Apart from the natural blinking of the quantum dot, it was observed that the pro- cess of charge variation can be visible even on the time scale of a few seconds. If series ofµ-PL spectra of the same dot are recorded sequentially with an integration time of 1s, it is possible to observe that the intensity of particular emission lines significantly changes from one spectrum to another.
Fig.10.9 illustrates two µ-PL spectra of the quantum dot taken at the same excitation conditions at two limits: when the X emission intensity is weakest and strongest in the whole series of measurements. It is observed that the emission lines gained in intensity in the same spectrum belong to the same ”family” of neutral or charged excitonic configuration recombination, as it was deduced from the photon correlation experiment (compare with discussion in previous section).
This process occurs, however, on a ”macro” time scale range in comparison to the one in the hundreds of ns range that was observed in the photon correlation experiment.
Fig.10.10 shows the calculated classical intensity cross-correlation coefficient for the emission lines at different energiesαandβ:
Γ = P
i(Iiα−Iα)(Iiβ−Iβ) q
P
i(Iiα−Iα)2P
i(Iiβ−Iβ)2
(10.3)
whereIiα,Iiαare the intensity of the signal in the following spectra imeasured at energyαandβ, respectively;IαandIβare the average intensities over all spectra at energiesαandβ, respectively. Γ=1, denoted in the plot by a red colour, represents
1.608 1.612 1.616 1.620 1.624 1.628 1.632 400
500 600 700 800 900
S1
P3 P1 "neutral state"
"charged state"
PL Intensity (arb. units)
Energy (eV) X1
A2 XX
S X*
X
XX T
Figure 10.9: The very long time blinking effect visible in the micro-PL spectrum of the single GaAs/AlAs quantum dot (different dot than the one whose emission spectrum is illustrated in Fig.10.1) at moderate excitation power and quasi-resonant excitation. The red curve illustrates the emission spectrum from the quantum dot mainly due to neutral states recombination and the green curve that due to charged states recombination. The most pronounced lines are marked in the picture.
X
P2 X*
XX A1
P1
Energy
X
P2 X*
XX A1
P1
Energy
Figure 10.10: The map illustrates the matrix of the Γ correlation coefficient ac- cording to eq.10.3 for a number of micro-PL spectra of a single quantum dot. Red and green colour at the cross points illustrates positive and negative correlations, respectively, between emission lines that correspond to the spectrum marked at the bottom and on the right hand side of the map. The spectrum is the average of all recorded spectra. The maps illustrated in the top and bottom panels represent the data recorded with slight variations of the exciting laser spot position and with a slight change of the excitation power, respectively.
the positively correlated signals and Γ=-1, denoted in the plot by a green colour, the anti-correlated signals. The diagonal in the map illustrates the auto-correlation of the same emission line. Thus at the diagonal Γ=1. The cross-correlations between different emission line intensities are found at respective coordinates. The map is thus symmetric with respect to the diagonal. For example, the red spot is found at the x-coordinate corresponding to X emission and y-coordinate corresponding to XX emission. This illustrates the positive correlation between X and XX emission lines.
Both maps illustrated in Fig.10.10 are represented in the same colour scale.
The experimental conditions differ slightly in both cases. In the case of the top plot the spectra were recorded while the position of laser spot on the sample was slightly changed. In the case of the bottom plot, the excitation intensity was varied to a very small degree. Therefore both maps reveal slightly different features. In the first one the positive correlations are better seen, in the latter the negative correlations are more pronounced.
The obtained results of positive and negative correlations are similar to those from the photon correlation experiment discussed in the previous section. The positive correlations are found in two groups of lines:
X, XXS, XXT, A2 and X*, P1, X1, P3, S1, A3.
The negative correlations are found between emission lines belonging to different groups. Additionally, the correlations of some less pronounced lines are visible, suggesting their charged or neutral origin.
The blinking effect has already been reported [10, 12, 14, 16, 20, 21, 22]. It was discussed mainly in terms of Auger type processes [12], the vicinity of the impurities and/or defects [16, 21] and/or generally, the interactions with the surroundings of the quantum dot. In the present experiment, by slightly scanning the dot with the laser beam or by small variations of the excitation power under the quasi-resonant excitation it was possible to change between the two spectra illustrated in Fig.10.9.
Thus it was possible to switch between more charged and neutral configurations of carriers in the dot. Most probably, this effect is caused by locally heating the surrounding matrix of the dot (possibly an impurity) and therefore enhancing the charge transfer. It is believed, however, that this long time effect is different in nature than the natural charge variation that was observed on thenstime scale.
The charge variation in quantum dots is visible in the ”macro” time scale, up to several seconds. This extremely long time is assigned to the change of the surroundings of the quantum dot, which can supply the quantum dot with free charges.