# Patent

## Title
Methods of forming highly p-type doped germanium tin films and structures and devices including the films

## Abstract

Methods of forming p-type doped germanium-tin layers, systems for forming the p-type doped germanium-tin layers, and structures including the p-type doped germanium-tin layers are disclosed. The p-type doped germanium-tin layers include an n-type dopant, which allows relatively high levels of tin and/or p-type dopant to be included into the p-type doped germanium-tin layers.

## Background
FIELD OF INVENTION
The present disclosure generally relates to techniques for forming layers including germanium tin and to structures and devices including such layers. More particularly, the disclosure relates to methods of forming p-type doped germanium-tin layers, to methods of forming structures and devices including the layers, to systems for forming the layers and structures, and to structures and devices including the layers.
BACKGROUND OF THE DISCLOSURE
Various electronic devices, such as semiconductor devices, and photonic devices, such as lasers and solar devices, may include or may desirably include germanium-tin (GeSn) layers, such as GeSn, GeSnSi, GeSnSiC, and the like. For example, GeSn layers can be used to form direct bandgap devices and/or may be used as a stressor to provide strain in an adjacent germanium layer to increase mobility in the germanium layer. Similarly, GeSnSi and/or GeSnSiC layers can be used to form tunable band gap devices as well as optical devices having tunable optical properties.
In many applications, it may be desirable to include relatively large amounts of Sn in GeSn films to increase the strain in the film. The increased strain can lead to, for example, better electron and/or hole mobility in devices including such films.
It may also be desirable to provide GeSn films with relatively high levels of a p-type dopant (e.g., boron) to form GeSn films having relatively low contact resistance. Generally, the higher the concentration of p-type dopant in the film, the lower the contact resistance.
Unfortunately, during p-type doped GeSn film formation, the p-type dopant and the Sn can compete with each other for inclusion in the film. As a result, as an amount of p-type dopant in the film increases, an amount of Sn that can be included in the film decreases. Similarly, as an amount of Sn in the film increases, an amount of p-type dopant that can be included in the film decreases.
Accordingly, improved methods of and systems for forming GeSn films that can allow relatively high levels of Sn and/or p-type dopant in the films are desired. Additionally, GeSn films having relatively high concentrations of p-type dopant and/or Sn, and structures and devices including such films, are desired.
SUMMARY OF THE DISCLOSURE
Various embodiments of the present disclosure relate to methods of forming p-type doped GeSn films, to structures and devices including p-type doped GeSn films, and to systems for forming the films. The methods described herein can be used to form p-type GeSn films suitable for a variety of applications, including films in semiconductor and photonic devices. While the ways in which various embodiments of the disclosure address the drawbacks of the prior art methods, films, structures, devices, and systems are discussed in more detail below, in general, the disclosure provides methods of forming p-typed germanium-tin layers (e.g., crystalline) capable of having a relatively high tin content and a relatively high p-type dopant content. Such films are particularly well-suited where low ohmic contact resistance is desired.
As used herein, germanium-tin (GeSn) layers (also referred to herein as films) or layers including germanium and tin are layers that include the elements germanium and tin. The layers may include additional elements, such as silicon (e.g., GeSnSi layers), carbon (e.g., GeSnSiC layers), and/or other elements.
In accordance with exemplary embodiments of the disclosure, methods of forming a p-type doped GeSn layer overlying a substrate include the steps of providing a gas-phase reactor having a reaction chamber, providing a germanium precursor to the reaction chamber, providing a tin precursor to the reaction chamber, providing a p-type dopant precursor to the reaction chamber, and providing an n-type dopant to the reaction chamber. The various precursors can be provided from one or more precursor sources, and need not all be provided separately. The inventors surprisingly and unexpectedly found that by adding n-type dopant during the film formation process, increased levels of one or both Sn and p-type dopant can be incorporated into the film.
