JPH06122595A - Growth of zinc oxide single crystal - Google Patents

Abstract

PURPOSE:To inhibit dissolution of ZnO seed crystals and improve the growth efficiency by charging ZnO and an alkaline solvent in a growth tank, dissolving Zn ions with heating, subsequently putting the seed crystals thereinto and growing ZnO single crystals at a high-temperature and a high-pressure. CONSTITUTION:Into a growth tank 10 the inside of which is coated with Ag, Pt, etc., ZnO sintered materials 1 having 1 to 2mm particle diameter are charged and a baffle plate 3 is set so as to divide the inside of the tank 10 into a raw material pack part 14 and a crystal growth part 16. ZnO seed crystals 7 are hung from a frame 5 and set to the crystal growth part 16. An alkaline solvent prepared by dissolving ZnO at a high-temperature and high-pressure of 360 to 400 deg.C and 500 to 700kg/cm<2> and containing ZnO and Zn ions is then poured into the tank 10 and H2O2 is added thereto so as to adjust the electric conductivity to 10<-3> to 10<-6>l/OMEGA.cm. The crystal growth part 16 and the raw material pack part 14 are subsequently heated respectively up to 360 to 400 deg.C and 380 to 420 deg.C and the objective growth of ZnO single crystals is carried out for 10 to 30day.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing a zinc oxide (ZnO) single crystal, and more particularly to a method for growing a ZnO single crystal which can be suitably used as an acoustoelectric effect element.

[0002]

2. Description of the Related Art Zinc oxide (ZnO) has a chemical composition of Zn.
It is an excess n-type semiconductor, and is a substance that has attracted attention as a piezoelectric body due to its crystal structure. Currently Zn
Although the use of O is carried out by a thin film, there is a problem that it cannot be used because the film is polycrystalline and is affected by the grain boundaries and the propagation loss becomes large. Therefore, high-purity ZnO is used.
Single crystallization is an important issue.

Regarding the single crystallization of ZnO, "high-purity Z
Hydrothermal growth of nO single crystal and evaluation of stoichiometry ”
(Noboru Sakagami, February 1988, Bulletin of the Akita National College of Technology No. 2
No. 3) has been reported. This document describes the growth of a ZnO single crystal by a hydrothermal synthesis method. According to this growth method, a ZnO sintered body is placed in the lower part of a crystal growing device, while a ZnO seed crystal is grown in the growing device. Each is placed on top and then filled with an alkaline solvent consisting of KOH and LiOH. In this state, the inside of the crystal growth apparatus is 370-4
The operation is performed at a growth temperature of 00 ° C. and a pressure of 700 to 1000 kg / cm ² , but here, the operation is performed so that the temperature of the lower part is 10 to 15 ° C. higher than the temperature of the upper part in the upper part and the lower part in the crystal growing device. By doing so, a ZnO single crystal is grown.

[0004]

However, in growing the ZnO single crystal as described above, the ZnO seed crystal may be dissolved in the alkaline solvent as the temperature in the crystal growing apparatus rises. That is, normally, the seed crystal is installed by suspending it by penetrating the noble metal wire, but the pores of the seed crystal penetrating the noble metal wire melt as the temperature rises, and as a result, the seed crystal falls. And
There is a problem that it becomes impossible to grow a single crystal.

Further, due to such dissolution, there is a problem that the seed crystal to be grown may become small or, in the worst case, may completely disappear. Further, in the growth by the hydrothermal synthesis method as described above, since the seed crystal hardly grows in the c-axis direction of the hexagonal crystal structure, if the c-axis direction of the seed crystal is shortened by the above-mentioned melting, a small crystal in the c-axis direction Only you can get it. Furthermore, in the conventional method, a crystal having a large thickness has to be used as a seed crystal, which is disadvantageous in cost.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to suppress the dissolution of a seed crystal when growing a single crystal by a hydrothermal synthesis method. It is to provide a method for growing a ZnO single crystal with improved efficiency.

[0007]

As a result of intensive studies aimed at achieving the above object, the present inventor has found that the above object can be achieved by dissolving ZnO in an alkaline solvent, thereby completing the present invention. .

Therefore, according to the method for growing a ZnO single crystal of the present invention, when growing a ZnO single crystal by using a hydrothermal synthesis method, (1) ZnO and an alkaline solvent are filled in a growth container, and (2) After heating the alkaline solvent to elute Zn ions, a ZnO seed crystal is placed in the above-mentioned growth container, (3) then this growth container is sealed, and a ZnO single crystal is grown under high temperature and high pressure. It is characterized by Further, the method for growing a ZnO single crystal of the present invention includes (1) Zn when growing a ZnO single crystal using a hydrothermal synthesis method.
An alkaline solution containing O and Zn ions and a ZnO seed crystal are placed in a growth container, and (2) this growth container is then sealed to grow a ZnO single crystal under high temperature and high pressure. To do.

