GaAs Quantum Dot Molecules Research Articles

Investigation of Exciton Dynamics to Confirm Coupling Effects Using a Comparison between Ensemble and Single Laterally‐Coupled GaAs Quantum Dot Molecules

Herein, time‐integrated and time‐resolved photoluminescence (PL) measurement experiments are performed on coupled GaAs quantum dot (QD) molecules with larger laser spot size and a single‐coupled QD molecule to investigate the dynamics of exciton states. A linearly polarized laser along the coupled direction of a laterally‐coupled GaAs QD molecule [10] is used to determine the coupling effects of the different exciton states. For the ensemble of coupled QD molecules, the exciton PL peak shifts to lower energy (≈70 meV) and shows a linear increase in the PL intensity, demonstrating the properties of exciton states as a function of the laser excitation power. In addition, a power‐dependent redshift is observed in the single‐coupled GaAs QD molecule as a function of the excitation power; however, the two distinguished exciton states become resolved and show different power factors when the laser excitation power is linearly increased. This result clearly indicates the presence of energy transfer between the two different exciton states, X1 and X2. Furthermore, when the polarized laser excitation power is increased, a coupled biexciton consisting of two different localized exciton states (X1 and X2) is clearly observed, and a power‐dependent spectral redshift is detected in the exciton and biexciton states. At a high excitation power (15 I0), the decay times of the two different localized exciton states clearly show different lifetimes; and the decay time difference due to the size difference can be ignored at a low excitation power. Theoretically, the decay times of the two different exciton states, wherein the size deviation can be ignored, should be the same. Based on the results of the time‐integrated and time‐resolved PL measurements and the comparison between the ensemble and single exciton PL peaks, it is concluded that the coupling process in laterally‐coupled GaAs QD molecules is strongly supported against long‐range separation.

Theoretical study of excitons in GaAs quantum dot molecules obtained by nanoholes filling

In this work we report a theoretical study of excitons behavior in self-assembled strain-free GaAs quantum dot molecules (QDMs) obtained by nanoholes filling. More precisely, we study a single molecule formed by a vertically coupled GaAs/AlGaAs quantum dots pair (QDP). The vertical coupling governed by varying the spacer thickness layer reveals how the excitons behave in a QDM. A model based on resolving the three-dimensional effective-mass Schrödinger equation is adopted to reliably provide a complete overview of the fine structure of neutral and complexes excitons and to explain how a pair of quantum dots interact in between while varying the spacer layer thickness mediating the coupling in the barrier. Our calculations can be adopted to improve the growth conditions and boost the optical properties of strain-free GaAs QDMs for using them in quantum information technologies and quantum computing.

Excited-state indirect excitons in GaAs quantum dot molecules

We demonstrate the fabrication of strain-free and widely adjustable GaAs quantum-dot molecules (QDMs) by filling of droplet etched nanoholes in AlGaAs. Gate-voltage dependent optical spectra of highly asymmetric QDMs exhibit anticrossings that clearly indicate strong coupling with delocalized molecule states. Furthermore, indirect excitons are observed that are related to recombinations of excited-state electrons and ground-state holes both located in different dots. Simple numerical simulations reproduce the electric-field dependent energy shifts of direct and indirect transitions and predict their radiative lifetimes. The visibility of excited-state indirect excitons even for strong off-resonant energy detuning indicates the presence of a phonon bottleneck which suppresses the relaxation of excited electrons into lower levels.

GaAs quantum dot molecules filled into droplet etched nanoholes

We fabricate self-aligned vertically stacked GaAs quantum dot molecules (QDMs) by filling of self-assembled nanoholes in AlGaAs. The tunable nanoholes are created using local droplet etching (LDE) combining conventional molecular beam epitaxy with self-assembled, lithography-free patterning. The optical emission from single, strain-free QDMs shows clear excitonic features with linewidths below 150µeV after optimizations of the fabrication process. This allows investigations of the coupling among the individual dots forming a QDM. In electric fields oriented along the axis of the QDM, luminescence emission from direct and indirect transitions can be clearly distinguished. Furthermore, an anti-crossing behaviour demonstrates inter-dot coupling in the QDM.

