We have observed an irreversible ultrafast crystalline-to-amorphous phase transition in Ge2Sb2Te5 chalcogenide alloy thin film using broadband single-shot imaging spectroscopy. The absorbance change that accompanied the ultrafast amorphization was measured via single-shot detection even for laser fluences above the critical value, where a permanent amorphized mark was formed. The observed rise time to reach the amorphization was found to be ∼130–200 fs, which was in good agreement with the half period of the A1 phonon frequency in the octahedral GeTe6 structure. This result strongly suggests that the ultrafast amorphization can be attributed to the rearrangement of Ge atoms from an octahedral structure to a tetrahedral structure. Finally, based on the dependence of the absorbance change on the laser fluence, the stability of the photoinduced amorphous phase is discussed.

Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices. These phase transitions are achieved by laser irradiation via thermal processes. Recent studies have suggested the potential of nonthermal phase transitions in the chalcogenide glass material Ge2Sb2Te5 triggered by ultrashort optical pulses; however, a detailed understanding of the amorphization and damage mechanisms governed by nonthermal processes is still lacking. Here we performed ultrafast time-resolved electron diffraction and single-shot optical pump-probe measurements followed by femtosecond near-ultraviolet pulse irradiation to study the structural dynamics of polycrystalline Ge2Sb2Te5. The experimental results present a nonthermal crystal-to-amorphous phase transition of Ge2Sb2Te5 initiated by the displacements of Ge atoms. Above the fluence threshold, we found that the permanent amorphization caused by multi-displacement effects is accompanied by a partial hexagonal crystallization.

No abstract

No abstract