JPH0570286A - Production of transparent zinc oxide - Google Patents

Abstract

PURPOSE:To produce the ZnO crystal having a large thickness and high transparency by rotating a substrate at the time of sublimating ZnO in the high-temp. part in a synthesizing pipe of a reducing atmosphere and depositing transparent ZnO on the substrate in a low-temp. part. CONSTITUTION:A sintered body B of ZnO powder is placed in the synthesizing pipe 1 made of mullite and is heated with a heater 9. A reducing gaseous mixture composed of gaseous CO and gaseous H2 is supplied into the synthesizing tube 1 from a gas introducing pipe 10 on the upstream side thereof and is passed to the downstream side and is discharged from a discharge port 11. A base body 4 made of a single crystal or polycrystal having crystal axes of pellet- shaped ZnO, alumina, sapphire, silicon, quartz glass, etc., is mounted to a revolving shaft 5 on the downstream side of the ZnO sintered body B and the base body 4 is rotated by a motor 7 and a reduction gear 8. The ZnO sintered body B of the high temp. sublimates by reacting with the reducing gas and comes into contact with the front surface of the base body 4 of the low temp. rotating on the downstream side. The transparent ZnO crystal body having the C-axis directing to the high-temp. part is thus deposited and formed at a high speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing transparent zinc oxide which does not use an oxidizing gas, and more particularly to a method for uniformly producing a thick transparent zinc oxide crystal without disturbing the crystal axis.

[0002]

2. Description of the Related Art Zinc oxide is one of the few substances that has semiconductivity, photoconductivity, or piezoelectricity, and is essentially transparent in the visible light region. It is expected to be applied to optoelectronic materials with To be used as these materials, transparency and crystal axis orientation are particularly required.

Transparent zinc oxide is conventionally manufactured by a method of sintering using hot pressing. For example, Japanese Unexamined Patent Publication (Kokai) No. 56-84367 discloses a method in which an oxalate coprecipitate of zinc and gadolinium is pyrolyzed into oxide fine powder, which is pressed and then sintered in air. There is. However, these methods are not sufficient to effectively utilize the performance of zinc oxide, because all of them have one or more additives and do not have crystal axis orientation.

The crystallographically oriented zinc oxide is produced by sputtering on a sapphire substrate or the like. For example, JP-A-54-120286 and JP-A-54
JP-A-162689 discloses a method of heating metallic zinc, oxidizing it in an oxygen atmosphere, and depositing it on a substrate. However, in these methods, the apparatus is complicated and the growth rate is slow, and the current industrialization is
Only a thin film having a thickness of several hundreds μm or less.

On the other hand, the present inventors have previously found a method for producing transparent zinc oxide simply and quickly as a means for solving such a problem, and filed a patent application (Japanese Patent Application No. 2-
81890). In this method, zinc oxide is heated and sublimated in a reducing gas, and the reducing gas is brought into contact with a low-temperature substrate to deposit (deposit) transparent zinc oxide on the substrate.
It is what makes them. According to this method, transparent zinc oxide having crystal axis orientation is obtained, and its growth rate is
It is faster than the growth rate of a zinc oxide film usually obtained by sputtering or CVD, and a thick film of about several mm can be easily obtained in a short time. However, the transparent zinc oxide obtained by this method is often found to have small cracks in the crystal, and in order to use it as a complete crystal, it is necessary to avoid it and cut it, and to obtain the desired size. There was a problem in obtaining the crystals of 1) with good reproducibility.

[0006]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a method for producing transparent zinc oxide using a reducing gas with good reproducibility and producing transparent zinc oxide without cracks.

[0007]

The present invention provides a method for producing transparent zinc oxide in which a reducing gas is used, zinc oxide is sublimated in the reducing gas, and the reducing gas is brought into contact with a low temperature substrate. This is based on the finding that when the substrate is rotated, cracks are less likely to form in the crystal of transparent zinc oxide deposited on the substrate, and a desired size can be produced with good reproducibility. Further, although the transparent zinc oxide is oriented along the c-axis, it was made based on the finding that the orientation can be improved by using the above method.

