Capacitance and conductance oscillations from electron tunneling into high energy levels of a quantum well in a p-i-n diode

Two-dimensional electron and hole gases separated by a few nm from each other are produced in p-i-n diodes based upon molecular beam epitaxy-grown GaAs/AlGaAs heterostructures. At such interlayer distances, the exciton formation and possibly Bose-Einstein condensation (BEC) is expected. We measure the capacitance between the layers and find it to oscillate as a function of the bias voltage. The peak values exceed the geometric capacitance by up to a factor of 2. The surprisingly regular periods of the oscillations are of the order of 10 to 30 mV and scale linearly with the inverse of the thickness, between 60 and 150 nm, of the GaAs layer placed between the barrier and the p-doped AlGaAs. The oscillations are related to the resonant electron tunneling into high energy levels of this GaAs layer acting as a quantum well. We suggest that long lifetimes of the electrons in these levels are the origin of the large peak values of the capacitance. The possible relation of the capacitance oscillations with BEC is discussed.