V-grooved GaAs solar cells promise the benefits of improved optical coupling, higher short-circuit current, and increased tolerance to particle radiation compared to planar cells. A GaAs homojunction cell was fabricated by etching a V-groove pattern into an n epilayer (2.1 x 10 to the 17th power per cu cm) grown by metalorganic chemical vapor deposition (MOCVD) on an n+ substrate (2.8 x 10 to the 18th power per cu cm) and then depositing and MOCVD p epilayer (4.2 x 10 to the 18th power per cu cm). Reflectivity measurements on cells with and without an antireflective coating confirm the expected decrease in reluctance of the microgrooved cell compared to the planar structure. The short circuit current of the V-grooved solar cell was 13 percent higher than that of the planar control

Arrison, A.

Bailey, S. G.

Fatemi, N. S.

Landis, G. A.

Thomas, R. D.

Wilt, D. M.

English

NASA Technical Reports Server

NASA Technical Memorandum 10 1970 
A V-Grooved GaAs Solar Cell 
S.G. Bailey 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 
N.S. Fatemi 
Sverdrup Technology, Inc. 
NASA Lewis Research Center Group 
Cleveland, Ohio 
G.R. Landis, D.M. Wilt, R.D. Thomas and A. Arrison 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 
N89-22 177 
unclas 
02043364 
Presented at the 
20th Photovoltaic Specialists Conference 
sponsored by The Institute of Electrical and Electronics EnglJeers 
Las Vegas, Nevada, September 26-30, 1988 
https://ntrs.nasa.gov/search.jsp?R=19890012806 2020-03-20T02:32:43+00:00Z
A V-GROOVED GaAs SOLAR CELL 
S . G .  B a i l e y  
N a t i o n a l  A e r o n a u t i c s  and Space A d m i n i s t r a t i o n  
Lewis Research Center  
C l e v e l a n d ,  O h i o  44135 
N . S .  Fatemi 
Sverdrup  Technology, I n c .  
NASA Lewis Research Center  Group 
C l e v e l a n d ,  O h i o  44135 
G.A. L a n d i s , *  D.M.  Wilt, R.D.  Thomas, and A .  A r r i s o n  
N a t i o n a l  A e r o n a u t i c s  and Space A d m i n i s t r a t i o n  
Lewis Research Center  
C l e v e l a n d ,  O h i o  44135 
SUMMARY 
V-grooved GaAs s o l a r  c e l l s  promise t h e  b e n e f i t s  o f  improved o p t i c a l  
c o u p l i n g ,  h i g h e r  s h o r t - c i r c u i t  c u r r e n t ,  and i n c r e a s e d  t o l e r a n c e  to  p a r t i c l e  
r a d i a t i o n  compared t o  p l a n a r  c e l l s .  
by  e t c h i n g  a V-groove p a t t e r n  i n t o  an n e p i l a y e r  ( 2 . 1 ~ 1 0 ~ ~  ~ m - ~ )  grown by  
MOCVD on an n+ s u b s t r a t e  ( 2 . 8 ~ 1 0 ~ 8  cm-3) and t h e n  d e p o s i t i n g  an MOCVD 
e p i l a y e r  ( 4 . 2 ~ 1 0 ~ 8  cm-3). R e f l e c t i v i t y  measurements on c e l l s  w i t h  and w i t h o u t  
an a n t i r e f l e c t i v e  c o a t i n g  c o n f i r m  t h e  expec ted  decrease i n  r e l u c t a n c e  of t h e  
mic rogrooved c e l l  compared to t h e  p l a n a r  s t r u c t u r e .  The s h o r t  c i r c u i t  c u r r e n t  
of the V-grooved s o l a r  c e l l  was 13 p e r c e n t  h i g h e r  t h a n  t h a t  o f  t h e  p l a n a r  
c o n t r o l .  
A GaAs homojunc t ion  c e l l  was f a b r i c a t e d  
03 W
