# Patent

## Title
Methods of forming films including germanium tin and structures and devices including the films

## Abstract

Methods of forming germanium-tin films using germane as a precursor are disclosed. Exemplary methods include growing films including germanium and tin in an epitaxial chemical vapor deposition reactor, wherein a ratio of a tin precursor to germane is less than 0.1. Also disclosed are structures and devices including germanium-tin films formed using the methods described herein.

## Background
FIELD OF INVENTION
The present disclosure generally relates to techniques for depositing films including germanium tin and to structures and devices including such films. More particularly, the disclosure relates to methods of forming films comprising germanium tin using germane as a precursor, to methods of forming structures and devices including the films, and to structures and devices including the films.
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 layers, such as GeSn, GeSiSn, and the like. For example, GeSn layers can be used to form direct band gap devices and/or may be used to provide strain in an adjacent germanium layer to increase mobility in the germanium layer. Similarly, GeSiSn layers can be used to form tunable band gap devices as well as optical devices having tunable optical properties. To obtain the desired device properties, the germanium-tin films generally have a crystalline structure, which generally follows the crystalline structure of the underlying layer.
GeSn layers may be deposited or grown using a variety of techniques. For example, vacuum processes, including molecular beam epitaxy and ultra-high vacuum chemical vapor deposition, have been used to form GeSn films. The germanium precursor for such processes typically includes digermane (Ge2H6) or trigermane (Ge3H8). When the film includes silicon, the silicon precursor may include a disilane, trisilane, or other higher order silane compounds, or hetero-nuclear Si—Ge compounds with the general formula of (H3Ge)xSiH4-x (x=1-4), (H3Si)xGeH4-x (x=1-4).
Although such processes have been used to deposit or grow crystalline GeSn and GeSiSn layers, use of digermane, trigermane, or higher order germane precursors, is problematic in several respects. For example, formation of films or layers including GeSn using digermane or higher order germane precursors, such as trigermane, is not selective when certain carrier gasses (e.g., hydrogen) and/or dopants (e.g., p-type dopants) are used with the precursor. Also, digermane is relatively unstable (explosive) in concentrated form; as a result, an amount of the precursor contained in a vessel may be limited, typically to less than 154 grams, which in turn, causes throughput of processes using such a precursor to be relatively low. In addition, digermane and higher order germanes are relatively expensive. Accordingly, improved processes for forming crystalline films including GeSn are desired.
SUMMARY OF THE DISCLOSURE
Various embodiments of the present disclosure relate to methods of forming GeSn films and to structures and devices including the films. The films formed using the methods described herein may be used, for example, for semiconductor, direct band gap, photonic, or any device including the film. While the ways in which various embodiments of the disclosure address the drawbacks of the prior art methods are discussed in more detail below, in general, the disclosure provides methods of forming germanium-tin layers (e.g., crystalline), using a germane (GeH4) precursor, on a surface of a substrate and structures and devices including such films.
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).
In accordance with various embodiments of the disclosure, methods of forming a layer including GeSn includes the steps of providing a gas-phase reactor, providing a germane precursor source coupled to the gas-phase reactor, providing a tin precursor source coupled to the gas-phase reactor, providing a substrate within a reaction chamber of the gas-phase reactor, providing a germane precursor and a tin precursor to the reaction chamber, and forming (e.g., epitaxially growing) a crystalline layer of germanium tin on a surface of the substrate. In accordance with various aspects of these embodiments, the step of providing a germane precursor and a tin precursor to the reaction chamber includes providing a mixture of the tin precursor and the germane precursor having a volumetric ratio of tin precursor to germane 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. A reaction chamber temperature and pressure during the step of forming a crystalline layer of germanium tin may vary in accordance with various factors. Exemplary reaction chamber temperatures 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 further aspects of these embodiments, the germanium-tin layer includes silicon. In these cases, the method further comprises a step of providing a silicon source precursor to the reaction chamber. Exemplary silicon source precursors include disilane, trisiliane, tetrasilane, neopentasilane, and higher order silane compounds. An amount of tin incorporated into the crystalline germanium-tin layer may be about greater than 1 at %, greater than 2 at %, or greater than 5 at %, or range from about 0 at % to about 15 at % tin, about 2 at % to about 15 at % tin, about 0.2 at % to about 5 at % tin, or about 0.2 at % to about 15 at % tin. When the layer of crystalline germanium-tin 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 20 at % silicon, about 2 at % silicon and about 16 at % silicon, or about 4 at % silicon and about 12 at % silicon. In accordance with further aspects of these embodiments, the substrate includes silicon. In accordance with other aspects, the substrate includes a layer of germanium—e.g., overlying silicon. Exemplary methods in accordance with these embodiments additionally include the steps of forming an insulating layer overlying the substrate, forming a via within the insulating layer, and forming (e.g., selectively) the (e.g., crystalline) layer of a germanium tin within the via and overlying the substrate, which may include one or more previously formed layers.
