WO2016020657A1 - Thin film semiconductor - Google Patents

Abstract

The invention provides a process for producing a semiconducting film, which process comprises: in a non-oxidising atmosphere, disposing a composition which is a liquid composition or a gel composition onto a substrate, wherein the composition comprises a compound which is a precursor to a semiconductor; heating the substrate in said non- oxidising atmosphere to form a film comprising the semiconductor on the substrate; and prior to removing the film from the non-oxidising atmosphere for the first time, irradiating the film in the non-oxidising atmosphere with ultraviolet light. The invention further provides semiconducting films obtainable by the process of the invention, including transparent conducting films which comprise polycrystalline zinc oxide. The invention also provides a coated substrate, which substrate comprises a surface, which surface is coated with a film of the invention, as well as a semiconducting device, such as for instance a thin film transistor, comprising a film of the invention.

Description

THIN FILM SEMICONDUCTOR

FIELD OF THE INVENTION

The invention relates to a process for producing a semiconducting film and to semiconducting films obtainable by that process.

BACKGROUND TO THE INVENTION

The combination of properties that define transparent conducting oxides (TCOs), namely, simultaneous transparency to visible light and highly tuneable electrical conductivity, enable a range of technologies that are essential to the modern world. As thin films, TCOs are used as transparent electrodes in electronic displays and photovoltaic cells, as the channel layer in transparent thin-film transistors (TFTs), and as low emissivity coatings on glass for a range of architectural and household applications.

The electrical performance of a TCO film is most directly related to its carrier concentration (ti) and its carrier mobility (j ). These two parameters come together to determine the electrical conductivity according to the relation σ = ηβμ, where e is the elementary charge. It is always important to realize that optimal values for n and μ can vary dramatically depending on the particular application for which a TCO film is intended. As two contrasting examples, low emissivity glass coatings require films with a high enough n to result in high infrared reflectivity, whereas transparent TFTs explicitly require a transparent channel layer with high μ, but low n.

Zinc oxide (ZnO) is a prototypical n-type TCO that has attracted a great deal of research and commercial attention because of its low cost, low toxicity, high earth- abundance, and suitability for deposition using scalable, solution-based techniques like spray pyrolysis. These attributes, in combination with the fact that the electrical and optical properties of ZnO are readily modified by deposition conditions and the addition of appropriate dopants, make it an attractive candidate for many TCO-related applications (C. W. Litton, D. C. Reynolds and T. C. Collins, Zinc Oxide Materials for Electronic and

Optoelectronic Device Applications , Wiley, Chichester, UK, 2011; C. F. Klingshirn, A. Waag, A. Hoffmann and J. Geurts, Zinc Oxide: From Fundamental Properties Towards Novel Applications, Springer, Heidelberg, Germany, 2010; Z. C. Feng, Handbook of Zinc Oxide and Related Materials, CRC Press, Boca Raton, FL, 2012).

The excitation and activation of ZnO by light is a well-studied phenomenon. Of particular interest have been so-called photoconductive effects, where the electrical conductivity of ZnO changes upon irradiation. These types of effects have been exploited in a variety of sensing applications (J. Suehiro, N. Nakagawa, S. Hidaka, M. Ueda, K.

Imasaka, M. Higashihata, T. Okada and M. Hara, Nanotechnology, 2006, 17, 2567-2573; Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li and C. L. Lin, Appl. Phys. Lett., 2004, 84, 3654-3656; R. Martins, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, M. Bender, N. Katsarakis, V. Cimalla and G. Kiriakidis, J. Appl. Phys., 2004, 96, 1398-1408) and to activate nanoparticle ZnO catalysts for the photodegradation of dissolved organics (R. Barnes, R. Molina, J. Xu, P. Dobson and I. Thompson, J. Nanopart. Res., 2013, 15, 1-11; A. Mclaren, T. Valdes-Sohs, G. Li and S. C. Tsang, J. Am. Chem. Soc, 2009, 131, 12540-12541). ZnO photoconductivity has thus far been found to have only a limited degree of persistence, although some effects of irradiation have been found to decay only slowly over a timescale of hours to days when the ZnO is stored in air (E. Molinari, F. Cramarossa and F. Paniccia, J. Catal, 1965, 4, 415-429).

SUMMARY OF THE INVENTION

The present invention relates to an even longer-lived photoconductive effect in semiconducting films produced from liquid or gel precursor compositions (i.e. by solution methods) in a non-oxidising atmosphere such as nitrogen. The effect results from UV irradiation of the films in the non-oxidising atmosphere before they are first exposed to air. Using this approach, films with an unexpectedly high Hall effect mobilities have been prepared, which would not have been expected for such films, particularly when deposited by solution methods, and which also compare favourably with mobilities reported for films of comparable carrier concentration deposited with the vacuum-based sputtering techniques commonly used by industry. In addition, high average conductivities were achieved at film deposition temperatures much lower than required to prepare the most conductive undoped films in the absence of irradiation. Such a reduction in processing temperature could facilitate the emergence of semiconductor applications requiring deposition onto flexible, temperature-sensitive substrates.

Accordingly, the invention provides a process for producing a semiconducting film, which process comprises:

in a non-oxidising atmosphere, disposing a composition which is a liquid composition or a gel composition onto a substrate, wherein the composition comprises a compound which is a precursor to a semiconductor; heating the substrate in said non-oxidising atmosphere to form on the substrate a film comprising the semiconductor; and

prior to removing the film from the non-oxidising atmosphere for the first time, irradiating the film in the non-oxidising atmosphere with ultraviolet light.

The invention also provides a semiconducting film which is obtainable by the process of the invention as defined above.

Also provided is a semiconducting film which is obtained by the process of the invention as defined above.

The process of the invention can be used to produce novel transparent conducting films having a particularly high μ, but low n, and which are therefore particularly suitable for use as the channel layer in a transparent thin-film transistor (TFT).

Accordingly, the invention also provides a transparent conducting film having: a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1;

an electrical conductivity of at least 10.0 Q^cm^"1; and

a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

The transparent conducting film of the invention typically comprises a metal oxide, a metal oxide halide, a metal oxide nitride, a metal oxide chalcogenide or a metal chalgogenide, any of which may be amorphous or polycrystalline. Metal oxides, for instance zinc oxide, are particularly preferred. These are transparent conducting oxides (TCOs). Thus, the transparent conducting film may comprise an amorphous zinc oxide- based TCO (e.g. InZnO or GalnZnO), or a polycrystalline zinc oxide-based TCO (e.g. polycrystalline ZnO). When zinc oxide is employed, it is often polycrystalline. Thus, in one embodiment the transparent conducting film comprises polycrystalline zinc oxide. The polycrystalline zinc oxide in this embodiment typically has a wurtzite structure.

The novel transparent conducting films of the invention, which may for instance comprise zinc oxide, may be undoped (i.e. not doped with any dopant element) or they may be mildly intrinsically doped, for example by a small degree of oxygen-deficient non- stoichiometry. In one embodiment, the transparent conducting film is not doped with any dopant element (i.e. it is not extrinsically doped) and is not intrinsically doped either. In another embodiment, the film is not doped with any dopant element (i.e. not extrinsically doped) but is intrinsically doped by oxygen-deficient non-stoichiometry. Alternatively, the film of the invention may be doped with a dopant element. The film of the invention may for instance be p-doped, for instance doped with an alkali metal element (such as Li), to reduce the carrier concentration, n, further whilst maintaining the high Hall mobility, μ. In this way, films particularly suitable for use in thin-film transistors (TFTs) may be produced.

Further provided is a coated substrate, which substrate comprises a surface, which surface is coated with a semiconducting film or transparent conducting film of the invention as defined above.

The invention also provides a semiconducting device comprising a semiconducting film or transparent conducting film of the invention as defined above.

The invention also provides a thin film transistor comprising a semiconducting film or transparent conducting film of the invention as defined above. Usually, the film of the invention is employed as a channel layer in the thin film transistor.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a schematic indicating the preparation, in situ UV treatment, optional ex situ UV treatment, and characterization steps for the ZnO thin films as detailed in the Example. In a nitrogen-filled deposition chamber, a zinc-containing precursor solution is decomposed by intereaction with a heated substrate to form a ZnO thin film (a). After deposition, the film is irradiated with UV in situ while it remains under the inert atmosphere of the deposition chamber (b). Once the sample is removed from the chamber and initially exposed to air (c), an optional ex situ UV irradiation (d) of the film can be performed just prior to electrical characterization of the sample (e).

Fig. 2 is a graph of the electrical conductivity of undoped ZnO thin films as a function of deposition temperature for films deposited in the dark (filled symbols) and with in situ UV irradiation (open symbols). Where shown, error bars represent standard deviations across the set of multiple samples deposited under the same conditions. The lowest temperature pair of data points are estimates based on 2-point resistance measurements.

Fig. 3 is a graph of the electrical conductivity of ZnO thin film deposited at 376 °C as a function of time since deposition. The linear trendline shows the persistence of the effect in situ irradiation with time when the film is stored in air. The dashed line indicates the average electrical conductivity of films deposited in the dark under the same conditions.

Fig. 4 (a) and (b) are graphs of the Hall effect (a) carrier mobility and (b) carrier concentration data as a function of deposition temperature for undoped ZnO films deposited with in situ UV irradiation (open symbols) and in the dark (filled symbols), collected without further treatment after sample removal from deposition chamber.

