High conversion efficiency and low weight are obviously desirable for solar cells intended for space applications. One promising structure is GaAs on Ge. The advantages of using Ge wafers as substrates include the following: they offer high efficiency by forming a two-junction tandem cell; low weight combined with superior strength allows usage of thin (3 mil) wafers; and they are a good substrate for GaAs, being lattice matched, thermal expansion matched, and available as large-area wafers

Bajgar, C.

Geoffroy, L. M.

Hart, R. E., Jr.

Haven, Victor E.

Lillington, D. R.

Tobin, S. P.

Vernon, S. M.

English

NASA Technical Reports Server

High Efficiency GaAs-Ge Tandem Solar Cells Grown by MOCVD 
S.M. Vernon, S.P. Tobin, C. Bajgar, V.E. Haven, and L.M. Geoffroy 
Spire Corporation 
Bedford, MA 01 780 
D.R. Lillington 
Specimlab, Inc. 
Sylmar, CA 91318 
R.E. Hart, J r .  
NASA Lewis Research Center 
Cleveland, 0 H 411 35 
Introduction 
High conversion efficiency and low weight are obviously desirable for solar cells intended for 
space applications. One promising structure is GaAs on Ge. The advantages of using Ge wafers as 
substrates include the following: 
0 high efficiency: by forming a two-junction tandem cell 
0 low weight: superior strength allows usage of thin (3 mil) wafers 
0 good substrate for GaAs: is lattice matched, thermal expansion matched, and available as 
large-area wafers. 
Experimental 
The cell structure studied in this work is shown schematically in figure 1. The GaAs and GaAlAs 
layers are grown by metalorganic chemical vapor deposition (MOCVD) in a SPI-MO CVDTM 450 
system. This reactor deposits onto five two-inch diameter or three three-inch diameter wafers per 
run, with a typical thickness uniformity of &4% over the entire batch. The substrates used in 
this work are two-inch Ge wafers, 0.008-inch thick, n-type, with a resistivity of 0.014 ohm-cm. The 
surface preparation before GaAs growth consists of scrubbing the surface with a modified commercial 
cleanser, etching the wafer in a NH~OH-HZO~-HZO mixture, rinsing in deionized water and a final 
blow dry in filtered Nz. The bottom cell (Ge p-n junction) is formed by in-diffusion of Ga and As 
from the GaAs layer growth. The As diffuses faster than the Ga, but since Ga has a higher solid 
solubility at the growth temperature, a p-type Ge top layer is formed. Measured and calculated Ga 
and As profiles in the Gc are shown in figure 2. The Ge junction profile is controlled by the time- 
temperature schedule used during growth of the top cell. The basic MOCVD growth parameters 
used are listed in table 1. The Ge wafers, before being subjected to growth of the GaAs top cell, are 
eREcf.xiisG PAGE BLANK NOT FILMED 
167 
https://ntrs.nasa.gov/search.jsp?R=19890015350 2020-03-20T01:43:27+00:00Z
first coated on the backside with MOCVD-grown GaAs to  minimize autodoping effects. The device 
fabrication tests are listed in table 2, and the cell size used in this work is 2 cm x 2 cm. 
Ge Autodoping 
The high vapor pressure of Ge causa the vapor transport of Ge atoms from hot, exposed Ge 
surfaces (typically the back of the wafer) to the growing film. In the MOCVD growth of GaAs 
on Ge these atoms are incorporated into the GaAs a8 donors, which can cause major problems by 
compensating the acceptors in the p type  regions of the cell (emitter, window, and cap layers). A 
straight-forward solution to  this problem is to coat the back of the Ge wafers before growing the 
cell structure. We have found that a few microns of MOCVD-grown GaAs is an effective coating 
for this purpose. The results of one of our early studies of this problem are displayed in figure 3. 
