Superlattice layers are effective in decreasing the density of dislocations in lattice mismatched heterostructures at least four orders of magnitude. Hence it was proposed to utilize this feature of superlattices to alleviate the problems due to misfit dislocations generated in the regions between two or more photovoltaic collecting junctions. A further advantage is that the possibility is presented for using silicon as a low cost substrate as well as for the low band gap junction. In the test case, a silicon low gap cell was connected to a GaAs.7P.3 high gap cell through a connecting region containing a GaAs/GaP superlattice

Blackslee, A. E.

Mitchell, K. W.

English

NASA Technical Reports Server

INCORPORATION OF SUPERLAWICE CRYSTAL LAYERS 
IN MULTIJUNCTION SOLAR CELLS 
A. E. Blackslee and K. W. Mitchell 
Solar Energy Research Insti tute 
Golden, Colorado 
SUMMARY 
This paper proposes the application of superlattice growth techniques in 
the fabrication of high efficiency multijunction solar cells in order to im- 
prove their crystalline perfection and hence enhance their output. Previous 
research has shown that superlattice layers are effective in decreasing the 
density of dislocations in lattice mismatched heterostructures at least four 
orders of magnitude. Hence it is proposed to utilize this feature of super- 
lattices to alleviate the problems due to misfit dislocations generated in 
the regions between two or more photovoltaic collecting junctions. 
A further advantage is that the possibility is presented for using silicon 
as a low cost substrate as well as for the low band gap junction. In the 
test case proposed here, a silicon low gap cell will be connected to a 
GaAs .7P 3 high gap cell through a connecting region containing a GaAs/GaP 
superlattice. 
INTRODUCTION . 
* 
This paper proposes the application of superlattice growth techniques 
for the fabrication of low cost substrates for high efficiency solar cells. 
Recent research has demonstrated the feasibility of achieving high photovoltaic 
conversion efficiepcies (potentially above 35%) using multiple junctions in -------__------ * 
A superlattice, as the term has come to be used in solid state electronics, 
is a tgin (-1pm) semiconductor structure comprised of many ultrathin layers 
(-100 A each) of two different semiconductors alternating with one another. 
A schematic diagram of a superlattice composed of two hypothetical semi- 
conductors A and B is shown in Fig. 1. This structure exhibits properties 
which tend to be intermediate between those of the two constituent materials 
but may be somewhat different from either one. It offers the possibility of 
creating semiconductors with made-to-order properties such as the band gap or 
optical absorption coefficient. It was first proposed by Esaki and Tsu (1) in 
1970 as a quantum mechanical curiosity which might some day give rise to de- 
vices with extremely high frequency response. Several other device proposals 
based on the superlattice concept have followed. Although most superlattice 
growth to date has been done by the MBE process, the technical feasibility of 
growing superfattices by a CVD process was demonstrated in 1969 by Blakeslee 
(2) and improved recently, using MO-CVD, by Dupuis and Dapkus ( 3 ) .  
131 
https://ntrs.nasa.gov/search.jsp?R=19810009027 2020-03-21T08:56:27+00:00Z
optimum band gap materials (4,5). In addition, multijunction converters are 
excellent candidates for hybrid photovoltaic/thermal systems. A hybrid con- 
verter of this type could have an electrical conversion efficiency of 25% 
while providing a thermal energy output in the range of 15OOC to 250°C (6). 
The GaAs substrate,which is used in structures currently being developed, 
is a drawback to widespread implementation of these high efficiency converters 
due to'its high cost and extensive consumption of gallium resources. In 
