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IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012
Metamorphic GaAsP and InGaP Solar Cells on GaAs
Stephanie Tomasulo, Student Member, IEEE, Kevin Nay Yaung, and Minjoo Larry Lee, Member, IEEE
Abstract—We have investigated wide-bandgap, metamorphic
GaAs1 −x Px and Iny Ga1 −y P solar cells on GaAs as potential subcell materials for future 4–6 junction devices. We identified and
characterized morphological defects in tensile GaAs1 −x Px graded
buffers that lead to a local reduction in carrier collection and a
global increase in threading dislocation density (TDD). Through
adjustments to the graded buffer structure, we minimized the formation of morphological defects and, hence, obtained TDDs ≈
106 cm−2 for films with lattice mismatch ≤1.2%. Metamorphic
Iny Ga1 −y P solar cells were grown on these optimized GaAs1 −x Px
graded buffers with bandgaps (Eg ) as high as 2.07 eV and opencircuit voltages (Voc ) as large as 1.49 V. Such high bandgap materials will be necessary to serve as the top subcell in future 4–6
junction devices. We have also shown that the relaxed GaAs1 −x Px
itself could act as an efficient lower subcell in a multijunction device. GaAs0 .6 6 P0 .3 4 single-junction solar cells with Eg = 1.83 eV
were fabricated with Vo c = 1.28 V. Taken together, we have demonstrated that GaAs1 −x Px graded buffers are an appropriate platform for low-TDD, metamorphic GaAs1 −x Px and Iny Ga1 −y P solar cells, covering a wide bandgap range.
Index Terms—Epitaxy, GaAsP, InGaP, metamorphic.
I. INTRODUCTION
ODAY’S highest efficiency solar cells are triple-junction
concentrator devices, with each junction efficiently collecting a different portion of the solar spectrum [1]. In order to
attain concentrated efficiencies surpassing 50%, a 4–6 junction
design will likely be necessary [2]. However, these devices require top subcells with bandgap (Eg ) energies of 2.0–2.2 eV [2],
[3]. Most III–V materials in this desired bandgap range are indirect, requiring thick absorber layers to fully collect the incident sunlight. A further challenge is that many of these materials contain Al, which leads to diminished efficiency due to
T
Manuscript received July 11, 2011; revised October 21, 2011; accepted
November 8, 2011. Date of publication January 16, 2012; date of current version January 30, 2012. This work was supported in part by the National Science
Foundation CAREER program under Grant DMR-09559616. The work of S.
Tomasulo was supported by an Award from the Department of Energy (DOE)
Office, Science Graduate Fellowship (SCGF) Program. The DOE SCGF Program was made possible in part by the American Recovery and Reinvestment
Act of 2009. The DOE SCGF program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by
Oak Ridge Associated Universities (ORAU) under DOE Contract DE-AC0506OR23100. The work of K. N. Yaung was supported by the Singapore Energy
Innovation Programme Office for a National Research Foundation Graduate
Fellowship. This work was carried out in part at the Center for Functional
Nanomaterials, Brookhaven National Laboratory, which is supported by the
U.S. Department of Energy, Office of Basic Energy Sciences, under Contract
DE-AC02-98CH10886.All opinions expressed in this paper are those of the
author’s and do not necessarily reflect the policies and views of DOE, ORAU,
or ORISE.
The authors are with the Department of Electrical Engineering, Yale
University, New Haven, CT 06511 USA (e-mail: stephanie.tomasulo@yale.edu;
kevin.nayyaung@yale.edu; minjoo.lee@yale.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JPHOTOV.2011.2177640
oxygen-related defects [4], [5]. Al-containing solar cells are also
known to result in increased bandgap-voltage offsets (Wo c =
Eg /q−Vo c ) as Al content is increased [6].
