JOURNAL OF APPLIED PHYSICS 100, 083110 共2006兲

Multiple stacking of self-assembled InAs quantum dots embedded
by GaNAs strain compensating layers
Ryuji Oshima,a兲 Takayuki Hashimoto, Hidemi Shigekawa, and Yoshitaka Okada
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan共Received 6 February 2006; accepted 18 July 2006; published online 26 October 2006兲
We have investigated a growth technique to realize high-quality multiple stacking of self-assembledInAs quantum dots 共QDs兲 on GaAs 共001兲 substrates, in which GaNxAs1−x dilute nitride material wasused as a strain compensation layer 共SCL兲. The growth was achieved by atomic hydrogen-assisted
rf molecular beam epitaxy, and the effect of strain compensation was systematically investigated byusing high-resolution x-ray diffraction measurements. By controlling the net average lattice strain toa minimum by covering each QD layer with a 40-nm-thick GaN0.005As0.995 SCL, we obtained asuperior QD structure with no degradation in size homogeneity. Further, no dislocations weregenerated even after 30 layers of stacking, and the area density of QDs amounted to as high as 3
⫻ 1012 cm−2. The photoluminescence peak linewidth was improved by about 22% for QDs
embedded in GaNAs SCLs as the accumulation of lattice strain with increasing growth of QD layers
was avoided, which would otherwise commonly lead to degradation of size homogeneity andgeneration of dislocations. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2359623兴
I. INTRODUCTION

Recently, a variety of optoelectronic device applicationsof quantum dots 共QDs兲 has been demonstrated such as QDlasers,1 optical amplifiers,2 and high-speed optical switches.3
In particular, most important device characteristics such as
low threshold currents,4,5 low chirping,6 and high characteristic temperatures7 have been demonstrated in QD lasers.
Moreover, the integration of GaAs-based long-wavelength
optoelectronics is also desirable in cost-effective opticalcommunication systems as well as in the next-generation
photovoltaic devices.8
In the case of InAs QDs/GaAs system, one of the challenging technological issues today is to develop a multiple
stacking technique in order to increase the total QD density,
because the effective active volume of QDs per layer is muchsmaller than an active layer of quantum well 共QW兲. To thisend, if the host material used for burying the QDs is the sameas the substrate, then the QDs in a stacked configuration
would be, in general, accompanied by degradation of size
homogeneity due to a buildup of internal strain with increasing number of QD layers.9,10 To overcome these problems,
several techniques, such as In-flush method11 and columnar
QD structures12 have been reported. More recently, a strain
compensation growth technique was demonstrated with InPbased system,13 and stacking of up to ten QD layers has beenachieved with GaAs-based system.14,15 However, a further
increase of stacked QD layers is required in order to realize amore practical QD superlattice structure. Here, we propose afabrication method to increase the number of stacked InAs
QD layers on GaAs 共001兲 substrates without any degradation
Author to whom correspondence should be addressed; FAX: ⫹81-29-8536902; electronic mail: bk981495@s.bk.tsukuba.ac.jpa兲0021-8979/2006/100共8兲/083110/4/$23.00

in the size uniformity and optical properties. Here, the strain
compensation scheme was achieved by using GaNAs as astrain compensation layer abbreviated as SCL in the following.

