Strain-induced Quantum Dots Research Articles

Wave-function mapping of InAs quantum dots by scanning tunneling spectroscopy.

Scanning tunneling spectroscopy is used to investigate the single-electron states and the corresponding squared wave functions of single and freestanding strain-induced InAs quantum dots grown on GaAs(001). Several peaks are found in dI/dV curves, which belong to different single-electron states. Spatially resolved dI/dV images reveal (000), (100), (010), (200), and (300) states, where the numbers describe the number of nodes in [11;0], [110], and [001] directions, respectively. The total number and energetic sequence of states is different for different dots. Interestingly, the (010) state is often missing, even when (200) and (300) states are present. We interpret this anisotropy in electronic structure as a consequence of the shape asymmetry of the dots.

Luminescence quantum beats of strain-induced GaAs quantum dots

Quantum beats of the strain-induced GaAs quantum dots were observed in the time-resolved photoluminescence in the magnetic field parallel and perpendicular to the growth direction. Quantum beats observed under the longitudinal magnetic field are caused by quantum interference of bright excitons showing Zeeman splitting. The oscillation period depends on the angle between the growth direction of the crystal and the magnetic field. Analysis based on the spin Hamiltonian for excitons explains the observed data and gives g factors 0.51, 0.17, and 0.34 to the exciton, electron, and heavy hole, respectively. Quantum beats coming from electron Larmor precession were observed under the transverse magnetic field. The isotropic electron g factor is observed in contrast to the anisotropic electron g factor for the corresponding quantum well and is ascribed to the strain-induced opposite energy shift of heavy- and light-hole bands.

Mechanisms of semiconductor nanostructure formation

We have examined the formation mechanisms of a variety of semiconductor nanostructures, including phase separation-induced alloy nanostructures and strain-induced self-assembled quantum dots. Using data from cross-sectional scanning tunneling microscopy, in conjunction with X-ray reciprocal space maps, we have developed new models for self-ordering of InAs/GaAs quantum dot superlattices and spontaneous lateral phase separation in InAlAs alloys. These models are likely to be applicable to a wide range of heteroepitaxial semiconductor nanostructures.

Electronic properties of GaN induced by a subsurface stressor

The influence of the strain field originating from a subsurface point source of dilatation (point stressor) on the electronic properties of nitride semiconductors is investigated. In the far field, real quantum dots can be viewed as such point stressors. The material surrounding the stressor was considered either to be a uniform GaN matrix or GaN matrix with an (In,Ga)N quantum well which is grown pseudomorphically between the stressor and the free surface. Isotropic elasticity was used to find the strain field around the stressor. A k⋅p perturbation theory approach was then applied to examine the shifts of the conduction and valence band edges caused by the stressor. We find lateral confinement for electrons and holes, which can be utilized to realize a strain-induced quantum dot in the quantum well.

Nano-Optics on Individual Quantum Objects - From Single to Coupled Semiconductor Quantum Dots

Some recent highlights of our optical studies on single and coupled semiconductor quantum dots are reviewed. In the first part, we concentrate on the role of spins and spin-spin interaction in nonmagnetic and magnetic single quantum dots. In the case of strictly resonant excitation of the ground state in self-assembled CdSe/ZnSe quantum dots, we find an exciton spin relaxation time, which exceeds the recombination lifetime significantly. Linear polarization has to be used for these experiments, as the electron-hole exchange interaction lifts the spin degeneracy and the eigenstates are linear combinations of spin-up and spin-down excitons. In a magnetic quantum dot, the exchange interaction between carrier spins and the spins of magnetic ions is shown to be responsible for giant magneto-optical effects. We demonstrate the formation of zero-dimensional magnetic polarons and we succeeded in measuring the magnetization on a scale of a few nanometers using the characteristic photoluminescence signal of individual quantum dots as experimental monitor. The second part is devoted to pairs of single quahtum dots. On one hand, single exciton tunneling within an individual quantum dot pair is demonstrated studying single pairs of vertically correlated strain-induced and self-organized quantum dots. On the other hand, we show that in a pair of lithographically defined single dots with strongly different g-factors the energy spacing between the dot ground states can be tuned in an external magnetic field by about 10 meV, giving access to a controlled coupling between two individual quantum dots.

