Note: Descriptions are shown in the official language in which they were submitted.
FIELD OF TI~E INVEN'rION
! This invention relates to an improved process for the produc- !
tion of electrically-conductive layers which are highly transparen
to visible light and highly reflective to infrared light, and to
the particularly advantageous coatings formed therewith. Such
layers are useful as transparent electrodes for solar photovoltaic¦
cells, photoconductive cells, liquid crystal electro-optical dis-
plays, photoelectrochemical cells, and many other types of optical
electronic devices. As transparent electrical resistors, such
layers are used for defrosting windows in airplanes, automobiles,
etc. As heat-reflecting transparent coatings on glass, these
layers enhance the efficiency of solar thermal collectors and of
windows in buildings, ovens, furnaces, and sodium-vapor lamps,
and of fiberglass insulation.
~, I
i'
- 1 -
:~- . ., - .
,, . ' ' ~ , ' '
-: . . ,
:- ~ .. , ., . :
-,
' ~ ' .
~lZ1666
~ACKG]~)UNI) Ul llll; INV~NlION
Various metal oxidcs, such as stannic oxide SnO2, indium
oxide In2O3, and ca~lmium stannate Cd2SnO4, have been the most
~widely used materials for forming transparent, electrically con-
5 1l ductive coatings and layers.
The earliest methods of applying these coatings were based
on spraying a solution of a metal salt (usually the chloride) on
a hot surface, such as glass. In this way, satisfactory trans-
parent, electrically resistive layers were first made for de-icing ~
10 ,~aircraft windows. However, the spray process produced rather cor- !
rosive by-products, hot chlorine and hydrogen chloride gases, which
~tended to attack the hot glass surface, producing a foggy appear-
~ance. U. S. Patent 2,617,745 teaches that this undesirable effect
can be mitigated by first applying a coating of pure silica on the
15 ,glass. I-lowever, a silica protective layer is not very effective
ion glass with a high alkali content and high thermal expansion
coefficient, such as common soda-lime glass. In addition, these
corrosive by-products attack metal parts of the apparatusJ and
ithe metallic impurities, such as iron, may then be deposited in
the coating, with deleterious effects on both the electrical
! conductivity and transparency of the coating.
Another problem has been a lack of uniformity and reproduci-
bility in the properties of the coatings. U. S. Patent 2,651,585
Iteaches that better uniformity and reproducibility are obtained
25 'when t}le humidity in the apparatus is controlled. The use of a
vapor, rather than a liquid spray, as described for example in
German Patent 1,521,239, also results in more uniform and repro-
ducible coatings.
-2-
, .
~ S~-UUl
Z~6~6
llven ~ith these improvements, more recent studies have been
made US:illg vacuum cleposition techniques, such as evaporation and
sputtering, in order to achieve cleaner and more rcproducible
! coatings. Despite the much higher cost of these vacuum processes, ,
5 l,the reduction of corrosive by-products and unwanted impurities
~introduced by tl~e spray methods is felt to be important particu-
larly in applications involving high-purity semiconductors.
The intentional addition of certain impurities is important
in these processes, in order to achieve high electrical conducti-
10 ,vity and high infrared reflectivity. Thus, tin impurity is incor-
porated in indium oxide, while antimony is added to tin oxide
(stannic oxide) for these purposes. In each case the function of
jthese desirable impurities ("dopants") is to supply "extra" elec-
trons which contribute to the conductivity. The solubility of
these impurities is high, and they can be added readily using all
lof the deposition methods referred to above. Fluorine has an ad-
,vantage over antimony as a dopant for tin oxide, in that the trans-
parency of the fluorine-doped stannic oxide films is higher than
lthat of antimony-doped ones, particularly in the red end of the
20 ,Ivisible spectrum. This advantage of fluorine is important in
potential applications to solar cells and solar thermal collectors.
