P. Frigeri

Thermally activated carrier transfer and luminescence line shape in self-organized InAs quantum dots

We investigated the temperature dependence (10–180 K) of the photoluminescence (PL) emission spectrum of self‐organized InAs/GaAs quantum dots grown under different conditions. The temperature dependence of the PL intensity is determined by two thermally activated processes: (i) quenching due to the escape of carriers from the quantum dots and (ii) carrier transfer between dots via wetting layer states. The existence of different dot families is confirmed by the deconvolution of the spectra in gaussian components with full width half maxima of 20–30 meV. The transfer of excitation is responsible for the sigmoidal temperature dependence of the peak energies of undeconvoluted PL bands.

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.

Quantum dot strain engineering for light emission at 1.3, 1.4 and 1.5μm

We designed and prepared by molecular beam epitaxy strain-engineered InAs∕InGaAs∕GaAs quantum dot (QD) nanostructures where we separately controlled: (i) the mismatch f between QDs and confining layers (CLs), and, then, the QD strain, by changing the thickness of a partially relaxed InGaAs lower CL and (ii) the CL composition x. The appropriate values of f and x to tune the emission energies at wavelengths in the 1.3–1.55μm range were calculated by means of a simple model. Comparing model calculations and activation energies of photoluminescence quenching, we also concluded that quenching is due to both intrinsic and extrinsic processes; we show that the structures can be designed so as to maximize the activation energy of the intrinsic process, while keeping the emission energy at the intended value in the 1.3–1.55μm range.

Growth patterns of self-assembled InAs quantum dots near the two-dimensional to three-dimensional transition

Self-assembled InAs quantum dots have been grown by molecular beam epitaxy in such a way as to obtain a continuous variation of InAs coverages across the wafer. Structured photoluminescence spectra are observed after excitation of a large number of dots; deconvolution into Gaussian components yields narrow emission bands (full width at half-maximum 20–30 meV) separated in energy by an average spacing of 30–40 meV. We ascribe the individual bands of the photoluminescence spectra after low excitation to families of dots with similar shapes and with heights differing by one monolayer, as strongly supported by numerical calculations of the fundamental electronic transitions in quantum dot structures.

Efficient room temperature carrier trapping in quantum dots by tailoring the wetting layer

The temperature dependence of carrier confinement in states of self-assembled In0.5Ga0.5As quantum dots (QDs) embedded in AlyGa1−yAs barriers has been investigated by means of photoluminescence (PL) measurements. We show that photoexcited carriers above the AlGaAs barriers have two recombination channels that contribute to the temperature quenching of the PL from QDs: (a) carrier losses in the AlGaAs layers during the relaxation process and (b) thermal evaporation of captured carriers out of QDs. The interplay between these two mechanisms determines the behavior of the nonresonantly excited photoluminescence as a function of temperature. Eliminating the first contribution by using resonant excitation of the QD PL, we demonstrate a definite enhancement of the carrier confinement at room temperature in InGaAs/AlGaAs QDs by increasing the Al content. We show that this effect is related to the increase in the energy separation between the electronic states in the QD and the wetting layer.

Carrier transfer and photoluminescence quenching in InAs/GaAs multilayer quantum dots

A detailed study of the carrier transfer and photoluminescence (PL) quenching in stacked InAs/GaAs quantum dots (QDs) is presented. Vertically aligned QD structures, grown by atomic layer molecular beam epitaxy, with different numbers N of dot planes and different spacer thicknesses (d) were prepared and studied. The dependencies of carrier transfer from the GaAs barriers to the InAs QDs and of the PL quenching channels on the design parameters N and d have been identified by performing continuous wave and time-resolved PL measurements. We have found that both the radiative recombination and capture efficiency into the QDs are reduced by increasing N and by decreasing d, as a consequence of the deterioration of the interdot GaAs spacers induced by stacking.

Electronic Coupling Effects on the Optical Properties and Carrier Dynamics of InAs Quantum Dots

Vertically aligned InAs/GaAs quantum dot structures were investigated. They were grown by atomic layer molecular beam epitaxy, with 10 layers and wedged spacer thickness d varying between 7.7 and 12.5 nm at steps of 0.2 nm. The effects of electronic coupling on the fundamental transition energy and decay dynamics were studied by time-resolved and continuous-wave photoluminescence (PL). The energy of the PL peak position decreases with increasing d, while the corresponding lifetime increases with d. This is attributed to the interplay of the electronic delocalization along the column and the Coulomb interaction between electrons and holes. Furthermore, a bell-shaped behaviour of the PL lifetime as a function of emission energy was observed inside the inhomogeneously broadened PL band.

Study of GaAs spacer layers in InAs/GaAs vertically aligned quantum dot structures

We investigated vertically aligned InAs/GaAs QD structures, grown by atomic layer molecular beam epitaxy, with a number N of layers and with spacer thicknesses d. QD alignment and structure quality were checked by transmission electron microscopy. The dependencies of carrier capture, decay dynamics and existence of quenching channels on the design parameters N and d were studied by time resolved photoluminescence (PL), PL excitation (PLE) and PL temperature-dependent measurements. Our results show that the carrier capture and the radiative efficiency of the QDs are negatively affected by increasing the number of QD layers and by reducing the spacer thicknesses; this effect is likely to be related to the increase of defect concentrations in GaAs spacers, due to relaxation of an increasingly large strain.

Characterization of hydrogen passivated defects in strain-engineered semiconductor quantum dot structures

The effects of hydrogen incorporation on carrier relaxation and recombination efficiencies in a large series of InAs self-assembled quantum dot structures deposited on InGaAs lower confining layers with different thicknesses and compositions have been addressed. With increasing H dose we observe an improvement in the radiative efficiency. By comparing steady state and time resolved photoluminescence measurements, it is established that the H passivation does not enhance the relaxation and capture efficiencies, but instead directly improves the emission yield from carriers in the dots. We therefore conclude that the H-passivated defects are located nearby, or even inside, the dots.

Vertical coupling effects in arrays of InAs quantum dots

We show that either a blueshift or a redshift of the fundamental transition energy can be observed in vertical arrays of InAs/GaAs quantum dots, depending on the spacer thicknesses, and explained by including strain, indium segregation, and Coulomb effects.