ARTICLE IN PRESS

Journal of Crystal Growth 301–302 (2007) 776–780
www.elsevier.com/locate/jcrysgro

Optical properties of stacked InAs self-organized quantum
dots on InP (3 1 1)B
Ryuji Oshimaa,, Kouichi Akahaneb, Masahiro Tsuchiyab,
Hidemi Shigekawaa, Yoshitaka OkadaaaInstitute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
National Institute of Information and Communication Technology, Basic and Advanced Research Department, 4-2-1 Nukui-kitamachi, Koganei,Tokyo 184-8795, Japanb

Available online 9 January 2007

Abstract
We have investigated the structural and optical properties of multi-stacked self-organized InAs quantum dot (QD) structures on InP(3 1 1)B substrates grown by molecular beam epitaxy. The reciprocal space mapping measured for asymmetric (4 0 0) reﬂections in highresolution X-ray diffraction revealed that satellite peaks originating from a periodic structure were not only observed along the growth
direction, but also along ½2̄ 3 3 direction within each QD plane. The mean in-plane spacing of neighboring QDs was 97 nm, which was ingood agreement with direct measurement of 100 nm by using atomic force microscope. A superior three-dimensional QD superlatticewith a total density of 1012 cm2 was achieved by our growth technique. We also observed a strong photoluminescence emission at
1.55 mm with a narrow linewidth of 56.07 meV at room temperature.
r 2006 Elsevier B.V. All rights reserved.
PACS: 81.05.Ea; 81.07.Ta; 81.15.Hi; 81.16.Dn
Keywords: A1. Low-dimensional structure; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials

1. Introduction
Recently, studies on the semiconductor quantum dots(QDs) have attracted intense research because of theirpotential for device applications in the quantum information technology [1,2], and optoelectronics such as QD
lasers [3], semiconductor optical ampliﬁers [4] and next
generation solar cells [5]. For this, fabrication of QDs witha sufﬁcient density and homogeneity in the active region isrequired. Until present, self-assembled QDs fabricated byspontaneous islanding by Stranski–Krastanov (S–K)
growth mode in lattice-mismatched heteroepitaxy wereintensely investigated, and S–K QDs show a high crystalline quality and good optical characteristics with few
defects and dislocations. In general, multiple stacking of
QDs by S–K growth is used to increase the total QD
density. However, as a consequence of gradual buildup ofCorresponding author. Tel./fax: +81 29 853 6902.

E-mail address: bk981495@s.bk.tsukuba.ac.jp (R. Oshima).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2006.11.063

