Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
9~6
This invention relates t:o a method of making
amorphous alloys having an increased band gap and
devices made therefrom. The invention has i~s
mos~ important application in making improved
photoresponsive alloys and devices having large
band gaps at least in a portion thereof for spe--
cific ~hotoresponsive applications including pho-
toreceptive devices such as solar cells of a p-i-n,
p-n, Schottky or MIS (metal-insulator-semicon~
ductor) type; photoconducting medium such as uti-
lized in xerography; photodetecting devices and
photodiodes including large area photodiode ar-
raysO
Silicon is the ba~is of the huge crystalline
semiconductor industry and is the material which
has produced expensive high efficiency (18 per
cent) crystalline solar cells for space applica-
tions. When crystalline semiconductor technology
reached a commercial state, it became the founda-
tion of the present huge semiconductor devicemanufacturing industry. This was due to the abil-
ity of the scientist to grow substantially defect-
free germanium and particularly silicon crystals,
and then turn them into extrinsic materials with
p-type and n-type conductivity regions therein.
--1--
~ , ~
This was accomplished by diffusing into such crys-
talline material parts per million of donor (n~ or
acceptor (p) dopant materials introduced as
substitutiona] impurities into the substantially
pure crystalline materials, to increase their
electrical conductivity and to control their being
either of a p or n conduction type. The fabri
cation processes for making p-n junction crystals
involve extremely complex, time consuming, and
expensive procedures. Thus, these crystalline
materials useful in solar cells and current con-
trol devices are produced ~nder very carefully
controlled conditions b~ ~rowing individual sinyle
silicon or germanium crystals, and when p-n junc-
tions are required, by doping such single crystalswith extremely small and critical amounts of dop-
ants.
These crystal growing processes produce such
relatively small crystals that solar cells require
the assembly of many single crystals to encompass
the desired area of only a single solar cell panel.
The amount of energy necessary to make a solar
cell in this process, the limitation caused by the
size limitations of the silicon crystal and the
necessity to cut up and assemble such a crystal-
--2~
line material, have all resulted in an impossible
economic barrier to the large scale use of crys-
talline semiconductor solar cells for energy con-
version. Further, crystalline silicon has an
indirect optical edge which results in poor liqht
absorption in the material. Because of the poor
light absorption, crystalline solar cells have to
be at least 50 microns thick to absorb the inci-
dent sunlight. Ev~n if the sin~le crystal mate-
rial is replaced by polycrystalline silicon withcheaper production processes, the indi~ect optical
edge is still maintained; hence the material thickness
is not reduced The polycrystalline material also
involves the addition o grain boundaries and
other problem defects.
An additional shortcoming of the crystalline
material, for solar applications, is that the
crystalline silicon band gap of about 1.1 eV in-
herently is below the optimum band gap of about
~ 1.5 eV. The admixture of germanium, while pos-
sible, further narrows the band gap which further
decreases the solar conversion efficiency.
In summary, crystal silicon devices have
fixed parameters which are not variable as de-
sired, require large amounts of material, are only
--3--
. 8 J~
producible in relatively small areas and are ex-
pensive and time consuming to produce. Devices
based upon amorphous silicon can eliminate these
crystal silicon disadvantages. Amorphous 5il icon
has an optical absorption edcle having properties
similar to a direct gap semiconductor and only a
material thickness of one micron or less is neces-
sary to absorb the same amount of sunlight as the
50 micron thick ~rystalline siliccn. Further,
amorphous silicon can be made faster, easier and
in larger areas than can crystal silicon.
Accordingly, a considerable effort has been
made to develop processes for readily depositing
amorphous semiconductor alloys or films, each of
which can encompass relatively large areas, if
desired, limited only by the si~e of the deposi-
tion equipment, and which could be readily doped
to form p-type and n-type materials where p-n
junction devices are to be made therefrom equiva-
lent to those produced by their crystalline coun-
terparts. For many years such work was substan-
tially unproductive. Amorphous silicon or ger-
manium (Group IV) films are normally four-fold
coordinated and were found to have microvsids and
S dangling bonds and other defects which produce a
--4--
high density o~ localized states in the energy gap
-thereof. The presence of a high density of local-
ized states in the energy gap o amorphous silicon
semiconductor films results in a low degree of
5 photocQnductivity and short carrier lietlme,
making such films unsuitable for photoresponsive
applications. Additionally, such films cannot be
successfully doped or otherwise modified to shift
the Fermi level cl~se to the conduction or valence
bands, making them unsuitable for making p-n junc-
tions for solar cell and current control device
applications.
In an attempt to minimize the aforementioned
problems involved with amorphous silicon and ger-
manium, W~E. Spear and P.G. LeComber of Carnegie
Laboratory o~ Physics, University of Dundee, in
Dundee, Scotland, did some work on "Substitutional
~oping of Amorphous Silicon", as reported in a
paper published in Solid State Communications,
Vol. 17, pp. 1193-1196, 1975, toward the end of
reducing the localized states in the energy gap in
amorphous 5il icon or germanium to make the same
approximate more closely intrinsic crystalline
silicon or germanium and o substitutionally dop-
ing the amorphous materials with suitable classic
--5--
dopants, as in doping crystalline materials, to
make them extrinsic and of p or n conduction types.
The reduction of the localizec] states was
accomplished by glow discharge deposition of amor
phous silicon films wherein a gas of silane (SiH~)
was passed through a reaction tube where the gas
was decomposed by an r.f. glow disçharge and de
posited on a substra~e at a substrate temperature
of about 500 600 ~ (2~7-327 C). The ma~erial so
deposited on the substrate was an intrinsic amor-
phous material consisting of silicon and hydrogen.
To produce a doped amorphous material a gas of
phosphine (PH33 for n-type conduction or a gas of
diborane (B2H6) for p~type conduction were pre-
mixed with the silane gas and passed through theglow discharge reaction tube under the same oper-
atiny conditions. The gaseous concentra~ion of
the dopants used was between about 5 x 10-6 and
10-2 parts per volume. The material so deposited
including supposedly substitutional phosphorus or
boron dopant and was shown to be extrinsic and of
n or p conduction type.
While it was not known by these researchers,
it is now known by the work of others that the
5 hydrogen in the silane combines at an optimum
--6--
temperature with many of the dangling bonds of the
silicon during khe glow discharge deposition, to
substantially reduce the density of the localized
states in the energy gap t~ard the end of making
the electronic properties of the amorphous mate-
rial approximate more nearly those of the cor-
respon~ing crystalline materlal.
