Patent Description:
Thin film transistors are widely used in displays, memory, and logic devices. Due to their possibly intensive long-term use, TFTs preferably comprise chemically stable materials. Furthermore, and more in particular, the material for the channel of a TFT preferably enables high on currents and low off currents, that is, a high current in the on-state and a low current in the off-state.

The process for forming TFTs is often in the back-end-of-line (BEOL) stage of integrated circuit fabrication. Furthermore, in the particular application of 3D memory devices, memory cells comprising transistors, e.g., thin film transistors, are typically stacked so as to form the 3D memory device. In the process of forming a new memory cell above an existing one, it is preferably to avoid temperatures above <NUM> so as not to damage any already formed memory cells. However, such a limitation on deposition temperatures severely limit the type of materials that may be deposited for fabricating TFTs. Many materials typically used as channel material require higher crystallization temperatures, which disqualifies them for use in TFTs.

On the contrary, the deposition of amorphous materials does not require reaching these high thermal treatment temperatures. However, the resulting films tend to be associated with significantly lower charge carrier mobilities than their crystalline counterparts. Moreover, to be compatible with silicon technology, the materials are preferably chemically stable against "forming gas" anneal. Amorphous materials are intrinsically less stable than their crystalline counterparts.

The current best amorphous oxide, the metal oxide a-InGaZnO<NUM>, i.e., IGZO, has, compared to typical amorphous oxides, relatively good electron mobility. This relatively good electron mobility is due to the interaction of s and d orbitals of the metal cations in a lower part of the conduction band. The strength of the bonds between the cations, and, in addition, the type of cations, drives the effective mobility of the system. The mobility of a- InGaZnO<NUM> and its chemical stability are, however, not sufficiently high. a-InGaZnO<NUM> has rather low electron mobility (approximately <NUM> to <NUM><NUM>/(V·s)) and low chemical stability.

<CIT> describes a p-type oxide semiconductor, which contains: a metal oxide containing thallium (Tl), where the metal oxide has been hole doped. <CIT> provides a TFT having high field effect mobility and a high ON/OFF ratio and improved to a normally off type, and describes a thin film field-effect transistor includes at least a gate electrode, a gate insulating film, an active layer containing amorphous oxide, a source electrode, and drain electrode, which are formed on a substrate. In the thin film field-effect transistor, a barrier layer is formed only in a region existing between the active layer and at least one of the source electrode and the drain electrode and brought into contact with at least one of the source electrode and the drain electrode. <CIT> describes a conductive oxide containing M and O, which are at least one element selected from the group consisting of In, Al, Zn, and Mg, and also contains crystalline AIMO. <CIT> describes a semiconductor device including a transistor having a high field-effect mobility, stable electrical characteristics, and a small current in an off state. A first electrode is formed over a substrate, a first insulating layer is formed adjacent to a side surface of the first electrode, and a second insulating layer is formed to cover the first insulating layer and be in contact with at least part of a surface of the first electrode. The surface of the first electrode is formed of a conductive material that does not easily transmit an impurity element. The second insulating layer is formed of an insulating material that does not easily transmit an impurity element. An oxide semiconductor layer is formed over the first electrode with a third insulating layer provided therebetween.

There is therefore a need in the art for a material and method for forming the material that solves one or more of the above problems.

It is an object of the present invention to provide a good first mixed metal oxide, which comprises Mg, Al, and Sb. It is a further object of the present invention to provide a good method for forming the first mixed metal oxide, and a transistor comprising the first mixed metal oxide.

It is further disclosed to provide a good transistor comprising a channel layer made of an amorphous second mixed metal oxide, which comprises Al and Zn. It is yet further disclosed to provide a good method for forming the transistor comprising the channel layer made of the amorphous second mixed metal oxide.

The above objectives are accomplished by methods and apparatuses according to the present invention.

It is an advantage of embodiments of the present invention that the mixed metal oxides (i.e., the first mixed metal oxide, and the second mixed metal oxide) may have a good electron mobility. It is an advantage of embodiments of the present invention that a channel in a transistor comprising one of these mixed metal oxides, may exhibit a high current in the on-state, and a low current in the off state.

It is an advantage of embodiments of the present invention that the chemical stability of the mixed metal oxides may be good. It is an advantage of embodiments of the present invention that the mixed metal oxides may have good stability against annealing in a forming gas atmosphere, which is a step that is often performed in semiconductor manufacturing processes.

