ARTICLE IN PRESS

Physica E 32 (2006) 77–80
www.elsevier.com/locate/physe

Long wavelength InAs self-assembled quantum dots embedded inGaNAs strain-compensating layers
Ryuji Oshima, Takayuki Hashimoto, Hidemi Shigekawa, Yoshitaka Okada
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
Available online 7 February 2006

Abstract
We investigated the effect of GaNAs strain-compensating layers (SCLs) on the properties of InAs self-assembled quantum dots (QDs)grown on GaAs (0 0 1) substrates. The GaNAs material can be used as SCL thereby minimizing the net strain, and thus is advantageous
for multi-stacking of InAs QDs structures and achieving long wavelength emission. The emission wavelength of InAs QDs can be tuned
by changing the nitrogen (N) composition in GaNAs SCLs due to both effects of strain compensation and lowering of potential barrierheight. A 1:3 mm photoluminescence emission at 77 K was clearly observed for sample with GaN0:024 As0:976 SCL. Further, we observed animprovement of optical properties of InAs QDs by replacing the more popular GaAs embedding layers with GaNAs SCLs, which is a
result of decreasing non-radiative defects owing to minimizing the total net strain.
r 2006 Elsevier B.V. All rights reserved.
PACS: 78.67.Hc; 81.15.Hi; 81.16.Dn
Keywords: Quantum dots; MBE; Strain compensation; GaNAs

1. Introduction
Self-assembled and self-organized quantum dots (QDs)have been extensively investigated for future applications
to high-performance QD lasers [1], single electron transistors [2], and high-efﬁciency compound solar cells [3]. Longemission wavelengths of 1.3 and 1:55 mm have been thefocus of intense research for application to future opticalﬁber communication systems. In the case of InAs
QDs/GaAs systems, the optical emission appears usuallyaround 1.0–1:1 mm due to a strong three-dimensional
quantum conﬁnement as well as compressive strain
accommodated around the QDs. To realize long wavelength emission from InAs QD structure, InGaAs [4,5] andGaAsSb [6] strain-reducing layers (SRLs) have been
recently used to bury InAs QDs. Recently, long wavelength
emission from QDs without SRLs, such as InGaSb QDsand In(Ga)AsN QDs, have been also proposed [7,8].
However, the accumulation of strain with multi-stacking
is still a limiting factor. On the other hand, if InAs QDs
Corresponding author. Tel./fax: +81 29 853 6902.

E-mail address: bk981495@s.bk.tsukuba.ac.jp (R. Oshima).
1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physe.2005.12.067

were embedded in GaNAs strain-compensating layers(SCLs), one can expect long wavelength emission while
retaining a high optical property as a result of net strain
reduction [9]. Consequently, multi-stacking of InAs QD
structure becomes possible [10,11]. In this work, we reporton the properties of self-assembled InAs QDs embedded in
GaNAs SCLs grown on GaAs (0 0 1) substrate.

2. Experiments
We fabricated self-assembled InAs QDs embedded in
GaNx As1x SCLs on GaAs (0 0 1) substrates by atomic
H-assisted RF molecular beam epitaxy (H-MBE) [12]. It ispossible to obtain higher quality Ga(In)NAs materials byatomic H irradiation during growth [13]. First, each GaAs
substrate was subjected to cleaning in the growth chamberat 580  C for 30 min with atomic H irradiation in order toobtain an atomically ﬂat surface. Then a 300 nm-thickGaAs buffer layer was grown at a growth rate of 1mm=h.
After lowering the substrate temperature to 480  C, 2.0
monolayers (MLs) of InAs QD layer were deposited at
0:1 mm=h. Subsequently, 20 nm-thick GaNx As1x SCL andARTICLE IN PRESS78R. Oshima et al. / Physica E 32 (2006) 77–80

20 nm-thick GaAs cap layer were further grown. Thegrowth rate of GaNAs layer was varied from 0.2 to
1:5 mm=h in order to control the N composition. The RFpower and N2 ﬂow rate were set to 200 W and 1.0 sccm,
respectively. The RF plasma was generated before theonset of QDs layer growth, and GaNAs SCL was grown
immediately after the QDs without growth interruption. In
this work, we varied the N composition of GaNAs SCL
from 0.15% to 2.4%. No post growth annealing wasperformed. The As4 and H2 back pressures during thegrowth were 1:2  106 and 5:0  106 Torr, respectively.
The growth process and surface morphology were studiedin situ by reﬂection high-energy electron diffraction(RHEED), and ex situ by using atomic force microscopy(AFM). The photoluminescence (PL) was measured byusing a standard lock-in technique at 77 K with the second
harmonic of a YAG laser at a wavelength of 532 nm as theexcitation source and InGaAs photodetector.
3. Results and discussion
Fig. 1 shows the AFM image of InAs self-assembled
QDs on GaAs (0 0 1) substrate without the top capping
layer. The average QD diameter, height and area density
were 35.3 nm, 7.3 nm, and 3:7  1010 cm2 , respectively.
This image shows that a good size uniformity in thediameter with dispersion of only 15%.
Fig. 2 shows the PL spectra at 77 K for single InAs QD
layers embedded in GaNx As1x SCLs with varying N
composition, (a) x ¼ 0% (GaAs cap), (b) 0.4%, (c) 1.6%,and (d) 2.4%, respectively. We also summarized the PLpeak wavelength as a function of N composition of GaNAs
SCL in the inset. First, PL peak is immediately red-shifted
upon replacing the GaAs embedding layer with GaNAs

Fig. 1. AFM image of typical InAs QD structure grown on GaAs (0 0 1)
substrate. Scan size is 1 mm  1 mm.

