Measurement and Thermodynamic Analysis of Carbon Solubility in Si–Cr Alloys at SiC Saturation

Abstract

Si–Cr alloy is one of the predominant solvents for rapid solution growth of 4H–SiC crystals. The solubilities of carbon in Si–40 mol%Cr alloy at SiC saturation at 1773–2273 K and in Si–Cr alloys of various chromium contents at 2073 K were measured by equilibrating the Si–Cr alloy with a 4H–SiC single crystal. Carbon solubility in Si–40 mol%Cr alloy increased with temperature from 0.22 mol% at 1773 K to 3.59 mol% at 2273 K. At 2073 K, carbon solubility at SiC saturation increased with the chromium content in the liquid from 0.18 mol% in Si–20 mol%Cr to 16.4 mol% in Si–80 mol%Cr. A thermodynamic analysis of the Si–Cr–C alloy was also conducted. Although the sub-regular solution model is often adopted to estimate phase relations in solution systems, this predicted a carbon solubility in Si-40 mol%Cr at SiC saturation more than two times higher than the measured value. In contrast, a quasi-chemical model that considered the competition between substitutional Si and Cr atoms bonding to interstitial carbon atoms reproduced the activity coefficient of carbon in Si–Cr alloys of 60–100 mol%Si composition, in which the carbon solubility at SiC saturation was less than 1.5 mol%, fairly well. This quasi-chemical model enabled the precise phase relation to be evaluated when designing the solution growth of SiC using a Si–Cr solvent.