Y. Ducommun

Few-Particle Effects in Semiconductor Quantum Dots: Observation of Multicharged Excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.

Structure and optical properties of semiconductor quantum nanostructures self-formed in inverted tetrahedral pyramids

We present a method for fabricating quantum dots using seeded self-organized growth of GaAs/AlGaAs heterostructures on substrates patterned with inverted pyramids. This method produces, at the tip of each inverted pyramid, highly uniform quantum dots whose size and position can be accurately controlled. In addition, a system of connected GaAs and AlGaAs two- and one-dimensional nanostructures is identified in the inverted pyramids using cross-sectional atomic force microscopy. A substrate removal technique is used to optimally prepare our samples for optical studies, allowing the increase of the luminescence efficiency of the quantum dots by up to three orders of magnitude. Micro-photoluminescence and cathodo-luminescence spectroscopy are used to study in detail the bandgap structure of the connected nanostructures identified in the pyramids, which constitute a complex, but controlled, barrier environment for the quantum dots.

Structure and Photoluminescence of Single AlGaAs/GaAs Quantum Dots Grown in Inverted Tetrahedral Pyramids

Arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition in inverted tetrahedral pyramids on {111}B GaAs substrates are investigated. Cross-sectional atomic force microscopy images evidence a pronounced thickening of the GaAs quantum well layer at the tip of the pyramid, giving rise to a lens-like QD structure. Low-temperature photoluminescence and cathodoluminescence spectra show distinct luminescence from the dots, exhibiting filling of QD states separated by 33 meV at increased carrier densities. Luminescence linewidths of 15 meV and line energy variations of less than 5 meV are obtained across mm2 sample areas.

Carrier transport and luminescence in inverted-pyramid quantum structures

Cathodoluminescence spectroscopy and wavelength-dispersive imaging were employed for investigating the carrier transport and recombination in GaAs/AlGaAs inverted-pyramid quantum dot (QD) heterostructures grown on patterned (111)B GaAs substrates. The spectra and images clearly evidence carrier recombination in quantum wells and quantum wires (QWR) and show potential variations in these structures. Luminescence from the lens-shaped QDs was identified and characterized as a function of the GaAs layer thickness. Furthermore, we show a tapering of the GaAs QWR that self-forms at the corners of the pyramids. Application of such tapered QWRs as “exciton accelerators” is discussed.

Optical Spectra of Single Quantum Dots: Influence of Impurities and Few-Particle Effects

Reference LPN-ARTICLE-2000-012doi:10.1002/1521-396X(200003)178:1 3.0.CO;2-MView record in Web of Science Record created on 2008-02-29, modified on 2017-05-12

Influence of the Shallow Impurity Environment on the Luminescence of Single Quantum Dots

Abstract The evolution of low-temperature photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed. In this weak excitation regime, a significant difference in the fine structure of single-QD spectra is observed not only among different, structurally identical QDs of the same sample, but also among spectra taken from the same single QD excited above and below the AlGaAs barrier. A time-resolved, two-color pump and probe PL experiment on a single QD indicates relaxation times between the different spectral shapes in the ms-range. A model, taking into account the influence of the shallow impurities in the environment of each QD, explains the experimental results.

Observation of Charged Few-Particle States in the Optical Spectra of Single Semiconductor Quantum Dots

A detailed discussion of the optical properties of single n-type modulation-doped semiconductor quantum dots is presented. We use the photo-depletion/back-hopping mechanism to optically control the number of surplus electrons in the dot. Comparison of the experimental data with calculated luminescence spectra, obtained within a direct-diagonalization approach for calculating electron-hole states, allows the identification of luminescence from the decay of up to fivefold charged excitons. Finally, we study the influence of fluctuating local electric fields, produced by ionized impurities in the surrounding of the dots, on the optical selection rules and on the linewidth of the emission peaks.

Carrier Capture and Recombination Dynamics in a Single Pyramidal Quantum Dot

Reference LPN-ARTICLE-2001-002doi:10.1002/1521-3951(200103)224:2 3.0.CO;2-6View record in Web of Science Record created on 2008-02-29, modified on 2017-05-12

Dynamic dipole-dipole interactions between excitons in quantum dots of different sizes

A model of the resonance dynamic dipole–dipole interaction between excitons confined in quantum dots (QDs) of different sizes at close enough distance is given in terms of parity inheritance and exchange of virtual photons. Microphotoluminescence spectra of GaAs–AlGaAs coupled QDs are proposed to be analyzed by this model, including features created by high-speed random switching, depending on the carrier configuration in and around the QD pair, between the dipole–dipole split states and the nonsplit states to give double peaks at both of the QDs.

Inverse ray-tracing method for nondestructive mapping of three-dimensional surfaces

A general method for nondestructive mapping of three-dimensional surfaces, particularly useful in cases where the surface is composed of a large array of identical depressions, is presented. The sample is first marked with a uniform flux of particles, which then act as isotropic sources of some detectable radiation. By measuring the emitted radiation from several directions, it is possible to reconstruct the surface using a parametrization coupled with a least squares fit procedure. As an example, we report on the experimental implementation of the algorithm to arrays of micron-scale inverted pyramids etched in GaAs substrates, using a particular sequence of alpha emitting atoms.