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Structure and optical anisotropy of vertically correlated submonolayer InAs/GaAs
quantum dots
Xu, Zhangcheng; Birkedal, Dan; Hvam, Jørn Märcher; Zhao, Z.Y.; Liu, Y.M.; Yang, K.T.; Kanjilal, A.;
Sadowski, J.
Published in:
Applied Physics Letters
Link to article, DOI:
10.1063/1.1581005
Publication date:
2003
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Xu, Z., Birkedal, D., Hvam, J. M., Zhao, Z. Y., Liu, Y. M., Yang, K. T., Kanjilal, A., & Sadowski, J. (2003).
Structure and optical anisotropy of vertically correlated submonolayer InAs/GaAs quantum dots. Applied Physics
Letters, 82(22), 3859-3861. https://doi.org/10.1063/1.1581005
Structure and optical anisotropy of vertically correlated submonolayer
InAsÕGaAs quantum dots
Zhangcheng Xu,
a)
Dan Birkedal, and Jørn M. Hvam
Research Center COM, Technical University of Denmark, DK-2800, Lyngby, Denmark
Zongyan Zhao, Yanmei Liu, and Kuntang Yang
Department of Physics, Anhui University, Hefei, 230039, People’s Republic of China
Aloke Kanjilal
Department of Physics, University of Aarhus, Ny Munkegde, DK-8000, Aarhus C, Denmark
Janusz Sadowski
Niels Bohr Institute, Copenhagen University, DK-2100, Copenhagen, Denmark
共Received 16 September 2002; accepted 21 April 2003兲
A vertically correlated submonolayer 共VCSML兲 InAs/GaAs quantum-dot 共QD兲 heterostructure was
studied using transmission electron microscopy, high-resolution x-ray diffraction 共HRXRD兲 and
polarization-dependent photoluminescence. The HRXRD 共004兲 rocking curve was simulated using
the Tagaki–Taupin equations. Excellent agreement between the experimental curve and the
simulation is achieved assuming that indium-rich VCSML QDs are embedded in a quantum well
共QW兲 with lower indium content and an observed QD coverage of 10%. In the VCSML QDs, the
vertical lattice mismatch of the InAs monolayer with respect to GaAs is around 1.4%, while the
lattice mismatch in the QW is negligible. The photoluminescence is transverse magnetic—polarized
in the edge geometry. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1581005兴
Much interest has been attracted to the growth of self-
assembled semiconductor quantum dots 共QDs兲, due to their
potential application in optoelectronic devices.
1
Submono-
layer 共SML兲 deposition is an alternative method to the
widely used Stranski–Krastanow 共SK兲 mode of growing
QDs.
2–4
Deposition of a SML InAs on a GaAs 共001兲 surface
leads to the formation of 1 ML high InAs islands elongated
along the
关
11
¯
0
兴
direction.
2
The density and the lateral sizes
of the islands depend on the deposition amount. Vertical cor-
relation of the SML QDs occurs when stacking the SLM
QDs with thin spacers.
5–9
High-power lasers with the
stacked SML InAs/GaAs QDs as the active region have re-
cently been demonstrated.
6–8
The structure and optical prop-
erties of the SK InAs/GaAs QDs have been intensively
studied,
1
but few papers have been reporting on the vertically
correlated 共VC兲 SML InAs/GaAs QDs.
4,9
In this letter, the
structure and the optical anisotropy of the VCSML InGaAs
QDs are investigated by plan-view transmission electron mi-
croscopy 共TEM兲, high-resolution x-ray diffraction
共HRXRD兲, and polarization-dependent photoluminescence
共PL兲 at low temperature in both the backscattering and the
edge emission geometries.
The VCSML InAs/GaAs QD sample was molecular-
beam epitaxy grown on a semi-insulating GaAs 共001兲 sub-
strate. After oxide desorption, a 500 nm GaAs buffer layer,
20 nm AlAs, and 82 nm GaAs were grown at 600 °C. After
the substrate temperature was lowered to 480 °C, 10 cycles
of InAs共0.5ML兲/GaAs共2.5ML兲 were deposited to form aVC-
SML QD layer, with 2 nm of GaAs covering the VCSML
QD layer. Then, the substrate temperature was again in-
creased to 600 °C to grow 20 nm of AlAs and a 106 nm
GaAs cap layer.
TEM studies were performed using a Philips CM20 in-
strument operating at 200 keV. Figure 1 shows a plan-view
TEM image of the VCSML InAs/GaAs QDs. The contrast is
mainly due to strain fields. 10% of the surface is covered by
QDs and the area density of QDs is around 5.2
⫻ 10
11
cm
⫺ 2
, much higher than the conventional SK InAs/
GaAs QDs. Most of the QDs are slightly elongated along
关
11
¯
0
兴
direction, and the size of QDs is around 5–10 nm in
diameter. However, the actual QD size could be smaller since
the strain field may extend beyond the QD boundary.
The determination of the strain in the QDs is useful not
only for understanding the QD growth but also for the cal-
culation of the energy diagram of the QDs.