In working with a similar method o~ glow
discharge fabricated amorphous silicon solar cells
utilizing silane, D.E. Carlson attempted to uti-
lize germanium in the cells to narrow the optical
gap toward the optimum solar cell value o~ about
1.5 eV from his best ~a~ricated solar cell mate-
rial which has a band gap of 1.65-1.70 eV. (D.E.
Carlson, Journal of Non Crystalline Solids, Vol.
35 and 36 (1980) pp. 707-717, given at 8th Inter-
national Conference on Amorphous and Liquid Semi-
Conductors, Cambridge, Mass., Aug. 27-31, 1979).
However, Carlson has further reported that the ad-
dition of germanium from germane gas was unsuc-
cessful because it causes significant reductions
in all of the photovoltaic parameters of the solar
cells. Carlson indicated that the degradation of
photovoltaic proper~ies indicates that defects in
the energy gap are being created in the deposi~ed
--7--
films. (D.E. Car~son, Tech. Dig. 1977 IEDM, Wash-
ington, D.C., p. 214)~
In the Tech. Dig. article, above referenced,
Carlson also reported the adclition of impurity
gases, such as N2 and CH4. C'arlson concludes that
these gases "have little effect on the photo-
voltaic properties eve~ when they constitute 10~
oE the discharge atmosphere," but 30~ of CH~ causes
degradation of the photovoltaic properties. No
suggestion is made by Carlson that the addition of
these gases can increase the band gap of the re-
sulting material. Carlson does state in the first
referenced article that the development of a bor-
on-doped "wide band gap, highly conductive p-type
material" is desirable, but made no suggestion as
to which of "several additives" should be utilized
to open the band gap. Carlson further stated that
"there is no evidence to date that the material
can be made highly conductive and p~type."
The incorporation of hydrogen in the above
silane method not only has limitations based upon
the fixed ratio of hydrogen to silicon in silane,
but/ most importantly, various Si:H bonding con-
figurations introduce new antibonding states which
5 can have deleterious consequences in these mate-
-8-
rials. Therefore, there are basic limitations inreducing the density of localized states in these
materials which are particularly harmful in terms
of effective p as well as n cloping. ~he resulting
density of states of the ~ilane deposited mate-
rials leads to a narrow depletion width, which in
turn limits the efficiencies of solar cells and
other devices whos~ operation depends on the drift
of free carriers. The method of making these
materials by the use of only sil~con and hydrogen
also results in a high density of surface states
which affects all the above parameters. Further,
the previous attempts to decrease the band gap of
the material while successful in reducing the gap
have at the same time added states in the ~ap.
The increase in the states in the band gap results
in a decrease or total loss in photoconductivity
and is thus counterproductive in producing photo~
responsive devices.
After the development of the glow discharge
deposition of silicon from silane gas was carried
out~ work was done on the sputter depositing of
amorphous silicon films in the atmosphere of a
mixture of argon (required by the sputtering depo-
sition process) and molecular hydroyen, to deter-
_g
mine the results of such molecular hydrogen on thecharacteristics of the depos:ited amorphous silicon
film. This research indicated that the hydrogen
acted as a compensating agent which bonded in such
a way as to reduce the localized states in the
energy gap. EI~wever, the degree to which the
localized states in the energy gap were reduced in
khe sputter deposition process was much less than
that achieved by the silane deposition process
described above. The above described p and n
dopant gases also were introduced in the sput-
tering process to produce p and n doped materials.
These materials had a lower doping efEiciency than
the materials produce~ in the glow discharge pro-
cess. Neither process produced efficient p-doped
materials with sufficiently high acceptor concen-
trations for producing commercial p-n or p-i-n
junction devicesO The n-doping efficiency was
below desirable acceptable commercial levels and
~0 the p-doping was particularly undesirable since it
reduced the width of the band gap and increased
the number of localized states in the band gap.
The prior deposition of amorphous silicon,
which has been altered by hydrogen from the silane
gas in an attempt to make it more closely resemble
--10--
crystalline silicon and which has been doped in a
manner like that of doping crystall.ine silicon,
has characteristics which in all important re-
spects are inferior to those of doped crystalline
silicon. Thus, inadequate doping efficiencies and
conductivity were achieved especially in the p-
type materialv and the photovoltaic qualities of
these silicon films left much to be desired.
Greatly improved amorphous silicon alloys
having significantly reduced concentrations of
localized states in the energy gaps thereof and
high quality electronic properties have been pre-
pared by glow discharge as fully described in U.S.
Patent No. 4,226,~98, Amorphous Semiconductors
Equivalent to Crystalline Semiconductors, Stanford R.
Ovshinsky and Arun Madan which issued October 7,
1980, and by vapor deposition as fully described
in U.S. Patent No. 4~217~374v Stanford R. Ovshinsky
and Masatsugu Izu, which issued on August 12,
1~80, under the same title. As disclosed in these
patents, fluorine is introduced into the amorphous
silicon semiconductor to substantially reduce the
density of localized states therein.
Activated 1uorine especially readily dif-
fuses into and bonds to the amorphous silicon in
--11--
the amorphous body, substantially to decrease the
density of localized defect states therein, be-
cause the small size of the lluorine atoms enables
them to be readily introducecl into the amorphous
body. The fluorine bonds to the dangling bonds oE
the silicon and forms what i'3 believed to be a
partially ionic stable bond with flexible bonding
angles, which results in a more stable and moLe
efficient compen~a~i~n or alteration than i5 form-
ed by hydrogen and other compensating or alteringagents. Fluorine is considered to be a more ef-
ficient compensating or altering element than
hydrogen when employed alone or with hydrogen
because of its exceedingly small size, hi~h re-
lS activity, specificity in chemical bonding, andhighest electronegativity. Hence, fluorine is
qualitatively different from other halogens and so
is considered a super-halogen.
As an example, compensation may be achieved
with fluorine alone or in combination with hydro-
gen with the addition of these element(s) in very
small quantities (e.g., fractions of one atomic
percent). However, the amounts of fluorine and
hydrogen mc>st desirably used are much greater than
5 such small percentages so as to form a silicon-
-12-
8~
hydrogen-fluorine alloy. Such alloying amounts of
fluorine and hydrogen may, for example, be in the
range of 1 to 5 percent or greater. It i8 be-
lieved that the new alloy so formed has a lower
density of defect states in l:he energy gap than
that achieved by the mere neutralization of dan-
gling bonds and similar defect states. 5uch larger
amount of fluorine, in particular, is believed to
participate substantially in a new struc~ural
configuration of an amorphous silicon-containing
material and facilitates the addition of other al-
loying materials. Fluorine, in addition to its
other characteristics mentioned herein, is be-
lieved to be an organizer of local structure in
the silicon-containing alloy through inductive and
ionic effects. It is believed that fluorine also
influences the bonding of hydrogen by acting in a
beneficial way to decrease the density of defect
states which hydrogen contributes while acting as
a density of states reducing element. The ionic
role that fluorine plays in such an alloy is be-
lieved to be an important factor in terms of the
nearest neighbor relationships.