That is, according to the present invention, zinc oxide is heated in a high-temperature heating region in a reducing gas to be sublimated, and then the reducing gas is brought into contact with a substrate having a temperature lower than that in the high-temperature heating region to be transparent on the substrate. A method for producing transparent zinc oxide, which comprises rotating the substrate in the method for depositing zinc oxide. In the method of the present invention, for example, zinc oxide is heated and sublimated upstream of a reducing gas stream having a temperature difference so that the upstream temperature is high and the downstream temperature is low, and a substrate provided downstream of the reducing gas stream is It can be efficiently carried out by a method of depositing transparent zinc oxide on the substrate while rotating.

The zinc oxide used as a raw material in the present invention may be zinc oxide powder, green compact, sintered body, or the like, but a higher density product can be charged in a fixed volume, so that a large product can be obtained. It is preferable for producing (transparent zinc oxide). That is, a sintered body is suitable. Further, zinc oxide may be used alone, but may contain various doping substances as desired.

Substrates used for depositing transparent zinc oxide in the present invention include zinc oxide, alumina, sapphire,
Any of various sintered bodies such as silicon and quartz glass, single crystals, glass, and materials made of metal may be used.
Among these, in order to produce a uniform and large transparent zinc oxide having no cracks, a single crystal or a polycrystal having an oriented crystal axis is particularly preferable. Like this,
From the viewpoint that it is currently available at low cost or can be easily synthesized, single crystals of zinc oxide, alumina, silicon, quartz, magnesium oxide, strontium titanate, titanium oxide, etc., oriented crystal axes Examples thereof include a polycrystal, a single crystal thin film, and a crystal axis oriented thin film. These substrates should be such that the solid form does not collapse during synthesis. The shape of the substrate is preferably such that the surface on which transparent zinc oxide is deposited and grown, that is, the surface facing the high temperature portion is made smooth, and a surface-treated material can be used if desired.

Compared with the case where a polycrystalline sintered body is used as the substrate, a high-quality oriented transparent zinc oxide having higher orientation and less cracks can be obtained by using the above-mentioned substrate. .. In the present invention, using a jig,
It is preferable to fix the base to the tip of the rotary shaft inserted from the downstream side of the base of the synthetic tube so that the base does not contact the wall of the synthetic tube.
The material of the rotating shaft is not particularly limited, but it needs to be mechanically stable in the use temperature and the reducing atmosphere. The rotating shaft rotates by connecting to a speed reducer connected to a low speed motor outside the synthesis tube. The rotation speed of the rotating shaft is
It is not particularly limited, but if it is too fast, vibration of the rotating shaft, shaft wobbling, etc. occur and it becomes difficult to obtain a homogeneous crystal. Therefore, it is preferably 10 rpm or less, more preferably 0.05 to 2
It is good to use rpm.

In the present invention, the temperature of the portion on which the substrate is placed is 1 to 600 ° C., preferably 50 to 400 ° C., as compared with the high temperature heating region.
It is better to lower the temperature by ℃. The distance between the high temperature portion and the low temperature portion on which the substrate is placed can be appropriately set in consideration of the size of the crystal to be generated and the crystal growth rate. Particularly, the temperature in the high temperature heating range is 800 to 1300 ° C., preferably 1000 to
1250 ℃, the temperature of the low temperature part to put the substrate 500 ~ 100
The temperature is 0 ° C, preferably 700 to 950 ° C.

The temperature of the high temperature heating range is 1000 to 125
0 ° C. is suitable because the sublimation rate of the raw material zinc oxide (powder or sintered body) is slower than this temperature, and the sublimation rate of the raw material zinc oxide is remarkably high or expensive when the temperature exceeds this temperature. This is because a heating device is required. Moreover, 700-950 degreeC is suitable as a temperature of a low temperature part,
This is because if the temperature is lower than this temperature, the crystal of zinc oxide grows on the inner wall of the device and the growth rate of the transparent zinc oxide crystal that grows on the substrate becomes slower. If the temperature is exceeded, the transparent zinc oxide crystal grows. This is because the coloration of is increased.

