Systematic improvements in fabrication yield were obtained by appropriate control of the liquid phase epitaxial growth process, contact fabrication and surface preparation. To improve radiation hardness, the junction depth was decreased while overcoming the penalty in decreased solar cell efficiency which tends to go hand-in-hand with the reduction of junction depth in (AlGa) As-GaAs solar cells. Cells were made with an AMO efficiency of 18% and a junction depth of 0.5 micrometers, as compared to junction depths on the order of 1.0 micrometers. With respect to the damage caused by proton irradiation, the nature of the observed damage was correlated to the energy and penetration depth of the damaging protons

Anspaugh, B. E.

Kamath, S.

Knechtli, R. C.

Loo, R.

English

NASA Technical Reports Server

FABRICATION OF HIGH EFFICIENCY AND RADIATION 
S. Kamath, R.C. Knech t l i  and R. Loo 
Hughes Research Labora to r i e s  
B. Anspaugh 
J e t  Propuls ion  Laboratory 
SUMMARY 
I n  our  development of t h e  technology f o r  making GaAs s o l a r  c e l l s  
s u i t a b l e  f o r  space a p p l i c a t i o n s ,  we a r e  concerned wi th  AM0 e f f i c i e n c y ,  
r a d i a t i o n  hardness  and r ep roduc ib l e  f a b r i c a t i o n .  Systematic  improvements i n  
f a b r i c a t i o n  y i e l d  were obtained by a p p r o p r i a t e  c o n t r o l  of t h e  l i q u i d  phase 
e p i t a x i a l  growth process ,  con tac t  f a b r i c a t i o n  and s u r f a c e  prepara t ion .  To 
improve r a d i a t i o n  hardness ,  we decreased t h e  junc t ion  depth whi le  overcoming 
the  pena l ty  i n  decreased s o l a r  c e l l  e f f i c i e n c y  which tends t o  go hand-in-hand 
wi th  t h e  r educ t ion  of j unc t ion  depth i n  (AlGa) As-GaAs s o l a r  c e l l s .  W e  have 
succeeded i n  making c e l l s  wi th  an  AM0 e f f i c i e n c y  of 18% and a j unc t ion  depth  
of 0.5 pm, as compared t o  j unc t ion  depths  on t h e  o rde r  of 1 .0 pm which had 
previous ly  been repor ted .  With r e s p e c t  t o  t h e  damage caused by pro ton  
i r r a d i a t i o n ,  we have been a b l e  t o  c o r r e l a t e  t h e  n a t u r e  of t h e  observed damage 
t o  t h e  energy and p e n e t r a t i o n  depth  of t h e  damaging protons.  
FABRICATION TECHNOLOGY 
I n  developing t h e  technology f o r  GaAs s o l a r  c e l l s  f o r  space a p p l i c a t i o n s ,  
we emphasized the  reproducib le  f a b r i c a t i o n  of l a r g e  a r e a  (2 cm x 2 cm) s o l a r  
c e l l s .  The i n f i n i t e  s o l u t i o n  l i q u i d  e p i t a x i a l  growth developed a t  HRL i s  
i d e a l l y  s u i t e d  f o r  t h e  growth of l a r g e  a r e a  l a y e r s  of GaAs and AlGaAs r equ i r ed  
f o r  t hese  c e l l s .  The growth system i l l u s t r a t e d  i n  Figure 1 has been descr ibed  
i n  p a s t  papers .  The f i g u r e  a l s o  shows a new modif ica t ion  of t h e  g r a p h i t e  
s u b s t r a t e  holder  t h a t  enables  u s  t o  grow on m u l t i p l e  s u b s t r a t e s  and proceed 
wi th  ba tch  process ing ,  l ead ing  t o  reduced c o s t  per  c e l l .  I r r a d i a t i o n  of t h e  
