Excitonic Cascades in Extra – low Density GaAs/AlAs Quantum Dots
B. Chwalisz-Piętka1,2, A. Wysmołek1, R. Stępniewski1, J. Suffczyński1, M. Goryca1,
T. Kazimierczuk1, W. Pacuski1, B. Piechal1, A. Trajnerowicz1, W. Maślana1, P. Kossacki1,
A. Golnik1, J. A. Gaj1, M. Potemski2, V. Thierry-Mieg3
1
Institute of Experimental Physics, Warsaw University, Poland
e-mail: Barbara.Chwalisz@fuw.edu.pl
2
3
Grenoble High Magnetic Field Laboratory, CNRS, Grenoble, France
Laboratoire de Photonique et de Nanostructures, Marcoussis, France
Counts [arb. units]
Spectroscopic studies of a unique structure whose active part consists of a type II
GaAs/AlAs double quantum well lead us to the observation of a new class of semiconductor
quantum dots with surface density typically as low as 106cm-2 [1, 2]. It is speculated that these
dots are naturally formed at the GaAs/AlAs interface during a growth-with-interruption due to
substantial gallium inter-diffusion in the AlAs layer. The optical properties of the dots
revealed that they are strongly confined systems which show a multiple zero-dimensional
shell structure. The type II system makes the dots easy to fill by carriers under non-resonant
excitation due to characteristic, very long-lived (up to ms) indirect two-dimensional excitons
which efficiently diffuse into zero-dimensional traps.
We performed photon correlation spectroscopy under non- and quasi- resonant cw
excitation of a single quantum dot (SQD) emission lines. We found that there are two different
time scales in the system (see the Fig.). First, in the range of hundred ns, we attribute to the
time required to change the charge state of the dot. Second, in the order of ns, is connected
with the radiative recombination process. Moreover, the cross-correlation profiles allowed us
to establish the lines related to differently charged states of SQD involving the carriers
confined on the ground and excited levels of SQD. Within these different charge states we
were able to determine several cascades.
In particular, the quasiX-X auto-corrrelation histogram taken under quasi-resonant excitation.
resonant excitation (below the X1200
symmetry energy levels in QW)
1000
revealed that the emission from SQD
800
under very low excitation power is
600
determined by the single exciton (X)
400
and bi-exciton (XX) recombination
cascades. The X-X auto-correlation
200
histograms show that the charge state
0
-300
-250
-200
-150
-100
-50
0
50
variation characteristic time is
Time [ns]
strongly affected by the excitation
power. Similar situation is achieved
when exciting above the barriers, where the long lived carriers confined at X-symmetry QW
states efficiently diffuse additionally supplying the dot with the carriers.
In the work we present various types of correlation histograms on a SQD emission
lines that allowed us to determine different processes responsible for the radiative
recombination and study the complex dynamics of this very unique system.
1.
2.
A. Wysmolek, B. Chwalisz, M. Potemski, R. Stepniewski, A. Babinski, S. Raymond, V. Thierry-Mieg, Acta
Physica Polonica A 106, 367 (2004)
B. Chwalisz, A. Wysmołek, R. Stępniewski, A. Babiński, M. Potemski, V. Thierry-Mieg, Int. J. of Modern
Physics B 18, 3807-3812 (2004)