Fig. 5 (a) and (b) are graphs of the Hall effect (a) carrier mobility and (b) carrier concentration data as a function of deposition temperature for undoped ZnO films made with in situ UV irradiation (open symbols) and in the dark (filled symbols), immediately after an additional ex situ irradiation. The corresponding data for the films without ex situ UV treatment is also provided on the dotted lines for comparison.

Fig. 6 shows normalized X-ray diffraction patterns of ZnO thin films deposited at a range of temperatures. The observed peaks are annotated with the corresponding crystalline plane of hexagonal wurtzite ZnO. The minor peak marked with asterisk * is an instrumental artefact related to tungsten contamination in the Cu x-ray source.

Fig. 7 shows scanning electron micrographs of undoped ZnO films deposited at a range of substrate temperatures from 250 to 417 °C. A clear change is morphology can be observed across this series, particularly between 292 °C and 334 °C.

Fig. 8 is a plot of variable temperature carrier mobility measurements of representative ZnO films deposited at 376 °C deposited with in situ irradiation (open symbols) and in the dark (filled symbols). Dashed fit lines to the higher temperature data are shown, as are the grain boundary potential barrier (Vb) and effective grain size (L) extracted from this fit for each sample.

Fig. 9 is a general schematic of changes in the near grain boundary band structure of air-exposed ZnO upon UV irradiation. Neutralization of filled acceptor states at the grain boundary results in increases in both carrier concentration and mobility. Whether these changes are reversible determines the persistence of the photoconductive effect.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for producing a semiconducting film. The film may be amorphous or polycrystalline. Often, however, the film is polycrystalline. Thus, the process is typically used to produce a semiconducting film, comprising a

polycrystalline semiconductor.

Of particular importance are transparent conducting films. Accordingly, the process is often a process for producing a transparent conducting film. Transparent conducting films produced by the process of the invention are, by definition, both transparent and conducting. The word "conducting" as used herein means that the film is electrically conductive. The word "transparent" as used herein means that the film has optical transmittance in the visible range of the spectrum, from about 400 nm to about 800 nm. Usually, the film produced by the process of the invention has a mean optical transparency in the visible range of the spectrum which is equal to or greater than about 50 %. More typically, the mean optical transparency is equal to or greater than about 70 %, or equal to or greater than about 75 %. Even more typically, the mean optical transparency in the visible range of the spectrum is equal to or greater than about 80 %. In one

embodiment, the transparency of the film is optimised to a value equal to or greater than about 90 %.

The semiconductor is generally inorganic. It typically comprises a metal ion, and may comprise a plurality of different metal ions. Thus, usually, the semiconductor comprises a metal compound. Often, the semiconductor comprises a metal oxide. The metal oxide may be a transparent conducting oxide (TCO). Zinc oxide is particularly preferred.

The process comprises: in a non-oxidising atmosphere, disposing a composition which is a liquid composition or a gel composition onto a substrate, wherein the composition comprises a compound which is a precursor to a semiconductor; heating the substrate in said non-oxidising atmosphere to form on the substrate a film comprising the semiconductor; and prior to removing the film from the non-oxidising atmosphere for the first time, irradiating the film in the non-oxidising atmosphere with ultraviolet light.

As mentioned above, the semiconductor typically comprises a metal compound. In that case, the compound which is a precursor to a semiconductor is usually a compound comprising the same metal. The metal compound may for instance be a metal oxide, a metal oxide halide, a metal oxide nitride, a metal oxide chalcogenide or a metal chalgogenide, any of which may be amorphous or polycrystalline, and the compound which is a precursor to the semiconductor may be a salt of the metal with, for instance an organic anion, such as acetate, or a halide anion, such as chloride. Usually, the compound which is a precursor is one which is soluble in a solvent that is also present in the liquid composition or gel composition.

Usually, the semiconductor comprises a metal oxide. The metal oxide may be amorphous or polycrystalline. The semiconducting film is typically a transparent conducting film. The transparent conducting film may for example comprise an amorphous zinc oxide-based TCO (e.g. InZnO or GalnZnO), or a polycrystalline zinc oxide-based TCO. Polycrystalline ZnO may for instance be used. Often, the metal oxide is a polycrystalline metal oxide, and the semiconducting film is often a transparent conducting film. The compound which is a precursor to a semiconductor is usually a compound comprising the same metal. The compound comprising the same metal is usually one which is soluble in a solvent that is also present in the liquid composition or gel composition. Typically, it is a halide salt of the metal, or a salt of the metal with an organic anion, for instance a chloride or acetate or citrate salt. More often than not it is a salt of the metal with an organic anion, for instance a metal acetate salt.

The metal oxide may for instance be zinc oxide. The zinc oxide may be amorphous or polycrystalline. Thus, the transparent conducting film may for example comprise an amorphous zinc oxide-based TCO (e.g. InZnO or GalnZnO), or a polycrystalline zinc oxide-based TCO. Polycrystalline zinc oxide is often employed. Polycrystalline zinc oxide having a wurtzite (hexagonal) crystal structure is a particularly preferred

semiconductor. The compound comprising the same metal in these cases is a zinc compound. Suitable zinc compounds would include zinc salts, for instance organic acid salts, for instance acetate and citrate salts, of zinc, or nitrate and halide salts of zinc.

Usually, the zinc compound is a zinc salt comprising an organic anion. For instance, the zinc compound may be zinc acetate or zinc citrate. The semiconducting film produced by the process may be amorphous or polycrystalline, but is often a polycrystalline ZnO film. Typically, this has a wurtzite (hexagonal) crystal structure. Also, often, the c-axis of the polycrystalline ZnO is perpendicular to the plane of the substrate.

The composition is typically a liquid composition, i.e. a composition in the liquid state. Such liquid compositions are typically solutions or dispersions of said compound which is a precursor to a semiconductor in a solvent. Usually, however, the composition is a solution which comprises (a) a solvent, and (b) said compound which is a precursor to a semiconductor dissolved in the solvent. Any suitable solvent may be employed. Typically, however, the solvent is a polar solvent. For instance, the solvent may comprise water, an alcohol, or a mixture of solvents comprising an alcohol and water. The precursor solution is therefore prepared by dissolving the appropriate amount of the compound which is a precursor to a semiconductor in an appropriate volume of a solvent or a mixture of solvents. Typically, the compound which is a precursor to a semiconductor is a salt, for instance a metal salt, such as for instance a zinc salt. Any suitable zinc salt soluble in polar solvents may be used, for instance acetates, nitrates, chlorides or zinc salts formed by other anions. Typically, the solvent comprises water and/or an alcohol mixed in the proportion between 0% and 100% of alcohol. Typically, between 0.5% and 10 vol.% of a mineral or organic acid is added to the precursor solution to prevent hydrolysis of metal salts, such as zinc salts.

The liquid composition need not contain a solvent, but could instead be a neat liquid. A suitable neat liquid would be one comprising or more liquid compounds which comprise Zn. Various zinc compounds in the liquid state are known, including organo- zinc compounds such as diethyl zinc.

In one embodiment, the composition is a gel composition, i.e. a composition in the gel form. A gel may be defined as a substantially dilute crosslinked system, which exhibits no flow when in the steady-state. Many gels display thixotropy - they become fluid when agitated, but resolidify when resting. In one embodiment, the gel composition used in the present invention is a hydrogel composition which comprises Zn. In another embodiment, the gel composition is an organogel composition which comprises Zn.

Gel compositions can advantageously be used in a sol-gel approach, wherein the step of disposing the composition onto a substrate comprises depositing a sol gel onto the substrate. The substrate is subsequently heated to form the film. The sol gel route is an inexpensive technique that allows for the fine control of the resulting film's chemical composition.

The semiconductor may, or may not, be doped with a dopant element. When the semiconductor is doped with a dopant element, the composition typically comprises a compound comprising said dopant element. Alkali metal dopant elements may be particularly suitable for suppressing the carrier concentration of zinc oxide films, for instance, to increase the suitability of the semiconducting film for TFT applications. Li, for instance, may be present as a dopant element. Dopant elements can be introduced, by including those elements in the composition too, in the desired concentration (for instance in the form of one or more precursor compounds). For example, in cases where the semiconductor comprises a doped zinc oxide, Zn and a dopant element, M, Zn and M are typically present in the form of two separate precursor compounds, namely a zinc- containing compound (typically a zinc salt, such as zinc diacetate or zinc citrate) and a compound containing M (typically a salt, for instance a salt with an organic anion such as acetate or citrate). Any suitable zinc salt and salt of M may be used. Typically, however, the salts must be soluble in a solvent (typically in a polar solvent). Thus, suitable salts include organic acid salts, for instance acetate and citrate salts of zinc, the nitrate and halide salts of zinc, and the organic acid salts of silicon, for instance silicon tetraacetate. In another embodiment, however, Zn and M are present in one and the same precursor compound in the composition.

Often, however, the semiconductor is not doped with a dopant element.

In the process of the invention, the liquid or gel composition can be disposed (or deposited) onto the substrate by any suitable method. Suitable methods include spraying, dip-coating and spin-coating.

