Geometrically structured surfaces have become increasingly important to solar cell efficiency improvements and radiation tolerance. Gallium arsenide solar cells with a V-grooved front surface which demonstrate improved optical coupling and higher short-circuit current compared to planar cells were fabricated. GaAs homojunction cells were fabricated by organometallic chemical vapor deposition (OMCVD) on an n+ substrate. The V-grooves were formed on the surface with an anisotropic etch, and an n-type buffer and p-type emitter were grown by OMCVD, followed by ohmic contacts. Reflectivity measurements show significantly lower reflectance for the microgrooved cell compared to the planar structure. The short circuit current of the V-grooved solar cell is consistently higher than that of the planar controls

Bailey, Sheila G.

Fatemi, N.

Landis, Geoffrey A.

Thomas, R. D.

Wilt, D. M.

English

NASA Technical Reports Server

NASA Technical Memorandum 102 104 
GaAs Solar Cells With 
V-Grooved Emitters 
[NASA-TH-102104) THE GaAs SOLA5 CELLS WXTH N89-25291 
V-GROOVED EH1TTr;HS [ N A S A .  L e w i s  B e s E a r c h  
Center) 6 p CSCL l o a  
Unclas 
G3/44 0217629 
S.G. Bailey 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 
N. Fatemi 
Sverdrup Technology, Inc. 
NASA Lewis Research Center Group 
Cleveland, Ohio 
D.M. Wilt, G.A. Landis, and R.D. Thomas 
National Aerokutics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
European Space Power Conference 
sponsored by the European Space Agency 
Madrid, Spain, October 2-6, 1989 
I 
https://ntrs.nasa.gov/search.jsp?R=19890016920 2020-03-20T01:28:53+00:00Z
S . G .  Ba i l ey  
Nat iona l  Aeronaut ics and Space Admin i s t ra t i on  
Lewis Research Center 
Cleveland, Ohio 44135 
N. Fatemi 
Sverdrup Technology, I n c .  
NASA Lewis Research Center Group 
Cleveland, Ohio 44135 
and 
D.M. W i l t ,  G . A .  Landis,* and R.D. Thomas 
Nat iona l  Aeronaut ics and Space Admin i s t ra t i on  
Lewis Research Center 
C1 eve1 and, Ohio 441 35 
ABSTRACT 
Geometrically structured surfaces have become 
increasingly important t o  solar  cel l  efficiency 
improvements and radiation tolerance. 
fabricated GaAs solar  ce l l s  with a V-grooved front  
surface which demonstrate improved optical 
coup1 ing and higher short-circui t  current compared 
t o  planar ce l l s .  GaAs homojunction c e l l s  were 
fabricated by organometallic chemical vapor 
deposition (OMCVD) on an n+ substrate.  
were formed on the surface with an anisotropic 
etch,  and an  n-type buffer and p-type emitter were 
grown by OMCVD, followed by ohmic contacts. 
Reflectivity measurements show signif icant ly  lower 
reflectance f o r  the microgrooved cel l  compared to  
the planar structure.  
of the V-grooved solar  cel l  i s  consistently higher 
than tha t  of the planar controls. 
We have 
V-grooves 
The short  c i r cu i t  current 
INTRODUCTION 
Significant advances in the efficiency of s i l icon 
solar c e l l s  have occurred due t o  the use of novel 
device geometries (Ref. 1 ,2 ) .  Similar benefits  in 
GaAs promise reduced reflectance, oblique passage 
of l i gh t  through the c e l l ,  and l igh t  trapping. An 
optimized microgrooved cel l  will have the 
advantages of increased total  absorptivity,  higher 
short-circui t  current, and increased radiation 
tolerance. 
Anlsotropic wet chemical etching has been shown t o  
provide a simple, inexpensive method t o  fabr icate  
structures with less  than two micron spacings 
(Ref. 3). Due to  the polar nature of the l a t t i c e ,  
anisotropic etching i n  GaAs i s  more complicated 
