United States Patent [191
[11]
4,107,723
Kamath
[45]
Aug. 15, 1978
Primary Examiner-Martin H. Edlow
Attorney, Agent, or Firm-William J. Bethurum; W. H.
MacAllister
[54] HIGH BANDGAP WINDOW LAYER FOR
GaAs SOLAR CELLS AND FABRICATION
PROCESS THEREFOR
[75] Inventor:
[73] Assignee:
[57]
G. Sanjiv Kamath, Malibu, Calif.
Hughes Aircraft Company, Culver
ABSTRACT
The speci?cation describes a semiconductor solar cell
and fabrication process therefor wherein a thin N-type
City, Calif.
gallium arsenide layer is deposited on a larger P-type
substrate layer which is selected from the group of
[21] Appl. No.: 792,840
[22]
Filed:
May 2, 1977
I
[51]
[52]
Int. Cl.2 ........................................... .. H01L 27/14
US. Cl. ...................................... .. 357/30; 357/16;
[58]
Field of Search ............................ .. 357/30, 16, 61
III-V ternary compounds consisting of aluminum phos
phide antimonide, AlPSb, and aluminum indium phos
References Cited
phide, AlInP. P-type impurities are diffused from the
substrate layer into a portion of the thin N-type gallium
arsenide layer to form P-type region therein which
de?nes a PN junction in the thin gallium arsenide layer.
Thus, the quantity of gallium arsenide required to pro
vide this PN photovoltaic junction layer in the cell is
U.S. PATENT DOCUMENTS
minimized, and the P-type substrate serves as a high
357/61
[56]
bandgap window layer for the cell. Such high bandgap
3,614,549
10/1971
Lorenz ............................... .. 317/234
3,914,136
10/1975
Krersel . . . . . .
. . . . ..
3,960,620
6/1976
Ettenberg .
.... .. 148/175
3,981,755
4,012,760
4,015,284
4,053,920
9/1976
3/1977
3/1977
10/1977
148/171
Gowers .
.. 148/171
Hara ............ ..
357/ 30
Hara ............ ..
357/16
Enstrom ............................... .. 357/30
of this window material is especially well suited for
ef?ciently transmitting the blue spectrum of sunlight to
the PN junction, thus enhancing the power conversion
efficiency of the solar cell.
6 Claims, 4 Drawing Figures
Sunlight
Insulator
P Contact
Support
kN Contact
U. S. Patent
4,107,723
Aug. 15, 1978
Fig.1.
/
/
//
/
/'/
_
/
/
/
/
/
/
/ p’ /
/
/
/
GuAs3
Fig. 2.
//
/
/
/
/
/
/
/
/
/
AU'ZD'AU
/
// AlInP
N Contact
\r
/
/
T0205
Au-Ge-Ni
Support
AlPSb or
/
/l3
/
/ /
/
/
//
\PN Junction
4,107,723
1
HIGHBANDGAPWQWLAYER FOR Gm
SOLAR CELLS AND FABRICATION PROCESS
I
2
THE INVENTION
Accordingly, the general purpose of this invention is
THEREFOR
_
to provide a new and improved gallium arsenide solar ,
5 cell and fabrication process therefor which process not
I
FIELD OF THE INVENTION
only greatly reduces the gallium quantity requirement
in solar cell manufacture, but at the same time improves
This invention relates generally to gallium arsenide
the transmission of the blue spectrum of sunlight in the
solar cells and more particularly to such solar cells
resultant solar cell relative to the above described prior
exhlblt
which an
areImproved
relativelypower
inexpensive
qonverslon
in cost
ef?clency'
and which 10 simultaneously
xegzlég?ugsi?ieé?ggilucrg
reducing theirszllzrmiilll?fgsiixxgs
production costs.
BACKGROUND
.
To accomplish this purpose, I initially provide a P
.
type substrate material selected from the group of ter
substhnml efforts have ,been made. m recent W“ 15 nary III-V compound semiconductors consisting of
toward lmprovmg the ef?mehcy an‘? ylelds ohsemlcoh'
aluminum phosphide antimonide and aluminum indium
dhctor solar cehs_a_hd reducing the“ Produchoh costs‘
As colhpared to “been, gihhhm arsenide has long been
phosphide. This substrate is’then exposed to an epitaxial
deposition process wherein a thin uniform N-type layer
l'ecoghlled as a material Wlth a bahdgap more favorahle
of gallium arsenide is grown to a predetermined thick
for maximizing solar cell ef?ciehcy and also a material 20 ness, typically on the order of about 1 micrometer.
better suited for operation at elevated temperatures.
