FAR-INFRARED MAGNETOOPTICAL STUDY OF 2-DIMENSIONAL ELECTRONS AND HOLES IN INAS ALXGA1-XSB QUANTUM-WELLS/

We present results of a detailed far-infrared magneto-optical study on a series of high-mobility InAs/AlxGa1-xSb (x=1.0, 0.8, 0.5, 0.4, 0.2, and 0.1) type-II single quantum wells. A wide range of phenomena aris ing from the unusual properties of two-dimensional (2D) electrons and holes and their Coulomb interaction in high magnetic fields has been r evealed. Semiconducting samples (x greater than or equal to 0.4), in w hich only 2D electrons exist in the InAs wells, exhibit cyclotron-reso nance (CR) splittings due to large conduction-band nonparabolicity. Se mimetallic samples (x=0.1 and 0.2), in which both 2D electrons (in InA s) and 2D holes (in AlxGa1-xSb) are present, show two additional lines (e and h-X lines) as well as electron and hole CR. The X-lines increa se 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 w hich is increased by the magnetic field. The electron CR shows strongl y oscillatory linewidth, amplitude, and mass, part of which are interp reted in the light of the unusual ''antinonparabolic'' band structure resulting from band overlap and coupling between conduction-band state s in InAs and valence-band states in AlxGa1-xSb; part of these results are qualitatively consistent with the predictions of Altarelli and co -workers. The X lines are attributed to internal transitions of correl ated electron e-h pairs (excitons) in high magnetic fields mediated by the excess electron density.