Exciton binding energy in type-II heterojunctions

The binding energy of the ground-state exciton in type-II heterojunctions in the presence of a static external electric field is calculated using a variational method. The anisotropy of the electron and hole effective mass and the carriers’ polarizability are considered. Polaron effects due to the coupling between exciton and LO phonons and interface phonons are included, and it is shown that these effects are important. The calculation was performed by using a generalization of the Lee-Low-Pines method, which is known to be valid for small and intermediate exciton-phonon coupling. We applied this theory to the system AlAs/GaAs, where the electron and hole are spatially separated, with the electron at the X point of the conduction band of AlAs and the hole at the Γ point of the valence band of GaAs. In this system it is observed that, due to the polaron effects, a minimum external electric field, ${\mathit{E}}_0$, applied perpendicular to the interface, is necessary to obtain a stable ground state for the exciton.