The distribution of electrons in two parallel two-dimensional electron sheets in close proximity at low temperatures is investigated. We find that due to the exchange interaction a symmetric structure may have a ground state in which all electrons are transferred to one of the sheets. This occurs if the separation between the two systems is sufficiently small and the two-dimensional electron gas density is sufficiently low. The exchange interaction consequently leads to a bistability with respect to electron population in the two-sheet system which may be useful for future electronic devices, such as memory elements.

We propose a model for quantum dot light emitting devices (QD-LEDs), which explores the most important parameters that control their electrical characteristics. The device is divided into a hole transport layer, several quantum dot layers, and an electron transport layer. Conduction and recombination in the central quantum dot region is described by a system of coupled rate equations, and the drift-diffusion approximation is used for the hole and electron transport layers. For NiO/Si-QDs/ZnO devices with suitable design parameter, the current and light output are primarily controlled by the quantum dot layers, specifically, their radiative and non-radiative recombination coefficients. Radiative recombination limits the device current only at sufficiently large bias. V C 2013 AIP Publishing LLC. [http://dx.

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Luminescence and Raman measurements on a new type of superlattice consisting of n-and/>-type doped GaAs layers grown by molecular-beam epitaxy confirm crucial predictions of theory. A strongly tunable energy gap is found in luminescence. Raman experiments provide the first observation of electronic subbands in purely space-chargeinduced quantum wells. A combined analysis of the luminescence and Raman data yields excellent agreement with self-consistent subband calculations based only on the design parameters of the sample.

Resonant tunneling transport was studied in GaAs-AlGaAs single well, double barrier structures. Negative differential resistance at 77 and 300 K was observed in devices grown by metalorganic chemical vapor deposition. The observed peak to valley ratios were 6 to 1 and 1.48 to 1, respectively. The presence of GaAs spacer layers was found to have a distinct effect on the 300 K current versus voltage characteristics. Microwave oscillation at frequencies up to 3.0 GHz was observed at 300 K. A short discussion of the oscillation frequency limits of our structure is presented.

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