Evidence for surface initiated solidification in Ge films upon picosecond laser pulse irradiation

Amorphous Ge films on Si films have been melted by single 30 ps laser pulse s at lambda =583 nm and a solidification process has been followed by means of real time reflectivity (RTR) measurements with nanosecond time resoluti on. Evidence is provided for the occurrence of surface initiated solidifica tion for films with thicknesses in the 80-130 nm range. This process occurs at high fluences following complete melting of the film and of a thin laye r of the Si substrate which undergoes mixing with the liquid Ge. The releas e of the solidification enthalpy of the latter layer together with its lowe r solidification temperature favored by constitutional undercooling are pro posed as the origin of the initial gradient inversion required for such a p rocess. This scenario leads also to the formation of a secondary solidifica tion front counterpropagating towards the film surface, as suggested by opt ical simulations of the experimental RTR transients. A transition from a su rface initiated solidification process towards a bulk solidification proces s is shown to occur when increasing the film thickness beyond 130 nm. The r esults further show that, besides the film thickness, the pulse duration ha s a major influence on the type of solidification process induced. (C) 2001 American Institute of Physics.