The n/p homojunction GaAs cell is found to be more radiation resistant than p/nheteroface GaAs under 10 MeV proton irradiation. Both GaAs cell types outperform conventional silicon n/p cells under the same conditions. An increase temperature dependency of maximum power for the GaAs n/p cells is attributed largely to differences in Voc between the two GaAs cell types. These results and diffusion length considerations are consistent with the conclusion that p-type GaAs is more radiation resistant than n-type and therefore that the n/p configuration is possibly favored for use in the space radiation environment. However, it is concluded that additional work is required in order to choose between the two GaAs cell configurations

Hart, R. E., Jr.

Swartz, C. K.

Weinberg, I.

English

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i,^, per
1. Weinberg, C.K. Swartz, and R.E. Hart, Jr.
Le>4s Research Center
Cleveland, Ohio
(NASA-T11-87136) PERPORN&NCE AND TEMPERATURE
DEPENDENCIES OF ESOTON IRRADIATED n/p GaAs
AND n/p SILICON CELLS (NASA) 10 p
HC A02/1!P A01	 CSCL 09C
M86-12509
Unclas
G3/33 27675
Prepared for the
Eighteenth Photovoltaic Specialists Conference
sponsored by the Institute of Electrical and Electronics Engineers
Las Vegas, Nevada, October 21-25, 1985
,,
PERFORMANCE AND TEMPERATURE OEPENDENCIFS OF PROTON IRRADIATED
nip AND p n GaAsANO n p SILICON CELLS
I. :veinberd, C.-K. Swartz, and R.E. Hart, Jr.
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
f
ABSTRACT
n/p homojunction GaAs cells are found to be
more radiation resistant than p/n heteroface GaAs
under 10 MeV proton irradiation. Both GaAs cell
types outperform conventional silicon n/p cells
under the same conditions. An increased tempera-
ture dependency of maximum power for the GaAs n/p
celis is attributed to differences in Voc between
the two GaAs cell types. These results and dif-
fusion length considerations are consistent with
the conclusion that p-type GaAs is more radiation
resistant than n-type and therefore that the n/p
configuration is possibly favored for use in the
space radiation environment. However, it is con-
Cluded that additional work is required in order
to choose between the two GaAs cell configur-
ations.
INTRODUCTION
The discovery, in the early days of the space
program, that p-type silicon was more radiation
resistant than n-type silicon led to the present
exclusive use of the n/p configuration for sili-
con solar cells in space (1). With regard to
GaAs, the available experimental data indicates
that the diffusion length for holes is signifi-
cantly greater than that observed for electrons
(2). Hence it would appear that the GaAs n/p
configuration may be preferrable in terms of
radiation resistance. Although most GaAs cells
have been of the p/n variety, some reasonably
good quality n/p cells have surfaced (3 and 4).
Hence cells of both types are available for com-
parison under laboratory conditions. In this
respect, GaAs p/n cells have previously been sub-
jected to both proton and electron irradiations
(5) while n/p cells have undergone electron irrad-
iations (3 and 6). However, to the best of our
knowledge, there have been no reported results in
the open literature concerning the performance of
r /p GaAs cells under proton irradiation. There-
fore, in an attempt to alleviate this apparent
deficiency, we have determined the performance of
n/p GaAs cells after proton irradiation. At the
same time, for comparison purposes, we have proton
irradiated p/n GaAs cells and conventional n/p
silicon cells. In addition, we have determined
the temperature dependencies of these cells in
both the irradiated and unirradiated states. Our
principal objective lies in contributing to a
data base for proton irradiated n/p GaAs cells
and at the same time, understanding and comparing
their performance an,1 temperature dependencies to
both p/n GaAs and conventional silicon solar
cells.
EXPERIMENTAL
Pertinent cell details are shown in Fig. 1
and Table I. The heteroface p/n GaAs cells were
fabricated by liquid phase epitaxy (5). The n/p
_ells were homojunctions fabricated using the
chloride method of vapor phase e p itaxy (3). The
silicon cells were conventional n/p, 10 ohm-cm
200 um thick. One set of silicon cells had both
a BSI• and RSR while the otner had only a RSR.
The p/n GaAs cells were obtained from the Huqhes
Research Laboratories, while the n/n cells were
obtained from the MIT Lincoln Laboratories. The
cells were irradiated by 10 MeV protons in the
Lewis Cyclotron. Temperature dependency runs,
from 0 to 80 ` C, were carried out p rior to, and
periodically, during the irradiations. Cell
electrical characteristics were determined before
aria after irradiation using an air mass zero Xenon
arc solar simulator.
RESULTS
From the plot of cell maximum power versus
proton fluence in Fig. 2 it is readily seen that
the n/p GaAs cells outperform all of the other
cell types over the indicated fluence range.
Variation of cell maximum power with temperature
was linear over the temperature range covered
(Fiq. 3). Pre-irradiation temperature variations
are shown in Table II while Fig. 4 displays the
variation of dPm/dT with fluence. It is noted
that the latter quantity is relatively constant
for GaAs while considerable change, with increas-
ing fluence, is noted for silicon. From Table
II, it is seen that dPm/dT is significantly
less for the p/n GaAs cells as compared to the
n/p cells. However, the homojunction cell still
maintains its superiority, at higher temperatures,
over most of the fluence ranqe (Fiq. 5).
I 
--C+j
DISCUSSION
Comparison of Radiation Resistance
Despite the indicated superiority of the n/p
cells, the data is not conclusive regarding the
possible superior radiation resistance of p-type
GaAs. This follows from the dependency of the
radiation resistance of heteroface p/n GaAs cells
on the p-type emitter depth (5). In this latter
research, a dedidea improvement in the radiation
resistance of these cells was observed when the
emitter de,
	
