Self-organized InAs quantum dots formation by As/P exchange reaction on (001) InP substrate

Article abstract of DOI:10.1063/1.121396

In this letter, we present the results of InAs quantum dots (QDs) prepared on a (001) InP substrate. As/P exchange reaction at the surface of InP buffer was used to form the InAs islands in the reactor of low pressure metalorganic chemical vapor deposition at 600°C. Preliminary characterizations of the InAs QDs have been investigated by using atomic force microscopy and photoluminescence (PL). Room temperature PL emission from the 0-dimensional system centers at 1520 nm and the full width at half maximum of the PL is 92 meV.

Full text of DOI:10.1063/1.121396

Self-organized InAs quantum dots formation by As/P exchange reaction on (001) InP
substrate
Benzhong Wang, Fanghai Zhao, Yuheng Peng, Zhi Jin, Yudong Li, and Shiyong Liu
Citation: Applied Physics Letters 72, 2433 (1998); doi: 10.1063/1.121396
View online: http://dx.doi.org/10.1063/1.121396
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/72/19?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 19
11 MAY 1998
Self-organized InAs quantum dots formation by As/P exchange reaction
on „001… InP substrate
Benzhong Wang,a) Fanghai Zhao, Yuheng Peng, Zhi Jin, Yudong Li, and Shiyong Liu
State Key Laboratory on Integrated Optoelectronics, Jilin University Region and Department of Electronic
Engineering, Jilin University, Changchun 130023, People’s Republic of China
͑Received 3 November 1997; accepted for publication 10 March 1998͒
In this letter, we present the results of InAs quantum dots ͑QDs͒ prepared on a ͑001͒ InP substrate.
As/P exchange reaction at the surface of InP buffer was used to form the InAs islands in the reactor
of low pressure metalorganic chemical vapor deposition at 600 °C. Preliminary characterizations of
the InAs QDs have been investigated by using atomic force microscopy and photoluminescence
͑PL͒. Room temperature PL emission from the 0-dimensional system centers at 1520 nm and the full
width at half maximum of the PL is 92 meV. © 1998 American Institute of Physics.
͓S0003-6951͑98͒01919-6͔
Three-dimensional ͑3D͒ confinement of carriers has re-
ceived much attention for quantum device applications as
well as for fundamental study in quantum physics.1 In order
to fabricate the structures showing the predicted quantum
effects, various techniques such as wet or dry etching, ion
beam implantation or milling, or regrowth on processed
samples have been investigated.2–5 However, these methods
result in large nonradiative recombination of carriers by
damage or impurities at the surface or the interface.
formed by using atomic force microscopy ͑AFM͒ and pho-
toluminescence ͑PL͒ spectra.
The samples used here were prepared on a ͑001͒ InP
substrate by low pressure MOCVD with a horizontal quartz
reactor. 100% arsine (AsH3), phosphine (PH3), and trimeth-
ylinium ͑TMIn͒ were used as source materials. The carry gas
was Pd-purified H2. Total H2 flow rate was 6 l /min. The
pressure of the reactor was kept at 76 Torr. After the sub-
strate treating at 650 °C under PH3 for 5 min, the InP buffer
layer with 200 nm was grown at 600 °C, and then the sample
was exposed to AsH3 at the same temperature with different
period of 2, 7, 10, and 30 s, respectively, which correspond
to samples A, B, C, and D. As/P exchange reaction occurs on
the InP surface during this stage to form InAs materials.
Subsequently, the InP cap layer with 50 nm was grown im-
mediately. To investigate a morphology of the InAs islands
formed during As/P exchange reaction, sample E without the
InP cap layer, but with reduced temperature under AsH3
from 600 to 350 °C, was performed. In all procedures the
flow rate of TMIn ͑17 °C͒, PH3, and AsH3 were 4.4
ϫ10Ϫ6, 2.0ϫ10Ϫ3, and 4.4ϫ10Ϫ4 mol/min, respectively.
The AFM measurements were performed by using
Nanoscope III ͑Digital Instruments͒. Photoluminescence
measurements were performed at room and 10 K temperature
by using a 632.8 nm line of He–Ne laser. The average exci-
tation density was about 300 mW/cm2.
As an in situ fabrication technique, Stranski-Krastanov
͑S-K͒ growth on lattice-mismatched substrates by molecular
beam epitaxy ͑MBE͒ or metalorganic chemical vapor depo-
sition ͑MOCVD͒ has been recently recognized as a promis-
ing way to eliminate such nonradiative recombination. More-
over, many reports have been presented to fabricate the
quantum dot ͑QD͒ structures by using this technique.6–10
The exchange of group V atoms on the surface of a III-V
semiconductor, due to their important function in the growth
of high quality quantum well heterostructures, has been stud-
ied by many authors.11–14 Direct evidence of the As–P ex-
change reaction on the surface of ͑001͒ InP under MOCVD
conditions has been observed by using surface photoabsorp-
tion ͑SPA͒, and it has been demonstrated that the reaction
can occur above 360 °C.12 The reaction under MBE condi-
tions has also been studied by using reflection high energy
13
electron diffraction ͑RHEED͒ and by reflectance anisot-
Figure 1͑a͒ shows the AFM image of sample E. One can
see clearly in Fig. 1͑a͒ that many large islands are formed by
the As/P exchange reaction at 600 °C as well as the process
of reducing the temperature. The mean size of the islands is
about 2000 nm, and the island shape is irregular. The lattice
mismatch between the InAs and InP is over 3%. Hence, the
formation of the InAs islands, like the S-K growth mode, is
driven by a strain of the InAs layer. However, the InAs layer
formed here is due to As/P exchange reaction rather than
deposition. The exchange reaction is easily saturated if the
reaction occurs homogeneously on the surface. However,
owing to the formation of islands, the layer in some regions
is thin enough that the reaction is not saturated and is con-
tinuous. Hence, although the time of the As/P exchange re-
action at 600 °C is only 10 s, the time of reducing the sam-
ropy spectroscopy ͑RAS͒ recently.14 The exposure of the InP
surface to As flux ͑or AsH3͒ results in the formation of a thin
pseudomorphic InAs layer on the InP surface.11–15 The struc-
tural properties of such an InAs layer have been investigated
in some detail.15 However, the previous reports have not
considered the effect of strain on the InAs layer, which will
result in the formation of InAs islands during As/P exchange
reaction. In fact, the islanding of the InAs layer plays an
important role in the As/P exchange reaction. In this letter,
we present the results of the formation of the InAs QDs on
the InP substrate by using As/P exchange reaction. Prelimi-
nary characterizations of the InAs QDs have also been per-
a͒
Electronic mail: wbzh@mail.jlu.edu.cn
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0003-6951/98/72(19)/2433/3/$15.00 2433 © 1998 American Institute of Physics
128.205.114.91 On: Tue, 17 Jun 2014 08:39:01

