U.S. patent application number 13/620930 was filed with the patent office on 2013-02-14 for plasma deposition of amorphous semiconductors at microwave frequencies.
The applicant listed for this patent is Stanford R. Ovshinsky. Invention is credited to Stanford R. Ovshinsky.
Application Number | 20130037755 13/620930 |
Document ID | / |
Family ID | 45565127 |
Filed Date | 2013-02-14 |
United States Patent
Application |
20130037755 |
Kind Code |
A1 |
Ovshinsky; Stanford R. |
February 14, 2013 |
Plasma Deposition of Amorphous Semiconductors at Microwave
Frequencies
Abstract
Apparatus and method for plasma deposition of thin film
photovoltaic materials at microwave frequencies. The apparatus
avoids deposition on windows that couple microwave energy to
deposition species. The apparatus includes a microwave applicator
with one or more conduits that carry deposition species. The
applicator transfers microwave energy to the deposition species to
energize them to a reactive state. The conduits physically isolate
deposition species that would react or otherwise combine to form a
thin film material at the point of microwave power transfer and
deliver the microwave-excited species to a deposition chamber.
Supplemental material streams may be delivered to the deposition
chamber without passing through the microwave applicator and may
combine with deposition species exiting the conduits to form a thin
film material. Precursors for the microwave-excited deposition
species include fluorinated forms of silicon. Precursors for
supplemental material streams include hydrogenated forms of
silicon.
Inventors: |
Ovshinsky; Stanford R.;
(Bloonfield Hills, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ovshinsky; Stanford R. |
Bloonfield Hills |
MI |
US |
|
|
Family ID: |
45565127 |
Appl. No.: |
13/620930 |
Filed: |
September 15, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12983203 |
Dec 31, 2010 |
8273641 |
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13620930 |
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13284912 |
Oct 30, 2011 |
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12983203 |
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13355541 |
Jan 22, 2012 |
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13284912 |
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Current U.S.
Class: |
252/501.1 |
Current CPC
Class: |
H01L 21/02592 20130101;
H01L 31/1804 20130101; H01L 31/20 20130101; Y02P 70/521 20151101;
H01J 37/32192 20130101; H01J 37/3244 20130101; H01L 21/02532
20130101; H01J 37/32357 20130101; H01J 37/32229 20130101; H01L
31/202 20130101; Y02P 70/50 20151101; H01L 21/0262 20130101; H01L
31/03767 20130101; Y02E 10/547 20130101; H01J 2237/3326 20130101;
H01L 31/035218 20130101 |
Class at
Publication: |
252/501.1 |
International
Class: |
H01B 1/04 20060101
H01B001/04 |
Claims
1. A photovoltaic material comprising silicon, fluorine, and
hydrogen, said fluorine having an atomic concentration between 0.1%
and 7%, said hydrogen having an atomic concentration between 1% and
8%.
2. The photovoltaic material of claim 1, wherein the atomic
concentration of fluorine is between 0.2% and 5% and the atomic
concentration of hydrogen is between 2% and 6%.
3. The photovoltaic material of claim 1, wherein the atomic
concentration of fluorine is between 0.5% and 4% and the atomic
concentration of hydrogen is between 3% and 5%.
4. The photovoltaic material of claim 1, wherein said material has
a first sub-bandgap absorption coefficient in the as-deposited
state and wherein the sub-bandgap absorption coefficient of said
material increases by less than 60% of said first sub-bandgap
absorption coefficient upon exposure of said material to AM1.5
solar radiation for 44 hours.
5. The photovoltaic material of claim 4, wherein said the
sub-bandgap absorption coefficient of said material increases by
less than 40% of said first sub-bandgap absorption coefficient upon
exposure of said material to AM1.5 solar radiation for 44
hours.
6. The photovoltaic material of claim 4, wherein said the
sub-bandgap absorption coefficient of said material increases by
less than 20% of said first sub-bandgap absorption coefficient upon
exposure of said material to AM1.5 solar radiation for 44
hours.
7. The photovoltaic material of claim 1, wherein said material has
a first sub-bandgap absorption coefficient in the as-deposited
state and wherein the sub-bandgap absorption coefficient of said
material increases by less than 70% of said first sub-bandgap
absorption coefficient upon exposure of said material to AM1.5
solar radiation for 115 hours.
8. The photovoltaic material of claim 7, wherein said the
sub-bandgap absorption coefficient of said material increases by
less than 45% of said first sub-bandgap absorption coefficient upon
exposure of said material to AM1.5 solar radiation for 115
hours.
9. The photovoltaic material of claim 7, wherein said the
sub-bandgap absorption coefficient of said material increases by
less than 30% of said first sub-bandgap absorption coefficient upon
exposure of said material to AM1.5 solar radiation for 115
hours.
10. The photovoltaic material of claim 1, wherein said material has
a first .mu..tau. product in the as-deposited state and wherein the
.mu..tau. product of said material decreases by less than 20% of
said first .mu..tau. product upon exposure of said material to
AM1.5 solar radiation for 44 hours.
11. The photovoltaic material of claim 10, wherein the .mu..tau.
product of said material decreases by less than 10% of said first
.mu..tau. product upon exposure of said material to AM1.5 solar
radiation for 44 hours.
12. The photovoltaic material of claim 10, wherein the .mu..tau.
product of said material decreases by less than 5% of said first
.mu..tau. product upon exposure of said material to AM1.5 solar
radiation for 44 hours.
13. The photovoltaic material of claim 10, wherein said material
has a first .mu..tau. product in the as-deposited state and wherein
the .mu..tau. product of said material decreases by less than 40%
of said first .mu..tau. product upon exposure of said material to
AM1.5 solar radiation for 115 hours.
14. The photovoltaic material of claim 13, wherein the .mu..tau.
product of said material decreases by less than 30% of said first
.mu..tau. product upon exposure of said material to AM1.5 solar
radiation for 115 hours.
Description
RELATED APPLICATION INFORMATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/983,203, entitled "Plasma Deposition of
Amorphous Semiconductors at Microwave Frequencies", and filed on
Dec. 31, 2010; the disclosure of which is incorporated by reference
in its entirety herein. This application is a continuation in part
of U.S. patent application Ser. No. 13/284,912, entitled "Plasma
Deposition of Amorphous Semiconductors at Microwave Frequencies"
and filed on Oct. 30, 2011; the disclosure of which is incorporated
by reference in its entirety herein. This application is a
continuation in part of U.S. patent application Ser. No.
13/355,541, entitled "Plasma Deposition of Amorphous Semiconductors
at Microwave Frequencies", and filed on Jan. 22, 2012; the
disclosure of which is incorporated by reference in its entirety
herein.
FIELD OF INVENTION
[0002] The invention establishes a new realm of plasma chemistry
and physics that enables the deposition of unique
atomically-engineered multi-element compositions for photovoltaic
applications that free the world from its dependence on fossil
fuels. More particularly, this invention solves the problem of
depositing silicon-containing semiconductors at high deposition
rates to achieve highly efficient photovoltaic materials with a low
density of states that exhibit no Staebler-Wronski degradation.
Most particularly, this invention relates to plasma deposition of
amorphous, nanocrystalline, microcrystalline, polycrystalline or
single crystalline semiconductors at microwave frequencies from
multiple source gases, one of which includes fluorine, in a process
that avoids undesirable coatings on microwave windows.
BACKGROUND OF THE INVENTION
[0003] Concern over the depletion and environmental impact of
fossil fuels has stimulated strong interest in the development of
alternative energy sources. Significant investments in areas such
as batteries, fuel cells, hydrogen production and storage, biomass,
wind power, algae, and solar energy have been made as society seeks
to develop new ways of creating and storing energy in an
economically-competitive and environmentally-benign fashion. The
ultimate objective is to minimize society's reliance on fossil
fuels and to avoid production of greenhouse gases.
[0004] A number of experts have concluded that to avoid the serious
consequences of global warming, it is necessary to maintain
CO.sub.2 at levels of 350 ppm or less. To meet this target, based
on current projections of world energy usage, the world will need
17 TW of carbon-free energy by the year 2050 and 33 TW by the year
2100. The estimated contribution of various carbon-free sources
toward the year 2050 goal are summarized below:
TABLE-US-00001 Source Projected Energy Supply (TW) Wind 2-4 Tidal 2
Hydro .sup. 1.6 Biofuels 5-7 Geothermal 2-4 Solar 600
Based on the expected supply of energy from the available
carbon-free sources, solar energy is clearly the most viable
solution for reducing greenhouse emissions and alleviating the
effects of global climate change. (See J. Esch, "Keeping the Energy
Debate Clean How Do We Supply the World's Energy Needs?", IEEE
Proc. 98(1), 39-41 (2010).)
[0005] Amorphous semiconductors are attractive materials for solar
energy applications. Among the amorphous semiconductors, amorphous
silicon is known to be a particularly promising solar energy
material. Unlike crystalline silicon, amorphous silicon is a direct
gap material that has strong absorption over much of the solar
spectrum. The strong absorption means that high efficiency solar
cells can be formed from thin layers of amorphous silicon. As a
result, solar panels based on amorphous silicon (or
chemically-modified or structurally-modified forms of amorphous
silicon, including composite forms of amorphous silicon that
include nanocrystalline phases) are lightweight, flexible, and
readily adapted to field use in a variety of installation
environments.
[0006] S. R. Ovshinsky has long recognized the advantages of
amorphous silicon and related materials as the active layer of
solar cells and has been instrumental through his inventions and
developments in advancing automated and continuous manufacturing
techniques for producing solar and photovoltaic devices based on
amorphous semiconductors or combinations of amorphous
semiconductors with nanocrystalline, microcrystalline,
polycrystalline or single crystalline semiconductors.
Representative discoveries of S. R. Ovshinsky in the field of
amorphous semiconductors and photovoltaic materials include U.S.
Pat. Nos. 4,400,409 (describing a continuous manufacturing process
for making thin film photovoltaic films and devices); 4,410,588
(describing an apparatus for the continuous manufacturing of thin
film photovoltaic solar cells); 4,438,723 (describing an apparatus
having multiple deposition chambers for the continuous
manufacturing of multilayer photovoltaic devices); 4,217,374
(describing suitability of amorphous silicon and related materials
as the active material in several semiconducting devices);
4,226,898 (demonstration of solar cells having multiple layers,
including n- and p-doped); 5,103,284 (deposition of nanocrystalline
silicon and demonstration of advantages thereof); and 5,324,553
(microwave deposition of thin film photovoltaic materials).
[0007] Current efforts in thin film photovoltaic material
manufacturing are directed at increasing the deposition rate
without impairing photovoltaic efficiency and, in the case of
silicon-containing materials, without exacerbating Staebler-Wronski
degradation. Higher deposition rates lower the cost of thin film
solar cells and can lead to a dramatic decrease in the unit cost of
electricity obtained from solar energy. As the deposition rate
increases, thin film photovoltaic materials become increasingly
competitive with fossil fuels as a source of energy. Presently,
PECVD (plasma-enhanced chemical vapor deposition) is the most
cost-effective method for the commercial-scale manufacturing of
amorphous silicon and related amorphous semiconductor photovoltaic
materials. Current PECVD processes provide uniform coverage of
large-area substrates with device quality photovoltaic material at
deposition rates of .about.1-5 .ANG./s. This deposition rate,
however, is insufficient to achieve cost parity with fossil
fuels.
[0008] In order to enhance the economic competitiveness of plasma
deposition processes, it is desirable to increase the deposition
rate. To effectively compete with fossil fuels, it is believed that
deposition rates of 100 .ANG./s or higher are needed. The
deposition rate of prevailing plasma deposition techniques is
limited by the high concentration of intrinsic defects that
develops in amorphous solar materials as the deposition rate is
increased. The intrinsic defects include structural defects such as
dangling bonds, strained bonds, unpassivated surface states,
non-tetrahedral bonding distortions, coordinatively unsaturated
atoms (e.g. two- or three-fold coordinated silicon or germanium).
The structural defects create electronic states in the bandgap and
near the band edges of amorphous semiconductors. The electronic
states detract from solar conversion efficiency by (1) promoting
nonradiative recombination processes that deplete the concentration
of free carriers generated by absorbed sunlight and (2) reducing
hole mobility. Intrinsic defects also contribute to degradation of
the solar conversion efficiency of amorphous silicon and related
materials through the Staebler-Wronski effect, an effect that leads
to a 15-30% reduction in photovoltaic efficiency with use over
time.
[0009] S. R. Ovshinsky has demonstrated that the concentration of
intrinsic defects formed in a plasma-deposited material depends on
the distribution of species present in the plasma. A plasma is a
complex state of matter that includes ions, ion-radicals, neutral
radicals and molecules in multiple energetic states. In particular,
S. R. Ovshinsky has shown that certain charged species can be
detrimental to the quality of as-deposited amorphous semiconductors
under conditions in which they promote the creation of defects.