internal lattice strain with stacking, QD size increases withincreasing number of stacked layers [6]. Recently, theconcept of strain compensation was ﬁrst demonstrated in
PbSe/PbEuTe QDs on BaF2 substrates [7]. By following
this technique, we previously reported on the stacking oflaterally ordered InAs self-organized QDs on InP (3 1 1)Bsubstrates by strain-compensation growth technique, inwhich Al atoms in InGaAlAs spacer layers played an
important role in preventing the segregation of In andinterdiffusion of In atoms between the spacer layer andQDs [8]. We also demonstrated 30 layer-stacked InAs
quantum dot solar cells for the ﬁrst time [9]. In this work,
we report on the structural and optical properties ofstacked InAs QDs on InP (3 1 1)B substrates with 100
multiple layers of QDs.
2. Experiments
For the fabrication of 100 multiple layers of selforganized InAs QDs on InP (3 1 1)B substrates, weARTICLE IN PRESS
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employed solid-source molecular beam epitaxy (MBE).
After the oxide removal and surface cleaning, a 150-nmthick In0.52Al0.48As buffer layer lattice matched to InP
substrate was grown at 500 1C. Then, four monolayers(MLs) of InAs QD layer and a 20-nm-thick In1xyAlxGayAs spacer layer, or strain-compensation layer
were consecutively grown in pair up to 100 multiple cycles
as reported previously [10]. The lattice constant ofIn1xyAlxGayAs layer was made smaller than that ofInP substrate in order to satisfy the strain-compensation
condition. The growth process and surface morphology
were studied in situ by reﬂection high-energy electron
diffraction (RHEED), and ex situ by atomic force
microscope (AFM) and cross-sectional scanning transmission electron microscope (STEM). A high resolution X-ray
diffraction (HR-XRD) was used to determine both thestrained state and periodicity of three-dimensional QD
superlattice structures. Photoluminescence (PL) measurements were performed at temperatures between 20 and300 K within a closed cycle helium cryostat. The signals
were detected by using a standard lock-in techniquewith cw 532 nm second harmonic generation (SHG)
Nd:YVO4 laser as an excitation source and a liquid
nitrogen-cooled InGaAs photodetector. The excitation
intensity was 6 W/cm2.
3. Results and discussion
Fig. 1 show the (a) topographic, and (b) two-dimensional fast Fourier transformation (2D-FFT) images,
measured for the topmost QDs surface in 100 layer-stacked
sample. The average diameter, height, and area density
were 72.1, 8.4 nm, and 2.88  1010 cm2, respectively. To be
noted was that no coalesced islands were observed, and asigniﬁcantly improved size dispersion of 9.4% in diameter
was obtained. Furthermore, sharp six-fold peaks andhigher-order peaks in the 2D-FFT image as shown inFig. 1(b) indicated a well deﬁned and laterally ordered QD
array structure. The ordered QD structure is known to berelated to the atomic structure of (3 1 1)B surface [11], andthe effect of strain ﬁeld in the underlying QD layers [12]. Asa result, the total density of QDs amounted to the order of1012 cm2, which could not be achieved by the conventional self-assembly growth technique. Fig. 2 shows thecross-sectional STEM image of stacked InAs QDs sample.
No dislocations were observed, which would otherwise be
generated if the build-up of lattice strain exceeded thecritical limit during stacking, and a vertical alignmentof QDs along growth direction was clearly maintained
without an increase in the size nonuniformity.
Next, we carried out reciprocal space mapping (RSM)
measurements around asymmetric (4 0 0) reﬂection in HRXRD as shown in Fig. 3. In contrast to a diffuse X-ray
scattering pattern that was observed for either a QW [13] ordisordered QD superlattice [14], several satellite peaks in
the RSM were observed not only along the growth
direction qy, but also along qx parallel to ½2̄ 3 3 direction

Fig. 1. (a) Topographic AFM image, and (b) 2D-FFT image of the
topmost dot layer of 100 layer-stacked InAs quantum dots on InP (3 1 1)B.
Scan size is 1 mm  1 mm.

within the QD planes. The 0th-order peak F0 in Fig. 3(a)showed a near perfect lattice-match with the peak S for InP
substrate. In this case, the averaged lattice constant ofquantum dot superlattice structure was nearly perfectly
matched to InP, i.e. the compressive strain in each QD
layer was compensated by introducing tensile strain by
InGaAlAs strain-compensating spacer layer. In addition,
the satellite peaks seen along qy suggested that no lattice
relaxation occurred during stacking.
The appearance of satellite peaks along qx such as F+1x,
F+2x and F1x in the ﬁgure indicated that the placement ofQDs were highly correlated within (3 1 1)B plane, that is,
QDs formed a periodic lattice structure at least along ½2̄ 3 3
direction. We calculated the mean period L between theneighboring QDs along ½2̄ 3 3 by using the followingequation,L ¼ ðni  nj Þl=2ðsin oi  sin oj Þ,(1)ARTICLE IN PRESS
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R. Oshima et al. / Journal of Crystal Growth 301–302 (2007) 776–780

Fig. 2. Cross-sectional STEM image of stacked InAs quantum dot structure.