The non op~imum spectral response of prior
art amorphous silicon photoresponsive devices is
overcome in accordance with t:he present invention
by adding one or more band gap increasing elements
to an amorphous photoresponsive alloy at least in
one or more regions thereof t:o adjust the band gap
to an increased utiliæation width for particular
applications without substantially increasing the
deleterious states in the gap. Thus, the high
quality electronic properties of the material are
not suhstantially affected in forming the new
increased band gap adjusted alloy~
The amorphous alloy preferably incorpora~es
at least one density of states reducing element,
fluorine. The compensating or altering element,
fluorine, and/or other elements can be added dur-
ing deposition or thereafter. The band gap in-
creasing element(s) can be activated and may be
added in vapor deposition) sputtering or glow
discharge processes. The band gap can be in-
creased as required for a specific application byintroducing the necessary amount of one or more of
the adjusting elements into the deposited alloy in
at least one region thereof. The band gap is in-
creased without substantially increasing the num-
-14-
2~
ber of states in the band gap of the alloy and
devices, because of the presence of fluorine in
the alloy.
The presence of fluorine in the alloy of the
invention provides a silicon alloy which differs
physically, chemically and e:Lectrochemically from
other silicon alloys because fluorine not only
covalently bonds to the silicon but also affects
in a positive manner the structural short range
order of the material. This allows increasing
elements/ such as carbon or nitrogen, effectively
to be added to the alloy, because fluorine forms
the stronger and more stable bonds than does hy-
drogen. Fluorine compensates or alters silicon as
well as the band increasing element(s) in the
alloy more efficiently than hydrogen, because of
the stron~er more thermally stable bonds and more
flexible bonding configura~ions due to the ionic
nature of the fluorine bonding. The use of fluo-
rine produces the alloy or film, described in U.S.Patent No. 4,217,374, in which the density of
states in the band gap are much lower than those
produced by a combination of silicon and hydrogen,
such as from silane. Since the band increasing
5 element~s) has been tailored into the material
15-
without adding substantial deleterious states,because of the influence of fluorine, the new
alloy maintains high quality electronic qualities
and photoconductivity when the adjusting ele-
ment(s) are added to tailor the wavelength thresh-
olcl for a specific photoresponse application.
Hydrogen further enhances the fluorine compensated
or al~ered alloy and can be added during deposi-
tion with fl-lorin~ or after deposition, as can
fluorine and other alterant elements. The post
deposition incorporation of hydrogen is advan-
tageous when it is desired to utilize the higher
deposition substrate temperatures allowed by Eluo-
rine.
While the principles of this invention apply
to each of the aforementioned deposition pro-
cesses, for purposes of illustration herein a
vapor and a plasma activated vapor deposition
environment are described. The glow discharge
system disclosed in U.S. Patent No. 4,226,898, has
other process variables which advantageously can
be utilized with the principles of this invention.
We have found that the above disadvantages
may be overcome if the silicon containing amor-
phous alloy includes at least fluorine to reduce
the density of states therein, to thereby allow
the inclusion of one ~r more band gap increasing
el~ments without substantially increasing the
states in the gap. The alloy thus may have a band
gap with an increased utilization width for use in
various devices including, for example/ p-n and
p-i-n, Schottky, or MIS solar cells, photo-detec-
tors and electrostatic image producing devices.
-17-
The preferred embodiment of this invention
will now be described by way of example with re-
ference to the drawings accompanying this specifi-
cation in which:
Fig. 1 is a dlagrammatic representation of
more or less conventional vacuum deposition equip-
ment to wHich has been added elements for carryi.ng
out the addition of fluorine (and hydrogen) by the
addition of molecular or ~luorine compounds ~on~
taining fluorine such as SiF~, and hydrogen inlets
and activated fluorine and hydrogen generating
units which decompose the molecular fluorine and
hydrogen within the evacuated space of the vapor
deposition equipment, to convert molecular fluo-
rine and hydrogen to activated fluorine and hy-
drogen and to direct one or both against the sub-
18-
~t~ 7
strate during the deposition of an amorphous alloycontaining silicon;
Fig. 2 illustrates vacuum deposition equip-
ment like that shown in FigO 1, with activated
Eluorine ~and hydrogen) ~enerating means com-
prising an ultraviolet light ~ource irradiatiny
the substrate during the process of depositing the
amorphous alloy, such light source replacing the
activat~d fluorine and hydrogen generator units
shown in Fig. 1 an~ increasing element generating
means;
Fig. 3 illustrates the vacuum deposition
equipment for Fig. 1 to which has been added addi-
tional means for doping the depositing alloy with
an n or p conductivity producing material;
Fig. 4 illustrates an application wherein the
deposition of the amorphous alloy and the applica-
tion of the activated fluorine and hydrogen may be
carried out as separate steps and in separate
enclosures;
Fig. 5 illustrates exemplary apparatus for
diffusing activated hydrogen into a previously
deposited amorphous alloy;
Fig. 6 is a fragmentary sectional view of an
embodiment of a Schottky barrier solar cell to
-19-
illustrate one application of the amorphous semi-
conductor photoreceptive alloys made by the pro-
cess of the invention;
Fig. 7 is a fragmentary sectional view of a
p-n junction solar cell device which includes a
doped amorphous semiconductor alloy made by the
process of the invention;
Fig. 8 is a fragment.ary sectional view of a
photodetection device which includes an amorphous
semiconductor alloy made by the process of the
nventlon;
Fig. 9 is a fragmentary sectional view of a
xerographic drum including an amorphous semi-
conductor alloy made by the process of the inven-
lS tion;
Fig. 10 is a fragmentary sectional view of a
p-i-n junction solar cell device;
Fig. 11 is a fragmen~ary sectional view of an
n-i-p junction solar cell device;
Fig. 12 is a diagrammatic representation of a
plasma activated vapor deposition system for depo-
siting the amorphous alloys with the increasing
element(s) of the invention incorporated therein;
and
-20-
Fig. 13 is a solar spectral irradiance chart
illustrating the standard sunlight wavelengths
available or various photoresponsive applica--
t.ions.