s o l a r  c e l l s  produced e a r l y  i n  t h e  program w i t h  junc t ion  depths  of about 1 pm 
l e d  u s  t o  conclude t h a t  shal lower junc t ions  would l e a d  t o  cons iderable  improve- 
ment i n  r a d i a t i o n  hardness .  The main o b j e c t i o n  t o  a shal low junc t ion  i n  t h e  
GaAs c e l l  i s  t h a t  it causes t h e  e l e c t r i c a l  j unc t ion  t o  approach t h e  (A1Ga)As- 
G a A s  heterophase boundary which is  a reg ion  of heavy s t r a i n  and d i s l o c a t i o n s ;  
t h i s  gene ra l ly  l e a d s  t o  a lower f i l l  f a c t o r  f o r  t h e  c e l l .  I n  our  case ,  lowering 
the  junc t ion  depth from 1 pm t o  less than  0.5 pm l e d  t o  a decrease  i n  e f f i c i e n c y  
*Par t s  of t h i s  p r o j e c t  were supported by A.F. Cont rac t  F33615-77-C-3150 and by 
NASA Cont rac t  NAS 1-14727 
https://ntrs.nasa.gov/search.jsp?R=19790024493 2020-03-21T21:02:48+00:00Z
from > 17% AM0 t o  < 15% AMO. A d e t a i l e d  s tudy  of e p i t a x i a l  growth parameters  
l e d  us  t o  decrease  t h e  growth temperature f o r  (A1Ga)As l a y e r s  t o  750°C and 
i n c r e a s e  t h e  growth t i m e  t o  over 5 minutes per  micron t o  achieve improved 
shal low junc t ions  of <0.5 pm. The p rog res s  achieved can be judged by t h e  
s t a t i s t i c s  i n  F igure  2. The performance of our  b e s t  4 cm2 c e l l  t o  d a t e  wi th  
0.5 pm junc t ion  depth i s  shown i n  F igure  3 .  
Since the  LPE growth tends  t o  be t h e  s lowes t  s t e p  i n  t h e  f a b r i c a t i o n  of 
c e l l s ,  u l t i m a t e  c o s t  r educ t ion  f o r  l a r g e  s c a l e  product ion would r e q u i r e  t h e  
adap ta t ion  of t h e  technique t o  ba tch  product ion.  We have succeeded i n  
demonstrating t h e  a d a p t a b i l i t y  of t h e  HRL technique f o r  t h i s  purpose by 
growing l a y e r s  on 4 s u b s t r a t e s  i n  a  s i n g l e  growth run,  us ing  a m u l t i p l e  s l i d e  
bar  assembly of t h e  type  i l l u s t r a t e d  i n  F igure  1. We a r e  p r e s e n t l y  extending 
t h i s  c a p a b i l i t y  t o  even l a r g e r  numbers. 
RESISTANCE TO EUYDIATION DAMAGE 
For space a p p l i c a t i o n s ,  t h e  a b i l i t y  of GaAs s o l a r  c e l l s  t o  withstand 
r a d i a t i o n  by h igh  energy p a r t i c l e s  i s  of g r e a t  importance i n  a d d i t i o n  t o  
supe r io r  beginning of l i f e  e f f i c i e n c y  vis-a-vis  s i l i c o n .  Our f i r s t  s t e p  t o  
determining t h e  r a d i a t i o n  r e s i s t a n c e  of t he  GaAs c e l l s  was t o  s tudy  t h e  
damage produced by 1 MeV e l e c t r o n s  a t  normal incidence.  This  s tudy  l e d  u s  t o  
formulate  a  model which showed t h e  c r i t i c a l  importance of decreas ing  junc t ion  
depth t o  i n c r e a s e  r a d i a t i o n  r e s i s t a n c e .  The r e s u l t s  on c e l l s  produced dur ing  
the  p a s t  year  w i th  0.5 ym junc t ion  depth  f u l l y  confirm our  model. Our 
t h e o r e t i c a l  c a l c u l a t i o n s  i n d i c a t e ,  however, t h a t  minimum r a d i a t i o n  damage is  