It is an advantage of embodiments of the present invention that elements making up the mixed metal oxides may be compatible with standard industrial silicon technology.

It is an advantage of the present disclosure that the mixed metal oxides may have good properties for use of the mixed metal oxides in transistors, such as in thin-film transistors (TFT), for example as channel material.

It is an advantage of embodiments of the present invention that the mixed metal oxides may be deposited at temperatures below <NUM>. It is an advantage of embodiments of the present invention that the mixed metal oxides may be deposited in the back-end-of-line (BEOL) stage of semiconductor manufacturing processing. For example, silicon devices, e.g., silicon logics, may degrade at temperatures above <NUM>. For example, this may further enable stacking memory cells comprising transistors, e.g., thin film transistors, comprising one of the mixed metal oxides. Indeed, any already formed memory cells may not be damaged when forming a memory cell on top of the already formed memory cells.

In a first aspect, the present invention relates to a first mixed metal oxide consisting of: a) a mixture consisting of <NUM> to <NUM> parts by mole Mg, <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Sb, and <NUM> to <NUM> parts by mole of other elements, other than Mg, Al, and Sb, selected from metals and metalloids, wherein the sum of all parts by mole of Mg, Al, Sb, and the other elements, other than Mg, Al, and Sb, selected from metals and metalloids amounts to <NUM>, b) oxygen, and c) less than <NUM> parts by mole of non-metallic and non-metalloid impurities, wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy.

In a second aspect, the present invention relates to a method for forming the first mixed metal oxide according to embodiments of the first aspect of the present invention, comprising depositing a magnesium oxide, an aluminium oxide, an antimony oxide, and optionally one or more other oxides, other than the magnesium oxide, the aluminium oxide, and the antimony oxide, selected from metal oxides and metalloid oxides on a substrate, so as to form the first mixed metal oxide consisting of: a) a mixture consisting of <NUM> to <NUM> parts by mole Mg, <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Sb, and <NUM> to <NUM> parts by mole of other elements, other than Mg, Al, and Sb, selected from metals and metalloids, wherein the sum of all parts by mole of Mg, Al, Sb, and the other elements, other than Mg, Al, and Sb, selected from metals and metalloids amounts to <NUM>, b) oxygen, and c) less than <NUM> parts by mole of non-metallic and non-metalloid impurities, wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy.

In a third aspect, the present invention relates to a transistor comprising the first mixed metal oxide according to embodiments of the first aspect of the present invention.

The present disclosure further relates to a transistor comprising a channel layer made of an amorphous second mixed metal oxide consisting of: a) a mixture consisting of <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Zn, and <NUM> to <NUM> parts by mole of other elements, other than Al and Zn, selected from metals and metalloids, wherein the sum of all parts by mole of Al, Zn, and the other elements, other than Al and Zn, selected from metals and metalloids amounts to <NUM>, b) oxygen, and c) less than <NUM> parts by mole of non-metallic and non-metalloid impurities, wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy.

Furthermore, the present disclosure relates to a method for forming the transistor according to embodiments of the disclosure, wherein forming the channel layer of said transistor comprises: depositing an aluminium oxide, a zinc oxide, and optionally one or more other oxides, other than the aluminium oxide, and the zinc oxide, selected from metal oxides and metalloid oxides on a substrate, so as to form the amorphous second mixed metal oxide consisting of: a) a mixture consisting of <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Zn, and <NUM> to <NUM> parts by mole of other elements, other than Al and Zn, selected from metals and metalloids, wherein the sum of all parts by mole of Al, Zn, and the other elements, other than Al and Zn, selected from metals and metalloids amounts to <NUM>, b) oxygen, and c) less than <NUM> parts by mole of non-metallic and non-metalloid impurities, wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy.

The word "comprising" according to the invention therefore also includes as one embodiment that no further components are present.

Reference will be made to transistors. These are devices having a first main electrode such as a drain, a second main electrode such as a source, a channel connecting the drain and the source, and a control electrode such as a gate for controlling the flow of electrical charges between the first and second main electrodes, through the channel.

In the context of the present invention, unless otherwise stated, when an amount, e.g., in parts by mole, of an element is mentioned, the amount is as measured by Rutherford Backscattering Spectroscopy.

In the context of the present invention, a metalloid is understood to be an element selected from Arsenic, Tellurium, Germanium, Silicon, Antimony, and Boron.

