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We propose a design for a semiconductor structure emitting broadband light in the infrared, based on InAs quantum dots (QDs) embedded into a metamorphic step-graded InxGa1-xAs buffer. We developed a model to calculate the metamorphic QD energy levels based on the realistic QD parameters and on the strain-dependent material properties; we validated the results of simulations by comparison with the experimental values. On this basis, we designed a p-i-n heterostructure with a graded index profile toward the realization of an electrically pumped guided wave device. This has been done by adding layers where QDs are embedded in InxAlyGa1-x-yAs layers, to obtain a symmetric structure from a band profile point of view. To assess the room temperature electroluminescence emission spectrum under realistic electrical injection conditions, we performed device-level simulations based on a coupled drift-diffusion and QD rate equation model. On the basis of the device simulation results, we conclude that the present proposal is a viable option to realize broadband light-emitting devices. [ABSTRACT FROM AUTHOR]

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In0.15Ga0.85As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 μm (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In0.15Ga0.85As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV is related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs. [ABSTRACT FROM AUTHOR]

In this work, we calculate the two-dimensional quantum energy system of the In(Ga)As wetting layer that arises in InAs/InGaAs/GaAs metamorphic quantum dot structures. Model calculations were carried on the basis of realistic material parameters taking in consideration their dependence on the strain relaxation of the metamorphic buffer; results of the calculations were validated against available literature data. Model results confirmed previous hypothesis on the extrinsic nature of the disappearance of wetting layer emission in metamorphic structures with high In composition. We also show how, by adjusting InGaAs metamorphic buffer parameters, it could be possible: (i) to spatially separate carriers confined in quantum dots from wetting layer carriers, (ii) to create an hybrid 0D-2D system, by tuning quantum dot and wetting layer levels. These results are interesting not only for the engineering of quantum dot structures but also for other applications of metamorphic structures, as the two design parameters of the metamorphic InGaAs buffer (thickness and composition) provide additional degrees of freedom to control properties of interest. [ABSTRACT FROM AUTHOR]

The structural and morphological features of bimodal-sized InAs/(In)GaAs quantum dots with density in the low 109 cm-2 range were analyzed with transmission electron microscopy and atomic force microscopy and were related to their optical properties, investigated with photoluminescence and time-resolved photoluminescence. We show that only the family of small quantum dots (QDs) is able to emit narrow photoluminescence peaks characteristic of single-QD spectra; while the behavior of large QDs is attributed to large strain fields that may induce defects affecting their optical properties, decreasing the optical intensity and broadening the homogeneous linewidth. Then, by using a rate-equation model for the exciton recombination dynamics, we show that thermal population of dark states is inhibited in both QD families capped by high In content InGaAs layers. We discuss this behavior in terms of alloy disorder and increased density of point defects in the InGaAs pseudomorphic layer. [ABSTRACT FROM AUTHOR]

We have investigated the temperature dependent recombination dynamics in two bimodally distributed InAs self assembled quantum dots samples. A rate equations model has been implemented to investigate the thermally activated carrier escape mechanism which changes from exciton-like to uncorrelated electron and hole pairs as the quantum dot size varies. For the smaller dots, we find a hot exciton thermal escape process. We evaluated the thermal transfer process between quantum dots by the quantum dot density and carrier escape properties of both samples. [ABSTRACT FROM AUTHOR]

In this work we study the properties of energy levels of the two-dimensional quantum system composed by wetting layers and thin capping layers in low density InAs/InGaAs quantum dot structures, that can be used as single photon sources at the fiber-optic wavelength of 1.3 μm. We show how, thanks to the low density of quantum dots, x-ray characterization of structures allows to extract thicknesses and compositions of the InAs wetting layer and the quantum well formed by the InGaAs capping layer, resulting in substantial deviations from the simplified picture of a wetting layer consisting of a 1.6 monolayer thick InAs square well. The agreement between model calculations of quantum confined energy levels based on x-ray data and photoluminescence peak energies substantiates the validity of this calculation, that also allows to investigate on carrier localization. The increase in In composition in the InGaAs capping layer results in reduced localization of heavy holes in the wetting layer, that are pushed into the InGaAs quantum well, while the electron, being already strongly delocalized in the quantum well, is relatively less affected. We discuss consequences on carriers' overlap in the wetting layer and on spatial separation between wetting layer and quantum dot carriers and we consider implications for control of properties of quantum dot structures suitable as single photon sources in the telecommunication windows. [ABSTRACT FROM AUTHOR]

In this work, we present a study of InAs quantum dots deposited on InGaAs metamorphic buffers by molecular beam epitaxy. By comparing morphological, structural, and optical properties of such nanostructures with those of InAs/GaAs quantum dot ones, we were able to evidence characteristics that are typical of metamorphic InAs/InGaAs structures. The more relevant are: the cross-hatched InGaAs surface overgrown by dots, the change in critical coverages for island nucleation and ripening, the nucleation of new defects in the capping layers, and the redshift in the emission energy. The discussion on experimental results allowed us to conclude that metamorphic InAs/InGaAs quantum dots are rather different nanostructures, where attention must be put to some issues not present in InAs/GaAs structures, namely, buffer-related defects, surface morphology, different dislocation mobility, and stacking fault energies. On the other hand, we show that metamorphic quantum dot nanostructures can provide new possibilities of tailoring various properties, such as dot positioning and emission energy, that could be very useful for innovative dot-based devices. [ABSTRACT FROM AUTHOR]

