Process for preparing ceramics, ceramics thus obtained and uses thereof, especially as a sputtering target
10144674 ยท 2018-12-04
Assignee
Inventors
Cpc classification
C04B2235/3286
CHEMISTRY; METALLURGY
C04B2235/3284
CHEMISTRY; METALLURGY
C04B2235/3293
CHEMISTRY; METALLURGY
C04B2235/76
CHEMISTRY; METALLURGY
C04B2235/761
CHEMISTRY; METALLURGY
C04B2235/3206
CHEMISTRY; METALLURGY
C04B35/453
CHEMISTRY; METALLURGY
C04B2235/3203
CHEMISTRY; METALLURGY
C23C14/3414
CHEMISTRY; METALLURGY
International classification
C04B35/628
CHEMISTRY; METALLURGY
C04B35/453
CHEMISTRY; METALLURGY
C04B35/626
CHEMISTRY; METALLURGY
Abstract
A method for preparing a ceramic from an inorganic base material in the form of a powder with a high boiling point, including a step in which the powder of the inorganic base material is mixed with a second inorganic component which is also in powder form and which serves as a dopant for the inorganic base material. The dopant comprises a single inorganic material or a mixture of at least two inorganic materials that have a dopant effect on the inorganic base material. The method also includes a sintering step performed at a high temperature. Owing to the high density thereof, the resulting ceramics are suitable for use as a target element. The films and electrodes obtained from said ceramics have particularly beneficial properties.
Claims
1. A ceramic having an inorganic base material doped by an element J and being represented by the formula (I)
E.sub.x.sup.kJ.sub.x.sup.mO.sub.x(km)/2.sup.2.sub.x(km)/2 wherein: E.sub..sup.kO.sub..sup.2 denotes the inorganic base material: E denotes at least one metal from groups I to VIII of the Periodic Table of the Elements, and k denotes the average degree of oxidation of E in the formula I; J denotes at least one metal from groups I to VIII of the Periodic Table of the Elements, and m denotes the average degree of oxidation of the element J, m<k; , k and are positive numbers being between 1 and 20, such that k2=0; x denotes a positive integer such that x<; and represents an anionic vacancy; wherein a size of the pores present in the ceramic, measured by the high-resolution SEM method, is between 0.1 and 0.8 micrometers; and wherein said ceramic is obtained by a process comprising: a step of mixing the inorganic base material in powder form with an inorganic dopant material in powder form; and a sintering step carried out at a temperature above 800 C., wherein the forces exerted on the powders, before the sintering step are less than or equal to 5 kg/cm.sup.2.
2. The ceramic according to claim 1, having a crystallinity which, measured according to the X-ray diffraction method, corresponds to a crystallite size between 100 and 200 nm.
3. The ceramic according to claim 1, having a conductivity which, measured according to the four-point method, and as a function of the temperature varying from 4.2 K to ambient temperature, is between 200 and 10000 siemens per cm.
4. The ceramic according to claim 1, having a charge mobility which, measured according to the Seebeck effect method, is between 0.01 and 300 cm.sup.2/vol.Math.s.sup.1.
5. The ceramic according to claim 1, containing In.sub.2O.sub.3 as inorganic base material, SnO.sub.2 and ZnO as inorganic dopant material, the amount of SnO.sub.2 and ZnO being between 3 and 15 mol. %, the amount of ZnO being greater than or equal to that of SnO.sub.2, said ceramic having: an electrical conductivity between 300 and 500 S/cm; a density between 6 and 7.1 g/cm.sup.3; a (total) surface area between 1 and 1000 cm.sup.2; and a percentage of irregularities between 5 and 20%.
6. A process for preparing a film, consisting in subjecting a target formed by a ceramic according to claim 1 to a RF or DC cathodic sputtering.
7. A film obtained by the process according to claim 6, from a target made of a conductive ceramic having the formula In.sub.1.805Sn.sub.0.095Zn.sub.0.10O.sub.3 with being between 0.001 and 0.03 or the formula In.sub.1.94Zn.sub.0.06O.sub.2.97, or from a target made of an insulating ceramic having one of the formulae Li.sub.4Ti.sub.4.5Mg.sub.0.5O.sub.11.5, Li.sub.4Ti.sub.4.5Zn.sub.0.5O.sub.11.5 or Li.sub.4Ti.sub.4.5Ni.sub.0.5O.sub.11.75.
8. The ceramic according to claim 1, wherein said ceramic has at least one of the following properties: a macroscopic electrical conductivity, measured according to the four-point method (four-probe measurements) with a Keithley device (model 2400 Source Meter), which is greater than 300 siemens per cm; an improved apparent density, measured according to the mercury porosimeter method, which is greater than 5 g/cm.sup.3; a (total) surface area greater than 5 cm.sup.2; an improved percentage of grain boundary irregularities, measured according to the high-resolution electron microscopy method, which is less than 30% of that of a corresponding ceramic prepared without addition of element.
9. A ceramic having an inorganic base material doped by an element J and being represented by the formula (I)
E.sub.x.sup.kJ.sub.x.sup.mO.sub.x(km)/2.sup.2.sub.x(km)/2 wherein: E.sub..sup.kO.sub..sup.2 denotes the inorganic base material: E denotes at least one metal from groups I to VIII of the Periodic Table of the Elements, and k denotes the average degree of oxidation of E in the formula I; J denotes at least one metal from groups I to VIII of the Periodic Table of the Elements, and m denotes the average degree of oxidation of the element J, m<k; , k and are positive numbers being between 1 and 20, such that k2=0; x denotes a positive integer such that x<; and represents an anionic vacancy; wherein the inorganic dopant material comprises SnO.sub.2 and ZnO, the amount of SnO.sub.2 and ZnO being between 3 and 15 mol. %, the amount of ZnO being greater than that of SnO.sub.2; wherein the molar ratio of the inorganic dopant material to the inorganic base material varies between 0.001 and 0.4; and wherein said ceramic is obtained by a process comprising: a step of mixing the inorganic base material in powder form with an inorganic dopant material in powder form; and a sintering step carried out at a temperature above 800 C., wherein the forces exerted on the powders, before the sintering step are less than or equal to 5 kg/cm.sup.2.
10. The ceramic according to claim 9, wherein the amount of ZnO is between 6 and 10 mol %.
11. The ceramic according to claim 9, wherein the ceramic has the formula In.sub.1.805Sn.sub.0.095Zn.sub.0.10O.sub.3 or the formula In.sub.1.812Sn.sub.0.090Zn.sub.0.098O.sub.3/2 with being between 0.001 and 0.03.
12. The ceramic according to claim 9, wherein the ceramic is characterized by a crystalline bixbyite structure.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
(35) Within the context of the present disclosure, the following aspects are considered.
(36) The inorganic material forming the target base element may be most particularly an oxide, but it may also be an oxyhalide such as an oxychloride and/or oxyfluoride and/or oxysulfide and, in particular, an oxide having one or other of the formulae E.sub..sup.kO.sub..sup.2 and M.sub.x.sup.qT.sub.y.sup.nO.sub.z.sup.2.