l i k e l y  t o  occur a t  about 0.2 ym. Recent ly we have f a b r i c a t e d  and t e s t e d  c e l l s  
wi th  a  j unc t ion  depth of 0.3 IIrn and AMO e f f i c i e n c y  of 16%. The r e s u l t s  a r e  
shown i n  Figure 4 and aga in  confirm our  model. W e  a r e  cont inuing  our systema- 
t i c  s t u d i e s  towards lowering t h e  junc t ion  depth t o  0.2 pm wi thout  adverse ly  
a f f e c t i n g  the  e f f i c i e n c y  of t h e  c e l l ,  
I n  p r a c t i c a l  space missions,  we have t o  be concerned wi th  i r r a d i a t i o n  
by pro tons  a s  w e l l  a s  e l e c t r o n s .  We have i n v e s t i g a t e d  t h e  behavior  of GaAs 
c e l l s  under proton i r r a d i a t i o n .  Data i n  F igure  5 r ep re sen t  t h e  e f f e c t  of 
both pro tons  and e l e c t r o n s  on a  number of our  c e l l s  on a  semilogari thmic p l o t .  
The c e l l s  a r e  a l l  similar wi th  a  j unc t ion  depth  of 0.5 pm. The e l e c t r o n s  were 
1 MeV and t h e  pro tons  were 100 and 290 KeV. Within measurement accuracy, 100 
KeV and 290 KeV pro tons  had t h e  same e f f e c t  on t h e  s o l a r  c e l l  power output .  
It is  i n t e r e s t i n g  t o  observe t h a t  i n  t h e  semilog p l o t ,  t h e  curves r ep re sen t ing  
e l e c t r o n  and pro ton  damage a r e  p a r a l l e l .  It lends  credence t o  t h e  concept of 
equ iva l en t  f luence  over extended ranges of f l uence  f o r  t hese  c e l l s .  
The r a d i a t i o n  damage caused by h igh  energy p a r t i c l e s  is obviously a  
func t ion  of t h e i r  energy. A s  f a r  as pro tons  a r e  concerned, they  produce most 
damage a t  t h e  end of t h e i r  t r a c k ,  a f t e r  p e n e t r a t i o n  i n  t h e  s o l a r  c e l l .  The 
n a t u r e  of t h e  damage which they produce i s  t h e r e f o r e  l a r g e l y  determined by 
t h e i r  depth of p e n e t r a t i o n  i n t o  the  s o l a r  c e l l .  This  l e d  us  t o  examine t h e  
damage caused i n  our c e l l s  a s  a  func t ion  of pro ton  pene t r a t ion  depth.  The 
r e s u l t s  are shown i n  Fig.  6. While we s t i l l  l a c k  measurements i n  t h e  i n t e r -  
mediate pro ton  energy range,  t h e  obse rva t ion  of t h e  damage caused by low 
energy pro tons  i s  e s p e c i a l l y  i n s t r u c t i v e .  A s  we s t a r t  w i th  very  low energy 
pro tons  (< 50 KeV), they  a r e  a r r e s t e d  w i t h i n  t h e  s u r f a c e  p-layer of t h e  GaAs, 
be fo re  even reaching  t h e  junc t ion .  The i r  major e f f e c t  is  t h e r e f o r e ,  t o  
i nc rease  bulk recombination i n  t h i s  l a y e r ,  t hus  decreas ing  t h e  s h o r t - c i r c u i t  
c u r r e n t  wi thout  a f f e c t i n g  t h e  junc t ion  q u a l i t y .  Cons i s t en t ly  w i th  t h i s ,  we 
observe i n  Fig. 6 t h a t  t h e  s h o r t - c i r c u i t  c u r r e n t  decreases  wi th  inc reas ing  
pro ton  energy be fo re  t h e  open-circui t  v o l t a g e  i s  much a f f e c t e d .  A s  t h e  pro ton  
energy is  increased  up t o  about 100 KeV, most pro tons  are a r r e s t e d  i n  t h e  