The inset shows a typical Polaron doping profile of autodoped GaAs grown on Ge. The doping level 
at  the GaAs surface is not a function of how thick the grown layer is. This figure shows that the 
autodoping level for GaAs on Ge is a strong function of growth temperature, but can be reduced 
dramatically by the GaAs back-side coating. 
Solar Cell Results 
The efficiency-measurement I-V curve for our best GaAs-Ge cell is shown in figure 4. This 
measurement was performed at  NASA Lewis Research Center through the courtesy of R.E. Hart, 
Jr. The 1-sun AM0 efficiency value of 21.7% is the highest reported to  date for a GaAs-Ge cell 
and for a two-terminal monolithic tandem cell. The AM1.5 1-sun measurement of this cell at  Spire 
yields an efficiency of 24.3%, and at SERI yields a value of either 24.4% or 20.2% depending on the 
solar simulator used. These data point out the extreme sensitivity of tandem cell efficiency to the 
exact spectrum of the incident radiation, and improved methods of testing tandem cells are currently 
being developed at the national laboratories. 
Discussion 
To understand the operation of these cells, we first examine the quantum-efficiency curve, figure 
5. The data were measured without light or voltage biases. The spectral response is identical to 
that of a good GaAs cell, with negligible response indicated from the Ge. The Ge cell does not 
contribute because at  wavelengths beyond 870 nm the high-quality GaAs junction is turned off, 
blocking current generated in the Ge cell. (In cases where the GaAs junction is short-circuited, 
we do see a long-wavelength response characteristic of Ge). The Ge junction, in contrast, is leaky, 
as indicated by its low V, (about 0.2 V), and does not interfere with measurement of the GaAs 
response. Efforts to  measure the Ge response by light-biasing the GaAs cell have not been successful 
yet, but are continuing. We believe that the Ge cell is in fact generating a large photocurrent, and 
this accounts for the relatively high measured fill factor. If the GaAs and Ge cells were seriously 
current-mismatched, then we could not measure both high short circuit current and high fill factor. 
168 
We have obtained several types of data showing the existence of a p/n junction in the Ge wafer, 
supporting our tandem cell hypothesis. These include spreading resistance doping profiles of the Ge, 
cross-sectional EBIC line scans, and temperature coefficient measurements. 
Figures 6 and 7 show doping profiles in the Ge substrate determined by spreading resistance 
after growth of the GaAs cell. A highly-doped n-type region extends about 6 microns into the Ge 
substrate, which has a background doping of about 5 x 10'7cm-3. The n-type doping profile is 
caused by rapid in-diffusion of arsenic during growth of the GaAs cell. A more detailed look at the 
profile near the GaAs interface (figure 7) reveals a very shallow but clear p-n junction in the Ge. As 
explained earlier, the p-type doping is from diffusion of Ga, which over-dopes the n-type As profile. 
This is the most direct evidence possible of a Ge p-n junction. 
We have also detected the positions of the GaAs and Ge junctions by Electron Beam Induced 
Current (EBIC) line scans of cross-sectioned cells. Figure 8 shows such a line scan superimposed 
on a simple diagram of the sample. The two peaks in the EBIC scan correspond to the GaAs and 
Ge junctions. From the scale on the original micrograph, these occur at  depths below the surface of 
about 0.7 and 6.2 microns respectively. (Note that the junction depth is about 0.2 microns shallower 
in the finished cell of figure 8 compared to the as-grown material of figures 6 and 7, due to removal 