Uncontrolled charged species tend to strike the deposition surface
with high kinetic energy and can damage a growing thin film
material through bond cleavage. Bond cleavage creates dangling
bonds and promotes the formation of locally strained coordination
environments that may contribute to electronic defect states. S. R.
Ovshinsky has shown that neutral species in a plasma, in contrast,
frequently promote more uniform bonding and lead to lower defect
concentrations in as-deposited material. S. R. Ovshinsky has
ultimately showed that the proper balance of charged and neutral
species is essential to maximizing deposition rate and minimizing
defects. He has further demonstrated that the optimal identity,
concentration, and charge of species in a plasma environment varies
with plasma conditions and can be constructively influenced through
chemical modification with agents such as fluorine.
[0010] To minimize the concentration of intrinsic defects, current
plasma deposition processes are performed at low deposition rates.
By slowing the deposition process, the intrinsic defects that form
in the as-deposited product material have the opportunity to
equilibrate to energetically-favored states that have more regular
bonding configurations. As a result, the concentration of intrinsic
defects is reduced. Unfortunately, the reduced deposition rate
impairs the economic competitiveness of the process and prevents
cost parity with fossil fuels.
[0011] In U.S. patent application Ser. Nos. 12/199,656; 12/209,699;
and 12/429,637; S. R. Ovshinsky described techniques for minimizing
the deleterious effect of uncontrolled charged plasma species on
the defect concentration. The patent applications describe
techniques for maximizing the presence of neutral species and
controlling the presence and activity of charged species at the
deposition surface through preferential formation of neutral
species in the plasma activation process, magnetic confinement to
regulate charged species, and/or separation of undesirable charged
species to form a charge-controlled deposition medium. Through
utilization of a charge-controlled deposition medium, high quality
amorphous or other silicon-containing semiconductors can be formed
at high deposition rates in a plasma deposition process.
[0012] Another strategy for increasing the deposition rate of
plasma-based processes is to increase the plasma frequency.
Conventional plasma deposition processes are typically completed at
radiofrequencies (e.g. 13.56 MHz). As the plasma frequency is
increased, the source gases used in plasma deposition are activated
more efficiently, more completely, and to higher energy states.
Plasma excitation at microwave frequencies (e.g. 2.45 GHz), for
example, leads to higher dissociation rates of source gases,
generates higher fluxes of ions and neutrals, and creates a higher
proportion of plasma species (ions, neutrals) sufficiently
energetic to participate in the deposition process. The high
dissociation rates and higher excitation energies associated with
microwave plasmas improve process efficiency by providing much
higher utilization of source gases than radiofrequency plasmas. The
high fluxes and energies of ions and neutrals produced by microwave
plasmas lead to significantly higher thin film deposition rates
than radiofrequency plasmas.
[0013] In addition to dissociation of a higher fraction of source
gases, the high deposition rate accompanying microwave deposition
of thin film precursors is also a consequence of the enhanced
reactivity of deposition intermediates. Enhanced reactivity of
deposition intermediates results from the higher energy of
activation available from microwave excitation. Microwave
excitation produces deposition intermediates with higher internal
energy by activating deposition precursors to higher energy
electronic and vibrational excited states. The higher internal
energy makes the deposition intermediates less stable and more
conducive to the structural rearrangements and reactions on the
deposition surface needed to form a thin film material.
[0014] Although enhanced reactivity of deposition precursors is
beneficial from the standpoint of deposition rate, it oftentimes
leads to unintended side effects. A common problem in microwave
deposition is the tendency of reactive deposition intermediates to
form thin films away from the substrate. Thin film coatings, for
example, may develop on the interior walls of the deposition
chamber and may serve as a source of contamination for subsequent
depositions.
[0015] Since the deposition chamber is normally operated under
vacuum or with a controlled atmosphere, it has a limited volume and
receives precursors, background gases, and energy from external
sources. Materials are generally delivered by conduits through
valves that pierce the boundaries of the chamber. Electrical energy
(such as the bias between electrodes needed to initiate a plasma or
the resistive dissipation used to heat a substrate) is typically
supplied by wires that connect an external power source through the
boundaries of the chamber to internal components. The formation of
thin film coatings on the openings or actuators of internal valves,
or on internal components such as electrodes or wires, may alter
deposition conditions, impair the uniformity of deposition or
prevent deposition altogether.
[0016] Unintended thin film coatings are particularly problematic
when they form on the windows of a deposition chamber through which
the electromagnetic energy used to activate a plasma from
deposition precursors is transmitted. In microwave deposition, for
example, the microwave generator is normally located remote from
the deposition chamber. The generator produces microwaves and
transmits them along a microwave waveguide to the deposition
chamber or a downstream applicator, where the microwaves pass
through a window to energize deposition intermediates or activate
deposition precursors to generate the reactive species used to form
a thin film material. To maximize the microwave energy coupled to
the deposition intermediates or precursors, it is necessary to
insure that the window is highly transparent to microwave
frequencies. If the reactive species generated by the microwaves
deposit the thin film material on the window and the thin film
material absorbs microwaves, the transparency of the window
decreases.
[0017] Decreased transparency of the window leads to two
detrimental effects. First, any decrease in transparency leads to a
reduction in the microwave energy coupled to the deposition
intermediates or precursors. Reduced microwave coupling means that
the deposition species are less dissociated, less energetic, less
reactive, and as a result, the deposition rate decreases. Second,
continued exposure of a microwave-absorptive thin film on the
window to microwave radiation leads to localized heating of the
thin film material that can cause thermal stresses and potentially
catastrophic failure of the window.
[0018] The detrimental consequences of thin film coatings on
microwave transmission windows do not arise if the coating is
transparent to microwave radiation. Most dielectrics (including
quartz, sapphire, diamond, boron carbide, SiO.sub.2, and
Si.sub.3N.sub.4) are highly transparent to microwave radiation and
may be formed safely at high deposition rates in a microwave plasma
process. Coatings made from lower bandgap materials (including
metals and most semiconductors), however, are much less transparent
to microwave radiation and present much more serious concerns over
safety and process consistency. Many desirable photovoltaic
materials, including amorphous silicon and silicon-germanium,
absorb microwave radiation and are difficult to manufacture in a
microwave plasma process because the high reactivity conditions
present in a microwave plasma promotes the formation of undesirable
coatings on the windows used to transmit microwave radiation to the
deposition environment. Accordingly, there is a need for a process
that permits microwave deposition of semiconducting photovoltaic
materials.
SUMMARY OF THE INVENTION
[0019] The amount of energy absorbed by the Earth's atmosphere,
oceans and land masses in one hour is more than the amount of
energy used by people on Earth in one year. This fact reveals that
solar energy is the ultimate solution to eliminating mankind's
dependence on fossil fuels. Implementation of solar energy on a
scale sufficient to meaningfully reduce fossil fuel consumption has
been hindered, however, by economics and concerns about cost. Dr.
Steven Chu, winner of a Nobel Prize in physics and presently the
Secretary of Energy, summed up the problem in a New York Times
article that appeared on Feb. 12, 2009, by stating that a
revolution in science and technology would be needed if the world
is to reduce its dependence on fossil fuels and curb the emissions
of carbon dioxide and other heat-trapping gases linked to global
warming. Dr. Chu also stated that a five-fold improvement in solar
technology was needed to adequately address global warming and
reduce the world's dependence on fossil fuels. This invention can
be summarized most simply as providing the revolution and
improvement in solar technology that Dr. Chu referred to.
[0020] With the invention, the unit cost of solar energy is at or
below the cost of fossil fuels. As a result, widespread
implementation of the instant invention will allow mankind to
reduce its dependence on fossil fuels and serves the higher goal of
democratizing energy by enabling all countries, regardless of
natural resources, to become self-sufficient in energy. Concerns
over the scarcity of fossil fuels, conflicts over sources of fossil
fuel will be eliminated, and national and worldwide security will
be enhanced.
[0021] The invention is predicated on a fundamental advance in
plasma chemistry and physics that allows for a tremendous increase
in the throughput and deposition rate of photovoltaic materials
containing Group IV elements (e.g. Si, Ge, Sn) in a continuous
manufacturing process. The fundamental advance in plasma chemistry
and physics enables a unique atomic engineering of multi-element
compositions that affords a method of controlling and forming thin
film photovoltaic materials in the presence of a microwave plasma.
With the invention, the deposition rate of thin film photovoltaic
materials based on silicon can be dramatically increased for the
first time without introducing the defects, the density of states
and Staebler-Wronski degradation that have heretofore diminished
photovoltaic efficiency and frustrated efforts to achieve cost
parity with fossil fuels.
[0022] The invention enables for the first time a gigawatt or more
of manufacturing capacity in a single machine of a size that fits
within an ordinary manufacturing plant. Because of this invention,
it will no longer be necessary to run multiple manufacturing
processes in multiple locations in parallel or to build multiple
machines in series to realize output on the gigawatt scale. The
tremendous cost reduction afforded by this invention will motivate
the development of new industries that will provide high-valued
jobs that stimulate the economy and promote the educational
system.
[0023] The foregoing benefits of the instant invention are more
particularly realized in the exemplary embodiments now
summarized:
[0024] This invention provides a method and apparatus for the
microwave deposition of atomically-engineered, multi-element,
silicon-containing photovoltaic materials with unique chemical
bonding and structural configurations, resulting in new physics.
The invention provides the high deposition rate advantage
associated with microwave deposition, while avoiding or minimizing
the problems of (1) forming unintended coatings in the deposition
chamber or on the windows used to transmit microwave radiation; (2)
creating electronic defect states in the bandgap that detract from
photovoltaic efficiency; and (3) degradation of photovoltaic
efficiency over time upon continuous exposure of the material to
incident radiation during operation due to the Staebler-Wronski
effect.
[0025] The silicon-containing photovoltaic material provided by the
instant invention is a thin film material that can be formed at
high speeds without compromising the quality of the material. The
invention solves the heretofore insurmountable problem of realizing
the benefit of high rate deposition from microwave plasma
excitation without creating a high concentration of structural or
electronic defects that produce a high density of states in the
bandgap. The thin film material of the instant invention features
new bonding relationships that provide a low concentration of
defects, a low density of states, a dense, non-porous structure,
and little or no Staebler-Wronski degradation. The instant
invention constitutes the advent of a new regime of deposition
conditions that exploits fundamentally new physics and chemistry to
achieve superior performance of photovoltaic materials based on
silicon (and other fourfold coordinate elements) in an ultrafast
deposition process. Silicon-based materials available from the
instant invention include materials that have an amorphous,
nanocrystalline, microcrystalline, polycrystalline, or single
crystalline structure as well as materials that combine two or more
of such structures.
[0026] This invention reduces or eliminates the Staebler-Wronski
effect by using fluorine in a microwave plasma to engineer the
deposition environment to insure formation of silicon-containing
photovoltaic materials with improved bonding and unique structural
configurations. Fluorine is active not only in the plasma, but also
within and on the surface of the product material. The beneficial
effect of fluorine enables the deposition of silicon-containing
photovoltaic materials with a low density of states and essentially
no Staebler-Wronski degradation at heretofore unattainable
rates.
[0027] The apparatus generally includes a microwave generator,
microwave waveguide, microwave applicator, and deposition chamber.
The microwave generator produces microwaves and launches them down
the waveguide toward the applicator. The applicator couples the
microwaves to deposition species flowing through one or more
conduits that pass through the applicator. The conduits are formed
from a material that transmits microwave radiation to permit
coupling of microwave energy to the deposition species. The
deposition species may be neutral precursors in a ground or excited
energetic state, ionized precursors, free radicals formed from a
neutral precursor, or constituents of a plasma. Coupling of
microwave energy to the deposition species energizes them to
promote reactivity and increase deposition rate. The energized
deposition species exit the one or more conduits, enter the
deposition chamber, and form a thin film material on a substrate.
The process may further include the introduction of one or more
supplemental material streams to the deposition chamber that have
not been subjected to microwave excitation. The supplemental
material streams may combine with the energized species exiting the
one or more conduits to provide a deposition medium from which a
thin film material is formed.
[0028] The thin film material is generally a semiconductor or
amorphous semiconductor material. The thin film material typically
includes silicon and/or germanium and may be an intrinsic
semiconductor or a semiconductor doped n-type or p-type.
Embodiments include silicon, germanium, and alloys of silicon and
germanium in amorphous, nanocrystalline, microcrystalline and/or
polycrystalline forms. The materials also include hydrogenated
and/or fluorinated variants.
[0029] Deposition species include silane, fluorosilanes, germane,
fluorogermanes, and mixtures thereof. Deposition species may also
include treatment gases that passivate or modify the surface of the
thin film material. Treatment gases may or may not provide elements
that are incorporated into the thin film material. Treatment gases
include hydrogen, hydrogen fluoride, fluorine, and noble gases.
Carrier gases such as argon, neon or helium may also be combined
with one or more deposition species or treatment gases in a conduit
of the applicator.