Iny Ga1−y P (0.27 ≤ y ≤ 0.40) is a direct-gap, Al-free material
with 2.05 ≤ Eg ≤ 2.26 eV but is lattice-mismatched to conventional substrates such as GaAs and GaP. Thus, the Iny Ga1−y P
active region must be grown on a graded buffer to obtain material
with low threading dislocation density (TDD ≤ 106 cm−2 ) and
long minority carrier diffusion lengths. GaAs1−x Px is an ideal
graded buffer material for this case, because it spans the lattice
constants of the desired Iny Ga1−y P compositions and does not
phase separate as Iny Ga1−y P graded buffers are known to do [7],
[8]. Low-TDD GaAs1−x Px graded buffers have recently been
demonstrated by both metal–organic chemical vapor deposition
(MOCVD) [9], [10] and solid source molecular beam epitaxy
(SSMBE) [11], [12].
In this study, we demonstrate GaAs1−x Px /GaAs graded
buffers to enable the growth of low-TDD, wide-Eg Iny Ga1−y P
(0.36 ≤ y ≤ 0.43) and GaAs0.66 P0.34 solar cells. We first show
that a morphological defect, referred to here as a faceted trench
(FT), leads to locally reduced carrier collection. To minimize
this detrimental effect, we optimized the GaAs1−x Px graded
buffer structure, resulting in solar cells completely free of FTs.
The optimized GaAs1−x Px graded buffer possessed low TDDs
of 0.7 × 106 –1.6 × 106 cm−2 for 0.16 ≤ x ≤ 0.34. Using this
low-TDD GaAs1−x Px platform, we demonstrate Iny Ga1−y P solar cells with Eg ≤ 2.07 eV and Vo c as high as 1.49 V, as well as
GaAs0.66 P0.34 solar cells with Eg = 1.83 eV and Vo c = 1.28 V.
II. EXPERIMENTAL DETAILS
All samples were grown on nominally on-axis, p-type GaAs
(001) substrates in a Veeco Modular Gen-II SSMBE chamber
containing Al, Ga, In, As, P, Be, and Si sources. The group Vs
were thermally cracked at 900 ◦ C to produce dimer beam fluxes,
whereas Be and Si acted as p- and n-type dopants, respectively.
All substrate temperatures Tsub reported here were measured
with a pyrometer, and all fluxes are beam equivalent pressures
measured with a beam flux monitor.
The GaAs1−x Px graded buffers were grown at Tsub = 680 ◦ C.
We have previously shown that GaAs1−x Px graded buffers
grown at this high temperature benefit from efficient P incorporation and minimized TDD [11], [12]. Formation of the stepgraded GaAs1−x Px buffer was achieved by maintaining a constant As2 overpressure, while raising the P2 flux. We chose the
Ga flux to give a 1 μm/h growth rate at Tsub = 580 ◦ C. However, at the elevated Tsub necessary for graded buffer growth, the
sticking coefficient for Ga is no longer unity, and the growth rate
decreases to ∼0.8 μm/h. For Iny Ga1−y P cells, we lowered Tsub
to 480 ◦ C to grow the Iny Ga1−y P active region lattice-matched
to the graded buffer cap. We also decreased the growth rate to
2156-3381/$26.00 © 2012 IEEE
TOMASULO et al.: METAMORPHIC GaAsP AND InGaP SOLAR CELLS ON GaAs
Fig. 1. Microscopy of FTs in In0 . 3 9 Ga0 . 6 1 P solar cells using (a) PVTEM,
(b) EBIC, (c) Nomarski, and (d) electroluminescence imaging. The dislocations piled up near FTs lead to locally reduced current collection in EBIC and
quenched radiative recombination under forward bias.
0.5 μm/h to facilitate high doping in the active region. Further
details on both Iny Ga1−y P and GaAs1−x Px /GaAs graded buffer
growth can be found in [11] and [12].
After growth, we determined material compositions using a
Bede D1 X-ray diffractometer. We then performed photoluminescence spectroscopy to estimate Eg of our material using a
527-nm neodymium-doped yttrium lithium fluoride laser and
an Ocean Optics USB 2000 spectrometer. To observe the effect of morphological defects on dislocation glide, planar-view
transmission electron microscopy (PVTEM) was performed in
a Tecnai T-12 microscope operated at 120 kV.
