Excitonic dynamics in CdTe/ZnTe quantum dots (original) (raw)
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Relaxation and Dephasing of Multiexcitons in Semiconductor Quantum Dots
Physical Review Letters, 2002
We measure the dephasing time of ground-state excitonic transitions in InGaAs quantum dots under electrical injection in the temperature range from 10 to 70 K. Electrical injection into the barrier region results in a pure dephasing of the excitonic transitions. Once the injected carriers fill the electronic ground state, the biexciton to exciton transition is probed and a correlation of the exciton and biexciton phonon scattering mechanisms is found. Additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.
ACS Nano, 2012
Multiple exciton generation and recombination (MEG and MER) dynamics in semiconductor quantum dots (QDs) are simulated using ab initio time-dependent density functional theory in combination with nonadiabatic molecular dynamics. The approach differs from other MEG and MER theories because it provides atomistic description, employs time-domain representation, allows for various dynamical regimes, and includes electronÀphonon interactions. MEG rapidly accelerates with energy, reflecting strong energy dependence of double exciton (DE) density of states. At early times, MEG is Gaussian rather than exponential. Exponential dynamics, assumed in rate theories, starts at a later time and becomes more important in larger QDs. Phonon-assisted MEG is observed at energies below the purely electronic threshold, particularly in the presence of high-frequency ligand vibrations. Coupling to phonons is essential for MER since higher-energy DEs must relax to recombine into single excitons (SEs), and SEs formed during MERs must lose some of their energy to avoid recreating DEs. MER simulated starting from a DE is significantly slower than MER involving an optical excitation of a SE, followed by MEG and then MER. The latter time scale agrees with experiment, emphasizing the importance of quantum-mechanical superpositions of many DEs for efficient MER. The detailed description of the interplay between MEG and MER coupled to phonons provides important insights into the excited state dynamics of semiconductor QDs and nanoscale materials in general.
Exciton Dephasing in Quantum Dot Molecules
Physical Review Letters, 2003
We have measured the exciton dephasing time in InAs=GaAs quantum dot molecules having different interdot barrier thicknesses in the temperature range from 5 to 60 K, using a highly sensitive four-wave mixing heterodyne technique. At 5 K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed. These results show how the quantum-mechanical coupling of the electronic wave functions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.
Picosecond scale dynamics of excitons in CdTe-based quantum wells and quantum dots
2009
The pump-probe technique was used to study the reflectance of CdTe based quantum wells. The energy of a pump beam was scanned across excitonic resonances. The observed dynamics for both the energetic position and oscillator strength of excitonic lines is caused by the changes of the population of charge carriers as well as by the spin dependent interactions. The polarization resolved resonance technique was applied to the system of single CdTe -based quantum dots. Observation of linear to circular polarization conversion for pair of the adjacent, anisotropic dots allowed determining the time of excitation transfer between them.
Trion, biexciton, and exciton dynamics in single self-assembled CdSe quantum dots
PHYSICAL REVIEW B, 2003
We present an analysis of time-and polarization-resolved data taken in microphotoluminescence experiments on individual CdSe/ZnSe quantum dots grown by molecular beam epitaxy. The identification of individual dots was performed by a spectral jitter correlation technique and by their polarization properties and density dependences. Decay times are given for exciton, trion, and biexciton states and evidence is shown for a spin-relaxation-limited energy relaxation of the trion. For the bright-exciton state the temperature dependence of the decay time is studied and a repopulation from the dark-exciton state is observed. Trion binding energies of 15-22 meV and biexciton binding energies of 19-26 meV are found.
Exciton spin relaxation in resonantly excited CdTe/ZnTe self-assembled quantum dots
Physical review. B, Condensed matter
We study the exciton spin relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. The experiments on single CdTe QDs and on large QD ensembles show that by combining LO phonon -assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of QDs. We find that for single symmetric QDs at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by QD shape asymmetry. Similar behavior is found in a large ensemble of CdTe QDs. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a QD become non-degenerate. Finally, due to strong electronic confinement in CdTe QDs, the large spin polarization of excitons shows no dependence on the number of LO phonons emitted between the virtual state and the exciton ground state during the excitation. J J π X π Y J J B=0 B=0 B≠0
Excitons and multi-excitons in single CdTe quantum dots probed by near-field spectroscopy
Solid State Communications, 2002
A near-®eld optical spectroscopy study of a single CdTe/ZnTe quantum dot at low temperatures is presented. While the photoluminescence spectrum at low excitation power reveals only one single sharp peak due to the radiative recombination of excitons (X) in the single dot, several additional sharp peaks appear with increasing excitation density. The dominant features are ascribed to exciton complexes and charged exciton complexes such as negatively charged excitons (X 2 ), neutral (2X and 3X) and negative (2X 2 and 3X 2 ) biexcitons and triexcitons. Exciton charging arises due to ef®cient hole trapping by residual acceptors in the barrier material. This partly inhibits the formation of biexcitons and triexcitons. A spectral feature appearing close to the X 2 peak is tentatively assigned to X 22 negative excitons. This feature is found to shift to the red with increasing power: two possible explanations for this unexpected behaviour are proposed. q
Excitons States in Semiconductor Quantum Dots
2016
It was found that within the band gap of the quantum dot of zinc selenide appears a zone of exciton states, located at the bottom of the conduction band. It has been shown that a decrease in the band gap width in this nanosystems conditioned by transition of the electron from quantum-dimensional level within the valence band of the quantum dot to the levels of the zone of exciton states. The dependence of the energy of a base state of an exciton from the radius of QD was obtained using a modified method of the effective mass.
Optical Properties of Excitons Confined in a Single CdTe Quantum Dot
Physica Status Solidi, 2002
Spectroscopy of CdTe single quantum dots reveals intrinsic properties of quantum dots, such as the acoustic phonon broadening mechanism and the fine structure of the exciton. Strong asymmetry of a dot induces both electron-hole exchange splitting of the bright exciton and an anisotropy of its linearly polarized components. Owing to the existence of a unique biexciton state, the biexciton transition reproduces the exciton fine structure in a reverse sense. At increasing excitation density, emission lines of a few exciton complexes corresponding to sand p-shell transitions are observed.