JP2007204324A - Manufacturing method of high purity zinc oxide single crystal, and high purity zinc oxide single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a high purity zinc oxide single crystal which enhances the yield of the zinc oxide single crystal, while not lowering the yield in the working process after heat treatment, by manufacturing a high purity zinc oxide single crystal satisfactorily low in Li concentration through the heat treatment of the zinc oxide single crystal. <P>SOLUTION: In this manufacturing method of a high purity zinc oxide single crystal, the temperature of the heat treatment of the zinc oxide single crystal is 1,100-1,400°C, and (a) when the temperature of the heat treatment is not lower than 1,100°C but not higher than 1,225°C, the time of the heat treatment satisfies formula (1), [time of heat treatment (hour)]≥-0.04×[temperature of heat treatment (°C)]+50, and (b) when the temperature of the heat treatment is over 1,225°C but not higher than 1,400°C, the time satisfies formula (2), [time of heat treatment (hour)]≥1. Also, the temperature reducing treatment from the temperature of the heat treatment is carried out at a temperature reducing rate of not greater than 350°C/hour. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a high-purity zinc oxide (hereinafter, used as a synonymous term having the chemical formula “ZnO” for zinc oxide) and a high-purity zinc oxide single crystal. Specifically, the present invention relates to a method for producing a high-purity zinc oxide single crystal by heat-treating the zinc oxide single crystal and a high-purity zinc oxide single crystal obtained thereby.

  A single crystal of zinc oxide (ZnO) is a semiconductor having a crystal structure of a hexagonal wurtzite compound and a large forbidden band width (Eg: 3.37 eV) by direct transition. Further, since the exciton binding energy (ZnO: 60 meV) is very large compared to other semiconductor materials (GaN: 21 meV, ZnSe: 20 meV), it is expected as a highly efficient light-emitting device material. In order to realize a light emitting device using ZnO, it is necessary to prepare ZnO to be p-type. However, ZnO easily generates defects such as oxygen vacancies or interstitial zinc, and tends to be n-type and not easily p-type. There is a nature.

  At present, many organizations are researching the conversion of ZnO to p-type, and if this is realized, it is expected that a revolution will occur in the photoelectronics and energy fields. In addition, GaN, which has been put into practical use as a blue light emitting diode (LED) for several years, has the same crystal structure and close lattice constant (lattice mismatch: about 2%), and may be manufactured at a low price in the future. Therefore, it is also attracting attention as a substrate for forming GaN instead of sapphire and SiC which are mainly used at present.

  In addition, single crystals of zinc oxide (ZnO) are used in various fields such as surface acoustic waves (SAW), gas sensors, piezoelectric elements, transparent conductors, and varistors.

  As a method for producing a single crystal of zinc oxide (ZnO), a hydrothermal synthesis method using a solvent containing Li is known. The hydrothermal synthesis method is industrially advantageous in that the crystal growth rate is large and a large single crystal can be obtained in a relatively short time. However, a lithium hydroxide (LiOH) aqueous solution is used as a solvent in the hydrothermal synthesis method. In some cases, the obtained ZnO single crystal necessarily contains Li as an impurity.

  In Patent Document 1, a zinc oxide (ZnO) single crystal produced by a hydrothermal synthesis method using a solvent of an alkaline aqueous solution composed of KOH and LiOH (hereinafter referred to as “alkaline solvent”) inevitably contains Li. It is described that it contains 0.1 to 30 ppm.

  Patent Document 2 discloses an alkali metal such as Li that forms an acceptor level when an appropriate donor atom is present in a zinc oxide single crystal substrate (wafer) produced by a hydrothermal synthesis method in the presence of an alkali metal such as Li. It is described that a weak yellow light emission that occurs in combination with the crystal is generated, and that the crystal after manufacture has a high resistance. And in order to prevent the occurrence of yellow emission of this crystal, the relative intensity of the ultraviolet emission peak in the range of 375 to 385 nm in the photoluminescence characteristics after the heat treatment is compared with the relative intensity of the ultraviolet emission peak seen in 400 to 800 nm. There is a description that the heat treatment is performed under such heat treatment conditions that a high light emission state is obtained. In order to prevent yellow light emission by removing the acceptor atoms, introduce donors, and reduce the resistance of the n-type semiconductor zinc oxide single crystal substrate by compensating the acceptor, the zinc oxide single crystal substrate (wafer) is 1200 ° C. in air. There is a description that it is preferable to heat-treat. In addition, it is described that heat treatment at higher temperature is possible because sublimation of zinc oxide is suppressed in oxygen.

