Stacked Quantum Dots Research Articles

Variation of the photoluminescence spectrum of InAs/GaAs heterostructures grown by ion-beam deposition.

This work reports on an experimental investigation of the influence of vertical stacking of quantum dots, the thickness of GaAs potential barriers, and their isovalent doping with bismuth on the photoluminescence properties of InAs/GaAs heterostructures. The experimental samples were grown by ion-beam deposition. We showed that using three vertically stacked layers of InAs quantum dots separated by thin GaAs barrier layers was accompanied by a red-shift of the photoluminescence peak of InAs/GaAs heterostructures. An increase in the thickness of the GaAs barrier layers was accompanied by a blue shift of the photoluminescence peak. The effect of isovalent Bi doping of the GaAs barrier layers on the structural and optical properties of the InAs/GaAs heterostructures was investigated. It was found that the Bi content up to 4.96 atom % in GaAs decreases the density of InAs quantum dots from 1.53 × 1010 to 0.93 × 1010 cm−2. In addition, the average lateral size of the InAs quantum dots increased from 14 to 20 nm, due to an increase in the surface diffusion of In. It is shown that isovalent doping of GaAs potential barriers by bismuth was accompanied by a red-shift of the photoluminescence peak of InAs quantum dots of 121 meV.

Carrier Transfer in Closely Stacked GaAs/AlGaAs Quantum Dots Grown by Using Droplet Epitaxy

We investigate the carrier transfer in stacked droplet epitaxially grown GaAs quantum dots (QDs) in experiments and calculations. While in the Stranski-Krastanov growth mode, QDs align due to stain propagation, droplet epitaxy QDs pose a difficulty for achieving coupled stacked QDs due to their random positioning. We demonstrate that carrier transfer is possible in such structures by designing their size and areal density. We achieve a significant geometrical overlap between stacked QDs by employing an areal density of 3.9×1010 dots/cm2 and an average QD diameter of 45.5 nm. A clear redshift in the position of the photoluminescence peak is observed when the separation layer’s thickness is reduced from 16 nm to 2.5 nm. Theoretical calculations of the electronic states of the stacked QDs with varying degrees of misalignment confirm that this red-shift is mainly caused by a lowering of the ground state energy due to coupling. To separately analyze the effect of vertical carrier transfer between QDs, we investigate samples with two layers of stacked QDs of different sizes. We demonstrates in photoluminescence experiments that carriers readily transfer to the larger QD when the barrier thickness is reduced to a degree where tunneling is possible.

Investigation of multimodality effect in quantum dots InGaAs/GaAs grown by MOVPE

Self-assembled InGaAs quantum dots with In content 80% grown on vicinal GaAs substrates by MOVPE method have been investigated by photoluminescence technique in a wide range of temperatures and excitation densities. Multimodality of distribution of size and (or) shape in array of vertically stacked quantum dots synthesized under different growth conditions as well as in a single layer structure was found on the ground of appearance of a set of peaks in photoluminescence spectra at lowered temperatures. Influence of growth conditions on density and radiation wavelength of quantum dot modes has been analyzed. Investigation of photoluminescence of samples as a function of temperatures has allowed determining activation energies of three channels of the losses that were attributed to different modes of quantum dots.

Guided-mode resonance gratings for enhanced mid-infrared absorption in quantum dot intermediate-band solar cells

Achieving strong absorption of low-energy photons is one of the key issues to demonstrate quantum dot solar cells working in the intermediate band regime at practical concentration factors and operating temperatures. Guided-mode resonance effects may enable large enhancement of quantum dot intraband optical transitions. We propose quantum dot thin-film cells designed to have significant field waveguiding in the quantum dot stack region and patterned at the rear-side with a sub-wavelength diffraction grating. Remarkable increase of the optical path length at mid-infrared wavelengths is shown owing to guided-mode resonances. Design guidelines are presented for energy and strength of the second-photon absorption for III-V quantum dots, such as InAs/GaAs and GaSb/GaAs, whose intraband and intersubband transitions roughly extends over the 2 - 8 µm range. The proposed design can also be applied to quantum dot infrared detectors. Angle-selectivity is discussed in view of applications in concentrator photovoltaic systems and infrared imaging systems.

