Decoupling Nucleation and Growth in Fast Crystallization of Phase Change Materials

Disentangling nucleation and growth in materials that crystallize on the nanosecond time scale is experimentally quite challenging since the relevant processes also take place on very small, i.e., sub-micrometer length scales. Phase change materials are bad glass formers, which often crystallize rapidly. Here systematic changes in crystallization kinetics are shown in pseudo-binary compounds of GeTe and Sb2Te3 and related solids subjected to short laser pulses. Upon systematic changes in stoichiometry, the speed of crystallization changes by three orders of magnitude concomitantly with pronounced changes in stochasticity. Resolving individual grains with electron backscatter diffraction (EBSD) permits to disentangle of the process of nucleation and growth. From these experiments, supported by multiphysics simulations of crystallization, it can be concluded that high crystallization speeds with small stochasticity characterize phase change materials with fast nucleation, while compounds that nucleate slowly crystallize much more stochastically. Nucleation and growth of prototypical phase change materials are disentangled by a combination of experiments with high temporal and high spatial resolution. Together with supporting multiphysics simulations of crystallization, it is shown that both the speed of crystallization and its stochasticity are governed by the material's nucleation while the influence of growth is rather small. image