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LWSA Contractor Kcport 159040

LOW ' ENERGY PROTON IATION DAMAGE TO

(AIGa)As-GaAs SOLAR CELLS (NASA .CP-lsq*.. "\ LOU E N E R G Y PROTON 879-25507 PqD1A'- ;ON D A 6 a 4 E TO ( A l G a ) As-Galls SOLAR CELLS ' h a 1 Report, 5 Jul. 1978 - S Jan. 1979 .;:ughes Research Labs.) 106 p n n c l a s EC hO6/?lF A01 CSCL 1 O A G3/44 26428

Robwt Loo, Sanjiv Kamath, and Rondd Knechtli

d u m Research Laboratories

3011 Malibu Canyon Road Malibu,CA 90265

h u a r y 1979

Coriiract NAS1-15443 Find Repon For period 5 Juiy 1978 through 5 January 1979

https://ntrs.nasa.gov/search.jsp?R=19790017336 2020-07-03T17:42:10+00:00Z

LOW ENERGY PROTON RADIATION DAMAGE TO ( A 1 G a ) A s - U SOLAR CELLS

Robert Loo, Sanjiv Kamath, and Ronald Knechtll

JAWARY 1979

Contract NAS1-15443 FINAL REPORT

For period 5 July 1978 through 5 January 1979

Prepartd for NASA

Langley Research Center Hampton, VA 23665

TABLE OF CONTENTS

Section Page

1 INTRODUCTION AND SUMMARY . . . . . . . . . . . . . . . . 1

2 PHOTO I-V CHARACTERISTICS . . . . . . . . . . . . . . . 11 3 SPECTRAL RESPONSE . . . . . . . . . . . . . . . . . . . 23 4 DARK I-V CHARACTERISTICS . . . . . . . . . . . . . . . . 31 5 CONCLUSION . . . . . . . . . . . . . . . . . . . . . . 47

APPENDICES

A ELECTRON AND PROTON DEGRADATION IN (A1Ga)As-GaAs SOLAR CELLS . . . . . . . . . . . . . . . 49

B PHOTO I-V CHARACTERISTICS OF (AI.Ga)As-GaAs SOLAR CELL BEFORE AND AFTER PROTON IRRADIATION . . . . . . . . . . . . . . . . . . . . . . 59

C PHOTO I-V CHARACTERISTICS OF HIGH EFFICIENCY SILICON SOL4R CELL BEFORE AND AFTER PROTON IRRADIATION . . . . . . . . . . . . . . 85

i i f

SECTION 1

INTRODUCTION A!W SUMMARY

The o b j e c t i v e o f t h i s con t r ac t was t o eva lua te the performance o f

(A1Ga)As-GaAs s o l a r ce l l s i r r a d i a t e d by low-energy protons (50 keV,

100 keV, and 290 keV) and to compare i t t o t h e performance of s i l i c o n

s o l a r c e l l s under the same condi t ions . This s tudy is an ex tens im o f

the work previously done under con t r ac t t o NASA Langley on high-energy

proton i r r a d i a t i o n a t 15.4 MeV and 40 MeV ( con t r ac t No. NASl-14727).

Tiie low-energy proton i r r a d i a t i o n w a s performed a t Hughes Research

'Laborat:,ries (HRL) us ing an ion-implantation machine. Both t h e GaAs and

S i s o l a r c e l l s were mounted s i d e by s i d e on aluminum p l a t e s . They were

i r r a d i a t e d i n vacuum (-10 Torr) a t roow temperature. The f lu rnce over

t h e t a r g e t p l a t e w a s uniform wi th in 55 percent .

-6

Figure 1 shows the (A1Ga)As-Gas s o l a r c e l l s t r u c t u r e used for t h i s

concent ra t ion f c r t he s u b s t r a t e is >5 x 1017 cm-3 wi th T e + study.

a s t h e dopant.

th ickness i s g r e a t e r than 10 um. The Be-doped p (A1Ga)As window layer

is grown by i n f i n i t e s o l u t i o n ep i raxy on Ga4s. Its c a r r i z r concent ra t ion

i s 1 x 1 0

is forrled by Be d i f f u s i o n from t h e (A1Ga)As layer i n t o the n buf fe r

layer. Thtl c a - - r i e r concent ra t ion of the p d i f f u s e d layer a l s o is

1 x 1OI8 cm-'. The th ickness of t he (A1Ga)As window layer is 0.5 pm

o r less , and the junc t ion depth f o r t hese cel ls is c o n t r o l l t b l y held

c iose t o 0.5 pm. A shal low junc t ion depth has been shown t o enhance t h e

r e s i s t a n c e t o r a d i a t i m damage (see Appendix A). The r e r a i n i n g p a r t s of

t he base l ine s t r u c t u r e a r e se l f -explana tory . The AuZn con tac t s are

about 3000 to 4000 h th ick wi th a s i l v e r over lay about A pm th i ck . The

n contac t i s A k C e N i (-5000 A ) wi th Ag over lay . The a n t i - r e f l e c t i o n ( A R )

coa t ing i s Ta 0 The c e l l s used f o r the proton damage s tudies have a 2 5' power conversion e f f i c i e n c y of approximately 16 percent with no cover

g l a s s p ro tec t ion .

The n

The n b u f f e r layer concent ra t ion i s 1 x 1017 cm-3 and the +

. During (A1Ga)As window l a y e r growth, a p-n junc t ion 18 cn-3

0

0

1

€ E a a u ! u !

v v 0 0

a x-

E a z A

c

0 N U v) K

2 if

a w

3 2

Q

H a 15

c a 0

t f 0 0 P

I I I 1 I I I I I I I I I I I

.. c 0

c 2 a

8 r

3 0 + C

I

z 0 n X

X I r

- a P

N

c 0 N 4 U X

N 91 4 rn

u1 91 &

v) 4

E,

n

E &

2

Table 1 g i v e s t h e f u l l mat r ix of tests performed on t h e G a A s and

s i l i c o n s o l a r c e l l s .

w i th 12-mil-thick cover g l a s s and then i r r a d i a t e d under the same condi-

t i o n s as t h e o t h e r s t o v e r i f y t h e s h i e l d i n g e f f e c t o f g l a s s a g a i n s t low-

energy pro tons.

In add i t ion , t h r e e GaAs s o l a r ce l l s were pro tec ted

Figure 2 shows t h e pene t r a t ion range of protons bombarding G a A s and

s i l i c o n a s a func t ion of proton energy. The low-energy protons do no t

pene t ra te f a r i n t o t h e c e l l , and t h e phys ica l damage they create is c l o s e

t o t h e sur face o f the c e l l . Also, l o r e n e r g y protons produce nonuniform

damage; when they are stopped i n a s o l a r ce l l , mst o f t h e damage is

concentrated a t t h e end of t h e proton t r ack . Since 50-keV pro tons are

stopped about 0.5 pm below the sur face , most of t h e damage is produced

i n t h e (A1Ga)As and i n t h e p

protons reach t h e junc t ion . The 100-keV pro tons are stopped about

1 .0 Frn below t h e su r face , c r e a t i n g damage c l o s e t o t h e junc t ion . The

290-keV protons pene t r a t e deeper i n t o t h e ce l l and are stopped -2.5 pm

below t h e sur face , causing damage throughout t h e a c t i v e region of t he

GaAs s o l a r cells . Since the S i s o l a r ce l l s have junc t ion depths between

0.2 p m and 0.3 pm, a l l t h e low-energy pro tons pene t r a t e beyond the

junc t ion and cause most of the damage i n the base p region.

+ Be-diffused G a A s l a y e r before t h e

The r e s u l t s of low-energy proton i r r a d i a t i o n tests are summarized

i n Figure 3. It shows t h e percentage of remaining maximum power output

versus proton f luence f o r t h e G a A s and s i l i c o n s o l a r ce l l s s p e c i f i e d i n

Table 1. The l o s s in power i n the s i l i c o n c e l l s ( t he do t t ed l i n e ) i s

mainly due t o the reduct ion i n the open-c i rcu i t vo l t age and f i l l f a c t o r

(FF). A s shown i n Sect ion 2 (Table 3) and discussed below, the G a A s

c e l l s a l s o s u f f e r l o s s e s i n V and FF but s u f f e r mostly from a reduc-

t ior . i n s h o r t c i r c u i t cu r ren t I . The t h r e e G a A s s o l a r c e l l s p ro tec t ed

by a cover g l a s s show no s lgn of degradat ion.

oc

s c

Table 2 g ives the c r i t i c . a l f luence f o r G a A s and s i l i c o n s o l a r c e l l s .

