Strain-induced Quantum Dots Research Articles

Tunable phononic coupling in excitonic quantum emitters.

Engineering the coupling between fundamental quantum excitations is at the heart of quantum science and technologies. An outstanding case is the creation of quantum light sources in which coupling between single photons and phonons can be controlled and harnessed to enable quantum information transduction. Here we report the deterministic creation of quantum emitters featuring highly tunable coupling between excitons and phonons. The quantum emitters are formed in strain-induced quantum dots created in homobilayer WSe2. The colocalization of quantum-confined interlayer excitons and terahertz interlayer breathing-mode phonons, which directly modulates the exciton energy, leads to a uniquely strong phonon coupling to single-photon emission, with a Huang-Rhys factor reaching up to 6.3. The single-photon spectrum of interlayer exciton emission features a single-photon purity >83% and multiple phonon replicas, each heralding the creation of a phonon Fock state in the quantum emitter. Due to the vertical dipole moment of the interlayer exciton, the phonon-photon interaction is electrically tunable to be higher than the exciton and phonon decoherence rate, and hence promises to reach the strong-coupling regime. Our result demonstrates a solid-state quantum excitonic-optomechanical system at the atomic interface of the WSe2 bilayer that emits flying photonic qubits coupled with stationary phonons, which could be exploited for quantum transduction and interconnection.

Formation of strain-induced quantum dots in gated semiconductor nanostructures

A long-standing mystery in the field of semiconductor quantum dots (QDs) is: Why are there so many unintentional dots (also known as disorder dots) which are neither expected nor controllable. It is typically assumed that these unintentional dots are due to charged defects, however the frequency and predictability of the location of the unintentional QDs suggests there might be additional mechanisms causing the unintentional QDs besides charged defects. We show that the typical strains in a semiconductor nanostructure from metal gates are large enough to create strain-induced quantum dots. We simulate a commonly used QD device architecture, metal gates on bulk silicon, and show the formation of strain-induced QDs. The strain-induced QD can be eliminated by replacing the metal gates with poly-silicon gates. Thus strain can be as important as electrostatics to QD device operation operation.

Ultra-low density InAs quantum dots

We show that InAs quantum dots (QDs) can be grown by molecular beam epitaxy (MBE) with an ultralow density of sin 107 cm−2 without any preliminary or post-growth surface treatment. The strain-induced QD formation proceeds via the standard Stranski-Krastanow mechanism, where the InAs coverage is decreased to 1.3–1.5 monolayers (MLs). By using off-cut GaAs (100) substrates, we facilitate the island nucleation in this subcritical coverage range without any growth interruption. The QD density dependences on the InAs coverage are studied by photoluminescence, atomic force microscopy, transmission electron microscopy, and are well reproduced by the universal double exponential shapes. This method enables the fabrication of InAs QDs with controllable density in the range 107–108 cm−2, exhibiting bright photoluminescence.

Green Emission from a Strain-Modulated InGaN Active Layer

Strain-induced quantum dots (QDs) like island formations are demonstrated to effectively suppress pits/dislocation generation in high indium content (26.8%) InGaN active layers. In addition to the strain redistribution in the QD-like islands, strain modulation on the InGaN active layers by using the GaN island capping is employed to form an increased surface potential barrier around the dislocation cores, which inhibits the carrier transport to the surrounding dislocations. Cathodoluminescence shows distinct double-peak emissions at 503 nm and 444 nm, corresponding to the In-rich QD-like emission and the normal quantum well emission, respectively. The QD-like emission becomes dominated in photoluminescence due to the carrier localization effect of In-rich InGaN QDs at relatively low “carrier injection current". Accordingly, green emission may be enhanced by the following origins: (1) reduction in pits/dislocations density, (2) carrier localization and strain reduction in QDs, (3) strain modulation by GaN island capping, (4) enhanced light extraction with faceted GaN islands on the surface.

