JPH06191845A - Electrically conductive oxide - Google Patents

Abstract

PURPOSE:To obtain an electrically conductive oxide having electric conductivity and transparency in a visible ray region by making a solid soln. having a spinel crystalline structure consisting of oxides consisting a specified mol ratio of MgO to In2O3 and a specified metal oxide. CONSTITUTION:This electrically conductive oxide is obtd. by producing such a solid soln. that has a spinel crystalline structure containing oxide (A) and oxide (B). The oxide (A) contains MgO and In2O3 by the mol ratio of (1+X): 1 (X is 0-0.1) and the oxide (B) is selected from ZrO2, HfO2, and GeO2. The electrically conductive oxide has higher electric conductivity than ITO (tin-doped In203) and almost equal or higher transparency in the visible ray region than ITO so that a transparent electrode having about 50OMEGA/cm<2> sheet resistance and >=90% transmittance in a visible ray region can be produced. This electrode can be used for a display of a large panel area requiring fine electode patterns.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrically conductive oxide having electrical conductivity and transparency in the visible region and used as a transparent electrode for displays, solar cells and the like.

[0002]

2. Description of the Related Art So-called transparent conductive materials, which are electrically conductive and transparent in the visible light region, are used as various displays such as liquid crystal displays (LCD) and electroluminescence displays (ELD), or as transparent electrodes for solar cells. Further, it is used as an anti-fog heater for freezing showcases, a heat ray reflection film for window glass of buildings and automobiles, and a coating for preventing static electricity and shielding electromagnetic waves of transparent materials.

A metal oxide semiconductor is generally used as a transparent conductive material of this type, and tin oxide (SnO ₂ ),
Indium oxide (ITO) doped with tin, CdSn
Various proposals have been made for ₂ O ₄ , ZnCd ₂ O _4, and among these, ITO is widely used. The ITO film has a relatively high electric conductivity, and at the same time, has an optical absorption edge in the vicinity of 350 nm and is transparent over almost the entire visible region except the region on the shortest wavelength side.

[0004]

However, as the area of a liquid crystal display (LCD) and the size of an electrode pattern are made finer, higher electrical conductivity and transparency are required. Then, there are emerging uses that cannot be adequately addressed. Therefore, the emergence of a material that has higher electrical conductivity and transparency and replaces ITO is desired. As one of the materials that responds to this, MgIn ₂ O _4-x having a spinel type crystal structure was proposed (Spring ₁₉₉₂ ).
The 39th Joint Lecture on Applied Physics, Lecture number 30p
-C-1 and 30p-C-2). The absorption edge of this material is I
It is around 300 nm, which is a shorter wavelength than TO, and higher visible transparency than that of ITO is confirmed. On the other hand, the electric conductivity is in the range of 10 ⁰ / Ω · cm, but this is because the concentration of conduction electrons as carriers is as low as 5 × 10 ¹⁷ / cm ^3, and improvement thereof is desired. It was

There are two possible methods for increasing the carrier concentration: introduction of oxygen vacancies and cation substitution.
The method is generally performed by subjecting the sample to a high temperature heat treatment in a hydrogen atmosphere. In the case of ITO, after forming a film by the sputtering method, the temperature is 300 ° C. in a hydrogen atmosphere.
There are methods such as heat treatment in the vicinity, or sputtering in a hydrogen atmosphere. On the other hand, the method of is Mg ²⁺
Alternatively, by using a metal ion that can have an oxidation number higher than that of In ³⁺ ions, that is, an oxidation number of 3 or 4 or more, Mg
^It replaces ²⁺ and / or In ³⁺ .