[0030] The presence of fluorine in the microwave deposition
environment (whether from a deposition precursor, treatment gas, or
supplemental material stream) is believed to facilitate new
structural organizations of the multiple elements present in the
environment at or adjacent to the deposition surface. The new
structural organizations are a new form of atomic engineering that
enables the high speed formation of silicon-containing photovoltaic
materials in a bonding configuration that avoids defects, improves
photovoltaic efficiency, and prevents Staebler-Wronski degradation.
The effective amount of fluorine incorporated into the product film
ranges from 0.1% up to or slightly above the etching threshold of
fluorine. The etching threshold of fluorine corresponds to the
concentration of fluorine at which etching of the product film
begins. The etching threshold of fluorine depends on the
characteristics of the deposition environment, including the
concentration of hydrogen. In one embodiment, the etching threshold
of fluorine is about 7%.
[0031] The conduits of the microwave applicator are transparent to
microwave radiation and are formed from a dielectric material, such
as an oxide or nitride. Representative dielectric materials include
SiO.sub.2, quartz, Al.sub.2O.sub.3, sapphire, transition metal
oxides, silicon nitride, and aluminum nitride.
[0032] In one embodiment, the applicator includes two or more
conduits, each of which carries a different deposition species. The
conduits may be physically separated or one conduit may be
concentric with or otherwise housed within another. The conduits
may be oriented in a direction aligned or non-aligned with the
direction of microwave propagation in the applicator. In one
embodiment, the conduits are oriented perpendicular to the
direction of microwave propagation.
[0033] One or more deposition species and/or carrier gases enter
each of the conduits of the applicator and are energized by
microwave radiation. In one embodiment, the deposition species or
carrier gases in each of two or more conduits are energized with a
common source of microwave radiation. In another embodiment,
separate sources of microwave radiation are used to energize
deposition species or carrier gases in two or more conduits. The
energized deposition species and/or carrier gases exit the conduits
and enter a deposition chamber. In the deposition chamber, the
energized deposition species are directed to a substrate and a thin
film material is formed. The substrate may be stationary or mobile.
In one embodiment, the substrate is a continuous web.
[0034] The deposition chamber may further include one or more
injection ports for delivering supplemental material streams to the
substrate. The supplemental material streams may include
precursors, intermediates, treatment gases, background gases or
carrier gases. The supplemental material streams may combine with
the energized deposition medium entering the chamber in the
vicinity of the substrate and may participate in the deposition
process by reacting or interacting with the energized deposition
medium to influence the composition or characteristics of the
deposited thin film material.
[0035] The deposition chamber may also include a supplemental
energizing source to prevent or slow relaxation or decay of the
energized species entering the deposition chamber from one or more
conduits of the microwave applicator. The supplemental energy
source can also be used to activate species in the deposition
chamber to form intermediate species through bond cleavage. The
supplemental energy source may energize supplemental material
streams added to the deposition through injection ports. Such
supplemental material streams will not have been activated or
energized by the microwave field used to excite deposition
precursors passing through the conduits of a microwave applicator.
The supplemental energy source may also excite or reduce the rate
of decay of the energy of deposition species exiting the conduits
and entering the deposition chamber. The energy boost provided by
the supplemental energy source can create new excited state
deposition species and/or preserve (or inhibit the decay of)
deposition species energized by microwave excitation in the
conduits. The presence of fluorine, for example, can be increased
by activating SiF.sub.4 to cleave an Si--F bond to liberate
fluorine.
[0036] In one embodiment, the supplemental energy source is an
electromagnetic source that includes an antenna or antenna array.
The antenna or antenna array may generate, induce, transfer, or
sustain an electromagnetic field and may further provide phase
control. In one embodiment, the electromagnetic field is a
microwave field. In one embodiment, the antenna couples microwave
energy from the source used to excite deposition species in the
applicator to the deposition chamber. In this embodiment, the
microwave energy that energizes the deposition medium in one or
more conduits is coupled to the deposition chamber by an antenna or
antenna array positioned within the deposition chamber. The antenna
or antenna array induces a transfer of microwave radiation from the
applicator to the deposition chamber to provide a supplemental
source of microwave energy in the deposition chamber. The
supplemental source can maintain (or inhibit the decay of) the
microwave-energized deposition medium delivered from the one or
conduits to the deposition chamber. The supplemental source may
also energize deposition precursors newly injected to the
deposition chamber that have not passed through the microwave
applicator used to energize species in the conduits.
[0037] The deposition chamber is further equipped with means to
meter, monitor, modulate and calibrate the presence and
distribution of species in the growth ambient. This capability
permits fine control over the ratios of the multiple elements in
deposition environment to insure optimum conditions for high speed
deposition. The relative amounts of fluorine and hydrogen are
particularly important to the success of the invention. It is
desirable to maximize the concentration of fluorine in the product
film, but the presence of too much fluorine in the growth ambient
may promote an undesirable etching effect that increases the
porosity of the product film. The presence of hydrogen in the
product film can aid in passivating defects, but too much hydrogen
may promote the Staebler-Wronski effect. In addition, fluorine and
hydrogen can interact with each other to deplete the concentration
of fluorine and/or hydrogen available to assist the process of
depositing the product film or to become incorporated into the
product film. Proper control of the ratio of fluorine to hydrogen
is important to realizing the superior photovoltaic materials
available from this invention. In one embodiment, it is desirable
to maximize the presence of fluorine in an energized state during
deposition and to minimize the presence of hydrogen in an energized
state during deposition.
[0038] The deposition chamber may be interconnected to one or more
additional deposition or processing units. Additional deposition
units permit the formation of multilayer thin film structures that
include materials of different composition. Thin film structures
include p-n junctions, p-i-n structures, tandem cells, or triple
junction cells, where at least one layer is formed according to the
instant invention. The thin film structures may be formed on
stationary, moving, or continuous substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 depicts a system for the microwave deposition of thin
film materials.
[0040] FIG. 2 depicts in side view an embodiment of a microwave
applicator with conduits delivering different deposition
species.
[0041] FIGS. 3A-3E depict in top view embodiments of a microwave
applicator with conduits delivering different deposition
species.
[0042] FIG. 4 depicts a system for the microwave deposition of thin
film materials that includes at least one energized deposition
medium stream and at least one non-energized deposition medium
stream.
[0043] FIG. 5 presents a compositional analysis of a
silicon-containing photovoltaic material in accordance with the
instant invention.
[0044] FIG. 6 presents a compositional analysis of a
silicon-containing photovoltaic material in accordance with the
instant invention.
[0045] FIG. 7 shows the optical absorption spectrum of the
silicon-containing photovoltaic material with the composition
depicted in FIG. 5.
[0046] FIG. 8 shows optical absorption spectrum of the
silicon-containing photovoltaic material with the composition
depicted in FIG. 6.
[0047] FIG. 9 shows the evolution of the optical absorption
spectrum of the silicon-containing photovoltaic material with the
composition depicted in FIG. 5 upon exposure to solar
radiation.
[0048] FIG. 10 compares the effect of solar radiation on
conventional amorphous silicon and an amorphous silicon material in
accordance with the instant invention.
[0049] FIG. 11 shows the dependence of the .mu..tau. product on the
ratio Si.sub.2H.sub.6/SiF.sub.4 for a series of samples.
[0050] FIG. 12 shows the dependence of deposition rate on the ratio
Si.sub.2H.sub.6/SiF.sub.4 for a series of samples.
[0051] FIG. 13 shows the dependence of the .mu..tau. product on
substrate temperature at a fixed Si.sub.2H.sub.6/SiF.sub.4 ratio
for a series of samples.
[0052] FIG. 14 shows the dependence of the .mu..tau. product on the
atomic concentration of fluorine for a series of samples.
[0053] FIG. 15 shows the dependence of .alpha..sub.CPM on the
atomic concentration of fluorine for a series of samples.
[0054] FIG. 16 depicts a portion of a deposition system that
includes a moving continuous web substrate.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0055] Although this invention will be described in terms of
certain preferred embodiments, other embodiments that are apparent
to those of ordinary skill in the art, including embodiments that
do not provide all of the benefits and features set forth herein
and including embodiments that provide positive benefits for
high-volume manufacturing, are also within the scope of this
invention. Accordingly, the scope of the invention is defined only
by reference to the appended claims.
[0056] The instant invention provides an apparatus and method for
microwave plasma deposition of thin film materials. The invention
is especially suited for the microwave plasma deposition of
materials that absorb microwave radiation. A schematic of a
microwave deposition system is depicted in FIG. 1. System 100
includes microwave generator 105 that creates a field of microwave
radiation and launches it through microwave waveguide 110 to
microwave applicator 115. Microwave generator 105 typically
includes a magnetron and delivers a field of microwave radiation at
a single frequency (e.g. 915 MHz, 2.45 GHz, 5.8 GHz). Applicator
115 couples the microwave radiation to deposition species passing
through conduits 120 and 125. Conduit 120 receives one or more
deposition species in stream 130 from source 140 and conduit 125
receives one or more deposition species in stream 135 from source
145. The microwave radiation couples to deposition species provided
to conduits 120 and 125 to produce streams 150 and 155,
respectively, containing energized deposition species that are
delivered to deposition chamber 160 for formation of a thin
film.
[0057] Although not shown, the deposition system may further
include an isolator directly after microwave generator 105 to
protect it from back-reflected microwave radiation. The isolator
includes a circulator and a dummy load to neutralize back-reflected
microwaves. The deposition system may also include a directional
coupler in the waveguide run to detect and monitor forward and
reflected microwave power, and a tuner to match the impedance of
the load with the impedance of the source. Adjustment of the tuner
minimizes the reflected power level. A termination device or
sliding short circuit may also be connected to the downstream end
of the applicator to assist with impedance matching or to establish
a standing wave condition that maximizes microwave power in the
vicinity of the conduits to increase the transfer of microwave
power to the deposition species.
[0058] Applicator 115 may include two or more conduits for
delivering deposition species to a region of microwave coupling
(power transfer). The conduits provide for physical separation of
two or more streams containing deposition precursors, while
permitting simultaneous microwave excitation of the individual
streams. The conduits receive deposition species from a source and
transport them to an interior cavity of the applicator for coupling
to the microwave radiation provided by waveguide 110. The coupling
transfers energy from the microwave radiation to the deposition
species to activate or otherwise energize them to a high energy
state. The energized deposition species are then delivered by the
conduits to the deposition chamber for deposition of a thin film
material.
[0059] The high energy state created by transfer of microwave power
is a reactive state and enhances reactions between deposition
species. The rates of reactions between deposition species that
occur in a non-energized state are generally increased when the
deposition species are placed in an energized state and reactions
that do not otherwise occur between deposition species may be
induced in the energized state. Physical separation of the
deposition species by the conduits provides the benefit of
preventing reactions between deposition species in the region where
microwave power (or energy) is transferred to the deposition
species. As a result, the formation of thin film materials in the
region of power transfer is avoided.
[0060] FIG. 2 shows an enlargement of applicator 115 in side view.
Microwave radiation from waveguide 110 enters cavity 117, which
couples microwave radiation to the deposition species in streams
130 and 135. In particular, cavity 117 is configured to transfer
microwave power or energy to the deposition species in streams 130
and 135 in regions 132 and 142, respectively. Regions 132 and 142
correspond to the regions of transfer of microwave power (or
energy) to streams 130 and 135, respectively, and coincide with the
interior portions of conduits 120 and 125, respectively, that pass
through the interior of applicator 115.
[0061] Microwave transfer region 132 includes boundary or window
134 that transmits microwave radiation through conduit 120 to
deposition species in stream 130. Microwave transfer region 142
includes boundary or window 144 that transmits microwave radiation
through conduit 125 to deposition species in stream 135. Transfer
of microwave power (or energy) to deposition species in stream 130
produces energized deposition species that exit applicator 115 in
stream 150. Transfer of microwave power (or energy) to deposition
species in stream 135 produces energized deposition species that
exit applicator 115 in stream 155.
[0062] To increase the deposition rate of a thin film material in
deposition chamber 160, it is desirable to maximize the transfer of
microwave power (or energy) to the deposition species transported
through conduits 120 and 125. Greater transfer of microwave power
(or energy) leads to more complete excitation or activation of the
deposition species, a higher overall energy for the deposition
species, greater dissociation, and greater reactivity. In one
embodiment, the efficiency of transfer of microwave power (or
energy) to the deposition species is increased by forming a
standing wave of microwave radiation in cavity 117 of applicator
115 and locating one or both of conduits 120 and 125 so that
deposition streams 130 and/or 135 pass through a region of maximum
or locally maximum intensity of the standing wave pattern. A
standing wave can be formed from the field of microwave radiation
that enters cavity 117 by adjusting the length of the cavity or by
attaching a termination device or sliding short to the cavity.
[0063] Physical separation of streams 130 and 135 in the region of
microwave power (or energy) transfer prevents reactions between
energized deposition species in stream 150 and energized deposition
species in stream 155 that might otherwise occur to form a coating
on the conduit windows. By delaying the interaction of the
energized deposition species in streams 150 and 155 until after
delivery into deposition chamber 160, the formation of a thin film
material occurs away from the region of microwave power (or energy)
transfer and the coating of conduit windows is avoided.