(37) The purpose of the doping created in the base material is to generate anion vacancies in the base material, thus promoting the production of dense ceramics (having a density between 70 and 100% and preferably greater than or equal to 90%). It is therefore necessary, in accordance with the invention, that the cation of the dopant has a degree of oxidation m (real number) that imperatively is below that of the cation in the base material, that is to say m<k in E.sub..sup.kO.sub..sup.2 and m<q and/or m<n in M.sub.x.sup.qT.sub.y.sup.nO.sub.z.sup.2. The formulae of the materials that are doped and that therefore have anion vacancies favorable to sintering without (or with low) prior compacting, may thus be written, with J denoting the dopant and m its degree of oxidation: 1) for E.sub..sup.kO.sub..sup.2 which denotes the base material: E.sub.-x.sup.kJ.sub.x.sup.mO.sub.-x(km)/2.sup.2.sub.x(km)/2 which denotes the doped material, with: the anion vacancies denoted by ; x which denotes the degree of substitution is less than (preferably greater than or equal to 0.005); and 2) for M.sub.x.sup.qT.sub.y.sup.nO.sub.z.sup.2 which denotes the base material: the formula M.sub.xx.sup.qJ.sub.x.sup.mT.sub.y.sup.nO.sub.zx(qm)/2.sup.2.sub.x(qm)/2 or M.sub.x.sup.qT.sub.yx.sup.nJ.sub.x.sup.mO.sub.zx(nm)/2.sup.2.sub.x(nm)/2 and/or a combination of the two formulae which denote the doped material, with: x which denotes the degree of substitution which is less than x in the formula M.sub.xx.sup.qJ.sub.x.sup.mT.sub.y.sup.nO.sub.zx(qm)/2.sup.2.sub.x(qm)/2 as indicated in the disclosure (preferably greater than or equal to 0.005x); x is less than y in the formula M.sub.x.sup.qT.sub.yx.sup.nJ.sub.x.sup.mO.sub.zx(nm)/2.sup.2.sub.x(nm)/2, preferably x being greater than or equal to 0.005y.
(38) It will be observed that the anion vacancies that are created do not bear a charge, whereas the other elements bear a charge, for example oxygen bears the negative charge 2; the cation M bears the positive charge q; the cation T bears the positive charge n; etc.
(39) The process according to the present invention makes it possible to prepare, from an inorganic base material, a target element for sputtering. It consists in adding, to the inorganic base material, another inorganic material of dopant type. This inorganic material advantageously contains one or more cations preferably having a degree of oxidation below the cation (or cations) constituting the inorganic base material.
(40) The powder mixture thus obtained is not subjected to any particular force or only to those necessary for carrying out a light compacting thereof.
(41) The atomic ratio of the dopant relative to that of the inorganic base material preferably varies between 0.005 and 0.2, and it is advantageously located between 0.05 and 0.06.
(42) The inorganic base material thus doped is simply placed in the form of a compacted or non-compacted powder in a suitable crucible or mold, that withstands high temperatures, preferably up to 1600 C. Such a crucible or mold may be, for example, based on alumina. The sintering of the doped inorganic material, thus positioned, takes place when the crucible or mold is brought to a high temperature (above 800 C. and below the melting point of the base material).
(43) It has been found that, unexpectedly, the sintering leads to a ceramic that is sufficiently dense to be able to be used as a target or target element for sputtering.
(44) Without being bound by theory, the formulae are presented in this disclosure as being a representation of the preferred ceramics obtained by the implementation of the processes of the invention.
(45) In the formulae, E is at least one metal from groups I to VIII of the Periodic Table of the Elements, for example Fe, Cu, Co, Ni, W, Mo, Ti, Cr, Sn and In. M and T denote at least two different metals from the Periodic Table, for example Li, Na, K, Ag, Cu and TI for M and Ni, Co, W, Mn, Cr, Fe, V and Ti for T. The symbols k, q and n denote the average degrees of oxidation of E, M and T respectively. The parameters and are positive integers that satisfy the formula k2=0 and x, y and z denote positive integers such that qx+ny2z=0.
(46) As examples of such oxides mention may be made, nonlimitingly, of TiO.sub.2, In.sub.2O.sub.3, Li.sub.4Ti.sub.5O.sub.12, MoO.sub.3, Cr.sub.2O.sub.3, Fe.sub.2O.sub.3, Li.sub.xNiO.sub.2, Li.sub.xCrO.sub.2, Li.sub.xCoO.sub.2 and LiFeO.sub.2.
(47) The dopant J, responsible for the densification, is preferably at least one metal from groups I to VIII of the Periodic Table but which imperatively has, in the cation state, a degree of oxidation m below that of one of the elements-cations of the base material, namely:
(48) 0<m<k for E.sub..sup.kO.sub..sup.2 and 0<m<q and/or 0<m<n for M.sub.x.sup.qT.sub.y.sup.nO.sub.z.sup.2.
(49) It has surprisingly been discovered that doping of the E.sub..sup.kO.sub..sup.2 compound with an oxide (or halide or oxyhalide) having the dopant element J.sup.m partially substituted for E, in the aforementioned proportions referred to here as x (with x strictly less than , i.e. 0<x<), generates the formation of anion vacancies y, according to E.sub.(-x).sup.kJ.sub.x.sup.mO.sub.-y.sup.2.sub.y; y is then strictly a positive number less than such that 2yx(km), i.e. yx(km)/2. For example, for TiO.sub.2, the dopant may be ZnO or MgO. Specifically, the zinc and magnesium have a degree of oxidation equal to +2, that is to say less than the degree of oxidation of +4 of titanium. In the case of MgO used as a dopant for example, it will be written: Ti.sub.1xMg.sub.xO.sub.2x.sub.x for TiO.sub.2 thus doped. The index x may vary between 0.0053 and 0.2 and preferably varies from 0.05 to 0.06, which corresponds substantially to a dopant level, measured per mole (or atom) of Ti, or per mole of TiO.sub.2, between 0.5 and 20%, and preferably from 5 to 6%.
(50) These anion vacancies which are neutral, that is to say uncharged, favor the densification of the final material obtained during its heating or annealing under the aforementioned conditions. Similarly, the doping of the M.sub.x.sup.qT.sub.y.sup.nO.sub.z.sup.2 compound with an oxide (or halide or oxyhalide) having the dopant element J.sup.m partially substituted for T, in the aforementioned proportions referred to here as x (with x strictly less than y), seems to generate the formation of anion vacancies y, according to M.sub.x.sup.qT.sub.(yx).sup.nJ.sub.x.sup.mO.sub.(xy).sup.2.sub.y; y is then strictly a positive number less than z such that 2y2x(nm), i.e. yx(nm)/2. Furthermore, and in a similar manner, the doping of the M.sub.x.sup.qT.sub.y.sup.nO.sub.z.sup.2 compound with an oxide (or halide or oxyhalide) having the dopant element J.sup.m partially substituted for M, in the aforementioned proportions referred to here as x (with x strictly less than x), seems to generate the formation of anion vacancies y, according to M.sub.(xx).sup.qJ.sub.x.sup.mT.sub.ynO.sub.(zx).sup.2.sub.y; y is then strictly a positive number less than x which satisfies the equation 2y=x(qm), i.e. yx(qm)/2. The J dopant (or dopants) may be partially substituted both for M and T. The corresponding formulae then result from the combination of the two aforementioned formulae. These anion vacancies favor the densification of the final material obtained during its heating or annealing under the aforementioned conditions.
(51) For example, for Li.sub.4Ti.sub.5O.sub.12, the dopant may be ZnO or MgO since zinc and magnesium have a degree of oxidation equal to +2, that is to say less than the degree of oxidation of +4 of the titanium.
(52) In the case of MgO used as a dopant for example, it will be written: Li.sub.4Ti.sub.5xMg.sub.xO.sub.12x.sub.x; for Li.sub.4Ti.sub.5O thus doped, x may vary between 0.025 and 1 and preferably is equal to 0.25-0.3, which corresponds substantially to a dopant level, measured per mole (or atom) of Ti between 0.5 and 20%, and preferably from 5 to 6%.
