Stability and dynamics of Pt-Si liquid microdroplets on Si(001)

The formation and dynamics of Pt-Si liquid droplets on Si (001) surfaces are explored with real-time ultraviolet photoelectron emission microscopy. PtSi islands of micrometer lateral diameter begin to melt and are transformed into molten Pt-Si alloy islands below the melting point of bulk PtSi. In particular, at similar to1100degreesC surface migration of the liquid microdroplets is observed, where the droplets move directionally from the cold to the hot regions of the surface following the temperature gradient across the substrate. It is proposed that the droplet surface migration is due to dissolution-diffusion-deposition flow of Si through the droplet driven by the Si concentration difference in the droplet. In addition, the migration rate of the droplet is measured as a function of temperature and droplet diameter. Above a minimum diameter, the migration velocity is independent of the droplet size, which indicates that Si diffusivity through the droplet is the primary factor determining the rate of migration. The activation energy for the Si diffusion in the droplet is found to be similar to0.57 eV. We conclude that the thermal and chemical stability of the droplet-substrate interface significantly affects the evolution and dynamics of the liquid island on the surface.