Coupled InAs Quantum Dots Research Articles

Formation of strain reduced In0.54Al0.34Ga0.12As layer of vertically coupled QDs arrays for O-band telecom single photon sources

We demonstrate the correlative research on the multi-stacked vertically coupled InAs quantum dots (VCQDs) capped by 15 nm combinational capping layer of In(0.21)Al(0.21)Ga(0.58)As and GaAs layer through Atom probe tomography (APT), Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), High-Resolution X-Ray Diffraction (HRXRD) and Reciprocal Space Mapping (RSM). From the comparative studies, we observed a strain reduction of ∼ 2.3 ± 0.05% in the system, leading to the formation of strain-reduced (SR) In0.54Al0.34Ga0.12As layer with a lattice constant of 5.88 Å, which will work as a shielding layer. We found that the middle portion of the 3rd to 8th layer QDs structure was constant in size, shape, composition, and density, which enhances the carrier confinement in QDs. The computed strain energy values contributed to the overall growth structure of the 30 - layers VCQDs were recorded to be 2.2 meV/atoms. We report the detailed analogy of the in-out flow of In/Ga adatoms in the In0.54Al0.34Ga0.12As layer leading to the evolution of fully developed bigger InAs QDs size. To support the APT results, we also present the outcomes studies of the HRXRD Out-plane and In-Plane symmetric RSM experiment at (002), (004), and (006) reflections to investigate the compressive strain contribution in the SR In0.54Al0.34Ga0.12As shielding layer. This 30-layer VCQDs structure can be employed to fabricate high thermal stability operating at 1.3 µm O-band telecom Single Photon Sources (SPSs).

Evaluation of In(Ga)As capping in a multilayer coupled InAs quantum dot system: Growth strategy involving the same overgrowth percentage

Strain-coupled multilayer Quantum Dot (QD) structures draw a great attention these days because of their superior optical and device performance. However, these coupled multilayer QD structures have limitations such as non-uniform QD size distribution, defects, and dislocations due to the propagation of strain in QD layers. Furthermore, the carrier relaxation lifetime must be improved in these coupled QD structures for sought-after device performance. This study serves for three main purposes. Firstly, we have utilized a growth strategy to maintain the overgrowth percentage such that the Stranski-Krastanov (SK) QD dimensions in every layer is same for all the multilayer structures. Secondly, these multilayer SK QDs are made to electronically couple with the Sub-monolayer (SML) QDs in such a way that the carrier relaxation lifetime can be increased, that could ameliorate the photoconductive gain and the responsivity. Lastly, the amount of cumulative strain generated inside QDs can be reduced by the incorporation of In0.15Ga0.85As strain reducing layer (capping layer) in a multilayer structure. Five different multilayer structures with single (x1), bi (x2), penta (x5), hepta (x7), and ten-layers (x10) of SK QDs electronically coupled to six stacks of SML QDs are used in this study. Photoluminescence (PL) studies reveal that the SK QD dimensions is maintained to be the same in all multilayer structures due to the proposed growth strategy, which is also observed through transmission electron microscopy (TEM) images. Photoluminescence excitation (PLE) analysis shows the match of excited states of SK QDs to the ground state of the SML QDs, aiding the carrier tunnel phenomena. The ω/2Ө measurements from high resolution X-ray diffraction (HRXRD) is carried out to calculate the overall strain in the grown heterostructures. The optical properties and the strain components obtained from the nextnano simulation tool are in good agreement with the experimental results. Thus, coupled multilayer QD structures with a growth strategy in the current study would be more suitable for the realization of quantum dot infrared photodetectors and intermediate band solar cell applications accommodated with high device efficiency.