[0064] FIG. 3A depicts cavity 117, conduits 120 and 125, and
deposition streams 130 and 135 as shown in FIG. 1 in top view. In
the embodiment of FIG. 3A, conduits 120 and 125 have a generally
circular cross-section. In other embodiments, the cross-section of
the conduits may have another cross-sectional shape, including
elliptical, oval, square, rectangular, polygonal, or other closed
contour. The cross-sectional shape and/or dimensions may also
differ for the different conduits introduced into cavity 117 of
applicator 115. In the embodiment of FIG. 3A, conduits 120 and 125
are generally aligned in the direction of microwave propagation. In
other embodiments, the positions of conduits 120 and 125 may be
non-aligned in the direction of microwave propagation. FIG. 3B, for
example, illustrates an embodiment in which conduits 120 and 125
are aligned in a direction generally orthogonal to the direction of
microwave propagation. The scope of the invention extends to
arbitrary placement or orientation of two or more conduits relative
to the direction of microwave propagation.
[0065] In the embodiments of FIGS. 3A and 3B, conduits 120 and 125
are physically displaced from each other and the cross-sections of
conduits 120 and 125 are non-overlapping. The objective of
maintaining a physical separation between two or more streams
containing different deposition species may also be achieved with
conduits having overlapping cross-sections. Two or more conduits
may, for example, be concentric with each other or one or more
conduits may be otherwise housed within another conduit. FIG. 3C
depicts an embodiment in which conduits 120 and 125 are concentric
(co-axial) with each other within applicator 115 and FIG. 3D
depicts an embodiment in which conduit 120 is housed within, but
not concentric with conduit 125. In FIGS. 3C and 3D, the boundary
of conduit 120 prevents intermixing of deposition species flowing
in conduits 120 and 125. Deposition stream 130 is delivered to the
interior of conduit 120 and deposition stream 135 is delivered to
the portion of the interior of conduit 125 that is exterior to
conduit 120. In the embodiment of FIG. 3C, for example, deposition
stream 135 experiences a generally annular flow as it passes
through applicator 115. Since the boundaries of conduits 120 and
125 transmit microwave radiation, microwave power (energy) can be
transferred to each of deposition streams 130 and 135 in the
embodiments of FIGS. 3C and 3D. FIG. 3E shows an embodiment in
which conduits 120 and 125 are housed within conduit 126. In the
embodiment of FIG. 3E, physical separation is maintained between
deposition streams 130, 135, and 136 within the interior of
applicator 115.
[0066] The number, size, shape, and relative positioning of two or
more conduits permits control over the electromagnetic field
provided by the microwave radiation. The relative intensity, for
example, of the microwave field varies spatially and conduits
carrying particular deposition species or precursors can be
positioned in regions of high or low electromagnetic intensity.
This flexibility affords a degree of control over the relative
reactivity of multiple deposition precursors and assists the
objective of engineering, on an atomic scale, the interaction and
spatial distribution of the multiple elements that make up the thin
film materials of the instant invention.
[0067] Conduits 120 and 125 are formed from a material that
transmits microwave radiation. Preferably, conduits 120 and 125 are
highly transparent to microwave radiation. Dielectric materials,
such as oxides and nitrides, are among the dielectric materials
that may be used to form conduits 120 and 125. Representative
dielectric materials include SiO.sub.2, quartz, Al.sub.2O.sub.3,
sapphire, transition metal oxides, silicon nitride, aluminum
nitride, and transition metal nitrides.
[0068] The embodiment shown in FIG. 1 includes an applicator having
two conduits for delivery of streams of deposition species. In the
embodiment of FIG. 1, the deposition species in the two conduits
are energized or activated by a common field of microwave
radiation. The instant invention extends generally to applicators
including two or more conduits for delivering two or more
independent streams of deposition precursors, where the independent
streams are energized or activated by a common field of microwave
radiation. The instant invention also includes embodiments in which
two or more streams of deposition precursors are provided to two or
more applicators, each of which includes one or more conduits. In
these embodiments, a separate microwave generator may be used for
each applicator to achieve independent control over the frequency
and/or power of the field of microwave radiation used to energize
or activate different streams of deposition precursors.
[0069] FIG. 4 depicts an embodiment in which supplemental material
streams are directed to the deposition chamber in combination with
an energized deposition medium. System 165 includes microwave
applicator 166 that receives input stream 168 and energizes it with
microwave radiation as described hereinabove to form energized
deposition medium 170 that is delivered to deposition chamber 172.
As described hereinabove, input stream 168 may include one or more
components, where each component is a deposition precursor,
intermediate, carrier gas, or diluent gas. System 165 further
includes inlets 174 and 176 that deliver supplemental material
streams 178 and 180 to deposition chamber 172. Supplemental
material streams 178 and 180 may be precursors, intermediates,
carrier gases, diluent gases, or background gases and are directly
delivered to deposition chamber 172 without being activated or
energized in microwave applicator 166. Supplemental material
streams 178 and 180 combine with energized deposition medium 170 in
the vicinity of substrate 182 positioned on mount 184. Supplemental
material streams 178 and 180 interact, dilute, or react with
energized deposition medium 170 at or on substrate 182 to form thin
film material 186. Transfer of energy may also occur between
energized deposition medium 170 and supplemental material streams
178 and 180. Energized deposition medium 170 may, for example,
excite or energize supplemental material streams 178 and 180.
Supplemental material streams may be delivered through inlets
coupled to the deposition chamber or through internal structures
within the deposition chambers such as a ring, manifold or
showerhead.
[0070] The embodiment shown in FIG. 4 depicts a deposition system
that includes two supplemental material streams in combination with
a microwave applicator that provides a single microwave-energized
deposition medium stream. In related embodiments, the microwave
applicator may provide two or more microwave-energized deposition
medium streams or two or more microwave applicators, each of which
provides one or more microwave-energized deposition medium streams
may be employed. The number of supplemental streams may be one or
more. In one embodiment, the one or more supplemental material
streams are introduced to the deposition chamber in an electrically
neutral form. In another embodiment, the one or more supplemental
material streams are introduced to the deposition chamber in an
ionized form. In a further embodiment, the one or more supplemental
material streams are introduced to the deposition chamber in their
electronic ground state.
[0071] The deposition chamber may also include an internal
energizing source to prevent or slow relaxation or decay of the
energized species entering the deposition chamber from the one or
more conduits of the one or more microwave applicators. As noted
hereinabove, microwave excitation of material streams passing
through a conduit of a microwave applicator may energize or ignite
a plasma therefrom. As the energized or ignited material stream
flows away from the region of microwave coupling and enters the
deposition chamber, it may no longer be influenced or excited by
the microwave field present in the applicator. As a result, the
excited, energized, activated or ignited species in the energized
deposition medium delivered to the deposition chamber may relax or
decay to lower energy states. When relaxation or decay occurs, the
distribution of species present may be altered and the
characteristics of the ultimate thin film may be compromised due,
for example, to a higher prevalence of defects or impurities. The
availability of higher energy forms of deposition species may
favorably influence formation of a thin film on the substrate by
preventing the formation of defects or removing defects that do
form. The extent of relaxation or decay in energy depends on the
separation between the substrate and point of entry of the
energized deposition medium into the deposition chamber, frequency
of collisions between species within the energized deposition
medium and the intrinsic decay rates of the individual species
present in the energized deposition medium.
[0072] In one embodiment, the supplemental energy source is an
electromagnetic source that includes an antenna or antenna array.
The antenna or antenna array may generate or sustain an
electromagnetic field and may further provide phase control. In one
embodiment, the electromagnetic field is a microwave field. In
another embodiment, the supplemental energy source is a passive
energy source that draws or induces a transfer of microwave
excitation energy from the applicator into the deposition chamber.
An antenna or antenna array, for example, may be positioned within
the deposition chamber and may pick up or receive a portion of the
microwave energy supplied to an applicator positioned outside of
the deposition chamber. By coupling to the microwave field produced
by the microwave generator, the antenna or antenna array provides a
source of excitation in the interior of the deposition chamber that
can be used to excite or energize supplemental materials streams
delivered to the deposition chamber that have not passed through a
conduit that was directly exposed to microwave radiation provided
by the microwave generator. The antenna or antenna array can also
supply energy to the microwave-energized deposition species
delivered to the deposition chamber from such conduits to prevent
or inhibit a decay in the energy of such species. Coupling of
microwave energy to the antenna or antenna array may be facilitated
by the presence of the ionized or activated deposition species in
the conduit. Ionized or activated deposition species may act as a
transmission waveguide to direct microwave energy to the antenna or
antenna array.
[0073] The deposition species that may be introduced into the
conduits of a microwave applicator or inlets providing supplemental
material streams include neutral gases, ionized gases,
pre-energized gases, plasmas, or combination thereof. The instant
invention provides a particular benefit for combinations of
deposition species that are capable of reacting (in either an
energized or non-energized state) to form a thin film material
capable of absorbing microwave radiation. Deposition species
generally include gas phase materials that contain silicon,
germanium, tin, fluorine, and/or hydrogen. Representative
deposition species include silane (SiH.sub.4), disilane
(Si.sub.2H.sub.6), fluorinated forms of silane (SiF.sub.4,
SiF.sub.3H, SiF.sub.2H.sub.2, SiFH.sub.3), germane (GeH.sub.4),
fluorinated forms of germane (GeF.sub.4, GeF.sub.3H,
GeF.sub.2H.sub.2, GeFH.sub.3), as well as ionized, energized, or
activated forms thereof, and combinations thereof. Deposition
species also include hydrogen gas, fluorine gas, NF.sub.3 gas, and
hydrogen fluoride gas, as well as carrier or background gases such
as argon, helium, krypton, or neon.
[0074] Deposition species may or may not contribute an element to
the intended thin film composition. A deposition species may, for
example, act as a surface treatment agent that improves the quality
of the deposited film. A fluorinated gas, for example, may function
as a surface treatment gas to passivate defects or saturate
dangling bonds at the surface of the deposited thin film material.
Alternatively, a deposition species may facilitate initiation of a
plasma or assist in establishing a particular deposition pressure
even though it does not contribute an element to the deposited
material.
[0075] Fluorinated deposition species are advantageous because
fluorine promotes regular tetrahedral coordination of silicon,
germanium and tin in thin film materials, relieves bond strain,
acts to passivate dangling bonds and other defects that produce
tail states or midgap states that compromise carrier mobility in
photovoltaic materials, and assists in the formation of
nanocrystalline, intermediate range order, or microcrystalline
phases of silicon and germanium. (For more information see, for
example, the following references by S. R. Ovshinsky: U.S. Pat. No.
5,103,284 (formation of nanocrystalline silicon from SiH.sub.4 and
SiF.sub.4); U.S. Pat. No. 4,605,941 (showing substantial reduction
in defect states in amorphous silicon prepared in presence of
fluorine); and U.S. Pat. No. 4,839,312 (presents several
fluorine-based precursors for the deposition of amorphous and
nanocrystalline silicon); the disclosures of which are incorporated
by reference herein).
[0076] Silane (SiH.sub.4) has been widely used as a deposition
precursor for amorphous silicon, but is known to produce material
that has poor electronic properties due to the presence of a
particularly high concentration of dangling and strained bonds.
Deposition of amorphous silicon from silane in the presence of high
hydrogen (H.sub.2) dilution has been shown to improve the
electronic properties of amorphous silicon. Inclusion of excess
hydrogen in the deposition process has the effect of passivating
dangling bonds and relieving bond strain to provide a material with
a lower concentration of defects, a lower density of states, and
better carrier transport properties. Hydrogen dilution provides
benefits similar to fluorine, but has the drawback of promoting a
time-dependent degradation of photovoltaic efficiency through the
Staebler-Wronski effect when present above a certain
concentration.
[0077] To date, efforts to increase the plasma deposition rate of
amorphous silicon from silane (alone or in the presence of hydrogen
dilution) by increasing the plasma frequency from the
radiofrequency range to the microwave frequency range have been
frustrated by an enhancement in the production of solid phase,
particulate silanaceous byproducts that has been observed as the
plasma frequency is increased. The silanaceous byproducts are
thought to be long chain or polymeric compounds of silicon and
hydrogen (e.g. polysilanes) and deposit throughout the deposition
chamber, including on the windows used to couple microwave energy
to silane and/or hydrogen. Since the silanaceous byproducts absorb
microwave radiation, microwave deposition of silane under
conditions of high hydrogen dilution has proved to be a
commercially impractical process. In addition, the presence of
silanaceous byproducts is thought to contribute to Staebler-Wronski
degradation. As a result, the benefits of high hydrogen dilution
have been commercially realized only in radiofrequency plasma
processes to avoid production of excess hydrogen and suppress
formation of silanecous byproducts and the deposition rates have
been accordingly low.
