Development of a technology for fabricating light-weight, high-efficiency, radiation-resistant solar cells for space applications is reported. The approaches currently adopted are to fabricate shallow homojunction n(+)/p as well as p/n AlGaAs-heteroface GaAs solar cells by organometallic chemical vapor deposition (OM-CVD) on single-crystal Si substrates using in each case, a thin Ge epi-interlayer first grown by CVD. This approach maintains the advantages of the low specific gravity of Si as well as the high efficiency and radiation-resistant properties of the GaAs solar cell which can lead to greatly improved specific power for a solar array. The growth of single-crystal GaAs epilayers on Ge epi-interlayers on Si substrates is investigated. Related solar cell fabrication is reviewed

Wang, K. L.

Yeh, Y. C. M.

Zwerdling, S.

Welsh 

NASA Technical Reports Server

PKCGliLSS T O W i W U  THIl\j-FILM CarZs SdLr l i t  CELLS USING A S l h  t iLE- 
CRYSTAL Si SUBSTKATE WITH H G e  INTEKLAYEK" 
Y.C.M. Yeh, K.L. Wang, and  S. Zwerdl ing 
Jet Propulsion Labora tory  
Cal i forn ia  Ins t i t u t e  of Technology 
Pasadena ,  Cal i forn ia  
EXTENDED ABSTRACT 
INTRODUCTION 
The o b j e c t i v e  o f  our  research program i s  t o  develop a technology f o r  
f a b r i c a t i n g  l i g h t - w e i g h t ,  h i g h - e f f i c i e n c y ,  r a d i a t i o n - r e s i s t a n t  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 .  The approaches c u r r e n t l y  adopted a r e  t o  f a b r i c a t e  
sha l low homojunct ion n+/p as w e l l  as p/n AlGaAs-heteroface GaAs s o l a r  c e l l s  
by organometal l  i c  chemical vapor d e p o s i t i o n  (OM-CVD) on s i n g l e - c r y s t a l  S i  
subs t ra tes  u s i n g  i n  each case, a t h i n  Ge e p i - i n t e r l a y e r  f i r s t  grown by CVD. 
T h i s  approach main ta ins  t h e  advantages o f  t h e  low s p e c i f i c  g r a v i t y  o f  S i  
as w e l l  as  t h e  h i g h  e f f i c i e n c y  and r a d i a t i o n - r e s i s t a n t  p r o p e r t i e s  o f  t h e  
GaAs s o l a r  c e l l  which can l e a d  t o  g r e a t l y  improved s p e c i f i c  power f o r  a. 
s o l a r  array. The i n t e r m e d i a t e  goals  o f  t h e  program a r e  t o  i n v e s t i g a t e  t h e  
growth o f  s i n g l e - c r y s t a l  GaAs e p i l a y e r s  on Ge e p i - i n t e r l a y e r s  on S i  s u b s t r a t e s  
and t o  develop re1 a ted  s o l  a r  c e l l  f a b r i c a t i o n  techno1 ogy. 
on our  progress toward these i n t e r m e d i a t e  goals.  
Th is  paper r e p o r t s  
EXPERIMENTAL 
I n  t h e  exper iment i n v o l v i n g  t h e  growth o f  a GaAs/Ge/Si s t r u c t u r e ,  Ge 
e p i - i n t e r l a y e r s  were f i r s t  grown on (100) s i n g l e - c r y s t a l  S i  s u b s t r a t e s  i n  
a v e r t i c a l  q u a r t z  CVD r e a c t o r  a t  700-725°C w i t h  a growth r a t e  o f  about 0.1 
micrometer/min by p y r o l y s i s  o f  GeHq i n  a H2 gas mixture.  Subsequently, t h e  
growth o f  GaAs l a y e r s  on t h e  Ge e p i - i n t e r l a y e r  were conducted i n  a h o r i z o n t a l  
q u a r t z  CVD r e a c t o r  a t  s i m i l a r  temperature range w i t h  a growth r a t e  o f  about 
0.2 micrometer/min by p y r o l y s i s  o f  t r i m e t h y l  g a l l i u m  (TMG) and a r s i n e  (AsH3) 
i n  a H2 gas mix tu re .  
*The research descr ibed i n  t h i s  paper was performed a t  t h e  J e t  P r o p u l s i o n  
Laboratory,  C a l i f o r n i a  I n s t i t u t e  o f  Technology, and was sponsored by t h e  
N a t i o n a l  Aeronaut ics  and Space A d m i n i s t r a t i o n ,  O f f i c e  o f  Aeronaut ics  and 