addition, dislocations due to lattice mismatch between the various composi- 
tional layers involved in these structures degrade the quality of the elec- 
trically active regions of the cells. If a low cost substrate such as silicon 
could be substituted for the GaAs, a major breakthrough would result not only 
for 111-V based solar cells, but for 111-V device technology in general., The 
application of superlattice growth techniques, which have already shown great 
promise in coupling non-lattice matched layers, is proposed for utilizing low 
cost substrates in high efficiency solar cell structures. The proposed 
approach would alleviate the problem of device degradation due to dislocation 
propagation into the active regions of the cell. Furthermore, it would 
demonstrate the possibility of using silicon as both the low cost substrate 
and the low band gap cell of a two-junction high efficiency photovoltaic 
converter. 
THE CONCEPT 
Presently, only the 111-V materials technology has been advanced 
sufficiently to address the materials requirements for growth of monolithic 
cascade cells, but the GaAs substrates which have been used thus far for the 
development of cascade devices are very expensive. Silicon substrates are a 
factor of twenty or so less expensive than GaAs, but, unfortunately, silicon 
does not have the proper lattice constant to be lattice-matched to the other 
candidate cells. 
We propose to use superlattice structures in the fabrication of high 
efficiencytandem solar cells, not for the active photoelectric conversion 
layers but rather as a means of alleviating one of the most difficult problems 
facing the construction of monolithic tandem cells, namely that of lattice 
mismatch. The mismatch occurs because the two cells of the tandem, a high 
band gap cell and a low band gap cell, will in general be created out of two 
semiconductor materials having different lattice parameters. 
indicates the lattice constant values and band gaps of a number of candidate 
semiconductors for multijunction converters. The crosshatched regions re- 
present the optimal range of band gaps for the top and bottom junctions of 
two-junction cascade cells. 
viable substrates for the cascade cells. Unfortunately, the band gap of Ge 
is too low to be optimal for the low band gap junction. 
proper band gap for the low band gap cell, it does not have the proper lattice 
constant to match the high band gap cell. Typically, a transitional, i.e., 
compositionally-graded, layer or series of layers is grown between the two cell 
regionsin order to reduce the density of dislocations that inevitably arise t o  
Figure 2 
Presently only Ge and GaAs are considered as 
Although Si has the 
132 
relieve t h e  m i s f i t  stress. A t  b e s t  t h e s e  grading techniques,  be  they  
continuous o r  by s t e p s ,  are only  p a r t i a l l y  success fu l ,  and a l a r g e  number of 
d i s l o c a t i o n s  remains. They propagate  through t h e  active s o l a r  cel l  l a y e r s  and 
cause recombination l o s s e s  and, when p resen t  i n  extreme d e n s i t i e s ,  c a n s e v e r l y  
d i s r u p t  t h e  c r y s t a l  growth process .  
t h a t  is  not  gene ra l ly  s u f f i c i e n t l y  apprec ia ted .  It has  been a major problem 
i n  achiev ing  t h e  necessary  device  q u a l i t y  l a y e r s  i n  t h e  p re sen t  cascade ce l l  
s t r u c t u r e s .  
This  i s  a ve ry  s e r i o u s  problem and one 
However, a way around t h i s  problem does e x i s t .  Matthews and Blakes lee  
(7) have demonstrated t h a t  growth of a s u p e r l a t t i c e  l a y e r  a f t e r  t h e  graded 
l a y e r  serves t o  conf ine  a l l  t h e  d i s l o c a t i o n  l i n e s  i n p l a n e s n o r m a l  t o  t h e  growth 
d i r e c t i o n  BO t h a t  thpy do no t  propagate  i n t o  t h e  active s o l a r  cel l  reg ion .  