of the GaAs contact layer and addition of the antireflection coating). This data would indicate a 
junction depth within the Ge of about 1 micron, larger than what was obtained from spreading 
resistance. At this time we do not understand the discrepancy. However, the important point is 
that we do see a second junction within the Ge wafer by both techniques. As a point of interest, 
it was necessary to forward bias the cell to 775 mV in order to detect the Ge response. This is 
the same problem encountered in the quantum efficiency measurement: unless the GaAs junction is 
turned on, its high resistance blocks current generated in the Ge. 
Temperature coefficient measurements made at  SERI also imply a GaAs/Ge tandem. Table 
3 summarizes the temperature coefficients of efficiency, V,,, I,,, and FF from 10 to 8OOC. Data 
are shown for the GaAs/Ge cell of figure 1 and for a GaAs/GaAs control cell. The temperature 
coefficient of the GaAs/Ge cell is more than twice that of the the GaAs cell, because Ge, with its 
small band gap, is much more temperature sensitive than GaAs. The V,, values extrapolated to 
zero Kelvin are 1.519 V for the GaAs cell and 2.299 V for the GaAs/Ge cell. The difference is 
roughly the band gap of Ge, again as expected for a series tandem. 
Given that we have a series-connected monolithic tandem cell of GaAs and Ge, it is somewhat 
surprising that the tandem does not appear to be seriously current-limited by the Ge subcell. An 
earlier report of a mechanically-stacked tandem achieved only 7.86 mA/cm2 from the Ge cell at  
AM1.5 [ref.l]. However, our calculations show that if all light absorbed in a single pass were collected 
in the Ge, a maximum one-sun AM0 photocurrent of 38.7 mA/cmZ (at 25OC and 39.7 mA/cm2 at 
75OC) is possible under a GaAs filter. The fact that our best cells are not current limited implies 
very good collection in the Ge subcell (or a very red-rich simulator). 
We have seen indications of Ge current-limiting in some cells, particularly under AM1.5 meit 
surement conditions. The reason is the infrared absorption bands in the AM1.5 spectrum, which do 
not affect the GaAs cell but reduce the current of the Ge cell. Figure 9 shows the efficiency curves 
of the same cell measured on two different simulators at  SERI. The V,, and J,, are nearly identical, 
but the fill factors are very different. The curve with the dent in the knee is for the simulat*or with 
less infrared light, giving a lower Ge photocurrent. The reason that I,, is not affected is that the Ge 
cell is leaky, and breaks down at  very low reverse voltages. Even though the Ge cell is driven into 
reverse bias by the current mismatch, it causes little voltage loss. 
169 
To summarize our understanding of the high-efficiency GaAs/Ge cell, a p n  junction is created 
in the Ge substrate as part of the GaAs cell growth process. This junction is in series with the GaAs 
junction, creating a monolithic tandem structure. The Ge junction is leaky enough in the reverse 
direction that it does not limit the short-circuit current of the series-connected tandem. However, 
it can limit the fill factor if the short-circuit current of the Ge subcell is too low. Finally, since the 
polarities of the GaAs and Ge junctions are the same, their open circuit voltages add. 
Summary 
A 4 cm2 GaAs-Ge two-terminal monolithic tandem cell grown by MOCVD has demonstrated 
21.7% efficiency measured at NASA (1-sun, AMO) and 24.3% measured at Spire (1-sun, AM1.5). The 
existence of a p-n junction in the Ge has been confirmed by spreading resistance probe, EBIC, and 
temperature coefficient data, and is explained by the diffusion properties of Ga and As in Ge. The 