$? 
p 
I 
INTRODUCTION 
G e o m e t r i c a l l y  s t r u c t u r e d  s u r f a c e s  have become i n c r e a s i n g l y  i m p o r t a n t  i n  
r a i s i n g  t h e  e f f i c i e n c y  o f  s i l i c o n  c e l l s  ( r e f s .  1 and 2 ) .  S i m i l a r  b e n e f i t s  a r e  
a n t i c i p a t e d  i n  GaAs solar c e l l s  i n c l u d i n g  reduced r e f l e c t a n c e ,  o b l i q u e  passage 
of l i g h t  through t he  c e l l ,  and l i g h t  t r a p p i n g .  The n e t  e f f e c t  o f  t h e  m i c r o -  
grooved s t r u c t u r e  i s  an i n c r e a s e  i n  t o t a l  a b s o r p t i v i t y ,  r a d i a t i o n  t o l e r a n c e ,  
and s h o r t - c i r c u i t  c u r r e n t  o f  t h e  s o l a r  c e l l .  
A n i s o t r o p i c  wet chemical  e t c h i n g  has been shown t o  p r o v i d e  a s i m p l e ,  
i n e x p e n s i v e  method f o r  f a b r i c a t i n g  s t r u c t u r e s  w i t h  l e s s  t h a n  2 pm spac ings  
( r e f .  3 ) .  I n  p r e v i o u s  work we demonstrated GaAs and AlGaAs MOCVD g r o w t h  on  
V-grooved s u r f a c e s  ( r e f s .  3 and 4 ) .  A n i s o t r o p i c  e t c h i n g  i n  GaAs i s  more 
c o m p l i c a t e d  t h a n  on s i l i c o n  due to  t h e  p o l a r  n a t u r e  o f  t h e  l a t t i c e  ( r e f .  5), 
t h u s  c a r e f u l  o r i e n t a t i o n  o f  t h e  photo1 i t h o g r a p h i c  mask i s  r e q u i r e d  ( r e f .  6). 
F i g u r e  1 shows a r e p r e s e n t a t i v e  s o l a r  c e l l  s t r u c t u r e .  
V-GROOVE FABRICATION 
S u b s t r a t e s  f o r  t h e  c e l l  f a b r i c a t i o n  were (100)  n 
Pr ior  t o  t h e  c a r r i e r  c o n c e n t r a t i o n  o f  2 . 8 ~ 1 0 ~ ~  ~ m - ~ .  
* N a t i o n a l  Research Counci 1 - NASA Research Assoc 
t y p e  w a f e r s  w i t h  a 
V-groove e t c h i n g  process ,  
a t e .  
t h e  s u b s t r a t e  was loaded as r e c e i v e d  from t h e  s u p p l i e r  i n t o  t h e  MOCVD r e a c t o r  
and s u b j e c t e d  t o  a h i g h  tempera ture  bake under a H2 and AsH3 ambient .  Follow- 
i n g  t h e  bake and e p i t a x i a l  n base l a y e r  o f  4 .5  pm t h i c k n e s s  w i t h  d o p i n g  
2 . 1 ~ 1 0 1 7  cm-3 was grown. 
The process  o f  e t c h i n g  V-grooves i n  t h e  n base e p i l a y e r  c o n s i s t s  o f  t h e  
f i r s t  masking t h e  s u r f a c e  w i t h  a p h o t o r e s i s t  p a t t e r n  o f  p a r a l l e l  4 pm l i n e s  and 
3 pm spaces, a l i g n e d  i n  t h e  (017) d i r e c t i o n .  
t r o n  mic rograph o f  t h e  s u r f a c e  a f t e r  t h e  p h o t o r e s i s t  s t e p .  Next  t h e  grooves 
were e tched u s i n g  t h e  H2S04:H202:H20 Caros e t c h a n t  ( r e f .  7 )  i n  t h e  r a t i o  of 
5 : l : l .  F u r t h e r  d e t a i l s  on  t h e  e t c h i n g  process  a r e  d i s c u s s e d  i n  r e f e r e n c e  3.  
F i g u r e  3 shows t h e  grooves formed by  e t c h i n g  f o r  150 sec i n  t h e  Caros e t c h a n t  
a t  24 "C. The m i c r o g r a p h  i n  F i g u r e  4 shows t h e  r e s u l t a n t  V-grooved s t r u c t u r e  