(53) When the process of doping with zinc Zn.sup.2+ or other cations (Mg.sup.2+, Cu.sup.2+ etc.) having a degree of oxidation lower than that of the base oxide is applied to the following oxides, they result in novel oxides, especially those which have doping levels between 0.5 and 20% and preferably between 5 and 6%: TiO.sub.2, MoO.sub.3, WO.sub.3, Cr.sub.2O.sub.3, Fe.sub.2O.sub.3, Li.sub.xNiO.sub.2 with x between 0.1 and 2 and preferably equal to 1, Li.sub.xCrO.sub.2.5 with x between 1 and 2, and preferably equal to 1, LiFeO.sub.2; and Li.sub.4Ti.sub.5O.sub.12, for which the titanium is in the average degree of oxidation of +4, doped with Ni.sup.2+ and/or Ni.sup.3+; a co-doping with Zn/Ni is also novel. The dopant level is the aforementioned (between 0.5 and 20% and preferably from 5 to 6%).
(54) According to one preferred embodiment of the invention, the films (or electrodes) are prepared by sputtering from ceramics (or targets) of novel composition In.sub.1.805Sn.sub.0.095Zn.sub.0.10O.sub.3. They give rise, including on a plastic carrier such as PET, to transparent (90% transmission in the visible range) and conductive (>1000 siemens/cm) electrodes that have remarkable performances.
(55) This material has a density of 6.57 g/cm.sup.3, which corresponds to 92% of the theoretical density, measured according to the mercury porosimeter method (Autopore IV 9500 Mercury Porosimeter). This density is amply sufficient so that the ceramics can be used as a target for sputtering. Below 70% of the theoretical density, the targets tend to crack during the sputtering and, furthermore, the plasma does not always remain homogeneous during the sputtering process leading to films that are inhomogeneous in composition, and therefore unreproducible. This is not the case for densities greater than 70%, and preferably greater than 80% as is the case here, since the density is greater than 90%.
(56) The density of a corresponding ceramic prepared without addition of dopant element via the conventional method (pressing of the powder at 1 t/cm.sup.2 approximately, and then annealing at a temperature of 1300 C.) is 5 g/cm.sup.3, i.e. 70% of the theoretical density. Under these conditions, it has been observed that it is impossible to prepare a ceramic without the aforementioned dopants by the method according to the invention.
(57) It is furthermore stated that the commercial ITO ceramics, which have a density of around 90%, are prepared by the heavy-duty and expensive hot-pressing technique of the prior art.
(58) The process of the present invention therefore makes it possible to prepare ceramics that have densities at least as high, in a manner that is much more flexible, simple and less expensive than the techniques known from the prior art.
(59) Another advantage is that it is thus possible to prepare, by the method of the invention, ceramics having a large surface area which may be greater than 100 cm.sup.2, on condition of using the aforementioned dopants.
(60) Finally the ceramics thus obtained have original intrinsic features especially including a pore size which is substantially greater (generally by 3 to 10%, preferably by 4 to 5%) than that of similar ceramics from the prior art, although having comparable electrochemical conductivities.
(61) It is possible that there are other explanations for explaining the high density of the ceramic materials obtained.
EXAMPLES
(62) The examples explained below are given by way of illustration only and should not be interpreted as constituting any limitation of the subject of the present invention.
Example 0
Preparation of a Commercial-type ITO Ceramic
(63) In order to prepare the circular ITO target (ceramic) (
(64) Step 1) 47.3173 g of In.sub.2O.sub.3 and 2.6827 g of SnO.sub.2 were mixed using the well-known technique of ball-milling with the 05.600 FRITSCH apparatus; for this purpose, the aforementioned mixture of powder was put into one of the two agate grinding bowls from
(65) Step 2) The powder was placed in a cylindrical stainless steel mold having an internal diameter of 60 mm and the powder was pressed at 25 tonnes/cm.sup.2 for 10 minutes. A compacted target was thus obtained.
(66) Step 3) The compacted target was gently (since it is very fragile) conveyed to an alumina carrier and the carrier with the target was heated at a rate of 300 C. per hour in a muffle furnace, shown on the left-hand side of the image, until the temperature of 1300 C. was reached, which was held for 12 hours; next it was cooled at a rate of 300 C. per hour. Then the desired ITO ceramic (
Example 1
ITZO Targets for the Preparation of Transparent and Metallic Electrodes for Optoelectronic Devices
(67) According to a first preferred embodiment of the invention, when the inorganic base material is the oxide In.sub.2O.sub.3 or tin-doped In.sub.2O.sub.3 (commonly known as ITO), for which indium is in the average degree of oxidation of +3, the dopant may advantageously be zinc oxide or magnesium oxide (preferable to a zinc or magnesium halide or oxyhalide, even if the latter have a certain advantage) for which the degree of oxidation of the zinc or of the magnesium is +2, that is to say lower than that of +3 of the indium.
(68) Even when the molar ratio of the dopant relative to that of the indium oxide is as low as 0.06, sufficiently dense ceramics for being able to be used as a target or target element for sputtering are obtained by simple heating (which may be in air) at temperatures above 1100 C. (ideally 1300 C.) following the aforementioned method and when said assembly is held at this temperature for a sufficient duration to convert the precursor system to said inorganic ceramic material.
(69) Furthermore, these ceramics have a sufficiently high electrical conductivity so that DC-mode sputtering (adapted to the industrial scale) can advantageously be used. Thus obtained by DC (or RF) sputtering from these targets are films having optoelectronic properties at least equal to those of commercial ITO ceramics.
Example 1
ITZO Degenerate Semiconductor for Preparing Transparent and Metallic Electrodes for Optoelectronic Devices
(70) In order to prepare the ITZO targets, in the example circular targets having a diameter of 5 cm and an In.sub.1.805Sn.sub.0.095Zn.sub.0.10O.sub.3 composition and having a weight equal to 50 g, three successive steps were followed according to the experimental protocol detailed below.
(71) In step 1) 45.8881 g of In.sub.2O.sub.3, 2.6217 g of SnO.sub.2 and 1.4902 g of ZnO were mixed using the well-known technique of ball-milling; for this purpose, the aforementioned mixture of powder was put into one of the two agate grinding bowls from
(72) In step 2), the powder was then placed, by pressing by hand with a 3 cm diameter stainless steel cylinder, in a flat-bottomed alumina container having a diameter of 75 mm (
(73) In step 3) the container was heated at a rate of 300 C. per hour in a muffle furnace, presented on the left-hand side of the image, until the temperature of 1300 C. was reached which was held for 12 hours; next it was cooled at a rate of 300 C. per hour. Then the desired ceramic (
(74) The density of the ceramics obtained in step 3, measured with a mercury porosimeter (AutoPore IV 9500), was 91% of the theoretical density. Their electrical resistivity that is advantageously very low, lower than that of the ITO ceramics, is illustrated in
(75) These particularly advantageous features confirm that the ITZO ceramics of the present invention may advantageously be used industrially in DC sputtering.
Example 2
Targets for the Preparation of Electrodes for Electrochemical Devices (Microgenerators, Electrochromic Devices)
(76) According to a second preferred embodiment of the invention, when the inorganic base material is the oxide Li.sub.4Ti.sub.5O.sub.12, for which the titanium is in the average degree of oxidation of +4, the dopant may advantageously be the zinc oxide ZnO or the magnesium oxide MgO or a transition metal oxide such as NiO or Ni.sub.2O.sub.3, for which the degree of oxidation of the zinc or of the magnesium is +2, that of the nickel+2 or +3, that is to say below that of +4 of the titanium. Even when the molar ratio of the dopant relative to that of the indium oxide is as low as 0.06, sufficiently dense ceramics for being able to be used as a target or target element for sputtering are obtained by simple heating (which may be in air) at temperatures above 1100 C. (ideally 1300 C.), following the aforementioned method. It is thus possible to obtain by RF sputtering of these targets films that can advantageously be used as electrodes for microgenerators (lithium batteries) or for electrochromic devices.