Of these, the method of introducing oxygen vacancies by heat treatment in hydrogen requires the use of explosive hydrogen gas, which increases the number of manufacturing steps, increases the number of factors to be controlled, and complicates the manufacturing steps. There are drawbacks such as. On the other hand, the cation substitution method can control the carrier concentration by the amount of added cations, which is advantageous in stabilizing the electric conduction characteristics.
There are no process drawbacks like hydrotreating. However, the added cation must be capable of substituting the cation site of the MgIn ₂ O ₄ spinel type crystal lattice, and its oxide must be a solid solution in the spinel. Further, it is a matter of course that it should not impair the visible transparency of MgIn ₂ O ₄ . There has been a demand for such a cation, that is, an appropriate cation capable of increasing the carrier concentration while maintaining visible transparency and improving the electrical conductivity.

[0007]

In order to solve such a problem, the present inventors searched for a metal ion that effectively acts in the cation substitution method, and in particular, Zr ⁺⁴ ,
The present inventors have found that three kinds of cations, Hf ⁺⁴ and Ge ⁺⁴ , are effective in improving the electric conductivity and do not reduce the visible transparency, and have reached the present invention.

That is, according to the present invention, the molar ratio of magnesium oxide and indium oxide is (1 + X): 1 (however,
X is 0 or more and 0.1 or less), and an oxide (A) containing
High electric conductivity and transparency in the visible region, which is characterized by being a solid solution having a spinel type crystal structure containing one or more oxides (B) selected from zirconium oxide, hafnium oxide and germanium oxide. It is an electrically conductive oxide.

MgIn ₂ O _4, which is the base material of the solid solution constituting the electrically conductive oxide of the present invention, is a composite oxide of magnesium oxide (MgO) and indium oxide (In ₂ O ₃ ) and has a spinel structure. Is a substance having a crystal structure known as. The spinel structure is a well-known crystal structure, and will be described in detail in, for example, “Crystal Chemistry of Illustrated Fine Ceramics” (FS Galassau, translated by Seiji Kato, Keizo Uematsu, published by Agne Technical Center (1984)). It has the following structure as described. That is, the chemical formula of a substance having a spinel structure is generally AB ₂ O.
It is represented by ₄ (A and B represent two kinds of cations). The unit cell of spinel is a cubic type, and 32 oxygen ions are contained in one unit cell, and the oxygen ions are closest packed in the <111> direction. The cubic closest packing of oxygen ions creates 64 tetrahedral coordination sites and 32 octahedral coordination sites in one unit lattice, of which 1/8 of the tetrahedral coordination sites One half of the octahedral coordination site is occupied by cations, and one unit cell has eight AB ₂ O.
Includes ₄ . The spinel structure is further divided into two types. A ion occupies the tetrahedral coordination site, and B ion occupies the octahedral coordination site. A positive spinel, half of the B ion is the tetrahedral coordination site, and the other half is the B ion. And the case where A ion occupies the octahedral coordination site is called reverse spinel. Mg
In ₂ O ₄ is an inverse spinel, and half of In ³⁺ ions are tetrahedrally coordinated and the other half, In ³⁺ and Mg ^2+, are octahedrally coordinated. The reason for this is that In ³⁺ is an ion that has a very small stabilization energy by coordinating to the octahedral coordination site of the spinel crystal lattice, and is an ion that does not easily enter the octahedral coordination site.

For the synthesis of MgIn ₂ O ₄ spinel, many ordinary synthesis methods of oxide ceramics can be used. As a starting material, oxides such as MgO and In ₂ O ₃ , carbonates such as MgCO ₃ and In that change into oxides at high temperatures, and In
Examples include various salts such as hydroxides such as (OH) ₃ . These starting materials are mixed in a predetermined mixing ratio,
It may be heat-treated in the air. Since the temperature and the holding time of the heat treatment change depending on the particle size of the raw material, the presence or absence of calcination, etc. and cannot be uniquely determined, only one example is given. Basic magnesium carbonate powder having an average particle size of 1 μm or less and an average particle size of 1
1 μm of indium oxide powder was used as the raw material in a mortar.
If the mixed powder that has been mixed for a period of time is calcinated in air at 900 ° C for 5 hours, then crushed in a mortar and further mold-molded at a pressure of 60 MPa, the reaction is sufficiently performed by heating at 1400 ° C for 2 hours. It progresses and becomes a spinel single phase. When using other raw materials, the optimum heat treatment conditions may be experimentally obtained by a usual method.