Carrier dynamics in hybrid nanostructure with electronic coupling from an InGaAs quantum well to InAs quantum dots

Carrier dynamics including carrier relaxation and tunneling within a coupled InAs quantum dot (QD) – In0.15Ga0.85As quantum well (QW) hybrid nanostructure are investigated using photoluminescence (PL) spectroscopy. This coupled hybrid nanostructure shows a single PL peak from the QD emission at low excitation intensity and a band filling behavior is observed as the excitation intensity increases, suggesting that there exists a channel to capture carriers from the QW to the QDs. Time resolved PL (TRPL) measurements extract a carrier tunneling time of 103.7 ps, which is only one third of the theoretical prediction. A double-channel resonant carrier tunneling mechanism from the QW to the wetting layer and to the fifth excited state of the QDs and then carrier relaxation into lower discrete QD energy states is proposed to explain this fast carrier tunneling. The double-channel resonant carrier tunneling mechanism is qualitatively supported through the analysis of the excitation-dependent PL spectra as well as the PL excitation spectra. These results enrich our understanding of carrier dynamics in coupled QD and QW hybrid structures.

Estimation of the effective electron-capture cross section in the emission processes from arrays of vertically coupled InAs quantum dots in the n-GaAs matrix

In the paper we investigate the processes of electron emission from arrays of vertically coupled InAs quantum dots in the space charge region of the Schottky barrier in the matrix of n- type of GaAs. We developed a model of thermally activated tunneling of charge carriers, which allows to calculate the parameters that characterize the processes of electron emission. The temperature dependence of the activation energy of the carriers, the ground state energy carrier in the quantum dot at different reverse bias voltage of the Schottky barrier, and the dependence of the electron-capture cross section on the binding energy of the quantum dot ground state were obtained. The comparison of the values performed for samples with different number of layers of quantum dots: 3, 6 or 10 layers.

Effect of barrier thickness on structural, optical, and spectral behaviors of vertically strain coupled InAs/GaAs quantum dot infrared photodetectors

The optical, electrical, and spectral properties of a strain coupled InAs quantum dot detector with a fixed quaternary capping of InAlGaAs and variable GaAs barrier thickness were investigated along with an equivalent uncoupled structure. Self-assembled quantum dots with a multimodal dot size distribution were achieved owing to vertical strain coupling. Strain and electronic coupling were utilized to improve the optical and electrical performance of the fabricated quantum dot infrared photodetector. The peak spectral response was tuned by varying barrier thickness, and a blue shift (almost 1 μm) was observed by increasing the capping thickness from sample A (90 Å capping) to E (500 Å capping). High responsivity and detectivity (∼1010 cm Hz1/2/W) were observed for all coupled samples as compared to the uncoupled sample. All coupled samples showed high thermal stability in the photoluminescence peak with high-temperature annealing.

Optical measurement and modeling of interactions between two hole spins or two electron spins in coupled InAs quantum dots.

Two electron spins in quantum dots coupled through coherent tunneling are generally acknowledged to approximately obey Heisenberg isotropic exchange. This has not been established for two holes. Here we measure the spectra of two holes and of two electrons in two vertically stacked self-assembled InAs quantum dots using optical spectroscopy as a function of electric and magnetic fields. We find that the exchange is approximately isotropic for both systems, but that significant asymmetric contributions, arising from spin-orbit and Zeeman interactions combined with spatial asymmetries, are required to explain large anticrossings and fine-structure energy splittings in the spectra. Asymmetric contributions to the isotropic Hamiltonian for electrons are of the order of a few percent while those for holes are an order of magnitude larger.

Using localized double-quantum-coherence spectroscopy to reconstruct the two-exciton wave function of coupled quantum emitters

Coherent multidimensional spectroscopy allows us to inspect the energies and the coupling of quantum systems. Coupled quantum systems—such as a coupled semiconductor quantum dot or pigments in photosynthesis—form delocalized exciton and two-exciton states. A technique is presented to decompose these delocalized wave functions into the basis of individual quantum emitters. This quantum state tomography protocol is illustrated for three coupled InAs quantum dots. To achieve the decomposition of the wavefunction, we combine the double-quantum-coherence spectroscopy with spatiotemporal control, which allows us to localize optical excitations at a specific quantum dot. Recently, a protocol was proposed for single exciton states (Richter et al 2012 Phys. Rev. B 86 085308). In this paper, we extend the method presented by Richter et al with respect to: the reconstruction of two-exciton states, a detailed analysis process of reconstruction and the effect of filtering to enhance the quality of the reconstructed wave function.