Figure 4 shows the percentage of remaining s o l a r c e l l power output ve r sus

proton energy f o r low-energy protons. For p r a c t i c a l space missions, the

e f f e c t of high-energy protons have t o be considered a s w e l l a s the eEfect

3

Table 1. Prcton I r r a d i a t i o n Tes t Matrix f o r t h e (A1Ga)As-GaAs and S i l i c o n Solar Cells

1 l o 9 1 x 10 lo

1 x 10l1

1 x 1 0 l2

1 x 10

1 x 1o1O

1 x 1 0

1 x 1 O I 2

1 lo9

1 x 1o1O

1 x 10l1

1 x 10 l2

50

100

2 90

2

2

3+1

3+1

9 2

2

3+1

3+1

2

2

3 t l

3+1

A1Ga)As-GaAs So la r Cells

Cell Number

2266, 2295

2266, 2295

2406, 2407, 2411 2402b

2426, 2427, 2428 2402b

2302, 2307

2302, 2307

2417 2419, 2420 24 306

2429, 2432, 2456 24 30b

2370, 2375

2370, 2375

2421 2422, 2424

2457 2477, 2311 2446A

2446 b

si1 icon Solar Cellsa

Number o f Cells

3

3

3

3

3

3

Cell Number

2 , 5 , 8

13, 14 , 15

1, 4 , 7

16, 1 7 , 18

a n onp,-0.2 t00.3 pm junc t ion depth, 2.2 c m x 2.0 cm, 12-mil- t h i c k h igh-ef f ic iency s i l i c o n s o l a r ce l l s wi th 3.0 AR coat- i ng , no cover g l a s s p ro tec t ion . X

b C e l l Numbers 2402, 2430, 2446 have 12-mil-thick cover g l a s s p ro tec t ion .

6483

4

3

2

2

5 w- (3 z 2 1

8252-1

0 50 100 1 50 200 250 300 350 PROTON ENERGY, keV

Figure 2 . Proton energy versus pno,jected range i n GaAs and S i .

5

8 8 8 $ R L-

% 'I f ldlf lO L13MOd 9NlNlVW3M 4 0 39VlN33M3d

9) 0

1 ri W

C 0 U 5 Id a rn

Id 9) > U a a U

0

L 9)

a d d 9) U h a

I 4

0 to

bo C d E 4

aJ b ru 0

QI M a U C aJ V 8.l aJ 01

8

2

a

;1

n Y L

.=I W -94 a

6

I I I I 3

7

of low-energy protons.

c e l l power output ve r sus pro ton energy a t both low and h igh energy proton

range.

p r i o r NASA Langley con t r ac t s . ) For comparison, t h e r e s u l t s obtained wi th

s i l i c o n c e l l s i r r a d i a t e d s imultaneously wi th our G a A s cells are a l s o

shown i n Figure 5.

t i o n f o r t h e in te rmedia te proton energy range.

sh ie lded a g a i n s t low-energy pro tons by a coverg lass (Curve I, Figure 5 ) ,

while no p r o t e c t i o n is a v a i l a b l e a g a i n s t t h e high-energy protons.

s h o r t , Figure 5 shows t h a t a t a l l ene rg ie s h igher than 2 5 MeV, GaAs

c e l l s show supe r io r r a d i a t i o n hardness as compared wi th s i l i c o n cells.

In t h e reg ions below 5 MeV where G A S c e l l s are s u s c e p t i b l e t o damage,

they a r e e f f e c t i v e l y sh ie lded by t h e irmal coverg lass p r o t e c t i o n used

w i t h t h e c e l l s i n a l l space missions (Curve 7, Figure 5).

Figure 5 shows t h e percentage of remaining s o l a r

(The high-energy proton i r r a d i a t i o n s t u d i e s were performed under

The do t t ed curve of Figure 5 shows t h e in t e rpo la -

The s o l a r cell is usua l ly

I n

More d e t a i l e d da t a and observa t ion an t h e tests o f t h e (A1Ga)As-

G a s and s i l i c o n s o l a r c e l l s a r e given below.

Proton Energy, keV

2 90

Tab!e 2. C r i t i c a l Proton Fluence f o r GaAs and S i So la r Cells

2 C r i t i c a l Fluence, p/cm

GaAs Solar Cells S i l i c c n Solar Cells

5 x l o l l 11 4 x 10

4.2 x 10

1 0 3 x IC!

11 4 x 1.0

1.7 x 10

10

11

8

8511-3A2

. _ .

1

0 Si SOLAR CELLS

9 GaAs SOLAR CELLS

PARAMETER: FLUENCE. IN PROTONS/CM~

I I I I I I - OL

0 0.1 1 10 100 PROTON ENERGY. MeV

Figure 5. Proton r a d i a t i o n damage: GaAs versus S i solar c c l l s .

9

SECTION 2

PHOTO I -V CHARACTERISTICS

Figures B-1 through B-24 i n Appendix B show t h e a i r mss zero (AMO)

photo I - V c h a r a c t e r i s t i c s of GaAs and S i s o l a r c e l l s before and a f c e r

proton i r r a d i a t i o n a t 50 keV, 100 keV, and 290 keV. Table 3 has been

compiled from t h e s e I-V c h a r a c t e r i s t i c s and g ives t h e fol lowing charac-

t e r i s t i c s of t h e ind iv idua l cel ls : Is,; Voc; FF; maximum power o u t p u t ,

pmx; and power conkersion e f f i c i e n c y , q.

t h i ckness D and t h e j u n c t i o n depth X j were measured by a scanning elec-

t r o n microscope (SEM) f o r each c e l l and are also shown on Table 3 t o

a s s i s t ou r understanding of t h e i r r a d i a t i o n r e s u l t s .

The (A1Ca)As window l a y e r

Figure 6 shows t h e percentage of remaining I a s a func t ion of s c proton f luences f o r 50-keV, 100-keV, and 290-keV proton ene rg ie s . Both

the 50-keV and 100-keV protons reduce t h e I less than t h e 290-keV

protons. This i s because most o f t h e danrage produced by 50-Lev and

100-keV protons i s c l o s e t o t h e junc t ion and on ly w i t h i n t h e p region,

whereas t h e 290-ke;' protons pcne t r a t e .ieep i.,to t h e c e l l , causing damage

on both s i d e s of t h e junc t ion (p acd n r eg ion ) . Figure 7 shows t h e mea-

silred V a s a fu i . - t ion of proton energy. For o u r GaAs s o l a r c e l l , V

degrades more wi th proton i r r a d i d t i o n a t 100 keV than a t 50 keV o r a t

290 keV. This i s exp l i cab le be . ise t h e 100-keV protons are stopped a t

t h e p-n j u n c t i . where they produc: s o a t of t h e damage.

sc

oc oc

Figures C-1 through (2-18 i n Appendix C show t h e photo I - V charac te r -

i s t i c s of t he s i l i c o n s o l a r cell before and a f t e r proton i r r a d i a t i o n ; t he

r e s u l t s are presented i n Table 4 . The S i s o i a r c e l l ' s I does n o t

change when the c e l l i s i r r a d i a t e d by 50 ksV and 100 keV up t o 1 x 10

cm proton fluence. However, i t i s d r a s t i r a l l y lower a t 290 keV a t

1 x 1 O I 2 c m 2 protcn flueiice.

junct ion ( l e s s than 0 .3 pm) and has no window l a y e r . Therefore , even a t

t h e lowest energy, 50 keV, protons pene t r a t e beyond the junc t ion . In

t h i s ca se , t h e Voc keeps decreasing wi th inc reas ing protc.1 ene rg ie s .

12 sc

-2

Also, t h e s i l i c c s s o l a r c e l l h a s a stiallow

11

I

U a 0

U U 0

m hl w 0 0

m rn d

C

* . c l U 0 \o L)c) 1A In In U u - e

d 0 0 0 n x

cu VI 0

u) 0 0 - o m * m r l r ( - 0 0 s o 0 o o m o m m m o m 0 0

r l d r ! 4 4 0 4 0 0 0 A d 4 4 . . . . . . . . . . . .

4 " I 0 0 l-4 rl 4 d

m r ( m r l d rl 4 4 0 0 0 0 0 0 0 0

o x x o x x o x o x o x o x r l r ( r l 4 4 rl 4 d

rl 4 4 r l d rl d rl

0 0 0 0 0 0 0 0 0 0 0 0 0 0 W l In V I I n VI VI u) m

G 9 hl cv

In m N N

h 0 U N

4 @N 4 0 U U cv N

12

69

C a

FL FL

m U N m 0 0

m Q) U 0 0

J.

u m m a 0 h h m a 0 * h 9 9 h h h h h h h h h h

0 0 0 0 0 0 0 0 . . . . . . . .

m r l m N u 0 0 o m o m o m 0 0 . . . . . . 4 0 4 2 A d r l r l

N N N hl 4 r( rl rl 0 0 0 0 rl rl rl rl

o x o x o l e o x rl rl rl rl

0 0 0 0 0 0 0 0 rn In m m

ID

0 rl M & 61

0

Y 0

V-I

I rl .rl

N rl

c U d 3

m

g

5

L:

m rl 4 9) u

(d -

1 3

w

C

In 00 tn tn N 4 l-l cv N

0 0 0 0 0

ul U

0

00 f- tn ul U U U U 0 0 0 0

f - 4 0 r l u l o 1 3 a m m c u m m . . . . . . . . . . . . u l u l u \ o m u 9 0 9 m e o t n t n r l 4 4 r l r l r l 4 4 r( r l r l r l r l

a a a m m U m m 0 0 h h e 9 u o o c v f - u r - I - h h h h h h r - h h r - h h r - r - . . . . . . . . . . . . 0 0 0 0 0 0 0 0 d d oc, 0 0

ul C O N r l C O N r l c v rl cv m m

o m m o m m 0 0 0 0 0 0 0 0 0 m m . . . . . . . . . . 4 0 0 d o c A d 4 0 d o d d