[0078] In one embodiment of the instant invention, materials with
properties comparable or superior to those available from a high
hydrogen dilution process are realized in a high rate microwave
plasma deposition process by delivering SiF.sub.4 to one conduit of
a microwave applicator and H.sub.2 to a second conduit of the same
or separate applicator. Separation of the source of silicon from
the source of hydrogen enables deposition of amorphous,
intermediate range order, nanocrystalline, and microcrystalline
forms of silicon in a microwave plasma process. Since SiF.sub.4 is
free from hydrogen, its excitation or activation by microwave
radiation does not lead to the production of polysilane or related
byproducts. As a result, the formation of unintended silanaceous
coatings on the microwave window is avoided and the severity of
Staebler-Wronski degradation is significantly reduced or even
eliminated. Similarly, microwave activation or excitation of
hydrogen in the absence of silicon occurs without the production of
undesirable solid phase byproducts. As a result, the distribution
of species needed to form high quality silicon-based photovoltaic
materials can be created in a continuous process without corrupting
the microwave windows. The species may then be transported away
from the region of microwave coupling and combined in the vicinity
of a substrate for deposition of a thin film material. The instant
invention thus provides the high deposition rate advantage afforded
by microwave plasma excitation while avoiding the drawback of
forming microwave-absorbing materials on the microwave windows used
to transfer the microwave energy needed to energize one or more
deposition precursors.
[0079] The relative amounts of SiF.sub.4 and H.sub.2 may be
adjusted by controlling the pressure or flow rate of each in their
respective conduits or by combining either or both of SiF.sub.4 and
H.sub.2 with a carrier or background gas. Inclusion of deposition
species such as F.sub.2 or HF provide further control over the
relative amounts of silicon, hydrogen and fluorine present in the
deposition environment established in the vicinity of the
substrate. Adjustment of the relative amounts of deposition species
containing silicon, germanium, hydrogen, and/or fluorine permits
control over the degree of crystallinity and microstructure of the
thin film material deposited on the substrate as well as control
over the density of states and severity of the Staebler-Wronski
effect.
[0080] In other embodiments, a fluorine-containing gas and a
hydrogen-containing gas may be delivered by separate conduits of
one or more microwave applicators. SiF.sub.4 and SiH.sub.4, for
example, may be delivered by separate conduits of a microwave
applicator. Similarly, SiH.sub.4 and a fluorine-containing gas
(e.g. F.sub.2, NF.sub.3, or a fluorinated germanium gas) may be
delivered by separate conduits of one or more microwave
applicators.
[0081] In still other embodiments, one or more of a
silicon-containing gas, germanium-containing gas,
fluorine-containing gas, or hydrogen-containing gas may be
delivered by a microwave applicator as an energized deposition
medium to a deposition chamber and one or more additional
silicon-containing gases, germanium-containing gases,
fluorine-containing gases, or hydrogen-containing gases may be
delivered as supplemental material streams to the deposition
chamber. For example, one or more of SiF.sub.4, SiH.sub.4, H.sub.2,
or F.sub.2 may be delivered by a microwave applicator as an
energized deposition medium to a deposition chamber and others of
SiF.sub.4, SiH.sub.4, H.sub.2, or F.sub.2 may be delivered as
supplemental material streams to the deposition chamber. As
indicated hereinabove, the supplemental material streams are
introduced to the deposition chamber without having first been
excited by microwave radiation. The supplemental material streams
may then combine with the microwave-energized deposition species
entering the deposition chamber from one or more conduits that have
been exposed to microwave radiation outside of the deposition
chamber.
[0082] In one embodiment, SiF.sub.4 is activated by microwave
energy in a conduit of an applicator and delivered to a deposition
chamber equipped to provide H.sub.2 as a supplemental material
stream, where the H.sub.2 stream has not been activated by
microwave radiation before entering the deposition chamber. In
another embodiment, SiF.sub.4 is activated by microwave energy in a
conduit of an applicator and delivered to a deposition equipped to
provide SiH.sub.4 as a supplemental material stream, where the
SiH.sub.4 stream has not been activated by microwave radiation
before entering the deposition chamber. In other embodiment,
fluorine is provided both in a supplemental, non-energized material
stream and as a microwave-energized material stream from an
applicator. The supplemental fluorine stream may include F.sub.2,
NF.sub.3 or HF diluted by a carrier gas such as a noble gas. The
supplemental fluorine stream may also include SiF.sub.4 or other
fluorinated form of silane or disilane.
[0083] The instant invention further contemplates delivery of
precursors in separate streams that are activated by
electromagnetic radiation at different frequencies. As noted
hereinabove, microwave activation of silane is believed to produce
materials with a particularly pronounced degree of Staebler-Wronski
degradation because of the high concentration of active hydrogen
released from silane. A lesser degree of active hydrogen is formed,
however, upon excitation of silane by a radiofrequency
electromagnetic field. In one embodiment of the instant invention,
the deposition process includes microwave activation of SiF.sub.4
and radiofrequency activation of silane. Radiofrequency activation
of silane provides a controlled source of hydrogen that allows for
management of the hydrogen-to-fluorine ratio in the deposition
environment.
[0084] The structural and compositional control afforded by the
instant invention further provides silicon-containing
semiconductors, including amorphous semiconductors, that exhibit
little or no Staebler-Wronski degradation. One of the drawbacks
associated with utilizing high hydrogen dilution in forming
amorphous silicon is a degradation of photovoltaic efficiency over
time known as the Staebler-Wronski effect. Although high hydrogen
dilution conditions form amorphous silicon materials with improved
photovoltaic efficiency, the effect is not permanent and
photovoltaic efficiency gradually decays over time with persistent
exposure to solar energy. The origin of the Staebler-Wronski effect
is not fully understood, but is believed to involve a
photogeneration of electronic defect states or carrier trapping
centers by incident sunlight. The degradation effect has been
observed to become more severe as the extent of hydrogen dilution
increases.
[0085] A pronounced Staebler-Wronski effect is one reason why
attempts in the prior art to prepare amorphous silicon in a
microwave deposition process have been unsuccessful. Although
microwave conditions have provided high deposition rates in the
prior art, the resulting amorphous silicon material has suffered
from an unacceptably high degree of Staebler-Wronski degradation.
It is believed that the more energetic conditions associated with
microwave plasmas (relative to radiofrequency plasmas) releases too
much hydrogen from the silane (SiH.sub.4) precursor to provide an
especially high degree of hydrogen dilution and an especially
severe Staebler-Wronski effect.
[0086] The instant inventor believes that inclusion of fluorine in
the composition of silicon-containing amorphous semiconductors can
remedy the Staebler-Wronski effect by strengthening bonds and
improving the structural integrity of the material to render it
less susceptible to light-induced defect creation. The instant
inventor recognizes, however, that direct inclusion of fluorine in
a prior art plasma deposition process (at microwave or
radiofrequency frequencies) leads to a further complication.
Specifically, microwave activation of a fluorine-containing
precursor leads to release of a high concentration of fluorine,
which can, in turn, promote deterioration of the structure of the
thin film product through etching. Etching creates pores in the
thin film product and leads to a low density material having a high
internal surface area. The high surface area includes a high
concentration of surface defect states that detract from
photovoltaic efficiency by promoting non-radiative recombination
processes. The high surface area is also reactive and promotes
contamination of the material with environmental agents such as
oxygen or nitrogen.
[0087] Management of the presence of fluorine provides a strategy
for minimizing the tendency of fluorine to etch the product film.
The presence of fluorine can be managed by controlling the timing
of fluorine introduction, the concentration of fluorine, the form
of fluorine in the deposition environment, and the energetic state
of fluorine-containing deposition species. Hydrogen, for example,
is one tool for controlling the form of fluorine because the
simultaneous presence of hydrogen and fluorine depletes the supply
of active, dissociated fluorine through the formation of HF. By
binding fluorine with hydrogen, the overall supply of active
fluorine can be regulated and controlled to provide enough fluorine
to promote favorable bonding configurations within the product thin
film while avoiding etching to facilitate formation of a dense,
non-porous product film at high deposition rates.
Example 1
[0088] In this example, selected compositional and optical
absorption characteristics of representative thin film materials
comprising amorphous silicon in accordance with the instant
invention are described. The materials are denominated Sample 547
and Sample 548 and were prepared in a deposition system similar to
that shown in FIG. 4 that included a single microwave applicator
with a single conduit passing therethrough and a single
supplemental inlet for delivering a non-energized supplemental
material stream. The conduit was made from sapphire and the
substrate was positioned about 4 inches from the interface of the
conduit with the deposition chamber. A mixture of 1 standard liter
per minute of SiF.sub.4 and 2 standard liters per minute of argon
was introduced to the conduit of the microwave applicator and
activated with microwave radiation at a frequency of 2.45 GHz and a
power of 600 W. SiH.sub.4 was introduced at a rate of 1 standard
liter per minute to the deposition chamber through the supplemental
delivery port in an electrically-neutral state. The energized
stream exiting the conduit of the microwave applicator and the
non-energized stream supplied by the supplemental delivery port
were directed to a substrate and a thin film product material was
formed therefrom. The substrate was maintained at a temperature of
about 400.degree. C. and positioned on a mount. An electrical bias
could be optionally provided to the mount. For Sample 547, the
substrate bias was maintained at ground and for Sample 548, the
substrate was AC-biased at 100 kHz with a 60V peak-to-peak biasing
signal. The deposition rates of Samples 547 and 548 were .about.140
.ANG./s.
[0089] FIGS. 5 and 6 show the results of a SIMS (secondary ion mass
spectrometry) analysis of the chemical composition of Samples 547
and 548, respectively. The SIMS profile is a measure of the
concentration of different elements in the composition as a
function of depth within the sample. FIG. 5 indicates that the
composition of Sample 547 was primarily silicon and also included
6.8% hydrogen, 0.3% fluorine, 0.05% oxygen, and 0.0003% nitrogen.
FIG. 6 indicates that the composition of Sample 548 was primarily
silicon and also included 6.0% hydrogen, 0.24% fluorine, 0.03%
oxygen, and 0.0003% nitrogen. The low level of oxygen in Samples
547 and 548 indicates that both materials are free from atmospheric
contamination.
[0090] FIGS. 7 and 8 show the optical absorption spectra of Samples
547 and 548, respectively, and provide comparisons with the optical
absorption spectra of two reference materials. The figures show the
dependence of the absorption coefficient .alpha. as a function of
photon energy (expressed in units of eV).
[0091] In FIG. 7, trace 305 corresponds to the optical absorption
spectrum of Sample 547. Trace 310 (labeled "NREL" and corresponding
to comparative Sample NREL) shows the optical absorption spectrum
of a high quality reference sample of amorphous silicon prepared by
a slow rate prior art deposition process. Sample NREL was
non-fluorinated and corresponds approximately to the current state
of the art for thin film amorphous silicon materials. Trace 315
(labeled "236" and corresponding to comparative Sample 236) shows
the optical absorption spectrum of a low quality reference sample
of fluorinated amorphous silicon prepared by a high deposition rate
process. Trace 310 for Sample NREL and trace 315 for comparative
Sample 236 are repeated in FIG. 8. FIG. 8 further includes trace
320 for Sample 548.
[0092] Characterization of comparative Sample 236 indicated that
the material was porous with a pore volume fraction of .about.10%
based on refractive index measurements. SIMS data indicated that
comparative Sample 236 had a composition that included 2.8%
hydrogen, 0.15% fluorine, 0.34% oxygen, and 0.0032% nitrogen. The
much higher oxygen concentration for comparative Sample 236
relative to Samples 547 and 548 is consistent with its high
porosity and greater susceptibility of environmental
contamination.
[0093] Comparative Sample NREL, comparative Sample 236, Sample 547,
and Sample 548 all exhibited a pronounced increase in optical
absorption at a photon energy of about 1.5 eV. The sharp increase
in absorption coefficient represents the onset of the transition
from the valence band to the conduction band for each of the
samples and the similarity of the photon energy at which the
transition occurs in each of the materials indicates that the
materials have similar bandgaps. The energy of the bandgap
indicates that Samples 547 and 548 are predominantly amorphous
phase materials.
[0094] The optical absorption spectrum of a semiconductor material
at energies below the bandgap provides a measure of the quality of
the material. In the absence of defects (structural, electronic, or
compositional), a semiconductor material should exhibit no
absorption at energies below the bandgap. When defects are present,
however, electronic states can form in the bandgap. These states
can participate in optical transitions between each other or with
either or both of the valence band and conduction band to provide
optical absorption features at energies below the bandgap energy.
The presence of midgap defects states is undesirable from a
performance perspective because they typically serve as
recombination or trapping centers that reduce photovoltaic
efficiency and carrier mobility. The intensity of optical
absorption in the below bandgap portion of the spectrum is a
measure of the density of defect states in the bandgap. Low optical
absorption at below bandgap energies signifies a low density of
defect states and a higher quality product material.
[0095] A comparison of the optical absorption spectra shown in
FIGS. 7 and 8 indicates that comparative Sample NREL shows only
weak absorption in the below bandgap spectral region (photon
energies of less than .about.1.5 eV), while comparative Sample 236
exhibits pronounced absorption in the below bandgap spectral
region. These results indicate that comparative Sample NREL has a
low concentration of defect states, while comparative Sample 236
includes a high concentration of defect states.