Mid-infrared Electroluminescence from Surface Plasmon Coupled InAs Quantum Dots

AbstractThe mid-infrared spontaneous emission from intersubband energy transitions in self-assembled InAs quantum dots is demonstrated with plasmonic top contact output couplers. Electrically pumped devices having subwavelength meshes designed to exhibit extraordinary optical transmission from 9 – 12 μm are measured and compared to a reference device with an open area contact. From additional patterning on the top contact, the signal-to-noise ratio was 4 times greater than the reference device. Beyond simply filtering the emission spectra of the quantum dot material, an emission null is observed which we link to the dots being in the near field region of the plasmonic coupler.

Electron emission from multilayer ensembles of vertically coupled InAs quantum dots in an n-GaAs matrix

Electron emission from multilayer arrays of vertically coupled InAs quantum dots into the n-GaAs matrix in Schottky-barrier structures (electron concentration n ≈ 2 × 1016 cm−3) is studied by admittance spectroscopy. It is established that, in the temperature region below ∼70 K, electron emission in a rate range of 3 × 104–3 × 106 s−1 proceeds via thermally activated tunneling through intermediate virtual states. As the number of layers in the quantum dot array increases from three to ten, a decrease in the electron emission rate is observed.

Polarization dependent photoluminescence of charged quantum dot molecules

AbstractWe present the polarization dependent photoluminescence from coupled InAs quantum dots. We find polarization memory signatures consistent with those reported for single quantum dots. We also observe a continuous change in the polarization memory of the positively charged exciton as it changes charge configuration due to a hole tunnelling as a function of applied bias. This may provide a method for electrical control of the anisotropic exchange interaction which would be useful for spin manipulation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Capacitance studies of multilayer ensembles of InAs QDs in a GaAs matrix

Quasi-static capacitance characteristics of multilayer arrays of vertically coupled InAs quantum dots (QDs) in a GaAs matrix were analyzed on the assumption of a Gaussian energy distribution of the ground state of the QDs. An array of InAs QDs with a characteristic base size of about 20 nm and height of ∼3 nm was ordered in the growth direction and had 3, 6, or 10 layers spaced by ∼5 nm. It was found that, as the number of layers increases from 3 to 10, the average binding energy of the ground electron state grows from ∼80 to ∼120 meV and the root mean square deviation characterizing the energy distribution of the levels of this state decreases from ∼30 to ∼15 meV.

Spin-state mixing in InAs double quantum dots

We quantify the contributions of hyperfine and spin-orbit-mediated singlet-triplet mixing in weakly coupled InAs quantum dots by electron transport spectroscopy in the Pauli spin-blockade regime. In contrast to double dots in GaAs, the spin-orbit coupling is found to be more than two orders of magnitudes larger than the hyperfine mixing energy. It is already effective at magnetic fields of a few mT, where deviations from pure hyperfine mixing are observed.

Optical Signatures of Coupled Quantum Dots

An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb interactions shift the molecular resonance of the optically excited state (charged exciton) with respect to the ground state (single charge), enabling light-induced coupling of the quantum dots. This result demonstrates the possibility of optically coupling quantum dots for application in quantum information processing.

Splitting of excitons and biexcitons in coupled InAs quantum dot molecules

Coupling in laterally coupled InAs quantum dot molecules has been observed by conventional microscopic photoluminescence spectroscopy. At low excitation intensity, splitting of exciton emission is observed, and this is ascribed to recombination of bonding and antibonding states when two quantum dots are coupled. With increasing excitation intensity, splitting of biexciton emission because of coupling is observed on both sides of the exciton peaks. The splitting of both exciton and biexciton peaks increases with increasing energy, which implies a large wave function overlap at high energy.

