Energy states and carrier transport processes in metamorphic InAsquantum dotsL. Seravalli, G. Trevisi, P. Frigeri, R. J. Royce, and D. J. Mowbray Citation: J. Appl. Phys. 112, 034309 (2012); doi: 10.1063/1.4744981 View online: http://dx.doi.org/10.1063/1.4744981 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i3 Published by the American Institute of Physics. Related ArticlesSpontaneous emission control of single quantum dots by electromechanical tuning of a photonic crystal cavity Appl. Phys. Lett. 101, 091106 (2012) High-frequency gate manipulation of a bilayer graphene quantum dot Appl. Phys. Lett. 101, 043107 (2012) Magnetovoltaic effect in a quantum dot undergoing Jahn-Teller transition Appl. Phys. Lett. 101, 023120 (2012) Fast detection of single-charge tunneling to a graphene quantum dot in a multi-level regime Appl. Phys. Lett. 101, 012104 (2012) Refrigeration effect in a single-level quantum dot with thermal bias Appl. Phys. Lett. 100, 233106 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
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Energy states and carrier transport processes in metamorphicInAs quantum dots
L. Seravalli,1 G. Trevisi,1 P. Frigeri,1 R. J. Royce,2 and D. J. Mowbray2
1CNR-IMEM Institute, Parco Area delle Scienze, 37/a, 43124 Parma, Italy2Department of Physics and Astronomy, University of Sheffield, Hicks Building, Sheffield S3 7RH,United Kingdom
(Received 1 March 2012; accepted 11 July 2012; published online 6 August 2012)
Photoluminescence excitation spectroscopy is used to probe energy states and carrier transport in
InAs quantum dot structures grown on InGaAs metamorphic layers, designed for room temperature
emission at 1.3, 1.4, or 1.5 lm. The dominant spectral feature is shown to arise from the partially
relaxed InGaAs confining layer. In structures with a low indium composition or thin InGaAs layer,
a clear wetting layer feature is observed which acts as the dominant reservoir for carriers thermally
excited from the quantum dots. Structures with high indium composition and/or thick InGaAs lack
a wetting layer and carriers escape directly to the InGaAs layers. VC 2012 American Institute ofPhysics. [http://dx.doi.org/10.1063/1.4744981]
I. INTRODUCTION
InAs quantum dots (QDs) grown on GaAs substrates
now allow the fabrication of 1.3 lm emitting lasers with sig-
nificantly improved performance in comparison to quantum
well lasers.1,2 A possible approach to extend emission
towards the main telecommunications wavelength of
1.55 lm is QD growth on a metamorphic InGaAs layer.3,4
Here, the relaxation of the InGaAs layer results in a decrease
of the lattice mismatch between the QDs and confining
layers (CLs) and hence of the QD strain (compared to growth
on GaAs), producing a large emission redshift. By varying
the composition and thickness of the metamorphic layer, it is
possible to separately control both the QD emission energy
and confinement potential.5 However, the precise nature of
the energy states related to the wetting layer (WL) and CLs,
and the processes of carrier transport in these complex struc-
tures remain unclear. An understanding of these factors is
vital for the optimisation of these structures. In addition, the
nature of the QD formation as the composition and thickness
of the InGaAs CL increases is unclear. In this paper, low
temperature photoluminescence (PL) and photoluminescence
excitation (PLE) spectroscopy are used to study a series of
InAs/InGaAs metamorphic QD structures. By studying struc-
tures with a systematic variation in both the thickness and
composition of the InGaAs CL, it is possible to obtain a
detailed understanding of the critical energy states, carrier
transport processes and the structural environment of the
QDs.
