Magneto-optics in GaAs-Ga1−xAlxAs quantum wells

The interband photoconductivity of GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As quantum wells has been studied in steady fields of up to 16 T, applied both parallel and perpendicular to the growth axis. In fields parallel to the growth axis, excitonic Landau-level transitions have been identified with Landau indices up to n=12 and energies up to 400 meV above the energy gap of bulk GaAs. Transition energies and diamagnetic shifts have been measured for both allowed (ΔN even) and forbidden (ΔN odd) subband exciton transitions, including a transition at high energy in the widest wells which we tentatively identify as involving a bound state in the spin-orbit split-off band. Landau levels are observed at fields as low as 2.5 T, permitting a determination of exciton binding energy from linear extrapolation. High-field results are fitted using a calculation of the binding energy of two-dimensional excitons as a function of magnetic field. It is found that the exciton binding energy obtained from fitting high-field results is slightly higher than that obtained from low-field measurements, giving exciton binding energies of 16–9 meV, for well widths from 22 to 110 Å. The heavy-hole effective mass for motion in the plane of the layers is found to vary with well thickness, and this is explained by decoupling of the bands.