SLOW INTERFACIAL REAMORPHIZATION OF GE FILMS MELTED BY PS LASER-PULSES

Melting and rapid solidification is induced in 50-nm-thick amorphous G e films on glass substrates by single laser pulses at 583 nm with a du ration of 10 ps. The solidification process is followed by means of re flectivity measurements with ns time resolution both at the air/film ( front) and the substrate/film (back) interfaces. Due to interference e ffects between the light reflected at the film-substrate and film-liqu id interfaces, the back side reflectivity measurements turn out to be very sensitive to the melt depth induced by the laser pulse and their comparison to optical simulations enables the determination of the sol idification dynamics. For low fluences, only a thin layer of the film is melted and solidification occurs interfacially leading to reamorphi zation of the molten material. The results provide a critical interfac e velocity for amorphization of similar to 4 m/s, much slower than the one that has widely been reported for elementary semiconductors. For high fluences, the molten layer depth approaches the film thickness an d the results are consistent with a bulk solidification process. In th is case, recalescence effects upon solid phase nucleation become impor tant and lead to the formation of crystallites distributed throughout the whole resolidified volume. (C) 1998 American Institute of Physics. [S0021-8979(98)05822-8].