Solar cells using polycrystalline films of gallium arsenid

Crossley, P. A.

Gill, R. B.

Hui, W. L. C.

Noel, G. T.

English

NASA Technical Reports Server

:. I 
D 
5 
PROGRESS I N  THIN FILII GaAs SOLAR CELLS 
i P.A. Crossley 
R.B. G i l l  
W.L.C. Hui 1 3  
I ~ --. 
N67- 2626.5 
G.T. Noel (ACCESSION NUMBER) (THR 
&CA Astro-Electronics  Divis ion /q 2
David Sarnoff Research Center 
Princeton, N. J. 
(NASA CR O R  TMX OR AD NUMBER) (CATEG 
INTRODUCTION 
G a A s  i s  a t t r a c t i v e  as an a c t i v e  material f o r  t h in - f i lm  s o l a r  cells  because 
of i t s  high o p t i c a l  absorpt ion c o e f f i c i e n t ,  and t h e  good match of t h e  bandgap 
t o  the  s o l a r  r a d i a t i o n  spectrum. 
have been under i n v e s t i g a t i o n  i n  our l a b o r a t o r i e s  f o r  some t i m e .  
t h e  f a b r i c a t i o n  of t h e  c e l l s  have been evolved, and t h e i r  ope ra t ing  cha rac t e r -  
i s t i c s  have been examined. This paper w i l l  review the  progress  which has  been 
made during t h e  las t  eighteen months. 
Cells using p o l y c r y s t a l l i n e  f i lms  of G a A s  
Methods f o r  
P-n junc t ions  formed i n  p o l y c r y s t a l l i n e  f i lms  of G a A s  have exh ib i t ed  low 
shunt r e s i s t ances .  Consequently, a b a r r i e r  contac t  s t r u c t u r e  has  been employed 
f o r  s o l a r  c e l l  f ab r i ca t ion .  Two ce l l  types have been evolved, as shown i n  
Figure 1. 
layer  of platinum, with an a n t i r e f l e c t i o n  layer  of SiO. 
have been made with a reas  up t o  10 cm , but  t h e  major i ty  of t h e  r e s u l t s  repor ted  
he re  were obtained with c e l l s  of 2 cm area. The o ther  c e l l  s t r u c t u r e  has an  
aluminum subs t r a t e ,  and a b a r r i e r  layer  of cuprous se len ide ,  and t h e  ce l l  areas 
have been i n  t h e  range 0.7 t o  1.0 cm . The r e s u l t s  obtained from t h e  development 
of t hese  two types of c e l l s  w i l l  be  discussed separa ' tely.  
The f i r s t  of these  uses  a s u b s t r a t e  of molybdenum, and a b a r r i e r  
Cells of t h i s  type 
2 
2 
2 
CELLS ON MOLYBDENUM 
Subs t ra te  Precoat 
A s  repor ted  earlier, ( l ) a  precoat  of Sn-Ge a l l o y  has  been developed 
which allows a low-resis tance contact  t o  be formed when G a A s  i s  depos i ted  on 
molybdenum. This  precoat  i s  appl ied by sequen t i a l  vacuum evaporat ion of t h e  
a l l o y  components. 
h a s  been inves t iga ted .  
ope ra t ion  of t h i s  precoat .  
4 
The e f f e c t  of t h e  a l l o y  composition on t h e  ce l l  performance 
Recent r e s u l t s  have provided an understanding of t he  
-~ ~ 
JI 
"Research repor ted  i n  t h i s  paper w a s  sponsored by t h e  Aero-Propulsion Laboratory, 
Research and Technology Division, A i r  Force Systems Command, Wright-Patterson A i r  
Force Base, Ohio, under cont rac t  No. AF33(615)-3486,  and by t h e  Nat ional  Aeronautics 
and Space Adminis t ra t ion,  Lewis Research Center,  Cleveland, Ohio, under Contract 
N o s .  NAS 3-6466 and NAS 3-8510. 
https://ntrs.nasa.gov/search.jsp?R=19670016936 2020-03-12T11:25:45+00:00Z
- 2 -  
It has been found that for low tin concentrations, a non-ohmic high- 
resistance contact is formed between the substrate and the-film. For high tin 
concentrations, a greater carrier concentration is found in the GaAs film 
surface region. 
