On the origin of recalescence in amorphous Ge films melted with nanosecond laser pulses

Rapid solidification phenomena have been studied in amorphous germanium films on silicon substrates by means of real time reflectivity measurements performed during irradiation with nanosecond laser pulses. The influence of the thermal response of the film/substrate system has been investigated by comparing the behavior of films with thicknesses in the range of 30-180 nm. Two different solidification scenarios are observed depending on the ratio between film thickness (d) and the thermal diffusion length (l) of amorphous germanium (l approximate to 80 nm for 12 ns laser pulses). In the thinner films (d<l), reamorphization occurs upon solidification. Recalescence is observed in the thicker ones (d greater than or equal to l) when the melt depth induced is above of approximate to 80 nm. Above this threshold, crystalline phases are nucleated upon solidification. The origin of this melt depth threshold is discussed in terms of the heat flow into the substrate, the supercooling prior to solidification, and the need of a minimum amount of initially solidified material. (C) 1997 American Institute of Physics.