Fabrication of InAs quantum dots on InP(100) by metalorganic vapor-phase epitaxy for 1.55 μm optical device applications

Article abstract of DOI:10.1063/1.1814442

A change in the density and wavelength of InAs quantum dots (QDs) on InGaAsP/InP(100) substrate grown by metalorganic vapor-phase epitaxy (MOVPE) in accordance with variation in the growth conditions was studied, aiming at optical device applications in the 1.55 μm region. In the moderate V/III ratio region, the size of QDs was found to decrease while the density increased as the group-V source was reduced, but on the other hand, both of them increased monotonously with increasing InAs supply. The combination of changing the V/III ratio and InAs supply allowed us to control the density and wavelength of QDs independently so that QDs with a density as high as 5.6 × 10¹⁰ and a 1.6 μm emission were obtained. The letter reports the MOVPE growth technique of QDs on InGaAsP/InP(100), which connects QDs with mature 1.55 urn device technology.

Full text of DOI:10.1063/1.1814442

Fabrication of InAs quantum dots on InP(100) by metalorganic vapor-phase epitaxy for
1.55 μm optical device applications
Kenichi Kawaguchi, Mitsuru Ekawa, Akito Kuramata, Tomoyuki Akiyama, Hiroji Ebe, Mitsuru Sugawara, and
Yasuhiko Arakawa
Citation: Applied Physics Letters 85, 4331 (2004); doi: 10.1063/1.1814442
View online: http://dx.doi.org/10.1063/1.1814442
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/85/19?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 19
8 NOVEMBER 2004
Fabrication of InAs quantum dots on InP„100… by metalorganic vapor-phase
epitaxy for 1.55 µm optical device applications
Kenichi Kawaguchi,a͒ Mitsuru Ekawa, Akito Kuramata, and Tomoyuki Akiyama
Fujitsu Limited, 10-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa, 243-0197, Japan
and Optoelectronic Industry and Technology Development Association (OITDA), 1-20-10, Sekiguchi,
Bunkyo-ku, Tokyo, 112-0014, Japan
Hiroji Ebe and Mitsuru Sugawara
NCRC, Institute of Industrial Science (IIS), The University of Tokyo, 4-6-1, Komaba, Meguro-ku,
Tokyo, 153-8505, Japan
Yasuhiko Arakawa
Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1, Komaba,
Meguro-ku, Tokyo, 153-8904, Japan and Optoelectronic Industry and Technology Development
Association (OITDA), 1-20-10, Sekiguchi, Bunkyo-ku, Tokyo, 112-0014, Japan1
͑Received 19 May 2004; accepted 6 September 2004͒
A change in the density and wavelength of InAs quantum dots ͑QDs͒ on InGaAsP/InP͑100͒
substrate grown by metalorganic vapor-phase epitaxy ͑MOVPE͒ in accordance with variation in the
growth conditions was studied, aiming at optical device applications in the 1.55 µm region. In the
moderate V/III ratio region, the size of QDs was found to decrease while the density increased as
the group-V source was reduced, but on the other hand, both of them increased monotonously with
increasing InAs supply. The combination of changing the V/III ratio and InAs supply allowed us to
control the density and wavelength of QDs independently so that QDs with a density as high as
5.6ϫ1010 and a 1.6 µm emission were obtained. The letter reports the MOVPE growth technique of
QDs on InGaAsP/InP͑100͒, which connects QDs with mature 1.55 µm device technology. © 2004
American Institute of Physics. ͓DOI: 10.1063/1.1814442͔
The last few years have seen much attention being paid
to the growth of quantum dots ͑QDs͒ using the Stranski–
Krastanow ͑SK͒ mode because the use of QDs makes it pos-
sible to drastically improve the characteristics of lasers or