It is preferable to provide a temperature gradient between the high temperature portion and the low temperature portion. The gradient may be irregular, such as providing several heating zones between the hot zone and the cold zone and providing different gradients independently between adjacent zones, but a constant gradient is preferred. The temperature gradient is 1 to 800 ° C./cm, preferably 50 to 250 ° C./cm. When the temperature gradient is smaller than this value, zinc oxide sublimated in the high temperature portion is condensed on the inner wall of the apparatus before reaching the low temperature portion, and it becomes difficult to obtain the target transparent zinc oxide in the low temperature portion. In addition, a gradient larger than this value requires a complicated device for providing a temperature gradient.

In the present invention, it is necessary to make a reducing atmosphere in both the high temperature portion and the low temperature portion, and it is preferable to pass the reducing gas. That is, in Japanese Patent Laid-Open No. 54-162689, an oxidizing gas is required to oxidize sublimated zinc and deposit it as zinc oxide on the substrate, but in the present invention, only a reducing gas is passed. Is good.
Examples of the reducing gas used in the present invention include ordinary reducing gases such as hydrogen and carbon monoxide alone or in a mixture,
Preferably, these are diluted with an inert gas such as nitrogen or argon. The content of the reducing gas in the inert gas is preferably 0.1 to 50%, and the oxygen partial pressure is 10 ⁻³ at
m or less, preferably 10 ^-7 atm or less.

With this reducing gas, the raw material zinc oxide placed in the high temperature portion is reduced and sublimated, transferred to the low temperature portion on which the substrate is placed, and again deposited as zinc oxide on the substrate to obtain transparent zinc oxide. It will grow. The linear velocity of the reducing gas can be arbitrary, but it is 0.1 cm / mi
It is preferably from n to 250 cm / min, more preferably from 1 cm / min to 100 cm / min, but below this, the sublimation of the raw material zinc oxide placed in the high temperature part does not proceed and the transfer becomes slow. As a result, the growth of the transparent zinc oxide becomes slower, and if it exceeds this, sublimation and transfer become violent, and as a result, the transparent zinc oxide is entrained and flows out of the system, and the growth of the transparent zinc oxide becomes slower.

The direction of flow of the reducing gas is not particularly limited. Normally, the reducing gas flows from the high temperature part to the low temperature part.
For example, when zinc oxide is reduced and sublimated in a high temperature part and is transferred to a low temperature part by diffusion, a reducing gas is flowed from the low temperature part to the high temperature part at a low speed, and transparent zinc oxide is deposited on the substrate placed in the low temperature part. It can also be deposited. In the method of the present invention, the time required for producing transparent zinc oxide is not particularly limited. That is, the growth rate of the transparent zinc oxide of the present invention is determined by the temperature of the material zinc oxide and the portion where the substrate is installed, the temperature gradient between them, and the linear flow velocity of the reducing gas. The reaction may be carried out until a product is obtained. The growth rate is sputtering or C
It is faster than the growth rate of the zinc oxide film obtained by VD, and usually 10 μm / hr or more, and in some cases, 100 μm / hr or more.

According to the method of the present invention, a transparent zinc oxide crystal having an orientation in which the c-axis faces the high temperature portion is grown on the substrate placed in the low temperature portion. Further, when the transparent zinc oxide is produced by rotating the substrate as in the present invention, the generation of cracks often seen when the substrate is not rotated is significantly suppressed, and a large crystal of transparent zinc oxide is stably produced. it can. The specific size is as follows.

Thickness in the c-axis direction: 2 mm or more, and further 10 mm or more is possible. Size in the direction perpendicular to the c-axis: Depends on the size of the substrate and the size of the device that accompanies it. By enlarging the substrate, a large area of transparent zinc oxide can be easily obtained. For example, a diameter of 1 mm or more, which is difficult to obtain with a single crystal grown in the c-axis direction, can be obtained, and a diameter of 10 mm or more is also possible.