TDDs in processed solar cells were estimated by planarview electron beam-induced current (EBIC) imaging using a
Helios NanoLab scanning electron microscope operated at
15 kV and 0.34 nA with a Stanford Research Systems SR-570
low-noise current amplifier. We measured the lighted current–
voltage (LIV) characteristics under approximate air mass 1.5G
conditions using an ABET Technologies LS 10500 solar simulator and a Keithley Instruments 2400 source meter. External
quantum efficiency (EQE) measurements were performed in a
PV Measurements QEX7 system.
III. RESULTS AND DISCUSSION
A. GaAs1−x Px Graded Buffer Optimization
FTs differ from the usual cross-hatch roughening encountered
in metamorphic growth and are defined by the following characteristics: [0-11] line direction, lengths of 100–1000 μm, depths
of ∼100–300 nm, and bounding facets between {1 1 3}A and
{1 1 4}A [11], [12]. FTs have previously been observed in tensile
growth on (0 0 1) substrates and have been shown to contribute
to the relaxation of elastic strain [13]. Fig. 1(a) reveals that FTs
halt dislocation glide and lead to threading dislocation pileups,
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causing the nucleation of additional threading dislocations to relax any remaining strain [14]. An additional consequence of the
threading dislocation pileups can be observed with EBIC [see
Fig 1(b)], where current collection is locally minimized near the
FT. Further evidence of the detrimental nature of FTs is given
in Fig. 1(c) and (d). Fig 1(c) shows a Nomarski micrograph of
an FT running down the center of a metamorphic In0.39 Ga0.61 P
solar cell. Applying a forward bias of 2.0 V causes the solar cell
to electroluminescence in Fig. 1(d), except near the FT, where
radiative recombination is quenched.
We previously showed that lowering the grading rate to
<0.20% μm−1 and lowering the change in P per step to 2%
for GaAs1−x Px buffers with x ≥ 0.30 minimized the FT density
(FTD) to <0.1 cm−1 [11], [12]. As noted in [11], a lower grading rate is necessary with increasing x to maintain low FTD;
at x = 0.20, a grading rate of 0.29% μm−1 is sufficient to obtain FTD < 0.1 cm−1 , while at x = 0.34, a grading rate of
0.16% μm−1 is required. An additional benefit of minimizing
FTD is a reduction in global TDD. Fig. 2 shows EBIC images of
processed Iny Ga1−y P solar cells on GaAs1−x Px graded buffers
with (a) x = 0.16, (b) x = 0.20, and (c) x = 0.34, revealing consistently low TDD for x ≤ 0.34. The TDDs of 0.7 × 106 –1.6 ×
106 cm−2 are comparable with those of recent record-breaking
metamorphic cells [15] and lower than those in MOCVD-grown
1.98–2.07 eV metamorphic Iny Ga1−y P [10]. A more complete
summary of EBIC results is given in Table I. EBIC imaging on a
GaAs0.66 P0.34 graded buffer without an In0.36 Ga0.64 P active region on top revealed a negligible change in TDD, resulting from
the additional steps associated with the In0.36 Ga0.64 P growth.
B. Metamorphic Iny Ga1−y P Solar Cells
A series of Iny Ga1−y P solar cells with different compositions
were grown on the optimized graded buffers discussed previously. GaAs1−x Px graded buffers for the metamorphic cells
were highly p-doped (NA = 1 × 1018 cm−3 ) to minimize the
effects of series resistance. Each solar cell then consisted of a
2.0-μm p-type Iny Ga1−y P:Be (NA = 1 × 1017 cm−3 ) base and
a 100-nm n-type Iny Ga1−y P:Si (ND = 1 × 1018 cm−3 ) emitter.
