Coherent optical phonons in different phases of Ge2Sb2Te5 upon strong laser excitation

Abstract

The transient reflectivity response of phase-change Ge 2 Sb 2 Te 5 films to intense femtosecond laser pulses is studied by ultrafast coherent phonon spectroscopy. The three different phases ͑amorphous, fcc-, and hcp-crystalline͒, as well as laser-crystallized films, are investigated, featuring different photoexcited carrier and coherent optical phonon dynamics. At least two main phonon frequencies are identified for each phase/material and their evolution for increasing pump fluences is investigated for the fcc-crystalline phase and the laser-crystallized material, revealing strong differences. We find evidence that a considerable fraction of amorphous phase remains in the laser-crystallized material, which features a different phonon frequency, not related to other phases. These results are important for emerging strategies aimed at driving ultrafast phase transitions via coherent phonon excitation for applications in data storage.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons

“…3a), and was slightly red-shifted from the literature value2225, but remained consistent with the optical mode due to the local Ge–Te bonds of the resonantly bonded structure2225. Similar phonon softening induced by single pump excitation has also been observed in conventional GST alloy films26. The choice of the value of Δ t is based on the period of the coherent phonon observed without prepump-pulse excitation; 3.48 THz corresponds to ≈290 fs and therefore multiples of 290 fs were used to excite the SET phase to coherently drive the vibrational amplitude beyond the threshold of transient structural change22272829 by the P 2 pulse.…”

Ultrafast crystalline-to-amorphous phase transition in Ge2Sb2Te5 chalcogenide alloy thin film using single-shot imaging spectroscopy

“…No Raman peak was observed at 1.5-1.6 THz in both crystalline and amorphous phase samples, which is a fingerprint of the hexagonal phase GST. 19,20 The observed phonon frequencies of the A 1 mode (3.1 THz in the fcc crystalline phase and 3.5 THz in the amorphous phase) were lower than those of the thicker GST samples previously reported ($3.6 THz in the fcc crystalline phase and $3.9 THz in the amorphous phase). 19,20 The origin of the frequency difference is unclear yet, but might come from a strain between the GST and a glass substrate and/or phonon confinement in nanosolid.…”

Anisotropic lattice response induced by a linearly-polarized femtosecond optical pulse excitation in interfacial phase change memory material

“…The phonon frequencies of these three modes are basically consistent with previously reported values20. The assignment of phonon modes in iPCM remains controversial, the 2.1 THz and 5.1 THz modes can be assigned to two different A 1 modes in the Sb 2 Te 3 sub-layers while the 3.4 THz mode may be a E g mode in the Sb 2 Te 3 sub-layers or the A 1 mode in the GeTe sub-layers2021222324. As seen in Fig.…”