Giant backscattering resonances in the magneto-resistance of GaAs/AlGaAs quantum dots

We discuss the observation of large resonant features, superimposed upon the quantum Hall plateaux of gated GaAs/AlGaAs quantum dots. The resonances correspond to a magnetically induced increase in the edge state backscattering, and under certain conditions can imply a complete reflection of the applied current. We demonstrate that the resonances are correlated to the depopulation of bulk Landau levels, and suggest they result from an increase in backscatterlng via confined Landau levels, as the latter depopulate in a magnetic field. The resonances are therefore analogous to the Shubnikov-de Haas oscillations, observed in two dimensional electron gas systems, and their temperature dependence is found to take the same functional form. We argue that the resonances are an intrinsic feature of edge state transport in quantum dots, since they result from scattering via Landau levels, controllably confined within the dot, and discuss our results in relation to recent theoretical and experimental studies, of edge state transport in small wires and dots.     

Electron dephasing of a GaAs/AlGaAs quantum well with self-assembled InAs dots

We study the magnetotransport of a GaAs/AlGaAs quantum well with self-assembled InAs quantum dots. Negative magnetoresistance is observed at low field and analysed by weak localization theory. The temperature dependence of the extracted dephasing rate is linear, which shows that the inelastic electron-electron scattering processes with small energy transfer are the dominant contribution in breaking the electron phase coherence. The results are compared with those of a reference sample that contains no quantum dots.     

AlGaAs capping effect on InAs quantum dots self-assembled on GaAs

The effects of AlGaAs capping on InAs quantum dots self-assembled on GaAs are investigated. It is observed that, the photoluminescence intensity becomes stronger up to twice when Al is incorporated into the cap layer. In the mean time, the full width at half maximum of the photoluminescence spectrum becomes narrower, the peak splitting between the ground and first excited exciton levels becomes wider, and the photoluminescence peak wavelength becomes longer. With considerations of the increased barrier height and the changed microstructures of the quantum dots induced by AlGaAs capping, the mechanisms of the observed improvements are discussed.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Optical properties of individual GaAs quantum dots embedded into AlGaAs nanowires

The photoluminescence (PL) spectra of semiconductor structures, namely, Al_0.3Ga_0.7As-based quasi-one-dimensional cylindrical nanowires (nanowhiskers), are measured. The diameter of a typical nanowire is 20–50 nm, and its length was 0.5–1.0 μm. Samples containing one or several GaAs-based quantum dots at the center of the quantum wire are studied. The dot thickness is 2 nm, and the dot diameter is 15–40 nm. Individual nanowhiskers, several nanowhiskers (3–4), and ensembles consisting of many nanowhiskers are studied. The PL spectra are measured for different optical-excitation intensities and in magnetic fields of up to 11 T.     

Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock–Darwin levels. In QDs with very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified.     

Carrier cooling and recombination heating in intermixed GaAs/AlGaAs quantum wires and dots

Il Nuovo Cimento D - The cooling of photoexcited hot carriers in 2D, 1D and 0D systems is studied experimentally. In comparison with a theoretical carrier relaxation model which holds for 2D and 1D...     

High-frequency magneto-oscillations in GaAs/AlGaAs quantum wells

We have observed very high-frequency, highly reproducible magneto-oscillations in modulation doped GaAs/AlGaAs quantum well structures. The oscillations are periodic in an inverse magnetic field (1/B) and their amplitude increases with temperature up to T approximately 700 mK. Being initially most pronounced around the filling factor nu=1/2, they move towards lower nu with increasing T. Front and back-gating data imply that these oscillations require a coupling to a parallel conducting layer. A comparison with existing oscillation models renders no explanation.     

Single quantum well transistor with modulation doped AlGaAs/GaAs/AlGaAs structures

Self-consistent calculations have been performed to obtain the wave functions and energy subbands of the two-dimensional electrons confined in a single quantum well of a Al_xGa_1?xAs/GaAs/Al_xGa_1?xAs heterostructure. The wave functions of the two-dimensional electron gas are found to be easily controlled by an external gate voltage applied between the AlGaAs-barriers, indicating a capability of fabricating a novel quantum well device, a modulation-doped single quantum well transistor.     

Quasi-continuous-wave operation of AlGaAs/GaAs quantum cascade lasers

Quasi-continuous-wave operation of AlGaAs/GaAs-based quantum cascade lasers (λ9 μm) up to 165 K is reported. The strong temperature dependence of the threshold current density and its higher value in high duty cycle is investigated in detail. The self-heating effect in the active region is explored by changing the operating duty cycles. The degradation of lasing performance with temperature is explained.     

Photoreflectance measurements in GaAs/AlGaAs asymmetric quantum wells

In this work we investigated the optical control of the bidimensional electron gas density in a single asymmetric quantum well using, for the first time, photoreflectance. We performed our measurements at 80 and 300 K as a function of the power density of the pump beam. Under strong illumination, the bidimensional electron gas density is washed out of the quantum well and under a dark condition, it reaches its maximum value. The variation of the optical transitions observed in our photoreflectance spectra was related to the induced changes of the band profile in between these two limiting cases.     

Self-assembled GaAs/AlGaAs quantum dots by molecular beam epitaxy and in situ AsBr3 etching

We report on a new method to produce self-assembled, unstrained, GaAs/AlGaAs quantum dots (QDs) with large confinement energy. First we create nanoholes on a GaAs surface by growing InAs islands on GaAs(0 0 1), overgrowing them with GaAs and etching the surface in situ with AsBr₃ gas. Then we transfer the holes to an AlGaAs surface, fill them with GaAs and overgrow them with AlGaAs. In this way we obtain GaAs inclusions in an AlGaAs matrix. We investigate the optical properties of such QDs by photoluminescence spectroscopy and their morphology by atomic force microscopy. We show that the QD size can be tuned and emission with inhomogeneous broadening down to 8.9 meV can be achieved.     

«Self-organized» InAs/GaAs quantum dots

Il Nuovo Cimento D - We discuss some specific properties of self-organized InAs-GaAs quantum dots grown by Molecular Beam Epitaxy. We report on the optical spectra obtained under resonant...     

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

Aluminium implantation-induced disordering of AlGaAs/GaAs quantum well structures

The process of Al implantation-induced disordering of AlGaAs/GaAs quantum well structures has been studied for optical waveguide applications. A study of the implanted samples using photoluminescence demonstrates that disordering is primarily a damage-based process and that this process may be suitable for the fabrication of surface gratings.