Space Techno1 ogy, under Contract  NAS7-100 w i t h  t h e  J e t  PropuJsion Laboratory ,  
and t h e  U n i t e d  Sta tes  A i r  Force ( a t  Wr ight -Pat terson AFB) th rough an agreement 
w i t h  t h e  Nat iona l  Aeronaut ics  and Space A d m i n i s t r a t i o n .  
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https://ntrs.nasa.gov/search.jsp?R=19830007546 2020-03-22T00:07:15+00:00Z
I n  t h e  development o f  t h e  n+/p GaAs shal  low-homojunct ion base1 i n e  s t r u c t u r e ,  
GaAs p- and n- type l a y e r s  were produced by a l t e r n a t e l y  adding d i m e t h y l z i n c  and 
hydrogen s u l f i d e ,  r e s p e c t i v e l y ,  t o  t h e  OM-CVD gas stream, as t h e  dopants. The 
a c t i v e  p- layer ,  about 2 micrometers t h i c k ,  was doped t o  1-2 x 1017/cm3, whereas 
t h e  n+ l a y e r  about  40 nm t h i c k  was doped t o  4 x 1018/cm3 f o r  low sheet res is tance.  
E l e c t r o p l a t e d  g o l d  was used f o r  g r i d  and back s u r f a c e  c o n t a c t  coa t ing ,  and t h e  
a n t i - r e f l e c t i o n  c o a t i n g  was made by vacuum d e p o s i t i o n  o f  Sb2O3. 
RESULTS AND D I S C U S S I O N  
D u r i n g  t h e  l a s t  Conference here  a t  NASA Lewis, we r e p o r t e d  t h e  f i r s t  
successfu l  growth o f  t h i n  e p i t a x i a l  l a y e r s  o f  GaAs by OM-CVD on to  a Ge e p i -  
i n t e r l a y e r  which was i n  t u r n  grown onto  a (100) S i  subst rate,  w i t h  a l l  e p i -  
growth done by t h e  CVD process. 
b r i g h t  and shiny.  However, some s t ress- induced cleavage l i n e s  and a h i g h  
s u r f a c e  m i c r o p i t  d e n s i t y  (about 105/cm2) were observed by SEM and o p t i c a l  
microscopy. 
o f  t h e  i n i t i a l  h i g h  m i c r o p i t  d e n s i t y  (about  6 x 107/cm2) o f  t h e  Ge e p i - i n t e r l a y e r .  
The s t r u c t u r e s  t h u s  produced were smooth, 
The h i g h  m i c r o p i t  d e n s i t y  o f  t h e  GaAs f i l m s  were a consequence 
I n  o r d e r  t o  improve t h e  Ge s u r f a c e  f o r  subsequent GaAs growth, p u l s e d  
l a s e r  annea l ing  exper iments were f i r s t  conducted. 
t h e  l a b o r a t o r i e s  o f  t h e  Lockheed M i s s i l e s  and Space Systems D i v i s i o n ,  Sunnyvale, 
C a l i f o r n i a  was used f o r  these annea l ing  s tud ies .  
performed on Ge/Si samples w i t h  energy d e n s i t i e s  rang ing  f rom 0.35 t o  0.79 
J/cm2 w i t h  a p u l s e  w i d t h  o f  l e s s  than 150 ns a t  a wavelength o f  1.06 micrometers.  
The r e s u l t s  a t  low energy d e n s i t y  (0.35 J/cm2) showed t h a t  t h e  annealed Ge 
e p i - l a y e r  possessed a smoother s u r f a c e  morphology, b u t  had a s u r f a c e  mic ro-p i  t 
d e n s i t y  s i m i l a r  t o  t h e  unannealed Ge f i l m s ,  On t h e  o t h e r  hand, a t  h i g h  energy 
d e n s i t y  (0.79 J/cm2)¶ some s u r f a c e  damage was ev ident .  
ob ta ined when a l a s e r  energy d e n s i t y  o f  about 0.64 J/cmz was used. 
t o  a 1 0 - f o l d  r e d u c t i o n  o f  t h e  s u r f a c e  m i c r o p i t  d e n s i t y  on t h e  Ge l a  e r  f rom t h e  
and a cons iderab ly  smoother sur face.  
morphology o f  Ge i s  s i g n i f i c a n t ,  t h e  m i c r o p i t  d e n s i t y  va lue  was s t i l l  t o o  h igh .  
Therefore,  exper iments were conducted w i t h  t h e  a im o f  improv ing  s u r f a c e  morphology 
by m o d i f y i n g  t h e  Ge i n t e r l a y e r  growth technique. 
A Nd:YAG l a s e r  system a t  
Anneal ing exper iments were 
The b e s t  r e s u l t s  were 
T h i s  l e d  
o r i g i n a l  va lue o f  about 107/cm2 (Fig.  1 )  t o  a va lue  o f  about 106/cm !? (Fig.  2)  
A l though t h e  improvement i n  s u r f a c e  
