Mater. Res. Soc. Symp. Proc. Vol. 891 © 2006 Materials Research Society0891-EE03-01.1

Strain compensation effect on stacked InAs self-assembled quantum dots
embedded in GaNAs layers
Ryuji Oshima, Takayuki Hashimoto, Hidemi Shigekawa, and Yoshitaka Okada
Institute of Applied Physics, University of Tsukuba
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
ABSTRACT
We have studied the effect of strain compensation in multiple stacking of InAs
self-assembled quantum dots on GaAs (001) substrates grown by atomic hydrogen assisted
RF-molecular beam epitaxy. The GaNxAs1-x material was used as a strain compensating spacerlayer. We confirmed by high resolution x-ray diffraction measurements that a 40 nm
GaN0.005As0.995 strain compensating layer provides compressive strain to compensate for tensile
strain induced by 2.0 ML InAs quantum dots. Consequently, we achieved a multiple stack ofInAs QDs up to 30 layers without formation of coalesced QDs, and the density of QDs exceeded
3×1012cm-2.

INTRODUCTION
Recently, studies on the self-assembled quantum dots (QDs) have attracted a strong attentionbecause QDs have the potential to improve the properties of optoelectronic devices such as QD
lasers [1], semiconductor optical amplifiers [2], and next generation photovoltaic device [3]. Inorder to achieve these improvements, the fabrication technique to obtain sufficient densities inactive region is required. It is well known that stacking is a powerful way of increasing thedensity. However, too many layers of stacking lead to degradation of quality because ofaccumulation of internal strain with an increase number of QDs layers.
In InAs QDs/GaAs systems, to overcome these problems, several novel techniques such as
In-flush methods [4] and columnar QD structures [5] have been reported. On the other, the strain
compensation technique has been proposed by using InGaP [6] or GaNAs [7] as strain
compensating spacer layers (SCL). However, this approach has been carried out to stack only
less than 10 QD layers and a further increase of QD stacking is required to realize a more useful
QD superlattice structure. In this letter, we propose a novel fabrication procedure in order tomultiply the number of InAs QDs on GaAs (001) substrates without degradation of size
uniformity, by strain compensation scheme using GaNAs as an embedding layer.

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EXPERIMENTAL DETAILSFor fabrication of multiple stacked self-assembled InAs QDs on GaAs (001) substrates, weemployed atomic hydrogen-assisted molecular beam epitaxy (H-MBE) [7,8] with a radio
frequency (RF) nitrogen plasma source. After native oxide desorption at 500°C by irradiating
atomic hydrogen, a 250 nm-thick GaAs buffer layer was grown at a growth rate of 1µm/h at580°C. Then, 2.0 monolayers (MLs) of InAs QD layer and 40 nm-thick GaNxAs1-x SCL wereconsecutively grown in stack from 10 up to 30 multiple cycles at 480°C. The growth rate of QD
and spacer layers were 0.1 and 1.2 µm/h, respectively. In a separate experiment, we determinedthat for the first QD layer, the average QD size was 23.8 nm in diameter, 3.1 nm in height,
13.4 % in diameter dispersion, and 1.0×1011 /cm2 in area density, respectively. The RF power
was fixed at 175 W in order to obtain 0.5 % N composition in GaNAs spacer layers. Thearsenic (As4), hydrogen, and nitrogen back pressure was set to 1.2×10-6 Torr, 5.0×10-6 Torr, and1.4×10-4 Torr, respectively.
Figure 1 shows the schematic concept of strain compensation technique in the growth ofstacked QD structure. In In(Ga)As QDs/GaAs system, the tensile strain with respect to GaAs
substrate generated by growth of QD layer can not be completely compensated after the growthof GaAs spacer layer, since the lattice constant of spacer layer is the same as substrate. Thisresidual strain affects the formation of QDs in the next stacked layer [9,10]. On the other, inIn(Ga)As QDs/GaNAs system, the lattice constant of GaNAs can be controlled to be smaller
than GaAs. Therefore, we can satisfy the strain compensation by capping InAs QDs with aGaNAs layer of suitable lattice constant.
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 scanningtransmission electron microscope (STEM). The strained state of superlattice structure in eachsample was determined by high resolution x-ray diffraction (HR-XRD).
Averagestrain

Averagestrain

HighCompensated

InAs QDs

InAs QDs

GaAs spacer layer

GaNAs spacer layer

GaAs substrate

GaAs substrate
Low

InAs / GaAs system

InAs / GaNAs system

Figure 1. Schematic concept of strain compensation techniquein stacking of self- assembled QDs.

