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

Hada M, Oba W, Kuwahara M, Katayama I, Saiki T, Takeda J, Nakamura KG - Sci Rep (2015)

Bottom Line: Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices.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.

View Article: PubMed Central - PubMed

Affiliation: Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan.

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.

No MeSH data available.


Related in: MedlinePlus

Near-UV pump and visible probe spectroscopy.(a) Two-dimensional plot of the time-resolved differential absorbance (ΔA/A) as a function of probe photon wavelength. (b) Averaged ΔA/A for wavelengths of 530–630 nm as a function of time delay. (c) Photographs after single-shot near-UV (400 nm) photoexcitation at various incident fluences (4, 6.5, 8 and 10 mJ cm–2). Legends are shown as insets in the figure. The permanent amorphization fluence threshold was determined to be approximately 8 mJ cm–2.
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f1: Near-UV pump and visible probe spectroscopy.(a) Two-dimensional plot of the time-resolved differential absorbance (ΔA/A) as a function of probe photon wavelength. (b) Averaged ΔA/A for wavelengths of 530–630 nm as a function of time delay. (c) Photographs after single-shot near-UV (400 nm) photoexcitation at various incident fluences (4, 6.5, 8 and 10 mJ cm–2). Legends are shown as insets in the figure. The permanent amorphization fluence threshold was determined to be approximately 8 mJ cm–2.

Mentions: Here we present results from single-shot time-resolved transmission spectroscopy of GST: the “on” and “off” states of the optical media materials are recognized by a change in optical index. Figure 1a,b present the near-UV (400 nm) pump and visible probe transmission spectra as a function of probe photon wavelength in the range of 530 nm to 630 nm. The changes in the optical index of GST were identical upon photoexcited by 400-nm or 800-nm light1214 suggesting that the opaque rock-salt GST is electronically stabilized into transparent amorphized-GST within 1 ps. The threshold for the permanent amorphization is determined as a laser incident fluence (F) of 8 mJ cm–2 (Fig. 1c) which is smaller than that obtained in the case of near-IR photoexcitation (>10 mJ cm–2).


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

Hada M, Oba W, Kuwahara M, Katayama I, Saiki T, Takeda J, Nakamura KG - Sci Rep (2015)

Near-UV pump and visible probe spectroscopy.(a) Two-dimensional plot of the time-resolved differential absorbance (ΔA/A) as a function of probe photon wavelength. (b) Averaged ΔA/A for wavelengths of 530–630 nm as a function of time delay. (c) Photographs after single-shot near-UV (400 nm) photoexcitation at various incident fluences (4, 6.5, 8 and 10 mJ cm–2). Legends are shown as insets in the figure. The permanent amorphization fluence threshold was determined to be approximately 8 mJ cm–2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4551965&req=5

f1: Near-UV pump and visible probe spectroscopy.(a) Two-dimensional plot of the time-resolved differential absorbance (ΔA/A) as a function of probe photon wavelength. (b) Averaged ΔA/A for wavelengths of 530–630 nm as a function of time delay. (c) Photographs after single-shot near-UV (400 nm) photoexcitation at various incident fluences (4, 6.5, 8 and 10 mJ cm–2). Legends are shown as insets in the figure. The permanent amorphization fluence threshold was determined to be approximately 8 mJ cm–2.
Mentions: Here we present results from single-shot time-resolved transmission spectroscopy of GST: the “on” and “off” states of the optical media materials are recognized by a change in optical index. Figure 1a,b present the near-UV (400 nm) pump and visible probe transmission spectra as a function of probe photon wavelength in the range of 530 nm to 630 nm. The changes in the optical index of GST were identical upon photoexcited by 400-nm or 800-nm light1214 suggesting that the opaque rock-salt GST is electronically stabilized into transparent amorphized-GST within 1 ps. The threshold for the permanent amorphization is determined as a laser incident fluence (F) of 8 mJ cm–2 (Fig. 1c) which is smaller than that obtained in the case of near-IR photoexcitation (>10 mJ cm–2).

Bottom Line: Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices.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.

View Article: PubMed Central - PubMed

Affiliation: Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan.

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.

No MeSH data available.


Related in: MedlinePlus