Temperature-dependent photoluminescence of vertically stacked self-assembled CdSe quantum dots in ZnSe

We studied the optical properties of multiple layers of self-assembled CdSe quantum dots (QDs) embedded in ZnSe, grown by molecular beam epitaxy. The ZnSe barrier thicknesses separating the QD layers ranged from 30 to 60 monolayers (ML). For stacks with thinnest ZnSe barriers photoluminescence (PL) measurements reveal blue shifts as large as 180 meV relative to PL observed for single QD layers. The amount of blue shift decreases with increasing barrier thickness, and for the 60 ML spacer the PL energy returns to that emitted by a single layer of QDs. Temperature dependence of the integrated intensity of the emission spectra reveals that the activation energy for PL quenching is largest for barrier thicknesses of 30 and 45 ML. We tentatively attribute these effects to a decrease in the size of the vertically stacked QDs when the thickness of the barrier layers is small.     

Landau levels in a novel two-dimensional electron system interacting with charged quantum dots

Landau levels have been theoretically investigated in a two-dimensional electron gas near a quantum dot (QD) layer. By a diagrammatical method, we have formulated the self-energy for the Landau level and deduced its relation to the AC conductivity σ_loc(ω) in the QD layer. As an example, we have examined the density of states in the case where σ_loc(ω) is described by Aω^S(S=0.8). It is found that the Landau levels are broadened due to the interaction with the localized electrons in the QDs.     

Determination of electron effective mass from optical transition energy in InGaAs/InAlAs quantum wells

Nonparabolic effective mass of conduction subbands in InGaAs/InAlAs quantum wells (QWs), lattice-matched to InP, was quantitatively obtained by analyzing interband-optical transition spectra. Thickness of InGaAs well was 5.3, 9.4, and . Thickness of InAlAs barrier was about , and each QW was independent. Excellent agreement was obtained between experimental mass and theoretical mass predicted by Kane's three-level band theory on bulk InGaAs, in a wide energy range of from the bandedge. Method of experimental analysis on a relation between eigen energy and effective mass was described.     

Influence of a lateral electric field on the optical properties of InAs quantum dots

We have performed single dot photoluminescence and time-resolved ensemble photoluminescence measurements on InAs quantum dots embedded in a lateral in-plane p–i–n or n–i–n device, respectively, which makes the application of lateral electric fields, i.e. field direction perpendicular to the growth direction, feasible. Time-resolved measurements show an increase in the radiative lifetime of up to 30% with increasing field. We attribute this to the reduced overlap between the electron and hole wave functions. Single dot spectroscopy revealed a small red-shift of the emission energies of maximum 0.5 meV. This shift can be explained by the quantum confined Stark effect taking into account that the red-shift due to the band-tilting is partly compensated by a decrease in exciton binding energy.     

Impurity effects in the optical absorption of quantum rings

We study the interplay between impurity scattering and Coulomb interaction effects in the absorption spectrum of neutral bound magnetoexcitons confined in quantum-ring structures. Impurity scattering breaks the rotational symmetry of the ring system, introducing characteristic features in the optical emission. Signatures of the optical Aharonov–Bohm effect are still present for weak scattering and strong Coulomb screening. Furthermore, an impurity-induced modulation of the absorption strength is present even for a strong impurity potential and low screening. This behavior is likely responsible of recent experimental observations in quantum-ring structures.     

Effect of inter-miniband tunneling on current resonances due to the formation of stochastic conduction networks in superlattices

We study the effects of inter-miniband electron tunneling and electric field domains on the current–voltage and conductance–voltage curves of biased semiconductor superlattices under the action of a magnetic field that is tilted relative to the plane of the layers. For this geometry, electrons in the superlattice minibands exhibit a unique type of stochastic semiclassical motion. At certain critical values of the electric field within the superlattice layers, the stochastic trajectories change abruptly from fully localized to completely unbounded, and map out an intricate web-like mesh of conduction channels in phase space. Delocalization of the electron paths produces a series of strong resonant peaks in the electron drift velocity versus electric field curves. We use these drift velocity characteristics to make self-consistent drift-diffusion calculations of the current–voltage and differential conductance–voltage curves of the superlattices, which reveal strong resonant features originating from the sudden delocalization of the stochastic single-electron paths. We show that this delocalization has a pronounced effect on the distribution of space charge and electric field domains within the superlattices. Inter-miniband tunneling greatly reduces the amount of space-charge buildup, thus enhancing the domain structure and both the strength and number of the current resonances.     

Separated AlxIn1−xN quantum dots grown by plasma-reactive co-sputtering

Separated Al_xIn_1−xN quantum dots (QDs) embedded in amorphous AlN films have been produced by radio-frequency co-sputtering technique on silicon (1 1 1) and quartz glass substrates. The mean size and density of Al_xIn_1−xN QDs can be conveniently monitored by deposition parameters. Transparent electron microscope, and X-ray diffraction were used to detect the structure of the Al_xIn_1−xN QDs system; field-emission scanning-electron microscope was adopted to measure the surface morphology and anticipate the size of the QDs; X-ray photoelectronic spectroscopy was used to measure the stoichiometric ratios of the QDs.     

Comparison of the band alignment of strained and strain-compensated GaInNAs QWs on GaAs and InP substrates

We present a comparison of the band alignment of the Ga_1−xIn_xN_yAs_1−y active layers on GaAs and InP substrates in the case of conventionally strained and strain-compensated quantum wells. Our calculated results present that the band alignment of the tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells on InP substrates is better than than that of the compressively strained Ga_1−xIn_xN_yAs_1−y quantum wells on GaAs substrates and both substrates provide deeper conduction wells. Therefore, tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells with In concentrations of x0.53 on InP substrates can be used safely from the band alignment point of view when TM polarisation is required. Our calculated results also confirm that strain compensation can be used to balance the strain in the well material and it improves especially the band alignment of dilute nitride Ga_1−xIn_xN_yAs_1−y active layers on GaAs substrates. Our calculations enlighten the intrinsic superiority of N-based lasers and offer the conventionally strained and strain-compensated Ga_1−xIn_xN_yAs_1−y laser system on GaAs and InP substrates as ideal candidates for high temperature operation.     

Observations of nascent superfluidity in a bilayer two-dimensional electron system at

Single-layer longitudinal and Hall resistances have been measured in a bilayer two-dimensional electron system at ν_T=1 with equal but oppositely directed currents flowing in the two layers. At small effective layer separation and low temperature, the bilayer system enters an interlayer coherent state expected to exhibit superfluid properties. We detect this nascent superfluidity through the vanishing of both resistances as the temperature is reduced. This corresponds to the counterflow conductivity rising rapidly as the temperature falls, reaching by . This supports the prediction that the ground state of this system is an excitonic superfluid.     

Magneto optics of single photons emitted from single InAs/GaAs self-assembled quantum dots in a planar microcavity

We fully characterize the fine spectral structure of neutral and negatively charged single microcavity quantum dot excitons, using polarization-sensitive magneto-photoluminescence spectroscopy. We show that the microcavity allows the simultaneous detection of both the bright and dark excitons using Faraday configuration. Thus, we were able to fully determine the fine structure and the g-factors of the neutral and negatively charged single exciton states within the same single quantum dot. Our measurements are in excellent agreement with novel, many carrier model calculations, which take into account Coulomb and exchange interactions among all the confined e–h pair states.