Robert et al. Nanoscale Research Letters 2012, 7:643
http://www.nanoscalereslett.com/content/7/1/643
NANO EXPRESS
Open Access
Theoretical and experimental studies of
(In,Ga)As/GaP quantum dots
Cedric Robert1*, Tra Nguyen Thanh1, Charles Cornet1, Pascal Turban2, Mathieu Perrin1, Andrea Balocchi3,
Herve Folliot1, Nicolas Bertru1, Laurent Pedesseau1, Mikhail O Nestoklon4, Jacky Even1, Jean-Marc Jancu1,
Sylvain Tricot2, Olivier Durand1, Xavier Marie3 and Alain Le Corre1
Abstract
(In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and
experimentally. The electronic band structure is simulated using a combination of k·p and tight-binding models.
These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical
properties are then studied in a low-In-content range through photoluminescence and time-resolved
photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types.
Keywords: Quantum dots, Tight-binding, k·p simulation, Time-resolved photoluminescence
PACS: 78.55.Cr, 78.47.jd, 78.67.Hc
Background
In the context of the monolithic integration of photonics
on silicon, the pseudomorphic approach, i.e., growing
lattice-matched compounds on Si, is a promising route
towards an efficient and long-term stable laser on Si [1].
It should overcome the issue of the dramatic number of
crystalline defects due to the large lattice mismatch
encountered in the growth of most III-V materials onto
Si substrates [2]. Among binary III-V materials, GaP presents the closest lattice constant to Si (0.37% at 300 K).
The perfect lattice matching can even be obtained by
introducing 2% of nitrogen in GaP. Recently, the epitaxial growth on Si substrate of GaP and GaPN0.02 has been
greatly improved by several groups [3-5]. Various active
zones grown on GaP substrate or on GaPN0.02/GaP/Si
have been proposed. The best results have been achieved
with compressive strained GaNAsP/GaP quantum wells
(QWs) in electrically pumped lasers operating up to 150
K (Si substrate) [6] or at room temperature (GaP substrate) [7]. However, the electron wave function at the
conduction band minimum has a special character [8]. It
is expected to limit the performances of laser devices
yielding high threshold current densities. Indeed, the
* Correspondence: cedric.robert@insa.rennes.fr
1
Université Européenne de Bretagne, INSA Rennes, France CNRS, UMR 6082
Foton-Ohm, 20 Avenue des Buttes de Coësmes, Rennes 35708, France
Full list of author information is available at the end of the article
conduction band of the GaAsP host material has a minimum at the XXY point on the edge of the Brillouin zone,
and partially localized electronic levels related to nitrogen incorporation lie at energies below this minimum.
The conduction band minimum of GaNAsP/GaP QWs
evidences a predominant localized N character [8].
Moreover, the maximum of the emission wavelength
reported for such structures with reasonable N content
is equal to 980 nm [7], which is not yet in the transparency window of Si.
Quantum dot (QD) lasers grown on GaAs or InP substrate display lower threshold currents due to the 0D
density of states when compared with QW lasers on the
same substrates [9]. (In,Ga)P QDs grown on GaP substrate have already been studied, and room temperature
electroluminescence has been obtained [10]. However,
theoretical studies have shown that the electronic band
lineups correspond to a borderline case between type I
and type II [11]. The (In,Ga)As(N)/GaP QDs system has
recently attracted much attention. Fukami et al. [12]
have claimed that the transparency window of silicon
may be reached with InGaAsN/GaP QDs when In composition is 50% to approximately 60% and N composition is 1% to approximately 2%. In the following,
InGaAs/GaP QDs are studied as a step toward
InGaAsN/GaP QDs system. Both room-temperature
photoluminescence (PL) [13] and electroluminescence
© 2012 Robert et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Robert et al. Nanoscale Research Letters 2012, 7:643
http://www.nanoscalereslett.com/content/7/1/643
[14] of InGaAs/GaP QDs have been recently reported.
