A. A. Quivy

InAs/GaAs quantum dots optically active at 1.5 μm

InAs quantum dots grown by molecular-beam epitaxy on GaAs substrates are demonstrated to be suitable structures to achieve an optical emission in the 1.3–1.5-μm range. Their tuning towards such long wavelengths was made possible by combining an extreme reduction of the InAs growth rate and a fast growth of the GaAs cap layer at low temperature. Our results create perspectives for the fabrication of GaAs-based devices operating in the most important telecommunications window.

Influence of the temperature and excitation power on the optical properties of InGaAs/GaAs quantum wells grown on vicinal GaAs(001) surfaces

Photoluminescence experiments were performed as a function of temperature and excitation intensity in order to investigate the optical properties of In0.1Ga0.9As/GaAs quantum wells grown on vicinal GaAs(001) substrates with different miscut angles. The misorientation of the surface played an important role and influenced the intensity, efficiency, energy, and full width at half maximum of the optical emission, as well as the segregation of indium atoms. It is shown that at high temperature the optical properties of InGaAs quantum wells grown on vicinal substrates are slightly inferior to ones of the same structure grown a nominal surface because of the faster escape of the carriers.

Influence of the temperature on the carrier capture into self-assembled InAs/GaAs quantum dots

Photoluminescence (PL) spectroscopy and atomic-force microscopy (AFM) were used to investigate the size evolution of InAs quantum dots on GaAs(001) as a function of the amount of InAs material. Different families of islands were observed in the AFM images and unambiguously identified in the PL spectra, together with the signal of the wetting layer. PL measurements carried out at low and intermediate temperatures showed a thermal carrier redistribution among dots belonging to different families. The physical origin of this behavior is explained in terms of the different temperature dependence of the carrier-capture rate into the quantum dots. At high temperatures, an enhancement of the total PL-integrated intensity of the largest-sized quantum dots was attributed to the increase of diffusivity of the photogenerated carriers inside the wetting layer.

Real-time determination of the segregation strength of indium atoms in InGaAs layers grown by molecular-beam epitaxy

The surface segregation of indium (In) atoms was investigated during the growth of InGaAs layers by reflection high-energy electron diffraction (RHEED). We observed that the decay constant of the RHEED-oscillation amplitude during growth depends on the growth conditions and is related, in a very simple way, to the segregation coefficient of the In atoms in the InGaAs layers.

The formation of self-assembled InAs∕GaAs quantum dots emitting at 1.3μm followed by photoreflectance spectroscopy

Photoreflectance (PR) modulation spectroscopy, supported by photoluminescence (PL) and atomic force microscopy, was applied to the study of the optical properties of InAs∕GaAs structures at the transition from the typical two-dimensional epitaxial growth to three-dimensional Stranski-Krastanov mode of InAs self-assembled quantum dot (QD) formation. Room temperature photoreflectance was measured on several molecular-beam epitaxy (MBE) grown structures, with growth conditions optimized for the 1.3μm emission (an important window for telecommunication applications), differing in the nominal thickness of InAs layer from 1 to 2.5 ML (monolayer). The evolution of optical transitions connected with energy levels confined in a very thin InAs∕GaAs quantum well was observed. For a small InAs nominal layer thickness (up to the critical thickness for the formation of three-dimensional islands), the heavy (light)-hole level to electron level transitions shift towards lower energy, indicating the increase in the quantu...

Influence of indium segregation on the RHEED oscillations during the growth of InGaAs layers on a GaAs(001) surface

The surface segregation of indium (In) atoms was investigated during the growth of InGaAs layers by reflection high-energy electron diffraction (RHEED). We showed that the strong damping of the RHEED oscillations usually observed during the deposition of InGaAs on GaAs was directly related to the presence of a population of In atoms at the surface of the sample originating from the segregation phenomenon in the InGaAs layers. We proposed a simple model to estimate the segregation coefficient R in situ and in real time from the RHEED oscillations. Our results were quantitatively confirmed by several RHEED measurements carried out under very different growth conditions and were in excellent agreement with data from the literature.

The influence of different indium-composition profiles on the electronic structure of lens-shaped InxGa1-xAs quantum dots

We present effective-mass calculations of the bound-state energy levels of electrons confined inside lens-shaped InxGa1−xAs quantum dots (QDs) embedded in a GaAs matrix, taking into account the strain as well as the In gradient inside the QDs due to the strong In segregation and In-Ga intermixing present in the InxGa1−xAs/GaAs system. In order to perform the calculations, we used a continuum model for the strain, and the QDs and wetting layer were divided into their constituting monolayers, each one with a different In concentration, to be able to produce a specific composition profile. Our results clearly show that the introduction of such effects is very important if one desires to correctly reproduce or predict the optoelectronic properties of these nanostructures.

Optical response at 1.3 μm and 1.5 μm with InAs quantum dots embedded in a pure GaAs matrix

We demonstrated that the growth of InAs on GaAs at rates as low as 0.003 ML/s can be used to form quantum dots (QDs) that are optically active at 1.3 and 1.5 μm. The emission at 1.5 μm (at 300 K) originating from individual InAs QDs embedded in a pure GaAs matrix is close to the important telecom window at 1.55 μm. Such emission is related to a narrow distribution of islands with a height peaked around 15 nm as shown by atomic-force microscopy.

Correlation between luminescence properties of AlxGa1−xAs∕GaAs single quantum wells and barrier composition fluctuation

The luminescence mechanism at low temperatures in AlxGa1−xAs∕GaAs single quantum wells grown by molecular-beam epitaxy with different aluminum concentrations in the barrier has been studied in detail using the photoluminescence spectroscopy (PL) as function of temperature (8K⩽T⩽90K) combined with the excitation intensity. The asymmetry presented by the PL spectra at the low-energy side, the blueshift behavior of the PL peak energy, and the PL line broadening with increasing temperature are explained through the exciton localization in confinement potential fluctuations. The exciton localization effects on the PL spectra are progressively reinforced with the increase of the Al concentration in the barrier constituent material. The PL peak energy dependence on temperature has been fitted through the expression proposed by Passler [Phys. Status Solidi B 200, 155 (1997)] adapted to systems with potential fluctuations, by subtracting the term σE2∕kBT, where σE is the standard deviation of the potential fluctua...

Ex-situ investigation of indium segregation in InGaAs/GaAs quantum wells using high-resolution x-ray diffraction

Calculations using the dynamical theory of diffraction together with a sample model which considers the segregation of indium atoms were employed to fit the high-resolution x-ray spectra of strained InGaAs/GaAs quantum wells grown by molecular-beam epitaxy. The segregation coefficients obtained from the best fits to the experimental data of samples grown at different temperatures are in excellent agreement with the expected values and confirm that x-ray diffraction is a valuable tool for the investigation of the segregation phenomenon.