The performance of gallium arsenide solar cells grown on Ge substrates is discussed. In some cases the substrate was thinned to reduce overall cell weight with good ruggedness. The conversion efficiency of 2 by 2 cm cells under AMO reached 17.1 percent with a cell thickness of 6 mils. The work described forms the basis for future cascade cell structures, where similar interconnecting problems between the top cell and the bottom cell must be solved. Applications of the GaAs/Ge solar cell in space and the expected payoffs are discussed

Chang, K. I.

Iles, P. A.

Morris, R. K.

Yeh, Y. C. M.

English

NASA Technical Reports Server

N87-26419
HETEROSTRUCTURE SOLAR CELLS*
K.I. Chang, Y.C.M. Yeh and P.A. Iles
Applied Solar Energy Corporation
City of Industry, California
and
R.K. Morris
Air Force Wright Aeronautical Laboratories
Wright Patterson AFB, Ohio
This paper presents the performance of GaAs solar cells grown on the Ge substrates, in
some cases the substrate was thinned to reduce overall cell weight with good ruggedness.
The conversion efficiency of 2x2cm cells under AMO reached 17.1% with the cell thickness
of 6 mils. The experience gained in this structure will be used to increase GaAs cell
efficiency. Also the work described forms the basis for future cascade cell structures,
where similar interconnecting problems between the top cell and the bottom cell must be
solved. The details include discussion of substrate properties, growth conditions of GaAs
cells, and cell construction including possible substrate thinning. A discussion will follow
regarding applications of the GaAs/Ge solar cell in space and expected payoffs over present
solar cell technologies.
INTRODUCTION
It has been known that GaAs solar cells have higher conversion efficiency and higher
radiation resistance than Si solar cells. However, GaAs solar cells are more than twice as
heavy as Si cells. For space applications it is desirable that the solar cell is lightweight,
have high efficiency, and high radiation resistance. In order to meet all these requirements
GaAs solar cells have to be fabricated on a substrate which is not only lighter or thinner
than GaAs, but also more rugged than GaAs. Both silicon and germanium were considered as
the starting substrates on which the GaAs solar cell may be fabricated. Because of the huge
lattice mismatch between Si and GaAs the silicon substrate was ruled out. Germanium was
selected because Ge and GaAs have very close lattice constants and thermal expansion
coefficients. Also, Ge is a very rugged material so that it can be thinned to reduce the
overall cell weight without introducing any mechanical problem. In this paper, we present
the results of p on n GaAs solar cells on n-type Ge substrates.
* This work was supported by the Department of Air Force under Contract F33615-$4-
C-02403.
45
https://ntrs.nasa.gov/search.jsp?R=19870016986 2020-03-20T09:51:04+00:00Z
CELL FABRICATION
The structure of the p on n GaAs/Ge solar cell is shown in Figure I. The z n-t_pe Ge
substrate is doped with Sb and the electron concentration is in the middle of I0 -cm- . The
starting Ge wafer is approximately 7 rail thick and the crystal orientation is _ degrees off
the (I00) direction. The heteroface GaAs/AIGaAs solar cell structure was deposited by the
low pressure organornetallic vapor phase epitaxy (OMVPE) technique. First an n-type GaAs
buffer layer doped with Se was grown on top of the n-type Ge wafer. Then a p-type GaAs
emitter and a thin p-type window layer were deposited and the p-type dopant was Zn.
During the OMVPE growth the chamber pressure was 130 torr and the growth temperatures
were 720 and 680°C_ The 720°C was used to take advantage of the Ge auto-doping effect so
that the electron concentration in the initial buffer layer was so high that it eases the
electric properties at the hetero-interface between the Ge substrate and the GaAs buffer
layer. This auto-doping effect was ceased by lowering the growth temperature to 680°C.