0 0 rl rl rl rl m 4 m r l rl rl rl d 0 0 0 0 0 0 0 0 r l r l r l r l rl rl rl 4

o x x o x x 0 % 0 % c x o x 4 4 4 4 rl rl rl rl

0 0 0 0 0 0 0 0 o c 0 0 0 0 0 2 4

0 0 c o 0 0 r i d r l r l rl 4

h 0 0 e4

(d h m 0 0 rl 4 N Pl U U U * N hl hl N

14

n 7) 0) 1 C rl U C 0 Y, C 0 r( U (D

-4 a (D L L

c 0 U 0 L c4

L aJ U u4

W

a a C a 9, L 0 u4 9, a m 0 r( U

4 L 9, U u Q L a L: V rl 4 aJ u L Q rt 0 [/I

m c I

n a

4

a

J z 2 W

n D

9

Q, I-4 .n CF w

v

\D d

0

9 h

0

ln

0 ?

h m 0

a0 m d Crr a s ? r - m 9 h h h

d d o c c o d d b h g r - 9 h h 1 - h . . . .

m - 9 a0 N m m m 0 9 0 9 o m m m

> 4 0 4 0 4 0 0 0 :+ . . . . . . . .

hl hl N N 4 rl I-l l-l 0 0 0 0 4 I-4 rl rl

o x o x o x o x l-i I-4 4 4

Q 9 0 N 9 0 N m In m U U U U N N N hl

3 $I 15

0 8

C !

k a Is.

U m 0

m In

0

In In 00 N N U

0 0 0

N 9

0

In

0

0 In

0

In In 00 U U U 0 0 0

N 9

0

m m m a 9 N I n In 9 9 0 0 m m 0 0 o o m h U \ o m -. . Io m a a c r l h h h h h h h h h h W - . h h h h h . . . . . . . . . d o 0 0 0 0 d d 0 0 0 0 d d d d

In In m u m u In 9 r l a 0 4 N 0 - m o m m o m 0 0 0 o m 0 0 O h . . . . . . . . . . . . . .

r 3 0 4 0 0 4 0 i d F ' O r l r l r l o

O h 9 a 9 u I n 0 U N 0 0 0 2 u uoo 4 o m o o m 4 0 0 . + m o m 0 0 r l u 4 4 r l r l rl 4 rl r l r l rl

.. rl rl rl rl N

rl rl rl rl 0 0 m r l m r l rl

4: 0 0 0 rl r( 4

0 0 0 0 0 0 0 X ; . c x x x X X X X

r r d 4 4 rl rl 4 4 rl

0 0 0 2 2 4 4 rl

0 0 0 0 0 0 0 0 m m m m hl N hl hl

O S o g o 0 0 0 m m m c\1N N N N

3 h 0 r-4

In h m cu

m rl cv U 9 hl N N U U u U U hl N N hl

h ln U hl

16

m (d

3 5 O U E a c 2

O N

C m

L4 L4

c X

u r n m o r l r l . . . . . . m u m u m m rl d r l d

9 m o m a m a r - . b h 9 h r - 0 0 0 0 0 0 . . . . . .

N a h 0 0

m a h l d N N 0 3 - O O N m m

. . . .

a0 A d O h O h 0 0

4 0 d o 4-4 . . . . . .

O d a m u u F l u 7 o u 0 0 d -4 4 4

N Y N 4 d rl 0 0 0 rl rl rl

X X X 0 0 0

4 d 4

0 0 0 0 0 0 Q\ m m N N hl

(D h 4 a h -4 u U rl U N N hl

17

Q) u

1 d vc c 0 rl U (d r( P a & Y r(

c 0 U 0 Lc a ID 1 ID b Q) > Y

Lc Lc 3 0

U r( J c) & 4 u u & 0 c rn

G

G

\o

01 & 1 M 4 Crr

18

Table 4. Si l icon Solar C e l l Charac te r i s t ics Before and After Proton I r rad ia t ion

C e l l Number

3

6

9

10

11

1 2

2

5

8

13

1 4

15

Energy, keV

0

50

0

50

0

50

0

50

0

50

0

50

0

100

0

io0

0

100

0

100

0

100

0

t 00

F1 uenc e,

p l cC2

0

1 x 10

0

1 x 1 0

0

1 x 10

0

1 x 10

0

1 x 10

0

1 x 10

0

1 x 10

0

1 x 10

0

11

11

11

1 2

1 2

12

11

11

1 x l o l l

1 x 10l2

0

0

1 x 10 1 2

I sc * mA

168

169

1 7 1

170

168

167

170.8

168.5

170

169

169.5

169

172

172

174

174

169

168

171.5

168

171.5

169

0 1 7 2

1 x 10 l2 1 169

voc , V

0.595

0.54

0.59

0.54

0.59

0.54

0.6

0.48

0.60

0.475

0.6

0.h75

0.60

0.54

0.595

0.54

0.59

0.54

0.6

0.45

0.6

0.445

0.6

0.45

pMAx, mA

73

65.1

75.2

65.5

72.9

63.8

76.2

54.4

76

54

76

54

75.7

66.4

76.1

68

73

63.6

77

48.9

77.6

48.5

76.1

48.7

FF

0.73

0.71

0.72

0.713

0.735

0.708

0.74

0.672

3.745

0.673

0.747

0.673

0.733

0.714

0.734

0.723

0.73

0.70

0.748

0.65

0.754 0.645

0.738

0.64

n , X

12

10.9

12.6

11.0

12.2

10.7

12.8

4.12

12 .8

9.1

12 .8

9.1

12.7

11.1

12.8

11.4

12.3

11.8

13.0

8.2

13.0

8.15

12.8

8.2

648

20

Cell Number

4

7

1

16

1 7

l a

0.39

0.60

0.39

Table 4. Si l icon Solar C e l l i h a r a c t e r i s t l c s Before and Af ter Proton I r r ad ia t ion (Continued)

24

76.1

26.5

J?J=wY, keV

0

2 90

0

2 90

0

2 90

0

2 90

0

2 90

0

2 90

Fluence,

p /cm2

0

1 x 10

0

11

1 x l o l l

1 x l o l l 0

@

1 x 10

0

12

1 x 10l2

0 1 x 10 1 2

I S C * mA

173

169

165

164

173

1C6

172

103

172 114

172

104

"OC ' V

0.59

0.52

0.585

0.52

0.6

0.52

0.59

pEIAx, mn

76.1

63.9

71.7

60.3

76.5

61.5

76

0.59

0.40 68.2

23.2

FF

0.75

0.727

0.743

0.707

0.737 0.712

0.749

0.60

0.737 0.60

0.67

0.56

n, x 12.8

10.7

12.0

10.1

12.8 10.3

12.8

4.0

12.8

4.46

11.5

3.9

648:

2 1

SECTION 3

SPECTRAL RESPONSE

Figures 8 through 10 show t h e average s p e c t r a l response o f t h e

(AlGa)As-GaAs s o l a r c e l l be fo re and a f t e r proton i r r a d i a t i o n wi th proton

ene rg ie s a t 50 keV, 100 keV, 290 keV, r e s p e c t i v e l y . A t 50 keV and

100 keV, t h e s h o r t wavelength region of t h e s p e c t r a l response shows more

degradat ion thdn does the long wavelength region.

long- and short-wavelength reg ions of t h e spectral response show

degradation. This is c o n s i s t e n t wi th t h e f a c t that t h e lower energy

protons, as does t h e s h o r t e r wavelength l i g h t , p e n e t r a t e less deeply

i n t o t h e GaAs than do t h e h igher energy protons. Thus, t h e lower energy

protons produce mst darnage where the s h o r t e r wavelength l i g h t genera tes

most of t h e e l ec t ron -ho le p a i r s .

produce most damage i n t h a t p a r t of t h e s o l a r cel l where t h e s h o r t wave-

l e n g t h l i g h t genera tes mst of t h e cu r ren t . .Th i s means, a s observed

above, t h a t lower energy pro tons should a f f e c t mostly t h e s p e c t r a l

response on t h e s h o r t e r wavelength s i d e . Conversely, t h e h ighe r energy

pro tons pene t r a t e more deeply i n t o t h e GaAs , as does t h e longe r wave-

l eng th l i g h t . The h igher energy pro tons w i l l t h e r e f o r e a l s o a f f e c t t h e

long wavelength p a r t o f t h e s p e c t r a l response. These obse rva t ions show

t h a t t h e s p e c t r a l response is s t r o n g l y a f f e c t e d by proton energy as w e l l

as by t h e proton f luence , j n agreement wi th our phys ica l model of t h e

GaAs s o l a r c z l l .