Spectroscopic signature of strain-induced quantum dots created by buried InAs quantum dots in an InGaAs quantum well

Spectroscopic study of self-assembled InAs/GaAs quantum dots (QDs) capped with an InxGa1−xAs quantum well (QW) is carried out under variable excitation intensity and temperature. The QW reduces strain in the QDs, and it is demonstrated that the cap layer shifts the transition energy of the QD lower, while maintaining the strong quantization of the QD electronic states which develops in photoluminescence as a series of excited state transitions at higher excitation intensity. An additional spectral band is also revealed below the energy of the InGaAs QW exciton, which is unambiguously assigned to the emission of a strain-induced QD layer created by the InAs stressors within the InGaAs QW.

General solution for Eshelby’s problem of 2D arbitrarily shaped piezoelectric inclusions

Eshelby’s problem of piezoelectric inclusions arises sometimes in exploiting the electromechanical coupling effect in piezoelectric media. For example, it intervenes in the nanostructure design of strained semiconductor devices involving strain-induced quantum dot (QD) and quantum wire (QWR) growth. Using the extended Stroh formalism, the present work gives a general analytical solution for Eshelby’s problem of two-dimensional arbitrarily shaped piezoelectric inclusions. The key step toward obtaining this general solution is the derivation of a simple and compact boundary integral expression for the eigenfunctions in the extended Stroh formalism applied to Eshelby’s problem. The simplicity and compactness of the boundary integral expression derived make it much less difficult to analytically tackle Eshelby’s piezoelectric problem for a large variety of non-elliptical inclusions. In the present work, explicit analytical solutions are obtained and detailed for all polygonal inclusions and for the inclusions characterized by Jordan’s curves and Laurent’s polynomials. By considering the piezoelectric material GaAs (110), the analytical solutions provided are illustrated numerically to verify the coincidence between different expressions, and to clarify the jump across the boundary of the inclusion and the singularity around the corner of the inclusion.

Cascaded exciton emission of an individual strain-induced quantum dot

Single strain-induced quantum dots are isolated for optical experiments by selective removal of the inducing InP islands from the sample surface. Unpolarized emission of single exciton, biexciton, and triexciton transitions are identified by power-dependent photoluminescence spectroscopy. Employing time-resolved experiments performed at different excitation powers, we find a pronounced shift of the rise and decay times of these different transitions as expected from cascaded single photon emissions. Good agreement is found for a rate equation model for a three step cascade.

Carrier Dynamics in Strain Induced Quantum Dots Modeled by Rate Equations and Gaussian Excitation Beam Distribution

Strain induced quantum dots are investigated by time resolved and continuous wave photoluminescence spectroscopy. Carrier dynamics is modeled with rate equations taking into account also the Gaussian intensity distribution of the excitation beam. It is shown that the Gaussian distribution has a clear effect on the saturation behaviour of the quantum dot peak intensities. The calculated intensities agree better with the measured luminescence intensities than results from the conventional rate equation approach.

Carrier dynamics of straininduced quantum dots and THz radiation‐induced exciton cooling

AbstractWe have modeled strain‐induced quantum dots and analyzed previous anomalous experimental observations of a cooling of radiative quantum dot (QD) excitons by THz radiation. We have made three‐dimensional (3D) modeling of the elastic strain and piezoelectric fields. The 3D electronic structure was simulated using multiband envelope function techniques and used as a basis for Monte‐Carlo simulations of the carrier dynamics.We have found that the THz radiation‐induced QD exciton cooling is due to an electron‐hole separation and a related THz radiation excitation of holes within the quantum dot. This phenomenon is maintained by an effective relaxation of holes into states which are well separated from the confined electrons. The relaxation of holes is attributed to acoustic phonon emission and, at high carrier densities, to Auger processes. The electron and hole populations of the quantum dot tend towards a highly polarized state that cannot be broken without external excitations of the holes (by e.g. THz radiation or a thermal bath). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier dynamics of straininduced quantum dots and THz radiation‐induced exciton cooling