To increase collection of short-wavelength photons, we grew a
20-nm n-InAlP:Si (ND ≈ 5 × 1017 cm−3 ) window layer latticematched to the underlying active region. A highly doped 52-nm
n-GaAs:Si (ND = 5 × 1018 cm−3 ) contact layer was grown to
facilitate ohmic contact to the device.
We then processed 2.1 mm × 2.1 mm solar cells via
standard photolithography and wet-etching techniques. Using
e-beam evaporation, we deposited Ni/AuGe (50 Å/1000 Å) as
the n-contact and Cr/Au (100 Å/1000 Å) as the p-contact. A
schematic of the processed Iny Ga1−y P solar cells is given in
Fig. 3.
We grew metamorphic In0.36 Ga0.64 P and In0.39 Ga0.61 P solar
cells on the optimized graded buffers as well as an In0.49 Ga0.51 P
control cell lattice-matched to GaAs. The Eg of these cells were
1.90, 2.00, and 2.07 eV for In0.49 Ga0.51 P, In0.39 Ga0.61 P, and
In0.36 Ga0.64 P, respectively. Fig. 4(a) shows the LIV characteristics of the three solar cells. By increasing Eg through a decrease
in In content, we see a proportional increase in Vo c to 1.42 V
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IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012
Fig. 2. EBIC of Iny Ga1 −y P solar cells on GaAs1 −x Px /GaAs graded buffers with (a) x = 0.16, (b) x = 0.20, and (c) x = 0.34, illustrating low TDD for
optimized graded buffers over a range of GaAs1 −x Px cap compositions.
TABLE I
SUMMARY OF EBIC MEASUREMENTS OF METAMORPHIC Iny Ga1 −y P n-ON-p SOLAR CELLS WITH GaAs0 . 8 4 P0 . 1 6 , GaAs0 . 8 0 P0 . 2 0 ,
AND GaAs0 . 6 6 P0 . 3 4 GRADED BUFFERS
Fig. 3.
study.
Schematic of the Iny Ga1 −y P solar cell structures discussed in this
(In0.39 Ga0.61 P) and 1.49 V (In0.36 Ga0.64 P) from 1.31 V in the
In0.49 Ga0.51 P control cell. Fill factors (FFs) remain constant for
all three cells at ∼0.8. A summary of the LIV results is given
in Table II. Fig. 4(b) shows that the long-wavelength cutoff in
EQE trends toward shorter wavelengths with increasing Eg , as
expected. A slight degradation in EQE is observed for short
wavelengths (λ). We attribute this to increased reflectivity for
λ < 370 nm as Ga-content is increased [16]. The addition of an
antireflection coating should mitigate this loss of EQE at short-λ
for increased Ga-content.
Dark IV measurements reveal an ideality factor of ∼2 for both
the metamorphic and lattice-matched Iny Ga1−y P solar cells. The
high ideality factor for both the lattice-matched and metamorphic cells indicates that both are dominated by recombination
in the depletion region, despite the wide disparity in TDD. We
Fig. 4. (a) LIV of In0 . 4 9 Ga0 . 5 1 P (E g = 1.90 eV), In0 . 3 9 Ga0 . 6 1 P (E g =
2.00 eV), and In0 . 3 6 Ga0 . 6 4 P (E g = 2.07 eV) solar cells. As In content is
decreased, E g increases, causing a proportional increase in V o c . (b) EQE
of In0 . 4 9 Ga0 . 5 1 P (E g = 1.90 eV), In0 . 3 9 Ga0 . 6 1 P (E g = 2.00 eV), and
In0 . 3 6 Ga0 . 6 4 P (E g = 2.07 eV) solar cells.
TOMASULO et al.: METAMORPHIC GaAsP AND InGaP SOLAR CELLS ON GaAs
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TABLE II
SUMMARY OF In y Ga 1 −y P SOLAR CELL LIV RESULTS
Fig. 6.
Schematic of the GaAs0 . 6 6 P0 . 3 4 solar cell structure.