The inventors have found that, surprisingly, the first mixed metal oxide of the invention may have a combination of good electrical conductivity, and good stability, e.g., chemical stability. Without being bound by theory, the first mixed metal oxide having amounts of Mg, Al, and Sb according to the present invention results in a conduction band that is strongly delocalized over cation sites within the first mixed metal oxide, similar to a conduction band of a-InGaZnO<NUM>. This results in good electrical conductivity. Furthermore, the first mixed metal oxide of the first aspect of the present invention has a preferred bandgap that is similar or slightly larger than the bandgap of a-InGaZnO<NUM>. Thereby, when, for example, the first mixed metal oxide is used as a channel in a transistor, a current through the channel in an off-state of the transistor may be small. Al provides good stability to the first mixed metal oxide. At the same time, increasing the amount of Al in the first mixed metal oxide may further increase the bandgap. An increase in bandgap may result in a reduction in dopability. Therefore, preferably, Al is present in an amount not larger than what is suitable to induce stability to the first mixed metal oxide. The presence of Sb may reduce the bandgap of the first mixed metal oxide. A reduction in bandgap may result in an increase in dopability. Furthermore, Sb may be used to compensate the effect of Al on the bandgap, as the presence of Al may increase the bandgap. At the same time, increasing the amount of Sb in the first mixed metal oxide may reduce the stability of the first mixed metal oxide. Similarly to a-InGaZnO<NUM>, doping of the first mixed metal oxide may be induced by an oxygen deficit, compared to a stoichiometric amount of oxygen, in the first mixed metal oxide.

In embodiments, the mixture of the first mixed metal oxide consists of <NUM> to <NUM> parts by mole Mg, <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Sb, and <NUM> to <NUM> parts by mole of other elements, other than Mg, Al, and Sb, selected from metals and metalloids, wherein the sum of all parts by mole of Mg, Al, Sb, and the other elements, other than Mg, Al, and Sb, selected from metals and metalloids amounts to <NUM>, wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy. In embodiments, the mixture of the first mixed metal oxide consists of <NUM> to <NUM> parts by mole Mg, <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Sb, and <NUM> to <NUM> parts by mole of other elements, other than Mg, Al, and Sb, selected from metals and metalloids, wherein the sum of all parts by mole of Mg, Al, Sb, and the other elements, other than Mg, Al, and Sb, selected from metals and metalloids amounts to <NUM>, wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy. In embodiments, the amount of antimony in the mixture is from <NUM> to <NUM>, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>, part by mole.

In embodiments, the oxygen is present in the first mixed metal oxide in an amount that is within <NUM> mol-%, preferably within <NUM> mol-%, such as within <NUM> mol-% of, most preferably equal to, a stoichiometric amount. In embodiments, the amount of oxygen, in moles, in the first mixed metal oxide is within <NUM> mol-%, preferably within <NUM> mol-%, such as within <NUM> mol-% of, most preferably equal to, the sum, in moles, of the stoichiometric amount of oxygen with respect to each metal, and possibly each metalloid, in the mixture. In other words, the combination of the mixture a) and the oxygen b) of the first mixed metal oxide consists of MgO, Al<NUM>O<NUM>, and Sb<NUM>O<NUM> and possibly one or more other stoichiometric metal oxides and/or one or more stoichiometric metalloid oxides, except for a possible margin of up to <NUM> mol-%, preferably up to <NUM> mol-%, such as <NUM> mol-%, in the amount of oxygen. In embodiments, oxygen is present in an amount in parts per mole that is within <NUM> mol-%, preferably within <NUM> mol-%, such as within <NUM> mol-% of, preferably equal to, the sum, in moles, of the amount of Mg, <NUM> times the amount of Al, and <NUM> times the amount of Sb. In this last embodiment, the first mixed metal oxide may additionally contain oxygen due to oxides of the other elements, other than Mg, Al, and Sb, selected from metals and metalloids. It is an advantage of embodiments of the present invention that the presence of oxygen may facilitate deposition of the first mixed metal oxide at a low temperature.

In embodiments, the amount of the other elements, other than Mg, Al, and Sb, selected from metals and metalloids in the mixture a) of the first mixed metal oxide is from <NUM> to <NUM> parts by mole.

In embodiments, the first mixed metal oxide is in an amorphous phase. It is an advantage of embodiments of the present invention that no crystallization of the first mixed metal oxide may be required. Crystallization may require annealing at very high temperatures, e.g., above <NUM>.