Dip coating typically refers to the immersing of the substrate into a tank containing the composition, removing the substrate from the tank, and allowing it to drain. Thus, dip- coating typically involves three stages: (i) immersion: the substrate is immersed in the composition at a constant speed, preferably without juddering the substrate; (ii) dwell time: the substrate remains fully immersed in the composition and motionless to allow for the coating material to apply itself to the substrate; and (iii) withdrawal: the substrate is withdrawn, again at a constant speed to avoid any judders. The faster the substrate is withdrawn from the tank the thicker the coating of the composition containing the compound which is a precursor to a semiconductor that will be applied to the substrate.

In spin coating, an excess amount of the composition comprising the compound which is a precursor to a semiconductor is placed on the substrate, which is then rotated at high speed in order to spread the fluid on the substrate thinly by centrifugal force. A spin coater or spinner is typically employed. Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of film is achieved. The applied composition is usually volatile, and simultaneously evaporates. Accordingly, the higher the angular speed of spinning, the thinner the film. The thickness of the film also depends on the concentration of the composition and the solvent. Spin coating can be used to create thin films with thicknesses below 10 nm.

Accordingly, in the process of the invention, the step of disposing the composition onto the substrate comprises spraying, dip-coating or spin-coating the composition onto said substrate.

Spraying is particularly preferred. Thus, the composition is typically disposed on the substrate by spray deposition. In spray deposition, a jet of fine droplets of the composition is sprayed onto the substrate, typically through a nozzle with the aid of a pneumatic carrier gas. Typically, in this embodiment, the composition is a liquid composition as opposed to a gel. More typically, it is a solution or a dispersion. Thus, a solvent is typically present. The composition is preferably a solution which comprises (a) a solvent, and (b) said compound which is a precursor to a semiconductor dissolved in the solvent. Any suitable solvent may be employed, as discussed above. Typically, however, the solvent is a polar solvent. For instance, the solvent may comprise water, an alcohol, or a mixture of solvents comprising an alcohol and water. Spray deposition has the advantages that the formation of fine droplets in the spray encourages the some or all of the unwanted solvent to evaporate as deposition onto the substrate occurs; it also allows a fine thin layer of film to be built-up gradually.

Typically, in the process of the invention, the steps of disposing the composition onto the substrate and heating the substrate are performed simultaneously. Simultaneous deposition onto the substrate and heating of the substrate is particularly preferable in embodiments where the composition is disposed onto the substrate by spraying it onto the substrate. Indeed, such embodiments embrace the production of semiconducting films by spray pyrolysis, wherein spraying said composition onto the heated substrate causes pyrolitic decomposition of the composition and formation of a layer of the film comprising the semiconductor on the substrate. Such embodiments of the invention are particularly advantageous because the two steps of (i) preparing the semiconductor and (ii) depositing the semiconductor in the form of a thin film are effectively performed simultaneously.

Accordingly, in one embodiment of the process of the invention, disposing the composition onto the substrate comprises spraying the composition onto the substrate. Typically, in this embodiment, the steps of spraying the composition onto the substrate and heating the substrate are performed simultaneously. Typically, in this embodiment, the process of the invention comprises spray pyrolysis, wherein spraying the composition onto the heated substrate causes pyrolitic decomposition of the composition and formation of said film comprising the semiconductor on the substrate.

Spray pyrolysis is a process in which a thin film is deposited by spraying a solution on a heated surface, where the constituents react to form a chemical compound which may be amorphous or crystalline. Both the amorphous and crystalline forms typically have important and characteristic optical and electrical properties. Typically, in the present invention, the constituents react to form a polycrystalline semiconductor as defined herein. The chemical reactants are selected such that the products other than the desired compound are volatile at the temperature of deposition.

A typical spraying apparatus for use in spray pyrolysis is described in Ann. Rev. Mater. Sci. 1982, 12:81-101, the contents of which are incorporated herein by reference. A propellant gas or carrier gas is introduced into a spray head, as is the liquid composition (the spray solution). Typically, the spraying apparatus provides for measurement of the flow of both the carrier gas and the liquid into the spray head. The spray head (also known as an atomiser or spray nozzle) also comprises an exit, which usually includes a nozzle through which the liquid or solution is propelled by the carrier gas to produce a spray of fine droplets. A pyrex glass or stainless steel spray head can be used, as can other atomizers, such as a resonant cavity or a piezoelectric transducer. The substrate heater is typically an electric heater which is controlled within +/-5 °C through a thermocouple located under the substrate and used as a sensor for a temperature controller.

Typically, in the process of the invention said spraying of the composition onto the substrate is performed with the aid of a carrier gas. The carrier gas propels the composition through the nozzle in the spray head to produce a fine spray of droplets, which are carried to the substrate by the carrier gas.

Significant variables in the spray pyrolysis process are the ambient temperature (which is typically room temperature), carrier gas flow rate, nozzle-to-substrate distance, droplet radius, solution concentration (when the liquid composition is a solution), flow rate of the liquid composition and, for continuous processes where large surface areas of substrate are covered by the transparent conducting oxide, substrate motion. Further factors are of course the chemical composition of the carrier gas and/or environment, and, importantly, substrate temperature.

Typically, the carrier gas comprises air, an inert gas or a mixture of gases, for example a mixture of argon and hydrogen. More typically, the carrier gas is compressed nitrogen, which is also used as a reactor gas.

Typically, in the process of the invention wherein spraying of the composition onto the substrate is performed with the aid of a carrier gas, the step of spraying the composition onto the substrate comprises (i) introducing said composition and said carrier gas into a spray head, wherein the composition is introduced at a first flow rate and the carrier gas is introduced at a second flow rate, wherein the first and second flow rates are the same or different, and (ii) spraying the composition onto said substrate from an exit of said spray head. Typically, the exit of the spray head comprises a nozzle.

Typically, the first flow rate, at which the composition is introduced into the spray head, is from 0.1 ml/min to 20 ml/min. More typically, the first flow rate is from 0.1 ml/min to 10 ml/min. The first flow rate may for instance be about 1 ml/min.

Typically, the second flow rate, at which the carrier gas is introduced into the spray head, is 1 1/min to 30 1/min. More typically, the second flow rate is about 15 1/min. Usually, the distance between said exit of said spray head (i.e. the nozzle) and the substrate is from 10 cm to 50 cm, more typically from 20 cm to 40 cm, for instance about 30 cm.

Typically, in the process of the invention, the step of spraying the composition onto said substrate comprises spraying a jet of fine droplets of said composition onto the substrate. It is thought that in some cases the droplets reach said substrate and reside on the surface of the substrate as the solvent evaporates, leaving behind a solid that may further react in the dry state. In other cases, the solvent may evaporate before the droplet reaches the surface and the dry solid impinges on the substrate, where decomposition then occurs. In both cases, the compound which is a precursor to a semiconductor reacts to form on the substrate a film comprising the semiconductor.

It is thought that the droplets will typically have a diameter in the order of micrometers, for instance from 1 to 100 μπι, or from about 1 to 50 μπι.

Small droplets, for instance droplets of from 1 μπι to 5 μπι in diameter or, more typically, droplets having a diameter of about 1 μπι, will produce smaller crystallites on the surface of the heated substrate. These small particles would likely sinter at significantly lower temperatures than larger crystallites, which allows for greater application of the process of the invention to complex structures and substrate geometries, and allows for lower-temperature deposition.

The step of spraying the composition onto said substrate may be performed for a particular duration to achieve a desired film thickness. For instance, in one embodiment, the step of spraying the composition onto said substrate is performed until a film thickness of from 100 nm to 1000 nm is achieved. More typically, it is performed until a film thickness of from 200 nm to 500 nm is achieved.

In one embodiment, the duration of the step of spraying the composition onto said substrate is from 5 minutes to 40 minutes. Such a duration typically leads to a film thickness of 100 nm to 1000 nm.

The substrate may be moved relative to the spray at a particular rate that results in a desired film thickness. As the skilled person will appreciate, the quicker the substrate is translated relative to the spray, the thinner the film deposition will be.

The non-oxidising atmosphere employed in the process of the invention is an inert atmosphere or a reducing atmosphere. Typically it is an inert atmosphere. The term "inert atmosphere", as used herein is meant unreactive with the semiconductor, including but not limited to minimising or preventing oxidation of the same. The non-oxidising atmosphere employed in the process of the invention, which is typically an inert atmosphere, is generally a gas which contains a significantly lower concentration of oxygen gas than air. Usually the non-oxidising atmosphere is therefore a gas which contains less than 8% oxygen by volume, and more typically less than 4% oxygen by volume, and preferably less than 1% oxygen by volume. Often, the non- oxidising atmosphere employed in the process of the invention is a gas which contains less than 0.5 % by volume of oxygen, for instance less than 0.2 % by volume of oxygen.

Preferably, the gas contains significantly less than water vapour than is typically present in air. Thus, it may for instance contain less than 0.5% by volume water vapour, less than 0.1% by volume water vapour, or for instance less than 500 ppm by volume of water vapour. In some embodiments, the gas contains less than 250 ppm by volume of water vapour. The balance may consist essentially of an inert gas, such as, for instance, nitrogen or a noble gas (e.g. argon or helium). Nitrogen is typically employed, as it is inexpensive.

Thus, inert atmospheres include but are not limited to gases which comprises at least 92% by volume of an inert gas. The gas which comprises at least 92% by volume of an inert gas may be at about atmospheric pressure. Alternatively, it could be at less than atmospheric pressure. The inert gas may be nitrogen or a noble gas, for instance argon or helium.