Despite this advantage of fluorine, most -- and perhaps all---
commercially available tin oxide coatings use antimony as a dopant.l
iPossibly this is because fluorine doping has only been demonstratedl
25 l~in the less satisfac~ory spray method, whereas the improved deposi-
tion methods (chemical vapor deposition, vacuum evaporation and
sputtering) are not believed to have been shown to produce fluorine
doping. In addition, a recent report by a committee of experts in
,
- 3-
i '
7 __ _ ___ _
-
~zi666
the American Institute of Physics Conference Proceedings No. 25,
p. 288 (1975), concludes that fluorine equilibrium solubility in
tin oxide is inherently lower than that of antimony. ~etherthe-
less, it is noted that the lowest resistivity tin oxide films
reported in the prior art are those of United States Patent
3,677,814 to Gillery. Using a spray method, he obtained fluor-
ine-doped tin oxide films with resistances as low as 15 ohms per
square by utilizing a compound, as a starting material, which
has a direct tin-fluorine bond. The lowest resistance in a
commercially available tin-oxide coated glass is presently in
the range of about 40 ohms per square. r~hen one wishes to ob-
tain coatings of as low as 10 ohms per square, one has hereto-
fore been forced to use the much more expensive materials like
indium oxide.
According to the invention, there is provided a pro-
cess for depositing transparent, fluorine-doped, tin-oxide films,
on a heated substrate said process comprising the steps of
(1) supplying a continuous stream of a reagent gas to
the vicinity of said substrate, said reagent gas containing
reagents which are convertible to a tin fluoride compound having
a direct tin-fluorine bond in the immediate proximity of said
heated substrate, and
(2) depositing said tin fluoride compound along with
a reaction product of the oxidizable tin component of said
reagent gas at the surface of said substrate and thereby achiev-
ing a fluorine-doped, tin oxide coating upon said surface.
The invention also provides an article of manufacture
comprising a substrate bearing a coating of fluorine-doped stannic
oxide, said substrate being selected from generally transparent
substrates and substrates of the type used in semi-conductors,
said coating being characterised by having a reflectivity of
about 85~ to 10 micron infra-red radiation when the substrate
B - 4 -
llZ16~6
is generally transparent and by having a resistance of less than
about 5 ohms per square and a bulk resistivity in said coating
of about 10 4 ohm-cm when the substrate is of the type used in
semi-conductors.
~ particular feature of the invention is to select
the reactants in such a way that the required tin-fluorine bond
is not formed until the deposition is imminent. Thus, the tin
fluoride material is better maintained in the vapor phase and at
temperatures low enough that oxidation of the compound occurs
only after the rearrangement to form a tin-fluorine bond. Films
of fluorine-doped tin oxide, thus formed, have extraordinarily
low electrical resistivity and extraorinarily high reflectivity
to infrared wavelengths.
., - 5 -
7~
11216~i6
Pre Fe ra b i~
S~J The process of the invention is~carried out utilizing a
gaseous mixture containing a volatile, organotin~
fluorine-bearing compound which is free of any direct
tin-fluorine bond. This mixture also contains a volatile
oxidizable tin compound and an oxidizing gas. This first
fluorine compound which is free of a fluorine-tin bond is
converted into a second organotin fluoride compound having such
a bond. Immediately after such conversion this second compound
is oxidized to form a fluorine dopant and the dopant is oxidized
along with the oxidizable tin compound to form a stannic oxide
film with a controlled amount of fluorine impurity on said solid
substrate.
k 52-O()l
.~ _
llZ1666
In a Eirst form of the invcntion, an organo-tin mono-fluoride
vapor is Lormed in the heated deposition region by the reformation
~of the vapor oÇ a more volatile compound containing both tin and
fluoroalkyl groups bonded to tin.
~ second advantageous embodiment of the invention utilizes an
organo-tin monofluoride formed at or near the gas-substrate inter-
face by reactions involving an organo-tin vapor and certain
fluorine-containing gases having fluoroalkyl and/or fluorosulfur
Igroups.
10 I The product layer in each case is a uniform, hard, adherent,
transparent coating whose electrical conductivity and infrared
reflectivity depend on the concentration of the fluorine-containing
dopant.
~IN Tl-IE DI~AWINGS
Figure 1 shows a schematic diagram of an apparatus suitable
for carrying out a process in wllich a fluorine dopant is an organo-
tin fluoroalkyl vapor, evaporated from its liquid form.
Figure 2 S1lows a similar diagram for the second embodiment,
in which the fluorine dopant is formed by reaction with certain
20 ! fluoroalkyl and/or fluorosulfur gases supplied from a compressed
gas cylinder.
