The characteristics of InP cells processed from thin layers of InP heteroepitaxially grown on GaAs, on silicon with an intervening GaAs layer, and on GaAs with intervening Ga(x)In(1-x)As layers are described, and the factors affecting cell efficiency are discussed. Under 10 MeV proton irradiations, the radiation resistances of the heteroepitaxial cells were superior to that of homoepitaxial InP cells. The superior radiation resistance is attributed to the high dislocation densities present in the heteroepitaxial cells

Brinker, David J.

Swartz, Clifford K.

Weinberg, Irving

English

NASA Technical Reports Server

NASA Technical Memorandum 103696
/N
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Heteroepitaxial InP Solar Cells
GaAs Substrates
on Si and
Irving Weinberg, Clifford K. S_va-rtz, and D-a-vid J. Brinker
Lewis Research Center
Cleveland, Ohio
Prepared for the
5th International Photovoltaic Science and Engineering Conference
cosponsored by the Japan Society of Applied Physics, The Institute of
Electrical Engineers of Japan, and The Foundation for the Advancement of
International Science
Kyoto, Japan, November 26-30, 1990
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https://ntrs.nasa.gov/search.jsp?R=19910011079 2020-03-19T18:34:28+00:00Z

HETEROEPlTAXIAL InP SOLAR CELLS IN Si AND GaAs SUBSTRATES
Irving Weinberg, Clifford K. Swartz, and David J. Brinker
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
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SUMMARY
The characteristics of InP cells processed from thin layers of InP hetero-
epitaxially grown on GaAs, on silicon with an intervening CaAs layer and on
GaAs with intervening CaxInl_xAs layers are described and the factors affecting
cell efficiency discussed. Under 10 MeV proton irradiations, the radiation
resistances of the heteroepitaxial cells were superior to that of homoepitaxial
InP cells. The superior radiation resistance is attributed to the high dislo-
cation densities present in the heteroepitaxial cells.
INTRODUCTION
InP solar cells are prime candidates for use in the space radiation envi-
ronment. This follows from their superior radiation resistance, annealibility
and theoretically predicted air mass zero efficiencies of over 22 percent
(ref. 1). To date, AMO, total area efficiencies over 19 percent are reprodu-
cibility achieved for cells 4 cm2 in area, InP cells produced by the Nippon
Mining Corporation are presently powering a small lunar orbiting satellite and
a NASA Lewis InP solar cell module shows no degradation after 3 years in space
(refs. 2 to 4). Despite these achievements, and others, the presently high InP
substrate cost presents a roadblock to the widespread use of these cells in
space. One solution to this problem lies in producing solar cells from thin
layers of InP epitaxially grown on cheaper, more durable substrates (ref. 5).
In the present case we describe the initial results emanating from our program
aimed at processing solar cells from thin layers of InP using either Si or GaAs
as the substrate material. Additional results emerging from a program, at the
Solar Energy Research Institute (SERI), are included for completeness in
describing major U.S. programs in this area.
CELL DETAILS
The research effort includes cells processed by OMCVD on silicon sub-
strates with an intervening GaAs layer (InP/GaAs/Si), InP deposited directly on
fiaAs (InP/GaAs) and InP deposited on GaAs with intervening layers of GaxInl_xAs
(InP/GaxInl_xAs/CaAs). The InP/GaAs and InP/GaAs/Si cells were processed at
the Spire Corporation in a NASA Lewis sponsored program, while the InP/GaxInl_x
As/GaAs cells were produced by the Solar Energy Research Institute (refs. 6
and 7). Pertinent cell details are shown in figure 1, while performance param-
eters of the best cells in each category are shown in table I.
FACTORS AFFECTING CELL PERFORMANCE
As initially shown by Yamaguchi et al. (ref. 5), high dislocation densi-
ties have a drastic effect on the performance of heteroepitaxialiy grown InP
solar cells. Lattice mismatch is one major factor contributing to the high
dislocation densities. This is qualitatively illustrated in table I, where the
best performance is exhibited by the InP/GaxInl_xAs/GaAs cell. This follows
from the fact that the latter cell with x = 0.47 is lattice matched to InP
while the mismatch between InP and GaAs is ~4 percent. Efficiencies approach-
ing 20 percent are anticipated when the InP/Ga x Inl_xAs/GaAs cell is optimized.
Dislocation densities have not been determined for this latter cell. On the
other hand, it has been determined that dislocation densities for the remaining