Exemplary methods can be used to form epitaxial layers of p-type doped GeSn overlying a substrate. In these cases, a reaction chamber temperature can range from about 200° C. to about 500° C., about 250° C. to about 450° C., or about 300° C. to about 420° C. Exemplary reaction chamber pressures during this step range from about 1 Torr to about 760 Torr, about 10 Torr to about 760 Torr, or about 50 Torr to about 760 Torr. In accordance with various aspects of these embodiments, a method includes a step of mixing one or more of the precursors at or near an inlet of the reaction chamber—e.g., at an inlet or injection manifold of the reactor.
Exemplary methods can include providing additional precursors, such as silicon and/or carbon precursors, to the reaction chamber; such additional precursor can be mixed with one or more of the other precursors at or near the inlet of the reaction chamber.
In accordance with exemplary aspects of various embodiments of the disclosure, a ratio of a flowrate of the p-type dopant precursor to the flowrate of the n-type dopant precursor during growth of the p-type doped GeSn film ranges from about 20 to about 1, about 50 to about 1, or about 100 to about 1. In accordance with further aspects, a ratio of a flowrate of the p-type dopant precursor to the flowrate of the tin precursor during growth of the p-type doped GeSn film ranges from about 1 to about 1, about 2 to about 1, or about 3 to about 1.
Exemplary p-type doped GeSn films formed using methods disclosed herein can include greater than 5 at %, or about 1 at % to about 15 at %, or about 5 at % or more to about 10 at % Sn, about 1×1018 cm−3 to about 1×1020 cm−3 p-type dopant, such as a group 13 element, and/or about 1×1017 cm−3 to about 5×1017 cm−3 n-type dopant, such as a group 15 element. The GeSn films can also include about 0 at % to about 35 at % silicon and/or about 0 at % to about 5 at % carbon.
In accordance with additional embodiments of the disclosure, a structure includes a p-type doped GeSn layer (e.g., crystalline) that can be formed in accordance with a method of the present disclosure. The structure may be used to form, for example, electronic (e.g., semiconductor) or photonic (e.g., solar or light-emitting) devices. In accordance with various aspects of these embodiments, an amount of tin incorporated into the p-type doped GeSn layer may be greater than 5 at %, or about 1 at % to about 15 at %, or about 5 at % or more to about 10 at % Sn. When the layer of p-type doped GeSn includes silicon, the layer may include greater than 0 at % silicon, greater than about 1 at % silicon, or between about 1 at % silicon and about 35 at % silicon, about 2 at % silicon and about 16 at % silicon, or about 4 at % silicon and about 12 at % silicon. Similarly, the p-type doped GeSn films can include greater than 0 at % carbon or greater than about 5 at % carbon. Exemplary p-type doped GeSn films also include about 1×1018 cm−3 to about 5×1019 cm−3 p-type dopant, such as a group 13 element, and about 1×1017 cm-3 to about 5×1017 cm−3 n-type dopant, such as a group 15 element. As set forth in more detail below, exemplary structures can include additional layers, such as layers typically used to form devices. For example, the structures can include a germanium layer, which can form a buffer layer and/or a fin layer as part of a FinFET device.
In accordance with yet additional exemplary embodiments of the disclosure, a device includes a p-type doped GeSn layer as described herein and that can be formed as described herein.
And, in accordance with yet additional exemplary embodiments of the disclosure, a system for forming a p-type doped GeSn layer includes a gas-phase reactor including a reaction chamber, a germanium precursor source coupled to the reaction chamber, a tin precursor source coupled to the reaction chamber, a p-type dopant source coupled to the reaction chamber, and an n-type dopant source coupled to the reaction chamber. The system can be configured to mix (e.g., have an operational control mechanism configured to cause mixing of) one or more of the precursors at or near an inlet of a reaction chamber (e.g., at an injection manifold). The exemplary systems are suitable for forming p-type doped GeSn structures as described herein and/or are suitable for performing the exemplary methods described herein.
Both the foregoing summary and the following detailed description are exemplary and explanatory only and are not restrictive of the present disclosure.