During this epitaxial growth step, a p_type impurity
Additionally, because of the small Optical absorption
(e.g. aluminum) in the substrate diffuses into a portion of
depth in direct transistion semiconductors like gallium
the thin N-type epitaxial layer to convert the conductiv
arsenide, short minority carrier diffusion lengths can be
ity of that portion to P-type and thus form the required
tolerated, and this leads to better resistance to radiation 25 photovoltaic PN junction for the cell. Finally,~ state-of
damage_
the-art metallization deposition and anti-re?ective coat
Qne state.°f.the_art type of gallium arsenide Solar C511
ing ?lm deposition techniques are used to provide the
which has evolved as a result of continuing efforts in
necessary front and back contact metallization for ex
solar cell research is a gallium arsenide solar cell com-
trashing POWer fro") the device and also the necessary
prising an N_type gahum arsenide substrate upon Which 3O anti-re?ection coating on the I?-type_ window layer
a P_type layer of gallium aluminum arsenide is epitaxh
thereof. The windowilayer for this device 18, of course,
ally deposited. One such structure is shown, for examth? P'typg shbstmte Instead of an epltaxlal layer as 1“
ple, in an article by J. DuBow in Electronics, Vol. 49,
pug; art. Willie's‘
d1
. d h h
.
No. 23 Nov. 11, 1976 at page 89. Another such struc. ’.
.
.
.
us’n w
e re? 1y appreciate. t at t 6' reqmre'
35 ment for the expensive element gallium has been sub
f‘ure 1S dlsclosed by H‘ J‘ Royal elal m an amcle :mltled
stantially reduced to only that required to form a thin
GaI-XAIXAS ' PPN neFero-luhhhoh Solar Cells ’ Jou"
epitaxial gallium arsenide layer, typically of one to ?ve
"al of the Electrochem'cal soc'ety' septfhhber 19.73 at
micrometers in thickness, instead of a much thicker
Pages 124E132’ and both of these arhcles are lhcor'
gallium arsenide substrate, typically of two hundred
Porated herein by reference- Yet ‘mother such Solar cell 40 micrometers or more. Simultaneously, the utilization of
structure is disclosed in a copehdhlg application of San‘
the above P-type substrate window material having a
.liv Kameth st 31-, Sef- NO- (PD-76165) assigned to the
higher band-gap energy than gallium aluminum arse
present assignee.
While the above type of gallium arsenide-gallium
nide substantially improves the power conversion effi
ciency of the solar cell by improving the transmission of
aluminum arsenide solar cell has proven to be a rela- 45 the blue spectrum of sunlight to the device’s PN junc
tively efficient device and generally satisfactory in sevtion.
eral respects, it nevertheless requires a relatively thick
Therefore, it is an Object of the Present invention to
bulk gallium arsenide substrate starting material for its
provide a new and improved gallium arsenide solar cell
manufacture. This requirement is partially responsible
and fahl’lcahoh pl'ficess thel'sfof_
_
for the expensive manufacturing cost for these solar 50 Another oblect 15 "Q provldeh galhlhh arsehlde solar
cells as a result of the high cost of gallium. However, it
ceh of the typehescnhed havmg ah Improved Power
is well known that major efforts are continually being
conversion efficiency relative to known state-of-the-art
made in both silicon and gallium arsenide solar cell
gagmhatsehghe S913‘ cells‘ .d
1
research to reduce solar cell manufacturing costs by
d
pr0“
less manufacturing cost than those state-of-the-art solar
.din hi h .eld and large Volume manufacturing 55
g
g yl
.
.
it‘? Cir-lift ‘5 at’) prov‘ 6 ago ardcell of the lype
escri e w 1c. may e mass pro uce at substantially
pr°ceSSes.s.for economlcauy makmg these solar cells’
cells utilizing a gallium arsenide substrate as the process
In addition to the above efforts to reduce solar cell
starting material_
costs’ there at? also cohhh‘hhg efforts to mcreas? the
A feature of the present invention is the provision of
Poweh cohversloh efhclehcy of these chjhs‘ In th1_s he‘ 60 a gallium arsenide solar cell of the type described which
gard, 1t has been observed that the gahmm alummhm
exhibits a good crystal lattice match between the vari
arsenide window material of the above state-of-the-art
type gallium arsenide ' gallium aluminum arsenide 5013!‘
cell is somewhat limited in its transmission of the blue
Gus layers that make up the cell_
Another feature of the invention is the provision of an
improved solar cell having a window material with a
spectrum of Sunlight This, of course, is a limiting factor 65 wide bandgap which permits good transmission of the
as regards the maximum attainable ef?ciency for these
blue spectrum of sunlight to the'device’s PN junction.
cells as well as the ultimate cost reduction of large scale
solar cell arrays.