A t 290 keV, both t h e

The lower energy pro tons t h e r e f o r e

Yigures 11 through 1 3 show t h e average s p e c t r a l response o f t he

s i l i c o i i s o l a r c e l l s before and a f t e r proton i r r a d i a t i o n . There is no

change in s i l i c o n s o l a r c e l l s p e c t r a l response a f t e r 50-keV o r 100-keV

proton i r r a d i a t i o n . However, a d r a s t i c reduct ion occurs , a s expected,

i n t h e long wavelength reg ion a f t e r i r r a d i a t i o n w i t h t h e higher energy

290 keV protons.

The change in t h e spectral response of t h e GaAs s o l a r ce l l s is i n good agreement w i t h t h e change i n I a f t e r i r r a d i a t i o n

Furthermore, as ind ica t ed above, t h e change in t h e s p e c t r a l response of

the c e l l s a f t e r i r r a d i a t i o n ag rees w i t h our model of t h e performance of

these c e l l s .

sc

23

WAVELENGTH, pm

Figure 8. (A1Ca)As-GaAs so lar c e l l spectral response before and a f t e r proton irradiation (proton energy = 50 keV).

1

a rri v)

H v)

w a

0.4 0.5 0.6 0.7 0.8 0.9 1 .o 1.1 WAVELENGTH, prn

F 'igure 9 . (A1Ga)As-GaAs so lar c e l l spectral response before and a f t e r proton irradiat ion (proton energy = 100 keV).

25

Figure 10. (.41Ga)As-GaAs so lar c e l l spectral respsiise before and a f t e r proton irradiatjsii (proton energy = 290 keV).

26

WAVELENGTH, pm

Figure 11. Spectral response of S i solar c e l l s before and a f t e r proton irradliation (proton energy = 50 keV).

27

WAVELENGTH, rn

Figure 12 . Spectral response of S i solar c e l l s before and after proton irradiatim (proton energy = 100 keV).

1

0.4 0.5 0.6 0.7 0.8 0.9 1 .o 1.1 WAVELENGTH, pm

Figure 13. Spectral response of Si solar cells before and after proton lrradiation (proton energy = 290 keV).

2 9

SECTION 4

DARK I-V CHARACTERISTICS

The dark I-V characteristics are an important measure of solar cell

perfornrance because they a f f e c t the cell 's f i l l f a c t o r and open-c i r cu i t

vo l tage .

e f f e c t o f t h e c u r r e n t t r a n s p o r t &chanisms:

currents . The t o t a l forward c u r r e n t vo l t age r e l a t i o n s h i p is t h e sum of t hese two c u r r e n t components:

The dark I - V characteristics are determined by t h e corsbineJ

d i f f u s i o n and recombination

where I is t h e diode leakage c u r r e n t , and n is the junc t ion q u a l i t y

f a c t o r , which v a r i e s between 1 and 2. The lower l i m i t of n (n= l ) impl ies

t h a t t h e diode is d i f f u s i o n l imi t ed . In t h i s case, t h e upper va lue n=2

corresponds t o t h e l i m i t i n g case c o n t r o l l e d by the generation-

recombination mechanism. Values of n g r e a t e r than 2 are a l s o poss ib l e i f

t h e space-charge recombination process involves more than one in te rmedia te

energy l e v e l .

0

Figures 14 through 23 shor t h e dark I-V characteristics be fo re and

a f t e r proton i r c a d l a t i o n f o r our GaAs cells. Before i r r a d i a t i o n , t h e

s lope of l o g I-V c h a r a c t e r i s t i c s o f t hese c e l l s has an n f a c t o r between

2.0 and 2.4. Thus, t he c u r r e n t t r a n s p o r t mechanism i s dominated by mre than one energy level a t t h e junc t ion .

Iog 'I-V c h a r a c t e r i s t i c s is lower, and the n f a c t o r is between 1.8 and

1.9. This i n d i c a t e s that the t canspor t mechanislu is dominated by a new

recombination c e n t e r introduced dur ing i r r a d i a t i o n . Also, t h e dark i -V

c h a r a c t e r i s t i c s a f t e r i r r a d i a t i o n (Figure 23) show t h a t 100-keV pro tons

damage the junc t ion more than do 290-keV protons.

ou r prev ious observa t ion i n the V measurements.

A f t e r i r r a d i a t i o n , t h e s lope of

This ag rees w e l l wi th

oc The d a r k I-V c h a r a c t e r j s t i c s f o i s i l i c o n s o l a r c e l l s were a l s o mea-

sured before and af ter i r r a d i a t i o n , as shown on Figures 24 through 26.

31

Figure 14. (A1Ga)As-GaAs solar cell dark I-v characteristics before and after proton irradiation (cell 2295). Proton energy = 50 keV; proton fluence = 1 x 101' p/cm2.

32

VOLTAGE, V

Figure 15. (A1Ca)As-GaAs solar cell dark 1-v characteristics before and after proton irradiation (cell 2407). Proton energy = 50 keV; proton fluence = 1 x 1011 p/cm2.

33

VOLTAGE, V

Figure 16. (A1Ga)As-GaAs solar cell dark I-V characteristics before and after proton irradiation (cell 2 4 2 7 ) . Proton energy = 50 keV; proton fluence = 1 x 1012 P/cmz.

34

Figure 17. (A1Ga)As-GaAs solar cell dark I-V characteristics before and after proton irradiation (cell 2302). Proton energy = 100 keV/ proton fluence = 1 x 1010 p/cm2.

35

VOLTAGE, J

Figure 18. (A1Ga)As-GaAs solar c e l l dark I-V characteristics before and after proton irradiation (ce l l 2420). Proton energy = 100 keV; proton fluence = 1 x 1011 P / C P * .

36

VOLTAGE, V

Figwe 19. (A1Ga)As-GaAs solar cell dark I-V characteristics before and after protan lrradistion (cell 2432). Proton energy = 100 keV; proton fluence = 1 x 1012 p/cm2.

37

VOLTAGE, V

Figure 20. (A1Ga)As-GaAs solar cell dark I-V characteristics before and after proton irradiation (cell 2375). Proton energy = 290 keV; proton fluence = 1 x 1010 p/cm2.

38

VOLTAGE, V

Figure 21. (A1Ga)As-GaAs solar cell dark I-V characteristics before and after proton irradiation (cell 2424). Proton energy = 290 keV; proton fluence = 1 x 1011 p/cm*.

39

VOLTAGE, V

Figure 22. (A1Ga)As-GaAs solar cell dark I-V characteristics (cell 2311). Proton energy = 290 keV; proton fluence = 1 x 1012 p/cm2.

40

I 1 I I I I 0.2 0.4 0.6 0.8 1 .G

VOLTAGE, V

Figure 2 3 . (A1Ga)As-GaAs solar cell dark I-V characteristics.

41

a

0.1 0.2 0.3 0.4 0.5 0.6 10-7

VOLTAGE, V

Figure 25. Si solar cell dark I -V characteristics after irradiation (proton energy =- LOO keV).

4 3

I - - - 3 - - - - - -

- - - - - - - - - - - - -

-

- - - - -

44

- - - - - - 3

- -

104 L - - - - - - - - - - - c - - PROTON ENERGY = 290 keV -

10-7 I I I 1 I 0.1 0.2 0.3 0.4 0.5 0.6

In the case of the silicon cells, although they became more leaky after

irradiation, the basic transport mechanism did not change. This conclu-

sion resulted from our observation that the two I-V curves before and

after irradiation stayed parallel to each other at the higher currents.

The increased leakage current after irradiation probably resulted from

an increase in the number of recombination centers at the junction, with-

out the generation of new types of recombination centers. In summary,

the change in the cells dark I-V characteristics after proton irradia-

tion is in good agreement with the change in V and fluence.

irradiation gives us further insight into the effects of proton

irrhdiation on the current transport mechanism in these cells.

for each proton energy oc In addition, the change in the log I-V slope after

45

SECTION 5

CONCLUSION

This i n v e s t i g a t i o n has given u s a mre d e t a i l e d understanding of t h e

e f f e c t of t h e non-uniform damage produced by lowenergy protons i n both

GaAs and S i sol?r cells. The damage in t h e s o l a r cell's s h o r t - c i r c u i t

c u r r e n t , open-circui t vo l tage , s p e c t r a l response, and dark I-V charac-

teristics are explained by t h e proton penet ra t ion p a t t e r n i n t o the solar

cell.

su r face of t h e cel l and have l i t t l e e f f e c t on t h e s o l a r cell's

performance.

reducing the open-circui t vo l tage by degrading t h e junc t ion q u a l i t y .

290-keV pro tons pene t r a t e deep i n t o the cell, c r e a t i n g heavy damage in

t h e base n region, t hus g r e a t l y reducing t h e s h o r t - c i r c u i t c u r r e n t and

causing g r e a t e r power l o s s .