Referring now more particularly to Fig. 1,
there is shown vapor deposition equipment general-
ly indicated by reference numeral 10, which may be
conventional vapor deposition equipment to which
is added an activated~ compensating or altering
material injecting means to be described. This
equipment, as illustrated, includes a bell jar 12
or similar enclosure enclosing an evacuated space
14 in which is located one or more crucibles like
crucible 16 containing the amorphous semiconductor
film-producing element or elements to be deposited
on a substrate 18. In the form of the invention
being described, the crucible 16 initially con-
tains silicon for forming an amorphous alloy con-
taining silicon on the substrate 18 which, for
example, may be a metal, crystalline or poly-
crystalline semiconductor or other material uponwhich it is desired to form the alloy to be depo-
sited by the process of the present invention~ An
electron beam source 20 is provided adjacent to
the crucible 16, which electron beam source, dia-
-21-
grammatically illustrated, usually includes a
heated filament and beam deflection means (not
shown) which directs a beam of electrons at the
silicon contained in the crucible 16 to evaporate
the same.
A high voltage DC power supply 22 provides a
suitable high voltage, for example, 10,000 volts
DC, the~ positive terminal of which is connected
through a control u~it 24 and a conductor 26 to
the crucible 16. The negative terminal of
which is connected through the control unit 24 and
a conductor 28 to the filament of the electron
beam source 20. The control unit 24 including
relays or the like for interrupting the connection
lS of thé power supply 22 to the conductors 26 and 28
when the film thickness of an alloy deposition
sampling unit 30 in the evacuated space 14 reaches
a given value set by operating a manual control 32
on a control panel 34 of the control unit 24. The
alloy sampling unit 30 includes a cable 36 which
extends to the control unit 24 which includes well
known means for responding to both the thickness
of the alloy deposited upon the alloy sampling
unit 30 and the rate of deposition thereof. A
5 manual control 38 on the control panel 34 may be
-~2-
'7
provided to fix the desired rate of deposition of
the alloy controlled by the amount of current fed
to the filament of the electron beam source through
a conductor 40 in a well known manner.
S irhe substrate 18 is ~a~ried on a substrate
holder 42 upon which a heater 44 is mounted. A
cable 46 feeds energizing current to the heater 44
which controls the temperature of the substrate
holder 42 and substrate 18 in accordance with a
temperature set~ing set on a manual control 48 onthe control panel 34 of the control unit 24.
The bell jar 12 is shown extending upwardly
from a support base 50 from which ~he various
cables and other connections to the components
within the bell jar 12 may extendO The ~upport
base 50 is mounted on an enclosure 52 to which
connects a conduit 54 connecting to a vacuum pump
56. The vacuum pump 56, which may be continuously
operated, evacuates the space 14 within the bell
jar 12. The desired pressure of the bell jar is
set by a control knob 58 on the control panel 34.
In this form of the invention, this setting con-
trols the pressure level at which the flow of
activated fluorine or fluorine and hydrogen into
5 the bell jar 12 is regulated. Thus, if the con-
-23-
2~
trol knob is set to a bell jar pressure of 10-4
Torr, the flow of fluorine or fluorine and hy-
drogen into the bell jar 12 will be such as to
maintain such pressure in the bell jar as the
vacuum pump 56 continues to operate.
~ ources 60 and 62 of molecular fluoritle and
hydrogen are shown connected through respective
conduits 64 and 66 to the control unit 24. A
pressure ~ensor 6~ in the bell jar 12 i5 connected
by a cable 70 to the control unit 24. ~low valves
72 and 74 are controlled by the control unit 24 to
maintain the set pressure in the bell jar. Con-
duits 76 and 78 extend from the control unit 24
and pass through the suppvrt base 50 into the
evacuated space 14 of tne bell jar 12. Conduits
76 and 78 r~spectively connect with activated
fluorine and hydrogen generating units 80 and 82
which convert the molecular fluorine and hydrogen
respectively to activated fluorine and hydrogen,
~ which may be atomic and/or ionized forms of these
gases. The activated f`luorine and hydrogen gen-
erating units 80 and 82 can be heated tungsten
filaments which elevate the molecular gases to
their decomposition temperatures or a plasma gen-
erating uni.t well known in the art for providing a
-24-
plasma of decomposed gases. Also, activated fluo-
rine and hydrogen in ionized forms Eormed by plas-
ma can be accelerated and injected into the de-
positing alloy by applying an electric field be-
S tween the substrate and the activating source. Ineither event, the activated fluorine and hydrogen
generator units 80 and 82 are preferably placed in
the immediate vicinity of the substrate 18, so
that the relativel~ ~hort-lived activated fluorine
and hydrogen delivered thereby are immediately in-
jected into the vicinity of the substrate 18 wherethe alloy is depositing. The activated fluorine
or fluorine and hydrogen ~s well as other com-
pensating or altering elements also can be pro-
duced from compounds containing the elements in-
stead of from a molecular gas source.
As previously indicated, to produce usefulamorphous alloys which have the desir~d charac~
teristics for use in photoresponsive devices such
as photoreceptors, solar cells, p-n junction cur-
rent control devices, etc.~ the compensating oraltering agents, materials or elements produce a
very low density of localized states in the energy
gap without changing the basic intrinsic character
of the film. This result is achieved with rela~
-25-
tively small amounts of activated fluorine and
hydrogen so that the pressure in the evacuated
bell jar space 14 can still be a relatively low
pressure (like 10~4 Torr). The pressure of the
gas in the generator can be higher than the pres-
sure in the bell jar by ad~u~sting the size of the
outlet of the generator.
The temperature of the substrate 18 is ad-
justed to obtain the maxim~m reduction in the
density of the localized states in the energy gap
of the amorphous alloy involved. The substrate
surface temperature will genera].ly be such that it
ensures high mobility of the depositing materials;
and preferably one belo~ the crystalliæation tem-
perature of the depositing alloy.
The surface of the substrate can be irra-
diated by radiant energy to further increase the
mobility of the depositinq alloy material, as by
mounting an ul~raviolet light source (not shown)
in the bell jar space 14. Alternatively, instead
of the activated fluorine and hydrogen generator
units 80 and 82 in Fig. 1, these units can be
replaced by an ultraviolet light source 84 shown
in Fig. 2, which directs ultraviolet energy against
5 the substrate 18. This ultraviolet light will
-25-
decompose the molecular fluorine or fluorine andhydrogen both spaced from and at the substrate 18
to form activated fluorine (and hydrogen) which
diffuses into the depositing amorphous alloy con-
densinq on the substrate 18. The ultravioletlight also enhances the surface mobility of the
depositing alloy materlal.
