The reduction of the temperatures required to restore cell performance after irradiation was investigated with emphasis on the annealing characteristics of two groups of cells containing different amounts of oxygen and carbon. Examination of defect behavior in irradiated boron doped silicon leads to the tentative conclusion that further reduction in annealing temperature could be achieved by decreasing the carbon concentration and either neutralizing the divacancy and/or minimizing its formation as a result of irradiation. A significant reduction in the temperature required to remove radiation induced degradation in 0.1 ohm centimeter silicon solar cells was achieved

Swartz, C. K.

Weinberg, I.

English

NASA Technical Reports Server

ANNEALING OF RADIATION DAMAGE IN LOW RESISTIVITY 
SILICON SOLAR CELLS 
I. Weinberg and C. K. Swartz 
NASA Lewis Research Center 
Cleveland, Ohio 
S i l i c o n  s o l a r  c e l l s  w i t h  base r e s i s t i v i t y  o f  0.1 ohm-centimeter a re  
p ro jec ted  t o  achieve 18 percent  a i r  mass zero  e f f i c i e n c i e s  ( re f ,  1). 
been found i n  the  pas t  t h a t  t hese ' l ow- res i s t i v i t y  c e l l s  r e q u i r e  anneal ing 
temperatures o f  approximately 500 C t o  res to re  performance a f t e r  
rad iat ion- induced degradat ion ( r e f .  2 ) .  However, i f  annealing i n  space i s  
t o  become a r e a l i t y ,  temperatures o f  200° C o r  lower are requ i red  t o  prevent  
i r r e v e r s i b l e  damage t o  o the r  p a r t s  o f  t h e  s o l a r  array.  
conduct ing a program t o  reduce the  temperatures requ i red  t o  r e s t o r e  c e l l  
performance a f t e r  i r r a d i a t i o n .  
behavior of r a d i  a t  ion- i  nduced de fec ts  produced i n  p-type s i  1 i c o n  i ndica te  
t h a t  oxygen and carbon are cons t i t uen ts  o f  t he  major de fec ts  observed a f t e r  
i r r a d i a t i o n  w i th  1-MeV e l e c t r o n s  ( r e f s .  3 and 4). Hence, our  c u r r e n t  
e f f o r t s  are concerned w i t h  t h e  anneal ing c h a r a c t e r i s t i c s  o f  two groups o f  
c e l  Is con ta in ing  d i f f e r e n t  amounts o f  oxygen and carbon. 
Junc t ion  fo rma t ion  i n  one group o f  c e l l s  was achieved by phosphorous 
i o n  imp lan ta t i on  i n t o  0.1-ohm-cm float-zone s i l i c o n  w i t h  oxygen and carbon 
concentrat ions <5~1015/cm3. 
phosphorous d i f f u s i o n  .into 0.1 ohm-centimeter f loat -zone s i l i c o n  whose 
oxygen and carbon concent ra t ions  were lO16/cm3. 
1-MeV e lec t rons  the  c e l l s  were i soch rona l l y  annealed, t i m e  a t  temperature 
being 20 minutes. 
f i g u r e  1 where anneal ing temperatures around 500 C are ind ica ted .  
isochronal  anneal ing d a t a  f o r  t he  ion-implanted c e l l s  are shown i n  f i g u r e  2, 
where i t  i s  seen t h a t  t h e  annea l in  temperatures are  f luence dependent and 
i s  observed. Wi th  i nc reas ing  f luence, the temperature increases u n t i  1, a t  
6 = 3 ~ 1 0 ~ ~ / c m ~ ,  anneal ing occurs around 500' C. 
o f  the  da ta  i s  the  appearance o f  reverse annealing, which occurs a t  
temperatures above 400' C as t h e  f luence increases above lO14/cm2. 
h i g h l y  s i g n i f i c a n t  b u t  n o t  s u f f i c i e n t  t o  achieve t h e  goal o f  anneal ing a t  
ZOOo C. Fu r the r  reduct i o n  appears poss ib le  by i den t  i f y i n g o a n d  removi ng the 
de fec ts  respons ib le  f o r  t h e  occurrence o f  annealing a t  300 C. The f i r s t  
s tep i n  t h i s  procedure i s  de fec t  i d e n t i f i c a t i o n ,  which we at tempt by 
consider ing some o f  t h e  p r o p e r t i e s  o f  known defec ts  i n  p-type (boron-doped) 
s i l i c o n ,  However, because o f  s e n s i t i v i t y  l i m i t a t i o n s ,  da ta  f rom deep-level 
