News from Nanotechjapan


Novel Crystal Growth Mechanism in Phase Change Materials Induced by Intense Terahertz Pulse Electric Field---Zener Tunneling Breakdown to Develop Novel Nanoscale Memory Device---

 On October 22, 2018, Kyoto University, University of Tsukuba, Tokai University, and Japan Science and Technology Agency (JST) announced that the research group led by Professor Hideki Hirori at the Institute for Chemical Research, Kyoto University discovered novel nanoscale crystal growth mechanism in phase change recording material Ge2Sb2Te5 (GST) induced by irradiation with an intense terahertz (THz) pulse.  This work was supported by JST, and details were published in Physical Review Letters*

 Chalcogenide compound semiconductor, including GST, has been used in recording devices, such as DVD, utilizing large change in electrical conductivity and refractive index of light due to the phase change between amorphous and crystalline.  The change in electrical conductivity (electrical switching) will be utilized to make power saving nonvolatile memory for information systems. However, diffusion of Joule's heat generated by electrical switching limits the memory size reduction as it extends the phase change area up to several microns.

 The research group systematically investigated the spatial and temporal dynamics of crystallization that occur in GST upon irradiation with an intense terahertz (THz) pulse. Amorphous GST film of 40 nm thick was sputter-deposited on a Si substrate and was covered with 20 nm thick ZnS-SiO2 film to form Au antenna on top to enhance THz electric field over the 5μm gap.  THz-pump-optical-probe spectroscopy revealed that Zener tunneling was induced by the THz field around 200 V/cm and caused local anisotropic heating for nanoscale crystallization along the direction of the electric field. 

*Yasuyuki Sanari, Takehiro Tachizaki, Yuta Saito, Kotaro Makino, Paul Fons, Alexander V. Kolobov, Junji Tominaga, Koichiro Tanaka, Yoshihiko Kanemitsu, Muneaki Hase, and Hideki Hirori, "Zener tunneling breakdown in phase-change materials revealed by intense terahertz pulses", Physical Review Letters, Vol. 121, p. 165702 - Published 19 October 2018, DOI: 10.1103/PhysRevLett.121.165702