The actual GaAs homojunction was grown at this temperature to obtain higher short-circuit
current. The typical thickness for the buffer layer 9 the emitter) and the window layer is 99
0.59 and 0.1urn) respectively.
After layer growth the front p-contact made of Au/Zn and Ag were deposited onto the
revealed p-emitter and the grid pattern was defined by the liftoff process. The multiple
layer AR coatings made of TiO and AI0 were next deposited on the window layer. At this
point the GaAs/Ge structure was further thinned to 3 mils by removing Ge from the
backside. Then the back n-contact made of Au/Ge, Ni) and Ag was deposited and sintered.
Finally) the cell was cut into size. The entire cell fabrication sequence is shown in Figure 2.
RESULTS
Doping Profiles
The doping profile of the typical GaAs/Ge solar cell was measured using a Polaron C-V
profiler. Figure 3 shows the doping profile of both the hole concentration in the emitter and
th_gelect3ron concentration in the buffer layer, iT._e hole concentration was about 1.3 x
l0 cm- . The electron _nce.r_ration was 2 x I0 cm for the first 3urn near the junction
and it increased to I x I0 cm- for the remaining 6urn. This higher electron concentration
was due to the auto-doping (out-diffusion) by Ge which is an n-type dopant in GaAs for the
OMVPE growth. The difference in the electron concentration in the buffer layer was due to
the growth temperatures. The growth temperature) Tg) was 720°C for the initial 6urn and it
was 685°C for the remaining 3urn and the p-layer.
Modeling
The reason that the GaAs/Ge cell had high open circuit voltage and lower fill-factor is
due to electric properties of the interface between the n-type Ge substrate and the n-type
GaAs buffer layer. An experiment was performed to analyze the interface properties. An
as-grown cell structure was etched in a 3:1:1 solution to remove the window layer and the p-
emitter. N-type ohmic contacts were deposited and sintered on the GaAs buffer layer and
the backside of the Ge substrate. The sample was cut into 5x5mm after sintering the n-
contacts. A typical dark I-V characteristics of the isotype heterojunction interface is shown
in Figure 4. The I-V characteristics are non-linear and the resistance of the 5x5mm sample
was approximately 2 ohms near the origin. Therefore the series resistance of the isotype
heterojunction between GaAs and Ge was high enough to result in low fill-factor.
46
Also a third terminal was made to the (n)GaAsbuffer layer of a GaAs/Ge cell to
evaluate the photovoltaic effect at the (n)GaAs/(n)Ge isotype heterojunction. It was found
that Voc was 0.035V and that the polarity of this photovoltage agreed with that of the p/n
GaAs cell. The polarity of this photovoltage suggests that the junction between the
(n+)GaAs and (n)Ge be an n-type Schottky barrier. Figure 5 shows the band structure of the
GaAs p/n junction and the (n+)GaAs/(n)Ge Schottky barrier. The degenerated (n+)GaAs acts
as a metal on the (n)Ge semiconductor. The magnitude of the barrier height affects the
collection photocurrent; i.e. Cff. The Schottky barrier height can be reduced by lowering
the resistivity of the Ge substrate. It has been verified that better CFF for GaAs/Ge cells
has been obtained by using low resistivity Ge wafers.
Light I=V Characteristics
The 2x2cm GaAs/Ge solar cells made on the 0.015 ohm-cm Ge substrates were tested
under an AMO simulator at 28°C. Figure 6 shows the light I-V characteristics of a 2x2cm
GaAs/Ge cell. The conversion efficiency of the cell was 17.1%; the open-circuit voltage
1.075V; the short-circuit current llS.9mA; and the fill-factor 0.742. The thickness of this
cell was approximately 7 mils.
Table I lists the performance of the GaAs/Ge cells fabricated in the same lot. All
cells have efficiency higher than 16% under AMO at 2g°c. At 50% cummulative yield the
cell efficiency was 16.g%; the open-circuit voltage 1.08gv; the short-circuit current
IlS.7mA; and CFF 0.722. The thickness of these cells was about 7 mils.
