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Journal of Crystal Growth 227–228 (2001) 1062–1068
Modiﬁcation of emission wavelength of self-assembled
In(Ga)As/GaAs quantum dots covered
by InxGa1xAs(04x40.3) layer
Zhichuan Niu*, Xiaodong Wang, Zhenhua Miao, Songlin Feng
National Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences,
Beijing 100083, People’s Republic of China
Abstract
Red shifts of emission wavelength of self-organized In(Ga)As/GaAs quantum dots (QDs) covered by 3 nm thick
InxGa1xAs layer with three diﬀerent In mole fractions (x ¼ 0:1, 0.2 and 0.3, respectively) have been observed.
Transmission electron microscopy images demonstrate that the stress along growth direction in the InAs dots was
reduced due to introducing the InxGa1xAs (x ¼ 0:1, 0.2 and 0.3) covering layer instead of GaAs layer. Atomic force
microscopy pictures show a smoother surface of InAs islands covered by an In0.2Ga0.8As layer. It is explained by the
calculations that the redshifts of the photoluminescence (PL) spectra from the QDs covered by the InxGa1xAs (x50:1)
layers were mainly due to the reducing of the strain other than the InAs/GaAs intermixing in the InAs QDs. The
temperature dependent PL spectra further conﬁrm that the InGaAs covering layer can eﬀectively suppress the
temperature sensitivity of PL emissions. 1.3 mm emission wavelength with a very narrow linewidth of 19.2 meV at room
temperature has been obtained successfully from In0.5Ga0.5As/GaAs self-assembled QDs covered by a 3-nm
In0.2Ga0.2As strain reducing layer. # 2001 Elsevier Science B.V. All rights reserved.
PACS: 81.15.Hi; 68.35.Bs; 78.66.Fd; 42.55.Px
Keywords: A1. Crystal morphology; A1. Quantum dots; A3. Molecular beam epitaxy; B2. Semiconducting gallium arsenide; B2.
Semiconducting indium gallium arsenide
1. Introduction
The study of semiconductor quantum dots
(QDs) has gained much interest for its fundamental physics as well as potential applications for
devices [1–6]. Particularly, the Stranski–Krastanow (SK) growth of InxGa1xAs islands on GaAs
*Corresponding author. Tel.: +86-10-82304268; fax: +8682305056.
E-mail address: zcniu@red.semi.ac.cn (Z. Niu).
substrates has been considered as a promising
method for the growth of high quality QDs.
Recently, the growth of InAs/GaAs QDs covered
by an InxGa1xAs or AlAs overgrowth layer or in
an InxGa1xAs quantum well has been carried out
in order to improve structural qualities or modify
energy band structures particularly for obtaining
longer wavelengths such as 1.3 or 1.55 mm [7–10].
It has been shown that strain reducing in the
overgrowth layer, suppression of In segregation
and compositional mixing between the cover layers
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 8 9 - 7
Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068
1063
and InAs islands play key roles for extending the
QDs emission wavelength to 1.3 mm. It is eﬀective
to modify energy band structures of self-assembled
QDs by overgrowth layers with diﬀerent lattice
constants or compositions such as InxGa1xAs
instead of GaAs matrix. At the early stage of
exploring the feasibility of long wavelength QDs
emission, it is very important to know clearly how
the structural and optical properties of InAs QDs
are inﬂuenced by the inserted InxGa1xAs or AlAs
covering layers.
In this paper, we have carried out a systematic
study on the inﬂuence of InxGa1xAs (04x40.3)
cap layer on the structural and optical properties
of InAs QDs grown on GaAs (1 0 0) substrates. It
was observed by the transmission electron microscopy (TEM) and atomic force microscopy (AFM)
measurements that the surface stress of the InAs
islands along growth direction was reduced by
overgrowth of InxGa1xAs layer on top of the
QDs. Photoluminescence (PL) measurements revealed that the red shift of PL peak energy of the
QDs was mainly due to the strain reduction of the
InAs QDs by introducing the InxGa1xAs layer. It
was further shown by calculations that the red
shifts of emission energy and variation of linewidth of the PL spectra of the QDs depend on the
contents of In mole fraction x of the InxGa1xAs
layer. The temperature characterzation of the QDs
was improved by the overgrowth of InxGa1xAs
layer which served as a layer for preserving the size
uniformity of the InAs islands. To further tune the
emission wavelength of the QDs to a longer range,
we performed systematic studies on the MBE
growth and PL measurements of In0.5Ga0.5As/
GaAs QDs which were covered with In0.2Ga0.8As
strain reducing layers. An emission wavelength of
1.3 mm with a very narrow linewidth of 19.2 meV
at room temperature has been obtained from the
In0.5Ga0.5As/GaAs QDs structures.
