Lateral Quantum Dot Molecules Research Articles

Structure of lateral two-electron quantum dot molecules in electromagnetic fields

The energy levels of laterally coupled parabolic double quantum dots are calculated for varying interdot distances. Electron-electron interaction is shown to dominate the spectra: In the diatomic m ...

Self-Assembled InAs Lateral Quantum Dot Molecules Growth on (001) GaAs by Thin-Capping-and-Regrowth MBE Technique

InAs lateral quantum dot molecules (QDMs) are grown on (001)-GaAs substrates. The self-assembled QDMs are formed in one continuous molecular beam epitaxial (MBE) growth via a thin-capping-and-regrowth technique. Lateral QDMs, each with 10-12 dots arranged in a specific pattern, are determined by the shapes of the underlying nanopropeller quantum dots (QDs). The nanopropeller QDs in turn are obtained by regrowth on nano-holes which have been previously created by capping the first InAs QD layer grown on (001)-GaAs substrate with a thin GaAs layer. The length of the propeller directly influences the number of QDs in a QDM. By varying the conditions for thin-capping, shorter or longer propellers can be achieved, allowing the number of QDs in each QDM to be controlled.

Extended electron states in lateral quantum dot molecules investigated with photoluminescence

InAs quantum dot molecules (QDMs) formed by molecular-beam epitaxy on GaAs (311)B substrates through self-organized anisotropic strain engineering are studied by excitation-power-density- and temperature-dependent macro- and microphotoluminescence (PL). An unusual asymmetric broadening, together with a continuous shift toward higher energies of the PL peak position, most prominent for the $p$-type modulation-doped QDMs, is observed with increasing excitation power density. The $n$-type modulation-doped QDMs exhibit a square-shaped PL spectrum, resembling that of modulation-doped quantum wells. In temperature-dependent macro-PL, two distinct minima of the full width at half maximum are observed, indicating thermally activated carrier redistribution within the QDMs through two different channels at lower and higher temperatures. The micro-PL spectra of the $p$-type modulation-doped QDMs exhibit discrete sets of sharp peaks on top of broad PL bands. The number and intensity of the sharp peaks increase with excitation power density. With increasing temperature, the number and intensity of the sharp peaks decrease while the intensity of the broad PL bands increases, in agreement with the carrier redistribution at lower temperatures. Only broad PL bands are observed for the $n$-type modulation-doped QDMs with similar behavior. These results are explained by state filling in the presence of extended electron states formed due to lateral electronic coupling of the quantum dots within the QDMs.

Topological Hunds rules and the electronic properties of a triple lateral quantum dot molecule

We analyze theoretically and experimentally the electronic structure and charging diagram of three coupled lateral quantum dots filled with electrons. Using the Hubbard model and real-space exact diagonalization techniques we show that the electronic properties of this artificial molecule can be understood using a set of topological Hunds rules. These rules relate the multielectron energy levels to spin and the interdot tunneling $t$, and control charging energies. We map out the charging diagram for up to $N=6$ electrons and predict a spin-polarized phase for two holes. The theoretical charging diagram is compared with the measured charging diagram of the gated triple-dot device.

Amorphous-Te-mediated self-organization of CdSe/ZnSe nanostructures

Deposition of amorphous Te on a strained epitaxial CdSe layer at room temperature, and its subsequent desorption in the temperature range of 200–310 °C results in novel morphological reorganization of the CdSe surface. Dashes, long chains of quantum dots (QDs), and lateral QD molecules have been observed in atomic force microscopy images of CdSe surfaces, after such a treatment. Te admixture of the QD-layer may not be ruled out. The morphology of the CdSe (Te) surface is strongly governed by the thickness of the deposited amorphous Te layer, but shows almost no dependence on the temperature at which this layer is desorbed. The observed self-ordering of nanostructures over length scales of ∼600 nm by this process strongly suggests that Te leads to an enormous enhancement of adatom migration. While at low temperature, photoluminescence (PL) consists of emissions from localized excitons in both the observed large QD-based nanostructures and a residual inhomogeneous CdSe(Te) layer, at room temperature the entire contribution is apparently from the former. This leads to a redshift of the spectra, with increase in temperature from 7 to 300 K, by as much as 600 meV.

Evolution of self-assembled lateral quantum dot molecules

Self-assembled lateral InAs quantum dot molecules (QDMs) are grown by solid-source molecular beam epitaxy (MBE) using the thin-capping-and-regrowth MBE process. Thin capping of GaAs on as-grown InAs quantum dots (QDs) at low temperatures leads to nanohole templates. The shape, size and depth of nanoholes are controlled by capping thickness. Subsequent regrowth with different amounts of InAs on the templates result in nano-propeller QDs with different blades’ dimensions. We showed that the length of the propeller blades is controlled by either the capping temperature or the thickness of capping layer. When the regrowth thickness reaches 1.2 ML, different self-assembled lateral QDMs are formed depending on the shape of nanoholes. The dot uniformity and the dot size of all QDM samples are confirmed by photoluminescence (PL) measurements at 77 K. In addition, by ramping the regrowth temperature while the In shutter is open after the deposition of the capping layer, we are able to investigate the formation of nano-propeller QDs at the beginning phases and to propose a model to explain the evolution.

