The response of cobalt 60 irradiated N/P silicon solar cells was measured as a function of the atomic number of the medium adjacent to the cell and the direction of the gamma ray beam. The interpositioning of various thicknesses of aluminum between the adjacent material and the cell had the effect of moving the cell to various locations in an approximate monatomic numbered medium. Using this technique the solar cell response was determined at various distances from the interface for gold and beryllium. The results were compared with predictions based upon ionization chamber measurements of dose perturbations in aluminum and found to agree within five percent. Ionization chamber data was then used to estimate the influence of various base contact materials

Burke, E. A.

Cappelli, J. R.

Lowe, L. F.

Wall, J. A.

English

NASA Technical Reports Server

SOLAR CELL RADIATION RESPONSE NEAR THE INTERFACE
OF DIFFERENT ATOMIC NUMBER MATERIALS
E. A. Burke, J. R. Cappelli, L. F. Lowe, and J. A. Wall
Air Force Cambridge Research Laboratories (AFSC)
Bedford, Massachusetts 01730
The response of Co-60 irradiated N/P silicon solar cells was measured as
a function of the atomic number of the medium adjacent to the cell and the direc-
tion of the gamma ray beam. The interpositioning of various thicknesses of
aluminum between the adjacent material and the cell had the effect of moving the
cell to various loe_tions in an approximate monatomic numbered medium. Using
this technique the solar cell response was determined at various distances from
the interface for gold and beryllium. The results were compared with predic-
tions based upon ionization chamber measurements of dose perturbations in alumin-
um and found to agree within five percent. Ionization chamber data was then used
to estimate the influence of various base contact materials. The directional effect
is most noticeable for gold contacts leading to a response when the beam enters the
cell face which is 40% greater than for the opposite direction. The situation is
reversed for beryllium which produces a response which is 17% greater when the
beam enters through the base than for the reversed direction.
INTRODUCTION
The prediction of solid state device response
to ionizing radiation requires a knowledge of
the energy absorbed (the dose or dose rate) in
active regions of the device. If the device is
adjacent to materials (packaging or.structural
supports) with atomic numbers different than
the device material and the primary radiation
produces electrons with ranges that are com-
parable to device dimensions, ionization cham-
ber studies indicate that the energy deposited
can vary significantly from that absorbed in the
absence of the surrounding materials. Methods
commonly used for predicting the dose neglect
the possible influence of adjoining materials.
Similarly, the materials, dimensions, and en-
capsulation of dosimeters rarely corresponds to
the test specimen so that experimental measure-
ments can be misleading. Little experimental
data exists concerning the nature and magnitude
of the dose perturbations produced and as a
result theoretical predictions have not been
adequately tested.
The greatest amount of work on the problem
of dose perturbations at interfaces has been
done by those with an interest in the biological
effects of radiation. They have been particular-
ly concerned about the enhanced dose in soft
tissue adjacent to bone under X irradiation.
Experimental and theoretical work on this pro-
blem has been reviewed by Spiers 1.
In view of the large differences in the photo-
electric cross sections for elements of different
atomic number and the resultant large difference
