Publications

LDRD final report on engineered superconductivity in electron-hole bilayers

Lilly, Michael L.; Bielejec, Edward S.; Seamons, J.A.; Dunn, Roberto G.; Lyo, S.K.; Reno, J.L.; Stephenson, Larry L.; Baca, Wes E.; Simmons, J.A.

Macroscopic quantum states such as superconductors, Bose-Einstein condensates and superfluids are some of the most unusual states in nature. In this project, we proposed to design a semiconductor system with a 2D layer of electrons separated from a 2D layer of holes by a narrow (but high) barrier. Under certain conditions, the electrons would pair with the nearby holes and form excitons. At low temperature, these excitons could condense to a macroscopic quantum state either through a Bose-Einstein condensation (for weak exciton interactions) or a BCS transition to a superconductor (for strong exciton interactions). While the theoretical predictions have been around since the 1960's, experimental realization of electron-hole bilayer systems has been extremely difficult due to technical challenges. We identified four characteristics that if successfully incorporated into a device would give the best chances for excitonic condensation to be observed. These characteristics are closely spaced layers, low disorder, low density, and independent contacts to allow transport measurements. We demonstrated each of these characteristics separately, and then incorporated all of them into a single electron-hole bilayer device. The key to the sample design is using undoped GaAs/AlGaAs heterostructures processed in a field-effect transistor geometry. In such samples, the density of single 2D layers of electrons could be varied from an extremely low value of 2 x 10{sup 9} cm{sup -2} to high values of 3 x 10{sup 11} cm{sup -2}. The extreme low values of density that we achieved in single layer 2D electrons allowed us to make important contributions to the problem of the metal insulator transition in two dimensions, while at the same time provided a critical base for understanding low density 2D systems to be used in the electron-hole bilayer experiments. In this report, we describe the processing advances to fabricate single and double layer undoped samples, the low density results on single layers, and evidence for gateable undoped bilayers.