Recent results of electron and proton irradiation and annealing of GaAs solar cells are presented along with some implications of these results. A comparison between the energy-levels produced by protons and by electrons which are not stopped in the material indicate that the damage produced by protons and electrons may be qualitatively different. Thus, annealing of proton damage may be very different from the annealing of electron damage

Conway, E. J.

Walker, G. H.

English

NASA Technical Reports Server

RADIATION DAMAGE IN GaAs SOLAR CELLS 
E. J .  Conway and G. H.  Walker 
National Aeronautics and Space Administration 
Langley Research Center 
INTRODUCTION 
Radiation damage of space borne equipment i s  an important consideration 
fo r  almost a l l  systems which f l y  higher than the bottom of the radiation 
be1 t s .  Without geomagnetic shielding, spacecraft face a radiation 
envi ronmnt which varies in par t ic le  type, intensi ty ,  energy, and time. 
Electron and proton radiation provide the greatest damage hazard. This paper 
will report recent results of electron and proton irradiation and annealing 
of GaAs solar c e l l s ,  and a1 so will speculate on some implications of these 
resul t s.  
Langley Research Center has been actively involved in GaAs solar cell 
research f o r  several years. Our space radiation effects  e f for t  began 
approximately two years ago. I t s  goal i s  t o  improve the radiation s t ab i l i t y  
of the ce l l s .  Thus, in general, unshielded shallow junction, p-GaAaAsl 
p-GaAs/n-GaAs ce l l s  and materials are studied. 
In the following sections, we will report on an inhouse ef for t  a t  LaRC, 
on contractual studies a t  Hughes Research Laboratory, and on grant research 
a t  University of Florida. 
I .  Electron I r radiat i  on Damage and Anneal i n g  
Langl ey Research Center 
Inhouse studies have emphasized 1 MeV electron damage ( i r radiat ions 
performed a t  LeRC) and thermal anneal i ng of GaAs solar cel l  s. We have 
studied both the damage and annealing as a function of electron fluence and 
of junction depth. Cell degradation was found to be less  severe for  shallow 
junction cel ls .  Also, the degree of recovery (due to  thermal annealing) was 
greater f o r  shallow junction ce l l s .  
In t h i s  report we will emphasize the annealing studies. O u r  results are 
in the form of short c i rcu i t  current ( I s c ) ,  normalized to  i n i t i a l  short 
c i rcu i t  current ( I sco ) ,  because t h i s  i s  a simple measurable quantity (not 
the product of several ) and i s  the more radiation and annealing sensit ive 
simple quantity . 
Figure 1 shows the effect  on relat ive short c i rcu i t  current of annealing 
0.8 pm junction depth ce l l s ,  a t  200°C for  10, 20, and 30 hours. For damage 
due to  f l  uences from 1014 and 1015 1 MeV electrons cm-2, recovery was 
essentially complete. The annealing recovery a f t e r  a fluence of 1016 elec- 
trons cm-2 appeared t o  cease upon reaching 0.83. These resul ts  suggested 
that  more complex damage occurs a t  h i g h  fluence and leads to  more complex 
anneal ing character is t ics  (Ref. 1). 
Figure 2 shows the results of annealing several shallow junction ce l l s  
a f t e r  i r radiat ion with 1015 lMeV electrons cm-2. Each set  of ce l l s  was 
https://ntrs.nasa.gov/search.jsp?R=19790024492 2020-03-21T21:02:42+00:00Z
i so the r i na l l y  annealed and t h e  data analyzed f o r  annea l ing  k i n e t i c s .  Anneal- 
i n g  was found t o  be f i r s t  o rde r  w i t h  an a c t i v a t i o n  energy o f  1.25 ev. A l l  
r e s u l t s  i n d i c a t e d  t h a t  Isc/Isco annealed t o  on l y  0.93, c o n f i r m i n g  an 
unco~npl e t e l y  recovered component. 
