A fast simulator for polycrystalline processes with application to phase change alloys

TL;DR

This paper introduces a fast stochastic simulation method for polycrystalline phase-change materials, effectively modeling complex annealing processes and crystallization dynamics in chalcogenide alloys like GST, with applications in memory devices.

Contribution

The paper presents a novel molecular-scale simulation approach based on a Gillespie algorithm for polycrystalline phase-change materials, capturing complex annealing behaviors and crystallization dynamics.

Findings

Accurately models incubation times at low temperatures.

Replicates non-trivial crystal size distributions.

Simulates melting dynamics at higher temperatures.

Abstract

We present a stochastic simulator for polycrystalline phase-change materials capable of spatio-temporal modelling of complex anneals. This is based on consideration of bulk and surface energies to generate rates of growth and decay of crystallites built up of `monomers' that themselves may be quite complex molecules. We perform a number of simulations of this model using a Gillespie algorithm. The simulations are performed at molecular scale and using an approximation of local free energy changes that depend only on immediate neighbours. The sites are on a lattice that neither correspond to the crystal lattice nor to individual monomers, but instead gives information about a two-state local phase $r$ (where $r=0$ corresponds to amorphous and 1 corresponds to crystalline) and a continuous crystal orientation $\phi$ at each site. As an example we use this to model crystallisation in chalcogenide GST ($GeSbTe$) alloys used for example in phase-change memory devices, where reversible changes between amorphous and crystalline regimes are used to store and process information. We use our model to simulate anneals of GST including ones with non-trivial spatial and temporal variation of temperature; this gives good agreement to experimental incubation times at low temperatures while modelling non-trivial crystal size distributions and melting dynamics at higher temperatures.