II. EXPERIMENTAL DETAILS
The samples were grown by molecular beam epitaxy
(MBE) on (100) GaAs substrates. Following a GaAs buffer
layer, an InxGa1� xAs metamorphic lower confining layer
(LCL) of thickness t and In composition x was deposited at
490 �C, followed by a single layer of InAs QDs deposited at
460 �C and capped with a 20 nm-thick InxGa1� xAs cap
layer. The LCL and capping layer have the same In composi-
tion. Sample parameters are given in Table I, with growth
and structural details are given in Refs. 5 and 6. By varying
the composition and thickness of the InGaAs LCL, it is pos-
sible to separately control both the strain state and confining
potential of the QDs. Optical spectroscopy was performed at
6 K in a He flow cryostat with PL excited with a red diode
laser and PLE excited by light from a 150 W tungsten-
halogen bulb dispersed by a 0.22 m monochromator. Detec-
tion was via a 0.75 m spectrometer and nitrogen cooled Ge
detector.
III. RESULTS AND DISCUSSION
PL spectra of the samples are shown in Figure 1 for tem-
peratures of 6 and 300 K (RT). Peak emission energies
extracted from the PL measurements at 6 K are given in
Table I. Samples A-D emit around 1.03 eV, with line widths
of �42 meV; samples E and F emit at 0.95 eV, with line
widths �65 meV. Sample G has a much larger line width
and exhibits a multimodal emission with a maximum at
0.91 eV. PL emission from the QDs is observed at RT for
samples A-D around 1.3 lm and at 1.42 lm for sample E, as
shown in Fig. 1. However, for samples F and G, the QD
emission is more strongly quenched and is no longer
observed at RT. This behaviour was previously studied and
interpreted as due to the thermal escape of confined carriers
out of the QDs due to the reduced confinement which occur
as the In composition, x, of the confining layers is
increased.5,6 For samples F and G, QD PL is observed up to
temperatures of 200 and 170 K, respectively, allowing ex-
trapolated RT band gaps of 0.885 eV for sample F and
0.825 eV for sample G to be inferred. These band gaps would
correspond to emission wavelengths of 1.4 lm and 1.5 lm at
RT if thermal carrier escape from the dots was reduced. This
can be achieved, for example, by embedding metamorphic
QDs in InAlAs barriers where we have shown previously
that it is possible to obtain RT emission as long as 1.59 lm.4
Figure 2 shows 6K PLE spectra for detection at the peak
of the QD PL. For comparison, the PLE of a reference
0021-8979/2012/112(3)/034309/5/$30.00 VC 2012 American Institute of Physics112, 034309-1
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sample with a layer of InAs QDs grown within an
In0.15Ga0.85As-GaAs quantum well (RT emission at 1.3 lm)
is included.7 Between 1.05 and 1.25 eV, the PLE of the refer-
ence sample exhibits features that shift rigidly with the
detection energy and are therefore attributed to carrier relax-
ation via the emission of multiple LO phonons, while no fea-
tures directly attributable to absorption into excited QD
states are observed.8 Elastically scattered light and the ab-
sence of a Stokes’ shift prevents the observation of the QD
ground state absorption. At higher energies (�1.35–
1.45 eV), two features are attributed to absorption by the
InGaAs QW and/or InAs WL. These two features are
believed to represent the light and heavy hole (LH and HH)
states which are split by confinement and strain effects. At
�1.5 eV, the step in the PLE represents the onset of strong
absorption by the bulk GaAs. Absorption at and above the
GaAs band gap gives a large positive feature in the PLE for
detection on the QD emission demonstrating, for this refer-
ence sample, the efficient transfer of carriers initially created
in the GaAs to the QDs.
The PLE spectra of the QDs grown on the metamorphic
layers, which are broadly similar to the only other reported
PLE characterization of metamorphic QD structures,9 show
some similarities with that of the reference sample but also
some noticeable differences. In three of the samples designed
to emit at 1.3 lm at RT (samples A, B—not clearly observed
in the spectrum used in Fig. 2—and D) multiple-LO phonon
features are observed. For example, in sample A, these are
observed to occur at fixed energies of 64, 96, and 132 meV
from the detection energy, suggesting 2, 3, and 4 multiples
of a �32 meV phonon energy (elastically scattered light pre-
vents the observation of any feature at the fundamental
energy of 32 meV). A similar fundamental energy is obtained
for samples B and D and also the reference sample. How-
ever, the resolution of the current PLE spectra does not allow
any small shifts in phonon energy between the different
TABLE I. Sample LCL parameters plus calculated values of the residual CL strain, QD-CL lattice mismatch, CL energy, and LH-HH splitting and experimen-
tally determined values of the PL energy and EQDþEact.