They w e r e  z b l e  t o  reduce t h e  d i s l o c a t i o n  d e n s i t y  i n  a G ~ A S / G ~ A S ~ . ~ P ~  
la t t ice  from 108 per  cm2 t o  almost zero.  
d i s l o c a t i o n  e l imina t ion  can be e f f e c t e d  are q u i t e  s u b t l e ;  they  are f u l l y  
set out  -in t h e  b a s i c  p a t e n t  f o r  t h e  process  granted t o  Blakes lee  and Matthews 
super- 
The d e t a i l s  as t o  why and how t h i s  
(8) * 
A method has  been suggested f o r  cons t ruc t ing  s o l a r  cells  on s i l i c o n  
s u b s t r a t e s  (9 ) .  
G e  l a y e r  through a graded a l l o y  l a y e r  of G e  S i  followed by growth of GaAs 
on t h e  G e ,  t o  which it is  lat t ice-matched.  A b e t t e r  approach is  t o  u s e  t h e  
s i l i c o n  as both  t h e  s u b s t r a t e  and t h e  low band gap junc t ion .  This  reduces 
t h e  complexity of t h e  cascade s t r u c t u r e  s u b s t a n t i a l l y ,  w i th  the b e n e f i t  of 
b e t t e r  y i e l d s  i n  a product ion l i n e .  
i n  t h i s  case would couple  t h e  h igh  band gap j u n c t i o n  t o  t h e  j u n c t i o n  formed 
i n  t h e  s i l i c o n .  
It c o n s i s t s  of connect ing t h e  S i  s u b s t r a t e  w i th  a depos i ted  
1-x x 3  
The grading l a y e r  and s u p e r l a t t i c e  l a y e r  
IMPLEMENTATION OF THE CONCEPT 
There are several tandem c e l l  concepts  which could b e n e f i t  from t h e  
inco rpora t ion  of a s u p e r l a t t i c e  t o  alleviate t h e  l a t t i ce  mismatch problem. 
One i n  p a r t i c u l a r  seems e s p e c i a l l y  a t t ract ive as a test v e h i c l e  because it 
no t  only o f f e r s  a good chance of r e l a t i v e l y  r a p i d  reduct ion  t o  p r a c t i c e  wi th  
t h e  least d i f f i c u l t y ,  b u t ,  i f  success fu l ,  could have a l a r g e  impact on t h e  
problem of c o s t  and a v a i l a b i l i t y  of s u b s t r a t e s  f o r  h igh  e f f i c i e n c y  s o l a r  cells. 
The p r o j e c t  s e l e c t e d  is  t o  l i n k ,  by us ing  a s u p e r l a t t i c e ,  a S i  wafer,  
which serves as both s u b s t r a t e  and low band gap ce l l ,  and a GaAs-based h igh  
band gap ce l l .  The s t r u c t u r e  w i l l  be  grown by t h e  method of metal-organic 
chemical vapor depos i t i on  (MOCVD) which w a s  shown t o  be  u s e f u l  f o r  growth of 
high e f f i c i e n c y  GaAs /GaAlAs  s o l a r  cel l  s t r u c t u r e s  by Dupuis et a l .  (10) i n  
1977. It has  a l s o  been used t o  make double h e t e r o s t r u c t u r e  lasers ( l l ) ,  FETs 
(12) and s u p e r l a t t i c e - l i k e  s t r u c t u r e s  known as Bragg r e f l e c t o r s  ( 3 ) .  Its 
p r i n c i p a l  advantage over o t h e r  techniques such as l i q u i d  phase ep i t axy  i s  t h e  
a b i l i t y  t o  grow very  t h i n  ( t e n s  of angstroms),  uniform l a y e r s  of c o n t r o l l a b l e  
composition. 
133 
It i s  necess r y  t o  go from S i  with a band gap of 1.11 e V  and a l a t t i c e  
constant of 5.43 !2 t o  some te rnary  a l l o y  of GaAs with a band gap i d e a l l y  
around 1.8 e V  while undergoing as s m a l l  a change i n  l a t t i ce  parameter as 
possible.  I n  v i e w  of t h e  lat ter requirement, t h e  Gap system is the  bes t  
choice f o r  forming a te rnary  with GaAs ,  s i nce  the  l a t t i ce  parameter d i f f e rence  
between S i  and GaP is  only 0.37% and acceptable heteroepitaxy of GaP on S i  
by M O O  has already been demonstrated (13). Thus, t h e  major s t e p s  of t h e  
process would be t h e  following: 
Star t  with a S i  s u b s t r a t e  and make a p-n junc t ion  by d i f fus ion  o r  by 
homoep it axy . 
Grow a t h i n  l aye r  of GaP on t h e  S i  from a vapor containing Ga(CH3)3 and 
PH3. 
Grow the  graded l aye r  by decreasing t h e  PH 