in the deposited energy it is not surprising to
find dose perturbations near interfaces at x-ray
energies. At gamma ray energies where the
Compton cross section is dominant, energy de-
position varies relatively little from one
material to another and small perturbations
might be expected.
The fact that significant changes in the dose
can occur at gamma ray energies was first
pointed out by Dutreix et al L and recently
examined for typical device materials expose_
to Co-60 gamma radiation by Wall and Burke-.
Energy deposition profiles in aluminum adjacent
to gold are shown in Fig. 1. The data was ob-
tained using a multiple cavity.parallel plate
ionization chamber technique_a. It shows that
the dose in aluminum is enhanced when the
gamma beam penetrates the aluminum before
entering the gold. However, the dose is ac-
tually reduced below the equilibrium value for
aluminum when the gamma beam is reversed.
There is a strong directional effect. The effect
is noticeable at a distance of 200 mg/cm 2 away
from the boundary which means that it extends
over typical device dimensions. For lower
atomic number materials the effect is similar
but somewhat reduced as shown in Fig. 2.
RELATIVE DOSE IN ALUMINUM
NEXT TO GOLD FROM Co6°7-RAYS
RELATIVE#//_L
DOSE
o IOO 200 300
THICKNESS (mg/cm 2)
FIGURE 1. - Air Ionization Measurements of the
Relative Dose in Lauminum next to Gold. Arrows
indicate the direction of the gamma beam.
.577
https://ntrs.nasa.gov/search.jsp?R=19720010026 2020-03-17T03:45:04+00:00Z
RELATIVES/A
DOSE _//
q
IO
°_,
A_CRL PHOTO
RELATIVE DOSE IN ALUMINUM FROM Co e° ),,RAYS
COPPER-ALUMINUM
RELATIVE_i/
DOSE
1.5
/,
M
//-
//
DISTANCE FROM CU(M.) / /
I I I I , I //
400 800 1200 / I
- i i I I I i I I I I i i i i J / ./
I00 200 300 "")
THICKNESS (mg/cm 2)
MOLYBDENUM -ALUM IN UM
i !
DISTANCE FROM Mo (#)
i I i I i I
400 800 1200
, i I J , I i I i i I i I
I00 200 500
THICKNESS (rng/cm 2)
120-406
FIGURE 2. - Air Ionization Measurements of the
Relative Dose in Aluminum Next to Copper and
M olybd enum.
Notice, however, that the minimum in the dose
profile appears to be greatest for materials of
intermediate atomic numbers s%ch as moly-
bdenum. It has also been found ° that beryllium
or carbon adjacent to aluminum reverses the
effect, i.e. the aluminum dose is enhanced when
the beam penetrates the beryllium first and
reduced for the opposite photon direction.
Finally, it has been found that micron layers of
high atomic number materials produce a readily
observable effect as shown in Fig. 3.
It is important to note that the results des-
cribed were obtained with ionization chambers.
We must be careful in assuming that other
phenomena, such as electron-hole pair produc-
tion in semiconductors will behave in the same
way in the transition zone near an interface. If
we are interested in applying ion chamber data
to solid state devices it is desirable to show that
the response of the latter can be predicted from
the former. That was the primary objective of
the work reported here.
Solar cells were selected for examining the
possibility of predicting device response. The
interface conditions studied were similar to
those studied with ionization chambers. The
results obtained indicated that ionization chamber
data could be employed to predict device re-
sponse. Calculations were then carried out for
solar cells with base contacts of several dif-
ferent materials.
578
1.9- EXPERIMENTAL CONFIGURATION
1.5
i.c
0.9
30o
IONIZATIONCHAMBER MEASUREMENTS
200 I00 OO tOO 200 3OO
DISTANCEFROM DISTANCEFROM
INTERFACE INTERFACE
ENERGYDEPOSITIONFOR THIN GOLDLAYER (6microns)
BETWEEN EQUILIBRIUMTHICKNESSESOF ALUMINUM
A_ABLE Z MATERIAL
:__//// fALUMINUM
::_? ENCAPSULATION
U
_E THICKNESS v'
