ARTICLE IN PRESS

Journal of Crystal Growth 301–302 (2007) 821–824
www.elsevier.com/locate/jcrysgro

MBE growth of InAs self-assembled quantum dots embedded inGaNAs strain-compensating layers
Takayuki Hashimoto, Ryuji Oshima, Hidemi Shigekawa, Yoshitaka OkadaInstitute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
Available online 29 December 2006

Abstract
We fabricated self-assembled InAs quantum dots (QDs) embedded with a combined set of a GaNAs strain-compensating layer (SCL)
and an InGaAs strain-reducing layer (SRL) by using atomic hydrogen-assisted molecular beam epitaxy (H-MBE) with a RF plasma
nitrogen source. By inserting an InGaAs SRL between the InAs QD layer and GaNAs SCL, we were able to achieve a signiﬁcant
improvement of optical properties. A 1:3 mm-range photoluminescence (PL) emission was clearly obtained at 300 K from QDs embedded
in a 5 nm-thick In0.15Ga0.85As SRL followed by a 20 nm-thick GaN0.008As0.992 SCL. Further, PL emission from the ﬁrst excited state ofQDs was also observed under a high excitation intensity of 20:0 W=cm2 .
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 structures; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials

1. Introduction
Self-assembled quantum dots (QDs) have been of greatinterest because of their potential applications to highperformance lasers [1], single electron transistors [2], andhigh-efﬁciency solar cells [3]. Recently, InAs/GaAs QDs
have been intensively investigated for the low-cost andhighly efﬁcient QD laser diodes operating at the opticalﬁber communication wavelength. However, the opticalemission from InAs/GaAs QDs often appears at awavelength of 1:0 mm, due to the strong three-dimensional quantum conﬁnement as well as compressive strain
accommodated around the QDs. In order to realize a longwavelength emission, several methods to partially reduce
the effect of compressive strain have been reported, whereInAs QD layer was embedded in InGaAs [4], and GaAsSb[5] strain-reducing layers (SRLs). In the case of using ahigh In or Sb composition in SRL, however, one has totake into account of the effect of critical thickness, which
Corresponding author. Tel./fax: +81 29 853 5293.

E-mail address: okada@ims.tsukuba.ac.jp (Y. Okada).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2006.11.059

would lead to the generation of dislocations and asigniﬁcant degradation of optical properties.
In our previous work, we proposed InAs self-assembled
QDs structures embedded in GaNAs strain-compensating
layers (SCLs) [6]. Since GaNx As1x alloy has a smaller
lattice constant than GaAs, it is possible to reduce thenet average strain by inducing a tensile strain by using
GaNAs. Further, by replacing the more common GaAs
embedding layer with GaNAs SCL of a suitable N
composition, we were able to observe, (1) photoluminescence (PL) peak shifting toward longer wavelengths,(2) a stronger PL intensity compared to the sample
embedded in GaAs, and (3) a decreased density of nonradiative recombination centers. However, increase of N
composition would generally result in a degradation ofoverall crystal quality of GaNAs, and also increases thelattice mismatch at the heterointerface. In this work, we
propose to insert an InGaAs SRL between the InAs QD
layer and GaNAs SCL in order to decrease the latticemismatch at the heterointerface. We show remarkably
improved PL characteristics, and a peak emission at 1:3 mm
was observed at RT.

ARTICLE IN PRESS822T. Hashimoto et al. / Journal of Crystal Growth 301–302 (2007) 821–824