Vertical strain-induced dot size uniformity and thermal stability of InAs/GaAsN/GaAs coupled quantum dots

We report, for the first time, 10-layer InAs/GaAsN/GaAs quantum dots (QDs) by varying the GaAs layer thickness and investigate their optical properties. Due to propagation of strain field from bottom to top, multiple stacking of self-assembled QDs helps special ordering of QDs on surface resulting in dot size uniformity. InAs coupled QDs with GaAsN/GaAs spacers of 2/10 nm exhibited a bimodal distribution with narrow linewidths and low As-related deep level defect peak intensity, attributable to good optical quality of QDs. N-like local vibrational mode in Raman spectroscopy was found to be consisted of ten oscillation fringes, possibly because of ten periods of GaAsN layer used in QD-heterostructures. Higher strain coupling resulted in lesser blue-shift with increasing annealing temperature as compared to the lower coupling, leading to thermal stability of coupled QDs. Coupled QDs exhibiting narrow linewidths and thermal stability could be employed in fabrication of photodetectors.

Light-trapping enhanced thin-film III-V quantum dot solar cells fabricated by epitaxial lift-off

We report thin-film InAs/GaAs quantum dot (QD) solar cells with n−i−p+ deep junction structure and planar back reflector fabricated by epitaxial lift-off (ELO) of full 3-in wafers. External quantum efficiency measurements demonstrate twofold enhancement of the QD photocurrent in the ELO QD cell compared to the wafer-based QD cell. In the GaAs wavelength range, the ELO QD cell perfectly preserves the current collection efficiency of the baseline single-junction ELO cell. We demonstrate by full-wave optical simulations that integrating a micro-patterned diffraction grating in the ELO cell rearside provides more than tenfold enhancement of the near-infrared light harvesting by QDs. Experimental results are thoroughly discussed with the help of physics-based simulations to single out the impact of QD dynamics and defects on the cell photovoltaic behavior. It is demonstrated that non radiative recombination in the QD stack is the bottleneck for the open circuit voltage (Voc) of the reported devices. More important, our theoretical calculations demonstrate that the Voc offset of 0.3V from the QD ground state identified by Tanabe et al., 2012, from a collection of experimental data of high quality III-V QD solar cells is a reliable – albeit conservative – metric to gauge the attainable Voc and to quantify the scope for improvement by reducing non radiative recombination. Provided that material quality issues are solved, we demonstrate – by transport and rigorous electromagnetic simulations – that light-trapping enhanced thin-film cells with twenty InAs/GaAs QD layers reach efficiency higher than 28% under unconcentrated light, ambient temperature. If photon recycling can be fully exploited, 30% efficiency is deemed to be feasible.

Large Red Shift of Emission of PbS Quantum-dot Superlattice with Butylamine Ligands

Background: Quantum-dot (QD) superlattice solar cells can attain quite high energy conversion efficiencies. The formation of epitaxial QD superlattice is very difficult since extremely delicate control is required for the QD stacking during the growth to avoid QD deformation and dislocation generation. It is well known that CQDs are self-assembled to form superlattice in electron microscope area after sedimentation in a solvent. To apply the colloidal QD superlattice to solar cell, superlattice size must be expanded from electron microscope order to centimeters or more. Method: Long-periodic packing was done by slowly depositing PbS QDs with short-chain butylamine ligands in a solvent into a pyramidal-hole array processed by anisotropic KOH etching of a (001)Si substrate. Results: QDs' ground state energy reduced by 116 meV together with 1.5-time increase in luminescence lifetime after the film formation. Conclusion: We demonstrate that the film prepared by close packing of colloidal QDs with shortchain ligands exhibited the large red shift of emission wavelength with the increase in emission lifetime. The results suggest that the carrier wavefunction was well delocalized in the long-periodic superlattice film on the template. Keywords: Quantum dot superlattice, photoluminescence, photovoltaic cells, PbS, butylamine ligands, large red shift.

