Electrochemical formation of a Ca-Si film in a molten CaCl 2 -KCl at 923 K was investigated. Potentiostatic electrolysis of a Si electrode at −0.10 V for 1 h resulted in the formation of a multiphase Ca-Si film having a thickness of about 30 μm. The Ca-Si film was converted into other alloy phases by anodic potentiostatic electrolysis after the cathodic electrodeposition of Ca metal at −0.10 V. The various transformation reactions and the corresponding equilibrium potentials were clarified. At 923 K, the equilibrium potential was found to be 0.18 V (CaSi 2 ). Reflectance measurements in the ultraviolet, visible, and near-infrared region clarified that the CaSi 2 film has a direct bandgap of 3.1 eV. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.1021504jes] All rights reserved.Manuscript submitted December 11, 2014; revised manuscript received January 26, 2015. Published February 14, 2015 Compound semiconductors are among the most promising materials for use in high-power electronic and photoelectric conversion devices because of their advantages.1 However, conventional processes for preparing semiconductors involve high energy consumption and the use of hazardous starting materials, and are hence unsuitable for mass production. Therefore, a new semiconductor preparation process that does not require harmful starting materials or high energy has been proposed.Recently, metal silicides, which are abundant, non-toxic materials, have attracted much interest for use as environment-friendly semiconductors, because they are expected to show excellent optical properties. In addition, the bandgap energy of metal silicides can be extended from the infrared to the visible region by appropriate choice of metal elements and phases of the compounds.
2-5Among the numerous metal silicides reported, Ca-Si metal silicides are well known semiconductor materials that exhibit superconductive properties. For example, Ca 2 Si is a semiconductor with an energy gap of 1.9 eV, 6 while CaSi is expected to show high hydrogen storage capacity. 7 In addition, superconductivity has been discovered at 14 K under pressure in a new polymorph of the CaSi 2 stoichiometry. 8,9 Recently, there has been increased interest in the use of intermetallic silicides to replace the conventional graphite anodes in Li-ion batteries. 10,11 Films fabricated from these alloys, with arbitrary shapes and sizes, may be used in various applications such as energy conversion devices.Nevertheless, the high vapor pressure of Ca makes the growth of a continuous film layer by deposition from the gas phase 12-17 difficult. Ca atoms are easily evaporated from the Si substrate, which prevents the formation of Ca-silicide by interdiffusion with the Si substrate.With this background, we proposed a novel el...