Example 2
Target Based on Li4Ti5O12 Doped with Zn for Preparing Electrodes for Microgenerators and for Electrochromic Devices
(77) The experimental protocol used for ITZO was used with the difference that the starting products were lithium carbonate, titanium dioxide and zinc or magnesium monoxide.
(78) Thus, in order to prepare the targets based on Zn-doped Li.sub.4Ti.sub.5O.sub.12, in the example circular targets having a diameter of 5 cm and an Li.sub.4Ti.sub.4.20Zn.sub.0.30O.sub.11.7 composition and having a weight equal to 50 g, three successive steps were followed according to the experimental protocol detailed below.
(79) In step 1) 16.07 g Li.sub.2CO.sub.3 (corresponding to 6.50 g of Li.sub.2O), 40.85 g of TiO.sub.2 and 2.65 g of ZnO were mixed using the well-known technique of ball-milling; for this purpose, the aforementioned mixture of powder was put into one of the two agate grinding bowls from
(80) In step 2), the powder was then placed, by pressing by hand with a 3 cm diameter stainless steel cylinder, in a flat-bottomed alumina container having a diameter of 75 mm.
(81) In step 3) the container was heated at a rate of 300 C. per hour in a muffle furnace, presented on the left-hand side of the image, until the temperature of 1300 C. was reached which was held for 12 hours; next it was cooled at a rate of 300 C. per hour. Then the desired ceramic (
(82) The density of the ceramics obtained in step 3, measured with a mercury porosimeter (AutoPore IV 9500), was 93% of the theoretical density. Their electrical resistivity was high, of the order of 10.sup.7 .Math.cm, confirming the insulating nature of the ceramics. Therefore, the ceramics will have to be used industrially in RF sputtering.
(83) The tests carried out demonstrate that the invention provides a simple, rapid and inexpensive process for preparing a target element of the ceramic type for sputtering constituted of an inorganic material having a melting point above 300 C. Such a process may be carried out by persons who do not have particular competencies in the ceramic or sintering art and it easily leads to the production of targets or target elements that especially make it possible to easily produce targets of large surface area.
(84) Industrialists prepare targets such as ITO by hot-pressing; this is one of the reasons why the targets are expensive. The products (ceramics) prepared according to the invention are therefore less expensive and also appear novel due to the fact that the accelerated sintering by the addition of elements such as, for example, ZnO in ITO or in indium oxide (described in the disclosure) substantially increases the electrical conductivity of the ceramics (when doped with 2 mol % of ZnO, the conductivity of ceramic ITO is approximately doubled); this is because, surprisingly, it appears that the percolation between the grains is improved and that grain boundary problems that impair the macroscopic conductivity are further avoided. The SEM photos from
Example 3
Ceramic of In1.862Sn0.098Zn0.04O3 Composition, Prepared under Similar Experimental Conditions Used in Example 1
(85) The ceramic of In.sub.1.862Sn.sub.0.098Zn.sub.0.04O.sub.3 composition is prepared as follows:
(86) Step 1) 46.7410 g of In.sub.2O.sub.3, 2.6704 g of SnO.sub.2 and 0.5886 g of ZnO were mixed using the well-known technique of ball-milling (see
(87) Step 2) The powder was then placed, by pressing by hand with a 3 cm diameter stainless steel cylinder, in a flat-bottomed alumina container having a diameter of 75 mm (
(88) Step 3) The container was heated at a rate of 300 C. per hour in a muffle furnace, presented on the left-hand side of the image, until the temperature of 1300 C. was reached which was held for 12 hours; it was then cooled at a rate of 300 C. per hour. The aforementioned low-density ceramic was thus obtained that cannot be used as a target for sputtering.
(89) The ceramic of In.sub.1.862Sn.sub.0.098Zn.sub.0.04O.sub.3 composition thus obtained has a density, measured by the aforementioned technique, of 2.76 g/cm.sup.3, which only represents 40% of the theoretical density which is considerably below the limit of 70% that corresponds to the possibility of using as a target for sputtering.
(90) The conductivity of this ceramic is equal to 50 siemens per cm only, measured by the aforementioned technique; it is thus 6 times lower than that of In.sub.1.805Sn.sub.0.095Zn.sub.0.10O.sub.3.
Example 4
Ceramic of In1.812Sn0.080Zn0.098O3 Composition, Prepared for the Application of Films Deposited by Sputtering onto Glass and PET Substrates
(91) The ceramic of In.sub.1.812Sn.sub.0.080Zn.sub.0.098O.sub.3 composition is prepared as follows:
(92) Step 1) 50 g of In.sub.2O.sub.3, SnO.sub.2 and ZnO powders, in suitable amounts, according to the optimized composition of the [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10 ceramic, were mixed using the well-known technique of ball-milling; for this purpose, the aforementioned mixture of powder was put into one of the two agate grinding bowls each containing 50 agate balls having a diameter of 8 mm; 30 ml of ethanol was added; it was covered with an agate lid as indicated in the figure; the ball-milling was then carried out for 3 hours at 250 rpm as indicated in
(93) Step 2) The powder was lightly pressed by hand in a flat-bottomed alumina container having a diameter of 82.56 mm (
(94) Step 3) The container was heated at a rate of 300 C. per hour in a muffle furnace, until the temperature of 1300 C. was reached which was held for 12 hours; next it was cooled at a rate of 300 C. per hour. Thus the ceramic having a relative density of around 92% was obtained. A ceramic having a final diameter of 50 mm was obtained after the sintering and polishing steps.
(95) The ITZO films deposited by sputtering on the PET substrates had higher optoelectronic performances than the commercial-type ITO films. In terms of optical properties, the ITZO films had a high visible transparency (greater than 86% for the films deposited on a glass substrate and greater than 80% for those deposited on a PET substrate). The resistivity of these
(96) films was low (around 4.410.sup.4 .Math.m for the films deposited on glass and of the order of 4.710.sup.4 .Math.m for the films deposited on PET) compared to those of the commercial-type ITO films deposited on the same substrates.
(97) Characterization of the Structures
(98) The characterizations carried out show, in particular, that the co-doping of In.sub.2O.sub.3 with Sn.sup.4+ and Zn.sup.2+ (ITZO) forms a solid solution, that makes it possible to prepare novel highly dense and conductive ITZO ceramics. It is thus established that the co-doping, in particular with the zinc, makes it possible to prepare a highly dense ceramic target and a large conductive surface area suitable for both types of DC and RF sputtering. The synthesis of such a target has thus been able to be carried out successfully by direct sintering of the powder mixture placed in a suitable container without using a cold or hot (expensive) pressing procedure. The ITZO thin films deposited on glass and plastic substrates were then deposited at ambient temperature using the ceramic target with the optimized composition. The influence of the sputtering conditions on the optoelectronic properties of the films was also established.
(99) Preparation of the ceramicsIn.sub.2O.sub.3 (99.99%, Aldrich), SnO.sub.2 (99.9%, Aldrich) and ZnO (99.9%, Aldrich) powders were used to prepare ITZO ceramics. Suitable amounts of selected oxides were milled by ball-milling for 30 min in an agate bowl containing agate balls and ethanol. The alcohol was then evaporated at 110 C. for 6 hours. After drying, the powder was ground in an agate mortar and a cylindrical crucible made of alumina having a diameter of 16 mm was filled with it, and it was then pressed by hand. The mixed powder, with which the crucible was filled, was finally sintered at 1300 C. in air for 12 hours. The dimensions of the granules obtained were measured with digital vernier calipers, and the granules were weighed using an analytical balance, these measurements allowing the apparent densities of the granules to be estimated.
