Functionalized Self‐Assembled InAs/GaAs Quantum‐Dot Structures Hybridized with Organic Molecules

InAs quantum dots grown on InP or InGaAsP are used for optical communication applications operating in the 1.3 – 1.55 μm wavelength range. It is generally understood that the optical properties of such dots are highly dependent on their structural and chemical profiles. However, morphological and compositional measurements of quantum dots using transmission electron microscopy can be ambiguous because the recorded signal is usually a projection through the thickness of the specimen. Here, we discuss the application of scanning transmission electron microscopy tomography to the morphological and chemical characterisation of surface and buried quantum dots. We highlight some of the challenges involved and introduce a new specimen preparation method for creating needle-shaped specimens that each contain multiple dots and are suitable for both scanning transmission electron microscopy tomography and atom probe tomography.

Capture, relaxation and recombination in quantum dots

Atomic-scale probing of metallic and semiconductor nanostructures

We investigated the effect of dot size distribution and interlayer thickness on the optical property of closely stacked self-assembled InAs/GaAs quantum dot (QD) structures with growth interruption for 30 s using an As2 source. The structural property was optimized by changing the growth parameters, such as growth temperature, growth time and group III/V ratio. As the stacking number was increased, the size of truncated pyramid-shaped QDs became larger in both height and width, maintaining an on-top vertical alignment with a dot density of ∼5.2–5.9 × 1010 cm−2. Compared to the single QD layer, five closely stacked QDs with the GaAs interlayer are found to exhibit a significant improvement of their photoluminescent (PL) intensity, indicating a slight shift of the PL peak position toward the high-energy side. The use of a thin GaAs interlayer of 3 nm in the QDs enhanced the blue shift, which is attributed to the dominant strain-induced intermixing or loss of indium atoms in the InAs QD layers. For the interlayer thicker than about 7 nm, the blue shifts are correlated to the dominant high-energy excited state transitions due to the successive state filling of the ground and higher excited states in the QDs. The energy separation of double PL peaks, originating from two different excited states, was kept at around 50 meV at room temperature. A possible mechanism concerning this phenomenon was also discussed.

Submicron Raman and photoluminescence topography of InAs/Al(Ga)As quantum dots structures

We have grown a stack of In‐flushed InAs quantum dot (QD) layers to realize a broadband emission centered at ∼1 µm with a Gaussian‐like spectrum as a light source for optical coherence tomography (OCT). The In‐flush process, which enables control of the height of the QD, was optimized to control the emission wavelength and intensity of self‐assembled InAs/GaAs QDs. A partial capping layer of GaAs was deposited on as‐grown InAs/GaAs QDs with various thicknesses (dcap), and a rapid annealing of the sample was executed with the proper annealing temperature. By optimizing dcap and the annealing temperature for each QD layer, a broadband Gaussian‐like spectrum with a 102‐nm‐bandwidth was obtained. The coherence function of the emission spectrum exhibits a 4.5‐µm axial resolution for OCT images without distinct side lobes. These results demonstrate the effectiveness of the In‐flushed QDs as a light source to obtain high‐quality OCT images. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaAs tunnel diodes (TDs) embedded with an InAs quantum dot (QD) layer were grown and their performance was compared with that of TDs without a QD layer. The TDs embedded with a QD layer showed enhanced peak tunnel current density and lower differential resistivity at zero bias compared with the TDs without a QD layer. The samples were then annealed to mimic the overlayer growth process. It was found that the performance degradation after annealing was smaller for the QD-layer-embedded TDs. The improved characteristics of the QD-layer-embedded GaAs TDs make them advantageous for interconnecting unit cells in tandem solar cells.

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-level spacing was largely related to the change in the QD shape and size, especially the aspect ratio (height/width), which was confirmed by cross-sectional TEM images.

Scintillation and optical properties of an integrated InAs/GaAs Quantum Dot (QD) Scintillation Detector were investigated to improve efficiency and reduce the decay time of luminescence. The photoluminescent properties of InAs QDs embedded in a GaAs matrix were studied as-grown by molecular beam epitaxy (MBE) on GaAs substrates, and on foreign substrates fabricated as thick (10-25 μm) waveguides. The luminescent efficiency of QDs as-grown on GaAs substrates were examined using photoluminescence (PL) measurements at room temperature and the thermal quenching of PL was analyzed by comparing the integrated PL intensities at 77K-400K. PL measurements were used to optimize QD AlAs capping, QD density (by varying spacing between QD layers), p-type modulation doping between QD layers, and the relative red-shift in QD luminescence from the GaAs band-edge. Room temperature PL revealed improved efficiency using QDs with >350 meV redshifted PL from the addition of a ~2 monolayer (ML) AlAs capping layer on top of the QDs and increased QD layer spacing (~800 nm). PL across a wide range of temperatures showed reduced PL thermal quenching with three distributed p-type modulation doping layers as compared to a single modulation doping layer.