Lang, e t  a1 . (Ref.  2) ,  had i n v e s t i g a t e d  1 PleV e l e c t r o n  i r r a d i a t i o n  and 
annea l ing  o f  CaAs us ing  deep l e v e l  t r a n s i e n t  spectroscopy (DLTS). He found 
s i x  l e v e l s ,  t h r e e  o f  which annealed a t  an a c t i v a t i o n  energy o f  1.4 eV, and 
two more a t  1.75 eV. Our values a re  i n  good agreement w i t h  those o f  Lang f o r  
t h e  low a c t i v a t i o n  energy annea l ing  stage, and (based on Lang 's  r e s u l t s )  we 
should have seen no f u r t h e r  annea l ing  f o r  t h e  t imes and teniperatures used. 
Thus, we b e l i e v e  t h a t  two impor tan t  annea l ing  stages e x i s t  f o r  GaAs, one 
assoc ia ted  w i t h  a  1.25 eV a c t i v a t i o n  energy and another w i t h  an a c t i v a t i o n  o f  
1.75 eV. 
Ile a re  c u r r e n t l y  f i n i s h i n g  a  c a l c u l a t i o n a l  s tudy on simultaneous 
i r r a d i a t i o n  and anneal i ng .  Using t h e  a c t i v a t i o n  energ ies and f requency 
f a c t o r s  f rom our  work and Lang's, we a re  e s t i m a t i n g  t h e  s i g n i f i c a n c e  o f  t h e  
expected degradat ion.  We expect t h e  r e s u l t s  t o  show t h a t  when i r r a d i a t i o n  
occurs a t  i t s  n a t u r a l  r a t e  i n  space, cont inuous thermal anneal ing can be very  
e f f e c t i v e  i n  reduc ing  t h e  r-esul t i n g  damage. 
11. Proton I r r a d i a t i o n  o f  GaAs So la r  C e l l s  
Hughes Research Labora to ry  
I n  space, s o l a r  c e l l s  a re  no t  o n l y  sub jec t  t o  e l e c t r o n  i r r a d i a t i o n ,  b u t  
a1 so t o  bombardment f rom pro tons  t rapped i n  t h e  r a d i a t i o n  be1 t s  and f rom 
those o r i g i n a t i n g  i n  s o l a r  f l a r e s .  The p ro ton  energy spectrum i s  very  wide, 
f rom l e s s  than  100 KeV t o  more t han  100 MeV. However, t h e r e  a re  many more 
p ro tons  a t  low energy than a t  h i gh  energy. 
For  t h e  1  a s t  two years,  Hughes Research Labora to ry  has been measuring 
t h e  e f f e c t  o f  va r ious  p ro ton  energ ies and f luences  on GaA%As/GaAs s o l a r  
c e l l  s. These c e l l  s  were general  l y  i r r a d i a t e d  unsh ie l  ded t o  p e r m i t  
unambiguous i n t e r p r e t a t i o n  o f  r e s u l t s  and t o  p rov ide  a  f i r m  bas i s  f o r  s h i e l d -  
i n g  s tud ies .  
C e l l s  w i t h  j u n c t i o n  depths l e s s  than 0.5 pm were i r r a d i a t e d  a t  h i gh  
energy (15 FleV and 40 MeV) and a t  low energy (50 KeV, 100 KeV and 290 KeV). 
S i  1  i c o n  s o l a r  c e l l  s  were a1 so i r r a d i a t e d  t o  permi t  a  d i r e c t  comparison 
between t h e  e f f e c t s  on GaAs c e l l s  and Si c e l l s .  F i gu re  3 summarizes t h e  
a v a i l a b l e  r e s u l t s .  A t  h i gh  p ro ton  energies,  t h e  performance o f  GaAs c e l l  s  
does n o t  degrade as r a p i d l y  w i t h  i n c r e a s i n g  f l uence  as does t h e  performance 
o f  S i  c e l l  s. However, a t  low energies,  GaAs c e l l  s  degrade inore r a p i d l y  than  
S i .  At  some i n te rmed ia te  energy, shown schemat ica l l y  by c ross ing  o f  dashed 
1  i nes, t h e  two k i nds  o f  c e l l  s  must be e q u a l l y  s e n s i t i v e .  l J i t h  a  12 m i l  cover  
g l ass  GaAs c e l l  s  show no degrada t ion  w i t h  energy ( t o  290 KeV) a t  a  f l u e n c e  
of 1 0 j 2  cn-2 as shown by t h e  100% 1  i ne a t  low energy i n  F i g u r e  3. Th i s  i s  
expected s ince  t h e  range o f  290 KeV pro tons  i s  much l e s s  than one m i l .  