In0.5Ga0.5As/GaAs QDs. After standard chemical
cleaning, the substrates were mounted on Mo
holders with indium. The growth sequence for the
ﬁrst four samples were as follows: ﬁrstly, a GaAs
buﬀer layer grown at 6008C, then 2 monolayer
(ML) InAs were deposited at 4508C to form the
QDs. After a 5 s interruption, a 3 nm InxGa1xAs
overgrowth layer (with diﬀerent In mole fractions
x ¼ 0:0 (i.e., GaAs), 0.1, 0.2 and 0.3 for the four
samples) was grown on the top of InAs QDs,
followed by a 50 nm GaAs cap layer. The growth
temperatures for the upper InxGa1xAs and GaAs
cap layers were kept at 4508C to minimize
intermixing of In and Ga atoms for preserving
the initial shape of the InAs islands. For the
second three In0.5Ga0.5As/GaAs samples, the
epitaxial layers were just the same as the above
samples except the 16 ML In0.5Ga0.5As layers
which were gorwn by cycled monolayer deposition
methods. For each cycle of (InAs)n/(GaAs)n,
n=1 ML, 1.5 ML and 2.0 ML for the three
samples, respectively.
In order to carry out AFM measurements, QD
samples were also grown with the same growth
sequence as above but without the 50 nm GaAs
cap layer. The growth rates of GaAs and InAs
were 1 mm/h and 0.11 mm/h, respectively. The
Arsenic (As4) pressure was kept at 3 107 Torr
during the growth.
The TEM images were taken using a JEM 200
CX electron microscope. The TEM samples were
mechanically polished to 70 mm. AFM measurements were performed ex situ using Nanoscope 3
from Digital Instrument with a silicon tip. For the
PL measurements, a He–Ne laser (524.5 nm) was
used as an excitation source. The excitation power
was 1 mW and the sample temperature could be
raised from 7 to 300 K. The signals from the
samples were detected by a liquid nitrogen cooled
Ge detector.
2. Experimental
3. Results and discussion
Two series of samples were grown by VG V80 H
MKII MBE system on semi-insulating (1 0 0)
GaAs substrates. The ﬁrst four samples were
InAs/GaAs QDs. The second three samples were
3.1. TEM and AFM investigation of the samples
Fig 1(a) and (b) show the cross-sectional dark
ﬁeld TEM pictures of the InAs QDs covered by
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Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068
Fig. 1. Cross-sectional dark-ﬁeld TEM image of 2 ML InAs
islands covered by (a): GaAs cap layers and (b): 3 nm-thick
In0.1Ga0.9As overgrowth and GaAs cap layers.
GaAs or 3 nm In0.1Ga0.9As overgrowth layer,
respectively. In Fig. 1(a) one can clearly identify
the InAs QDs and the GaAs capping and buﬀer
layers through the contrast of the images and the
dark contrast expands obviously to the capping
and buﬀer layers. In Fig. 1(b), however, the
contrast of the image is weaker than that in
Fig. 1(a). The dimensions of the QDs are estimated
to be about 18 nm in base diameter and 3 nm in
height. In-rich clusters are clearly seen but no
misﬁt dislocations are observed.
Fig. 2(a) and (b) show AFM images (scanning
ﬁeld 1 mm 1 mm) for both the samples with In
mole fractions x ¼ 0:2 and 0.3, respectively. The
morphology of the 2.0 ML InAs QDs covered by a
3 nm thick In0.2Ga0.8As overgrowth layer is quite
ﬂat with the waviness being less than 1 nm, as
shown in Fig. 2(a). However, in the case of the
QDs covered by the In0.3Ga0.7As overgrowth
layer, the surface morphology shows larger islands
(about 60 nm in bottom diameters) indicating the
formation of the InxGa1xAs islands on the top of
InAs QDs, as shown in Fig. 2(b).