AbstractThe cover picture of this issue of physica status solidi (c) has been taken from the article [1]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fractal analysis of InGaN self-assemble quantum dots grown by MOCVD

Recently, it has been shown that nitride nanostructures can be self-assembled using growth interruption during the metal-organic chemical vapor deposition (MOCVD) growth. In this work, strain-induced InGaN-GaN self-assembled quantum dot (SAQD) samples were grown by MOCVD. A fractal dimension processing has been applied to characterize the surface roughness of uncapped InGaN-GaN SAQDs. The fractal dimension D can be used to describe the influence of different annealed conditions on surface characterization. Fractal analysis reveals the surface roughness of nanostructure surfaces is decreased after the annealing process. It can be seen that a smoother surface was obtained under an annealing temperature at 800°C, and it implies that the surface roughness of this case is minimum in all tests. The results of this paper also described a mathematical modeling method for the observation of the fractal dimension in semiconductor nanostructure films.

Cooling of radiative quantum-dot excitons by terahertz radiation: A spin-resolved Monte Carlo carrier dynamics model

We have developed a theoretical model to analyze the anomalous cooling of radiative quantum dot (QD) excitons by THz radiation reported by Yusa et al [Proc. 24th ICPS, 1083 (1998)]. We have made three-dimensional (3D) modeling of the strain and the piezoelectric field and calculated the 3D density of states of strain induced quantum dots. On the basis of this analysis we have developed a spin dependent Monte Carlo model, which describes the carrier dynamics in QD's when the intraband relaxation is modulated by THz radiation. We show that THz radiation causes resonance transfer of holes from dark to radiative states in strain-induced QD's. The transition includes a spatial transfer of holes from the piezoelectric potential mimima to the deformation potential minimum. This phenomenon strongly enhances the QD ground state luminescence at the expense of the luminescence from higher states. Our model also reproduces the delayed flash of QD ground state luminescence, activated by THz radiation even $\sim1$ s after the carrier generation. Our simulations suggest a more general possibility to cool the radiative exciton subsystem in optoelectronic devices.

Photon echo study of excitons and excitonic complexes in self-assembled quantum dots

The authors have studied the excitons and excitonic complexes in two kinds of self-assembled quantum dots (QDs) using photon echo measurements. In GaAs strain-induced quantum dots (SIQDs), a pronounced biexcitonic beat with a period of 1 ps is observed. The biexciton binding energy in SIQDs is obtained from the beat period, and its magnetic field dependence is investigated. It is found that the biexciton binding energy is remarkably increased by the lateral confinement and they are almost independent of the applied magnetic field up to 8 T. A theoretical calculation of the biexciton binding energy in SIQDs is presented to explain the observed magnetic field dependence. In charge-tunable InP QDs, the photon echo signal shows dramatic changes depending on the electric bias. The decay profile of the echo intensity is not a single exponential but Gaussian-like function, which indicates non-Markovian nature of the dephasing process in this system. Theoretical calculation is done assuming tunneling induced dephasing mechanism, and it reproduces the experimental results quite well.

Spin relaxation mechanism of strain-induced GaAs quantum dots studied by time-resolved Kerr rotation

We observed electron spin precession under magnetic field in single-layer quantum dots (QDs) by highly sensitive time-resolved Kerr rotation measurement. The spin lifetime is longer than that for the quantum well (QW). This is a result of the additional spatial confinement of electrons in QDs. Below $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the spin lifetime is almost constant, and is 7 times shorter than the carrier lifetime. This suggests that the strong electron-hole exchange interaction dominates over the electron spin lifetime in QDs at low temperature.

Highly sensitive time-resolved Kerr rotation measurements on single-layer quantum dots

We construct the highly sensitive Kerr rotation system to observe electron spin precession in single-layer quantum dots. The system consists of a photoelastic modulator, a balanced detector and tandem double lock-in amplifiers. The angle resolution achieves 5×10 −6 degree, and is highest among the reported so far. To evaluate the possibility of measuring spin relaxation in single-layer quantum dots, we studied the strain-induced GaAs quantum dots. We observe successfully the electron spin precession in the single-layer quantum dots under resonant excitation.