In Figs. 1 and 2, the band gap increasing
elements can be added in gaseous form i~ an iden-
tical fashion to the fluorine and hydrogen byreplacing the hydrogen generator 82 or by adding
one or more activated increasing element gene-
rators 86 and 88 (Fig. 2). Each of the generators
86 and 88 typically will be dedicated to one o
the increasing elements such as carbon or nitro-
gen. For example, the generator 86 could supply
carbon as in the form of me~hane gas (CH4).
Referring now to Fig. 3, it is illustrated
the additions to the equipment shown in Fig. 1 for
adding other agents or elements to the depositing
alloy. For example, an n-conductivity dopant,
like phosphorus or arsenic, may be initially added
to make the intrinsisally modest n-type alloy a
more substantially n-type alloy, and then a p-
S dopant like aluminum, gallium or indium may be-27-
2~:~'7
added to form a good p-n junction within the al-
loy. A crucible 90 is shown for receiving a dop~
ant like arsenic which is evaporated by bombarding
the same with an electron beam source 92, like the
beam source 20 previously de~3cribed. The rate at
which the dopant evaporates :into the atmosphere of
the bell jar 12, which is determined by the in-
tensity of the ele~tron beam produced by the elec-
tron beam source 9~, i5 set by a manual control 94
on the control panel 34, which controls the cur~
rent fed to tlle filament forming part of this beam
source to produce the set evaporation rate. The
evaporation rate is measured by a thickness sampl-
i~g unit 96 upon which the d~pant material de-
posits and which generates a signal on a cable 98extending between the unit 96, and control unit
24, which indicates the rate at which the dopant
material is deposited on the unit 96.
After the desired thickness of amorphous
alloy having the desired degree of n-conductivity
has been deposited, evaporation of silicon and the
n-conductivity dopant is terminated and the cru-
cible 90 (or another crucible not shown) is pro-
vided with a p conductivity dopant described, and
the amorphous alloy and dopant deposition process
-28-
~3~
then proceeds as before to increase the thickness
of the amorphous alloy with a p-conductivity re-
gion therein.
The band increasing element~s) also can be
added by a similar process to that described for
the dopant by utilizing another crucible similar
to t:he crucible 90.
In the case where the amorphous alloys com-
prise two or more elements which are s~lid at room
temperature, then it is usually desirable to sepa~
rately vaporize each element placed in. a separate
crucible, and control the deposition rate thereof
in any suitable manner, as by setting controls on
the control panel 34 which, in association with
the deposition rate and thickness sampling units,
controls the thickness and composition of the
depositing alloy.
While activated fluorine (and hydrogen) are
believed to be the most advantageous compensating
agents for use in compensating amorphous alloys
including siliconl in accordance with broader
aspects of the invention, other compensating or
altering agents can be used. For example, oxygen
may be use:Eul in reducing the density of localized
states in the energy gap when used in small amounts
_~9_
so as not to change the intrinsic characteristic
of the alloy.
As previously indicated, altho~gh it i5 pre-
ferred that compensating and other agents be in-
corporated into the amorphous alloy as it is de-
posited, in accordance with ano~her aspect of the
lnventiorl, the amorphous alloy deposition process
and the pro¢ess of injecting the compensating and
other agents into the semiconductor all~y can be
done in a completely separate environment from the
depositing of the amorphous alloy. This can have
an advantage in certain applications since the
conditions for injecting such agents are then
completely independent ~f the conditions for the
alloy deposition. Also, as previously explained,
if the vapor deposition process produces a porous
alloy, the porosity of the alloy, in some cases,
is more easily reduced by environmental conditions
quite different from that present in the vapor
deposition process. To this end, reference should
now be made to Figs. 4 and 5 which illustrate that
the amorpho~s deposition process and the com-
pensating or altering agent diffusion process are
carried out as separat~ steps in completely dif-
ferent envi:ronments, Fig. 5 illustrating appara~us
-30-
3~
for carrying out the post compensation diffusion
process.
As there shown, a low pressure container body
100 is provided which has a ;Low pressure chamber
102 having an opening 104 at the top thereof.
This openirlg 104 is close~ by a cap 106 having
threads 108 which thread around a corresponding
threaded portion on the exterior of the ontainer
body 100. ~ sealing O-ring 110 is sandwiched
between the cap 106 and the upper face of the
container body. A sample-holding electrode 112 is
mounted on an insulating bottom wall 114 of the
chamber 100. A sub~trate 116 upon which an amor-
phous semiconductor alloy llB has already been
deposited is placed on the electrode 112. The
upper face of the substrate 116 contains the amor-
phous alloy 118 to be altered or compensated in
the manner now to be described.
Spaced above the substrate 116 is an elec-
trode 120. The electrodes 112 and 120 are con-
nected by cables 122 and 124 to a DC or RF supply
source 12~ which supplies a voltage between the
electrodes 112 and 120 to provide an activated
plasma of the compensating or altering gas or
5 gases, such as fluorine, hydrogen, and the like,
-31-
fed into the chamber 107. For purposes of sim-
plicity, Fig. 5 illustrates only molecular hydro-
gen being fed into the chamber 102 by an inlet
conduit 128 passing through t:he cap 106 and ex-
tending from a supply tank 1-l0 of molecular hydro-
gen. Other compensating or altering gases (such
as fluorine and the like) also may be similarly
fed into the chamber 102. The conduit 128 is
shown connected to a valve 132 near the tank 130.
A flow rate indica~ing gauge 134 is shown con-
nected to the inlet conduit 128 beyond the valve
132.
Suitable means are provided for heating the
interior of the chamber 102 so that the substrate
temperature is elevated pref~rably to a temper-
ature below, but near the crystallization temper-
ature of the film 118. For e~cample, coils of
heating wire 136 are shown in the bottom wall 114
of the chamber 102 to which coils connect a cable
(not shown) passing through the walls of the con-
tainer body 100 to a source of current for heating
the same.
The high temperature togethér with a plasma
of yas contai.ning one or more compensating ele-
ments developed between the electrodes 112 and 120
~32-
'7
achieve a reduction of ~he localized states in the
band gap of the alloy. The compensating or alter-
ing of the amorphous alloy 1.18 may be enhanced by
irradiating the amorphous al.loy 118 with radiant
energy from an ultraviolet l.ight source 138, wh.ich
is shown outside of the container body 100 di-
recting ultraviolet light between the elec-trodes
112 and 120 through a quartz window 140 mounted in
the side wall of the container body 100.