BRIEF DESCRIPTION OF THE DRAWING FIGURES
A more complete understanding of exemplary embodiments of the present disclosure may be derived by referring to the detailed description and claims when considered in connection with the following illustrative figures.
FIG. 1 illustrates a system for forming a p-type doped GeSn layer in accordance with exemplary embodiments of the disclosure.
FIG. 2 illustrates a method of forming a p-type doped GeSn layer in accordance with further exemplary embodiments of the disclosure.
FIG. 3 illustrates another exemplary method of forming a p-type doped GeSn layer in accordance with embodiments of the disclosure.
FIG. 4 illustrates a structure in accordance with exemplary embodiments of the disclosure.
FIG. 5 illustrates another structure in accordance with exemplary embodiments of the disclosure.
FIG. 6 illustrates yet another structure according to yet additional exemplary embodiments of the present disclosure.



It will be appreciated that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of illustrated embodiments of the present disclosure.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE
The description of exemplary embodiments of methods, systems, structures, and devices provided below is merely exemplary and is intended for purposes of illustration only; the following description is not intended to limit the scope of the disclosure or the claims. Moreover, recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features or other embodiments incorporating different combinations of the stated features.
Exemplary methods in accordance with the present disclosure relate to methods of forming p-type doped GeSn (e.g., crystalline) layers overlying a substrate. The p-type doped GeSn layers may include additional elements, such as silicon and/or carbon, which form part of a crystalline lattice with the p-type doped GeSn layer. As set forth in more detail below, the p-type doped GeSn layer also includes an n-type dopant. Inclusion of the n-type dopant allows increased concentrations of Sn and/or the p-type dopant in the film. It is thought that the n-type dopant may act as a surfactant and hold the Sn in place while the film/layer forms. By way of examples, p-type doped GeSn layers that also include an n-type dopant can have higher concentrations of both the p-type dopant and Sn, whereas without the n-type dopant, the p-type dopant and the Sn are thought to compete with each other for inclusion in the lattice structure and therefore limit the concentration levels of each in a crystalline layer. Exemplary p-type doped GeSn films can have, for example, Sn concentrations greater than 5 at % and resistivities below 1 milliohm.
As used herein, a “substrate” refers to any material having a surface onto which material can be deposited. A substrate can include a bulk material (e.g., single crystal silicon, single crystal germanium, or other semiconductor wafer) or may include one or more layers overlying the bulk material. Further, the substrate can include various topologies, such as trenches, vias, lines, and the like formed within or on at least a portion of a layer of the substrate. Exemplary substrates include a silicon wafer, a layer comprising germanium overlying a silicon wafer, and a layer comprising germanium silicon tin overlying a silicon wafer.
Turning now to the figures, FIG. 1 illustrates a cut-away side view of an exemplary system 100 suitable for forming p-type doped GeSn films. System 100 includes an optional substrate handling system 102, a reactor 104, a gas distribution system 106, and optionally a wall 108 disposed between reactor 104 and substrate handling system 102. System 100 also includes a first gas source 112, a second gas source 114, a third gas source 116, a fourth gas source 118, and an exhaust source 110. By way of examples, first gas source 112 can include a germanium precursor, second gas source 114 can include a tin precursor, third gas source 116 can include a p-type dopant precursor, and fourth gas source 118 can include an n-type dopant precursor. One or more of first gas source 112, second gas source 114, third gas source 116, and fourth gas source 118 can also include a carrier gas. Additionally or alternatively, one or more of the precursors can be mixed with a carrier gas prior to or within gas distribution system 106. System 100 can also include additional precursor sources (e.g., silicon and/or carbon precursor sources, which may be mixed with a carrier gas), purge gas sources, and/or carrier gas sources coupled to gas distribution system 106. Exemplary gasses suitable as carrier and purge gasses include nitrogen, argon, helium, and hydrogen.
During operation of system 100, substrates (not illustrated) are transferred from, e.g., substrate handling system 102 to reactor 104. Once substrate(s) are transferred to reactor 104, one or more gasses, such as precursors, carrier gasses, and/or purge gasses, are introduced into a reaction chamber of reactor 104 via gas distribution system 106.
Reactor 104 can be a standalone reactor or part of a cluster tool. Further, reactor 104 can be dedicated to a particular process, such as a deposition process, or be configured to perform multiple processes. Reactor 104 can be a single-substrate, horizontal-flow reactor, which enables laminar flow of reactants over a substrate, with low residence times, which in turn facilitates relatively rapid sequential substrate processing. Alternatively, reactor 104 can be a vertical flow reactor with, for example, one or more precursor flows emanating from a showerhead and flowing substantially downward onto a substrate. By way of example, reactor 104 includes a reactor typically used for epitaxial chemical vapor deposition (CVD) processing, such as an Epsilon® 2000 Plus, available from ASM, which can include direct plasma, and/or remote plasma apparatus (not illustrated), and/or various heating systems, such as radiant, inductive, and/or resistive heating systems (also not illustrated). Using a plasma may enhance the reactivity of one or more precursors. An exemplary CVD reactor suitable for system 100 is described in U.S. Pat. No. 7,476,627, issued to Pomarede et al. on Jan. 13, 2009, the contents of which are hereby incorporated herein by reference, to the extent such contents do not conflict with the present disclosure.