The magnesium oxide / indium oxide molar ratio is important in obtaining the spinel single phase. The results of the present inventors' studies show that for MgIn ₂ O ₄ spinel, I
n ₂ O ₃ hardly dissolves in solid solution, and I
Even if the excess amount of n ₂ O ₃ is only 1%, MgIn ₂
A mixture of O ₄ spinel and In ₂ O ₃ is formed. Meanwhile, M
It has been found that gO forms a solid solution with spinel up to 10 mol%. Since Mg ²⁺ can enter both the tetrahedral coordination site and the octahedral coordination site in the spinel structure, excess Mg ²⁺ becomes the tetrahedral coordination site in the reverse spinel structure MgIn ₂ O ₄ . By entering, it seems that the structure is stabilized. In the present invention, in the oxide (A) containing magnesium oxide and indium oxide, the molar ratio of the former to the latter is (1 + X): 1 (X = 0 to 0).
This is the reason why it is limited to 0.1).

The oxide (B) which forms a solid solution with the mother spinel and is effective for the generation of carrier electrons and does not impair the visible transparency is at least one of ZrO ₂ , HfO ₂ and GeO _2. Limited.

Focusing on the formation of a solid solution, a cation capable of substituting either tetrahedral coordination or octahedral coordination of the spinel crystal lattice has an ionic radius of about 0.5 to 1.0. Expected to be in the Angstrom range. Further, for carrier injection, cations having an oxidation number of 4 or more must be used. If substituted selectively Mg ²⁺ trivalent cations, career is injected, the result of the present inventors have studied the substitution of trivalent ions such as Ga is Mg ²⁺ rather In ³⁺ It was found that there was no effect on the improvement of electric conductivity as a result. That is +4 for carrier injection
A cation having an oxidation number higher than the valence is required. Many cations that simultaneously satisfy this condition and the above-described ionic radius condition are conceivable. Among them, the visible transparency of MgIn ₂ O ₄ is not impaired, and it is effective for carrier injection and enhances the electrical conductivity. Zr ⁺⁴ , Hf ⁺⁴ , Ge having effect
There are only 3 types of ⁺⁴ . Other than this, slightly Nb ⁺⁵ , T
It was found that only a ⁺⁵ and Sn ⁺⁴ produced a visible transparent spinel, but the carrier concentration was rather lowered, which was harmful for increasing the electric conductivity. Zr ⁺⁴ , Hf ⁺⁴ , Ge
^The reason why carrier generation behavior is completely different between ⁺⁴ and Nb ⁺⁵ , Ta ⁺⁵ , and Sn ⁺⁴ has not been clarified yet, but the latter that lowers carrier concentration is The following ions can take the oxidation states of Nb ⁺⁴ , Ta ⁺⁴ , Sn ^+2, etc., while the former ion is stable only at +4 valence, so the latter ion has an electron in the spinel crystal lattice. It is presumed that it has been captured and is in a lower oxidation state. That is, Nb ⁺⁵ , Ta ⁺⁵ ,
It is possible that Sn ⁺⁴ acts as a carrier trap.

In order to achieve both practically preferable conductivity and transparency, the content of the oxide (B) selected from ZrO ₂ , HfO ₂ and GeO ₂ is preferably in the range of 1 to 20 mol%. If it is less than 1 mol%, it is difficult to improve the electric conductivity. When it exceeds 20 mol%, the carrier concentration becomes 1
It is thought that plasma resonance of conduction electrons occurs in the visible light region, reaching the level of 0 ²¹ / cm ^3, but transparency is likely to be lost, and mobility may decrease and electrical conductivity may not increase so much. Because. The content of oxide (B) is 5 to 1
It is particularly preferably 0 mol%. Zr ⁺⁴ , Hf
The optical absorption edge of the spinel solid solution was almost unchanged from the case of MgIn ₂ O ₄ even by the addition of ⁺⁴ and Ge ^+4.
It is at 0 nm.

Starting materials for the spinel solid solution of the present invention include:
Pre-synthesized Mg _{1 + x} In ₂ O _{4+ x} , ZrO ₂ and H
fO ₂ , GeO ₂ or a mixture of various salts such as carbonates which change into these oxides at high temperature in air at a predetermined mixing ratio, or Mg, In, and Zr, Hf, Ge
It is sufficient to use a mixture of the respective oxides or salts described above in a predetermined mixing ratio. These can be heated in air to form a spinel solid solution by a solid phase reaction. The treatment temperature is preferably 1200 ° C. or higher and 1600 ° C. or lower. 12
This is because the reaction rate is slow below 00 ° C, and the In content from the spinel begins to evaporate above 1600 ° C.

Next, as a method for practically using the spinel solid solution of the present invention as a film, a method adopted for an ITO film or the like can be used. For example, in order to form a spinel solid solution film by a sputtering method, (i) a target composed of a sintered body of the spinel solid solution synthesized by the above method or a powder compact thereof is used, (ii) Mg
It is possible to use each target of O, In ₂ O ₃ , and MO ₂ (M = Zr, Hf, Ge), or (iii) to use oxygen plasma or the like together with each metal or its alloy target. . It is also possible to form a spinel solid solution film by forming a complex oxide by coating with a spray method or a sol-gel method using a compound that produces an oxide of these metals by thermal decomposition and heating. In this way, a solid solution film can be formed on a transparent substrate such as glass to form a transparent electrode.

[0017]

EXAMPLES The present invention will be described in more detail with reference to examples. Examples 1 to 3 and Comparative Examples 1 to 17 Indium oxide powder having an average particle size of 1 μm and a purity of 4N and basic reagent grade basic magnesium carbonate powder were mixed at a molar ratio of In / Mg = 2, and alumina was mixed. The powder was placed in a crucible, heated in the air at 900 ° C. for 5 hours to be calcined, and then pulverized to obtain a starting material. This powder is X
As a result of analysis by line diffraction, it was a mixture of a component very close to MgIn ₂ O ₄ spinel and a small amount of In ₂ O ₃ . It was confirmed that the powder obtained by heating this powder in air at 1400 ° C for 2 hours showed only X-ray diffraction lines corresponding to MgIn ₂ O ₄ and was a calcined powder raw material corresponding to the spinel of stoichiometric composition. Was done.

After adding the oxides shown in Table 1 to the calcined powder so that the amount of the constituent metal M added was M / Mg = 5/95 in a molar ratio, and dry pulverizing and mixing in a mortar for 1 hour. , About 7
A disc having a diameter of 20 mm and a thickness of about 2 mm was die-molded at a pressure of 0 MPa, and this was sintered in air at 1400 ° C. for 2 hours to obtain a sintered body. For comparison, a sintered body was also prepared in which only MgIn ₂ O ₄ was treated under the same conditions (Comparative Example 1).
The relative density of the cooled and recovered sintered body was between 80 and 85%. This sample was diamond ground to a diameter of 15 m
It processed into the disk sample of m and thickness 0.6mm. The results of examining the color tone of these samples are shown in Table 1. It is the white or white-green sample that can be expected to have transparency when a dense film is formed. Besides, ZrO ₂ , HfO ₂ , GeO ₂ , Nb ₂ O ₅ , Ta ₂
There were only 6 types, O ₅ and SnO ₂ . So these 6
The electrical conductivity and the carrier concentration of each sample were determined by the van der Pauw method, and the carrier type was determined from the Hall measurement, and the carrier concentration of each sample was determined. It was confirmed that the carriers were electrons in all of the six samples, and various parameters relating to the electric conduction characteristics were as shown in Table 2. Comparing the MgIn ₂ O ₄ single-phase sample (Comparative Example 1) as a reference, in the samples to which ZrO ₂ , HfO ₂ , and GeO ₂ of Examples 1 to 3 are added, the carrier mobility does not change, An increase in carrier concentration and an increase in electric conductivity reflecting this increase were observed, and Comparative Example 1
While the electric conductivity of the above did not reach 100 Ω ⁻¹ cm ⁻¹ , the electric conductivity of all of them showed significantly higher than 100 Ω ⁻¹ cm ⁻¹ . On the other hand, Nb ₂ O ₅ and Ta ₂ O of Comparative Examples 2 to 4
_5, in the sample of SnO ₂ added either electric conductivity further lower than the Comparative Example 1, SnO ₂ is about half, Nb ₂ O
₅ and Ta ₂ O ₅ were lower than two digits. Note that X
In the line diffraction, only the diffraction line of spinel was detected in all the samples, and it was confirmed that the added oxide was in solid solution in the spinel.

