Abstract: The relationship between the reflectivity of the Ge₂Sb₂Te₅ thin films, which are as-deposited
and melt-quenched states separately, and the pulsed laser time was investigated by the focused pulse laser, and it was found that
the reflectivity changes for the two kinds of samples are different. On the basis of the principle of the droplet formation in the gas-liquid
system and the basic ideas of the statistical physics, the crystallization mechanism and course of the Ge₂Sb₂Te₅ thin films, which are in
as-deposited and melt-quenched states separately, were analyzed and studied in detail. The crystalline nucleus can’t form if Ge₂Sb₂Te₅ is in
below-saturation or saturation state, different size crystalline nucleus can form if it is in super-saturation. While only the crystalline nucleus
which are more than the critical size can grow into crystalline. In addition, the stress, decreasing the crystallization energy barrier and increasing
the super-saturation degree of the amorphous Ge₂Sb₂Te₅, is the leading factor causing the different relationships of reflectivity change
with pulsed laser width between the as-deposited and melt-quenched Ge₂Sb₂Te₅ thin films. At the same time, on the basis of the
analytical results, the reflectivity change characters and laws of the two kinds of the samples under the focused pulse laser were explained in detail.

Key words: Ge₂Sb₂Te₅, thin films, crystallization, pulsed laser, super-saturation degree