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Jake Scoggin1 Helena Silva1 Ali Gokirmak1

1, University of Connecticut, Storrs, Connecticut, United States

Phase change memory (PCM) is a novel nonvolatile memory technology that stores data in a highly resistive amorphous or highly conductive crystalline phase of a chalcogenide such as Ge2Sb2Te5 (GST). PCM devices switch by melting and rapidly quenching crystalline material to amorphous (reset) or heating amorphous material until it crystallizes (set). Accurate modeling of reset and set requires accurate material parameterization over the temperatures experienced during device operation (300 K~1000 K), but high temperature parameters are often difficult to obtain due to rapid (as fast as ~ns) transitions from amorphous to cubic and cubic to hexagonal phases. Where high temperature data is not known, it is extracted from low temperature measurements. For example, room temperature specific heats of amorphous and cubic GST have been measured to be approximately equal, and this room temperature value is often treated as a temperature independent parameter for both phases. However, the measured heat released during crystallization and heat absorbed during melt are not equal, and thus the specific heats of amorphous and cubic GST are thermodynamically required to diverge at some point (likely at the glass transition temperature) [1]. We model high temperature specific heats in GST by enforcing the thermodynamic relationship between specific heat and enthalpy. We use the resultant specific heats coupled with the measured heat of fusion to implement a temperature and phase transition rate dependent heat source which captures at once both a variable heat of crystallization and the heat of fusion. Using our finite element phase change model [2], [3] fully coupled with electrothermal physics, we analyze the effects of a variable heat of crystallization on grain maps in GST due to the exponential dependence of nucleation and growth rates on temperature. We also simulate reset and set operations on mushroom and pillar cell geometries and show more crystallization during reset, longer set times, higher set energies, and lower thermal crosstalk than predicted by temperature independent specific heats and heat of crystallization.
[1] J. Kalb, “Crystallization kinetics in antimony and tellurium alloys used for phase change recording,” 2006.
[2] Z. Woods and A. Gokirmak, “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part I—Effective Media Approximation,” IEEE Trans. Electron Devices, vol. 64, no. 11, pp. 4466–4471, Nov. 2017.
[3] Z. Woods, et. al, “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part II--Discrete Grains,” IEEE Trans. Electron Devices, pp. 1–7, 2017.

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