Structural dependency of the optical properties of coupled GaAs quantum qots and rings

By using droplet epitaxy, we fabricated asymmetrically-coupled GaAs quantum dot molecules (AQDMs) and symmetrically-coupled GaAs concentric quantum-double rings (SCQRs). The geometrical coupling distances, dot-to-dot for the AQDMs and ring-to-ring for the SCQRs, were 30 nm and 28 nm, respectively. The photoluminescence (PL) measurement was performed with a femtosecond laser as an excitation source to investigate the structure-dependent optical properties of the AQDMs and the SCQRs as functions of the excitation intensity (corresponding to the injection carrier density). The excitation-power-dependent PL showed that the emission peak of the AQDMs was systemically red-shifted with increasing excitation intensity while the emission peak of the SCQRs was blue-shifted. The anomalous behavior in the shift of the emission peak for the AQDMs can be explained by using the inhomogeneous carrier distribution due to the geometrical asymmetry of the coupled QDs.

Fabrication of coupled GaAs quantum dots and their optical properties

AbstractThe coupled GaAs quantum dot molecules (QDMs), which were laterally coupled along the [1$ \bar 1 $0] direction, were formed by droplet epitaxy. The optical properties of QDMs were investigated by femtosecond laser excited photoluminescence (PL) measurement. When the laser polarization is parallel to [1$ \bar 1 $0] direction on the QDMs, the main PL peak is red‐shifted by 55 meV from the PL peak measured with the laser polarization parallel to [110] direction. By changing the excitation power, we found that the emission wavelength of the QDMs was strongly depending on the excitation laser polarization. From this result, we carefully suggested that the confined states in QDMs could be tuned by optical field. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study of phononassisted singlet‐singlet relaxation in two‐electron semiconductor quantum dot molecules

AbstractPhonon‐assisted singlet‐singlet relaxation in semiconductor quantum dot molecules is studied theoretically. Laterally coupled quantum dot structures doped with two electrons are considered. We take into account interaction with acoustic phonon modes via deformation potential and piezoelectric coupling. We show that piezoelectric mechanism for the considered system is of great importance and for some ranges of quantum dot molecule parameters is the dominant contribution to relaxation.It is shown that the phonon‐assisted tunneling is much faster (down to ∼ 6 ps even at zero temperature) in comparison with other decoherence processes. The influence of Coulomb interaction is discussed and its consequences are indicated. We calculate the relaxation rates for GaAs quantum dot molecules and study the dependence on quantum dot size, distance and offset between the constituent quantum dots. In addition the temperature dependence of the tunneling rates is analyzed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Zero-strain GaAs quantum dot molecules as investigated by x-ray diffuse scattering

The authors report on x-ray diffuse scattering at nominally strain-free GaAs(001) quantum dot molecules (QDMs). Al0.3Ga0.7As deposited by molecular beam epitaxy on GaAs(001) acts as barrier layer between the GaAs(001) substrate and subsequently grown QDMs; the adjusted thickness of 50nm preserves the in-plane lattice parameter. Pairs of lenselike quantum dots are created with preferential orientation along [11¯0] placed on shallow hills. Grazing incidence diffraction along with kinematical scattering simulations indicate completely strain-free QDs which prove a strongly suppressed intermixing between QDMs and the underlying AlGaAs barrier layer.

Publisher's Note: Broken symmetry and quantum entanglement of an exciton inInxGa1−xAs∕GaAsquantum dot molecules [Phys. Rev. B71, 075325 (2005)

Publisher's Note: Broken symmetry and quantum entanglement of an exciton in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi mathvariant="normal">In</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">Ga</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi mathvariant="normal">As</mml:mi><mml:mo>∕</mml:mo><mml:mi mathvariant="normal">Ga</mml:mi><mml:mi mathvariant="normal">As</mml:mi></mml:math>quantum dot molecules [Phys. Rev. B<b>71</b>, 075325 (2005)