50% germanium. 
It appears that the optimum alloy proportions are 50% tin, 
It is interesting to see why this should be so. 
The phase diagram for the Ge-Sn alloy system is shown in Figure ( 2 ) .  
We find that at the temperature of the substrate during GaAs film growth - 8OO0C - 
the 50% - 50% alloy lies on the liquidus curve. 
reduced, therefore, some of the germanium will be precipitated as a separate 
phase on the surface of the molybdenum substrate. 
for the high-resistance contact which is produced under these circumstances. 
The fact that the precoat alloy is liquid at the film growth temperature also 
implies that the GaAs grows by precipitation from a solution in the alloy, and 
that the growing face is covered by tin and germanium at the start of the 
deposition process. 
with tin and germanium, and this is thought to provide the low-resistance contact 
to the molybdenum. 
will be lost from the substrate by thermal evaporation. This was checked 
experimentally, and it was confirmed that both the tin and the germanium leave 
the substrate: .the germanium may be transported as germane by reaction with 
the hydrogen furnace ambient. 
method for the formation of a GaAs film with low resistivity material against 
the substrate, and a progressively decreasing impurity concentration as the 
film thickness increases. 
If the tin proportion is 
This is thought to account 
Consequently the initially grown material will be saturated 
Calculations also show that at the growth temperature, tin 
Hence the precoating technique provides a 
GaAs film growth 
Although various alternative GaAs film growth processes have been 
investigated,the majority of the cells have been made with GaAs films formed 
by the vapor transport process using GaAs source wafers, with oxygen providing 
transport by the reaction: (3) 
high temp. ,-’ Ga20 4- As2  + H2 
low temp. 
2 GaAs + H20 
A new type of furnace has been developed for the GaAs film growth: 
shown schematically in Figure 3.  
furnace are that it uses only quartz components, and that comparatively large- 
area films can be grown from small-area source wafers. 
this is 
The principal advantages of this type of 
In operation, the 
- 3 -  
temperature g rad ien t  e x i s t i n g  i n  the  furnace causes  t r a n s p o r t  of GaAs from 
source a t  900°C t o  s u b s t r a t e  a t  800 C. 
form between 80 and 100 microns of  GaAs on two 2 cm c e l l s  simultaneously.  
0 
The growth per iod  i s  16 hours, t o  
2 
B a r r i e r  Layer S tud ie s  
The platinum b a r r i e r  l aye r  i s  appl ied  by vacuum evaporation, to  form 
a f i lm  giving about 70% o p t i c a l  transmission. A l t e r n a t i v e s  t o  t h i s  procedure 
have been examined. 
The d i f fus ion  of  z inc from a vacuum-deposited l aye r  t o  form a p-n junc t ion  
has  been found t o  y i e l d  opera t ing  c e l l s ,  bu t  t h e  e f f i c i e n c i e s  have been lower 
than those obtained from the  platinum b a r r i e r  cells ,  t h e  main problem being 
poor diode c h a r a c t e r i s t i c s .  The use of copper i n  p lace  o f  platinum a s  a b a r r i e r  
layer ,  t o  g ive  a lower shee t  res i s tance ,  y ie lded  c e l l s  which degraded upon 
s to rage  i n  room ambient: Figure 4 shows the  c e l l  c h a r a c t e r i s t i c s ,  and the 
degradation. 
Current c o l l e c t i o n  gr ids ,  with an i n t e g r a l  busbar, a r e  app l i ed  by vacuum 
evaporat ion of  copper. With care,  a f i n e  w i r e  can be so ldered  t o  t h i s  busbar 
t o  provide a reasonably s t rong  low-resistance contac t .  The c e l l  is completed 
with an a n t i r e f l e c t i o n  l aye r  o f  S i 0  appl ied  by vacuum evaporation. 