a f t e r  removal  o f  t h e  p h o t o r e s i s t .  
F i g u r e  2 shows a scann ing  e l e c -  
R e f l e c t a n c e  o f  t h e  V-grooved s u b s t r a t e  a f t e r  AR c o a t i n g  was measured on 
a Perk in-Elmer Lambda 9 u v / v i s / n i r  s p e c t r o p h o t o m e t e r .  A compar ison o f  t h e  
r e f l e c t a n c e  o f  t h e  V-grooved s u r f a c e  w i t h  a p l a n a r  sample shows t h e  expec ted  
r e d u c t i o n  i n  r e f l e c t i v i t y  o v e r  t h e  s p e c t r a l  range o f  i n t e r e s t  f o r  t h e  m i c r o -  
grooved s t r u c t u r e .  F i g u r e  5 shows t h e  r e f l e c t i v i t y  as a f u n c t i o n  of wave- 
l e n g t h  from 400 to  900 nm. 
To r e a c h  l i m i t  e f f i c i e n c i e s  on  v e r y  t h i n  m a t e r i a l ,  f u t u r e  GaAs s o l a r  
c e l l s  may need t o  i n c o r p o r a t e  g e o m e t r i c a l  l i g h t - t r a p p i n g  s t r u c t u r e s .  To demon- 
s t r a t e  t h e  f e a s i b i l i t y  o f  f o r m i n g  o p t i c a l  con f inement  s t r u c t u r e s  on  GaAs, we 
f a b r i c a t e d  a c ross-grooved s t r u c t u r e  ( r e f .  2 ) ,  where V-grooves a r e  fo rmed on 
b o t h  f r o n t  and back s i d e s  o f  t h e  w a f e r ,  w i t h  t h e  back-s ide  grooves perpend icu-  
l a r  t o  t h e  f r o n t  g rooves .  T h e o r e t i c a l  a n a l y s i s  shows t h i s  s t r u c t u r e  t o  be 
h i g h l y  e f f e c t i v e  i n  i n c r e a s i n g  t h e  o p t i c a l  p a t h l e n g t h  w i t h  the m a t e r i a l  
( r e f .  8 ) .  Tab le  I shows t h e  e x p e r i m e n t a l  i n c r e a s e  i n  a b s o r p t i o n  measured for 
weakly  absorbed (950 nm) l i g h t  f o r  t h e  c ross-grooved s t r u c t u r e  compared w i t h  
s p e c u l a r  ( p o l i s h e d  b o t h  s i d e s )  and l a m b e r t i a n  ( p o l i s h e d  f r o n t ,  roughened back)  
samples from t h e  same w a f e r .  The samples had no  AR c o a t i n g  or back sur face  
r e f l e c t o r .  A b s o r p t i o n  by  t h e  c ross-grooved s t r u c t u r e  has been enhanced b y  
n e a r l y  a f a c t o r  o f  seven compared t o  t h e  p l a n a r  c o n t r o l .  
CELL FABRICATION 
The a c t i v e  c e l l  l a y e r s  w e r e  e p i t a x i a l l y  grown i n  a h o r i z o n t a l ,  c o l d  w a l l ,  
sub-atmospher ic p r e s s u r e  MOCVD r e a c t o r .  Growth t e m p e r a t u r e  was 620 "C;  t h e  
V / I I I  r a t i o  was 46; t h e  dopant  was a 500 ppm m i x t u r e  o f  H2S i n  UHP H 2 ;  and t h e  
chamber p r e s s u r e  was 100 t o r r .  A t h i n  b u f f e r  l a y e r  was f i rs t  d e p o s i t e d  and 
t h e n  a t h i c k  n base l a y e r  was grown w i t h  a c a r r i e r  c o n c e n t r a t i o n  o f  
2 . 1 ~ 1 0 1 7  cm-3. 
The e m i t t e r  g rowth  was a g a i n  preceded by  a h i g h  tempera ture  bake under  a 
H2 and AsH3 ambien t .  
and DEZn was t h e  p dopant .  A l l  o t h e r  r e a c t o r  parameters were i d e n t i c a l  t o  