However, Patent Document 2 describes that the yellow light emission is determined by the relationship between the Li equivalent concentration forming the acceptor order and the donor atom, but the Li concentration is unknown.
In addition, Li has a small atomic radius and is easy to move in the crystal. When used in various applications, Li diffusion in the zinc oxide single crystal is a concern, so a zinc oxide single crystal having a lower Li concentration is desired. ing.

However, the conventional methods disclosed in Patent Document 1, Patent Document 2, and the like cannot sufficiently reduce the Li concentration.
Further, the inventors generate cracks when polishing is performed after heat treatment at 1200 ° C. in the atmosphere, which is the condition described in Patent Document 2, and the production yield of the zinc oxide single crystal substrate (wafer) is greatly reduced. I found that there is a lot to do.
JP 2004-315361 A JP 2005-39131 A

  The present invention manufactures a high-purity zinc oxide single crystal having a sufficiently low Li concentration, which is a problem in the prior art as described above, and does not reduce the yield in the processing step after the heat treatment. The purpose of this is to improve the yield.

  As a result of intensive studies to solve the above problems, the present inventors have conducted a heat treatment temperature, a heat treatment time, a temperature drop rate of a zinc oxide single crystal in a method for producing a high-purity zinc oxide single crystal by heat treating a zinc oxide single crystal. It has been found that a zinc oxide single crystal can be produced in which the Li concentration is sufficiently reduced by controlling the amount of the material and cracks are unlikely to occur in the processing step after the heat treatment.

That is, the first gist of the present invention is a method for producing a high-purity zinc oxide single crystal by heat-treating a zinc oxide single crystal, wherein the heat treatment temperature is 1100 to 1400 ° C., and the heat treatment time is (a) heat treatment. When the temperature is 1100 ° C. or more and 1225 ° C. or less, the formula (1) is satisfied, and when the heat treatment temperature exceeds 1225 ° C. and is 1400 ° C. or less, the formula (2) is satisfied. The present invention resides in a method for producing a high-purity zinc oxide single crystal, which is carried out at a temperature lowering rate of not more than ° C / hour.
[Heat Treatment Time (Time)] ≧ −0.04 × [Heat Treatment Temperature (° C.)] + 50 Formula (1)
[Heat treatment time (hours)] ≧ 1 Formula (2)

The second gist of the present invention resides in a high-purity zinc oxide single crystal having a Li concentration of 1 × 10 ¹⁶ atoms / cm ³ or less.

  According to the present invention, a large high-purity zinc oxide single crystal with a reduced Li concentration in the single crystal can be obtained. Further, by reducing the Li concentration, when the high-purity zinc oxide single crystal is an n-type semiconductor, the resistance of the single crystal can be reduced. In addition, when a semiconductor element is formed using high-purity zinc oxide as a substrate, it is possible to prevent deterioration of the semiconductor element and improve characteristics.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. However, the description of the constituent elements described below is an example (representative example) of an embodiment of the present invention, and the present invention is not limited to the gist of the present invention. The content of is not specified.

[High purity zinc oxide (ZnO) single crystal]
The high-purity zinc oxide (ZnO) single crystal of the present invention has a Li concentration of 1 × 10 ¹⁶ atoms / cm ³ or less, preferably 5 × 10 ¹⁵ atoms / cm ³ or less, more preferably 1 × 10 ¹⁵ atoms. / Cm ³ or less. The amount of Li in the high-purity zinc oxide (ZnO) single crystal of the present invention is preferably as small as possible, but is usually 1 × 10 ¹³ atoms / cm ³ or more in relation to the production method described later.

The Li concentration of the high-purity zinc oxide (ZnO) single crystal is determined according to the intended use.
For example, since Li acts as an acceptor element, it is a substance that affects the conductivity type and conductivity of a zinc oxide semiconductor, and it is necessary to define the conductivity type and the resistance value within a target range in relation to other impurity elements. is there. In addition, since Li is an atom having a small atomic radius and easy to move in the crystal, when a zinc oxide single crystal having too much Li content is used for a semiconductor element, a change in characteristics due to long-term energization or failure Therefore, an appropriate concentration range depending on the application is required.

The Li concentration in the high-purity zinc oxide (ZnO) single crystal can be measured by a secondary ion mass spectrometer (SIMS). As analysis conditions at this time, O ²⁺ may be used as primary ions, and the acceleration voltage may be 8.0 kV.

The high-purity zinc oxide (ZnO) single crystal of the present invention has a size of 5 mm square (= 5 × 5 mm ² ) × thickness of 0.2 mm or more, preferably 5 cm in diameter (= 2.5 × 2.5 × 3. 14 cm ² ) × thickness of 0.3 mm or more, particularly preferably diameter of 5 cm × thickness of 0.5 mm or more.
The lower limit of the size of the high purity zinc oxide (ZnO) single crystal is determined by the intended use application.
For example, when it is used as a substrate (wafer) for a semiconductor element, it is difficult to use unless it has a size of about 5 mm square × thickness 0.2 mm. It is necessary to be.

