Effect of subband mixing on exciton binding energies in type-II quantum-well structures

We have calculated the binding energies of the heavy-hole and the light-hole excitons in the GaAs-AlAs type-II quantum-well structures, taking into account the coupling of the confined electron subband states to the hole subband states, both confined and unconfined. Realistic finite potential barriers are used in the calculations. The inherent lack of reflection symmetry in the type-II quantum wells is shown to lead to a strong mixing of the electron and hole subband wave functions, and to an ∼0.5–2 meV increase in the binding energy of the heavy-hole exciton, and an ∼1–4 meV increase in that of the light-hole exciton. Such increases in the binding energies are attributed to the smaller average electron-hole distance caused by the modified electron and hole charge distributions in the vicinity of the GaAs-AlAs interface, which in turn is the result of mixing of the first electron and hole subbands with the spatially less confined wave functions of higher subbands.