  Using atomistic pseudopotential and configuration-interaction many-body
calculations, we predict a metal-nonmetal transition and an excitonic ground
state in the InAs/InSb quantum dot (QD) system. For large dots, the conduction
band minimum of the InAs dot lies below the valence band maximum of the InSb
matrix. Due to quantum confinement, at a critical size calculated here for
various shapes, the single-particle gap $E_g$ becomes very small. Strong
electron-hole correlation effects are induced by the spatial proximity of the
electron and hole wavefunctions, and by the lack of strong (exciton unbinding)
screening, afforded by the existence of fully discrete 0D confined energy
levels. These correlation effects overcome $E_g$, leading to the formation of a
bi-excitonic ground state (two electrons in InAs and two holes in InSb) being
energetically more favorable (by $\sim$ 15 meV) than the state without
excitons. We discuss the excitonic phase transition on QD arrays in the low dot
density limit.