In accordance with further exemplary embodiments of the disclosure, a method of forming a structure comprising a germanium-tin (e.g., crystalline) layer includes the steps of providing a gas-phase reactor, providing a substrate within a reaction chamber of the gas-phase reactor, and forming a layer comprising germanium tin (e.g., crystalline) on a surface of the substrate using one or more precursors comprising germane (GeH4). In accordance with various aspects of these embodiments, the step of forming a layer comprising germanium tin includes providing a mixture of a tin precursor and the germane having a volumetric ratio of tin precursor to germane 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. In accordance with further aspects, a reaction chamber temperature during the step of forming a crystalline layer comprising germanium tin ranges from about 200° C. to about 500° C., about 250° C. to about 450° C., or about 300° C. to about 420° C. 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. In accordance with yet further aspects of these embodiments, the step of forming a layer comprising germanium tin includes forming a layer comprising silicon germanium tin. In these cases, a silicon precursor is provided to the reaction chamber. Exemplary silicon source precursors include disilane, trisiliane, tetrasilane, neopentasilane, and higher order silane compounds. An amount of tin incorporated into the germanium-tin layer may be about greater than 1 at %, greater than 2 at %, or greater than 5 at %, or range from about 0 at % to about 15 at % tin, about 2 at % to about 15 at % tin, about 0.2 at % to about 5 at % tin, or about 0.2 at % to about 15 at % tin. When the layer of crystalline germanium-tin layer 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 20 at % silicon, about 2 at % silicon and about 16 at % silicon, or about 4 at % silicon and about 12 at % silicon. In accordance with various aspects of these embodiments, the substrate includes silicon. In accordance with other aspects, the substrate includes a layer of germanium—e.g., overlying silicon. Exemplary methods in accordance with these embodiments additionally include the steps of forming an insulating layer overlying the substrate, forming a via within the insulating layer, and forming (e.g., selectively) the layer of germanium tin within the via and overlying the substrate.
In accordance with yet further embodiments of the disclosure, a structure includes a crystalline germanium-tin layer formed in accordance with a method of the present disclosure. The structure may be used to form electronic (e.g., semiconductor) or photonic (e.g., solar or light-emitting) devices.
And in accordance with yet additional exemplary embodiments of the disclosure, a device includes a crystalline germanium-tin layer formed in accordance with a method 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 layer of crystalline germanium tin in accordance with exemplary embodiments of the disclosure.
FIG. 2 illustrates a method of forming a layer comprising germanium tin in accordance with further exemplary embodiments of the disclosure.
FIG. 3 illustrates another method of forming a layer comprising germanium tin in accordance with exemplary embodiments of the disclosure.
FIG. 4 illustrates a structure in accordance with exemplary embodiments of the disclosure.
FIG. 5 illustrates a rocking scan of a structure in accordance with exemplary embodiments of the disclosure.
FIG. 6 illustrates another structure according to yet additional exemplary embodiments of the present disclosure.
FIG. 7 illustrates yet another structure according to 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, 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.
The present disclosure relates, generally, to methods of forming layers, such as a crystalline layer, including germanium and tin, overlying a substrate. The germanium-tin layers may include additional elements, such as silicon, which form part of a crystalline lattice with the germanium-tin layer.
As used herein, a “substrate” refers to any material having a surface onto which material can be deposited. A substrate may include a bulk material such as silicon (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 may 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 silicon, and a layer comprising germanium silicon tin overlying silicon.
FIG. 1 illustrates a system 100 suitable for forming germanium-tin using the methods described herein. In the illustrated example, system 100 includes a reactor 102, a germane precursor source 104, a tin precursor source 106, an optional third precursor source 108 (e.g., for inclusion of silicon or other element(s) in a formed layer). a purge and/or carrier gas source 110, an optional mixer 112, an optional intake plenum 114, and a vacuum source 116. Sources 104-110 may be coupled to mixer 112 or reactor 102 using lines 118-132 and valves 134-140. Although not illustrated, a system, such as system 100, may include additional sources and corresponding delivery lines for dopants (e.g., n-type dopants such as phosphorous or arsenic or p-type dopants such as boron). Additionally or alternatively, the dopants may be included in one or more of the precursor sources 102-108.
Reactor 102 may be a standalone reactor or part of a cluster tool. Further, reactor 102 may be dedicated to a particular process, such as a deposition process, or reactor 102 may be used for other processes—e.g., for layer passivation and/or etch processing. For example, reactor 102 may include a reactor typically used for epitaxial chemical vapor deposition (CVD) processing, such as an Epsilon® 2000 Plus, available from ASM, and may 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. The illustrated reactor is a single-substrate, horizontal-flow reactor, which enables laminar flow of reactants over a substrate 142, with low residence times, which in turn facilitates relatively rapid sequential substrate processing. 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. Although illustrated as a horizontal-flow reactor, reactor 102 in accordance with alternative embodiments may include a vertical flow, for example, flow emanating from a showerhead and flowing substantially downward onto a substrate.
An operating pressure of a reaction chamber 144 of reactor 102 may vary in accordance with various factors. Reactor 102 may be configured to operate at near atmospheric pressure or at lower pressures. By way of 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.
Source 104 includes germane (GeH4) and may optionally include one or more dopant compounds, such as compounds typically used to fabricate photonic and/or semiconductor devices. Exemplary p-type dopant compounds include B2H6 and exemplary n-type dopant compounds include AsH3.
Use of germane is advantageous over other precursors, such as digermane, trigermane, and other higher-order germanes, used to form germanium-tin layers, because germane is relatively selective when mixed with various carrier gasses (e.g., hydrogen, nitrogen, or the like) and is also relatively selective, even when dopants (e.g., p-type dopants) are used with the precursor. Also, germane is relatively safe, compared to higher order digermanes, and thus can be used and/or transported in higher quantities, compared the higher order germanes. Also, germane is used as a precursor for other layers, such as germanium, and is more readily available and is less expensive, compared to higher-order germane compounds.
Tin precursor source 106 includes 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; ZSn(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 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.
Optional third precursor source 108, when used, includes a precursor for additional elements or compounds that may be included in a deposited layer. For example, precursor source 108 may include a silicon precursor, such as disilane, trisilane, tetrasilane, neopentasilane, and higher order silanes, or may include one or more compounds suitable as a dopant precursor/compound. If source 108 includes a silicon precursor, then, as noted above, system 100 may include additional dopant sources and corresponding supply lines.
Gas source 110 may include any suitable purge or carrier gas. Exemplary gasses suitable as carrier and purge gasses include nitrogen, argon, helium, and hydrogen.
System 100 may include a gas distribution system. An exemplary gas distribution system, which allows for fast switching between gasses (e.g., from sources 104-110) is set forth U.S. Pat. No. 8,152,922 to Schmidt et al., issued Apr. 10, 2012, entitled “Gas Mixer and Manifold Assembly for ALD Reactor,” the contents of which are hereby incorporated herein by reference, to the extent the contents do not conflict with the present disclosure. The gas distribution system may be used to, for example, mix one or more precursor gasses and a carrier gas (which may be the same or different from a purge gas from gas source 108) prior to the gasses reaching plenum 114 or reactor 102.
Turning now to FIG. 2, an exemplary method 200 of forming a crystalline germanium-tin layer is illustrated. Method 200 includes the steps of providing a gas-phase reactor (step 202), providing a germane source coupled to the gas-phase reactor (step 204), providing a tin precursor source coupled to the gas-phase reactor (step 206), providing a substrate within a reaction chamber of the gas-phase reactor (step 208), providing germane and a tin precursor to the reaction chamber, wherein a ratio of the tin precursor to the germane is less than 0.1 (step 210), and forming a crystalline layer of germanium tin on a surface of the substrate (step 212).
During step 202, a gas-phase reactor, such as a CVD reactor suitable for epitaxial growth is provided. The reactor may be a single-substrate, laminar flow reactor. Such reactors are available from ASM, such as the Epsilon® 2000 Plus.
During steps 204 and 206, suitable germane (GeH4) and tin precursor sources are coupled to the reactor. As noted above, the germane source may include dopant compounds, such as p-type dopant compounds. Or, dopants may be supplied from additional sources (not illustrated). The tin precursor source may include, for example, tin chloride and/or tin deuteride and/or other tin precursors noted herein.
During step 208, a substrate is loaded into a reaction chamber of a reactor. The substrate may be received from a loading load lock of a reactor system and transported to the reaction chamber using a suitable transfer mechanism.
At step 210, the tin precursor and germane are provided to a reaction chamber of a reactor. The tin precursor and germane may be mixed (e.g., at mixer 112) prior to entering the chamber. Additionally, the germane and tin precursor may be individually or in combination mixed with one or more carrier gasses. The germane and/or tin precursor may be mixed with a carrier upstream of the reaction chamber, such as at a mixer, or upstream of a mixer.
