Abstract-Terahertz generation measurements of multilayered GeTe–Sb2Te3 phase change materials

Kotaro Makino, Kosaku Kato, Yuta Saito, Paul Fons, Alexander V. Kolobov, Junji Tominaga, Takashi Nakano, and Makoto Nakajima

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-6-1355

Multilayered structures of GeTe and Sb2Te3${\mathrm{Sb}}_2{\mathrm{Te}}_3$ phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always 𝑝$p$ polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and Bi2Te3${\mathrm{Bi}}_2{\mathrm{Te}}_3$, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in 𝑝$p$-type semiconducting iPCM materials.

© 2019 Optical Society of America

Abstract-Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses

Yasuyuki Sanari, Takehiro Tachizaki, Yuta Saito, Kotaro Makino, Paul Fons, Alexander V. Kolobov, Junji Tominaga, Koichiro Tanaka, Yoshihiko Kanemitsu, Muneaki Hase, and Hideki Hirori

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.165702

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ upon irradiation with an intense terahertz (THz) pulse. THz-pump–optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

Abstract-Significant Volume Expansion as a Precursor to Ablation and Micropattern Formation in Phase Change Material Induced by Intense Terahertz Pulses

Kotaro Makino, Kosaku Kato, Keisuke Takano, Yuta Saito, Junji Tominaga, Takashi Nakano, Goro Isoyama, Makoto Nakajima,

https://www.nature.com/articles/s41598-018-21275-3

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge2Sb2Te5phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called “LIPSS”), which are of potential importance for laser material processing.

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

Kotaro Makino

, Yuta Saito

, Paul Fons

, Alexander V. Kolobov

, Takashi Nakano

, Junji Tominaga

 & Muneaki Hase

http://www.nature.com/articles/srep19758

Optical excitation of matter with linearly-polarized femtosecond pulses creates a transient non-equilibrium lattice displacement along a certain direction. Here, the pump and probe pulse polarization dependence of the photo-induced ultrafast lattice dynamics in (GeTe)2/(Sb2Te3)4interfacial phase change memory material is investigated under obliquely incident conditions. Drastic pump polarization dependence of the coherent phonon amplitude is observed when the probe polarization angle is parallel to the c–axis of the sample, while the pump polarization dependence is negligible when the probe polarization angle is perpendicular to the c–axis. The enhancement of phonon oscillation amplitude due to pump polarization rotation for a specific probe polarization angle is only found in the early time stage (≤2 ps). These results indicate that the origin of the pump and probe polarization dependence is dominantly attributable to the anisotropically-formed photo-excited carriers which cause the directional lattice dynamics.