CERTIFIED TRANSLATION OF PCT/EP2009/006037
METHOD FOR LOCAL CONTACTING AND LOCAL DOPING OF A
SEMICONDUCTOR LAYER
The invention relates to a method for local contacting and local doping of a
semiconductor layer and a semiconductor structure having at least one local doping.
It is known to contact a surface of a semiconductor layer, covered with at least one
passivating dielectric layer, such that a metal layer is applied upon the dielectric
layer and the metal layer is briefly heated locally via a radiation source. This
heating leads to a local melt-mixture of the metal layer, the dielectric layer, and the
semiconductor, so that after the melt-mixture has solidified an electric contact
develops between the semiconductor and the metal layer.
Such a method is particularly used in the production of solar cells and described in
DE 100 46 170 A1, for example.
The present invention is based on the object of improving the known method such
that the contact features are improved particularly with regards to the
recombination features of the semiconductor surface in the area of the contact so
that another optimization of the effectiveness of the solar cell is achieved and/or the
production costs are further reduced.
This object is attained in a method for the local contacting and local doping of a
semiconductor layer according to claim 1 as well as a semiconductor structure
according to claim 18. Advantageous embodiments of the method according to the
invention are found in claims 2 through 17; an advantageous embodiment of the
semiconductor structure according to the invention is found in claims 19 through
20.
The method according to the invention for locally contacting and locally doping a
semiconductor layer comprises a processing step A, in which a layer structure is
created on the semiconductor layer.
The semiconductor layer typically comprises a semiconductor wafer, such as a
silicon wafer. However, the method according to the invention may also be used for
any other semiconductor layer, such as a semiconductor layer at the surface of a
laminated structure, for example.
The processing step A of the method comprises the processing steps i and ii, with in
the processing step i at least one intermediate layer being applied onto one side of
the semiconductor layer. Subsequently, in the processing step ii, at least one metal
layer is applied upon the intermediate layer most recently applied in step i, with the
metal layer at least partially covering the most recently applied intermediate layer.
When using the method according to the invention for producing a rear contact,
typically the intermediate layer will essentially cover the side of the semiconductor
layer in its entirety and the metal layer will essentially cover the entire
intermediate layer. However, the scope of the invention also includes, for example,
an embodiment with frontal contacts of a solar cell, in which the intermediate layer
only partially covers the side of the semiconductor layer and/or the metal layer only
partially covers the intermediate layer.
In a processing step B, the layer structure is locally heated such that in a section
briefly a melt-mixture develops comprising at least partial areas of at least the
metal layer, the intermediate layer, and the semiconductor layer, and after the
melt-mixture has solidified a contact forms between the metal layer and the
semiconductor layer. If the solar cell according to the invention comprises several
intermediate layers at the location of the local heating, the melt-mixture is
preferably formed from partial areas of all intermediate layers, the metal layer, and
the semiconductor layer.
Therefore, an electrically conductive connection exists between the metal layer and
the semiconductor layer in the area the melt-mixture has solidified.
It is essential that in the method according to the invention at least one
intermediate layer represents a doping layer. This doping layer includes a dopant,
with the dopant showing a higher solid-matter solvency in the semiconductor layer
than the solid-matter solvency of the metal of the metal layer in the semiconductor
layer.
The invention is based on the knowledge of the inventor that during
recrystallization, due to the use of a doping layer, the dopant is integrated in a
substituting fashion into the crystalline grid of the semiconductor in a higher
concentration based on to its solid-matter solubility being greater than the one of
the metal of the metal layer and thus, after the melt-mixture has solidified, a locally
high doping develops in the area of the electric contact between the metal layer and
the semiconductor layer due to the dopant.
In order to create high-efficiency solar cells it is known to create locally high doping
areas in those sections of the semiconductor layer, using several photo-lithographic
steps and inward diffusion, in which during subsequent processing steps the electric
contacting shall occur between the metal layer and the semiconductor layer.
By using the method according to the invention, it is possible for the first time with
a local heating of the layer structure preferably via a radiation source, particularly
a laser, to simultaneously produce locally high doping as well as electric contacting
between the metal layer and the semiconductor layer. The method according to the
invention particularly comprises the advantage that the locally high doping always
develops in the section of the semiconductor layer in which the electric contacting
occurs between the metal layer and the semiconductor layer. Any local adjustment
between the areas of the locally high doping and the electric contacting is therefore
excluded.
In reference to methods for locally high doping of prior art, the method according to
the invention additionally shows the advantage that any removal of the doping
layer can be avoided. Rather, both the doping layer and the metal layer remain on
the semiconductor layer and also on the finished solar cell for example so that no
additional processing steps are necessary for removing the doping layer.
The contacting features are considerably improved by the locally high doping with
the dopant, particularly the contact resistance between the semiconductor layer and
the metal layer is reduced and the boundary area between the semiconductor
surface and the metal layer is considerably better protected from the recombination
of carriers of minority charges and thus the electric features are improved.
Particularly in the application of the method according to the invention for
producing solar cells these improvements lead to an increase in effectiveness and/or
to a reduction of costs during manufacturing because no additional processing steps
are necessary to produce the locally high doping.
The object is further attained in a semiconductor structure according to the
invention as shown in claim 18. The semiconductor layer comprises a
semiconductor layer, at least one intermediate layer on one side of the
semiconductor layer, and at least one metal layer, which at least partially covers
the intermediate layer or, in case of several intermediate layers, the most recently
applied intermediate layer and/or the intermediate layer located farthest away from
the semiconductor layer, with the semiconductor structure comprising at least one
local area, which represents a solidified melt-mixture of sections of at least the
metal layer, the first layer, and the semiconductor layer, so that the metal layer and
the semiconductor layer are connected in an electrically conducting fashion at the
location of the solidified melt-mixture. The solidified melt-mixture is the result of a
short local heating, which briefly causes the existence of a local melt-mixture of the
above-mentioned layers.
It is essential that at least one intermediate layer is a doping layer, which
comprises a dopant, with the dopant having a higher solubility in the semiconductor
layer than the metal of the metal layer.
The semiconductor layer according to the invention is preferably produced via the
method according to the invention.
A minimum concentration of the dopant in the doping layer is advantageous in
order to achieve a sufficiently high concentration of the dopant after the melt-
mixture has solidified.
Beneficially the concentration of the dopant in the doping layer is equal or greater
than 1 x 1021 cm-3, particularly beneficial is a concentration equal or greater than 5
x 1021 cm-3.
It is particularly advantageous if the concentration of the dopant for a selected
thickness of the doping layer is standardized for the unit of area of the boundary
semiconductor layer/doping layer and amounts to at least 2.5 x 1014 cm-2,
particularly at least 1 x 1015 cm-2. If the doping layer is applied upon an
intermediate layer, the above-mentioned values per unit of area are advantageous
for the boundary intermediate layer/doping layer.
Experiments of the applicant have shown that advantageously such elements are
used as dopants which are part of the main Group III or V of the periodic table
and/or compounds showing such elements as their components. In particular, it is
advantageous that the dopant is boron or phosphorus or gallium.
The applicant was able to produce very good contacting results in experiments
using the method according to the invention in which the doping layer was
embodied from boron-silicate glass.
For a further improvement of the effectiveness of solar cells it is advantageous that
the first intermediate layer applied upon the semiconductor layer has a passivating
effect with regards to the speed of surface recombination at the boundary of said
semiconductor layer towards said first intermediate layer. This way, the
recombination of the carriers of the minority charge is avoided by the locally high
doping at the areas of the electric contacting but also at the areas between the
locally high dopings due to the passivating effect of the first intermediate layer
applied upon the semiconductor layer.
Here, it is within the scope of the invention that only the doping layer is applied
between the semiconductor layer and the metal layer and the doping layer is
embodied such that it achieves the above-described passivating effect. In
particular, it is advantageous, though, first to apply a layer upon the surface of the
semiconductor layer particularly suitable for passivating the surface, subsequently
the doping layer upon said passivating layer, and finally to apply the metal layer
upon the doping layer.
The doping layer is advantageously thinner than 1 um, particularly thinner than
500 nm. This way, sufficient heat conductivity is ensured during a local
introduction of heat in order to create the melted layer.
In another advantageous embodiment of the method according to the invention the
local melting occurs in an essentially punctual or linear area.
In particular it is advantageous to use punctual contacts for the creation of contacts
at the rear of a solar cell. However, for the creation of frontal contacts at a solar cell
it is advantageous to create linear contact, because typically solar cells are
contacted at the front by linear metallic structures connected to each other in a
comb-like fashion.
Advantageously the local area in which the layers are melted shows a diameter of
less than 500 µm, particularly less than 200 µm. This ensures that in the adjacent
areas, in which no contacting occurs, the crystalline structure of the semiconductor
is not damaged and thus its electric features are not compromised.
Advantageously a multitude of local contacts and locally high dopings are created
via the method according to the invention. In particular, it is advantageous in solar
cells that the portion of the overall area of all melted sections in reference to the
total surface area of the semiconductor layer amounts to less than 20%, particularly
to less than 5%. An excessive portion of the areas with high doping and electric
contacting would lead to an increased combination of carriers of minority charges;
the above-stated percentages ensure an optimal ratio between the contacting
sections with locally high doping and the areas with increased surface passivation.
As described above, in the processing step B a local heating of the laminate occurs
such that a melt-mixture forms.
Advantageously, in step B the local heating is executed such that at least the
temperature of the eutectic point of the melt-mixture is reached, particularly that
the laminate structure is locally heated to at least 550 degrees centigrade.
As described above, the method according to the invention has the advantage that
no removal of the doping layer is required. The transportation of the charge
carriers thus occurs, beginning at the semiconductor layer, from the semiconductor
layer to the area of the solidified melt-mixture in the metal layer, and from there
into potentially connected external circuits and/or a neighboring solar cell in case of
modular wiring.
Typically, the metal layer is embodied to minimize loss due to ohmic resistance.
Thus it is advantageous for the doping layer to show a resistance greater than the
resistance of the metal layer by at least a factor of 10, particularly at least by a
factor of 100, preferably by at least a factor of 1000 so that the current flow occurs
parallel to the surface of the semiconductor layer essentially within said
semiconductor layer and in the metal layer, however not in the doping layer.
In particular, it is advantageous that the doping layer is electrically insulating.
This way, additionally a barrier is formed against any undesired contacts between
the metal layer and the semiconductor layer.
In order to improve the optic features of the solar cell it is advantageous for at least
the first of the layers applied on the semiconductor layer to be an optically
transparent layer, particularly a layer transparent in the range of wavelengths
from 300 nm to 1500 nm.
This is necessary for the use of the method according to the invention to create
frontal contacts of a solar cell, because in this case the electromagnetic radiation is
received via the front of the semiconductor layer and thus transparency is necessary
particularly in the spectral range relevant for solar cells.
However, it is also advantageous to embody the intermediate layer in a transparent
fashion, as described above, when the method according to the invention is used to
create rear contacts at a solar cell, because this way the reflective features of the
rear of the solar cell are improved and the electromagnetic radiation impinging the
solar cell and reaching its rear is reflected and thus the overall absorption of
radiation in the solar cell, and thus the effectiveness of the solar cell, is increased.
Another increase of the effectiveness of the solar cell can be achieved with the
method according to the invention in an advantageous embodiment by applying an
additional intermediate layer between the doping layer and the metal layer, with
this intermediate layer being embodied without any corrosive features in reference
to the metal layer. This way, a reduction of the effectiveness due to corrosion of the
metallic conductor layer is avoided or at least reduced and consequently the
degradation of the effectiveness of the solar cell due to environmental influences is
reduced.
Such layers are preferably produced from the materials silicon dioxide or silicon
nitride or silicon carbide.
In another preferred embodiment of the method according to the invention an
additional intermediate layer is applied between the semiconductor layer and the
doping layer. This intermediate layer preferably comprises silicon dioxide or
amorphous silicon or amorphous silicon nitride or aluminum oxide. Similarly, the
scope of the invention also comprises for such an intermediate layer to be made
from a combination of the above-mentioned materials, such as described in M.
Hofmann et al, Proceedings of the 21st EU-PVSEC, Dresden, 2006.
These layers particularly show very good passivating effectiveness with regards to
features of surface recombination of the semiconductor layer.
Experiments of the applicant have shown that particularly the following laminate
system is advantageous for the method according to the invention.
A passivation layer having a thickness from approximately 10 nm to 30 nm is
applied onto a silicon wafer (semiconductor layer), subsequently a doping layer
having a thickness from approximately 100 nm to 200 nm, thereupon an
intermediate layer free from any corrosive features, for example a layer of silicon
nitride having a thickness of approximately 30 nm, and finally a metal layer, for
example an aluminum layer having a thickness from 0.5 µm to 10 µm, preferably a
thickness of approximately 2 µm.
In the following, additional features and preferred embodiments of the method
according to the invention are explained using the figures. Shown here:
Fig. 1 is a schematic structure of a solar cell 1,
Fig. 2 is a section of the resulting layer structure at the rear of the solar cell
according to Fig. 1 prior to the local melting step, and
Fig. 3 is the section according to Fig. 2 after the melting and solidification of the
melt-mixture.
For purely competitive reasons, the industrial production of solar cells is subject to
the goal of producing solar cells with the highest possible effectiveness, i.e. with the
electric current yielded being as high as possible from the solar energy input
received by the solar cell, and simultaneously keeping the production expenditures
and thus the costs as low as possible.
The following explanations of measures to be observed shall serve for a detailed
understanding of the measures for an optimal production of solar cells:
Solar cells are structural parts that transform light into electric energy. Usually
they are made from a semiconductor material; most solar cells are made from
silicon, comprising the n- and/or p-conducting semiconductor ranges. The
semiconductor ranges are called emitters and/or bases, as known per se. Inside the
solar cell, positive and negative charge carriers are created by the light impinging
the solar cell, which are spatially separated from each other at the boundary area
between the n-(emitter) and p-doped (base) - semiconductor area, the so-called p-n
junction. By way of metallic contacts, connected to the emitters and the bases,
these charge carriers separated from each other can be connected.
In the simplest form, solar cells comprise a full-area base 2 and emitter sections 3,
with the emitter 3 being located at the side facing the light, i.e. the front of the solar
cell. For the purpose of illustration, here reference is made to Fig. 1 showing a solar
cell 1 of prior art.
For electrically contacting the base 2, usually the rear of the solar cell 1 is provided
with a metal layer 4 extending over the entire area, upon which in turn suitable
rear contacting - conductor circuits 5 are applied, for example made from AlAg.
The emitter range 3 contacts a metal grid 6, with here the object is given to lose as
little light as possible for the solar cell due to reflection at the metal contact, i.e. the
metal grid 6 comprises a finger structure in order to cover as little of the area of the
solar cell as possible. In order to optimize the performance yielded from the solar
cell 1 it is additionally attempted to keep the optic loss due to reflection as little as
possible. This is achieved by the deposit of so-called anti-reflection coating 7 (ARC)
on the frontal surface of the solar cell 1. The layer thickness of the anti-reflection
coating 7 is selected such that straight, destructive interference of the reflected
light results in the spectral range most important with regards to energy. The anti-
reflection materials used are titanium dioxide, silicon nitride, and silicon dioxide,
for example. Alternatively or additionally thereto, a reduction of reflection can be
achieved by producing a suitable surface texture using etching and/or mechanical
processing, as also discernible from the solar cell shown in Fig. 2. Here, the emitter
area 3 as well as the anti-reflection coating 7 applied upon the emitter, are
embodied with such a structure that the light impinging the structured surface of
the solar cell 1 has an increased probability of coupling at the structures embodied
like pyramids. Even in case of a solar cell according to Fig. 2, the electric contacting
of the emitter 3 occurs with a metal grid 6 as delicate as possible, with only one
small contacting finger being shown in Fig. 2. Furthermore, the anti-reflection
coating 7 can also serve as a passivating layer, which serves as a mechanical
surface protection but additionally also shows intrinsic effects with regards to
reducing surface-recombination processes, which is explained in greater detail in
the following.
With regards to the electric contacting of a solar cell, it must be differentiated
between the front and the rear. While at the rear of the solar cell it is attempted to
create a contact, which is primarily characterized in a low contacting and
conducting resistance, the front must additionally allow the reception of as much
light as possible to the solar cell. Thus, usually a comb-shaped structure is created
at the front, as discernible from Fig. 1, in order to keep losses from both resistance
as well as shadowing as little as possible. Usually, either fully covering or
structured, e.g., grid-like, contacts are used at the rear of the solar cell.
A section of the resulting laminate structure at the rear of the solar cell is shown in
Fig. 2, with the sequence of layers being inversed in Figs. 2 and 3, i.e. the layer
positioned at the bottom in the solar cell is shown at the top in Figs. 2 and 3.