  On the other hand, the upper limit of the size of the high-purity zinc oxide (ZnO) single crystal is not particularly limited, but the size of the zinc oxide single crystal that can be manufactured is determined by the size of the single crystal manufacturing apparatus and the heat treatment apparatus described later. .

High-purity zinc oxide (ZnO) single crystals are those that do not generate cracks in the processing step, preferably those that do not generate cracks in the processing step after heat treatment, and particularly preferably, such as lapping and polishing after heat treatment Cracks do not occur in the polishing process.
For example, when a high purity zinc oxide (ZnO) single crystal is used as a semiconductor element substrate (wafer), a very accurate surface flatness may be required, but cracks will not occur after heat treatment. If it exists, there exists an advantage which can implement polishing which contributes to the improvement of surface flatness after heat processing.

  Details of the method for producing the high-purity zinc oxide (ZnO) single crystal of the present invention will be described later, but the method for producing the zinc oxide single crystal as a raw material is not particularly limited. However, the zinc oxide single crystal before heat treatment needs to be a method for producing a zinc oxide single crystal in which Li is contained. For example, in the case of a hydrothermal synthesis method using a solvent containing Li, Li as an impurity is necessarily included.

[Production method of high purity zinc oxide (ZnO)]
The high purity zinc oxide (ZnO) single crystal of the present invention is produced by heat-treating the zinc oxide single crystal as a raw material under specific conditions.

<Production of raw material zinc oxide single crystal>
The zinc oxide single crystal, which is a raw material for producing the high-purity zinc oxide (ZnO) single crystal of the present invention, may contain impurities other than Li derived from the production method. For example, an autoclave of a single crystal production apparatus used in a hydrothermal synthesis method of a zinc oxide single crystal is formed of high-tensile steel or the like whose main material is iron. , It is contained in the zinc oxide single crystal as the raw material.

As a method for producing a zinc oxide single crystal as a raw material, a hydrothermal synthesis method using a solvent containing Li is preferable because large crystals can be obtained in a short time. In this case, for example, it may be manufactured according to the description in Patent Document 1.
That is, a ZnO single crystal is used as a seed crystal and a high-purity ZnO powder of 99.999% or more is used as a raw material, and usually 1 to 6 mol / l, preferably 3 mol / l KOH and usually in an autoclave of a single crystal production apparatus. Crystals are produced in the presence of an alkaline solvent containing 1 to 3 mol / l, preferably 1 mol / l LiOH. The filling rate of the alkaline solvent is preferably 60 to 90% of the free internal volume in the autoclave.

  The production of the zinc oxide single crystal as the raw material is preferably performed in a supercritical state at a high temperature and a high pressure (usually 300 to 400 ° C., 500 to 1000 atm). At this time, convection is generated by lowering the temperature of the growth region by about 15 to 50 ° C. below the temperature of the dissolution region, and the raw material melted in the dissolution region rises to the growth region and precipitates in the seed crystal to produce a crystal. .

  Specifically, the growth temperatures of the growth region and the dissolution region are preferably 300 to 360 ° C. and the melting region temperature of 340 to 400 ° C. And a crystal | crystallization is manufactured by carrying out steady operation for 30 to 200 days with this state, Then, a single-crystal manufacturing apparatus is stopped, it lowers to room temperature, and a zinc oxide single crystal is taken out.

The zinc oxide single crystal that is the raw material for producing the high-purity zinc oxide (ZnO) single crystal of the present invention thus produced contains Li, but the Li concentration of the zinc oxide single crystal that is the raw material is The upper limit is usually 1.5 × 10 ¹⁹ atoms / cm ³ , preferably 1 × 10 ¹⁹ atoms / cm ³ , more preferably 5 × 10 ¹⁸ atoms / cm ³ . A lower Li concentration in the zinc oxide single crystal as a raw material is preferable from the purpose of the present invention in which the Li concentration is lowered by heat treatment. For the same reason, a lower Li concentration is preferable for the lower limit, but from the viewpoint of availability of raw materials, it is usually 5 × 10 ¹⁶ atoms / cm ³ , preferably 1 × 10 ¹⁷ atoms / cm ³ , more preferably 5 × 10 ¹⁷ atoms / cm ³

<Heat treatment>
In the present invention, a commercially available general electric furnace capable of heating at 1100 ° C. to 1400 ° C. can be used as a heat treatment apparatus used for heat treatment of the raw material zinc oxide single crystal. For example, a material that employs silicon carbide or Kanthal (a registered trademark of Sandvik Intellectual Property Acte Borg) can be used as the material of the heating element.
Moreover, a high purity alumina fiber etc. can be used for a refractory material.

