Stability and crystallization of amorphous clusters in crystalline Si

We have simulated using molecular dynamics the thermal stability and crystallization kinetics of nanometre-sized clusters of amorphous Si embedded in crystalline Si, which are of interest for phase-change memory devices. We have calculated the interfacial and bulk excess energies of the amorphous clusters, and studied their crystallization kinetics at 700–1500 K. At temperatures below (above) 1150 K the activation energy is 0.73 ± 0.04 eV (1.52 ± 0.07 eV), indicating a change of mechanism at 1150 K. We predict the stability of much larger amorphous clusters by extrapolating our simulation data using an analytic model. The stability with respect to crystallization of small amorphous clusters embedded in crystals is of both fundamental and technological interest. The creation of amorphous clusters, and their elimination through crystallization, is the basis of phase-change memory devices such as rewritable compact discs [1]. The principal requirements of the technology are the stability of the amorphous clusters against crystallization at ambient temperatures,and an understanding of the kinetics of crystallization as a function of temperature and cluster size. At the fundamental level the kinetics of crystallization of embedded amorphous clusters involves a determination of the activation energy for migration of the interface surrounding the cluster. In this report we present molecular dynamics simulations of the crystallization of amorphous clusters of Si embedded in crystalline Si. Although Si is not one of the materials used for phase-change memory devices we have selected it because the availability of reasonable interatomic potentials for Si enables us to develop a methodology for investigating the processes involved in these devices. In particular we show how molecular dynamics simulations of the crystallization of relatively small clusters may be used to generate data that can be inserted into an analytic model 0953-8984/05/274263+08$30.00 © 2005 IOP Publishing Ltd Printed in the UK 4263 4264 S von Alfthan et al