(100) Chemical composition and apparent densityAccording to the literature [11, 28, 31-35], the best conductivity results were obtained for an amount of Sn.sup.4+ which varied from 6-10 mol % in In.sub.2O.sub.3, depending on the synthesis conditions. The Sn.sup.4+ content in the present ceramics was set at 10 mol % and the initial Zn.sup.2+ content varied in the co-doped ceramic from 0-10 mol %. For reasons of clarity, a simplified sample identification underlining the influence of the doping of Zn in ITO has been adopted (Table I).
(101) The EPMA results, reported in Table I, show that there is good agreement between the final compositions of the ceramic after sintering and the nominal starting compositions. The Zn content in the final composition of the ceramic having the nominal composition In.sub.2O.sub.3:Zn.sub.0.02 (IZO) reaches 1.4 mol %. It should be noted that this value is consistent with the reported solubility limit of ZnO in In.sub.2O.sub.3 (1-2 mol %) [24, 36]. A slight loss of SnO.sub.2 that varies from 0.5-1 mol % (which corresponds to 0.27 to 0.54 wt %) is also observed for the two ITZO and ITO ceramics (Table I).
(102) Table I presents: chemical composition of the ceramic and apparent density for the (ITZO) ceramics [In.sub.2O.sub.3:Sn.sub.0.10]: Zn.sub.y, 0y0.10. The reported apparent densities were deduced by measuring the dimensions and the weights of the granules. It should be noted that the granules are prepared by pressing by hand the powder mixture in an alumina crucible. The data for In.sub.2O.sub.3:Zn have only been given by way of comparison. /2 indicates the neutral oxygen vacancy created by doping with Zn, the value of /2 varying with the Zn content.
(103) TABLE-US-00001 TABLE I Composition of the Sample Starting ceramic determined identification mixture by EPMA 0.005 ** In.sub.2O.sub.3:Zn.sub.0.02 (In.sub.2O.sub.3).sub.0.99 + (ZnO).sub.0.02 In.sub.1.986Zn.sub.0.014O.sub.2.993/2 3.03 In.sub.2O.sub.3:Sn.sub.0.10 (ITO) (In.sub.2O.sub.3).sub.0.95 + (SnO.sub.2).sub.0.1 In.sub.1.910Sn.sub.0.090O.sub.3 2.52 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.04 [(In.sub.2O.sub.3).sub.0.95 + (SnO.sub.2).sub.0.1].sub.0.98 + (ZnO).sub.0.04 In.sub.1.866Sn.sub.0.089Zn.sub.0.045O.sub.3/2 3.50 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.06 [(In.sub.2O.sub.3).sub.0.95 + (SnO.sub.2).sub.0.1].sub.0.97 + (ZnO).sub.0.06 In.sub.1.847Sn.sub.0.091Zn.sub.0.063O.sub.3/2 3.92 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.08 [(In.sub.2O.sub.3).sub.0.95 + (SnO.sub.2).sub.0.1].sub.0.96 + (ZnO).sub.0.08 In.sub.1.827Sn.sub.0.090Zn.sub.0.083O.sub.3/2 4.87 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10 [(In.sub.2O.sub.3).sub.0.95 + (SnO.sub.2).sub.0.1].sub.0.96 + (ZnO).sub.0.10 In.sub.1.812Sn.sub.0.090Zn.sub.0.098O.sub.3/2 6.57 ** Apparent density (g/cm.sup.3) 0.05
(104) These results are confirmed by thermogravimetric analysis (TGA) obtained for the ITO and ITZO ceramics (
(105) The IZO ceramic having the nominal composition In.sub.2O.sub.3:Zn.sub.0.02 (Table I) has a low density: 3.03 g/cm.sup.3; it corresponds to only 42% of the theoretical density of In.sub.2O.sub.3. This indicates that the concentration of Zn in IZO, corresponding to the solubility limit of Zn in In.sub.2O.sub.3, is not sufficient to induce a high densification when the granule is prepared by the method in question (pressed by hand) [36]. However, for the ITZO ceramics, it has emerged that the apparent density increases from 2.52 to 6.57 g/cm.sup.3 (reaching 92% of the theoretical density) when the Zn concentration increases from 4-10 mol % [37]. From Table I and
In.sub.2-x-y-.sup.3+Sn.sub.x.sup.4+Zn.sub.y+.sup.2+O.sub.3(/2).sup.2.sub./2[(xy)e.sub.C.B.sup.](a)
(106) Specifically, as was observed for AZTO, the neutral oxygen vacancies promote mass transfer at the grain boundary resulting from the densification of the ceramic. However, the presence of Zn.sup.2+ in the substitutional position will compensate for the free carriers produced by the doping with Sn.sup.4+[according to the formula (a)] resulting from the net charge concentration per unit of formula equal to xy.
(107) Structural characterizationIndium oxide has the bixbyite-type cubic structure (also known as c-type rare-earth oxide structure) which has a unit cell of 80 atoms (In.sub.32O.sub.48) with the space group Ia3 and a lattice parameter equal to 10.117 [39]. This structure may be derived from the structure related to fluorite (CaF.sub.2) by removing a quarter of the anions and by allowing small changes of the ions [40]. The indium cations are located at two non-equivalent sextuple positions, referred to as b and d (
(108) ITO (In.sub.2O.sub.3:Sn)The X-ray diffraction patterns for In.sub.2O.sub.3 and ITO (nominal composition In.sub.2O.sub.3:Sn.sub.0.10 powders annealed at 1300 C. are shown in
(109) ITZO (In.sub.2O.sub.3:Sn:Zn)The X-ray diffraction pattern for the ITZO powders sintered (annealed according to GDT) at 1300 C. (
(110) Table II presents the change in the cell parameter with the Zn content for sintered ITZO powders. The ITO cell parameter is added as a reference.
(111) TABLE-US-00002 TABLE II Sample identification a () In.sub.2O.sub.3:Sn.sub.0.10 (ITO) 10.123 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.04 10.114 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.06 10.107 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.08 10.104 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10 10.097
(112) The change in the surface morphology of the ceramic with the Zn content is presented in the SEM micrographs (
In.sub.1-x-y-.sup.3+Sn.sub.x.sup.4+Zn.sub.y+.sup.2+O.sub.2(/2).sup.2.sub./2[(xy)e.sub.C.sup.(formula (a))
which allows a mass transfer to the grain boundaries and thus to the grain percolation, resulting in an increase of the ceramic density [37].
(113) Electrical measurementsIn.sub.2O.sub.3 is a non-stoichiometric n-type semiconductor or even semimetal, with a wide energy band of semiconductor or even semimetal gap (3.5 eV) for high doping levels. The origin of such conductivity is due to the charged oxygen vacancy (V.sub.o) and/or to the doping with Sn.sup.4+. Fan and Goodenough [11] developed a model that shows that the bottom of the conduction band is essentially composed of In:5s states and the top of the valence band is composed of O:2p states (
(114) The ITZO ceramics demonstrate electrical resistivities that are lower compared with that of ITO (
(115)
in which S is the Seebeck coefficient measured in V/K. The charge carrier concentration may then be deduced using the following equation for a degenerate semiconductor:
(116)
N is the charge carrier concentration and m* is the effective mass of the electron (the assumption was made that m* is equal to 0.4 m.sub.e [42]). All the electrical data deduced from Seebeck and the resistivity measurements are listed in Table III. At first, the charge concentration decreases with the amount of Zn in the ceramic. This may be explained by the increase in the substitution of In.sup.3+ by Zn.sup.2+ in the In.sub.2O.sub.3 structure, which is confirmed by the displacement toward a higher angle of the various peaks of the XRD (X-ray diffraction) patterns (
(117) Table III presents the values of E.sub.F-E.sub.c, of the mobility, of the charge concentration and of the resistivity for the ITO ceramics and various ITZO ceramics. The charge concentration was deduced using the Seebeck coefficient measurements.