  There is no particular limitation on the shape of the zinc oxide single crystal serving as a raw material to be charged into the heat treatment apparatus. For example, a bulk single crystal produced by the hydrothermal synthesis method may be used, or a shape processed according to the application may be used. Preferably, it is processed into a plate shape, and for example, a material sliced into a plate shape to produce a substrate (wafer) can be mentioned. This is because Li is separated from the surface of the zinc oxide single crystal by the heat treatment, so that the one processed into a plate shape having a large surface area with respect to the volume is superior. In addition, since the flatness of the processed surface is not particularly limited, lapping and polishing are not essential before the heat treatment.

  Moreover, when processing according to a use, what was processed by arbitrary crystal orientations can be used. For example, when slicing into a plate shape to produce a substrate (wafer), not only slicing perpendicularly to the c-axis and processing into a plate shape, but also slicing to a plane parallel to the c-axis, etc. Slicing can be performed with any crystal orientation and is not limited to a specific plane orientation.

  There is no particular limitation on the size of the zinc oxide single crystal used as a raw material to be input into the heat treatment apparatus for the same reason as the size of the high-purity zinc oxide (ZnO) single crystal described above.

  In the furnace of the heat treatment apparatus, a zinc oxide substance can be present together with a zinc oxide single crystal as a raw material. Although it is not essential that the zinc oxide material be present in the heat treatment furnace, since zinc oxide is easily sublimated due to the high vapor pressure of zinc, if zinc oxide material is present in the heat treatment apparatus, it is the raw material zinc oxide This is preferable because it has an effect of preventing sublimation of zinc oxide from a single crystal.

  The zinc oxide substance to be present may be in any form such as powder, polycrystal, and sintered body. However, a sintered body is preferable. This is because it is easy to handle such as easy installation in the heat treatment apparatus.

  The purity of the zinc oxide substance coexisting with the raw material zinc oxide single crystal is usually 99.9% or more, preferably 99.99% or more, more preferably 99.999% or more. Moreover, it is preferable that the Li concentration in the zinc oxide substance is usually 0.1 ppm or less. For example, when a solution in which a zinc oxide substance is dissolved in nitric acid and hydrochloric acid is quantified by a standard addition method using an inductively coupled plasma mass spectrometer (ICP-QMS), a solution whose Li is below the detection limit may be used. This is to prevent impurities in the zinc oxide material from contaminating the zinc oxide single crystal.

  In addition, the zinc oxide substance may or may not be in contact with the zinc oxide single crystal that is the raw material, but it is preferable that a non-contact portion is present. This is because there is an effect of preventing reattachment of Li detached from the zinc oxide single crystal to the zinc oxide single crystal.

  Specifically, as will be described later, a zinc oxide single crystal wafer 41 is placed in a zinc oxide sintered body box 42 as shown in FIG. 2, and this box 42 is placed in a heat treatment apparatus as shown in FIG. A more preferred example is to do this.

  In the present invention, by controlling the heat treatment conditions within the scope of the present invention, the Li concentration of the obtained high purity zinc oxide single crystal can be adjusted. It can be obtained in concentration.

Hereinafter, the heat treatment conditions according to the present invention will be described.
The heat treatment atmosphere may be a gas that does not react with zinc oxide, and is preferably performed in a gas containing oxygen in order to suppress sublimation of zinc oxide. For example, in the atmosphere and in oxygen, both are possible, but the atmosphere is preferable from the simplicity of the apparatus, and the effect on the reduction of the Li concentration is sufficient in the atmosphere. However, heat treatment in an atmosphere not containing oxygen is not preferable at 1000 ° C. or higher because zinc oxide sublimation becomes active.

The minimum of heat processing temperature is 1100 degreeC normally, Preferably it is 1200 degreeC, and an upper limit is 1400 degreeC normally, Preferably it is 1300 degreeC. The upper limit of the heat treatment temperature is sufficient if the heat treatment time required for setting the target Li concentration to 1 × 10 ¹⁶ atoms / cm ³ or less is 1 hour. In addition, when performing heat treatment at a higher temperature than this, the materials of the furnace material and the heating element that can be used in the heat treatment apparatus are limited, resulting in high costs.

On the other hand, the lower limit of the heat treatment temperature is about 6 hours at 1100 ° C., which is necessary for making the Li concentration 1 × 10 ¹⁶ atoms / cm ³ or less, which is a practical range for manufacturing. by.