Ce l l  c h a r a c t e r i s t i c s  
Calcu la t ions  have been made on the  deple t ion  region width and c a r r i e r  
concent ra t ion  i n  the  a c t i v e  region of t h e  c e l l ,  us ing capaci tance-vol tage 
2 
measurements. (4-6)The procedure used has  been t o  evaporate  an a r r a y  of 1 
plat inum d o t s  on to  the  GaAs f i l m  surface,  t he  d o t s  l y ing  on t h a t  p a r t  o f  t h e  
f i l m  t o  be covered by t h e  busbar on completion of t he  c e l l .  When the capac i tance  
measurements were f inished,  t he  c e l l  was completed by evaporat ion of t he  b a r r i e r  
layer ,  grid-busbar, and SiO, without any e t ch ing  t o  remove t h e  platinum dots .  
Hence the  deple t ion  reg ion  widths and c a r r i e r  concent ra t ions  which a r e  measured 
correspond c l o s e l y  t o  those of the completed cel ls .  The r e s u l t s  e x h i b i t  a 
c o r r e l a t i o n  between N 
Mechanisms which would g ive  r ise t o  an e f f e c t  of t h i s  type have been considered. 
The s h i f t  i n  the  Fermi l e v e l  caused by t h e  change i n  c a r r i e r  concent ra t ion  i s  i n  
t h e  wrong d i r e c t i o n  t o  account for  t h e  observed V v a r i a t i o n .  The e f f e c t  o f  
a combination of  shunt and s e r i e s  r e s i s t a n c e s  on V have been considered: 
i n  t h i s  ca se  a l so ,  the  observed vol tage  change is l a r g e r  than 
and Voc for  t h e  c e l l s :  t h i s  is  shown i n  Figure 5. d 
oc 
oc 
- 4 -  
can be accounted for by this mechanism. 
arises from changes in trapping levels at the active surface, but this has 
not been confirmed. 
It appears possible that the effect 
A correlation between I and depletion region width is also observed in 
the measurements which have been made. This is of the expected form, and has 
been combined with the results of a theoretical analysis in Figure 6 .  
experimental points are shown by circles: the scatter is thought to be the 
result of surface recombiration rate variations, since this is particularly 
sensitive to changes in surface treatment. 
operation under 100 mW cm 
are l o s t  on transmission through the grid and barrier layer. The parameter aL 
is the product of optical absorption coefficient ( a ) ,  which may be taken as 
approximately 10 cm for light of higher energy than the GaAs energy gap, 
and the minority carrier diffusion length (L) in the base material. 
earlier work, the diffusion contribution to the cell output current was assumed 
to be negligible, because of the small minority-carrier lifetime in GaAs. The 
figure shows that this assumption is not wholly valid: 
diffusion lengths of up to 1/3 micron. 
-8 2 of around 4 x 10 Compared to the figure of 2 x 
GaAs, this seems reasonable, and makes evident the effect of the polycrystalline 
nature of the films. 
sc 
The 
The curves are calculated for 
-2 air mass one sunlight, from which 27% of the photons 
4 -1 
In 
the results indicate 
This implies a mobility lifetime product 
cm . for single-crystal 
2 
An efficiency of 3.8% has been achieved in a 2 cm cell. The I - V  
characteristic for such a cell is shown in Figure 7. This characteristic was 
obtained under tungsten illumination giving the same I value as sunlight of 
100 mW cm-’, as extrapolated from sunlight measurements. 
sc 
Earlier work on the cells showed that an etching step, which produced 
initially higher cell efficiencies, also caused cell degradation. This has 
been eliminated by elimination of the etching process, and the cell efficiency 
reported above approaches the best seen previously only in the etched cells. 
CELLS ON ALUMINUM 
The cell structure is shown in section in Figure 8. 
