  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.