The heat treatment time depends on the heat treatment temperature and the Li concentration in the target zinc oxide single crystal. The lower limit of the heat treatment time for obtaining a high-purity zinc oxide single crystal having a target Li concentration of 1 × 10 ¹⁶ atoms / cm ³ or less is as follows: (a) When the heat treatment temperature is 1100 ° C. or higher and 1225 ° C. or lower, When (b) the heat treatment temperature exceeds 1225 ° C. and is 1400 ° C. or less, (1) is determined so as to satisfy the following formula (2).
[Heat Treatment Time (Time)] ≧ −0.04 × [Heat Treatment Temperature (° C.)] + 50 Formula (1)
[Heat treatment time (hours)] ≧ 1 Formula (2)

Further, the lower limit of the heat treatment time for obtaining a high purity zinc oxide single crystal having a target Li concentration of more preferably 1 × 10 ¹⁵ atoms / cm ³ or less is (c) when the heat treatment temperature is 1100 ° C. or higher and 1300 ° C. or lower. Is determined so as to satisfy the following formula (4) when the heat treatment temperature exceeds 1300 ° C. and is 1400 ° C. or lower.
[Heat Treatment Time (Time)] ≧ −0.045 × [Heat Treatment Temperature (° C.)] + 59.5 Formula (3)
[Heat treatment time (hours)] ≧ 1 Formula (4)

Note that as shown in the equations (1) to (4), heat treatment is performed for at least one hour. This is for ensuring the temperature stability in the furnace immediately after the temperature rise of the heat treatment apparatus and performing a stable heat treatment.
The upper limit of the heat treatment time is not particularly defined, but a heat treatment time that can achieve the target Li concentration in the crystal is sufficient.

  Regarding the temperature lowering rate after the heat treatment, the temperature lowering treatment from the heat treatment temperature is performed at 350 ° C./hour or less, preferably 300 ° C./hour or less. This is because if the temperature lowering process is further performed and the cooling is rapidly performed, the thermal strain in the zinc oxide single crystal increases, and cracks are likely to occur in a processing step after the heat treatment (polishing such as polishing). The lower limit of the cooling rate is not particularly limited. The smaller the temperature lowering rate, the smaller the thermal strain, which is preferable. However, since the temperature lowering time becomes longer, an industrially appropriate range may be used.

  The pressure during the heat treatment is not particularly limited. However, under reduced pressure, in order to promote sublimation of the zinc oxide single crystal, it is preferably under atmospheric pressure or under pressure, and preferably under atmospheric pressure from the simplicity of the apparatus.

  There is no particular limitation on the gas introduction into the heat treatment apparatus during the heat treatment. When the gas is introduced, the introduced gas is preferably a gas that does not react with zinc oxide and does not contain impurities. For example, when the heat treatment is performed in the atmosphere, a method of introducing the atmosphere into the apparatus may be employed in order to replace the atmosphere in the apparatus containing Li or the like released from the zinc oxide single crystal.

By such a method for producing a high purity zinc oxide single crystal of the present invention, a high purity zinc oxide single crystal of the present invention having a Li concentration of 1 × 10 ¹⁶ atoms / cm ³ or less can be produced.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples as long as the gist thereof is not exceeded.

[Production Example 1: Production of raw material zinc oxide single crystal used in Examples and Comparative Examples]
A zinc oxide single crystal was manufactured by a method according to the hydrothermal synthesis method described in Patent Document 1 using a single crystal manufacturing apparatus having a structure shown in FIG.

  A single crystal production apparatus 11 shown in FIG. 1 includes an autoclave 12 capable of applying a temperature and pressure necessary for producing a ZnO single crystal by hydrothermal synthesis, and an inside of the autoclave 12. It is comprised from the manufacturing container 20 accommodated and used. The autoclave 12 is, for example, covered with a lid body 14 with a packing 17 sandwiched between the container body 13 of the autoclave 12 formed of high-strength steel mainly composed of iron, and fixed by the fixing portion 15. The inside is hermetically sealed. The manufacturing container 20 accommodated and used in the autoclave 12 is made of platinum (Pt), and the shape thereof is a substantially cylindrical container. And the bellows 30 which acts as a pressure adjustment part is attached to the upper part in the state which sealed the inside of the manufacturing container 20. As shown in FIG.