Source 112 can include germane (GeH4), digermane (Ge2H6), trigermane (Ge3H8), tetragermane (Ge4H10) and/or other germanium halides and can optionally include one or more dopant compounds, such as p-type and/or n-type compounds typically used to fabricate photonic and/or semiconductor devices. As noted above, source 112 can also include a carrier gas.
Source 114 can include any compound suitable for providing tin to a germanium-tin layer. Exemplary tin precursors include tin chloride (SnCl4), deuterated stannane (SnD4), and methyl and/or halide substituted stannanes, such as compounds having a formula Sn(CH3)4-nXn, in which X is H, D (deuterium), Cl, or Br, and n is 0, 1, 2, or 3; Z2Sn(CH3)3-nXn in which Z is H or D, X is Cl or Br, and n is 0, 1, or 2; Z2Sn(CH3)2-nXn in which Z is H or D, X is Cl or Br, and n is 0 or 1; or SnBr4. Some exemplary tin precursors suitable for use with the present disclosure are discussed in more detail in U.S. application Ser. No. 13/783,762, filed Mar. 4, 2013, entitled TIN PRECURSORS FOR VAPOR DEPOSITION AND DEPOSITION PROCESSES, the contents of which are hereby incorporated herein by reference, to the extent such contents do not conflict with the present disclosure.
Sources 116, 118 can include suitable p-type and n-type dopant precursors. Alternatively or additionally, sources 112 and/or 114 can include p-type and/or n-type dopant precursors. Exemplary p-type dopant precursors include diborane (B2H6) and exemplary n-type dopant precursors include arsine (AsH3), phosphine (PH3), and the like.
Optional additional precursor sources, when used, can include one or more precursors for additional elements or compounds that may be included into, for example, a crystalline lattice of a deposited layer. For example, the additional precursor sources can include a silicon precursor, such as silane, disilane, trisilane, tetrasilane, neopentasilane, and higher order silanes, a carbon precursor, such as monomethyl silane, and/or additional dopant sources.
Turning now to FIG. 2, an exemplary method 200 of forming a p-type doped GeSn layer overlying a substrate is illustrated. Method 200 includes the steps of providing a substrate within a reaction chamber of a gas-phase reactor (step 202), providing precursors to the reaction chamber (step 204), and forming a p-type doped GeSn film overlying the substrate (step 206).
During step 202, a substrate, such as a wafer, is loaded into a gas-phase reactor, such as a CVD reactor suitable for epitaxial growth. The reactor may be a single-substrate, laminar flow reactor, such reactors as the Epsilon® 2000 Plus reactor system available from ASM. During step 202, the reaction chamber may be brought to a desired operating temperature and pressure. An operating temperature and pressure of the reaction chamber can vary in accordance with various factors. For example, the reaction chamber can be configured to operate at near atmospheric pressure or at lower pressures. By way of specific examples, an operating pressure of reactor 102 during layer formation steps ranges from about 1 Torr to about 760 Torr, about 10 Torr to about 760 Torr, or about 50 Torr to about 760 Torr. Similarly, an operating temperature of reactor 104 can range from, for example, about 200° C. to about 500° C., about 250° C. to about 450° C., or about 300° C. to about 420° C.
During step 204, one or more precursors are provided to the reaction chamber. As noted above, one or more precursors can be derived from a common source (e.g., a germanium precursor can also include n-type and/or p-type dopant precursors). Alternatively, each precursor can be provided from a separate source. By way of example, a germanium precursor, such as digermane, a tin precursor, such as tin chloride, a p-type dopant precursor, such as diborane, and an n-type dopant, such as phosphine, are provided to a reaction chamber during step 204. Flowrates of one or more of the precursors can be selected to obtain desired concentrations of Ge, Sn, p-type dopant, and n-type dopant as set forth herein. Further, additional precursors, such as silicon and/or carbon precursors, can be provided to the reaction chamber during this step.
During step 204, one or more of the reactants can be mixed at or near an inlet to the reaction chamber, such as at gas distribution system 106—e.g., within an injection manifold of a gas distribution system.
At step 206, a p-type doped GeSn layer (e.g., a p-type doped GeSn crystalline layer) is formed overlying a substrate. During this step, reactor conditions can be desirably maintained to allow for epitaxial growth of the p-type doped GeSn layer.
Method 200 may also include steps of forming an insulating layer overlying a substrate and forming a via within the insulating layer. Exemplary techniques of forming an insulating layer and a via within the insulating layer are described in more detail below. In these cases, the p-type doped germanium-tin layer may be selectively formed on the substrate within the via.
FIG. 3 illustrates another method 300 in accordance with additional embodiments of the disclosure. Method 300 includes the steps of providing a gas-phase reactor (step 302), providing a substrate within a reaction chamber of the gas-phase reactor (step 304), and forming a crystalline p-type doped GeSn layer on a surface of the substrate.