top cell-bottom cell interconnect is accomplished by the GaAs-Ge tunnel junction which appears to  
be adequate for one-sun operation. The testing methodology for determining the absolute efficiency 
of twcFtermina1 tandem cells needs improvement. Finally, the use of Ge substrates has been shown 
to be a viable route to achieving high-efficiency solar cells for space applications. 
Reference 
[ 11 L.D. Partain, M.S. Kuryla, R.E. Weiss, R.A. Ransom, P.S. McLeod, L.M. Fraas and J.A. Cape, 
“26.1% Solar Cell Efficiency for Ge Mechanically Stacked Under GaAs,” J. Appl. Phys., 62, pp. 
3010-3015, 1987. 
Tab le  1. Growth Parameters 
Dopants : 
P r e s s u r e  : 
Main Flow: 
Growth Rate: 
Temperature:  
V-111 R a t i o :  
TMGa, TMA1, AsH3 
1 atm. 
5 slpm H2 
4 pm/hr  
700 O C 
15: 1 
Table 2. Process  Sequence f o r  GaAs-Ge So la r  C e l l  
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
11. 
1 2 .  
Remove G a A s  Back Coating 
Evaporate Back Contact (AuAg) 
Alloy Back Contact 
Image-reversal Photoli thography f o r  Front Grid 
Evaporate Front Grid (CrAuAg) 
L i f t o f f  
S i n t e r  Contacts 
Plate Metal (Au) 
Photoli thography f o r  Mesa Etch 
Mesa Etch 
S e l e c t i v e  Etch t o  Remove G a A s  Cap 
Evaporate AR coa t ing  (ZnS + MgF2) 
Table 3. Temperature Coef f i c i en t s  (ppm/deg C )  of Ef f i c i ency ,  
Voc , Is, , and FF Measured f o r  G a A s / G e  and G a A s / G a A s  
s o l a r  Cells 
Pa ramet er G a A s  /Ge G a A s / G a A s  
oc V 
I S C  
FF 
Ef f 
-3702 
+ 445 
- 175 
-3067 
-1669 
+ 246 
- 29 
-1466 
171 
n 
c9 
I 
0 
E 
z 
I- 
Q: 
I- z 
W 
0 
z 
0 
0 
d 
II! 
Q: 
Q: a 
0 
U 
0 
a 
10l8- 
10’’- 
LAYER 
FRONT CONTACT 
CONTACT LAYER 
0 
=n-TYPE 
o=p-TYPE 
I I I I I 
p GaAs 
AR COATING 
WINDOW 
EMITTER 
-- 
BASE - n GaAs 
BUFFER n+GaAs 
p+Ge ____-------- ‘-I- EMITTER -- 
SUBSTRATE --__I 1 n G e  
BACK CONTACT 
Figure 1. Structure of GaAs-Ge Tandem Solar Cell 
THICKNESS (pm) 
4.0 
0.3 
0.16 
0.03 
0.5 
2.6 
1.7 
- 1  
200. 
4.0 
M0587-3 
AlGaAs CELL 
SPREADING RESISTANCE PROBE DATA 
DEPTH (microns) 
Figure 2.  Doping Profile in a Ge Substrate af ter  Growth of an AlGaAs Solar 
Cell a t  80OOC. 
172 
m 
- k I 
z 
z a 
a 
6 
W z 
/ 
I o" L- 600 640 680 ~ 
720 760 800 
GROWTH TEMPERATURE ('C) 
Figure 3. Autodoping of GaAs from Ge Substrate. 
VOLTAGE (V)  
Figure 4. GaA8-Ge Tandem Cell Efficiency Measurement at NASA-Lewis. 
173 
1.0 I I I I I I I 
0.8 
0.6 
0.4 
0.2 
0.0 
300. 400 500 600 700 800 900 1000 
Wavelength (nm) 
Figure 5. Measured Spectral Response of GaAs-Ge Tandem Cell. 
' I  
I '  
I 
~ 12 
Figure 6. Doping Profile of the Ge Substrate after Growth of a GaAs solar 
Cell, Obtained by Spreading Resistance. The heavy n-type doping is 
caused by in-diffusion of arsenic from the GaAs. 
174 
GaAs Ge 
I 
Figure  7. Spreading Res is tance  Probe P r o f i l e  of G e  Bottom Cell .  
I 
Ge 
BOTTOM 
CELL 
I 
I 
I 
N I P  
I 
I 
TOP CELL 
I 
N I I  
I 
J / 1 I
GaAs 
MEASUREMENT COURTESY OF R. MATSON, SERl 
h v  - 
4- 
Figure 8. E B I C  Line Scan on Cross-Section of GaAs-Ge Tandem Cell. 
175 
S P I R E  G a A s  on Ge. X 2 5  new bulb. X T l O .  188-2 
120.0 I I I I I I I I I I 1 I I 
t 
100.0 
8 0 . 0  
h a 
E - 
8 0 . 0  
Y 
C 
&I 
c 
L 
J 
0 
40.0 
2 0 . 0  
: 
X T i O  102.3 1.209 77.5 2 4 . 4  
X 2 5  New Bulb 102.3 1.206 64.3 20.2 
(NASA AM0 120.84 1.225 78.7 21.7) \ 
\ 
0.0 I I I I I I I I I I I R I I 
0 N t rn m 0 v 
0 0 0 0 0 r( 8.4 r( 
'u 
Voltage (volts) 
Figure 9. AM1.5 Efficiency Curves of a GaAs/Ge Solar Cell Measured on Two 
Different Simulators at SERI. 
176 


High efficiency GaAs-Ge tandem solar cells grown by MOCVD

High efficiency GaAs-Ge tandem solar cells grown by MOCVD

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server