A passivated, V-grooved GaAs solar cell offers important advantages in terms of improved optical coupling, higher short circuit current, and increased tolerance to particle radiation when compared to the planar cell configuration. An AlGaAs epilayer has been deposited on a p-type GaAs epilayer grown on an n-type V-grooved GaAs surface using MOCVD. A wet chemical etching process was used to produce a V-pattern with a 7.0 micron periodicity. Reflectivity measurements substantiate the expected decrease in solar reflectance. Scanning electron microscopy techniques were used to confirm the presence of the AlGaAs layer and verify the existence of a pn junction

Arrison, Anne

Bailey, Sheila G.

Leon, Rosa P.

English

NASA Technical Reports Server

NASA Technical Memorandum 100138 
A V-Grooved AlGaAslGaAs 
r Passivated PN Junction 
? 
8 
P 
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Sheila G. Bailey, Rosa P. Leon, and Anne Arrison 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
19th Photovoltaic Specialists Conference 
sponsored by the American Institute of Electr,;al and Electronics Engineers 
New Orleans, Louisiana, May 4-8, 1987 
8 ' L  
IBASA-Ta-100138) B V-66iCGVEC AlGaAs/GaAs N87-2754 1 
PASSIVATED PN J U E C T X G Z  [NASA) 12 F A v a i l :  
IIIS BC AOZ/BIP B O 1  CSCL 20L 
Unclas 
63/76 0092894 
https://ntrs.nasa.gov/search.jsp?R=19870018108 2020-03-20T09:34:16+00:00Z
A V-GROOVED AlGaAs/GaAs PASSIVATED PN JUNCTION 
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Sheila G. Bailey, Rosa P. Leon, and Anne Arrison 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
SUMMARY 
A passivated, V-grooved GaAs solar cell offers important advantages in 
terms of improved optical coupling, higher short-circuit current, and increased 
tolerance to particle radiation when compared to the planar cell configuration. 
An AlGaAs epilayer has been deposited on a p-type GaAs epilayer grown on an 
n-type V-grooved GaAs surface using MOCVD. A wet chemical etching process was 
used to produce a V-pattern with a 7.0 pm periodicity. 
ments substantiate the expected decrease In solar reflectance. Scanning elec- 
tron microscopy techniques were used to confirm the presence o f  the AlGaAs 
layer and verify the existence o f  a pn junction. 
Reflectivity measure- 
INTROOUCl ION 
Recent advances in silicon solar cell performance (ref. 1 )  indicate the 
importance of improved optical design coupled with enhanced electronic charac- 
teristics in increasing solar cell efficiencies. A passivated V-grooved GaAs 
solar cell offers important advantages in terms of improved optical coupling, 
higher short-circuit current, and increased tolerance to particle radiation 
when compared to the planar cell configuration. The microgrooved structure has 
recently been achieved for a GaAs pn junction (ref. 2). The next step in fab- 
ricating an efficient GaAs V-grooved solar cell is to passivate the front sur- 
face of the structure with an AlGaAs layer. Planar AlGaAs/GaAs heteroface 
solar cells have the highest efficiency o f  any GaAs solar cells reported to 
date (ref. 3). In this paper results are presented confirming the metalorganic 
chemical vapor deposition (MOCVD) epitaxial growth of a passivating AlGaAs 
layer on a microgrooved GaAs pn junction. 
The microgrooved structure has several advantages when compared to the 
planar cell structure. The improved optical coupling reduces reflective losses 
by 70 percent (ref. 4). In addition to this, the sawtooth geometry permits 
absorption o f  light closer to the location of the pn junction which results in 
a larger effective absorption coefficient, Q (ref. 5). An increase in the pro- 
duct aL, where L is the minority-carrier diffusion length, Increases the 