Typically, the non-oxidising atmosphere employed is therefore a gas which comprises at least 92% by volume, more typically at least 96% by volume, of an inert gas, for instance at least 99% by volume of an inert gas. The non-oxidising atmosphere employed may for instance be a gas which comprises at least 99.5% by volume of an inert gas, for instance at least 99.8% by volume of an inert gas. The inert gas may be nitrogen or a noble gas, such as argon or helium. Usually, however, it is nitrogen.

In one embodiment, the non-oxidising atmosphere employed consists essentially of an inert gas, which may for instance be nitrogen or a noble gas, such as argon or helium. The non-oxidising atmosphere employed may in some embodiments consist of said inert gas

Typically, the non-oxidising atmosphere employed is therefore a gas which comprises an inert gas (which may for instance be nitrogen, or a noble gas such as argon or helium, or a mixture of any of these, but is typically nitrogen), less than 1% by volume of oxygen, and less than 500 ppm by volume of water vapour. The non-oxidising atmosphere may for instance comprise said inert gas, less than 0.5 % by volume of oxygen, and less than 250 ppm by volume of water vapour. The step of heating the substrate typically comprises maintaining the substrate at an elevated temperature for the duration of the step of disposing (e.g. spraying) the composition onto said substrate. Typically, the elevated temperature is from 100 °C to 1000 °C, more typically from 100 °C to 800 °C, and even more typically from 200 °C to 500 °C.

More typically, in the process of the invention for producing a semiconducting film, for instance when the film comprises zinc oxide, the step of heating the substrate comprises maintaining the substrate at an elevated temperature, wherein the elevated temperature is from 250 °C to 500 °C.

The elevated temperature may for instance be from 200 °C to 500 °C, or for instance from 200 °C to 400 °C, or more typically from 250 °C to 400 °C, or even from 290 °C to 400 °C. In some embodiments, the elevated temperature is from 300 °C to 400 °C. It may for instance be from 320 °C to 400 °C, or for example from 325 °C to 400 °C, or from 330 °C to 400 °C. The elevated temperature may for instance be from 325 °C to 385 °C, or for example from 335 °C to 385 °C .

Usually, heating the substrate comprises maintaining the substrate at a temperature of from 330 °C to 400 °C.

The energy of the ultraviolet light with which the film is irradiated in the non- oxidising atmosphere is typically equal to or greater than the electronic band gap of the semiconductor.

In the case of zinc oxide, the band gap is approximately 3.4 eV.

Accordingly, the ultraviolet light usually comprises ultraviolet light having an energy of at least about 3.4 eV.

The ultraviolet light with which the film is irradiated in the non-oxidising atmosphere may comprise ultraviolet light having a wavelength of from 10 nm to 380 nm. Usually, it comprises ultraviolet light having a wavelength of from 100 nm to 380 nm, or for instance ultraviolet light having a wavelength of from 200 nm to 380 nm, or from 280 nm to 380 nm. For instance, it may comprise ultraviolet light having a wavelength of from 100 nm to 365 nm, or for instance from 200 nm to 365 nm. Often, the ultraviolet light with which the film is irradiated comprises ultraviolet light having a wavelength of from about 300 nm to about 365 nm, for instance ultraviolet light with a wavelength of about 365 nm.

The irradiating often comprises irradiating the film with said ultraviolet light at a total radiative flux of from 100

Usually, for instance, it comprises irradiating the film with said ultraviolet light at a total radiative flux of less than 150 μλν/cm², for example from 120 ΐο 150 μλν/cm², e.g. about 140 μλν/cm².

The process of the invention may further comprise, during the step of irradiating the film in the non-oxidising atmosphere with ultraviolet light: cooling the substrate and film. The term "cooling", in this context, is intended to cover both actively cooling the substrate and film, and passively allowing the substrate and film to cool. Typically, the process further comprises, during the step of irradiating the film in the non-oxidising atmosphere with ultraviolet light: cooling the substrate and film to a temperature of less than 50 °C. More typically, it is cooled to a temperature of equal to or less than 30 °C during the step of irradiating, e.g. to about room temperature.

The process may further comprise, after irradiating the film in the non-oxidising atmosphere with ultraviolet light, removing the film from the non-oxidising atmosphere for the first time and exposing the film to air.

In some embodiments, the process of the invention further comprises a second step of irradiating the semiconducting film thus produced with ultraviolet light, which second step is performed after the step of removing the film from said non-oxidising atmosphere for the first time and exposing the film to air. This is termed herein ex-situ irradiation.

In some embodiments, the process of the invention further comprises annealing the semiconducting film thus produced. This step is typically performed after the step of removing the film from the non-oxidising atmosphere for the first time and exposing the film to air. Thus, the semiconducting film that has already been formed may be annealed in a further step together with the substrate.

The film may for instance be annealed at a temperature of from 150 °C to 1000 °C, more typically from 200 °C to 800 °C, and even more typically from 200 °C to 500 °C. The film may for instance be annealed at a temperature of from 350 °C to 400 °C. Typically, the film is annealed for about 30 to 60 min.

The annealing step is usually performed in the presence of nitrogen gas.

Alternatively, the annealing step may be performed in the presence of a noble gas, such as argon. In another embodiment, the annealing step is performed in the presence of an inert gas and hydrogen.

Typically, therefore, the step of annealing the substrate is performed in a nitrogen atmosphere, or in a mixture of an inert gas (e.g. a noble gas) and hydrogen.

Any suitable substrate may be employed in the process of the invention. Typically, though, the substrate is transparent in the visible range of the spectrum. Suitable substrates include substrates that comprise glass, silicon, oxidised silicon, a polymer, a plastic, sapphire, silicon carbide, alumina (AI2O3), zinc oxide (ZnO), yttrium- stabilised zirconium (YSZ), zirconium oxide (Zr0₂), fused silica or quartz.

In one embodiment, the substrate is glass, a silicon wafer, an oxidised silicon wafer or a plastic material (for instance, kapton, PET, polyimide, etc.). Usually, glass and SiC /Si substrates are used. Typically, the substrate is a polymer or glass.

In one embodiment, the substrate is a polymer. Typically, the polymer is flexible. The polymer may be any suitable polymer and is typically a conjugated polymer, for instance PET (polyethylene terephthalate). Such coated polymers are useful in flexible electronics applications.

The semiconducting films of the present invention can be produced having patterned structures, by employing various patterning techniques. These include, for instance, etching the film, lithography, screen printing or ink jet printing. In this way, the resulting film can have any desired two-dimensional or three-dimensional pattern.

A patterned film structure is useful in many applications, including in the design of printed electrodes or circuit boards, for instance, where the transparent conductive film is only desired in certain specific places.

In order that the semiconducting film is deposited on only a portion of the substrate, the substrate surface may be masked before the step of disposing the film on the substrate. In this way the film is only formed on the unmasked areas of the substrate, and does not form on the masked areas. Additionally or alternatively, patterning techniques such as inkjet printing, screen printing, or lithography can be applied to control exactly on which parts of the surface the film is formed. For example, by direct-writing or inkjet printing onto the surface of the substrate in certain places only, film formation occurs only at those places. The resulting film will then have a specific two-dimensional pattern.

Accordingly, in one embodiment of the process of the invention, the film is disposed on only a portion of the surface of the substrate to form a patterned film.

Typically, this is achieved by using a patterning technique (for instance by direct writing) or by masking one or more portions of the substrate prior to film formation.

Advantageously, ZnO is an etchable material, so etching can also be used to pattern the transparent conducting ZnO films described herein.

Accordingly, in another embodiment of the process of the invention, the process further comprises subjecting the film to an etching process, thereby producing a patterned film. Any suitable etchant can be used, for instance HBr, HC1, HF and HF/NH4. In one embodiment, the etchant is an HBr, HC1, HF or HF/ H4 etch bath.

Such patterning and etching techniques can be performed more than once and/or in combination with one another, leading to the build-up of a complex two- or three- dimensional film pattern.

The process of the invention may further comprise producing a semiconducting device comprising the semiconducting film thus produced.

In particular, the process may further comprise producing a thin film transistor (TFT) comprising the semiconducting film thus produced. Usually, the semiconducting film is a channel layer in a TFT, for instance a transparent channel layer.

The invention also provides a semiconducting film which is obtainable by a process as defined in any one of the preceding claims.

The semiconducting film may be as further defined hereinbefore. For instance, the semiconducting film is typically a transparent conducting film. Typically, the transparent conducting film comprises a metal oxide (i.e. a transparent conducting oxide, or TCO), a metal oxide halide, a metal oxide nitride, a metal oxide chalcogenide or a metal chalgogenide, any of which may be amorphous or polycrystalline, although metal oxides are particularly preferred. The metal oxide may for instance be zinc oxide. Thus, the transparent conducting film may for example comprise an amorphous zinc oxide-based TCO (e.g. InZnO or GalnZnO), or a polycrystalline zinc oxide-based TCO.

Polycrystalline zinc oxide is often employed. When the metal oxide is polycrystalline zinc oxide, possible crystal structures include, but are not limited to wurtzite. Typically, the polycrystalline zinc oxide has a wurtzite structure.