[0096] The results show that Samples 547 and 548 exhibit
significantly less absorption in the below bandgap spectral region
than comparative Sample 236. This is an indication that Samples 547
and 548 possess a much lower concentration of defects and a much
lower density of states than comparative Sample 236. The defect
concentrations and density of states of Samples 547 and 548 are
only slightly greater than that of comparative Sample NREL.
[0097] The results indicate that the instant invention provides a
silicon-containing amorphous semiconductor material at high
deposition rate that has a defect concentration comparable to the
state of the art found in low deposition rate amorphous silicon.
Relative to other high deposition rate processes, the instant
invention provides a denser, less porous material that has a much
lower concentration of midgap defects.
[0098] FIG. 9 shows the evolution of the optical absorption
spectrum of Sample 547 with time upon continuous exposure to
incident electromagnetic radiation. The optical absorption spectrum
of Sample 547 was measured before exposure to incident radiation.
Sample 547 was then subjected to continuous exposure to incident
radiation and the optical absorption spectrum was measured again
following various times of exposure. Sample 547 was first exposed
to radiation that simulates the solar spectrum (AM-1) for 14 hours
and the optical absorption spectrum was measured. Sample 547 was
next exposed to the sun for an additional 12 hours and the optical
absorption spectrum was measured again. Finally, Sample 547 was
exposed to the AM-1 simulated solar spectrum for a further 59 hours
(for a total 85-hour exposure time) and the optical absorption
spectrum was measured again. FIG. 9 presents the results of the
measurements and shows that the optical absorption spectrum of
Sample 547 was virtually unchanged upon exposure to incident
radiation. Constancy of the optical absorption spectrum indicates
that density of states of Sample 547 did not increase during the
period of exposure and demonstrates that Sample 547 is essentially
free from degradation due to the Staebler-Wronski effect.
[0099] In one embodiment, the thin film material of the instant
invention is free from Staebler-Wronski degradation after exposure
to the solar spectrum for at least 14 hours. In another embodiment,
the thin film material of the instant invention is free from
Staebler-Wronski degradation after exposure to the solar spectrum
for at least 26 hours. In a further embodiment, the thin film
material of the instant invention is free from Staebler-Wronski
degradation after exposure to the solar spectrum for at least 85
hours.
[0100] FIG. 10 compares the effect of solar radiation on Sample 547
with the effect of solar radiation on the NREL comparative sample
of conventional amorphous silicon referred to in FIG. 7. Sample 547
is referred to as the "Ovshinsky Solar" sample and the NREL sample
is referred to as "Standard a-Si" in FIG. 10. As indicated in FIG.
9, Sample 547 exhibited little or no Staebler-Wronski degradation.
The comparative sample of conventional amorphous silicon, however,
exhibited pronounced degradation upon exposure to solar radiation.
Before exposure to solar radiation, Sample 547 exhibited slightly
greater absorption at below bandgap energies than conventional
amorphous silicon (See FIG. 7). The density of states in the
bandgap for Sample 547 was therefore slightly higher than the
density of states in the bandgap for conventional amorphous silicon
before exposure to solar radiation. After exposure to solar
radiation, however, conventional amorphous silicon exhibited
significantly greater absorption at below bandgap energies than
Sample 547. Based on the data shown in FIG. 10, it is estimated
that the density of states in Sample 547 after exposure to solar
radiation is less by a factor of five than the density of states in
conventional amorphous silicon. Unlike conventional amorphous
silicon, materials prepared in accordance with the instant
invention exhibit stable optical properties without photogeneration
of midgap or near edge defect states when exposed to solar
radiation. Convention amorphous silicon, in contrast, exhibits
appreciable Staebler-Wronski degradation.
[0101] The higher density of states resulting from the
photodegradation of conventional amorphous silicon affects not only
optical properties, but also electrical properties. The higher
density of defect states in conventional amorphous silicon function
as carrier traps that reduce carrier mobility and conductivity.
Mobility and conductivity of materials prepared in accordance with
the instant invention remain stable upon exposure to
electromagnetic radiation. The low density of states and superior
electrical properties of the instant materials indicate that the
instant materials have utility beyond photovoltaics into a broader
array of electronic applications. The instant materials, for
example, provide excellent prospects for transistors and diodes
based on amorphous silicon. The instant invention motivates a true
silicon-based thin film electronics technology.
[0102] This example demonstrates the remarkable result that a
silicon-containing photovoltaic material that exhibits a low
density of states and essentially no Staebler-Wronski degradation
can be deposited at a rate .about.140 .ANG./s.
Example 2
[0103] In this example, the effect of process gas ratio on the
deposition rate and photoconductivity of representative materials
comprising amorphous silicon in accordance with the instant
invention is described. The samples were prepared using the
deposition system described in Example 1 hereinabove. A mixture of
1 standard liter per minute of SiF.sub.4 and 2 standard liters per
minute of argon was introduced to the conduit of the microwave
applicator and activated with microwave radiation at a frequency of
2.45 GHz and a power of 600 W. Instead of SiH.sub.4, however,
disilane (Si.sub.2H.sub.6) was introduced to the deposition chamber
through the supplemental delivery port and delivered as an
electrically-neutral supplemental material stream. The flow rate of
disilane was systematically adjusted to provide a series of samples
for which the ratio of the flow rate of disilane to the ratio of
the flow rate of SiF.sub.4 ranged from 0.3 to 2.0. The energized
stream of SiF.sub.4 and argon exiting the conduit of the microwave
applicator and the non-energized stream of disilane supplied by the
supplemental delivery port were directed to a substrate and a thin
film product material was formed therefrom. The substrate was
maintained at a temperature of 400.degree. C. and positioned on a
grounded mount.
[0104] The bandgap, deposition rate, and .mu..tau. product were
measured for each sample. The bandgap generally increased with
increasing Si.sub.2H.sub.6/SiF.sub.4 flow rate ratio and varied
from 1.59 eV at a flow rate ratio of 0.3 to 1.64 eV at a flow rate
ratio of 2.0. .mu. and .tau. correspond to carrier mobility and
carrier lifetime, respectively, upon photoexcitation at a
particular wavelength. The .mu..tau. product was measured at two
above-bandgap wavelengths: 565 nm and 660 nm. As is known in the
art, the .mu..tau. product correlates with the photoconductivity of
each sample. A higher value for the .mu..tau. product indicates
better photoconductivity and a higher quality material. The
presence of defects deteriorates the quality of the material by
providing trap states that capture photogenerated carriers and
reduce carrier mobility. Deposition rate was determined from the
thickness of the deposited material and the time of deposition.
[0105] FIG. 11 shows the variation of the .mu..tau. product as a
function of the Si.sub.2H.sub.6/SiF.sub.4 flow rate ratio. The
.mu..tau. product is expressed in units of cm.sup.2/V. The results
indicate that the .mu..tau. product was higher upon excitation at
660 nm than upon excitation at 565 nm, but that the .mu..tau.
product at both excitation wavelengths was maximized at a
Si.sub.2H.sub.6/SiF.sub.4 flow rate ratio of about 1.25.
[0106] FIG. 12 shows the variation of deposition rate with the
Si.sub.2H.sub.6/SiF.sub.4 flow rate ratio. The deposition rate
increased from 93 .ANG./s at a Si.sub.2H.sub.6/SiF.sub.4 flow rate
ratio of 0.3, peaked at 139 .ANG./s at a Si.sub.2H.sub.6/SiF.sub.4
flow rate ratio of 1.0 and gradually declined at
Si.sub.2H.sub.6/SiF.sub.4 flow rate ratios above 1.0.
[0107] The data show that the highest values of the .mu..tau.
product and the highest deposition rates both occurred over the
range of Si.sub.2H.sub.6/SiF.sub.4 flow rate ratio between 1.0 and
1.5. This example therefore demonstrates that the highest quality
samples are formed at the highest deposition rates.
Example 3
[0108] In this example, the effect of substrate temperature on the
deposition rate and photoconductivity of representative materials
comprising amorphous silicon in accordance with the instant
invention is described. The samples were prepared using the
deposition system described in Example 1 hereinabove. A mixture of
1 standard liter per minute of SiF.sub.4 and 2 standard liters per
minute of argon was introduced to the conduit of the microwave
applicator and activated with microwave radiation at a frequency of
2.45 GHz and a power of 600 W. Instead of SiH.sub.4, however,
disilane (Si.sub.2H.sub.6) was introduced at a rate of 1 standard
liter per minute to the deposition chamber through the supplemental
delivery port. The disilane was delivered in an
electrically-neutral state. The Si.sub.2H.sub.6/SiF.sub.4 flow rate
ratio was fixed at 1.0 in these experiments. The energized stream
of SiF.sub.4 and argon exiting the conduit of the microwave
applicator and the non-energized supplemental stream of disilane
supplied by the supplemental delivery port were directed to a
substrate and a thin film product material was formed therefrom.
The substrate was placed on a grounded mount. A series of samples
was prepared for which the substrate temperature was varied between
300.degree. C. and 495.degree. C.
[0109] The bandgap, deposition rate, and .mu..tau. product were
measured as described in Example 2 hereinabove as a function of
substrate temperature. The bandgap was 1.78 eV at a substrate
temperature of 300.degree. C. and decreased in an approximately
linear manner to 1.52 eV at a substrate temperature of 495.degree.
C. The deposition rate was relatively constant, but showed a slight
decrease from about 150 .ANG./s for substrate temperatures of
375.degree. C. or less to about 135-140 .ANG./s for substrates
temperatures of 400.degree. C. or higher.
[0110] The variation of the .mu..tau. product as a function of
substrate temperature is shown in FIG. 13. The .mu..tau. product
was measured using excitation wavelengths of 565 nm and 660 nm. The
data indicate that the .mu..tau. product was highest for substrate
temperatures between 375.degree. C. and 475.degree. C. and was
lower for substrate temperatures above and below this range.
[0111] The index of refraction was also measured for this series of
samples and showed a progressive increase with increasing substrate
temperature from 3.02 at a substrate temperature of 300.degree. C.
to 3.59 at a substrate temperature of 495.degree. C. The refractive
index was above 3.5 for substrates at or above 425.degree. C. The
results indicate that the samples became increasingly dense and
non-porous as the substrate temperature was increased. For
comparison purposes, the refractive index of fully densified
amorphous silicon is about 3.6.
[0112] This example shows that the optimum substrate temperature
for depositing a silicon-containing photovoltaic material from
Si.sub.2H.sub.6 and SiF.sub.4 at a flow rate ratio of 1.0 is
between 375.degree. C. and 475.degree. C. The results show that
high quality samples (as judged by the .mu..tau. product and
refractive index) can be prepared at extremely high deposition
rates using the principles of atomic engineering annunciated
herein.
Example 4
[0113] In this example, the characteristics of a series of samples
that differed in the concentrations of hydrogen and fluorine are
compared. The samples were deposited using the apparatus described
in Example 1 hereinabove. In each of the samples, a mixture of
SiF.sub.4 and argon was introduced to the conduit passing through
the microwave applicator and excited with microwave energy. The
microwave-energized mixture of SiF.sub.4 and argon was directed to
a deposition chamber and combined with a supplemental stream of
either SiH.sub.4 or Si.sub.2H.sub.6. As noted hereinabove, the
supplemental stream was added directly to the deposition chamber
and was not passed through the region of microwave excitation in
the applicator. Relevant deposition conditions for each of the
samples are summarized below, where the flow rates of SiF.sub.4,
Ar, SiH.sub.4 and Si.sub.2H.sub.6 are reported in units of standard
liters per minute, the substrate temperature (T.sub.substrate) is
reported in units of .degree. C., the microwave power at 2.45 GHz
is reported in units of kW, and the deposition rate is reported in
units of .ANG./s:
TABLE-US-00002 Sample 638 639 656 683 699 700 SiF.sub.4 1 1 1 1 1 1
Ar 2 2 1.5 2 3 2 SiH.sub.4 1 1 1 1 Si.sub.2H.sub.6 1 1
T.sub.Substrate 350 450 400 400 365 350 Power 2.2 1.2 2.2 1.5 2.2
1.5 Deposition 193 80 169 178 245 302 Rate
[0114] For each of the samples, the following characteristics were
measured: atomic concentration of hydrogen, fluorine, oxygen, and
nitrogen (using SIMS), sub-bandgap optical absorption coefficient
(.alpha..sub.CPM (cm.sup.-1) (using the constant photocurrent
method (CPM)), and .mu..tau. product (cm.sup.2/V). The results are
summarized below:
TABLE-US-00003 Sample 638 639 656 683 699 700 Hydrogen 8.4% 1.9%
4.0% 5.2% 3.2% 4.2% Fluorine 1%.sup. 0.02% 0.08% 0.10% 0.19% 0.4%
Oxygen 0.008% 0.003% 0.005% 0.002% 0.01% 0.004% Nitrogen 0.001%
0.002% 0.002% 0.001% 0.002% 0.001% .alpha..sub.CPM (cm.sup.-1)
12.8.sup. 43.8 12.8 5.8 5.8 5.4 .mu..tau. (cm.sup.2/V) 3.5 .times.