Preferably, the first mixed metal oxide is transparent in the visible region of the electromagnetic spectrum. The other elements selected from metals and metalloids, other than Mg, Al, and Sb, may comprise any metal or metalloid, different from Mg, Al, and Sb.

In embodiments, the first mixed metal oxide contains less than <NUM> parts by mole of non-metallic and non-metalloid impurities. In preferred embodiments, the first mixed metal oxide contains less than <NUM> parts by mole of each non-metallic and non-metalloid impurity, e.g., hydrogen. It is an advantage of embodiments of the present invention that a high purity for the first mixed metal oxide may provide the first mixed metal oxide with good electrical properties.

Any features of any embodiment of the first aspect may be independently as correspondingly described for any embodiment of the second or third aspect of the present invention.

In embodiments, the deposition is performed at a temperature of at most <NUM>, preferably in a temperature range of from <NUM> to <NUM>. It is an advantage of embodiments of the present invention that the first mixed metal oxide may be deposited in the BEOL stage of a manufacturing process of a semiconductor structure, and with forming stacks of transistors, e.g., in 3D memory devices, without damaging other components of the semiconductor structure. In embodiments, the substrate comprises a semiconductor device.

In embodiments, the magnesium oxide, the aluminium oxide, the antimony oxide, and the optional one or more other oxides, other than the magnesium oxide, the aluminium oxide, and the antimony oxide, are deposited using physical vapour deposition. It is an advantage of embodiments of the present invention that physical vapour deposition may result in a homogenous, uniform mixture of the oxides. It is an advantage of embodiments of the present invention that physical vapour deposition is compatible with the BEOL stage of semiconductor manufacturing, and with forming 3D memory devices. In embodiments, the substrate comprises silicon. In embodiments, the substrate comprises a monocrystalline silicon wafer.

In embodiments, the physical vapour deposition is performed by sputtering using a magnesium oxide target, an aluminium target, and an antimony oxide target. In embodiments, a first AC potential field is applied to the magnesium oxide target, an AC potential field is applied to the antimony oxide target and a DC potential field is applied to the aluminium target. In preferred embodiments, the DC potential field applied to the aluminium target is a pulsed DC potential field. It is an advantage of embodiments of the present invention that the deposition of the first metal oxide may be efficient. When other elements, other than the magnesium oxide, the aluminium oxide, and the antimony oxide, selected from metal and metalloids are present in the mixture, the deposition of the oxides of these other elements may be performed by sputtering using a corresponding metal or metalloid target or using a corresponding metal oxide or metalloid oxide target.

Any features of any embodiment of the second aspect may be independently as correspondingly described for any embodiment of the first or third aspect of the present invention.

In embodiments, the first mixed metal oxide according to embodiments of the first aspect forms a channel layer. It is an advantage of embodiments of the present invention that a charge mobility in the on-stage of the transistor may be high. It is an advantage of embodiments of the present invention that a charge mobility in the off-stage of the transistor may be low.

In embodiments, the transistor is a thin film transistor. In embodiments, the thin film transistor comprises the first mixed metal oxide over, e.g., on, a substrate, e.g., glass or a silicon substrate or a polymer substrate. In embodiments, the thin film transistor comprises a gate material, e.g., indium tin oxide, over, e.g., on, the substrate. The substrate may comprise a semiconductor device. Preferably, the first mixed metal oxide is over the gate material, or the gate material is over the first mixed metal oxide. The gate material is typically separated from the first mixed metal oxide by an insulator material, such as SiO<NUM>, Al<NUM>O<NUM>, silicon nitride, or HfO<NUM>. The first mixed metal oxide typically contacts a drain and a source electrode, which preferably comprise a metal. The process for making thin-film transistors, e.g., channels of thin-film transistors, typically requires particularly low temperatures of formation. It is an advantage of embodiments of the present invention that the combination of good charge mobility, low formation temperature, and chemical stability, may make the first mixed metal oxide according to embodiments of the first aspect of the present invention particularly well-suited for applications in thin film transistors.

Any features of any embodiment of the third aspect may be independently as correspondingly described for any embodiment of the first or second aspect of the present invention.