Often, the semiconducting film has: a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1; and an electrical conductivity of at least 10.0 Q^cm^"1.

Preferably, the semiconducting film has a Hall mobility of at least 33.0 cm²V^"1s^"1. It may for instance have a Hall mobility of at least 35.0 cm²V^"1s^"1 or for instance at least 35.0 crr^V ¹. In some embodiments, the film has a Hall mobility of at least 37.0 cm²V^" V¹, or for example 40.0 crr^V ¹. The film may for example have a Hall mobility of at least 45.0 cn^V ¹.

Usually, the film has an electrical conductivity of at least 11.0 Q^cm^"1. It may for instance have an electrical conductivity of at least 13.0 Q^cm^"1, or for instance at least 15.0 The film of the invention, which typically comprises zinc oxide, is often not doped with a dopant element (i.e. it is often not extnnsically doped). The film typically therefore has a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3. It may for instance have a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

In some embodiments, however, the film, which typically comprises zinc oxide, is p-doped, e.g. in order to suppress the carrier concentration. It may for instance be p-doped with an alkali metal dopant element, e.g. with Li. The p-doped film may have a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3. It may for instance have a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

The process of the invention can be used to produce novel transparent conducting films having a particularly high μ, but low n, and which are therefore particularly suitable for use as the channel layer in a thin-film transistor (TFT).

Accordingly, the invention also provides a transparent conducting film having: a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1; an electrical conductivity of at least 10.0 Q^cm^"1; and a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

The transparent conducting film of the invention typically comprises a metal oxide or a metal chalgogenide, which may be amorphous or polycrystalline. Metal oxides, for instance zinc oxide, are particularly preferred. These are transparent conducting oxides (TCOs). When zinc oxide is employed, it may be polycrystalline or amorphous. It may for example be an amorphous zinc oxide-based TCO (e.g. InZnO or GalnZnO), or polycrystalline zinc oxide.

In one embodiment the transparent conducting film comprises polycrystalline zinc oxide. The polycrystalline zinc oxide in this embodiment typically has a wurtzite structure. Accordingly, in one embodiment, the invention provides a transparent conducting film comprising polycrystalline zinc oxide having a wurtzite structure, the film having: a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1; an electrical conductivity of at least 10.0 Q^_1cm^{" 1}; and a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

The novel transparent conducting film of the invention, which may for instance comprise zinc oxide as discussed above, may be undoped (i.e. not doped with any dopant element) or it may be mildly intrinsically doped, for example by a small degree of oxygen- deficient non-stoichiometry. In one embodiment, the transparent conducting film is not doped with any dopant element (i.e. it is not extnnsically doped) and is not intrinsically doped either. In another embodiment, the film is not doped with any dopant element (i.e. is not extrinsically doped) but is intrinsically doped by oxygen-deficient non- stoichiometry. Alternatively, the film of the invention may be doped with a dopant element. The film of the invention may for instance be p-doped, for instance with an alkali metal element, such as Li, to reduce the carrier concentration, n, further whilst maintaining the high Hall mobility, μ. In this way, films particularly suitable for use in thin-film transistors (TFTs) may be produced.

Preferably, the film of the invention has a Hall mobility of at least 33.0 crr^VV¹. It may for instance have a Hall mobility of at least 35.0 cm²V^"1s^"1 or for instance at least 35.0 crr^VV¹. In some embodiments, the film has a Hall mobility of at least 37.0 cm²V^" V¹, or for example 40.0 crr^VV¹. The film may for example have a Hall mobility of at least 45.0 cn^V ¹.

Usually, the film also has an electrical conductivity of at least 11.0 Q^cm^"1. It may for instance have an electrical conductivity of at least 13.0 Q^cm^"1, or for instance at least 15.0 Q- ¹.

The film of the invention may not be doped with a dopant element (i.e. it may not be extrinsically doped). For instance, when the film comprises zinc oxide, the zinc oxide may not be doped with a dopant element. The film may therefore have a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3. It may for instance have a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3. Thus, the film may have: a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 45.0 cm²V^"1s^"1; an electrical conductivity of at least 15.0 Q^cm^"1; and a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

The film is often not doped with a dopant element. Thus, often, when the film comprises zinc oxide, said zinc oxide is not doped with a dopant element. The undoped film may have a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3. It may for instance have a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

In other embodiments, however, the film is p-doped, e.g. in order to suppress the carrier concentration. Thus, often, when the film comprises zinc oxide, said zinc oxide is p-doped. The film, or when the film comprises zinc oxide, the zinc oxide, may for instance be p-doped with an alkali metal dopant element. The alkali metal is typically selected from Li, Na, K, Rb and Cs, and is often Li. The p-doped film may have a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3. It may for instance have a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3, or for instance less than or equal to 1.0 x 10¹⁸ cm^"3, or for instance less than or equal to 5.0 x 10¹⁷ cm^"3.

The invention also provides a coated substrate, which substrate comprises a surface, which surface is coated with a film of the invention as defined herein. The substrate is typically transparent in the visible range of the spectrum. Suitable substrates include substrates that comprise glass, silicon, oxidised silicon, a polymer, a plastic, sapphire, silicon carbide, alumina (AI2O3), zinc oxide (ZnO), yttrium- stabilised zirconium (YSZ), zirconium oxide (Zr0₂), fused silica or quartz.

In one embodiment, the substrate is glass, a silicon wafer, an oxidised silicon wafer or a plastic material (for instance, kapton, PET, polyimide, etc.). Usually, glass and Si0₂/Si substrates are used. In one embodiment, the substrate is a polymer. Typically, the polymer is flexible. The polymer may be any suitable polymer and is typically a conjugated polymer, for instance PET (polyethylene terephthalate). Such coated polymers are useful in flexible electronics applications.

The invention also provides a semiconducting device comprising a semiconducting film of the invention as defined herein.

The invention also provides a thin film transistor (TFT) comprising a

semiconducting film of the invention as defined herein. Usually, the semiconducting film is a channel layer in the TFT, for instance a transparent channel layer.

Typically, the film is polycrystalline zinc oxide, has a wurtzite (hexagonal) crystal structure, and a c-axis perpendicular to the plane of a substrate on which the film is disposed.

The present invention is further illustrated in the Example which follows: EXAMPLE

Experimental: Film Deposition

The basic spray pyrolysis procedure and apparatus used to prepare ZnO films is as follows.

A 0.225 M solution of anhydrous zinc acetate (99.99% metals basis, Aldrich) was prepared in a 7:93 (by volume) mixture of glacial acetic acid (Fischer Scientific, 99.81%) and deionized water (Ondeo Purite Select Analyst). The mixture was stirred with a PTFE- coated magnetic stirbar for 5 min and then gravity filtered through qualitative filter paper, resulting in a clear, homogeneous solution. For each deposition run, 4 mL of this solution was transferred to a second glass vial equipped with a slirbar. While stirring, anhydrous ethanol (99.8%, 8 mL, Aidrich) was poured quickly into the aqueous solution.

24 mm x 32 mm, No. 1 borosilicate glass cover slips (Menzei-Glaser) uitrasomcally cleaned in acetone and blown dry with nitrogen were used as substrates. The substrate was heated from below by a stainless steel block with two embedded 100W cartridge heaters. The temperature of the system was regulated with a type J thermocouple inserted into the centre of the heating block and a Eurotherm PiD temperature controller. Samples were deposited at a substrate surface temperature as specified herein, of for instance 340 °C. The sy stem was heated to the operating temperature and held there for 10 min before the start of spraying to allow thermal equilibration. Deposition was carried out inside an acrylic- chamber with a positive internal pressure of nitrogen. For each run, 10 mL of precursor solution was supplied to the BETE XA PR-050 fog nozzle at a rate of 0.7 mL-min-1 using a syringe pump. Nitrogen carrier gas was supplied to the nozzle at a flow rate of 14-15 L-min-1. The nozzle was located at the top of the chamber, approximately 30 cm above the heated substrate. After spraying, the system was held at operating temperature for a further 5 min before being cooled to a rate of 20 °C-min^~f .

For dark deposition without irradiation, precautions were taken to exclude ambient light from the spray chamber.

For in situ irradiation, a modified chamber front with a 65 mm diameter quartz window was used to permit the UV irradiation of substrate and growing film over various portions of the spraying process. A UVP EL Series blacklight with emission centred at 365 nm (3.4 eV) illuminated the growing film at an angle of 45° at a distance of 10 cm, resulting in a total radiative flux of -140

as measured by a Newport 842-PE power/energy meter. Unless otherwise noted, the light source was on from the end of solution spraying until just before the chamber was opened to ambient atmosphere at < 50 °C at the end of a deposition procedure.

Thermal feedback from the system is taken from the centre of the heater block, however the reported deposition temperatures are of the substrate surface, based on a series of calibration measurements using an optical pyrometer. Temperature uniformity across the substrate surface was approximately ± 7 °C. Experimental: Film Characterisation

Temperature dependent electrical transport properties of the films were analysed by the van der Pauw method (L. J. van der Pauw, Philips Tech. Rev., 1958, 20, 220-224) using an Ecopia HMS-3000 Hall effect measurement system with a 0.55 T permanent magnet. Indium solder was used to create four Ohmic electrical contacts on the corners of a 7 mm x 7mm piece of film. For samples deposited at lower heater temperatures that were not of sufficient conductivity to allow reliable carrier concentration and mobility measurements, only electrical resistivity is reported. Measurements at temperatures below (to -80 K) and above (to -350 K) ambient temperature were achieved using a custom-designed heating and cooling system. Sample temperature was measured with a Type T thermocouple embedded in the sample mount and in contact with the film substrate.

