M. Grassi Alessi

Stokes shift in quantum wells: Trapping versus thermalization

Low temperature photoluminescence and photoluminescence excitation measurements have been performed in a set of

Self-aggregated InAs quantum dots in GaAs

{\mathrm{In}}_{\mathit{x}}

InxGa1_xAs/GaAs interfaces: from2D islands to quantum dots

{\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}

Carrier Transfer between InGaAs/GaAs Quantum Wells Separated by Thick Barriers

As/GaAs samples with a different indium molar fraction, well width, growth conditions, and post-growth treatment. This has allowed to change in a controlled way the degree and source of disorder in the samples, thus resulting in an excitonic absorption linewidth varying between 1 and 18 meV, and an ensuing Stokes shift changing between zero and 8 meV. The conditions of validity of two different models relating the Stokes shift to the linewidth broadening have been established in terms of different regimes of disorder and temperature. A continuous transition between those regimes has been demonstrated. \textcopyright{} 1996 The American Physical Society.

Carrier thermal escape and retrapping in self-assembled quantum dots

We present a detailed study, both structural and optical, of GaAs/InAs/GaAs heterostructures for InAs nominal coverages (L) ranging from 0.6 to 3 ML. Planar transmission electron microscopy (TEM) provides direct evidence of the presence of InAs quantum dots (QDs) for all values of L, with an increase in their density at high values of L. Transverse TEM shows also that those QDs have mostly small base angles. Accordingly, the evolution of the optical properties of InAs/GaAs is investigated by photoluminescence (PL) and PL excitation measurements (PLE). A broad PL band is observed in all samples, which is ascribed to the recombination of heavy-hole excitons in the InAs quantum dots, observed with TEM. For thin coverages (L⩽1.6ML), a narrow PL band is also observed, which is attributed to recombination of heavy-hole excitons in a two-dimensional (2D) InAs layer. The two bands shift to lower energy for increasing L. For L⩾1.6 ML, the QD band has a faster shift and exhibits a complex structure, while the excit...

OPTICAL PROPERTIES OF INAS QUANTUM DOTS : COMMON TRENDS

The transfer of carriers between two adjacent InxGa1−xAs wells with different thicknesses and separated by a thick GaAs barrier has been measured as a function of temperature for different indium concentrations x. The efficiency of the carrier transfer has been determined by photoluminescence excitation measurements. It is roughly constant at low temperature, and increases for increasing temperatures, going through a maximum between 50 and 70 K. At higher temperatures, where the photoluminescence shows a drastic quenching, also the carrier transfer efficiency decreases rapidly. The thermal escape of carriers out of the narrow well mediates the transfer, quenched at higher temperatures by an increased role of nonradiative recombination centres in the well and/or in the barrier. A comparison with a simple rate equation model supports an ambipolar escape of carriers in cw measurements, a unipolar escape of carriers in time-resolved measurements. This is explained in terms of the different excitation conditions in two types of experiments.

Optical spectroscopy of quasimonolayer InAs at the onset of quantum-dot nucleation

SummaryIn this paper we try to give a unified picture of InxGa1-xAs/GaAs quantum systems (0 ≤x ≤ 1), as deduced primarily by optical measurements at 10 K. This has been made possible by the systematic investigation of photoluminescence and photoluminescence excitation of InGaAs/GaAs strained heterostructures differring for indium molar fraction and well width. The large number of samples used has allowed us to change in a controlled way the degree and source of disorder in the samples. The evolution from a two-dimensional growth, with formation of InGaAs and GaAs interconnected two-dimensional islands at the interfaces, to a three-dimensional growth, with self-aggregation of InGaAs dots, has been monitored, also with the help of structural measurements.

Competition between radiative decay and energy relaxation of carriers in disorderedInxGa1−xAs/GaAsquantum wells

We have studied the carrier transfer between two adjacent wells of different width separated by a 100 nm thick barrier. At low temperatures (T 30 K, the transfer strongly increases, denoting that the thermal escape out of the QW becomes the main mechanism. Finally, a drastic quenching of the transfer is found for temperatures higher than 50 to 70 K, suggesting the activation of a further non-radiative channel different from the thermal escape.