5,10,11
HRXRD is
a兲
Electronic mail: zxu@com.dtu.dk
FIG. 1. 共011兲 bright-field plan-view TEM image of the structure with
VCSML InAs/GaAs QDs.
APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 22 2 JUNE 2003
38590003-6951/2003/82(22)/3859/3/$20.00 © 2003 American Institute of Physics
Downloaded 26 Mar 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
an important nondestructive method of determining the inter-
face morphology and the strain of the buried films. The mea-
sured rocking curves are the ‘‘fingerprints’’ of the investi-
gated structure and can be analyzed by using the dynamical
x-ray diffraction theory, i.e., the Tagaki–Taupin
equations.
11,12
Due to the structural complexity of a QD het-
erostructure, the average strain of the QD plane rather than
of the QDs themselves has been determined for the SK
QDs.
13–15
In the case of the VCSML QDs, the contribution
from the QD parts and the lateral non-QD parts to the finally
measured reflection rocking curve can be considered sepa-
rately, as shown in the inset of Fig. 2. According to the
growth mechanism of stacked SML InGaAs QD
heterostructure,
7
the VCSML QD in our sample can be de-
scribed as 10 cycles of GaAs共2ML兲/InAs共1ML兲, while the
lateral non-QD part can be described as a 30 ML GaAs and
50% of the surface should be covered by the VCSML QDs.
However, QDs only fill about 10% of the surface in the plan-
view TEM image 共Fig. 1兲. This means that 40% of the SML
InAs is in the lateral non-QD part, possibly due to In–Ga
intermixing or because vertical correlation does not occur for
all the SML QDs. Therefore, the lateral non-QD part is an
InGaAs alloy rather than pure GaAs, and our sample is a
QD–quantum-well 共QW兲 heterostructure. The average In
composition in the lateral QW is calculated to be around
15%, according to the 10% QD coverage on the surface.
The HRXRD rocking curve near the 共004兲 reflection
peak of GaAs 共001兲 substrate was measured using a MAC
Science HRXRD instrument with a Cu K
␣
x-ray source and
aGe共004兲 monochromator. Figure 2 shows the experimental
data and simulated results based on Tagaki–Taupin equa-
tions. The total reflection coefficient R(⌬
) of the sample
can be written as
R
共
⌬
兲
⬇xR
QD
共
⌬
兲
⫹
共
1⫺ x
兲
R
QW
共
⌬
兲
, 共1兲
where ⌬
is the angular deviation from the Bragg angle of
the 共004兲 peak of the GaAs substrate, x is the QD coverage
percentage on the surface, R
QD
(⌬
) and R
QW
(⌬
) are the
reflection coefficients from the part of the sample with only
the VCSML QDs and the part with only the lateral QW,
respectively. Note that Eq. 共1兲 is valid only when xR
QD
(⌬
)
is so small that the multiple scattering of x rays between the
QD and the QW parts is negligible. In the angular range from
⫺ 1000 to ⫹ 500 arcs 共range I兲, the reflectivity is much
higher than outside this range and mainly depends on the
R
QW
(⌬
) because QW covers 90% of the surface. Only
when the lattice mismatch is negligible between the lateral
8.48 nm In
0.15
Ga
0.85
As QW and GaAs, do the calculated in-
terference fringes 关curve b in Fig. 2共b兲, lines 1 to 3兴 match
the experimental ones in range I. At the same time, no infer-
ence fringes in the range from ⫺ 1000 to ⫺ 4000 共range II兲 in
curve b can be comparable to the experimental data. There-
fore, the interference fringes in range II are mainly from
R
QD
(⌬
). By varying the lattice constant a
InAs
⬜
of the InAs
monolayer inside the VCSML QDs, R
QD
(⌬
) is calculated
and found to match the interference fringes 关curve a in Fig.
2共a兲, lines 4 to 6兴 only when a
InAs
⬜
⬇5.7324 Å, i.e., a
InAs
⬜
is
around 1.4% higher than the lattice constant of GaAs. With
x⫽ 10% determined from the TEM image 共Fig. 1兲, curve c in
Fig. 2共c兲 is calculated from Eq. 共1兲. It can be seen that all the
interference fringes in curve c in Fig. 2共c兲 can match the
experimental ones very well. The macroscopic continuum
elasticity theory 共MCET兲 predicts that for a pseudomorphic
InAs layer buried in GaAs 共001兲, the strain normal to the
共001兲 plane would be 7.26%, corresponding to a
InAs
⬜
⬇6.4981 Å.
16,17
However, the validity of the MCET in the
monolayer limit is a long debated issue.
18–20
In the case of
SML InAs QDs, the InAs islands with 1 monolayer in height
and around 5–10 nm in diameter are surrounded by a GaAs
matrix, and the MCET cannot be applied directly. Our
HRXRD gives an experimental determination of the strain in
the SML InAs QDs.
Shape anisotropy effects on the electronic properties of
VCSML InAs/GaAs QDs are investigated here by measuring
the polarization dependence of the optical transitions. The PL
was excited at 10 K by a He–Ne laser at 632.8 nm with an
excitation density ⬃10 W/cm
2
. A 64 cm monochromator
with a Si charge coupled device was used to detect the PL.