Ana lys is  o f  spec t ra l  response measurements i n d i c a t e  t h a t  n e i t h e r  50 KeV 
( range IL 0 - 4  pm) nor  100 KeV ( range IL 0.8 pm) p ro tons  cause much damage below 
t h e  j u n c t i o n .  Th is  i s  t h e  case because bo th  t h e  window th i ckness  and t h e  
j u n c t i o n  depth (measured frorn t h e  i n s i d e  o f  t he  window) p rov ide  s h i e l d i n g  t o  
t h e  j u n c t i o n .  Th is  observa t ion  suggests t h a t  a  b e n e f i t  might be de r i ved  f rom 
us ing  a  t h i c k  window. For  example, a  2 m i l  window cou ld  s top  protons up 
t o  5 CIeV. Windows have been kep t  t h i n  i n  t h e  pas t  t o  permi t  extended b lue  
response f o r  enhanced beginn ing-of -1  i f e  (BOL) e f f i c i e n c y .  P re l  im ina ry  i n d i  - 
c a t i o n s  a re  t h a t  w i t h  a  t h i c k  window and reduced BOL e f f i c i e n c y ,  a  h igh  end- 
o f -1  i f e  e f f i c i e n c y  cou ld  be ma in ta ined  w i t h  1  ow weight and g rea t  simp1 i c i t y  
i n  comparison w i t h  cover g lass sh ie l d i ng .  The major  problems appear t o  be 
t h e  e f f e c t s  o f  r a d i a t i o n  on t h e  o p t i c a l  and e l e c t r i c a l  p r o p e r t i e s  o f  t h e  
conduct i ve GaARAs window. 
111. E l e c t r o n i c  P rope r t y  Changes o f  GaARAs and GaAs S o l a r  C e l l  
M a t e r i a l s  due t o  Pro ton  and E l e c t r o n  I r r a d i a t i o n  - 
Univ. o f  F l o r i d a  
T h i s  fundamental e f f o r t  suppor ts  t h e  more empi ri c a l  r a d i a t i o n  s tud ies  
performed on c e l l  s. Independent measurements a re  made o f  lnacroscopic para- 
meters, such as e f f i c i e n c y  and s h o r t  c i r c u i t  cu r ren t .  Fundamental 
m i  c roscopic-1 eve1 measurements o f  r a d i  a t i  on i nduced de fec t  c h a r a c t e r i  s t i c s ,  
such as t r a p  energy, d e n s i t y  and cap tu re  cross sec t ion ,  a re  c o r r e l a t e d  w i t h  
macroscopic measurements and impo r tan t  new i n f o r n a t i  on i s  developed. 
The U n i v e r s i t y  o f  F l o r i d a  and Hughes Research Labora to ry  performed some 
coopera t i ve  s tud ies  on low energy p ro ton  ( .05, .10 and .29 MeV) i r r a d i a t e d  
d iodes and c e l l  s. Deep l e v e l  t r a n s i e n t  spectroscopy (DLTS) rneasurernents were 
made o f  r a d i  a t i  on induced t r a p  d e n s i t y  and c a r r i e r  cap tu re  c ross  s e c t i  ons. 