For Hall effect measurements after ex situ irradiation, film pieces were mounted to a sample holder and irradiated with the 365 nm EL Series blacklight at a distance of -10 cm until a multimeter showed the sample was at constant resistance (after about 5 minutes). The samples were then quickly loaded into the HMS-3000, with measurements being taken within 15 seconds of the sample being removed from the light.

Optical transmission measurements were made for wavelengths between 320 and 2000 nm using a Perkin-Elmer Lambdal9 UV-Vis-NIR spectrophotometer. Film thicknesses were estimated from this data using the envelope method of Swanepoel (R. Swanepoel, J. Phys. E Sci. Instrum., 1983, 16, 1214-1222).

The surface morphology of the thin films was characterized using a JEOL JSM-840F field emission scanning electron microscope at an accelerating voltage of 5 kV. Samples were sputter coated with a -2.5 nm layer of platinum before observation in the SEM to reduce the effect of surface charging and attendant artefacts.

X-ray diffraction measurement of samples was performed using a PANalytical X'Pert PRO diffractometer with a Bragg-Brentano geometry and a fixed X-ray source at an emission current of 40 mA and an anode voltage of 45 kV. The spectra were recorded using monochromated Cu-Kal radiation (λ = 1.5406 A) or Cu-Κα (λ = 1.5418 A) over a time of 115 minutes. Samples were placed on a 60 rpm spinner to improve averaging of crystallite orientation.

Results

When undoped ZnO thin films prepared by spray pyrolysis are exposed to UV with energy exceeding the ZnO electronic bandgap (-3.4 eV) before the films are first removed from the N2-filled deposition chamber (termed "in situ irradiation", Figure lb), an increase in electrical conductivity is observed compared to films deposited in the dark (Figure 2). The relative improvement in electrical conductivity increases consistently with decreasing deposition temperature. The electrical conductivities of films deposited using in situ irradiation are similar for deposition temperatures of 334 °C or greater, with the highest average conductivity of 13.3 ± 3.0 Ω-l cm-1 achieved at 334 °C. Considering that the maximum conductivity for films deposited in the dark is 8.9 ± 1.3 Ω^"1 cm^"1 for films deposited at 417 °C, in situ irradiation effectively reduces the maximum processing temperature needed to produce the highest conductivity of undoped ZnO films by over 80 °C.

For the films deposited at higher temperatures, the electrical conductivity increase reported here is considerably longer lived than that reported in previous reports of ZnO

photoconductivity, remaining largely unchanged even if the samples are stored in open air for several months. Repeated measurements of the electrical conductivity at different times after deposition are presented in Figure 3 for a sample deposited at 376 °C. The small variability in the conductivity with time can be likely ascribed to classic photoconductive effects, as the ambient lighting conditions before and during resistivity measurements were not strictly controlled.

Room temperature Hall effect measurements to determine the carrier concentration and mobility were carried out on all samples of with conductivity high enough for reliable measurements. The Hall carrier mobility for films deposited at various temperatures and measured directly after removal from the deposition chamber is shown in Figure 4a. In situ irradiation leads to a statistically significant improvement in Hall mobility at all deposition temperatures for which a direct comparison is available. The largest absolute improvement in mobility occurs for films deposited at 376 °C, where a maximum value of 44.3 cm2/Vs was achieved (the average over 18 samples was 37.4 ± 3.5 cm2/Vs). This is a 78% (greater than 3σ) improvement compared to films deposited at the same temperature in the dark. For both irradiated and dark cases, carrier mobility is a strong function of temperature, with all films deposited at 292 °C and lower having a carrier mobility less than 1 cm²/Vs, which was the lower limit for reliable measurements with available equipment.

The corresponding carrier concentration data is shown in Figure 4b. For films deposited at 334 °C, in situ irradiation results in an approximate doubling of the carrier concentration relative to both higher temperature films and films deposited in the dark at 334 °C. This increase in carrier concentration adds to a substantial mobility enhancement. It is this higher carrier concentration that allows in situ irradiated films deposited at 334 °C to have a similar conductivity as films deposited at higher temperatures, despite having lower carrier mobility (Figure 2).

Other photoconductive effects are also present in ZnO, including a well-studied temporary effect related to the chemisoprtion of air-derived, conductivity-limiting chemical species at grain boundaries and their subsequent light-induced desorption (D. B. Medved, J. Chem. Phys., 1958, 28, 870-873; D. A. Melnick, J. Chem. Phys., 1957, 26, 1136-1146). To minimize the effect of these adsorbed species and make the new effect under study more apparent, two series of films (with and without in situ UV treatment) that had already been exposed to air were then irradiated with UV ex situ (Figure Id). This treatment is intended to remove weakly-adsorbed species from the ZnO grain boundaries, revealing the electrical properties of the underlying material as a function of in situ irradiation and temperature- dependent film micro structure.

Hall effect mobility and carrier concentration measurements taken immediately after such treatment are displayed in Figures 5a and 5b, respectively. A systematic increase in carrier concentration due to in situ irradiation is observed for all deposition temperatures.

Additionally, at moderate deposition temperatures (ca. 292-376 °C), in situ irradiation improves the electron mobility, with the largest relative improvement occurring at a deposition temperature of 376 °C.

The ZnO thin films prepared in this study were also characterized using X-ray diffraction, scanning electron microscopy, and UV-Vis-NIR transmission spectrophotometry. No structural differences were observed between in situ irradiated and dark-deposited films prepared at the same deposition temperature by these techniques. However, as has been reported elsewhere, there were significant changes in film morphology as a function of deposition temperature (D. F. Paraguay, L. W. Estrada, N. D. R. Acosta, E. Andrade and M. Miki-Yoshida, Thin Solid Films, 1999, 350, 192-202; J. L. van Heerden and R.

Swanepoel, Thin Solid Films, 1997, 299, 72-77).

The X-ray diffraction patterns of all films revealed only one crystalline phase, consistent with ZnO in its hexagonal wurtzite form (Figure 6). All films are polycrystalline and composed of crystallites with a preferred orientation relative to the substrate. The degree and direction of orientation depends on the deposition temperature, ranging from almost purely (0 0 2) oriented at high deposition temperatures to a crystallite orientation completely lacking this reflection at low temperatures. While this transition appears to occur continuously across the temperature range studied, a sharp change in preferred orientation occurs between deposition temperatures of 292 and 334 °C. A structural transition with changing deposition temperature is also plainly visible in SEM images (Figure 7). At the higher deposition temperatures (ca. 376 °C and above) the films consist almost entirely of thin hexagonal platelets stacked mostly parallel to the substrate surface, consistent with the predominance of the (0 0 2) reflection observed by XRD. For the samples deposited at 334 °C, hexagonal platelets are still clearly visible, but they are visibly less uniform than their higher temperature counterparts, and an irregular granular material also appears among the stacks. As the deposition temperature is further reduced, a dramatic morphological change occurs between 334 and 292 °C. This temperature range is marked by the complete disappearance of the hexagonal plates and the initial appearance of the feather-like grains of ZnO that dominate at low deposition temperatures. These trends also correlate well with the changes observed in the X-ray diffraction patterns of Figure 6.

UV-Vis-NIR transmission measurements reveal that film transparency generally increases with decreasing deposition temperature. Once again, the most significant inflection in this parameter point occurs between 334 °C and 292 °C. It may additionally be noted that the film thickness deposited per amount of precursor solution per unit time (i.e. specific the deposition rate) also decreases as deposition temperature decreases, though not entirely in concert with the changing visible transparency. These properties, as well as the electrical properties discussed earlier, are summarized in Table 1 for each of the deposition conditions studied.

Thus, Table 1, below, provides a summary of the properties of ZnO thin films deposited in this Example using spray pyrolysis at various deposition temperatures, with and without in situ UV irradiation. Where listed, bounds are the standard deviations over a population of samples deposited under those conditions. NM indicates conditions where the conductivity is too low for the property to be reliably measured using equipment available.

Transmittance data are uncorrected for the glass substrates, which average 92%

transparency in the visible range.

Discussion

Spray pyrolysis, in concert with this newly described UV treatment, can produce undoped films possessing carrier mobilities that compare favourably with those of the best films produced by other more complex and capital-intensive techniques, such as sputtering, pulsed laser deposition, and organometallic chemical vapour deposition (K. Ellmer, J. Phys. D, 2001, 34, 3097-3108). This is true in particular because the performance of transparent thin film transistors used for active-matrix displays and touchscreens is largely dependent on a semiconducting channel layer with high carrier mobility and low carrier concentration. Such high mobility, solution-deposited ZnO films may have substantial

26

SUBSTITUTE SHEET RULE 26 promise in TFT applications, especially when combined with an appropriate approach for suppressing carrier concentration.

The ZnO films in the present study are polycrystalline, meaning that their macroscopic behaviour results from a combination of the distinct properties of both the crystalline ZnO grain interiors and of the grain boundaries joining neighbouring grains. It is proposed herein that the conductivity improvement from in situ irradiation results from principally from changes in the grain boundary properties, rather than any modification of the ordered bulk regions of the ZnO grains.

