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Perpendicular motion of photoexcited electron and hole pairs assisted by phonon scattering isinvestigated in a novel step-graded staircase heterostructure consisting of strainedInX(Al0.17Ga0.83)1-XAs multiple quantum wells (QWs) with similar widths but different five x valuesby cw and time-resolved photoluminescence (PL) experiments. From the temperature dependence ofPL spectral intensity of distinct five peaks corresponding to the QW layers, we find that, astemperature increases, the PL peaks decrease their relative intensities progressively from shorterwavelength sides after increasing the signal amplitude in the intermediate temperature range. Thesevariations reveal that the photoexcited carriers directionally move from shallower to deeper QWs viaphonon-assisted activation above the barrier band edge state. The PL dynamics directly indicate theperpendicular flowing of photoexcited carriers and the capture by the deeper QW, thus providing firmevidences for the dynamical carrier flow and capture processes.

The confinement of electrons in semiconductor quantum dots (QDs) enables the realisation of many quantum optical phenomena in semiconductors including quantum infonnation processing. Self-assembled QDs grown by the Stranski Krastanov (SK) method can produce QDs with high optical quality, which is needed for such experiments. They are, however, randomly distributed on the surface on which they are grown. Many quantum optics experiments require lateral ordering of the QDs. This can be achieved by growing the QDs on a strained surface made by a superlattice whereby the QDs nucleate in small clusters in a well defined lattice [lj. An intriguing aspect of these clusters is the formation of coupled QD molecules that will enable new functionalities such as generation of entangled photons and two qubit gates. An open question has been to what extent the structuring influences the radiative properties of the QDs. This question is addressed here using time- and energy-resolved spectroscopy on QDs [21. We conclude that the radiative properties are preserved for the ordered QDs. To investigate the optical quality of ordered QDs detailed tinne-resolved measurements of the spontaneous emission of ordered InAs/GaAs QDs grown on a (In,Ga)As/GaAs superlattice have been carried out. The results are compared with the regular SK grown InAs/GaAs QDs that are randomly placed and known to have a high optical quality. An AFM image of the ordered QDs is shown in the left part of figure 1. Time- and energy-resolved spontaneous emission has been measured using a streak camera and the resulting decays are modelled well by a single exponential decay. The resulting decay rates are shown in the right part of figure 1 together with measurements on randomly distributed QDs. The emission spectra of the QDs (not shown) are centered at —1.15 eV for the ordered QDs and at —1.22 eV for the randomly positioned QDs corresponding to the excitonic ground states. The decay rate of the ground state of the ordered QDs levels off at the same value as for the randomly distributed QDs. At higher emission energies the decay rates increase due to non-radiative contribution from the excited state for both types of QDs - except for the decrease in decay rate at high emission energies, which is originating from the superlattice. We model the ground state decay rate by a simple model using Fenni’s "golden rule". The decay rates of tho ordered QDs demonstrate that the optical quality has not been deteriorated by growing the QDs on the superlattice, i.e. the quantum efficiency of the ground state is high. This establishes the capabilities of ordered QDs for quantum optics experiments.

The dynamics of perpendicular transport of photoexcited carriers assisted by phonon scattering is investigated in a novel step-graded Inx(Al0.17Ga0.83)1-xAs quantum-well heterostructure by measuring the temperature dependence of spectrally and temporally resolved photoluminescence (PL). When builtin potential gradients are present in the quantum-well heterostructure due to variations in the In mole fraction (x) in the well, carriers that are thermally released by the particular well move unidirectionally from shallower to deeper wells. That is, asymmetric unidirectional motion of photoexcited carriers is possible via phonon-assisted activation above the barrier band-edge state. We have directly measured this perpendicular motion of photoexcited carriers by monitoring the transient PL signals from the different wells, which are spectrally separated. A rate equation analysis rigorously explains the dynamical changes of the PL signal intensities from the quantum wells as a function of lattice temperature. Our study of PL dynamics proves the asymmetric perpendicular flow of photoexcited carriers and the capture by the deeper quantum wells, providing firm evidence for the dynamical carrier flow and capture processes in the novel heterostructure.