Fabrication of Multi-layer Self-assembled InAs Quantum Dotsfor High-Efficiency Solar Cells
Ryuji Oshima*, Hiroyuki Komiyama, Takayuki Hashimoto, Hidemi Shigekawa, and Yoshitaka Okada
University of Tsukuba, Institute for Applied Physics, Tsukuba, Ibaraki 305-8573, Japan*
tel: +81-29-853-6902, fax : +81-29-853-5205, email: bk981495@s.bk.tsukuba.ac.jp
ABSTRACT: We have investigated the characteristics of quantum dot (QD) solar cells with stacked multilayers of self-organized QDs. The GaAs-based p-i-n solar cells with 20 stacked InAs QD layers were
fabricated by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) on GaAs (001) substrates, andmultiple stacking of InAs QDs was achieved by strain-compensation growth technique, in which the tensile
strain with respect to GaAs substrate induced by InAs QDs is compensated by a compressive strain
induced by GaNAs spacer layers. By controlling the net average lattice strain to a minimum by covering
each QD layer with a 40nm-thick GaN0.005As0.995 strain compensating layer (SCL), we were successful inobtaining a superior QD stacked structure with no degradation in size uniformity. Further, no dislocations
were observed even after 30 layers of stacking, and the area density of QDs amounted to the order of
12-2
10 cm . The External quantum efficiency characteristics show a spectral response up to 1150nm. The
enhanced absorption performance of the cell in the lower photon energies is attributed to InAs QDs
inserted in the i-layer.
Keywords: Quantum Dots, III-V Semiconductors, Fundamentals

1. Introduction
Recently,lowdimensional
semiconductor
structures such as quantum wells (QWs) andquantum dots (QDs) have attracted significant
attention as a possible means of exploiting the belowbandgap photons to generate additional photocurrents
beyond that corresponding to band-to-band transitionin the bulk [1-5]. In QD solar cells, dots should beperiodically distributed in space leading to theformation of an intermediate band rather than amultiplicity of discrete levels. Though efficiency
enhancements are theoretically predicted in suchintermediate band solar cells, which ideallyincorporate a three-dimensional (3D) quantum dot
superlattice in the active region [6-8], the experimental
demonstration of these cells are challenging due totechnological difficulties in the fabrication of highly
uniform and spatially-ordered QD arrays required in a
multiple stack configuration.
The most popular fabrication technique of QDs is
to take advantage of spontaneous self-assembly or
self-organization mechanism of coherent 3D islanding
during growth, known as Stranski-Krastanov (S-K)
growth mode in lattice-mismatched epitaxy. In order toachieve these improvements, the fabrication
technique to obtain sufficient densities in active region
is required. It is well known that stacking is a powerful
way of increasing the dot density. However, too manylayers of stacking lead to degradation of quality
because of accumulation of internal strain withincreasing number of QDs layers. Previously, we
reported on the preliminary results of InP-based solar
cells with 30 multi-layers of self-organized QDs [9,10].
In this work, we have carried out basic studies on thecharacteristics of GaAs-based solar cells with 20
stacked InAs QDs grown on GaAs (001) substrates.
Multiple stacking of QDs was achieved by straincompensation growth technique, in which GaNAs
alloy was used as a strain compensating spacer layer[11,12].

1-4244-0016-3/06/$20.00 ©2006 IEEE

AveragestrainAveragestrainCompensated

High

InAs QDs

InAs QDs

GaAs spacer layer

GaNAs layer

GaAs substrate

GaAs substrate
InAs / GaAs system

Low

InAs / GaNAs system

Figure 1. Schematic concept of strain compensation
technique in stacking of self-assembled
QDs.
AuZn contact
MgF2 ARC
50 nm p+- GaAs (5 ´ 1018 cm -3)
150 nm p - GaAs (2 ´ 1018 cm-3)
40 nm GaNAs

20 multi-layers of InAs
QDs and 40 nm thickGaN0.005As0.995 SCL.

40 nm GaNAs
40 nm GaNAs
40 nm GaNAs
1000 nm n - GaAs (1 ´ 1017 cm-3)
250 nm n+- GaAs (1 ´ 1018 cm-3)

GaAs (001)
In contact

Figure 2. Schematic structure of multi-stacked InAs
QD solar cells fabricated in this work.