The f o l l o w i n g  s teps were taken t o  improve Ge-crys ta l  growth: 1 )  improv ing  
t h e  l e a k - t i g h t n e s s  o f  t h e  CVD Ge growth system, 2)  reduc ing  t h e  s u r f a c e  boundary 
l a y e r  t h i c k n e s s  d u r i n g  Ge f i l m  growth by m o d i f i c a t i o n  o f  t h e  r e a c t o r  design, 
and 3)  a more e f f e c t i v e  i n i t i a l  p u r g i n g  o f  t h e  system t o  min imize r e s i d u a l  
02 and H20 vapor  contaminat ion.  
by f o u r  o rders  o f  magnitude f o r  t h e  Ge i n t e r l a y e r s  was achieved. A photomicrograph 
(Fig. 3)  o f  t y p i c a l  Ge/Si Sam l e s  t h u s  produced shows a s u r f a c e  m i c r o p i t  d e n s i t y  
Thus a r e d u c t i o n  o f  s u r f a c e  m i c r o p i t  d e n s i t y  
corresponding t o  about 5 x 10 s /cin2. 
Subsequently, GaAs f i l m s  were depos i ted  on those improved Ge/Si samples by 
When a Ge/Si sample w i t h  a 0.5-micrometer t h i c k  Ge i n t e r l a y e r  was used OM-CVD. 
as t h e  s u b s t r a t e  wafer,  a smooth epi-GaAs f i l m  w i t h  low m i c r o p i t  d e n s i t y  ( l e s s  
than 5 x lO3/cm2) and w i t h o u t  c leavage l i n e s ,  and hav ing  a l a r g e  s u r f a c e  area 
(2cm x 2cm) can be made r o u t i n e l y  by OM-CVD. 
GaAs/Ge/Si sample i s  shown i n  F ig .  4. T h i s  low m i c r o p i t  d e n s i t y  va lue  shou ld  
A photomicrograph o f  such a 
100 
be adequate f o r  s o l a r  c e l l  (and o t h e r )  a p p l i c a t i o n s ,  i n  view o f  t h e  f a c t  t h a t ,  
t y p i c a l l y ,  a good s i n g l e - c r y s t a l  GaAs e x h i b i t s  s i m i l a r  va lues o f  s u r f a c e  e t c h -  
p i t  dens i ty .  
technology, f u r t h e r  improvement i n  energy convers ion e f f i c i e n c y  has been achieved. 
The s o l a r  c e l l  samples grown showed AM0 e f f i c i e n c i e s  between 15 and 16%. 
F ig.  5. The AM0 energy convers ion e f f i c i e n c y  f o r  t h i s  GaAs s o l a r  c e l l  measured 
a t  28°C i s  about 16% u s i n g  a Spect ro lab  model XT-10 AM0 s imu la to r .  
e f f i c i e n c y  va lue  i s  comparable t o  t h a t  r e p o r t e d  by o t h e r s  employing t h e  same 
nt/p shal low-homojunct ion GaAs s o l a r - c e l l  s t r u c t u r e .  The AM0 values o f  Voc, 
Jsc and FF f o r  t h i s  c e l l  were about 0.99V, 26.6 mA/crn2 and 0.81, r e s p e c t i v e l y .  
I n  t h e  development o f  t h e  n+/p s h a l l  ow-homojunction GaAs base1 i ne s o l a r  c e l l  
L i g h t  I - V  c h a r a c t e r i s t i c s  o f  t h e  bes t  c e l l  w i t h  an area o f  1 cm h i s  shown i n  
Th is  
101 
Fig.  1. SEM photomicrograph o f  a t y p i c a l  Ge f i l m  grown a t  700°C on 
a (100)  S i  subs t ra te ,  b e f o r e  l a s e r  anneal ing.  
F i g .  2. SEM photomicrograph o f  t h e  Ge/Si smaple annealed a t  an average 
energy -dens i t y  o f  about  0.64 J/cm2. 
102 
F i g .  3. Photomicrograph o f  a t y p i c a l  Ge f i l m  grown a t  700°C on (100) 
S i  s u b s t r a t e  u s i n g  t h e  improved Ge-growth technology. 
Fig. 4. Photomicrograph o f  a GaAs/Ge/Si sample w i t h  OM-CVD grown GaAs 
on a CVD-grown Ge i n t e r l a y e r .  
103 
30 
N n+/p GaAs/GaAs SOLAR CELL 
AM0 (XT-10 SIMULATOR) 5 i! 20 
i., 
). 
ln z w 
0 
t; 725 8 Sb203 AR-COATING 
10 w 
nz 
3 
nz FF = 0.81 
V EF’F = 15.8% 
0 
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 
VOLTAGE, V 
Fig. 5. L i g h t  I - V  character is t ics  o f  a lcm x lcm - area n+/p GaAs/GaAs 
solar ce l l .  
104 


Progress toward thin-film GaAs solar cells using a single-crystal Si substrate with a Ge interlayer

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