The weaker contrast in the TEM image of
Fig. 1(b) directly evidences that the strain ﬁeld in
the QDs covered by the In0.1Ga0.9As layer along
the growth direction is smaller than that of the
QDs covered by GaAs layers, since the dark
contrast in the TEM images around the islands has
been proved to be due to the strain eﬀect induced
Fig. 2. Atomic force microscope top graphical images of InAs
QDs covered by 3 nm-thick (a): In0.2Ga0.8As and (b): In0.3Ga0.7As cover layers. Scanning ﬁxed: 1.0 mm 1.0 mm.
in the capping and buﬀer layers [11]. We have
known that [8] the stress component and consequent strain in the growth direction can be reduced
by reducing the mismatch of the lattice constant
between the layers; therefore, further reduction of
the strain can be realized by increasing the In
content in the InxGa1xAs cover layers. In our
experiment, the In0.2Ga0.8As layer with less indium
has large lattice mismatch to InAs dots and
preferred to grow around the dots ﬂattening the
regions besides the dots, as shown in Fig. 2(a).
However, if the In content x of the InxGa1xAs
layer is above 0.3, the overgrowth InxGa1xAs
layer will easily nucleate to bigger islands on the
top of InAs dots, leading to unsymmetrical
distribution of the stress on the top of InAs
islands and inevitably resulting in size ﬂuctuation
Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068
of the InAs QDs. These will be proved by the
following PL measurements.
3.2. PL measurements of the samples
The PL spectra taken at 10 K are shown in
Fig. 3, in which the inset shows the spectra of PL
integrated intensity versus full width at half
maximum (FWHM) for all samples with diﬀerent
In contents of the InxGa1xAs layers. In the case
of the InAs QDs covered by a GaAs layer
(x=0.0), the PL peak energy is positioned at
0.97 mm (1.27 eV) with an FWHM of 62 meV. In
all cases of the QDs covered by a 3-nm-thick
InxGa1xAs (x50.1) strain-reducing layer, considerable red shifts of the PL peak energies with
enhanced integrated intensity were observed. The
minimum FWHM of 36 meV is obtained for the
case of x=0.2, while the FWHM is 39 meV for the
case of x=0.3.
Generally, the red shifts of the PL peak of the
InAs QDs covered by InxGa1xAs layers are
mainly due to the following reasons [8]: (1) the
strain in the growth direction is decreased by the
overgrowth of InxGa1xAs layer. (2) InAs/GaAs
intermixing during the growth can be eﬀectively
reduced. Here we separately consider how the two
factors modify the energy band gaps of the InAs
QDs.
Firstly, we consider the eﬀect of strain reduction
in the overgrowth layer which can modify the
energy gap of the QDs. We could assume
simplistically that the QDs are spherical in shape
and covered by InxGa1xAs or GaAs. In the case
of QDs covered by In0.2Ga0.8As layer, the strain
component and pressure within the QDs can be
expressed as [12]
P
; ery ¼ eyf ¼ efr ¼ 0;
err ¼ ð1Þ
3l þ 2m
P¼
4ð3l þ 2mÞða 1Þ
;
4a þ 8
ð2Þ
where l and m are Lame’s constants, and a
(=1.0565) is the ratio of the lattice constants of
InAs and In0.2Ga0.8As. So the changes of conduction- and valence-band edges are
dEc;v ¼ ac;v ð3err Þ ¼ 0:055ac;v ðeVÞ;
ð3Þ
where ac;v in Eq. (3) are the deformation potentials
of InAs. Using ac=5.08 eV, av=1.00 eV [13], so
a=(acav)=6.08 eV, we obtain an energy-gap
change of
dE ¼ dEc dEv ¼ 0:337 eV:
ð4Þ
Similarly, the energy change of the band gap of
the QDs covered by a GaAs layer can be
calculated (by the same equations as above) as
follows:
dE 0 ¼ 0:425 eV:
Fig. 3. PL spectra of InAs QDs covered by 3 nm-thick
InxGa1xAs (x=0, 0.1, 0.2, and 0.3) strain-reducing layer at
10 K with an excitation intensity of 1 mW. In the inset, the
linewidth of the PL spectra versus In contents of the
InxGa1xAs layer is shown.