II. EXPERIMENTS

For the fabrication of multiple layers of self-assembled
InAs QDs on GaAs 共001兲 substrates, we employed atomic
hydrogen-assisted molecular beam epitaxy 共H-MBE兲 with aradio frequency 共rf兲 nitrogen plasma source.16,17 After oxide
removal and surface cleaning of each GaAs substrate byatomic H irradiation at 500 ° C, a 250-nm-thick GaAs bufferlayer was grown at a growth rate of 1 ␮m / h at 580 ° C.
Then, 2.0 ML 共monolayer兲 of InAs QD layer and a40-nm-thick GaNxAs1−x SCL were consecutively grown inpairs up to 20 or 30 multiple cycles at 480 ° C. No growthinterruption was employed during the stacking process. The
growth rate of QD layer and spacer layer were 0.1 and1.2 ␮m / h, respectively. In a separate experiment, we determined that in the first InAs QD layer prior to stacking, theaverage QD size was 23.8 nm in diameter, 3.1 nm in height,
13.4% in diameter dispersion, and ⬃1.0⫻ 1011 / cm2 in areadensity. The arsenic 共As4兲, hydrogen, and nitrogen back pressures during growth were kept constant at 1.2⫻ 10−6, 5.0⫻ 10−6, and 1.4⫻ 10−4 Torr, respectively. The rf power wasvaried from 150 to 200 W in order to control the N composition in GaNxAs1−x SCLs. The growth process and surface
morphology were studied in situ by reflection high-energy
electron diffraction 共RHEED兲, ex situ by atomic force microscope 共AFM兲, and scanning transmission electron microscope 共STEM兲. The strained state of the QD superlattice
structure in each sample was determined by using highresolution x-ray diffraction 共HR-XRD兲, and the optical properties were investigated by photoluminescence 共PL兲 mea-100, 083110-1© 2006 American Institute of Physics

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具␧⬜典 =FIG. 1. 共a兲 Symmetric 共004兲 x-ray diffraction patterns for samples with 20
stacked layers of InAs QDs structure embedded in GaNxAs1−x SCLs withx = 0%, 0.3%, 0.5%, and 0.7%. 共b兲 Simulated 共solid line兲 and measured
共closed circle兲 strains in InAs/ GaNAs system perpendicular to the growthaxis on GaAs 共001兲 substrate.

surements by using a standard lock-in technique at room
temperature 共RT兲 with a cw 532 nm second harmonic generation 共SHG兲 Nd: YVO4 laser as an excitation source and aliquid nitrogen cooled InGaAs photodetector.

III. RESULTS AND DISCUSSIONFigure 1共a兲 shows the measured XRD spectra of ␻ / 2␪
共004兲 scans for samples with 20 stacked pairs of InAs QDswith x = 0% 共GaAs兲, 0.3%, 0.5%, and 0.7% of GaNxAs1−x
SCLs. Satellite peaks originating from the periodic superlattice structure can be clearly observed in each sample andthese spectra were characterized by the zeroth-order peaks asindicated by arrows in the figure, which were located at32.9544° for x = 0, 33.0004° for x = 0.3%, 33.0196° for x= 0.5%, and 33.053 77° for x = 0.7%.
The perpendicular lattice strain 具␧⬜典 accumulated aroundthe superlattice layer can be calculated bysin ␪GaAs− 1,sin共␪GaAs + ⌬␪兲