First principles theoretical calculations, within the framework of density functional theory using the PBEsol functional, were performed to assess the electrical properties and the stability of a range of mixed metal oxides. Herein, the electrical properties are defined by the magnitude of the bandgap, and the inverse state weighted overlap (ISWO) parameter. The ISWO parameter may define the overlap of orbitals between atoms in a material. A low ISWO value represents a delocalized molecular orbital, whose atomic orbitals are continuously connected between the different atomic sites. A high ISWO value represents a highly localized and poorly connected molecular orbital. The ISWO parameter, and how it may be calculated, is further described in <NPL>.

In this example, calculations were performed for primary, one metal and oxygen, and binary, two metals and oxygen amorphous oxides, for <NUM> metals and metalloids (Mg, Al, Si, Ti, Zn, Ga, Zr, Ag, Cd, In, Sn, and Sb). Machine learning (support vector machines) was used to develop predictor functions for oxides containing up to all <NUM> element and oxygen. A single objective function F(x) (shown below) was developed, that, through minimalization, may be used to predict promising materials.

By varying the weights and target gap in the objective function, it was found that first mixed metal oxides consisting of Mg, Al, and Sb as metal, have very promising properties. Hereinbelow, results are shown for an amorphous first mixed metal oxide consisting of Mg, Al, Sb, and oxygen in a stoichiometric amount, as a function of the amount of Mg, Al, and Sb in the first mixed metal oxide. Herein, x is for the amounts of Mg, Al, and Sb in the first mixed metal oxide.

Reference is made to Table A, which contains values for the ISWO of the conduction band of the first mixed metal oxide, as dependent on the amount of Mg, Al, and Sb in the first mixed metal oxide. Herein, an ISWO of the conduction band of the first mixed metal oxide is defined with respect to an ISWO of the conduction band of a-InGaZnO<NUM>, i.e., Δ-ISWOC = Ic(x). Preferably, the ISWO of the conduction band is as low as possible. A low ISWO for the conduction band may correspond to a continuous molecular orbital for the conduction band, and a potentially high charge mobility in the conduction band. For a transistor channel consisting of a material having a low ISWO for the conduction band, there may be a high current through the channel in an on-state of the transistor. The ISWO for the conduction band is found to increase with an increasing concentration of antimony and to decrease with an increasing concentration of Al in the first mixed metal oxide.

Reference is still made to Table A, which, furthermore, contains values for the ISWO of the valence band of the first mixed metal oxide as dependent on the amount of Mg, Al, and Sb in the first mixed metal oxide. Herein, an ISWO of the valence band of the first mixed metal oxide is defined with respect to the ISWO of the valence band of a-InGaZnO<NUM>, i.e., Δ-ISWOV = Iv(x). Preferably, the ISWO of the valence band is as high as possible. A high ISWO for the valence band may correspond to a discontinuous molecular orbital for the valence band, and a potentially low charge mobility in the valence band. For a transistor channel consisting of a material having a high ISWO for the valence band, there may be a low current through the channel in an off-state of the transistor. The ISWO for the valence band is found to increase with decreasing concentrations of Sb in the first mixed metal oxide.

Reference is still made to Table A, which, furthermore, contains values for the bandgap of the first mixed metal oxide, as dependent on the amount of Mg, Al, and Sb in the first mixed metal oxide. Herein, a bandgap is defined with respect to the bandgap of a-InGaZnO<NUM>, i.e., Δ-gap = G(x). Preferably, the bandgap of the first mixed metal oxide is similar to, or a bit, e.g., <NUM> eV, higher than the bandgap of a-InGaZnO<NUM>. It may be observed that the bandgap increases with an increasing concentration of Al, and with a decreasing concentration of Sb. For a transistor comprising a channel comprising the first mixed metal oxide, the large bandgap means that the charge mobility may be low in the off state, and high in the on state of the transistor. However, doping may be increasingly difficult for larger band gaps.

Reference is still made to Table A, which, furthermore, contains values for the energy of formation of the first mixed metal oxide, as dependent on the amount of Mg, Al, and Sb in the mixed metal oxide. A relative energy of formation of the mixed metal oxide is provided that is the difference between: the energy of formation with respect to the energy of formation of isolated atoms of the mixed metal oxide in the gas phase; and the energy of formation with respect to the energy of formation of isolated atoms of a-InGaZnO<NUM> in the gas phase. The relative energy of formation is given as Δ-Eform = Ef(x). Herein, a negative value means that the mixed metal oxide is calculated to be more stable than a-InGaZnO<NUM>. It may be observed that the energy of formation reduces, and, correspondingly, the stability increases, with increasing amount of Al, and decreasing amount of Sb.

