Optical properties of CdTe=ZnTe quantum dot

Physica E 12 (2002) 503 – 506
www.elsevier.com/locate/physe
Optical properties of CdTe=ZnTe quantum dot superlattices
S. Ma(ckowskia; ∗ , G. Karczewskia , J. Kossuta , G. Sek
; b , J. Misiewiczb , G. Prechtlc ,
c
W. Heiss
a Laboratory
of Low-Dimensional Structures, Institute of Physics, Polish Academy of Sciences, 32=46 Lotnikow Avenue,
02-668 Warszawa, Poland
b Institute of Physics, Wroclaw University of Technology, Wybrzeze
˙ Wyspianskiego 27, 50-370 Wroclaw, Poland
c Institut fur Halbleiter- und Festk-orperphysik, Universit-at Linz, Altenberger Stra0e 69, 4040 Linz, Austria
Abstract
We report on the optical properties of CdTe=ZnTe quantum dot superlattices with di3erent thickness of the spacer between
neighboring layers of quantum dots. It is found that for a spacer width larger than 20 monolayers (ML) of ZnTe, only one
emission band is observed which is ascribable to excitonic recombination in isolated CdTe islands. On the other hand, if
the ZnTe spacer width is smaller than 20 ML, a second luminescence band evolves on the low energy side of the main PL
peak, which we attribute to recombination of excitons in spatially correlated CdTe quantum dots. This additional emission
is characterized by a small oscillator strength and by a very long (∼20 ns) decay time, as evidenced by the photore8ectance
and the time resolved luminescence spectroscopy. Thus, it is interpreted as being due to recombination of electrons and holes
localized in two spatially separated but neighboring CdTe quantum dots. ? 2002 Elsevier Science B.V. All rights reserved.
PACS: 71.35.−y; 78.67.Hc; 81.15.Hi
Keywords: Quantum dots; Optical properties; Indirect excitons
Correlation in both vertical and lateral directions
between positions of quantum dots (QDs) in epitaxially grown semiconductor structures has been a subject of both theoretical [1,2] and experimental [3–8]
studies in di3erent materials systems. In particular, it
has been shown that various kinds of vertical and=or
lateral arrangement are associated with an anisotropy
of the elastic energy of the spacer (barrier) material [2]. In this communication we present the optical spectroscopy results obtained on CdTe=ZnTe QD
superlattices with varying width of the ZnTe spacer
layer. From our earlier reports [6,9] it is known that
∗ Corresponding
author. Tel.: +48-22-8431331;
+48-22-843-0926.
E-mail address: [email protected] (S. Ma(ckowski).
fax:
for ZnTe spacers thicker than 20 monolayers (ML),
only single luminescence (PL) line is observed. This
line, in a view of cross-sectional transmission electron
microscopy (TEM) data, is ascribed to excitonic recombination in isolated CdTe islands [6]. On the other
hand, as evidenced by TEM results [9], for ZnTe spacers thinner than 20 ML the islands tend to nucleate
◦
along the lines, which are tilted by approximately 40
with respect to the growth direction. The presence of
this spatial correlation is accompanied by the appearance of the second emission line on the low energy
side with respect to the main (isolated QDs related)
PL transition [6]. This emission line gains the intensity as the thickness of the spacer is getting smaller.
Simultaneously, the linewidth of the main PL transition narrows substantially (by the factor of two),
1386-9477/02/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 3 4 3 - 5
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S. Mackowski et al. / Physica E 12 (2002) 503 – 506
probably due to improved uniformity of CdTe QDs. It
is important to note, that the zero-dimensional character of the excitons in the studied structures is evidenced by the sharp emission lines visible in their
microluminescence spectra [6].
The close spatial positions of the QDs make
possible a coupling between electronic degrees of
freedom of the carriers localized in two separate
islands. In particular, as it was shown in Ref. [5],
forming a stack parallel to the growth direction affects the linear polarization of the QDs emission
from such a structure. For other directions of the
vertical correlation spatially indirect excitons can
be formed from an electron and hole localized in
two di3erent but closely spaced QDs. Optical results
shown below indicate that spatially indirect excitons are present in the investigated CdTe=ZnTe QD
superlattices.
In order to investigate the in8uence of the spacer
width on the electronic properties of QDs, a set of superlattices consisting of one hundred repetitions of 2
monolayer (ML) wide CdTe quantum dot layer separated by ZnTe barriers was grown by molecular beam
epitaxy. The width of the ZnTe spacers was varied
between 3 and 75 ML. The superlattices were deposited on (1 0 0)-oriented GaAs substrates. The substrate temperature during all growth processes was
◦
equal to 380 C. Prior to the deposition of the superlattices 4 m wide CdTe bu3er layer was grown to
diminish an in8uence of misMt dislocations due to the
lattice mismatch between ZnTe and GaAs. It has to be
pointed out that because of entirely di3erent growth
conditions in the present case, the obtained structures
are not comparable with those reported earlier [10].
In particular, as revealed by TEM, the present structures do not contain a quasi two-dimensional wetting
layer [9].
Optical properties of the samples were examined
by several techniques including di3erent types of photoluminescence (PL) as well as by photore8ectance
(PR) spectroscopy.
The PL was excited either non-resonantly above
the ZnTe barrier energy ( = 457 nm line of an argon
laser) or in a selective way (dye laser with Rhodamine 6G). The emission was analyzed by a Jobin–
Yvon spectrograph equipped with a LN2 -cooled
charge-coupled-device detector. The exciton dynamics was studied by time resolved PL (TRPL) utilizing
the time correlated single photon counting technique.