Inter-dot electron transport in coupled InAs quantum dots under a magnetic field

We studied electrical transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. A resonance peak in the current–voltage characteristics was observed in the low temperature region. When the magnetic field was applied, a linear shift of the resonance voltage was observed. As a result of the g-factor estimation, the resonance is attributed to the current corresponding to the electron transport through coupled InAs QDs.

Optical and transport studies in coupled InAs quantum dots embedded in GaAs

We studied optical and electron transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. Photoluminescence (PL) from the high dot density samples indicated asymmetry in the PL spectra when the ambient temperature is lower than about 50 K . Comparing this result with theoretical calculations, it is shown that this phenomenon is explained by the inter-dot electronic coupling effect. In the photo-conductance measurement, resonance peaks in the current–voltage characteristics were observed in the low-temperature region. The dependence of the resonance voltage on the magnetic field intensity was studied to extract the g-factor. It is also shown that the resonances are attributed to the current corresponding to the electron transport through QDs. According to these results, it is concluded that the inter-dot electronic coupling in the self-assembled InAs/GaAs QD systems occurs when the inter-dot spacing is as low as several nanometers and the ambient temperature is less than about 50 K .

Charge separation in coupled InAs quantum dots and strain-induced quantum dots

We present an InAs self-assembled quantum dot structure designed to spatially separate and store photo-generated electron-hole pairs. The structure consists of pairs of strain-coupled quantum dots. Separation of electron-hole pairs into the quantum dots and strain-induced quantum dots has been observed using power-dependant photoluminescence and bias-dependent photoluminescence.

CTR and DCXRD Studies of Lateral Ordering of Quantum Dots in Multilayer Periodic Structures

Containing an array of vertically coupled InAs quantum dots (QDs) in GaAs matrix, multilayer periodic InAs-GaAs structures grown by molecular beam epitaxy, were investigated by SR-CTR (synchrotron radiation-crystal truncation rods) and double-crystal X-ray diffraction (DCXRD). X-ray diffraction is very sensitive to the formation of vertically coupled QDs array, which changes spatial distribution of the scattered radiation. The appreciable elongation of superstructural and substrate spots in lateral directions is explained by occurrence of an additional long-range lateral ordering of scattering objects and quasi-periodic deformation relief in the top GaAs layer. Perfect coherently-connected InAs and GaAs clusters are introduced as a novel qualitative structural model of scattering layers. Based on this model the main features of spatial distribution of diffracted intensity have been explained.

Resonant tunneling through coupled InAs quantum dots

We report the observation of electron tunneling through coupled quantized states in stacked InAs quantum dots embedded in AlAs single barrier for the first time. We speculate that the series of observed dips in the second derivative of the current corresponds to four current peaks with relative coupling energy separation of 5–8 meV. Comparison of these results with calculational results based on the device structure revealed that the data fit reasonably well if we assume the effective mass of electrons in the dots to be (0.085±0.005) m 0. This result of energy dependent effective mass of electrons is consistent with the previous experiments on infrared spectroscopy and quasi-1D transport.

Thermal stability of vertically coupled InAs-GaAs quantum dot arrays

The effect of high-temperature annealing on the optical properties of vertically coupled InAs quantum dots in a GaAs matrix and on the performance of a quantum-dot laser are studied. A strong blue shift of the photoluminescence peak and lasing line, as well as changes in the photoluminescence intensity and temperature dependence of the threshold current density are observed. The reason for this behavior is probably a reduction in the carrier localization energy due to a partial mixing of the In and Ga atoms as well as an improvement in the quality of the low-temperature grown Ga(Al)As layers achieved by high-temperature annealing.