Molecular Dynamics Studies of Impurity Segregation and Trapping.
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MOLECULAR DYNAMICS STUDIES OF IMPURITY SEGREGATION AND TRAPPING.
G. H. Gilmer, Christopher Roland and R. P. U. Karunasiri, AT&T Bell Laboratories, Murray Hill, New Jersey 07974
The ability to make highly doped 8-layers in semiconductors depends on the rate of interchange of atoms between layers at the crystal surface. We have simulated molecular beam epitaxy on a silicon (100) surface covered with a monolayer of impurity atoms. The kinetics of impurity segregation to the surface was examined for various growth conditions and segregation energies. We find that segregation is facilitated by appreciable inter-layer diffusion of atoms in the top several layers. The amount of diffusion is much greater during deposition than it is when the beam is off. 1. INTRODUCTION Recent improvements in interatomic potentials and computer technology have stimulated the development of realistic models of crystal growth [1-4]. These models provide a good test of crystal growth theories in cases where the simulations can span enough space and elapsed time to encompass the relevant growth mechanisms. Computer limitations restrict molecular dynamics (MD) models to extremely fast crystal growth. Nevertheless, the model gives useful information on the atomic mobilities and mechanisms that affect impurity redistribution during growth. The MD simulations fill an important gap in crystal growth calculations. They are the most accurate simulations currently available of the atomic-level dynamics, and yet they can generate enough data for meaningful statistics. Impurity trapping during crystal growth is an important process that is not very well understood. Solidification experiments have shown that alloys can be made with impurity concentrations that exceed the equilibrium values by several orders of magnitude, provided that solidification is fast [5]. In such alloys, the chemical potential of the impurity is greater than the value that it would have in precipitates of the impurity-rich phase. These alloys can form because the rapid growth does not allow the impurities to aggregate or diffuse away from the growing crystal. The large undercooling of the melt provides a strong driving force, so that a solid with a relatively high free energy can be formed. A detailed understanding of the crystallization of such alloys requires a model that accounts for the mobility of the impurity through the solid-liquid interface. A similar situation exists in the case of molecular beam epitaxy (MBE) with the co-deposition of dopants. The control of doping concentrations in the films is often limited by the segregation of the dopant to the surface [6,7]. The free energy of the system is usually lower with the dopant located mainly at the surface. But it is sometimes possible to inhibit the motion of the impurity to the surface by appropriate choice of the growth conditions. In this paper we study the growth of a thin film with a 8-layer, a heavily doped buried layer that has a thickness on the order of one monolayer [6,8]. Layers rich in Sb deposited during S
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