Control of the nucleation during directional solidification of solar cell silicon is important in order to be able to control the growth and number of grains formed. A certain amount of undercooling is required to obtain dendritic growth with faceted twins (which has shown promising results for structure control), but a too high undercooling will lead to extensive nucleation which will oppose the positive effect of a small number of large grains with controlled growth directions. In the present experiments, the nucleation undercooling of silicon on Si3N4 coated SiO2 with variation in coating parameters has been investigated. Experiments were performed with the sessile drop method, and with differential thermal analysis, with a cooling rate of 20 K/min. There were no significant differences in nucleation undercooling between the different variations in coating. The undercooling does not seem to be dependent on coating thickness, oxygen concentration, wetting angle or roughness at the given cooling rate. (C) 2011 Elsevier B.V. All rights reserved.