  In such a single crystal manufacturing apparatus 11, the ZnO seed crystal 3 is suspended using the frame 21 and the platinum wire 22 on the upper side in the manufacturing container 20, and the raw material 26 is disposed on the lower side to dispose the seed crystal 3. A ZnO single crystal is manufactured by growing. An internal baffle plate 24 for controlling thermal convection is provided between the ZnO seed crystal 3 and the raw material 26, and the inside of the production container 20 is divided into a melting region and a growth region by the internal baffle plate 24. Yes. A plurality of holes are formed in the internal baffle plate 24, and the opening area of the baffle plate 24 determined by the number of holes is set to 10%. The flow rate to the growth region can be controlled, and the rate of crystal growth is affected. Further, an external baffle plate 25 is provided outside the production container 20, and the convection outside the production container 20 is restricted by the external baffle plate 25, so that the seed crystal 3 grows between the regions in the production container 20. The temperature difference required for this is obtained.

  Using the single crystal production apparatus 11 as described above, a single crystal of ZnO can be produced from a seed crystal by a hydrothermal synthesis method. A ZnO single crystal having a caliber size that can be used for industrial purposes can be produced by selecting almost no impurities in the production vessel 20 and selecting the number of production days according to the intended use.

A ZnO powder having a purity of 99.9999% was pressed and hardened in a mold container, and then fired at 1100 ° C. for 24 hours, and the solidified product was filled in the production container 20. Next, 80% of the free volume of pure water in which 1 mol / l LiOH and 3 mol / l KOH are dissolved as mineralizers is injected into the production vessel 20 and 0.05 mol / l of H ₂ O ₂ is further injected. did. Thereafter, the space between the production container 20 and the bellows 30 was welded, and the inside of the production container 20 was completely sealed and welded. Moreover, 80% of free volume was filled with pure water for heat transfer between the autoclave 12 (φ200 × 300 mm) and the production container 20. The autoclave 12 includes a container main body 13 and a lid body 14. The container main body 13 and the lid body 14 are covered with a packing 17 interposed therebetween, and are fixed by a fixing portion 15 so that the inside thereof can be hermetically sealed.

  Thereafter, the melting region and the growth region were heated by the heater 16. During the heating, the temperature of the melting region was raised 15 to 50 ° C. higher than the temperature of the growth region, and finally the temperature was raised so that the melting region was about 360 ° C. and the growth region was about 310 ° C. The raw material melted in the melting region rises by convection and precipitates from the vicinity of the seed crystal 3 in the growth region, so that the seed crystal grows and a ZnO single crystal is manufactured. In this state, a steady operation is performed for 60 days, and growth is performed at a production rate of about 0.2 mm / day in both the c-axis direction and the a-axis direction. A single crystal was taken out to obtain a zinc oxide single crystal used in Examples and Comparative Examples of the present invention.

[Examples 1 to 5]
A c-plane wafer having a thickness of 1 mm was cut out from the zinc oxide single crystal obtained by the hydrothermal synthesis method in Production Example 1 and processed into a circular wafer having a diameter of 5 cm and a thickness of 1 mm, and then both surfaces were lapped and polished to finish a mirror surface ( The wafer thickness after processing is 0.8 mm).

  The wafer 41 is put in a zinc oxide sintered body box 42 as shown in FIG. 2 and covered, and a zinc oxide sintered body box 42 in which the wafer is placed is installed in a heat treatment apparatus 51 as shown in FIG. did. Here, as the zinc oxide sintered body constituting the box 42 to be used, when Li was quantified by standard addition using ICP-QMS (Yokogawa Analytical Systems HP4500), a solution dissolved in nitric acid and hydrochloric acid, What was below a detection limit (0.01 ppm) was used.

  A desktop fast heating furnace (model MSFT-1530) manufactured by Yamada Electric Co., Ltd. was used as the heat treatment apparatus. After raising the temperature from room temperature to the heat treatment temperature shown in Table 1 in the air over 2 hours, heat treatment was performed in the air at the heat treatment temperature and heat treatment time shown in Table 1. This heat treatment time is a heat treatment time that satisfies the formula (1) or the formula (2). The wafer after the heat treatment was cooled to room temperature at a temperature drop rate of 300 ° C./hour.

The heat-treated wafer was washed with pure water and then polished on one side by 10 μm. None of the wafers were cracked during polishing. A 10 mm square sample was cut out from the finished mirror surface wafer, and the Li concentration was quantified with a secondary ion mass spectrometer (SIMS) (model number: CAMEA ims4f) manufactured by CAMEA. At this time, O ²⁺ was used as the primary ion, and the acceleration voltage was analyzed at 8.0 kV.

