Blue luminescence from ultrathin GaAs layers embedded in AlAs

Our investigations focus on low-temperature luminescence experiments on a set of type-II GaAs/AlAs multiple-quantum-well (MQW) samples grown by low-pressure metal-organic vapor-phase epitaxy. The layered structures consists of 50 periods of either 2 monolayers (ML), 4, 5, 6, or 7 ML GaAs embedded in 28 ML AlAs. For (001) GaAs substrates, 6 degrees misoriented towards the nearest (111) plane of group-V atoms, monolayer steps at the AlAs/GaAs interfaces with regular terrace widths (2.7 nm) can be seen by high-resolution transmission-electron microscopy. In the photoluminescence spectra of these MQW samples, type-I luminescence is found to be dominant even at room temperature. The peak wavelength of the type-I emission depends strongly on the GaAs layer thickness; it ranges from about 620-440 nm. The intense type-I emission seems to be connected with the interface peculiarities. Our astonishing observation might be explained as follows: (i) The perfect interface structure pl events the loss of photoexcited carriers from GaAs layers to the surrounding AlAs materials, i.e., the energy loss by optical-phonon scattering is reduced. (ii) For our well thicknesses two-dimensional (2D) phonons must be coupled with 3D electrons leading also to a reduction of the electron-phonon interaction. (iii) The regular interface steps should favor a coherent interaction (quantum interferences) of excitons and/or electrons confined in the GaAs wells with energetically resonant continuum states of the AlAs barriers. The experimentally observed optical transition energies of the type-I and type-II recombination are compared with model calculations applying an effective-mass approach and empirical tight-binding Green's-function scheme.