N + e + h
N INITIAL
FINAL
ground ground
ex cite d ex cite d
T = 4.2K T > ~30K
Figure 8.9: Scheme of the proposed recombination diagram when the temperature is increased (see text for details).
due to X* decay caused by charge fluctuations in the dot. The additional low energy emission lines, XX3 and XX4, that appear in the spectra above ∼30K are most probably due to transitions leaving behind a hole in the excited state.
0 10 20 30 40 50 60 70 80 0.0
0.1 0.2 0.3 0.4 0.5
X Lorentzian shape
FWHM (meV)
Temperature (K)
-1 0 1
0.01 0.1 1
PL Intensity (arb. units)
∆Energy (meV)
4.5K
0.23meV
Temp.
FWHM
-1 0 1
25K
0.21meV
-1 0 1
50K
0.16meV
-1 0 1
75K
0.29meV
-1 0 1
0.0 0.2 0.4 0.6 0.8 1.0
PL Intensity (arb. units)
∆Energy (meV) 4.5K
0.23meV
Temp.
FWHM
-1 0 1
25K
0.21meV
-1 0 1
50K
0.16meV
-1 0 1
75K
0.29meV
a)
b)
c)
Figure 8.10: a) Examples of a Lorentzian fit to single X emission lines at temper- atures 4.2K (a), 25K (b), 50K (c) and 75K (d) for the quantum dot which emission spectrum is illustrated in Fig.8.1a). b) The same picture as a) but in logarith- mic intensity scale. The good agreement of the fit with the data confirms that the emission lines are homogenously broadened. c) Resulting width (FWHM) of the X emission lines for all temperatures. The lines do not broaden significantly in the whole range of the investigated temperatures before disappearing from the spectrum.
emission lines exhibit no significant broadening even at high temperatures, up to 70K, as illustrated in Fig.8.10c).
A similar effect is observed for the XX1 and XX3 emission lines. No broaden- ing is observed even at high temperatures of 100K. They are, contrary to X, better fit by a Gaussian shape line. This can suggest their identification as the emission of an ensemble of excitons and/or more complicated excitonic state recombination (leaving the residual carriers in an excited state).
The lack of broadening, typical for other quantum dots, shows that in the case of the investigated quantum dots the system is very insensitive to inelastic scatter- ing by phonons. This can be due to the absence of appropriate states between the dot discrete energy levels (bottleneck effect).
However, if looking carefully for the phonon-sidebands in X emission line at high temperatures it can be found that for some quantum dots the effect is indeed visible at high temperatures. Fig.8.11 illustrates the Lorentzian fit of the X emis- sion line for the quantum dot whose emission spectrum is illustrated in Fig.8.1b).
The phonon-sidebands are observed to appear in the spectra at temperatures as high as 75K. In this case also a broadening of the emission line is observed from 0.2meV at 4.K up to 0.5meV at 95K.
-2 -1 0 1 2
0.0 0.2 0.4 0.6 0.8 1.0
FWHM = 0.28meV
PL Intensity (arb. units)
Energy (meV)
T = 75K
-2 -1 0 1 2
0.0 0.2 0.4 0.6 0.8 1.0
PL Intensity (arb. units)
Energy (meV)
T = 90K
FWHM = 0.48meV
Figure 8.11: Examples of a Lorentzian fit to a single X emission line at tempera- tures 75K (left panel) and 90K (right panel) for the quantum dot which emission spectrum is illustrated in Fig.8.1b). Dots: experimental data; solid line: Lorentzian fit.
The thermal broadening of the X emission line is hardly observed up to a temperature of 70K. This can be due to weak interaction of electrons with acoustic phonons in the case of the investigated quantum dots.
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Chapter 9
Multiexcitonic Emission Decay Times
Dans ce chapitre, nous pr´esentons la m´ethode de spectroscopie, r´esolue en temps, appliqu´ee `a une boˆıte quantique unique. Le temps de relaxation de l’´emission to- tale de boˆıtes quantiques sous excitation non r´esonante est tr`es long, de l’ordre de quelques dizaines de microsecondes. Cette observation est en contradiction avec les temps typiquement mesur´es sur d’autres systemes de boˆıtes quantiques, qui sont plutˆot de l’ordre de la nanoseconde. Dans le cas d’une excitation r´esonante, nous observons aussi un temps de relaxation de l’ordre de la nanoseconde. Nous don- nons une interpr´etation de cet effet comme r´esultant de l’interaction du syst`eme d’excitons indirect dans le double puits quantique pr´esentant de tr`es longs temps de relaxation, avec le syst`eme de boˆıtes quantiques. Dans ce cadre, la structure `a double puits quantique agit comme un r´eservoir d’excitons pour les boˆıtes. Ce mod`ele per- met d’´evaluer certaines propri´et´es des excitons bidimensionnels indirects comme leur constante de diffusion et leur mobilit´e.
The time evolution of the photoluminescence spectra of a single quantum dot was observed.
The following chapter discusses how the excitation power and the temperature from 4K up to 60K influence the derived characteristic times.
In the first section the general excitation power and temperature effects on time-resolved spectra are discussed.
In the second section, a more detailed analysis of the observed decay times is given.
The experimental setup for time resolved experiments is described in 4.5.