2008 NSTI Nanotechnology Conference and Trade Show - Nanotech 2008 - 11th Annual

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TechConnect Summit
Clean Technology 2008

Current-voltage characteristics of phase-change nanostructured-particles for high density non-volatile memory applications

A.R. Joeng, Y.J. Oh, W. Jo
Ewha Womans University, KR

phase-change memory, current-voltage characteristics, nanoparticles

Phase-change materials, such as the chalcogenide Ge2Sb2Te5 (GST), are very important for application to read-write optical and electrical storage, since they can be switched fast between amorphous and crystalline phases by applying Joule heating of electrical pulses. For most of applications, a small feature size of GST is indispensable. One of the practical ways is to study physical properties of nanoparticles in terms of phase formation and crystallization. Nanometer-size GST particle can be a model system for high density integrated devices to look into their reliability characteristics. Besides reducing the GST size, it is reported that nitrogen doping is one of attractive approaches, which has the smallest reset current and enhances the endurance with the grain growth suppression effect of the incorporated nitrogen. We have been in-situ synthesized GST nanoparticle-clusters by pulsed-laser ablation method. Their microstructure and phase formation was confirmed using scanning and high resolution transmission electron microscopy. Fourier transformation analysis of micrographs also shows the crystal structure of the GST phase. The GST nanoparticle-clusters have a lattice constant with ~6Å like bulk value, face-centered cubic and hexagonal structure. According to our previous work, we proposed that GST nanoparticles are able to reduce down operating power of phase-change nonvolatile memory devices in comparison films or bulk. We made a metal–dot capacitor structure, which can transform amorphous into crystalline phases at very low voltage. However, this structure was layered by several nanoparticles and measured the current-voltage (I-V) characteristics of several nanoparticles, so we introduced a few GST nanoparticle-clusters for finding out I-V characteristics of nanoparticle in itself. We have fabricated GST nanoparticles directly on Pt/Ir-coated atomic force microscopy (AFM) tips. Using AFM, we can exactly measure the I-V characteristics of only a few nanoparticle-clusters due to decrease of thermal resistance. We investigated the I-V characteristics of the GST nanoparticle-clusters using a contact mode AFM. The bias voltage was applied from -10V to 10V with various initial pulses which can alter the phase formation of the GST nanoparticles. For once DC sweep, GST nanoparticle-clusters showed good phase change property, which is the amorphous phase of GST nanoparticles was changed to crystalline, on the other hand, the crystalline GST nanoparticles was transformed to amorphous phase according to the electric field applied to the AFM tip. Additionally, we synthesized nitrogen-doped GST nanoparticle-clusters which have larger resistance than that as grown. Therefore, we guess that nitrogen-doped GST nanoparticle-clusters also reduce down the reset current and to enhance the endurance with the grain growth suppression effect. Judging from this result, nanoparticle-clusters GST can be applied to the high density high-density GST-based nonvolatile random access memory.

Nanotech 2008 Conference Program Abstract