Impurity accumulation in an adsorption layer during MBE doping

Abstract

A theoretical model of capture and surface segregation of an impurity during MBE doping is proposed. It is assumed that an impurity is incorporated in the surface layer of a crystal doped due to both the blocking of the impurity atom by host atoms in kinks on steps and the exchange between the impurity atoms adsorbed and the host atoms in the surface layer. The surface segregation is considered as an accumulation of an impurity in the adsorption layer due to the jumps of impurity atoms adsorbed over the steps and the migration of impurity atoms from the surface layer into the adsorption layer. It is shown that an increase in the supersaturation near a step with a drop in the temperature and an increase in the growth rate suppresses the surface segregation, due to the more effective impurity blocking in the kinks. The formation of 2D islands on the terraces and nonequilibrium kinks at the island edges results in a partial drop of the supersaturation and weakening of the temperature and growth rate effects on the surface segregation. When impurity atoms intensively migrate from the surface layer to the adsorption one, the blurring of the doping profile sharply (superexponentially) increases with the temperature. This is connected with the exponential dependence of the probability of an impurity immuring in the surface layer by a moving step on the rate constant of the impurity migration from the surface layer into the adsorption one. The model reproduces typical dependences of the width of the doping transition’s concentration region on the temperature and growth rate obtained in experiments on Sb-doping of Si.