t r a n s i e n t  spectroscopy (DLTS) are n o t  a v a i l a b l e  f o r  0.1-ohm-centimeter 
boron-doped s i l i c o n .  
avh i  l a b l e  f rom prev ious  work f o r  0.3-ohm-cent imeter s i  1 i con  ( r e f .  3) 
F igu re  3 shows t h e  DLTS da ta  f o r  t h i s  low r e s i s t i v i t y  boron-doped s i l i c o n ,  
It has 
Hence, we have been 
Prev ious i n v e s t i g a t i o n s  i n t o  t h e  anneal ing 
The second group o f  c e l l s  was f a b r i c a t e d  by 
A f t e r  i r r a d i a t i o n  by 
Resu l ts  f o r  t h e  d i f f u s e d  junc$ ion  c e l l s  are shown i n  
The 
where, a t  a f l uence  ( 6 )  of 1014/cm 8 , an annealing temperature o f  300' C 
An a d d i t i o n a l  f e a t u r e  
The p resen t l y  observed 200° reduct  i'bn i n  anneal i ng temperature i s 
On the o the r  hand, ae fec t  spectra f rom DLTS are 
181 
https://ntrs.nasa.gov/search.jsp?R=19810009032 2020-03-21T08:56:23+00:00Z
and t a b l e  I summarizes the  i n fo rma t ion  a v a i l a b l e  on t h e  de fec ts  shown i n  the  
f i g u r e .  
E, + 0.26 eV are u n i d e n t i f i e d .  For t h e  remaining defects,  a l t hough .  
i d e n t i f i c a t i o n s  are  ava i l ab le ,  many a re  ambiguous. 
E v  + 0.38 eV has been ambiguously i d e n t i f i e d  as a vacancy-oxygen-carbon 
complex (V-04)  ( r e f .  3) o r  a c a r b o n - i n t e r s t i t i a l  carbon-subs t i tu t iona l  p a i r  
(CI-Cs) ( r e f s .  5 and 6). 
temperature f o r  t h i s  d e f e c t  favors t h e  V - 0 4  i d g n t i f  i c a t i o n .  However, t he  
simultaneous presence o f  bo th  de fec ts  below 300 C i s  no t  precluded. The 
de fec t  a t  E, + 0.23 eV w i t h  anneal ing temperature a t  300' C has been 
unambiguously i d e n t i f i e d  as t h e  divacancy ( r e f s .  3 and 5).  As seen from t h e  
tab le,  t h e  d e f e c t  a t  E, - 0.27 eV i s  a boron r e l a t e d  de fec t  ( r e f s .  3 
and 5).  
growth o f  t h e  defect  a t  E, + 0.30 eV ( r e f .  3 ) .  
been t e n t a t i v e l y  i d e n t i f i e d  as e i t h e r  a boron-oxygen vacancy complex 
( r e f .  3) o r  t h e  s i l i c o n  d i - i n t e r s t i t i a l  ( re f .  5) .  However, f rom f i g u r e  3 i t  
i s  seen t h a t  t h i s  d e f e c t  anneals ou t  a t  a temperature s l i g h t l y  above 400' K, 
w h i l e  t h e  s i l i c o n  d i - i n t e r s t i t i a l  anneals o u t  a t  500° C ( r e f .  7 ) .  
preceding d iscuss ion  i n d i c a t e s  t h a t  the  poss ib le  de fec ts  t h a t  anneal o u t  a t  
300° C are t h e  divacancy and t h e  s i l i c o n  d i - i n t e r s t i t i a l .  I f  these were the  
o n l y  s i g n i f i c a n t  de fec ts  present  a t  the  lower f luence, they cou ld  be 
respons ib le  f o r  t h e  anneal ing observed a t  300° C. Th i s  cou ld  be poss ib le  i f  
a f luence dependence e x i s t e d  f o r  the  produc t ion  r a t e  o f  t h e  remaining 
de fec ts  such t h a t  t h e i r  concent ra t ion  was very low a t  (5 = lO14/cm2 and 
increased s i g n i f i c a n t l y  w i t h  inc reas ing  f luence.  
exper imental  evidence t o  support  t he  pos tu la ted  f l uence  dependence. With 
respect  t o  the  f luence dependent reverse anneal observed between 400' and 
500' C, i t i s  d i f f i c u l t  from t h e  present  da ta  t o  ass ign t h i s  e f f e c t  t o  a 
s p e c i f i c  d e f e c t  o r  defects .  
boron-doped s i l i c o n  leads t o  t h e  t e n t a t i v e  conc lus ion  t h a t  f u r t h e r  reduc t ion  
i n  annealing temperature cou ld  be achieved by decreasing the  carbon 
concent ra t ion  and e i t h e r  n e u t r a l i z i n g  the divacancy and/or min imiz ing  i t s  