Fig. 2. PL spectra at 77 K from InAs QDs embedded in GaNx As1x SCLs
with varying N composition. Inset shows center peak wavelength as afunction of N composition.

SCL, which is primarily due to the effect of strain
compensation. Second, PL peak shifts to longer wavelength with increasing N composition. This is due tolowering of quantum energy levels as the barrier height ofGaNAs SCLs is reduced. The band gap energy of GaNAs
as a function of N composition is also plotted as a referencein the inset of Fig. 2. It can be seen that the slope shows agood ﬁt to that of experimental results in the higher N
composition (0.8–2.4%) range. Since the addition ofnitrogen increases mainly the conduction band offset [14],lowering of quantum energy level is expected mainly tooccur at the conduction band. Here, in order to investigate
interface diffusion of N atoms into InAs QDs layer, we
performed post-growth anneal at 600  C for 5 min in N2
ambient. The emission wavelength from InAs QDs showedslight blueshift. This suggests that the diffusion of In
atoms to GaNAs layer occurs more easily than that of N
atoms to InAs QDs layer. Therefore, we attribute the main
mechanism of redshift to strain compensation effect and/orreduction of barrier height. Further, a 1:3 mm PL emission
at 77 K was clearly observed for sample with 2.4% nitrogencomposition of GaNAs SCL. Therefore, the emission
wavelength of InAs QDs is shown to be controllable bychanging the N composition of GaNAs SCLs. But further
investigation is still needed in order to achieve longer
wavelength of 1:55 mm range.
Fig. 3 plots the integral PL intensity and full-width at
half-maximum (FWHM) as a function of N compositionof GaNAs SCL. We notice that PL intensity becomesstronger up to 0.8% compared to GaAs cap ðx ¼ 0Þ,
although the potential barrier height becomes lower.ARTICLE IN PRESS
R. Oshima et al. / Physica E 32 (2006) 77–80

79Fig. 3. Integral PL intensity (open square) and full width at halfmaximum (FWHM) (circle) as a function of N composition of GaNAs
SCL.

This indicates that generation of defects near the QDs iseffectively suppressed due to reduction of total latticestrain. However, PL intensity starts to decrease withincreasing of N composition up to 2.4% as a result of,(1) lowering of potential barrier height of GaNAs SCLs,
and/or (2) degradation of GaNAs material quality
caused by phase separation and localization of N atoms,
which become strongly apparent at low growth rates inGa(In)NAs matrix. On the other hand, the FWHM
decreases with increasing N composition up to 0.8%. This
improvement suggests that QD shape and size homogeneity are modiﬁed by the net strain compensation by covering
the InAs QDs with GaNAs SCLs. Similar improvement of
optical characteristics has been observed by other growthmethods [9].
To compare the radiative emission efﬁciency of InAs
QDs covered by GaAs and GaNx As1x , the PL properties
were evaluated as a function of temperatures from 20to 250 K. Fig. 4 shows the temperature dependence on
PL intensity from InAs QDs covered by (a) GaAs,(b) GaN0:004 As, and (c) GaN0:016 As. The experimental
data can be ﬁtted with the equation below,
I ¼ I 0 =f1 þ C expðE a =kB TÞg,(1)

where E a and kB are the activation energy of conﬁned
carriers and Boltzman constant, and I 0 and C are theconstants independent of temperature, respectively. C is
proportional to the density of non-radiative recombination
centers and it is deﬁned as the ratio of the probability of
non radiative to radiative recombination process at hightemperatures [15]. Although the activation energy ofthe sample with GaNAs SCLs obtained from the ﬁts areslightly smaller than that of GaAs capped sample(120 meV), which is due to lowering of potential barrierheight, we note that C constants related to the nonradiative recombination probability becomes smaller by aFig. 4. Temperature dependence of PL intensity from InAs QDs coveredby (a) GaAs, (b) GaN0:004 As, and (c) GaN0:016 As, respectively.

factor of 4–30. Therefore, the optical quality of the InAs
QDs buried in GaNAs SCLs is much improved byadopting net strain compensation growth technique.

4. Summary
In summary, we investigated the effect of GaNAs SCLs
on the optical properties of InAs self-assembled QDsgrown on GaAs (0 0 1) substrates. By replacing the GaAs
embedding layer with GaNAs SCL, (1) PL peak shifts
toward longer wavelength and 1:3 mm emission wassuccessfully obtained from InAs QDs embedded inGaN0:024 As SCL, (2) PL intensity becomes stronger up
compared to GaAs cap peak, although actual potential
barrier becomes lower, and (3) density of the non-radiative
recombination centers is decreased for samples embeddedwith GaNAs SCLs. These features are as a consequenceof strain compensation. Therefore, GaNAs SCLs may
be useful in realizing high performance long wavelengthdevices.

Acknowledgement
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), Japan,and Nihon Shoken Shogaku Zaidan. One of the authors(R. Oshima) is supported by Research Fellowships of theJapan Society for the Promotion of Science (JSPS) forYoung scientists.

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