(118) TABLE-US-00003 TABLE III Charge concentration Resistivity Sample E.sub.F E.sub.C Charge mobility (10.sup.20 e.sup. cm.sup.3) (10.sup.3 .Math. cm) identification (eV) (cm.sup.2V.sup.1s.sup.1) 5% 5% 5% In.sub.2O.sub.3:Sn.sub.0.10 (ITO) 0.67 0.16 6.30 64 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.04 0.62 0.18 5.63 61 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.06 0.61 0.23 5.42 51 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.08 0.55 2.30 4.68 5.8 [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10 0.47 10.09 3.65 1.7
(119) Using the EPMA results and the electrical measurements, it is possible to calculate the exact final formula for the ITO and ITZO ceramics. In the case of the ITO ceramic, there is only a substitution of In.sup.3+ by Sn.sup.4+ in the In.sub.2O.sub.3 lattice producing free electron carriers in the conduction band according to the formula:
In.sub.2x.sup.3+Sn.sub.x.sup.4+O.sub.3.sup.2[xe.sub.C.B.sup.](B)
x was deduced from the charge carrier concentration (Table III) and found to be equal to 0.04 per unit of formula. Thus, the following formula for ITO should normally be written:
In.sub.1.96.sup.3+Sn.sub.0.04.sup.4+O.sub.3.sup.2[0.04e.sub.C.B.sup.](C)
(120) However, the formula (c) differs from that determined using EPMA: In.sub.1.91Sn.sub.0.09O.sub.3 which is more precise. In fact, it is recalled that the amount 0.09 Sn is divided into three parts: (i) one part will go to form the additional rutile SnO.sub.2 phase as shown previously by the XRD (X-ray diffraction) analysis, (ii) another part substitutes In.sup.3+ producing free electrons in the conduction band according to the formula (c) and (iii) the remaining Sn are very probably segregated at the grain boundaries when structural disorder predominates.
(121) For ITZO, both Sn.sup.4+ and Zn.sup.2+ substitute In.sup.3+ in In.sub.2O.sub.3 according to the formula (a)
(In.sub.2-x-y-.sup.3+Sn.sub.x.sup.4+Zn.sub.y+.sup.2+O.sub.3(/2).sup.2.sub./2[(xy)e.sub.C.B.sup.]).
(122) The parameters calculated (x, y and ) and the corresponding final formula for the ITZO ceramics are listed in Table IV.
(123) Table IV presents the parameters and final formula for ITZO calculated using the EPMA results and charge concentration determined by the Seebeck measurements.
(124) TABLE-US-00004 TABLE IV Sample identification x y Final formula [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.06 0.091 0.057 0.006 In.sub.1.847.sup.3+Sn.sub.0.091.sup.4+Zn.sub.0.063.sup.2+O.sub.2.997.sup.2.sub.0.003 [(0.034)e.sub.C.B..sup.] [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.08 0.092 0.062 0.020 In.sub.1.826.sup.3+Sn.sub.0.092.sup.4+Zn.sub.0.082.sup.2+O.sub.2.990.sup.2.sub.0.010 [(0.030)e.sub.C.B..sup.] [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10 0.090 0.066 0.032 In.sub.1.812.sup.3+Sn.sub.0.090.sup.4+Zn.sub.0.098.sup.2+O.sub.2.984.sup.2.sub.0.016 [(0.024)e.sub.C.B..sup.]
(125) ConclusionsThe granules of the ITO, IZO and ITZO ceramics were prepared without using a hot or cold pressing procedure. They were obtained simply by mixing lightly pressed (pressed by hand) powder in a cylindrical crucible made of alumina and then by sintering at 1300 C. The idea was to be capable of preparing large-scale targets that could be used for industrial applications in a vapor phase deposition process.
(126) It has been found that the final composition of the IZO ceramic has a Zn content of 1.4 mol %, which corresponds to the solubility limit in In.sub.2O.sub.3. The density of the IZO ceramic obtained is low (3.03 g/cm.sup.3) compared to the theoretical density of In.sub.2O.sub.3 (7.16 g/cm.sup.3). For the ITO ceramic, a good agreement between the final composition of the ceramic and the starting mixture was observed with a very small loss of Sn.sup.4+ (1 mol %) and its density is low (35% of the theoretical density). For ITZO, the final compositions of the ceramic are also in good agreement with their starting mixtures, also with a very small loss of Sn.sup.4+ (0.5-1 mol %). However, the density of the ITZO ceramic prepared gradually increases when the Zn content increases, due to the increase of neutral oxygen vacancies that promote mass transfer to the grain boundaries and, in this way, facilitate the percolation between the grains. The highest density (92% of the theoretical density) is observed for the ceramic having the nominal composition [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10.
(127) In good agreement with the literature [35], the solubility of SnO.sub.2 in In.sub.2O.sub.3 reached 6 mol % as was shown on the X-ray diffraction patterns. However, it emerges that the solubility of Sn and Zn increases when they co-substitute In in In.sub.2O.sub.3. Specifically, this was shown by the X-ray diffraction pattern analysis. An additional peak corresponding to Sn or Zn oxide phases was not observed for the ceramics having a Zn content 6 mol %. Furthermore, the small change of the peaks from the X-ray diffraction pattern toward higher angles takes into account the reduction in the cell parameter due to the substitution of some In.sup.3+ by Zn.sup.2+.
(128) More advantageously, the electrical resistivities of the ITZO ceramics are lower than those of their ITO homolog due to a higher density and a lower porosity and consequently to a higher mobility. The lowest resistivity (1.710.sup.3 .Math.cm) was observed for that having the nominal composition [In.sub.2O.sub.3:Sn.sub.0.10]Zn.sub.0.10. To conclude, by using the simple sintering of a lightly pressed mixed ITZO powder, a highly dense and conductive ceramic has successfully been prepared, which is suitable for sputtering. It is recalled that the nominal composition [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10 corresponds to the initial powder mixture:
[(In.sub.2O.sub.3O.sub.3).sub.0.95+(SnO.sub.2).sub.0.1].sub.0.95+(ZnO).sub.0.10
(129) This mixture will be used for preparing a ceramic target which is suitable for depositing a thin film via the sputtering technique.
(130) Thin filmsITZO thin films were deposited using the RF sputtering deposition technique. The sputtering machine (Leybold L560) which was previously used for the deposition of ATO and AZTO thin films was used. This work was carried out in collaboration with J. P. Manaud, from the Centre de Ressources Couches Minces de l'ICMCB (Thin Film Resource Center of ICMCB).
(131) Target preparationAn ITZO ceramic target having a diameter of 50 mm was prepared using the optimized ceramic composition. A batch of 50 g of appropriate amounts of In.sub.2O.sub.3, SnO.sub.2 and ZnO powders were milled using balls for 3 hours in an agate bowl containing agate balls and ethanol. Then, after evaporation of the ethanol, the powder was ground in an agate mortar, and then a cylindrical crucible made of alumina having a diameter of 82.56 mm was filled with the powder (see
(132) The powder mixture in the crucible was lightly pressed (by hand) and then sintered at 1300 C. in air for 12 hours. The ITZO ceramic target having a relative density of 0.92 was then obtained. It emerged that the diameter of the target was 52.5 mm after heat treatment which corresponds to 36.4% shrinkage in the diameter due to the densification process. The target having a final diameter of 50 mm was obtained after polishing.
(133) Optimized sputtering parametersBy using the target already prepared, the ITZO thin films were deposited by RF magnetron sputtering in a sputtering chamber equipped with a turbopump (Leybold L560). After depositing the films, the pressure of residual gases was around 5-910.sup.5 Pa. Before each deposition process a pre-sputtering was carried out systematically for 20 min for the purpose of cleaning the surface of the target. The deposition of the films was carried out at ambient temperature without heating the substrate. They were deposited onto substrates made of glass or made of PET (polyethylene terephthalate), over various deposition times. The RF power density for the deposition was varied from 0.5 to 2.5 W/cm.sup.2. This was carried out at a total gas pressure set at 1 Pa under a mixture of argon (99.999%) and oxygen (99.99%), with a partial pressure of oxygen varying between 0 and 2%.
