Fermi-level effect and junction carrier concentration effect on p-type dopant distribution in III-V compound superlattices
The pronounced segregation phenomenon in the distribution of p-type dopants Zn and Be in GaAs and related III-V compound heterostructures has been explained quantitatively by treating simultaneously the processes of dopant atom diffusion, segregation, and the effect of heterojunction carrier concentrations on these two aspects. Segregation of a dopant species between two semiconductor heterostructure layers is described by a model incorporating (i) a chemical effect on the neutral species; and (ii) in addition, a Fermi-level effect on the ionized species. The process of Zn and Be diffusion in GaAs and related compounds is governed by the doubly-positively-charged group III element self-interstitials (IIII2+), whose thermal equilibrium concentration and hence also the Zn and Be diffusivities exhibit also a Fermi-level dependence, i.e., in proportion to p2. A heterojunction is consisting of a space charge region with an electric field, in which the hole concentration is different from those in the bulk layers. This influences the junction region concentrations of IIII2+ and of Zn- or Be-, which in turn influence the distribution of the ionized acceptor atoms. The overall process involves diffusion and segregation of holes, IIII2+, Zn- or Be-, and an ionized interstitial acceptor species. The junction electric field also changes with time and position.