was reduced from 1 to 0.5 um. On
the other hand, the improvement was relatively
small when the emitter depth was decreased from
0.5 to 0.3 um (5). 8y analogy it is inferred
that the effect of decreasing emitter depth on
radiation resistance is relatively small for
depths less than 0.3 um. Extrapolating to the
Present case, it is assumed that the increased
radiation resistance of the homojunction cell is
not predominantly due to the large reduction in
emitter depth.
An additional argument arises from relative
diffusion length considerations. Using published
literature values (7, 8 and 9) Weiser and
Godlewski have summarized the measured data for
both conductivity types in GaAs (Fig. 6) (2).
The data shows that, for a given dopant concen-
tration, the minority carrier diffusion length
for p-type GaAs is significantly greater than for
n-type. Usinq the curve fitting formulae of
Goradid anc Curtis (10), it is found that Ln,
the minority carrier diffusion length, is 28 um
in the p-region of the present n/p cell while
L	 is 6 um in the n-region of the present GaAs
p9n cell. The preceding would tend to favor the
increased radiation resistance of p-type over
n-type GaAs under 10 MeV proton irradiation.
However, it is noted that different radiation
induced defects are formed in n-type as compared
to p-type GaAs. Thus consideration of pre-
irradiation diffusion lengths alone is not
entirely conclusive in favoring one conductivity
type over another. However, the present results
are not inconsistent with the conclusion that,
for approximately equal doping concentrations,
p-type GaAs is more radiation resistant than
n-type.
Variation With Temperature at Constant Fluence
The temperature variation of Voc can be
discussed by considering the relation (11)
Voc - E
dVoc/dT - 
--^—`Zo - qk
	(1)
where Eo is the extrapolated value of the
semicondgctor band gap at absolute zero, expressed
in volts, and k is Boltzmann's constant.
Calculated values of dVoc/dT from Ea. (1) are
in fair agreement with measured values (Table
III). In general for cells with the same value
of Eo, smaller values of Voc lead to
increRsingly negative values of dVoc/dT. This
follows from the dominance of the first term in
Eq. (1) and the fact that Voc < Ego.
With regard to Pma , from the usual rela-
tion for fill factor it fs shown that
dPm/dT . Voc FF (dIsc/dT) + Voc Isc (dFF/dT)
+ FF Isc (dVoc/dT)	 (2)
At constant fluence, the preceding relation
predicts a linear relation, over a limited temp-
erature range, between dPm/dT and the tempera-
ture variation of Isc, Voc and FF respectively.
This is shown for dVoc/dT in Fig. 7. Similar
plots are obtained for dIsc/dT and dFF/dT.
Substitution of numerical values in Eq. (2)
reveals that, for GaAs, the magnitude of the term
in dVoc/dT is approximately three times the
magnitude obtained for either of the remaining
two terms. Although the third term in Eq. (2) is
not overwhelmingly predominant, the preceding can
be interpreted as indicating that the temperature
variation of Voc is the largest contributor to
dPm/dT. This being the case and considering
Eq. (1), the difference in Voc behavior between
the two GaAs cell types would tend toward explain-
ing the difference in dPm/dT observed between
the p/n and n/p GaAs cells (Table II). The pre-
ceding argument applies to the present GaAs cells.
However it breaks down for the present silicon
cells where Voc differences exist yet the Pmax
temperature variations are approximately equal.
However, in the latter case, the voltage differ-
ence is essentially due to an effective back sur-
face field.
Temperature Effects at Variable Fluence
Previous results for the heteroface p/n GaAs
cells indicate that dPm/dT decreases with
increasing fluence under 1 MeV electron irradi-
ation (12). For similar cells, in the present
case, this quantity is relatively constant, under
proton irradiation, over the entire proton fluence