We report a detailed study of InAs/GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15 nm and 2×1011 cm-2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25 nm and densities ranging from 2×108 to 1.5×109 cm-2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: (i) free carrier concentration is reduced near the QD plane, (ii) the QD photoluminescence intensity is significantly quenched, and (iii) deep levels show up with typical features related to extended structural defects. [ABSTRACT FROM AUTHOR]

The temperature dependence of carrier confinement in states of self-assembled In[sub 0.5]Ga[sub 0.5]As quantum dots (QDs) embedded in Al[sub y]Ga[sub 1-y]As barriers has been investigated by means of photoluminescence (PL) measurements. We show that photoexcited carriers above the AlGaAs barriers have two recombination channels that contribute to the temperature quenching of the PL from QDs: (a) carrier losses in the AlGaAs layers during the relaxation process and (b) thermal evaporation of captured carriers out of QDs. The interplay between these two mechanisms determines the behavior of the nonresonantly excited photoluminescence as a function of temperature. Eliminating the first contribution by using resonant excitation of the QD PL, we demonstrate a definite enhancement of the carrier confinement at room temperature in InGaAs/AlGaAs QDs by increasing the Al content. We show that this effect is related to the increase in the energy separation between the electronic states in the QD and the wetting layer. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Single indium arsenide quantum dot emission spectra have been recorded using a four-core, crosstalk-free, multicore fiber placed at the collection arm of a confocal microscope. We developed two different measurement setups depending on the relative configuration of the excitation and collection spots. In the single-matched mode, the emission from the excited area is collected by a single core in the multicore fiber, whereas the three remaining cores capture the emission from neighboring, non-excited areas. This procedure allows for the recording of the quantum dot emission from carrier diffusion between sample positions separated by more than 6 \mu \textm . In the multiple-matched mode, the excitation spot overlaps the four-core emission area. This configuration permits the acquisition of the micro-photoluminescence spectra at different sample positions without scanning. These results show the possibilities offered by multicore fibers for the spectroscopic analysis of single semiconductor quantum dot optical emission. [ABSTRACT FROM PUBLISHER]

Tensile strained Ge is considered as an enabling material for the integration of light emitting sources on Si because it shows unique indirect to direct bandgap transition and very high carrier mobilities (> 10.000 cm2/Vs). In the framework of this goal we aim to realize tensile germanium on In(Ga)As quantum dots (QD) stressors grown on GaAs and in this paper we focus on the study of deposition at different temperatures of Ge layers by Metal Organic Vapor Phase Epitaxy (MOVPE) on QD grown by Molecular Beam Epitaxy (MBE). The structures were characterized by Photoluminescence (PL) and Atomic Force Microscopy (AFM) before and after Ge epitaxy. Cross sectional TEM analysis shows that the Ge overlayer is epitaxially grown on the InGaAs/GaAs QD underlayer, with the same orientation. No structural defects are observed in the whole structures. Intermixing and In desorption occur at higher deposition temperature. [ABSTRACT FROM AUTHOR]

Photoluminescence and time resolved photoluminescence from single metamorphic InAs/GaAs quantum dots (QDs) emitting at 1.3 μm have been measured by means of a novel fibre-based characterization set-up. We demonstrate that the use of a wavelength tunable fibre Bragg grating filter increases the light collection efficiency by more than one order of magnitude as compared to a conventional grating monochromator. We identified single charged exciton and neutral biexciton transitions in the framework of a random population model. The QD recombination dynamics under pulsed excitation can be understood under the weak quantum confinement potential limit and the interaction between carriers at the wetting layer and QD states. [ABSTRACT FROM AUTHOR]

In this work we study the properties of wetting layers in InAs/InGaAs/GaAs quantum dot (QD) structures suitable for single photon emission; the mandatory low density of QDs is obtained by an molecular beam epitaxy (MBE) approach based on the deposition of sub-critical InAs coverages followed by post-growth annealing. Such a growth regime is fundamentally different from the Stranski-Krastanow (SK) one commonly used for the deposition of QDs. By fitting x-ray diffraction (XRD) spectra, ten-steps composition profiles were derived and used as inputs of model calculations of the two-dimensional quantum energy system: model results were validated by comparison with photoluminescence spectra. A strong reduction of In molar fraction in wetting layers in comparison with SK structures was found, causing a larger wavefunction delocalization for carriers that populate the wetting layer energy levels. Moreover, by considering the limits for strain relaxation when InxGa1–xAs capping layers are used, we grew structures with the highest possible values of x to study the odifications of the energy system. When x = 0.20 the electron-heavy hole overlap is almost halved and the carriers' probability of being in the first nanometre of the wetting layer is reduced by 60%. These results will be useful for advanced design of QD nanostructures for single photon sources. [ABSTRACT FROM AUTHOR]

The article presents research which discussed the properties and growth of low-density InAs/GaAs quantum dot structures through the use of Molecular Beam Epitaxy (MBE). The researchers offered an interesting method to the creation of telecommunication wavelengths single-photon sources. They found that the semiconductor quantum dots (QD) have very low density and their light emission characteristics can be assessed by growth of the dots at low growth rate and high growth temperature.

