Lateral Quantum Dot Molecules Research Articles

Electronic coupling of single lateral strained InGaAs quantum dot molecule based on nanohole structure

The electronic properties of laterally coupled InGaAs/GaAs quantum dot molecule are studied theoretically under in-plane electric field. The quantum dot molecule energy spectrum and envelope functions are calculated by solving one-band effective-mass Schrödinger equation with considering strain effect by employing finite element method in three dimensions. The obtained results indicate that the electron's coupling energy strongly depends on the In mole fractions of the nanohole, such that quantum dots act as a molecule in case that In mole fractions of dots are larger than that of nanohole. It can be also observed that electrons perceive the double-dots structure composing quantum dot molecule in close distances(less than 7nm), while the holes discern two single dots structure. The effect of an in-plane electric field on the energy spectrum is investigated and it can be demonstrated that the coupling energy can be tuned by applying a low-intensity static electric field.

Diamagnetic and paramagnetic shifts in self-assembled InAs lateral quantum dot molecules

We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response.

Inter-dot strain field effect on the optoelectronic properties of realistic InP lateral quantum-dot molecules

We report on numerical simulations of InP surface lateral quantum-dot molecules on In0.48Ga0.52 P buffer, using a model strictly derived by experimental results by extrapolation of the molecules shape from atomic force microscopy images. Our study has been inspired by the comparison of a photoluminescence spectrum of a high-density InP surface quantum dot sample with a numerical ensemble average given by a weighted sum of simulated single quantum-dot spectra. A lack of experimental optical response from the smaller dots of the sample is found to be due to strong inter-dot strain fields, which influence the optoelectronic properties of lateral quantum-dot molecules. Continuum electromechanical, k→·p→ bandstructure, and optical calculations are presented for two different molecules, the first composed of two dots of nearly identical dimensions (homonuclear), the second of two dots with rather different sizes (heteronuclear). We show that in the homonuclear molecule the hydrostatic strain raises a potential barrier for the electrons in the connection zone between the dots, while conversely the holes do not experience any barrier, which considerably increases the coupling. Results for the heteronuclear molecule show instead that its dots do not appear as two separate and distinguishable structures, but as a single large dot, and no optical emission is observed in the range of higher energies where the smaller dot is supposed to emit. We believe that in samples of such a high density the smaller dots result as practically incorporated into bigger molecular structures, an effect strongly enforced by the inter-dot strain fields, and consequently it is not possible to experimentally obtain a separate optical emission from the smaller dots.

Strain-Induced Formation of Fourfold Symmetric SiGe Quantum Dot Molecules

The strain field distribution at the surface of a multilayer structure with disklike SiGe nanomounds formed by heteroepitaxy is exploited to arrange the symmetric quantum dot molecules typically consisting of four elongated quantum dots ordered along the [010] and [100] directions. The morphological transition from fourfold quantum dot molecules to continuous fortresslike quantum rings with an increasing amount of deposited Ge is revealed. We examine key mechanisms underlying the formation of lateral quantum dot molecules by using scanning tunneling microscopy and numerical calculations of the strain energy distribution on the top of disklike SiGe nanomounds. Experimental data are well described by a simple thermodynamic model based on the accurate evaluation of the strain dependent part of the surface chemical potential. The spatial arrangement of quantum dots inside molecules is attributed to the effect of elastic property anisotropy.

Coulomb interaction signatures in self-assembled lateral quantum dot molecules

We use photoluminescence spectroscopy to investigate the ground state of single self-assembled InGaAs lateral quantum dot molecules. We apply a voltage along the growth direction that allows us to control the total charge occupancy of the quantum dot molecule. Using a combination of computational modeling and experimental analysis, we assign the observed discrete spectral lines to specific charge distributions. We explain the dynamic processes that lead to these charge configurations through electrical injection and optical generation. Our systemic analysis provides evidence of interdot tunneling of electrons as predicted in previous theoretical work.

Chirped InGaAs quantum dot molecules for broadband applications.

Lateral InGaAs quantum dot molecules (QDMs) formed by partial-cap and regrowth technique exhibit two ground-state (GS) peaks controllable via the thicknesses of InAs seed quantum dots (x), GaAs cap (y), and InAs regrowth (z). By adjusting x/y/z in a stacked QDM bilayer, the GS peaks from the two layers can be offset to straddle, stagger, or join up with each other, resulting in multi-GS or broadband spectra. A non-optimized QDM bilayer with a 170-meV full-width at half-maximum is demonstrated. The temperature dependencies of the emission peak energies and intensities from the chirped QDM bilayers are well explained by Varshni's equation and thermal activation of carriers out of constituent quantum dots.

Excited-state spectroscopy of single lateral self-assembled InGaAs quantum dot molecules

We perform excited-state spectroscopy of single self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity structure, which are grown using a combination of metal-organic vapor phase and solid-source molecular beam epitaxy. The individual quantum dot molecules (QDMs) consist of two single dots that are coupled along the $[1\overline{1}0]$ crystal direction via electron tunneling. By microphotoluminescence excitation spectroscopy, we investigate the presence and structure of the excited states confined in the system using a wide-band tunable Ti:sapphire laser source. We show that the excited states are very similar even for different molecules. We attribute this to the presence of a potential basin below the quantum dots that levels the differences among various QDMs.