Tunneling of zero-dimensional excitons in a single pair of correlated quantum dots

We have studied single pairs of self-assembled and strain-induced quantum dots by means of highly spatially resolved photoluminescence and photoluminescence excitation spectroscopy. A spectrally narrow resonance in the excitation spectrum of a single self-assembled quantum dot at the emission energy of the corresponding strain-induced quantum dot indicates an efficient interdot transfer of a single zero-dimensional exciton.

Spatial carrier-carrier correlations in strain-induced quantum dots

The electron-hole correlation effects on the energy levels and the wave functions of the electrons and holes in a strain-induced quantum dot containing one to ten carrier pairs have been studied using large-scale configuration-interaction calculations. The present calculations show the formation of excitons and biexcitons in the quantum dot. By increasing the number of carrier pairs, one observes a transition from a strongly correlated system to a quantum dot system for which the electron-electron and hole-hole correlations are dominated by exchange interaction and are relatively well described at the Hartree-Fock level, while for an accurate description of the electron-hole correlations configuration-interaction calculations are necessary. Ring-shaped carrier distributions emerge with increasing number of carrier pairs.

Spectroscopy of strain-induced quantum dots inGaAs/AlxGa1−xAsquantum well structures

The lattice distortion surrounding buried self-assembled InAs stressors can create strain-induced quantum dots (SIQD's) in a nearby $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well (QW). The optical properties of these new zero-dimensional systems are studied experimentally and compared to a simple theoretical model. We demonstrate that the photoluminescence energy of the SIQD's can be tuned in a wide energy range by the QW width as well as by the spacing between the QW and InAs quantum dot layer.

Full Configuration Interaction Calculations of Electron-Hole Correlation Effects in Strain-Induced Quantum Dots

The electron–hole correlation effects on the energy levels and the wave functions of a strain-induced quantum dot containing 1 to 10 electron–hole pairs have been studied using large scale configuration-interaction calculations. The results show the importance of including the correlations in order to reproduce the experimentally measured quantum dot spectra and to obtain bound multi-exciton complexes. Increasing the number of electron–hole pairs in the quantum dot a transition from a strongly correlated system to the one approximated by the Hartree-Fock theory is observed.

Full Configuration Interaction Calculations of Electron–Hole Correlation Effects in Strain-Induced Quantum Dots

Full Configuration Interaction Calculations of Electron–Hole Correlation Effects in Strain-Induced Quantum Dots

Spectroscopy and Mechanical Modification of Single Strain-Induced Quantum Dots

We report the observation of very sharp lines in the photoluminescence spectra of single strain-induced GaAs quantum dots. By removing self-assembled GaSb stressors from the sample surface with an atomic force microscope, we have demonstrated that only the largest GaSb stressors are responsible for the sharp strain-induced quantum dot lines. Spectral line shifts are observed as the quantum dot potential is tuned by modification of the large stressors. The variation of the spectra with laser power density is consistent with emission from multiexciton complexes and excited dot orbital states.

Optical Study of Strain-Induced GaAs Quantum Dots

The excitation-intensity-dependent nonlinear luminescence as well as the luminescence of strain-induced GaAs quantum dots (SIQDs) was studied. The luminescence spectrum of SIQDs was composed of three well-resolved transitions and each transition had a different saturation excitation intensity. The increase and saturation of luminescence was more clearly seen in the nonlinear luminescence, where the saturation excitation intensities for each energy level were demonstrated to be proportional to the degeneracies of each level. The nonlinear luminescence spectra were simulated by rate equations, taking account of the relaxation rate, the recombination rate, and the state-filling effect caused by Pauli blocking. As a result of fitting, the relaxation rate was estimated to be about 30 ps.

Correlation Effects in Strain-Induced Quantum Dots

We report on Coulomb correlation effects in the luminescence of strain-induced quantum dots. In single dots, under low power excitation, we observe the rising of sharp lines associated to the formation of excitonic molecules. In the grand-ensemble, in magnetic fields up to 45 T, we observe Darwin-Fock states of the dots to merge into a unique Landau level, with a considerable reduction in the total diamagnetic shift due to the enhanced electron–hole correlation caused by the increased degeneracy of the state.