where ␪GaAs is the Bragg angle for GaAs substrate and ⌬␪ isthe angle difference between the reflection peak of GaAs
substrate and that of the zeroth-order peak. The XRD results
and other QD parameters are summarized in Table I. It wasclearly seen that the perpendicular strain 具␧⬜典 gradually reduced with increasing N composition, and most importantly,the zeroth-order peak for x = 0.5% showed a nearly perfectlattice match with GaAs. In this case, the averaged lattice
constant of InAs/ GaN0.005As0.995 almost matched that ofGaAs, i.e., the tensile strain with respect to GaAs substrateaccumulated around each QD layer has been canceled out byintroducing a compressive strain generated by a 40-nm-thick
GaN0.005As0.995 SCL. Furthermore, the small oscillation
fringes observed indicated that abrupt heterointerfaces between QD and spacer layers have been obtained. The dependence of 具␧⬜典 on N composition for samples with 20 pairs ofInAs/ GaNxAs1−x structure are shown in Fig. 1共b兲. The simulated strain curve as drawn by a solid line was calculated byusing the lattice constant of 5.6536 Å for GaAs, 6.0590 Åfor InAs, and 4.503 Å for cubic-GaN. Here, the QD layer
was approximated by 2.0 ML of uniform InAs layer for simplicity. From the simulation results, it can be seen that theperpendicular strain 具␧⬜典 became almost zero at x ⬃ 0.5%,and all the experimental data were in good agreement withthe simple simulation result. In other words, the N composition in GaNAs SCLs has been precisely controlled in ourgrowth system.
Figure 2 shows the cross-sectional STEM image for asample with 20 stacked layers of InAs QDs embedded in 共a兲
GaAs spacer layers and 共b兲 GaN0.005As0.995 SCLs. Only thetop portions of the stacked structures are shown. A larger QD
diameter of ⬃25 nm in the top layers of sample 共a兲 wasobserved compared to ⬃20 nm for 共b兲. This can be possibly
explained by a larger residual internal strain in 共a兲 than in共b兲. However, in STEM measurements, it was difficult toobtain accurate information on the strained state of QDs, andthus the compositional changes due to possible interdiffusion
at the heterointerface between QD and spacer layers werestudied. A significant outdiffusion of In and/or N atoms andhence intermixing would be commonly expected if thegrowth was performed at high growth temperatures. In sucha case, the highly strained structure would also cause a generation of dislocations,18 which in turn accelerates furtherintermixing of constituent atoms.19 Though the actual latticemismatch at the heterointerface between QD layer and GaNAs SCL was larger than that for InAs/ GaAs heterointerface, we observed no strain-induced defects or dislocations
as in Fig. 2共b兲, which would otherwise have led to an enhanced intermixing at the interface. Therefore, we believethat at least under our growth conditions, the outdiffusion ofIn and/or N atoms was minimal though further analysis will
be required.
In a stacking of QD layers in self-assembled growth,
QDs are usually vertically aligned if the spacer layer thickness is relatively thin, typically 10– 20 nm in InAs/ GaAs
system.10 However, it is also known that the strain field ex-

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TABLE I. The QD parameters for samples with 20 stacked layers of InAs QDs embedded in GaNxAs1−x spacerlayers. The mean diameter, height, density, and size fluctuation are quoted, and also HR-XRD results including
the difference ⌬␪ of diffraction angle between GaAs substrate peak and zeroth order satellite peak and net
perpendicular strain 具␧⬜典 are summarized.
20 stacked pairs of InAs QDs/ GaNxAs1−x

Diameter 共nm兲Height 共nm兲Sheet density 共cm−2兲
Total density 共cm−2兲
Fluctuation共%兲
⌬␪ 共deg兲⌬␧Single InAs
QDs

x = 0%

x = 0.3%

x = 0.5%

x = 0.7%

23.8
3.1
1.0⫻ 1011
¯13.4
¯¯

26.7
3.8
1.3⫻ 1011
2.5⫻ 1012
25.6
0.07310.001 967

25.6
2.8
1.0⫻ 1011
2.0⫻ 1012
16.4
0.02710.000 728

24.7
2.3
1.0⫻ 1011
2.0⫻ 1012
14.2
0.00790.000 210

25.4
3.2
9.0⫻ 1010
1.8⫻ 1012
15.7−0.0263−0.000 704

tends over an even longer length. The pairing probability ofthe two neighboring InAs QDs in the vertical direction is still
as high as ⬃64% as in Fig. 2共a兲, while the pairing probability was smaller and ⬃48% for strain controlled sample as inFig. 2共b兲.
Figure 3 shows the AFM images measured for the topmost QD layer of 30 layer stacked structures embedded in 共a兲
GaAs spacer layers and 共b兲 GaN0.005As0.995 SCLs. The
sample 共a兲 with no strain compensation 共uncontrolled兲
showed coalesced and large islanded structures, whilesample 共b兲 showed a significantly improved size dispersion