OF ALUMINUM
a FcnL PHOTO 121-50
&FCRL PHOTO | 2 0 - 2 8 5
FIGURE 3. - Air Ionization Measurements of Dose
Perturbations Near a Thin Gold Film.
FIGURE 4. - Experimental Configuration Used in
Solar Cell Measurements.
EXPERIMENTAL
The devices employed were I x 2 cm, N/P
silicon solar cells with 0.5 U m junction depth
and a 2.5 _crn base rcsistivity. The diffusion
length of minority carriers in the base was
determined by exposing the cells encapsulated
in aluminum (to establish equilibrium dose
conditions) to a Co-60 gamma source and
measuring the short circuit current. The dose
rate was established with a calibrated ionization
chamber and the minority carrier production
rate calculated assuming that 3.6 eV are re-
quired to produce an electron-hole pair _.
Typical diffusion lengths for undamaged cells
were in the vicinity of 140 Um.
The sample holder employed in the solar cell
exposure is shown in Fig. 4. The cell was
mounted in a recessed aluminum plate whose
thickness exceeded the range of the highest
energy electrons generated by Co-60 gamma
radiation. In the experiments reported here
the base contact was adjacent to the aluminum
plate. Aluminum foils of variable thickness
could be placed between the face of the cell and
the gold or beryllium plates used in the inter-
face studies, Rotation of the sample mount
through 180o permitted the response to be de-
termined for both beam directions.
A 16 kilocurie Co-60 source was employed in
the irradiations. Both collimated and uncol-
limated photon beams yielded the same results
within the uncertainty of the measurements. In
order to normalize the data the cell response
was measured with aluminum replacing the gold
or beryllium end plate.
COMPARISON OF CELL RESPONSE
WITH IONIZATION CHAMBER RESULTS
The short circuit current as a function of
aluminum foil thickness was normalized to the
current observed with the cell completely en-
capst_lated in aluminum. Results are shown for
both gold and beryllium end plates and for two
beam directions in Figs. 5 and 6. As can be
seen the respons e is qualitatively similar to the
ion chamber results. The difference can be
attributed to the solar cell thickness (0. 038 crn)
which represents a significant fraction of the
distance over which the dose perturbation occurs.
If t he ion chamber results are a true indication
of the dose rate the carrier generation rate will
not be uniform through the cell. We can make
a quantitative comparison by adopting an
approach similar to that used in calculating the
spectral response of solar cells 5.
For electrons in the p type base the form of
the continuity equation applicable is
1 _n
G n
-U n+ --_ div _n = _ (1)
where n is the excess minority carrier concen-
tration in the base, :l n the current density,
G n and U n are the minority carrier generation
and recombination rates, and q is the charge on
an electron. For steady state conditions, zero
electric fields, low injection and one dimensional
geometry Eq (1) simplifies to
n = B2n
G n 2,n - D n "_2 (2)
579
SOLAR CELL RESPONSE NEAR A Be-At
INTERFACE IRRADIATED WITH
Co-60 GAMMA RAYS
FIGURE 5. - Measured Solar Cell Response Near a FIGURE 6. - Measured Solar Cell Response Near a
Au-AI Interface. Be-A1 Interface.
where TT_ is the lifetime of minority carriers
and D n tg_ diffusion coefficient. The normalized
ionization chamber data can befitted with an
exponential series to within a few percent. If
we assume that the generation rate is given by
the same function we have
_am( x + x 1 )= e (3)G n G o A m
('n
where the values of the coefficients for beryllium,
copper and gold are given in Table I, xl is the
thickness of aluminum between the end plate and
the face of the cell and x any point within the cell
