Direct-energy-gap dependence on Al concentration in AlxGa

Excitonic Aharonov-Bohm effect in QD-on-ring nanostructures

We show by the first-order perturbation theory and the configuration interaction method that the Coulomb interaction in quantum rings mixes electron-hole pair states with the same total angular momentum, which makes it difficult to observe a clear excitonic Aharonov-Bohm (A-B) effect. To avoid this situation, we propose the use of a combined structure of a quantum dot on the top of a quantum ring with an applied static electric field. Under moderate experimental conditions with respect to the applied electric and magnetic fields, we show that we can observe the excitonic A-B effect due to the reduction of the Coulomb interaction and an increase in the difference between the average radii of the electron and hole trajectories.

Molecular spectrum of laterally coupled quantum rings under intense terahertz radiation

We study the influence of intense THz laser radiation and electric field on molecular states of laterally coupled quantum rings. Laser radiation shows the capability to dissociate quantum ring molecule and add 2-fold degeneracy to the molecular states at the fixed value of the overlapping size between rings. It is shown that coupled to decoupled molecular states phase transition points form almost a straight line with a slope equal to two. In addition, the electric field direction dependent energy spectrum shows unexpected oscillations, demonstrating strong coupling between molecular states. Besides, intraband absorption is considered, showing both blue and redshifts in its spectrum. The obtained results can be useful for the controlling of degeneracy of the discrete energy spectrum of nanoscale structures and in the tunneling effects therein.

Spontaneous alloy composition ordering in GaAs-AlGaAs core-shell nanowires

By employing various high-resolution metrology techniques we directly probe the material composition profile within GaAs-Al0.3Ga0.7As core-shell nanowires grown by molecular beam epitaxy on silicon. Micro Raman measurements performed along the entire (>10 μm) length of the [111]-oriented nanowires reveal excellent average compositional homogeneity of the nominally Al0.3Ga0.7As shell. In strong contrast, along the radial direction cross-sectional scanning transmission electron microscopy and associated chemical analysis reveal rich structure in the AlGaAs alloy composition due to interface segregation, nanofaceting, and local alloy fluctuations. Most strikingly, we observe a 6-fold Al-rich substructure along the corners of the hexagonal AlGaAs shell where the Al-content is up to x ~ 0.6, a factor of 2 larger than the body of the AlGaAs shell. This is associated with facet-dependent capillarity diffusion due to the nonplanarity of shell growth. A modulation of the Al-content is also found along the radial [110] growth directions of the AlGaAs shell. Besides the ~10(3)-fold enhancement of the photoluminescence yield due to inhibition of nonradiative surface recombination, the AlGaAs shell gives rise to a broadened band of sharp-line luminescence features extending ~150-30 meV below the band gap of Al0.3Ga0.7As. These features are attributed to deep level defects under influence of the observed local alloy fluctuations in the shell.

Optically programmable excitonic traps

With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits.

Quantum confinement and magnetic-field effects on the electron g factor in GaAs-(Ga, Al)As cylindrical quantum dots

We have performed a theoretical study of the quantum confinement (geometrical and barrier potential confinements) and axis-parallel applied magnetic-field effects on the conduction-electron effective Landé g factor in GaAs-(Ga, Al)As cylindrical quantum dots. Numerical calculations of the g factor are performed by using the Ogg-McCombe effective Hamiltonian-which includes non-parabolicity and anisotropy effects-for the conduction-band electrons. The quantum dot is assumed to consist of a finite-length cylinder of GaAs surrounded by a Ga(1-x)Al(x)As barrier. Theoretical results are given as functions of the Al concentration in the Ga(1-x)Al(x)As barrier, radius, lengths and applied magnetic fields. We have studied the competition between the quantum confinement and applied magnetic field, finding that in this type of heterostructure the geometrical confinement and Al concentration determine the behavior of the electron effective Landé [Formula: see text] factor, as compared to the effect of the applied magnetic field. Present theoretical results are in good agreement with experimental reports in the limiting geometry of a quantum well, and with previous theoretical findings in the limiting case of a quantum well wire.

Theoretical and experimental limits of the analysis of III/V semiconductors using EELS

The optimisation of acquisition conditions for EELS spectroscopy of Al(x)Ga(1-x)As heterostructures permits one to find the absolute concentration with a precision of better than delta = +/-0.02 and to detect changes in concentration of +/-0.01 for x = 0-0.5. In order to achieve this concentration precision, we investigated ways to reduce the influence of three major sources on the inaccuracies of the measurement: the effect of electron channelling which biases the ionisation probabilities on the different atomic sites, the contribution of the sample surface layers and the accuracy in the spectral analysis. An optimal specimen orientation that maximises the stability of the electron densities on Al and As sites without introducing an unacceptable loss of spatial resolution due to sample tilt is found by computing the electron channelling intensity as a function of the specimen tilt angle. The influence of a Ga enriched surface layer on the analysis is demonstrated. Two methods for the extraction of the edge intensity from the spectra are compared. These methods are shown to give the upper and the lower limit of the Ga concentration.

Optimization of semiconductor quantum devices by evolutionary search

A novel simulation strategy is proposed for searching for semiconductor quantum devices that are optimized with respect to required performances. Based on evolutionary programming, a technique that implements the paradigm of genetic algorithms in more-complex data structures than strings of bits, the proposed algorithm is able to deal with quantum devices with preset nontrivial constraints (e.g., transition energies, geometric requirements). Therefore our approach allows for automatic design, thus avoiding costly by-hand optimizations. We demonstrate the advantages of the proposed algorithm through a relevant and nontrivial application, the optimization of a second-harmonic-generation device working in resonance conditions.

Characterization of an AlGaAs rib waveguide using a grating in a Fabry-Perot étalon configuration

We characterize an AlGaAs rib waveguide using a structure based on a third-order Bragg reflector fabricated by electron-beam lithography. The grating forms a resonant cavity with the sample's uncoated output facet, and we observe a Fabry-Perot étalon behavior superimposed on the Bragg reflector characteristics. We present transmitted intensity measurements as a function of wavelength and temperature. We derive results for waveguide refractive-index profile, loss coefficient, group effective index, effective-index wavelength dispersion, and effective-index temperature dispersion. An auxiliary prism on top of the grating allows us to couple light out of the waveguide into the air. Angular measurements of the outcoupled orders at different wavelengths confirm the wavelength dispersion measurements made on the Fabry-Perot étalon formed by the grating and the cleaved mirror.