In this case the signal was detected by a fast microchannel plate. As an excitation source for TRPL
experiments the frequency-doubled Ti: Sapphire laser
giving 6 ps long pulses with 80 MHz repetition was
used.
Photore8ectance (PR) measurements were performed at bright conMguration at T = 10 K as well
as at room temperature. For these measurements the
sample was mounted on a cold Mnger in a closed-cycle
helium cooler. The sample was illuminated simultaneously by normal incidence white light from a halogen lamp (probe beam) and by an argon laser pump
beam (488 nm line) chopped at frequency of 125 Hz.
The latter provides the variation of the dielectric
function and hence of the re8ectivity coeQcient of
the structure [11]. The re8ected light after being dispersed by a monochromator was detected by a silicon
photodiode.
On the basis of the results of PL and PR, all QD superlattice structures studied here can be divided qualitatively into two groups. A single spectral line, which
is associated with isolated CdTe islands [6], characterizes the Mrst group, which includes the samples with
the ZnTe spacer wider than 20 ML. To the second
group belong the samples with thinner ZnTe spacers.
Their spectra consist of two emission lines. As an example of the latter, the PL and PR spectra obtained
at low temperatures for the structure with the ZnTe
spacer equal to 5 ML are presented in Fig. 1. The
two spectral features could be identiMed in both, PL
and PR data. In accordance with the data obtained for
samples with wide ZnTe spacers [6], the high-energy
(E∼2:13 eV) line is ascribed to excitons in isolated
CdTe QDs. On the other hand, the much broader low
energy (E∼2:0 eV) line indicates the presence of another type of excitons, which, in a view of TEM results, is related to the vertically and=or laterally correlated CdTe QDs. The appearance of the low-energy
line in the samples containing spatially correlated islands suggests that these correlated dots may be also
electronically coupled.
The most striking di3erence between the PR and
PL data shown in Fig. 1 is the relative intensity
of these two spectral lines corresponding to isolated and correlated QDs. Namely, in the case of
PL, the dominant emission is that related to correlated QDs, while the isolated ones become much
S. Mackowski et al. / Physica E 12 (2002) 503 – 506
isolated QDs
photoreflectance
T=10K
0
10
Log PL intensity [a. u.]
Light Intensity [a. u.]
LZnTe=5ml
505
correlated QDs
e
e
e
e
hh
hh
hh
hh
LZnTe=12 ml
T=1.8K
luminescence
T=4.2K
-1
10
0
1
2
3
0
1
2
3
4
5
6
Time [ns]
1.8
1.9
2.0
2.1
2.2
2.3
Energy [eV]
Fig. 1. Low temperature PL and PR spectra for the structure with
5 ML thick ZnTe spacer.
more pronounced when measuring PR. Such situation
was met for all structures in which the low-energy
band was observed. The very weak intensity of the
PR line at the energy position corresponding to the
correlated QD emission implicates that the oscillator strength of this transition is quite small. This
suggests that the excitons in electronically coupled
QDs are built from carriers localized in two separate
islands since then their wave functions overlap is
smaller.
This suggestion is supported by selectively excited
PL experiments as well as by TRPL results. Indeed,
when exciting above the ZnTe barrier or resonantly
into isolated QD energy level, both isolated and correlated QD emissions are observed. In contrast to that,
if the excitation energy is between these two lines or
is resonant with the PL band related to the recombination in correlated QDs, no measurable signal can
be detected. This indicates, that indeed the recombination visible on the low energy side of the isolated
QDs PL band originates from the indirect excitons
localized in electronically coupled QDs. Moreover,
a decay time characteristic for the recombination in
correlated islands is found to be almost two orders
of magnitude longer than that typically observed in
similar structures with isolated dots only [12], and
it approaches 20 ns. On the other hand, the value of
the decay time obtained for the high energy emission (∼400 ps) agrees well with the recombination
Fig. 2. Low temperature TRPL spectra for isolated (left panel)
and correlated (right panel) QDs emission bands. The schematic
diagram of the recombination process in each case is also given.
dynamics observed for CdTe isolated QDs [12]. The
latter proves additionally the initial assignment of the
spectral features observed in both PL and PR data. The
example of the temporal evolution of the PL intensity
in the sample with ZnTe spacer 12 ML wide is shown
in Fig. 2. The inset shows a schematic representation of the transitions involved in both types of the
recombination.
Both the long decay time and the absence of
the correlated QDs emission when exciting resonantly evidence that the low energy PL band
originates from the recombination of indirect excitons formed by electron and hole localized by
two di3erent but spatially correlated neighboring
islands.
The optical properties of CdTe=ZnTe QD superlattices with di3erent ZnTe spacer thickness were
studied by means of PL and PR spectroscopy. It was
found that the signal related to correlated QDs is
characterized by small oscillator strength and by a
very long decay time approaching 20 ns. Thus we
conclude, that this emission is caused by the recombination of excitons consisting of electrons and holes
localized in two di3erent (most probably neighboring) CdTe QDs. The obtained results prove also that
by varying the width of the spacer between QD layers
one can in8uence not only the structural properties
of these superlattices but also a3ect their optical
characteristics.
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S. Mackowski et al. / Physica E 12 (2002) 503 – 506
Acknowledgements
This work was performed with Mnancial support by
the Committee for ScientiMc Research in Poland under
Grants No. 5P03B02120 and 8 T11B 010 18 and by the
R
OAD
with the project Nr. 5=2001. S. Ma(ckowski and
G. S;ek acknowledge the support from The Foundation
for Polish Science. Partial support of EU RTN under
contract HPRN-CT-2000-00134 is acknowledged.
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