range. The difference between the results of
electron and proton irradiations could possibly
be due to the generation of different defects by
the two radiation types. Aside from this, Figs.
8 and 9 indicate a rough correlation between the
fluence dependent behavior of dPm/dT and both
disc/dT and dVoc/dT for the present GaAs cells.
In the case of silicon, the fluence dependent
behavior of dPm/dT correlates with dIsc/dT
only (Fig. 9). It is speculated that, for sili-
con, this behavior is due to a fluence dependent
defect property in proton irradiated p-type sili-
con. At the present time, insufficient informa-
tion exists to make a more definitive statement.
CONCLUSION
The present results would, at first glance,
appear to favor the contention trat, for approxi-
mately equal dopant concentrations, p-type GaAs
is more radiation resistant than n-type. Hence,
it would appear to follow that n/p GaAs cells are
2'^
3	 ^
preferable to p/n cells. However, despite the
evidence for increased diffusion lengths in the
p-type semiconductor, the radiation induced
defects in this material could possibly be more
effective lifetime killers than those formed in
n-type GaAs. Exercising caution, it seems best
to state that the present results do not contra-
dict the conclusion that, for the appropriate
doping concentrations, p-type GaAs is more radi-
ation resistance than n-type and that the nip
configuration is preferable.
The larger temperature variation observed
for the nip GaAs cell appears to be dependent, to
a large extent, on Voc which in turn is depend-
ent on cell design and processing. It is our
understanding that improved cells of the present
nip configuration exhibit Voc values equal to
or greater than those measured for the present
heterojunction p/n cells. Unfortunately these
cells were not available to us at the time that
this research was completed. In view of the pre-
sent results it would be of great interest to
determine the temperature dependencies of the
improved nip cells. In addition, future compari-
sons would most fruitfully be made between nip
and p/n cells of the same design. Such cells are
now becoming available (4) and one can anticipate
more definitive comparative radiation damage
studies in the near future. In lieu of such
studies, it is concluded that;
In addition to improved efficiencies, the
radiation resistance of GaAs cells are greater
than that of conventional silicon cells.
The present results are consistent with the
conclusion that, given approximately equal doping
concentrations, p-type GaAs is more radiation
resistant than n-type. However, additional effort
is required to definitively determine the relative
radiation resistance and thus to chose between the
pin and nip solar cell configurations for space
use.
The difference in dPm /dT between the two
types of GaAs cells is due largely to differences
in open circuit voltage.
For silicon, it is speculated that the large,
fluence dependent variation in dPm/dT is due
to a fluence dependent defect property.
REFERENCES
1. J. Mandelkorn, C. McAfee, J. Kasperis,
L. Schwartz and W. Pharo, "Fabrication and
Characteristics of Phosphorous-Diffused
Silicon Solar Cells," J. Electro chem. Soc.,
Vol. 109, pp. 313-318,-1762--
2. V.G. Weizer and M.P. Godlewski, "A 25.5% AMO
Gallium Arsenide Grating Solar Cell," to be
presented at the 18th Photovoltaic Special-
ists Conference, Las Vegas, NV, Oct. 21-25,
1985.
3. J.C.C. Fan, G.W. Turner, R.P. Gale and
C.O. flozler, "Gallium Arsenide Shallow-
Homojuoction Solar Cells," in 14th IEEE
Photovoltaic Seecialists Conference; flew
York:	 , pp •	 -	 r-
4. J.G. Werthen, H.C. Hamaker, G.F. Virshup and
C.W. Ford, "18.1% Efficient (1-Sun, AMO)
Large-Area GaAs Solar Cells," A
Lett., Vol. 46, pp. 776-778, 1
5. R. Loo, G.S. Kamath and R.C. Knechtli,
"Radiation Damage in GaAs Solar Cells," in
14th IEEE Photovoltaic Specialists Conference,
New or	 , pp.
	
