Asymmetric stark shifts of exciton in InAs/GaAs pyramidal quantum dots

Within the framework of the single-band effective-mass envelope-function theory, the effect of electric field on the electronic structures of pyramidal quantum dot is investigated. Taking the Coulomb interaction between the heavy holes and electron into account, the quantum confined Stark shift of the exciton as functions of the strength and direction of applied electric field and the size of the quantum dot are obtained. An interesting asymmetry of Stark shifts around the zero field is found. (C) 1997 Elsevier Science Ltd.

Carrier relaxation and thermal activation of localized excitons in self-organized InAs multilayers grown on GaAs substrates

We have investigated the temperature dependence of photoluminescence (PL) properties of a number of self-organized InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 to 3 ML. The temperature dependence of InAs exciton emission and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML. The fast redshift of PL energy and an anomalous decrease of linewidth with increasing temperature were observed and attributed to the efficient relaxation process of carriers in multilayer samples, resulting from the spread and penetration of the carrier wave functions in coupled InAs quantum dots. The measured thermal activation energies of different samples demonstrated that the InAs wetting layer may act as a barrier for the thermionic emission of carriers in high-quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to escape thermally from the localized states.

GSMBE growth and PL investigation of lattice-matched InGaAs/InP quantum wells

Using a home-made gas-source molecular beam epitaxy system, high quality InGaAs quantum wells with different well widths lattice-matched to a (001) InP substrate have been obtained. Sharp and intense peaks for each well can be well resolved in the PL spectra for the sample. For well widths larger than similar to 60 Angstrom, the exciton energies are in good agreement with those of calculation. For wells narrower than 40 Angstrom, our line widths are below the theoretical values of line width broadening due to one monolayer interface fluctuation, showing that the interface fluctuation of our sample is within one monolayer.

Energy relaxation processes of hot quasi-two-dimensional excitons in very thin GaAs/AlGaAs quantum wells by exciton-acoustic-phonon interaction

The rising time of the excitonic luminescence in GaAs/AlGaAs quantum wells is studied as a function of the well width. For well thickness below approximately 20 Angstrom, we find an increase of rising time with decreasing well width. We explain the dependence of the rising time on well width in very thin quantum wells by the slow-down energy relaxation and/or exciton migration processes due to the decrease of the scattering rate of the exciton-acoustic-phonon interaction. (C) 1996 American Institute of Physics.

DETERMINATION OF THE X-CONDUCTION-SUBBAND ENERGIES IN TYPE-II GAAS/ALAS/GAAS QUANTUM-WELL BY DEEP-LEVEL TRANSIENT SPECTROSCOPY

Using deep level transient spectroscopy (DLTS) the X conduction-subband energy levels in an AlAs well sandwiched by double GaAs layers were determined. Calculation gives eight subbands in the well with well width of 50 Angstrom. Among them, five levels and the other three remainders are determined by using the large longitudinal electron effective mass m(1)(1.1m(0)) and transverse electron effective mass m(t)(0.19m(0)) at X valley, respectively. Two subbands with the height energies were hardly detectable and the other six ones with lower energies are active in the present DLTS study. Because these six subbands are close to each other, we divided them into three groups. Experimentally, we observed three signals induced from the three groups. A good agreement between the calculation and experiment was obtained. (C) 1995 American Institute of Physics.

PHOTOLUMINESCENCE STUDY OF GAAS/ALGAAS QUANTUM-WELL HETEROSTRUCTURE INTERFACES

Photoluminescence (PL) is used to study the interface properties of GaAs/AlGaAs quantum well (QW) heterostructures prepared by molecular beam epitaxy with growth interruption (GI). The discrete luminescence lines observed for the sample with GI are assigned to the splitting of the heavy-hole exciton associated with heterointerface islands with the lateral size greater than exciton diameter and mean height less than one monolayer, and the spectra have the Gaussian lineshapes. The results strongly support the microroughness model. We also study the temperature dependence of the exciton energies and find that excitons are localized at the interface roughness at low temperature even in the sample with GI. The lateral size of the microroughness of the GI sample is estimated to be less than 5 nm from the exciton localization energy.

Exciton Localization and Delocalization in GaNAs/GaAs Quantum Wells

We have studied exciton localization and delocalization effect in GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL) and timeresolved PL measurements. Studied results suggest that, at low temperature and under a conventional CW excitation, measured PL spectra were dominated by localized exciton (LE) emission caused by potential fluctuations in GaNAs layer. However, under short pulse laser excitation, it is different. An extra high-energy PL peak comes out from GaNAs/GaAs QWs and dominates the PL spectra under high excitation and/or at high temperature. By investigation, we have attributed the new PL peak to the recombination of delocalized excitons in QWs. This recombination process competes with the localized exciton emission, which, we believe, constitutes the "S-shaped" temperature-dependent emission shift often reported in ternary nitrides of InGaN and AlGaN in the literature.

Mechanism on exciton-mediated energy transfer in erbium-doped silicon

Exciton-mediated energy transfer model in Er-doped silicon was presented. The emission intensity is related to optically active Er concentration, lifetime of excited Er3+ ion and spontaneous emission. The thermal quenching of the Er luminescence in Si is caused by thermal ionization of Er-bound exciton complex and nonradiative energy back-transfer processes, which correspond to the activation energy of 6.6 and 47.4 meV, respectively. Er doping in silicon introduces donor states, a large enhancement in the electrical activation of Er (up to two orders of magnitude) is obtained by co-implanting Er with O. It appears that the donor states are the gateway to the optically active Er. (C) 2000 Elsevier Science B.V. All rights reserved.