Order of theXconduction-band valleys in type-II GaAs/AlAs quantum wells

The optically detected magnetic resonance of electrons and excitons in type-II GaAs/AlAs quantum wells has been studied as a function of the orientation of the quantum well with respect to the direction of the magnetic field. An analysis of the anisotropy reveals that for quantum wells with AlAs layers thinner than ∼55 Å the $X_z$ conduction-band valley has the lowest energy while for quantum wells with thicker AlAs layers the $X_x$ and $X_y$ valleys are lowest in energy. This is a consequence of the lattice-mismatch strain splitting of the AlAs X conduction band which dominates the confinement splitting for AlAs layers thicker than ∼55 Å.