10.sup.-8 5.3 .times. 10.sup.-9 3.0 .times. 10.sup.-8 4.4 .times.
10.sup.-8 NA 4.9 .times. 10.sup.-8
[0115] The data for the .mu..tau. product and .alpha..sub.CPM as a
function of the atomic concentration of fluorine are presented in
FIG. 14 and FIG. 15, respectively, where the sample number
corresponding to each data point is listed. The data in FIG. 15
indicate that the .mu..tau. product increases with increasing
fluorine over the series of samples 639, 656, 683 and 700. The data
in FIG. 15 further indicate that the sub-bandgap absorption
coefficient decreases with increasing fluorine concentration over
the series of samples 639, 656, 683, 699, and 700.
[0116] Although sample 638 has a higher fluorine concentration than
sample 700 and exhibits a smaller .mu..tau. product than sample
700, the data indicate that sample 638 has a particularly high
concentration of hydrogen and a higher absorption coefficient in
the sub-bandgap regime than sample 700. The instant inventors
believe that the high hydrogen concentration counteracts the
beneficial effect of fluorine and that the improvements in
.mu..tau. product and sub-bandgap absorption coefficient will
continue as the fluorine concentration is increased to 1% or higher
if the hydrogen concentration is properly controlled.
[0117] In further experiments, the effect of light exposure on the
.mu..tau. product and sub-bandbap absorption coefficient
.alpha..sub.CPM of samples 683 and 699 were determined. In the
experiments, samples 683 and 699 were exposed to sunlight. The
effect of sunlight on the .mu..tau. product and sub-bandbap
absorption coefficient .alpha..sub.CPM of each sample was monitored
over time for up to 115 hours. The incident solar radiation was
estimated to correspond to an AM1.5 spectrum. Results from the
experiments are summarized below:
TABLE-US-00004 Sample 699 .alpha..sub.CPM .mu..tau. t.sub.ex (hr)
(cm.sup.-1) .alpha..sub.CPM/.alpha..sub.CPM, 0 (cm.sup.2/V)
.mu..tau./.mu..tau..sub.0 (.mu..tau.).sup.-1
(.mu..tau.).sup.-1/(.mu..tau.).sup.-1.sub.0 0.025 5.80 1 1.18E-07 1
8.44E+06 1 44 8.05 1.39 6.61E-08 0.56 1.51E+07 1.79 115 8.15 1.41
5.68E-08 0.48 1.76E+07 2.09
TABLE-US-00005 Sample 683 .alpha..sub.CPM .mu..tau. t.sub.ex (hr)
(cm.sup.-1) .alpha..sub.CPM/.alpha..sub.CPM, 0 (cm.sup.2/V)
.mu..tau./.mu..tau..sub.0 (.mu..tau.).sup.-1
(.mu..tau.).sup.-1/(.mu..tau.).sup.-1.sub.0 0.025 5.81 1 3.99E-08 1
2.50E+07 1 44 6.89 1.19 3.95E-08 0.99 2.53E+07 1.01 115 7.27 1.25
2.96E-08 0.74 3.38E+07 1.35
The exposure time of 0.025 hr corresponds to 1.5 min and represents
the initial measurement of the sample. The initial measurement
corresponds to the performance of the sample in its as-deposited
state. In the tables, t, is the time of exposure, .alpha..sub.CMP,0
is the initial value of .alpha..sub.CPM (the value after 0.025 hr
of exposure), .mu..tau..sub.0 is the initial value of .mu..tau.
(the value after 0.025 hr of exposure); and
(.mu..tau.).sup.-1.sub.0 is initial value of the reciprocal of the
.mu..tau. product (the value after 0.025 hr of exposure).
(.mu..tau.).sup.-1 is reported because it is often used as a
measure of the concentration of sub-bandgap defects. A higher value
of (.mu..tau.).sup.-1 correlates with a higher concentration of
sub-bandgap defects and provides a measure of the light-induced
change in defect concentration.
[0118] The light exposure results for samples 683 and 699 were
compared to prior art results published by Stradins, Fritzsche, and
Tran (Materials Research Society Symposium Proceedings, vol. 336,
p. 227-239 (1994)) and Stradins & Fritzsche (Philosophical
Magazine B, vol. 69, p. 121-139 (1994)). These publications
considered the effect of light exposure on samples of amorphous
silicon prepared from silane (in the absence of fluorine and
without microwave excitation) using various deposition techniques
(rf glow discharge, hot wire method, and rf plasma deposition using
a heated mesh).
[0119] The light exposure results indicated that although samples
683 and 699 exhibited a lower .mu..tau. product and a higher
absorption coefficient at short times than the comparative
non-fluorinated prior art samples, the samples exhibited
comparatively little variation in any of the .mu..tau. product,
inverse .mu..tau. product (.mu..tau.).sup.-1, or sub-band gap
absorption .alpha..sub.CPM over time. The non-fluorinated prior art
samples exhibited a pronounced degradation in both .mu..tau.
product and sub-bandbap absorption. Both the inverse .mu..tau.
product (.mu..tau.).sup.-1 and sub-bandgap absorption of each of
the prior art samples increased by a factor of about ten (1000%)
over an exposure time of 115 hours with a comparable solar source.
In sample 699, the inverse .mu..tau. product (.mu..tau.).sup.-1
increased by only a factor of about two, while the sub-bandgap
absorption increased by only a factor of 1.4 over an exposure time
of 115 hours. Sample 683 exhibited essentially no increase in
inverse .mu..tau. product (.mu..tau.).sup.-1 over an exposure time
of 44 hours and an increase in inverse .mu..tau. product
(.mu..tau.).sup.-1 of only a factor of 1.35 over an exposure time
of 115 hours. Sample 683 also exhibited an increase in sub-bandgap
absorption of only a factor of 1.25 over an exposure time of 115
hours.
[0120] Based on the wide difference in the effect of exposure, the
results showed that samples 638 and 699 exhibited better
performance (higher .mu..tau. product and lower sub-bandgap
absorption) than the prior art samples after an exposure time of
only about 100 min. Above 100 min. of exposure time, the
performance of the prior art samples continued to degrade at an
appreciable rate, while samples 683 and 699 continued to exhibit
only slight degradation in performance. The variations in the
performance of samples 683 and 699 between exposure times of 44 hr
and 115 hr, for example, were negligible.
[0121] The sun exposure results indicate that fluorinated samples
in accordance with the instant invention exhibit stable
photoconductivity and sub-bandgap absorption characteristics over
time and avoid the pronounced degradation in performance observed
for conventional non-fluorinated amorphous silicon-based
photovoltaic materials. The consistency in the performance of the
instant fluorinated amorphous silicon materials upon exposure to
light demonstrates that the instant materials are only weakly
susceptible to light-induced defect creation. Exposure of the
instant materials to light for a prolonged period of time did not
appreciably increase the defect concentration above the initial
concentration.
[0122] Consistency of performance is important from a manufacturing
standpoint because it avoids the expense and time required to
stabilize the performance of thin film amorphous silicon
photovoltaic materials. To balance the power load and configure the
electronics, it is important that the output of the photovoltaic
material remain steady over time. As a result, before assembling a
photovoltaic module with prior art amorphous silicon materials, it
is necessary to subject the as-deposited material to a prolonged
conditioning step to insure stable performance. The results of this
example indicate that the condition step can be avoided using the
instant materials.
[0123] The instant materials provide a further advantage in that
their photoconductivity and sub-bandgap absorption characteristics
remain stable on annealing at elevated temperatures. In one
experiment, the effect of annealing samples 683 and 699 at
350.degree. C. for 30 min on the .mu..tau. product and sub-bandgap
absorption was measured. The experiment showed that annealing had
little or no effect on the performance of either sample. It is
known in the art that conventional, non-fluorinated amorphous
silicon exhibits an increase in photoconductivity upon annealing.
It is also known that annealing of amorphous silicon occurs in the
field as seasons change. High summertime temperatures lead to
annealing and an increase in the photoconductivity of amorphous
silicon relative to cooler seasons. Variability in
photoconductivity during the year as seasons change is undesirable
because it leads to variability in the output power of solar
modules. Such variability can affect the performance of devices
powered by the solar module. Expensive supplemental electronics is
needed to counteract variability in output. The consistency in the
performance of the instant fluorinated materials with annealing
indicates that seasonal variations in output power will not occur
in photovoltaic modules formed from the instant materials. As a
result, the module design can be simplified and module costs can be
minimized.
[0124] This example illustrates that an increase in fluorine
concentration leads to higher photoconductivity (as measured by the
.mu..tau. product) and lower sub-bandgap absorption. The higher
photoconductivity and lower sub-bandgap absorption indicate
fluorine improves the quality of silicon-containing photovoltaic
materials formed at high deposition rates by lowering the
concentration of defects in the material. This example also
illustrates that the beneficial effects of fluorine may be
sensitive to the presence of hydrogen and that the presence of
excess hydrogen should be avoided. This example further
demonstrates the consistency in the performance of amorphous
silicon materials in accordance with the instant invention upon
prolonged light exposure or annealing.
[0125] Light-induced degradation due to the Staebler-Wronski effect
leads to a 15% or greater decrease in the initial conversion
efficiency of conventional amorphous silicon solar cells due to
pronounced light-induced defect creation and its effect on the
inverse .mu..tau. product and sub-bandgap absorption as noted
hereinabove. The significant time-variability of the performance of
conventional amorphous silicon makes the qualification and
utilization of conventional amorphous silicon solar cells
difficult, time-consuming, and expensive as it is necessary to
condition the solar cells for a prolonged period of time after
manufacturing in order to stabilize performance. With the instant
invention, the performance of amorphous silicon is sufficiently
stable in the as-deposited state to avoid post-manufacturing
conditioning. (At worst, only a brief conditioning step may be
required). The ability of the instant invention to deposit
amorphous silicon in a stabilized as-deposited state at a rate of
150 .ANG./s or higher (which is two orders of magnitude greater
than the current industry standard) represents a major
technological achievement that will revolutionize the thin film
photovoltaic industry.
[0126] The foregoing discussion demonstrates that the instant
invention permits simultaneous realization of the benefits of
hydrogen and fluorine. As noted hereinabove, both hydrogen and
fluorine passivate dangling bonds and relieve bond strain. Since
the bond strengths of hydrogen and fluorine with silicon differ,
however, hydrogen and fluorine may exhibit a preferential
effectiveness for remediating energetically distinct defects within
the spectrum of defects known to exist in the various structural
forms of silicon (amorphous, intermediate range order,
nanocrystalline, and microcrystalline). As a result, material of
particularly high quality can be expected through the combined
effects of fluorine and hydrogen. With this invention, such
material can be formed continuously at high deposition rates in a
microwave plasma deposition process for the first time and unique
bonding configurations can be achieved that minimize the density of
states and suppress the Staebler-Wronski effect through careful
control of the relative amounts of hydrogen and fluorine.
[0127] One objective is to maximize the amount of fluorine in the
product material, but to do so from a deposition environment in
which the concentration of active fluorine is not so high as to
cause detrimental etching. High concentrations of fluorine can be
tolerated in the deposition environment provided that the
fractional concentration of activated fluorine is controlled to
prevent deleterious etching. A slight degree of etching may be
acceptable because it may aid in removing impurities or atoms that
are irregularly bonded at the surface. An appreciable degree of
etching, however, must be avoided to prevent a reduction in the
deposition rate. The presence of fluorine in an inactive state is
advantageous because inactive fluorine can be converted to active
fluorine through, for example, a supplemental energy source in the
deposition chamber as described hereinabove. Conversion of inactive
fluorine active fluorine compensate for the depletion of fluorine
in the deposition environment as it becomes incorporated into the
product film.
[0128] In one embodiment, the atomic concentration of fluorine in a
silicon-containing photovoltaic material is between 0.1% and 7%. In
another embodiment, the atomic concentration of fluorine in a
silicon-containing photovoltaic material is between 0.2% and 5%. In
a further embodiment, the atomic concentration of fluorine in a
silicon-containing photovoltaic material is between 0.5% and 4%. In
one embodiment, the atomic concentration of fluorine is as
indicated above and the atomic concentration of hydrogen is between
1% and 8%. In another embodiment, the atomic concentration of
fluorine is as indicated above and the atomic concentration of
hydrogen is between 2% and 6%. In a further embodiment, the atomic
concentration of fluorine is as indicated above and the atomic
concentration of hydrogen is between 3% and 5%.
[0129] Deposition species may also include precursors designed to
achieve n-type or p-type doping. Doping precursors include gas
phase compounds of boron (e.g. boranes, organoboranes,
fluoroboranes), phosphorous (e.g. phosphine, organophosphines, or
fluorophosphines), arsenic (e.g. arsine or organoarsines), and
SF.sub.6. One or more deposition or doping precursors may be
introduced to one or more conduits individually, sequentially, or
in combination.
[0130] The instant microwave deposition apparatus and method may be
used to form amorphous, nanocrystalline, microcrystalline, or
polycrystalline materials, or combinations thereof as a single
layer or in a multiple layer structure. In one embodiment, the
instant deposition apparatus includes a plurality of deposition
chambers, where at least one of the deposition chambers is equipped
with a remote plasma source having the capabilities described
hereinabove. The different chambers may form materials of different
composition, different doping, and/or different crystallographic
form (amorphous, nanocrystalline, microcrystalline, or
polycrystalline).
[0131] The instant deposition process provides thin film materials
having compositions within the scope of the instant invention at
high deposition rates with a low density of defect states and
little or no Staebler-Wronski effect. In one embodiment, the thin
film deposition rate is at least 20 .ANG./s. In another embodiment,
the thin film deposition rate is at least 50 .ANG./s. In still
another embodiment, the thin film deposition rate is at least 100
.ANG./s. In a further embodiment, the thin film deposition rate is
at least 150 .ANG./s. In one embodiment, thin film materials formed
at the foregoing deposition rates exhibit essentially no
Staebler-Wronski degradation after exposure to an AM-1 solar
spectrum for at least 14 hours. In another embodiment, thin film
materials formed at the foregoing deposition rates exhibit
essentially no Staebler-Wronski degradation after exposure to an
AM-1 solar spectrum for at least 26 hours. In still another
embodiment, thin film materials formed at the foregoing deposition
rates exhibit essentially no Staebler-Wronski degradation after
exposure to an AM-1 solar spectrum for at least 85 hours.
[0132] The degree of Staebler-Wronski degradation may be assessed
in terms of the photoconductivity (as measured by the .mu..tau.
product) of the photovoltaic material upon exposure to light
relative to the photoconductivity of the photovoltaic material in
its as-deposited state. In one embodiment, the .mu..tau. product
decreases by less than 20% upon exposure to an AM1.5 light source
for 44 hours. In another embodiment, the .mu..tau. product
decreases by less than 10% upon exposure to an AM1.5 light source
for 44 hours. In still another embodiment, the .mu..tau. product
decreases by less than 5% upon exposure to an AM1.5 light source
for 44 hours. In one embodiment, the .mu..tau. product decreases by
less than 40% upon exposure to an AM1.5 light source for 115 hours.
In another embodiment, the .mu..tau. product decreases by less than
30% upon exposure to an AM1.5 light source for 115 hours.
[0133] The degree of Staebler-Wronski degradation may also be
assessed in terms of the inverse .mu..tau. product (which is one
measure of sub-bandgap defect concentration) of the material upon
exposure to light relative to the inverse .mu..tau. product of the
photovoltaic material in its as-deposited state. In one embodiment,
the inverse .mu..tau. product increases by less than 20% upon
exposure to an AM1.5 light source for 44 hours. In another
embodiment, the inverse .mu..tau. product increases by less than
10% upon exposure to an AM1.5 light source for 44 hours. In still
another embodiment, the inverse .mu..tau. product increases by less
than 5% upon exposure to an AM1.5 light source for 44 hours. In one
embodiment, the inverse .mu..tau. product increases by less than
80% upon exposure to an AM1.5 light source for 115 hours. In
another embodiment, the inverse .mu..tau. product increases by less
than 60% upon exposure to an AM1.5 light source for 115 hours. In
another embodiment, the inverse .mu..tau. product increases by less
than 40% upon exposure to an AM1.5 light source for 115 hours.
[0134] The degree of Staebler-Wronski degradation may also be
assessed in terms of the sub-bandgap absorption coefficient
.alpha..sub.CPM (which is a second measure of sub-bandgap defect
concentration) of the material upon exposure to light relative to
the sub-bandgap absorption coefficient .alpha..sub.CPM of the
photovoltaic material in its as-deposited state. In one embodiment,
.alpha..sub.CPM increases by less than 60% upon exposure to an
AM1.5 light source for 44 hours. In another embodiment,
.alpha..sub.CPM increases by less than 40% upon exposure to an
AM1.5 light source for 44 hours. In still another embodiment,
.alpha..sub.CPM increases by less than 20% upon exposure to an
AM1.5 light source for 44 hours. In one embodiment, .alpha..sub.CPM
increases by less than 70% upon exposure to an AM1.5 light source
for 115 hours. In another embodiment, .alpha..sub.CPM increases by
less than 45% upon exposure to an AM1.5 light source for 115 hours.
In a further embodiment, .alpha..sub.CPM increases by less than 30%
upon exposure to an AM1.5 light source for 115 hours.
[0135] In one embodiment, the temperature of the substrate is
between 300.degree. C. and 500.degree. C. In another embodiment,
the temperature of the substrate is between 325.degree. C. and
475.degree. C. In one embodiment, the temperature of the substrate
is between 350.degree. C. and 450.degree. C. In one embodiment, the
substrate is electrically grounded. In another embodiment, the
substrate is electrically biased. In a further embodiment, the
electrical bias is an AC bias.
[0136] In one embodiment, the molar or volumetric flow rate of
disilane to SiF.sub.4 is between 0.3 and 2.0. In another
embodiment, the molar or volumetric flow rate of disilane to
SiF.sub.4 is between 0.5 and 1.75. In still another embodiment, the
molar or volumetric flow rate of disilane to SiF.sub.4 is between
0.75 and 1.5. Since disilane includes 2 moles of silicon per mole
of precursor, the flow rate ratios will be doubled when using
silane as a precursor in conjunction with SiF.sub.4. In one
embodiment, the molar or volumetric flow rate of silane to
SiF.sub.4 is between 0.6 and 4.0. In another embodiment, the molar
or volumetric flow rate of silane to SiF.sub.4 is between 1.0 and
3.5. In still another embodiment, the molar or volumetric flow rate
of disilane to SiF.sub.4 is between 1.5 and 3.0.
[0137] In one embodiment, SiF.sub.4 or other fluorinated silicon
precursor is activated or energized with microwave radiation remote
from a deposition chamber and delivered to a deposition chamber
equipped to receive silane or disilane through a supplemental
delivery line, where the silane or disilane is not activated or
energized with microwave radiation before entering the deposition
chamber. In one embodiment, the silane or disilane is not exposed
to the microwave radiation that activates or energizes the
SiF.sub.4 or other fluorinated silicon precursor before entering
the deposition chamber. As noted hereinabove, however, once the
silane or disilane is inside the deposition chamber, either may be
activated or energized with a microwave source internal to the
deposition chamber. The SiF.sub.4 is typically mixed with an inert
gas (e.g. a noble gas) in the region of microwave excitation. In
one embodiment, the molar ratio of inert gas to SiF.sub.4 is
between 0.5 and 5. In another embodiment, the molar ratio of inert
gas to SiF.sub.4 is between 1 and 3.
[0138] The instant deposition apparatus is adapted to deposit one
or more thin film materials on a continuous web or other moving
substrate. In one embodiment, a continuous web substrate or other
moving substrate is advanced through each of a plurality of
deposition chambers and a sequence of layers is formed on the
moving substrate. The individual deposition chambers within the
plurality are operatively interconnected and environmentally
protected to prevent intermixing of the deposition species
introduced into the individual chambers. Gas gates, for example,
may be placed between the chambers to prevent intermixing. A
variety of multiple layer or stacked cell device configurations may
be obtained.
[0139] FIG. 16 shows a portion of a deposition system in accordance
with the instant invention that includes a continuous web
substrate. The deposition system includes deposition chamber 260
equipped with continuous web substrate 230. Continuous web
substrate 230 is in motion during deposition and is delivered to
deposition chamber 260 by payout roller 265 and received by take up
roller 270 after deposition of thin film material 275. Continuous
web substrate 230 enters and exits deposition chamber 260 through
isolation devices 280. Isolation devices 280 may be, for example,
gas gates. Deposition chamber 260 receives streams 250 and 255
containing energized or activated deposition species from separate
conduits (not shown) of a microwave applicator (not shown) as
described hereinabove. Streams 250 and 255 enter deposition chamber
260 through inlets 220 and 225. Inlets 220 and 225 may correspond
to outlets of conduits that pass through a microwave applicator.
Streams 250 and 255 are directed to the surface of substrate 230
and combine or other react or interact to form thin film material
275. Deposition chamber may optionally be equipped with independent
means for generating a plasma to further energize or activate
streams 250 and 255. Additional deposition chambers may be
operatively connected to deposition chamber 260 to permit formation
of a multilayer thin film structure or device.
[0140] One important multilayer photovoltaic device is the triple
junction solar cell, which includes a series of three stacked n-i-p
devices with graded bandgaps on a common substrate. The graded
bandgap structure provides more efficient collection of the solar
spectrum. In making an n-i-p photovoltaic device, a first chamber
is dedicated to the deposition of a layer of an n-type
semiconductor material, a second chamber is dedicated to the
deposition of a layer of substantially intrinsic (i-type)
semiconductor material, and a third chamber is dedicated to the
deposition of a layer of a p-type semiconductor material. In one
embodiment, the intrinsic semiconductor layer is an amorphous
semiconductor that includes silicon, germanium, or an alloy of
silicon and germanium. The n-type and p-type layers may be
microcrystalline or nanocrystalline forms of silicon, germanium, or
an alloy of silicon and germanium. The process can be repeated by
expanding the deposition apparatus to include additional chambers
to achieve additional n-type, p-type, and/or i-type layers in the
structure. A triple cell structure, for example, can be achieved by
extending the apparatus to include six additional chambers to form
a second and third n-i-p structure on the web. Tandem devices and
devices that include p-n junctions are also within the scope of the
instant invention.
[0141] Bandgap grading of multiple junction device structures may
be achieved by modifying the composition of the intrinsic (i-type)
layer in the separate n-i-p subunits. In one embodiment, the
highest bandgap in the triple junction cell results from
incorporation of amorphous silicon as the intrinsic layer in one of
the n-i-p structures. Alloying of silicon with germanium to make
amorphous silicon-germanium alloys leads to a reduction in bandgap.
The second and third n-i-p structures of a triple junction cell may
include intrinsic layers comprising SiGe alloys having differing
proportions of silicon and germanium. In this way, each of the
three intrinsic layers of a triple cell device has a distinct
bandgap and each bandgap can be optimized to absorb a particular
portion of the incident solar or electromagnetic radiation.
[0142] In one device configuration, the incident radiation first
encounters an n-i-p structure that includes an amorphous silicon
intrinsic layer. The amorphous silicon intrinsic layer absorbs the
shorter wavelength fraction of the incident radiation (e.g. shorter
wavelength visible and ultraviolet wavelengths) and transmits the
longer wavelength fraction (e.g. middle and longer wavelength
visible and infrared wavelengths). The longer wavelength fraction
next encounters a second intrinsic layer that includes a
silicon-germanium alloy having a relatively lower germanium
content. The second intrinsic layer absorbs the shorter wavelength
portion (e.g. middle wavelength visible portion) of the longer
wavelength fraction transmitted by the amorphous silicon intrinsic
layer and transmits the longer wavelength portion (e.g. long
wavelength visible and infrared wavelengths) to a third intrinsic
layer having an intrinsic layer that includes a silicon-germanium
alloy with a relatively higher germanium content. By grading the
bandgaps of the intrinsic layers, more efficient absorption of the
incident radiation occurs and better conversion efficiency is
achieved.
[0143] In addition to compositional variation, bandgap modification
may also be achieved through control of the microstructure of the
intrinsic layer. Polycrystalline silicon, for example, has a
different bandgap than amorphous silicon and multilayer stacks of
various structural phases may be formed with the instant continuous
web apparatus. The nanocrystalline and intermediate range order
forms of silicon can provide bandgaps between the bandgap of
crystalline silicon and the bandgap of amorphous silicon.
[0144] Another important multilayer structure is the p-n junction.
In conventional amorphous silicon or hydrogenated amorphous
silicon, the hole mobility is too low to permit efficient operation
of a p-n junction. The low hole mobility is a consequence of a high
defect density that leads to efficient trapping of charge carriers
before they can be withdrawn as external current. To compensate for
carrier trapping, an i-layer is often included in the structure.
With the material prepared by the instant invention, the defect
concentration in n-type or p-type material is greatly reduced and
efficient p-n junctions can be formed from silicon, germanium, and
silicon-germanium alloys. Alternatively, p-i-n structure can be
formed in which the i-layer thickness necessary for efficient
charge separation is much smaller that is required for current
devices.
[0145] Those skilled in the art will appreciate that the methods
and designs described above have additional applications and that
the relevant applications are not limited to the illustrative
examples described herein. The present invention may be embodied in
other specific forms without departing from the essential
characteristics or principles as described herein. The embodiments
described above are to be considered in all respects as
illustrative only and not restrictive in any manner upon the scope
and practice of the invention. It is the following claims,
including all equivalents, which define the true scope of the
instant invention.
* * * * *