U.S. patent application number 15/175511 was filed with the patent office on 2016-10-06 for methods of growing heteroepitaxial single crystal or large grained semiconductor films and devices thereon.
This patent application is currently assigned to Solar-Tectic LLC. The applicant listed for this patent is Ashok Chaudhari, Karin Chaudhari, Pia Chaudhari. Invention is credited to Praveen Chaudhari.
Application Number | 20160293790 15/175511 |
Document ID | / |
Family ID | 43535112 |
Filed Date | 2016-10-06 |
United States Patent
Application |
20160293790 |
Kind Code |
A1 |
Chaudhari; Praveen |
October 6, 2016 |
METHODS OF GROWING HETEROEPITAXIAL SINGLE CRYSTAL OR LARGE GRAINED
SEMICONDUCTOR FILMS AND DEVICES THEREON
Abstract
A method of growing a semiconductor film including the following
steps: providing a substrate, depositing a metal thin film on the
substrate, depositing an initial semiconductor film on the metal
thin film to form a semiconductor film on the substrate, and
depositing a different semiconductor material on the semiconductor
film. An electromagnetic device and the relevant elements is also
included. Such methods and decies being used in tandem cells.
Inventors: |
Chaudhari; Praveen;
(US) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chaudhari; Karin
Chaudhari; Ashok
Chaudhari; Pia |
Briarcliff Manor
Briarcliff Manor
Briarcliff Manor |
NY
NY
NY |
US
US
US |
|
|
Assignee: |
Solar-Tectic LLC
Briarcliff Manor
NY
|
Family ID: |
43535112 |
Appl. No.: |
15/175511 |
Filed: |
June 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13929085 |
Jun 27, 2013 |
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15175511 |
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12903750 |
Oct 13, 2010 |
8491718 |
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13929085 |
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12774465 |
May 5, 2010 |
9054249 |
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12903750 |
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12154802 |
May 28, 2008 |
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12774465 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/02546 20130101;
Y02E 10/547 20130101; H01L 21/02625 20130101; H01L 21/02653
20130101; C30B 25/183 20130101; H01L 21/02439 20130101; H01L
21/02532 20130101; H01L 31/18 20130101; H01L 21/02488 20130101;
H01L 21/0256 20130101; H01L 31/0475 20141201; H01L 21/02425
20130101; H01L 31/0725 20130101; H01L 21/02598 20130101; C30B
23/025 20130101; H01L 31/1804 20130101; C30B 29/06 20130101; H01L
31/0687 20130101; H01L 21/0262 20130101; Y02E 10/544 20130101; C30B
11/12 20130101; H01L 21/02623 20130101; H01L 31/02366 20130101;
H01L 31/02363 20130101; H01L 31/0216 20130101; H01L 21/02603
20130101; H01L 21/02557 20130101; C30B 25/02 20130101; H01L
21/02422 20130101; H01L 21/02645 20130101; H01L 21/02595
20130101 |
International
Class: |
H01L 31/18 20060101
H01L031/18; H01L 31/0687 20060101 H01L031/0687; H01L 31/0236
20060101 H01L031/0236; H01L 31/0725 20060101 H01L031/0725; H01L
31/0216 20060101 H01L031/0216; H01L 31/0475 20060101
H01L031/0475 |
Claims
1. A method of growing a semiconductor film comprising the
following steps: providing a substrate; depositing a metal thin
film on said substrate; depositing an initial semiconductor film on
said metal thin film to form a semiconductor film on said
substrate; and depositing a different semiconductor material on
said semiconductor film.
2. The method as recited in claim 1 further comprising applying the
semiconductor film to tandem solar cells.
3. The method as recited in claim 1 further comprising depositing a
buffer layer on the substrate prior to depositing the metal thin
film.
4. The method as recited in claim 3 wherein the buffer layer is
textured.
5. The method as recited in claim 4 wherein the different
semiconductor film is deposited heteroepitaxially.
6. The method as recited in claim 4 wherein the semiconductor film
is textured.
7. The method as recited in claim 4 wherein the semiconductor film
forms epitaxially.
8. An electromagnetic device comprising: a substrate; a metal thin
film deposited on said substrate; an initial semiconductor film
deposited on said metal thin film to form an semiconductor film;
and a different semiconductor material deposited on said
semiconductor film.
9. The electromagnetic device as recited in claim 8 wherein
electromagnetic device is a tandem solar cell.
10. The method as recited in claim 8 further comprising a buffer
layer between the substrate and the metal thin film.
11. The method as recited in claim 10 wherein the buffer layer is
textured.
12. The method as recited in claim 11 wherein the different
semiconductor film is heteroepitaxial.
13. The method as recited in claim 8 wherein the semiconductor film
is formed epitaxially.
14. The electromagnetic device as recited in claim 8 wherein said
different semiconductor film is a silicide.
15. The electromagnetic device as recited in claim 8 wherein said
initial semiconductor film is silicon rich.
16. The electromagnetic device as recited in claim 8 wherein said
initial semiconductor film is a conductive layer for a photovoltaic
device to be built.
17. The electromagnetic device as recited in claim 8 wherein the
substrate is glass, metal tape or plastic.
18. The electromagnetic device as recited in claim 8 wherein the
substrate is crystalline.
19. The electromagnetic device as recited in claim 8 wherein the
substrate is non crystalline.
Description
[0001] The present invention is a continuation of U.S. patent
application Ser. No 13/929,085 filed Jun. 27, 2013, which is
continuation of U.S. patent application Ser. No 12/903,750 filed
Oct. 13, 2010, which is a continuation-in-part of U.S. patent
application Ser. No. 12/774,465 filed May 5, 2010, which is a
continuation of U.S. patent application Ser. No. 12/154,802 filed
May 28, 2008 both of which are hereby incorporated by reference in
its entirety.
REFERENCES CITED
TABLE-US-00001 [0002] U.S. Patent Documents 4,717,688 January 1987
Jaentsch 148/171 5,326,719 July 1994 Green et al. 427/74 5,544,616
August 1996 Ciszek et al. 117/60 6,429,035 B2 August 2002 Nakagawa
et al. 438/57 6,784,139 B1 August 2004 Sankar et al. 505/230
[0003] Other Publications
[0004] Kass et al, Liquid Phase Epitaxy of Silicon:
Potentialialities and Prospects", Physica B, Vol 129, 161
(1985)
[0005] Massalski et al, "Binary Alloy Phase Diagrams", 2.sup.nd.
edition, (1990), ASM International
[0006] Findikoglu et al, "Well-oriented Silicon Thin Films with
High Carrier Mobility on Polycrystalline Substrates", Adv.
Materials, Vol 17, 1527, (2005)
[0007] Teplin et al, "A Proposed Route to Thin Film Crystal Si
Using Biaxially Textured Foreign Template Layers" Conference paper
NREL/CP-520-38977, November 2005
[0008] Goyal et al., "The RABiTS approach: Using Rolling-assisted
Biaxially Textured Substrates for High-performance YBCO
Superconductors," MRS Bulletin, Vol. 29, 552, (2004)
[0009] Nast et al, "Aluminum Induced Crystallization of Amorphous
Silicon on Glass Substrates Above and Below the Eutectic
Temperature", Appl. Phys. Lett., Vol 73, 3214, (1998)
[0010] Girault et al, "Liquid Phase Epitaxy of Silicon at very low
Temperatures", J. Crystal Growth, Vol 37, 169 (1977)
[0011] Kayes et al, "Comparison of the Device Physics Principles of
Planar and Radial
[0012] p-n junction Nanorod Solar Cells", J. Appl. Phys., Vol 97,
114302, (2005)
FIELD OF THE INVENTION
[0013] The present invention is related to producing large grained
to single crystal semiconductor films, such as silicon films, for
producing articles such as photovoltaic and other electronic
devices.
FEDERAL FUNDING
[0014] None
BACKGROUND OF THE INVENTION
[0015] It is widely known that radiation from the sun striking
earth provides enough energy to supply all of mankind's needs for
energy for the indefinite future. Such a source of energy can be
clean and environmentally benign.
[0016] It is also widely known that global warming is associated
with the use of fossil fuels, such as coal, oil, and natural gas.
It is accepted by the scientific community that global warming can
have severe adverse effects around the planet. There are numerous
efforts around the world, combined with a sense of urgency, to cut
down emissions from the usage of fossil fuels. A dominant factor in
favor of the continual use of fossil fuels is their cost per unit
of available energy. If, for example, the cost of producing
photovoltaic cells can be reduced by a factor of approximately
three while maintaining efficiency of conversion, the photovoltaic
technology would become cost competitive with fossil fuels.
[0017] A major cost component in photovoltaic cells is the cost of
the substrate on which the semiconductor film capable of converting
sunlight into electricity is placed. The most widely used substrate
is single crystal silicon (Si). These substrates developed for the
microelectronics industry have been modified for application in
photovoltaic technology. If a silicon film could be deposited on an
inexpensive substrate, such as glass, and with comparable quality
as that found in silicon single crystals used in the
microelectronics industry, the cost of photovoltaic technology
would drop significantly.
[0018] Epitaxial growth of thin films is a very well established
process. It has been investigated by hundreds of researchers.
Epitaxial deposition provides a very viable way of growing very
good quality films. Many single crystal semiconductors and
insulator surfaces are used to study the epitaxial growth of
metallic films; for example, the growth of silver on silicon,
sapphire, or a mica surface. Epitaxial metallic films have also
been grown on other metallic films, such as gold on silver. In
contrast to metals, semiconductors, such as silicon, are difficult
to grow epitaxially. For example, heteroepitaxial films of silicon
have been successfully grown only on sapphire but at temperatures
that are relatively high for the applications we disclose here,
such as the growth of silicon on glass substrates.
[0019] In order to take advantage of highly textured large grained
films for photovoltaic technology two problems need to be solved:
inexpensive growth of high quality films and the availability of an
inexpensive substrate on which desirable properties can be
achieved. Here, we disclose a method for growing semiconductor
films, such as silicon, satisfying the two requirements listed
above and suitable for photovoltaic technology and other electronic
applications.
[0020] The thermodynamic stability and formation temperature of two
or more elements is described by a composition versus temperature
diagram, called a phase diagram. In this invention we shall make
use of phase diagrams. These phase diagrams are available in the
scientific literature (Massalski et al). The phase diagram provides
information on the behavior of different phases, solid or liquid as
a function of temperature and composition. For example, the
liquidus in a simple binary eutectic system, such as Au and Si,
shows how the relative composition of the liquid and solid, it is
in equilibrium with, changes with temperature. It is therefore
possible to choose an average composition, different from the
eutectic composition, and cool the mixture in such a way as to
precipitate out one phase or the other. If the composition is
chosen to be richer in silicon than the eutectic composition then
on cooling through the liquidus boundary between the single phase
liquid and the two phase liquid plus solid, silicon will nucleate
and form a solid phase. If on the other hand it is gold rich
relative to the eutectic composition the first solid phase to
nucleate is gold rather than silicon.
[0021] At and below the eutectic temperature the two components, in
this case, Au and Si solidify from the liquid phase to phase
separate into the two components Au and Si. The interface energy
between the two components is generally positive and therefore
drives the two components to aggregate into distinct phases with a
minimum of surface area between the two rather than a fine mixture
of the two. There is, however, the energetics of two other
interfaces to consider also: one with the substrate and the other
with vacuum or gas. In considering energetics it is not only the
chemical interaction of the metal or Si with the substrate that is
important but also its crystallographic orientation, for the
surface or interface energy depends upon orientation of the grains.
Another concern is the difference in lattice match between the
nucleating film and the substrate which can lead to strain induced
energy that is minimized by either inducing defects or not growing
uniformly in thickness across the substrate surface. These factors
determine if silicon is likely to deposit on the substrate
(heterogeneous nucleation) or nucleate and forms small crystals in
the liquid (homogeneous nucleation).
[0022] An advantage of using eutectics compositions is that the
eutectic temperature is lower than the melting temperature of the
constituent elements. For example, the eutectic temperatures of Au,
Al, and Ag with Si are 363, 577, and 835 degrees Centigrade
(.degree. C.), respectively. In contrast the melting temperatures
of the elements are 1064, 660, and 961.degree. C., respectively.
The melting temperature of silicon is 1414.degree. C. The eutectics
then offer the possibility of nucleating a silicon crystal from the
liquid far below the temperature at which pure liquid silicon
crystallizes. By a proper choice of the substrate surface exposed
to the nucleating silicon, it is possible to nucleate and grow
single crystal or large grained silicon films.
[0023] We have discussed silicon eutectics using elements such as
Au, Ag, and Al. However, it is possible to replace the elements by
silicon based compounds. For example, the compound nickel silicide
forms a eutectic with Si. There are numerous other examples of
silicide compounds forming a eutectic with Si (Massalski et al). An
advantage of using a silicide is that frequently the electrical
contact of the silicide with silicon has very desirable properties,
such as a good ohmic contact or a Schottky barrier. Some silicides
are also known to have an epitaxial relationship with silicon. In
this case, by appropriately choosing either a silicide rich or
silicon rich melt either the silicon can be induced to grow
epitaxially on the silicide or the silicide on silicon. A
disadvantage in this approach is the eutectic temperature, which is
generally high.
[0024] Low temperature solutions can also be formed with some
elements, For example, gallium (Ga) and Si have a eutectic
temperature of less than 30.degree. C., very close to that of the
melting point of Ga. There are other elements, such as indium or
tin that form low temperature liquid solutions with silicon. Si can
be nucleated from these solutions at very low temperatures relative
to pure silicon (Girault et al, Kass et al). These temperatures are
sufficiently low that it opens up the possibility of using organic
materials as substrates on which large grained to single crystal
films can be grown. While this is an advantage, there is also a
serious disadvantage; at these low temperatures, the silicon film
can contain defects and hence are not very useful as a photovoltaic
material. However, these very low temperature deposits can be used
to initiate the nucleation of a very thin silicon film, which is
subsequently thickened by using higher temperature processes to
optimize its photovoltaic properties.
[0025] The choice of a particular system (phase diagram) is not
only determined by temperature and energetics of the interfaces,
but also by the solubility of the second element in Si. It is
desirable to have precise control of the doping of Si in order to
optimize its semiconductor properties for photovoltaic
applications. It is also important to select the composition of the
substrate and temperature of processing such that there is minimal
or no chemical interaction between the silicon film and the surface
of the substrate on which it is being deposited.
[0026] From the preceding description, we can extract five common
points which are relevant to this invention. First, one end of the
phase diagram always has the semiconductor we wish to nucleate and
use to produce a film, we have used silicon in the preceding
examples but it could be germanium or a compound such as gallium
arsenide or cadmium selenide. Second, the thermodynamically
predicted concentration of the second element or phase in the
semiconductor is minimal. If there is solubility then it must be a
desirable dopant. For example aluminum (Al) in silicon behaves as a
p-type dopant and experience in the semiconductor industry has
shown that trace amount of Al can be desirable. Third, the liquidus
curve has the highest temperature on the semiconductor side. In
other words, the melting point of the semiconductor is greater than
the liquidus for all compositions in equilibrium with the
semiconductor. Fourth, the homogeneous nucleation energy of silicon
crystal from the melt is greater than that for heterogeneous
nucleation on the substrate. This latter condition promotes
heterogeneous nucleation. And, fifth, the temperature for epitaxial
growth is low enough to use inexpensive substrates such as glass
but high enough to promote a good quality silicon film. For
example, a growth temperature above approximately 550 degrees
Centigrade (550.degree. C.) is desirable to make a good quality
silicon film. The softening temperature of ordinary glasses is
around 600.degree. C. The softening temperature of borosilicate
glasses is higher. However it is not high enough to use
conventional deposition temperature of greater than 750 degrees
Centigrade for silicon on insulator, such as a sapphire
substrate.
[0027] In order to take full advantage of the invention disclosed
here the semiconductor material has to be deposited on a substrate
material which is inexpensive, and the surface of which enables
heterogeneous nucleation and growth. In the following we shall
discuss two specific methods for producing substrates suitable for
heterogeneous deposition of films for photovoltaic technology. Both
of these methods have been described in the scientific literature
and we do not claim to invent them. We include them here for
completeness.
[0028] The use of rolled and textured Ni and Ni-alloy sheets has
been proposed as substrate material for superconducting films and,
more recently, for films for photovoltaic devices (Findikoglu et
al). In order to facilitate the growth of epitaxial superconducting
films on such substrates, there have been two approaches described
in the scientific literature: in one the sharp rolling texture
produced in a rolled and annealed Ni alloy is used as a template on
which various epitaxial buffer layers are deposited followed
finally by an epitaxial film of a high temperature cuprate
superconductors (Goyal et al). In the second approach (Findikoglu
et al), the nickel ribbon is used as a substrate for ion beam
assisted deposition of a wide variety of highly textured ceramics,
for example, magnesium oxide (MgO). The ion beam aligns the growing
MgO film, which provides a template for the subsequent deposition
of the cuprate superconductor. The latter approach is not limited
to using metal tapes but can be extended to other inexpensive
substrates such as glass (Teplin et al). It has been found that
texture can also be induced in MgO by depositing the film on a
substrate that is inclined to the normal from the oncoming vapor of
MgO.
[0029] One limitation of the use of glass as a substrate has been
its softening temperature, which is generally lower than the
conventional processing temperatures required for the growth of
large grained or single crystal films of silicon. With the method
of depositing silicon films at low temperatures, described in this
invention, the use of buffered glass becomes an option for we can
deposit highly textured and large grained silicon on MgO at or
below the softening temperature of glass. Similarly, researchers
have grown crystalline aluminum oxide (Al.sub.2O.sub.3) on
inexpensive substrates (Findikoglu et al). We shall use MgO and
Al.sub.2O.sub.3 as illustrative examples. However, it is understood
to those skilled in the art that a variety of other materials can
also work. Both Findikoglu et al and Goyal et al describe other
buffer layers, including conducting ceramic layers, such as
TiN.
OBJECTS OF THE INVENTION
[0030] It is an object of the present invention to provide single
crystal or highly textured relatively large grained good quality
semiconductor films and, in particular silicon films, for
photovoltaic technology or other semiconductor devices, such as
field effect transistors used, for example, in displays.
[0031] It is yet another object of this invention to provide single
crystal or highly textured relatively large grained good quality
semiconductor films and, in particular silicon films, at low
temperatures. For example, if silicon films are used, the growth
temperature is between 450 and 750 degrees Centigrade.
[0032] It is yet another object of this invention to provide single
crystal or highly textured relatively large grained good quality
semiconductor films and, in particular silicon films, on
inexpensive substrates, for example, substrates such as glass on
which buffer layers such as MgO and/or Al.sub.2O.sub.3 have been
deposited.
SUMMARY OF THE INVENTION
[0033] In accordance with one aspect of the present invention, the
forgoing and other objects can be achieved by alloying a
semiconductor and, in particular silicon, with elements or
compounds that form an eutectic system, and increasing slowly the
concentration of the semiconductor, such as silicon, through the
liquidus line to reach the two phase region in which the
semiconductor, in particular silicon, nucleates out of the melt and
on the surface of a substrate.
[0034] In accordance with another aspect of the present invention,
the forgoing and other objects can be achieved by alloying a
semiconductor and, in particular silicon, with elements or
compounds that form an eutectic system, and increasing slowly the
concentration of the semiconductor, such as silicon, through the
liquidus line to reach the two phase region in which the
semiconductor, in particular silicon, nucleates on the surface of a
substrate to produce a highly textured relatively large grained or
single crystalline film.
[0035] In accordance with yet another aspect of the present
invention, the forgoing and other objects can be achieved by
alloying a semiconductor and, in particular silicon, with elements
or compounds that form an eutectic system, and increasing slowly
the concentration of the semiconductor, such as silicon, through
the liquidus line to reach the two phase region in which the
semiconductor, in particular silicon, nucleates on the surface of a
substrate made of a buffered tape in which texture is produced by
mechanical deformation and the buffer layers are epitaxial to the
texture of the metal tape. The buffer layer exposed to the melt
comprises of compounds, such as Al.sub.2O.sub.3 or MgO.
[0036] In accordance with yet another aspect of the present
invention, the forgoing and other objects can be achieved by
alloying a semiconductor and, in particular silicon, with elements
or compounds that form an eutectic system, and increasing slowly
the concentration of the semiconductor, such as silicon, through
the liquidus line to reach the two phase region in which the
semiconductor, in particular silicon, nucleates on the surface of a
substrate made of a buffered tape, a glass substrate, or any other
material suitable for inexpensive manufacture of photovoltaic cells
in which strong texture is produced by ion beam assisted
deposition. The final layer, which is exposed to the silicon melt,
comprises of compounds, such as Al.sub.2O.sub.3or MgO.
[0037] In accordance with still another aspect of the present
invention, the forgoing and other objects can be achieved by using
a solid phase composition comprising a semiconductor and, in
particular silicon, with elements or compounds that form an
eutectic system, and in which a thin film of the element or
compound is deposited first followed by the semiconductor, such as
silicon, and depositing at a temperature where the semiconductor
atoms diffuse through the element or compound to heterogeneously
nucleate on the substrate and propagate this crystallinity to the
semiconductor film remaining on top of the element or compound.
[0038] The method of manufacture of materials suitable for
photovoltaic technologies described in this invention are much less
expensive in the conversion of sunlight into electricity than those
practiced in the prior art.
BRIEF DESCRIPTION OF DRAWINGS
[0039] FIG. 1 shows the phase diagram of the eutectic system
Au--Si, taken from the literature (Massalski et al). The melting
points of the two elements Au and Si, as well as the eutectic
temperature are shown in the figure. The eutectic composition is
also indicated. The liquidus line, which defines the boundary
between the liquid gold-silicon alloy and solid silicon and a
gold-silicon liquid alloy, and on the silicon rich side of the
phase diagram, is marked. The figure also shows the change in
phases as the composition is changed by depositing silicon on a
film of gold held at constant temperature. As the silicon is
evaporated on to the gold film, the film comprises of gold solid
and a liquid gold-silicon alloy which changes from the point marked
by 11 towards 12. Further deposition of silicon results in the film
entering the liquid phase region between the points marked 12 and
13. As the silicon deposition continues beyond the point 13, the
liquidus boundary, solid silicon nucleates from the liquid which is
in equilibrium with a silicon-gold liquid alloy. The solid silicon
is deposited on a MgO substrate, forming a highly textured and
relatively large grained heterogeneously nucleated film. The
thickness of the solid silicon film increases till the deposition
is stopped. As it cools Si continues to deposit from the melt while
the Au--Si liquid solution becomes richer in gold. This process
continues till the eutectic temperature is reached, at which point
the liquid solidifies and phase separates into gold and silicon
solids.
[0040] We have used the phase diagram of the Au--Si eutectic. The
Al--Si eutectic is very similar. Here we can heterogeneously
nucleate silicon from the Al--Si melt on a single crystal sapphire
substrate to form a single crystal heteroepitaxial silicon
film.
DETAILED DESCRIPTION OF THE INVENTION
[0041] As described above, we have disclosed a method to produce
low cost single crystal or large grained epitaxially aligned good
quality semiconductor films, in particular silicon, for
photovoltaic technology. We have also suggested the use of tapes or
glass slabs as substrate materials. The tapes provide strong
texture on which buffer layers suitable for silicon growth are
present. Our method can produce silicon epitaxy at substantially
lower temperatures than those commonly practiced, hence not only
minimizing interaction with the surface of the substrate but also
enabling the use of glass substrates.
[0042] We shall be using the eutectics of silicon with gold and
aluminum in describing the details of the invention. It is,
however, understood that one skilled in the art can extend the
methodology to other semiconductors such as germanium, gallium
arsenide, or the cadmium selenide class of photovoltaic
materials.
[0043] FIG. 1 shows the phase diagram of the eutectic system
Au--Si. The eutectic composition is nominally 18.6 atomic percent
pct Si and the rest being gold. A thin gold film is first deposited
on the buffered substrate. This is followed by silicon deposition.
As the silicon concentration increases the film first forms a two
phase mixture of gold and liquid gold-silicon. The composition of
the latter is determined by the choice of the deposition
temperature. With further increase of silicon, the liquid phase
region, marked 12, is reached and the remaining gold is dissolved.
With still further increase of the amount of silicon, the second
liquidus phase boundary, marked 13, is reached and subsequent
deposition of silicon atoms results in a solid phase of silicon in
equilibrium with the silicon-gold liquid. If the substrate surface
is suitably chosen, for example MgO crystals, the solid silicon
nucleates heterogeneously onto the surface. The choice of the
temperature of deposition is determined by balancing two
considerations: quality in terms of defects of the epitaxial film;
too low a temperature or too rapid a growth rate of the film at
that temperature can introduce defects versus too high a
temperature when chemical interaction or mechanical integrity of
the substrate limit the usefulness of the material.
[0044] We have started with vapor deposition of the metallic film
and added silicon to it to traverse the phase diagram from point
marked 11 in the figure. However, the metallic element and silicon
can be co evaporated to reach any concentration between the points
marked 12 and 13 in the figure and subsequently silicon added to
reach the desired thickness, before cooling to room
temperature.
[0045] When the desired thickness of the silicon film is obtained,
the substrate with the film is cooled to room temperature. Even
though the amount of gold required to catalyze a silicon film is
small, it can be further reduced by etching the gold away, for
example, by using iodine etch, available commercially. This gold
can be recycled
EXAMPLES OF THE INVENTION
[0046] The following non-limiting examples are used as
illustrations of the various aspects and features of this
invention.
i. Example 1
[0047] A good high vacuum system with two electron beam guns, is
used to deposit gold and silicon independently. A glass substrate
coated with ion beam assisted deposited MgO film is held at
temperatures between 575 and 600.degree. C. These are nominal
temperatures. It is understood to one skilled in the art that lower
or higher temperatures can also be used depending upon the
softening temperature of the glass substrate or the reaction
kinetics of either gold or silicon with the metallic tape or its
buffer layers when used as substrates. A thin gold film of
approximately 10 nm thickness is deposited first. This is followed
by a silicon film deposited at a rate of 2 nm per minute on top of
the gold film. The ratio of the thickness of the gold and silicon
films is chosen such that the final composition ensures that a
point, marked 13, in FIG. 1 is reached. This point lies at the
boundary between the two phase region of solid Si and a liquid
Si--Au mixture. For example, for a 10 nm gold film followed a 100
nm silicon film satisfies this condition. Additional silicon film
nucleates heterogeneously on the MgO surface to form the desired
thin film. The film can now be cooled to room temperature, where
the film now comprises of two phases: gold and a relatively large
grained and highly textured film of silicon on MgO.
[0048] By relatively large grained it is understood to imply a
grain size larger than would have been achieved if a silicon film
had been deposited under the same conditions but without Au. In the
example discussed above the crystallographic texture is strongly
[111]. Instead of an insulating substrate such as MgO, it is
possible to choose stable and electrically conducting nitrides,
such as TiN.
[0049] The gold diffuses to the surface of the silicon film, driven
by its lower surface energy relative to the silicon surface. The
film is etched in a solution, such as a commercially available
iodine based chemical, which removes the gold from the two phases,
gold and silicon, leaving behind a silicon film.
[0050] This silicon film can now be used as the surface on which a
thicker silicon film appropriately doped to form a p-n junction,
suitable for applications such as photovoltaics, can be deposited.
Alternatively, the thin silicon film can be used for
heteroepitaxial deposition of other semiconductors, which might be
more efficient convertors of sunlight to electricity.
[0051] We have used two electron beam guns as an illustrative
example. It is understood to one skilled in the art that other
methods such as a single gun with multiple hearths, chemical vapor
deposition, thermal heating, or sputtering can also be used.
Example 2
[0052] A good high vacuum system with two electron beam guns is
used to deposit aluminum and silicon independently. A glass
substrate or a Ni based substrate coated with a buffer layer of
Al.sub.2O.sub.3 is held at temperatures between 600 and 615 degree
.degree. C. These are nominal temperatures. It is understood to one
skilled in the art that lower or higher temperatures can also be
used depending upon the softening temperature of the glass
substrate or the reaction kinetics of either aluminum or silicon
with the metallic tape or its buffer layers when used a substrates.
The eutectic Al--Si is used instead of the Au--Si example above. A
thin Al film 6 nm thick is deposited on the Al.sub.2O.sub.3
followed by a 100 nm thick silicon deposition, and as described in
example 1, above, the two phase region comprising of solid silicon
and a liquid Si--Al mixture is reached. The deposition is stopped
and the sample is slowly cooled to room temperature. Aluminum
diffuses through the silicon film, driven by its lower surface
energy relative to silicon. The silicon film is heteroepitaxially
aligned by the Al.sub.2O.sub.3 surface. The aluminum film on the
surface can be etched chemically by well known processes to leave
behind a silicon film. The surface of this film can now be used for
further growth of epitaxial films either for photovoltaic devices
or for field effect transistors.
[0053] We note, as stated earlier, that silicon can be grown
epitaxially on sapphire but at temperatures higher than 750.degree.
C. This is a well established commercial process. However, in the
absence of aluminum, silicon deposition at, say, 600.degree. C.
produces a fine grained film rather than a heteroepitaxial film, as
described above.
Example 3
[0054] We describe in this example how different methods of
deposition can be combined to take advantage of highly textured
films as described in example 1, above. The Si film produced from
the deposition of example 1 is etched to remove the Au and then
placed back into the vacuum chamber and p.sup.+-Si is deposited on
this film. This latter layer serves two purposes: it provides a
conducting layer for a photovoltaic device to be subsequently built
on it and can be the starting point for a variety of differently
configured photovoltaic devices as, for example, a nanowire
photovoltaic device. Here a 2-3 nm thick gold film is deposited on
the silicon using an electron gun. This 2-3 nm thick gold film
breaks up into nanoparticles and is the starting point used by a
number of investigators to use chemical vapor deposition to grow
nanowires and use these nanowires for photovoltaic devices. The
difference is that we show how an inexpensive buffered glass can be
used rather than a relatively expensive single crystal Si
substrate.
[0055] A second possibility is to deposit a Au film of thickness 5
nm as islands on a MgO buffered glass substrate, using lithographic
or other means known in the art. A heavily doped silicon (p.sup.+
or n.sup.-) film is now deposited on the surface followed by a p-
or n-type silicon using electron beam deposition, as described in
example 1. The thickness of the heavily doped film is in the micron
range whereas the lightly doped film is of the order of 100 nm. The
deposition process is now changed and chemical vapor deposition is
used for subsequent deposition of suitably doped films of silicon,
practiced in the art to grow silicon nanowire photovoltaic devices.
The heavier doped silicon film serves the purpose of a conducting
layer. Using gold islands has the advantage of controlling the
nanowires diameter and length in order to maximize the efficiency
of the photovoltaic cell (Kayes et al). Instead of using the
insulating MgO buffer layer, a conducting material such as TiN can
be used.
i. Example 4
[0056] We describe how different methods of deposition and
temperature can be combined to take advantage of films grown as
described in Examples 1 and 2, above to produce desirable device
structures. Instead of depositing the Al and Si films described in
Example 2 above, by two electron beam heated sources in a vacuum,
we deposit the Si by decomposition of silane using a chemical vapor
deposition chamber. This is a well known industrial process. As in
Example 2, we deposit a 6 nm thin film of Al on to a sapphire
substrate held at 600.degree. C. We then introduce silane gas into
the chamber. At these temperatures, the silane decomposes to form a
Si film which reacts with the Al to produce a eutectic solution and
when this solution is saturated with Si (the equivalent of point
marked 13 in FIG. 1 for an Au--Si alloy) the Si precipitates to
heterogeneously nucleate to form an epitaxial film on the surface
of the sapphire substrate. This film is continuous and can be doped
by adding borane or phosphene gases to silane to obtain p or n type
semiconductor behavior. This film can now be used as a basis to
construct thin film photovoltaic cells or, alternatively, grow
nanowires of Si on top of it by simply lowering the temperature of
the substrate below the eutectic temperature of Al--Si (577.degree.
C.). For example, if the temperature of deposition is 500.degree.
C., Si nanowires will grow on top of the Si film. The Al particles
that precipitate out of the Al--Si solution once the temperature of
the substrates is below the eutectic temperature now catalytically
reduce the silane gas to form nanowires, as described in the
literature. These nanowires can be used to build electronic
devices, including photovoltaic cells.
[0057] While the principles of the invention have been described in
connection with specific embodiments, it should be understood
clearly that the descriptions, along with the examples, are made by
way of example and are not intended to limit the scope of this
invention in any manner. For example, a variety of suitable
substrates different from the examples given above can be utilized
or a different variety of deposition methods and conditions can be
employed as would be understood from this invention by one skilled
in the art upon reading this document.
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