Temperature effects on the radiative recombination in InAlAs/GaAlAs quantum dots

Full text

Temperature effects on the radiative recombination in InAlAs/GaAlAs
quantum dots
A. Ben Daly
a,
n
, F. Bernardot
b,c
, T. Barisien
b,c
, A. Lemaître
d
, M.A. Maaref
a
, C. Testelin
b,c
a
Laboratoire Matériaux, Molécules et Applications, Institut Préparatoire aux Études Scientifiques et Techniques, Université de Carthage, BP 51,
2070 La Marsa, Tunis, Tunisia
b
Sorbonne Universités, UPMC Université Paris 06, UMR 7588, Institut des NanoSciences de Paris, F-75005 Paris, France
c
CNRS, UMR 7588, INSP, F-75005 Paris, France
d
Laboratoire de Photonique et Nanostructures, CNRS, UPR 20, Route de Nozay, F-91460 Marcoussis, France
article info
Article history:
Received 15 August 2015
Received in revised form
3 October 2015
Accepted 15 November 2015
Available online 28 November 2015
Keywords:
Quantum dots
Photoluminescence
Time-resolved photoluminescence
Carrier lifetime
abstract
The influence of the temperature has been studied in self-assembled InAlAs/GaAlAs quantum dots (QDs)
using photoluminescence (PL) and time-resolved PL (TRPL). With increasing temperature, the exciton
retrapping in QDs, after a thermal activation, is evidenced and confirmed by a narrowing of the PL
spectrum width, and an increase of the PL decay time. From the temperature dependence of the integrate
PL signal, the activation energy is estimated at 110 meV, in agreement with the electronic state in QD and
wetting layer (WL) determinate by PL spectroscopy measurements. The influence of the QD size on the
QD confinement energy, is also observed in the evolution of the decay time with temperature and
detection energy.
&2015 Elsevier Ltd. All rights reserved.
1. Introduction
The physical properties of materials undergo dramatic changes
if the sizes become sufficiently small. When electrons and holes
are limited to regions smaller than a few tens of nanometers,
quantum effects become apparent. Then the material properties
no longer depend on the material composition alone but also on
the size of the structure. A nanostructure that confines an electron
or a hole in all three dimensions and which is sufficiently small to
cause quantization of the carrier energy is named a quantum
dot (QD).
The atomlike properties of QDs have made these nanos-
tructures very attractive candidates for use in many quantum
devices, such as single-electron devices [1], detectors [2], single-
photon sources [3] and spin-qubit [4,5]. Most studies so far con-
centrated on material systems such as InAs/GaAs and InGaAs/GaAs
with emission in the infrared. For applications requiring visible
emission such as high-density optical storage or display and illu-
mination sources, shorter wavelength systems [6–8] such as red-
emitting InAlAs QDs embedded in a GaAlAs matrix are desired.
The InAlAs/GaAlAs QD system features a variety of interesting
characteristics of zero-dimensional systems, such as extremely
sharp homogeneous linewidths [9], invariant linewidths and
lifetimes for temperatures up to the onset of thermionic emission
[10], state-filling and excited-state emissions [11], distinctive car-
rier dynamics and phonon interactions [12] and spin-polarization
features [13]. Typical emission spectra from a self-assembled QD
ensemble have a Gaussian distribution line shape at low excitation
intensity, caused by small variations in the parameters of the dif-
ferent QDs probed, such as size, composition and strain [14–16],so
that InAlAs QDs often display a relatively large inhomogeneous
broadening (a few tens of meV) in their emission spectrum.
The electronic and optical properties of the QDs are commonly
studied by photoluminescence (PL) techniques, but also by time-
resolved PL (TRPL), which is a powerful tool for the investigation of
carrier dynamics in semiconductors: electrons and holes created in the
material by ultrashort laser pulses recombine and emit light, which is
subsequently detected; the temporal development of the PL signal
allows one to draw conclusions about the dynamical processes in the
sample. In this paper, we have investigated the temperature depen-
dence of PL and TRPL spectra of a visible-emitting InAlAs/GaAlAs QD
sample grown on a GaAs substrate. The thermal activation of excitons
and their retrapping in QDs is observed, and its influence on exciton
dynamics and PL spectra is discussed.
2. Material and methods
The QD sample was prepared by molecular-beam epitaxy, and
is similar to previously studied samples [17]: the growth of
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Solid State Communications
http://dx.doi.org/10.1016/j.ssc.2015.11.011
0038-1098/&2015 Elsevier Ltd. All rights reserved.
n
Corresponding author. Tel.: þ216 23 642 756; fax: þ216 71 746 551.
E-mail address: amenibendaly@gmail.com (A. Ben Daly).
Solid State Communications 227 (2016) 9–12