Fig. 3. RSM of diffracted X-ray intensity around (4 0 0) reﬂection
measured for 100 layer-stacked InAs quantum dots on InP (3 1 1)B. qx
and qy are along ½2̄ 3 3 and [3 1 1] azimuth, respectively.

where ni and nj are the degrees and oi and oj are the Bragg
angles of the ith and jth order satellite peaks, respectively. lis the wavelength of X-ray (CuKa11.5456 Å). The mean
spacing between the dots was L ¼ 97 nm, which was ingood agreement with the direct measurement of 100 nm
obtained from AFM measurements.
Fig. 4 show the PL spectra measured at (a) 20 K and (b)300 K, respectively. The PL peak corresponding to emission from the fundamental state of QDs was clearlyobserved around 1.55 mm at 300 K. An asymmetric broadening of PL spectra on the high-energy side was also
observed, and we believe that this tail shape was due toFig. 4. PL spectra measured for 100 layer-stacked InAs QDs attemperatures between 20 and 300 K.

additional emission from the ﬁrst excited state. Fig. 5
shows the temperature dependence of integral peakintensity. We observed a strong quenching of PL intensity
above 160 K which was due to the onset of thermally
activated nonradiative recombinations. The experimental
Arrhenius plot was ﬁtted with the following equation andthe activation energy Ea of conﬁned carriers in QDs wasdetermined,I ¼ I 0 =f1 þ C expðE a =kB TÞg(2)where kB is Boltzmann’s constant, I0 and C are theconstants independent of temperature. We obtained a good

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Fig. 6. PL peak linewidth of quantum dot emission as a function oftemperature.
Fig. 5. PL integral intensity (closed squares) of quantum dot emission as afunction of temperature. Solid line shows a ﬁtted curve.

4. Conclusion
ﬁt to the experimental data as shown in Fig. 5, and Ea wascalculated to be 150 meV.
Finally, Fig. 6 shows the temperature dependence of fullwidth at half-maximum (FWHM) of PL peak as a functionof temperature. It can be seen that a relatively narrow PL
linewidth of 56.07 meV was obtained at 300 K. The
penetration depth of laser beam was estimated to be about
150–200 nm and thus the number of QD layers that wasprobed by the laser was about 8–10 layers from thetopmost surface. Although PL signal represented only thecontributions from several QD layers in our measurements,
the narrow linewidth was comparable to that for a single
layer of InAs QDs on InP substrate reported previously[15,16]. This indicates that a superior in-plane size
uniformity of QDs was obtained even after 100 layers ofstacking. In addition, an anomalous decrease in PL
linewidth with increasing temperature up to 100 K wasobserved. There have been several reports on the carrier
dynamics of thermally activated carriers that migrate or
tunnel from smaller sized QDs to larger dots via wetting
layer. Such carrier dynamics is commonly observed inbimodal QD structures and is known to show a minimum
PL linewidth at an intermediate temperature range [17],and exhibit two components following the Varshni law[18]. Here we further believe that the carriers can betransferred between QDs more easily in our stacked
structure since carrier scattering would be reduced in an
ideal three-dimensional superlattice structure. The observation that the integral PL intensity remained constant forintermediate temperatures as shown in Fig. 5 supported
that the carrier transfer between QDs was the dominating
mechanism instead of other relaxation mechanism such as
non-radiative recombination.

We successfully fabricated 100 layers of multi-stacked
self-organized InAs quantum dots on InP (3 1 1)B substrates, which showed a good laterally ordered structureswith clear six-fold symmetry. The QDs were aligned along½2̄ 3 3 within the plane as well as along the growthdirection. Further, we observed PL emission at 1.55 mm
with a narrow linewidth of 56.07 meV at room temperaturethat is suitable for ﬁber-optic communication as well as
next-generation photovoltaic applications.
Acknowledgments
This work was supported in part by the Incorporated
Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade and Industry (METI). One of theauthors (RO) is supported by Research Fellowships of theJapan Society for the Promotion of Science (JSPS) for
Young scientists.
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