To derive an optimum with respect to each of the parameters, an objective function is calculated according to the following formula:<MAT> wherein G(x) = Δ-gap, Ic(x) = Δ-ISWOC, Iv(x) = Δ-ISWOV, and Ef(x) = Δ-Eform, each as a function of composition, i.e., amount of Mg, Al, and Sb. Herein, Gt is a target bandgap, and A, B, and C are weight factors. The optimum with respect to material properties corresponds to a minimum in F(x). Herein, for each of Gt and weight factors A, B, and C, the following values were used: Gt: [<NUM>, <NUM>, <NUM>, <NUM>, <NUM>]; A: [<NUM>, <NUM>]; B: [<NUM>, <NUM>]; and C: [<NUM>, <NUM>, <NUM>, <NUM>]. For all these 4x2x2x4 combinations, F(x) is optimized. The values summarized in Table A are all unique solutions of this optimization.

A minimum in F(x) corresponds to a balance between good electrical properties and good stability. According to the present calculations, these properties compare well with, and may be better than, the corresponding properties of InGaZnO<NUM>, i.e., IGZO, that is at present generally used in the field of thin-film transistors. Preferred embodiments of the present invention correspond to first mixed metal oxides having an amount of Mg, Al, and Sb, close to that of the optimum.

In addition, it was found from similar calculations as above that second mixed metal oxides consisting of Al and Zn as metal have very promising properties. Hereinbelow, results are shown for an amorphous second mixed metal oxide consisting of Al and Zn, and oxygen in a stoichiometric amount, as a function of the amount of Al and Zn in the second mixed metal oxide. Herein, x is for the amounts of Al and Zn in the second mixed metal oxide.

Reference is made to Table B, which shows results of similar calculations as above, but now for a second mixed metal oxide.

An ISWO of the conduction band of the second mixed metal oxide, defined with respect to an ISWO of the conduction band of a-InGaZnO<NUM>, i.e., Δ-ISWOc = Ic(x), increases with an increasing concentration of Al, and with a decreasing concentration of Zn. It may be observed that an ISWO of the valence band of the second mixed metal oxide, defined with respect to the ISWO of the valence band of a-InGaZnO<NUM>, i.e., Δ-ISWOV = Iv(x), increases with an increasing concentration of Al, and with a decreasing concentration of Zn. A bandgap, defined with respect to the bandgap of a-InGaZnO<NUM>, i.e., Δ-gap = G(x), increases with an increasing concentration of Al and with a decreasing concentration of Zn. A relative energy of formation Eform = Ef(x). of the second mixed metal oxide, which is the difference between the energy of formation with respect to the energy of formation of isolated atoms of the mixed metal oxide in the gas phase, and the energy of formation with respect to the energy of formation of isolated atoms of a-InGaZnO<NUM> in the gas phase, decreases with an increasing concentration of Al, and with a decreasing concentration of Zn.

Reference is made to <FIG>, which is a schematic representation of a combinatorial physical vapour deposition system that may be used for performing a method according to embodiments of the second aspect of the present invention, to form a first mixed metal oxide according to embodiments of the first aspect of the present invention. The combinatorial physical vapour deposition system of this example may deposit oxides, i.e., MgO, Al<NUM>O<NUM>, and Sb<NUM>O<NUM>. For the deposition, in the example, three sputter targets <NUM>, <NUM>, and <NUM> are mounted, each on an individual cathode <NUM>, <NUM>, and <NUM>. Each of the sputter targets <NUM>, <NUM> and <NUM> is aimed towards a substrate <NUM>. In this example, the substrate <NUM> is a <NUM> wide Si wafer. Heating of the substrate <NUM> may be enabled by clamping the substrate <NUM> to an electrically heated rotating chuck <NUM>. The deposition is typically performed in an Ar atmosphere, although also other gases may be used. Ar may be ionized and accelerated towards each of the targets <NUM>, <NUM>, and <NUM>, by application of a potential to each of the corresponding cathodes <NUM>, <NUM>, and <NUM>. The impact of the Ar ions on a target <NUM>, <NUM>, or <NUM> induces release of atoms or atom clusters from the target <NUM>, <NUM>, or <NUM>. This process is referred to as 'sputtering'.

