A. Vinattieri

Controlled tuning of the radiative lifetime in InAs self-assembled quantum dots through vertical ordering

Multilayer structures of InAs quantum dots have been studied by means of photoluminescence techniques. A strong increase of the radiative lifetime with increasing number of stacked dot layers has been observed at low temperatures. Moreover, a strong temperature dependence of the radiative lifetime, which is not present in the single layer samples, has been found in the multistacked structures. The observed effects are nicely explained as a consequence of the electronic coupling between electrons and holes induced by vertical ordering.

Thermal ionization of excitons in GaAs/AlGaAs quantum well structures

Photoluminescence and photoluminescence excitation spectra have been performed on GaAs/AlGaAs quantum well structures in the temperature range 4–300 K. Sharp exciton resonances are present up to room temperature and can be ascribed to localized excitons for T≤50–70 K and to free excitons at higher values of T. Nevertheless, a line‐shape analysis of the PL spectra clearly shows the presence of band‐to‐band recombination. A fit based on a simple statistical model reproduces with high accuracy the photoluminescence spectrum line shape and allows to evaluate the relative densities of excitons and free carriers generated by the excitondissociation. We find that the ratios of the relative densities can be interpreted on the basis of the law of mass action for describing the thermal equilibrium between excitons,electrons, and holes.

Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy

We realized ultra-narrow excitonic emission from single GaAs/AlGaAs quantum dots (QDs) grown by a refined droplet epitaxy technique. We found that uncapped quantum dots can be annealed at 400 degrees C without major changes in their morphology, thus enabling an AlGaAs capping layer to be grown at that temperature. Consequently, we demonstrate a fourfold reduction of the linewidth of the emission together with an increased recombination lifetime, compared to the conventional droplet epitaxial QDs. The averaged linewidth of neutral excitons measured by micro-photoluminescence on single quantum dots was around 35 microeV.

GaAs solid state detectors for particle physics

Abstract We report on progress with Schottky diode and p-i-n diode GaAs detectors for minimum ionising particles. The radiation hardness and potential speed of simple diodes is shown to be more than competitive with silicon detector. A discussion is given of the present understanding of the charge transport mechanism in the detectors since it influences their charge collection efficiency. Early results from microstrip detectors are also described.

Spectral diffusion and line broadening in single self-assembled GaAs∕AlGaAs quantum dot photoluminescence

We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs∕AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment.

Volume versus surface-mediated recombination in anatase TiO2 nanoparticles

We present an experimental study of the radiative recombination dynamics in size-controlled anatase TiO2 nanoparticles in the range 20–130 nm. From time-integrated photoluminescence spectra and picosecond time-resolved experiments as a function of the nanoparticle size, excitation density, and temperature, we show that photoluminescence comes out from a bulk and a surface radiative recombination. The spectral shift and the different time dynamics provide a clear distinction between them. Moreover, the intrinsic nature of the emission is also proven, providing a quantitative evaluation of volume and surface contributions.

Reduction of the internal electric field in wurtzite a-plane GaN self-assembled quantum dots

We present a study of the emission of GaN∕AlN self-assembled quantum dots grown on a-plane 6H-SiC showing evidence of the suppression of the internal electric field. The strain in dots and barriers is determined by means of Raman scattering and the induced piezoelectric polarizations are estimated. These reveal a compensation of the spontaneous polarization and justify the lack of a quantum confined Stark effect found in the photoluminescence spectra. Strain effects and strong confinement are responsible for the partial depolarization of the emission and its energy dependence.

Picosecond dynamics of resonantly-excited excitons in GaAs quantum wells

Abstract We have simultaneously investigated various processes contributing to the initial dynamics of resonantly-excited excitons in quantum wells. A unified analysis of the results provides quantitative information on different scattering rates and exciton recombination rate. Our results clearly show the important role of enhanced recombination of exciton-polaritons in quantum wells and provide new insights into the initial dynamics of excitons.

Temperature Dependence of Exciton Lifetimes in GaAs/AlGaAs Quantum Well Structures

The photoluminescence decay times in GaAs/AlGaAs single quantum well structures with layer thickness between 20 A and 80 A have been investigated in the temperature range from 4 K up to room temperature. It is shown that the increase in the PL decay time usually ascribed to radiative free-excitons recombination is present even in conditions where other mechanisms such as recombination of localized excitons and free carriers, exciton ionization and thermal activation of nonradiative processes are effective.

Poissonian statistics of excitonic complexes in quantum dots

We report a detailed experimental investigation of the power dependence of excitonic complexes (neutral exciton, neutral biexciton, and charged exciton) confined in single self-assembled GaAs∕AlGaAs strain-free quantum dots grown by droplet epitaxy. By using the random population theory we show that, under stationary excitation, the power dependence of the excitonic complexes precisely follows the Poissonian statistics. This result allows us to determine with great accuracy the state filling condition of the quantum dots (QDs) and therefore to estimate the capture volume of the QDs.