fo rmat ion  as a r e s u l t  o f  i r r a d i a t i o n .  we emphasize t h e  specu la t i ve  nature 
of t h i s  conclusion. The f a c t  remains, however, t h a t  the  present work has 
demonstrated a s i g n i f i c a n t  reduc t i on  i n  t h e  temperature requ i red  t o  remove 
rad iat ion- induced degradat ion i n  0.1-ohm-centimeter s i l i c o n  s o l a r  c e l  I s .  
Re fe r r i ng  t o  t a b l e  I and f i g u r e  3, the  de fec ts  a t  Ev  + 0.48 and 
The de fec t  a t  
We note  f rom f i g u r e  3 t h a t  t he  anneal ing 
It i s  observed t h a t  disappearance o f  t h i s  de fec t  i s  fo l lowed by 
Th is  l a t t e r  de fec t  has 
The 
However, there  i s  no 
The preceding cons ide ra t i on  o f  de fec t  behavior i n  i r r a d i a t e d  
REFERENCES 
1. Godlewski, M. P.; Brandhorst, H. W., Jr.; and Baraona, C. R . :  Effect  
o f  High Doping Levels  on S i l i c o n  So la r  C e l l  Performance. E leventh 
Photovo l ta ic  S p e c i a l i s t s  Conference - 1975, IEEE, 1975, pp. 32-35. 
2. Weinberg, I.; and Swartz, C. K.: Annealing o f  Rad ia t i on  Damage i n  0.1 
and 2 ohm-cm S i l i c o n  So lar  Cel ls .  NASA TP-1559, 1979. 
3. Mooney, P. M.; e t  a l . :  Defect  Energy Levels  i n  Boron Doped S i l i c o n  
I r r a d i a t e d  w i t h  1-MeV Electrons. Phys. Rev. B, vol.' 15, no. 8, 
Apr. 15, 1977, pp. 3836-3843. 
182 
4.. Weinberg, ,I.; and Swartz, C, K.: O r i g i n  o f  Reverse Annealing i n  
5. K imer l ing,  L. C.: De fec t  S ta tes  i n  Electron-Bombarded S i l i c o n :  
Radiation-Damaged S i l i c o n  So la r  Cel ls .  Appl. Phys. Lett., vol. 36, 
no. 8, Apr. 15, i980, pp. 693-695. 
Capacitance Trans ient  Analysis.  I n t e r n a t i o n a l  Conference on Rad ia t ion  
E f f e c t s  i n  Semiconductors, Dubrovnik, Yugoslavia, 1976, N. B. U r l i  and 
T. W. Corbett ,  eds. I n s t i t u t e  o f  Physics (London) Conference Ser ies 
no. 31, 1977, pp. 221-230. 
6. Brower, K. L . :  EPR o f  a John T e l l e r  D i s t o r t e d  <111> Carbon I n t e r s t i t i a l c y  
i n  I r r a d i a t e d  S i l i c o n .  Phys. Rev. B,.vol. 9, 1974, pp. 2607-2617. 
7. Brower, K. L.: EPR o f  a <001> Si  I n t e r T t i t i a l  Complex i n  I r r a d i a t e d  
S i l i con .  Phys. Rev. B, vol .  14, pp. 872-883 (1976). 
183 
TABLE I .  - ANNEALING AND CAPTURE CROSS SECTIONS OF MAJOR DEFECTS 
I N  BORON-DOPED SILICON 
Anneal Capture cross 
tempErat ure, sect ion, 
2 
Cill 
Defect Energy 
level ,  
eV 
Detection 
method 
~~ 
E + 0.38 
E + 0.23 
V 
E - 0.27 
C 
E + 0.30 
V 
E + 0.26 
V 
v-0-c 
or  
c -c 
I S  
Divacancy 
I n  Out 
0 0 
n P 
-1 3 -16 
30 400 0 .3~10 2x10 EPR, DLTS 
-- 300 ------ EPR, DLTS 
-13 -16 -- 300 4x10 3x10 EPR, DLTS 
I I i 
-13 
B O  --- 200 2x10 
B -6 --- 180 
I 1  
or 
I S  
------ 
-13 
6-0-V 170 40Oe 3.6~10 
DL TS ------- 
DL TS ------- 
-16 
2x10 DL TS 
1 I I 1 I 
184 
FIGURE 1: - ANNEALING TEMPERATURES OBSERVED IN RADIATION DAMAGED S IL ICON SOLAR CELLS 
(CELL P-N JUNCTION F O R K D  BY DIFFUSION; RESIST IV ITY = 0.1 OHM-CM) 
50 
40 
PERCENT 
DEGRADATION 
I N  SHORT- 
CIRCUIT 
CURRENT 
30 
20 
10 
0 -  
- CB = I MEV ELECTRON 
FLUENCE CCM-2) 
- 
- 
- # 
f 
+ = 1014 ---J 
- 
185 
FIGURE 2: - REDUCED ANNEALING TEMPERATURES I N  RADIATION DAMAGED SILICON SOLAR CELLS 
(CELL P-N JUNCTION FORMED BY ION-IMPLANTATION; RESISTIVITY = 0.1 OHM-CM) 
30 PERCENT 
DEGRADATION 
I N  SHORT- 
20 CIRCUIT 
CURRENT 
9 = 1 MEV ELECTRON 
FLUENCE (CW-2) 
0 100 200 300 400 500 600 
TEMPERATURE; OC 
FIGURE 3: - DLTS DATA FOR 0,3 n-CM P-SI- ISOCHRONALLY 
ANNEALED AFTER 1 ~ I E V  ELECTRON IRRADIATION 
rE,  - 0.27 
I 
RELATIVE 
DEFECT 
CONCENTRATIONS 
0 100 200 300 400 
TEMP, OC 
26 
I 
500 
186 


Annealing of radiation damage in low resistivity silicon solar cells

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server