Excitation mechanisms of individual $\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}$ quantum dots studied by photon correlation spectroscopy

Systematic measurements of auto- and cross-correlations of photons emitted from individual $\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}$ quantum dots under pulsed excitation were used to elucidate nonresonant excitation mechanisms in this self-assembled system. Memory effects extending over a few excitation pulses have been detected in agreement with previous reports and quantitatively described by a rate equation model, fitting a complete set of correlation and PL intensity results. The important role of single carrier trapping in the quantum dot was established. An explanation was suggested for the unusually wide antibunching dip observed previously in $X\text{−}X$ autocorrelation experiments on quantum dots under cw excitation.

Figure 1

(Color online) (a) Time-integrated $(0–1.6\mathrm{ns})$ PL spectrum of the $\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}$ sample at $T=7\mathrm{K}$. Structures originating from acceptor-bound (AX), donor-bound (DX), and free (FX) excitons from the ZnTe barrier as well as from excitons in the wetting layer are indicated. Emission from the QD layer forms a wide band centered at $2.25\mathrm{eV}$. Markers (1) and (2) indicate energies corresponding to temporal profiles presented in Fig. 1b. (b) PL transients of free excitons in ZnTe barriers (1) and quantum dots (2) at $T=7\mathrm{K}$. Points: experimental data and solid lines: fitted curves. Respective decay times indicated.

Figure 2

Photoluminescence spectrum of the selected quantum dot, excited with a frequency-doubled pulsed beam of a Ti:sapphire laser at $402\mathrm{nm}$ $\left(3\mathrm{eV}\right)$. The averaged excitation power was $700\mathrm{nW}$.

Figure 3

Integrated photoluminescence intensity at $T=1.7\mathrm{K}$ as a function of excitation power for $X$, $CX$, and $XX$. Solid lines show model calculation (see Sec. 4B).

Figure 4

Intensity of $X$, $CX$, and $XX$ photoluminescence as a function of time following excitation pulse. Solid lines represent exponential decays with indicated lifetime values. Excitation power $700\mathrm{nW}$, temperature $1.7\mathrm{K}$.

Figure 5

(Color online) Integrated intensity of $X$, $CX$, and $XX$ lines vs delay between two excitation pulses in ECS experiment. Excitation power of a single beam $230\mathrm{nW}$, temperature $1.7\mathrm{K}$.

Figure 6

(Color online) Histograms of correlated counts as a function of time interval $\tau ={\tau }_{\mathit{\text{STOP}}}-{\tau }_{\mathit{\text{START}}}$ between photon registration events in start and stop detectors, tuned to indicated transitions (start-stop order). The average excitation power $700\mathrm{nW}$, repetition period $6.6\mathrm{ns}$, and time bin $733\mathrm{ps}$. Acquisition time $1\mathrm{h}$ (a, d, e) or $2.5\mathrm{h}$ (b, c, f). Single count rates 9300/s, 5800/s, and 4600/s (first detector) and 7200/s, 4500/s, and 3900/s (second detector) for $X$, $CX$, and $XX$, respectively. Solid lines represent fits of rate equation model (see Sec. 4B), calculated for each peak and joined with a line for better visibility.

Figure 7

$X\text{−}X$ (a) and $CX\text{−}X$ (b) second order correlation functions determined with two different repetition periods $T_{\mathit{rep}}=6.6\mathrm{ns}$ (gray bars) or $13.2\mathrm{ns}$ (white bars), plotted as a function of peak consecutive number $n=\tau ∕T_{\mathit{rep}}$.

Figure 8

Scheme of energy levels and optical transitions considered in the model. $\alpha \left(t\right)$, $\beta \left(t\right)$, and $\xi \left(t\right)$ are the time dependent capture rates of oppositely charged single carriers and exciton, respectively. Constants ${\tau }_X$, ${\tau }_{CX}$, and ${\tau }_{XX}$ are decay times of respective states.