Growth, characterizations, theory and lasing of vertically stacked quantum dots (original) (raw)
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Optimizing the spacer layer thickness of vertically stacked InAs/GaAs quantum dots
Materials Science and Engineering: C, 2006
Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties. D
Nanotechnology, 2007
This work systematically investigated the optical and structural properties of multilayer electronic vertically coupled InAs/GaAs quantum dot (QDs) structures grown by molecular beam epitaxy for long-wavelength applications. A significant energy blue-shift in the photoluminescence (PL) spectra from 30-period InAs/GaAs QDs structures was observed as the GaAs spacer thickness was decreased. Transmission electron microscopy (TEM) and PL measurements indicated that the abnormal blue-shift can be attributed to the strain-driven In/Ga intermixing between QDs and spacer layers, which overcompensates for the effects of electronic and structural couplings between QD layers. Moreover, this study demonstrates that increasing the growth rate of InAs QDs can prevent intermixing. A PL emission wavelength of 1320 nm with strong luminescence at room temperature, which corresponds to an energy red-shift of 50 meV from that of the single QD layer sample, was achieved in a 10-period InAs/GaAs QD superlattice with a spacer thickness of 16 nm.
The vertical coupling effect on the electronic states in self-assembled InAs/GaAs quantum dots
Solar Energy Materials and Solar Cells, 2006
A series of self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy to investigate the dependence of transition energy on GaAs spacer layer thickness. The latter was varied of 60, 45, 30, 15, and 10 monolayers (MLs) for the five different samples. The photoluminescence (PL) measurements were carried out. The electronic states in coupled selfassembled InAs/GaAs QDs are investigated through PL properties with the aid of the theoretical calculation. First the energy levels of electrons and holes are calculated by solving the threedimensional Schro¨dinger equation by considering the vertical coupling effect between a finite numbers of QDs. Based on the results the energies transitions between electrons and holes levels are calculated. Modification of PL spectra by increasing number of layers was found and attributed to an increasing vertical coupling. The PL full-width at half-maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 MLs. Moreover, the observed behavior PL lines is analyzed theoretically. r
Applied Physics A: Materials Science & Processing, 2004
Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 µm wavelength range (at room temperature) with growth interruption after InAs QD deposition.
InAs/GaAs SK quantum dots stacking: Impact of spacer layer on optical properties
In this paper, we report the optical properties of vertically stacked multilayers of self-organized InAs/GaAs quantum dots QDs with different GaAs spacer layer thicknesses of 10 and 15 nm, using photoluminescence PL measurement. The 10 K PL spectrum exhibits double-emission peaks ambiguously identified in PL spectra where the excitation power dependence reveals that these emission peaks are attributed to fundamental ground state GS of large quantum dots in the low energy side, and first excited state 1ES transitions of large quantum dots in coincidence with fundamental ground state GS of smaller ones, in the high energy side. Both the excitation power dependence and the temperature dependence measurement results exhibited competition between the latter's optical transitions in the QDs, which becomes more prevalent at higher excitation powers. The main PL peak is quenched above 190 K, giving rise to "a 1D miniband", ascribed to the electronic coupling between QDs along the stacking direction and tuning the emission wavelength around 1.3 μm. Our results emphasize the Key role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layers in optimizing design for long-wavelength devices.
High optical property vertically aligned InAs quantum dot structures with GaAsSb overgrown layers
Journal of Crystal Growth, 2011
This study investigates the feasibility of growing high quality columnar InAs/GaAsSb quantum dots (QDs) on a GaAs (1 0 0) substrate using a molecular beam epitaxial system. Structural and photoluminescence (PL) studies are conducted on vertically aligned, ten-period InAs quantum dot (QD) stacks with two different overgrown layer designs. Experimental results indicate an increased dot density of 5 Â 10 10 cm À 2 with completely suppressed coalescences in vertically aligned InAs quantum dots capped by a GaAsSb layer. This finding demonstrates a columnar dot structure with an enhanced luminescent intensity, activation energy, and narrow spectral line width of 22 meV.
Evidence of lateral coupling in long wavelength vertically stacked InAs/GaAs(001) quantum dots
The European Physical Journal Applied Physics, 2005
1.3 µm room temperature emitting multiple-stacked InAs/GaAs(001) quantum dots (QDs) are grown by molecular beam epitaxy (MBE) and investigated by photoluminescence (PL), polarized photoluminescence (PPL), photoluminescence excitation (PLE), time resolved photoluminescence (TRPL) and atomic force microscopy AFM. The PL measurement shows that two distinct sets of QDs coexist in the sample. The AFM image of the tenth QDs layer not only confirms the bimodal size distribution of the QDs but also shows that the large QDs are elongated along the [1-10] direction. The former structural information has been verified by PPL. Through the excitation density dependent PL and the detection energy dependent PLE we have evidenced two kinds of QDs within the small size dots population: isolated QDs and laterally coupled QDs with vertically coupled large size QDs. The large size dot population is found to possess a long PL decay time confirming that they are electronically coupled. In the meanwhile the PL decay time of the small QDs is found to be similar to that of single layer QDs. These results would help improve understanding some fundamental properties of an interesting structure for optoelectronic applications.
Comparison of single-layer and bilayer InAs/GaAs quantum dots with a higher InAs coverage
Opto-Electronics Review, 2010
Epitaxially grown self-assembled InAs quantum dots (QDs) have found applications in optoelectronics. Efforts are being made to obtain efficient quantum-dot lasers operating at longer telecommunication wavelengths, specifically 1.3 μm and 1.55 μm. This requires narrow emission linewidth from the quantum dots at these wavelengths. In InAs/GaAs single layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size ...
Superlattices and Microstructures, 1998
In this work, we present substrate orientation effects on optical properties in vertically stacked In 0.5 Ga 0.5 As layers grown by molecular beam epitaxy on (311)A/B and reference (100) GaAs substrates. Samples were grown for different GaAs spacer thicknesses. The spacer thickness variation shows the influence on PL spectra for all planes. The differences in peak shape, peak position, amplitude and integrated luminescence have been observed for all surfaces. These differences suggest that indium migration in spacer layers is caused by the strain fields induced by islands buried below, and is different at the three surfaces. Vertical electronic coupling between quantum dots is confirmed by photoluminescence temperature dependence.