1065
ð5Þ
So the value of the red shift of the emission
energy deduced from Eqs. (4) and (5) is 88 meV.
Secondly, we consider the reduction eﬀect of
InAs/GaAs intermixing due to the In segregation
during the growth which also leads to some red
shifts of the PL emission energy. In fact, the
compositional mixing of the InAs/GaAs interfaces
not only degrades the size uniformity of the islands
but also modiﬁes the energy band structures of the
QDs. It has been estimated theoretically that the
red shifts of the emission energy of the QDs caused
by the In segregation are approximately 20–
30 meV [14].
Based on the calculations discussed above,
one can estimate approximately all PL energy
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Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068
shifts for the samples with diﬀerent In mole
fractions of InxGa1xAs cover layers as shown
in Fig. 4, in which the experimental results are
presented for comparison, revealing the fact that
the main contribution to the red shift of PL peak
energy is due to the eﬀect of the InxGa1xAs
layers.
The narrowing of the PL linewidth of the QDs
covered by InxGa1xAs layer is possibly due to the
InxGa1xAs overgrowth layers which suppress the
composition mixing or In segregation eﬀectively
during the growth of covering layers preserving the
size ﬂuctuations of the InAs/GaAs islands [8]. The
initial shape of the InAs islands covered by
InxGa1xAs layer can be preserved at a relatively
earlier stage during the overgrowth of InxGa1xAs
layer than during the overgrowth of GaAs capping
layer. The In segregation during the growth of
InxGa1xAs covering layer on the top of the InAs
islands will be suppressed, because the In elemental source is supplied during the growth of
InxGa1xAs covering layer. In our experiment, for
the case of InxGa1xAs cover layer with the In
mole fraction x=0.2, the size uniformity of the
QDs was improved as shown in the AFM images
of Fig. 2(a) and the PL spectra of Fig. 3. While for
the case of the InxGa1xAs covering layer having
higher In contents x50.3, the bigger In0.3Ga0.7As
islands were nucleated on the top of the InAs
islands as shown in AFM pictures of Fig. 2(b). The
nucleation of In0.3Ga0.7As islands might create
unsymmetrical distribution of stress on the top of
InAs islands, leading to an increment of the
FWHM from 36 emV for the case of x=0.2–
39 emV for the case of x=0.3. It will be further
discussed in the following that the InxGa1xAs
cover layer also plays a key role for the improvement of temperature dependent characterization of
the InAs QDs.
The temperature dependent spectra of the PL
peak energy for the four samples are compared as
shown in Fig. 5. The red shifts of the PL peak
energy correspond to the In mole fraction x of the
InxGa1xAs cover layers. The red shift of the
temperature dependent PL signals for the cases of
x=0.1, 0.2 and 0.3 is smaller than that of x=0.0,
revealing less dependency or sensitivity of PL
emission to the variation of temperatures. In
contrast, there is no diﬀerence in the red shifts of
the PL peak between the cases of x=0.2 and 0.3.
It can accordingly be concluded that the red
shift of the temperature dependent PL peak energy
is eﬀectively preserved mainly due to the overgrowth of InxGa1xAs layer leading to improvements of the size uniformity and relaxation of the
surface strain of the QDs, and partly due to the
bandgap narrowing eﬀect at higher temperature. It
has been well-recognized [15] that the self-organized InAs QDs system can be regarded as a
coupled system in which the wave functions of
Fig. 4. Calculated (dashed line) and experimental (solid line)
data of the PL peak energy shifts vs In content of the
InxGa1xAs (04x40.3) cover layers.
Fig. 5. Temperature dependent spectra of the red shifts of the
PL peak energy of the QDs corresponding to diﬀerent In mole
fractions from 0.0 to 0.3.
Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068
carriers in an individual QD penetrate into
adjacent dots. The coupling and relaxation eﬀects
of the carriers will be enhanced with temperature
increase due to increased electron–photon interaction in the QDs. As a result, photogenerated
carriers transfer and relax into energetically lowlying states, giving rise to the fast red shift of the
exciton energy. For the samples of InAs QDs
covered by InxGa1xAs (0.3>x50.1), the size
uniformity of the QDs was improved by the
overgrowth of InxGa1xAs layer, resulting in
better uniformity of energy states. Therefore, the
shifts of the PL peak energy caused by the
temperature increase will be smaller due to better
uniformity of the energy states of the InAs QDs
covered by InxGa1xAs layers. It is reasonable
that the suppression of intermixing or In segregation for the case of x=0.3 should be stronger than
that for the case of x=0.2. But the size distribution of the QDs is obviously worse due to the
nucleation of InxGa1xAs on the top of InAs
islands as evidenced by the AFM images and PL
measurements. These two contrary aspects lead to
the same energy shifts for x=0.2 and 0.3 cases as
shown in our spectra of the PL peak energies
versus temperatures. In precise treatment for the
explanation, the temperature dependence of the
bandgap narrowing eﬀect should be taken into
account. It has been shown [14] that the nonlinear
decrease of the sublevel separation of the QDs
with InGaAs overgrowth layer is certainly due to
the strain lattice distortion in the embedded dot
structures, while the QDs without overgrowth
layer change linearly during the increase of
temperature. For complete understanding of the
strain ﬁeld dominating eﬀects on the temperature
dependence of the PL peak of the InAs QDs
covered by the InGaAs layers, we need further
investigation for the relationship between sublevel
and strain ﬁeld distribution or lattice distortion
around the dots.
In order to further increase the red shifts of the
PL emission wavelength, the In0.5Ga0.5As/GaAs
QDs have been grown. From the above discussion,
we know that within three kinds of strain reducing
layers with diﬀerent In mole fractions, the
In0.2Ga0.2As layer is the best one to modify
the structural and optical properties of the InGaAs
1067
QDs. So the In0.2Ga0.2As cover layer was used
for the In0.5Ga0.5As/GaAs QDs samples. Their
optical properties were studied by PL measurements. As shown in Fig. 6, 1.3 mm ranged emission
wavelengths at room temperature are obtained
from all three samples. Particularly, the linewidth
of the PL spectra of the sample n=1.0 is 19.2 meV
which is one of the best results, to our knowledge,
so far. The inset of Fig. 5 shows PL spectra
dependency on laser excitation power intensity
taken at 10 K from the sample of n=1.0, showing
clearly the excited states at higher excitation
power.
Those PL results indicate that the three
In0.5Ga0.5As/GaAs quantum dots have very good
optical properties with a useful emission wavelength of 1.3 mm. Although the deposition cycle
(n=1.0, 1.5, 2.0) for the growth of In0.5Ga0.5As/
GaAs QDs is diﬀerent, which might cause diﬀerent
strain states to remain in the InGaAs/GaAs
Fig. 6. Room temperature PL spectra for three In0.5Ga0.5As/
GaAs QDs samples grown via cycled (InAs)n/(GaAs)n (n=1.0,
1.5, 2.0, respectively) monolayer deposition, in which the
narrowest FWHM of 19.2 meV is shown for the sample
n=1.0. In the inset, the excitation power dependent PL spectra
are shown for the sample n=1.0, the excited energy states are
clearly resolved at higher excitation power intensity.
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Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068
interfaces and lead to variation of energy band
structure of the QDs [16], the similar emission
wavelength between the three samples reveals that
the In0.2Ga0.2As covering layer modiﬁes the stress
in the interfaces of the InGaAs and GaAs layers
leading to better uniformity in strain distribution.
So this strain reducing eﬀect induced by InGaAs
overgrowth layer on top of InGaAs/GaAs QDs
structure is conﬁrmed again as a very important
factor for reproducibility of growth of QDs with
good optical quality.
obtained successfully, showing good reproducibility in the growth of such QD structures.
4. Summary
References
The inﬂuence of InxGa1xAs covering layer on
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demonstrate that the stress in the InAs dots along
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Acknowledgements
This work was partly supported by the National
Natural Science Foundation of China, Hundred
Talents Program of Chinese Academy of Science
and State Climbing Research Project.