The zinc oxide crystal thus grown on the substrate may be used together with the substrate used, but it is also possible to cut the substrate into pieces, cut and polish it into any size and shape. is there. The zinc oxide crystal obtained by the method of the present invention is transparent, but is colored slightly brown. It is considered that this is because oxygen deficiency occurs in zinc oxide due to the action of the reducing gas used in the production. Therefore, in order to obtain a transparent body which is colorless or close thereto, it is sufficient to carry out heat treatment in an oxygen atmosphere after manufacturing by the method of the present invention. For example, in an inert gas mixed with air or oxygen, 500 to 1000 ° C., 1
It should be done under the condition of time or more. At this time, in addition to the usual electric furnace, it is possible to perform the treatment under pressure such as HIP.

The transparent zinc oxide obtained by the method of the present invention has the following characteristics. 1) It is transparent in the visible light region. The transparent zinc oxide produced by the method of the present invention was cut at a right angle to the c-axis direction and polished to a thickness of 1 mm, and the average light transmittance in the visible light region was 65% or more. Further, when this is heat-treated in an oxygen atmosphere, 80% or more is obtained. 2) Oriented in the c-axis direction.

When X-ray diffraction is measured by applying X-rays to the c-axis direction of the crystal, 101-plane (d = 2.476A), which has a large peak intensity observed in ordinary zinc oxide (polycrystal), 1
Almost no peak is observed on the 00 plane (d = 2.816A), and the 002 plane (d = 2.602A) or 004 perpendicular to the c-axis.
A peak with a large intensity on the plane (d = 1.301A) is observed, which indicates that the orientation is on the c-axis. 3) Properties of Crystalline The transparent zinc oxide of the present invention is a hexagonal polycrystal of zinc oxide, and its purity depends on the raw material zinc oxide and the mixing of impurities from the apparatus. can get.

Crystals doped with various elements can be obtained by adding a doping substance to the raw material zinc oxide in order to change the electrical characteristics. Such doping can also be performed by post-treating the obtained transparent zinc oxide. For example, in the case of doping with Li, the cut and polished transparent zinc oxide is put into an aqueous solution of lithium carbonate having a predetermined concentration, taken out and dried, and the surface is coated with lithium carbonate, and then at 800 to 1000 ° C. for 24 hours. The heat treatment may be performed in the air to some extent. 4) Electrical characteristics Zinc oxide alone shows semiconductivity, but Al, Ga, Ge, I
Those doped with trivalent or tetravalent elements such as n and Sn, F and the like show conductivity, and those doped with monovalent elements such as Li, Na and K show insulating properties.

[0024]

According to the present invention, it is possible to produce, at a high speed, a relatively thick, relatively large and uniform transparent zinc oxide crystal body with a simple apparatus and a simple operation. Further, according to the method of the present invention, high-purity transparent zinc oxide can be obtained, while a desired element can be doped by selecting a doping agent, and the characteristics can be arbitrarily changed.

Therefore, the transparent zinc oxide produced by the method of the present invention has new applications in the fields of electronics and optoelectronics. For example, if the semiconductivity, conductivity, and transparency of zinc oxide are used, it can be expected to be used for display substrates and transparent electrodes. If it is insulated by doping with Li or the like, it can be widely used as a piezoelectric material. In addition, it is also useful as an optical material that absorbs ultraviolet rays and transmits visible light.

Next, the present invention will be described with reference to examples.

[0027]

【Example】

Example 1 High temperature part 2 of mullite tube (internal diameter 34 mm) of the apparatus shown in FIG.
In addition, about 5 g of zinc oxide was placed in the low temperature part 3 by placing 6 sintered bodies B, each of which was fired in air at 1000 ° C. for 1 hour after rubber pressing.
Rotating shaft 5 with smooth surface A of base body 4 made of pelletized zinc oxide sinter having a diameter of 12 mm and a thickness of 5 mm facing the high temperature part.
Was installed by using a ceramic adhesive. The rotating shaft 5 was connected to a speed reducer 8 connected to a motor 7 through a shaft sealing portion 6 and rotated at a rotation speed of 0.2 rpm. It is heated by a heating device 9 connected to a temperature controller to reach a high temperature state from room temperature for a heating time of 2 hours, after which the temperature distribution in the device is set as shown in FIG. Chilled During this period, 6% hydrogen as reducing gas
Using nitrogen containing nitrogen, the amount measured at room temperature was introduced into the synthesis tube 1 from the introduction tube 10 at a linear flow rate of 3 cm / min, discharged from the discharge port 11 and constantly circulated from the temperature rise to the furnace cooling. The obtained oriented transparent zinc oxide crystal had a thickness in the growth direction of about 12 mm in any part, and when this crystal was cut and polished from the substrate, a 7 mm thick crystal free from cracks was obtained. It was