Figure 3 shows a simplified version of the apparatus for
practicing either the first or the second embodiments of the
linvention.
Figure 4 is a schematic section of a solar cell and illustrates
llone use of the invention in a semicoIlductor application.
~ igure 5 shows window 120 coated with layer 118 according to
thc inveIltioll.
i, .
,, '
K 5
~lZ1~66
Figures 6 ancl 7 are graphs illustrative of varying conducti- ¦
vity allcl re~lectivi~y with concelltrations of fluorine dopant. The
process of this invelltion has two main steps: (1) forming a re-
active vapor mixture whicll will produce, on,heating, a compound
5 Ihaving a tin-fluorine bond, and (2) bringing this vapor mixture
to a heated surface, on which fluorine-cloped tin oxide deposits.
Tlle eml)odimellts clescribed below cliffer in the chemical source of
the fluorine dopant in the reactive vapor mixture, and also in
'the means by which the vapor mixture is made. I`he second step
10 ~I(deposition on the heated surface) is largely the same in each
l~example.
i l`he tin is supplied by a volatile, oxidizable tin compound,
~such as tetramethyltin, tetraetllyltin, dibutyltin diacetate, di-
j!methyltin dihydride, dimethyltin dichloride, etc. The preferred
15 ~compound is tetramethyltin, since it is sufficiently volatile at
¦room temperature, non-corrosive, stable and easily purified. This
volatile tin compound is placed in a bubbler marked 10 in the
T:igures, and an inert carrier gas, such as nitrogen, is bubbled
througll the tin compound. ~or the very volatile compouncds, such asl
20 Itetramethyltin and climethyltin dihydride, the bubbler can be at
room temperature, while for the other less volatile compounds, the
bubbler and the tubing must be heated appropriately, as will be
;understood by those skilled in the art. It is one advantage of
l~the instant invention that high-temperature apparatus can be
25 ~avoided and that simple cold-wall supplies can be used.
l`he vapor mixture must contain an oxidizing gas, such as
oxygen, nitrous oxide, or the like. Oxygen is the preferred gas,
since it is reaclily available and works just as well as the more
expensive altcrnate oxiclizers.
7-
llZ16~;6
'l`he pressures of the gases are Lixed by the regulators 25,
and thc ~low ratcs of the oxygen from tank 20, and of the carrier
gas rom tank 21, are controlled by metering vales 30, and measured
by 1Owllleters 40. The gas streams then pass through one-way check
5 Ivalves 50 into a mixing tube 60 and funnel-shaped chamber 70. A l,
tin oxide film deposits on the hottest surface 80, which is heated ¦
by the heater 90, typically to temperatures about 4000 to 6000C.
The general type of process just described is commonly known
lin the art as chemical vapor deposition. Various modifications,
10 Isuch as having the substrate surfaces vertical and rotating or
below the reaction chamber and rotating, are known to those skilled
in the art, and may be particularly suitable for use depending upon
the geometry of the substrate or other conditions affecting a given
jiapplication.
15 ~ Rotation of the substrate is recommended in order to best move
,i
,the sample through any convection currents which may occur in the
apparatus and thereby best assure the uniformity of the deposited
layers. Ilowever, it has now been discovered that, by placing the
iheated substrate facing downwardly, highly uniform coatings may be
obtained more simply without rotation, because the gas, when heated
from above, does not set up troublesome convection currents.
Another advantage of having the substrate above the reactive vapors
;is that any dust or dirt, or powder by product formed by homogeneous
'nucleation in the gas, does not fall onto the growing film.
25 ! An invention described herein is an improved process by which !
controlled amounts of fluorine impurity may be introd-lced into the
growing tin oxide film. In the simplest aspect of this invention,
the fluorine dopant is a vapor containing one tin-fluorine bond in
;
... . . . . . .
112~..6ti6
each molecule. The other three tin valences are satisfied by
organic groups and/or halogens other than fluorine. Typical of
such compounds is tributyltin fluoride. It has been discovered
that the fluorine thus bound, and made available to a hot surface
in vapor form, is not cleaved from the tin during oxidation at a
hot surface.
Unfortunately, all known compounds with such a direct tin-
fluorine bond are not significantly volatile near room temperature.