Another feature is the provision of a process for mak
ing gallium arsenide solar cells which requires an abso
3
4,107,723
4
ber 22 such as copper or aluminum, and in actual prac
tice many of these solar cell structures are connected in
lute minimum of the expensive element gallium in the
fabrication of these solar cells.
series and in parallel on a much larger support member
These and other objects and features of the invention
(not shown) in the fabrication of large array solar cells
will become more readily apparent in the following
description of a preferred embodiment of the invention. 5 operative for the generation of relatively large amounts
of electrical power.
DRAWINGS
Various modi?cations may be made in the above
described embodiment of the invention without depart
FIGS. 1, 2, and 3 illustrate schematically a preferred
ing from the true scope thereof. For example, the pres
processing sequence for practicing the invention; and
ent invention is not limited to the particular aluminum
FIG. 4 is an isometric view of the completed solar
cell structure of FIG. 3.
phosphide antimonide and aluminum indium phosphide
Referring now to the drawings, there is shown in
III-V ternary substrate window materials and may also
FIG. 1 a P-type substrate starting material 10 which
be fabricated using other aluminum-containing ternary
or binary III-IV compounds, such as aluminum arsenide
monide or aluminum indium phosphide and which is 15 or aluminum phosphide. Additionally, the present in
vention may be extended to the manufacture of semi
appropriately doped with a P-type impurity such as Be
conductor devices such as laser diodes and light emit
or Ge. The substrate 10 will typically be on the order of
ting diodes to which the objects and features herein
200-250 micrometers in thickness and has a large band
preferrably consists of either aluminum phosphide anti
apply. Also, vapor phase epitaxy (V PE) can be used
gap greater than 2 electron volts to provide good light
transmission to the PN junction of the solar cell. Addi 20 instead of liquid phase epitaxy in forming the thin GaAs
layer 12 previously described.
tionally, the composition of the P-type substrate 10 will
The following process and materials values are appli
be adjusted to provide a good crystal lattice match for
cable to one process and one device embodiment of the
the thin epitaxial layer 12 of N-type gallium arsenide
present invention, but should not be deemed as restric
which is deposited on the upper surface of the substrate
10 as shown in FIG. 2.
25 tive of the invention as claimed.
Preferably, this epitaxial deposition process used to
form the thin N-type later 12 is carried out using the
EXAMPLE
A substrate of beryllium doped aluminum phosphide
antimonide of AlP,25Sb_75 composition is initially
cleaned and polished using standard wafer processing
vertical graphite slide bar and saturated solution growth
LPE techniques which are described in allowed co
pending application Ser. No. 717,806 by Sanjiv Kamath
et al entitled “Method for Growing Thin Semiconduc
techniques and is then submerged into a saturated solu
tor Epitaxial Layers”. The gallium arsenide epitaxial
tion of gallium arsenide in gallium using the above iden
ti?ed vertical slide bar LPE epitaxial deposition tech
niques of S/N 717,806. The Be doping concentration in
layer 12 will typically be from 1 to 5 micrometers in
thickness and will have a resistivity on the order of
about 10-3 ohm centimeters. During the epitaxial 35 the AlP_25Sb,75 substrate is preferably on the order of
about 1x1018 atoms per cubic centimeter, and the GaAs
growth step to form the layer 12, a P-type impurity such
as germanium or beryllium is diffused from the substrate
10 into a portion 13 of the epitaxial layer 12 to form the
PN junction 14 therein, as indicated by the dotted line in
FIG. 2. As compared to the relatively scarce and expen 40
sive element gallium required of conventional GaAs
substrate starting materials, aluminum is a widely avail
able and inexpensive element. Therefore, the use of
aluminum as a principal element of the compound semi
conductor substrate (instead of gallium) greatly reduces
in GA solution is initially doped with either Te or Sn to
a concentration of approximately 1017 atoms per cubic
centimeter or greater. During this LPE growth process,
a thin N-type 5 micrometer GaAs epitaxial layer of
10*3 ohm cm resistivity is grown on the P-type
AlP_25Sb_75 substrate, which substrate has a bandgap of
greater than 2.3 electron volts. By reducing the initial
solution growth temperature of about 750° C or greater
45 at a rate of 02° C per minute for about 5 minutes, a thin
N-type GaAs layer of about 5 micrometers can be
grown on the Be doped aluminum phosphide antimon
ide substrate, and the Te or Sn N-type doping of the
substrates as a starting material for the process.
GaAs in Ga solution provides a PN junction in the thin
The structure in FIG. 2 is then transferred to a con
ventional ohmic contact deposition station wherein an 50 GaAs epitaxial layer at a depth of between 0.2 and 0.5
micrometers from the substrate -epi layer interface.
