Vertically-coupled Quantum Dots Research Articles

Effect of magnetic field on energy and entanglement of an exciton in coupled quantum dots

Using the one-dimensional effective potential method, we theoretically study the properties of an exciton in vertically coupled quantum dots (VCQDs) under the influence of an applied magnetic field. The ground state transition energies of the heavy-hole exciton are calculated in a single quantum dot and in VCQDs, respectively. Our results are in good agreement with those of experiments. In symmetrical VCQDs, the entanglement of the exciton state monotonously increases with increasing barrier width, and can even attain the value of the maximally entangled state. When the symmetry of the VCQDs is broken, the entanglement of the exciton state almost entirely breaks down under the combined influences of tunneling and the Coulomb interactions.

Optical properties of modulation-doped InGaAs vertically coupled quantum dots

Optical spectroscopy including photoluminescence, electroluminescence, photocurrent, and differential absorption, have been investigated for the triple-layer InGaAs vertically coupled quantum dots (VCQDs) by adding modulation doping (MD) in the 5 nm GaAs spacer layers. In addition to the QDs fundamental and excited transitions, a coupled-state transition is observed for the VCQDs. For the VCQDs of p-type MD, the optical transitions at ground state and coupled state are enhanced by the improvement of hole capture for the valence subbands. For the VCQDs of n-type MD, the main absorption change occurs at the coupled state, consistent with the dominant emission peak observed in EL spectra.

Donor impurity in vertically-coupled quantum-dots under hydrostatic pressure and applied electric field

In this work we make a predictive study on the binding energy of the ground state for hydrogenic donor impurity in vertically-coupled quantum-dot structure, considering the combined effects of hydrostatic pressure and in growth-direction applied electric field. The approach uses a variational method within the effective mass approximation. The low dimensional structure consists of three cylindrical shaped GaAs quantum-dots, grown in the z-direction and separated by Ga1-xAlxAs barriers. In order to include the pressure dependent Γ – X crossover in the barrier material a phenomenological model is followed. The main findings can be summarized as follows: 1) for symmetrical and asymmetrical dimensions of the structures, the binding energy as a function of the impurity position along the growth direction of the heterostructure has a similar behavior to that shown by the non-correlated electron wave function with maxima for the impurity in the well regions and minima for the impurity in the barrier regions, 2) for increasing radius of the system, the binding energy decreases and for R large enough reaches the limit of the binding energy in a coupled quantum well heterostructure, 3) the binding energy increases for higher Aluminum concentration in the barrier regions, 4) depending of the impurity position and of the structural dimensions of the system (well width and barrier thickness) – and because changing the height of the potential barrier makes possible to induce changes in the degree of symmetry of the carrier-wave function –, the electric field and hydrostatic pressure can cause the impurity binding energy increases or decreases, and finally 5) the line-shape of the binding energy curves are mainly given by the line-shape of the Coulomb interaction.

Stark effect in a multilayer system of coupled InAs/GaAs quantum dots

Electron emission in a system of vertically coupled quantum dots (VCQDs) in InAs/GaAs p-n-heterostructures obtained by molecular beam epitaxy has been studied by means of deep-level transient spectroscopy (DLTS) as a function of the number of quantum dot (QD) rows and the reverse bias voltage. For a GaAs spacer thickness of d GaAs = 40 A, the system occurs in a molecular state, irrespective of the number of QD rows. An increase in this number leads to a decrease in the Stark shift, which is probably related to a decrease in the lattice strain potential in the vicinity of VCQDs.

Spin hot spots in vertically coupled few-electron isolated quantum dots

The effects of spin-orbit (SO) coupling arising from the confinement potential in single and two vertically-coupled quantum dots have been investigated. Our work indicates that a dot containing a single electron shows the lifting of the degeneracy of dipole-allowed transitions at B=0 due to the SO coupling which disappears for a dot containing two electrons. For coupled dots with one electron in each dot, the optical spectra is not affected by the coupling and is the same as the dot containing one electron. However, for the case of two coupled dots where one partner dot has two interacting electrons while the other dot has only one electron, a remarkable effect is observed where the oscillator strength of two out of four dipole-allowed transition lines disappears as the distance between the dots is decreased.

On the nucleation of GaP/GaAs and the effect of buried stress fields

ABSTRACTWe have recently shown that spatial ordering for epitaxially grown InP dots can be obtained using the periodic stress field of compositional modulation on the InGaP buffer layer. The aim of this present work is to study the growth of films of GaP by Chemical Beam Epitaxy (CBE), with in-situ monitoring by Reflection High Energy Electron Diffraction (RHEED), on layers of unstressed and stressed GaAs. Complementary, we have studied the role of a buried InP dot array on GaP nucleation in order to obtain three-dimensional structures. In both cases, the topographical characteristics of the samples were investigated by Atomic Force Microscopy (AFM) in non-contact mode. Thus vertically-coupled quantum dots of different materials have been obtained keeping the in-place spatial ordering originated from the composition modulation.

