Trion, biexciton, and exciton dynamics in single self-assembled CdSe quantum dots
B. Patton, W. Langbein, and U. Woggon*
Experimentelle Physik IIb, Universita ¨t Dortmund, Otto-Hahn-Str. 4, 44221 Dortmund, Germany
Received 7 February 2003; published 18 September 2003
We present an analysis of time- and polarization-resolved data taken in microphotoluminescence experi-
ments on individual CdSe/ZnSe quantum dots grown by molecular beam epitaxy. The identification of indi-
vidual dots was performed by a spectral jitter correlation technique and by their polarization properties and
density dependences. Decay times are given for exciton, trion, and biexciton states and evidence is shown for
a spin-relaxation-limited energy relaxation of the trion. For the bright-exciton state the temperature dependence
of the decay time is studied and a repopulation from the dark-exciton state is observed. Trion binding energies
of 15–22 meV and biexciton binding energies of 19–26 meV are found.
DOI: 10.1103/PhysRevB.68.125316 PACS numbers: 78.67.Hc, 78.55.Et, 78.66.Hf
I. INTRODUCTION
The original interest in self-assembled wide-gap semicon-
ductor nanostructures was driven by their potential applica-
tions in light-emitting or lasing devices for the green, blue,
or ultraviolet spectral ranges.
1–4
Nowadays, a further pecu-
liarity attracts attention to wide-gap materials: Coulomb cor-
relation energies have been observed in quantum dots which
are up to 10 times higher than those in III-V semiconductor
materials.
5–9
It makes, e.g., II-VI semiconductor quantum
dots interesting candidates in the study of few-particle states
such as trions, biexcitons, or charged biexcitons, yielding
binding energies, lifetimes, and fine structure of these
electron-hole complexes with both integer and half-integer
total spin states.
Much of the initial work on the optical properties of self-
assembled II-VI quantum dots has focused on ensembles of
dots.
10–14
The spectroscopy of such groups exhibits an inho-
mogeneous broadening of spectral features due to variance in
the geometry and composition of different dots within the
same sample. More recent times have seen the publication of
data on single CdSe/ZnSe quantum dots.
6–8,15–20
An advan-
tage of performing measurements on individual dots is that
the homogeneous broadening for different transitions in the
dots should be accessible. However, when single-dot spec-
troscopy was initially attempted it was found that the spectral
position of optical transitions in the dot varied with time.
This spectral jitter reflects the effect of fluctuating electro-
static fields on the energy levels within the dot.
21–25
The
mechanism which generates the fields is the trapping of
charge carriers in the vicinity of individual dots leading to a
local electric field and a resulting Stark shift in the position
of spectral features corresponding to that dot. The magnitude
and dynamics of the shift due to this quantum confined Stark
effect differ between different dots, since they are located at
different positions. By correlating the dynamic shift in the
spectral position of several emission peaks one is therefore
able to identify emission peaks from transitions within the
same dot.
In this paper we present measurements on self-assembled
CdSe quantum dots whence we perform a correlation analy-
sis on the spectral jitter which allows us to group spectral
transitions from individual dots. With this information we are
able to label individual spectral features according to the
quasiparticles which generate them. In doing so we have
focused on properties of excitons with an additional charge
trions, a quasiparticle in which a single electron hole in-
teracts with a spin-singlet state of two holes electrons, re-
spectively.
The trion is an interesting elementary excitation because
its radiative decay can be used to monitor properties of
single electrons or holes which are usually optically inacces-
sible. It is expected that the number of charge carriers within
a quantum dot will affect both the lifetime of the state and its
other spectroscopic properties such as exchange splitting and
binding energy. The existence of trions was first reported
almost ten years ago
26–28
and predominantly studied in quan-
tum well structures see, e.g., Ref. 29–31 and references
therein. Only recently were trion states studied in self-
assembled quantum dots using magnetophotoluminescence
and microphotoluminescence ( PL) experiments.
32–39
In
parallel the problem of charged nanocrystals has been ad-
dressed through experiments on colloidal CdSe quantum
dots.
40
While detailed theoretical work predicts binding and
charging energies,
41–44
there is a substantial lack of experi-
mental data with which to be compared.
In the work presented here, we have measured the binding
energies of trions and biexcitons relative to the exciton. We
give a systematic study of both the fine structure and the
dynamics of all three quasiparticles. The observed energy
shift of the trion transition due to the presence of an addi-
tional charge carrier is compared with data obtained for col-
loidal CdSe dots
40
and those predicted in theory.
41
We also
give data on the lifetimes of trion states and show evidence
for a spin-relaxation-limited energy relaxation of the trion.
Finally, we present a temperature-dependent study of the
bright-exciton decay and discuss the influence of a thermally
activated repopulation from the dark-exciton state.
II. EXPERIMENT
The results presented in this paper were obtained from a
sample containing CdSe quantum dots embedded in ZnSe.
Previous work has characterized such systems.
5,13
The
sample consists of three monolayers of CdSe grown between
ZnSe layers of 25 and 50 nm thickness. In Ref. 5 we found
PHYSICAL REVIEW B 68, 125316 2003
0163-1829/2003/6812/1253169/$20.00 ©2003 The American Physical Society 68 125316-1