A particular advantage of the invention is achieved by form-
ing the fluorine dopant from volatile compounds which do not have
the required tin-fluorine bond, but which will rearrange on heating
to form a direct tin-fluorine bond. This rearrangement advanta-
geously occurs at temperatures high enough (e.g. > 100C) so that
the tin fluoride thus formed remains in the vapor phase, but also
low enough (e.g. < 400C) so that the oxidation of the compound
occurs only after the rearrangement. One example of such a com-
pound is trimethyl trifluoromethyltin, (CH3)3SnCF3. On heating to
a temperature of about 150C in a heated zone adjacent to the
deposition surface 80, this compound rearranges to form a direct
tin-fluorine bond, in (CH3)3SnF vapor, which then reacts as the
fluorine donor or dopant. Other compounds which undergo similar
rearrangements at temperatures which will, of course, differ some-
what from compound to compound, have the general formula R3SnRF,
where R is a hydrocarbon radical, and RF is a fluorinated hydro-
carbon radical having at least one fluorine atom bonded to that
carbon atom which is bonded to the tin. The main advantage of
these fluorine dopants is that they are volatile liquids, so that
they can easily supply sufficient vapor pressure when evaporated
.
K 5~-U01
.~ ._
11~16~6
at room temperature. Illis simplifies thc desigll of thc apparatus,
as sllown in ligure 1, by elimillatillg the need for maintaining a
warm zone between tlle bubblcr 15 and the reaction chamber 70, to
keep the 1uorine dopallt in the vapor phase. Thus the apparatus
5 Ican be o the type wllicll is usually called a "cold-wall chemical
vapor deposition reactor," which is widely used, for example, in
¦ the semicon~uctor industry to deposit silicon, silicon dioxide,
silicon nitride, etc. Anotller important feature of the "cold-wall
reactor" for semiconductor applications is that it minimizes un-
10 Iwanted impurities at a low level in both the substrate and the
deposited film. Similarly, in glass manufacture, the gas mixture
can be added to the annealing and cooling oven at the stage when
the glass is at the appropriate temperature, e.g. about 470C for
Isoft gIass. In this way, highly uniform films can be achieved in
15 Ithe normal glass-production equipment.
The preferred compound for use in the embodiment of Figure 1
is (Cl13)3SnCF3, since it is more volatile than the compounds with
more carbon atoms. It is a stable, colorless, non-corrosive
liquid, W]liC]I does not decompose in air at room temperature, and
only reacts extremely slowly with water.
A particular advantageous second embodimellt of the invention
luses a fluorine-containing gas which reacts with an organo-tin vapor
! on heating, to produce a tin fluoride vapor. For example,
,;~-fluoroalkyl halides, preferably wherein the alkyl group has 4
carbons or fewer, such gases as iodotrifluoromethane, CF3I,
CF3CF2I, C3F7I, and tlle like, can be mixed Wit]l organo-tin vapors
such as tetrametllyltin vapor ~CH3)4Sn, at room temperature, i.e.
to 90F, and more preferably to temperatures of 150F, without
any reaction. Moreover, fluoroalkyl bromides like C~3Br, C2~sBr
-10-
i
.
1121666
~and ~lle like are llsoruL as ~luorille-~olltaillillg gases. Ihey are
:Iess re.lctive al~(l abo~ o 2~ tilnes nlolc alc re(~ e(l il~ tlle re-
actant gas, I)ut they are mucll less expensive. lhis is particularly
surprising because of tlle reputed inertness for such compounds.
Iluoroal~yl chlorides are not favored for use because their reacti-
vity is substantially lower than even the bromides.
IVhen such a vapor mixture approaches the heated surface, re-
action takes place in the gas phase to, event~ually, produce the
desired tin-fluoriJIe bonds. Although the reaction sequence is
complex, it is believed to begin by reactions such as
CF3I + R~SII ~ 3SIlCF3 + RI
to yield the organo-tin fluoroalkyl R3SnCF3 vapors in the region
near the inter-face of the hot surface, where they serve as fluorine
dopants for the growing tin oxide film, just as in the first embodi!-
ment.
Certain other fluorine-containing gases also function in
this second embodiment of the invention. For example, sulfur
chloride pentafluoride, SF5Cl, is an effective fluorine donor gas,
as is sulfur bromide pentafluoride SF5Br.