Sample
LCL In
composition (x)
LCL thickness
(nm)
Residual CL
strain (%)
QD-CL lattice
mismatch (%)
6 K PL
energy (eV)
LH-HH splitting
(eV)
CL energy
(eV)
EQDþEact
(eV)
A 0.09 1000 �0.232 6.74 1.049 0.015 1.389 1.319 6 0.014
B 0.12 165 �0.547 6.84 1.021 0.034 1.347 1.296 6 0.014
C 0.15 60 �0.876 6.96 1.028 0.053 1.332 1.285 6 0.018
D 0.18 31 �1.185 7.08 1.023 0.070 1.300 1.191 6 0.010
E 0.24 145 �0.517 5.92 0.943 0.033 1.183 1.175 6 0.028
F 0.28 37 �0.998 6.14 0.954 0.061 1.168 1.110 6 0.010
G 0.31 1000 �0.188 5.04 0.911 0.012 1.089 1.083 6 0.031
FIG. 1. PL spectra at 6 K (samples A-G) and RT (samples A-D) showing the
QD emission.
FIG. 2. PLE spectra recorded for a sample temperature at 6 K and detection
at the peak of the QD emission. A spectrum of a reference InAs-InGaAs dot-
in-a-well sample is included for comparison. Solid boxes indicate the calcu-
lated CL heavy and light hole energies. Solid horizontal lines indicate the
values of EQDþEact with associated errors. Vertical arrows indicate features
attributed to WL absorption. P indicates multiple phonon features.
034309-2 Seravalli et al. J. Appl. Phys. 112, 034309 (2012)
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samples to be determined. Phonon energies between 28 and
34 meV have been reported previously in PLE studies of
InAs QDs grown within a GaAs matrix.10 The majority of
the samples also show a broad WL related feature which will
be discussed in detail below.
One significant difference between the reference sample
and the metamorphic structures is the absence of a GaAs-
related feature in any of the PLE spectra. All samples contain
a 100 nm GaAs buffer layer and the thickness of the metamor-
phic InGaAs layers in all but samples A and G should be suffi-
ciently thin to permit optical excitation of the GaAs buffer;
under high power laser excitation at 6 K GaAs emission is
observed for samples B-F. The absence of a GaAs signal in
the PLE indicates that carriers excited in the GaAs buffer are
unable to migrate efficiently to the QDs, suggesting that they
are either trapped in the GaAs or recombine non-radiatively at
misfit dislocations within the InGaAs layer; these are known
to exist at high concentrations at the InGaAs-GaAs interface.
The strain relaxation of the InGaAs capping layer in the
present samples has been analyzed in detail by x-ray diffrac-
tion (XRD).11 It is found that the capping layer is pseudo-
morphic to the InGaAs LCL, hence in terms of energy levels
the upper CL and LCL can be effectively considered as iden-
tical in the following discussions.
The left hand edge of the solid boxes in Fig. 2 is the cal-
culated CL band gaps as given by the model of Ref. 5, based
on the approach of strain relaxation developed by Maree
et al.12 These results have been confirmed by x-ray diffrac-
tion, photo-reflectance (PR), and Raman characterization of
the current samples.11 The calculated CL band gap energies
agree very well with the onset or initial maximum of the
strongest features in the PLE spectra, allowing these to be
attributed to absorption by the uniformly strained InGaAs
layer. The calculated and experimentally determined ener-
gies are plotted as a function of In composition in Fig. 3. The
residual strain in the CL will split the LH and HH states.
