of 3
GaInP / GaAs dual junction solar cells on Ge / Si epitaxial templates
Melissa J. Archer,
1,
a

Daniel C. Law,
2
Shoghig Mesropian,
2
Moran Haddad,
2
Christopher M. Fetzer,
2
Arthur C. Ackerman,
3
Corinne Ladous,
3
Richard R. King,
2
and
Harry A. Atwater
1
1
California Institute of Technology, Pasadena, California 91125, USA
2
Spectrolab, Inc., Sylmar, California 91342, USA
3
Aonex Technologies, Pasadena, California 91106, USA

Received 10 January 2008; accepted 4 February 2008; published online 11 March 2008

Large area, crack-free GaInP
/
GaAs double junction solar cells were grown by metal organic
chemical vapor deposition on Ge
/
Si templates fabricated using wafer bonding and ion implantation
induced layer transfer. Photovoltaic performance of these devices was comparable to those grown on
bulk epi-ready Ge, demonstrating the feasibility of alternative substrates fabricated via wafer
bonding and layer transfer for growth of active devices on lattice mismatched substrates. ©
2008
American Institute of Physics
.

DOI:
10.1063/1.2887904

State-of-the-art high-performance multijunction solar
cells are tandem monolithic devices. As such, these devices
are limited in their ultimate performance by lattice matching
requirements for monolithic growth processes. Though
lattice-mismatched multijunction solar cells have been
grown with metamorphic growth techniques,
1
5
they are lim-
ited to a relatively small range of lattice mismatch in the
metamorphic buffer layers.
In multijunction solar cells, monolithic epitaxial integra-
tion involves several requirements. First, all materials in the
structure must be approximately lattice matched. This en-
sures that the quality of the active regions are high enough to
enable efficient carrier extraction. In addition, the monolithic
nature requires these devices to have only two terminals, and
based on Kirchhoff’s Law, all subcells must operate at the
same current. Therefore, an ideal device would divide the
solar spectrum power between the subcells evenly. Unfortu-
nately, optimal band gaps for spectrum splitting are not gen-
erally in lattice matched materials. As a result, current
designs compromise ideal spectrum splitting to achieve lat-
tice matched devices.
In lattice matched GaInP
/
GaAs
/
Ge multijunction cells,
the Ge cell is significantly overpowered. A third junction
with a band gap of approximately 1 eV would enable signifi-
cantly higher efficiency. This is the primary motivation be-
hind the metamorphic growth efforts in multijunction solar
cells.
2
4
Improving current matching between subcells en-
hances the performance of the entire device. Thick buffer
layers are required in metamorphic growth to achieve full
relaxation of lattice mismatch induced strain enabling the
growth of high quality active regions. The current solar cell
with the world record efficiency is a metamorphic triple
junction GaInP
/
GaAs
/
Ge cell.
6
Alternatively, wafer bonding can accommodate any
amount of lattice mismatch at the bond interface because this
interface is incoherent rather than the forced coherency of an
epitaxially grown interface. Wafer bonding and layer transfer
is an enabling technology for the realization of a four-
junction solar cell with band gaps close to the calculated
optimal band gaps. A prototypical wafer bonded four-
junction
would
consist
of
GaInP
/
GaAs
/
InGaAsP
/
InGaAs

1.9
/
1.42
/
1.05
/
0.72 eV

using layer transfer and
wafer bonding to combine the GaInP
/
GaAs dual junction
grown on a GaAs or Ge template with the InGaAsP
/
InGaAs
grown on an InP
/
Si template. For this structure to be viable,
we must have Ohmic contacts at the bonded interfaces and
good quality epitaxial growth on the bonded templates.
In the present work, we will discuss progress on the top
dual junction cell grown on Ge
/
Si epitaxial templates. High
performance bottom cells grown on InP
/
Si templates and
low-resistance bonded interfaces have been discussed
previously.
7
,
8
The bonded templates were fabricated with wa-
fer bonding and ion implantation induced layer transfer.
9
,
10
This technique allows careful control of transferred film
thickness and guarantees high-quality single crystal thin
films in direct contact with the handle substrate. This is in
contrast to previous work to grow III-V devices on Si sub-
strates, which required buffer layers to minimize the defect
generation in the active devices.
11
13
The first step in fabrication of these epitaxial templates
is to implant a Ge wafer with H
+
at 180 keV and a dose of
1

10
17
cm
−2
. Next, wet chemical cleaning removes organic
and particulate contaminants from both the oxidized Si and
Ge wafers. We employ a SiO
2
bonding layer for thermal
stability of the transferred film. Just before initiating the
bond, both substrates are plasma activated. A Suss Microtech
SB-6e bonder initiates the bond at a temperature of 200 ° C.
a

Electronic mail: griggs@caltech.edu.
FIG. 1.

