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Kotaro Makino2 Kosaku Kato1 Keisuke Takano1 Yuta Saito2 Junji Tominaga2 Takashi Nakano2 Paul Fons2 Goro Isoyama3 Makoto Nakajima1

2, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan
1, Osaka University, Suita, Osaka, Japan
3, Osaka University, Ibaraki, Osaka, Japan

Terahertz (THz) wave is a powerful experimental tool for studying a metal-insulator transition in phase change materials [1]. In addition, a pulse of THz wave can be used for fast material control technique by exploiting unique light-matter interaction without interband electronic transition. Indeed, sub-picosecond threshold phase switching has been realized in Ag-In-Sb-Te by application of intense THz field [2]. In this study, we irradiated thin films of crystalline (cubic) Ge2Sb2Te5 (GST) and GeTe/Sb2Te3 interfacial phase change memory (iPCM) material with intense THz pulse train by employing a free electron laser (FEL). This FEL emits intense THz macropulses which consist of train of ∼150 micropulses with 4 THz center frequency, 10 ps duration, and 37 ns interval. It was found that a significant volume expansion due to peculiar damage or precursor of ablation was formed at the center of the THz-irradiated area in GST samples. A surface profile measurement revealed that this volume expansion is completely different from conventional amorphization in the context of the phase change material study. Furthermore, a fine ripple patterns, whose spatial periods are smaller than excitation wavelength, were observed at the outside of the expanded area. This ripple pattern is thought to be a sort of laser induced periodic surface structure (LIPSS). Among LIPSSs, the ripples overbid in this study can be classified as low-spatial-frequency LIPSS (LSFL) by considering the fact that the refractive index of cubic GST is relatively high and hence the effective wavelength at the sample is estimated to be comparable to the spatial period of ripples. In general, the threshold fluence of laser-induced phase change is lower than that of a damage. However, any consequence of phase change was not induced when the fluence was lower than the damage threshold. This result manifests that the application of intense electrical field introduced by a THz pulse train cannot induce amorphization nor further crystallization in cubic GST under the current experimental condition and implies that electrically- and photo-excited incubation states play an important role for amorphization in GST as previously reported [3,4]. On the other hand, in iPCM sample, a different THz-induced pattern without ripple was observed. We attributed this result to the difference in the in-plane electrical conductance.

[1] V. Bragaglia et al., Scientific Reports 6, 28560 (2016).
[2] P. Zalden et. al. Phys. Rev. Lett. 117, 067601 (2016).
[3] P. Fons et. al. Phys. Rev. B 82, 041203 (R) (2010).
[4] D. Loke et. al. Science 336, 1566 (2012).

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