Contact Integrity
The contact integrity is defined by low contact resistance, good adhesion)
solderability, and weldability. The Ag-plated tabs were soldered onto the p-contact ohmic
pads of a GaAs/Ge solar cell with 16% efficiency. The light I-V curves of the cell before
and after soldering the tabs are shownin Figure 7. No degradation in electrical output of
the cell was introduced by soldering process. The soldered tabs were pulled at 45 degrees
with respect to the cell surface. The pull strength was 925 and 475 grams. The separation
between the tabs and the cell was caused by divots in GaAs. Figure g shows the
microphotograph of the large divots in GaAs after pulling off the tab on the p-contact ohmic
pad. This kind of failure mode is acceptable because the pull strength was higher than 250
grams, which was the criteria for GaAs solar cells, and because the separation was due to
large divots in GaAs.
Applications
It has been recognized that development of a lightweight GaAs solar cell is critical for
achieving arrays with specific power approaching 300 W/kg (Ref. I). The work presented
here is a major step in approaching this type of performance. With the thin GaAs/Ge solar
cell, a 50% improvement over silicon in EOL output can be realized with about the same cell
weight. Table 2 outlines the relative performance of three cell technologies) silicon,
gallium arsenide) and gallium arsenide on germanium. All values represent solar cells with
150 micron (6 mil) coverglasses in a 5 year, mid altitude orbit.
47
Further weight reduction and thermal survivability could be achieved by the use of
deposited integral coverglasses. The relatively low tensile strength of standard GaAs ceils
inhibits use of integral covers due to cell bowing (Ref. 2). Use of a germanium substrate,
with its higher strength, should allow for integral covers to be deposited on the thin GaAs
cells, eliminating the need for adhesives. Further work is required in this area to ensure
that the cover deposition temperature does not cause germanium diffusion into the GaAs
buffer layer.
Other areas which warrant further research include fabrication and processing of the
germanium substrate, larger area (5 #cm2) device fabrication, and a cell interconnect
process. Optimization of these processes would result in a planar solar array specific power
of about 290 W/kg using present array blanket technology. This value compares to about I85
W/kg for a silicon array of the same basic design.
Use of the thin GaAs/Ge solar cells would not have to be limited to planar arrays or
single junction applications. These cells could be used in concentrator arrays as well,
although their advantages over conventional GaAs cells diminishes in this configuration.
One remaining advantage) which could become significant) would be a reductiion in cost due
to the substrate. The present cost of germanium is about half that of galIium-arsenide.
Finally, the design of the thin GaAs cells lends itself well to multi junction cell applications.
The materials technology developed for this cell could be applied to an AIGaAs/Ge
monolithic multi-juncton cell design promising even higher efficiencies. In addition) the thin
cell technology would yield a lightweight multi-junction solar cell.
CONCLUSION
In conclusion) the conversion efficiency of 2x2cm GaAs/Ge solar ceils under AM, at
28°C reached 17.1% and a lot average of 16.8% has been demonstrated. It was found that
the fill factor of the cell could be improved by reducing the barrier height at the is.type
heterojunction between the Ge substrate and the (n) GaAs buffer layer.
This solar cell design presents many significant options for application to future space
power systems. The most obvious use would be in ultra lightweight arrays) where the thin
c,ell's high efficiency and radiation resistance would greatly improve power densitites. The
GaAs/Ge solar cell could also be used in concentrators, where the biggest contribution would