20 1 In a similar way, tri fluoromethyl sulfur pentafluoride CF3SF5
gas acts to form tin-fluoricle bonds by gas phase reactions.
I}le advantage of this second embodiment is that the fluorine
donor is a gas, and the process is further shown in Figure 2. The
Ipreferred gases are CF3I and CF3Br, which are non-corrosive, non-
25 ~flammable, not appreciably toxic, and readily available commerciall?.S1-5Cl and SF5Br and highly toxic, and thus are less desirable for
use. Cl3SF5 is noll-toxic, but somewhat less reactive thaJI CF3I.
.
`iF~ ~
,~
llZ1666
l`llc dcpos:itioll process may be rurther simplified, as shown
;n l~igure 3, if thc gas mixtures are pre-mixed and stored in a
compresse(l gas cylinder 19. For safe storage and use, the oxidiz-
able compoulld must of course be kept at a concentration such that
S 'it cannot form an explosive mixture. For e~am~le, the lower explo-
`sion limit Or~ tetramethyltin in air is about . The concentra-
tions whicll have used for the chemical vapor depositions are
less than a ~ ~ o-f this level. In addition, the use of CF3I
or CF3Br as a fluorine dopant incidentally acts as a flame suppres-
sant.
Films prepared according to the invention are found to haveinfrared reflectivities of 90% and more measured, as is known in
the art, at the convention 10-micron wave length of light which is
characteristic of thermal infrared radiation at room temperature.
Tllis 90% reflectivity is to be compared to the 80~ reflectivity
,whicll is heretofore achieved using tin oxide coatings. In usual
practice, these inLrared reflective layers will be from about 0.2
to 1 micron in thickness; thicknesses of 0.3 to 0.5 microns are
typical.
In order to characterize more quantitatively the fluorine
doping levels in the films, the infrared reflectivity was measured
over the wavelengtll range of 2.5 microns to 40 microns. By fitting
these data with theoretical curves, as described in detail by
R. Groth, E. Kauer and P. C. van den Linden, "Optical Effects of
~ree Carriers in SnO2 l.ayers," Zeitschrift fur Naturforschung,
Volume 179, pages 7~9 to 793 ~1962), values were obtained for the
free electron concentratioll in the films. The values obtained were
in the range from 102 cm~3 to 1021 cm~3, and increased regularly
with incrcasing concentrations of the fluorine (~opant. Theoreti-
cally, OIIC free clectron should be released for each fluorine atom
~ ~ 2 () ~) 1
112i666
wlli.c]l replaces an oxyge]l atom in the lattice. This hypothesis wasveriied by ~uger l.lectron Spectroscopic measurements of the total
fluorinc concentratioll in some oE the films, which gave fluorine
concentrations in agreement with the free electron concentrations,;
to within the experimental uncertainties. This agreement is indi- ,
cative that most of the incorporated fluorine is electrically
active.
The infrared reflectivity at 10 microns and also the bulk
lelectrical conductivity of the films, were found to be maximum at
10 Il`a doping level of about 1.5-2% fluorine substitution for oxygen.
The maxima are very broad, and almost maximum conductivities and
reflectivities are shown by films with 1% to 2.5% fluorine. There
is also a weak, broad absorption throughout the visible wavelength
range, which increases directly with fluorine concentration.
Therefore, to prepare films with high electrical conductivity and
,high visible transparency, a fluorine concentration in the film
of about 1% ~i.e., fluorine to oxygen ratio .Ol in the film) is
most desirable. I~owever, this optimum will vary somewhat, depend- ,
ling on the spectral distribution of interest in a given applica-
tion. By varying the fluorine dopant concentration, routine ex-
perimentation can easily establish the optimum percentage for any
given application.
Fluorine doping levels exceeding 3% can easily be achieved in
the films, using the methods of the instant invention. Prior art
25 ,results had not exceeded 1%, and the opinion, cited above, was
that this was the solubility limit for fluorine. I~hile such high
doping levels are not needed to produce optimum infrared reflecti-
vity or electrical conductivity, the gray films produced at doping
levels of 2% or more may be useful on architectural glass, for
-l3
: -
.,, . -
llZi666
~limitillg solar heat gain in air-conditioned buildings. In such
applica~ions, the doping level at the surface of the film advan-
tageously is reduced to about 2~, in order to have maximum infra-
red reflectivity.