In
0.62
Al
0.38
As QDs was performed on a 100 nm-thick Ga
0.67
Al
0.33
As
epilayer, and was monitored by reflection high-energy electron
diffraction (RHEED). The QDs were covered by a Ga
0.67
Al
0.33
As
epilayer of same composition and same thickness.
Fig. 1 presents a sketch of the sample structure. The growth
temperature is 560 °C. The critical thickness at which the growth
turns from two-dimensional (deposition of In
0.62
Al
0.38
As layers) to
three-dimensional (appearance of In
0.62
Al
0.38
As QDs) is detected
on the RHHED pattern; it is measured during the growth process
at about 3.7 monolayers. During the PL and TRPL measurements,
the sample was in a variable-temperature He-cryostat (4–150 K),
on a cold finger. For the PL measurements, a 405 nm laser diode
was used and focused on the sample in a 100 mm-diameter spot;
the resulting PL was dispersed by a monochromator and detected
by a silicon avalanche photodiode.
In the TRPL studies, the sample was excited by ps-laser pulses,
at 408 nm, generated by doubling a mode-locked Ti:sapphire laser
with a repetition rate of 76 MHz. The TRPL signal was then
detected by a synchroscan streak camera; the spectral bandwidth
was about 0.5 meV. The PL spectra of the sample obtained at low
(T¼10 K) and high (T¼100 K) temperatures are shown in Fig. 2.
The studied sample shows PL emission around 1.7 eV. At T¼10 K,
one sees a broad QD ground state (GS) emission spectrum, cen-
tered at 1.742 eV, with a full width at half maximum (FWHM)
equal to about 90 meV and associated to the inhomogeneous QD
size and confinement distributions.
We performed the experiments in the limit of low optical flux,
in which the PL emission is purely due to the exciton GS; this is
confirmed by the symmetric Gaussian profile of the PL spectra. The
narrow line at 1.969 eV, with a FWHM 13.6 meV, is attributed to
the PL of the Ga
0.67
Al
0.33
As symmetric barrier. Because of the rapid
capture of the electron–hole pairs and of their thermalization, the
wetting layer (WL) emission is not observed. At T¼100 K, one
observes a redshift of the QD GS emission (with a PL maximum at
1.677 eV) and the appearance of the WL emission line at 1.753 eV,
favored by the carrier thermal activation. We stress that the fea-
ture attributed to the WL PL, at 100 K, is not a instrumental noise,
as experimentally checked: this feature is present for all acquisi-
tion runs, and it does not shift with temperatures between 100 K
and 130 K. One can then estimate an energy difference of the order
of 80 meV between QD and WL GS excitons (220 meV between WL
and barrier excitons).
3. Results and discussion
The InAlAs/GaAlAs QDs obtained through the Stranski–Krasta-
nov growth mode have potential barriers with finite heights, and
therefore, at elevated temperatures, carriers can escape the con-
fined region by thermionic emission into the WL or barrier
material [18]. This important carrier-loss process is especially
significant in structures with shallow barriers [19].
The carrier distribution between the QDs and the confining
layers (barrier or WL) is controlled by capture processes in the QDs
by phonon diffusion and the escape process from QDs to WL
through thermionic emission. The probability per unit time for a
carrier to absorb a phonon and be emitted above the barrier is
greatly enhanced at high temperatures, due to a large phonon
population in the lattice. In order to have deeper insight into the
optical properties of our QDs, the GS emission energy of the QD
structure as a function of temperature is studied in Fig. 3.As
usually observed in semiconductors [20], a monotonous redshift of
the PL peak position was observed with increasing temperature.
The temperature dependence can be well reproduced by the
following Varshni empirical (1)[21] and Bose–Einstein (2) [22]
equations:
E
g
TðÞ¼E
g
0ðÞ
αT
2
βþTð1Þ
E
g
TðÞ¼E
g
0ðÞ λ
exp
ℏ
ω
ph
k
B
T