probably be in cell cost reduction. Additionally, the technology developed for this work
represents an important step in the development of high efficiency monolithic multi-
junction solar ceils.
REFERENCES
I. Scott-Monck, 3.; Stella) P.: Recent Developments in High Performance Planar Solar
Array Technology. Proc. 19th IECEC, Vol. 1, August 198t_.
. Fodor, 3.S.: High Efficiency Solar Panel (HESP)-Final Report. AFWAL-TR-81-2050,
June 1981.
. Wolff) G.: Kamath, S.; Vedura, G.3.; HESP-Phase II) Gallium Arsenide. AFWAL=TR=
80-2128) March 1981.
Anspaugh, B.E.; Downing, R.G: Radiation Effects in Silicon and Gallium Arsenide Solar
Cells Using Is.tropic and Normally Incident Radiation. 3PL Pub. 8#-61, September
198#.
48
. Gurev, H.S.: Integral Covers Ior Solar Cells - Interim Report. AFWAL-TR-85-2021_,
May 1985.
TABLE 1
PERFORMANCE OF 2x2cm GaAs/Ge CELLS UNDER AMO AND 28°C
INDIVIDUAL CELL PERFORMANCE
CELL Voc Isc CFF Vm EFF
# (mY) (mA) (%) (mY) (%)
12 1073 113.9 741.2 8_ 17.1
3 I101 113.1 72.0 8_; 1(.9
2 1091 116.1; 71.7 860 1(.8
6 1092 113,2 72.3 ......Ol_l l Ik6
7 1083 !16.1 71.8 8_k_ 1(.,.7
11 10911 I 13.3 71.2 821) 1(.6
1 1092 113.3 71.1 868 16.t;
9 1098 116.11 70.0 8_ I (.$
I1 1061 116.3 72.0 818 I(.Q
I0 1096 I 13.3 70._1 8t;11 16.3
'3 1080 116,* 693 828 1(.2
8 1088 113.1 69.1 8211 16.0
CUMMULATIVE CELL PERFORMANCE
Yield Voc kc CFF Vm EFF
(%) (mY) (mA) (%) (mV) (%)
8.3 1073 113.9 711,2 831; 17.1
1(.7 1088 I 13.3 73.1 8_ 17.0
23.0 1089 113.9 72.6 8J7 16.9
33.3 """nuTu 113.7 72.6 oa,"" 16.9
111.7 1088 113.8 72.11 838 1(._
_0.0 1089 113.7 72.2 833 1_.8
_8.3 1090 113.7 72.1 833 16.8
66.7 1091 113.8 71.8 833 14.8
73.0 1087 113.8 71.8 832 16.7
83.3 1088 113.6 71.7 8_3 1(.7
_1.7 1088 I 13.7 71.3 831 I(.6
100.0 1088 113.6 71.3 849 16.(,
49
TABLE 2
COMPARISON OF SOLAR CELL TECHNOLOGY PERFORMANCE
CELL TYPE
Si
GaAs
GaAs/Ge
CELL
THICKNESS
(MICRONS)
100
305
78
TOTAL
WEIGHT
(GRAMS)
0.351
0.822
0.35
"_%
BOL
15 5.4
17 9.5
18.5
EOL 3
10.4
I From Ref. 2
2 From Ref. 3
3
EOL values based on data in Ref.4; values for GaAs/Ge cells are calculated
assuming agreement with data for GaAs performance.
GRID LINES
WINDOW
ANTI-REFLECTIVE COATING
JUNCTION
BUFFER
ELEMENT
GRID (p-CONTACT)
AR COATING
WINDOW (p+)
JUNCTION (p)
BUFFER (n)
SUBSTRATE (n 4)
n - CONTACT
THICKNESS
q.II
e.!
e.i
e. qs
9
"Is
3.31
COMPOSITION
Xg -Au Zn Au
TI OxlA/2 03
Ga_ (Zn)
G_s ISe)
C,e (Sb)
Ag-AuGeNI Au
FIGURE 1
GaAs/Ge SOLAR CELL STRUCTURE
DOPANT
CONCENTRATION
(x I| II ca-]l
m
ITO I
2
e.l TO e.S
m
5O
1. Thin Ge Substrates to 7-8 Mils.
2. Prepare the surface of Gewafers.
3. GrowGaAsand AIGaAsby MOCVDtechnique.
4. Etch window layer.
5. Deposit front p-contacts.
6. Deposit ARcoating.
7. Thin GaAs/Gecells to 3 mils.
8. Deposit back n-contacts.
9. Cut cells to size.
10. Test
FIGURE 2
PROCESS STEPS FOR THIN GaAs/Ge SOLAR CELLS
19 ! • ! ! ' • 1 • !
I
E
u
v
z
o 17
r
/
/
16 ! , I, ! I I & I I !
0 1 2 3 4 5 6 7 8 9
DEPTH, X(um)
FIGURE 3
10
DOPING PROFILE OF A GaAs P/N JUNCTION GROWNON
AGe SUBSTRATE
51
SLIm
-lJ.
--sa_. II
DARK l-V CHARACTERISTICS OF A
Eg = .11
([) ) A1GaAs
1.43
(p)GaAs
OF THE
/
/
/
/
/
/
I I/
I1 5L_. El
VF ]LEIR. I;$dlv _V3
FIGURE 4
(n)GaAs/(n)Ge ISOTYPE HETEROJUNCTION.
DEVICE IS 5mm SQUARE
(n)GaAs (n+)GaAs
0.65
(n)Ge
THE SIZE
i EF
p/n
BAND STRUCTURE
FIGURE 5
OF THE GaAs/Ge
Schottky
SOLAR CELL
52
Voc- IB?5.N lsc- ll5,9ma C'YY" 74.2X [:¢Y- I?.IX
129
-I
0.0 _.5
_4.THGI_
FIGURE 6
LIGHT I_V CURVES OF A 2x2cm P ON N GaAs/Ge SOLAR CELL (#12) TESTED
UNDER AMO AND 28°C, THIS CELL IS 6 MIL THICK,
l,e
(a) (b)
120
i,"
ee
I
" ,i
DoO
Gl_t.t
W,_- llle6mv lnc= IIS.?mo ¢4rt- GO.Ok [4'(= IS.aJ_
B.5 .I .U
VOL.TRGC
Ill
Gnt.t
roe= Ile0av isc* llS.tme ¢(f- $0.|_ [((* ,IS,|k
5"
I= eo
G
.J
• •
B,I! B.5
_'Tnlt[
F!GVRE7
LIG._HTIz_.__.A_A_TER!S_T__[.CS__O[_.AGa_A§G_Z_G.__SOLAR_ELL#8 (a)BEFOREAND
(b)AFTERSOLDERINGTABSTO THE FRONTP-OHMICCQNTA_TS_
53
FIGURE 8 
MICROPHOTOGRAPHS (50X) OF ( a )  THE PAD AND (b )  THE TAB AFTER 
PULL TEST FOR CELL #8. 


Heterostructure solar cells

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