Electrodeposition of Si Films from SiO₂ in Molten CaCl₂-CaO: The Dissolution-Electrodeposition Mechanism and Its Epitaxial Growth Behavior

Molten salt electrodeposition of crystalline silicon (Si) films from silicon dioxide (SiO₂) in molten calcium chloride (CaCl₂)-calcium oxide (CaO) has been systematically investigated. The dissolution-electrodeposition mechanism was studied by cyclic voltammetry (CV), in situ X-ray diffraction (XRD), and in situ Raman spectroscopy. The results show that different silicate ions, including SiO₃²⁻, SiO₄⁴⁻, would be generated in molten salt and could be influenced by the molar ratios of additive SiO₂ and CaO, as well as the electrolytic parameters. In particular, with the increase of electrodeposition time, SiO₄⁴⁻ increased as the dominated silicate ions in molten salt. Furthermore, different current densities, time and substrates would also have vital influences on the electrodeposition process and the electrodeposited Si products. Si products with tunable morphology have been deposited on different substrates by adjusting the electrodeposition conditions. The deposited crystalline Si films exhibit homogeneous epitaxial structures, in particular, the epitaxial Si film grown on the 110-oriented Si wafer possesses uniform inverted pyramid structure. The ohmic resistivity test and microstructure analysis results show that the electrodeposited epitaxial crystalline Si films have the similar properties and characteristics as their single crystal Si wafer substrates. In general, the investigation of the dissolution-electrodeposition mechanism and its epitaxial growth behavior helps the progress of this one-step CaO-assisted dissolution-electrodeposition process for the production of epitaxial Si films.