Solar cells made of Ge, Ge(0.93)Si(0.07) alloys, GaAs and Al(0.08)Ga(0.92)As were irradiated in two experiments with 1-meV electrons at fluences as great as 1 x 10(exp 16) cm(exp-2). Several general trends have emerged. Low-band-gap Ge and Ge(0.93)Si(0.07) cells show substantial resistance to radiation-induced damage. The two experiments showed that degradation is less for Al(0.08)Ga(0.92)As cells than for similarly irradiated GaAs cells. Compared to homojunctions, cells with graded-band-gap emitters did not show the additional resistance to damage in the second experiment that had been seen in the first. The thickness of the emitter is a key parameter to limit the degradation in GaAs devices

Chu, C. L.

Iles, P. A.

Timmons, M. L.

Venkatasubramanian, R.

English

NASA Technical Reports Server

N91-30240
Radiation Resistance of Ge, Geo.03Sio.07, GaAs, and Alo.osGao.02As Solar Cells*
M.L. Timmons and R. Venkatasubramanian
Research Triangle Institute
Research Triangle Park, NC
P.A. Iles and C.L. Chu
Applied Solar Energy Corporation
City of Industry, CA
Solar cells made of Ge, G%93Si007 alloys, GaAs and AloosGao 92As have been irra-
diated in two experiments with 1-meV electrons at fluences _ great as 1 X 1016 cm -2.
Several general trends have emerged. Low-band-gap Ge and Ge093Sio07 ceils show
substantial resistance to radiation-induced damage. AloosGaoq2As cells have shown in
the two experiments that degradation is less than for GaAs cells similarly irradiated.
Compared to homojunctions, cells with graded-band-gap emitters did not show the
additional resistance to damage in the second experiment that had been seen in the
first. The thickness of the emitter is a key parameter to limit the degradation in GaAs
devices.
Introduction
Radiation damage to devices is a key factor for space photovoltaics since the end-
of-life (EOL)/beginning-of-life (BOL) ratios largely determine how much extra array
area must be launched to meet EOL mission requirements. At the previous SPRAT,
we presented preliminary radiation exposure data using 1-meV electrons at fluences as
high as 5 X 1015 cm -2 for A1008Gao92As and GaAs solar cells[I]. In that study, the
A1Ga,_ emitter configurations were varied and included homojunctions, grade compo-
sitions, and heterojunctions. GaAs cells grown on Ge (with inactive GaAs/Ge inter-
faces) were included. All of these cells were grown in production-type reactors.
The preliminary data showed that AIGaAs cells, regardless of emitter configura-
tion, degraded less than GaAs cells. But beyond this, the graded-emitter cells
degraded the least of all in the experiment.
In this paper, we describe a second experiment that extends the results presented
previously. The 1-meV electron fluence has been increased to 1 X 1016 cm -2, and Ge
and Ge093Si0o7 alloy cells have been added. Again, AIGaAs and GaAs cells form the
major focus of the study.
This work was funded by the Department of the Air Force under Contract Nos.
F33615-87-C-2804 and F33615-88-C-2847. Mr. K. Reinhardt and Mr. J. Scofield are
the respective Project Engineers.
37-1
https://ntrs.nasa.gov/search.jsp?R=19910020926 2020-03-19T17:03:10+00:00Z
Experimental
All tile AloosGao92As cells used in this study were grown at RTI in a research-
type reactor. This reactor is horizontal and operates at atmospheric pressure.
Trimethylgallium (TMG), trimethylaluminum (TMA), and ursine (ASH4) are the
sources of Ga, AI, and As, respectively, and Se from a 50 ppm H2Se/H 2 mixture and Zn
from dimethylzine (DMZ), and diethylzinc (DEZ) are the n- and p-type dopants,
respectively. The AlOaAs layers were grown at 780°C using a V/III ratio of about 35
to 40. The RTI GaAs cells were grown at 700 ° C.
The Ge and Ge09aSi007 cells were grown in a vertical low pressure reactor. The
reactor was operated at 250 Torr and at 700°C for Ge and 900°C for Ge093Si007.
Mixtures of germane (GeH4) , disilane (SigH6) , ASH4, and diborane (B2H6) in H 2 were the
source and dopant gases. The junctions evaluated in this study, using p-on-n polarity,
were all epitaxially grown.
Tile RTI-grown devices were also processed at RTI. The current-voltage (I-V)
characteristics were measured under a xenon-lamp solar simulator. The devices were