158

2. Fabrication of multi-stacked Quantum
Dots
For successful implementation of self-assembled
QDs to practical devices including solar cells, we firststudied a fabrication technique for multi-stacking of
InAs QD superlattice structures on GaAs (001)substrates. All growths were done by atomic
hydrogen-assisted solid-source molecular beam
epitaxy (H-MBE) with a radio frequency (RF) nitrogenplasma source. Figure 1 shows the schematicconcept of strain compensation technique for thegrowth of stacked QD structures. In In(Ga)AsQDs/GaAs system, the tensile strain with respect to
GaAs substrate generated by growth of QD layer can
not be completely compensated after the growth of
GaAs spacer layer, since the lattice constant of
spacer layer is the same as substrate. This residualstrain affects the formation of QDs in the next stacked
layer [13,14]. On the other, in In(Ga)As QDs/GaNAs
system, the lattice constant of GaNAs can becontrolled to be smaller than GaAs. Therefore, we cansatisfy the strain compensation by capping InAs QD
layer with a GaNAs layer of suitable lattice constant.
Figure 2 shows the schematic structure of amultilayer stacked QD solar cell fabricated in this work.
After the substrate cleaning, 0.25 µm-thick n+-GaAs
buffer layer (n = 1 ´ 1018 cm -3 ), followed by 1 µm-thickn-GaAs base layer (n = 1 ´ 1017 cm -3 ) were grown at
580 °C. Then, 2.0 monolayers (MLs) of InAs QD layerand 40 nm-thick GaN0.005As0.995 SCL wereconsecutively grown in stack from 10 up to 30 multiple
cycles at 480°C. This stacked configuration was an
optimized condition reported elsewhere, in which thetensile strain accommodated around QDs are near
perfectly compensated by compressive strain
generated by GaNAs layer [11, 12]. The growth rates
of QD and spacer layers were 0.1 and 1.2 mm/h,
respectively. After the growth of intrinsic QD region,
0.15 µm-thick p+-GaAs emitter layer (p = 2 ´10 18 cm3), followed by 50 nm-thick p+-GaAs contact layer (p =5 ´ 1018 cm -3 ) were grown.
The properties of QDs were studied by using
atomicforce
microscope(AFM),
scanning
transmission electron microscope (STEM), andphotoluminescence (PL) measurements. The PL
measurements were done in a temperature-controlled
cryostat using a 532 nm diode pumped solid-statelaser at output power of 1 W/cm2 and liquid-nitrogencooledInGaAsphotodetector.
For
P-Vcharacterization, AuZn alloy was used for the top
contact and In for bottom contact. Standard antireflection coating (ARC) with MgF2 was employed in2
0.25 cm -sized solar cells.

Fluctuation (%)

Figure 3. AFM images for the top 30-layer stacked InAs
QDs samples embedded with (a) GaAs, and(b) GaN0.005As0.995 SCLs, respectively. Scan
size is 500 nm ´ 500 nm.

30

w/o SCLs
with SCLs

20

Height (nm)10
4
3Diameter (nm)

2
28
26
24
22 0

10

20

30

Stack number (layer)

Figure 4.

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

Figure 5. Cross sectional STEM image for 20-layer
stacked InAs QDs grown by strain
compensation technique.

3. Results and Discussion

density of as high as ~3.0 ´1012 cm-2 after 30 layer
stacking from Fig. 3 (b). An improved size uniformityof stacked QDs is also evident from the dependenceof average QD lateral size, height, and lateral sizefluctuation on stack number embedded in GaAs
spacer layers and GaN0.005As0.995 SCLs as shown in
Fig. 4. In the case of samples with GaAs spacer
layers, as the stack number increases, the dot size
significantly increases and size dispersion becomesdegraded as caused by accumulation of internal strain

First, AFM images of the topmost QD layer of 30
stacked samples with (a) GaAs spacer layers, and (b)GaN0.005As0.995 SCLs prior to embedding in p-njunction are shown in Fig. 3. The sample with nostrain compensation in Fig. 3 (a) shows a higher
coalesced island density of 3.0 ´ 109 cm-2 compared7-2
to 4.8 ´ 10 cm observed in sample with GaNAs
SCL as shown in Fig. 3 (b). Thus, use of GaNAs SCL
results in a significantly reduced size fluctuation andbetter homogeneity. We can calculate a total QD

