Materials grown by plasma-enhanced chemical vapour deposition

Abstract

A method of growing a material using plasma-enhanced chemical vapour deposition 10, the method comprising: providing a seeding layer to a reactor 12, wherein the seeding layer comprises a seeding prec

Description

TECHNOLOGICAL FIELD Embodiments of the present disclosure relate to growing semiconductive materials by a plasma-enhanced chemical vapour deposition technique / method. BACKGROUND Silicon is the most important material for use within the semiconductor industry. This stems from the relative ease at which crystalline Silicon (c-Si) can be produced, and its compatibility with insulating materials such as Silicon Dioxide, thus enabling the fabrication of transistor structures for various electronic applications. Germanium or other materials are sometimes used as an equivalent to Silicon because they are also intrinsically semiconductive. There are various methods for producing crystalline Silicon. A method for producing monocrystalline Silicon for the semiconductor industry is the Czochralski method. However, this is a high-energy process. A method for producing polycrystalline Silicon is excimer laser annealing. However, achieving homogeneity over large areas is a challenge. Another method for producing polycrystalline Silicon is metal-induced crystallisation (MIC) / metal-Induced lateral crystallisation (MILC). However, this requires a predeposited film of amorphous Silicon. A method for producing thin films of hydrogenated amorphous Silicon (a-Si:H) is Plasma-enhanced chemical vapour deposition (PECVD). The process involves the dissociation of Silane gas (SiH4) using a radio frequency (RF) power supply, to create a plasma within a reactor. The plasma contains film-forming radicals that react at the surface of a substrate, resulting in the formation of a thin film of a-Si:H. BRIEF SUMMARY Embodiments of the disclosure provide a method of making a material, and the resulting material. According to various, but not necessarily all, embodiments there is provided a method of growing a compound semiconductor using plasma-enhanced chemical vapour deposition, the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex; and exposing the seeding layer to a plasma phase of at least one gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the compound semiconductor, wherein the compound semiconductor comprises elements of the at least one gaseous precursor and of the seeding precursor. An advantage is that a low-energy and spatially-homogeneous method (PECVD) can be used for producing binary compound semiconductors, or even compound semiconductors with more than two elements. The coordination centre element of the coordination complex of the seeding layer acts as a catalyst for promoting crystal growth in PECVD conditions, and bonds with the material. The result is a thin film of a compound semiconductor. Many compound semiconductors have better electrical and physical properties than elemental semiconductors. The method operates at low temperatures, can provide uniform deposition rates over large areas, and requires no pre-existing thin film of amorphous Silicon. The compound semiconductor comprises more than 1000 parts per million of each element. An element in a lower concentration is defined herein as a dopant. In some examples, the element from the coordination centre from the seeding layer (e.g. Gallium, Zinc) may bond with an element of the gaseous precursor (e.g. Oxygen, Nitrogen, Arsenic, Phosphorous) and. Examples include Gallium(lll) Oxide, Gallium Nitride, Gallium Arsenide, Gallium Phosphide, or Zinc Oxide. The compound may further be doped in the same growth step, by a dopant precursor in the seeding layer such as Aluminium, to improve electrical conductivity. For example, Aluminium-doped Zinc Oxide may be produced. Alternatively, elements from two or more gaseous precursors may bond with each other at nucleation sites formed by the seeding layer. For example, a Carbon-containing gas and a Silicon (or Germanium)-containing gas may produce Silicon Carbide (or Germanium Carbide). In the above examples, the provision of a suitable gas (e.g. Carbon, Oxygen, Arsenic or Nitrogen gas) in conjunction with a metal or metalloid in the gas or seeding layer (e.g. Zinc, Gallium, Silicon or Germanium) can grow a binary compound semiconductor in PECVD conditions. In the above method, various coordination complex types may be used. A coordination complex with a metal-Carbon bond to an organic ligand is referred to as an organometallic complex. If the coordination centre element is a nonmetal, the coordination complex is an organo-nonmetallic complex. If the coordination centre element is a metalloid, the coordination complex is an organo-metalloid complex. If the coordination centre element does not directly bond to a Carbon atom of the organic ligand, the coordination complex is a metal-organic complex. According to various, but not necessarily all, embodiments there is provided a method of growing a material such as a compound semiconductor described above, or a doped or other crystalline material, using plasma-enhanced chemical vapour deposition, the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex, wherein the coordination complex comprises a coordination centre element bound directly to a Carbon atom of a ligand; and exposing the seeding layer to a plasma phase of a gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the material. In the above embodiment, the coordination complex is an organometallic complex, an organo-nonmetallic