Effect of an electric field on electronic excitations in double quantum wells

We have measured photoluminescence (PL) and inelastic light scattering spectra of GaAs/AlGaAs double quantum wells (DQW). The structures contain a high-mobility two-dimensional (2D) electron gas produced by modulation doping. The electron density as well as the intersubband spacings of the DQWs are changed by applying a DC voltage between a top and a bottom contact. Band-gap renormalization effects are observed in PL spectra when an excited subband becomes populated with electrons, as the 2D density increases. Inelastic light scattering yields information on the bias dependence of collective intersubband excitations of the 2D electron gas. The applied voltage reduces the coupling between layers leading to an energy shift of the excitations in each well in different directions. (C) 2002 Elsevier Science B.V. All rights reserved

Exchange instability of the two-dimensional electron gas in semiconductor quantum wells

A two-dimensional (2D) electron gas formed in a modulation- doped GaAs/AlxGa1-xAs single quantum well undergoes a first- order transition when the first excited subband is occupied with electrons, as the Fermi level is tuned into resonance with the excited subband by applying a dc voltage. Direct evidence for this effect is obtained from low-temperature photoluminescence spectra that display the sudden renormalization of the intersubband energy E-01 upon the abrupt occupation of the first excited subband. Calculations within density-functional theory, which treat the 2D exchange potential exactly, show that this thermodynamical instability of the electron system is mainly driven by intersubband terms of the exchange Coulomb interaction, thus being a unique but fundamental property of an electron system with more than one occupied subband

Identification of bound exciton complexes in ZnO

An identification of shallow bound exciton centers in ZnO is presented based on magneto-optical measurements and diffusion experiments. The thermalization behavior of the Zeeman split components confirms that the I 4, I6, I8 and I9 exciton lines stem from donor bound exciton complexes. The results are supported by theoretical analysis of shallow bound exciton complexes revealing the Γ7 symmetry of the upper valence band. The presence of two-electron satellites related to the respective transitions is further evidence for the donor bound complexes and enabled the determination of donor binding energies. Hydrogen, aluminum, gallium and indium were identified to origin the I4, I 6, I8 and I9 lines by doping, diffusion and annealing experiments combined with photoluminescence and secondary ion mass spectrometry. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim