U.S. patent application number 14/499788 was filed with the patent office on 2016-03-31 for epitaxial growth of czt(s,se) on silicon.
The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Nestor A. Bojarczuk, Talia S. Gershon, Supratik Guha, Byungha Shin, Yu Zhu.
Application Number | 20160093755 14/499788 |
Document ID | / |
Family ID | 55450245 |
Filed Date | 2016-03-31 |
United States Patent
Application |
20160093755 |
Kind Code |
A1 |
Bojarczuk; Nestor A. ; et
al. |
March 31, 2016 |
Epitaxial Growth of CZT(S,Se) on Silicon
Abstract
Techniques for epitaxial growth of CZT(S,Se) materials on Si are
provided. In one aspect, a method of forming an epitaxial kesterite
material is provided which includes the steps of: selecting a Si
substrate based on a crystallographic orientation of the Si
substrate; forming an epitaxial oxide interlayer on the Si
substrate to enhance wettability of the epitaxial kesterite
material on the Si substrate, wherein the epitaxial oxide
interlayer is formed from a material that is lattice-matched to Si;
and forming the epitaxial kesterite material on a side of the
epitaxial oxide interlayer opposite the Si substrate, wherein the
epitaxial kesterite material includes Cu, Zn, Sn, and at least one
of S and Se, and wherein a crystallographic orientation of the
epitaxial kesterite material is based on the crystallographic
orientation of the Si substrate. A method of forming an epitaxial
kesterite-based photovoltaic device and an epitaxial
kesterite-based device are also provided.
Inventors: |
Bojarczuk; Nestor A.;
(Poughkeepsie, NY) ; Gershon; Talia S.; (White
Plains, NY) ; Guha; Supratik; (Chappaqua, NY)
; Shin; Byungha; (Daejeon, KR) ; Zhu; Yu;
(West Harrison, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Family ID: |
55450245 |
Appl. No.: |
14/499788 |
Filed: |
September 29, 2014 |
Current U.S.
Class: |
136/256 ;
136/258; 438/95 |
Current CPC
Class: |
H01L 21/02557 20130101;
Y02E 10/50 20130101; H01L 21/02483 20130101; H01L 21/02433
20130101; H01L 31/0326 20130101; H01L 21/02488 20130101; H01L
21/02658 20130101; H01L 21/02631 20130101; H01L 21/02568 20130101;
H01L 21/0256 20130101; H01L 21/02381 20130101; H01L 21/02609
20130101 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/0368 20060101 H01L031/0368; H01L 31/18
20060101 H01L031/18 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0001] This invention was made with Government support under
Contract number DE-EE0006334 awarded by Department of Energy. The
Government has certain rights in this invention.
Claims
1. A method of forming an epitaxial kesterite material, comprising
the steps of: selecting a silicon (Si) substrate based on a
crystallographic orientation of the Si substrate; forming an
epitaxial oxide interlayer on the Si substrate to enhance
wettability of the epitaxial kesterite material on the Si
substrate, wherein the epitaxial oxide interlayer comprises a
material that is lattice-matched to Si; and forming the epitaxial
kesterite material on a side of the epitaxial oxide interlayer
opposite the Si substrate, wherein the epitaxial kesterite material
comprises copper (Cu), zinc (Zn), tin (Sn), and at least one of
sulfur (S) and selenium (Se), wherein the epitaxial oxide
interlayer is configured to prevent etching of the Si substrate
during the step of forming of the epitaxial kesterite material, and
wherein a crystallographic orientation of the epitaxial kesterite
material is based on the crystallographic orientation of the Si
substrate such that the crystallographic orientation of the
epitaxial kesterite material is controlled by selecting the Si
substrate having a particular crystallographic orientation.
2. The method of claim 1, further comprising the step of: selecting
the Si substrate from one of a Si(001) substrate and a Si(111)
substrate.
3. The method of claim 1, wherein the epitaxial oxide interlayer is
formed having a thickness of from about 5 nanometers to about 30
nanometers, and ranges therebetween.
4. The method of claim 1, wherein the epitaxial oxide interlayer
completely covers a surface of the Si substrate on which the
epitaxial kesterite material is formed.
5. The method of claim 1, wherein the epitaxial oxide interlayer
comprises erbium oxide (Er.sub.2O.sub.3), lanthanum oxide
(La.sub.2O.sub.3), or yttrium oxide (Y.sub.2O.sub.3) each of which
is lattice-matched to Si.
6. The method of claim 1, wherein the step of forming the epitaxial
oxide interlayer and the step of forming the epitaxial kesterite
material are performed by evaporation in a vacuum chamber in-situ
without breaking vacuum between deposition of the epitaxial oxide
interlayer and the epitaxial kesterite material.
7. The method of claim 6, further comprising the steps of: cleaning
the Si substrate; and placing the Si substrate, once cleaned, in
the vacuum chamber.
8. The method of claim 6, wherein the step of forming the epitaxial
oxide interlayer is performed in the vacuum chamber at a pressure
of from about 1.times.10.sup.-6 Torr to about 1.times.10.sup.-9
Torr, and ranges therebetween, at a temperature of from about
500.degree. to about 750.degree., and ranges therebetween.
9. The method of claim 6, wherein the step of forming the epitaxial
kesterite material is performed in the vacuum chamber at a pressure
of from about 1.times.10.sup.-6 Torr to about 1.times.10.sup.-9
Torr, and ranges therebetween, at a temperature of from about
350.degree. C. to about 550.degree. C., and ranges
therebetween.
10. A method of forming an epitaxial kesterite-based photovoltaic
device, comprising the steps of: selecting a Si substrate based on
a crystallographic orientation of the Si substrate; forming an
epitaxial kesterite material on the Si substrate which serves as an
absorber layer of the photovoltaic device, wherein the epitaxial
kesterite material comprises Cu, Zn, Sn, and at least one of S and
Se, and wherein a crystallographic orientation of the epitaxial
kesterite material is based on the crystallographic orientation of
the Si substrate such that the crystallographic orientation of the
epitaxial kesterite material is controlled by selecting the Si
substrate having a particular crystallographic orientation; forming
a buffer layer on a side of the epitaxial kesterite material
opposite the Si substrate; and forming a transparent front contact
on a side of the buffer layer opposite the epitaxial kesterite
material.
11. The method of claim 10, further comprising the step of: forming
an epitaxial oxide interlayer on the Si substrate such that the
epitaxial oxide interlayer is present between the Si substrate and
the epitaxial kesterite material, wherein the epitaxial oxide
interlayer enhances wettability of the epitaxial kesterite material
on the Si substrate, wherein the epitaxial oxide interlayer is
configured to prevent etching of the Si substrate during the step
of forming of the epitaxial kesterite material, and wherein the
epitaxial oxide interlayer comprises a material that is
lattice-matched to Si.
12. The method of claim 11, wherein the epitaxial oxide interlayer
is formed having a thickness of from about 5 nanometers to about 30
nanometers, and ranges therebetween.
13. The method of claim 11, wherein the epitaxial oxide interlayer
completely covers a surface of the Si substrate onto which the
epitaxial kesterite material is formed.
14. The method of claim 11, wherein the epitaxial oxide interlayer
comprises Er.sub.2O.sub.3, La.sub.2O.sub.3, or Y.sub.2O.sub.3 each
of which is lattice-matched to Si.
15. The method of claim 10, further comprising the step of:
selecting the Si substrate from one of a Si(001) substrate and a
Si(111) substrate.
16. The method of claim 10, wherein the buffer layer comprises
cadmium sulfide (CdS), a cadmium-zinc-sulfur material of the
formula Cd.sub.1-xZn.sub.xS (wherein 0<x.ltoreq.1), indium
sulfide (In.sub.2S.sub.3), zinc oxide, zinc oxysulfide, or aluminum
oxide (Al.sub.2O.sub.3).
17. The method of claim 10, wherein the transparent front contact
comprises at least one of indium-tin-oxide (ITO) and aluminum doped
zinc oxide (AZO).
18. An epitaxial kesterite-based device, comprising: a Si
substrate; an epitaxial oxide interlayer on the Si substrate,
wherein the epitaxial oxide interlayer comprises a material that is
lattice-matched to Si; and an epitaxial kesterite material on a
side of the epitaxial oxide interlayer opposite the Si substrate,
wherein the epitaxial oxide interlayer serves to enhance
wettability of the epitaxial kesterite material on the Si
substrate, wherein the epitaxial kesterite material comprises Cu,
Zn, Sn, and at least one of S and Se, wherein a crystallographic
orientation of the epitaxial kesterite material is based on the
crystallographic orientation of the Si substrate, and wherein the
epitaxial oxide interlayer completely covers a surface of the Si
substrate onto which the epitaxial kesterite material is formed and
serves to prevent etching of the Si substrate.
19. The epitaxial kesterite-based device of claim 18, wherein the
epitaxial oxide interlayer comprises Er.sub.2O.sub.3,
La.sub.2O.sub.3, or Y.sub.2O.sub.3 each of which is lattice-matched
to Si, and wherein the epitaxial oxide interlayer has a thickness
of from about 5 nanometers to about 30 nanometers, and ranges
therebetween.
20. The epitaxial kesterite-based device of claim 18, further
comprising: a buffer layer on a side of the epitaxial kesterite
material opposite the Si substrate; and a transparent front contact
on a side of the buffer layer opposite the epitaxial kesterite
material.
Description
FIELD OF THE INVENTION
[0002] The present invention relates to epitaxial CZT(S,Se)
materials and more particularly, to epitaxial growth of CZT(S,Se)
materials on silicon (Si) including techniques for controlling
CZT(S,Se) crystallographic orientation and for enhancing
wettability of the CZT(S,Se) on Si using a thin film epitaxial
oxide interlayer.
BACKGROUND OF THE INVENTION
[0003] Kesterite materials, such as those containing copper (Cu),
zinc (Zn), tin (Sn) and at least one of sulfur (S) and selenium
(Se) (abbreviated herein as "CZT(S,Se)"), are inexpensive and
earth-abundant. CZT(S,Se) is of current interest for use as a solar
cell absorber material.
[0004] The most common form of CZT(S,Se) in high efficiency solar
devices is polycrystalline. While the role of grain boundaries is
not well understood, it is thought that grain boundaries in
polycrystalline CZT(S,Se) absorber materials can undesirably lead
to voltage problems in solar devices.
[0005] Thus, solar devices employing grain boundary-free forms of
CZTS would be desirable.
SUMMARY OF THE INVENTION
[0006] The present invention provides techniques for epitaxial
growth of CZT(S,Se) materials on silicon (Si) including techniques
for controlling CZT(S,Se) crystallographic orientation and for
enhancing wettability of the CZT(S,Se) using a thin film epitaxial
oxide interlayer. In one aspect of the invention, a method of
forming an epitaxial kesterite material is provided. The method
includes the steps of: selecting a Si substrate based on a
crystallographic orientation of the Si substrate; forming an
epitaxial oxide interlayer on the Si substrate to enhance
wettability of the epitaxial kesterite material on the Si
substrate, wherein the epitaxial oxide interlayer is formed from a
material that is lattice-matched to Si; and forming the epitaxial
kesterite material on a side of the epitaxial oxide interlayer
opposite the Si substrate, wherein the epitaxial kesterite material
includes copper (Cu), zinc (Zn), tin (Sn), and at least one of
sulfur (S) and selenium (Se), and wherein a crystallographic
orientation of the epitaxial kesterite material is based on the
crystallographic orientation of the Si substrate.
[0007] In another aspect of the invention, a method of forming an
epitaxial kesterite-based photovoltaic device is provided. The
method includes the steps of: selecting a Si substrate based on a
crystallographic orientation of the Si substrate; forming an
epitaxial kesterite material on the Si substrate which serves as an
absorber layer of the photovoltaic device, wherein the epitaxial
kesterite material includes Cu, Zn, Sn, and at least one of S and
Se, and wherein a crystallographic orientation of the epitaxial
kesterite material is based on the crystallographic orientation of
the Si substrate; forming a buffer layer on a side of the epitaxial
kesterite material opposite the Si substrate; and forming a
transparent front contact on a side of the buffer layer opposite
the epitaxial kesterite material.
[0008] In yet another aspect of the invention, an epitaxial
kesterite-based device is provided. The epitaxial kesterite-based
device includes: a Si substrate; an epitaxial oxide interlayer on
the Si substrate, wherein the epitaxial oxide interlayer is formed
from a material that is lattice-matched to Si; and an epitaxial
kesterite material on a side of the epitaxial oxide interlayer
opposite the Si substrate, wherein the epitaxial oxide interlayer
serves to enhance wettability of the epitaxial kesterite material
on the Si substrate, wherein the epitaxial kesterite material
includes Cu, Zn, Sn, and at least one of S and Se, and wherein a
crystallographic orientation of the epitaxial kesterite material is
based on the crystallographic orientation of the Si substrate.
[0009] A more complete understanding of the present invention, as
well as further features and advantages of the present invention,
will be obtained by reference to the following detailed description
and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a diagram illustrating an exemplary methodology
for forming an epitaxial CZT(S,Se) material on a silicon (Si)
substrate according to an embodiment of the present invention;
[0011] FIG. 2 is a cross-sectional diagram illustrating a starting
structure (e.g., a Si substrate) for an exemplary implementation of
the present techniques to form an epitaxial CZT(S,Se)-based
photovoltaic device according to an embodiment of the present
invention;
[0012] FIG. 3 is a cross-sectional diagram illustrating an
epitaxial oxide interlayer having been deposited onto the Si
substrate according to an embodiment of the present invention;
[0013] FIG. 4 is a cross-sectional diagram illustrating an
epitaxial CZT(S,Se) layer having been formed on a side of the
epitaxial oxide interlayer opposite the Si substrate according to
an embodiment of the present invention;
[0014] FIG. 5 is a cross-sectional diagram illustrating a buffer
layer having been formed on a side of the epitaxial CZT(S,Se) layer
opposite the epitaxial oxide interlayer according to an embodiment
of the present invention;
[0015] FIG. 6 is a cross-sectional diagram illustrating a
transparent front contact having been formed on a side of the
buffer layer opposite the epitaxial CZT(S,Se) layer according to an
embodiment of the present invention;
[0016] FIG. 7A is a diagram illustrating that a Si(001) starting
substrate for the present process yields epitaxial
CZT(S,Se)(004)/(200) and CZT(S,Se)(008)/(400) according to an
embodiment of the present invention;
[0017] FIG. 7B is a diagram illustrating that a Si(111) starting
substrate for the present process yields epitaxial CZT(S,Se)(112)
according to an embodiment of the present invention;
[0018] FIG. 8 is a diagram illustrating how with the inclusion of
an epitaxial oxide interlayer between the epitaxial CZT(S,Se) and
the Si substrate the CZT(S,Se) remains epitaxial to the underlying
Si substrate according to an embodiment of the present invention;
and
[0019] FIG. 9 is a diagram illustrating how epitaxial CZT(S,Se)
films have difficulty wetting Si substrates according to an
embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Provided herein are techniques for producing grain
boundary-free epitaxial CZT(S,Se) materials on silicon (Si)
substrates. As provided above, CZT(S,Se) materials are kesterites
containing copper (Cu), zinc (Zn), tin (Sn), and at least one of
sulfur (S) and selenium (Se). Epitaxy involves the growth of a
crystalline material (in this case CZT(S,Se)) on a crystalline
substrate (in this case a Si substrate). To be epitaxial, the atoms
in the growing film must align themselves with those in the
substrate so as to form a continuous atomic arrangement across the
interface (i.e., the atomic planes in the film are a continuation
of the atomic planes in the substrate).
[0021] Grain boundaries are the interfaces between grains in a
polycrystalline material and have been correlated with
recombination in other polycrystalline material systems. Grain
boundaries in a CZT(S,Se) material are undesirable as they are
thought to impede carrier mobility and/or introduce recombination
centers, thus affecting the operation of corresponding
polycrystalline CZT(S,Se)-based devices. The present epitaxial
CZT(S,Se) materials on the other hand generally have a much lower
density of two-dimensional boundaries and can be free of grain
boundaries. This is based on the nature of the epitaxial growth
process employed herein. Specifically, as will be described in
detail below, formation of the present epitaxial CZT(S,Se) material
involves heating the substrate to a high temperature (examples
provided below) which gives the Cu, Zn, Sn, and S and/or Se atoms
sufficient kinetic energy to find the perfect crystallographic
lattice site for each atom. The result is the formation of larger
crystallites with fewer two-dimensional boundaries. Therefore, the
problems associated with two-dimensional boundaries such as grain
boundaries can be largely avoided. As will be described in detail
below, the present techniques involve epitaxial growth of CZT(S,Se)
on a lattice-matched substrate such as Si.
[0022] Further, during epitaxy CZT(S,Se) does not cover the
substrate well. Specifically, if deposited directly onto a Si
substrate, epitaxial CZT(S,Se) tends to cover some areas while
leaving others bare, resulting in voids in the solid layer.
According to the present techniques, a thin film oxide interlayer
may be used to enhance wettability of the CZT(S,Se) on the Si
substrate. The oxide interlayer is also epitaxial in order to
insure lattice matching between the Si substrate and the CZT(S,Se).
While beneficial, it is notable that use of the oxide interlayer is
optional. Namely, if grown thick enough, the originally-isolated
CZT(S,Se) islands will eventually merge into a single solid layer.
Ideally however, wettability is improved without changing film
thickness. This can be achieved through the use of the present
epitaxial oxide interlayer.
[0023] Any surfaces of the Si substrate exposed during the
epitaxial CZT(S,Se) growth process can be subject to sulfur
etching. Specifically, silicon sulfide (Si--S) is a volatile
compound at the temperatures used herein to grow the CZT(S,Se) on
Si. Thus, Si can be lost in the form of Si--S during deposition. As
provided above, sulfur may be a component of the CZT(S,Se). Sulfur
etching is undesirable since regions of the substrate surface which
have been sulfur etched will not allow uniform nucleation of
CZT(S,Se). Thus, another notable advantage to use of the present
oxide interlayer is that it physically covers and protects the Si
substrate from sulfur etching.
[0024] An overview of the present techniques is now provided by way
of reference to methodology 100 of FIG. 1. As provided above, the
starting structure for the process is a Si substrate. Unless
otherwise noted, the Si substrates employed herein are
mono-crystalline Si substrates. Further, the Si substrate can be
one of multiple layers present. Thus the starting Si substrate can
be, for example, the top layer of a multi-layer stack (i.e., a
multi-layer stack wherein the top layer is a (mono-crystalline) Si
layer). Si is lattice matched to CZT(S,Se) and thus is an ideal
substrate for epitaxial CZT(S,Se) growth. The term
"lattice-matched" as used herein refers generally to the condition
where two materials have the same, or very similar (within 5%
difference), lattice constants as one another. Lattice-matching
ensures that the growth is epitaxial (see above) and that
structural defects such as dislocations in the growing thin film
are kept to a minimum. As is known in the art, lattice constants
denote the physical dimensions of a crystal (e.g., a crystal in
three-dimensions typically has three lattice constants a, b, and
c). The lattice constants of two materials do not have to match up
exactly for the materials to be lattice-matched. According to the
present techniques, materials having up to a 5% mismatch in their
lattice constants are still considered herein to be lattice-matched
to one another. For instance, the lattice constants a, b, and/or c
of a first material may each differ by up to about 5% from the
lattice constants a, b, and/or c, respectively, of a second
material yet the first and second materials are considered herein
to be lattice-matched. Lattice mismatches greater than 5% introduce
an unacceptably high level of strain into the epitaxial material
which inevitably leads to structural defects such as
dislocations.
[0025] The crystallographic orientation of the CZT(S,Se) can
advantageously be controlled by the crystallographic orientation of
the starting Si substrate. By way of example only, starting with a
Si(001) substrate, epitaxial CZT(S,Se)(004)/(200) and
CZTS,Se(008)/(400) are produced. By comparison, starting with a
Si(111) substrate epitaxial CZT(S,Se)(112) is produced. See FIGS.
7A and 7B, described below. The atomic plane of a crystal can be
denoted using three value notations known as miller indices (e.g.,
(001), (200), etc.). In the instant case, the miller indices used
designate the atomic plane in a Si or CZT(S,Se) crystal. From
"(004)" for example, an atomic plane can be visualized that
intersects the x, y, z axes at infinity for x and y and 1/4 of the
unit cell in the "z" direction, i.e., at the reciprocal of the
"miller indices." In contrast, "(200)" represents an atomic plane
that intersects the unit cell at 1/2 in the x direction and
infinity in y and z.
[0026] The CZT(S,Se) crystal structure is anisotropic, i.e., not
isotropic. Thus if a, b, and c represent the lattice constants in
the x, y, and z directions, respectively, then in CZT(S,Se)
a=b.noteq.c, where c is larger than a and b by approximately
2.times.. If the CZT(S,Se) crystal is oriented so that its "c" axis
is perpendicular to the substrate, the planes will be arranged
differently than if the c-axis is parallel to the substrate. Both
orientations of CZT(S,Se) on Si (i.e., where the CZT(S,Se) c-axis
is perpendicular or parallel to the underlying Si substrate) are
possible and expected to form. The (004) plane from the regions of
material with a c-axis perpendicular to the Si and the (200) plane
from the regions of material with a c-axis parallel to the Si are
indistinguishable in the diffraction pattern. Thus, the
corresponding peak is labeled herein with both sets of indices
(e.g., "(004)/(200)").
[0027] Thus, in step 102, the starting Si substrate is selected
based on its crystallographic orientation (e.g., either Si(001) or
Si(111)) so as to control the crystallographic orientation of the
CZT(S,Se). It is advantageous to be able to control the
crystallographic orientation of the CZT(S,Se) because different
crystallographic orientations can yield different properties in the
material. For instance, with "anisotropic" materials (like
CZT(S,Se), where some of the atomic planes are polar and some are
non-polar), different surface properties can be obtained depending
on what orientation the crystal has (resulting in either a
non-polar or a polar surface). Other times, carrier transport is
better/worse in some crystallographic directions compared to
others. This is expected to be the case for CZT(S,Se) also.
Usually, controlling crystal orientation is of special importance
when material properties vary in different crystallographic
directions. By way of example only, the carrier "effective mass" of
kesterites such as CZT(S,Se) is thought to vary according to
crystallographic direction. See, for example, Clas Persson,
"Electronic and optical properties of Cu.sub.2ZnSnS.sub.4 and
Cu.sub.2ZnSnSe.sub.4," Journal of Applied Physics 107, 053710
(2010) (hereinafter "Persson"), the contents of which are
incorporated by reference as if fully set forth herein. The
findings in Persson suggest that the effective mass is larger in
the (001) direction, meaning that the mobility would be lower in
this direction--a sign of worse transport. In that case, selection
of the (111) direction would be preferable.
[0028] In step 104, a pre-clean of the Si substrate is performed.
According to an exemplary embodiment, the pre-clean involves a
standard RCA cleaning (to remove organic contaminants, oxides, and
ionic or heavy metal contaminants from the substrate) followed by a
hydrofluoric acid (HF) dip (e.g., in 10:1 H.sub.2O:HF for 1
minute). The present coevaporation process for forming the
epitaxial CZT(S,Se) (see below) will be carried out in a vacuum
chamber. Thus, immediately after the HF dip, the Si substrate is
transferred to a vacuum chamber.
[0029] In step 106, the Si substrate is preferably heated in the
vacuum chamber to remove surface hydrogen from the pre-clean.
According to an exemplary embodiment, the Si substrate is heated in
step 104 to a temperature of from about 600 degrees Celsius
(.degree. C.) to about 750.degree. C., and ranges therebetween.
[0030] In step 108, an epitaxial oxide interlayer is formed on the
Si substrate. The epitaxial oxide interlayer is lattice-matched to
both Si and CZT(S,Se). As provided above, while use of an epitaxial
oxide interlayer is optional, the epitaxial oxide interlayer
advantageously increases wettability of the epitaxial kesterite
CZT(S,Se) on the Si substrate and prevents undesirable etching of
the Si substrate surface by sulfur (from the CZT(S,Se) deposition
process). As further provided above, the use of a thin epitaxial
material for the oxide interlayer maintains lattice matching
between the Si substrate and the CZT(S,Se).
[0031] Only a thin layer of the epitaxial oxide interlayer is
needed to enhance wettability of the CZT(S,Se) on the Si substrate.
By way of example only, in step 108 the epitaxial oxide interlayer
is formed on the substrate to a thickness of from about 5
nanometers (nm) to about 30 nm, and ranges therebetween. Using a
thin epitaxial oxide interlayer that is lattice-matched to both Si
and CZT(S,Se) ensures that the CZT(S,Se) is still epitaxial to the
underlying oxide interlayer/Si substrate. While thin, it is
preferable that the epitaxial oxide interlayer fully/completely
covers the surface of the Si substrate onto which the epitaxial
CZT(S,Se) material will be deposited. In addition to increasing
adherence of the epitaxial CZT(S,Se) to the Si substrate, an
unbroken layer of the epitaxial oxide interlayer serves to protect
the Si substrate against degradation by the sulfur in the
CZT(S,Se). It is however notable that while an unbroken epitaxial
oxide interlayer that fully covers the Si substrate is ideal in
terms of protecting the Si substrate against sulfur degradation, it
is not absolutely necessary for enhancing wettability.
Specifically, a discontinuous layer of the epitaxial oxide
interlayer will still serve to enhance wettability of the CZT(S,Se)
on the Si substrate surface.
[0032] Thus, the oxide interlayer is formed from an epitaxial
material that is lattice-matched to Si. Suitable materials for
forming the epitaxial oxide interlayer include, but are not limited
erbium oxide (Er.sub.2O.sub.3), lanthanum oxide (La.sub.2O.sub.3),
or yttrium oxide (Y.sub.2O.sub.3). Epitaxial Er.sub.2O.sub.3,
La.sub.2O.sub.3, and Y.sub.2O.sub.3 are all lattice-matched to Si.
According to an exemplary embodiment, the epitaxial oxide
interlayer is formed using an evaporation process in the vacuum
chamber. By way of example only, the epitaxial oxide interlayer is
formed on the Si substrate by evaporating a source material (e.g.,
Er.sub.2O.sub.3, La.sub.2O.sub.3, or Y.sub.2O.sub.3) in the vacuum
chamber at a temperature of from about 500.degree. C. to about
750.degree. C., and ranges therebetween, at a reduced pressure of
from about 1.times.10.sup.-6 Torr to about 1.times.10.sup.-9 Torr,
and ranges therebetween.
[0033] In step 110, an epitaxial kesterite CZT(S,Se) layer is
formed on a side of the epitaxial oxide interlayer opposite the Si
substrate (or directly on the Si substrate if the (optional)
epitaxial oxide interlayer is not present). As described above, the
CZT(S,Se) layer is epitaxial to the underlying epitaxial oxide
interlayer/Si substrate. Thus, the CZT(S,Se) will be relatively
free from two-dimensional structural defects with a crystal
structure templated from the underlying Si substrate.
[0034] According to an exemplary embodiment, the epitaxial
kesterite CZT(S,Se) layer is formed on the epitaxial oxide
interlayer (or directly on the Si substrate) by co-evaporating the
source materials (e.g., Cu, Zn, Sn, and at least one of S and Se)
in the vacuum chamber at a temperature of from about 350.degree. C.
to about 550.degree. C., and ranges therebetween, at a reduced
pressure of from about 1.times.10.sup.-6 Torr to about
1.times.10.sup.-9 Torr, and ranges therebetween. Since the
epitaxial oxide interlayer and the epitaxial kesterite CZT(S,Se)
layer may both be deposited onto the Si substrate in the vapor
chamber, according to an exemplary embodiment both the epitaxial
oxide interlayer and the epitaxial kesterite CZT(S,Se) layer are
deposited in-situ, without removing the Si substrate from the
vacuum chamber and without breaking vacuum between deposition of
the layers.
[0035] Kesterite CZT(S,Se) materials contain a number of volatile
components. For example, Zn, Sn, S and Se, are volatile at the
temperatures employed during CZT(S,Se) productions. For instance,
when kesterite samples are heated above 400.degree. C.,
re-evaporation of Sn(S,Se) occurs causing desorption and loss of Sn
from the samples. See, for example, Mitzi et al., "The path towards
a high-performance solution-processed kesterite solar cell," Solar
Energy Materials & Solar Cells 95 (January 2011) 1421-1436, the
contents of which are incorporated by reference as if fully set
forth herein. Thus, care must be taken to control the concentration
of these volatile components during fabrication. It is notable that
the Sn lost due to desorption is however generally not elemental
Sn, but occurs primarily in the form of tin sulfide (SnS). See, for
example, A. Weber et al., "On the Sn loss from thin films of the
material system Cu--Zn--Sn--S in high vacuum," Journal of Applied
Physics 107, 013516 (January 2010) (hereinafter "Weber"), the
contents of which are incorporated by reference as if fully set
forth herein. It has been found that carrying out high temperature
anneals (even at temperatures of 550.degree. C. or above) in a
sulfur environment (e.g., H.sub.2S) can thus mitigate the loss of
Sn. See Weber. Additionally, the vapor pressure of elemental Zn is
reasonably high at the substrate temperatures commonly employed for
CZT(S,Se) deposition. Accordingly, the Zn, Sn, S, and Se fluxes are
key controls for epitaxial growth of the epitaxial CZT(S,Se)
material. Specifically, the fluxes of Zn, Sn, S, and Se are
increased compared to that of Cu so as to ensure sufficient
incorporation of these elements into the structure. The precise
fluxes used depend on the substrate temperatures and therefore the
rates at which these species are expected to be expelled from the
material. Given the teachings provided herein, one skilled in the
art would be able to, for a given set-up, adjust the Zn, Sn, S, and
Se fluxes relative to one another and/or to that of the Cu to
account for volatility. By way of example only, according to U.S.
Patent Application Publication Number 2012/0100663 by Bojarczuk et
al., entitled "Fabrication of CuZnSn(S,Se) Thin Film Solar Cell
with Valve Controlled S and Se," the contents of which are
incorporated as if fully set forth herein, the sources for one or
more components of a kesterite material are placed in a vapor
chamber along with a substrate, and thermal evaporation is used to
co-evaporate the components onto the substrate. Advantageously, the
S and Se fluxes can be precisely regulated (by introducing the S
and Se to the vapor chamber from individual S and Se cracking
cells).
[0036] Following deposition of the epitaxial kesterite CZT(S,Se)
layer, the structure is then removed from the vacuum chamber. A
brief cooling period can be employed prior to removing the
structure from the chamber. Further processing may be then
performed to create a functioning device. See, for example, the
method described below that employs the present techniques to
forming a photovoltaic device having an epitaxial kesterite
CZT(S,Se) absorber layer.
[0037] An exemplary implementation of the present techniques to
form an epitaxial kesterite CZT(S,Se)-based photovoltaic device is
now described by way of reference to FIGS. 2-6. As shown in FIG. 2,
the process begins with a Si substrate 202. As described in
conjunction with the description of step 102 of methodology 100 (of
FIG. 1) above, Si is lattice matched to CZT(S,Se) and thus is an
ideal substrate for epitaxial CZT(S,Se) growth.
[0038] Further, the crystallographic orientation of the CZT(S,Se)
can advantageously be controlled by the orientation of Si substrate
202. See, for example, FIGS. 7A and 7B, described below. According
to an exemplary embodiment, Si substrate 202 is either a Si(001) or
a Si(111) substrate.
[0039] Prior to placing the Si substrate 202 in a vacuum chamber, a
pre-clean of the substrate is preferably performed. As described in
conjunction with the description of step 104 of methodology 100 (of
FIG. 1) above a standard RCA cleaning may be performed (to remove
organic contaminants, oxides, and ionic or heavy metal contaminants
from the substrate) followed by a HF dip (e.g., in 10:1 H.sub.2O:HF
for 1 minute). Immediately after the HF dip, the Si substrate is
transferred to a vacuum chamber. The configuration of a vacuum
chamber is well known to those of skill in the art. Thus, for ease
and clarity of depiction, the vacuum chamber itself is not shown in
the figures. As described in conjunction with the description of
step 106 of methodology 100 (of FIG. 1) above, once the Si
substrate 202 is placed in the vacuum chamber the Si substrate 202
is preferably heated (e.g., to a temperature of from about
600.degree. C. to about 750.degree. C., and ranges therebetween) to
remove surface hydrogen from the pre-clean step.
[0040] As shown in FIG. 3, an epitaxial oxide interlayer 302 is
then deposited onto the Si substrate 202. The epitaxial oxide
interlayer 302 is lattice-matched to both Si and CZT(S,Se). While
use of an epitaxial oxide interlayer is optional, as provided above
the epitaxial oxide interlayer advantageously increases wettability
of the CZT(S,Se) on the Si substrate 202 and prevents undesirable
sulfur etching of the Si substrate surface. According to an
exemplary embodiment, the oxide interlayer 302 is formed from
epitaxial Er.sub.2O.sub.3, La.sub.2O.sub.3, or Y.sub.2O.sub.3.
While each of these epitaxial materials is lattice-matched to Si,
it is still preferable that the epitaxial oxide interlayer 302 is
thin (e.g., the oxide interlayer 302 is formed having a thickness
of from about 5 nm to about 30 nm, and ranges therebetween). As
noted above, while thin, it is preferable that the epitaxial oxide
interlayer 302 fully covers the surface of the Si substrate onto
which the epitaxial CZT(S,Se) material will be deposited. A solid
layer of the epitaxial oxide interlayer serves to protect (as well
as increase adherence to) the Si substrate against sulfur etching.
While an unbroken epitaxial oxide interlayer that fully covers the
Si substrate is ideal in terms of protecting the Si substrate
against sulfur degradation, it is not absolutely necessary for
enhancing wettability. Specifically, a discontinuous layer of the
epitaxial oxide interlayer will still serve to enhance wettability
of the CZT(S,Se) on the Si substrate surface.
[0041] As described in conjunction with the description of step 108
of methodology 100 (of FIG. 1) above the epitaxial oxide interlayer
302 can be formed in the vacuum chamber using an evaporation
process. By way of example only, the epitaxial oxide interlayer 302
is formed on the Si substrate 202 by evaporating a source material
(e.g., Er.sub.2O.sub.3, La.sub.2O.sub.3, or Y.sub.2O.sub.3) in the
vacuum chamber at a temperature of from about 500.degree. C. to
about 750.degree. C., and ranges therebetween, at a reduced
pressure of from about 1.times.10.sup.-6 Torr to about
1.times.10.sup.-9 Torr, and ranges therebetween.
[0042] As shown in FIG. 4, an epitaxial kesterite CZT(S,Se) layer
402 is then formed on a side of the epitaxial oxide interlayer 302
opposite the Si substrate 202 (or directly on the Si substrate 202
if the (optional) epitaxial oxide interlayer is not present). The
epitaxial kesterite CZT(S,Se) layer 402 will serve as an absorber
layer of the photovoltaic device. As described above, the kesterite
CZT(S,Se) layer 402 is epitaxial to the underlying epitaxial oxide
interlayer 302/Si substrate 202. Thus, the epitaxial kesterite
CZT(S,Se) 402 will be relatively free from two-dimensional
structural defects with a crystal structure templated from the
underlying Si substrate 202.
[0043] As described in conjunction with the description of step 110
of methodology 100 (of FIG. 1) above the epitaxial kesterite
CZT(S,Se) layer 402 can be formed on the epitaxial oxide interlayer
302 (or directly on the Si substrate 202) by co-evaporating the
source materials (e.g., Cu, Zn, Sn, and at least one of S and Se)
in the vacuum chamber at a temperature of from about 350.degree. C.
to about 550.degree. C., and ranges therebetween, at a reduced
pressure of from about 1.times.10.sup.-6 Torr to about
1.times.10.sup.-9 Torr, and ranges therebetween. As described
above, the epitaxial oxide interlayer 302 and the epitaxial
kesterite CZT(S,Se) layer 402 may be deposited in-situ, i.e.,
without removing the Si substrate 202 from the vacuum chamber and
without breaking vacuum between deposition of these layers. As also
described above, due to the volatility of Zn, Sn, and the chalcogen
(i.e., S and Se) components at processing temperatures, care must
be taken to control the fluxes of these volatile components during
fabrication.
[0044] Following deposition of the epitaxial kesterite CZT(S,Se)
layer 402, the structure is removed from the vacuum chamber. A
brief cooling period can be employed prior to removing the
structure from the chamber. Further processing may be then
performed to complete the device. By way of example only, a buffer
layer 502 may then be formed on a side of the epitaxial kesterite
CZT(S,Se) layer 402 opposite the epitaxial oxide interlayer 302 (or
if the epitaxial oxide interlayer 302 is not present the buffer
layer 502 is formed on a side of the epitaxial kesterite CZT(S,Se)
layer 402 opposite the Si substrate 202). See FIG. 5.
[0045] Suitable materials for forming the buffer layer 502 include,
but are not limited to, cadmium sulfide (CdS), a
cadmium-zinc-sulfur material of the formula Cd.sub.1-xZn.sub.xS
(wherein 0<x.ltoreq.1), indium sulfide (In.sub.2S.sub.3), zinc
oxide, zinc oxysulfide (e.g., a Zn(O,S) or Zn(O,S,OH) material),
and aluminum oxide (Al.sub.2O.sub.3). According to an exemplary
embodiment, the buffer layer 502 is formed on epitaxial kesterite
CZT(S,Se) layer 402 using standard chemical bath deposition.
[0046] Finally, as shown in FIG. 6, a transparent front contact 602
is formed on a side of the buffer layer 502 opposite the epitaxial
kesterite CZT(S,Se) layer 402. Suitable materials for forming the
transparent front contact 602 include, but are not limited to,
transparent conductive oxides (TCO) such as indium-tin-oxide (ITO)
and/or aluminum doped zinc oxide (AZO). According to an exemplary
embodiment, the transparent front contact 602 is formed on the
buffer layer 502 by sputtering.
[0047] As provided above, according to the present techniques, the
overall orientation of the epitaxial kesterite CZT(S,Se) material
can be controlled by using different Si substrate orientations.
This concept is further illustrated in FIGS. 7A and 7B.
Specifically, as shown in FIG. 7A, when the starting substrate for
the present process is a Si(001) substrate, then the epitaxial
CZT(S,Se)(004)/(200) and CZT(S,Se)(008)/(400) peaks are visible in
the x-ray diffraction pattern, indicating preferential alignment of
the CZT(S,Se) in the same direction as the substrate. FIG. 7A is an
x-ray diffraction pattern of epitaxial CZT(S,Se) produced
(according to the present techniques) on a Si(001) substrate. In
FIG. 7A, the diffraction angle is plotted on the x-axis and signal
intensity (measured in counts per second (cps)) is plotted on the
y-axis.
[0048] By comparison, as shown in FIG. 7B, when the starting
substrate for the present process is a Si(111) substrate, then the
epitaxial CZT(S,Se)(112) peak is observed. FIG. 7B is an x-ray
diffraction pattern of epitaxial CZT(S,Se) produced (according to
the present techniques) on a Si(111) substrate. In FIG. 7B, the
diffraction angle is plotted on the x-axis and signal intensity
(measured in cps) is plotted on the y-axis.
[0049] The samples shown in FIGS. 7A and 7B were prepared without
the epitaxial oxide interlayer. However, as shown in FIG. 8, with
the inclusion of the present epitaxial oxide interlayer which is
lattice-matched to Si and CZT(S,Se) (i.e., between the CZT(S,Se)
and the Si substrate) the CZT(S,Se) layer is still epitaxial to the
underlying Si substrate. Compare the spectrum in FIG. 8 with that
shown in FIG. 7B, and described above. For illustrative purposes,
only a sample formed on a Si(111) substrate is depicted in FIG. 8.
However commensurate results would also be obtained with a Si(001)
starting substrate. Further, the epitaxial oxide interlayer used in
the example shown in FIG. 8 is Er.sub.2O.sub.3, however the same
results would be obtained using any of the epitaxial oxide
interlayer materials described herein. FIG. 8 is an x-ray
diffraction pattern of epitaxial CZT(S,Se) produced (according to
the present techniques) on a Si(111) substrate coated with an
epitaxial oxide interlayer of Er.sub.2O.sub.3. In FIG. 8, the
diffraction angle is plotted on the x-axis and signal intensity
(measured in cps) is plotted on the y-axis.
[0050] As described above, epitaxial CZT(S,Se) films have
difficulty wetting Si substrates. See FIG. 9. As shown in FIG. 9,
there are uncovered portions of the Si substrate between the
CZT(S,Se) grains. While growing a thicker film will improve the
wetting, it would be ideal if wetting is improved without having to
change the film thickness. Advantageously, use of the present
epitaxial oxide interlayer enhances wettability of the CZT(S,Se) on
Si, while permitting the CZT(S,Se) to remain epitaxial to the
underlying Si substrate. See above.
[0051] Although illustrative embodiments of the present invention
have been described herein, it is to be understood that the
invention is not limited to those precise embodiments, and that
various other changes and modifications may be made by one skilled
in the art without departing from the scope of the invention.
* * * * *