collection efficiency o f  generated carriers and thus the short-circuit current 
of the cell. The high effective a also helps to mitigate the effects of 
decreased diffusion length on the short-circuit current when the cell is 
exposed to particle radiation. It is therefore expected that this configura- 
tion will show improved radiation resistance. 
grooved structure is then an increase in the total absorptivity, radiation 
tolerance, and short-circuit current of the device. By orienting the grid 
fingers perpendicular to the grooves a small emitter series resistance can be 
ma 1 nta i ned . 
The net effect of the micro- 
F A B R I C A T I O N  
The s t ruc tu re  o f  the  microgrooved heteroface dev ice i s  shown i n  f i g u r e  1. 
Fabr i ca t i on  o f  the device began w i t h  a s i l icon-doped GaAs subs t ra te  w i t h  a 
c a r r i e r  concentrat ion o f  2xl018/cm3. A standard pho to l i t hog raph ic  process was 
used t o  t r a n s f e r  a pho to res i s t  p a t t e r n  c o n s i s t i n g  o f  p a r a l l e l  4 - p m  l i n e s  and 
3-prn spaces from a chromium and quar tz  mask t o  the  GaAs n-type subst rate.  
F igure  2 i s  a photograph o f  a sect ion,  i n c l u d i n g  a p o r t i o n  of the contac t  f i n -  
ger,  of t h e  photores is t  p a t t e r n  on the  subs t ra te  p r i o r  t o  e tch ing.  
Several wet chemical etchants were i nves t i ga ted  under var ious e tch ing  con- 
d i t i o n s .  
tem I n  the  r a t i o  o f  5 : l : l .  F igure 3 shows the grooves formed by e tch ing  f o r  
2 min i n  the  above s o l u t i o n  a t  28 O C  w i t h  manual a g i t a t i o n .  
The best grooves were obtained us ing  the  H2S04:H202:H20 etchant  sys- 
The scanning e l e c t r o n  microscope (SEM) micrograph ind i ca tes  t h a t  the  
F igure 3 i l l u s t r a t e s  the  r e s u l t s  o f  an i so t rop i c  
The 
etchant  undercut t he  pho to res i s t  l i n e s  t o  produce a very regu la r  sawtooth sur-  
face w i th  a per iod o f  7 pm. 
e t ch ing  consis tent  w i th  those obtained by o ther  i n v e s t i g a t o r s  ( r e f .  6 ) .  
SEM micrograph i n  f i g u r e  4 shows incomplete e tch ing  r e s u l t i n g  i n  a f l a t  (001) 
o r i en ted  top. 
A y e p i l a y e r  o f  0.22 pm thickness w i t h  a c a r r i e r  concent ra t ion  o f  1017 t o  
10l8/cm was grown us ing a standard S P I - M O C V D  450 reac tor .  
n ide  f i l m  was produced b the  decomposition o f  t r i m e t h y l g a l l i u m  i n  the  presence 
o f  an a rs ine  atmosphere a t  700 O C .  Zinc, the p- type dopant, was in t roduced as 
d imethy l  z inc .  I n  the same run, a p - ty  e A1.85Ga 15As l a y e r  o f  0.17 pm t h i c k -  
ness w i t h  a c a r r i e r  concentrat ion o f  1078 t o  1019)cm-3 was grown on the  GaAs 
p-e I l a y e r ,  fo l lowed by a 570 A GaAs cap l a y e r  w i t h  a c a r r i e r  concent ra t ion  o f  
nlm. This i s  t h e  f i r s t  repor ted simultaneous growth o f  AlGaAs on the (111) Ga, 
(111) Ga, and (100) 2' towards the  (110) GaAs c r y s t a l  planes. The micrographs 
i n  f i g u r e s  5 and 6 show the  d i f f e r i n g  con t ras t  o f  the p and n regions a f t e r  
growth of the  GaAs and AlGaAs ep i layers .  The micrograph i n  f i g u r e  7 shows a 
grea ter  growth r a t e  on the f l a t  surfaces, (001) o r i e n t a t i o n ,  produced by incom- 
p l e t e  e tch ing  o f  a subst rate as 4n f i g u r e  4. 
The g a l l i u m  arse- 
10 l!i / ~ m - ~ .  The aluminum was produced by the  decomposition o f  t r i m e t h y l  a lumi-  
ANALYSIS 
Ref lectance measurements made on a Perkins-Elmer Lambda 9 UV/V IS /N IR  spec- 
trophotometer conf i rm the reduc t ion  i n  r e f l e c t i v i t y  f o r  the  microgrooved s t ruc -  
tu re .  The t o t a l  re f l ec tance  i n  percent  i s  shown i n  f i g u r e  8 as a func t i on  of 
wavelength f rom 200 t o  2500 nm. 
( r e f .  7) i s  0.353 f o r  the  p lanar  GaAs subs t ra te  and 0.155 f o r  t he  AlGaAsIGaAs 
V-groove s t ruc tu re .  The re f l ec tance  o f  the  V-grooved s t r u c t u r e  would be 
decreased even fu r the r  if an a n t i - r e f l e c t i v e  coa t ing  were added. 
The in teg ra ted  AM0 t o t a l  so la r  re f l ec tance  
I n  f i g u r e  9 an SEM micrograph shows a c r o s s ' s e c t i o n  o f  the device;  the 
plane o f  the  p i c t u r e  i s  the  (011) cleavage p lane which i s  perpendicu lar  t o  the 
plane where c r y s t a l  growth occurred. Measurements f rom t h i s  type o f  micrograph 
g i v e  a j u n c t i o n  depth of 0.45 pm. The e l e c t r o n  beam induced cu r ren t  (EBIC) 
i n t e n s i t y  f o r  the p o s i t i o n  noted by the  h o r i z o n t a l  l i n e  i n  f i g u r e  10 confirms 
the  presence o f  a pn j unc t i on .  The micrograph i n  f i g u r e  11 conta ins a l i n e  
scan o f  both secondary e lec t ron  i n t e n s i t y  and EBIC  i n t e n s i t y  superimposed on 
2 
the V-groove structure. 
mit analysis of the AlGaAs/GaAs interface by use of the superposition tech- 
nique. 
Interface. The micrograph in figure 13 indicates passivation of the emitter 
reflecting no change in the EBIC magnitude at the interface. 
Higher magnification micrographs of the V-groove per- 
Figure 12 shows a change in the EBIC magnitude at the AlGaAs/GaAs 
CONCLUSION 
We have demonstrated the growth of an AlGaAs epilayer on a p-type GaAs 
epilayer which was grown on an n-type GaAs microgrooved substrate. Epitaxial 
growth on microgrooved surfaces could have an important impact on photovoltaic 
technology by allowing for more innovative device designs. This advance per- 
mits fabrication of solar cells that utilize advantageous optical geometries 
while maintaining the best electronic and materials properties of MOCVD grown 
GaAs cells. 
ACKNOWLEDGMENTS 
The authors would like to acknowledge Spire Corp., Bedford, Massachusetts 
for their expertise in growing the MOCVD epilayers. 
express their gratitude to Dr. W.D. Williams for the use of the microlitho- 
graphy facilities, to Dr. D.A. Gulino for his participation in the reflectance 
measurements, and to N. Fatemi for depositing contacts for EBIC measurements. 
They would also like to 
REFERENCES 
1. A.W. Blakers and M.A. Green, "20% Efficiency Silicon Solar Ce 
Phys. Lett., vol. 48, pp. 215-217, 1986. 
2. R.P. Leon, S.G. Bailey, 6.A. Mazaris, and W.D. Williams, "ForL.ation of a 
pn Junctlon on an Anisotropically Etched GaAs Surface Using Metalorganic 
Chemical Vapor Deposition,' Appl. Phys. Lett., vol. 49, pp. 945-947, 1986. 
3. J.G. Werthen, G.F. Virshup, C.W. Ford, C.R. Lewis, and H.C. Hamaker, "21A 
(One Sun, Air Mass Zero) 4 cm2 GaAs Space Solar Cells," A w l .  Phys. 
-.f Lett vol. 48, pp. 74-75, 1986. 
4. P.M. Holm and R.J. Roedel, "The Design and Fabrication of Anisotropically- 
Etched GaAs Solar Cells,' Seventeenth IEEE Photovoltaic Specialists 
Conference Record, New York: IEEE, 1984, pp. 921-926. 
5. A.L. Fahrenbruch and R.H. Bube, Fundamentals of Solar Cells, New York: 
Academi c Press, 1983. 
6. S. Adachi and K. Oe, Vhemical Etching Characteristics of  (001) GaAs," 3. 
Electrochem. SOC., vol. 130, pp. 2427-2435, 1983. 
7. H.S .  Rauschenbach, Solar Cell Array Design Handbook, New York: Van Nostrand 
Reinhold, 1980. 
3 
UNETCHED SPACE FOR FRONT CONTACT 
F BACK I/ CONTACT 
t V 
FIGURE 1. - SCHEMATIC DIAGRAM OF A MICROGROOVED PASSIVATED 
