Number Of Quantum Dot Layers Research Articles

Growth of high optical quality InAs quantum dots in InAlGaAs∕InP double heterostructures

Two methods were studied to improve the optical quality of InAs quantum dot nanostructures grown in lattice-matched InAlGaAs∕InP double heterostructures of a center wavelength around 1.55μm. By either inserting InAlAs∕InGaAs superlattices between the InAlGaAs waveguide and the upper InAlAs cladding layer, or depositing a tensile-strained InGaAs strain-balance layer after the quantum dot formation, the optical quality of the quantum dot samples is greatly improved and a strong room-temperature photoluminescence is observed. The InGaAs strain-balance layer can be used to reduce the overall strain of the heterostructure, which makes it possible to stack a large number of quantum dot layers to improve the uniformity of the dot size distribution and increase the optical gain volume for high performance photonic device applications.

Polarized Raman spectroscopy of multilayer Ge∕Si(001) quantum dot heterostructures

Polarized Raman spectroscopy in backscattering geometry has been applied here for the investigation of Ge∕Si(001) quantum dot multilayer structures (with the number of layers ranging from 1 to 21) grown by the Stranski-Krastanov technique. The characteristic Raman spectra of the dots have been obtained by taking the difference between the Raman spectra of the dot sample and the reference Si substrate, taken under the same excitation/scattering conditions. We found that the Raman spectra of Ge∕Si dots obtained in such a manner are strongly polarized, in particular, for the Ge-Ge (at ∼295cm−1) and Si-Ge (at ∼413cm−1) vibrational modes. The dependence of peak intensity and peak position of the Ge-Ge and Ge Raman bands versus the number of dot layers has been analyzed. It was found that studied quantum dot (QD) systems possess prominent anisotropic intermixing. This results in the Si content in the dots being high and this increases with the number of QD layers. At the same time, the increase of the number of layers was followed by a reduction in the compressive stress within the dots.

Photoinduced Fluorescence Enhancement in CdSe/ZnS Quantum Dot Submonolayers Sandwiched between Insulating Layers: Influence of Dot Proximity

We report on photoinduced fluorescence enhancement (PFE) in a thin film of CdSe/ZnS core/shell quantum dots (QDs) sandwiched between a glass substrate and a silicon oxide layer and the dependence on the degree of proximity between the QDs. CdSe/ZnS QDs capped with tri-n-octylphosphine oxide (TOPO) were colloid-chemically synthesized, and the QD thin films of various thicknesses were then fabricated on glass substrates using a spin-coating technique. A SiOx protective layer was subsequently sputtered on the QD thin film to prevent photoadsorption of water molecules and photooxidation. The fluorescence intensity monotonically increased under continuous excitation except for the case of the thinnest sample which exhibited intensity decay. Interestingly, the increasing rate of fluorescence intensity increases as a function of the number of QD layers, θ, to the extent of θ ≤ 3.2 and then decreases at θ ≥ 3.2. Fluorescence lifetime measurements indicate that the band-edge radiative relaxation probability becomes relatively higher as the fluorescence intensity is enhanced. Considering the experimental results with respect to QD submonolayers (θ ≤ 1), and in accordance with the theoretical model, PFE is elucidated by the charging effect of trapped electrons.

Investigation of CdSe/ZnSSe quantum dot ordering by grazing incidence small angle X‐ray scattering

AbstractThe spatial correlation of CdSe quantum dots in stacked CdSe/ZnSSe quantum dot layers has been investigatedusinggrazing incidence small angle X‐ray scattering. Lateral and vertical ordering has been found by the presence of satellite spots, and its dependence on the number of quantum dot layers and on the spacer layer thickness has been studied. Significant ordering occurs, if the number of quantum dot layers is not too small, and if the ZnSSe spaces thickness is not too large. These findings are interpreted in terms of a strain‐driven, self‐organized ordering process. In addition, an anisotropy of two‐fold symmetry has been observed, with the weakest correlation signal along 〈110〉. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier Distribution, Gain, and Lasing in 1.3->tex<$mu hboxm$>/tex<InAs–InGaAs Quantum-Dot Lasers

In this paper, we present the results of a theoretical model built to describe the temperature-dependent lasing characteristics of InAs-InGaAs quantum-dot (QD) lasers operating at 1.3 /spl mu/m. From the model, we find that traditional carrier distribution theories are inadequate to describe the performance of these lasers. We therefore introduce an improved model that allows for both free carriers and excitons in the dots. The new model provides threshold current and characteristic temperature T/sub 0/ values that are in good agreement with experimental data. The results of our modeling reveal that, while it is the excitons that mainly contribute to the gain, the ratio of excitons to free carriers significantly affect the T/sub 0/ of QD lasers. Our model results also indicate that the wetting layer current plays little role in QD laser performance. In addition, the model correctly predicts other experimental observations such as; increased T/sub 0/ for increased number of QD layers and p-doped structures, and the oscillatory behavior of T/sub 0/, lending further credibility to the model.

Resonant Raman scattering by acoustic phonons in self-assembled quantum-dot multilayers: From a few layers to superlattices

We report on resonant Raman scattering (RS) by acoustic phonons in self-assembled Ge/Si quantum-dot (QD) multilayers. In previous studies the observation of doublet features in the low-frequency Raman spectra was attributed to superlattice effects, i.e., Brillouin-zone folding, despite the often small number of QD layers. We propose a model which accounts for the low-frequency resonant RS, whatever the number of QD layers, i.e., from a few layers to superlattices. It is shown that the features in the low-frequency spectra (peak frequencies and intensities, doublet splittings and intensity ratios) can all be consistently understood within the resonant RS interference model. RS interferences occur when acoustic phonons interact with an ensemble of localized electronic states. Calculations and experiments were carried out in order to investigate how the RS depends on the number of QD layers and on the multilayer location with respect to the surface. Indeed, spectra are shown to depend on these finite-size effects: Reliable assignments cannot be made when the analysis is restricted to the peak frequencies. RS intensities have been calculated in order to identify the relevant scattering mechanism.

High efficiency (ηD>80%) long wavelength (λ>1.25 μm) quantum dot diode lasers on GaAs substrates

Diode lasers based on several layers of self-organized quantum dots (QD) on GaAs substrates were studied. The lasing wavelength lies in the range λ=1.25–1.29 μm, depending on the number of QD layers and optical losses. A record external differential efficiency of 88% and the characteristic temperature of threshold current, 145 K, were obtained. The internal losses, and also threshold and spectral characteristics, are correlated with the optical gain and radiative recombination efficiency, which are strongly dependent on the design of the active region and growth modes.

Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.