Impurity segregation in Lennard-Jones A/AB heterostructures. II. The effect of impurity size

Abstract

Segregational phenomena in Lennard-Jones heterostructures of the type A/A1−xBx were investigated using nonequilibrium molecular dynamics computer simulation techniques. In this paper, the ratio of the size of the ‘‘impurity’’ B atoms relative to that of the substrate A atoms was varied, covering ranges where the B atoms were either smaller than or larger than the A atoms. Ratios of the collision diameter, σR=σB/σA, of 1.20, 1.15, 0.90, 0.85, 0.80, 0.75, and 0.5 were considered. The concentration of the B atoms was also varied. The tendency for segregation of the B atoms into a surface-melted thin film was studied and compared to the segregational behavior exhibited at a moving solid/(bulk) liquid interface, the latter emulating liquid phase epitaxy. When the B atoms are significantly larger than the substrate atoms, as here, either misfit dislocations are nucleated or an amorphous phase is produced depending on the concentration of B atoms and the degree of lattice mismatch. When the B atoms are smaller than the substrate A atoms, there is a more pronounced tendency to maintain crystallinity and to occupy interstitial lattice positions rather than substitutional sites. This asymmetry of segregational behavior depending on whether the impurities are larger than or smaller than the substrate is suggested to be due to the inherent asymmetry of the Lennard-Jones potential. The results of previous vapor phase epitaxial simulations are reviewed in the larger context of the present results for liquid phase epitaxy.