Fabrication of InAs quantum dots in AlAs∕GaAs DBR pillar microcavities for single photon sources

We report the molecular beam epitaxy growth of low-density strain-induced InAs quantum dots (QD) embedded in an AlAs∕GaAs distributed Bragg reflector structure for a triggered photon source. By optimal selection of growth temperature, InAs deposited thickness and other experimental parameters, it is possible to grow low density (10∕μm2) InAs quantum dots with a suitable emission wavelength for a triggered photon source. The empirical formulas for the refractive indices of AlAs and GaAs materials at high temperature over a wide wavelength range are constructed by combining high resolution x-ray diffraction, dynamic optical reflectivity, and optical reflectivity spectrum techniques. Utilizing the electron-beam lithography and electron-cyclotron-resonance plasma etching techniques, a micropost microcavity with the top diameter of 0.6μm and the post height of 4.2μm has been fabricated. Narrow, spectrally limited single QD emission embedded in a micropost microcavity is observed in the photoluminescence.

Buried stressors in nitride semiconductors: Influence on electronic properties

An analysis is presented on the effect of the strain field originating from a subsurface stressor (point source of dilatation or a dilatating ellipsoidal inclusion) on the electronic properties of nitride semiconductors. With good accuracy, real quantum dots can be modeled as such stressors. We consider the following material structure design: a uniform semi-infinite GaN matrix with a buried stressor or a GaN matrix with a single (In,Ga)N quantum well, which is grown pseuodomorphically between the stressor and the free surface. We utilize isotropic elasticity to determine the strain field in the structures under investigation. We then apply a k∙p perturbation theory approach 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 proposed for the realization of strain-induced quantum dots in the quantum well.

Observation of biexcitonic quantum beat in strain-induced GaAs quantum dots

Four-wave mixing in strain-induced GaAs quantum dots formed in a GaAs quantum well is studied using a highly sensitive heterodyne technique. A clear oscillation with a period of 1 ps is observed, recognized as a biexcitonic quantum beat based on its polarization dependence. The period of the beat is about 1.5 times shorter than that from the quantum well in the same sample. The beat represents direct evidence of an increase in the biexciton binding energy in the dots due to lateral confinement potential originating from the stressors.

Structural and optical properties of GaInNAs/GaAs quantum structures

Structural and optical properties of GaInNAs/GaAs quantum structures grown bymetalorganic vapour phase epitaxy (MOVPE) are studied. The growth of arsenide–nitridesby MOVPE is reviewed and compared to the other major growth technique, molecularbeam epitaxy. Post-growth thermal annealing and laser irradiation are employed and foundto affect GaInNAs quantum wells differently: with laser treatment no undesired blue-shiftof the photoluminescence peak is observed. The critical thickness for misfit dislocationformation of GaAsN on GaAs is found to be about twice as large as the theoreticalprediction. GaAsN epilayers are also found to contain Ga vacancies in defect complexes.The current status of experimental research on GaInNAs quantum dot structures isreviewed. Self-organized and strain-induced GaInNAs quantum dots are grownand their formation and optical properties are studied. The emission wavelengthof InGaAs quantum dots is extended by using InGaAs and GaInNAs barrierlayers.

Quantum beat of strain-induced GaAs quantum dots

We report the observation of two kinds of quantum beats in the time-resolved photoluminescence of the strain-induced GaAs quantum dots in the magnetic field parallel and perpendicular to the growth direction. They come from the Zeeman splitting of bright excitons in the Faraday configuration and electron Larmor precession in the Voigt configuration. Analysis of the oscillation period depending on the magnetic field strength, its orientation and the polarizations of excitation and luminescent light gives g-factors of the exciton, the electron and the heavy hole depending on the crystal orientation. The electron g-factor obtained by the analysis of the quantum beat observed in the Faraday configuration agrees with that obtained directly from the electron Larmor precession in the Voigt configuration. 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.