Ultrafast time-resolved electron diffraction revealing the nonthermal dynamics of near-UV photoexcitation-induced amorphization in Ge2Sb2Te5.

Masaki Hada,Kazutaka G Nakamura,Masashi Kuwahara,Ikufumi Katayama,Toshiharu Saiki,Jun Takeda,Wataru Oba

doi:10.1038/srep13530

Abstract

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.

Highlights

Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices
We investigate the mechanism of the crystal-to-amorphous phase transition process followed by near-UV (400 nm) femtosecond laser excitation on 20-nm-thick polycrystalline GST films using time-resolved electron diffraction (Tr-ED) measurements and single-shot time-resolved optical spectroscopy
We found that the local amorphization evolves to more complicated permanent amorphization via multi-displacement effects above this fluence threshold when more than 25% local amorphization occurs in the system

Summary

Introduction

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. The thermal nature of the laser-induced amorphization of GST deduced from crystallographic methods conflicts with the results obtained from time-resolved x-ray and optical spectroscopy[9,15] These observations are in line with the energy difference of ~1 eV between the incident photon (1.55 eV) and the bandgap in GST (0.5–0.6 eV)[18]. We investigate the mechanism of the crystal-to-amorphous phase transition process followed by near-UV (400 nm) femtosecond laser excitation on 20-nm-thick polycrystalline GST films using time-resolved electron diffraction (Tr-ED) measurements and single-shot time-resolved optical spectroscopy. The permanent amorphization is accompanied by partial hexagonal recrystallization, which may be one of the damage processes in optical storage

Methods

Results

Conclusion