complex, or an organo-metalloid complex. An advantage over metalorganics is fewer undesired impurities. In specific examples, the coordination complex may be an organo-nonmetallic complex or an organo-metalloid complex. The coordination centre element may have a lower electrical conductivity than Silicon (e.g. Selenium, Phosphorous, Boron, Sulphur). If the elements coagulate, there is a reduced chance of a localised short circuit when the material is in use, compared with use of an electrically conductive element such as metal. The method can be used to grow p-type and n-type doped semiconductors. For example, to grow a p-type semiconductor, the coordination complex may be an organotrivalent complex such as an organoboron complex (e.g. Triphenylborane). To grow an n-type semiconductor, the coordination complex may be an organopentavalent complex such as an organophosphorous complex (e.g. Tris(2-carboxyethyl)phosphine hydrochloride, C9H16CIO6P). The coordination centre element therefore not only acts as a catalyst for crystal growth in PECVD conditions, but also self-dopes the structure with n-type or p-type impurities. The method can be used to grow a semiconductor in which the catalyst is also semiconductive, to improve the uniformity of semiconductive properties. For example, an organoselenium complex such as Diphenyl Selenide contains Selenium which is slightly semiconductive, while also being an effective catalyst for crystal growth (particularly nanowires) in PECVD conditions. Sulphur is another effective catalyst for the growth of Silicon or Germanium nanostructures in PECVD conditions. The Sulphur may be provided as an organosulphuric complex such as Phenyl Disulfide. Sulphur advantageously creates energy states within the bandgap of Silicon. These energy states can be exploited for infrared detection and Electronic Memory Devices. According to various, but not necessarily all, embodiments there is provided a method of growing a material using plasma-enhanced chemical vapour deposition, the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex, wherein the coordination complex comprises a coordination centre element having an electrical conductivity lower than Silicon (e.g. Selenium, Phosphorous, Boron, or Sulphur); and exposing the seeding layer to a plasma phase 5 of a gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the material. According to various, but not necessarily all, embodiments there is provided a method of growing a p-type or n-type doped semiconductor material using plasma-enhanced chemical vapour deposition, the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex, wherein the coordination complex comprises a coordination centre element that is pentavalent or trivalent; and exposing the seeding layer to a plasma phase of a gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the material comprising the pentavalent or trivalent element in dopant concentrations. According to various, but not necessarily all, embodiments there is provided a method of growing an optically active material using plasma-enhanced chemical vapour deposition, the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex, wherein the coordination complex comprises a coordination centre element of Yttrium or Erbium; and exposing the seeding layer to a plasma phase of a gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the material. Yttrium and Erbium have applications in optoelectronics. According to various, but not necessarily all, embodiments there is provided a material grown according to one or more of the methods. BRIEF DESCRIPTION Some examples will now be described with reference to the accompanying drawings in which: FIG.1 schematically illustrates an example of a method; FIG. 2 schematically illustrates an example of a substrate; FIG. 3 schematically illustrates an example of a seeded substrate; FIG. 4 schematically illustrates an example of a PECVD apparatus; and FIG. 5 is a micrograph illustrating an example of Silicon nanopillars grown using PECVD. DETAILED DESCRIPTION FIG.1 illustrates a method 10 of growing the material using PECVD. At block 11, the seeding layer is prepared, for example by deposition onto a substrate. At block 12, the seeding layer is provided to a PECVD reactor. At block 13, the material is grown. First, at block 11 of the method 10 of FIG. 1, the seeding layer is prepared. The seeding layer comprises the seeding precursor coordination complex. Preparation of the seeding layer may comprise depositing the seeding precursor directly or indirectly onto a substrate of material with a high melting point, such as glass or ceramic. FIG. 2 shows an example of the substrate. FIG. 3 shows a coated substrate 30 comprising the substrate 20 and the seeding layer 31. The particular deposition method depends on the desired results. For a patterned electrode and / or moderate precision, printing (e.g. inkjet, screen-print, k-bar coating) or paint-brushing may be used. For uniform coverage over large areas, dip-coating, wire-bar coating, or spin-coating methods may be used. If high precision or a thin layer is not a requirement, spray coating or drop-casting may be used. K-bar coating, inkjet printing, screen printing and paint-brushing are readily scalable. Dip-coating and spin-coating enable precise control of the seeding layer thickness by varying immersion time (spinning speed in case of the spin-coating), the concentration of seeding precursor in aqueous solution, temperature, dip speed, and choice of solute. The required seeding layer thickness depends on the desired ratio of elements