than in s i l icon.  By careful orientation of the 
photolithographic mask, we obtained the desired 
s t ructure  shown i n  Figure 1 (Ref. 4) .  
we have reported a microgrooved GaAs pn junction 
and AlGaAs passivated pn junction (Ref. 5, 6 ) .  
h 
Previously 
*Nat ional  Research Counci l  - NASA Research 
Associate.  
FRONT CONTACT A A A A 
METALLIZATION / \ / \ \ 1 \ 
t 
A 
- 3.5 
Figure 1. Schematic of V-grooved gallium arsenide 
solar  cel l  (not t o  scale) .  
V-GROOVE FABRICATION 
Fabrication of the solar  cel l  began with (100) 
n-type substrates with a ca r r i e r  concentration of 
2.8 X lO18cm-3. 
received from the supplier into the MOCVD reactor 
and subjected t o  a h i g h  temperature bake under a 
H2 and AsH3 ambient. 
epitaxial  n+ buffer layer of .3pm thickness w i t h  
doping of 1.5 X lO18cm-3 was grown followed by a 
4pm epitaxial  n base layer with doping of 2.1 X 
10 17cm-3. 
The substrate was loaded as 
Following the bake an 
The wafers were then removed from the  MOCVO 
reac to r  and a standard pho to l i t hog raph ic  process 
was used t o  form a pho to res i s t  p a t t e r n  o f  p a c a l l e l  
4pm l i n e s  and 3pm spaces, a l i gned  i n  the  (001)  
d i r e c t i o n .  F igure  2 shows a scanning e l e c t r o n  
microscope (SEM) micrograph o f  the sur face a f t e r  
the  pho to res i s t  step. 
Figure 2. SEM micrograph of  photoresist pattern 
on GaAs base epilayer 
V-grooves were etched in the GaAs using the 
H~S04:H202:H20 Caros etchant (Ref. 7 )  i n  the r a t i o  
of 5: l : l .  
are discussed i n  R e f .  3. Figure 3 shows the 
grooves formed by etching fo r  150 seconds in the 
Caros etchant a t  2 4 O C .  The etchant undercut the 
photoresist l ines  t o  produce a very regular 
sawtooth surface w i t h  sharp peaks and a period of 
7pm. 
Further de ta i l s  on the etching process 
Figure 3. Micrograph of  GaAs and photoresist 
a f t e r  150s in Caros etchant a t  24OC 
The micrograph in Figure 4 shows the resul tant  
V-grooved s t ructure  a f t e r  removal o f  the 
photoresist. 
Figure 4. Micrograph of V-grooved GaAs surface 
before emitter growth 
REFLECTANCE A N D  LIGHT TRAPPING 
Reflectance of the V-grooved substrate  a s  a 
functlon of wavelength both before and a f t e r  an 
an t i re f lec t ive  ( A R )  coating of Ta2O5 was measured 
on a Perkin-Elmer Lambda 9 uv/vis/nir 
spectrophotometer. A comparison of the 
reflectance of the V-grooved surface w i t h  a planar 
sample shows the expected reduction i n  
re f lec t iv i ty  over the spectral range o f  in te res t .  
The re f lec t iv i ty  a s  a function of wavelength from 
400-900nm i s  shown in Figure 5. 
50 p 
" '\\ PLANAR 
I 
I 
w V-GROOVED '\ I I 
f '\ '. 
I 
400 500 600 700 800 900 
WAVELENGTH, NM 
Figure 5. Reflectivity versus wavelength for 
AR coated Dlanar and V-grooved GaAs 
ORIGINAL PAGE JS 
OF POOR QUALITY 
2 
Future improvements i n  GaAs s o l a r  c e l l s  may w e l l  
i nco rpo ra te  the  t h i n ,  1 i g h t - t r a p p i n g  s t r u c t u r e s  
t h a t  have r e c e n t l y  been demonstrated on s i l i c o n .  
The a b i l i t y  t o  make double h e t e r o s t r u c t u r e  c a r r i e r  
confinement i n  GaAs makes u l t r a t h i n ,  
l i g h t - t r a p p i n g  s t r u c t u r e s  e s p e c i a l l y  a t t r a c t i v e .  
I f  the  o p t i c a l  pa th leng th  i s  increased, the  a c t i v e  
th ickness o f  t he  c e l l  can be correspondingly  
decreased, r e s u l t i n g  i n  an increase i n  the open 
c i r c u i t  vo l tage  and g r e a t l y  improved r a d i a t i o n  