Stranski–Krastanov InAs/GaAsSb quantum dots coupled with sub-monolayer quantum dot stacks as a promising absorber for intermediate band solar cells

The optical properties of the Stranski–Krastanov (S–K) grown InAs/GaAsSb quantum dots (QDs) coupled to sub-monolayer (SML) InAs QD stacks are investigated using photoluminescence (PL) spectroscopy. The PL emission peak of the S–K QDs shifts to shorter wavelengths with increasing the number of SML stacks (NSML) due to the increasing strain fields from the SML QDs. The PL peak energy is linearly increased with increasing the cube root of excitation power, with a different ratio of the absorption coefficient to radiative recombination rate for all the QD samples. The total carrier lifetime for the S–K QDs is increased with increasing NSML, most probably caused by the increase in the ground-state transition energy of the S–K QDs. The nonmonotonic behavior of the thermal activation energy of electrons in the S–K QDs is observed due to the NSML-dependent variation of the strain and Coulombic interaction within the QDs.

Polaron resonances in two vertically stacked quantum dots

In this work, we present a theoretical study of polaron states in a double quantum dot system. We present realistic calculations which combine 8 band \kp model, configuration interaction approach and collective modes method. We investigate the dependence of polaron energy branches on axial electric field. We show that coupling between carriers and longitudinal optical phonons via Fr\"{o}hlich interaction leads to qualitative and quantitative reconstruction of the optical spectra. In particular, we study the structure of resonances between the states localized in different dots. We show that $p$-shell states are strongly coupled to the phonon replicas of $s$-shell states, in contrast to the weak direct $s$-$p$ coupling. We discuss also the dependence of the phonon-assisted tunnel coupling strength on the separation between the dots.

Double stack layer structure of SiNx/pm-Si thin films for downshifting and antireflection properties

In this work, we propose a double stack layer design of SiNx/pm-Si thin films by using dichlorosilane in plasma enhanced chemical vapor deposition (PECVD). Adequate deposition condition has been found to attain the average minimal reflectance corresponding to two different layers of SiNx and pm-Si at the suitable refractive index and thickness of the respective films. The average reflectance of the structure is reduced to ∼3% at normal incidence for a wavelength range from 190 to 1100nm as in comparison to single layer of SiNx thin film structure. Furthermore, using transmission electron microscopy (TEM) presence of double layer stack and formation of silicon quantum dots (QD’s) in the size regime of 3–4nmhas been found. Photoluminescence (PL) diminishment in the double layer structure as in comparison to the single layer of SiNx thin film confirms the effective absorption and total internal reflection induced due to the gradient refractive index. However, visible photoluminescence observed in the present work also remarks the downshift property from the thin film which could subsequently improve the efficiency in silicon solar cells. Present work highlights the prospective of novel structure for downshifting, antireflection and as an passivation coating in new generation of silicon solar cells.

In/Ga intermixing effect on high-stacked InAs/In0.11Ga0.89As/GaAs self-assembled quantum-dots solar cells using photoluminescence spectroscopy and scanning near-field optical microscopy

By using photoluminescence spectroscopy and scanning near-field optical microscopy, the local effect of In/Ga intermixing between InAs quantum-dot layers and InGaAs and/or GaAs layers has been investigated in 5 and 20 stacked InAs/In0.11Ga0.89As/GaAs self-assembled quantum-dot solar cells. The photoluminescence spectra show two peaks due to a bimodal size distribution of the quantum dots. By increasing the number of quantum-dot stacks, a red shift of the ground-state emission energy of 70meV (100meV) for the entity of quantum dots with small (large) size and a decrease of the intensity in combination with an increase of the width of the peaks is observed. The red shift is attributed to three effects: (i) formation of a mini band caused by the electronic coupling of islands in the vertical direction, (ii) increase of the quantum dots size caused by strain-field coupling, and (iii) In/Ga intermixing. The increases of the peak width and the reduction of the intensity is explained by the creation of non-radiative recombination centers. Using near-field optical microscopy to map the topography and to measure the local photocurrent, the non-radiative recombination centers can be tentatively identified.