as measured from the cell face. The solution
to Eq. 2 is
n = C 1
where
x/L -x/L _ e-am(Xl + x)
e + C 2 e +Go Bm (4)
m
A
( 1 a 2 )-1 (5)
Bm = D n Ln 2 m
and L n is the diffusion length which is equal to
_'_n D_ . If we assume tt_at the excess minority
I1
carrier concentration zs zero at the junction and
at the base contact, i.e., n = o at x = _ and
x = b, then
b-4/L 45-b/L -1
C 1 = (e -e ) (6)
( e-am(Xl + Q-b/L -am(Xl+b)-45/L
--e )
and
C 2
b-45/L 4-b/L-I
= -(e -e ) -
(7)
-_rn(Xl+_) + b/L -C_m(Xl+b) -_/L
TB m ( e we )G o
_n
The diffusion current from the base is given by
dnId'-_- (8)J = qD n
x=_
580
TABLEI. Coefficients for the Empirical Fit to Ionization Chamber
Measurements of Energy Deposition Profiles in Aluminum
Adjacent to Beryllium, Copper, and Gold.
Relative Dose = _ Am e m
In all cases A 1 = I.0 aI = o
Beam Direction High Z - Low Z
!End
i Plate A 2 A 3 A4 ('2 33 _4I
I Be -0.159 -0. 075 0 0. 0213 0. 137 0
Cu -0.183 0.192 0 0.0094 0.0749 0
Au
-0. 262 0. 255 0. 284 0. 0097 0. 0408 0. 351
Beam Direction Low Z - High Z
iE_d_
Plate A 2 A3 A4 if2 33 _4
e 0. 283 -0. 836 0 0. 0122 0, 0325 0
Cu 0. 204 0. 293 0 0. 0172 0. 334 0
u 0,526 0. 231 0. 249 0. 0144 0. 0998 0. 346
When _<< L the ratio of the diffusion current
J(x l) to that for uniform ionization Jo' is given
by
D
J(x) _ n 1
Jo L2 _ "
Bm e-am(xl+t) 8-ae -am
where 8 = coth (b/L)
c_ csch (b/L)
The results obtained from Eq. (9) are com-
pared with the normalized solar cell response
of a cell which had a measured diffusion length
of 140 +15 Um in Figs. 7 and 8 for gold and
beryllium the normalized results agree within a
few percent. The calculated relative sensitivity
of the result to the diffusion length for these
cases is shown in Figs. 9 and 10.
6L1.5
1.4
1,3
1.2
z
o
a-
1.0
_0.9
_r
SOLAR CELL RESPONSE NEAR A
Au-AI INTERFACE IRRADIATED
WITH Co-60 GAMMA RAYS
i
o8f0.70.6
0.5
EX PERIMENTAL
-- CALCULATED
, , , -_40 80 ':>0 '60 200 2zK) 280 320 3
ALUMINUM mg/cm 2
.=
FIGURE 7. - Comparison of Measured Solar Cell
Response Near a Au-A1 Interface and Calculations
Based Upon Ionization Chamber Measurements.
tu
1.6
1.5
1.4
1.3
1.2
SOLAR CELL RESPONSE NEAR A
Be-AI INTERFACE IRRADIATED
WITH Co-60 GAMMA RAYS
i I.I _=_=_
¢:
O.8
0.7
0.6
0.5 _-
40
EXPERIMENTAL
-- CALCULATED
, , , _, , _ ; , !80 120 160 2 0 240 2 0 :5 0 560
ALUMINUM mq/cm 2
FIGURE 8. - Comparison of Measured Solar Cell
Response Near a Be-A1 Interface and Calculations
Based Upon Ionization Chamber Measurements.
581
1.9
1.8
1.7
1.6
1.5
1.4
a: 1.3
_ 1.2
w
I.I
CALCULATED DEPENDENCE ON DIFFUSION
LENGTH OF SOLAR CELL RESPONSE NEAR A Au'AI
INTERFACE IRRADIATED WITH Co'60 GAMMA RAYS
ATION CHAMBER RESULTS
1.0111 DIFFUSION LENGTH(MICRONS)
0.9 TION CHAMBER RESULTS
0.81 )'_
0.7 [ I i = _ = _ i , ,
40 80 120 160 200 240 280 320 360
ALUMINUM mo/cm 2
FIGURE 9. - Calculated Influence of Diffusion Length
Upon So!__v Cell Response NPar a Au-A! Interface.
0.9
0.8
CALCULATED DEPENDENCE ON DIFFUSION LENGTH
1.3[- OF SOLAR CELL RESPONSE NEAR A Be-At
1,2_ INTERxFAC.__E IRRADIATED WITH Co-6O GAMMA RAYS
/
I.I I" _ION CHAMBER RESULTS
|.0 DIFFUSION LENGTH (MICRONS)
/_IONIZATION CHAMBER RESULTS
7
0.7
0.6
o._ ,_ 8_ ,;o _o 2%o z_ z_o 3;'0
ALUMINUM rag/ore 2
A_*CRL PHOTO IZ1-4l
/(./o x3o 2 .3oY
CALCULATED SOLAR CELL
RESPONSE FOR VARIOUS BASE MATERIALS
Since calculations of solar cell response
based upon ionization chamber data agreed well
with measurements when the interface was ad-
jacent to the cell face, they were extended to the
more realistic situation where the material was
adjacent to the base contact. This requires a
modification of Eq. (9) as follows
J(x) _ Dn 1
% "
_B m -am(Xl+b-t) 18 -_m(6-b) L1e - ae -a m
m
(I0)