However, the description of the electronic band structure of this QD system is still lacking.
In this paper, we investigate (In,Ga)As/GaP QDs in a
low-indium-content range both from the theoretical and
experimental points of view. The effects of both indium
composition and QD geometry is analyzed through a
combination of k·p and tight-binding (TB) simulations.
Optical properties are then studied by temperaturedependent photoluminescence and time-resolved photoluminescence (TRPL).
Methods
(In,Ga)As QDs are grown on n-doped GaP(001) substrate using gas-source molecular beam epitaxy. After
the growth of a 450-nm buffer layer and a 4-monolayer
(ML) (In,Ga)As deposition with 30 s of annealing under
As (see the work of Nguyen Thanh et al. [13] for more
information), a 30-nm GaP capping layer is finally
deposited to prevent surface non-radiative recombinations. The growth temperature is set at 580°C. The nominal composition of indium is set at 30%, but because of
high growth temperature, indium effective composition
is assumed to be below or equal to 15%.
Temperature-dependent PL experiments are carried
out by exciting samples with a 405-nm continuous-wave
laser diode. Power density is roughly estimated at 80
W·cm−2. Samples are set in a helium bath closed-cycle
cryostat to study PL from 10 K to room temperature.
Measurements are also performed above room temperature using a hot plate. Attention is given to avoid the
red luminescence of the deep centers in n-doped GaP
substrates. Actually, the penetration length of the
405-nm beam is lower than the thickness of the GaP
buffer layer, avoiding the substrate to be excited. Secondly, similar PL spectra are obtained on the same
structures on non-doped GaP substrate, thus excluding
any significant contribution from the GaP deep center
luminescence.
For TRPL measurements, the sample is excited by a
frequency-doubled Ti/sapphire laser at the wavelength
of 405 nm. The repetition rate is 80 MHz. The PL signal
is analyzed by an S20 streak camera, and measurements
are performed at 10 K to overcome non-radiative recombinations channels.
Results and discussion
Band structure calculations
The eight-band k·p method has been extensively used to
accurately simulate the electronic band structure of QDs
with type I band alignment and direct optical transition
(InAs/GaAs, InAs/InP. . .) [15,16]. The case of InGaAs/
GaP QDs in the low-In-content range is expected to be
trickier because of the coupling of zone center conduction
Page 2 of 5
band states with conduction band states located on the
edge of the Brillouin zone [13]. To deal with this issue, we
simulate the direct optical transition with the eight-band
k·p method. To get an estimation of X-like and L-like state
energies in the dot, we consider the TB sp3d5s* model [17]
for a QW with a thickness equal to the height of the dot.
Thus, the lateral quantum confinement effect is disregarded but is assumed to have negligible effects on lateral
valleys with large effective masses.
To consider realistic QD geometries for the simulation, the morphology of InGaAs/GaP QDs are imaged
by plane-view scanning tunneling microscopy (STM).
The 75 × 75-nm2 STM image shown on Figure 1a exhibits InGaAs/GaP QDs with approximately a cone shape.
The in-plane anisotropic ratio (between length and
width) is indeed measured in the range of 1 to 1.5. The
statistical analysis of diameter and height distributions is
presented in Figure 1b. The k·p simulation is performed
using the geometry defined on Figure 2. A C∞v symmetry is considered for QD geometries, and strain calculations are performed using elasticity and parameters of
Vurgaftman et al. [18] and the finite element method for
numerical computation. Three typical dimension sets
representative of the inhomogeneous size distributions
are summarized in the table of Figure 2. The A, B, and
C geometries correspond to real QDs typically found in
the sample (see Figure 1b). The D geometry is chosen to
study theoretically larger QDs in order to address the
problem of lowering the emission energy. A typical
wetting layer of 1-ML thick is added in the model to account for the Stranski-Krastanov growth mode. Deformation potentials and Luttinger parameters used in the
k·p model are those extracted from the TB calculation
for bulk InGaAs and GaP [17]. The valence band offsets
are taken from recent ab initio calculations [19].