The increase in Wo c versus Eg in the MM-InGaP cells is instead
attributed to an increase in TDD from 2 × 106 –8 × 106 cm−2
due to nonideal dislocation glide kinetics [10]; note from
Table I that these TDDs are 1.3–11.4 times higher than that
of this study. In contrast, due to our constant TDD with increasing Eg and lack of Al, our cells have a constant, yet slightly
higher Wo c versus Eg . We expect that the incorporation of a
back surface field (BSF) [17], [18] and improvements in the
Iny Ga1−y P growth conditions could help lower our Wo c overall, since we are neither limited by increasing Al content nor by
increasing TDD with increasing Eg .
A comparison of our cells with those in [6] and [10] reveals
a much steeper increase in Vo c versus Eg , as a result of the
constant Wo c with Eg [see Fig. 5(b)]. If this trend continues, we
can expect to surpass the Vo c of the widest Eg AlInGaP cells as
we increase beyond 2.15 eV.
C. GaAs0.66 P0.34 Solar Cells
Fig. 5. (a) Comparison of W o c for this study (red diamonds) with previously
published AlInGaP [6] and MM-InGaP [10] data (black squares). Dotted line
represents the King et al. radiative limit for W o c [6]. While W o c for this study
is slightly higher, it remains constant with E g , presumably due to the lack of
Al and low TDD. (b) Comparing V o c for this study (red diamonds) against the
same previously published cells in (a) (black squares) reveals a much steeper
trend in V o c versus E g as a result of our constant W o c .
therefore believe that improvements in the growth conditions
and solar cell structure could lead to better performance in both
cases and will investigate this in future work.
In Fig. 5(a), we compare Wo c (=Eg /q-Vo c ) of our metamorphic Iny Ga1−y P solar cells with those of published AlInGaP [6]
and metamorphic InGaP (MM-InGaP) solar cells [10]. Lower
Wo c is desirable as it indicates larger quasi-Fermi level splitting
and, thus, larger Vo c . King et al. showed that the theoretical
radiative limit for Wo c is ∼0.4 V for a wide range of bandgaps
[see the dotted line in Fig 5(a)] [6]. Due to increasing Al content
in the AlInGaP cells, Wo c increases from 0.42 to 0.52 V with
increasing Eg , indicating a degradation in material quality [6].
In addition to serving as a low-TDD platform for metamorphic Iny Ga1−y P growth, we hypothesized that the GaAs1−x Px
graded buffer cap could potentially act as a lower subcell in a
future multijunction device. As proof of this concept, we grew
a GaAs0.66 P0.34 solar cell with Eg = 1.83 eV, utilizing the optimized graded buffer described previously. The active region consisted of a 2.0-μm p-GaAs0.66 P0.34 :Be (NA = 1 × 1017 cm−3 )
base and a 100-nm n-GaAs0.66 P0.34 :Si (ND = 3 × 1018 cm−3 )
emitter. It should be noted that the GaAs0.66 P0.34 solar cells lack
the window and contact layers present in the Iny Ga1−y P solar
cell structure. As a result, we increased the doping of the emitter
such that it could also act as a contact layer. We added a BSF
by grading the doping between the highly doped graded buffer
and the lower doped base. The GaAs0.66 P0.34 solar cells were
processed using the same techniques and contact metals as in the
Iny Ga1−y P case. Fig. 6 gives a schematic of the GaAs0.66 P0.34
solar cell structure.
Fig. 7 shows the LIV characteristics of the GaAs0.66 P0.34
solar cell. The Vo c for this 1.83 eV solar cell is 1.28 V, yielding a Wo c of 0.55 V. The decrease in Wo c , compared with
the Iny Ga1−y P solar cells, may stem from the addition of a
BSF. These GaAs0.66 P0.34 cells have an FF of ∼0.8 but exhibit
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Fig. 7.
IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 1, JANUARY 2012
LIV characteristics of the GaAs0 . 6 6 P0 . 3 4 solar cell.
relatively poor current collection due to the lack of a window
layer. Considering the high Vo c and FF obtained in these cells,
we conclude that metamorphic GaAs1−x Px could act as an efficient lower subcell in a metamorphic multijunction solar cell.