in the grown compound. The molar gas concentration of each gas, and the mass of the seeding precursor depends on the material being grown. Then, at block 12 of the method 10 of FIG. 1, the coated substrate 30 is added to the PECVD reactor. FIG. 4 schematically illustrates an example of a PECVD apparatus 40 comprising the reactor 41. Only the main components are shown. FIG. 4 represents just one example of a variety of usable PECVD designs. The illustrated PECVD apparatus 40 comprises a sealable reactor chamber 41, a gas inlet 43 for receiving the gaseous precursor(s), a radio frequency source 42, and a pair of electrodes 44, 45 between which is generated the radio frequency field which excites the gaseous precursor(s) to a plasma state. The coated substrate 30 is placed between the electrodes 44, 45, for exposure of the seeding layer 31 to the plasma. The radio frequency source 42 is configured to generate sufficient radio frequency energy to dissociate the gases (e.g. Silane gas) into a plasma comprising film-forming radicals (e.g. elemental Silicon). The plasma-generating excitation frequency may be from tens of kilohertz to tens of megahertz. A non-limiting example is approximately 13.6MHz which is reserved for industrial, scientific and medical (ISM) purposes. Then, at block 13 of the method 10 of FIG. 1, the method comprises exposing the seeding layer to the plasma phase of the gaseous precursor(s), at a temperature above the cracking temperature of the seeding layer, to grow the material. The temperature may be controlled using a heater configured to heat the coated substrate 30. Experiments have found the suitable growth-temperature range to be between approximately 200 Celsius and approximately 400 Celsius, wherein the precise temperature depends on the cracking temperature of the compound used in the seeding layer. This is significantly lower than the temperature required by Low-pressure CVD (around 600 Celsius). The compound grown by the method may comprise nanocrystals, nanowires, and / or nanoparticles. The term ‘nanowire’ herein refers generally to nanostructures that have a constrained nano-scale thickness or diameter, and an unconstrained length. This covers nanostructures that would be described as nanorods, nanopillars (low length:width ratio) and nanowires (high length:width ratio). Experiments have found that the reactor pressure may be between approximately 100mTorr and approximately 900mTorr, depending on the desired nanostructure shapes. Lower pressures within the range can produce planar nanocrystals. Higher pressures within the range have been found to produce nanowires, which become straighter as pressure increases. Experiments have found that a radio frequency power of between approximately 5 Watts and approximately 25 Watts will form a plasma containing film-forming radicals. Higher powers within the range have been found to create smaller diameter nanowires, at a slower growth rate. In an experiment, Silicon nanopillars were grown by PECVD at a temperature of approximately 350 Celsius, a radio frequency power of approximately five watts, and a pressure of approximately 400mbar. FIG. 5 illustrates an example of Silicon nanopillars grown by PECVD. Examples are provided below of precursors that can grow wide-bandgap compound semiconductors as thin films in PECVD. In order to produce Gallium-based compounds, a Gallium-containing coordination complex may form the seeding precursor. In an example implementation, the coordination complex may comprise Gallium(l 11) acetylacetonate or Gallium(l 11) nitrate. The thermal decomposition (cracking) temperature of these coordination complexes is within the range supported by PECVD (200-400 Celsius). If the gaseous precursor is Oxygen-containing (e.g. 02), Gallium(lll) Oxide can be grown. If the gaseous precursor is Nitrogen-containing (e.g. N2, NH3), Gallium Nitride can be grown. If the gaseous precursor is Arsenic-containing (e.g. AsH3), Gallium Arsenide can be grown. If the gaseous precursor is Phosphorous-containing (e.g. PH3), Gallium Phosphide can be grown. In order to produce a Zinc Oxide compound, a Zinc-containing coordination complex may form the seeding precursor. In an example implementation, the coordination complex may comprise Zinc Acetate, which thermally decomposes at less than 400 Celsius. If the gaseous precursor is Oxygen-containing, Zinc Oxide can be grown. In some implementations, the compound may be doped. The seeding layer may comprise a dopant precursor. For example, Zinc Oxide may be doped with an Aluminium-containing coordination complex (e.g. Aluminum nitrate hydrate) to produce an n-type semiconductor. During preparation of the seeding layer, the dopant 9 precursor and the seeding precursor may be mixed to form a solution, prior to deposition (e.g. dip-coating or spin-coating). In order to produce a Carbide-based compound, two gaseous precursors may be reacted. For example, a Silicon-containing gas may be reacted with a Carbon-containing gas, to produce Silicon Carbide. Examples of Silicon-containing gases include Silane, Silylene, Trichlorosilane, or Disilene. If Germanium is used instead of Silicon, the gas may be Germane, Germylene, or Digermane. Other gases are possible. Examples of Carbon-containing gases include Methane or Acetylene. The reaction occurs at nucleation sites on the seeding layer. The nucleation sites may be created by thermal decomposition of a seeding layer coordination complex at below 400 Celsius. Examples of coordination complexes suitable for decomposing in PECVD and promoting Silicon Carbide (or Germanium Carbide) growth include: Zinc Acetate; Indium (III) Acetylacetonate; Copper (II) Acetylacetonate; Gallium (III) acetylacetonate; Silver Acetate; Nickel (II) acetate tetrahydrate; Nickel (II) formate dihydrate; Nickel (II) formate; Tin (II) Acetylacetonate; Diphenyl selenide; Tin (II) Acetate; Chromium(lll) nitrate nonahydrate; Platinum(ll) 2,4-pentanedionate; Copper(ll) ethylacetoacetate; Molybdenum(VI) oxide bis(2,4-pentanedionate); Magnesium nitrate hydrate; Iron(lll) nitrate hydrate; Indium(lll) nitrate hydrate; Aluminum nitrate hydrate; Silver nitrate; Palladium(ll) nitrate; Cadmium acetate dihydrate; Gallium(lll) nitrate solution; or Bismuth(lll) nitrate hydrate. Examples of compound semiconductors comprising more than three elements include Aluminium Gallium Nitride, and Silicon Germanium Carbide. Aspects of the disclosure also relate to the use of coordination complexes containing a direct Carbon bond to the coordination centre element, as the seeding precursor. This enables the growth of a variety of materials in PECVD conditions. When growing an n-type doped semiconductor, Silicon or Germanium-containing gas may be excited into a plasma state. The coordination complex in the seeding layer may comprise a pentavalent element as the coordination centre, bound to a Carbon atom of an organic ligand. If the pentavalent element (dopant) is Phosphorous, for example, the coordination complex may be Tris(2-carboxyethyl)phosphine hydrochloride) (e.g. C9H16CIO6P). This organophosphorous compound avoids phosphine gas used for n-type doping in Si. When growing a p-type doped semiconductor (e.g. Silicon or Germanium-based), the seeding layer may instead comprise a trivalent element as the coordination centre element. For example, If the trivalent element (dopant) is Boron, the coordination complex may be Triphenylborane. A semiconductor (e.g. Silicon or Germanium-based) may be grown in which the coordination centre of the seeding precursor is semiconductive. Selenium is an example of a semiconductor which promotes the formation of nanostructures. Pure Selenium is volatile and may evaporate from the seeding layer prematurely, however, an organoselenium complex such as Diphenyl Selenide will stabilize the Selenium until the process begins. Further, an example of a Sulphur-containing complex is Phenyl Disulfide. Aspects of the disclosure also relate to optically active materials. Elements such as Erbium or Yttrium may be useful dopants in the production of optically active Silicon or Germanium materials. An example of an Yttrium-containing complex is Yttrium(lll) acetate hydrate or Yttrium trifluoroacetate. An example of an Erbium-containing complex is Erbium (III) Acetate. Example use cases for the materials described above include, but are not limited to battery electrodes, thin-film transistors, electronic memory and photovoltaics. Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one..” or by using “consisting”. Although references are made herein to targeted 11 properties such as nanocrystalline structures, nanowires, and particular compounds, these have an inclusive not an exclusive meaning. For example, the material is substantially, but not exclusively nanocrystalline, and substantially, but not exclusively comprises nanowires. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance it should be understood that the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon. The following clauses do not form part of the claimed invention. 1. A method of growing a compound semiconductor using plasma-enhanced chemical vapour deposition, the method comprising: providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex; and exposing the seeding layer to a plasma phase of at least one gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the compound semiconductor, 13 wherein the compound semiconductor comprises elements of the at least one gaseous precursor and of the seeding precursor. 2. The method of clause 1, wherein the compound semiconductor is a binary compound or a ternary compound. 3. The method of clause 1 or 2, wherein the compound is of an element of the gaseous precursor and an element corresponding to a coordination centre of the coordination complex of the seeding precursor. 4. The method of clause 3, wherein the coordination centre of the seeding precursor is Gallium, and wherein the element of the gaseous precursor is one of: Oxygen, so that the compound semiconductor is Gallium(l 11) Oxide; Nitrogen, so that the compound semiconductor is Gallium Nitride; Phosphorous, so that the compound semiconductor is Gallium Phosphide; or Arsenic, so that the compound semiconductor is Gallium Arsenide. 5 The method of clause 3, wherein the element of the gaseous precursor is Oxygen, wherein the coordination centre of the seeding precursor is Zinc, and wherein the compound semiconductor is Zinc Oxide. 6. The method of clause 3, 4 or 5, wherein the seeding layer further comprises a dopant. 7. The method of clause 6, wherein the element of the gaseous precursor is Oxygen, wherein the coordination centre of the seeding precursor is Zinc, wherein the dopant is Aluminium, and wherein the compound semiconductor is Aluminium-doped Zinc Oxide. 8. The method of clause 1 or 2, wherein the compound is of elements of two gaseous precursors. 9. The method of clause 8, wherein the elements of the two gaseous precursors are Carbon and either Silicon or Germanium, and wherein the compound 14 semiconductor is Silicon Carbide or Silicon Germanium Carbide or Germanium Carbide. END OF DESCRIPTION 5 l / we claim:

Claims

1. A method of growing a material using plasma-enhanced chemical vapour deposition, the method comprising:providing a seeding layer to a reactor, wherein the seeding layer comprises a seeding precursor, wherein the seeding precursor comprises a coordination complex, wherein the coordination complex comprises a coordination centre element bound directly to a Carbon atom of a ligand; andexposing the seeding layer to a plasma phase of a gaseous precursor, at a temperature above a cracking temperature of the seeding precursor, to grow the material.

2. The method of claim 1, wherein the coordination centre element has a lower electrical conductivity than Silicon.

3. The method of claim 1 or 2, wherein the coordination complex is an organo-nonmetallic complex or an organo-metalloid complex.

4. The method of claim 1, 2 or 3, wherein the coordination complex is an organotrivalent complex, and wherein the material exhibits p-type semiconductivity.

5. The method of claim 4, wherein the organotrivalent complex is an organoboron complex.

6. The method of claim 5, wherein the organoboron complex is Triphenylborane.

7. The method of claim 1, 2 or 3, wherein the coordination complex is anorganopentavalent complex, and wherein the material exhibits n-type semiconductivity.

8. The method of claim 7, wherein the organopentavalent complex is an organophosphorous complex.

9. The method of claim 8, wherein the organophosphorous complex is Tris(2-carboxyethyl)phosphine hydrochloride).

10. The method of claim 1, 2 or 3, wherein the coordination complex is an organosulphuric complex.5 11. The method of claim 10, wherein the organosulphuric complex is PhenylDisulfide.

12. The method of claim 1, 2 or 3, wherein the coordination complex is an organoselenium complex.1013. The method of claim 12, wherein the organoselenium complex is Diphenyl Selenide.

14. The method of any preceding claim, wherein the element of the gaseous 15 precursor is semiconductive, and optionally wherein the semiconductive element of the gaseous precursor comprises Silicon or Germanium or Gallium.

15. A material grown according to the method of any one of the preceding claims.

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