[0019]

[Table 1]

[0020]

[Table 2]

Examples 4 to 12 By the same method as in Examples 1 to 3, samples were prepared in which only the amount of oxide added to MgIn ₂ O ₄ spinel was changed, and the color tone and electric conductivity were examined. The results are shown in Table 3. According to Table 3, it was confirmed that the electric conductivity increased with the addition amount in the addition amount range of 1 to 20 mol%.

[0022]

[Table 3]

Examples 13 to 15 and Comparative Example 18 Examples 1 to 3 were carried out except that the calcination powder molding die had a large inner diameter of 60 mm, and after the die molding, CIP molding was further carried out at a hydrostatic pressure of 200 MPa. Under the same conditions as above, a sputtering target of a sintered body having a diameter of 50 mm was prepared for each spinel solid solution. With this target,
A film having a thickness of about 1000 Å was formed on a glass substrate by a DC magnetron sputtering method to prepare a transparent electrode. The sputtering conditions were a sputtering gas pressure of 0.4 MPa and a substrate temperature of 200 ° C. For comparison, an ITO target was used to prepare a sample having an ITO film of the same thickness. The visible light transmittance and the sheet resistance of each sample were measured. The results are shown in Table 4.
In the table, the visible light transmittance is represented by the transmittance at a wavelength of 500 nm. In each sample, I
A transparent film having a visible transmittance equal to or higher than that of TO and a surface resistance much lower than that of ITO and having electrical conductivity exceeding that of ITO was formed. From the results of thin film X-ray diffraction, all the films of the examples were crystalline and showed a diffraction pattern close to that of MgIn ₂ O ₄ spinel, which confirmed that a film of spinel solid solution was formed.

[0024]

[Table 4]

[0025]

EFFECT OF THE INVENTION The electrically conductive oxide of the present invention has electrical conductivity exceeding ITO and visible transparency equal to or higher than that of ITO. Therefore, for example, it is possible to manufacture a transparent electrode having a low sheet resistance value of about 50 Ω / cm ² and a transmittance of 90% or more in the visible region, and having a large panel area,
In addition, it can be applied to displays and the like that require fine electrode patterns.

Claims (3)

[Claims] 1. A molar ratio of magnesium oxide to indium oxide of (1 + X): 1 (where X is 0 or more and 0.
A solid solution having a spinel type crystal structure, containing an oxide (A) containing 1 or less) and one or more oxides (B) selected from zirconium oxide, hafnium oxide and germanium oxide. And an electrically conductive oxide. 2. The electrically conductive oxide according to claim 1, wherein the content of the oxide (B) is 1 to 20 mol% based on all oxides. 3. A transparent electrode, characterized in that the electrically conductive oxide according to claim 1 or 2 is used as a film forming material and is formed on a transparent substrate.