t h e  base d e p o s i t i o n .  A 0.1 pm t h i c k  p e p i l a y e r  w i t h  a c a r r i e r  c o n c e n t r a t i o n  
o f  4 . 2 ~ 1 0 ~ 8  cm -3 was grown on t h e  V-grooves. 
l a y e r  g rowth  on  t h e  V-grooves. 
t h e  b o t t o m  t o  t h e  t o p  o f  t h e  groove.  
The V-grooves were o r i e n t e d  p a r a l l e l  t o  t h e  gas flow 
F i g u r e  6 i l l u s t r a t e s  t h e  e p i -  
The e p i l a y e r  t h i c k n e s s  v a r i e s  s l i g h t l y  from 
2 
E-beam evapora ted  Au-Zn c o n t a c t s  were a p p l i e d  a t  a s l i g h t  a n g l e  t o  t h e  grooves 
i n  t h e  p e m i t t e r .  Au-Ge-Ni c o n t a c t s  were u t i l i z e d  on t h e  n base. The as 
d e p o s i t e d  c o n t a c t s  were ohmic and no subsequent s i n t e r i n g  was r e q u i r e d .  
ANALYSIS 
The p l a n a r  and V-groove c e l l s  which a r e  compared were f a b r i c a t e d  s i m u l t a -  
neous ly .  However, t h e  MOCVD g rowth  r a t e s  a r e  s l i g h t l y  d i f f e r e n t  on t h e  (100)  
and t h e  (111) c r y s t a l l o g r a p h i c  p l a n e s .  We e x p e c t  an u n c e r t a i n t y  of  a few 
p e r c e n t  due t o  t h i s  v a r i a t i o n  i n  f a b r i c a t i o n .  V-grooved s u r f a c e s  appear more 
s u s c e p t i b l e  t o  i m p e r f e c t i o n s  i n  t h e i r  s t r u c t u r e ,  p o s s i b l y  due t o  t h e  more 
e x t e n s i v e  p r o c e s s i n g  necessary  t o  produce a V-groove s o l a r  c e l l .  
l a r g e  a rea  g r i d  f i n g e r  c o n t a c t s  on o u r  V-grooved s t r u c t u r e s  have o f t e n  tended 
t o  c r e a t e  a shunt  p a t h  from t h e  e m i t t e r  t o  t h e  base, e f f e c t i n g  t h e  open c i r c u i t  
v o l t a g e  o f  o u r  c e l l s .  Consequent ly ,  o u r  b e s t  V has o n l y  approached 800 mV. 
I n  c o n t r a s t ,  most o f  t h e  smal l  a rea  t e s t  d iodes  (40 by  40 pm) l o c a t e d  around 
each o f  o u r  s o l a r  c e l l s  do n o t  e x h i b i t  s h u n t i n g .  T h i s  i s  an i n d i c a t i o n  t h a t  
w i t h  o p t i m i z e d  p rocess ing ,  we w i l l  be a b l e  t o  a v o i d  t h i s  s h u n t i n g  prob lem i n  
o u r  l a r g e r  a rea  f r o n t  c o n t r a c t s .  
As a r e s u l t ,  
Examinat ion  o f  t h e  d a r k  c u r r e n t - v o l t a g e  c h a r a c t e r i s t i c s  o f  t h e  t e s t  
d iodes  has r e v e a l e d  t h a t  a r e v e r s e  s a t u r a t i o n  c u r r e n t  d e n s i t y  (Jo> o f  about  
l x l O - l 8  A/cm2 ( t o t a l  a rea )  i s  r e a d i l y  a c h i e v a b l e  w i t h  these  s o l a r  c e l l s ,  assum- 
i n g  a d i o d e  i d e a l i t y  f a c t o r  of one. We have a l r e a d y  been a b l e  t o  a t t a i n  a 
s h o r t  c i r c u i t  c u r r e n t  d e n s i t y  o f  27.5 mA/cm2, w i t h  Ta205 as t h e  AR c o a t i n g ,  
w i t h  o u r  V-grooved s o l a r  c e l l s .  The t h i c k n e s s  o f  t h e  AR c o a t i n g  has n o t  been 
o p t i m i z e d .  T h i s  r e p r e s e n t s  a 13 p e r c e n t  i n c r e a s e  o v e r  t h e  comparable p l a n a r  