Interconnection between gain spectrum and cavity mode in a quantum-dot vertical-cavity laser

The effect of self-adjustment of the cavity mode in vertical cavity surface-emitting lasers containing three-period InGaAs-GaAs vertically-coupled quantum dots has been observed. The effect originates from a strong modulation of the refractive index near the gain peak, caused by excitons in quantum dots. The possibility of single quantum dot lasing is demonstrated.

Lateral association of vertically-coupled quantum dots

The structural and optical properties of vertically coupled In0.5Ga0.5As quantum dots (QDs) in a GaAs matrix with a large number of QD stacks has been studied. We show that for large numbers of QD deposition cycles (N), in addition to vertical coupling, lateral coupling effects between the laterally neighboring QDs influence the electronic spectrum of the structure. Lateral coupling of vertically-coupled QDs or even geometrical merging of QDs in the upper rows result in an appearance of a new photoluminescence (PL) line associated with a radiative recombination of excitons via the states of laterally coupled QDs, which dominates the PL spectrum for large N.

Low-threshold injection lasers based on vertically coupled quantum dots

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during successive deposition of several sheets of (In,Ga)As quantum dots separated by thin GaAs spacers. VECODs are introduced in the active region of a GaAs-A1GaAs GRIN SCH lasers. Increasing the number of periods (N) in the VECOD leads to a remarkable decrease in threshold current density ( ~ 100 A/cm 2 at 300 K for N = 10). Lasing proceeds via the ground state of the quantum dots (QD) up to room temperature. Placing the QD array into an external AIGaAs--GaAs quantum well allows us to extend the range of thermal stability of threshold current density (To = 350 K) up to room temperature. Using (In,Ga)As-(A1,Ga)As VECODs in combination with high temperature growth of emitter and waveguide layers results in further reduction of threshold current density (60-80 A/cm 2, 300 K) and increase in internal quantum efficiency (70%). Room temperature continuous wave operation (light output 160 mW per mirror) and lasing via the states of QDs up to I = (6-7) Ith have been demonstrated.

3D Arrays of Quantum Dots for Laser Applications

AbstractWe have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during alternate short-period GaAs-InAs (InGaAs) depositions after InAs (or InGaAs) pyramids are formed on a GaAs (100). The resulting arrangement represents laterally ordered array of nanoscale structures inserted in a GaAs matrix, where each structure is composed of several vertically merging InAs (or InGaAs) parts. VECODs are introduced in the active region of GaAs-AlGaAs double heterostructure laser. The threshold current density remarkably decreases with increase in number of periods (N) of the VECOD (down to 90 A cm-2 at 300K for N=10). The differential efficiency increases with N and the lasing occurs through ground state of quantum dot exciton up to room temperature (λ=1.05 μm).

Ordered arrays of quantum dots: Formation, electronic spectra, relaxation phenomena, lasing

Elastic relaxation on facet edges, renormalization of the surface energy of the facets, and interaction between islands via the strained substrate are the driving forces for self-organization of ordered arrays of uniform coherent three-dimensional islands on crystal surfaces. For a (100) surface of a cubic crystal, two-dimensional square lattice of pyramid-like islands (quantum dots) with the periodicity along the directions of the lowest stiffness [010] and [001] has the minimum energy among different one-dimensional and two-dimensional arrays. For the InAs GaAs(100) system, an equilibrium array of dots of the lateral size ∼ 120–140 Å exists in a fixed range of growth parameters. The main luminescence peak at 1.1 eV, as well as peaks of excited states coincide in energy with the peaks revealed in the calorimetric absorption spectra regardless of the amount of InAs deposited (2–5 ML). Raman spectra indicate significant strain in InAs dots. The “phonon bottleneck” effect is bypassed via multi-phonon exciton and carrier relaxation. Ultranarrow lines (>0.15 meV) are observed in cathodoluminescence spectra up to high temperatures. Low threshold current density operation via zero-dimensional states and ultrahigh temperature stability of the threshold current ( T 0=450 K) are realized for a quantum dot injection laser. Increase in the gain and significant reduction in the radiative lifetime are possible via the self-organization of vertically-coupled quantum dots (VECODs) arranged in a well ordered artificial three-dimensional tetragonal lattice.