Closely spaced, independently contacted electron–hole bilayers in GaAs–AlGaAs heterostructures

We describe a technique of fabricating closely spaced electron–hole bilayers in GaAs–AlGaAs heterostructures. Our device incorporates a novel method of making shallow ohmic contacts, to a low density (View the MathML source) 2-DEG, using MBE grown doped InAs. These contacts do not require annealing and are completely free from “spiking”. Independent four terminal measurements on both layers (25 nm apart) are possible. Hole mobilities View the MathML source and electron mobilities μe>106cm2V-1s-1 are simultaneously attained at 1.5 K. Preliminary Coulomb drag measurements made down to View the MathML source are consistent with the Onsager reciprocity relation for four terminal conductance and indicate an enhancement of Coulomb interaction over the values obtained from a “Thomas–Fermi” calculation. The technique lends itself fully to conventional uniform MBE growth and does not require shadow masking or ion implantation. In the simplest mode of operation a single external voltage, slightly higher than the bandgap of GaAs, is required for simultaneous accumulation of electrons and holes separated by the 25 nm Al0.3Ga0.7As barrier. Both carrier densities increase with the applied interlayer bias and the measured densities are reproducible over several cooldowns. A backgate can be added to the structure to get independent control of the carrier densities. Our process is independent of the precise depth of the 2DEG/2DHG or the width of the barrier and will be useful in studying novel phases resulting from a possible excitonic pairing between electrons and holes.