(134) For the purpose of having films with good optoelectronic properties, the sputtering conditions were first optimized. After that, the influence was studied of the power density (P) and the partial pressure of oxygen (p.sub.O2) on the deposition rate, on the optical and electrical properties of the ITZO thin films [43]. In order to have sputtered particles of low energy (which are suitable for a PET substrate), the distance from the target to the substrate (d.sub.ts) was set at 7 cm, which is the maximum distance that makes it possible to maintain the plasma in the sputtering chamber having a low sputtering power density of 0.5 W/cm.sup.2.
(135) Influence of the sputtering parameters on the deposition rateThe determination of the deposition rate was carried out, as is customary, by depositing a film for a certain period of time onto the glass substrate and then by measuring the thickness of the film using a profilometer. According to
(136) As expected, unlike the power density, the deposition rate decreases with an increase in the amount of oxygen in the plasma (
(137) Influence of the sputtering parameters on the optical properties
(138) The optical energy of the forbidden band (E.sub.g) was determined by extrapolating the linear portion of the plotted curve (
(139) The influence of the partial pressure of oxygen (p.sub.O2) on the transmission was studied for thin films prepared under the lowest power density (0.5 W/cm.sup.2) which give the best transparency in the visible range. A low visible transparency (77%) was observed for the film deposited at p.sub.O2=0.1% (insert from
(140) E.sub.g decreases from around 3.89 to 3.66 eV when the partial pressure of oxygen in the sputtering chamber changes from 0.1 to 1% (
(141) Influence of the sputtering parameters on the electrical propertiesTable V indicates the change in the carrier concentration, the mobility and the resistivity as a function of the power density. The resistivity of ITZO thin films increases gradually from 4.610.sup.4 .Math.cm to 5.110.sup.3 .Math.cm, when the power density increases from 0.5 W/cm.sup.2 to 1.5 W/cm.sup.2 (
(142) Table V presents: Carrier concentration (determined from Hall measurements), mobility and resistivity for various ITZO thin films deposited at various power densities.
(143) TABLE-US-00005 TABLE V Power Mobility Resistivity density Carrier concentration (cm.sup.2/V .Math. s) (10.sup.3 .Math. cm) (W/cm.sup.2) (10.sup.20 e.sup. cm.sup.3) 5% 5% 5% 0.5 5.54 24.1 0.46 1 2.82 29.1 0.76 1.5 0.331 36.8 5.1 2 0.67 21.6 4.2 2.5 2.11 18.7 1.6
(144) The change in the resistivity was monitored as a function of the partial pressure of oxygen for ITZO thin films deposited at a power density of 0.5 W/cm.sup.2.
(145) The values of the carrier concentration, of the mobility and of the resistivity for various partial pressures of oxygen are collated in Table VI. The lowest resistivity (4.410.sup.4 .Math.cm) is obtained for the films deposited at p.sub.O2=0.2% (
(146) Table VI presents: Carrier concentration determined by Hall measurement, calculated mobility, and resistivity measured for various ITZO thin films deposited at various partial pressures of oxygen.
(147) TABLE-US-00006 TABLE VI Mobility Partial pressure Carrier concentration (cm.sup.2/V .Math. s) Resistivity of oxygen (%) (10.sup.20 e.sup. cm.sup.3) 5% 5% ( .Math. cm) 5% 0.1 5.36 17.8 6.55 10.sup.4 0.2 4.89 28.8 4.44 10.sup.4 0.3 3.41 23.3 7.85 10.sup.4 1 0.923 0.40 1.70 10.sup.1
(148) Influence of the sputtering parameters on the structure and morphologyThe change in the X-ray diffractograms (
In.sub.2-x-y-.sup.3+Sn.sub.x.sup.4+Zn.sub.y+.sup.2+O.sub.3(/2).sup.2.sub./2[(xy+2z)e.sub.C.B.sup.](d)
(149) In order to have a high carrier concentration, it is better to preferably have Zn.sup.2+ in an interstitial position (z).
(150) In a crystalline structure, Zn.sup.2+ will preferably occupy the substitutional position for the purpose of minimizing the energy and of reducing the steric effects while the creation of interstitials will be favored in the case of a disordered (amorphous) structure. Furthermore, Park et al. [24] have shown that the existence of Zn in an interstitial position in the structure of In.sub.2O.sub.3 leads to an increase in the cell parameter. When the positions of the peaks of ITO thin films obtained with such characteristics are compared, a change toward lower angles is always observed, which indicates an increase in the cell parameter. This change is minimized in the case of the better crystallized compound (corresponding to the power of 1.5 W/cm.sup.2). Thus, it is possible to expect to have a higher proportion of interstitials in the disordered structure, resulting from a higher carrier concentration. The SEM (scanning electron microscope) photographs prepared at various power densities are presented in
(151) The surface roughness was also studied using the atomic force microscopy (AFM) technique (
(152) Table VII presents: Change in the average surface roughness with power density.
(153) TABLE-US-00007 TABLE VII Power density (W/cm.sup.2) R.sub.a (nm) 0.5 0.24 1.5 0.87 2.5 3.42
(154) Optimized sputtering parametersThe preceding results that relate to the influence of the sputtering parameters on the thin films, make it possible to conclude that: i) the lowest resistivity, in addition to the highest transparency, were observed for the thin film deposited at the power density (P) of 0.5 W/cm.sup.2; ii) the highest transparency was observed for films deposited at p.sub.O2>0.1%; and iii) the lowest resistivity was obtained for films deposited at p.sub.O2=0.2%.
(155) Hence, the optimized sputtering conditions for ITZO thin films resulting in high transparency and the lowest resistivity are the following:
P=0.5W/cm.sup.2, p.sub.tot=1Pa, p.sub.O2=0.2% and d.sub.ts=7cm
(156) Specifically, these sputtering parameters result in films with a structure that is amorphous to X-rays where Zn.sup.2+ preferably occupies the interstitial position improving, in this way, the carrier concentration [43].
(157) ITZO thin films prepared according to the optimal conditionsThe optimized sputtering conditions were used to deposit ITZO thin films onto substrates made of glass (ITZO-glass) or of plastic (ITZO-PET). Next, the composition, structure, roughness and also the optical and electrical properties of the thin films were thoroughly studied.
(158) CompositionAs is customary, the EPMA (Electron Probe Microanalysis) technique was used to determine the composition of the thin films. The composition of the ITZO thin films deposited under optimized sputtering conditions onto glass or plastic substrates and also the composition of the ceramic target for deposition are indicated in Table VIII. The final composition of the films deposited on the glass or plastic substrates is the same.
(159) However, there is a small loss of Sn and Zn compared to the composition of the ceramic target. This difference may be due to the different sputtering yields of the various species present in the target [43].
(160) Table VIII presents the compositions of the ITZO ceramic and thin film determined by the EPMA technique.
(161) TABLE-US-00008 TABLE VIII Identification of the ceramic ITZO and of the thin film Final composition of the In.sub.1.821Sn.sub.0.090Zn.sub.0.098O.sub.3 ceramic 0.005 Composition of the thin film In.sub.1.838Sn.sub.0.084Zn.sub.0.078O.sub.3 (on glass) 0.005 Composition of the thin film In.sub.1.839Sn.sub.0.082Zn.sub.0.079O.sub.3 (on plastic) 0.005
(162) Morphology and structure
(163) The two films, ITZO-glass and ITZO-PET, display a structure that is amorphous to X-rays (
(164) Optical propertiesThe change in the transmittance with the wavelength for the ITZO films deposited on glass and plastic substrates is shown respectively in
(165) For ITZO-glass (insert from
(166) A high IR reflectivity was obtained for both the ITZO-glass and ITZO-PET thin films (
(167)
The ITZO films always have a higher reflectivity in the IR range than commercial ITO films due to the high values of the carrier mobility (Table IX).
