Crystal Growth Rates in Doped Sb x Te Fast-Growth Phase-Change Films Studied with Transmission Electron Microscopy
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Crystal Growth Rates in Doped SbxTe Fast-Growth Phase-Change Films Studied with Transmission Electron Microscopy Bart J. Kooi, Ramanathaswamy Pandian, and Jeff Th. M. De Hosson Department of Applied Physics, Zernike Institute for Advanced Materials, Univeristy of Groningen, Nijenborgh 4, Groningen, NL-9747AG, Netherlands ABSTRACT Isothermal crystallization of doped SbxTe fast-growth phase-change films was investigated using transmission electron microscopy with in situ heating. SbxTe films with four different values for the Sb/Te ratio, x=3.0, 3.3, 3.6 and 4.2, were analyzed and the films were sandwiched between two types of dielectric layers. One dielectric layer type is based on 80at.%ZnS-20at.%SiO2, the other on (Ge,Cr)N. The crystal growth rates reduce if the phasechange films are sandwiched between amorphous dielectric layers. The reduction is very pronounced at the lowest measured temperatures (150 oC), becomes smaller at higher temperatures and probably disappears at around 200 oC. The crystal growth rates increase with increasing Sb/Te ratio, but the activation energy for crystal growth is not significantly affected by the Sb/Te ratio. Finally a systematic study of the effect of the electron beam of the TEM on the crystal growth rates is performed showing accelerated growth rates. The present work shows that particularly at relative low temperatures, just above the glass-transition temperature, the growth rates as limited by the atomic mobilities are sensitive to various (boundary) conditions, e.g. capping layers and irradiation.
1. INTRODUCTION Phase-change data storage technology is based on the reversible switching between two (meta-)stable phases, amorphous and crystalline, and exploits the differences in physical properties between the phases. Doped alloys derived from eutectic SbxTe, so-called fast-growth materials, hold strong cards for both high-speed and high-density storage requirements [1-6]. This material type is currently used in (the rewritable) optical disk formats including digital versatile disk (DVD), Blu-ray disk [7], and high-density DVD (HD-DVD) [8] and also proposed for the line concept “phase-change random access memory” (PRAM) [9]. The present work focuses on the crystal growth rates in films based on these fast-growth materials at relative low temperatures (150- 200 oC). Effects of various Sb/Te ratios within the phase-change films and dielectric layers surrounding the phase-change films have been scrutinized. The isothermal crystallization has been studied by Transmission Electron Microscopy (TEM) using in situ heating. The advantage of this technique is that it provides detailed information with a high spatial resolution, allowing nucleation (rates) and growth (rates) to be monitored separately. Most techniques for the determination of crystallization kinetics measure the overall crystallization rate, but are unable to unravel these separate contributions of nucleation and growth. A disadvantage of TEM is that the crystallization process can be clearly modified by ele
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