The low pressure or vacuum in the chamber 102
can be developed by a vacuum pump (not shown) such
as the pump 56 in Fig. 1. f~he pressure oE the
chamber 102 can be on the order of O3 to 2 Torr
with a substrate temperature on the order of 2Q0
to 450C. ~he activated fluorine (and hydrogen)
as well as other compensating or altering elemen~s
also can be produced from compounds containing the
elements instead of from a molecular gas source,
as previously mentioned.
Various applications of the improved amor-
phous alloys produced by the unique processes of
the invention are illustrated in Figs. 6 through
11. Further, the alloys and devices of the pre
sent invention can be utilized with or in other
devices or configurations such as, for example, in
a multiple cell device.
-33-
Fig. 6 shows a Schottky barrier solar cell
142 in fragmentary cross-section. The solar cell
142 includes a substrate or electrode 144 of a
material having good electrical conductivity prop-
erties, and the ability o~ making an ohmic contactwith an amorphous alloy 146 compensated or altered
to provide a low density of localized states in
the energy gap and with a band yap optimized by
the processes of the present invention~ The sub-
strate 144 may comprise a low work function metal,such as aluminum, tantalum, stainless steel or
other material matching with the amorphous alloy
146 deposited thereon which preferably includes
silicon, compensated or altered in the manner of
the alloys previously described. It is most pre-
ferred that the alloy have a region 148 next to
the electrode 144, which region forms an n~ con-
ductivity, heavily doped, low resistance interface
between the electrode and an undoped relatively
high dark resistance region 150 which is an in-
trinsic, but low n-conductivity region.
The upper surface of the amorphous alloy 146
as viewed in Fig. 6, joins a metallic region 152,
an interface between this metallic region and the
5 amorphous alloy 146 forming a Schottky barrier
-34-
154. The metallic region 152 is transparent orsemi-transparent to solar radiation, has good
electrical conductivity and is of a high w~rk
function (for example, 4.5 eV or greater, pro-
duced, for example, by gold, platinum, palladium,etc.) relative to that oE the amorphous alloy 146.
The metallic region 152 may be a single layer of a
metal or it may be a multi-layer. The amorphous
alloy 146 may have a thickness of about .5 to 1
micron and the metallic region 152 may have a
thickness of about 100A in order to be semi-trans-
parent to solar radiation.
On the surface of the metallic region 152 is
deposited a grid electro~e 156 made of a metal
having good electrical conductivity. The grid may
comprise orthogonally related lines of conductive
material occupying only a minor portion of the
area of the metallic region, the rest of which is
to be exposed to solar energy. For example, the
grid 156 may occupy only about from 5 to 10% of
the entire area of the metallic region 152. The
grid electrode 156 uniformly collects current from
the metallic region 152 to assure a good low series
resistance for the device.
35-
An anti~ref.lection layer 158 may be applied
over the grid electrode 156 and the areas of the
metallic region 152 between the grid electrode
areas. The anti-reflectiorl layer 158 has a solar
radiation incident surface 160 upon which impinges
the solar radiation. For exalDple/ the anti-re-
flection la~er 158 may have a thickness on the
order of magnitude of the wavelength of the max-
imum energy point of the solar radiation spectrum,
divided by four times the index oE refraction of
the anti-reflection layer 158 If the metallic
region 152 is platinum of lOOA in thickness, a
suitable anti-reflection layer 158 would be zirco-
nium oxide of about 500A in thickness with an
index of refraction of 2.1.
The band increasing element(s) are added to
the photocurrent generating region 150 at least in
a portion thereof such as adjacent the region 152.
The Schottky barrier 154 formed at the interface
between the regions 150 and 15~ enables the pho-
tons from the solar radiation to produce current
carriers in the alloy 146! which are collected as
current by the grid electrode 15~. An oxide layer
~not shown) can be added between the layers 150
and 152 to produce an MIS (metal insulator semi-
conductor) solar cell.
-36-
In addition to the Schottky barrier or MIS
solar cell shown in Fig. 6, there are solar cell
constructions which utilize p-n junctions in the
body of the amorphous alloy Eorming a part thereof
formed in accordance with successlve deposition,
compensating or altering and doping steps like
that previously described. These other forms of
solar cells are generically illustrated in Fig. 7
as well as in Figs. 10 and 11.
These constru~t,ions 162 generally include a
transparent electrode 164 tnrough which the solar
radiation energy penetrates into the body of the
solar ce]l involved. Between this transparent
electrode and an opposite electrode 166 is a de-
posited amorphous alloy 168, preferably including
silicon, initially compensated in the manner pre
viously described. In this amorphous alloy 168
are at least two adjacent regions 170 and 172
where the amorphous alloy has respectively oppo-
sitely doped regions, region 170 being shown as an-conductivity region and region 172 being shown
as a p-conductivity region. The' doping of the
regions 170 and 172 is only sufficient to move the
Fermi levels to the valence and conduction bands
involved so that the dark conductivity remains at
-37-
a low value. The alloy 168 has high conductivity,
highly doped ohmic contact interface regions 174
and 176 of the same conductivity type as the adja-
cent region of the alloy 168. The alloy regions
174 and 176 contact electrodes 164 and 166, re
spectively. The increasing element(s) are added
to regions 174 and also be added to region 172.
Referring now to Fig. 8, there is illustrated
another applicatioh of an am~rphous alloy utilized
in a photo-detector devic~ 178 whose resistance
varies with the amount of light impinging thereon.
An amorphous alloy 180 thereof is band gap in-
creased and compensated or altered in accordance
with the invention, ha~ no p n junctions as in the
e~bodiment shown in Fig. 7 and is located between
a transparent electrode 182 and a substrate elec-
trode 184. In a photo-detector device it is de~
sirable to have a minimum dark conductivity and so
the amorphous alloy 180 has an undoped, but com-
pensated or altered region 186 and heavily dopedregions 188 and 190 of the same conductivity type
forming a low resistance ohmic contact with the
electrodes 182 and 184, which may form a substrate
for the alloy 18Q. The increasing element(s) are
5 added at least to the region 188.
-38-
LL~2~
Referring to Fig. 9 an electrostatic imageproducing device 192 (like a xerography drum) is
illustrated. The device 192 has a low dark con-
ductivity, selective wavelength threshold, undoped
or slightly p-doped amorph~us alloy 194 deposited
on a suitable substrate 196 !~uch as a drum. The
increasing ele~ent(s) are added to the alloy 194
at least near the outer region thereof.
