A. Patanè

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}}

Photoinduced structures in the exciton luminescence spectrum of InGaAs/GaAs quantum well heterostructures

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

Binding Energy and Lifetime of Excitons in InxGa1—xAs/GaAs Quantum Wells

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.

Excitation transfer through thick barriers in asymmetric double-quantum-well structures

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...

InxGa1_xAs/GaAs interfaces: from2D islands to quantum dots

Photoreflectance measurements have been performed in a number of InAs/GaAs single-quantum wells with nominal thickness L ranging from 0.6 to 2.0 ML. The InAs growth mode was investigated by analyzing the evolution, with increasing coverage, of the optical response associated with the InAs layer. For L⩽1.6 ML, the experimentally derived energies for the optical transition originating in the InAs are consistent with those evaluated in a simple square-well envelope-function scheme. The dependence of the photoreflectance line shape broadening on L is well described up to L=1.4 ML in terms of a disordered InAs/GaAs interface made by interconnected InAs and GaAs islands with a typical size of order 2 nm. For L=1.6 ML, the quantum well spectral features broaden abruptly and vanish for L=2 ML, suggesting the disappearance of the InAs 2D layer in favor of a predominant nucleation of large quantum dots.

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

A large set of InGaAs/GaAs quantum well structures was investigated by means of continuous wave photoluminescence(PL) and photoluminescence excitation spectroscopy. Strong photomodulation effects are observed in PL, namely, a strong sensitivity to the excitation energy and strong changes in the line shape when resonant and nonresonant excitations are used together. Correspondingly, the exciton emission exhibits a doublet structure and the excitation spectra, as detected by monitoring the emission at the two peak energies of the PL doublet, show quite different profiles, with peaks and/or dips not directly related to absorption resonances. On the grounds of time‐resolved experiments it is shown that band‐bending modifications, due to trapping of free carriers at interface defects, account for the observed photomodulation.

Excitation energy dependence of the optical properties of InGaAs/GaAs quantum well heterostructures

We report a systematic study of exciton binding energies and lifetimes in InGaAs/GaAs quantum wells. The experimental binding energies have been deduced from photoluminescence excitation measurements taking into account the contribution of the 2s state of the exciton and the line broadening. The experimental results have been compared with accurate calculations in a four-band model, where exciton energies take into account the polaron correction. The theory accounts for all the experimental observations and provides a good quantitative agreement with the experimental values.

Linewidth analysis of the photoluminescence of InxGa1-xAs/GaAs quantum wells (x=0.09, 0.18, 1.0).

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.