L. Bouzaïene

Improvement of the electron density in the channel of an AlGaAs/GaAs heterojunction by introducing Si δ doping in the quantum well

The electronic subband of δ-doped AlGaAs/GaAs heterostructure has been studied theoretically by the finite differential method. We use an efficient self-consistent analysis to solve simultaneously the Schrodinger and Poisson equations. The results show the possibility to increase the electron density in the channel by the introduction of the silicon δ doping in a quantum well, where the Al concentration is smaller than in the barrier. The effect of the quantum well width is studied on the electron density. To test the validity of our calculation, we have grown, by molecular beam epitaxy, a series of δ-doped AlGaAs/GaAs heterojunctions having various alloy compositions seen by the silicon. If we consider the spreading of the silicon in space during the δ-layer growth, we show that the theoretical model explains well the experimental Hall data for all aluminum concentrations.

Optical anisotropy in self-assembled InAs nanostructures grown on GaAs high index substrate

We present a study of the optical properties of InAs self-assembled nanostructures grown by molecular beam epitaxy on GaAs(11N)A substrates (N = 3-5). Photoluminescence (PL) measurements revealed good optical properties of InAs quantum dots (QDs) grown on GaAs(115)A compared to those grown on GaAs(113)A and (114)A orientations substrate. An additional peak localized at 1.39 eV has been shown on PL spectra of both GaAs(114)A and (113)A samples. This peak persists even at lower power density. Supporting on the polarized photoluminescence characterization, we have attributed this additional peak to the quantum strings (QSTs) emission. A theoretical study based on the resolution of the three dimensional Schrödinger equation, using the finite element method, including strain and piezoelectric-field effect was adopted to distinguish the observed photoluminescence emission peaks. The mechanism of QDs and QSTs formation on such a high index GaAs substrates was explained in terms of piezoelectric driven atoms and the equilibrium surfaces at edges.

Optical investigation of InAs quantum dots inserted in AlGaAs/GaAs modulation doped heterostructure

Optical properties of InAs quantum dots (QDs) inserted in AlGaAs/GaAs modulation doped heterostructure are investigated. To study the effect of carrier transfer behavior on the luminescence of self-assembled quantum dots, a series of sample has been prepared using molecular beam epitaxy (Riber 32 system) in which we have varied the thickness separating the delta dopage and the InAs quantum dots layer. Photoluminescence spectra show the existence of two peaks that can be attributed to transition energies from the ground state (E1-HH1) and the first excited state (E2-HH2). Two antagonist effects have been observed, a blue shift of the emission energies result from electron transferred from the AlGaAs/GaAs heterojunction to the InAs quantum dots and a red shift caused by the quantum confined Stark effect due to the internal electric field existing In the AlGaAs/GaAs heterojunction.

Temperature dependence of optical properties of InAs/GaAs self-organized quantum dots

Self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy. The photoluminescence, its power, and temperature dependences have been studied for the ensembles of InAs QDs embedded in GaAs matrix to investigate the interband transition energies. Theoretical calculations of confined electron (heavy-hole) energy in the InAs/GaAs QDs have been performed by means of effective mass approximation, taking into account strain effects. The shape of the InAs QDs was modeled to be a convex-plane lens. The calculated interband transition energies were compared with the results of the photoluminescence spectra. The calculated interband transition energy from the ground electronic subband to the ground heavy-hole state was in reasonable agreement with the transition energy obtained by the photoluminescence measurement.

Effect of carriers transfer behavior on the optical properties of InAs quantum dots embedded in AlGaAs/GaAs heterojunction

In this paper, we have investigated the optical properties of InAs quantum dots (Qds) embedded near the channel of a delta-doped AlGaAs/GaAs high electron mobility transistor. In order to study the influence of the two-dimensional electron gas (2DEG) on the luminescence of QDs, we have prepared different structures in which we varied the thickness (d) separating the interface of AlGaAs/GaAs heterojunction from the InAs quantum dot layer. Various photoluminescence (PL) behaviors are observed when d decreases. PL spectra show the existence of two peaks which can be attributed to transition energies from the ground state (E1-HH1) and the first excited state (E2-HH2). A blueshift, a decrease in the PL intensity and an increase in the full width at half maximum of the PL peaks are observed, when the InAs QDs layer is closer to the 2DEG.

