One- and two-phonon capture processes in quantum dots

Abstract

Multiphonon capture processes are investigated theoretically and found to contribute efficiently to the carrier injection into quantum dots. It is shown that two-phonon capture contributes where single-phonon capture is energetically inhibited and can lead to electron capture times of a few picoseconds at room temperature and carrier densities of 10 17 cm Ϫ3 in the barrier.

Photoneutralization and slow capture of carriers in quantum dots probed by resonant excitation spectroscopy

“…The antibunching at zero delay is the signature that the RE signal corresponds to the single photon emission from the QD [26]. Apart from zero delay, on all spectra, a strong bunching appears, up to values of g (2) (τ ) = 8 for P gate = 145 nW. We note that this bunching decays with a time constant τ B ∼ 0.1 µs (for P gate = 145 nW), much longer than the radiative lifetime T 1 = 330 ps of the QD.…”

“…In order to compare the capture rates used in our fits to the ones given in literature [2][3][4], the charge density has to be considered instead of the optical excitation power. Considering the number of electron-hole pairs created by the optical gate in the GaAs barrier N = √ C γGaAs √ P gate [13,37], with C the photo-generation coefficient and γ GaAs the radiative recombination rate of the electronhole pairs in the GaAs barrier which are respectively about 10 10 nW −1 s −1 (for a HeNe laser at 1.96 eV) and 1 ns −1 [41], the density of charges photo-created in the GaAs barrier is given by n (m −2 ) ≈ 10 12 √ P gate(nW) (for a laser spot diameter of 2 µm).…”

“…The carriers can be then captured in the QD and the defect in its vicinity, either by the emission of optical phonons, or by Auger processes. The capture of a carrier in the QD by the emission of an optical phonon is a single charge process and is therefore proportional to the number of electrons or holes in the continuum [1,2,39], and in other words to √ P gate . Contrarily to this latter process, the Auger-assisted capture of a carrier involves two charges, where the incident charge in the continuum interacts with a target charge either in the continuum (Auger effect of type (I)), or in the QD (Auger effect of type (II)).…”

“…Within this hypothesis, the variations of the QD charge state is essentially governed by the two capture processes (1), (2), and its radiative recombination.…”

“…Then, intra-dot relaxation processes between the different QD discrete states lead to electron and hole population of the ground states from which the QD photoluminescence occurs. Theoretical works show that the capture mechanism relies either on emission of optical phonons [1,2] with typical capture times between 100 fs and 100 ns, or on electron-electron and electron-hole Auger scattering [3,4] with typical capture times between 1 ps and 1 µs. These calculations are in perfect agreement with timeresolved experiments performed in QDs ensemble [5] and in single QDs [6] where capture times of the order of 100 ps have been measured.…”