2434
Appl. Phys. Lett., Vol. 72, No. 19, 11 May 1998
Wang et al.
FIG. 2. The PL spectra of the InAs QDs of samples B, C, and D measured
at 10 K temperature, which were formed by using As/P exchange reaction
on the InP surface.
ture is relatively stronger; ͑2͒ the reaction can be controlled
by varying the reaction time and reaction temperature; ͑3͒
the reaction still occurs at least in the coverage of nominal 3
MLs InAs.
Figure 2 shows the 10 K PL spectra of samples B, C, and
D, respectively. For sample B as shown in the Fig. 2, three
peaks at 925, 1020, and 1440 nm are seen clearly. They can
be attributed to the InP bulk, the InAs wetting layer ͑WL͒,
which corresponds to the effects of InAs/InP strained quan-
tum well ͑SQW͒, and the InAs QDs, respectively. In this
method, like the S-K growth mode, the InAs islands also
were formed on the top of two-dimensional ͑2D͒ InAs WL.
Because the density of the InAs islands is not high enough,
the photogenerated carriers cannot be wholly captured by
InAs QDs. The recombination of some carriers in the 2D
InAs WL can be also observed at the lower temperature. The
full width half maximum ͑FWHM͒ of the PL is about 23
meV, and it is almost the same as that of the InAs/InP SQW
grown by MOCVD.16 Other samples without the InP cap
layer or the process of As/P exchange reaction, but with
other preparing conditions being constant, were also mea-
sured at 10 K temperature. However, any signal has not been
taken except for the peak at 925 nm. It confirms that the
peaks at 1440 nm and around 1020 nm are due to the As/P
exchange reaction.
FIG. 1. The AFM image of the samples without the InP caplayer: ͑a͒ After
As/P exchange reaction of 10 s at 600 °C, the sample’s temperature was
cooled down under AsH3. ͑b͒ After deposition of the InAs with nominal 3
MLs at 490 °C, the sample’s temperature was cooled down under AsH3.
Sample C ͑shown in the Fig. 2͒ also shows 1440 and
1060 nm peaks with emission from the QDs and WL respec-
tively. We have noted that the peak of the QDs is much
stronger than that of sample B. It indicates that the InAs
island density of sample C is higher than that of sample B.
With increasing island density, more and more photogener-
ated carriers relax from a 2D WL into zero-dimensional QD
levels, which results in a decrease of the PL intensity and of
the WL and an increase of the PL intensity of the QDs.
However, the QD PL intensity of sample D is lower than that
of sample C. It suggests that the bigger incoherent islands
were formed due to the longer reaction time ͑30 s͒ which
results in nonradiative recombination of carriers. On the
other hand, the peak at 1116 nm ͑1.111 eV͒, corresponding
to 4 MLs InAs WL,16 was much broaden and two shoulders
appear on the lower energy side. This indicates that the
thickness of the InAs layer is not uniform and there is a
ple’s temperature is very long ͑about 10 min from 600 to
350 °C͒. The reaction during the stage of reducing the tem-
perature makes the InAs islands very large. Figure 1͑b͒
shows the AFM image of another sample that InAs with
nominal 3 monolayers ͑MLs͒ was deposited on the InP
buffer at 490 °C, and then its temperature was lowered under
AsH3. The InAs islands ͑about 200 nm wide͒ as well as
many small islands are seen clearly, and the island shape is
round. As/P exchange reaction during the stage of reducing
the temperature still increases the InAs islands size. How-
ever, compared with sample E, the size of the islands is
much smaller even though the InAs with nominal 3 MLs was
deposited on the InP surface before reducing the tempera-
ture. It is obvious that the amount of InAs in sample E is
more than that in the sample shown in Fig. 1͑b͒. The results
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suggest that: ͑1͒ As/P exchange reaction at higher tempera-
grade of composition in the InAs/InP interface. In addition,
128.205.114.91 On: Tue, 17 Jun 2014 08:39:01