` 5 ' Using the measured electron concentrati~ons and electrical
~conductlvities, the electron drift mobilities can be obtained.
~For various films, values from 50 to 70 cm2/Volt-sec were calcula-
ted in this way. Previously obtained mobilit~ values for tin oxide
Ifilms have ranged from 5 to 35 cm2/Volt-sec. It is believed the
lO linstant films are the first to have suc]l mobilities exceeding
i40 cm2/Volt-sec. These values illustrate, in another way, the
superior quality of the process of this invention and of the films
prepared therewith.
The process of the invention is also highly desirable for
15 Iuse in making novel devices such as those having electroconductive
,layers in semiconductor manufacture (e.g. integrated circuits and
the like), and also the manufacture of heat-reflective transparent
; objects like windows. - j
l`he most advantageous mode of the invention is that wherein the
organo-tin fluQride compound having a tin-fluorine bond is decom-
posed at the substrate immediately after form~tion. I`his decompo- !
sition is preferably in a narrow reaction zone which is largely
heated to the decomposition temperature by heat from the substrate
itself.
"
' ' ,
-14-
, - , - . : , ~
llZ16~i6
I ILLU~'l`Rl\'~'IVl~ Ol)[~ll.N'I' Ol 'l'lll. INVI~N'I'ION
In or~er to point out more fully the nature of the present
invelltlon, the following examples arc given as illustrative embodi-
lments of the present process and products p~oduced thereby.
' Unless otherwise specified, the specific examples disclosed
below are carried out according to the following general procedure:
.I~.xa
T}le process is exempli~ied by an cxperiment using the appa-
l'ratus of ~igure 1 to produce a gas stream which contains 1%
¦tetrametllyltin ~CI-l3)4Sn, o.n2OO trimcthyl trifluoromethyltin
(Cll3)3snc~3~ 10o nitrogen carrier gas, and balance oxygen gas.
~he resulting stream is passed over a pyrex glass platc which iS
~15 Clll in diameter and maintained at 500C for about a 5 minute
ideposition period. Ihe flow rate of the gas stream is about 400 cc
'per minute. This flow rate is such that the gas turnover rate in
funnel 70 is al)out once each two minutes. A transparent film
ahout l micron thick is de~osited. It shows electrical res-istance
I
of 2 ohms per square, corresponding to a volullle resistivity o-f
l0.0002 ohm-clll. Ihis film is measured to have a Iluorine to oxygen
ratio o~ abollt .nl7 and a drift mobility of about 50 cm2/Volt-sec.
_xamplc 2
IVhell the process of Example 1 is repeated using a sodium free
silicon substrate, the resistance value dropped to about 1 ohm per
Isquare, i.e. about one-hal~ the value of the resistivity achieved
Wit}l a sodium-bearing substrate.
!
!
1, i
."
.. . .
, '.
-15-
:
. . .
...
~ ... : . . . .
.:....... . . :.
11216~6
lc 3
~ n adValltagCOUS pl`OCCSS is illustratcd by a process utilizing ¦
th~ ap1);1r~1tl1s o~ urc 2. Thc rcsulting gas mixturc consists of
l~ tetr11mcthylti1l (C113)4Sn, 0.2% iodotrifluoromethane C~31, 20~ i
nitrogen carrier gas, balance oxygen. T:ilms grown on pyrex glass
substrates showed the same clectrical charac~eristics as in
Examp]e l.
Example 4
1he simplified apparatus in Figure 3 is ~sed by forming the
mixture describecl in Example 3, in a compressed gas cylinder l9.
T11e results are identical to those of Example 3. After a month of
~storage in tlle gas cylinder, the experiment was repeated, giving
~ identical results. This demonstrates the stability and shelf life
; of this mixture.
, i
Exam~lc 5
Example 3 is repeated, except that when the stan1lic oxide
film is 0.5 microns thick, deposition is stopped. The re- ¦
sulting stannic oxide fil1n llas an infra-red reflectivity Or
otlt 90%.
i~ .
