Molecular Dynamics Studies of the Ion Beam Induced Crystallization in Silicon
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ABSTRACT We have studied the ion bombardment induced amorphous-to-crystal transition in silicon using molecular dynamics techniques. The growth of small crystal seeds embedded in the amorphous phase has been monitored for several temperatures in order to get information on the effect of the thermal temperature increase introduced by the incoming ion. The role of ion-induced defects on the growth has been also studied. INTRODUCTION The amorphous to crystal transformation in silicon is of considerable technological interest because of its potential applications in the field of microelectronic processing [1]. The crystallization of amorphous silicon starts with the nucleation of small crystal clusters. Initially, these clusters are unstable because of the large surface-to-volume ratio and therefore tend to shrink. However, from thermodynamic considerations a few of them will become large enough for growth to be favoured. The nucleation and growth of small crystal clusters are thermally activated processes that have been described theoretically [2] and experimentally in silicon [3]. Ion irradiation has been shown to dramatically enhance the solid phase crystallization of silicon with respect to the pure thermal process [4,51. In particular, when amorphous silicon is bombarded by xenon ions at 1.5 MeV, the nucleation and growth rates can be enhanced by eight and four orders of magnitude, respectively, with respect to the pure thermal processes [4]. These experimental studies showed that the ion bombardment induced crystallization is controlled by beam parameters such as dose rate and average energy deposited in the solid by elastic collisions [5]. However, some of the aspects of the ion beam induced recrystallization are not yet fully understood. While molecular dynamics (MD) simulation is a powerful tool that can be used to study ion-induced dynamical processes at the atomic scale, the conditions of the experiment (doses of ]013 cm- 2s-1 and energies above 1.5 MeV) are far beyond the scope of the MD method. Nevertheless simple situations, such as defect-induced motion of a crystal-amorphous interface [6], can be simulated and some qualitative and quantitative data can be obtained. We have carried out molecular dynamics simulations in order to get some insight into the phenomenon of the growth of small crystal clusters embedded in an amorphous matrix when it is ion bombarded. MODEL The most important feature in an MD simulation is the selection of the potential that describes the interactions between the atoms. We have used the Stillinger-Weber (SW) potential for silicon [7]. This potential is widely used since it is quite simple (a combination of two-body and three-body terms) and describes fairly well the properties of both crystal and 201 Mat. Res. Soc. Symp. Proc. Vol. 396 ©1996 Materials Research Society
liquid phases of silicon. Of special interest in our study is the fact that the melting point of crystal silicon predicted by the potential (TcL=1 69 1 K) is nearly the same as the value obtained experimentally.
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