to lerance.  For a c e l l  w i t h  an o p t i c a l  pa th leng th  
enhancement o f  a f a c t o r  o f  50, near the  
t h e o r e t i c a l  i s o t r o p i c  l i m i t ,  an open c i r c u i t  
vo l tage improvement o f  kT l n (50 ) ,  o r  100mV, cou ld  
be obta ined a t  optimum th ickness.  
wafer t h i ckness  could be very t h i n ,  p o s s i b l y  l e s s  
than a micron. While such t h i n  subs t ra tes  a re  n o t  
feasable us ing  c u r r e n t  technology, new 
technologies such as pee led - f i lm  GaAs (Ref. 8), o r  
t h i n  CLEFT m a t e r i a l  (Ref. 9), may make such 
devices poss ib le .  
One o f  t h e  most e f f e c t i v e  l i g h t - t r a p p i n g  
geometries i s  t he  cross-grooved s t r u c t u r e  (Ref. 
l o ) ,  where pe rpend icu la r  grooves a r e  formed on 
bo th  t h e  f r o n t  and back surface. Theore t i ca l  
a n a l y s i s  show 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  the o p t i c a l  pa th leng th  i n  
the  m a t e r i a l  (Ref. 2) .  We f a b r i c a t e d  the  
cross-grooved s t r u c t u r e  on a GaAs water  t o  t e s t  
t h e  e f f e c t i v e n e s s  o f  l i g h t  t rapp ing .  The l i g h t  
t r a p p i n g  f o r  weakly absorbed li h t  was measured 
and compared w i t h  a Lambertian Qrandom) 
l i g h t - t r a p p i n g  geometry and w i t h  a p lana r  
subst rate.  None o f  t he  s t r u c t u r e s  measured had AR 
coa t ings  o r  back-surface r e f l e c t o r s  (BSR). 
1 shows t h e  measured r e s u l t s ,  The r e s u l t s  
conf i rmed t h e  e f f e c t i v e n e s s  o f  l i g h t  t rapp ing  i n  
t h e  cross-grooved s t ruc tu re ,  w i t h  abso rp t i on  
enhanced by 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 .  
The requ i red  
Table 
S t r u c t u r e  Transm., 
p e r c e n t  
Specular 44.1 
Cross-grooved 3.8 
Lambert i an 9.8 
Elect rochemical  capaci tance vo l tage  (ECV) 
measurements us ing  a Polaron PN4200 p r o f i l e  
p l o t t e r  were performed on an adjacent  p lana r  
s e c t i o n  a f t e r  complet ion o f  the V-grooved c e l l .  
The c a r r i e r  concen t ra t i on  p r o f i l e  i s  shown i n  
F igure 6. 
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 
’’ F , r p  + EMITTER 
16 7 Z  0 1 2 DEPTH, 3 pM 4 5 6 
F igu re  6. Measured c a r r i e r  concen t ra t i on  ECQ 
p r o f  i 1 e 
F igu re  7 i l l u s t r a t e s  the e p i l a y e r  growth on the  
V-grooves. 
from the  bottom t o  the top  o f  t he  goove. 
The e p i l a y e r  th i ckness  v a r i e s  s l i g h t l y  
CELL FABRICATION 
The a c t i v e  c e l l  l a y e r s  were 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  wa l l ,  sub-atmospheric pressure 
MOCVD reac to r .  Gowth temperature was 620%; the  
V/III r a t i o  was 46; the n dopant was a 500 ppm 
m i x t u r e  o f  H2S i n  UHP Hp; and t h e  chamber pressure 
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 r s t  
deposi ted and then 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  concen t ra t i on  o f  2.1 X 1017cm-3. 
The e m i t t e r  growth was again preceded by a h igh  
temperature bake under a H2 and AsH3 ambient. 
V-grooves were o r i e n t e d  p a r a l l e l  t o  the  gas f l o w  
and DEZn was t h e  p dopent. 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  the  base deposi t ion.  
A O.lpm 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  
concen t ra t i on  o f  4.2 X lO18cm-3 was grown on the 
The 
V-grooves. ORIGINAL PAGE tS 