2. Experimental procedure

3. Results and discussion

Self-assembled InAs QD structures were grown on GaAs
(0 0 1) substrates by atomic hydrogen-assisted molecularbeam epitaxy (H-MBE) with a RF plasma nitrogen source.
Atomic H was generated in passing the hydrogen gas byusing a cracking cell with tungsten ﬁlament heated at1500  C by direct current. After the oxide removal andsurface cleaning of GaAs substrate by atomic H irradiation
at 500  C for 30 min, a 300 nm-thick GaAs buffer layer wasﬁrst grown at a growth rate of 1 mm=h at 580  C. Next, asingle layer of 2.0 monolayers (MLs) of InAs QD wasdeposited at a growth rate of 0:1 mm=h at 480  C. Then,four different types of embedding layers were used to bury
the QDs as shown in Fig. 1 in order to investigate the effectof strained state of QD layer on the optical properties, (a) a
40 nm-thick GaAs layer as a reference, (b) a single layer of
20 nm-thick GaN0.016As0.984 SCL, (c) a single layer of5 nm-thick In0.25Ga0.75As SRL, and (d) a combined set of a
5 nm-thick In0.15Ga0.85As SRL and a 20 nm-thick GaN0.008As0.992 SCL. Finally, a 40 nm-thick GaAs cappinglayer was grown in all samples. Atomic H was irradiatedduring the entire growth sequence in order to obtain an
atomically smooth surface and passivation of defect centers
in GaAs and GaNAs [7]. The RF plasma power was set at200 W. The growth rate of GaNAs layer was changed from
1:0 mm=h to 0:5 mm=h in order to control the N composition, since N composition is known to vary in inverse
proportion to the growth rate [8]. No growth interruption
was employed during growth, and the arsenic (As4),hydrogen and nitrogen back pressures were set to
1:4  106 , 4:5  106 and 1:5  104 Torr, respectively.
The growth mode was monitored in situ by reﬂection highenergy electron diffraction (RHEED) and the surface
morphology was studied ex situ by atomic force microscope (AFM). The PL measurements were performed
within a closed cycle helium cryostat, and the signals weredetected by using a standard lock-in technique with a cw
532 nm second harmonic generation (SHG) Nd:YVO4 laser
as an excitation source and a liquid nitrogen-cooled
InGaAs photodetector.

Prior to covering the QDs with a capping layer, weconﬁrmed that the average size of InAs QDs was 35.3 nm
in diameter and 7.3 nm in height with an area density of3:7  1010 =cm2 . A good size uniformly in the diameter witha dispersion of only 15% was obtained in our sample [6].
Fig. 2 shows the PL spectra measured at 77 K for InAs
QDs embedded with a (a) GaAs, (b) GaN0.016As0.984 SCL,(c) In0.25Ga0.75As SRL, and (d) combined set of In0.15Ga0.85As SRL and GaN0.008As0.992 SCL, respectively. The
excitation intensity was 2 W/cm2. In Fig. 2(b)–(d), weobserved a long wavelength emission at around 1:20 mmcompared with 1:03 mm in (a). However, the PL intensities
were strongly dependent on the actual structure ofembedding layers. A redshift of PL peak for the samplewith GaNAs SCL as of Fig. 2(b) can be explained by areduction of the net strain and potential barrier heightaround QDs. However, a decrease of PL intensity waspossibly due to the effect of both a larger lattice mismatchin the heterointerface between InAs QDs and GaNAs SCL
and a poor crystalline quality of GaNAs material. We
previously reported that increasing the N composition
above 0.8% in GaNAs SCL would result in a monotonous
degradation of PL properties [6]. In the case of Fig. 2(c),the observed redshift was likely due to a reduction of thelocal strain accommodated around QDs by use of anIn0.25Ga0.75As SRL. By using a combined set of In0.15Ga0.85As SRL and GaN0.008As0.992 SCL, a strong PLemission at 1:2 mm was achieved as shown in Fig. 2(d). Theshoulder peak observed at a higher energy was from thecontribution of an excited state as will be discussed later.
The narrower full width at half maximum (FWHM) ofFig. 1. Schematic structure of QD samples fabricated in this work.

Fig. 2. PL spectra measured at 77 K for InAs QDs embedded with (a)GaAs (reference), (b) GaN0.016As0.984 SCL, (c) In0.25Ga0.75As SRL, and(d) In0.15Ga0.85As/GaN0.008As0.992, respectively.

ARTICLE IN PRESS
T. Hashimoto et al. / Journal of Crystal Growth 301–302 (2007) 821–824

45.5 meV for (d) was obtained by a Gaussian ﬁtting for theground state emission compared to 57.1 meV for (b) and64.4 meV for (c), respectively. Furthermore, a 1:3 mm PLemission at 300 K with a FWHM of 41.1 meV was observed
for the sample (d) as shown in Fig. 3, while the other three
showed much weaker PL emission.
Next, we evaluated the effect of embedding layer
structure on radiative emission efﬁciency of InAs QDs.
Fig. 4 shows the temperature dependence of PL integral
intensity for the sample with (a) GaAs, (b) GaN0.016As0.984
SCL, (c) In0.25Ga0.75As SRL, and (d) In0.15Ga0.85As/

Fig. 3. PL spectrum measured at 300 K for InAs QDs embedded with acombined set of In0.15Ga0.85As/GaN0.008As0.992 layers.