Spectroscopic signatures of many-body interactions and delocalized states in self-assembled lateral quantum dot molecules

Lateral quantum dot molecules consist of at least two closely spaced InGaAs quantum dots arranged such that the axis connecting the quantum dots is perpendicular to the growth direction. These quantum dot complexes are called molecules because the small spacing between the quantum dots is expected to lead to the formation of molecular-like delocalized states. We present optical spectroscopy of ensembles and individual lateral quantum dot molecules as a function of electric fields applied along the growth direction. The results allow us to characterize the energy level structure of lateral quantum dot molecules and the spectral signatures of both charging and many-body interactions. We present experimental evidence for the existence of molecular-like delocalized states for electrons in the first excited energy shell.

Greenberger-Horne-Zeilinger states in a quantum dot molecule

We present a microscopic theory of a lateral quantum dot molecule in a radial magnetic field with a Greenberger-Horne-Zeilinger (GHZ) maximally entangled three particle ground state. The quantum dot molecule consists of three quantum dots with one electron spin each forming a central equilateral triangle. The antiferromagnetic spin-spin interaction is changed to the ferromagnetic interaction by additional doubly occupied quantum dots, one dot near each side of a triangle. The magnetic field is provided by micromagnets. The interaction among the electrons is described within an extended Hubbard Hamiltonian which is solved by using exact diagonalization techniques. The set of parameters is established for which the ground state of the molecule in a radial magnetic field is well approximated by a GHZ state.

Bimodal optical characteristics of lateral InGaAs quantum dot molecules

Lateral InGaAs quantum dot molecules (QDMs) formed by thin-cap-and-regrowth molecular beam epitaxial technique comprise large, central quantum dots (cQDs) and small, satellite quantum dots (sQDs) in close proximity. Temperature-dependent photoluminescent (PL) measurements show that the bimodal size distribution gives rise to bimodal optical characteristics: the cQDs ground-state (GS) emissions vary slowly with temperature while the full-width at half maximum (FWHM) remains almost constant; the sQDs GS emissions, on the other hand, exhibit a sigmoidal temperature shift while the FWHM shows an anomalous temperature behaviour. The bimodal optical characteristics are well described in the existing framework of spatially localised excitons in QDs and inter- and intramolecular carrier redistributions in each and among the QDMs via non-resonant multi-phonon assisted mechanisms.

Influence of the charge carrier tunneling processes on the recombination dynamics in single lateral quantum dot molecules

We report on the charge carrier dynamics in single lateral quantum dot molecules and the effect of an applied electric field on the molecular states. Controllable electron tunneling manifests itself in a deviation from the typical excitonic decay behavior in dot molecules. It results in a faster population decay and can be strongly influenced by the tuning electric field and intermolecular Coulomb energies. A rate equation model is developed and compared to the experimental data to gain more insight into the charge transfer and tunneling mechanisms. Nonresonant (phonon-mediated) electron tunneling which changes the molecular exciton character from direct to indirect, and vice versa, is found to be the dominant tunable decay mechanism of excitons besides radiative recombination.

Strain engineering in Si via closely stacked, site-controlled SiGe islands

The authors report on the fabrication and detailed structural characterization of ordered arrays of vertically stacked SiGe/Si(001) island pairs. By a proper choice of growth parameters, islands which have both large sizes and high Ge fraction are obtained in the upper layer. Finite element method calculations of the strain distribution reveal that (i) the Si spacer between a pair of islands can act as a lateral quantum dot molecule made of four nearby dots for electrons and (ii) the tensile strain in a Si cap deposited on top of the stack is significantly enhanced with respect to a single layer.

Temperature-dependent photoluminescent characteristics of lateral InGaAs quantum dot molecules

Lateral InGaAs quantum dot molecules (QDMs) are grown by molecular beam epitaxy. Each QDM comprises a large, central quantum dot (cQD) and several small, satellite quantum dots (sQDs). Temperature-dependent photoluminescent measurements show low-energy (∼1–1.2 eV) emissions resulting from cQDs and mid-energy (∼1.2–1.3 eV) emissions from sQDs. The ground-state and excited-state energy peaks and spacings of the QDMs simply follow bandgap temperature dependencies as shown in effective one-dimensional simulations of coupled Schrödinger–Poisson equations. Transfers of luminescence from sQDs to cQDs observed at low- (20–77 K) or mid- (50–100 K) temperatures for samples with near-merged or well-separated sQD–cQD QDMs are attributed to strong or weak sQD–cQD electronic coupling through the wetting layer, respectively.

Tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule

We present a theory of tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule. The theory combines exact treatment of an isolated many-body system with the rate equation approach when the quantum dot molecule is weakly connected to the leads subject to arbitrary source-drain bias. The tunneling spectroscopy of the many-body complex is analyzed using the spectral functions of the system and applied to holes in a quantum dot molecule. Negative differential conductance is predicted and explained as a result of the redistribution of the spectral weight between transport channels. It is shown that different interference effects on singlet and triplet hole states in a magnetic field lead to spin-selective Aharonov-Bohm oscillations.