As noted above, use of germane has several advantages over use of typical precursors used to form layers including germanium and tin. It was surprisingly and unexpectedly found that using relatively high partial pressures of germane relative to the tin precursor formed high-quality, crystalline germanium-tin layers. In accordance with various aspects of exemplary embodiments of the disclosure, the step of providing germane and a tin precursor to the reaction chamber includes providing a mixture of a tin precursor and the germane having a volumetric ratio of tin precursor to germane 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.
During step 212, a crystalline layer (e.g., an epitaxial layer) is formed overlying a substrate. Exemplary reaction chamber temperatures during step 212 may range from about 200° C. to about 500° C., about 250° C. to about 450° C., or about 300° C. to about 420° C. And, exemplary reaction chamber pressures during this step may range from about 1 Torr to about 760 Torr, about 10 Torr to about 760 Torr, or about 50 Torr to about 760 Torr.
Step 212 may include forming a layer comprising germanium silicon tin. In these cases, a silicon precursor is provided to the reaction chamber. Exemplary silicon source precursors include disilane, trisiliane, tetrasilane, neopentasilane, and higher order silane compounds. The silicon precursor or other suitable precursor may be mixed with a carrier gas and optionally mixed with one or more other precursors as described herein.
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 germanium-tin layer may be selectively formed on the substrate within the via, as described below.
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 layer comprising germanium tin on a surface of the substrate using one or more precursors comprising germane (step 306).
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 208 of method 200.
At step 308, a crystalline layer comprising germanium tin is formed. In accordance with various aspects of exemplary embodiments of the disclosure, the step of forming a layer comprising germanium tin includes providing a mixture of a tin precursor and the germane having a volumetric ratio of tin precursor to germane 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. In accordance with further aspects, a reaction chamber temperature during the step of forming a crystalline layer comprising germanium tin 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 may include forming a layer comprising silicon germanium tin. In these cases, a silicon precursor is provided to the reaction chamber. Exemplary silicon source precursors include disilane, trisiliane, tetrasilane, neopentasilane, and higher order silane compounds.
Method 300 may also include optional steps 308 and 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. As noted above, use of a germane precursor is advantageous because it is relatively selective when using a variety of carrier gasses, such as hydrogen and when the layer includes one or more dopants, such as p-type dopants.
The layers formed using method 200 or method 300 (e.g., during steps 212 or 306) may include, for example, greater than 1 at %, greater than 2 at %, or greater than 5 at %, or between about 0 at % and about 15 at % tin, about 2 at % and about 15 at % tin, about 0.2 at % and about 5 at % tin, or about 0.2 at % and about 15 at % tin. When the layer of crystalline germanium tin 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 20 at % silicon, about 2 at % silicon and about 16 at % silicon, or about 4 at % silicon and about 12 at % silicon.
FIG. 4 illustrates a transmission electron microscope image of a structure 400 formed in accordance with various embodiments of the invention, such as method 200 or method 300. Structure 400 includes a silicon substrate 402, a germanium buffer layer 404 overlying substrate 402, and a germanium-tin layer 406 formed using germane as a precursor. In the illustrated example, layer 406 includes about 8 at % tin. No threading defects were observed in the illustrated structure.
FIG. 5 illustrates a (004) rocking scan of a structure, such as structure 400. The scan illustrates discrete peaks associated with the germanium-tin layer, the germanium layer, and the silicon substrate. As illustrated, the germanium-tin peak is associated with Pendellosung fringes, indicating a high degree of crystallinity in the germanium-tin layer.
Turning now to FIG. 6, a structure 600 in accordance with additional exemplary embodiments of the disclosure is illustrated. Structure 600 includes a substrate 602, an insulating layer 604, a via 606 formed within layer 604, a germanium layer 608 (e.g., epitaxially formed overlying substrate 602), and a germanium-tin layer 610 (e.g., epitaxially formed overlying layer 608). Layers 608 and/or 610 may be selectively formed within via 606—e.g., using method 200 or method 300.
FIG. 7 illustrates yet another structure 700 in accordance with additional embodiments of the disclosure. Structure 700 includes a substrate 702, a germanium-silicon-tin layer 704, a germanium-tin layer 706, and a germanium-silicon-tin layer 708. In the illustrated example, germanium-tin layer 706 is between two layers of germanium-silicon-tin 704 and 708. Layers 704-708 may be formed according to methods described herein. Further, 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. 6.
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. The specific routines or methods described herein may represent one or more of any number of processing strategies. Thus, the various acts illustrated may 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, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.