The results are shown in Table 1.
From Table 1, when the heat treatment temperature was 1100 ° C., the heat treatment time was 6 hours, and the heat treatment time was 1200 ° C. for 2 hours, the Li concentration became 1.0 × 10 ¹⁶ atoms / cm ³ or less. When the heat treatment temperature was 1100 ° C., the heat treatment temperature was 12 hours, 1250 ° C. for 3 hours, and 1300 ° C. for 2 hours, the Li concentration became 1.0 × 10 ¹⁵ atoms / cm ³ or less.

For Example 2, four samples were equally cut out from the center and outer peripheral parts as shown in FIG. 4 from the finished mirror surface of the wafer, and the measurement points were points on the circumference of the wafer center and a radius of 20 mm from the center. When the SIMS measurement was performed, the Li concentration was 1.0 × 10 ¹⁶ atoms / cm ³ or less at all the measurement points.
The Li concentration is almost uniform from the finished mirror surface to a depth of 10 μm, and the Li concentration does not change even if the wafer is polished 500 μm from the surface and then measured in the depth direction from the polished surface. It was confirmed.

[Comparative Examples 1 and 2]
From the zinc oxide single crystal obtained by the hydrothermal synthesis method in Production Example 1, from the position adjacent to the position where the wafer was cut out in Examples 1 and 2 in Comparative Example 1, in Comparative Example 2, in Examples 3 and 4. Each wafer was cut out from a position adjacent to the position where the wafer was cut out and processed into a circular wafer having a diameter of 5 cm and a thickness of 1 mm in the same manner as in Examples 1 to 5, and then both surfaces were lapped and polished to finish a mirror surface. This wafer was heat-treated in the atmosphere at the heat treatment temperature and heat treatment time shown in Table 1. This heat treatment time is a condition that does not satisfy the formula (1) and the formula (2), and is shorter than the heat treatment temperature in Examples 1 to 5. Other conditions were the same as in Examples 1 to 5, and the Li concentration was quantified in the same manner.

The results are shown in Table 1.
From Table 1, the Li concentration exceeds 1.0 × 10 ¹⁶ atoms / cm ³ in both the heat treatment temperature of 1100 ° C. and the heat treatment time of 1 hour and 2 hours, which is higher than those in Examples 1 to 5. showed that.

In Comparative Example 1, as in Example 2, four samples were equally cut out from the center and outer peripheral portions as shown in FIG. 4 from the finished mirror surface of the wafer, and the measurement points had a radius from the center and the center of the wafer. When SIMS measurement was performed so as to be a point on the circumference of 20 mm, the Li concentration exceeded 1.0 × 10 ¹⁶ atoms / cm ³ at all the measurement points.
The Li concentration is almost uniform from the finished mirror surface to a depth of 10 μm, and the Li concentration does not change even if the wafer is polished 500 μm from the surface and then measured in the depth direction from the polished surface. It was confirmed.

  From the results of Examples 1 to 5 and Comparative Examples 1 and 2, the relationship between the heat treatment temperature and heat treatment time and the Li concentration is summarized in FIG.

[Comparative Example 3]
From the zinc oxide single crystal obtained by the hydrothermal synthesis method in Production Example 1, the wafer was cut out from a position adjacent to the position where the wafer was cut out in Example 2, and the diameter was 5 cm × thickness 1 mm as in Examples 1-5. After processing into a circular wafer, both sides were lapped and polished to a mirror finish. A 10 mm square sample was cut out from this wafer, the Li concentration was quantified by SIMS without heat treatment, and the results are shown in Table 1.
As shown in Table 1, the Li concentration was 6.5 × 10 ¹⁷ atoms / cm ³ , which was higher than those of Examples 1 to 5 and Comparative Examples 1 and 2 where heat treatment was performed.

Further, in the same manner as in the second embodiment, four samples are equally cut out from the center and the outer peripheral portion as shown in FIG. 4 from within the wafer surface, and the measurement points are points on the circumference having a radius of 20 mm from the wafer center and the center. When SIMS measurement was performed, the Li concentration exceeded 2.0 × 10 ¹⁷ atoms / cm ³ at all measurement points.
The Li concentration was substantially uniform from the wafer surface to a depth of 10 μm, and it was confirmed that the Li concentration did not change even when the wafer was polished from the surface by 500 μm and measured in the depth direction from the polished surface.

[Comparative Example 4]
From the zinc oxide single crystal obtained by the hydrothermal synthesis method in Production Example 1, the wafer was cut out from a position adjacent to the position where the wafer was cut out in Example 1, and the diameter was 5 cm × thickness 1 mm as in Examples 1-5. After processing into a circular wafer, both sides were lapped and polished to a mirror finish. As shown in Table 1, this wafer was heat-treated at 1300 ° C. for 2 hours in the atmosphere. This heat treatment time satisfies the formula (2) and is the same as that of the first embodiment.

