Selective breakdown of quantum Hall edge states and non-monotonic Coulomb drag in a GaAs-AlGaAs electron-hole bilayer

The attractive Coulomb interaction between a two-dimensional electron gas (2DEG) and hole gas (2DHG) confined to quantum wells separated by a thin insulating barrier (similar to 20 nm) may give rise to several interesting phenomena. The possible bosonic behaviour of these bilayer excitons have been anticipated for many years. We have developed a technique of fabricating closely spaced electron-hole bilayers in GaAs-AlGaAs heterostructures. Our device incorporates a novel method of making negative Schottky barrier based ohmic contacts, to a low density( 5 x 10(10)cm(-2)) 2-DEG, using MBE grown doped InAs. These contacts do not require annealing and are completely free from spiking. Electrons and holes accumulate on opposite sides of a 25 nm Al0.3Ga0.7As barrier, when suitably biased. Only in devices with very low barrier leakage, we find that at electron densities N-e > 10(11) cm(-2), instabilities start appearing in the quantum Hall plateaux in the electron layer. The effect first appears in the v = 2 plateau. At higher densities, lower temperatures and tilted magnetic fields the effect starts appearing in the odd integer plateaus as well. Hall drag measurements strongly indicate that the instabilities result from interlayer interaction. At the lowest densities measured so far, the Coulomb drag in zero magnetic field shows an upturn at T < 1 K, which is a possible indication of electron-hole pairing fluctuations. (c) 2007 Elsevier B.V. All rights reserved.