c e l l .  T h i s  w i t h  t h e  Jo v a l u e  c i t e d  above t r a n s l a t e s  t o  a Voc o f  about  
980 mV. The c u r r e n t  s o l a r  c e l l  s t r u c t u r e  w i t h  t h e  i n c r e a s e d  V-grooved a r e a  
does n o t  appear t o  e f f e c t  t h e  open c i r c u i t  v o l t a g e  o f  these  c e l l s  s i g n i f i -  
c a n t l y .  
V-groove c e l l s .  
F i g u r e  7 shows t h e  quantum e f f i c i e n c y  compar ison for  t h e  p l a n a r  and 
CONCLUSIONS 
The work p r e s e n t e d  demonstrates t h e  f e a s i  b i  1 i t y  and p o t e n t i a l  f o r  
V-grooved GaAs s o l a r  c e l l s .  The V-grooved geometry p e r m i t s  u t i l i z a t i o n  o f  
advantageous o p t i c a l  c h a r a c t e r i s t i c s  w h i l e  m a i n t a i n i n g  t h e  b e s t  e l e c t r o n i c  and 
m a t e r i a l s  p r o p e r t i e s  o f  MOCVD grown GaAs s o l a r  c e l l s .  
ACKNOWLEDGMENTS 
The a u t h o r s  would l i k e  t o  express  t h e i r  g r a t i t u d e  t o  D r .  R. Leonard and 
t h e  s t a f f  o f  t h e  m i c r o l i t h o g r a p h y  f a c i l i t y  f o r  t h e i r  l i t h o g r a p h y  a s s i s t a n c e ,  
to  R .  H a r t  and D r .  D .  B r i n k e r  for  t h e  e l e c t r i c a l  measurements, and t o  V .  Weiz- 
e r  for s u g g e s t i o n s  c o n c e r n i n g  t h e  a n a l y s i s  o f  c e l l  b e h a v i o r .  
REFERENCES 
1 .  M . A .  Green, S.R.  Wenham, and A.W. B l a k e r s ,  "Recent Advances i n  H igh  
E f f i c i e n c y  S i l i c o n  S o l a r  C e l l s , "  i n  N i n e t e e n t h  I E E E  P h o t o v o l t a i c  
S p e c i a l i s t s  Conference,  I E E E ,  1987, pp. 6-12. 
3 
2. 
3. 
S t r u c t u r e  Transm., 
p e r c e n t  
Specu lar  44.1 
Cross-grooved 3.8 
Lambert ian 9.8 
4. 
R e f l e c t . ,  Absorp., 
p e r c e n t  p e r c e n t  
45.1 10.8 
24.6 73.5 
36.5 53.7 
5. 
6. 
7. 
8. 
G.A. Landis, "Design and Manufacture of Cross-Grooved Solar Cell," U.S.  
Patent No. 4,608,451, Aug. 1986. 
S.G. Bailey, G.A. Landis, and D.M. Wilt, "Anisotropic Etching and OM-CVD 
Growth on GaAs," presented at the State of  the Art Program on Compound 
Semiconductors-IX, Electrochemical Society Fall Meeting, Chicago, IL, 
Oct. 1988. 
S.G. Bailey, R.P. Leon, and A. Arrison, "A V-grooved AlGaAs/GaAs 
Passivated- pn Junction ,I' Nineteenth IEEE Photovol tai c Speci a1 i sts 
Conference, IEEE, 1987, pp. 421-426. 
R.P. Leon, S.G. Bailey, G.A. Mazeris, and W.D. Williams, "Formation of 
pn Junction on an Anisotropically Etched GaAs Surface Using Metalorganic 
Chemical Vapor Deposition," Appl. Phys. Lett., vol. 49, pp. 945-947, 1986 
S.G. Bailey, G.A. Landis, and D.M. Wilt, "A V-grooved GaAs Solar Cell," 
in: Space Photovoltaic Research and Technology Conference, 1988, 
pp. 625-628. 
S. Adachi and K .  Oe, "Chemical Etching Characteristics of (001) GaAs," 
J. Electrochem. SOC., vol. 130, pp. 2427-2435, 1983. 
P. Campbell and M.A. Green, "Light Trapping Properties of Pyramidally 
Textured Surfaces. . .of Solar Cell Materials," J .  A p p l .  P h y s . ,  vo l .  62,  