In this case x 1 is the thickness of aluminum inter-
posed betweefi the base contact and the interface.
.&ll other quantities are the same as those em-
ployed in Eq. (9).
Calculations based on Eq. (10) are shown in
:Figs. 11-14. As might be expected the perturba-
tion is largest for a thick (> 0.02 cm) gold base.
When the beam enters through the cell and exits
from the gold the response is 23% larger than it
would be for an aluminum base contact. The
reversed beam yields a result which is 12%
lower than it would be for aluminum. Even 6 _m
of gold produces an effect. The influence of
copper and beryllium are smaller than for thick
gold but readily noticeable. It is interesting to
note that when the beam enters through the
beryllium contact the response is approximately
12% greater than it would be for an aluminum
base contact. Low atomic number elements can
enhance the response if the beam traverses them
before entering the device.
SUMMARY
It has been found that ionization chamber
measurements of dose perturbations in aluminum
adjacent to gold and beryllium can be used to pre-
dict the response of silicon solar cells near the
same materials. The influence of gold, copper,
and beryllium base contacts was then calculated
for N/P silicon solar cells exposed to Co-60
gamma rays. Strong directional effects are
evident in all cases. The response can be
enhanced by both high and low atomic number
materials depending upon the direction of the
gamma beam. For the cells examined here the
response as a function of beam direction varied
as much as 40%. Ionization chamber measure-
ments indicate that at Co-60 gamma energies
(-- 1. 25 MeV) the interface effects could be as
large as a factor of two.
FIGURE 10. - Calcula{ed Influence of Diffusion Length
Upon Solar Cell Response Near a Be-A1 Interface.
582
CALCULATED SOLAR CELL RESPONSE
1"3 I- TO Co-60 GAMMA RAYS
1.2 _(GOLDTBASE CONTACT)
I.I
wm 1.0
z
0
0.9
- 0.8
o: 0.7
0.6
0.5 t
0 40
A_c_BL PHOTO I 2 I - 55
1,2
I.I
uJ 1.0
_ 0.9
_ 0,8
_ 0.7
0.6
0.5
A_I:RL Pt',01_
I I I I I I
80 120 160 200 240 280
ALUMINUM mg/cm 2
FIGURE II Calculated Response with the Au-AI In-
terface Near the Base Contact. The thickness of
aluminum indicated lies between the gold and the
base. Arrows indicate the beam direction relative
to the cell face.
I
320
CALCULATED SOLAR CELL RESPONSE
TO CO-60 GAMMA RAYS
(6_M GOLD BASE CONTACT)
),
Y
I
4O
121-5l
[ I I I I I
80 120 160 200 240 280
ALUMINUM mQ/cm 2
FIGURE 12, Calculated Response for a Thin Gold
Film Near the Base Contact.
I
360
1.2
CALCULATED SOLAR CELL RESPONSE
1.0
z
o
0.9
bJ
g¢
_0.8
LIJ
=: 0.7
0.6
0.5
0
M*CML PH010
1.2
TO Co-60 GAMMA RAYS
(COPPER BASE CONTACT)
),
y
I I I I I I i
40 80 120 160 200 240 280
ALUMINUM mQ/cm 2
I 21 - 48 FIGURE 13. - Calculated Response for a Cu-A11nter_
face Near the Base.
I
320
CALCULATED SOLAR CELL RESPONSE
TO Co-60 GAMMA RAYS
(BERYLLIUM BASE CONTACT)
I.I
w 1.0
o3
_n 0.9uJ
- 0.8
.J
_ 0.7
0.6
I d 0.5
320 360 0
A_RL _01nD
T
I I I I I I I
40 80 120 160 200 240 280
ALUMINUM mg/cm 2
I 2 FIGURE 14. Calculated Response w|th a Be-A1 Inter-
face Near the Base Contact.
I
320
REFERENCES
1. SPIERS, F.W., in Radiation Dosimetry
Vol. III, Ahix, Roesch and Tochilin,
editors (Academic Press, Inc. , N.Y.
(1969) p 809-867.
2. DUTREIX, J. and BERNARD, M., Br. 5.
Radcol. 39, 205 (1966).
3. WALL, J.A. and BURKE, E. A., IEEE
Trans. Nuc. Sci., ]WS17, 305(1970)
4. KLEIN, C. A., J. Appl. Phys. 39, 2029,
(1968)
5. RAPFAPORT, P. and WYSOCKI, J. J.,
in Photoelectronic Materials and Devices,
Larach, S., editor D. Van Nostrand Co.,
Inc. {1965)p. 239-275
583
I
360
-I
360


Solar cell radiation response near the interface of different atomic number materials

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