InAs precoat 
A precoat layer of I d s  is applied to the aluminum foil, to provide 
a low-resistance ohmic contact from the substrate to the GaAs film. The thickness 
- 5  - 
of t h i s  layer  i s  5 microns, and it i s  formed by vapor t r a n s p o r t  from a 
p o l y c r y s t a l l i n e  wafer of I d s ,  by a process similar t o  t h a t  used f o r  t h e  G a A s  
f i lm  growth. 
known. However, chemical ana lys i s  of material deposi ted on t h e  furnace w a l l s  
r evea l s  t h a t  t he  indium i s  mobile during t h e  GaAs f i l m  growth, which impl ies  
t h a t  a l loy ing  occurs  a t  t h e  Ids-GaAs i n t e r f ace .  
t h a t  t h e  overlying f i l m  c r y s t a l l i t e s  form e p i t a x i a l l y  on those  of t h e  precoat .  
This e l imina te s  the  l aye r  of very small c r y s t a l l i t e s  of G a A s  which are formed 
immediately ad jacent  t o  t h e  aluminum i n  t h e  absence of t h e  precoat .  It i s  
thought t o  be t h e  junc t ions  a t  t h e  gra in  boundaries of t hese  c r y s t a l l i t e s  which 
a r e  t h e  cause of t h e  blocking contac ts  when G a A s  i s  formed d i r e c t l y  on e i t h e r  
aluminum o r  molybdenum subs t r a t e s .  
The d e t a i l s  of the  mechanism of opera t ion  of t h e  precoa t  are not  
Sect ioning a l s o  i n d i c a t e s  
G a A s  f i l m  growth. 
G a A s  f i lms  f o r  t hese  c e l l s  are a l s o  formed by vapor t r a n s p o r t ,  t h e  
p r i n c i p l e  being t h a t  descr ibed above f o r  f i lms  grown on molybdenum s u b s t r a t e s .  
The furnace d e t a i l s  are s l i g h t l y  d i f f e r e n t ,  and the  opera t ing  temperatures are 
lower. 
and t h e  source wafers are approximately 100 C higher  i n  temperature.  
The s u b s t r a t e s  are he ld  j u s t  below the  melt ing po in t  of aluminum, 64OoC, 
0 
Control of t h e  oxygen concent ra t ion  during t h e  GaAs f i l m  growth has  proved 
t o  be c r i t i c a l  f o r  euccess.  The oxygen i s  introduced as w a t e r  vapor, and the  
necessary c o n t r o l  i s  achieved by mixing pure hydrogen and hydrogen sa tu ra t ed  
with water vapor before  passing t h e  gas  through t h e  furnace.  
of t h e  grown l a y e r s  i s  governed by t h e  oxygen concent ra t ion ,  h igher  oxygen 
concent ra t ions  producing h igher - res i s tance  f i lms.  This e f f e c t  i s  presumably 
due t o  t h e  in t roduc t ion  of t rapping levels by t h e  oxygen impuri ty ,  producing 
t h e  "semi-insulating" form of G a A s  by t h e  mechanism descr ibed by Blanc and 
Weisberg. (7) Reduction of oxygen concentrat ion slows t h e  G a A s  f i l m  growth rate, 
s o  t h a t  a compromise must be reached where a reasonably t h i c k  f i l m  can be  formed 
wi th  a r e s i s t a n c e  s u f f i c i e n t l y  l o w  not  t o  impair c e l l  performance. 
growth per iod i s  normally employed, t o  produce a f i l m  15 microns i n  thickness .  
The r e s i s t i v i t y  
A 22-hour 
This  f i l m  i s  pol ished by etching i n  a d i l u t e  s o l u t i o n  of bromine i n  methanol, 
t o  prepare  t h e  su r face  f o r  t h e  b a r r i e r  l aye r  depos i t ion .  
Barrier layer formation. 
The barrier layer is formed by flash evaporation, under vacuum, of a 
non-stoichiometric form of cuprous selenide. The starting material has a 
composition of Cu 
of very thin layers depends critically upon the flash evaporation conditions. 
This point has been analysed by Ellis(8), and we shall not pursue the matter 
Se, determined by chemical analysis. The actual stoichiometry 1.85 
further at this time. 
Specimens of cuprous selenide formed on glass slides have been used for 
measurements of optical transmission and electrical resistance. 
fabrication, these monitor specimens show optical transmission of 60 to 70% to 
1.5 eV radiation, and sheet resistances down to 12  ohms per square. 