and homogeneity. In a separate experiment, we confirmedthat the density of large coalesced islands in a stacked QD
structure increased with increasing stacked layers and became strongly apparent after about 30 stacks or more inInAs/ GaAs system. The QD parameters in the topmost QDlayer for 20 layer stacked samples are also summarized inTable I. The QD size became smaller with increasing N composition and reached a minimum at x = 0.5% and was almost
the same as that obtained for the single layer QD sample.
Furthermore, the size fluctuation was minimal under theFIG. 2. Cross-sectional STEM image of top portion of 20 stacked layers ofInAs QDs sample embedded in 共a兲 GaAs spacer layers and 共b兲
GaN0.005As0.995 SCLs.

FIG. 3. AFM images of the top QD layer of 30 layer stacked InAs QDs
embedded in 共a兲 GaAs spacer layers and 共b兲 GaN0.005As0.995 SCLs. Scan size
is 500⫻ 500 nm2 in both cases.

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083110-4

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Oshima et al.

induced by 2.0 ML of InAs QDs. The reduced overall strain
not only resulted in suppression of generation of coalescedislands and dislocations, but also improved the PL linewidth.
Though more studies on the optical properties of multistacked QD samples are required and currently under investigation, the results reported herein would suggest that the useof GaNAs SCLs is a promising way of realizing high-quality
optoelectronic devices on cost-effective GaAs substrates,
which can be applied to long-haul optical communication
lasers as well as high efficiency multijunction tandem solarcells.
ACKNOWLEDGMENTS

FIG. 4. PL spectra measured at room temperature from 20 stacked layers ofInAs QDs embedded in GaAs spacer layers 共dashed line兲 andGaN0.005As0.995 SCLs 共solid line兲.

strain compensated condition. In Fig. 3共b兲, we calculated atotal QD density of as high as ⬃3.0⫻ 1012 cm−2 after 30
pairs of stacking.
Finally, Fig. 4 shows the room temperature PL spectrafrom a sample with 20 pairs of stacked QDs with minimized
overall strain by using GaN0.005As0.995 SCLs 共solid line兲 andfrom an uncontrolled sample with GaAs spacer layers
共dashed line兲. We observed that a single emission peak and astrong PL intensity was clearly observed in the stacked
sample, while an InAs QD sample with a single layer of dots
showed a weak PL emission at room temperature. On theother hand, by burying the QDs with GaNAs SCLs, we observed several important features such as 共1兲 PL emission
wavelength
from
InAs
QDs
redshiftedfrom1120.8 to 1144.9 nm, and 共2兲 full width at half maximum
共FWHM兲 of the emission peak was reduced from65.2 to 53.0 meV. The redshift of the emission wavelength
can be explained by the effect of strain compensation andreduction of potential barrier of InAs QDs embedded in GaNAs SCLs compared to GaAs spacer layers.20 The observed
reduction of FWHM was attributed to the improvement ofsize homogeneity in the growth direction.
IV. CONCLUSION

In conclusion, we have characterized the effect of strain
compensation in multiple stacked InAs QD structures grownby atomic H-assisted rf-MBE. The samples were fabricatedwith 40-nm-thick GaNxAs1−x which was used as a strain
compensation layer 共SCL兲, and 20 or 30 layers of InAs QDs
were fabricated on GaAs共001兲 substrates. From XRD measurements, 40-nm-thick GaN0.005As0.995 SCLs provided sufficient compressive strain to compensate for tensile strain

This work was supported in part by the Incorporated
Administrative Agency New Energy and Industrial Technology Development Organization 共NEDO兲 under Ministry ofEconomy, Trade and Industry 共METI兲, Japan, and the Thermal and Electric Energy Technology Foundation. One of theauthors 共R. O.兲 is supported by Research Fellowships of theJapan Society for the Promotion of Science 共JSPS兲 for young
scientists. The authors also acknowledge Dr. K. Akahane ofNational Institute of Information and Communication Technology, Tokyo, for valuable discussions.
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