823

GaN0.008As0.992, respectively. The experimental data canbe ﬁtted with the following equation:I ¼ I 0 =f1 þ C 1 expðE 1 =kB TÞ þ C 2 expðE 2 =kB TÞg(1)which is characterized by the two thermally activatedprocesses. kB is Boltzmann constant, and I 0 , C 1 and C 2 are
the constants independent of temperature. The activationenergy E 1 shows the thermal escape of carriers from theQDs to the potential barrier, while E 2 shows the escape ofcarriers from the QDs to the shallow levels induced by
defects and dislocations at the heterointerface. C is known
to be proportional to the density of non-radiativerecombination centers [9]. It can be seen that good ﬁtting
curves were obtained and Table 1 summarizes the ﬁtting
parameters E 1 , E 2 , C 1 and C 2 for each sample, respectively. It can be seen that the use of GaNAs SCL waseffective for reducing the value of C 1 compared to the caseof GaAs capping as of (a), which was due to the effect ofstrain-compensation. However, a lower E 1 and higher C 2
were observed as a result of the reduction of potential
barrier and larger lattice-mismatch at the heterointerface.
On the other, the use of InGaAs SRL showed a marginal
reduction of C 1 and E 1 compared to GaAs capping. By
using the combined set of InGaAs/GaNAs layer, we wereable to obtain a higher E 1 value of 140 meV and lower C 1 .
Therefore, we conﬁrmed that a combination of InGaAs
SRL/GaNAs SCL signiﬁcantly improves the opticalproperties of InAs QDs. The optical properties of QDs
depend strongly on the factors such as the barrier height,
strain effect, and strain-induced interdiffusion.
Here we should also consider the effect of both theinterdiffusion at the interface and the segregation ofnitrogen leading to a high quantity of nitrogen being
accumulated toward the surface [10]. However, we thinkthese effects are much smaller than the effect of local andnet average strain, and the interdiffusion should be
minimal because no growth interruption was employedduring the growth of QDs and embedding layers. A partial
reduction of compressive strain around QDs by use of
InGaAs SRL as well as a reduction of net average strain byusing GaNAs SCL are both important in improving theoptical properties of InAs QDs.
Finally, we investigated the excitation intensity dependence of PL characteristics for the sample embedded withTable 1
Activation energies E 1 , E 2 and C 1 , C 2 constants obtained from the ﬁttingby using Eq. (1) for each sample

Fig. 4. Temperatures dependence of PL integral intensity for sample with(a) GaAs, (b) GaN0.016As0.984 SCL, (c) In0.25Ga0.75As SRL, and (d)In0.15Ga0.85As/GaN0.008As0.992, respectively. The ﬁtted curves are shown
as solid lines.

Cap layer structureE1(meV)

C1E2(meV)C2(a) GaAs(b) GaN0.016As0.984/GaAs(c) In0:25 Ga0:75 As/GaAs(d) In0.15Ga0.85As/GaN0.008As0.992/
GaAs1157011014025 000
1200
10 0002500

20202525

1510.3

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T. Hashimoto et al. / Journal of Crystal Growth 301–302 (2007) 821–824

which can be attributed to an improved quality of theheterointerface and reduced non-radiative recombination. Thus, the use of InGaAs SRL/GaNAs SCL isconsidered a powerful way to improve the optical properties of InAs QDs.
4. Summary
In summary, we investigated the optical properties ofInAs self-assembled QDs embedded in InGaAs/GaNAs
layers on GaAs (0 0 1) substrates by atomic H-assisted RFMBE. From PL measurements, a strong emission ataround 1:3 mm with a narrow linewidth of 41.1 meV at
300 K was achieved by using InGaAs/GaNAs combined
layer. We attribute these improvements to the reduction of
non-radiative recombination due to reduction of both localand net strain. Though more studies are required such asthe effect of interdiffusion, the results found here may bevery useful in realizing high-quality long-wavelength lasers
on cost effective GaAs substrates.
Fig. 5. Dependence of PL characteristics on excitation intensity forsample embedded with a combined set of In0.15Ga0.85As/GaN0.008As0.992
layers. The excitation intensity was varied from 0.2 to 20.0 W/cm2. S-peak
indicates an emission from the ground state, while p-peak shows that fromthe ﬁrst excited state of QDs, respectively.

InGaAs/GaNAs combined layer and the results are shownin Fig. 5. The excitation intensity was varied from 0.2 to20:0 W=cm2 . We clearly observed an emission from the ﬁrstexcited state (p) as the excitation was increased. Though
not shown, the emission from the excited state was notobserved in the sample with a single layer of GaNAs SCLor InGaAs SRL even at a high excitation intensity of20.0 W/cm2. The energy spacing between the ground state(s) and p-state was about 66.6 meV, which agreed well withthe reported value for a typical InAs/GaAs system [11].
The appearance of a p-state indicates that the use of
In0.15Ga0.85As/GaN0.008As0.992 embedding layer increases
the recombination lifetime and hence eases the state ﬁlling,References[1] Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939.
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