junc t ion  reg ion .  The r e s u l t i n g  d e t e r i o r a t i o n  of t h e  junc t ion  q u a l i t y  l e a d s  t o  
i nc reas ing  leakage c u r r e n t  and correspondingly lower open-circui t  vo l t age .  A s  
t h e  pro ton  energy is  f u r t h e r  increased  above 100 KeV, most pro tons  p e n e t r a t e  
beyond t h e  junc t ion  and do t h e i r  damage i n  t h e  n-doped base  region.  This  
l e a d s  t o  a d d i t i o n a l  bu lk  recombination and f u r t h e r  r educ t ion  of t h e  s h o r t  
c i r c u i t  c u r r e n t .  A s  t h e  pro ton  energy is  f u r t h e r  increased  above 300 KeV, t h e  
pro tons  are a r r e s t e d  a t  depths  p a s t  t h e  a c t i v e  p a r t  of t h e  c e l l .  A s  t h e  energy 
of a l l  p ro tons  t r a v e r s i n g  t h e  a c t i v e  p a r t  of t h e  s o l a r  c e l l  keeps inc reas ing ,  
t h e  damage which they cause i n  t h i s  p a r t  of t h e  c e l l  decreases  and t h e  e f f e c t  
on both s h o r t - c i r c u i t  c u r r e n t  and open-c i rcu i t  vo l t age  decreases  wi th  f u r t h e r  
i n c r e a s e s  i n  pro ton  energy. I n  summary, t h e  measurements of F ig .  6 show t h a t  
t h e  pro tons  a r r e s t e d  i n  t h e  v i c i n i t y  of t h e  junc t ion  have a most d e l e t e r i o u s  
e f f e c t  on t h e  open-circui t  vo l t age ,  wh i l e  t h e  pro tons  a r r e s t e d  w i t h i n  t h e  
a c t i v e  p a r t  of t h e  c e l l ,  b u t  f u r t h e r  away from t h e  junc t ion  a f f e c t  mostly t h e  
s h o r t - c i r c u i t  cu r r en t .  An i n t e r e s t i n g  p r a c t i c a l  r e s u l t  is  t h e  f a c t  t h a t  t h e  
shal low a c t i v e  depth  of t he  G a A s  s o l a r  c e l l s  (< 2 u m ,  because GaAs i s  a d i r e c t  
bandgap m a t e r i a l )  limits t o  a very  low and narrow energy range (< 0.5 MeV) t h e  
pro tons  which cause most damage. Th i s  l e a d s  t o  most e f f e c t i v e  coverg lass  
s h i e l d i n g  of GaAs s o l a r  c e l l s  f o r  p r o t e c t i o n  a g a i n s t  p ro ton  damage i n  a  real 
space environment. I n  p a r t i c u l a r ,  t h i s  compares favorably  wi th  S i  s o l a r  c e l l s  
where most damage i s  caused by pro tons  wi th  an  energy on t h e  o rde r  of 2 MeV, as 
compared t o  200 KeV f o r  GaAs s o l a r  c e l l s .  
To compare t h e  r a d i a t i o n  damage caused by protons on GaAs and on S i  solar 
c e l l s  r e s p e c t i v e l y ,  w e  have summarized i n  Fig.  7 t h e  r e s u l t s  of a  number of 
our  measurements. W e  show t h e  damage caused by a given f luence  t o  both types  
of c e l l s ,  as a func t ion  of pro ton  energy. While very  low energy pro tons  cause 
more damage t o  GaAs than  t o  S i  s o l a r  c e l l s ,  a t  h ighe r  ene rg i e s  t h e  r eve r se  is  
t rue .  I n s o f a r  as these  s o l a r  c e l l s  are r e a d i l y  p ro t ec t ed  a g a i n s t  low energy 
pro tons ,  b u t  remain s u s c e p t i b l e  t o  t h e  h igher  energy pro tons ,  F ig .  7 shows 
t h a t  G a A s  s o l a r  c e l l s  compare favorably  wi th  S i  s o l a r  c e l l s  i n  t h e i r  a b i l i t y  t o  