From the X-ray diffraction of a small piece cut from this crystal, the degree of c-axis orientation was calculated as follows. That is,
Intensity ratio of (101) plane to (002) plane and (103)
The intensity ratio of the (004) plane and that of the (004) plane were compared with those of the zinc oxide powder measured separately. When the result of the comparison is shown by the ratio (%) of the crystal strength ratio to the powder strength ratio, the former is 0%,
The latter was 2%.

Example 2 Example 1 was repeated except that a transparent zinc oxide having a diameter of 12 mm and a thickness of 3 mm and oriented to the c-axis was used as the substrate 4, and the orientation axis (c-axis) was parallel to the rotation axis 5. Transparent zinc oxide was grown in the same manner as in. From the obtained oriented transparent zinc oxide crystal, a portion free from cracks was cut and polished to have a thickness of 9
mm was obtained.

Further, as a result of evaluating the degree of c-axis orientation as a result, the strength ratio between the (101) plane and the (002) plane is 0% with respect to that of the powder, and the strength between the (103) plane and the (004) plane is high. The ratio was 1.5%. Comparative Example 1 The same procedure as in Example 1 was carried out except that the rotary shaft 5 was removed and the substrate was placed directly in the synthesis tube 1. The thickness of the obtained crystal in the growth direction was 9 mm at the upper part of the crystal and 6 mm at the lower part, and uniform growth was not achieved. Further, cracks were formed in the crystal perpendicularly to the growth direction, and even if the crystal was cut from the substrate and polished, only a thickness of 2.5 mm was obtained if the crack was avoided.

As a result of evaluating the degree of c-axis orientation of this crystal, the strength ratio of the (101) plane to the (002) plane was 0% with respect to that of the powder, and the strength of the (103) plane to the (004) plane. The ratio was 7%. Example 3 9 × each from the crystals obtained in Example 1 and Comparative Example 1.
9 × 2 mm polished crystals were cut out, put into an aqueous lithium carbonate solution (4.98 g / 1), and dried to coat the sample surface with lithium carbonate. These were heated in air at 900 ° C. for 24 hours to thermally diffuse lithium, and the surface was polished to prepare a sample. All samples were high resistance materials having a resistance of 10 ⁸ Ωcm or more.

An In-Ga electrode was applied to the obtained sample,
The electromechanical coupling coefficient was obtained by measuring the impedance in the range of 10 kHz to 10 MHz. The results are shown in Table 1 in comparison with the value of the single crystal. The electromechanical coupling coefficient of Example 1 is larger than that of Comparative Example 1, and the reason why the value is close to the value of the single crystal is that the degree of orientation is high. Thought to be a thing.

[0033]

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus for producing transparent zinc oxide used in the method of the present invention.

2 shows a temperature distribution in the manufacturing apparatus of FIG.

Claims (4)

[Claims] 1. Zinc oxide is heated in a high-temperature heating zone in a reducing atmosphere to sublimate, and then the reducing gas is brought into contact with a substrate whose temperature is lower than in the high-temperature heating zone to deposit transparent zinc oxide on the gas. The method for producing transparent zinc oxide, characterized in that the substrate is rotated. 2. The manufacturing method according to claim 1, wherein one kind selected from the group consisting of a single crystal and a polycrystalline body having an oriented crystal axis is used for the substrate. 3. The method according to claim 2, wherein the substrate is a transparent zinc oxide oriented in the c-axis. 4. A transparent zinc oxide is formed on a substrate provided downstream of the reducing air stream by heating and sublimating zinc oxide upstream of the reducing air stream having a temperature difference so that the upstream temperature is high and the downstream temperature is low. The method according to any one of claims 1 to 3, wherein the is precipitated.