After the heat treatment of the wafer, the wafer was cooled with the furnace of the heat treatment apparatus opened. At this time, after 2 hours from the start of cooling, the cooling was performed to such an extent that it could be touched by hand, and the cooling rate was 500 ° C./hour or more. About other conditions, it carried out similarly to Examples 1-5.
The heat-treated wafer was washed with pure water in the same manner as in Examples 1 to 5 and further polished on one side by 10 μm. As a result, cracks occurred during polishing.

From the results of Comparative Example 3 described above, the sample of the hydrothermal synthesis method without heat treatment showed that the Li concentration exceeded 1.0 × 10 ¹⁷ atoms / cm ³ , and the Li concentration was high before the heat treatment. .

Further, from the comparison between Examples 1 to 5 and Comparative Examples 1 and 2, in order to obtain a high-purity zinc oxide single crystal having a Li concentration of 1.0 × 10 ¹⁶ atoms / cm ³ or less, the heat treatment required depending on the heat treatment temperature. The time is different, and it is necessary to satisfy the formula (1) or (2) as shown in FIG. 5, and furthermore, a high-purity zinc oxide single crystal having a Li concentration of 1.0 × 10 ¹⁵ atoms / cm ³ or less is obtained. Therefore, the necessary heat treatment time differs depending on the heat treatment temperature, and it is shown that the formula (3) or the formula (4) needs to be satisfied as shown in FIG.

  Further, when a high-purity zinc oxide single crystal wafer having a diameter of 5 cm and a thickness of 1 mm is obtained by comparing Examples 1 to 5 with Comparative Example 4, not only the heat treatment temperature and the heat treatment time but also the cooling of the wafer after the heat treatment. Therefore, it has been shown that it is necessary to set the temperature lowering rate within the conditions of the present invention in order to prevent the occurrence of cracks in the polishing step after the heat treatment.

It is typical sectional drawing which shows the structure of the single-crystal manufacturing apparatus used in order to manufacture a ZnO single crystal by the hydrothermal synthesis method in the manufacture example 1. FIG. In the heat processing in Examples 1-5, it is typical sectional drawing which shows the state which put the zinc oxide single crystal wafer in the box of a zinc oxide sintered compact. It is typical sectional drawing which shows the state which installed the box of the zinc oxide sintered compact which put the heat processing apparatus and the zinc oxide single crystal wafer which were used by the heat processing in Examples 1-5. 6 is a schematic plan view showing sample locations cut out for in-plane distribution measurement in Example 2, Comparative Example 1, and Comparative Example 3. FIG. It is a graph which shows the relationship between the heat processing temperature and heat processing time in Examples 1-5 and Comparative Examples 1 and 2, and Li concentration.

Explanation of symbols

3 Seed Crystal 11 Single Crystal Production Device 12 Autoclave 13 Container Body 14 Lid 15 Adhering Part 16 Heater 17 Packing 20 Production Container 21 Frame 22 Platinum Wire 24 Internal Baffle Plate 25 External Baffle Plate 26 Raw Material 30 Bellows 41 Zinc Oxide Single Crystal Wafer 42 Zinc oxide sintered body box 51 Heat treatment device 52 Heating element

Claims (6)

In a method for producing a high-purity zinc oxide single crystal by heat-treating a zinc oxide single crystal,
The heat treatment temperature is 1100 to 1400 ° C.
The heat treatment time (a) When the heat treatment temperature is 1100 ° C. or more and 1225 ° C. or less, the formula (1) is
(B) When the heat treatment temperature exceeds 1225 ° C. and is 1400 ° C. or less, the formula (2) is satisfied,
A method for producing a high-purity zinc oxide single crystal, wherein the temperature lowering treatment from the heat treatment temperature is performed at a temperature lowering rate of 350 ° C./hour or less.
[Heat Treatment Time (Time)] ≧ −0.04 × [Heat Treatment Temperature (° C.)] + 50 Formula (1)
[Heat treatment time (hours)] ≧ 1 Formula (2)   The method for producing a high-purity zinc oxide single crystal according to claim 1, wherein the heat treatment is performed in air.   The method for producing a high-purity zinc oxide single crystal according to claim 1 or 2, wherein heat treatment is performed in the presence of a zinc oxide substance other than the zinc oxide single crystal. A high-purity zinc oxide single crystal having a Li concentration of 1 × 10 ¹⁶ atoms / cm ³ or less.   The high-purity zinc oxide single crystal according to claim 4, which has a size of 5 mm square or more and a thickness of 0.2 mm or more.   The high-purity zinc oxide single crystal according to claim 4 or 5, which is produced by a hydrothermal synthesis method containing Li in the reaction system.

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