## Claims

1. A structure comprising a crystalline layer of germanium tin formed according to a method comprising the steps of: providing a gas-phase reactor; providing a substrate within a reaction chamber of the gas-phase reactor; and selectively forming a crystalline layer comprising germanium tin within a via formed within an insulating layer and on a surface of the substrate using one or more precursors comprising germane, wherein the germanium tin layer is between two layers of germanium silicon tin.

2. The structure of claim 1, wherein the one or more precursors comprise a tin precursor selected from the group consisting of: SnCl4, SnD4, and a methyl and/or halide substituted stannate.

3. The structure of claim 1, wherein the one or more precursors comprise a silicon precursor selected from the group consisting of: disilane, trisilane, tetrasilane, and neopentasilane.

4. The structure of claim 1, wherein the crystalline layer comprising germanium tin comprises about 2 at % to about 15 at % tin.

5. The structure of claim 1, wherein during the step of selectively forming, a ratio of a tin precursor to the germane is less than 0.05.

6. The structure of claim 1, wherein the substrate comprises a layer comprising germanium overlying silicon.

7. The structure of claim 1, wherein the two layers of germanium silicon tin comprise from about 1 at % to about 20 at % silicon.

8. The structure of claim 1, wherein the substrate comprises a layer comprising germanium silicon tin overlying silicon.

9. A structure comprising a crystalline layer of germanium tin, formed according to a method comprising the steps of: providing a gas-phase reactor; providing a substrate within a reaction chamber of the gas-phase reactor; and forming a crystalline layer comprising germanium tin on a surface of the substrate using one or more precursors comprising germane, wherein the germanium tin layer is between two layers of germanium silicon tin.

10. The structure of claim 9, wherein the one or more precursors comprise a tin precursor selected from the group consisting of: SnCl4, SnD4, and a methyl and/or halide substituted stannate.

11. The structure of claim 9, wherein the one or more precursors comprise a silicon precursor selected from the group consisting of: disilane, trisilane, tetrasilane, and neopentasilane.

12. The structure of claim 9, wherein the crystalline layer comprising germanium tin comprises about 2 at % to about 15 at % tin.

13. The structure of claim 9, wherein during the step of selectively forming, a ratio of a tin precursor to the germane is less than 0.05.

14. The structure of claim 9, wherein the substrate comprises a layer comprising germanium overlying silicon.

15. The structure of claim 9, wherein one or both of the two layers of germanium silicon tin comprise from about 1 at % to about 20 at % silicon.

16. The structure of claim 9, wherein the substrate comprises a layer comprising germanium silicon tin overlying silicon.

17. A method of forming a structure comprising a germanium-tin layer, the method comprising the steps of: providing a gas-phase reactor; providing a substrate within a reaction chamber of the gas-phase reactor; forming a crystalline layer comprising germanium tin on a surface of the substrate using one or more precursors comprising germane, wherein the germanium tin layer is between two layers of germanium silicon tin.

18. The method of claim 17, wherein the step of forming a crystalline layer comprising germanium tin comprises selectively forming the crystalline layer comprising germanium tin within a via within an insulating material.

19. The method of claim 17, wherein the germanium tin layer comprises from about 2 at % tin to about 15 at % tin.

20. The method of claim 17, wherein one or both of the two layers of germanium silicon tin comprise from about 1 at % to about 20 at % silicon.