Far-infrared magneto-optical study of the two dimensional electrons and holes in indium arsenide/aluminum(x) gallium(1-x) antimony quantum wells

Over the past three decades there has been considerable interest in correlation effects in narrow-gap semimetals/semiconductors in which electrons and holes coexist. Various possible collective ground states have been predicted for systems with electron-hole (e-h) excitonic binding energies comparable with or larger than the band-gap. However, in spite of extensive experimental studies of bulk materials at low temperatures and at high magnetic fields/pressures near the semimetal-semiconductor transition, direct evidence of these ground states have not been given to date. Recent attention has been paid to spatially-separated two-dimensional (2D) e-h systems, which have been successfully fabricated owing to the advancement of semiconductor nanotechnology.; Here I present results of a detailed far-infrared magneto-optical study on a series of high-mobility InAs/Al{dollar}sb{lcub}x{rcub}{lcub}rm Ga{rcub}sb{lcub}1-x{rcub}{lcub}rm Sb{rcub} (0.1le x le1.0){dollar} type-II quantum wells. A wide range of new phenomena arising from the unusual properties of 2D electrons and holes and their Coulomb interaction in high magnetic fields has been revealed. Semiconducting samples ({dollar}xge0.4),{dollar} in which only 2D electrons exist in the InAs layers, exhibit cyclotron resonance (CR) splittings due to the large conduction-band nonparabolicity. Semimetallic samples ({dollar}xle0.2),{dollar} in which both 2D electrons (in InAs) and 2D holes (in Al{dollar}sb{lcub}x{rcub}{lcub}rm Ga{rcub}sb{lcub}1-x{rcub}{dollar}Sb) are present, show two new features (e- and h-X-lines) in addition to e- and h-CR. The X-lines increase in intensity at the expense of CR either with increasing e-h pair density or decreasing temperature, strongly suggesting that they are associated with e-h binding. The e-CR shows strongly oscillatory linewidth, amplitude, and mass, only when holes coexist with electrons. Part of the results are qualitatively consistent with the prediction of Altarelli and coworkers on resonant coupling between conduction-band states in InAs and valence-band states in AlGaSb.