Fine structure of excitons in type-II GaAs/AlAs quantum wells

Optically detected magnetic resonance in zero field as well as in a finite magnetic field has been used to study the excitons in type-II GaAs/AlAs quantum wells. The spectra are analyzed using the appropriate spin Hamiltonian for the quasi-two-dimensional indirect excitons. The electron-hole exchange interaction and the g factors for the electron and hole are obtained for several thicknesses of the GaAs and AlAs layers. Good agreement exists between the trend in the exchange interaction and the effective-mass theory of Rejaei Salmassi and Bauer. The anisotropy of the electron g factor is in accordance with a lifting of the threefold degeneracy of the AlAs X conduction-band minimum by the quantum-well potential giving the ${\mathit{X}}_{\mathit{z}}$ valley the lowest energy in the thin-layer quantum wells studied. The effective heavy-hole g value of ∼2.5 is much smaller than in the bulk and depends on the GaAs well thickness. This is probably a consequence of the valence-band mixing in quantum-well structures. Two classes of excitons are observed, each with a symmetry that is lower than the anticipated ${\mathit{D}}_{2\mathit{d}}$ point-group symmetry for excitons in quantum wells. The actual symmetry of the type-II excitons and the width of the exciton resonances are related to the microscopic structure of the GaAs/AlAs interface.