Correlation Effects in Strain-Induced Quantum Dots

Correlation Effects in Strain-Induced Quantum Dots

Longitudinal Stark Effect in Parabolic Quantum Dots

We present an experimental and theoretical investigation of the longitudinal quantum confined Stark effect in strain-induced parabolic InGaAs quantum dots. The electric field is applied in the plane perpendicular to the growth axis (quantum well plane), through a sub-micron gap opened in a metallic contact, which contains just a few dots. The changes in the microphotoluminescence (µ-PL) spectra were measured versus the bias, at low temperature. Due to the device geometry, the electric field profile in the gate is not constant resulting in an asymmetric Stark effect as a function of the applied field polarity. Calculations of the field profile in the conatct gap and of the distorsion of the parabolic-like potential as a function of the applied field have been made in order to correlate the observed changes in the electro-optical properties of the dots with the carrier spill over and wavefunction modifications under electric field.

Carrier capture processes in strain-inducedInxGa1−xAs/GaAsquantum dot structures

We investigate carrier capture processes in strain-induced quantum dot structures. The quantum dots consist of a near-surface InGaAs/GaAs quantum well in which a lateral confining potential is generated by the strain from InP stressor islands grown on the sample surface. Using photoluminescence spectroscopy, we show that the rate of carrier capture into the quantum dots increases dramatically when the energetic depth of the confinement potential is reduced by enlarging the quantum well/surface separation D. While carriers in the quantum well region between the quantum dots are found to experience D-dependent nonradiative surface recombination, this process seems to be negligible for carriers in the quantum dots, presumably due to the protecting InP islands.

Piezoelectric potentials and carrier lifetimes in strain-induced quantum well dots

The piezoelectric effect and its influence on the electronic structure of strain-induced quantum dots has been analyzed by elastic continuum and $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ methods. The piezoelectric effect gives rise to side minima for electrons and holes. These minima are located a few tens of nanometers apart from each other and from the deformation-potential minima. The carriers confined in these minima have radiative lifetimes that are $2\ifmmode\times\else\texttimes\fi{}{10}^{3}$ longer than the lifetime of carriers in the quantum dot ground state. Under quasiequilibrium conditions confinement of a free electron and a hole in the piezominima is energetically \ensuremath{\sim}105 meV more favorable than exciton formation followed by confinement in the deformation potential minima. This suggests that with increasing photoexcitation intensity confinement of electrons and holes in the piezominima will largely cancel the piezoelectric polarization of the structure.

Carrier-Carrier Correlations in Strain-Induced Quantum Dots

Carrier-Carrier Correlations in Strain-Induced Quantum Dots

Effects of few-particle interaction on the atomiclike levels of a single strain-induced quantum dot

We investigate the effects of few-particle population of a single strain-induced quantum dot by optical excitation. The low-power photoluminescence spectra consist of sharp lines with energy separation of a few meV, associated to the formation of excitonic molecules in the single dot. With increasing photoexcitation intensity, the population of higher states is observed; however, we also observe a clear intensity dependence of the transition energies, inconsistent with a simple filling of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the dot, we obtain good agreement with experiment and show that excitonexciton interactions control the spectral changes as the number of pairs is increased. The realization of high-quality semiconductor quantum dots has recently lead to the demonstration of artificial atoms, in which addition energies 1 and Zeeman splitting 2 associated with three-dimensionally confined electron states mimic the characteristics of natural atoms. Similarly, the optical excitation of a single quantum dot ~QD! results in optical spectra reminiscent of atomic transitions with sharp and spectrally narrow lines. 3‐7 The understanding of the complex optical spectra from single dots and their dependence on photoexcitation intensity is, however, still quite poor. The atomiclike lines in the spectra were found to exhibit intriguing red- and blue-shifts, and new features are seen to appear and disappear depending on the number of injected electronhole pairs. 8‐10 Such a complicated dynamics originates from the Coulomb correlation among pairs of carriers ~excitons! confined in the dot, which is often neglected in the analysis of these experiments. In fact, we expect that whenever an electron-hole ~e-h! pair is added to the dot, the energy states must change because of the resulting additional Coulomb interactions. In this paper, we report a complete study of the single-dot spectra, both experimental and theoretical, which elucidates the atomiclike nature of the optical transitions and the important role of Coulomb correlations in an isolated quantum dot. As a prototype nanostructurewe have chosen a single strain-induced quantum dot physically isolated from the surrounding environment in a nanomesa defined by highresolution lithography. This system has never been investigated by local spectroscopy so far, but for an intriguing nearfield luminescence experiment in which no atomiclike

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.