N-type contact layer 16, such as a gold-germanium
After the formation of the basic PN junction structure
nickel alloy, is evaporated on the exposed surface of the
described above, the contacts and coating layers 16, 18
epitaxial layer 12. Finger-like electrodes 18 of P-type
solar cell processing costs as compared to the costs of
those prior art processes which utilize gallium arsenide
and 20 of FIGS. 3 and 14 are applied to the outer sur
metallization, such as a gold-zinc-gold alloy, are then
deposited using known and appropriate resist masking
55
faces thereof using standard metallization techniques.
techniques to cover only selected areas of the exposed
surface of the P-type window layer 10. A suitable anti
As an alternative to the above aluminum phosphide
antimonide substrate in the above example, an alumi
retlective (AR) coating 20, such as tantalum oxide,
num indium phosphide substrate of Al_40I_60P composi
tion and with a bandgap of greater than 2 electron volts
TA205, is deposited as shown in FIG. 3 between the
electrode members 18 and typically has an index of 60 can be used as the substrate starting material. Addition
ally, other binary compound substrate starting materials
refraction of about 2.1. Finally, the stucture of FIG. 3 is
such as aluminum nitride or boron nitride can be substi
encapulated in a suitable quartz glass (not shown) and
tuted for either of the above Al-including ternary com
this cover glass will preferably include a coating of
pounds withoutvdeparting from the scope of the present
magnesium ?uoride, MgFz, on the surface thereof to
minimize light re?ection at the outer surface of the 65 invention. That is, either the Al or B in the last named
binary compounds would provide the required P-type
device.
substrate conductivity for subsequent conversion to N
The structure of FIG. 3 may be bonded, as shown in
type by the Sn or Te doping of the GaAs in Ga solution.
FIG. 4, to a suitable metallic conducting support mem
5
4,107,723
Furthermore, the use of B and N in addition to Al in the
substrate starting material ‘can result in a higher sub
strate bandgap energy than that of the above ternary
6
3. The device de?ned in claim 2 which further in
cludes anti-re?ective coating means disposed between
selected contact metallization areas on the surface of
compounds. However, good crystal lattice matching
said substrate layer for minimizing re?ectivity thereat,
should be maintained between the alternative binary
compound substrate window material and the GaAs
5 and means adjacent said contact means for supporting
said solar cell in a desired position or array.
epitaxial layer deposited theron for optimum solar cell
4. A PN junction semiconductor device including:
performance. Thus, the universal abundance of B and N
a. a gallium-free P-type substrate layer selected from
the group of materials consisting of aluminum
makes the above binary compound substrate window
phosphide antimonide, aluminum arsenide, alumi
materials economically attractive for low cost solar cell
num indium phosphide, aluminum nitride and
fabrication for the same reason that the abundance of Al
' boron nitride, and
makes the Al-containing ternary compounds attractive
for this manufacture.
What is claimed is:
1. A semiconductor solar cell including, in combina 15
tion:
a. a P-type substratelayer selected from ,the group
consisting of aluminum phosphide antimonide, alu
minum arsenide,e aluminum indium» phosphide,
boron nitride and aluminum nitride, and I
b. a thin layer of Nv-type gallium arsenide disposed on
said substrate layer and having a photovoltaic PN
junction thereinfor generating power in response
to sunlight inpinging on said solar cell, whereby 25
said substrate layer serves as an ef?cient light trans
b. a thin layer of N-type gallium arsenide disposed on
said substrate layer' and having a PN junction
therein, whereby said device requires a minimum
amount of gallium in its fabrication, and solar cells,
light emitting diodes, laser diodes and other PN
junction devices may be formed as PN junction
structures utilizing ,a minimum of gallium.
‘ 5. The device de?ned in claim 4 which includes
means for providing electrical contacts at the outer
surfaces of said substrate layer and said gallium arsenide
layer, respectively.
i 6. A PN junction device comprising a gallium-free
B-type substrate window layer of a material selected
from the group consisting of aluminum phosphide anti
mitting window layer for said cell .and having a _ monide, aluminum arsenide, aluminum indium phos
high bandgap energy, and the gallium required to ' phide, boron nitride and aluminum nitride, and a thin
provide a photovoltaic gallium arsenide PN junc . layer of N-type gallium arsenide disposed on said sub
tion layer is minimized.
30 strate layer and having a PN junction therein, whereby
2. The device de?ned in claim 1 which further in
said device requires a minimum amount of gallium in its
cludes electrical contact means on the surfaces of said
fabrication, and solar cells, light emitting diodes, laser
substrate layer aridv said thin gallium arsenide layer,
respectively, for transmitting power from said solar cell
in response to sunlight inpinging thereon.v
diodes and other PN junction devices my be formed as
PN junction structures utilizing a minimum of gallium.
35
45
50
55
*
i
it
1!
t