-	 .
6. D.J. Flood, D.J. Brinker, C.K. Swartz,
R.E. Hart Jr., and J.C.C. Fan, "Radiation
Damage and Annealing in Large Area n+/p/p+
GaAs Shallow Homojunction Solar Cells," in
Space Photovoltaic Research and Technology
A	 -	 , Washington, D.0
Rational Aeronautics and Space Administration,
1982, pp. 179-184.
7. L. Jastrzebski, J. Lagowski, H.C. Gatos, and
W. Walukiewicz, "Minority Carrier Lifetime in
Gallium Arsenide at Elevated Temperatures:
Implications for Solar Cell Performance," in
Gallium Arsenide and Related Compounds,
C. M. Wolfe, Ed., Inst. Physics Conf. Series
No. 45, Bristol:	 Institute of Physics, 1979,
pp. 437-444.
8. G.J. Rees and W.I. Milne, "Modulation
Efficiency in Gallium Arsenide LEDs," in
Gallium Arsenide and Related Compounds,
Wolfe, 	 nsPhysics Cont. eries
No. 45, Bristol: 	 Institute of Physics, 1979,
pp. 362-368.
R.J. Nelson, "Measurements of 100 um Minority
Carrier Di f fusion Lengths in p-Gallium
Arsenide by a New Photoluminescence Method,"
in Gallium Arsenide a nd Related Compounds,
C.M. Wolfe, Ed., Inst. Physics Conf. Series
No. 45, Bristol:	 Institute of Physics, 1979,
pp. 256-262.
10. C. Goradia, M.G. Goradia and H. Curtis,
"Near-Optimum Design of GaAs-Based
Concentrator Space Solar Cells for 80 `C
Operation," in 17th IEEE Photovoltaic
Specialists Conference, New York: IEEE,
1984, pp. 56-62.
11. E.S. Yang, Fundamentals of Semiconductor
Devices, New York: McGraw Hill, 1978,
pp. 103-106.
12. C.K. Swartz and R.E. Hart Jr., "Temperature
and Intensity Dependence of the Performance
of an Electron-Irradiated (A1Ga)As/GaAs Solar
Cell," in Solar Cell High Ef ficient and
Radiation ama e	 —	 , as rng on,
^.: a^Aeronautics and Space
Administration, 1979, pp. 217-226.
*1
. w
TABLE I. - PRE-IRRADIATION ELECTRICAL CHARACTERISTICS
T . 25 °C
Cell Number of
cells
Efficiency,
%
Voc,
v
Isc
ma/cm2
FF,
%
GaAs (p/n) 4 16.4 1.022 28.5 77.3
GaAs (n/p) 2 16.6 0.96 29 81.8
Si	 (BSF/BSR) 2 14.6 0.616 42.43 76.6
Si	 (BSR) 2 12.7 0.559 39.58 78.8
TABLE II. - PRE-IRRADIATION VARIATION OF CELL
ELECTRICAL CHARACTERISTICS WITH TEMPERATURE
Cell	 dF NT,
mw/cm 	 °C
dVoc/dT,
mV/°C
dlsc/dT,
ma/cm2 °C
dFF/dT,
%/°C
p/n-GaAs -4.4x10- -2.03 2.03x10
-2
-5.25x10-2
n/p-GaAs -6.04r.10 2 i	 -2.3 2.06x10-2 -7.35x10-2
BSF/BSR-Si -9.17x10-2
I
-2.4 1.9410-2 -12.2x10-2
BSR-Si -9.11x10-2 -2.59 2.3x10-2 -14x10-2
TABLE III. - CALCULATED AND MEASURED
Voc TEMPERATURE VARIATION
(PRE-IRRADIATION VALUES)
Cell Voc,
V
dVoc!dT,
mViK
Measured Calculated
p/n GaAs 1.022 -2.03 -1.98
n/p GaAs 0.960 -2.3 -2.21
Si BSF/BSR 0.616 (	 -2.4 -2.17
Si	 BSR 0.559 -2.59 -2.39
NE
_U
E
Ln
w
0
W
O
CL
XQ
AIXGaI_xAs
N	 t	 r---
lcm -31 14µn1
1018	 0.5
1017 10
r AR COAT
/r-Au-Zn-Ag
P - GaAs (Be)
n-GaAs	 (S n)
n - GaAs (Te)
(SUBSTRATE)
AR COAT
	 S n	 N	 t
(cm 3 )	 1µm1
n - GaAs (S)
F
I
P - GaAs (Zn)
P - GaAs IZn)
P -GaAs
(SUBSTRATE)
(Zn)
54018 0.05
2.64017 3.4
44018 3.3
Au-(',e-Ni-Ag	 ^ Au
Pin	 n/P
Figure 1. - GaAs cell configurations.
0	 101V	 1011
10 MeV PROTON FLUENCE (cm-2)
Figure 2. - Maximum power density versus 10 MeV
proton fluence.
101 c
i
-ti
95,—
90
W
OC L 85
8C5z
75
70
0 20	 40	 60	 80
TEMPERATURE, OC
Figure 3. - Temperature variation of cell Pmax (pIn GaAs).
10
-10
	