The t o t a l  cap tu re  c ross  s e c t i o n  [Z;(TOT)] was computed by r n u l t i p l y i n g  each 
t r a p  d e n s i t y  by i t s  e l e c t r o n  o r  ho le  c ross  s e c t i o n  and adding. F i g u r e  4  
shows t h e  re1 a t i  ve sho r t  c i  r c u i  t c u r r e n t  and r e l a t i v e  e f f i c i e n c y  p l o t t e d  
aga ins t  z (TOT). The p o i n t s  a r e  1  abeled f o r  t h e  p ro ton  i r r a d i a t i o n  energy and 
f 1  uence . 
Although F i  gure 4  i s  based on some p r e l i m i n a r y  data, i t  suggests t h a t  
one rneans f o r  cornpari ng i r r a d i a t i o n  e f f e c t s  a t  d i f f e r e n t  energ ies and d i  f f e r -  
e n t  f l uences  i s  by means o f  t h e  c a r r i e r  t o t a l  cap tu re  c ross  sec t ion .  Z; (TOT) 
i s  a  macroscopic parameter and should be c o r r e l a t a b l  e  w i t h  macroscopic para- 
meters o t h e r  than  those shown i n  F i g u r e  4, such as d i f f u s i o n  leng th .  
The e l e c t r o n  b i n d i n g  energy o f  r a d i a t i o n  induced de fec t s  has been 
measured f o r  some se t s  o f  r a d i a t i o n  and m a t e r i a l  parameters. For  low energy 
p ro ton  i r r a d i a t i o n ,  these l e v e l s  a re  shown i n  the f i r s t  t h r e e  colu~nns o f  
F i gu re  5. For 1 MeV p ro ton  i r r a d i a t i o n  o f  n-GaAs, two e l e c t r o n  t r a p s  a t  
Ec - 0.14 eV and E c  - 0.46 eV have been measured a1 so (F ig .  5, column 4) .  
An i n c r e a s i n g  number o f  l e v e l s  were measured as t h e  p ro ton  energy was 
inc reased  up t o  0.29 MeV. However, a t  a  p ro ton  energy o f  1 MeV on l y  two 
e l e c t r o n  t r a p s  were found ( t h e  deeper o f  which became deeper w i t h  i nc reas ing  
f l uence )  . Th is  r e s u l t  suggests t h a t  qual  i t a t i v e l y  d i f f e r e n t  de fec t s  cou ld  be 
i n t r oduced  when low energy protons a r e  stopped i n  GaAs s o l a r  c e l l s .  
The l a s t  column o f  F i g u r e  5  i l l u s t r a t e s  schemat ica l l y  t h e  s ix-energy 
1  eve ls  which Lang, e t  a1 . (Ref. 2 )  found t o  r e s u l t  f rom 1 FleV e l e c t r o n  
i r r a d i a t i o n .  Lang i d e n t i f i e d  f i v e  e l e c t r o n  t r a p s  and one h o l d  t r a p .  Th is  i s  
s i g n i f i c a n t l y  d i f f e r e n t  f rom t h e  number and d i s t r i b u t i o n  produced by 1 MeV 
pro tons .  
A l though t h e  r e s u l t s  i l l u s t r a t e d  i n  F igu re  5  do no t  s t r o n g l y  a f f e c t  our 
view o f  r a d i a t i o n  degradat ion,  they  do a f f e c t  our  approach t o  anneal ing. I f  
d i f f e r e n t  p a r t i c l e s  and d i f f e r e n t  energ ies  produce qual i t a t i v e l y  d i f f e r e n t  
damage, f o r  example bound aggregates o f  de fec t s  f o r  low energy protons and 
i s o l a t e d  s imp le  de fec ts  f o r  h i g h  energy e l ec t rons ,  then annea l ing  k i n e t i c s  
may be d i f f e r e n t  a lso.  As a  f i r s t  approximat ion, many o f  us have assumed 
t h a t  a l l  damage anneals l i k e  e l e c t r o n  damage; horrever, t h i s  must be checked 
c a r e f u l  l y  . 
U n i v e r s i t y  o f  F l o r i d a  s tud ies  w i l l  be focuss i r lg  on t h i s  i n  t h e  corning 
year.  