EMITTER AIGaAs/GaAs HETEROFACE pn JUNCTION. 
FIGURE 2. - PHOTORESIST PATTERN ON A GaAs SUBSTRATE 
PRIOR TO THE WET ETCHING PROCESS. 
4 
FIGURE 3. - ETCHED GROOVES ON A GaAs SUBSTRATE. 
___ ~~ 
f IGURE 4. - GROOVES ON A GaAs SUBSTRATE SHOWING INCOM- 
PLETE ETCH I NG . 
5 
FIGURE 3.  - ETCHED GROOVES ON A GaAs SUBSTRATE. 
FIGURE 4. - GROOVES ON A GaAs SUBSTRATE SHOWING INCOM- 
PLETE ETCH I NG 
5 
SAWTOOTH. 
6 
FIGURE 7. - AIGaAs/GaAs EPILAYER GROWTH ON AN INCON- 
PLETELY ETCHED GaAs SUBSTRATE. 
H 
A IGaAs/GaAs V-GROOVE 
0 500 1000 1500 2000 
WAVELENGTH, NM 
FIGURE 8. - TOTAL REFLECTANCE AS A FUNCTION OF WAVE- 
LENGTH FOR A PLANAR GaAs SUBSTRATE AND A V-GROOVE 
A I GaAs/GaAs STRUCTURE. 
7 
FIGURE 9. - HIGH MAGNIFICATION SEM MICROGRAPH SHOWING 
PORTION OF V-GROOVE WITH EPILAYERS. 
FIGURE 10. - ELECTRON BEAM INDUCED CURRENT SUPERIM- 
POSED ON THE V-GROOVE STRUCTURE AT THE POSITION 
NOTED BY THE HORIZONTAL L I N E .  
8 
FIGURE 11. - LINE SCANS OF SECONDARY ELECTRON INTENSITY 
AND EBIC INTENSITY SUPERIMPOSED ON V-GROOVE STRUCTURE. 
FIGURE 12. - SEM MICROGRAPH OF V-GROOVE AND LINE SCAN 
OF EBIC MAGNITUDE SHOWING A CHANGE I N  THE COLLECTED 
CURRENT AT THE A I GaAsIGaAs INTERFACE . 
9 
FIGURE 13. - V-GROOVE MICROGRAPH WITH LINE SCAN OF EBIC 
MAGNITUDE INDICATING PASSIVATION OF THE P-TYPE GaAs 
EM I TTER . 
10 
National Aoronaubcs and 
1. Report No. 
NASA TM-1 001 38 
~ 
Report Documentation Page 
2. Government Accession No. 
U n c l a s s i f i e d  
A V-Grooved AlGaAs/GaAs Passivated PN Junc t ion  
Unclass i f 1 ed 11 A02 
7. Author@) 
She i l a  G. Bai ley,  Rosa P. Leon, and Anne A r r i s o n  
9. Performing Organization Name and Address 
Na t iona l  Aeronaut ics and Space Admin is t ra t ion  
Lewis Research Center 
Cleveland, Ohio 44135 
2. Sponsoring Agency Name and Address 
Na t iona l  Aeronaut ics and Space Admin is t ra t ion  
Washington, D.C. 20546 
3. Recipient's Catalog No. 
5. Report Date 
6. Performing Organization Code 
506-41 -1 1 
0. Performing Organization Report No. 
E-3688 
10. Work Unit No. 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Tec hn i ca 1 Memorandum 
14.'Sponsoring Agency Code 
5. Supplementary Notes 
Prepared f o r  t he  1 9 t h  Photovo l ta ic  Spec ia l i s t s  Conference sponsored by t h e  
American I n s t i t u t e  o f  E l e c t r i c a l  and E lec t ron ics  Engineers, New Orleans, 
Louis iana, May 4-8, 1987. 
6. Abstract 
A pass ivated,  V-grooved GaAs so la r  c e l l  o f f e r s  impor tan t  advantages i n  terms o f  
improved o p t i c a l  coupl ing,  h igher  s h o r t - c i r c u i t  cu r ren t ,  and increased to le rance  
t o  p a r t i c l e  r a d i a t i o n  when compared t o  t h e  p lanar  c e l l  con f i gu ra t i on .  An AlGaAs 
e p i l a y e r  has been deposi ted on a p-type GaAs e p i l a y e r  grown on an n- type 
V-grooved GaAs sur face us ing  MOCVD. A wet chemical e t ch ing  process was used t o  
produce a V--pattern w i t h  a 7.0 pm p e r i o d i c i t y .  
s t a n t l a t e  the  expected decrease i n  solar re f l ec tance .  Scanning e l e c t r o n  mic ro-  
scopy techniques were used t o  conf i rm the  presence o f  t he  AlGaAs l a y e r  and v e r i f y  
t h e  ex i s tence  o f  a pn j u n c t i o n .  
R e f l e c t l v i t y  measurements sub- 
7. Key Words (Suggested by Author(s)) 
Ga l l i um arsenide 
Aluminum g a l l i u m  arsenide 
E tch ing  
HOCVD 
18. Distribution Statement 
U n c l a s s l f l e d  - u n l i m i t e d  
STAR Category 76 
9. Security Classif. (of this report) 120. Security Classif. (of this page) 121. NO of pages 122. Price' 


A V-grooved AlGaAs/GaAs passivated PN junction

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