Figure 7. Micrograph o f  c leaved cross-sect ion o f  
e p i t a x i a l  pn j u n c t i o n  on V-grooved GaAs 
sur face 
E-beam evaporated Au-Zn con tac ts  were a p p l i e d  a t  a 
s l i g h t  angle t o  the  grooves i n  the  p e m i t t e r .  
Au-Ge-Ni con tac ts  were u t i l i z e d  on the n base. 
The as deposi ted contacts  were ohmic and no 
subsequent s i n t e r i n g  was requi red.  
OF POOR QUALITY 3 
ANALYSIS CONCLUSIONS 
The V-grooved solar ce l l s  were fabricated 
simultaneously with control planar solar  c e l l s  in 
our MOCVD reactor. 
in the growth ra te  on the (111) and the (100) 
planes, however, these cel l  s t ructures  a re  not 
identical .  In addition, the V-grooved surfaces 
are  more susceptible t o  imperfections due t o  the 
more extensive and more complicated processing 
steps required in the i r  fabr icat ion,  as compared 
to  the planar surfaces. 
speculate tha t  the microlithographic imperfections 
on the surface of the ce l l s  upon etching can 
resul t  in strange geometric surface shapes. 
in t u r n  may lead t o  non-uniform MOCVD growth of 
the cel l  structure.  As a resu l t ,  the large area 
front  g r i d  contacts of the V-grooved c e l l s  have 
often created s h u n t  paths from the emitter t o  the 
base, l imiting the best observed Voc t o  about 
800mV. 
t e s t  diodes located on the grooved emitter j u s t  
outside the active area of the V-grooved c e l l s  
often show no sign of shunting, which indicates 
that  the problem with shunting caused by the front  
grid contacts i s  not inherent with the V-grooved 
s t ructure ,  and t h a t  optimized processing should 
a l lev ia te  i t .  
Due t o  the s l i gh t  difference 
Part icular ly ,  we 
This 
On the other hand, small area (44 by 44pm) 
We have observed a short  c i r cu i t  current density 
of 27.5 mA/cm2 with non-opimized A R  coating for  
our V-grooved ce l l s .  This represents a 13 percent 
increase over the p l a n a r  control ce l l s ,  which 
demonstrates the superior optical  coupling of the 
V-grooved structure.  The measured q u a n t u m  
eff ic iency comparison fo r  planar and V-grooved 
so lar  c e l l s  i s  shown i n  Figure 8. In order t o  be 
able t o  study the e f fec t  of increased surface area 
(by a fac tor  of a) on the open c i r cu i t  voltage of 
the V-grooved s t ructure ,  we have estimated the 
reverse saturation current density (J,) fo r  these 
c e l l s ,  u s i n g  the dark current-voltage 
character is t ics  of the t e s t  diodes which did n o t  
exhibi t  shunting. Our calculations show tha t  Jo 
values as  low as  8E-19 A/cm2 ( to t a l  area) are 
readily available w i t h  V-grooved ce l l s .  The Jo 
t rans la tes  t o  a Voc of about 980 mV fo r  the 
V-grooved s t ructure ,  which i s  comparable to  the 
Voc of our control planar ce l l s .  
l . O y  
.8 
.6 
.4 
PLANAR CELL w/AR 
---4--- V-GROOVE CELL 
\ . 2 f -  -b-- PLANAR CELL 
O U   . 6  .8 1 .o 
The resu l t s  presented demonstrate the feasi  bi 1 i t y  
and potential f o r  V-grooved GaAs solar  ce l l s .  The 
V-grooved geometry permits u t i l i za t ion  of 
advantageous optical character1 s t l c s  while 
maintaining the best electronic and materials 
properties of MOCVD grown GaAs so lar  ce l l s .  
ACKNOWLEDGEMENTS 
The authors would l i ke  t o  express the i r  grati tude 
to  Dr. R. Leonard and the s ta f f  of the 
microlithography f a c i l i t y  for  t he i r  assistance,  t o  
R. Hart and Dr. D. Brinker fo r  the e lec t r ica l  
measurements, and to  V. Weizer fo r  informative 
discussions. 