The rear contacts of these solar cells, shown in Fig. 1, are advantageously created
by the method according to the invention. In the following, this creation is
explained using Figs. 2 and 3, which show a section of a local area at the rear of the
solar cell shown in Fig. 1, in which a local electric contact and a locally high doping
is created. In this exemplary embodiment, a silicon disk (or silicon wafer) 8
represents a semiconductor layer, from which the solar cell is produced, shown in
Fig. 1. A passivating layer 9 made from silicon dioxide, approx. 10 nm thick, is
applied upon the silicon disk 8. Subsequently a thin layer of high-doped boron-
PCT/EP2009/006037
silicate glass is applied, approximately 80 nm thick. This doping layer 10 comprises
the dopant boron in a concentration of approximately 2 x 1021 cm-3.
An anti-reflection coating 11, approx. 10 nm thick, is embodied as a silicon-dioxide
layer and applied upon the doping layer 10.
In this exemplary embodiment therefore a total of 3 intermediate layers are applied
upon the silicon disk 8 (processing step A i.)
Subsequently, a metal layer 12, made from aluminum and having a thickness from
approximately 2 µm to 3 µm (processing step A ii.) is applied upon the last
intermediate layer, i.e. the silicon-dioxide layer.
Subsequently, using a brief local radiation at the location 13 of the aluminum layer
a melt-mixture created from aluminum, the thin intermediate layers located
underneath it, and an area of a depth of a few pm of the semiconductor layer, i.e.
the silicon disk 8, is produced. The radiation occurs for a period from approx. 50 to
5000 ns. After the end of the local radiation an area with a thickness of a few µm
recrystallizes from the previously formed melt-mixture. This is schematically
shown in Fig. 3 by the area 14 for the contact formed.
The dopant boron shows a solubility of approximately 3 x 1019 cm-2 in silicon,
compared to the considerably lower solubility of aluminum amounting to 3 x 1018
cm2 in silicon. Therefore, during recrystallization the boron is integrated in the
crystalline grid with a much higher concentration when the silicon structure forms
during solidification due to the considerably higher solubility compared to
aluminum. The solidified area therefore comprises a local boron-high doping and
additionally an electric contact is created between the metal layer 12 and the silicon
disk 8 (processing step D).
Therefore, the method according to the invention is advantageous for the local
contacting of a solar cell in reference to methods of prior art according to DE 100 46
170 A1: Due to the higher doping with boron in the area of the electric contacting a
significantly lower recombination rate is realized at the contacts. This way, an
increased number of contact points can be implemented, i.e. an increased overall
area of electric contacts, without reducing the effectiveness of the solar cell due to
increased recombination. However, due to the increased overall area of the electric
contacting the electric output resistance is reduced when charged carriers are
conducted from the silicon disk via the metal layer, so that overall the effectiveness
of the solar cell is increased.
The above-described exemplary embodiment relates to the production of the rear
contacts of the solar cell shown in Fig. 1. However, the scope of the invention also
covers using the method according to the invention for the creation of frontal
contacting and/or for a n-doped semiconductor layer.
CLAIMS
1. A method for local contacting and local doping of a semiconductor layer,
comprising the following processing steps:
A) creating a layer structure on the semiconductor layer by
i) applying at least one intermediate layer (9, 10, 11) on one side of the
semiconductor layer, and
ii) applying at least one metal layer (12) on the intermediate layer (11)
applied last in the step i., with the metal layer at least partially
covering last applied the intermediate layer (11),
B) locally heating the layer structure such that in a local area briefly a melt-
mixture is formed of at least sections comprising the layers: the metal layer
(12), the intermediate layer (9, 10, 11), and the semiconductor layer, and
after solidification of the melt-mixture a contacting forms between the metal
layer (11) and the semiconductor layer,
characterized in that in step A) i), at least one of the intermediate layers
applied comprises a doping layer (10), which includes a dopant, with the
dopant showing greater solubility in the semiconductor layer than the metal
of the metal layer.
2. A method according to claim 1, characterized in that a concentration of the
dopant in the doping layer (10) is equal or greater than 1 x 1021 cm-3,
particularly equal or greater than 5 x 1021 cm-3.
3. A method according to claim 1, characterized in that the concentration of the
dopant in reference to a boundary area of the doping layer (10) to the
semiconductor layer or the intermediate layer is equal or greater than 2.5 x
1014 cm2, particularly equal or greater than 1 x 1015 cm-2.