159

due to large lattice mismatch. On the other, QD size insamples with GaN0.005As0.995 SCLs are nearly
constant even after 30-layer stacking process. These
results suggest that GaNAs SCLs reduce the net
strain and also act to suppress the increase of QD
size during the stacking process.
Figure 5 shows the cross-sectional STEM image
for sample with 20 stacked layers of InAs QDs
embedded in GaN0.005As0.995 SCLs. Only the top
portion of the stacked structures is shown. In STEM
measurements, it was difficult to obtain information on
the strained state of QDs, but the compositionalchanges due to possible interdiffusion at theheterointerface between QD and spacer layer werestudied. Although the actual lattice mismatch at theheterointerface between QD layer and spacer layer
becomeslargerthan
that
of
InAs/GaAs
heterointerface, Fig. 5 shows an improved overall
crystalline quality with no strain-induced defects anddislocations. It should also be added that interdiffusionof In and/or N atoms was not observed. In stacking
process 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 [9]. However, it is known that thestrain field extends over an even longer length. It canbe seen from Fig. 5 that the pairing probability of thetwo neighboring InAs QDs in the vertical direction isstill as high as ~48% even for GaNAs spacer layer
thickness of 40 nm. In order to achieve high efficiency
QD solar cells, three-dimensional (3D) quantum dot
superlattice is desired. However, it is not quite optimal
for high-efficiency solar cell applications andadjustments are currently undertaken.
Figure 6 shows the room temperature PL spectrafrom sample with 20 layers stacked QDs withminimized overall strain by using GaN0.005As0.995 SCLs
(solid line), and uncontrolled sample with GaAs
spacer layers (dashed line), respectively. We
observed a single emission peak with a strong PL
intensity in the stacked sample, while an InAs QD
sample with a single layer of dots showed a weak PLemission at room temperature which we measured ina separate experiment. On the other, by burying the
QDs with GaNAs SCL, we observe several importantfeatures, (1) PL emission wavelength from InAs QDs
redshifts from 1120.8nm to 1144.9nm, and (2) full
width of half maximum (FWHM) of emission peak isreduced from 65.2meV to 53.0meV. The redshift ofemission wavelength can be explained by the effect ofstrain compensation and/or reduction of potential
barrier of InAs QDs embedded in GaNAs SCLs
compared to GaAs spacer layers [16]. The observed
reduction of FWHM is clearly attributed to theimprovement of size uniformity in growth direction due
to the effect of strain compensation as discussed
herein.
Figure 7 show the external quantum efficiencies(EQEs) measured for a 20-multilayer QD solar cellwith 40 nm-thick GaN0.005As0.995 strain-compensating
spacer layers as a function of reverse bias voltage. It
can be seen that a solar cell with embedded InAs
QDs into the intrinsic region of GaAs-based p-i-n
diode do actually increase the EQE response beyond
longer wavelengths and Further, we can see a
photoabsorption peak at 910 nm which corresponds
to contribution from GaNAs spacer layer. The

Figure 6.

PL spectra measured at room temperaturefrom 20 stacked layers of InAs QDs
embedded in GaAs spacer layers (dashedline), and GaN0.005As0.995 SCLs (solid line),
respectively.

Figure. 7 EQE curves for 20 multi-layers QD solar cellwith 40 nm-thick GaN0.005As0.995 spacer layers
as a function of reverse bias voltage. A
reversebias
largely
improves
thephotoabsorption in 500 ~ 900 nm wavelength
range, but EQE in the near-infrared region is
not much affected.

enhanced absorption performance in the lower photon
energies > ~ 1050 nm is attributed to contribution fromInAs QDs, and this result is in good agreement with
PL peak wavelength in Fig. 6. On the other,photoabsorption peak at 1000nm may be emission
from localized states in GaNAs or excited state in
InAs QDs, and these are currently under investigation.
It can be further seen that the application of a reverse
bias largely affects the photoabsorption in 500 ~ 900
nm wavelength range, but EQE in the near-infrared
region is not much affected. This suggests that theelectron-hole pairs generated in InAs QDs are
extracted out of the dots and collected as output
current quite efficiently, at least in this sample
structure. The optimization of the QD structure leading
to the formation of minibands and the cell structure is160currently underway
performance.

tofurther

improvethe4. Summary
Preliminary studies on the characteristics of
GaAs-based QD solar cells with 20 multi-layers ofself-assembled InAs quantum dots are presented.
The p-i-n solar cells with stacked InAs dot layers were
fabricated by atomic H-assisted RF-MBE on GaAs
(001) substrates, and multiple stacking of dots was
successfully achieved by using GaNAs as strain
compensating spacer layer. The uniformity of dot size12
was 14% and total dot density on the order of 10
cm -2 has been achieved after stacking 20 multi-layersof InAs QDs. No dislocations were generated insamples with 40 nm-thick GaNAs spacer layers. The
EQE showed a spectral response up to 1.15 mm. The
enhanced absorption performance of the cell in thelower photon energies is clearly attributed toabsorption in InAs QDs in the i-layer. However, further
increase in EQE in the near-infrared region is desiredfor improvement of the QD solar cell, and stacking QD
structure seems to play an important role in theoverall performance of the cell.

