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

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 sub-stitutional Si and Cr atoms bonding to interstitial carbon atoms reproduced the activity coefficient of carbon in Si-Cr alloys of 60100 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.