amplifiers.1–6 QDs offering these advantages are suitable for
use in 1.55 µm optical telecommunication devices. One of
the attractive material combinations for fabricating QDs with
a 1.55 µm region emission is InAs and InP. A quantum-dot
laser fabricated on InP͑311͒B,7 and a quantum-dash laser8
and amplifier9 fabricated on InP͑100͒ have been reported as
1.55 µm InAs quantum-dot devices fabricated on InP sub-
strates. All these devices were grown by molecular-beam ep-
itaxy ͑MBE͒. Metalorganic vapor-phase epitaxy ͑MOVPE͒
and chemical-beam epitaxy ͑CBE͒, however, are more suit-
able for fabricating buried heterostructure ͑BH͒ devices that
are to be used with high current densities, since it remains
difficult to grow phosphorus-containing materials such as
InGaAsP by MBE. Some attempts for realizing 1.55 µm InAs
QDs on InP substrates grown by these growth techniques
have been reported,10–14 and preliminary work on quantum-
dot laser using InAs QDs embedded in InGaAsP grown by
CBE was reported.15 The use of MOVPE to fabricate
quantum-dot devices also has another advantage in terms of
device integration because most commercially available op-
tical devices used in the 1.55 µm region are grown by
MOVPE. However, those reports related to InAs QDs on an
InGaAsP layer grown by MOVPE, which is used for actual
devices, have been limited to the likes of optical properties,
such as the luminescence wavelength and lifetime.12 It
should be also noted that when designing devices, there is a
need to control the QD density and emission wavelength
independently because high density and a 1.6 µm emission
are required to obtain a large gain in the 1.55 µm region.
In this work, the behavior of the density and wavelength
of InAs QDs on InGaAsP/InP͑100͒ substrate grown by
MOVPE was studied. It was found that the V/III ratio has an
opposite trend for the density in the moderate V/III ratio
region, which allows one to fabricate high-density QDs with
an emission of 1.6 µm.
The MOVPE growth was performed using trimethyl-
indium ͑TMI͒, triethyl-gallium ͑TEG͒, arsine ͑AsH3͒, and
phosphine ͑PH3͒ as sources. On InP͑100͒ n-type substrates,
InP buffers with a thickness of 100 nm were grown at
630 °C, followed by 200-nm-thick In0.85Ga0.15As0.33P0.67 lay-
ers at 550 °C. Then, InAs QDs were grown at temperatures
between 460 and 550 °C. For those samples to be used for
photoluminescence ͑PL͒ measurements, InGaAsP capping
layers with a thickness of 40 nm were grown at the same
temperature as that used for growing InAs QDs. The nominal
InAs supply was varied between 1 and 3 monolayers ͑ML͒
by changing switching-on times, and the V/III ratio was var-
ied between 5 and 20 by changing the AsH3 flow, respec-
tively. The density and the size of the QDs were obtained
using an atomic force microscope ͑AFM͒. All of the PL mea-
surements were performed at room temperature.
Figure 1 shows four AFM images of InAs QDs grown at
480 °C on InGaAsP. It was observed that the density of the
QDs increases from 1.1 to 3.5ϫ1010 cm−2 as the nominal
InAs supply was increased from 1 to 3 ML, as shown in
Figs. 1͑a͒–1͑c͒. The average height of the QDs also exhibited
monotonous increase from 6.7 to 9.7 nm, and the change in
the lateral size exhibited the same trend as the change in
a͒
Electronic mail: k_kawaguchi@jp.fujitsu.com
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0003-6951/2004/85͑19͒/4331/3/$22.00 4331 © 2004 American Institute of Physics
137.189.170.231 On: Sat, 20 Dec 2014 17:43:21