This work reports on theoretical and experimental investigation of the impact of InAs quantum dots (QDs) position with respect to InGaAs strain reducing layer (SRL). The investigated samples are grown by molecular beam epitaxy and characterized by photoluminescence spectroscopy (PL). The QDs optical transition energies have been calculated by solving the three dimensional Schrödinger equation using the finite element methods and taking into account the strain induced by the lattice mismatch. We have considered a lens shaped InAs QDs in a pure GaAs matrix and either with InGaAs strain reducing cap layer or underlying layer. The correlation between numerical calculation and PL measurements allowed us to track the mean buried QDs size evolution with respect to the surrounding matrix composition. The simulations reveal that the buried QDs’ realistic size is less than that experimentally driven from atomic force microscopy observation. Furthermore, the average size is found to be slightly increased for InGaAs capped QDs and dramatically decreased for QDs with InGaAs under layer.

Optical transitions of InAs/In 0.36Ga 0.64As/GaAs(311B) surface quantum dots clearly identified by the piezoreflectance technique

Optical studies on InAs/InGaAs/GaNAs strain-compensated quantum dots grown on GaAs (0 0 1) by molecular beam epitaxy

InAs quantum dots in a GaAs matrix are studied. Those quantum dots are used in applications to enhance the overall efficiency of multi-junction solar cells beyond 40%. Photoluminescence measurements at 77 K using a 532 nm laser have been performed on an epitaxially grown structure of self-assembled InAs quantum dots in a GaAs matrix upon a Ge substrate, where three energy levels are determined at E_n=0=1.01 eV, E_n=1=1.07 eV and E_n=2=1.13 eV. Theoretical calculations of the energy levels determine the quantum dots to be 7 nm high and have a 37 nm base diameter, which is close to atomic force microscopy measurements performed on the samples. Intensity dependant photoluminescence measurements reveal the saturation of the first excited energy level at 5×10⁶ W/m². A general model for the saturation of the first quantum dot excited energy level is then developed. This saturation model is applied to the AM1.5D solar spectrum at 297 K to determine the concentration of solar energy needed to saturate the first excited energy level within a multi-junction solar cell. Saturation was determined to be at ~1.56×10⁴ suns (where 1 sun = 1000 W/m²). Since current solar concentrations are between 500-1000 suns concentration, the saturation of such quantum dots will not occur.

In this paper we report on the growth, using MOVPE (Metal Organic Vapor Phase Epitaxy), of a layer of InAs quantum dots (QDs) on top of quaternary layers of In0.53AlyGa0.47−yAs lattice‐matched to InP substrates, with a variable Stoichiometry due to the variation in the Al concentration. It has been recently reported that as the Al concentration in the quaternary layer increases, smaller InAs QDs are obtained, and also that the peak of the luminescence emission shifts to longer wavelengths up to around 2.1 microns. In this work we present results on the PL characterization for a set of InAs QDs/In0.53AlyGa0.43−yAs samples, for aluminum concentrations of y = 0, 0.058, 0.11 and 0.165. PL spectra were measured changing the laser excitation power and the sample temperature in the range from 16K up to room temperature. From the analysis of the PL spectra we observe that at low temperatures the emission band is composed of two contributions centered at around 0.64 and 0.673 eV which shift to higher energies as the Al concentration in the layer increases as a result of the increase in the energy barrier height between the dot and the InAlGaAs layer. We discuss the nature of the transitions involved in the PL bands and their behavior with the laser power and temperature, using a 1D theoretical model that takes into account the energy position of the quantized electronic levels in the InAs QDs due to the band offsets and to the strain effects at the InAlGaAs‐InAsQD‐InP interfaces.

The strain-mediated evolution of epitaxial ZnTe/ZnSe quantum structures is studied at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy, coupled with electronic properties characterized by photoluminescence spectroscopy. The growth development of these buried quantum dots clearly demonstrates a homogeneous profile with similar pyramidal geometry rather than bi-modal distribution; contradicting prior reports on ZnTe/ZnSe quantum dots. The result is consistent with atomistic theoretical calculations on strain distribution and electronic structure of a modeled quantum dot of similar geometry using a valence force field model. It is also found that the transition from 2-D islands to 3-D quantum dots involves thermally activated carrier transfer process and follows up with formation of extended defects at the quantum dot surface, acting as an effective source for remnant misfit strain relaxation. The new physical understanding concerning the growth of self-assembled ZnTe/ZnSe quantum dots embedded in the active regions provides important information for the measures to control the properties of buried ZnTe quantum dots, setting up a key footstep in developing novel materials of energy conversion.