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phys. stat. sol. (c) 3, No. 11, 3916–3919 (2006) / DOI 10.1002/pssc.200671621
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Biexcitonic complexes and the decay dynamics of trion
and exciton in a single CdSe quantum dot
M. Koch1, K. Kheng*,1, I. C. Robin 2, and R. André2
1 CEA-CNRS, Université J. Fourier – Grenoble group “Nanophysique et Semiconducteurs”,
Grenoble, 17 rue des Martyrs, 38054 Grenoble, France 2 Laboratoire de Spectrométrie Physique/CNRS, BP 87, 38402 Grenoble, France
Received 1 May 2006, revised 14 June 2006, accepted 18 July 2006
Published online 24 November 2006
PACS 71.35.Pq, 73.21.La, 78.67.Hc
We report dynamical studies of a single CdSe quantum dot for various excitation densities. Exciton and
trion states refilling by recombination of neutral and charged biexciton is revealed by the observation of a
delayed decay for the sharp trion and exciton lines respectively for increasing excitation intensity. The
biexcitonic complexes lifetimes are then estimated from the rise time of the excitonic decays.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Studies of multiexcitonic complexes in semiconductor quantum dots (QDs) are par-
ticularly relevant to emerging quantum technologies. Exciton or trion recombination provide the single
photon emission for single photon sources while biexciton can be used as a quantum bit pair for quantum
processing. As compared to the more extensively investigated III-V QDs, II-V QDs such as CdSe QDs
present the advantage of a large exciton binding energy which allows separation of the exciton and biex-
citon emission lines up to higher temperature. For single source application the dynamic of exciton and
trion is of particular importance since it fixes the generation rate of the single photons emission. In this
paper we report time-resolved photoluminescence measurements of a single CdSe quantum dot for vari-
ous excitation densities and show the effect of biexcitonic (neutral and charged) complexes recombina-
tion on the decay dynamics of the exciton and the trion.
2 Experiment and details The QDs are formed from 3 monolayers of CdSe deposed on ZnSe barrier
by atomic layer epitaxy. We induce the QD formation by the technique of deposition/desorption of
amorphous Se [1]. Single dots spectroscopy is carried out at low temperature (5 K) using aluminum
shadow masks with 0.2-1.0 µm apertures and a large numerical-aperture microscope objective. The QDs
photoluminescence (PL) are excited with the 488 nm line of an cw argon laser (below barrier excitation)
for time-integrated measurements and with a mode-locked, frequency doubled Ti:Sapphire laser (above
barrier excitation) delivering (1.2 ps-long pulses with a repetition rate of 76 MHz) for time-resolved
measurements. The emitted light in this case is dispersed by a 30 cm-focal length monochromator and
recorded by a streak camera system. The effective time resolution of our set-up is 4 ps. The spectral
resolution is about 1.5 meV and 0.1 meV for the time-resolved and time-integrated mode respectively.
Figure 1 shows the emission spectra of a QD where the exciton (X), trion (T) and biexciton (XX) lines
have been clearly identified. Indeed, these power dependent spectra show that the biexciton intensity
increases quadratically with the excitation density while exciton intensity increases linearly. Moreover, X
and XX lines present the typical symmetrical doublet structure [2] induced by the electron-hole exchange
* Corresponding author: e-mail: [email protected], Phone: +33 438 78 47 01, Fax: +33 438 78 51 97
phys. stat. sol. (c) 3, No. 11 (2006) 3917
www.pss-c.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
interaction in asymmetric QD, that is, each line X or XX is composed of two components linearly polar-
ized along two orthogonal directions with a symmetrical sequence between X and XX.
As for the trion line T, it only shows a singlet structure, as expected, even in case of QD asymmetry (in
contrast to excitons). This is because the two electrons in the ground state of the negative trion are paired
so that electron–hole exchange interaction counterbalance. Very clear identification of the trion state T
can be given by magneto-PL measurements in Voigt geometry [3]. Thus, this line T splits into four com-
ponents due to the non-zero value of the hole g-factor [4]. The biexciton and trion binding energies in
this dot are 21 meV and 17 meV respectively.
We want now to focus on the time-resolved study of the QD. Because of the limited spectral resolution
and of the appearance of additional spectral lines due to the above barrier excitation we were able to
analyze accurately the decay dynamics only for the exciton and the trion (the biexciton line is superposed
by another very intense line for above barrier excitation).
Fig. 1 Exciton (X), Trion (T) and biexciton (XX) emission lines from a single CdSe QD for increasing excitation
power (from bottom to up).
Fig. 2 Streak camera images of a single CdSe QD emission at three different excitation powers P = I0, 6I0 and
16I0. At high excitation power a continuous background emission appears and the trion line (T) decay is delayed
due to state refilling effects.
3918 M. Koch et al.: Biexcitonic complexes and decay dynamics of trion and exciton in a CdSe QD
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com
X
T
X
T
X
T
Fig. 3 Exciton (X) and trion (T) decay for various excitation powers.
Figure 2 shows the images of the streak camera integrated over 200 s at three different excitation powers.
The spectra cover a range of 10 nm in the horizontal axis and are centered at 533 nm (2.329 eV). The
vertical axis corresponds to a time range of 2.2 ns.