Power dependent photoluminescence of lateral quantum dot molecules: Indication of extended electron states

AbstractExcitation power density dependent photoluminescence (PL) measurements of an ensemble of ordered lateral quantum dot (QD) molecules containing on average 4 and 8 QDs are carried out. Asymmetric broadening of the PL spectra is observed with increasing excitation power density which is stronger for 8 QDs per molecule than for 4 QDs. In addition, a shift of the PL peak to higher energies occurs. In p‐type modulation‐doped QD molecules the asymmetric broadening and high‐energy shift are enhanced. N‐type modulation‐doped QD molecules exhibit a square‐shaped PL spectrum, resembling that of modulation‐doped quantum wells. These findings are consistently explained by state filling in the presence of extended electron states due to lateral electronic coupling of the QDs within the molecules forming a quasi two‐dimensional density of states. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum Hall Ferrimagnetism in Lateral Quantum Dot Molecules

We demonstrate the existence of ferrimagnetic and ferromagnetic phases in a spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall regime. The spin phase diagram is determined from the Hartree-Fock configuration interaction method as a function of electron number N and magnetic field B. The quantum Hall ferrimagnetic phase corresponds to spatially imbalanced spin droplets resulting from strong interdot coupling of identical dots. The quantum Hall ferromagnetic phases correspond to ferromagnetic coupling of spin polarization at filling factors between nu=2 and nu=1.

Quantum Light Emission of Two Lateral Tunnel-Coupled(In,Ga)As/GaAsQuantum Dots Controlled by a Tunable Static Electric Field

Lateral quantum coupling between two self-assembled (In,Ga)As quantum dots has been observed. Photon statistics measurements between the various excitonic and biexcitonic transitions of these lateral quantum dot molecules display strong antibunching confirming the presence of coupling. Furthermore, we observe an anomalous exciton Stark shift with respect to static electric field. A simple model indicates that the lateral coupling is due to electron tunneling between the dots when the ground states are in resonance. The electron probability can then be shifted to either dot and the system can be used to create a wavelength-tunable single-photon emitter by simply applying a voltage.

Self-assembled lateral InAs quantum dot molecules: Dot ensemble control and polarization-dependent photoluminescence

By proper control of underlying templates, lateral InAs quantum dot molecules (QDMs) can be grown with a desired number of quantum dots (QDs) per molecule. We demonstrate that by capping as-grown InAs QDs with lattice-mismatched GaAs layer and by varying the capping temperature between 430 and 470 °C, we are able to control the span of nanoholes template to between 150 and 300 nm. InAs regrowth on this template result in nanopropellers (0.6 ML regrowth) or QDMs (1.2 ML), the latter with 4–12 QDs per molecule depending on the span of the underlying nanoholes template. All QDM samples grown share a characteristic feature with a clear preference to orient along the [ 1 1 ¯ 0 ] crystallographic direction. Room-temperature photoluminescence measurements show a strong polarization-dependent characteristic, in good agreements with the QDM’s geometry.

Far-infrared spectra of lateral quantum dot molecules

We study effects of electron–electron interactions and confinement potential on the magneto-optical absorption spectrum in the far-infrared (FIR) range of lateral quantum dot molecules (QDMs). We calculate FIR spectra for three different QDM confinement potentials. We use an accurate exact diagonalization technique for two interacting electrons and calculate dipole transitions between two-body levels with perturbation theory. We conclude that the two-electron FIR spectra directly reflect the symmetry of the confinement potential and interactions cause only small shifts in the spectra. These predictions could be tested in experiments with non-parabolic quantum dots (QDs) by changing the number of confined electrons. We also calculate FIR spectra for up to six non-interacting electrons and observe some additional features in the spectrum.

Thin-capping-and-regrowth molecular beam epitaxial technique for quantum dots and quantum-dot molecules

A thin-capping-and-regrowth molecular beam epitaxial technique is proposed and demonstrated to be a suitable approach for the growth of lateral quantum-dot molecules (QDMs). By regrowing on top of nanoholes, previously formed from as-grown quantum dots (QDs) via a thin-capping process, nanopropeller QDs are formed. By repeating the thin-capping-and-regrowth process for several cycles at the regrown thickness of 0.6 ML, nanopropeller QDs are linked along the [11¯0] crystallographic direction, leading to the alignment of QDs. The thin-capping-and-regrowth process is repeated for 1, 3, 5, 7, and 10cycles on different samples for comparison purposes. It is found from ex situ atomic force microscopy that at 7cycles of thin capping and regrowth of QDs, the best alignment of QDs is achieved. This is due to the strain having an optimum condition. The samples that undergo three and five thin-capping-and-regrowth cycles show some randomness of QD formation. When the process is repeated for 10cycles⁠, QDs become randomly distributed, but with a higher dot density than the as-grown sample. The high dot density results in a strong photoluminescence at room temperature. It is also shown that when self-aligned QDs are used as templates, aligned QDMs can be obtained at a regrowth thickness of 1.2 ML.