and rep lac ing  i t  with ASH Continue t h i s  process u n t i l  t h e  top of t h e  3' graded l aye r  has approximately t h e  composition G a A s  
gap of 1.80 e V .  
content of t h e  r eac t an t  vapor 
This grading should be accomplished7wi&n 1Oum o r  less. 
3 
P with a band- 
Remove t h e  threading d i s loca t ions  by i n s e r t i n g  a t h i n  asymmetric GaAs/GaP 
s u p e r l a t t i c e  region containing 10  o r  so o s c i l l a t i o n s .  
thickness of t h e  r e spec t ive  cons t i t uen t  l aye r s  GaAs and GaP sh6uld be i n  
t h e  r a t i o  of 7 t o  3. 
I n  order t o  l a t t i c e  
match the  s u p e r l a t t i c e  t o  t h e  underlayer with composition G a A s  7P 3' t h e  
Grow a 2-3pm laye r  of e s s e n t i a l l y  d is loca t ion- f ree  GaAs P atop t h e  
s u p e r l a t t i c e  and f a b r i c a t e  a junc t ion  the re in  by epitax? o r  d i f fus ion .  7 .3 
In  carrying out  t h e  above procedure, two of t h e  operations are pa r t i cu la r -  
l y  c r i t i c a l .  One i s  t h e  need f o r  very p rec i se  con t ro l  of t he  composition of 
t he  graded l aye r  and of t h e  top c e l l  l aye r .  This can be e f fec ted  mainly by 
ca re fu l  con t ro l  of t h e  input flow of r eac t an t  gases but might eventually 
incorporate i n  s i t u  o p t i c a l  measurement of t h e  s o l i d  composition. The o ther  
c r i t i ca l  point i s  t h e  a b i l i t y  t o  switch back and f o r t h  r ap id ly  and reproduc- 
i b l y  from G a A s  t o  GaP i n  c rea t ing  t h e  s u p e r l a t t i c e .  Both of these  po in t s  
can be accommodated within t h e  scope of present technology by making mod- 
i f i c a t i o n s  t o  present MOCVD systems. Other po in t s  requi r ing  ca re fu l  con- 
s ide ra t ion  are meticulous s u b s t r a t e  sur face  treatment i n  order t o  assure  
good i n i t i a l  epitaxy of GaP on S i  and ca re fu l  experimentation t o  determine 
theabso lu te  minimum thickEess f o r  each l a y e r ,  s ince  it is  very important t o  
eliminate as much of t h e  expensive Ga-containing material as poss ib le .  
134 
REFERENCES 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
11. 
12. 
13. 
L. Esaki and R. Tsu, IBM J. Res. Develop., 14 61 (1970). 
A.E. Blakeslee, J. Electrochem. SOC., - 118 1459 (1971). 
R.D. Dupuis and P.D. Dapkus, Appl. Phys. Lett., - 33 68 (1978). 
R.L. Moon, L.W. James, et al., Proc. 13th IEEE Photovoltaic Specialists 
Conf. (1978), pp. 859-867. 
S.M. Bedair, M.F. Lamorte, J.R. Hauser, K.W. Mitchell, Proc. Inter- 
national Electron Device Meeting, Washington, D.C., December 1978, 
pp 250-253. 
K.W. Mitchell, Proc. International Electron Device Meeting, Washington, 
D.C., December 1978, pp. 254-257. 
J.W. Matthews and A.E. Blakeslee, J. Crystal Growth 27, 118 (1974); 
J. Crystal Growth - 32, 265 (1976). 
A.E. Blakeslee and J.W. Matthews, U.S. Patent 4,088,515, May 9, 1978. 
Y.C.M. Yeh, K.L. Wang, B.K. Shin and R.J. Stirn, Proc. 14th IEEE 
Photovoltaic Specialists Conf. (1980), pp. 1338-42. 
R.D. Dupuis, P.D. Dapkus, R.D. Yingling and L.A. Moudy, Appl. Phys. 
Lett;, 31 201 (1977). 
R.D. Dupuis and P.D. Dapkus, Appl. Phys. Lett., 2 839 (1977). 
J. Hallais, J.P. Andre, P. Baudet and D. Baccon-Gibod, Gallium 
Arsenide and Related Compounds 1978, (Inst. Phys. Conf. Ser. No. 45) 
p. 361. 
H.B. Pogge, B.M. Kemlage and R.W. Broadie, J. Cryst. Growth, - 37 (1977). 
135 
A 
Layers 
Substrate 
. . 
Top Layer 
B 
Layers 
Superlattice tructure 
FIGURE 1 
Bandgag and Lattice Constant Values for the 
Ill-V Semiconductors 
2 5  ' eAIP ' I I 1 
2.0 
1.S 
1 .o 
Od t K: \ 
k lnSl 
I I I I I I 
Lattlce Parameter (A) 
5.2 5.4 5.6 5.0 6.0 6.2 6.4 
0 
e 
FIGURE 2 
i 
.6 
136 


Incorporation of superlattice crystal layers in multijunction solar cells

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