Negligible Temperature Dependence of Nitrogen Solubility in Molten Silicon–Chromium Alloys at Middle Composition Range

Abstract

N solubility in molten Si-based alloys is an important property for controlling the concentration of N in SiC crystals, fabricated by the solution growth method. In this study, N solubility in Si–Cr alloys, which is the commonly used solvent for solution growth, was measured at 1953, 2023, and 2073 K, and then evaluated in terms of thermodynamics. The measured N solubilities in Si–40, 55, and 72.5 mol% Cr obeyed the Sieverts’ law, and increased with the increase in Cr concentration. For Si–40 and 55 mol% Cr alloys, the temperature dependence of the N solubility was negligibly small, or even slightly positive, which is opposite to those for pure Si and Cr. This reverse feature in the middle composition range was reproduced by the estimated activity coefficient of N by the quasi-chemical model, which assumes N as interstitial atoms in Si–Cr solvents. In addition, another feature was found for the estimated activity coefficient of N, that is, an upward convex against the Si–Cr composition. These characteristics arise from the negatively large heat of mixing of Si–Cr alloys and its relatively large temperature dependence.