IV. CONCLUSION
We have demonstrated the growth of metamorphic Iny Ga1−y P
with Eg as high as 2.07 eV through the optimization of tensile
GaAs1−x Px graded buffers on GaAs. By minimizing FTD in the
tensile graded buffers, we achieved metamorphic Iny Ga1−y P
and GaAs1−x Px solar cells with TDD comparable with recordbreaking metamorphic solar cells. Building on our optimized
graded buffer, we produced 2.00 and 2.07 eV Iny Ga1−y P solar
cells with Vo c = 1.42 and 1.49 V, respectively. We expect further
improvements in Vo c through the addition of a BSF and optimized Iny Ga1−y P growth conditions. Moreover, we have found
that low-TDD, metamorphic GaAs1−x Px could potentially act
as a lower Eg subcell in a multijunction device. The work
presented here shows that SSMBE-grown GaAs1−x Px graded
buffers are an ideal platform for metamorphic GaAs1−x Px and
Iny Ga1−y P, providing two potential subcells in future 4–6 junction solar cells.
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ACKNOWLEDGMENT
The authors would like to thank J. Simon, M. Steiner, and
F. Camino for their assistance.
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Stephanie Tomasulo (S’11) received the B.S. degree
in physics and the M.S. degree in electrical engineering from Rensselaer Polytechnic Institute, Troy, NY,
in 2008 and 2009, respectively. She is currently working toward the Ph.D. degree in electrical engineering
with Yale University, New Haven, CT.
While working toward the M.S. degree, she was
funded through a National Science Foundation GK12 Fellowship, which allowed her to mentor two local high school physics classes. In 2010, she accepted
the Department of Energy Office of Science Graduate
Research Fellowship. Her research interests include compound semiconductor
materials and devices for energy-related applications.
Ms. Tomasulo received the Best Student Poster Presentation at the North
American Molecular Beam Epitaxy Conference in 2010 and was nominated
for the Best Student Presentation Award at the 2011 Photovoltaic Specialists
Conference.
TOMASULO et al.: METAMORPHIC GaAsP AND InGaP SOLAR CELLS ON GaAs
Kevin Nay Yaung received the B.Eng. degree in engineering Science from the National University of
Singapore, Singapore, in 2010.
He is currently with the Department of Electrical Engineering, School of Engineering and Applied
Science, Yale University, New Haven, CT. His research interests include epitaxial growth, integration,
and characterization of III–V materials on Si for photovoltaic devices.
Mr. Yaung received the National Research Foundation Graduate Fellowship given by the Singapore
Energy Innovation Programme Office in 2010.
61
Minjoo Larry Lee (M’08) received the B.Sc. degree (Hons.) in materials science and engineering
from Brown University, Providence, RI, in 1998
and the Ph.D. degree in electronic materials from
the Massachusetts Institute of Technology (MIT),
Cambridge, in 2003.
From 2003 to 2006, he was a Postdoctoral Researcher with the Microsystems Technology Laboratory, MIT. From 2006 to 2007, he was a Research
Engineer with the Center for Thermoelectrics Research, RTI International, Durham, NC. In 2008, he
joined Yale University, New Haven, CT, as an Assistant Professor of electrical
engineering. He is the author or coauthor of more than 70 technical papers and
refereed conference proceedings and holds six patents. His previous research
has been on chemical vapor deposition growth techniques and high-mobility
strained Si, SiGe, and Ge field-effect transistors. His current research interests include III–V molecular beam epitaxy, photovoltaic materials, and self-assembled
nanostructures.
Dr. Lee is a member of the Minerals, Metals and Materials Society, the
Materials Research Society (MRS), and the American Vacuum Society. He
has received numerous recognitions, including the Defense Advanced Research Projects Agency Young Faculty Award, the National Science Foundation
CAREER award, the MRS Gold Award for graduate student research, and the
IEEE Electron Devices Society George E. Smith Award.