  Excitonic fine structure splitting in quantum dots is closely related to the
lateral shape of the wave functions. We have studied theoretically the fine
structure splitting in InAs quantum dots with a type-II confinement imposed by
a GaAsSb capping layer. We show that very small values of the fine structure
splitting comparable with the natural linewidth of the excitonic transitions
are achievable for realistic quantum dot morphologies despite the structural
elongation and the piezoelectric field. For example, varying the capping layer
thickness allows for a fine tuning of the splitting energy. The effect is
explained by a strong sensitivity of the hole wave function to the morphology
of the structure and a mutual compensation of the electron and hole
anisotropies. The oscillator strength of the excitonic transitions in the
studied quantum dots is comparable to those with a type-I confinement which
makes the dots attractive for quantum communication technology as emitters of
polarization-entangled photon pairs.