As used herein, the terms cornpensating agents
or materials and altering agents, elements or
materials mean materials which are incorporated in
the amorphous alloy for altering or changing the
structure thereof, such as, activated flucrine
(and hydrogen) incorporAted in the amorphous alloy
containing silicon to form an amorphous sili-
con/fluorine/hydrogen composition alloy, having a
desired band gap and a low density of localized
states in th~ energy gap. The activated fluorine
(and hydrogen) is bonded to the silicon in the
alloy and reduces the density of localized states
therein and due to the small size of the fluorine
and hydrogen atoms they are both readily intro-
duced into the amorphous alloy without substantial
dislocation of the silicon atoms and their rela
5 tionships in the amorphous alloy~ This is true
-39-
most particularly because of the extxeme electro~neyativity, specificityr sma]1 size and reactivity
of fluorine, all of which characteristics help
influence and organize the local order of the
alloys. In creating this new alloy the strong
inductive powers of fluorine and its ability to
act as an organizer of short range order is of im-
portance. The ability of fluorine to bond with
both silicon and hydrogen results in the formation
of new and superior alloys with a minimum of lo-
calized defect states in the energy gap. Hence,
fluorine and hydrogen are introduced without sub-
stantial formation of other localized states in
the energy gap to form the new alloys~
Referring now to Fig. 10, a p-i-n solar cell
198 is illustrated having a substrate 200 which
may be glass or a flexible web formed from stain-
less steel or aluminum. The substrate 200 is of a
width and length as desired and preferably at
least 3 mils thick. The substrate has an in-
sulating layer 202 deposited thereon by a conven-
tional process such as chemical deposition, vapor
deposition or anodizing in the case of an aluminum
substrate. The layer 202 for instance, about 5
5 microns thick can be made of a metal oxide. For
-40-
an aluminum substrate, it preferably is aluminum
oxide (A12O3) and for a stainless steel substrate
it may be silicon dioxide (SiO2) or other suitable
glass.
An electrode 204 is deposited in one or more
layers upon the layer 202 to form a base electrode
for the cell 198. The electrode 204 layer or
layers is deposited by vapor deposition, which is
a relatively fast ~position process. The elec-
trode layers preferably are reflective metal elec-
trodes of molybdenum, aluminum, chrome or stain-
less steel for a solar cell or a photovoltaic
device. The reflective electrode is preferable
since, in a solar cell, non-absorbed light which
passes throu~h the semiconductor alloy i5 re-
flected from the electrode layers 204 where it
again passes through the semiconductor alloy which
then absorbs more of the light energy to increase
the device efficiency.
The substrate 200 is then placed in the de-
position environment. The specific examples shown
in Figs. 10 and 11 are illustrative of some p-i-n
junction devices which can be manufactured uti-
lizing the improved methods and materials of the
5 invention. Each of the devices illustrated in
-41-
Figs. 10 and 11, has an alloy body haviny an over-
all thickness of betwe~n about 3,000 and 30,000
angstroms. This thickness ensures that there are
no pin holes or other physical defects in the
structure and that there is maximum light ab-
sorption efficiency. A thicker material may ab-
~vrb more light, but at some thickness will not
generate more current since ~he greater thickness
allows more recombination of the light generated
electron-hole pairs. (It should be understood
that the thicknesses of the various layers shown
in Figs. 6 through 11 are not drawn to scaleO)
~ eferring first to forming the n-i-p device
198, the device is formed by first depositing a
heavily doped n+ alloy layer 206 on the electrode
20A. Once the n+ layer 206 is deposited, an in
trinsic (i) alloy layer 208 is deposited thereon.
The intrinsic layer 208 is followed by a highly
doped conductive p+ alloy layer 210 deposited as
the final semiconductor layer. The alloy layers
206, 208 and 210 form the active layers of the
n-i-p device 198.
While each of the devices illustrated in
Figs. 10 and 11 may have other utilities, they
5 will be now described as photovoltaic devices.
-~2-
Utilized as a photovoltaic device, the selectedouter p~ layer 210 is a low :Light absorption, high
conductivity alloy layer. The intrinsic alloy
layer 208 which can have an :increased band gap
near the p~ layer 210 for ~ solar photoresponse,
high light absorption~ low dark conductivity and
high photoconductivity incluc3ing sufficient amounts
of the increasing element(s) to widen the band gap
as desired. The bottom alloy layer 204 is a low
1~ light absorption, high conductivity n+ layer. The
overall device thickness ~etween the inner surface
of the electrode layer 206 and the top surface of
the p+ layer 210 is, as stated previously, on the
order of a~ least about 3,000 angstroms. The
thickness of the n+ doped layer 206 is preferably
in the range of about 50 to 500 angstroms. The
thickness of the intrinsic alloy 208 is preferably
between about 3,000 angstroms to 30,000 angstroms.
The thickness of the top p~ contact layer 210 also
is preferably between about 50 to 500 angstroms.
Due to the shorter diffusion length of the holes,
the p+ layer generally will be as thin as possible
on the order of 50 to 150 angstroms. Further, the
outer layer (here p+ layer 210) whether n+ or p~~
will be kept as thin as possible to avoid absorp-
tion of light in that contact layer.
-43-
'7
In this device as well as the p~n junctiondevice of Fig. 7, the p~ layer (174 or 210) is
utilized as a contact layer and not to absorb
sunlight. Therefore it should function as a win-
dow to allow the sunlight to pass through to beabsoebed in the deple-tion region of the device.
In addition to makin~ the layer thin it also should
have a large ban~ gap. By adding the increasing
element(s) also to the region adjacent the p~
contact layer the devices increase sunlight ab-
sorption and hence Jsc and also provide some in-
crease in VOC. A second type of p-i~n junction
device 212 is illustxated in Fig. 11. In this
device a first p+ layer 214 is deposited on the
electrode layer 204' followed by an intrinsic
amorphous alloy layer 216 an n amorphous alloy
layer 218 and an outer n~ amorphous alloy layer
220. The layers 216, 21~ and 220 each can contain
~he band gap increasing element (5) . Further,
although the intrinsic alloy layer 208 or 216 (in
Figs. 10 and 11) i5 an amorphous alloy the other
layers are not so restricted and may be poly-
crystalline, such as layer 214. (The inverse of
the Figs. 10 and 11 structure not illustrated,
also can be utilized.)