Polarization fine structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity

Single lateral InGaAs quantum dot molecules have been embedded in a planar microcavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine structure splitting of the molecular neutral excitonic states is reported which leads to two polarization split classically correlated biexciton-exciton cascades. The fine structure splitting is found to be on the order of 10 μeV.

Gate-controlled spin-spin interactions in lateral quantum dot molecules

We present a method of changing the effective spin-spin interactions of electrons localized in quantum dot molecules from antiferromagnetic to ferromagnetic using electrical means. We show that starting with antiferromagnetic interaction of two electrons localized in two quantum dots, the effective spin-spin interaction can change to ferromagnetic by allowing interaction with a third dot containing two spin singlet electrons. The total spin of this four-electron complex can be tuned by changing the third dot potential from singlet to triplet. This is demonstrated using the linear combination of harmonic orbitals combined with the configuration-interaction method mapped onto the Hubbard and an effective $t\text{\ensuremath{-}}J$ model for the triple dot system. All three approaches predict singlet-triplet spin transition, i.e., a mechanism for changing the magnitude and sign of the effective Heisenberg interaction between two localized spins.

Spin transitions induced by a magnetic field in quantum dot molecules

We present a theoretical study of magnetic field driven spin transitions of electrons in coupled lateral quantum dot molecules. A detailed numerical study of spin phases of artificial molecules composed of two laterally coupled quantum dots with N=8 electrons is presented as a function of magnetic field, Zeeman energy, and the detuning using real space Hartree-Fock Configuration Interaction (HF-CI) technique. A microscopic picture of quantum Hall ferromagnetic phases corresponding to zero and full spin polarization at filling factors $\nu=2$ and $\nu=1$, and ferrimagnetic phases resulting from coupling of the two dots, is presented.

Towards deterministically controlled InGaAs/GaAs lateral quantum dot molecules

We report on the fabrication, detailed characterization and modeling of lateral InGaAs quantum dot molecules (QDMs) embedded in a GaAs matrix and we discuss strategies to fully control their spatial configuration and electronic properties. The three-dimensional morphology of encapsulated QDMs was revealed by selective wet chemical etching of the GaAs top capping layer and subsequent imaging by atomic force microscopy (AFM). The AFM investigation showed that different overgrowth procedures have a profound consequence on the QDM height and shape. QDMs partially capped and annealed in situ for micro-photoluminescence spectroscopy consist of shallow but well-defined quantum dots (QDs) in contrast to misleading results usually provided by surface morphology measurements when they are buried by a thin GaAs layer. This uncapping approach is crucial for determining the QDM structural parameters, which are required for modeling the system. A single-band effective-mass approximation is employed to calculate the confined electron and heavy-hole energy levels, taking the geometry and structural information extracted from the uncapping experiments as inputs. The calculated transition energy of the single QDM shows good agreement with the experimentally observed values. By decreasing the edge-to-edge distance between the two QDs within a QDM, a splitting of the electron (hole) wavefunction into symmetric and antisymmetric states is observed, indicating the presence of lateral coupling. Site control of such lateral QDMs obtained by growth on a pre-patterned substrate, combined with a technology to fabricate gate structures at well-defined positions with respect to the QDMs, could lead to deterministically controlled devices based on QDMs.

Theory of spin, electronic, and transport properties of the lateral triple quantum dot molecule in a magnetic field

We present a theory of spin, electronic, and transport properties of a few-electron lateral triangular triple quantum dot molecule in a magnetic field. Our theory is based on a generalization of a Hubbard model and the linear combination of harmonic orbitals combined with configuration interaction method for arbitrary magnetic fields. The few-particle spectra obtained as a function of the magnetic field exhibit Aharonov-Bohm oscillations. As a result, by changing the magnetic field, it is possible to engineer the degeneracies of single-particle levels, and thus, control the total spin of the many-electron system. For the triple dot with two and four electrons, we find oscillations of total spin due to the singlet-triplet transitions occurring periodically in the magnetic field. In the three-electron system, we find a transition from a magnetically frustrated to a spin-polarized state. We discuss the impact of these phase transitions on the addition spectrum and the spin blockade of the lateral triple quantum dot molecule.

Spin-transitions in a triple lateral quantum dot molecule in a magnetic field

We present a theory on the effects of the magnetic field on the electronic structure and spin-transitions of lateral triple quantum dot molecule filled with a few electrons. We use a simple Hubbard model and a microscopic model based on linear combination of harmonic orbitals (LCHO) method combined with configuration interaction (CI) approach for many electron systems. It is shown that, in the presence of a magnetic field, periodic spin singlet–triplet transitions occur for two- and four-electron systems due to the Aharonov–Bohm (AB) oscillations of single-electron energy levels. This leads to the possibility of controlling the total spin of the many-electron system. For three-electron system we find a transition from a magnetically frustrated to the spin-polarized state which is a result of the competition between the Zeeman energy and the effects due to topology and Fermi statistics. We discuss the impact of these magnetic field induced phase transitions on the addition spectrum.