-9	 IL
Si (BSR)
-8E
S i(BS F, BSR)
E	 N%
-7
X GaAs, n1p
	
E -6	
— — — — — --
CL
—0
	
-5	 GaAs, p/n
--0 — — — — — — — 0
0	
Y
	 1010	 1011	 1012
10 MeV PROTON FLUENCE (cm-2)
Figure 4. Temperature variation of maximum power
density at different fluences.
q
i
1
t
i
r
22
T•6OoC
2
 ,-GaAs, nlp
^^\	 O^cv
3 18
"%0`^
E GaAs, On
v 16
\bW ^.
`^W
0 14
Si (BSF, BSR)	 O
^^^^
**0\
x 12 Si (BSR)
10 ^^
	
8 1 	 1	
'	 '
	
0	 1010
	 loll	 1012
10 MeV PROTON FLUENCE (cm-2)
Figure 5. - Maximum power density at different fluences.
1000
-q 100
c^
zW
z 10
0
N
ti 1
G
1
13	 14	 15	 16	 17	 18
LOG DOPING CONCENTRATION, cm-3
Figure 6. - Electron and hole diffusion lengths in
GaAs. (Weizer and Godlewski (ref. 2)1
19
nip GaAs
7
U0
N 6
Ev3
E
5
X
E
CA_
*74
t
3 1 	 1	 1	 1	 1	 I
1.0
	 2.0	 2.1	 2.2	 2.3	 2.4
-dVoc MT (mV I°CI
Figure 7. - Pre-irradiation P m
 temperature variation
versus Voc temperature variation (GaAs).
3
Si-BSFIBSRi Si-BSR
2.5 —^- ---- i---_— --------^
E	 —-----t---_
n Ip GaAs
Pin GaAs
1.5 
0
	
1010	 1011
10 MeV PROTON FLUENCE (cm-2)
	 1012
Figure 8. - dVocldT versus fluence for all cells.
^Y
f
For sale by the National Technical Information Service, Springfield, Virginia 22161
1
L ., .e►s _
e
i, Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM-87136
4. Title and Subtitle 5. Report Date
Performance and Temperature Dependencies of Proton
Irradiated n/p and p/n GaAs and n/p Silicon Cells .	 'Loorming Organization Code
506-41-42
7. Aut!ior(s) 8. Performing Organization Report No.
I.	 Weinberg, C.K.	 Swartz, and R.E.	 Hart, Jr. E-2753
10. Work Unit No.
9. Performing Organization Name and Address
Contract or Grant No.
National Aeronautics and Space Administration 	
11.
Lewis Research Center
Cleveland, 
	 Ohio	 44135	 13. Type or Report and Period Covered
Technical Memorandum12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
	 14.
Washington,	 D.C.	 20546
Sponsoring Agency code
15, Supplementary Notes
Prepared for the Eighteenth Photovoltaic Specialists Conference sponsored by the
Institute of Electrical
	 and Electronics	 Engineers,	 Las Vegas, Nevada,
October 21-25,
	 1985.
116. Abstract
n/p homojunction GaAs cells are found to be more radiation resistant than p/n
heteroface GaAs under 10 MeV proton irradiation.
	 Both GaAs cell types out-
perform conventional	 silicon n/p cells under the same conditions.
	 An increased
temperature dependency of maximum power for the GaAs n/p cells is attributed
largely to differences in	 Voc	 between the two GaAs cell types.
	 These results
and diffusion length considerations are consistent with the conclusion that
p-type GaAs is more radiation resistant than n-type and therefore that the n/p
configuration is possibly favored for use in the space radiation environment.
However,	 it is concluded that additional work is required to order to choose
between the two GaAs cell configurations.
I
17 Key Words (Suggested by Author(s)) 18. Distribution Statement
Solar cells;
	
Radiation damage; Unclassified	 - unlimited
Temperature dependency STAR Category 33
19. Security Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21 No. of pages 22. Price'
Al


Performance and temperature dependencies of proton irradiated n/p GaAs and n/p silicon cells

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