Grain boundaries often contain a large population of distorted or dangling bonds, chemisorbed species, and other defects. In the cases typically reported for undoped, as- deposited ZnO, grain boundary defects are dominated by those capable of accepting and trapping electrons from the nearby grain interior. The build-up of negative charge in these surface acceptor states and the corresponding depletion of electrons from the nearby grain interior creates a potential barrier that inhibits electron transport across grain boundaries. Surface donor states, which have, for example, been invoked in films annealed in reducing environments, would produce the opposite effect and enhance film electrical conductivity.

The effect of grain boundary potential barriers on the macroscopic electrical properties of thin films can be analyzed by the model presented by Seto, and Orton and Powell, which treats grain boundaries as back-to-back Schottky barriers in a low voltage regime (J. W. Orton and M. J. Powell, Rep. Prog. Phys., 1980, 43, 1263-1307; J. Y. W. Seto, J. Appl. Phys., 1975, 46, 5247-5254). Near room temperature, where charge transport over the potential barrier is dominated by thermionic emission, the temperature-dependent Hall mobility is given

where L is an effective grain size, m* is the electron effective mass, Vb is an activation energy related to the grain boundary barrier height, and k is the Boltzmann constant. When the Hall effect mobilities of films deposited at 376 °C with and without in situ UV irradiation were measured as a function of temperature between 80 and 350 K and fitted using this model (Figure 8), the principal difference observed was that films receiving in situ UV treatment had a substantially reduced grain boundary potential barrier compared to

27

SUBSTITUTE SHEET RULE 26 films deposited in the dark. Notably, the model parameter related to the grain interiors - L, the effective grain size - was not significantly changed by the in situ irradiation process.

Additionally, the amount of incident UV light required to achieve a conductivity enhancement is very small; a radiant flux of less than <150 μλν/cm² from a handheld laboratory UV lamp is enough to produce the significant response observed. Furthermore, a conductivity improvement from in situ irradiation still occurs even when a film is exposed to UV only at temperatures below 100 °C while cooling under N₂ after deposition. It seems unlikely that such a modest addition of energy at these temperatures would be enough to cause any gross structural rearrangement or annealing of the bulk ZnO lattice within the grains, something which normally proceeds at an appreciable rate only at a substantial fraction of the melting point (Tm = 1975 °C). That the primary area of effect is the grain boundaries is further corroborated by the sets of structural data obtained by XRD and SEM, neither of which shows any observable difference in gross morphology as a function of film irradiation.

Working under the hypothesis that in situ irradiation affects the grain boundaries of polycrystalline ZnO thin films, prior and related work on ZnO photoresponse can be considered for insights into the possible origins of the in situ UV effect. The fact that the electrical properties of ZnO can be affected by radiation with energy higher than its bandgap has been studied and reported on for nearly 60 years (D. A. Melnick, J. Chem. Phys., 1957, 26, 1 136-1146; E. Mollwo, in Photoconductivity Conference, ed. R. G.

Breckendridge, B. R. Russell and E. E. Hahn, John Wiley and Sons, New York, 1954, p. 509). To date, the aspect of this that has attracted the most attention is the fact that the observed conductivity changes can persist for hours or days after the irradiation ceases, if the sample is in air. In fact, if the sample is kept under a good vacuum or a rigorously inert atmosphere, the conductivity improvement essentially does not decay (G. Heiland, J. Phys. Chem. Solids, 1961, 22, 227-234).

This phenomenon of so-called persistent photoconductivity has been attributed to light- driven changes in chemical speciation at grain boundaries. It is generally accepted that one or more oxygen-containing species derived from the interaction of ZnO surfaces with air are at play, even though the precise roles played by various candidates remains unresolved. Proposals for relevant species include chemisorbed O^" and O^2", 0₂ ^" and 0₂ ^2", as well as CO2^". It has been found in this work that "persistent" photoconductivity decays faster when a film is kept under wet nitrogen than when it is kept under dry nitrogen, suggesting that surface hydroxide may also be involved in this grain boundary chemistry.

Regardless of their precise chemical identities, surface acceptor states resulting from such species can be neutralized by the holes that are generated by photo-excitation of the bulk ZnO (Figure 9a-c). This reduces height of the grain boundary potential barrier and has the net effect of increasing the population of conduction band electrons; hence, electrical conductivity is increased (Figure 9d). The neutralization of these states also formally represents the breaking of chemisorption bonds between the acceptor species and the ZnO lattice, allowing the former to readily desorb. When irradiation ceases with the sample in air, the grain boundary speciation returns to its pre-irradiation state over the course of hours (or days) as species from the atmosphere re-adsorb and reform electron-accepting defects. Thus, while the oft-reported photoconductivity in ZnO is called "persistent" because equilibrium is achieved via a relatively slow chemical process as opposed to a fast electronic one, it is nevertheless unstable under ambient conditions.

In stark contrast, in the present work it has been discovered that the conductivity enhancement resulting from in situ UV treatment is much longer lasting. Nevertheless, the general principles behind "persistent" photoconductivity can still be applied, as any change in grain boundary chemistry that reduces the number of surface acceptor states or introduces compensating surface donor states will simultaneously lower the grain boundary potential barriers inhibiting carrier mobility and increase the population of electrons able to participate in electronic conduction (Figure 9).

Thus, the in situ UV effect can be ascribed to light-induced formation of stable or metastable surface defects that only occurs before a recently deposited sample is exposed to air. On the other hand, "persistent" photoconductivity is related to the reversible desorption and adsorption of weakly bound chemical species introduced by air exposure. The data presented in Figure 5 shows that the effects of in situ irradiation can be observed in addition to those of "persistent" photoconductivity, implying that there are multiple processes acting to modify the donor/acceptor density at the ZnO grain boundaries. While there is presently not enough information to make atom-level mechanistic proposals, it is possible to suggest examples of defect systems that fit these qualitative constraints. For instance, theoretical calculations by Li, et al. suggest that an oxygen-deficient ZnO surface would lead to the presence of surface donor states (H. Li, L. K. Schirra, J. Shim, H. Cheun, B. Kippelen, O. L. A. Monti and J. Bredas, Chem. Mater., 2012, 24, 3044-3055). Their calculations also show that under oxygen-poor conditions (as in a N2-filled deposition chamber) the formation of near-surface oxygen vacancies (Vo) is only slightly disfavoured at thermal equilibrium, suggesting that a substantive population of surface Vo could be formed under the hole-rich conditions of UV irradiation. The observation that in situ UV treatment is most effective when ZnO films are cooled to room temperature under irradiation is consistent with the expectation that defect-neutralizing reactions are most facile at higher temperatures; in effect, in situ UV treatment causes a metastable Vo population to be frozen in.

Under the oxygen-rich conditions in air, surface Vo formation is highly energetically unfavourable. In such a case, irradiation likely can only induce desorption of the less strongly bound species present due to interaction of the ZnO with air, giving rise to "persistent" photoconductivity. Indeed, thermal desorption measurements have previously been used to experimentally distinguish various oxygen-containing species that bind to the ZnO surface with differing strengths (E. Molinari, F. Cramarossa and F. Paniccia, J.

Catal, 1965, 4, 415-429; T. I. Barry and F. S. Stone, P. Roy. Soc. Lond. A Mat., 1960, 255, 124-144; A. M. Peers, J. Phys. Chem., 1963, 67, 2228-2229). To explain why the effect of in situ irradiation cannot be achieved after initial air exposure, it is necessary to assume that a surface modification occurs that further decreases the likelihood of Vo formation even if the sample is returned to nitrogen.

Consideration of how these photoconductive effects vary as a function of deposition temperature allows an essential link to be made between these microscopic phenomena and the challenges facing ZnO thin films as practical engineering materials. It has been shown here that in situ irradiation has a significant positive effect on the electrical conductivity of films made at all temperatures, and that this process allows the deposition of ZnO thin films with superior electrical properties, as long as the deposition temperature is at least -334 °C. However, when the deposition temperature is further reduced, for example, into ranges that are comfortably compatible with inexpensive polymer substrates, it is found that the mobility reduction caused by weakly adsorbed species at grain boundaries is the primary limiting factor to electrical conduction. The ZnO films prepared at the lowest deposition temperatures have as deposited electrical conductivities many (at least 4-5) orders of magnitude lower than those deposited above 334 °C. However, when the effect of grain boundary acceptors is temporarily relieved by ex situ irradiation (Figure Id), the films in the present study deposited at 292°C and above only show electrical conductivity differences of at most a few tens of percent. Even the film deposited below the reported onset temperature of zinc acetate decomposition (-251 °C) was only -5.5 times more resistive than its higher T counterparts, after such treatment (T. Arii and A. Kishi,

Thermochim. Acta, 2003, 400, 175-185).

