Status of atomic-level simulations of solid phase epitaxial recrystallization of amorphous Si

In semiconductor technology amorphous Si layers are formed by ion implantation, and during subsequent annealing the solid phase epitaxial recrystallization (SPER) of the amorphous material takes place. In order to simulate the SPER process and to understand its atomic-level mechanisms, classical molecular dynamics calculations are employed since they allow the consideration of several thousand atoms and a time scale up to some hundreds of nanoseconds. In the last decade different authors investigated SPER in Si by this type of simulations, but the critical review shows that their results are not consistent with the experimental data. In most cases the SPER rate was strongly overestimated. Moreover, the results obtained by different groups under virtually equal conditions do not agree. This may be due to the different approaches used to prepare the initial state consisting of an atomic system with a nearly planar amorphous-crystalline interface. The main cause for the disagreement with experimental data is the inaccuracy of the interatomic potentials used in the different studies. The improvements considered in the present work are based on a better description of the amorphous phase using a modified potential without changing the established potential for the single-crystalline material. It is found that amorphous Si with realistic structural and thermodynamic properties can be obtained by certain modifications of known interatomic potentials, but these modifications do not yield the correct SPER rate. However, it is shown that the value of the SPER rate is strongly correlated with the melting temperature of amorphous silicon obtained by the corresponding modified potential. Obviously, this dependence can be explained by the fact that both melting and SPER are essentially determined by the flexibility of atomic bonds. The atomic mechanism of SPER consists in sequential local arrangements of atomic bonds and positions, preferentially along {111} facets or terraces.