InAs/GaAs quantum dot (QD) structures were grown by molecular beam epitaxy (MBE) with InAs coverages θ continuously graded from 1.5 ML to 2.9 ML. A critical coverage of 2.23 ML is found, above which the islands undergo ripening, which causes a fraction of quantum dots to increase in size and to eventually relax through the formation of pure, edge-type misfit dislocations which propagate towards the surface in the form of V-shaped defects. Concomitant with ripening, extended-defect related traps with activation energies of 0.52 and 0.84 eV were observed, and regarded as the cause of the significant worsening of the optical and electrical properties in high coverage structures. Their relationship with the observed dislocations is discussed. [ABSTRACT FROM AUTHOR]

We studied the dependence of the photoluminescence emission energy of InGaAs/AlGaAs quantum dot (QD) structures grown by molecular beam epitaxy as a function of the Al and In content in barriers and QDs, respectively. We show that emissions are blue-shifted by increasing both the Al content in the 0 to 0.30 range and, unexpectedly, the In composition in the 0.4 to 0.7 range; we suggest that such results stem from significant changes in QD sizes, shapes, and composition profiles. This research led to the preparation of structures with efficient light emission in the 980 nm window of optoelectronic interest. [ABSTRACT FROM AUTHOR]

This work deals with the strain relaxation mechanism in InGaAs metamorphic buffers (MBs) grown on GaAs substrates and overgrown by InAs quantum dots (QD). The residual strain is measured by using Raman scattering and X-ray diffraction, both in Reciprocal Space Map and in single ω-2θ scan modes (ω and θ being the incidence angles on the sample surface and on the scattering planes, respectively). By relating the GaAs-like longitudinal optical phonon frequency ωLO of InGaAs MBs to the in-plane residual strain ε measured by means of photoreflectance (PR), the linear ε-vs.-ωLO working curve is obtained. The results of Raman and XRD measurements, as well as those obtained by PR, are in a very satisfactory agreement. The respective advantages of the techniques are discussed. The measurements confirm that strain relaxation depends on the thickness t of the buffer layer following a ~t-1/2 power law, that can be explained by an energy-balance model. [ABSTRACT FROM AUTHOR]

We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 108 cm-2 dot density and metamorphic InxGa1-xAs (x=0.15,0.30) confining layers result in emission wavelengths at 1.3 μm. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows. [ABSTRACT FROM AUTHOR]

We report on Raman scattering experiments on InAs/AlxGa1-xAs quantum dot heterostructures with 0≤x≤0.6. The samples were prepared by using molecular beam epitaxy (MBE) and atomic layer MBE for the growth of different layers. For x>0, we detected several lines originating from the AlxGa1-xAs alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q≅0, and weaker scattering from disorder-activated phonons with q≠0. In particular, we identified a line at ∼250 cm-1 as due to disorder-activated longitudinal optical phonons in the alloy. This conclusion is different than the attribution of this line to scattering from dots and, consequently, we do not recognize the possibility of deriving any information about the actual composition of the dots from an analysis of this line as proposed by other authors. [ABSTRACT FROM AUTHOR]

We present design, preparation by molecular beam epitaxy, and characterization by photoluminescence of long-wavelength emitting, strain-engineered quantum dot nanostructures grown on GaAs, with InGaAs confining layers and additional InAlAs barriers embedding InAs dots. Quantum dot strain induced by metamorphic lower confining layers is instrumental to redshift the emission, while a-few-nanometer thick InAlAs barriers allow to significantly increase the activation energy of carriers’ thermal escape. This approach results in room temperature emission at 1.59 μm and, therefore, is a viable method to achieve efficient emission in the 1.55 μm window and beyond from quantum dots grown on GaAs substrates. [ABSTRACT FROM AUTHOR]

Photoluminescence excitation spectroscopy is used to probe energy states and carrier transport in InAs quantum dot structures grown on InGaAs metamorphic layers, designed for room temperature emission at 1.3, 1.4, or 1.5 μm. The dominant spectral feature is shown to arise from the partially relaxed InGaAs confining layer. In structures with a low indium composition or thin InGaAs layer, a clear wetting layer feature is observed which acts as the dominant reservoir for carriers thermally excited from the quantum dots. Structures with high indium composition and/or thick InGaAs lack a wetting layer and carriers escape directly to the InGaAs layers. [ABSTRACT FROM AUTHOR]