During step 302, a reactor suitable for growing a crystalline layer comprising germanium tin is provided. The reactor may include any reactor described herein, such as a horizontal-flow epitaxial CVD reactor.
During step 304, a substrate is provided within a reaction chamber of a reactor. Step 304 may be the same as or similar to step 202 of method 200.
At step 306, a crystalline layer of p-type doped GeSn is formed. In accordance with various aspects of exemplary embodiments of the disclosure, the step of forming the p-type doped GeSn layer includes providing a volumetric ratio of the tin precursor and the germanium of about 0.001 to about 0.1, about 0.005 to about 0.05, less than about 0.1, or less than about 0.05. Additionally or alternatively, a flowrate of the p-type dopant precursor to the flowrate of the n-type dopant precursor during growth of the p-type doped GeSn film ranges from about 20 to about 1, about 50 to about 1, or about 100 to about 1. In accordance with further aspects, a ratio of a flowrate of the p-type dopant precursor to the flowrate of the tin precursor during growth of the p-type doped GeSn film ranges from about 1 to about 1, about 2 to about 1, or about 3 to about 1. In accordance with further aspects, a reaction chamber temperature during the step of forming a p-type doped crystalline GeSn layer ranges from about 200° C. to about 500° C., about 250° C. to about 450° C., or about 300° C. to about 420° C. And, in accordance with yet further aspects, a reaction chamber pressure during the step of forming a layer comprising germanium tin ranges from about 1 Torr to about 760 Torr, about 10 Torr to about 760 Torr, or about 50 Torr to about 760 Torr.
Step 306 can include forming a layer including GeSnSi. In these cases, a silicon precursor can be additionally provided to the reaction chamber. Exemplary silicon source precursors include disilane, trisilane, tetrasilane, neopentasilane, and higher order silane compounds. Similarly, step 306 can include forming a layer including GeSnSiC. In these cases, a carbon source, such as a source including monomethyl silane, can be additionally provided to the reaction chamber.
Method 300 can also include optional steps 308 and/or 310 of forming an insulating layer overlying a substrate (step 308) and forming a via within the insulating layer. During step 308, any suitable insulating layer, such as silicon oxide or silicon nitride may be deposited onto the substrate. Then, during step 310, one or more vias may be formed within the insulating layer. Reactive ion etching or other suitable technique may be used to form the one or more vias. In the cases where steps 308 and 310 are performed, the crystalline layer formed during step 306 may be selectively formed within the vias.
The p-type doped GeSn layers formed using method 200 or method 300 (e.g., during steps 206 or 306) can include greater than 5 at %, or about 1 at % to about 15 at %, or about 5 at % or more to about 10 at % Sn, about 1×1018 cm−3 to about 1×1020 cm−3 p-type dopant, such as a group 13 element, and/or about 1×1017 cm−3 to about 5×1017 cm−3 n-type dopant, such as a group 15 element.
When the p-type doped GeSn layers include silicon, the layers may include greater than 0 at % silicon, greater than about 1 at % silicon, or between about 1 at % silicon and about 35 at % silicon, about 2 at % silicon and about 16 at % silicon, or about 4 at % silicon and about 12 at % silicon. Similarly, when the p-type doped GeSn layers include carbon, the layers can include greater than 0 at % carbon or greater than about 5 at % carbon.
Turning now to FIGS. 4-6, exemplary structures 400, 500, and 600, including a p-type doped GeSn layer as described herein, are illustrated. FIG. 4 illustrates structure 400, including a substrate 402, a buffer layer 404, and a p-type doped GeSn layer 406. Substrate 402 can include any suitable substrate, such as a silicon substrate that can have one or more layers formed therein and/or thereon. Buffer layer 404 can include a layer of germanium that is epitaxially grown overlying substrate 402. P-type doped GeSn layer 406 can include any of the p-type doped GeSn layers described herein and can be formed, for example, according to the methods disclosed herein.
Structures in accordance with additional examples of the disclosure can include additional layers—e.g., below layer 404 and/or above layer 406. By way of example, a structure can include a germanium layer overlying layer 406.
Turning now to FIG. 5, structure 500 includes a substrate 502, an insulating layer 504, a via 506 formed within layer 504, a buffer (e.g., germanium) layer 508, and a p-type doped GeSn layer 510. Layers 508 and 510 are epitaxially formed overlying substrate 502 and can be selectively formed within via 606—e.g., using method 200 or method 300.
FIG. 6 illustrates yet another structure 600 in accordance with additional embodiments of the disclosure. Structure 600 includes a substrate 602, a buffer layer 604 (e.g., a germanium buffer layer), a p-type doped GeSn layer 606, and a germanium layer 608. Although not illustrated, one or more layers 704-708 may be formed within a via of an insulating material, as described above in connection with FIG. 5.
It is to be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense. In the case of exemplary methods, specific routines or steps described herein can represent one or more of any number of processing strategies. Thus, the various acts illustrated can be performed in the sequence illustrated, performed in other sequences, performed simultaneously, or omitted in some cases.
The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, layers, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.