cell configurations range between 3xlO 8 and 8x108 cm-2 (refs. 6 and 8). It is
noted that a reduction in dislocation density of several orders of magnitude is
required in order to reach efficiencies over 18 percent (ref. 5).
Results obtained from transmission electron microscopy indicate that the
dislocation density is greatest at the semiconductor interfaces and decreases
markedly as one progresses toward the cell junction (refs. 6 and 8). As seen
in table II, the values obtained for I01 and J02, the diffusion and recombi-
nation components of the cell's dark current, respectively, are consistent with
the observed dislocation density distribution in the heteroepitaxial ceils.
This follows from the observation that Jo1 and J02 are significantly greater
for the heteroepitaxial cells when compared to the values obtained for these
parameters in the homoepitaxial cell shown in table II. Furthermore the great-
est such difference occurs in I01, the diffusion component. Due to the dislo-
cation density distribution, the recombination component, which is attributable
to recombination in the depletion region is affected by a relatively low dislo-
cation density while the diffusion component is predominantly due to dark cur-
rent in the cell's high dislocation density base region.
It is believed that the reduced efficiency of theInP/GaAs/Si cell is due
to a high series resistance. In fact it was found that the series resistance
of the InP/GaAs/Si cell was 4.75 fl-cm 2 compared to 0.8 _-cm 2 for InP/GaAs.
The high series resistance of the InP/GaAs/Si cell originates from the neces-
sity to short out a reverse diode formed, during processing, by diffusion of
the n-dopant Si into the GaAs. This is accomplished by shorting out the GaAs
buffer layer in regions outside the cell's active area thus creating a high
resistance path to the cell's back contact (ref. 6).
PROTON IRRADIATIONS
To date, no lnP/GaxInl_xAs/GaAs cells were available for exposure to radi-
ation. However, the remaining cells were irradiated by 10 MeV protons to a
fluence slightly in excess of 1013 cm-2. The results, in terms of cell maxi-
mum power, are shown in figure 2. The excellent radiation resistance of these
cells is further illustrated by a comparison of the InP/GaAs cell with a homo-
epitaxial n+p InP cell (fig. 3). A similar result applies to the remaining
cell parameters (ref. 9).
Carrier removal was determined from plots of 1/C 2 versus V both before
and after irradiation. It was found that, after irradiation to a fluence of
1.1xlO 13 cm-2, the cells p-base carrier concentration decreased from a pre-
irradiation value of 3xlO 16 to lxlO 16 cm-3 hence the carrier removal rate is
1.8x103 cm-1. In comparison, the carrier removal rate for 1MeV electrons is
2.2 cm-1 (ref. 10). It should be noted that the latter value was obtained for
2
a homoepitaxial n+p InP cell (ref. 10). Aside from cell differences, con-
sidering the much higher energy and the tendency of protons to form defect
clusters, one expects relatively higher values for the present proton irradia-
tions. However, aside from these differences, the basic mechanism of carrier
removal is uncertain in both cases.
The effect of carrier removal on series resistance was determined using:
AR
S Aq
o
where L is the cell's p-base width, A is the cell area, and the sub-
scripts o and _ denote pre- and post-irradiation values, respectively.
Using L = 3 pm, A : 0.25 cm2, and 150 and 170 cm2 V-1 sec -1 for the pre- and
post-irradiation mobilities, respectively, one finds that _R s = 0.004 _.
Therefore carrier removal has a negligible effect on cell series resistance.
CONCLUSION
Although the radiation resistances of the InP/GaAs and InP/GaAs/Si cells
are significantly greater than those observed for homoepitaxial InP cells, the
present results are not intrinsic to InP. In the present case, the high dis-
location densities lead to extremely short pre-irradiation diffusion lengths.
Hence, dislocations dominate in both the pre- and post-irradiation cell per-
formance. Aside from the obvious need to reduce dislocation densities by
several orders of magnitude, it is most desirable to process the cells using
silicon substrates. In this case, in addition to considerable reduction in
cost, the greater mechanical strength of Si facilitates chemical thinning of
the resultant cell. Unfortunately Si is an n-dopant in InP and GaAs. Hence,
during the processing, diffusion of Si into the p-type buffer layer forms a
reverse diode which drastically degrades cell performance. This problem