1
ð2Þ
where E
g
(0) is the band gap at 0 K,
α
is a temperature coeffi-
cient (electronvolt per kelvin),
β
is close to the Debye temperature,
λ
is the constant of the electron–phonon coupling strength, and
finally ℏω
ph
refers to the phonon energy. The used values for the
fits of Fig. 3 are
α
¼170.1 meV/K,
β
¼46715 K,
λ
¼51 79 meV
and ℏω
ph
¼4.770.7 meV. Eq (2) gives a slightly better fit of the
GaAs
buffer +
substrate
GaAs 5nm
Ga0.13Al0.87As 20 nm
Ga0.67Al0.33As 100 nm
Ga0.67Al0.33As 100 nm
Ga0.13Al0.87As 20 nm
In0.62Al0.38As
Fig. 1. Structure of the studied sample, consisting of a single layer of
In
0.62
Al
0.38
As QDs.
1500 1600 1700 1800 1900 2000
PL intensity (arb.unit)
wetting layer
Energy (meV)
T=100K
T=10K
barrier
quantum dots
Fig. 2. PL spectra of the sample of In
0.62
Al
0.38
As/Ga
0.67
Al
0.33
As QDs at 10 K and
100 K .
020
40 60 80 100 120 140 160
1620
1640
1660
1680
1700
1720
1740
1760
Bose-Einstein law
Varshni law
Energy (meV)
Temperature (K)
Fig. 3. (Color online) GS PL peak energies measured at various temperatures.
Symbols are the experimental data, and the lines are calculated using Varshni
(continuous green line) and Bose–Einstein (dashed red line) laws, see text.
A. Ben Daly et al. / Solid State Communications 227 (2016) 9–1210

experimental data than Eq. (1), especially in the low-temperature
region; indeed, the PL position dependence on the temperature is
probably due to the electron–phonon scattering rather than the
thermal expansion. One can note that the phonon energy is very
close to the transverse acoustic mode TA(L) in In
0.62
Al
0.38
As esti-
mated at 5.6 meV, from an interpolation between the InAs and
AlAs TA(L) energies equal to 4.7 and 7 meV, respectively [23]. The
role of the transverse acoustic modes in the temperature depen-
dence of the band gap energy has already been evidenced in GaAs
bulk and InGaAs QDs [24,25].
We have also analyzed the FWHM of the QD PL spectra as a
function of the temperature (see Fig. 4). At low temperature (from
10 to 100 K), the FWHM is reduced, and is followed by an increase
at higher temperature. This behavior has been previously observed
for InAs and InGaAs QDs [26–29]. At low temperature (T¼10 K),
the thermal escape of the carriers from the QDs into the WL
cannot occur, because the radiative rate in the QDs is large as
compared to the thermal activation rate. So, after a random carrier
capture in the QDs, the electron–hole pairs recombine and the PL
spectrum is directly related to the QD emission energy distribu-
tion, caused by size fluctuations. When the temperature increases
(up to 100 K), the carriers in the smallest QDs (and in the confined
states with the lowest confinement energies) are thermally acti-
vated into the WL or barrier states, where they recombine or are
captured by QDs with larger confinement energies.
This leads to a decrease of the emission by small QDs, to a
narrowing of the PL spectra on the high-energy side, and to a
reduction of the FWHM. At high temperatures (above 100 K), most
of the QDs are concerned by the thermal activation: the excitons
are again redistributed among the QDs, hence the PL spectra tend
to the QD emission distribution, and the FWHM increases, as
observed.
To estimate the average activation energy, we have analyzed
the temperature dependence of the integrate PL signal (see Fig. 5).
In the high-temperature regime, the integrate PL intensity
decreases steeply for increasing temperatures; from this behavior,
one can extract an activation energy E
a
¼110 meV assuming an
exponential law exp 
E
a
k
B
T

