Undercooling measurement and nucleation study of silicon droplets on various substrates

The heterogeneous nucleation of solid silicon is studied when molten droplets solidify on various substrates. An experimental installation has been developed in order to record in real time the melting-solidification process, measure the undercooling temperature and look at the solidification of the droplets. Three different categories of substrate materials are studied: oxides (silica, zyarock and sapphire), nitrides (silica+oxidized Si3N4 coating, zyarock+oxidized Si3N4 coating, sintered Si3N4, PBN and HIP-BN) and carbon-containing (isostatic graphite, glassy carbon and SiC). Higher undercooling values are measured in the case of oxide substrates where the solidified droplet is found to be mainly composed of a single-crystal. In the case of nitride substrates, a dissolution/precipitation process takes place and β-phase Si3N4 precipitates are found to act as nucleation centers for the silicon solidification. The nucleating power of Si3N4 is attributed to the good epitaxial fit with silicon. Oxidation of Si3N4 powder at a higher temperature increases the undercooling of the droplet. When the silicon droplet is solidified on BN substrates, BN particles are detected on the surface of the droplet as well as a Si3N4 layer at the substrate/Si interface which promotes nucleation. Carbon-containing substrates are found to favor the nucleation of silicon due to the creation of a SiC layer at the substrate/silicon interface and precipitation of SiC particles in the droplets. However, no explanation of the important nucleating effect of SiC has been found.