sent to ASEC where the I-V measurements were repeated; generally, there was good
agreement between the two sets of measurements. Several AIGaAs cells were
evaluated by deep-level-transient spectroscopy (DLTS) at SERI, under the direction of
Dr. R.K. Ahrenkiel, to establish a baseline deep defect level prior to irradiation. The
only observed level was the DX center, present in most A1GaAs DLTS data, and the
defect densities were low.
As in the first experhnent, ASEC added several GaAs cells grown on Ge with inac-
tive GaAs/Ge interfaces. These cells have an emitter thickness of about 0.5 pm com-
pared to the 0.25-Fire-thick emitters found in the GaAs and A1GaAs cells grown at
RTI.
The irradiation ot" the devices was effected at JPL under the direction of Dr. B.E.
Anspaugh. The cells, those grown and processed at RTI and GaAs/Ge cells from the
ASEC process line, were divided into three groups. One group was irradiated at a flu-
ence of 1015cm -2, the second at 5 X 1015cm -2, and the third at 1016era -2. Unfor-
tunately, there was no sequential irradiation with measurement after each level of
exposure.
After irradiation, workers at ASEC remeasured the I-V characteristics and the
quantum efficiencies at wavelengths of 450 and 800 nm. The cells were then returned
to I_TI, where they were also remeasured. There was again good agreement between
the two sets of measurements. Several of the A1GaAs and GaA_s samples were returned
to SEFH I'or remcasurement of the defect content by DI,TS.
The BOL et'fici(,ncies for the cells used in the second experiment were typically
higher than those of the cells from the first experiment. The Ge-cell efficieneies
37-2
ranged from 4 to 6 percent, and Ge093Si007 efficiencies were about 5 percent. These
values were measured under a xenon-lamp simulator and may be slightly high because
of high currents. Aloo8Gao92As homojunction cells varied from 15 to 19 percent, and
graded-emitter cells varied from 14 to 17 percent. A1GaAs cells for which the emitter
changed composition in discrete steps had BOL efficiencies of 14 to 17 percent. GaAs
cell efficiencies were 17 to 20 percent, and cells with A1GaAs graded-composition
emitters and GaAs bases had 13 to 19 percent 'efficiencies. The BOL efficiencies of
these cells are high enough to extract useful information and are important because
cells with lower efficiencies will generally show less effects of radiation damage.
Results
While, overall, the experiment went smoothly, some difficulties were encountered.
A decision to test separate groups of cells at different fluences, rather than using
sequential irradiation, was made to reduce the risk of damaging the cells during
repeated measurements between irradiations, but this decision also reduced the sample
size for each fluence. These smaller sample sizes prevented resolution of some
discrepancies in the observed degradation data. Attempts to extract quantitative dam-
age coefficients were hampered by the spread in the initial quantum efficiency values.
The relatively small changes in quantum efficiencies at lower fluences made determina-
tion of the diffusion lengths L n difficult. And since the damage coefficient K and L n
are related usually by the expression
1 1
L5
+ K¢
where Ln¢ and Lno and the diffusion lengths after and before irradiation, respectively,
at the fluence of ¢, uncertainty in L n produces uncertainty in K.
Despite these difficulties, several conclusion about the trends in the data can be
nlade.
I. Ge and Ge093Sio07 cells show substantial resistance to radiation damage even at
the highest fluences, confirming predictions made in our original proposal.
. Although having lower beginning values for the product of Jsc and Voc , the
AIGaAs cells, regardless of emitter configuration, degrade less than the GaAs and
GaAs-on-Ge cells.
. \¥ithin the AtGaAs group, the graded-band-gap cells fail to show the perfor-
mance advantage that was observed in the first experiment, i.e., shallow homo-
junction cells performed as well or better.
37-3
The GaAs cells with tile 0.25-#m-thick emitters degrade less than the cells with
0.5-lira-thick emitters, which is a trend generally observed ['or cells.
These trends carl be seen in Figure 1 that shows tlle post-irradiation efficiencies