Efficient Heterotransfer between Visible Quantum Dots

We fabricated green–red (GR) and blue–red (BR) bilayer stacked quantum dots (QDs) using electrospray deposition. Along with steady state and time-resolved photoluminescence (PL), subnanosecond donor PL decay and corresponding acceptor PL rise signals were observed, which are ascribed to the energy transfer between different visible QDs (heterotransfer). The heterotransfer rates were estimated as (0.57 ± 0.01 ns)−1 and (0.65 ± 0.02 ns)−1 for GR and BR systems, respectively, which agree well with theoretical calculations. Owing to their geometrical proximity, mixed QD layers with GR and BR showed qualitatively higher heterotransfer efficiencies of 64% and 81%, compared to stacked QD layers, which have efficiencies of 23% and 64%, respectively.

A modified gradient approach for the growth of low-density InAs quantum dot molecules by molecular beam epitaxy

Two vertically stacked quantum dots that are electronically coupled, so called quantum dot molecules, are of great interest for the realization of solid state building blocks for quantum communication networks. We present a modified gradient approach to realize InAs quantum dot molecules with a low areal density so that single quantum dot molecules can be optically addressed. The individual quantum dot layers were prepared by solid source molecular beam epitaxy depositing InAs on GaAs(100). The bottom quantum dot layer has been grown without substrate rotation resulting in an In-gradient across the surface, which translated into a density gradient with low quantum dot density in a certain region of the wafer. For the top quantum dot layer, separated from the bottom quantum dot layer by a 6nm thick GaAs barrier, various InAs amounts were deposited without an In-gradient. In spite of the absence of an In-gradient, a pronounced density gradient is observed for the top quantum dots. Even for an In-amount slightly below the critical thickness for a single dot layer, a density gradient in the top quantum dot layer, which seems to reproduce the density gradient in the bottom layer, is observed. For more or less In, respectively, deviations from this behavior occur. We suggest that the obvious influence of the bottom quantum dot layer on the growth of the top quantum dots is due to the strain field induced by the buried dots.

Dynamic characteristics of 20‐layer stacked QD‐SOA with strain compensation technique by ultrafast signals using optical frequency comb

In this article, we evaluated the static and dynamic characteristics of 20‐layer stacked quantum dot semiconductor optical amplifiers (QD‐SOAs) grown on an InP(311)B substrate with a strain compensation technique by ultrafast signals using an optical frequency comb. The gain peak wavelength of the fabricated QD‐SOA was 1520 nm, and a maximum gain of approximately 15.8 dB was obtained when the injection current was 350 mA. By using optical bit rate multiplier modules, an optical pulse, which was generated by the use of the optical frequency comb, was multiplexed. We observed the dynamic behavior of the QD‐SOA with the pulse trains. The minimum interval between pulses was 4.2 ps, and we measured using an optical sampling oscilloscope. It was found the QD‐SOA could respond to ultrafast pulses that were equivalent to the signals of 220 Gb s−1 class speed without any large pattern distortions. In addition, we could obtain a rather clear optical pulse waveform. We also estimated the gain recovery time of the QD‐SOA to be 6.0 ps by using a pump probe‐like method. One of the reasons that the fabricated QD‐SOA could respond to ultrafast signals was its ultrashort gain recovery time.

Lateral excitonic switching in vertically stacked quantum dots

We show that the application of a vertical electric field to the Coulomb interacting system in stacked quantum dots leads to a 90° in-plane switching of charge probability distribution in contrast to a single dot, where no such switching exists. Results are obtained using path integral quantum Monte Carlo with realistic dot geometry, alloy composition, and piezo-electric potential profiles. The origin of the switching lies in the strain interactions between the stacked dots hence the need for more than one layer of dots. The lateral polarization and electric field dependence of the radiative lifetimes of the excitonic switch are also discussed.

A Nanowire-Based Plasmonic Quantum Dot Laser.

