Excimer laser-induced temperature field in melting and resolidification of silicon thin films

The liquid/solid interface motion and temperature history during excimer laser annealing of 50-nm-thick Si films on fused quartz substrates are investigated by in situ nanosecond time-resolved electrical conductance, optical reflectance, and transmittance at visible and near-IR wavelengths, combined with thermal emission measurements. The temperature response, melt propagation and evolution of the recrystallization process are fundamentally different in the partial-melting and the complete-melting regimes. Because it is necessary to balance the latent heat across the propagating phase-change interface, the maximum induced temperature in the partial-melting regime remains close to the melting point of amorphous Si. The peak temperature rises in the complete-melting regime, but the nonparticipating nature of the liquid Si/fused quartz interface allows substantial supercooling (>200 K), followed by spontaneous nucleation into fine-grained material. These phase transformations are consistent with the recrystallized polycrystalline Si morphologies that indicate grain enhancement in the near-complete-melting regime. It is also found that melting of polycrystalline Si occurs close to the melting point of crystalline Si. This temperature is by approximately 140 K higher than the melting point of amorphous Si.