten fluorides and that the design of the molten salts bath is a key technology for fabricating high-quality Si layers with high current efficiency.