Appl. Phys. Lett., Vol. 72, No. 19, 11 May 1998
Wang et al.
2435
FIG. 4. The room temperature PL spectrum of sample C.
FIG. 3. The position of 10 K PL spectra as function of the As/P exchange
reaction time: ͑᭿͒ InAs wetting layer; ͑᭹͒ InAs quantum dots.
In conclusion, As/P exchange reaction on the surface of
InP, as a promising way, can be used to in situ fabricate the
InAs self organized QDs. In our preparation, the strongest
PL spectrum emission from the InAs QDs was obtained from
sample C, in which the time of the As/P exchange reaction
was 10 s. The room temperature PL spectrum centers at
1520, and the FMWH of the PL is about 92 meV. In addi-
tion, the PL measured at 10 K temperature suggests also the
emission from the WL. When the reaction time varies from 2
to 30 s, the peak positions change in the region of 1.327–
1.111 eV. The narrowest peak was obtained from sample B
with 7 s of As/P exchange reaction.
for the PL spectra of sample A ͑not shown here͒, there was a
very weak peak around 1440 nm, which indicates that the
density of InAs islands is very low due to the short time of
anion exchange reaction. However, the peak centered at 934
nm ͑1.327 eV͒ is strong and narrow, which can be attributed
to InAs/InP SQW with 1 ML InAs layer.16
Figure 3 shows an energetic position of the PL peak
emission from the InAs WL and QDs as a function of the
reaction time. One can see clearly in Fig. 3 that the positions
of the QD PL peaks are similar except for sample A, whose
reaction time is 2 s. This might indicate that the lateral size
of islands formed in the initial stage is large, and the island
density is very low as discussed previously. With the in-
crease of reaction time in the initial stage, the island size will
become small, and the density increases rapidly, which re-
sults in an increase of the PL intensity. However, by further
increasing the reaction time, which corresponds to the in-
creasing amount of InAs, the islands will become large
again, and some incoherent islands are formed, resulting in a
decrease of the PL intensity. The change process of the is-
lands agrees with our experiment’s results. Similar results
have also been observed by using in situ AFM measurements
while depositing InAs on GaAs substrate under MBE
conditions.17 In addition, although the islands formed in the
initial stage will change with increased reaction time, the
formation of more new islands makes the distribution of the
island size unchanged. It results in similar positions of the
QD PL. In Fig. 3, we also found that the thickness of the WL
increases with reaction time. However, the rate of increase in
the thickness will decrease with the increase in reaction time.
It suggests that the rate of As/P exchange reaction decreases
with the increase in reaction time due to the influence of the
thickness InAs WL.
The authors would like to thank the State Key Labora-
tory on integrated optoelectronics of China for their support.
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Figure 4 shows a room temperature PL spectrum of
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