In GaAs s o l a r cells, t h e 50-keV pro tons are stopped a t t h e f r o n t

The 100-keV pro tons are stopped c l o s e t o t h e junc t ion , t h u s

The

The 5O-keV, 100-keV, and 290-keV protons pene t r a t e beyond t h e junc-

t i o n in t h e s i l i c o n c e l l s (xj = 0.2 t o 0.3 pm). The power l o s s in t h e

s i l i c o n c e l l after low-energy proton i r r a d i a t i o n is most ly r e f l e c t e d i n

t h e change in t h e open-circui t vo l t age and f i l l f a c t o r .

l o s s i n s h o r t - c i r c u i t cu r ren t i n these s i l i c o n cells u n t i l they were

There was no

2 bombarded by 290-keV protons a t a 1 x lo1' p/cm f luence l e v e l . The

change in t h e open-c i rcu i t vo l tage , sho r t - c i r cu i t c u r r e n t , s p e c t r a l

response, and dark I-V c h a r a c t e r i s t i c s after i r r a d i a t i c n a t each proton

energy and f luence were found t o be c o n s i s t e n t with oa r explanat ion O F the e f f e c t of protons.

show t h a t a new recombination c e n t e r dominates t h e c u r r e n t t r a n s p o r t

mechanism a f t e r i r r a d i a t i o n .

Also, t h e GaAs cells' dark I-V c h a r a c t e r i s t i c s

The GaAs cel ls are more a f f e c t e d by low-energy pro tons than is t h e

s i l i c o n cel l because the low-energy protons are stopped wi th in the s h o r t

a c t i v e region of t he GaAs c e l l and produce a high amount of e f f e c t i v e

darnage.

glass, as demonstrated i n F igc re 3.

However, these ce l l s can be e f f e c t i v e l y sh ie lded by the cover

To complete our proton damage s t u d i e s on G a A s c e l l s a s a funct ion

of proton energy and f luence , it is important t o study the damage a t

47

in te rmedia te proton ene rg ie s (from 50 keV to 1 5 MeV). w i l l improve understanding of proton damage characteristics in GaAs c e l l s

and provide t h e da t a requi red for determining t h e optimum cover glass

p ro tec t ion t h a t can be provided f o r t hese solar cel ls in s p e c i f i c space

missions, wi th given proton energy spectra and f luences.

be i n s t r u c t i v e , as another f u t u r e study, t o study the anneal ing charac-

t e r i s t i c s of t h e var ious damage c e n t e r s produced by protons of i ' f f e r e n t

ene rg ie s and f luences.

c h a r a c t e r i s t i c s could provide a d d i t i o n a l means f o r i nc reas ing the l i f e

of GaAs s o l a r c e l l s i n space missions.

This information

It w i l l also

A d e t a i l e d understanding of t hese annea l ing

48

APPENDIX A

This paper was presented to the 13th Photovoltaic Specialists

Conference, June 1978 a t Washington, D.C.

49

APPENDIX A

BLBCTDOW AND PBoTolS DKGMDATIOU 111 (AlGdhr-Gdr SOLI0 CELLS

R. Lao, L. Coldhammer,* B. Anspaugh,** R.C. Knechtli and C.S. RaMth

Bugbee Reoeorch hboratorieo ~ l b u . C a l i f o m l a 90265

Reeults ou radiation d-8 in (A lb )k -CCdr oqlar cells by 1 MeV e lec t ron fluences up to 1 x l0l6 fluencae up to 5 x lo f l P cm-- are presented. The damage is compared v i t h data on state-of-the-art ei'licon c e l l s vtiich were i r rad ia ted aloag v i t h the gallium arsenide ce l le . Va v e r i f i e d experiment- a l l y our theore t ica l expectation that the j u n c t h depth ham t o be kept re la t ive ly shallow, to mlni- d z e rad ia t ion damage. The Z-ge to the c.Ae cells as a function of i r rad ia t ion . is correlated v i t h the change in t h e i r spectral response and dark I - V characteristics. The e f f e c t of t h e n u l arnuling on the (AlCa)As-cds solar c e l l s was also investigated. Ibis data is used to predict fur ther avenues of optimlieation of the cds cells.

and b 15, 20) 30 and 00 UeV proton

INTRODUCTION

The behavior of solar cells under rad ia t ion avironment is of grea t Importance f o r space application. Prevloue s tudies have shoun that the (AlCe)Ae-GaAe so la t cells have achieved an ef f ic iency of 18.5% MD (1) v i t h a radiation res i s tance equal to or b e t t e r than that obeerved in v i o l e t s i l i c o n calls (21. In t h i s paper, we report the radiat ion e f f e c t on large-area (2 cm x 2 cm) ( A 1 C e ) A a - c l i k eolar cello fabricated a t Hughes Research Laboratorlee (HRL) wiry the i n f i n i t e m e l t l iqu id phase e p i t a x i a l (WE) grovth system. Our best cell t o da te has an AH0 e f f i - - ciency of 18%. and our improved shallow-junction cells show more radiat ion res i s tance than s i l i c o n cells.

CaAS has a la rge o p t i c a l absorption conecant and a shor t d i f fus ion length; essent ia l ly . a l l the photovoltaic response is close t o the GaAB surface. region has a negl ig ib le e f f e c t on c e l l p e r f o w n c e . Consequently, the reduction i n the required minority c a r r i e r diffusion length and t h e rehtive s h a l l m e s s of the a c t i v e region a r e the key factore that can be exploited t o make GaAe solar c e l l a more radiat ion res i s tan t . Data consis tent with these observations is presented beiow.

The radiat ion domege beyond t h i s a c t i v e

*Hughcn , \ i r , . r , I t t L.rmp,iny, F1 Scgiiiidci, (:ailfornia ** 1.1'.: ... IJ,I.,~~L.,?.I, h i lit>rni:i

mmmmmL -0 1 ehorn tho (luC;.)b-Gub eolar coll

etructu?a. The n+ concentration for the nubetrate l o f i n d at 5 x 1017 c r 3 v i t h Te ae the dopant. The n buffer layer concentration la 1 x 1017 -3. ( A t thio dop- level. the open-circuit voltage la 1 V.) The thickneu of t h i o layer v u f ixad a t 10 um or more became reoul te in t icared that t h e subotrate w l e i b l l i t y l a r i n f . i r e d at e buffer layer thicknemo of 10 m. A thicknear l e u than t h l e is not aluaye ouf f ic ien t to remove the e f f e c t of the subs t ra te on cell performance.

AR COATING

\ pCOMACT

LIGHT \

(AI&)&

GaA8

'p-n JUNCTION 'n CONTACT

NUMBER OF FINGERS- 24 pCONTACT. Au-Z*& nCONTACT: Au-GdUPM AR COATING: T e s pAIx&l,Ar X 2 0 s CELL SIZE - 2 x 2 CM2

Figure 1. The (A1Ca)As-CaAe solar cell

50

The viadow layer of (A&Gal-&s is gram by LPB on GaAs. Our layer has x > 0.90. making t b baridgap and hence the o p t i c a l tr&emlssion as high ne possible. Ihe dopant is b ryllium (Be). The

vhdw l ayer gtosrth. a p-n holPDjuuction 1s fo-d by Re diffusion Prom t h e (AlGa)Aa layer into the n buffer layer . The earrler concentra loa of the

amcent ra t ion is 1 x 1018 an- 3 . Durw ( A I ~ ) A S

p d i f f w e d region is a l s o 1 x 10l8 cm' 5 . The remain- parts of the basel ine s t ruc ture

are self-explanatory. 3OOO to 4000 A with a silver overlay about 9 urn thick, and the n contact 1s AuGeNi (s 5000 A) with an Ag overlay.

The Au-Zn contacts are,about

The AR coat ing is Ta2O5.

The oynamltron p a r t i c l e accelerator at JPL V(LB u e d as the electron source f o r high-energy e lec t ron i r rad ia t ion ; the i r rndla t loae were perfolred in vacuum a t rooin temperature. d f o r d t y over the test plane was f OX with no areas of discontinuity. Paaeured Vith a Faraday cup t h e current of which waa integrated t o es tab l i sh e lec t ron fluences and t o automatically s top the i r rad ia t ion a t the desired fluence levels.

The

Fluxes and f luences were

Mgh-energy proton i r r a d i a t i o n vas performed at the Crocker Nuclear Laboratory a t the University of Cal i fornia at Davis. duce a primary proton beam a t energies between approximately 8 and 68 MeV. oounted with emall pieces of double-€ace masking tape t o aluminum plates . separately in air at s p e c i f i c proton energies and fluences.* waa uniform v i t k i n 25%. The c e l l teaperature during i r r a d i a t i o n vas kept at 3OOC.

This cyclotron can pro-

The solar cells were

Each p l a t e was i r rad ia ted

The fluence over the t a r g e t plane

'Iba f u l l matrix of tests performed on the ( A l G m ) b G a h s o l a r .:ells and on several representative s i l i c o n s o l a r cells are given inTablee 1 a d 2.