The polarization of the PL was analyzed using a fixed polar-
izer and a broadband /2 plate. The PL polarization anisot-
ropy is defined as P⫽ (I
储
⫺ I
⬜
)/(I
储
⫹ I
⬜
), where I
⬜
is the
vertically polarized intensity and I
储
is the horizontally polar-
ized intensity in the laboratory coordinate system.
Figure 3 shows the degree of linear polarization of PL
detected in both the backscattering 关curve 共a兲 in Fig. 3共a兲兴
and the edge geometries 关curves 共b兲 and 共c兲 in Figs. 3共b兲 and
3共c兲, respectively兴. In the backscattering geometry, the light
propagates in the 关001兴 direction 共surface emission兲, I
储
and
I
⬜
are polarized along the
关
11
¯
0
兴
and 关110兴 directions, re-
spectively. The broad peak 关curve 共d兲 in Fig. 3共d兲兴 centered
at 1.26 eV with a linewidth of 55 meV is assigned to the
ground-state transitions of the VCSML InAs/GaAs QDs. PL
emission from the QDs is predominantly polarized along the
关
11
¯
0
兴
direction in the whole energy range, P⬇0.22 关curve
共a兲 in Fig. 3共a兲兴, due to the elongation of the QDs along the
FIG. 2. HRXRD rocking curves in the vicinity of the GaAs 共004兲 refection,
共a兲 the simulated curve of R
QD
, assuming a
InAs
⬜
⫽ 5.7324 Å, 共b兲 the simu-
lated curve of R
QW
, assuming a
In
0.15
Ga
0.85
As
⬜
⫽ 5.6533 Å, and 共c兲 the combi-
nation of 共a兲 and 共b兲, assuming the QD coverage percentage is 10%, and 共d兲
the experimental data. The curves are shifted vertically for clarity. The inset
shows the different contributions from the QD parts and the lateral non-QD
parts to the reflection coefficient of a VCSML QD sample. Lines 1 to 6 are
guides for the eyes.
3860 Appl. Phys. Lett., Vol. 82, No. 22, 2 June 2003 Xu
et al.
Downloaded 26 Mar 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
关
11
¯
0
兴
direction, as shown in the plan-view TEM observa-
tion.
In the edge geometry, the spectra were obtained exciting
and detecting at the sample edges, the light propagates in
关110兴 or
关
11
¯
0
兴
direction, I
储
and I
⬜
are polarized along the
关001兴 and
关
11
¯
0
兴
or 关110兴 directions, respectively. These po-
larizations are, respectively, referred to as transverse mag-
netic 共TM兲 and transverse electric 共TE兲 modes. To suppress
the PL emission from the 关001兴 sample surface, the edge
emission was spatially selected in near and far field. From
curves 共b兲 and 共c兲 of Figs. 3共b兲 and 3共c兲, respectively, the PL
emission was found to be TM polarized in the VCSML
InGaAs/GaAs QDs, while a single-layer InGaAs/GaAs SK
QD structure shows a TE polarization, measured in the same
geometry and using similar equipment. This indicates that
the electronic states are elongated along the growth direction
for VCSML InGaAs/GaAs QDs. A similar result has been
observed in the case of vertically coupled CdSe/ZnSSe SML
QDs and InGaAs/GaAs SK QDs.
4,21
A slightly higher anisot-
ropy of P⬇0.17 for emission along the
关
11
¯
0
兴
direction
关curve 共b兲 in Fig. 3共b兲兴 than that of P⬇0.09 along the 关110兴
direction 关curve 共c兲 in Fig. 3共c兲兴 is due to the fact that the
lateral dimensions of VCSML QDs along
关
11
¯
0
兴
direction are
nearer to the QD heights. This feature is not only of the
fundamental interest for electronic structure and optical
properties, but also important in its applications in
polarization-independent devices.
In summary, the structure and optical anisotropy of
VCSML InAs/GaAs QDs, are studied using plan-view TEM,
HRXRD, and polarization-dependent PL at low temperature
in both the backscattering and edge geometries. The method
of calculating the reflection coefficient from a VCSML QD
heterostructure was demonstrated, using the Tagaki–Taupin
equations, the structure of VCSML QDs, and the QD cover-
age percentage on the surface. The separate contributions
from the QDs and the lateral QW to the total x-ray diffrac-
tion signal have been extracted. The lattice mismatch in the
InAs monolayer inside the VCSML is found to be around
1.4% with respect to GaAs, and that in the lateral QW is
negligible. In the edge geometry, the PL emission is TM
polarized, which is important for the application of VCSML
QDs in optoelectronic devices.
This work was supported by the Danish Technical Sci-
ence Research Council.
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FIG. 3. The degree of linear polarization of PL measured at 10 K, 共a兲 in the
backscattering geometry, collecting light in the 关001兴 direction, 共b兲 in the
edge geometry, collecting light in the
关
11
¯
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兴
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3861Appl. Phys. Lett., Vol. 82, No. 22, 2 June 2003 Xu
et al.
Downloaded 26 Mar 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp