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BACKGROUND OF THE INVENTION
The present invention relates to a Shottky barrier type solid-state element
for use in a film-shaped solar battery or the like and a method of
producing the same.
In a method heretofore employed for producing a semiconductor solid-state
element for use in a solar battery or the like, a solid-state element is
usually formed by polishing a single-crystal bulk of, for instance,
silicon (Si). Therefore, in the conventional method, it is very difficult
to form a thin film of semiconductor the thickness of which is on the
order of microns; and even if such a thin film is produced, the amount of
material that can be utilized will be as low as several percent or less of
the total. Moreover, it is very difficult for any conventional technique
to provide an excellent Schottky barrier highly qualified for use in a
solar battery. For instance, it is considered to be impossible or almost
impossible for the conventional C.V.D. process, vacuum evaporation process
or sputtering process to produce a high-quality crystalline film and, when
a Schottky barrier is to be formed, to control and optimize its
microstructure and composition and to make its density sufficiently high
and its resistance sufficiently low.
SUMMARY OF THE INVENTION
Therefore, the primary object of the present invention is to eliminate the
above-mentioned disadvantages of the prior art and to provide a Schottky
barrier type solid-state element high in quality and productivity,
thin-film-shaped, light in weight, easy to transport, and suitable for use
in a solar battery, and a method of producing the same.
According to the present invention, there are provided a Shottky barrier
type solid-state element and a method of producing the same, the Schottky
barrier type solid-state element having a Schottky barrier type element
portion consisting of a metallic board and a semiconductor film layer
provided on the surface of the metallic board, the metallic board being
formed of such a metal as can form a Schottky barrier between itself and
the semiconductor film layer, the semiconductor film layer being provided
on the metallic board so as to form a Schottky barrier therebetween, and a
semiconductor-side terminal electrode at least provided on the external
surface of the semiconductor film layer of the Schottky barrier type
element portion so as to obtain an ohmic contact therewith, wherein at
least the semiconductor film layer of the Schottky barrier type element
portion is formed by the ionized-cluster-beam deposition process which
vaporizes a material (a semiconductor, in this case) to be deposited to
form a vapor, injects the vapor into a vacuum region of about 10.sup.-2
Torr or less to form aggregates of atoms of the vapor called clusters,
bombards the clusters with electrons to ionize at least a part of the
clusters thereby producing ionized clusters, and accelerates the ionized
clusters by an electric field to make them impinge on a substrate thereby
forming a film layer thereon.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages will become more apparent from the
following description taken in conjunction with the accompanying drawings,
in which:
FIG. 1 is a schematical sectional view of an example of the evaporation
apparatus for carrying out the ionized-cluster-beam deposition process for
use in the method of producing a Schottky barrier type solid-state element
according to the present invention, showing the principle thereof;
FIG. 2 is a schematical sectional view of the Schottky barrier type
solid-state element according to one embodiment of the present invention,
showing the essential part thereof for the explanatory purpose; and
FIG. 3 is a schematical sectional view of the Schottky barrier type
solid-state element according to another embodiment of the present
invention, showing the essential part thereof for the explanatory purpose.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, description will be hereinafter made on one
embodiment of the present invention to which the ionized-cluster-beam
deposition process is applied.
Reference numeral 1 designates a substrate on the surface 2 of which a
single-crystal semiconductor film layer 3 made of, e.g., p-type or n-type
silicon is deposited. Numeral 4 designates a closed-type crucible having
at least an injection nozzle 5. A material 6 of the semiconductor film
layer 3 to be deposited on the substrate 1, that is, a p-type or n-type
semiconductor material, for instance, p-type or n-type silicon is placed
in the crucible 4, which is then heated up to a high temperature by
suitable heating methods such as resistance heating and electron
bombardment heating (as shown in FIG. 1) to vaporize the material 6
therein to form a vapor 7 the pressure of which is about 10.sup.-6 to
several Torrs. The vapor 7 is then injected from the crucible 4, through
the nozzle 5, into a vacuum region 8 kept at a pressure 1/100 or less of
the vapor pressure in the crucible 4 and also at about 10.sup.-2 Torr or
less. At this time, the vapor 7 is converted into aggregates of atoms
called clusters 9 due to supercooling phenomenon caused by adiabatic
expansion. One cluster usually consists of about 100 to 2,000 atoms. If
one of the atoms constituting each cluster 9 is ionized, an ionized
cluster can be formed. Therefore, a filament 10 is provided as a therionic
emission source to emit electrons, which impinge on the clusters 9 to
produce ionized clusters 12. The ionized clusters 12, while flying
together with the non-ionized neutral clusters 9 towards the substrate 1
provided in the route of the clusters, are accelerated by an electric
field created by an acceleration power source 13 connected to electrodes
positioned at and/or near the substrate 1, and thereby are brought into
collision with the surface 2 of the substrate 1 to form a film layer 8
thereon.
In the above-mentioned ionized-cluster-beam deposition process which forms
ionized clusters of a material-to-be-deposited and accelerates them to let
them impinge on a substrate thereby depositing a film layer thereon, the
surface 2 of the substrate 1 is at all times kept clean due to continuous
sputter-cleaning action produced by bombardment of the ionized clusters
12, and therefore a very clean and highly adhesive deposition can be
achieved. In addition, the ionized clusters 12 are accelerated with a
proper high energy given by a high-voltage electric field applied, and
therefore, when they impinge on the surface 2 of the substrate 1, their
kinetic energy is partly converted into thermal energy, which causes a
local temperature rise and thereby enables the film layer 3 of the
depositing material to epitaxially grow on the surface 2 of the substrate
1. As mentioned above, the self-heating effect of the surface of the
depositing film layer due to the conversion of the kinetic energy of the
clusters into thermal energy can independently achieve an excellent
crystal growth without any particular external heating of the substrate 1.
However, a suitable combination of an increase in the kinetic energy of
clusters and application of external heating to the substrate will further
increase the single-crystal area and therefore can made to grow a more
excellent epitaxial crystal film layer.
Referring now to FIG. 2, description will be hereinafter made on the first
embodiment of the Schottky barrier type solid-state element used as a
solar battery according to the present invention and a method of producing
the same.
Reference numeral 14 designates a thin-sheet-shaped metallic board 14 made
of such a metal as can form a Schottky barrier between itself and a
semiconductor to be deposited thereon. Numeral 16 designates a p-type or
n-type semiconductor crystalline film layer deposited on the surface 15 of
the metallic board 14 by the above-mentioned ionized-cluster-beam
deposition process so that a Schottky barrier may be formed therebetween.
Thus an element portion 17 of a Shottky barrier type solid-state element
20 used as solar battery is formed, which consists of the metallic board
14 and the crystalline semiconductor film layer 16. In this case, the
metallic board 14 and the semiconductor film layer 16 correspond,
respectively, to the substrate 1 and the depositing material film layer 3
described in the case of the ionized-cluster-beam deposition process with
reference to FIG. 1. The reference numeral 18 designates a
metallic-board-side terminal electrode coated at a suitable portion on the
metallic board 14 where the crystalline film layer 16 is not coated. In
addition, a semiconductor-side terminal electrode 19 is provided at a
suitable portion on the semiconductor film layer 16 coated on the metallic
board 14 so as to form a Schottky barrier therebetween, by depositing
thereon a metallic film layer made of such a metal as can establish an
ohmic contact with the material of the semiconductor film layer 16 using
the above-mentioned ionized-cluster-beam deposition process. The stage for
forming the metallic-board-side terminal electrode 18 on the metallic
board 14 may be carried out either before or after the stage for forming
the semiconductor film layer 16 on the metallic board 14, as a matter of
course. Moreover, the metallic board 14 may be formed so that a part
thereof can be used as the metallic-board-side terminal electrode 18,
depending upon the size and shape thereof.
The metallic board 14, at least the upper layer thereof, may be preferably
made of gold, chromium, etc. The semiconductor film layer 16 provided on
the metallic board 14 so as to form a Schottky barrier therebetween may be
formed of a p-type or n-type semiconductor and may preferably have a
thickness of several microns to several hundred microns. The metallic film
layer of the semiconductor-side terminal electrode 19 provided on the
upper surface of the semiconductor film layer 16 so as to establish an
ohmic contact therebetween may be preferably made of a metal containing
aluminum, indium and others when the semiconductor to be in contact
therewith is, for instance, p-type silicon; and a metal containing
antimony, etc. when the semiconductor to be in contact therewith is, for
instance, n-type silicon.
As mentioned above, the Schottky barrier type element portion 17 is
provided at the opposite surfaces thereof with terminal electrodes by the
ionized-cluster-beam deposition process disclosed in the above embodiment.
Accordingly, when the ionized clusters impinge on the deposition surface,
the kinetic energy of the ionized clusters is partly converted into
thermal energy, and therefore a very good contact can be established and,
in addition, a sufficient ohmic contact can be obtained between the metal
and the semiconductor of the element portion coming into contact with each
other in a heat processing carried out at a temperature far lower than
that applied in the case of the prior art such as the conventional vacuum
evaporation process. From the standpoint of the process procedure, this
heat processing has an advantage in that it can be performed either during
or after the deposition stage using the ionized-cluster-beam deposition
process.
Reference numeral 21 designates a reflection preventive film layer properly
formed on the upper surface of the element portion 17. The reflection
preventive film layer 21 provides a light-receiving layer for effectively
absorbing rays incident thereon from the outside, and may be formed by the
ionized-cluster-beam deposition process or other various conventional
methods. The metallic film layer of the semiconductor-side terminal
electrode 19 formed on the upper surface of the element portion 17 can be
provided with the function of the reflection preventive film layer 21
concurrently, if the material, conditions, etc. are properly selected.
Thus a Schottky barrier type solid-state element 20 used as a solar battery
very high in quality can be produced.
In each stage mentioned above in which the above ionized-cluster-beam
deposition process is carried out, it is just the matter of course that
the processing condictions such as substrate temperature, intensity of
electron current for ionization and the acceleration voltage for ionized
clusters should be properly selected so that each deposited film layer may
be optimized in adhesion, strength, etc.
As to the stage in the above embodiment where the ionized-cluster-beam
deposition process is applied, description has been made on the case where
the metallic board 14, on which the semiconductor film layer is deposited
so as to form a Schottky barrier therebetween, is made to correspond to
the substrate 1 referred to in the ionized-cluster-beam deposition process
described with reference to FIG. 1 and where the semiconductor film layer
16 is deposited on this metallic board 14 to form a Schottky barrier;
however, on the contrary, the semiconductor film layer 16 may be made to
correspond to the above substrate 1, and the material of the metallic
board 14 may be deposited on this semiconductor film layer 16.
In the above embodiment, a method of producing a Schottky barrier type
solid state element used as a solar battery having a mono-Schottky-barrier
type element portion is shown which comprises the steps of depositing a
semiconductor film layer on the surface of a metallic board by the
ionized-cluster-beam deposition process to form a mono-Schottky barrier
type element portion, the metallic board being made of metal at least at
the surface thereof, and fixing terminal electrodes on the metallic board
and the semiconductor film layer, respectively. However, the present
invention is not limited to such embodiment, as a matter of course. For
instance, a plurality of laminated element portions similar to the above
element portion may be provided between the electrodes in ohmic contact
therewith to form what is called the multi-Schottky-barrier type
solid-state element, i.e, solar battery, which can be made very thin and
highly effective.
Reference is now made to FIG. 3, which is a sectional side view of the
essential part of a Schottky barrier type solid-state element used as a
solar battery according to another embodiment of the present invention.
Description will be hereinafter made on this solid-state element used as a
solar battery and a method of producing the same.
Reference numeral 22 designates a film-speed or thin-sheet-shaped or
flexible film-shaped substrate board made of various organic substances
such as polyimide and Mylar, or inorganic insulating materials such as
glass and ceramics, or metals. On the upper surface of this substrate
board 22, such a metallic film layer as can establish an an ohmic contact
with a semiconductor film layer to be subsequently deposited thereon is
deposited by the ionized-cluster-beam deposition process as described with
reference to FIG. 1 to form a semiconductor-side terminal electrode 23.
After the semiconductor-side terminal electrode 23 is formed, a
semiconductor film layer 24 (the thickness of which is, for instance,
about several thousand angstroms to several microns) made of n-type or
p-type silicon is deposited on the upper surface of the above-mentioned
semiconductor-side terminal electrode 23 in a laminated manner by the
ionized-cluster-beam deposition process similar to the above-mentioned. In
this deposition stage, the substrate board 22 with the semiconductor-side
terminal electrode 23 thereon is made to correspond to the substrate 1
referred to in the description on the ionized-cluster-beam deposition
process with reference to FIG. 1, and the semiconductor film layer 25
corresponding to the film layer 3 of FIG. 1 is deposited on the
semiconductor-side terminal electrode provided on the substrate board 22,
by the ionized-cluster-beam deposition process.
After the above deposition stage is completed, a metallic board 25, the
thickness of which is about several hundred angstroms to several microns,
formed of such a metal film layer as can form a Schottky barrier between
itself and the semiconductor film layer 24 is deposited on the upper
surface of the semiconductor film layer 24 by the ionized-cluster-beam
deposition process similar to the above-mentioned. Thus a Schottky barrier
type element portion 29, consisting of the metallic board 25 and the
semiconductor film layer 24, is formed.
After the above deposition process is completed, a current-collecting
metallic-board-side terminal electrode 26, comb-shaped, wire-shaped or the
like, is provided on the upper surface of the metallic board 25, and a
reflection preventive film layer 27 is provided at a proper portion on the
upper surface of the metallic board 25.
As mentioned above, the silicon film layer is formed by the
ionized-cluster-beam deposition process, in which, when the ionized
clusters impinge on the deposition surface, its kinetic energy is partly
converted into thermal energy. Therefore, the silicon film layer thus
deposited shows good crystalline properties, and can provide a sufficient
ohmic contact with the metallic film layer being in contact therewith, by
a heat processing carried out at a temperature far lower than that applied
in the conventional method.
In each stage mentioned above in which the ionized-cluster-beam deposition
process is carried out, it is just the matter of course that the
processing conditions such as substrate temperature, intensity of electron
current for ionization, and acceleration voltage for ionized clusters
should be properly selected according to the substrate material, the
surface condition and depositing material of each film layer, etc. so that
each deposited film layer may be optimized in quality, adhesion, strength,
etc.
The embodiment shown above is concerned with a Schottky barrier type
solid-state element having a set of laminated film layers. More
particularly, as mentioned above, the method of producing such a
solid-state element comprises the steps of forming a semiconductor-side
terminal electrode, formed of such a metal film layer as can establish an
ohmic contact with a semiconductor to be subsequently deposited thereon,
on a substrate board; forming a semiconductor film layer on the
semiconductor-side terminal electrode; and providing a metallic board,
formed of such a metallic film layer as can form a Schottky barrier
between itself and the semiconductor film layer, on the semiconductor film
layer. However, the present invention is not limited to this embodiment,
and the above sets of steps may be repeated to form a plurality of sets of
laminated film layers, as a matter of course. In this manner, a
solid-state element having a further increased photo-electric conversion
efficiency, etc. can be produced.
The Schottky barrier type solid-state element thus produced has a
construction in which a very high-quality and film-shaped Schottky barrier
type semiconductor element is provided on the substrate board 22.
Therefore, if the substrate board 22 is formed of a flexible film, the
solid-state element as a whole can be made sufficiently flexible and
therefore free from fatigue due to folding or rolling-up.
In the above embodiment of the present invention, the ionized-cluster-beam
deposition process is used for each of the deposition stages of the
semiconductor-side terminal electrode 23, the semiconductor film layer 24
and the metallic board 25, and thereby the adhesion of each film layer is
remarkably increased and the product quality is much improved. In this
case, it is necessary that the ionized-cluster-beam deposition process
should be applied at least to the deposition stage of the semiconductor
film layer 24.
As mentioned above, the substrate board 22 is preferably made of
thin-sheet-shaped or flexible film-shaped organic substance such as Mylar
or Polyimide, or thin-sheet-shaped inorganic substance such as glass or
ceramics, or film-shaped or thin-sheet-shaped metal. Of these various
shapes and materials, a proper one may be selected according to the
purpose and use of the Schottky barrier type solid-state element to be
produced.
The metallic film of the semiconductor-side terminal electrode 23 is
preferably made of a metal containing, for instance, aluminum and indium
when the semiconductor film layer 24 in contact therewith is of p-type
silicon, and a metal containing, for instance, antimony when the
semiconductor film layer 24 is of n-type silicon.
The metallic board 25 is preferably made of gold, chromium, etc.
The semiconductor-side terminal electrode 23 and the metallic board 25 are
in ohmic contact with the semiconductor film layer 24 and the
metallic-board-side terminal electrode 26, respectively.
The reflection preventive film layer 27 shown in the above embodiment is
provided to form a light-receiving surface for effectively absorbing rays
incident thereon from the outside, when the solid-state element is used as
a solar battery; and is formed on the upper surface of the solid-state
element by the ionized-cluster-beam deposition process of the present
invention or other various methods. Instead of providing the reflection
preventive film layer 27, the metallic-board-side terminal electrode 26
may be so formed that it can concurrently perform the reflection
preventive function.
It will apparent from the foregoing description that the present invention
is concerned with the Schottky barrier type solid-state element which is
produced by depositing at least the semiconductor film layer in a
laminated manner by the ionized-cluster-beam deposition process and is
highly suitable for use in a solar battery or others.
Various features and effects of the present invention are enumerated as
follows:
(1) In the ionized-cluster-beam deposition process used in the present
invention, the substrate surface is at all times kept clean due to the
sputter-cleaning action of ionized clusters during deposition, and
therefore a deposited film can be made very high in adhesion and quality.
(2) Since the deposition process of the present invention accelerates the
ionized clusters with a suitable high energy created by a high-voltage
electric field applied, it can produce what is called the self-heating
effect of the surface of the depositing film layer which creates a local
temperature rise due to partial conversion of the kinetic energy of the
ionized clusters into thermal energy when the ionized clusters impinge on
the substrate, and also it can produce what is called the migration effect
which breaks up the ionized and non-ionized clusters into individual
atomic particles and spreads them over the surface of the depositing film
layer by the energy they have at the time of impact. Therefore, the
deposition process of the present invention can achieve an excellent
crystal growth of the depositing material.
Furthermore, in the present invention, the crystal growth of the depositing
film layer is carried out while being controlled by the crystalline
properties of the substrate, and therefore a Schottky barrier type
solid-state element excellent in quality and crystalline properties can be
produced.
(3) The deposition process of the present invention can control the
acceleration voltage and current during deposition so that the
microstructure and composition of the deposition boundary may be
optimized. Accordingly, it can increase the deposition density and
decrease the resistance at the deposition surface, and therefore can
produce a solid-state element whose construction and composition is most
suitable for use in a solar battery.
(4) The thickness of the film layer to be deposited on the substrate so as
to form a Schottky barrier therebetween can be controlled by properly
adjusting the processing conditions during deposition. Therefore, the
thickness of the semiconductor film layer positioned upper than the
junction, i.e., the Schottky barrier portion can be made less than that
formed by the conventional methods. As a result, the wavelength
sensitivity range for incident rays is widened and the photovoltaic
conversion efficiency is improved.
(5) The deposition process of the present invention for forming the n-type
or p-type semiconductor film layer can achieve the concentration control
of the depositing material during deposition, which has been difficult to
achieve by the prior art. As a result, the present invention can produce a
high-performance solid-state element which can effectively take out
charged particles produced by light irradiation.
(6) If the terminal electrode provided on the element portion is formed
also by the ionized-cluster-beam deposition process as mentioned in the
above embodiment, heat processing for obtaining an ohmic contact
therebetween can be carried out at a temperature far lower than that
applied in the prior art, and thereby can easily provide a connection
therebetween with a sufficient ohmic contact, resulting in a high-quality
product.
(7) As shown in the embodiment of the present invention, a flexible
Schottky barrier type solid-state element, almost impossible to obtain by
the prior art, can be produced by forming a film-shaped semiconductor
element on a flexible organic film using the ionized-cluster-beam
deposition process. The solid-state element thus produced can save its raw
materials, and is light in weight, small in size and flexible; and can be
folded or rolled up into a compact size, and therefore is easy to handle,
transport, store, etc.
(8) The present invention can achieve metal-to-metal multi-layer bonding
with strong adhesion, which is considered impossible to achieve by the
conventional evaporation processes because of weak adhesion. Especially
when the solid-state element is to be used as a solar battery, a
multi-layer film suitable for preventing the reflection of incident rays
can be formed.
(9) In the above-mentioned embodiment, description was made on the case
where silicon was used as semiconductor. However, the semiconductor that
can be used in this invention is not limited to silicon alone. Besides
silicon, this invention can use other element semiconductors and compound
semiconductors such as Ge, GaAs, InP and CdTe to produce a Schottky
barrier type solid-state element. When a compound semiconductor is used,
the compound semiconductor itself is not necessarily required to be put in
the closed type crucible, and a suitable mixture of component elements of
the compound semiconductor may be put therein.
Obviously, numerous modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the invention may
be practiced otherwise than as specifically described.
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