Color online

Optical micrographs of a full 50 mm Ge
/
Si template
made with layer transfer and wafer bonding

left

and GaInP
/
GaAs solar
cells grown on a Ge
/
Si template

right

.
APPLIED PHYSICS LETTERS
92
, 103503

2008

0003-6951/2008/92

10

/103503/3/$23.00
© 2008 American Institute of Physics
92
, 103503-1
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The bonded pair was then annealed at 250 – 350 ° C under

1 MPa pressure to induce exfoliation and strengthen the
bond between the two wafers. The Ge layer transferred to the
Si substrate is approximately 1.4

m thick. Thus far, we
have shown up to full 2 in. wafer layer transfer of Ge on Si,
as shown in Fig.
1
.
The RMS roughness of these films after layer transfer is
approximately 25 nm and the ion implantation induced dam-
aged layer extends approximately 200 nm into the film.
Removal of the damaged material and abatement of the
surface roughness are crucial to enabling high quality
epitaxial growth on these substrates. A dilute CP-4

HF : HNO
3
:CH
3
COOH

wet etch removes the damaged
layer. Touch polish with a Logitech PM5 chemical mechani-
cal polisher minimizes the surface roughness further. Final
RMS roughness of the Ge
/
Si templates is

0.5 nm. Figure
2
shows cross-sectional transmission electron microscopy

X-
TEM

images of Ge homoepitaxy on Ge
/
Si templates with
and without damage removal. Removal of the ion implanta-
tion induced lattice damage produced substrates that are vi-
able for high quality epitaxial growth.
To examine the potential of these substrates for use in
heteroepitaxy of high quality III-V materials, dual junction
GaInP
/
GaAs solar cells were grown using Ge
/
Si epitaxial
templates. Figure
3
shows a schematic of the structure.
Spectrolab performed all cell growth and processing. Light
current-voltage

I
-
V

performance was measured under
AM1.5D illumination before and after an antireflective

AR

coating was applied to the devices

Fig.
4

. The light
I
-
V
data
show comparable short circuit current between some control
devices grown on a bulk Ge substrate and some devices
grown on a Ge
/
Si template. However, open circuit voltage is
slightly lower

1.97 – 2.08 V versus 2.16 V

in the devices
FIG. 3. Schematic cross section of the dual junction solar cell grown and
processed by Spectrolab. The bonded interface is shown by a dashed line.
FIG. 4. Photovoltaic
I
-
V
curves

top

and spectral response

bottom

for the
GaInP
/
GaAs solar cells grown on Ge
/
Si epitaxial templates and on a bulk
epi-ready Ge substrate.
FIG. 2. Cross-sectional transmission electron microscopy images of Ge homoepitaxy on a Ge
/
Si template without damage removal

left

and with damage
removal

right

. The white line is at the interface of the substrate and the homoepitaxy.
103503-2 Archer
etal.
Appl. Phys. Lett.
92
, 103503

2008

Downloaded 14 Mar 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
grown on the Ge
/
Si template. Overall, the device perfor-
mance is comparable to the control with no loss in fill factor

FF

compared with the control

FF = 0.79

. After AR coat-
ing, the control cell showed an efficiency of 17.2%–19.9%,
whereas the Ge
/
Si templates had an efficiency of 15.5%–
15.7%.
Spectral response measurements

Fig.
4

indicate the
GaInP cell band gap has shifted approximately 60 meV from

1.74 to

1.8 eV. This shift in the band gap is due to a
change in GaInP composition. The Ge substrate used for the
control sample in these growths was

100

oriented with a
miscut of 6° toward the

011

orientation, whereas the Ge
wafer used to make the Ge
/
Si template was

100

oriented
with a miscut of 9° toward the

011

orientation. Higher
miscut substrates have lower In composition for the same
growth conditions.
14
Shown in Fig.
5
is the high resolution
X-ray diffraction data for the control sample and the Ge
/
Si
template sample. The scan on the control sample shows the
top cell to be compressively strained 691 s, which corre-
sponds to an indium composition of about 53%, assuming it
is 100% strained. On the other hand, the Ge
/
Si sample is
lattice matched, which corresponds to an indium composi-
tion of 49.5%. Increasing indium composition by 3.5% de-
creases the band gap by

64 meV,
15
which correlates well
with spectral response measurements.
We have demonstrated high efficiency GaInP
/
GaAs dual
junction solar cells on Ge
/
Si templates. In combination with
the results from bottom cells grown on InP
/
Si templates
and the low-resistance bonded interfaces, these results
enable the fabrication of the full four junction bonded
GaInP
/
GaAs
/
InGaAsP
/
InP on Si solar cell.
The authors would like to acknowledge the National Re-
newable Energy Laboratory through Subcontract No. XAT-
4-33624-10 for support of the Caltech and Spectrolab por-
tions of this work. One of us

M.J.A.

acknowledges
fellowship support from the National Science Foundation.
Support for TEM work was provided by the Caltech Kavli
Nanoscience Institute and Materials Science TEM facilities
supported by the MRSEC Program of the National Science
Foundation under Award No. DMR-0520565.
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FIG. 5.
High-resolution x-ray diffraction rocking curves for the
GaInP
/
GaAs solar cells grown on Ge
/
Si epitaxial templates and on a bulk
epi-ready Ge substrate.
103503-3 Archer
etal.
Appl. Phys. Lett.
92
, 103503

2008

Downloaded 14 Mar 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp