Amorphous and crystalline phases of Ge 2 Sb 2 Te 5 films are investigated by coherent phonon spectroscopy. By heating amorphous films above specific temperatures, the coherent phonon signatures exhibit pronounced changes due to the crystallization of the amorphous phase into a cubic lattice and the transition from the cubic to a hexagonal crystal structure. The phonon modes observed are identified by comparison with coherent phonon spectra of the binary Sb 2 Te 3 and GeTe constituents.

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We present reflectivity measurements allowing the quantitative determination of crystallization times from 103 s down to 10-8 s. For the slow speeds the crystallization of a written mark at constant temperatures is monitored. For the fast speeds time resolved measurements are employed. The presented experimental methods show a high potential to yield information on stoichiometry induced variation of the crystallization speed and on the physical mechanism of phase change processes.