## Claims

1. A method of forming a p-type doped germanium-tin layer, the method comprising the steps of:
providing a substrate within a reaction chamber of a reactor;
providing one or more precursors to the reaction chamber; and
forming a p-typed doped germanium-tin layer on a surface of a substrate within the reaction chamber,
wherein the p-type doped germanium-tin layer comprises about 1×1018 cm−3 to about 1×1020 cm−3 p-type dopant comprising a group 13 element, and
wherein the p-type doped germanium-tin layer comprises about 1×1017 cm−3 to about 5×1017 cm−3 n-type dopant comprising a group 15 element.

2. The method of claim 1, further comprising a step of mixing a p-type dopant precursor and an n-type dopant precursor at an inlet of the reaction chamber.

3. The method of claim 1, wherein the step of providing one or more precursors to the reaction chamber comprises:
providing a germanium precursor to the reaction chamber;
providing a tin precursor to the reaction chamber;
providing a p-type dopant precursor to the reaction chamber; and
providing an n-type dopant precursor to the reaction chamber.

4. The method of claim 3, further comprising a step of providing a silicon precursor to the reaction chamber.

5. The method of claim 4, wherein the p-type doped germanium-tin layer comprises silicon.

6. The method of claim 1, wherein a ratio of a flowrate of a p-type dopant precursor to a flowrate of an n-type dopant precursor during growth of the p-type doped germanium-tin layer ranges from about 100 to about 1.

7. The method of claim 1, wherein a ratio of a flowrate of a p-type dopant precursor and a flowrate of a tin precursor ranges from about 3 to about 1.

8. The method of claim 1, wherein the step of providing one or more precursors to the reaction chamber comprises providing a carbon precursor to the reaction chamber.

9. The method of claim 1, wherein the p-type doped germanium-tin layer further comprises carbon.

10. The method of claim 1, wherein the p-type doped germanium-tin layer comprises greater than five atomic percent tin.

11. The method of claim 1, wherein the p-type doped germanium-tin layer comprises about one to about fifteen atomic percent tin.