4332
Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004
Kawaguchi et al.
FIG. 3. Relationship between QD density and PL wavelength when the V/III
ratio was varied.
the ideal SK QDs. This is supported by the fact that no QDs
were formed under the same 1 ML growth condition when
InAs was supplied directly on InP.
FIG. 1. AFM images of InAs QDs grown at 480 °C, ͑a͒1 ML, ͑b͒ 2 ML, ͑c͒
3 ML with a V/III ratio of 20 and ͑d͒ 2 ML with a V/III ratio of 8.
The change in the density and height in accordance with
variation in the V/III ratio can be classified into three cases,
namely, a sufficient group-V source region such as V/III ӷ1,
a moderate region, and extremely sparse region such as V/III
Ͻ1. In the first case, it is expected that the formation and
growth of QDs depends solely on the amount of the group-
III source. In the second case, however, those of QDs begin
to depend on the amount of the group-V source as well as
that of the group-III source, and the group-V source amount
only dominates in the last case.
It can be said that the first region does not exist in this
study, since QDs depend on the V/III ratio in the whole re-
gion shown in Fig. 3. The region where the density increased
while height decreased is considered to be the moderate re-
gion. This trend in the moderate region indicates that the
nucleation is enhanced but that the island volume growth rate
is reduced because the sources are consumed as part of the
nucleation. Since an increase in the nucleation rate requires
an increase in the atomic density of In, In atoms seems to be
able to migrate on the surface avoiding QDs as the group-V
source is reduced. It is noted that the V/III ratio dependence
here cannot be explained if the group-V flux affects only the
migration length of the group-III atoms. In this simple
model, a reduction in the V/III ratio results in an increase in
the surface migration length of the In atoms and, therefore,
the size of the QDs should increase while the density should
decrease.16 The sparse region where both the density and
height fell with a reduction in the group-V source was ob-
served even at V/III Ͼ1, and, therefore, the effective V/III
ratio is regarded as being smaller than the V/III ratio calcu-
lated from the supply amount. It is also noted that such
growth limitation of QD size is also observed in MBE-grown
InAs QDs on InP.17
height. It was also found, however, that the QDs showed a
different dependence on the V/III ratio. The density increases
from 2.7 to 3ϫ1010 cm−2 when the V/III ratio was decreased
from 20 to 8 as shown in Figs.1͑b͒ and 1͑d͒, while the height
falls from 7.8 to 6.2 nm.
Figures 2 and 3 show the relationship between the den-
sity and PL wavelength as the InAs supply amount and the
V/III ratio are varied. As shown in Fig. 2, the wavelength
shows blueshift and the density falls monotonously with a
reduction in the InAs supply. The wavelength, on the other
hand, becomes shorter while the density initially increases
and then falls with a reduction in the V/III ratio, as shown in
Fig. 3. It is noted that the shift in the wavelength is due to the
change in the height given that the height has the greatest
effect on the quantum confinement. This is consistent with
the results of AFM observation.
In the study of InAs supply dependence, since the
amount was varied by adjusting the deposition time with a
fixed TMI flow rate, the QDs can grow in size by incorpo-
rating the supplied source after nucleation. The density can
also increase in situations where the nucleation continues
during the sources being supplied. It should be noted that the
height and density increase simultaneously even though the
two phenomena compete for the sources. The increase in the
density with an increase in the supply is not surprising, while
there is a possibility of the nucleation of QDs being en-
hanced by local fluctuations in the alloy composition and/or
strain distribution in the InGaAsP buffer layers, relative to
With the usual way of tuning the wavelength by chang-
ing the InAs supply, the density decreases as the wavelength
is shortened because the wavelength is defined by the height
of the QDs. The opposite trend, however, is observed when
the amount of V-group source satisfies the second case,
which was discussed earlier. The density and wavelength
can, therefore, be controlled independently by using this re-
gion. QDs with a density as high as 5.6ϫ1010 cm−2 and a 1.6
µm emission were obtained as a result, overcoming the shift
toward a longer wavelength as the density increases ͑Fig. 4͒.
The thermal stability and the stacking of the InAs QDs
FIG. 2. Relationship between QD density and PL wavelength when the InAs
supply amount was varied.
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embedded in InGaAsP were confirmed, as required for the
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Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004
Kawaguchi et al.
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The authors would like to acknowledge O. Aoki and Y.
Kinoue for their technical support. This work was supported
by NEDO and IT program, MEXT.
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