requires solution before high efficiency heteroepitaxial InP cells can be
formed using Si substrates. Use of the p+n InP cell configuration is one
solution to this problem. This, and other possible solutions, appear to be
more workable than the quick fix obtained by shorting out the GaAs layer in
the InP/GaAs/Si cell. This procedure, although resulting in increased effi-
ciency, leads to a high cell series resistance. An additional problem although
of a more basic and academic nature, lies in determining the basic mechanisms
of carrier removal due to both proton and electron irradiations.
REFERENCES
1. Goradia, C.; Thesling, W.; and Weinberg, I.: Key Factors Limiting the Voc
of n+pp + InP Solar Cells. Twenty-First Photovoltaic Specialists Confer-
ence; Proceedings, IEEE, 1990, pp. 386-393.
2. Keavney, C.J.; Haven, V.E.; and Vernon, S.M.: Surface Recombination and
High Efficiency in InP Solar Cells. Second International Conference on
Indium Phosphide and Related Materials; Proceedings, IEEE, 1990,
pp. 435-446.
3. Yamaguchi,M., et al.: First Space Flight InP Solar Cells. Twenty-First
Photovoltaic Specialists Conference; Proceedings, IEEE, 1990, pp. 198-202.
4. Brinker, D.J.; and Weinberg, I.: Recent Results from the InP Homojunction
Cell Module on the LIPS III Spacecraft. Twenty-First Photovoltaic
Specialists Conference; Proceedings, IEEE, 1990, pp. 1167-1171.
5. Yamaguchi, M., et al.: A New Approach for Thin Film InP Solar Cells.
Sol. Cells, vol. 19, Nov. 1986, pp. 85-96.
6. Keavney, C.J., et aI.: Fabrication of n÷/p InP Solar Ceils on Sili-
con Substrates. Appl. Phys. Lett., vol. 54, Mar. 20, 1989, pp. 1139-1141.
7. Wanlass, M.g.; and Coutts, T.J.: Private Communication. Solar Energy
Research Institute, Golden, CO, 1990.
8. Pearton, S.J., et al.: Characterization of InP/GaAs/Si Structures Grown by
Atmospheric Pressure Chemical Vapor Deposition. J. Appl. Phys., vol. 65,
no. 3, Feb. 1, 1989, pp. 1083-1088.
9. Weinberg, I., et al.: Heteroepitaxially Grown InP Solar Cells. Second
International Conference on Indium Phosphide and Related Materials; Pro-
ceedings, IEEE, 1990, pp. 66-72.
10. Yamaguchi, M., et al.: Electron Irradiation Damage in Radiation Resistant
InP Solar Cells. jap. J. Appl. Phys., vol. 23, no. 3, Mar. 1984,
pp. 302-307.:
4
TABLE I.
Cell
InP/GaInAs/GaAs
I nP/GaAs
I nP/CaAs/S i
- CELL PERFORMANCEPARAMETERS
Efficiency, Voc, Jsc,
percent mV mA/cm 2
13.7 783 31.7
10.8 705 28.9
7.1 644 26
FF,
percent
75.7
72.7
58.2
TABLE I I.
Cell
- DIFFUSION
A1
InP
InP/GaAs
InP/GaAs/S i
1.03
1.27
1.26
AND RECOMBINATION PARAMETERS
A2 JOl,n
A/cm z
2.24 1.4xlO -16
2.12 6.6x10 -11
2.15 1.7xlO -10
J02,,
A/cm _
4. IxlO -9
1.0xlO -7
1.5xlO -7
InP/GaAs
n+ InP
p InP
p+ InP
GaAs Substrate
InP/GalnAs/GaAs
--N N--
n+InP
p InP
p+ InP
p+ Ga.4 7 In.53 As
p+ Gaxlnl_ x As
p+ GaAs
GaAs Substrate
Figure 1.--Heteroepitaxial InP cell detail&
InP/GaAs/Si
n+ InP
p InP
p+ InP
p+ GaAs
Si Substrate
_E 14
_o
E
r=..
o_ 10
O.
E
E
}-¢
InP/Ga.As
InP/GaAs/Si
_¢ I I I
0 1011 1012 1013
10 MeV proton fluence, cm -2
Figure 2,_11 maximum power versus 10 MeV proton
fluence.
N
1,0 m
InP/GaAs
I I I
1010 1011 1012 1013
10 MeV proton fluence, cm-2
Figure 3.--Normalized efficiencies after proton irradiation.
Report Documentation PageNational Aeronautics and
Space Administration
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM- 103696
5. Report Date4. Title and Subtitle
Heteroepitaxial InP Solar Cells on Si and GaAs Substrates
7. Author(s)
Irving Weinberg, Clifford K. Swartz, and David J. Brinker
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
6. Performing Organization Code
8. Performing Organization Report No.
E-5918
10. Work Unit No.
506-41-11
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the 5th International Photovoltaic Science and Engineering Conference cosponsored by the Japan
Society of Applied Physics, The Institute of Electrical Engineers of Japan, and The Foundation for the
Advancement of International Science, Kyoto, Japan, November 26-30, 1990.
16. Abstract
The characteristics of InP cells processed from thin layers of InP heteroepitaxially grown on GaAs, on silicon
with an intervening GaAs layer and on GaAs with intervening Gaxlnl_xAs layers are described and the factors
affecting cell efficiency discussed. Under 10 MeV proton irradiations, the radiation resistances of the heteroepitaxial
cells were superior to that of homoepitaxial lnP cells. The superior radiation resistance is attributed to the high
dislocation densities present in the heteroepitaxial cells.
17. Key Words (Suggested by Author(s))
]nP
Heteroepitaxial
Solar cells
Radiation effects
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