Acknowledgements
This work was supported in part by the
Incorporated Administrative Agency New Energy andIndustrial Technology Development Organization(NEDO) under Ministry of Economy, Trade andIndustry (METI), Japan, and The Thermal and Electric
Energy Technology Foundation. One of the authors(R.O) is supported by Research Fellowships of theJapan Society for the Promotion of Science (JSPS)
for Young scientists.

References
[1]
[2]
[3]

[4]

M. Wolf, “Limitations and possibilities for
improvement of photovoltaic energy converters”,
Proc. IRE 48 (1960) 1246.
K. W. J. Barnham and G. Duggan, “A new
approach to high-efficiency multi-band-gap solar
cells”, J. Appl. Phys. 67 (1990) 3490.
N. J. Ekins-Daukes, K. W. J. Barnham, J. P.
Connolly, J. S. Roberts, J. C. Clark, G. Hill, M.
Mazzer,“Strain-balanced
GaAsP/InGaAsquantum well solar cell”, Appl. Phys. Lett. 75(1999) 4195.
Y. Okada, N. Shiotsuka, and T. Takeda,“Potentially modulated multi-quantum wells for

161

high-efficiency solar cell applications”, Solar
Energy Materials & Solar Cells 85 (2005) 143.
[5] Y. Okada and N. Shiotsuka, “Fabrication ofpotentially modulated multi-quantum well solar
cells”, Proceedings of 31st IEEE Photovoltaic
Specialists Conference, Florida (Jan. 2005).
[6] A. J. Nozik, “Quantum dot solar cells”, Physica E
14 (2002) 115.
[7] A. Luque and A. Marti, “Increasing the efficiency
of ideal solar cells by photon induced transitionsat intermediate levels”, Phys. Rev. Lett. 78 (1997)
5014.
[8] M. Green, “Multiple band and impurity
photovoltaic solar cells: general theory andcomparison t tandem solar cells”, Prog. Photovolt.
Res. Appl. 9 (2001) 137.
[9] K. Akahane, N. Ohtani, Y. Okada, and M.
Kawabe, “Fabrication of ultra-high density InAsstacked quantum dots by strain-controlled growth
on InP (311)B substrate”, J. Crystal Growth 245(2002) 31.
[10] Y. Okada, N. Shiotsuka, H. Komiyama, K.
Akahane, and N. Ohtani, “Multi-Stacking of Highly
Uniform Self-Organized Quantum Dots for Solar
Cell Applications”, Proceedings of 20th European
Photovoltaic Solar Energy Conference, June
2005, 1AO.7.6.
[11] R. Oshima, T. Hashimoto, H. Shigekawa, and Y.
Okada, “Multiple stacking of self-assembled
InAs quantum dots embedded by GaNAs strain
compensating layers” J. Appl. Phys. to be
published.
[12] R. Oshima, T. Hashimoto, H. Shigekawa, and Y.
Okada, “Strain compensation effect on stacked
InAs self-assembled quantum dots embedded in
GaNAs layers”, Material Research Society
Symposium Proceedings (Fall 2005), to be
published.
[13] G. S. Solomon, J. A. Trezza, A. F. Marshall, andJ. S. Harris, Jr., “Vertically aligned andelectronically coupled growth induced InAs
islands in GaAs”, Phys. Rev. Lett. 76 (1996) 952.
[14] Q. Xie, A. Madhukar, P. Chen, and N. P.
Kobayashi,” Vertically Self-Organized InAs
Quantum Box Islands on GaAs (100)” Phys.
Rev. Lett. 75 (1995) 2542.
[15] R. Oshima, T. Hashimoto, H. Shigekawa, and Y.
Okada, “Long wavelength InAs self-assembled
quantum dots embedded in GaNAs straincompensating layers” Physica E, in press.