The first thing we observe is that at low excitation powers all spectral lines rise very quickly and attain
their maximal intensity all at the same time directly after the excitation. This is no more the case when
we increase the excitation power. The streak camera images show for example that the maximum of the
trion emission is obviously more and more retarded when the excitation power P increases (retardation of
200 ps and 500 ps for the second and third image respectively). This is clear evidence for a refilling
process, that is the trion state is refilled by the recombination of charged biexcitons.
Finally, when the excitation power is high, a continuous background emission, with a short decay time,
superimposes with the discrete lines. This background emission can be attributed to the multiple possi-
bilities of electron-hole recombination when the dot is filled with few electrons and few holes [5] just
after the pulsed excitation.
To get now a more quantitative discussion we plot in Fig. 3 the time dependence of the intensity of the
exciton (X) and the trion (T) integrated over a narrow spectral range. Because of the semi-logarithmic
scale a mono-exponential decay corresponds to a straight line with negative slope which gives the decay
time. We have to distinguish between three different regimes:
- I0: At the lowest excitation power the exciton decays mono-exponentially with a decay constant of 252
ps. We did not observe the bi-exponential component, which may due to the refilling of the bright ex-
citon state from the spin flip of the dark exciton. The behavior of the trion line gives already evidence
for a refilling process, since it is best fitted by a bi-exponential decay with one rising and one decreas-
phys. stat. sol. (c) 3, No. 11 (2006) 3919
www.pss-c.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ing component. The rising component, which should correspond to the lifetime of the charged biexci-
ton from which the refilling occurs, was determined to be 205 ps, whereas the subsiding component
has a decay constant of 230 ps. This decay constant determines the asymptotic behavior of the decay
for large delay t and is thus equal to the lifetime of the trion.
- 2I0 < I < 3.2 I0: For these excitation powers both exciton and trion decayed bi-exponentially indicating
a state refilling from the recombination of neutral and charged biexcitons respectively.
- 3.2 I0 < I < 20 I0: Now the continuous background emission we already mentioned becomes signifi-
cant and has to be considered. This emission with a short decay constant is responsible for the sharp
peak at the beginning of each
decay curve. For the trion line,
we obtained a good agreement
with the measured curves by
simply applying a tri-expo-
nential fit. The third exponential
decay component can be attrib-
uted to the continuous emission
whose decay constant was
found to be around 150 ps. All the decay constants result-
ing from the exponential fitting
of the experimental data are
summarized in Table 1. The
statistical average is 250 ± 25 ps
for the exciton lifetime and 245
± 25 ps for the trion. In addi-
tion, the bi-exponential be-
havior of the trion line allows us
to estimate a charged biexciton
lifetime of 190 ± 30 ps.
The decay dynamics of single CdSe quantum dots were first studied by Bacher et al. [6] who report a
longer lifetime for the biexciton (310 ps) than for exciton (290 ps). Patton et al. [7] observed a larger
number of single dots and found exciton lifetimes varying between 200 and 600 ps. In these experiments
the biexciton lived usually significantly shorter than the exciton whereas the trion had a similar lifetime.
This seems to be confirmed by our results, which lead to similar lifetimes both for exciton and trion and
for neutral and charged biexcitons.
3 Conclusion The dynamical studies of a single CdSe QD reveal a delayed decay for the sharp trion
and exciton lines when the excitation intensity increases. This observation is an evidence of exciton and
trion states refilling by radiative (cascade) emission of neutral and charged biexciton respectively. The
bi-exponential fit of the decay dynamics gives almost the same lifetime for exciton and trion (≈250 ps) in
one hand and for the neutral and charged biexciton (≈190 ps) on the other hand.
References
[1] I. C. Robin et al., Physica E 26, 119 (2005).
[2] V. D. Kulakovskii et al., Phys. Rev. Lett. 82, 1780 (1999); L. Besombes et al., Phys. Rev. Lett. 85, 425 (2000).
[3] A. V. Koudinov, I. A. Akimov, Yu. G. Kusrayev, and F. Henneberger, Phys. Rev. B 70, 241305 (2004).
[4] S. Moehl, I. C. Robin, Y. Léger, R. André, L. Besombes, and K. Kheng, phys. stat. sol. (b) 243, 849 (2006).
[5] E. Dekel et al., Phys. Rev. B 61 11009 (2000); K. Kheng et al., Physica E 26, 262 (2005).
[6] G. Bacher, R. Weigand, J. Seufert, V. D. Kulakovskii, N. A. Gippius, and A. Forchel, Phys. Rev. Lett. 83, 4417
(1999).
[7] B. Patton, W. Langbein, and U. Woggon, Phys. Rev. B 68, 125316 (2003).
Table 1 Fitting parameters of the exponential fit at various excita-
tion powers. τ1 is the decay constant of the exciton or trion, τ2 is the
rising exponential component due to the refilling process, τ3 is the de-
cay constant of the continuous background emission. (From 6I0, the
background emission starts to dominate the exciton emission and the
exciton rise time cannot be obtained with accuracy)
Exc. Power Exciton Trion
I [I0] τ1 [ps] τ2 [ps] τ1 [ps] τ2 [ps] τ3 [ps]
1 252 230 205
2 203 185 237 225
3 229 187 310 130
6 240 253 211
8 245 237 170 115
10 275 248 173 120
13 288 254 209 171
16 257 207 188 172
20 249 247 196 154
<τ>: 249 186 247 190 146