-44-
Following the deposition of the various semi-
conductor alloy layers in the desired order for
the devices 198 and 212, a further deposition step
is performed, preferably in a separate deposition
environment. Desirably, a ~por deposition envi-
ronment is utilized since it is a fast deposition
process. In ~his stepJ a TCO layer 222 (trans-
parent conductive oxide) is added which, for exam-
ple, may be indium tin oxide jITO)~ cad~ium stan-
nate (Cd2SnO~, or doped tin oxide (SnO2~. TheTCO layer will be added following the post com-
pensation of fluorine (and hydrogen) if the films
were not deposited with one or more of the desired
compensating or altering elements therein. Also,
the other compensating or altering elements, above
described, can be added by post compensation.
Rn electrode grid 224 can be added to either
of the devices 198 or 212 if desired. For a de-
vice having a sufficiently small area, the TCO
layer 222 is generally sufficiently conductive
such that the grid 224 is not necessary for good
device efficiency. If the device is of a suf-
ficiently large area or i the conductivity of the
TCO layer 222 is insufficient, the grid 224 can be
5 placed on the layer 222 to shorten the carrier
-45-
path and increase the conduction efficiency of thedevices.
Referring now to Fig. 12, one embodiment of a
plasma activated vapor deposition chamber 226 i9
illustrated in which the semiconductor and band
increasing element(s) of the invention can be
depo~ited. A control unit 228 is utilized to
control the deposition parameters, such as pres-
sure, flow rates, e-~c., in a manner similar to
that previously described with respect to the unit
24 (Fig. l). The pressure would be maintained at
about 10 3 Torr or less.
One or more reaction gas conduits, such as
230 and 232, can be u~ilized to supply gases such
as silicon tetrafluoride (SiF4) and hydrogen (~12)
into a plasma region 234. The plasma region 234
is established between a coil 236 fed by a DC
power supply (not illustrated) and a plate 238.
The plasma activates the supply gas or gases to
supply activated fluorine (and hydrogen) to be
deposited on a substrate 240. The substrate 240
may be heated to the desired deposition temper-
ature by heater means as previously described.
The band increasing element(s) and silicon
5 can be added from two or more evaporation boats,
-~6-
such as 242 and 244. The boat 242 could for ex-
ample contain carbon and the boat 244 would con-
tain silicon. The elements in boats 242 and 244
can be evaporated by electroll beam or other heat~
ing means and are activated by the plasma.
If it is desired to layer the band increasing
element(s) in ~he photogeneratin~ region of the
film heing deposited, a shutter 246 can be uti-
lizedO The shutter could rotate layering separate
band increasing elements from tw~ or more of the
boats or can be utilized ~o control the depositing
of ~he band increasing element from the boat 242
(or others) to provide layers in the film or to
vary the amount of band increasing element de-
p~sited in the film. Thus, the band increasingelement(s~ can be added discretely in layers, in
substantially constant or in varying amounts.
Fig. 13 illustrates the available sunlight
spectrum. Air Mass O (AMO~ being the sunlight
available with no atmosphere and the sun directly
overhead. AMl corresponds to the same situation
after filtering by the earth's atmosphere. Crys
talline silicon has an indirect band gap of about
1.1 to 1.2 eV, which corresponds to the wavelength
of about 1.0 micrometer (microns). This equates
~47~
to losing, i.e. not generating useful photons, for
substantially all the light wavelengths above 1.0
microns. As utilized herein, band gap or E opti-
cal is defined as the extrapolated intercept of a
plot of (~ ~)1/2, where ~ is the absorption co-
efficient and ~ L~ ~or e) is t:he photon energy.
For light having a wavelength above the threshold
deEined by the band gap, the photon energies are
not sufficient to generate a photocarrier pair and
hence do not add any current to a specific device.
Each of the device semiconductor alloy layers
can be glow discharge deposited upon the base
electrode substrate by a conventional glow dis-
charge chamber described in the aforesaid U.S.
15 Patent No. 4,226,898. The alloy layers also can
be deposited in a continuous process. In these
cases, the glow discharge system initially is
evacuated to approximately 1 mTorr to purge or
eliminate impurities in the atmosphere from the
deposition system. The alloy material preferably
is then fed into the deposition chamber in a com-
pound gaseous form, most advantageously as silicon
tetrafluoride (SiF~) hydrogen (H2) and methane
(CH4). The glow discharge plasma preferably is
5 obtained from the gas mixture. The deposition
-4~-
system in U.S. Patent No. 4,226,898 preferably is
operated at a pressure in the range of about 0.3
to 1.5 Torr, preferably between 0.6 to 1.0 Torr
such as about 0.6 Torr.
The semiconductor mate~ial is deposited from
a sel~-sustained plasma onto the substrate which
is heated, preEerably by infrared means to the
desired deposition temperature for each alloy
layer. The doped layers of ~he devices are de-
posited at various temperatures in the range of
200C to about 1000C, depending upon the form of
the material used. The upper limitation on the
substrate temperature in part is due to the type
of metal substrate utilized. For aluminum the
upper kemperature should not be above about 600C
and for stainless steel it could be above about
1000C. For an initially hydrogen compensated
amorphous alloy to be produced, such as to form
the intrinsic layer in n-i p or p-i-n devices, the
substrate temperature should be less than about
400C and preferably about 300C.
The doping concentrations are varied to pro-
duce the desired p, p+, n or n+ type conductivity
as the alloy layers are deposited for each device.
For n or p doped layers, the material is doped
-49-
with 5 to 100 ppm of dopant material as i~ is
deposited. For n+ or p+ doped layers the material
is doped with 100 ppm to over 1 per cent of dopant
material as it is deposited. The n dopant mate~
rial can be deposited at their respective optimum
substrate temperatures and preferably to a thick
ness in the range of 100 ppm to over 5000 ppm for
the p~ material.
The glow discharge deposition can Include an
AC signal generated plasma into which the mate-
rials are introduced. The plasma preferably is
sustained between a cathode and substrate anode
with an AC signal of about lkHz to 13.6 MHz.
Although the band increasing method and ele-
ment(s) of the invention can be utilized in de-
vices with various amorphous alloy layers, it is
preferable that they are utilized with the fluo-
rine and hydrogen compensated glow discharge de-
posited alloys. In this case, a mixture of sili-
con tetrafluoride and hydrogen is deposited as anamorphous compensated alloy material at or below
about 400C, for the n-type layer. The intrinsic
amorphous alloy layer and the p+ layer can be
deposited upon the electrode layer at a higher
substrate temperature above about 450C which will
provide a material which is fluorine compensated.
-50-
As previously mentioned, the alloy layersother than the intrinsic alloy layer can be other
than amorphous layers, such ,as polycrystalline
layers. ~By the term "amorphousil is meant an
alloy or material which has long range disorder,
although it may hav~ short or :intermediate order
or even contaln at times som,e crystalline inclu-
sions3.