resist pro ton  r a d i a t i o n  damage i n  a  real space environment. 
EFFICIENCY 
The i n i t i a l  mot iva t ion  f o r  developing G a A s  s o l a r  c e l l s  w a s  t h e i r  poten- 
t i a l  f o r  b e t t e 5  AM0 e f f i c i e n c y  than  S i  s o l a r  c e l l s .  Our p re sen t  s ta te-of- the-  
a r t  f o r  a 4 cm shal low junc t ion  GaAs s o l a r  c e l l  has  t h e  c h a r a c t e r i s t i c s  shown 
i n  Fig.  3 .  The f i l l  f a c t o r  of t he  c e l l  i s  s t i l l  r e l a t i v e l y  poor,  even though 
i ts  AM0 e f f i c i e n c y  is  18%. The t h e o r e t i c a l  upper l i m i t  f o r  the  f i l l  f a c t o r  of 
such a  c e l l  i s  0.88. Some of our  deeper j unc t ion  c e l l s ,  have a  f i l l  f a c t o r  as 
h igh  as 0.86. On t h i s  b a s i s ,  we cons ider  r e a l i s t i c  an  e x t r a p o l a t i o n  t o  a  f i l l  
f a c t o r  of 0.85, a s  a  r e s u l t  of f u r t h e r  progress  i n  c o n t r o l l i n g  t h e  c r y s t a l  
q u a l i t y  a t  t h e  i n t e r f a c e  and i n  t h e  junc t ion  region.  Such an  e x t r a p o l a t i o n  
l e a d s  t o  a n  AM- e f f i c i e n c y  of 20.3%. Under i d e a l  condi t ions ,  Fig.  8  shows t h a t  
an AM0 e f f i c i e n c y  of 24% can be achieved wi th  a  GaAs s o l a r  c e l l .  This  
i d e a l i z e d  c a l c u l a t i o n  makes however, no allowance f o r  any l o s s e s  a s soc i a t ed  
wi th  c o n t a c t  shadowing, r e f l e c t i o n ,  o r  f o r  j unc t ion  imperfec t ions  leading  t o  a  
lower f i l l  f a c t o r .  The i d e a l  c a l c u l a t i o n  a l s o  assumes t h e  p-doping l e v e l  t o  be 
a t  t h e  l i m i t  of degeneracy (p = 7.1018 holes/cm3). For t h e s e  reasons ,  we 
cons ider  ou r  estimate of an  AM0 e f f i c i e n c y  of 20%, based on our experimental  
r e s u l t s ,  t o  be  more r e a l i s t i c .  
LIQUID EPITAXY 
GROWTH SYSTEM 
GRAPHITE WAFER HOLDER 
1 CELLd HR 32 CELLSIHR 
Figure 1. - Batch fabrication of GaAs solar cells. 
JUNCTION DEPTH: xi = 0.5 pm I 
SEPT DEC MARCH JUNE SEPT DEC 
77 77 78 78 78 78 . . . . . - 
IMPROVEMENT IN CELL YIELD 
Figure 2. - Improvement of GaAs solar cel l  yield with time. 
VOLTAGE, V 
Figure 3. - High efficiency GaAs solar cell. 
lo1* IF?? loT4 loq5 
1 MeV ELECTRON FLUENCE, ELECTRONSICM* 
Figure 4. - Effect of junct ion depth on radiation damage. 
Figure 5. - Proton versus electron radiation damage. 
I."" 
0.90 
0.80 
/- I OPEN CIRCUIT 
= VOLTAGE BEFORE 
-\, - I 
0' 
) lo = CURRENT 
FLUENCE = 1011 PROTONSICM~ 
xi = 0.5 p m  = JUNCTION DEPTH 
I 
PROTON ENERGY, MeV 
(PROTON PENETRATION DEPTH, pm) 
Figure 6. - Effect of proton energy on radiation damage. 
Figure 7. - Proton radiation damage on GaAs versus Si solar cells. 
Figure 8. - Theoretical l imi t  for GaAs solar cell efficiency. 
PARAMETER 
SHORT-CIRCUIT CURRENT 
OPEN CIRCUIT VOLTAGE 
FILL FACTOR 
EFFICIENCY 
BEST 
ESTIMATE 
32 ~ A / C M ~  
1.01 V 
0.85 
20.3% 
THEORETICAL 
MAXIMUM 
35 ~ A / C M ~  
1.05 V 
0.88 
23.9% 


Fabrication of high efficiency and radiation resistant GaAs solar cells

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