normalized to pre-irradiation values as a function of fluence. The data points are the
averages of the cells irradiated at that fluence, and because of dividing the samples
that is mentioned above, each point represents usually no more than three to four
cells, which, unfortunately, is a small sampling size.
t,'rom the data shown in Figure 1, the performance of the Ge and Ge0oaSi007 cells
is clearly outstanding. Even at a fluence of 1016 electrons/era 2, these cells have
rolained 8.5 percent of the BOL efficiency. And a single Ge cell, upon which a 3-/_m-
t lfick A100sGa0o2As layer had been grown, performed even better, retaining about 94
percent of the BOL efficiency. This latter cell represents the structure of an
AIGaAs/Ge cascade cell and clearly indicates that this cascade cell, or any other using
(;e as the bottom junction, will likely by limited by the radiation resistance of the top
cell.
Since cell fill factors changed by no more than two to five percent for all of the
cells in the test, the differences in the normalized GaAs and AIGaAs data must reflect
differences in Voc and/or Jsc. For the GaA.s and A1GaAs cells, the ratios of pre- and
post-irradiated V,3_ and as( are plotted versus the fluence in Figure 2. The data show
that, t,he volt, age degrades comparably for both Gm_Ys and AIGaAs, indicating that, the
changes in the normalized efficiencies must Colne from changes in the current collee-
t i<m, i.e., changes in diffusion length and minority-<mrrier lifetime. The AIGaAs cells
ret:_in a},out. 7() to 7.5 percent of the current.. The thickness (>f'the GaAs cell emitters
._ignificantly impacts current collection. The 0.25-1zm-t.hicl¢-emil, ter ceils retain about
5() percent more current than the cells with the thicker emitters at. 1016 electrons/era 2.
The change in the current collection in the GaAs and AIGa._gs cells was examined
furt.hcr hy considering the spectral response of Chese cells. This is shown in Figures 3
and 4. The data in Figure 3 were gathered wi'& ASI_;C's two-source (tungsten and
xt'n()ll) sirl/ll]ator, using the sources separately to illuminate the samples. For the
AIC,:IAs cells, the "red" and "blue" responses from t ilngsten and xenon, respectively,
degrade at about, the same rate although the normalized ratio is lower for the "red".
[;'(_l' l.he (_nAs cells with thin emitters, the degradation is ol_ly slightly greater than for
tho Al(;nAs ('ells. The thick-emitter GaAs cells show :_rl inii, ial rapid decrease in both
the "red" and "blue" regions followed by a slowing of the degradation rate, but still
great.er th:/lt for the AIGaAs cells.
l"ig,lt'(, t shows measlire,I qunnt,ln_ ,ft'iciencies at wave, lengths of,150 and 800 nm
as :_ function of Huencc for t,he .\l(',aAs _iltt (_a:\s cells and cont'ir_i_s lhe simulator
d_t_ sh(_wn i_ l"i_<ure 3. The higher starting value> t'_," the (;aAs-tm.>_d (:ells indicate
hmger ditTusi_m I,nglh>, 1_1_1the slower rate of decr(_nse t'{_r the Al(;a,,\s-based cells
SUpl_()rls the <'<)Ii('},ls]<)l_ th:lt lho _lifl'usiorl length cl / g_,s loss, l,r()_lu<q g tile observed
37-4
higher current ratios for those cells. And since this is the ease for both short and long
wavelengths, we infer qualitatively that the degradation coefficient is lower in AIGaAs
than GaAs for both the base and emitter regions.
Normalized data present only part of the necessary information to evaluate radia-
tion damage characteristics. Efficiency data must also be considered. Since the fill
factor changes were small, plotting the Jsc Voc product provides the additional informa-
tion. This is shown in Figure 5. The key result, here is that, while initially having
lower Jsc Voc products than GaAs cells, the AIGaAs-cell performance equals the GaAs-
cell performance at an EOL fluenee of 10 is electrons/era 2, and, because the A1GaAs
degradation proceeds at a slower rate, will outpei'fornl GaAs at higher fluenees. This
may be pivotal for long-duration flights or high-radiation-intensity orbits. In Figure 6
also note the stability of the output of the Ge and GeSi cells.
The DLTS data for the cells are currently being analyzed and will be reported at a
later date.
Conclusions
In summary, a second radiation-resistance experiment has shown qualitatively
