In situ SEM Observation of Grain Formation and Growth Induced by Electrical Pulses in Lateral Ge 2 Sb 2 Te 5 Phase-chang
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0997-I12-05
In situ SEM Observation of Grain Formation and Growth Induced by Electrical Pulses in Lateral Ge2Sb2Te5 Phase-change Memory You Yin1, Daisuke Niida1, Kazuhiro Ohta2, Akihira Miyachi1, Masahiro Asai1, Naoya Higano1, Hayato Sone1, and Sumio Hosaka1 1 Department of Nano-Material Systems, Gunma University, 1-5-1 Tenjin, Kiryu, Gunma, 3768515, Japan 2 Department of Electronic Engineering, Gunma University, 1-5-1 Tenjin, Kiryu, Gunma, 3768515, Japan
ABSTRACT A study of electric-pulse-induced crystallization of Ge2Sb2Te5 (GST) was conducted by in situ scanning electron microscopy observation and resistance measurement. A lateral phasechange memory with a top GST channel connected by two separate underlying electrodes was adopted in this study to easily observe the crystallization process. At a low voltage pulse, randomly distributed nuclei were initiated. At the first growth stage, these nuclei grew fast with the pulse amplitude at a rate of around 60 nm/V and then growth rate slowed down to around 14 nm/V when the grain diameter was closed to film thickness. Device resistance during crystallization dropped by around one order of magnitude, which should be due to amorphous to face-centered-cubic transition. INTRODUCTION There is growing demand for nonvolatile memories characterized by fast write speed and high endurance with the rapid development of information society. As the mainstream in nonvolatile memory market today, flash memory, however, has many demerits such as long write/erase time, low endurance, high programming energy, high voltage, and limited scalability by tunnel oxide. Phase-change memory (PCM) attracted much attention as an emerging nonvolatile memory due to its almost perfect properties such as excellent endurance, nondestructive read, direct overwrite, low programming energy, fast speed, high performance, multistate storage capability [1-6]. Chalcogenides such as Ge2Sb2Te5 (GST) in PCM are characterized by the fast reversible switching between amorphous and crystalline phases, accompanying large differences in both optical and electrical properties [1, 2]. The fast phase change can be induced by either laser irradiation or electrical pulses. The former method has been widely used for optical rewritable storage media. The latter is adopted in PCM, which is recently studied intensely over the world. Chalcognide in PCM can be amorphized via melting and quenching by applying a short but high electrical pulse so PCM enters a highly resistive state. On the other hand, it can be poly-crystallized by applying a long but low electrical pulse and PCM correspondingly enters a lowly resistive state. Crystallization and amorphization mentioned above are two phase change processes in both optical media and PCM. In order to study the kinetics of the amorphous-to-crystalline transition,
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FIG. 1. (a) Schematic cross sectional diagram of a vertical PCM, in which GST is well surround by other parts of the device. (b) Schematic top view of our late
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