Table 1. Electron I r r a d l a t i m Experiments 7646-4

PROTON ENERGY

Y I V

1 TYPE AN0 NUMBER OF CELLS

~ W O N FLUENCE vcd

XECTRON ELECTRON EMERGY FLUENCE (A,OI)AD-

MeV

15.4 15.4 22 22 30 so 40 40

5 x 1010 5x10" 5 x 10" 5 x 10l1

6 x lo1'

5 x 10''

5 x 1010

5 x 1010

Table 2. UQh-Energy Proton I r rad ia t ion Experiments

3 3 3 3 3 3 3 3

5 4 4 4 2 3 2 3 2 3 2 3 2 3 2 3

Si CONVEN- TIONAL

3 3 3 3 3 -

7686-

si HIGH

EFFlClENCV

3 3 3 3 3

TYPE AND NUYOER OF CELLS

I I

RESULTS AND DISCUSSION

A group of cells were fabricated e a r l y in the These cells program f o r e lectron rad ia t ion tests.

were designed to have high eff ic iency with no attempt at optimlring the design parameters t o increase rac'lation hardness. maximum power obtained from the cells p lo t ted against 1 MeV electron radiat ion f lwnce . r e s u l t s vere then compared v i t h those f o r two types of s i l i c o n cells aa shown in Table 1. This showed the need t o Improve these eer ly c e l l s f o r b e t t e r res is tance t o e lec t ron radiat ion damage a t fluences in excess of 4~1014/&.

Figure 2 sham the

These

Figure 3 shave the spec t ra l reaponee before and e f t e r e lectron i r rad ia t ion . The results shov tha t in these cells the s p e c t r a l response in the shor t wave1eng:h region shcws greater damage compared t o the longvavelength region. o p t i c a l absorption coef f ic ien t is greater f o r shor t vave lengths, most of the absorption In t h i s region w i l l be close t o the surface of the cell. Ihe photo generated carriers, therefore. must travel fa r ther to reach the junction than do those generated by longer wevelengths. We sa- . pected from the spec t ra l response of the damaged cells t h a t t h e i r junct ions had t o be r e l a t i v i l y deep compared t o the minority c a r r i e r diffurrlon length in the damaged layer . Our suspicion vas confirmed by the measured junction depth of _> 1 urn. These observation led us t o examine the influ,ance of t h e junction depth on radiat ion damage more carefu l ly .

Since the

*By cmpsrlrlg the reaul t s from previous s o l a r cells I r rad ia ted both i n a i r and in vacuum. we found that ttw lonlred nsses i n a i r nurru~rnding tho c e l l durinq l r r e d i s t h ~ tisvr no effei .1 o n Llir vt -1 I N .

51

-1 120 I I I I 1

100 ip

2 8 0 - 4 2 B o - Y W f 40- 2 3 2 0 - 0

0

€ 1 4

1

I RRADlATlON

IRRADIATION

I I I

40 :: o (AIGaJA&84s SOLAR CELL 0 HIGH EFFICIENCY Si SOLAR CELL n Si CONVENTIONAL CELL

WAVE LENGTH 4, p m

7 # b B 1

40 L 20 0

I RRADIATION

IRRADIATION

0.2 0.4 0.6 0.8 1 .o

WAVELENGTH A , p m

Figure 3. (A1Ca)As-CeAs molar cell spectral response before and after 1 MeV el-ctron irradiation

To correlate theory and experiments, Figure 4 shows the (A1Ga)Ae-CaAe solar cell ohort circuit current density at3 a function of 1 EleV elrctroa radiatim fluence. The continuous curve reprerenta the normalized experimental values. The dotted

c

EXPERIMENTAL

THEORETICAL 20

15

10

5 \-

Figure 4 . Short circuit current density versus 1 %(a0 election radiation

fluence, electronstan2

line is the theoretical c u m . Both curves cor- respona to M (AlC.)Ae layer thickness of 1 lm and a junction depth of 1 um. theoretical curve, the minority carrier diffusion length L WM related to the fluence 4 by the usual relation:

For CalCdating the

The initial diffusion lengths for holes (Lpo) and electron8 (bo) were assumed to be 2 and 5 Ira. respectively in theee calculation8. The damage constant KL for the diffusion length used for both p- and a-type GaAa was deduced by matching the theoretical curve to the experhenta. xrve as shown in Figure 4. It vas found to be KL = 7 x assuming the same value of KL for the n- and pdoped GaAa.

U s i n g thia value for XL. the short-circuit density VM calculated for several junction depths M a function of a 1 HeV electron fluence. The results, shown in Figure 5, show that radiation damage decreases as junction depth decreases.

Based on this a~lysis, we proceeded to fabri- cate a second-generation of (A1Ca)As-CaAa solar cells with the goal of decreased sensitivity to the radiatjon environment. The windm layer thickneso WM made at 0.5 rn while the junctic?! depth waa decreqerd to % 0.5 um by readjusting the LPE layer growth parameters.

Figure 6 shows the measured short-circuit current of these shallower junction cells versua 1 MeV electron fluence. improved radiation resistance is in good agreement

The experimentally observed

52

lo t

FLUENCE, 1 MeV ELECTRON§/CM2

Figu.e 5 Predicted (A1Ga)As-Ga~s solar cell ehort circuit current density versus 1 Me? electron radiation fluence ((Alr'h)As layer thickness = 1.0 m, initial diffusion length + = 2 pm, ho - 5 m, and diffusion length damage constant KL = 7 x lo'*)

-i 1 MeV ELECTRON FLUENCE LEVEL, E/CM2

Figure 6. Short circult current versus electron fluence level (1 MeV)

vith the predictions of the theory. Figure 7 shovs the experimental results for both tA1Ca)As-CaAs solar cells and nevly developed high-efficiency Si solar cells as a function of 1 MeV electron irradiation. Also shown for reference are the results of our prcvioue set of irradiated (A1Ca)Ae-CaAs drep-junc.t Ion Molar tell^.

€

160

i lW

3

s r w x

IAIGdAs-GnAs SOLAR CELLS SHALLOW xi JUNCTION (0.6 Frn)

I IAIGo)+GaAsSOLAR CELL' . I u ~~

~

1012 1013 1014 1016 1016

1 MoV ELECTRON RADIATION FLUENCE Elm2

Figure 7. Maximum output power versus 1 MeV electron fluence

The spectral response of the deep-junction and shallover junction cells, both before and after irradiation. are given in Figure 8(a and b). figure shows that the radiation damage in the deeper junction cells is concentrated in the short- wavelength region, whereas the shallover junction causes the damage to shift to the longer wavelength. This is consistent vith our observation that the collection of minority carriers in the p region is not much affected up to the fluence nt whirh the electron diffusion lenRth is redurcd to Icss thnn the p layer thlckneus.

The

Figure 8(c) show the spectral response of the shallover juncti n solar cells irradiated at flLmces 1 x lo1! e l m 2 vith electron energies varying from 0.7 MeV to 1.9 MeV. higher energies. these cells shov more degradation, probably because KL increases vith increasing electron energy. Figure 9 shovs typical dark current-voltage (I-V) characteristics before and after electron irradiation. Although solar cells become motc leaky after irradiation, the basic transport mechanism remains the same (as shown by the I-V curves, which remain parallel to each other). This increased leakaRe current probably results from an increase in the number of recombination centers at the junction.

High Energy Proton Damage

As expected at

Twenty-four (A1Ca)As-CaAs solar cells and several representative silicon cells were irradiated vith 15.4 MeV to LO V protons at fluences of 5 x 1O1O and 5 x 101'p/cm2 (Table 2).

The baseline structure of the solar cell used for 1 roton irradisttnn W ~ I R the same as ihtlt o f thc Hhnllow-lunciion nolrir t .c-1 In lined f o r t * l c c ~ r t ~ i

53

7644611 100 I I I I 1

0.4 0.5 0.6 0.7 0.8 0.9

WAVELENGTH, p m

Plgure 8(a). (A1Ca)AB-GaAs solar cell spectral response versus 1 MeV electron radiation fluences

7M5-13 loo

90

80

70

60

5c

IRRADIATION)

40

30

?o FLUENCE = 1 1015 E/CM~/

LUENCE = 5 x 1014 E K M ~

FLUENCE = 5 x 1015 E / C M ~

FLUENCE = 1 x 1016 EKM* 10

" 0.4 0.5 0.6 0.7 0.8 0.e

WAVELENGTH, p m

Figure 8(b). (A1Ca)Aa-CaAs solar cell spectral response versua 1 MeV electron radiation f luence

90

80

70

60

50

40

30

20

lo 0 0.4 0.5 0.6 0.7 0.8 0.9

WAVELENGTH A p m

Figure "(c). (A1Ce)As-GaAs solar cell spectral response for several electron energies

YW-16

o 0.1 0.4 0.8 0.8 1.0

VOLTAGE, V

FSgurc 9 . Dark 7-V charecterietics before end and after electron irradiation

54

irradiation, except that no cover g h 8 0 vaa apelied to these cells.

The r e a u l t s of theme proton i r r a d i a t i o n teota are o-rited in Figure LO (a ond b) , which a h a n the mxhum molar cel l output power verew proton irradiation f l w n c e f o r t h e three types of c e l l o mp.cif1.d L Table 2. are more r e s i e t a n t t o high-energy proton radiat ion damage than t h e a l l i c o n cells. p lo t ted in F-e 10 are .*

t e o t n o u l t a ; these show t..d a- fac t expected from proton fluance on 8 t h improved (AlC.)Ae-C.Ae mifar cell with 8 beginning-of-life AM0 power-convetah e f f ic iency of 16%. This extrapolat ion is pert inent mince the f s a e i b i l l t y of an 18% eff ic iency has already been demonstrated for t h i s type of cell.

