Far-infrared magneto-optical study of two-dimensional electrons and holes in InAs/Al_x Ga_1-x Sb quantum wells

We present results of a detailed far-infrared magneto-optical study on a series of high-mobility InAs/Al_x Ga_1-x Sb (x=1.0, 0.8, 0.5, 0.4, 0.2, and 0.1) type-II single quantum wells. A wide range of phenomena arising from the unusual properties of two-dimensional (2D) electrons and holes and their Coulomb interaction in high magnetic fields has been revealed. Semiconducting samples (x⩾0.4), in which only 2D electrons exist in the InAs wells, exhibit cyclotron-resonance (CR) splittings due to large conduction-band nonparabolicity. Semimetallic samples (x=0.1 and 0.2), in which both 2D electrons (in InAs) and 2D holes (in Al_x Ga_1-x Sb) are present, show two additional lines (e and h-X lines) as well as electron and hole CR. The X-lines increase in intensity at the expense of CR with increasing electron-hole (e-h) pair density, decreasing temperature, or increasing magnetic field (at low field), suggesting that they are associated with e-h binding which is increased by the magnetic field. The electron CR shows strongly oscillatory linewidth, amplitude, and mass, part of which are interpreted in the light of the unusual 'antinonparabolic' band structure resulting from band overlap and coupling between conduction-band states in InAs and valence-band states in Al_x Ga_1-x Sb; part of these results are qualitatively consistent with the predictions of Altarelli and co-workers. The X lines are attributed to internal transitions of correlated electron e-h pairs (excitons) in high magnetic fields mediated by the excess electron density.