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DISCUSSIONFirst, we measured symmetric scans around (004) reflection in ω-2θ geometry by HR-XRD
in order to investigate the strain compensation effect of GaNxAs1-x SCLs. The XRD spectra
around (004) reflection in ω-2θ geometry for 30 layers of stacked InAs QDs samples with (a)GaAs spacer layer, and (b) GaN0.005As0.995 SCLs are shown in Figure 1. Several satellite peaks
originating from superlattice structure can be observed and these spectra are characterized by thezeroth order peak located at 32.9544 ° for sample (a), and 33.03725 ° for sample (b), respectively.
The zeroth order peak for sample (a) is located at a lower angle than GaAs peak. This indicatesthat the tensile strain generated from QD layer is still remaining. Most importantly, we note thatzeroth order peak for sample (b) shows a near perfect match with GaAs peak. In this case, theaveraged lattice constant of 2.0 ML-InAs QDs/40 nm-GaN0.005As is almost matched to GaAs, i.e.
the tensile strain accumulated around each QDs layer has been successfully cancelled out by an
introducing compressive strain generated by 40nm-thick GaN0.005As0.995 SCLs.

(a)Intensity (arb. units)(b)w/o SCL(GaAs spacer layer)with SCL

32.6

32.8

33.0

33.2

33.4

ω (degree)

Figure 2. Symmetric (004) x-ray diffraction patterns for 30 stacked InAs QDs
structure (a) without and (b) with GaN0.005As0.995 SCL, respectively.

0891-EE03-01.4[001][1-10][110][1-10]

[110]
(a)

(b)

(c)

Figure 3. (a) and (b) show AFM images of 30 layer stacked InAs QDs samples covered
with GaAs and GaN0.005As0.995 SCLs, respectively. Scan size is 500 nm × 500nm. (c) shows the cross sectional STEM image of 20 layer stacked InAs QDsgrown by strain compensation technique.
The AFM images of the top QD layer of 30 stacked sample with (a) GaAs spacer layers, and(b) GaN0.005As SCLs are shown in Figure 3. The sample with no strain compensation in Fig. 3 (a)shows many coalesced islands, while sample with GaNAs SCL results in a significantly reduced
size fluctuation and better homogeneity as shown in Fig. 3 (b). We calculate a total QD densityof as high as ~3.0 ×1012 cm-2 after 30 layer stacking from Fig. 3 (b). Figure 3 (c) shows the crosssectional STEM image of 20 layers of InAs QDs embedded in GaN0.005As SCLs. This image
shows an overall good crystalline quality without any dislocation and defects. Several features
can be also noted; (1) only a few groups of QDs are vertically aligned, and (2) QD size in an
upper layer is almost the same as that in the lower layers.
An improved size uniformity of stacked QDs is also evident from the dependence ofaverage QD lateral size, height, and size fluctuation in the lateral size on stack number embeddedin GaAs spacer layers and GaN0.005As0.995 SCLs as shown in Figure 4. In the case of sample withGaAs spacer layers, as the stack number increases, dot size significantly increases and size
dispersion becomes degraded due to accumulation of internal strain caused by a build up latticemismatch. On the other, QD size in sample with GaN0.005As0.995 SCLs is nearly constant evenafter 30-layer stacking process. These results suggest that GaNAs SCLs reduce the net strain andalso acts to suppress the increase of QD size in the stacking process.

Fluctuation (%)

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30w/o SCLs
with SCLs

20Height (nm)10432Diameter (nm)28262422 0102030

Stack number (layer)

Figure 4. Dependence of average QD lateral size, height, and size fluctuation of stacked
InAs QDs samples on stack number for w/o (closed circle) and with (opencircle) GaN0.005As0.995 SCLs, respectively.

CONCLUSION
We have characterized the effect of strain compensation in multiple stacked InAs QDs
structures grown by atomic H assisted RF-MBE. Samples were fabricated with 40 nm-thickGaN0.005As0.995 for SCLs and 10 ~ 30 layers of InAs QDs were stacked on GaAs(001) substrates.
From XRD measurements, GaN0.005As0.995 SCLs provide compressive strain to compensate fortensile strain induced by 2.0 ML InAs QDs. The reduced overall strain leads not only to suppress
the generation of coalesced islands due to a build up of lattice mismatch, but also no degradationin size uniformity is observed. These results suggest that use of GaNAs SCL is powerful inrealizing high quality optoelectronic device such as long communication wavelength lasers and1eV junction in ultra high efficiency tandem solar cells.
ACKNOWLEDGMENTS
This work was supported in part by the Incorporated Administrative Agency New Energyand Industrial Technology Development Organization (NEDO) under Ministry of Economy,
Trade and Industry (METI), Japan, and Nihon Shoken Shogaku Zaidan. One of the authors (R.O)
is supported by Research Fellowships of the Japan Society for the Promotion of Science (JSPS)

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for Young scientists. We also thank Dr. K. Akahane of National Institute of Information andCommunication Technology (NICT) for valuable discussion.
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