(168) Electrical propertiesAlthough in the presence of a higher carrier concentration for ITZO-PET films than for ITZO-glass films that have the same thickness (Table IX), the resistivity of the ITZO-PET films is barely higher (Table IX and
(169) Table IX presents: Carrier concentration, mobility and resistivity for various thicknesses of ITZO-PET and ITZO-glass. The data for commercial ITO thin films (ITO-PET and ITO-glass) are given by way of comparison.
(170) TABLE-US-00009 TABLE IX Carrier concentration Mobility Resistivity Sheet Thickness (10.sup.20 e.sup. cm.sup.3) (cm.sup.2V .Math. s) (10.sup.4 .Math. cm) resistance Sample (nm) 20 0.05 0.05 0.05 (/) 5% ITZO-PET 260 5.30 25.2 4.68 18.1 ITZO-PET 480 5.41 16.2 5.62 14.0 ITZO-Glass 260 4.89 28.8 4.44 17.2 ITZO-Glass 500 5.04 26.5 4.67 9.1 Commercial ITO-PET 200 5.00 10.7 1.17 58.5 ITO-Glass 100 8.43 18.6 3.99 39.9
(171) Table IX also shows that the resistivity for the ITZO films (260 nm) deposited on unheated glass and/or on plastic substrates is close to that observed for commercial ITO thin films that have been deposited at 200 C. on a glass substrate (ITO-glass), this temperature resulting in well crystallized films. The carrier concentration of the ITZO films is lower than that of the ITO-glass films, but they have a higher mobility (Table IX). More advantageously, the ITZO-PET thin films bring out lower resistivities, and consequently lower sheet resistances, than the commercial ITO-PET thin films that have been deposited in a similar manner at ambient temperature. This may be explained by the higher carrier concentration and mainly by the higher carrier mobility that take place in the ITZO-PET films [43].
(172) ConclusionsITZO thin films, deposited from the optimized ITZO ceramic target, were prepared by RF magnetron sputtering. More advantageously, the ITZO films deposited on PET polymer substrates have greater optoelectronic performances than their commercial ITO homologs. Their amorphous nature permits the fact that Zn.sup.2+ is in interstitial positions that lead to an increase in the carrier concentration, and consequently in the conductivity. In crystalline ITZO films, Zn.sup.2+ is in substitutional positions leading to a reduction in the conductivity. This behavior is different from that observed for ITO, for which the conductivity increases when the crystallinity increases. This study shows the advantage of such thin films on plastic substrates.
(173) The optimized sputtering parameters in order to have high optoelectronic performances are the following:
P=0.5W/cm.sup.2, p.sub.O2=0.2%, P.sub.tot=1Pa, and d.sub.ts=7cm
(174) The amorphous films obtained on both the glass and plastic substrates have the same chemical composition and they are in good agreement with the composition of the target. A slight loss of Sn and Zn has also been observed in the films due to the different sputtering yields of the various elements present in the target. The morphology of the thin films is dense with a very smooth surface.
(175) In terms of optical properties, the ITZO thin films have brought out a high visible transparency. It is 86% for ITZO-glass and 80% for ITZO-PET; these values are close to the transmittance value observed for the commercial ITO film. Due to their high carrier mobilities, the resistivity of the ITZO films, deposited on a glass or plastic substrate, is as low as that observed for commercial ITO-glass. The lowest resistivity value reached 4.410.sup.4 .Math.cm for ITZO-glass, whereas it reached 4.710.sup.4 .Math.cm for ITZO-PET. Advantageously, the ITZO thin films have lower resistivities, and consequently lower sheet resistances, than the commercial ITO-PET thin films, due to the higher carrier concentration and mainly to the higher carrier mobility of the ITZO-PET films. Furthermore, the IR reflectivity of the ITZO films is always higher than that observed for the commercial ITO films due to the higher carrier mobility that occurs in ITZO.
(176) Since the ITZO thin films deposited on plastic substrates (ITZO-PET) have higher performances than their commercial ITO homologs (ITO-PET), these are good candidates for polymer-based optoelectronic devices, such as flexible ECDs (Electrochromic devices), OLEDs, flexible solar cells, etc.
(177) Particular advantages of the process and of the ceramics thus obtainedThe process makes it possible to obtain ceramics with high densities (greater than or equal to 90%, and preferably of around 91%). They can, therefore, advantageously be used on an industrial scale (and a fortiori in the laboratory) as a target for DC sputtering (in the case of conductive ceramics such as ITZO of In.sub.1.805Sn.sub.0.095Zn.sub.0.10O.sub.3 composition) or RF sputtering (in the case of insulating ceramics such as Zn-doped Li.sub.4Ti.sub.5O.sub.12 of Li.sub.4Ti.sub.4.70Zn.sub.0.30O.sub.11.7 composition). The process has one very important advantage in the sense that the steps of hot-pressing or of pressing at ambient temperature (of the order of one tonne per cm.sup.2) used to date for producing industrial or laboratory ceramics are avoided here. Therefore with this process there is a significant gain in terms of preparation time (divided by at least a factor of 3) and therefore of personnel cost.
(178) With this process there is also a significant financial gain in terms of equipment cost, since here there is no longer any need for the industrial hot-pressing equipment which is expensive and difficult to maintain; the same applies for the ambient-temperature pressing equipment. Furthermore, although the ceramics proposed here are slightly less dense than the industrial ceramics (the latter often being very close to the theoretical density, that is to say greater than or equal to 95%), their density, of around 90%, or even slightly higher, is amply sufficient in order to be able to be used as targets for sputtering, as emphasized above.
(179) Furthermore, in the case of the conductive ceramics of the invention, the electrical conductivity advantageously remains very high, despite a slightly lower density, that has also been reported.
(180) It has furthermore been demonstrated that the conductivity of the aforementioned ITZO ceramics is slightly greater than those of the ITO ceramics commonly used.
(181) As regards the electrical resistivity of the ITZO ceramics, it has surprisingly been discovered that these ceramics possess low electrical resistivities in comparison with that of ITO. It gradually decreases with the Zn content and reaches its minimum for ceramics that nominally contain 10 mol % of Zn. This is partially due to the difference in the density of the ceramic. The lowest resistivity was observed for the ceramic having the highest density. The charge concentration decreases with the Zn content in the ceramic. This can be explained by the increase in the substitution of In.sup.3+ by Zn.sup.2+ in the In.sub.2O.sub.3 structure.
(182) As regards the charge mobility, it has surprisingly been discovered that a significant increase in the mobility is associated with a high increase in grain percolation. A low mobility is obtained for ceramics having a semiconductor behavior, whereas a high mobility is observed for ceramics having a metallic behavior.
(183) The highest density (around 93% of the theoretical density) is observed for ceramics having the nominal composition [In.sub.2O.sub.3:Sn.sub.0.10]:Zn.sub.0.10.
(184) Surprisingly, the ITZO ceramic resistivities are lower than those of ITO, due to a higher density and a lower porosity and due to a greater mobility.
(185) The X-ray diffraction pattern for the ITZO powders sintered (annealed according to GDT) at 1300 C. (
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(187) Although the present invention has been described using specific implementations, it is understood that several variations and modifications may be grafted to said implementations, and the present invention aims to cover such modifications, uses or adaptations of the present invention that in general follow the principles of the invention and that include any variation of the present description which will become known or conventional in the field of activity in which the present invention lies.