Modeling Heterogeneous Melting in Phase Change Memory Devices

TL;DR

This paper develops thermodynamic models for phase change in memory devices, showing how grain size and heterogeneity influence melting, crystallization, and switching times, with simulations demonstrating ~1 ns reset/set times in nanocrystalline cells.

Contribution

It introduces a comprehensive thermodynamic and kinetic model for heterogeneous melting in phase change memory, emphasizing the role of grain size and boundary effects on device performance.

Findings

Smaller grains melt at lower temperatures.

Heterogeneous melting significantly reduces switching times.

Simulations show ~1 ns reset/set times in nanocrystalline cells.

Abstract

We present thermodynamic crystallization and melting models and calculate phase change velocities in $Ge_2 Sb_2 Te_5$ based on kinetic and thermodynamic parameters. The calculated phase change velocities are strong functions of grain size, with smaller grains beginning to melt at lower temperatures. Phase change velocities are continuous functions of temperature which determine crystallization and melting rates. Hence, set and reset times as well as power and peak current requirements for switching are strong functions of grain size. Grain boundary amorphization can lead to a sufficient increase in cell resistance for small-grain phase change materials even if the whole active region does not completely amorphize. Isolated grains left in the amorphous regions, the quenched-in nuclei, facilitate templated crystal growth and significantly reduce set times for phase change memory cells. We demonstrate the significance of heterogeneous melting through 2-D electrothermal simulations coupled with a dynamic materials phase change model. Our results show reset and set times on the order of ~1 ns for 30 nm wide confined nanocrystalline (7.5 nm - 25 nm radius crystals) phase change memory cells.