The influence of In content is presented on Figure 3a
for a QD with the C geometry. The first electronic levels
in the Γ, X, and L valleys and the first heavy-hole level of
the QD are represented as a function of the In content.
The electronic Γ and the heavy-hole levels are calculated
with the k·p method, whereas the X and L electronic
levels are calculated with the TB model. For low In content (below 30%), the ground optical transition is type I
but is indirect with the first electron level of the X type.
For very low In content (below 15%), the Γ-type conduction band level in the QD is even located at an energy
above the one of the X-type conduction band of the GaP
barrier. Nevertheless, a direct and type I ground state
transition is predicted for In composition above 38%.
This is a necessary condition in order to obtain a very
efficient optical transition for such a QD geometry.
The influence of QD geometry is shown on Figure 3b
for a medium In content of 30%. For small QDs, the first
Γ electronic level undergoes an important quantum
Robert et al. Nanoscale Research Letters 2012, 7:643
http://www.nanoscalereslett.com/content/7/1/643
Page 3 of 5
Figure 1 (In,Ga)As QD image and statistical correlation. (a) A 75 × 75-nm2 STM 3D plane view of (In,Ga)As QDs. (b) Statistical correlation
between diameter and height on a 800 × 800-nm2 image.
confinement effect which lifts up this level above both X
and L levels, which are less affected by confinement
effects. An indirect to direct type crossover is predicted
for large QDs.
In conclusion, an increase of In content and an enlargement of QD size are expected to lower the first
Γ-type conduction band level and thus yield an efficient
optical transition. Moreover, the indirect to direct type
crossover should be induced by strain relaxation associated to In content increase and QD enlargement.
Optical properties
Continuous-wave PL spectra are presented in Figure 4
for various temperatures. At 12 K, the PL spectrum
exhibits a single peak centered at 1.78 eV. The peak
shape undergoes a strong evolution from low to high
temperatures. At 260 K, a shoulder appears on the
high-energy side of the spectrum. At 300 K, a second
Figure 2 QD morphologies used for the eight-band k·p
calculations.
optical transition clearly appears. When increasing the
temperature above 300 K, the maximum of PL intensity
switches from the low-energy (LE) transition to the
higher-energy (HE) transition. This behavior may indicate that the HE optical transition is more efficient than
the LE one. At 300 K, the LE transition is reported at
1.74 eV and the HE transition at 1.84 eV.
To understand the nature of these two optical transitions, the dynamics of the recombination of carriers are
investigated through TRPL spectroscopy. Experiments
are performed at 10 K to overcome non-radiative recombination channels. The radiative lifetimes are deduced
from the measured PL decay times. The sample is first
excited with a low-incident power density equal to 70
W·cm−2. The LE optical transition is only detected in accordance with the spectrum shown on Figure 4 at low
temperature. The evolution of the emission as a function
of time is shown on Figure 5a. The LE optical transition
exhibits a very long decay time which is greater than the
repetition period of the laser (12 ns) and is not easily
measurable with this experimental setup. Such a long
lifetime can be interpreted on the basis of the theoretical
results of the previous section. The energy position of
the LE PL peak at low temperature (ELE = 1.78 eV)
is consistent with the calculated indirect transition
(between 1.74 and 1.79 eV) for medium-sized dots in
the 0% to 15% In content range.
The sample is then excited with a large power density
equal to 4,000 W·cm−2 in order to fill the low-energy
electronic levels and allow the HE optical transition to
occur. The PL dynamics at selected energies, ELE and
EHE, are respectively shown on Figure 5b,c. The timeresolved emission related to the LE transition can be fitted by the sum of a shorter exponential decay with a
lifetime of 770 ps and a constant associated with the
very long lifetime of the indirect transition. Many-body
Auger effects leading to an enhancement of intradot carrier relaxation may lower the optical transition lifetime.