pp.  243-249, 1987. 
4 
. 
ORlGINAL PAGE IS 
OF POOR QUALITY 
FIGURE 1. - SCHEMATIC OF V-GROOVE GEOMETRY GALLIUM ARSENIDE SOLAR 
CELL (NOT TO SCALE). 
FIGURE 2. - SEM MICROGRAPH OF PHOTO-RESIST PATTERN 
ON GaAs BASE EPILAYER. 
5 
FIGURE 3. - MICROGRAPH AFTER 150s ETCH I N  CAROS F IGURE 4. - MICROGRAPH OF V-GROOVED GaAs  SURFACE 
ETCHANT A T  24 'C. BEFORE EMITTER GROWTH. 
50 
40 
c 
PLANAR 
I 
400 500 600 700 800 900 
WAVELENGTH. NM 
FIGURE 5. - R E F L E C T I V I T Y  VERSUS WAVELENGTH FOR PLANAR AND 
V-GROOVED GaAs  (AFTER AR COATING) .  
ORIGINAL PAGE IS 
OF POOR QUALITY 
ORtGiMAL PAG€ IS 
OF POOR QUALITY 
FIGURE 6. - MICROGRAPH OF CLEAVED CROSS-SECTION 
OF EPITAXIAL p-n JUNCTION ON V-GROOVED GaAs 
SURFACE. 
V-GROOVE CELL w/AR 
PLANAR CELL w/AR 
V-GROOVE CELL 
PLANAR CELL 
0 
.4 .6  .8  1 .o 
WAVELENGTH, PM 
FIGURE 7. - QUANTUM EFFICIENCY AS A FUNCTION OF WAVELENGTH 
FOR THE V-GROOVE AND PLANAR CELL WITH AND WITHOUT AN AR 
COAT I NG . 
7 
N/\s/\ 
National Aeronautics and 
1. Report No. 
NASA TM-101970 
Report Documentation Page 
2. Government Accession No. 
17. Key Words (Suggested by Author@)) 
4. Title and Subtitle 
A V-Grooved GaAs Solar Cell 
18. Distribution Statement 
7. Author(+ 
S.G. Bailey, N.S. Fatemi, G.R. Landis, D.M. Wilt, R.D. Thomas, 
and A. Arrison 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 
Unclassified Unclassified 8 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135-3191 
22. Price' 
A02 
12. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, D.C. 20546-0001 
3. Recipient's Catalog No. 
5. Report Date 
6. Performing Organization Code 
8. Performing Organization Report No. 
E-4668 
10. Work Unit No. 
506-4 1 - 1 1 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
15. Supplementary Notes 
Presented at the 20th Photovoltaic Specialists Conference sponsored by The Institute of Electrical and Electronics 
Engineers, Las Vegas, Nevada, September 26-30, 1988. S.G. Bailey, D.M. Wilt, R.D. Thomas, and A. Arrison, 
NASA Lewis Research Center. N.S. Fatemi, Sverdrup Technology, Inc., NASA Lewis Research Center Group, 
Cleveland, Ohio 44145. G.R. Landis, National Research Council-NASA Research Associate. 
6. Abstract 
V-grooved GaAs solar cells promise the benefits of improved optical coupling, higher short-circuit current, and 
increased tolerance to particle radiation compared to planar cells. A GaAs homojunction cell was fabricated by 
etching a V-groove pattern into an n epilayer (2.1 x 1017cm-3) grown by MOCVD on an n+ substrate 
(2.8 x lO18cm-3) and then depositing an MOCVD p epilayer (4.2 x lO18cm-3). Reflectivity measurements on 
cells with and without an antireflective coating confirm the expected decrease in reluctance of the microgrooved 
cell compared to the planar structure. The short circuit current of the V-grooved solar cell was 13 percent higher 
than that of the planar control. 
V-groove 
GaAs 
Solar cell 
Unclassified - Unlimited 
Subject Category 44 
*For sale by the National Technical Information Service, Springfield, Virginia 221 61 NASA FORM 1626 OCT 86 


A V-grooved GaAs solar cell

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