Recombination kinetics of InAs quantum dots: Role of thermalization in dark states

We present time-resolved photoluminescence measurements of $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots showing a relevant interplay between recombination kinetics and thermalization processes. By time-resolved and steady-state measurements we experimentally demonstrate that the thermal population of optically inactive states fully accounts for the increase of the intrinsic radiative lifetime in quantum dots when the lattice temperature raises. Experimental data are compared with model predictions for the electronic structure of the quantum dot, and the dark optical transitions involved in the thermalization nicely correspond to the first-hole-excited levels. In addition, the observation of thermalization between the electronic levels allows us to estimate the relaxation time to the ground state, which turns out to be in the ps time scale.

Figure 1

Typical cw-PL spectra at $10\mathrm{K}$ for increasing excitation power density from $0.02\mathrm{W}∕{\mathrm{cm}}^2$ at step of factor of 2.

Figure 2

(a) Ratio ${\mathrm{PL}}_{\text{e.s.}}∕{\mathrm{PL}}_{\text{g.s.}}$ between the spectrally integrated emission of the e.s. and g.s. as a function of $1∕kT$, for fixed cw excitation power. The solid line refers to the Boltzmann factor expected for an energy splitting of $58\mathrm{meV}$. (b) Arrhenius plot of the spectrally integrated emission from the QD. The inset shows two cw spectra at different temperatures, indicating the quenching of the emission from the wetting layer. The solid line is the fit of the data performed using the model reported in Ref. 7.

Figure 3

TR-PL time decays at $T=10\mathrm{K}$ for the g.s. and e.s. (b) PL time decay of the WL emission at $10\mathrm{K}$. ETR indicates the experimental time resolution.

Figure 4

TR-PL time decays at different temperatures for the g.s. (solid lines) and e.s. (open circles) recombinations.

Figure 5

cw-PL spectra at different temperatures, keeping constant the detected signal from the g.s.

Figure 6

Comparison between the experimental g.s.-PL lifetime and the predictions for $\Delta E=8\mathrm{meV}$ (dotted line), $\Delta E=16\mathrm{meV}$ (solid line), and $\Delta E=58\mathrm{meV}$ (dashed line). In all cases $g=2$. The inset shows the comparison between the calculated spectrum as discussed in the text (solid line) and the cw experimental spectrum at $10\mathrm{K}$ in a band-filling condition (dashed line).