Observation of decoupled heavy and light holes in GaAs-Ga1−xAlxAs quantum wells by magnetoreflectivity

Abstract

Interband reflectivity measurements have been performed on a variety of GaAs-${\mathrm{Ga}}_{0.64}$ ${\mathrm{Al}}_{0.36}$As multiple-quantum-well structures, in magnetic fields up to 9.5 T, applied perpendicular to the layers. Sharp features are observed in the reflectivity spectra at 1.8 K, which have been identified as excitonic Landau-level transitions between the hole subbands and electron subbands of the wells. The relative positions of the excitonic resonances are obtained from a comparison of the calculated reflectivity of the structures with the experimental data. The Landau-level transitions seen in the reflectivity data persist down to very low magnetic fields (∼0.3 T). Results for both high- and low-field limits are fitted with use of a calculation of the exciton-energy levels as a function of magnetic field, giving an exciton reduced mass around 30% higher at high fields than the low-field limit close to B=0 T. This change is thought to reflect a change in the heavy-hole effective mass: At low fields, the heavy- and light-hole bands are fully decoupled within the Luttinger Hamiltonian, so that the in-plane heavy-hole mass will be small, while at higher fields coupling will occur, leading to the relatively large heavy-hole masses measured in earlier magnetooptical experiments.