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-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.