Depending on the conductance of the target <NUM>, <NUM> or <NUM>, the potential applied to the cathode <NUM>, <NUM>, or <NUM> may be oscillated. When the target <NUM>, <NUM>, or <NUM> comprises an oxide material, e.g., MgO, Al<NUM>O<NUM>, or Sb<NUM>O<NUM>, the applied potential may oscillate at a frequency inside the radio frequency domain. When the target <NUM>, <NUM>, or <NUM> is an elemental target, e.g., Mg or Al, a DC potential may be applied. In this example, a MgO target <NUM> and a Sb<NUM>O<NUM> target <NUM> are powered with an oscillating potential, and an elemental Al target <NUM> is powered with a pulsed DC potential. To obtain a fully oxidized material with elemental targets, O<NUM> may be added to the sputtering gas. The O<NUM> gas may oxidize the target during the sputtering process, thereby forming an insulating top layer on the target. In that case, a pulsed DC potential is preferably used. Pressure is regulated by the total flow, i.e., Ar flow and O<NUM> flow, and is typically in the range of a few, e.g.,<NUM> to <NUM>, Pa.

A uniform deposition, i.e., a uniform first mixed metal oxide <NUM>, may be achieved by optimization of the aiming angle of the cathodes <NUM>, <NUM>, and <NUM>, and by rotation of the substrate <NUM> at a high rate. Typically, a deposition rate is low enough to facilitate random mixing of the elements during deposition. Thereby, the deposition may result in a uniform film of the first mixed metal oxide <NUM>. The composition of the film <NUM>, i.e., the amounts of Sb, Mg, and Al in the mixed metal oxide <NUM>, may be regulated by adjusting the potential that is applied to each cathode <NUM>, <NUM>, and <NUM>.

Reference is made to <FIG>, which is a schematic representation of a combinatorial physical vapour deposition system that may be used for performing a method according to embodiments of the fifth aspect of the present invention, to form a second mixed metal oxide according to embodiments of the fourth aspect of the present invention. The combinatorial physical vapour deposition system of this example may deposit oxides, i.e., Al<NUM>O<NUM>, and ZnO. For the deposition, in the example, two sputter targets <NUM> and <NUM> are mounted, each on an individual cathode <NUM> and <NUM>. Each of the sputter targets <NUM> and <NUM> is aimed towards a substrate <NUM>. In this example, the substrate <NUM> is a <NUM> wide Si wafer. Heating of the substrate <NUM> may be enabled by clamping the substrate <NUM> to an electrically heated rotating chuck <NUM>. The deposition is typically performed in an Ar atmosphere, although also other gases may be used. Ar may be ionized and accelerated towards each of the targets <NUM> and <NUM>, by application of a potential to each of the corresponding cathodes <NUM> and <NUM>. The impact of the Ar ions on a target <NUM> and <NUM> induces release of atoms or atom clusters from the target <NUM> and <NUM>. This process is referred to as 'sputtering'.

Depending on the conductance of the target <NUM> and <NUM>, the potential applied to the cathode <NUM> and <NUM> may be oscillated. When the target <NUM> or <NUM> comprises an oxide material, e.g., ZnO or Al<NUM>O<NUM>, the applied potential may oscillate at a frequency inside the radio frequency domain. When the target <NUM> or <NUM> is an elemental target, e.g., Al, a DC potential may be applied. In this example, a ZnO target <NUM> is powered with an oscillating potential, and an elemental Al target <NUM> is powered with a pulsed DC potential. To obtain a fully oxidized material with elemental targets, O<NUM> may be added to the sputtering gas. The O<NUM> gas may oxidize the target during the sputtering process, thereby forming an insulating top layer on the target. In that case, a pulsed DC potential is preferably used. Pressure is regulated by the total flow, i.e., Ar flow and O<NUM> flow, and is typically in the range of a few, e.g., <NUM> to <NUM>, Pa.

Claim 1:
A first mixed metal oxide (<NUM>) consisting of:
a) a mixture consisting of <NUM> to <NUM> parts by mole Mg, <NUM> to <NUM> parts by mole Al, <NUM> to <NUM> parts by mole Sb, and <NUM> to <NUM> parts by mole of other elements, other than Mg, Al, and Sb, selected from metals and metalloids, wherein the sum of all parts by mole of Mg, Al, Sb, and the other elements, other than Mg, Al, and Sb, selected from metals and metalloids amounts to <NUM>,
b) oxygen, and
c) less than <NUM> parts by mole of non-metallic and non-metalloid impurities,
wherein the parts by mole are as measured by Rutherford Backscattering Spectroscopy.