We are  per fo rming  a  c a l c u l a t i o n a l  study o f  t h e  simultaneous 1 MeV e l e c -  
t r o n  i r r a d i a t i o n  and annea l ing  e f f e c t s  i n  GaAs s o l a r  c e l l s .  Separate e x p e r i -  
ments suggest t h a t  ope ra t i on  o f  t he  c e l l s  near 200°C should con t i nuous l y  
anneal t h e  damage and s i g n i f i c a n t l y  reduce t h e  e f f e c t  o f  l o n g  t e r n  e l e c t r o n  
i r r a d i a t i o n .  
Under i r r a d i a t i o n  by h i gh  energy protons,  GaAs c e l l s  do no t  degrade as 
r a p i d l y  as do S i  c e l l s .  For lohi energy protons, GaAs c e l l  s  degrade more 
r a p i d l y ;  however, i f  t h e  c e l l s  a re  p rov ided  w i t h  s u f f i c i e n t l y  t h i c k  cover  
g lass r a d i a t i o n  sh ie l ds ,  then no c e l l  damage occurs. A  t h i c k  GaARAs window 
m i  ght  p rov ide  a  1  i ght-wei ght  , i n t e g r a l  s h i e l  d, bu t  severe ly  damaged windows 
would r e q u i r e  e f f e c t i v e  annea l ing  t o  makc t h i s  concept useabl e. 
Degradat ion o f  s o l a r  c e l l  performance parameters appears t o  c o r r e l a t e  
w e l l  w i t h  i nc reas ing  t o t a l  c ross  s e c t i o n  f o r  c a r r i e r  capture.  Deep l e v e l  
t r a n s i e n t  spectroscopy (DLTS) has i d e n t i f i e d  numerous t r a p  energy-1 eve1 s  
produced by protons. A  comparison between t h e  energy- leve ls  produced by 
p ro tons  and by e l e c t r o n s  which a re  no t  stopped i n  t h e  m a t e r i a l  i n d i c a t e  t h a t  
t h e  darnage produced by protons and e l  ec t rons  may be qua1 i t a t i  v e l y  d i f f e r e n t .  
Thus, annea l ing  o f  p ro ton  damage may be very  d i f f e r e n t  f rom the  anneal i n g  o f  
e l e c t r o n  darnage. 
REFERENCE 
1. Walker, G. H.; Conway, E. J.; J. Electrochem. Soc. 25, 10, p. 172G, 1978. 
2. Lang, D. V.; Logan, R. A.; and Ki rner l ing,  L. C.: Phys. Rev. B 15, 10, 
p. 4874, 1977. 
Isc 
- 
I 
SCo No annealing 
Figure 1. - Annealing recovery of short  c i rcu i t  cur rent  for one MeV electron 
irradiated cells. 
RELAT l VE 
CURRENT 
.6 1 I I I I 
0 2 4 6 8 
ANNEALING TIME (hours 
Figure 2. - Recovery of short  c i rcu i t  cur rent  as a funct ion of annealing time. 
REMA IN lNG 
POWER. % 
&lxlo12 0 - s :  
- GaAe 
0 
-
0.1 1 10 100 
PROTON ENERGY, MeV 
Figure 3. - Proton radiation damage: GaAs and S i  solar cells. 
290 kev 100 kev 
1 0 c m - ~  1012 ,, -2 
290 kev 
10 l2 cm -2 
'tot ='HT + =ET 
(cm 
Figure 4. - Total carr ier  cross section effects for proton irradiated GaAs solar cells. 
0.05 Mev 0.10 Mev 0.29 Mev 
PROTON PROTON PROTON 
--- 
CB C B CB 
--- 
- e  
- e e 
- e  
- h  
1 M e v  
1 Mev ELECTRON 
PROTON LANG, ET. AL. ) 
--
CB C B 
-- 
Figure 5. - Comparison of radiation induced trap levels i n  GaAs. 


Radiation damage in GaAs solar cells

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