REFERENCES 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
Green M A e t  a1 1987, Recent advances in h i g h  
efficiency s i l icon solar  ce l l s ,  Nineteenth IEEE 
Photovoltaic Special is ts  Conference, 6-12. 
Campbell P e t  a1 1987, L i g h t  trapping 
properties of pyramidally textured surfaces, 
Bailey S G e t  a1 1988, Anisotropic etching and 
OMCVD growth on GaAs, State  of the Art Program 
on Compound Semiconductors-IX, Electrochemical 
Society Fall Meeting. 
Bailey S G e t  a1 1988, Chemical etching and 
OMCVD on varied geometries of GaAs, Space 
Photovoltaic Research and Technology Conf., 
Bailey S G e t  a1 1987, A V-grooved AlGaAs/GaAs 
passivated pn junction, Nineteenth I E E E  Photo- 
vol ta ic  Special is ts  Conference, 421-426. 
Leon R P e t  a1 1986. Formation of a D n  
J. Appl. PhyS. 62 ( l ) ,  243-249. 
27 2- 287. 
junction on an anisotropically etched GaAs 
surface using MOCVD, Appl. Phys. Lett. 49, 
945-947. 
Adachi S e t  a1 1983, Chemical etching 
character is t ics  of (001) GaAs, J. Electro- 
chemical SOC. 130, 2427-2435. 
Yablonovitch E e t  a1 1987, Extreme se lec t iv i ty  
in the l i f t -o f f  of epi taxial  GaAs fi lms, Appl. 
Phys. Lett. 51 (26), 2222-2224. 
Gale R P e t  a1 1983. Lateral eDitaxial over- 
growth of GaAs and GaAlAs OMCVD, Inst. Phys. 
Conf. Ser. No. 65, 101-108. 
Landis G A 1986, Cross grooved solar  ce l l ,  U.S. 
Patent 4,608,451. 
. .  
WAVELENGTH. PM 
Figure 8. Quantum efficiency as a function of 
wavelength for  the V-groove and planar 
cel l  w i t h  and without an A R  coating 
4 
i 
m National Aeronautics and Report Documentation Page I 
1 .  Report No. 
NASA TM-102104 
2. Government Accession No. 
4. Title and Subtitle 
GaAs Solar Cells With V-Grooved Emitters 
21. No of pages 
6 
9. Security Classif. (of this report) 20. Security Classif. (of this page) 
Unclassified Unclassified 
7. Author(s) 
S.G. Bailey, N. Fatemi, D.M. Wilt, G.A. Landis, and R.D. Thomas 
- 
22. Price' 
A02 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135-3191 
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-4870 
10. Work Unit No. 
506-41-1 1 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
15. Supplementary Notes 
Prepared for the European Space Power Conference sponsored by the European Space Agency, Madrid, Spain, 
October 2-6, 1989. S.G. Bailey, D.M. Wilt, and R.D. Thomas, NASA Lewis Research Center; N. Fatemi, 
Sverdrup Technology, Inc., NASA Lewis Research Center Group, Cleveland, Ohio 44135; G.A. Landis, 
National Research Council-NASA Research Associate. 
_- 
16. Abstract 
Geometrically structured surfaces have become increasingly important to solar cell efficiency improvements and 
radiation tolerance. We have fabricated GaAs solar cells with a V-grooved front surface which demonstrate 
improved optical coupling and higher short-circuit current compared to planar cells. GaAs homojunction cells 
were fabricated by organometallic chemical vapor deposition (OMCVD) on an n+ substrate. V-grooves were 
formed on the surface with an anisotropic etch, and an n-type buffer and p-type emitter were grown by OMCVD, 
followed by ohmic contacts. Reflectivity measurements show significantly lower reflectance for the microgrooved 
cell compared to the planar structure. The short circuit current of the V-grooved solar cell is consistently higher 
than that of the planar controls. 
*For sale by the National 1-echnical Informalion Service, Springfield, Virginia 22161 NASA FORM 1626 OCT 86 


The GaAs solar cells with V-grooved emitters

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