. This value, averaged on the QD
ensemble, is in good agreement with the energy difference
between the QD GS and the WL, which comes from the T¼100 K
PL spectra (see Fig. 2).
In order to further confirm the capture and emission processes
in our InAlAs/GaAlAs QDs, we have performed TRPL measure-
ments at different temperatures ranging from 10 to 140 K. Fig. 6
shows the temporal profiles of the PL signal at the peak energy, for
different temperatures. The decay traces of Fig. 6 are single-
exponential I
PL
TðÞ¼A
1
exp 
t
τ
1

þA
2

over two orders of mag-
nitude, and the observed decay times obtained by single-
exponential fits at different detection energies are summarized
in Fig. 7. As previously discussed [30], an increase of the recom-
bination time with temperature is not expected in QDs, where the
radiative emission rate should be constant at low temperature
owing to the nominally zero-dimensional character of the density
of states [31,32]. Nonetheless, as shown in Fig. 7, the PL decay time
increases in the low temperature range, as in previous studies
[32,33], then decreases above some critical temperature.
The critical temperatures, at which the decay time starts to
steeply decrease with temperature, are close to 80, 100 and 110 K
respectively, for high, medium and low energy of detection. This
critical temperature shifts with the QD confinement energy, which
suggests that the temperature dependence of the decay time is
directly related to thermal activation processes [34]. For a fixed
detection energy, and a fixed carrier confinement energy (or QD
size), when the temperature starts to increase, the thermal
020
40 60 80 100 120 140 160
55
60
65
70
75
80
85
90
FWHM (meV)
Temperature (K)
Fig. 4. FWHM of the PL spectra of the studied QDs, as a function of temperature.
1
10
0.1 0.2 0.3 0.4 0.5 0.6
Ea = 110 meV
Integrate PL Intensity (arb. unit)
1/kBT (meV-1)
Fig. 5. Integrate PL intensity from the QDs versus inverse temperature.
-500 05001000 1500 2000
100
1000
10000
120K
10K
PL intensity
Time (ps)
80K
Fig. 6. (Color online) Time evolution of the PL intensity at the peak energy, at
temperatures 10 K (black line), 80 K (red line), and 120 K (blue line).
20 40 60 80 100 120 140 160
0
0
100
200
300
400
500
600
700
E1
Er
E2
Decay time (ps)
Temperature (K)
Fig. 7. (Color onlie) PL decay times as a function of temperature, at low (E
1
; green
circles), medium (E
r
; red squares) and high (E
2
; blue triangles) detection energies.
Inset: spectral positions of the used detection energies E
1
,E
r
and E
2
.
A. Ben Daly et al. / Solid State Communications 227 (2016) 9–12 11

activation becomes efficient first in the QD with a smaller activa-
tion energy (smaller size): the carriers can escape from the QDs to
the WL before capture by larger QDs. The decay increases then,
due to refilling from smaller QD [32], as long as the observed QD
are insensitive to thermal activation. At higher temperature, the
decay time decreases when the thermal activation processes are
efficient and contribute to the total decay rate; this happens at a
critical temperature which increases with increasing confinement
(in agreement with the temperature dependence observed in
Fig. 7).
4. Conclusions
We have investigated the PL and TRPL properties of a visible-
emitting InAlAs/GaAlAs QD sample grown on a GaAs substrate.
Three PL peak patterns were observed, attributed to the islands
with inhomogeneously broadened emissions, the WL, and the
symmetric barrier. In TRPL experiments, we did not observe the
expected constant decay time as a function of temperature.
Instead, the observed decay times increase at low temperature up
to a critical temperature which increases with decreasing emission
energy, then decrease steeply with increasing temperatures. We
attribute the temperature dependence of the PL decays to thermal
activation of excitons from the QDs to the WL and to exciton
retrapping. Signatures of this thermally activated process are also
clearly evidenced in FWHM and integrate PL dependence on
temperature. Such results may be important in view of red-
emitting QD devices, at high temperature.
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