that A1GaAs solar cells degrade more slowly the GaAs cells. We were somewhat disap-
pointed that clear evidence favoring graded-band-gap emitters over homojunction
emitters could not be concluded from the data. Because of the slower degradation, the
performance of the A1GaAs cells equalled that of GaAs at a 1-MeV electron fluence of
101acre -2. Because of some discrepancies in the data, we were unable to determine
quantitative damage coefficients for AlGa_&s to compare with the published data for
GaAs.
Ge and GeSi cells, added for this experiment, showed remarkable resistance to
damage. Both Oe and Ge003Si007 cells demonstrated BOL/EOL ratios of about, 0.85 at
10 is electrons/era 2. AGe junctioi_ with a 3-1_m-thick A1Ga_s cap showed a BOL/EOL
rat,io of 0.94 at that ftuenee.
And, finally, thin emitter GaAs ('.ells outperformed thicker-emitter cells. While
this is not unexpected, it may suggest that there are processing optimizations yet to be
done to examine the trade-offs between emitter thickness influence on yield versus the
resistance to radiation-in(hleed damage. It was clear that the thinner emitter ceils pro-
dueed at RTI were more susceptible t.o 1)robiJ)g damage during I-V and spectral meas-
urements.
References
M.L. Timmons, R. Venl<at:_sut)r:_l,_-_nl_n, T.S.C.olpitts, J.S. Hills, J.A. Hutchby,
P.A. lies, and C.I_. (?h_2, (h'aded I]oe_d(jal_ Al(ht, ts ,Solar Cells for AIGaAs/Ge
Cascade Cells, Pro(:c(_(tillgs of the Space t)h()l()v()llaic Research and Technology
Conference, 1989, (NASA (:oi_t'erence I',ll)li('ati()tl 3107), p. 48.
37-5
¢.a
°_
t=
ID
t'q
0_
I.
©
Z
0
Ge with AIGaAs-Cap -0
• "-Ge (ells' s
-- GeSi Cells
ttomojunedon AIG a.As
G raded-Emitter AIGaAs
Step-Emitter AIGa._
Grade-Emitter GaAs
RTI GaAs (0.2,5 gm Emitter)
ASEC Ga.As (0.50 _m Emitter)
t i I i
2 4 6 8
1-MeV Electron Fluence (X 1015 cm -2)
I
I
10
Figure 1. Normalized effieiencies versus fluence t'or tile AIGaAs, GaAs, Ge, and
GS"e, l solar cells after irradiat, ion with 1-MeV electrons.
37-6
1.0
0.9
0.8
"O
0.7
tq
E
Z
0.5
0.5
0.4
I I.... I
_ "%, _---,,.,_-- -a I
v-__ _:__ _ -_._1
\
% %
,%
%
,%
_1. "%-
%.
D-I|omojunction AI(; ,_,_
• .. Graded-Emitter AIGaAs
• o Step-Emitter AIG_s
O" RTI GaAs (0.25 l_m Emitter)
if'l- ASEC Ga_s (0.50/_m Emitter)
%.
%.
%.
"%
_11 I II1| II I I | ..... IIIII| w !
0 2 4 6 8
1-MeV Electron Fluence (× 10 Is cm
m
I III |
10
._)
Voc (GaJcs and Al(;aAs Cells)
Js¢ (AlGa.As Cells)
Jsc (RTI GaAs Cells)
Jsc (ASEC G'aAs Cells)
Figure 2. Normalized values of Jsc and Vo_, versus fluence for the GaAs and A1GaAs
solar cells that were part of this exp(,rim(ml. The dashed lines show
current, and solid lines show voltag(,.
37-7
ac
c
e--
t-
"5
r,
,%
.=
Z -- AI G aAs Homoj ur/c tion
--Graded-Emitter AIG a_
--Step-Emitter AIG aAs
.... ! 1 ! t
2 4 6 8
1-MeV Electron Fluence (/ 1015 cm -2)
.,
I
10
1.0
0.9
i
--
c
0.8 "
0.7
,%
0--
-_ 0.6
C
Z 0.5
0.4
A, A -- Graded-Emitter GaAs
-S,O--RI'I (;aAs
0)0 -- ASEC GaAs
• t -I I . t
0 2 4 6 8
1-MeV Electron Fluence (X lO is cm-2)
i
10
Figure 3. Normalized photocurrents versus fluence from a two-source solar simula-
tor. using the xenon and tungsten sources seI>arately to illuminate the
cells. This technique produces qualitative "t)lue" (closed symbols) and
"red" (open symbols) responses.
37-8
ouANTUM EFFICIENCY vs. FLUEHCE
"_,=0.45vm ______----
_._..-.-------.-- 'l -
1 I GaAs
O c;
8
0 2 FLUENCE (X10 E 15) Gp. 7 Ga/m_ Gp. 9 Gaa_,_e
O_S_me p _.S_
)., :0.8: tam
0.8
AIGaAS
0.8
GaAs
lo
8 a
2
FLUENCE (X10E ',5)
Figure 4.
lO
Quantum efficiency me_urements as a function of fluence made
wavelengths of 450 and 800 nm for G_s and AlGa2_s solar cells.
ut
37-9
JscVoc vs. FLUENCE
3O
25
20
15
10
i
_ 3(- --_ .........
Gp. 1 Ge
-----X----
Gp. 5 AIGaAe step
graded emitter
I i I I
2 4 6 8 10
FLUENCE (Xt0 E 15)
Gp. 2 Ge Si Gp, 3 AIGa/ul Gp. 4 AIGa/_
homoJunctlon graded emitter
I • !!
Gp. 6 GalAs Gp. 7 GaAs
Gp. g Ga/Ls/Ge
graded emitter
.... 0 ........ [] ........ f_---
Figure ,5. The product of Jsc and Voc as a function of fluence for GaAs and AIGaAs
solar cells. The a fluence of 1016 cm -2, the performance of the GaAs and
A1GaAs cells are equal.
37-10


Radiation resistance of Ge, Ge0.93Si0.07, GaAs and Al0.08Ga0.92 as solar cells

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