This splitting has been calculated using the equation given in
Ref. 13 and material parameters from Ref. 14, with relevant
values given in Table I. The LH energies are indicated by
the right hand edge of the solid boxes in Fig. 2 so that the
width of these boxes indicates the calculated LH-HH split-
ting. None of the PLE spectra exhibit a feature that correlates
with the expected position of the LH state (assuming the
onset represents the HH state) and in general the width of the
main PLE feature is significantly broader than the calculated
LH-HH splitting. As PLE represents not only the sample
absorption but also the relaxation efficiency for carriers from
their initial energy to the detection energy, the decrease of
the PLE strength at high energies indicates that carriers cre-
ated with energies high in the CL relax inefficiently into the
QDs; instead they are preferentially lost to non-radiative
centres. Evidence for such centres in QD structures grown
on metamorphic InGaAs layers has been reported in Refs. 15
and 16. It is noticeable that in the majority of structures the
CL PLE signal falls to zero above �1.48 eV, indicating effi-
cient carrier transport to non-radiative centres above this
energy.
Previous studies of the temperature dependence of the
PL intensity have allowed the activation energy, Eact, to be
extracted for the PL quenching processes.5,6 This energy rep-
resents the potential barrier for carrier escape from the QDs,
thus the sum of this energy and the QD emission energy,
EQD, gives the energy of the state to which carriers first
escape. EQDþEact values are given in the final column of
Table I and are indicated in Fig. 2 by the horizontal lines, the
length of which indicates the experimental errors in the
determined values.
For samples A-D and F, the values of EQDþEact coin-
cide fairly well with an obvious feature in the PLE (indicated
by the vertical arrows in Fig. 2). This feature is spectrally
distinct from the CL feature and is hence attributed to the
WL which has previously been identified as the critical state
controlling carrier escape from the QDs. The EQDþEact
values and energies of features observed in the PLE spectra
are plotted in Fig. 3. Also, included in Fig. 3 is PR data for
one sample (data of Ref. 6) which has been used to deter-
mine the WL energy of the x¼ 0.15 sample C: this agrees
well with the feature observed in the PLE spectrum of this
sample. Hence, it appears that for samples with a low In
composition or small thickness, the PLE exhibits a feature
that can be attributed to the WL and that it is this state that
acts as the reservoir to which carriers are thermally excited
from the QDs.
The exceptions to the observation of a probable WL
PLE feature corresponding to the EQDþEact energy are the
two samples with thick and high In composition LCLs; sam-
ples E and G. In both samples, the calculated EQDþEact val-
ues are close to the onset of the feature attributed to the CL
absorption. This agrees with previous preliminary studies6
which suggested that WL states may not be effective for
structures with high In composition CLs. It is found that in
structures with x� 0.28 and a LCL thickness exceeding
100 nm (consistent with the parameters for samples E and
G), the value of EQDþEact is significantly larger than for
FIG. 3. Experimentally determined CL energies (open squares �) and WL
energies (open upward triangles D) by PLE, and EQDþEact (open circles O)
by PL, with calculated CL energies (open downward triangles r) as a func-
tion of the In composition x of the InGaAs CLs. The solid diamond indicates
PR data from Ref. 6.
034309-3 Seravalli et al. J. Appl. Phys. 112, 034309 (2012)
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structures with LCLs of the same composition but smaller
thicknesses. This suggests a different state acts as the reser-
voir for carrier escape in these structures. However, in the
previous PR spectroscopic study of high energy states, spec-
tra were not reported for samples with x> 0.15,6 leaving
open, the question of what happens to the WL in metamor-
phic samples with large x.
In metamorphic structures, it is possible to study the
separate effects of QD-CL mismatch (proportional to the QD
strain) and the confinement potential for QD carriers on the
relevant sample parameters. In Refs. 5 and 6, we have dis-
cussed in detail how the change in the QD-CL mismatch
affects the activation energies when x is kept constant. In the
current work, we focus on the effect of varying the confine-
ment. QD confinement potentials were calculated as the sum
of the electron and heavy hole band discontinuities between
the InAs QDs and InGaAs CLs. In Fig. 4, the experimentally
determined PLE values of the CL and WL are plotted against
these calculated confinement potentials, in addition, to the
QD peak energy emission at 6 K derived from the PL spectra
of Fig. 1. As the confinement is reduced, due to the decrease
of the InGaAs band gap (as experimentally verified by the
CL PLE), the levels to which carriers initially in the QDs are
thermally excited to are lowered, for both escape to WL
states (samples A–D and F) and escape to CL states (samples
E and G). Fig. 4 shows that the QD emission in samples A-D
(all engineered for RT emission at 1.3 lm) is not affected by
a reduction in confinement as x is increased, due to the con-
comitant increase of the QD mismatch, obtained by using
thinner LCLs (see Table I). In contrast, it is noticeable that
the WL energy in samples A-D is much more sensitive to the
QD-CL mismatch than is the QD emission. A similar behav-
ior is observed for samples E and F (engineered for RT emis-
sion at 1.4 lm). This pair of samples demonstrates an
additional interesting behavior in that for sample E (smaller
x, thicker LCL) no WL feature is observed, resulting in ther-
mally activated carriers escaping directly to the CL, as veri-
fied by the coincidence of EQDþEact with the energy of the
CL (see Figure 3). This results in a larger thermal activation
energy for the QD PL in this sample, allowing the observa-
tion of emission up to RT (Figure 1). In contrast in sample F
carrier’s escape to the WL which results in a smaller activa-
tion energy and the complete quenching of the PL at 200 K.