Robert et al. Nanoscale Research Letters 2012, 7:643
http://www.nanoscalereslett.com/content/7/1/643
Page 4 of 5
Figure 3 Influence of In content and QD geometry on the electronic levels. (a) Electronic levels of InxGa1−xAs QD with geometry C.
(b) Electronic levels of In0.3Ga0.7As QDs for the four geometries defined in Figure 2. The Γ electronic level and the heavy-hole level in the QD are
calculated with the k·p method. The X and L electronic levels are calculated with the TB model.
The density of electron-hole pairs is indeed estimated to
be high (above 10 per QD). Such effects have been
observed in InAs/InP QDs [20]. For the HE transition,
the emission shows a biexponential decay with short lifetimes of 340 and 1,700 ps, respectively. Both times are
consistent with a direct type-I electronic transition in
QDs and a better overlap of electron and hole wave
functions. The EHE-ELE difference is also in reasonable
agreement with that of theoretical calculations. For
large-sized dots and In content of 15%, an energy difference of 100 meV is indeed calculated between both direct and indirect optical transitions.
the low-In-content range. In agreement with theoretical
results, TRPL measurements are consistent with a
ground optical transition of indirect type. A direct optical transition can be observed for high-power density
or at room temperature where electrons get enough
thermal energy to partially fill the Γ-type conduction
band state.
Conclusions
The (In,Ga)As/GaP QD system is studied both theoretically and experimentally. The simulation results of k·p
and TB methods are coupled and predict an indirect to
direct crossover with the increase of In content and the
ripening of QDs. Optical properties are then studied in
Figure 4 Temperature-dependent PL spectra of (In,Ga)As/GaP
QDs. The black thin dashed lines show the fit of the two transitions
by two Gaussian peaks.
Figure 5 PL dynamics at 10 K at selected energies, LE and HE.
For power densities of (a) 70 and (b, c) 4,000 W·cm−2. Red lines
show biexponential fits.
Robert et al. Nanoscale Research Letters 2012, 7:643
http://www.nanoscalereslett.com/content/7/1/643
Page 5 of 5
Competing interests
The authors declare that they have no competing interests.
14.
Authors' contributions
CR performed the optical property measurements and theoretical
calculations. TNT, CC, and NB performed the MBE growth and the analyses
of quantum dot geometry. PT and ST performed the STM measurements.
MP, AB, HF, and XM supervised the measurements of optical properties. LP,
MON, JE, and JMJ developed the simulation of electronic properties. OD and
ALC managed the team. All authors read and approved the final manuscript.
Acknowledgments
This research is supported by ‘Région Bretagne’ through the PONANT project
including FEDER funds. This work was performed using HPC resources from
GENCI CINES and IDRIS 2012-c2012096724. The work is also supported
through the participation of the SINPHONIC JC JC ANR project N° 2011 JS03
006 and NANOTRANS C’NANO research program.
15.
16.
17.
18.
19.
Author details
1
Université Européenne de Bretagne, INSA Rennes, France CNRS, UMR 6082
Foton-Ohm, 20 Avenue des Buttes de Coësmes, Rennes 35708, France.
2
Equipe de Physique des Surfaces et Interfaces, Institut de Physique de
Rennes UMR UR1-CNRS 6251, Université de Rennes 1, Rennes Cedex F-35042,
France. 3Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 avenue de
Rangueil, Toulouse 31077, France. 4Ioffe Physico-Technical Institute, Russian
Academy of Sciences, St. Petersburg 194021, Russia.
Received: 19 July 2012 Accepted: 24 October 2012
Published: 23 November 2012
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doi:10.1186/1556-276X-7-643
Cite this article as: Robert et al.: Theoretical and experimental studies of
(In,Ga)As/GaP quantum dots. Nanoscale Research Letters 2012 7:643.
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