Preparation of Co-Added β-FeSi₂ by Cation-Exchange Reaction between Molten Salts and Si

Abstract

A β-Fe_(1−x)Co_xSi₂ film was prepared by a cation-exchange reaction between molten NaCl-KCl-FeCl₂-CoCl₂ salts and a Si substrate at 1173 K. At low CoCl₂ concentrations of molten salts, the β-FeSi₂ phase was formed. However, when the CoCl₂ concentration was higher than 0.015 mol%, α-FeSi₂ and FeSi phases were formed in addition to the β-FeSi₂ phase. The α-FeSi₂ and FeSi phases changed to β-FeSi₂ when the sample was annealed at 1123 K for 24 h. The Co content of the reaction layer increased with the CoCl₂ concentration of molten salts. The electrical resistivity of β-FeSi₂ was measured by the four-probe method, and the band gap (E_g) of the sample was roughly determined from the temperature dependence of electrical resistivity by using the equation ρ=ρ₀exp(E_g⁄2kT). The electrical resistivity decreased and the band gap slightly decreased with the increase in the Co content of β-FeSi₂.