PQpres 11 and 12 show -:le average apec t ra l reopome of the (AlGa)Ae-Cde soia? telle before and a f t e r proton i r r a d i a t i o n w i t h ,roton energies of 15.b UeV and 40 MeV. r e q e c t i v e l y . The spec t ra l response in t h e short wavelength region of theee e h l l o r - j u n c t i o n solar c e l l s i e almost insensi t ive to t h e proton i r rad ia t ion . A s l i g h t decrease in the S O h r cel l apec t ra l responae occurs only i n the long wavelength region.

Tbe (AlCA)Am-CaA. ookr c e l l o

The dotted line. -polatioaa of our

Figure 13 shove the dark I -V c h s r a c t e r i e t i c before and a f t e r i r rad ia t ion . Again, j u s t as i o the caoe of e lectron i r rad ia t ion . the so la r c e l l Junction becomas s l i g h t l y leaky due t o the increes- ing nlnber of recombination centers produced by promn i r rad ia t ion although the basic t ransport mechanism remains unchanged.

t

i f B

z 8 3

67ri6-17

I D -

0

(AIGdAs-GaAa CELLS

----

Si CELLS CONVENTIONAL

0 1 x 1010 1 x 1011 1 x 10'2

16.4 MeV PROTON IRRADIATION FLUENCE P/CM2

Figure 10(a). Solar c e l l mn%imum output power versum 15.4 MeV proton i r rad ia t ion fluence.

20 1 , I l r l , , >3;l,; Si BARE CELLS

0 1010 10'1 1012

40 MoV PROTON IRRADIATION FLUENCE P/CMZ

Figure 10(b). Solar c e l l maximum output power versus 40 Ur7 proton irradiation fluence

a4 0.6 0.0 0.7 0.8 0.9

WAVELENGTH,pm

Figure 11. (A1Ca)Ae-CsAe S O h t c e l l apr r t rn l reaponee before and a f t e r 15.4 MeV proton

i r rad is t ion

Radiation Annealing Studies

CaAa so lar c e l l s dama~cd by radfat ion recover t h e i r eff ic iency when annealed a t lnu tempererrirco

55

WAVELENGTH, pm

Figure 12. (A.lCa)Ae-CaAe eclar cell spectral responee before and after 60 MeV proton

irrad ia t ion

7545-18

a E -

10-2

10-3

10-4

10-5

104

IRRACi4TION

AFTER PROTON I R RADIATI0:J 15.4 MeV, 5 x lo1' PICMZ (CELL NO. 1052)

\ BEFORE A 'RRACi4TION

10-7 1-11 I 1 I I I I I 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

VOLTAGE

Figure 13. Dark I-V characterletice before and after proton irradiation

Figure 13. Dark I-V characterietire before and after proton irradiation

on the order of 200'C to 300.C ( 3 , 6 ) . Some pro- liminary t h e m 1 annealing experlmente an the r6dlation-damaged ( A ~ G ~ ) A ~ - C ~ A S solar cells were

performed in (rur laborator!. The cella were irrndi- atad nt flucncae of 1 x 10 5/cd WACh electron ener-

.gieo wrying frou 0.7 HeV through 1.0 MeV to L9IlaV. Subrequently, they were annealed in vacuum at t a r peraturea of over 200°C. Figure 14 shove cha effect of annealing ae a function of annealing time and t-pcraturee.

Figure 15 c.mpnres the epeccral teeponee of theme cells after L:.: annealing step with the apec- tral reeponae before and after electron irradiation. The long wavelength region shove significant recovery. eignificnnt recovery in the mipority carrier diffusion length in C a s after radiation dami.de.

Thin nU8geetS that the annealing leade to a

Figure 16 ehwe the dark X-V characteristics of these cella. These cella show leaky p-n junctions after irradiation; however, they almost completely recover to their pre-irradiation condition after annealing at 210'C. These results indicate that (A1Ca)As-GaAs nolnr cellp can be annealed at ;;rat- tical temperatures to rer>ve radiation damage. could be exploited for longer space miesions.

This

CONCLUSION AND SUMMARY

Several 2 cm x 2 cm (A1Ca)Ae-GaPe cells were subjected to radiation damage ctudies using both electrons and proton:. The results show that:

0 (A1Ca)Aa-CaAs solar cells can be mnde more raeietant to radiation damage than can eilicon cella for both electron and proton irradiation.

0 '%e junction depth is a sensitive poram- eter la determining radiation resistance.

0 The (A1Ca)As-CaAe solar cells suffer only a moderate amount of degradation at proton energies above 15.6 MeV.

0 The efficicnc J of electron-radiation- damaged (AlCabAa-CaAs solar cells recover when annealed at temperatures 88 low aa 203OC :o 3OG"C.

ACKNOWLEDCK 4 *

The r a d i a t i o n d.rmage SLIIUII'L. r t * p i r ~ ~ * + II,*I-C. vere supported in p.irt h y i .viLr. ic ' ts i rcm N.\S.\ L.?nglzy, Ciintr.ii L NAY 1 - 1 4 7 2 '

1.

2.

3.

4.

l ~ l . l ' l ' K l SI I h

.I.M. U ~ I ~ I ~ I . I [ I ,111tl 11. I. I I . * V I . I , " . \ I I I - ~ r t l l , II'I,II l : t , . l 1 l> i i k - H t * G : r o w t l l L l ~ . t l t ~ l d l , i r III,.II 4 I I I-. I LI*rc.v (:.IAIAs-(:.I,I+ h~ I .ir t 1' I I \, ' ' , \ ~ ~ l ) I . l ' l , \ . R.I. Moon, et 81.. "Performance of (A1Ca)As- CaAe Solar Celle 11: the Space F,nvironment.'* 12th IEEE Photovoltaic Speclalists Cmrferenr-. 255 (1975). R.S. Nlllr abrd J S . Harris. "Gallium Arsenide Concentratioa Syetem," presented at the A I M Conference on the Futurc of Aerospace Power Syrtem, March 1977 . C.H. Walker rnd E.!. Ponway, "Annealing of CaAe Solar Cells Damaged by Electron Irrudia- tion," J. of El-c, -ice1 Chemical Society, Vol. 125, No. 6, : '6. 1978.

i.tslt. 10. ~ Y Z ( 1 ~ 7 7 ) .

56

i Y)

a 5 0

@ AFTERTHQRMAL ANNEALING

0 I I I I P 0.4 O S 0.6 0.7 0 . 8 0 8

0 10 100 ANNEALING na, HR

10. onamuld fraction ver- m n u l h g tina

la,

I I I I I 1 0.4 0.5 0.0 0.7 0.8 0.8 0.4 0.6 a6 a7 0.8 0.0

WAVELENGTH, ptn WAVELENGTH. pm

?gyre 15(n). Spectrnl reaponme before and nf tar thermal nnnealhq

call #1222

Piwe U(c). Spectral rsrpoare before nod aftir arrndbtion nnd after

amenling cell ll278

57

0

IRRADIATION

IRRADIATION (0.7 MeV; 1 x 1015 UCd

I 1 AFTER THERMAL ANNEALING

Q AFTER ELECTRON -- 0.5 1.0

W

Figure lafa). Dark I-V characteristic before and after thermal annealing cell #1222

la4

1 6

lab

lad

0 0.2 &4 a6 b S 1.0 VOLTS

Figure 16(b). Dark I -V characteristic before and after thermal annealing cell 11008

-2!i

O 7 - 1 1

10-2

10-3 a -

10-5

10-6

10-7

-

OBEFORE ELECTRON - DAFTER ELECTRON

I RRADl ATlON

IRRADIATION

OAFTER THERMAL

0.5 1 .o

W Fqure l6(c). Dark I-V characteristic before and sfter thermal annealing cell 11278

58

APPENDIX B

PHOTO I-V CHARACTERISTICS OF (AlGe)As-caAs SOLAR CELL BEFORE AND AFTER PROTON

IRRADIATION

59

lo0

4 E -

50

0

,BEFORE AND ASTER IRRADIATION - _ _ ._. AT 1 x l@P/CM FLUENCE

- FLUENCE = 1 x l O l o P d /

0.5 VOLTAGE, V

1 .o

Figure B-1. Photo I-V characteristics before and after proton irradiation. Proton energy = SO keV (cell 2266).

60

100

U E -

50

0

JFLUENCE = 1 x 10" P/CM'

-- -. .

FLUENCE = 1 x 10" P/CM 2 #

0.5 VOLTAGE, V

1 .o

Figure B-2. Photo I 4 characteristics before and after proton irradiation; proton energy - SO keV (cell 2235).

61

8162-27

a E -

loo

3

AFTER

0 0.5 VOLTAGE, V

1 .o

Figure B-3. Photo I-V characteristics before and after proton irradiation; proton energy = 50 keV; 1 x 1011 P/cm (cell 2406).

62

ia

a E -

!Y

BEFORE

0 0.5 VOLTAGE, V

1

Figure B-4 . Photo I-V characteristics before and after roton irradiation; proton energy = 50 keV; 1 x lop1 P/cm (cell 2407) .