At least qualitatively, both the trends in as deposited electrical conductivity and the magnitude of the ZnO photoresponse correlate extremely well with the changes in film morphology that occur as a function of deposition temperature. These electrical properties show an inflection around a deposition temperature of 334 °C, near where the film morphology transitions from the hexagonal platelets characteristic of high deposition temperatures to a feather-like low temperature structure (Figure 7). The appearance of higher porosity and surface-to-volume ratio could mean that low temperature films have more surface area (possibly with a different distribution of exposed crystallographic faces) upon which atmospheric species may adsorb and form acceptor states. Further, the featherlike low temperature morphology consists of smaller, less uniform grains in less intimate mutual physical contact than the hexagonal platelets, which is also consistent with a lower underlying mobility. Thus, while it has been demonstrated here that in situ UV is an effective way to improve the electrical properties of ZnO films prepared at moderate temperatures, the primary challenge to producing stable, highly conductive ZnO at lower temperatures is in the understanding and mitigation of air-dependent grain boundary effects.

In conclusion, in this Example, a new in situ UV irradiation treatment has been shown to produce a substantial and long-lasting improvement in ZnO electrical conductivity relative to films deposited in the dark. Using spray pyrolysis at moderate deposition temperatures (334-417 °C), ZnO thin films can be produced with record high carrier mobilities, which is of significant interest for applications in transparent thin film transistors. This treatment also effectively lowers the deposition temperature required to deposit the highest conductivity films. It has also been shown that this new in situ irradiation effect operates in addition to and largely independent of classic "persistent" photoconductivity. While both of these processes influence electrical transport by modifying the defect density at ZnO grain boundaries, their ability to do so is affected differently by deposition temperature-dependent changes in film microstructure. The results presented here highlight the overriding importance of grain boundary effects in determining the electrical properties of polycrystalline transparent conducting films.

Claims

1. A process for producing a semiconducting film, which process comprises:

in a non-oxidising atmosphere, disposing a composition which is a liquid composition or a gel composition onto a substrate, wherein the composition comprises a compound which is a precursor to a semiconductor;

heating the substrate in said non-oxidising atmosphere to form on the substrate a film comprising the semiconductor; and

prior to removing the film from the non-oxidising atmosphere for the first time, irradiating the film in the non-oxidising atmosphere with ultraviolet light.

2. A process according to claim 1 wherein the semiconductor comprises a metal compound, and wherein the compound which is a precursor to a semiconductor is a compound comprising the same metal.

3. A process according to claim 1 wherein the semiconductor comprises a metal oxide, and wherein the compound which is a precursor to a semiconductor is a compound comprising the same metal.

4. A process according to claim 3 wherein the semiconductor comprises a

polycrystalline metal oxide and the semiconducting film is a transparent conducting film.

5. A process according to claim 3 or claim 4 wherein the metal oxide is zinc oxide, and the compound comprising the same metal is a zinc compound.

6. A process according to any one of claims 3 to 5 wherein the compound comprising the same metal is a salt of the metal comprising an organic anion.

7. A process according to claim 5 wherein the zinc compound is zinc acetate.

8. A process according to any one of the preceding claims wherein the composition is a solution which comprises (a) a solvent, and (b) said compound which is a precursor to a semiconductor dissolved in the solvent.

9. A process according to any one of the preceding claims wherein the semiconductor is, or is not, doped with a dopant element, provided that when the semiconductor is doped with a dopant element, said composition comprises a compound comprising said dopant element.

10. A process according to any one of the preceding claims wherein the steps of disposing the composition onto the substrate and heating the substrate are performed simultaneously.

11. A process according to claim 10 wherein disposing the composition onto the substrate comprises spraying the composition onto the substrate.

12. A process according to claim 11 which comprises spray pyrolysis, wherein spraying the composition onto the heated substrate causes pyrolitic decomposition of the composition and formation of said film comprising the semiconductor on the substrate.

13. A process according to any one of the preceding claims wherein the non-oxidising atmosphere is an inert atmosphere.

14. A process according to any one of the preceding claims wherein the non-oxidising atmosphere is a gas which comprises at least 96% by volume of an inert gas.

15. A process according to any one of the preceding claims wherein the non-oxidising atmosphere is a gas which comprises at least 99% by volume of an inert gas.

16. A process according to claim 14 or claim 15 wherein the gas comprises said inert gas, less than 1% by volume of oxygen, and less than 500 ppm by volume of water vapour.

17. A process according to any one of claims 14 to 16 wherein the gas comprises said inert gas, less than 0.5 % by volume of oxygen, and less than 250 ppm by volume of water vapour.

18. A process according to any one of claims 14 to 17 wherein the inert gas is nitrogen.

19. A process according to any one of the preceding claims wherein heating the substrate comprises maintaining the substrate at an elevated temperature for the duration of the step of disposing the composition onto said substrate.

20. A process according to any one of the preceding claims wherein heating the substrate comprises maintaining the substrate at a temperature of from 250 °C to 500 °C.

21. A process according to any one of the preceding claims wherein heating the substrate comprises maintaining the substrate at a temperature of from 330 °C to 400 °C.

22. A process according to any one of the preceding claims wherein the energy of the ultraviolet light is equal to or greater than the electronic band gap of the semiconductor.

23. A process according to any one of the preceding claims wherein the ultraviolet light comprises ultraviolet light having an energy of at least 3.4 eV.

24. A process according to any one of the preceding claims wherein the ultraviolet light comprises ultraviolet light having a wavelength of from 10 nm to 365 nm.

25. A process according to any one of the preceding claims wherein said irradiating comprises irradiating the film with said ultraviolet light at a total radiative flux of at least

26. A process according to any one of the preceding claims which further comprises, during the step of irradiating the film in the non-oxidising atmosphere with ultraviolet light: cooling the substrate and film, or allowing the substrate and film to cool.

27. A process according to any one of the preceding claims which further comprises, during the step of irradiating the film in the non-oxidising atmosphere with ultraviolet light: cooling the substrate or allowing the substrate to cool, to a temperature of less than 50 °C.

28. A process according to any one of the preceding claims which further comprises, after irradiating the film in the non-oxidising atmosphere with ultraviolet light, removing the film from the non-oxidising atmosphere for the first time and exposing the film to air.

29. A process according to any one of the preceding claims wherein the substrate is transparent in the visible range of the spectrum.

30. A process according to any one of the preceding claims wherein the substrate comprises glass, silicon, oxidised silicon, a polymer, a plastic, sapphire, silicon carbide, alumina (AI2O3), zinc oxide (ZnO), yttrium- stabilised zirconium (YSZ), zirconium oxide (ZrC ), fused silica or quartz.

31. A process according to any one of the preceding claims which further comprises producing a semiconducting device comprising the semiconducting film thus produced.

32. A process according to any one of claims 1 to 30 which further comprises producing a thin film transistor comprising the semiconducting film thus produced.

33. A semiconducting film which is obtainable by a process as defined in any one of the preceding claims.

34. A semiconducting film according to claim 33 which is a transparent conducting film, the transparent conducting film having:

a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1; and

an electrical conductivity of at least 10.0 Q^cm^"1.

35. A semiconducting film according to claim 34 which has a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

36. A semiconducting film according to claim 34 having a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

37. A semiconducting film according to any one of claims 34 to 36 wherein the film comprises a metal oxide.

38. A semiconducting film according to claim 37 wherein the metal oxide is polycrystalline zinc oxide or an amorphous zinc oxide-based transparent conducting oxide.

39. A semiconducting film according to claim 38, wherein the zinc oxide is

polycrystalline zinc oxide having a wurtzite structure.

40. A semiconducting film according to claim 38 or claim 39 wherein said zinc oxide is not doped with a dopant element and the film has a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

41. A semiconducting film according to claim 40 having a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

42. A semiconducting film according to claim 38 or claim 39 which is p-doped with an alkali metal dopant element and the film has a carrier concentration, n, as defined in claim 40 or claim 41.

43. A transparent conducting film having:

a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1;

an electrical conductivity of at least 10.0 Q^cm^"1; and

a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

44. A transparent conducting film according to claim 43 which comprises a metal oxide.

45. A transparent conducting film according to claim 44 wherein the metal oxide is zinc oxide.

46. A transparent conducting film according to claim 45 wherein the zinc oxide is polycrystalline zinc oxide having a wurtzite structure.

47. A transparent conducting film according to claim 45 or claim 46 wherein said zinc oxide is not doped with a dopant element.

48. A transparent conducting film according to claim 45 or claim 46 wherein said zinc oxide is p-doped with an alkali metal dopant element.

49. A transparent conducting film according to claim 47 or claim 48 having a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.

50. A coated substrate, which substrate comprises a surface, which surface is coated with a film as defined in any one of claims 33 to 49.

51. A coated substrate according to claim 50 wherein the substrate is as defined in claim 29 or claim 30.

52. A semiconducting device comprising a film as defined in any one of claims 33 to 49.

53. A thin film transistor comprising a film as defined in any one of claims 33 to 49.

54. A thin film transistor according to claim 53 which comprises a transparent conducting film comprising zinc oxide, the film having:

a mean optical transparency in the visible range of the spectrum of at least 70 %; a Hall mobility, μ, of at least 31.0 cm²V^"1s^"1;

an electrical conductivity of at least 10.0 Q^cm^"1; and

a carrier concentration, n, of less than or equal to 1.0 x 10¹⁹ cm^"3.

55. A thin film transistor according to claim 54 wherein said zinc oxide is not doped with a dopant element, or is p-doped with an alkali metal dopant element.

56. A thin film transistor according to claim 54 or claim 55 wherein the transparent conducting film has a carrier concentration, n, of less than or equal to 5.0 x 10¹⁸ cm^"3.