Two-electron lateral quantum-dot molecules in a magnetic field

Laterally coupled quantum dot molecules are studied using exact diagonalization techniques. We examine the two-electron singlet-triplet energy difference as a function of magnetic field strength and investigate the magnetization and vortex formation of two- and four-minima lateral quantum dot molecules. Special attention is paid to the analysis of how the distorted symmetry affects the properties of quantum-dot molecules.

Shape, strain, and ordering of lateral InAs quantum dot molecules

The results of an x-ray study on freestanding, self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots grown by molecular beam epitaxy are presented. The studied samples cover the range from statistically distributed single quantum dots to quantum dot bimolecules, and finally to quantum dot quadmolecules. The x-ray diffraction data of the single quantum dots and the bimolecules, obtained in grazing incidence geometry, have been analyzed using the isostrain model. An extended version of the isostrain model has been developed, including the lateral arrangement of the quantum dots within a quantum dot molecule and the superposition of the scattering from different parts of the dots. This model has been applied to the scattering maps of all three samples. Quantitative information about the positions of the dots, the shape, and the lattice parameter distribution of their crystalline core has been obtained. For the single dot and the bimolecule, a strong similarity of the shape and lattice parameter distribution has been found, in agreement with the similarity of their photoluminescence spectra.

Ordering of quantum dot molecules by self-organization

Lateral InAs quantum dot (QD) molecules are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (3 1 1)B by molecular beam epitaxy. During stacking the SL template self-organizes into a two-dimensionally ordered strain-modulated network on a mesoscopic length scale. InAs QDs preferentially grow on the nodes of the network due to local strain recognition. The QDs form a lattice of separated groups of closely spaced ordered QDs, whose number is controlled by the upper GaAs separation layer thickness on the SL template and the SL growth temperature. The QD molecules exhibit strong photoluminescence emission up to room temperature.

Ordered quantum dot molecules and single quantum dots formed by self-organized anisotropic strain engineering

An ordered lattice of lateral InAs quantum dot (QD) molecules is created by self-organized anisotropic strain engineering of an (In,Ga)As∕GaAs superlattice (SL) template on GaAs(311)B by molecular-beam epitaxy, constituting a Turing pattern in solid state. The SL template and InAs QD growth conditions, such as the number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers, are studied in detail for an optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy. The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain-correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution x-ray diffraction. Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature-dependent PL measurements exhibit an unusual behavior of the full width at half maximum, indicating carrier redistribution solely within the QD molecules.

Ordering of quantum dot molecules by self-organization

Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B by molecular beam epitaxy. During stacking the SL template self-organizes into a highly ordered two-dimensional (In,Ga)As and, thus, strain field modulation on a mesoscopic length scale, constituting a Turing pattern in solid state. InAs QDs preferentially grow on top of the SL template nodes due to local strain recognition, forming a lattice of separated groups of closely spaced ordered QDs. The SL template and InAs QD growth conditions like the number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers are studied in detail for optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy. The average number of InAs QDs within the molecules is controlled by the thickness of the upp...

Self-organized lattice of ordered quantum dot molecules

Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As∕GaAs superlattice (SL) template on GaAs (311)B in molecular-beam epitaxy. During stacking, the SL template self-organizes into a two-dimensionally ordered strain modulated network on a mesoscopic length scale. InAs QDs preferentially grow on top of the nodes of the network due to local strain recognition. The QDs form a lattice of separated groups of closely spaced ordered QDs whose number can be controlled by the GaAs separation layer thickness on top of the SL template. The QD groups exhibit excellent optical properties up to room temperature.

Role of interactions in the far-infrared spectrum of a lateral quantum-dot molecule.

We study the effects of electron-electron correlations and confinement potential on the far-infrared spectrum of a lateral two-electron quantum-dot molecule by exact diagonalization. The calculated spectra directly reflect the lowered symmetry of the external confinement potential. Surprisingly, we find interactions to drive the spectrum towards that of a high-symmetry parabolic quantum-dot. We conclude that far-infrared spectroscopy is suitable for probing effective confinement of the electrons in a quantum-dot system, even if interaction effects cannot be resolved in a direct fashion.

Formation of lateral quantum dot molecules around self-assembled nanoholes

We fabricate groups of closely spaced self-assembled InAs quantum dots (QDs)—termed lateral QD molecules—on GaAs (001) by a combination of molecular-beam epitaxy and AsBr3 in situ etching. An initial array of homogeneously sized nanoholes is created by locally strain-enhanced etching of a GaAs cap layer above InAs QDs. Deposition of InAs onto the nanoholes causes a preferential formation of the InAs QD molecules around the holes. The number of QDs per QD molecule ranges from 2 to 6, depending on the InAs growth conditions. By decreasing the substrate temperature, the number of QDs per QD molecule increases, but the statistical distribution is wider due to a reduced In atom diffusion length. Our photoluminescence investigation documents the nanohole and QD molecule formation step by step and confirms the high crystal quality of these structures. An analysis of the nanohole geometry as a function of annealing time and InAs filling allows us to propose a model for the QD molecule formation process.