Metamorphic samples, hence, allow QD structures emitting
at the same wavelength but with different WL and CL
energies.
Using the present data, obtained from PLE spectra, it is
possible to conclude more directly that WL states in samples
with high-x and thick LCLs appear to be absent. This behav-
ior cannot be due to the band discontinuities becoming too
small so as not to confine the WL states as in Fig. 4 it is evi-
dent that the WL is observed for sample F despite having a
lower confinement potential than sample E where the WL is
absent. This possible absence of WL states is tentatively
attributed to an increase of surface roughness with increasing
metamorphic layer thickness. This would affect the WL
properties and possibly inhibit the formation of a two-
dimensional system. It should be noted that QDs are still
well formed in structures with high In composition, as they
have been experimentally observed by AFM and other tech-
niques, as widely discussed in Refs. 5 and 17. It should also
be noted that this conclusion of an absence of WL states act-
ing as an escape channel for QD confined carriers is based
not only on the absence of a feature in the PLE spectra but
also, more importantly, on the concomitant agreement of
EQDþEact with the CL energy states.
The WL is expected to affect not only the thermal
escape of carriers but also confined carrier properties, for
example, oscillator strengths, inter-dot coupling, and dipole
polarization via Coulomb interactions.18 A modified or
absent WL could have important consequences in metamor-
phic InGaAs/InAs structures with a low density of QDs, as it
has been recently shown that such structures with high In
composition can act as single photon sources with emission
at long wavelengths.19
IV. CONCLUSIONS
In conclusion, PLE has been used to identify the main
energy levels in InAs QDs grown on metamorphic InGaAs
layers. The dominant feature is attributed to the partially
relaxed InGaAs CL. Carriers generated with high energy in
the CLs do not relax into the QDs but are preferentially lost
to non-radiative centres, most probably related to disloca-
tions in the metamorphic layer. By comparing PLE spectra
with data from temperature-dependent PL it is found that in
samples with thin or low-In composition InGaAs layers, WL
states are the main channel for thermal escape of confined
carriers. In contrast, in samples with thick and high In com-
position InGaAs layers there are no WL states acting as
escape channels for confined carriers or relaxation paths for
excited carries. By analyzing data as a function of the QD
FIG. 4. Experimentally determined 6 K CL energies (open squares �) and
WL energies (open upward triangles D) by PLE, and QD emission (open
circles O) by PL as a function of the calculated confinement potentials (the
sum of the QD-CL band discontinuities for electrons and heavy holes).
034309-4 Seravalli et al. J. Appl. Phys. 112, 034309 (2012)
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confinement energy, it has been shown how in metamorphic
QD structures it is possible to control the WL and CL levels
whilst keeping the QD emission constant, with important
consequences in terms of thermally activated carrier trans-
port processes.
ACKNOWLEDGMENTS
The work has been supported by the Italian Ministry of
University and Research through FIRB Contract No.
RBAP06L4S5, by the UE through the “SANDiE” Network
of Excellence, Contract No. NMP4-CT-2004-500101 and by
the U.K. Engineering and Physical Sciences Research Coun-
cil (EPSRC), Grant No. GR/S49308/01.
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