For cell 
Multiple-beam interferometry methods have been used to measure the refractive 
index of the cuprous selenide films on glass. 
refractive index of 2.4 at 5460g. 
layer has been designed. 
Hea~ens(~)show that a h/4 layer of cuprous selenide, followed by a A/4 layer of 
magnesium fluoride, should give the desired result. 
These measurements indicate a 
Working from this value, an antireflection 
Appropriate calculations based on the nethodc of 
This has been verified 
experimentally, with barrier-layer cells on single-crystal GaAs showing I 
increases of 25% over cells without MgF2. 
sc 
Cell stability. 
In some instances, cell degradation has been observed in cells stored 
in air. The characteristics of a degraded cell are shown in Figure 10. 
effect is apparently caused by the presence of two series diodes in opposition. 
It has been found that these characteristics can be returned to normal by etching 
the cells in dilute nitric acid, the results also being shown in the figure. 
The 
Ellis(8)has postulated that cuprous selenide films with a stoichiometry 
between Cu 
remaining cuprous selenide going to the Cu 
by the results of Heyding'"). 
believe that the cell degradation which we have seen is due to the formation of 
such an oxide layer. The excess copper in the films would be the result of the 
loss of selenium during flash evaporation, by the accommodation coefficient 
effects considered by Ellis. 
restoring the cell to its normal state. 
can be prevented by etching the cells immediately after their fabrication, but 
we have not yet had an opportunity to run the necessary life tests to conclusively 
establish this point. 
Se and Cu Se tend to form a surface layer of cuprous oxide, the 1.85 2 
Se stable form. This is confirmed 1.85 
The effect is illustrated in Figure 10. We 
The acid etch apparently removes this oxide layer, 
We also have evidence that the degradation 
- 7 -  
Characteristics and efficiency. 
The characteristic of a typical cell is shown in Figure 11. Perhaps the 
most striking thing about this characteristic is the rectification ratio, a 
value of 1000 to 1 at 1 volt being typical. The efficiency of 3% is also the 
mean figure for our present production process, the best value obtained to date 
being 4.2%. 
The cells are very light-weight, the best cell having a power-to-weight 
-2 ratio of 135 watts per pound under 140 mW cm illumination. The cells are some- 
what flexible, and can be cut with scissors or a scalpel, such an operation being 
part of our normal processing. 
DISCUSSION 
The cell types which have been described are in different states of 
development. 
practical range, and the cell configuration and contacting techniques have been 
developed. 
more advanced design hold greater promise for providing low-weight power systems. 
The cells on molybdenum have areas which are approaching the 
The cells on aluminum have more modest areas, but because of their 
One problem is possessed in common by these cells and by most other thin- 
This centers on the low V film polycrystalline cells. 
compared with the V 
It would appear that this problem is a most fruitful field for further study, 
and if suitable surface treatments can be developed to improve the V 
thin-film cell efficiencies will be increased considerably. 
values which are obtained, oc 
values for single-crystal cells of the same materials. oc 
values, oc 
- 8 -  
REFERENCES 
1. P. Vohl et al. "GaAs thin-film solar  cells" Photo-voltaic Specialists' 
2. 
3 .  
4 .  
5. 
6. 
7. 
8. 
9. 
Conference Sf5, October 1965. 
M. Hansen, Binary Alloys (McGraw-Hill Book Co., Inc., New York, 1958). 
C.D. Thurmond and C.J. Frosch, J. Electrochem. SOC. 111, 184 (1964). 
A.M. Goodman, J. App. Phys. 34, 329 (1963). 
A.K. Jonscher, Principles of semiconductor device operation (John Wiley and 
Sons, Inc., New York, 1960). 
H.K. Henisch, Rectifying Semiconductor Contacts (Oxford University Press, 
London, 1955). 
J. Blanc and L.R. Weisberg, Nature 192, 155 (1961). 
S.G. Ellis, J. Appl. Phys. (Submission for publication). 
O.S. Heavens, Optical properties of thin solid films, (Dover Publications Inc., 
New York, 1965). 
10. R.D. Heyding, Can. J. Chem. 44, 1233 (1966). 
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Progress in thin film GaAs solar cells

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