~ ~ 20 ~xamples 6-13
, ~ ,
~ ~ I ` lhe following gases each are substituted, in equi-molecular
, .
portions, for Cl3I in the procedure of Example 3 (excepting that
the concentration of fluorine dopants is incrcased l5 times in
examples 6, 7, 8 and 13.) Excellent conductivity and infra-red
- 25 relÇectivity are achieved: 1
. i
,, ~ . I
't ::
~. .
,, I
-16-
I
~ '
'.".~ . ' ' " ' ' ~ ~ ' '
, ~, . : . ':
llZ1666
EY~ t' (,-ls 1.. ~ )1c (,~s
6 Cl~31~r lO c3l7
7 C2l~5]~1 11 Sl7513r
8 C31:713r 12 S~5Cl
9 C2J:5 r . 13 C~3S~5
Conventional silicon photovoltaic cells,("Solar cells")
have heretoore comprise~ typical surface resistances of 50 to 100 ¦
ohms per s~!uare. In order to have an acceptably low total cell
resistallce, a metallic gri~ witll a spacillg oE.l or 2 millimeters
is deposite~l on tlle silicon surface. By ~epositing a fluorine-
~ope~ tin-oxide layer witll a sheet resistance of about 0.5 ohms
per square (about 2 microns thick) on the cell surface, the metallic
grid spacing can be increased to about lO millimeters, with a
correspollding reduction in the cost of t]le grid. Alternatively,
the grid size can be kept small, and the cell is able to function
eEEiciently even when t]le sunlight has been concentrated by a
factor of ahout lO0, provided aclequate cooling of the cell is
maintained.
A schematic section lO0 of such a cell is s11own in Figure 4
wherein a 2-~icron layer 102 of n-Sn~2 (the fluorine-doped
material of the invention is used), a 0.4 micron layer 104 ol` n-
silicon ~phospllorous-dope~ silicon as ~nown to the art), a O.lmm
p-silicon layer lOG (~oron-doped silicon as known to the art) are
joined with an aluminum layer 108 serving as an electrode.
iMetallic grids llO are speced about lO millimeters apart. Yet an
excellent performance is achievod.
. , ~
-17-
I
' ' , '' , ' '` ~
11;~ ;6
The deposited layers can be used in manufacture of other
semiconductor articles, e.g. conductors or resistors. Tin-oxide
coatings have been so used in integrated circuits before. The
improved conductivity will allow wider application of this material
in the future. Not only is the sheet resistance range extended
; to much lower values (e.g. about 5 ohms per square or less? than
herefore possible, but also deposition of the layer can be achieved
within the same apparatus which is used, for example, to grow
epitaxial silicon. This eliminates the costly and cumbersome un-
lO ; loadlng, cleaning, and loading steps between depositions.
The resistivity values obtained for the fluorine-doped tin-
oxide on silicon substrates is about 10 4 ohm-cm, which is ;~
comparable to that of evaporated tantalum metal, which is sometimes
.
used for connections in integrated circuits. The good match be-
tween thermal expansion coefficients of tin-oxide and silicon
allows deposition of thick layers without significant strains.
Figure 6 shows the electrical conductivity of the fluorine-
doped stannic oxide films as a functïon of measured fluorine to
oxygen ratio in the films, for deposition temperatures of 480C and
500C.
Figure 7 shows the infra-red reflectivity of the fluorine-
doped stannic oxide films as a function of measured fluorine to
oxygen ratio in the films, for deposition temperatures of 480C
and 500 & .
Also indicated on Figures 6 and 7 are ~1) the conductivity of
the expensive indium-oxide materials known to the art and as described
in Philips Technical Review, Vol. 29, Page 17 ~1968) by van Boort
and Groth and ~2) the best alleged prior art values for conductivity
and reflectivity of doped stannic oxide coatings.
:
- 18 -
X
` `-`.~ 11216~;6
~ lthougll scvcral emho(l~ cllts o~ tllc prescnt invention havc
beell ~Icscrihccl an~l illustratc~l, it will he apparent to those
skille(l in thc art that various cllangcs and further modiEications
may be ma(lc thereill Wit]lout departure from the s~irit of the in-
vention or from the scol~e of the appen~e~ claims.
, I
` I
~ : .
:
: ~ :
:
, ~
;~; :
~: :
, ~ ,
,
.
~ I
~ !
~ . I
1.
~ . " .
.. ,.... . . , ``
.. - ~ , ~. .