4. A method according to at least one of the previous claims, characterized in
that the dopant includes a Group III or Group V element, particularly that
the dopant represents boron or phosphorus or gallium.
5. A method according to at least one of the previous claims, characterized in
that the doping layer (10) is embodied from boron-silicate glass.
6. A method according to one of the previous claims, characterized in that the
first intermediate layer (9) applied on the semiconductor layer has a
passivating effect with regards to the speed of the surface recombination at a
boundary area of the semiconductor layer / intermediate layer.
7. A method according to at least one of the previous claims, characterized in
that the doping layer (10) is thinner than 1 µm, particularly thinner than 500
nm.
8. A method according to at least one of the previous claims, characterized in
that the local area in which the layers are melted has a diameter smaller
than 500 µm, particularly smaller than 200 µm.
9. A method according to at least one of the previous claims, characterized in
that the local melting occurs in an essentially punctual or linear area.
10. A method according to one of the previous claims, characterized in that a
multitude of local areas are melted, with an overall portion of the area of all
sections in reference to the total surface of the semiconductor layer is less
than 20 %, particularly less than 5 %.
11. A method according to at least one of the previous claims, characterized in
that in step B locally a heating occurs to at least 550 °C.
12. A method according to at least one of the previous claims, characterized in
that the doping layer (10) has a layer resistance, which is greater than the
layer resistance of the metal layer (12) by at least a factor of 10, particularly
by at least a factor of 100, preferably by at least a factor of 1000.
13. A method according to at least one of the previous claims, characterized in
that the doping layer (10) is electrically insulating.
14. A method according to at least one of the previous claims, characterized in
that at least the first layer applied on the semiconductor layer is an optically
transparent layer, particularly a layer essentially transparent in wave
lengths ranging from 300 nm to 1500 nm.
15. A method according to at least one of the previous claims, characterized in
that an additional intermediate layer (11) is applied between the doping layer
(10) and the metal layer (12), with the additional intermediate layer (11) has
no corrosive features in reference to the metal layer.
16. A method according to at least one of the previous claims, characterized in
that an additional intermediate layer (9) is applied between the
semiconductor layer and the doping layer (10), preferably comprising silicon
dioxide or amorphous silicon or amorphous silicon nitride or aluminum oxide.
17. A method according to at least one of the previous claims, characterized in
that the doping layer (10) is applied via a chemical vapor deposition or via
evaporation deposition or cathode-sputtering or in the form of a spin-on layer.
18. A semiconductor structure, comprising a semiconductor layer, at least one
intermediate layer (9, 10, 11) on one side of the semiconductor layer, and at
least one metal layer (12), which at least partially covers the intermediate
layer or, in case of several intermediate layers the intermediate layer applied
last, with the semiconductor structure comprising at least one local area,
which represents a solidified melt-mixture of sections of at least the metal
layer, the first layer, and the semiconductor layer, so that the metal layer
and the semiconductor layer are connected at a location of the solidified melt-
mixture in an electrically conducting fashion, characterized in that at least
one intermediate layer represents a doping layer (10), which includes a
dopant, with the dopant having a greater solubility in the semiconductor
layer than a metal of the metal layer (12).
19. A semiconductor layer according to claim 18, characterized in that the
semiconductor structure comprises a doping via the dopant in the area of the
solidified melt-mixture.
20. A semiconductor structure according to at least one of claims 18 and 19,
characterized in that the semiconductor structure is produced by a method
according to at least one of claims 1 through 17.

A method for local contacting and local doping of a semiconductor layer including
the following process steps: A) Generation of a layer structure on the semiconductor
layer through i) application of at least one intermediate layer on one side of the
semiconductor layer, and ii) application of at least one metal layer onto the
intermediate layer last applied in step i), wherein the metal layer at least partly
covers the last applied intermediate layer, B) Local heating of the layer structure in
such a manner that in a local region a short-time melt-mixture of at least partial
regions of at least the layers: metal layer, intermediate layer and semiconductor
layer, forms. After solidification of the melt-mixture, a contacting is created
between metal layer and semiconductor layer. It is essential that in step A) i) at
least one intermediate layer designed as dopant layer is applied, which contains a
dopant wherein the dopant has a greater solubility in the semiconductor layer than
the metal of the metal layer.