63

4 E -

50

82629

1- .- - .- BEFORE

A F E 8

0 0.5 1 .o VOLTAGE. V

2 Figure B-5. Photo 1-V characteristics before and after proton

irradiation; proton energy = 50 keV; 1 x 1011 P/cm (cell 2411).

64

100

a E -

!a

0

AFTER

0.5 VOLTAGE, V

1 .o

Figure B-6. Photo I-V characteristics before and after roton , irradiation; proton energy = 50 bell; 1 x lop2 P/cm' (cel l 2426) .

65

0

8262-3

BEFORE

0.5 VOLTAGE, V

1 .o

2 Figure B-7. Photo I-V characteristics before and after proton irradiation; proton energy = 50 keV; 1 x 1012 P/cm (cell 2427).

66

101

a E -

Y

82623 I 1 - !

I , ,

8 l : I : !

!

I

VOLTAGE, V


67

1 O(

a E -

5(

I

1 -

I

i

I

... i

i ! : -

I

L.

!

I I

1 i

0 I

I !

i

I

;

!

i I

I !

I

I I I ! I 1 1

I ' 1 . I I

1

0.5

. ..

..

i i

.- 4.. . .

! - ,

I

I ! ! I I

I I i

i

VOLTAGE, V

Figure 8-9. Photo I-V characteristics before and after proton irradiation; proton energy = 100 keV (cell 2302).

D

68

a E -

51

i

; - - I

- 4

..

I

I I I

!

!

1. i I t

I

I

i ! 1 -

!

i- ! !

I

I

I

1 i

I

!

!

D

Figure B-10. Photo I-v characteristics before and after proton irradiation; proton energy = 100 keV (cell 2307).

69

100

U E -

50

0 1

0.5

VOLTAGE, V

0

Figure B-11. Photo I -V characteristics before and a f t e r proton ., irradiation; proton energy = 100 keV; 1 x 10'1 P/cm' ( c e l l 2417).

70

a E -

!3

l

,

0.5

VOLTAGE, V

3

Figure B-12. Photo I-V characteristics before and after proton irradiation; proton energy ( ce l l 2419).

130 keV; 1 x 10l1 P/cm

7 1

0 0.5 VOLTAGE, V

D

Figure B-13. Photo I -V characteristics before and after proton irradiation; proton energy = 100 keV; 1 x 1011 P/cm (cell 2420) .

72

-BEFqtE'---T ...

VOLTAGE. V

2 Figure B-14. Photo I-V characteristics before and after proton irradiation; proton energy = 100 keV: 1 x 1012 Plcm ( ce l l 2 4 2 9 ) .

73

a E

VOLTAGE. V

2 Photo I-V characteristics before and after proton irradiation; proton energy = 100 keV; 1 x 1012 P/cm (cell 2432).

Figure B-15.

74

ia

U E -

51

a2624 -:

-. . -

-. . . ..

1 .

VOLTAGE, V 0

Figure B-16. Photo I-V characteristics before and after proton irradiation; proton energy = 100 keV; 1 x 10l2 P/cm (cell 24563.

75

loa

U E -

5a

a VOLTAGE, V

Figure B-17. Photo I - V characteristics before and after proton irradiation; proton energy = 290 keV (cell 2370).

76

826242

U E -

8

I

i - - .

.. ..

..

VOLTAGE, V

Figure B-18. Photo I-v characteristics before and after proton liradiation; proton energy = 290 keV (cell 2375).

77

10C

c E -

5(

0.6 VOLTAGE, V


78

VOLTAGE, W

D

2 Figure B-20. Photo I-V characteristics before and after proton irradiation; proton energy = 290 keV; 1 x 1011 P/cm (cell 2 4 2 2 ) .

79

825224 i

1 I . . ._. i I

I ! ! . I I _ _ . . _. .

a E -

50

a 0.5

VOLTAGE, V

2 Figure B-21. Photo I-V characteristics before and after proton ii.-adiation; proton energy = 290 keV; 1 x 10l1 P/cm (cell 2424) .

80

100

U E -

54

0 0.5

VOLTAGE, V

I 0

Figure B-22. Photo I-V characteristics before and after proton irradiation; proton energy - 290 keV; 1 x 1012 P/cm (cell 2 4 5 7 ) .

81

lo(

a E -

51

82824

BEFORE

I 0

VOLTAGE, V

Figure B-23. Photo I-V characteristics before and after proton irradiation; proton energy = 290 keV; 1 x 1OI2 T/cm (cell 2477).

82

1M:

a E -

5(

C I

VOLTAGE, V

2 Figure B-24, Photo I-V chzracteristics before and a f t e r proton

irradiation; proton energy = 290 keV; 1 x 1Ol2 P/cm (cell 2311).

83

APPENDIX c PHOTO I-V CHARACTERISTICS OF HIGH EFFICTZNCY SILlCON

SOLAR CELL BEFORE AND AFTER PROTON IRRADIATION

85

a E

826249

150

100

50

0 0.5 1 .o VOLTAGE. V

;tire C - I . Photo 1-'J c h a r a c t e r i s t i c s of Si solar c e l l kcidre and after proton irradiation; ,, proton er.ergy = 56 k. . I ; 1 x lo11 P/cm- (i.t.11 .%j.

a E -

8182 80

150

100

50

0 1 .o 0.5

VOLTAGE, V

Figure C-2. Photo I--V charac ter i s t i c s of S i so lar c e l l

2 before and a f t e r proton irradiation; proton energy = 50 keV; 1 x 10l1 P/cm ( c e l l 6 ) .

87

8252-51

100

C E -

50

0 0.5 1 .o VOLTAGE, V

kigure C-3. Photo I-V characteristics of Si solar cell before an3 after proton irradiation; proton energy = 50 keV; 1 x ,0l1 F/cm (cell 9).

2

88


tgure C-4. Photo I - V characteristics of S i solar ce before and after proton irradiation; proton energy = 50 keV; 1 x 1012 P/cm (cell 10).

11

89

0 0.5 VOLTAGE, V

1 .o

Figure C-5. Photo I -V characteristics of Si solar cell before and after proton irradiation; ~

proton energy = 50 keV; 1 x lo1* P/cmL (ce l l 11).

90

0 0.5

VOLTAGE, V 1 .o

Figure C-6. Photo I - V charactertstics of Si solar ce

2 before and after proton irradiation; proton energy = 50 keV; 1 x 10l2 P/cm (cell 12).

11

91

U E -


Figure C-7. Photo I-V characteristic8 of Si solar ce before and after proton irradiation; proton energy = 100 keV; 1 x 10l1 P/cm (cell 2).

111

92

a E

Fi

0262-66

100

50


gure C-8. Photo I-V characteristics of Si solar CL

before and after proton irradiation: proton energy = io0 keV; 1 x 1011 P/cm (cell 5).

93

8262-67

150

100

a E -

50


Figure C-9. Photo I-V characteristics of Si solar c*

2 before and after proton irradiation; proton energy = 100 keV; 1 x 10l1 P/cm (cell 8).

?. 1

94

a E

VOLTAGE. V

Figure C-10. Photo I-V rharac ter i s t i c s of S i so lar ce l l before and a f t e r proton irradiat ion; oroton energy = 100 keV; 1 x 1012 P/cm (cel l 1 3 ) .

95

VOLTAGE, V

Figure C-11. Photo I-V charac ter i s t i c s 8 ) : S i aol.ar c e l l befP?e and a f t e r proton irradiat ion; proton energy = 100 keV; 1 x 1012 P/cm (cell 14).

96

628280

a E -


Figure C-12. Photo 1-V characteristics of Si solar c before and aft.:c proton irradiation; proton energy = 100 keV; 1 x 1012 P/cm (cell 15).

ell

97

.O

Figure C-13. Fhoto I-V charac ter i s t i c s of S i solar c e l l

2 before and a f t e r protm irradidtion; proton energy = 290 keV; 1 :i 1011 P/cm ( ce l l 1).

98

U E

VOLTAGE, V

Figure C-14. Photo 1-l: cbracte'istics of 31 solar ce before awl after proten irrsdiation; proton energy - 290 keV; 1 x 1,011 P/cm (cell 4).

11

93

0 0.5

VOLTAGE, V

1 .o

Figure C-1s. Photo I-V characteristics of Si solar cell

2 before and after proton irradiation; proton energy = 290 keV; 1 x 1011 P/cm ( c e l l 7).

100

a E

Figure C-16. Photo I-V characteristics of Si solar ce

2 before and after proton irradiation; proton energy = 290 keV; 1 x 1012 P/cm (cell 16).

11

101

Figure C-17. Photo 1-v characteristics of Si solar ce before and after proton irradiation; proton energy = 290 keV; 1 x 10l2 P/cm (cell 17).

2

102

FJ

825266

150

100

50

0 0.5 t.0

VOLTAGE, V

igure C-18. Photo I-V characteristics of Si solar c 2 before and after proton irradiation;

proton energy = 290 keV; 1 x 1012 P/cm (cell 18).

ell

103

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