Quantum dots enable strong carrier confinement and exhibit a delta-function like density of states, offering significant improvements to laser performance and high-temperature stability when used as a gain medium. However, quantum dot lasers have been limited to photonic cavities that are diffraction-limited and further miniaturization to meet the demands of nanophotonic-electronic integration applications is challenging based on existing designs. Here we introduce the first quantum dot-based plasmonic laser to reduce the cross-sectional area of nanowire quantum dot lasers below the cutoff limit of photonic modes while maintaining the length in the order of the lasing wavelength. Metal organic chemical vapor deposition grown GaAs-AlGaAs core-shell nanowires containing InGaAs quantum dot stacks are placed directly on a silver film, and lasing was observed from single nanowires originating from the InGaAs quantum dot emission into the low-loss higher order plasmonic mode. Lasing threshold pump fluences as low as ∼120 μJ/cm(2) was observed at 7 K, and lasing was observed up to 125 K. Temperature stability from the quantum dot gain, leading to a high characteristic temperature was demonstrated. These results indicate that high-performance, miniaturized quantum dot lasers can be realized with plasmonics.

Characteristics of highly stacked InAs quantum-dot laser grown on vicinal (001)InP substrate

We fabricate broad-area laser diodes consisting of 30-layer stacks of InAs quantum dots by using a strain-compensation technique on a vicinal (001)InP substrate. These laser diodes exhibit ground-state lasing at 1576 nm in the pulsed mode with a high characteristic temperature of 111 K at around room temperature (20–80 °C).

Influence of the inverse Auger process on the performance of InxGa1−xN/GaN quantum dot solar cells

It has been proposed that the use of self-assembled nano sized quantum dot (QD) arrays can break the Shockley–Queisser efficiency limit by extending the absorption of solar cells into the low-energy photon range while preserving their output voltage. This would be possible if the infrared photons are absorbed in the two sub-bandgap QD transitions simultaneously and the energy of two photons is added up to produce one single electron–hole pair, as described by the intermediate band model. This paper indicates the energy conversion efficiency of a quantum dot multilayer solar cell considering impact ionization effect. A p–i–n InxGa1−xN/GaN quantum dot solar cell structure has been taken into account in the calculation. It is shown that the efficiency of a cell strongly depends on the impact ionization in stacked quantum dots at i-region of the cell. In our proposed structure it is demonstrated that, if averaged probability of impact ionization, P, varies from zero to one, maximum efficiency increases by more than 12% (from 43 percent in P=0 to 55 percent in P=1). Also it is demonstrated that by decreasing θ, maximum efficiency increases and reaches to its maximum, 59%, in θ=2.

Atom probe tomography analysis of InAlGaAs capped InAs/GaAs stacked quantum dots with variable barrier layer thickness

In this work, we analyse by Atom Probe Tomography (APT) the composition distribution of InAs/GaAs stacked quantum dots (QDs) with InAlGaAs capping layers. Two different GaAs barrier layer thicknesses have been considered, 20 nm and 30 nm. Our results have shown the vertical alignment of QDs in the structure with reduced barrier layer and the unexpected formation of InGaAs quantum rings (QRings) surrounded by an Al-rich area in this sample. The APT data show that the QRings contain an amount of In atoms that is double the In composition in the QDs. The reduced strain measured in the QRings with regard to the QDs suggests that this change in morphology is promoted by the reduction in the strain of the nanostructure. The behaviour of Al during the process of formation of the QRings is also discussed.

Measurement Back-Action in Stacked Graphene Quantum Dots.

We present an electronic transport experiment in graphene where both classical and quantum mechanical charge detector back-action on a quantum dot are investigated. The device consists of two stacked graphene quantum dots separated by a thin layer of boron nitride. This device is fabricated by van der Waals stacking and is equipped with separate source and drain contacts to both dots. By applying a finite bias to one quantum dot, a current is induced in the other unbiased dot. We present an explanation of the observed measurement-induced current based on strong capacitive coupling and energy dependent tunneling barriers, breaking the spatial symmetry in the unbiased system. This is a special feature of graphene-based quantum devices. The experimental observation of transport in classically forbidden regimes is understood by considering higher-order quantum mechanical back-action mechanisms.