Broken symmetry in laterally coupled InAs/GaAs quantum dots molecule

Laterally coupled quantum dot (QD) structures (artificial molecules) are investigated using one layer composed of InAs islands grown by molecular beam epitaxy. We have elaborated, at different InAs growth rates, a series of samples. Those elaborated at lower InAs growth rate present a weak and inhomogeneous QD density. The atomic force microscopy images show that the QDs are gathered by pair having a variable interdot separation. This is what enables us to study the lateral coupling. At 10 K, microphotoluminescence (micro-PL) measurements (realized on a design of 0.2 μm diameters) present either one, two, or four peaks. To understand the origin of these peaks, we have calculated the excitonic transitions of a system of two identical laterally coupled QDs according to the distance that separates them. The results of this model do not explain PL spectra. Then, we have considered a system of two QDs having slightly different sizes (asymmetric system). We have shown that when the interdot distance is ranging ...

Sensitivity enhancement of AlGaAs∕InGaAs∕GaAs quantum well-based Hall device

We have investigated the consequences of a modified design of modulation doping pseudomorphic AlGaAs∕InGaAs∕GaAs and we propose a quantum well structure for a Hall device with the goal of improving its performances. From self-consistent calculations we find that the electron concentration ns in the interface region is increased. This implies that one can have a wider spacer layer and still have the same ns with the result that the mobility is improved. This result should be valuable for many types of devices. We specifically consider Hall sensors, where it is desirable to have a low electron concentration and a high mobility.

Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength

With the help of photoluminescence Spectroscopy (PLS), we have investigated the optoelectronic properties of two different families of InAs quantum dashes (QDashes) grown on misoriented InP(001) substrate with 2∘off miscut angle toward the [110] direction (2∘F type). The lowest full width at half maximum (FWHM) of the PL spectrum measured at 12 K indicates the good self organization of InAs QDashes. The weak ratio of the integrated PL measured in 12–300 K temperature range denotes the good spatial confinement of the photogenerated carriers in InAs QDashes. The fast redshift of the PL peaks energy and the anomalous decrease of the FWHM with the increase of the temperature are attributed to an efficient thermal relaxation process of photogenerated carriers in the vicinal sample. This result is highlighted with the help of theoretical modeling of the PL peak energy as a function of the temperature, using three models (Varshni, “Vina, Logothetidis and Cardona” and Passler). From experimental and theoretical results, we have evidenced the contribution of longitudinal acoustic-phonons (LA-phonons) in the PL of InAs/InP QDashes, via the deformation potential, especially in high temperatures range. We have attributed this behavior to the strained InAs/InP QDashes and/or to the topography of the vicinal InP(001) substrate which favors the presence of stepped phonons polarized along the steps. These vibrational modes can further interact with the excitons at high temperatures. The measured thermal activation energies of each family of InAs QDashes demonstrate that the InAs wetting layer act as a barrier for the thermoionic emission of photogenerated carriers. This result confirms the good spatial confinement of excitons in this sample.

Analysis and design of microwave devices with inhomogeneous and stratified dielectric layers at oblique incidence

Abstract. In this paper, we explore the effects of polarization, incidence angle, layer thickness, and permittivity profiles on reflection and transmission characteristics. Hill’s method used in this work is an efficient tool to obtain an analytical solution of the propagation equations and can be applied to all arbitrary profiles of permittivity and permeability. Using Hill’s method and an arbitrary profile of permittivity, numerical simulations for both polarizations transverse electric and transverse magnetic waves are performed to compute the reflection and transmission coefficients of spectrum analyzer, frequency selective surfaces (FSS), and electromagnetic bandgap (EBG) structures having optimized thickness.

Strain Effects on Optical Properties of (In,Ga)As-Capped InAs Quantum Dots Grown by Molecular Beam Epitaxy on GaAs (113)A Substrate

We have investigated the optical properties of InAs/GaAs (113)A quantum dots grown by molecular beam epitaxy (MBE) capped by (In,Ga)As. Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A broadening of the PL emission due to size distribution of the dots, when InAs dots are capped by GaAs, was observed. A separation between large and small quantum dots, when they are encapsulated by InGaAs, was shown due to hydrostatic and biaxial strain action on large and small dots grown under specifically growth conditions. The PL polarization measurements have shown that the small dots require an elongated form, but the large dots present a quasi-isotropic behavior.