[0009]

In the growing method of the present invention, the mechanism of action of ZnO dissolved in an alkaline solvent is not clear, but it is presumed to be as follows. That is, the solubility of ZnO in the alkaline solvent increases as the temperature rises. Therefore, in the conventional hydrothermal synthesis method, as the temperature in the crystal growth container rises, not only the ZnO that is the growth raw material dissolves, but also the ZnO seed crystal begins to dissolve. The dissolution of this seed crystal can be achieved by dissolving Z in an alkaline solvent.
It is considered that the amount of dissolved nO does not proceed until it becomes saturated or supersaturated.

Therefore, in the growing method of the present invention, the dissolution amount of ZnO in the alkaline solution is increased by dissolving ZnO in the alkaline solvent. As a result, the above-mentioned state of supersaturation is reached sooner,
It is presumed that the dissolution of O seed crystal is suppressed. Therefore, the ZnO seed crystal is hardly dissolved during the growth process, and the ZnO single crystal can be obtained with high growth efficiency.

Next, the method of growing the ZnO single crystal of the present invention will be described in detail. First, as ZnO used as a raw material for growing a ZnO single crystal, powder or granular one can be used. However, non-ionized Zn
When the O powder is suspended in the alkaline solution, the suspended powder may adhere to the ZnO seed crystal and mix with the seed crystal to be grown. Therefore, in the case of using ZnO powder, if the ZnO powder is filled in a porous or perforated container and this container is installed in the growth container, it is possible to prevent the ZnO powder from floating and avoid the adverse effect on the seed crystal. be able to. From the above, it is usually preferable to use a ZnO sintered body as a raw material, and this sintered body can be manufactured by firing according to a conventional method. It is preferable to select one having a diameter of about 1 to 2 mm from the obtained sintered bodies.

Next, the ZnO sintered body is filled in a growth container having Ag or Pt or the like coated inside or in a growth container made of Ag or Pt or the like. Then, if necessary, a baffle plate is installed in the container to divide into a raw material filling portion filled with a ZnO sintered body and a crystal growth portion in which a ZnO seed crystal is arranged. Next, the ZnO seed crystal is placed in the upper part of the container (in the case of using a baffle plate, a crystal growth part). As this seed crystal, a single crystal prepared by a vapor phase growth method can also be used.
Since the single crystal does not contain Li, if the crystal is grown by the hydrothermal synthesis method, the matching between the seed crystal and the growing portion where the crystal grows deteriorates. It is better to use the obtained crystals.

Next, an alkaline solution in which ZnO is dissolved in an alkaline solvent composed of KOH and LiOH, that is, an alkaline solution containing Zn ions is poured into the container. The injection rate is preferably about 80% of the volume of the container. The dissolution method of ZnO is not particularly limited, but is, for example, 360 to 400 ° C., 500 to 700 kg / c.
It can be performed under high temperature and pressure of m ² . Alternatively, ZnO and the above-mentioned alkaline solvent may be filled in a growth container, and the ZnO seed crystal may be placed in the growth container after heating and pressurizing under the above conditions to elute Zn ions. In any case, it is preferable that ZnO is sufficiently dissolved to bring it into a saturated or supersaturated state. When the alkaline solution is injected, H ₂ O ₂ may be further injected in order to highly purify the obtained ZnO single crystal. However, in this case, and taking into consideration the nature of the H ₂ 0 ₂ oxidizing agent, the internal growth vessel or coated with Pt, it is necessary to produce a growth vessel itself Pt.

Next, the growing container is placed in another container, for example, an autoclave, a pressure medium is filled in the autoclave, and the container is immersed. The pressure medium may be any substance that is weakly corrosive under high temperature and high pressure, and distilled water is preferable. The pressure medium generates pressure at the growth temperature according to the filling rate with respect to the internal volume (hereinafter, referred to as "free internal volume") remaining when the growth container is installed in the autoclave. Adjusts the filling rate of the pressure medium so that the pressure becomes equal to or slightly higher than the pressure in the growth container, thereby fulfilling the function of protecting the growth container. When distilled water is used as the pressure medium in the above solvent and solvent concentration, its filling rate is preferably about 60 to 80% of the free internal volume of the autoclave.

Next, the autoclave is placed in a heating furnace, the temperature of the growth container is raised, and the crystal growth part and the raw material charging part are heated to a predetermined temperature. At this time, it is preferable that the temperature of the crystal growth part is lower by about 5 to 25 ° C. than the temperature of the raw material filling part. That is, the temperature of the crystal growth part is 360 to 400.
C., and the temperature of the raw material filling part is preferably 380 to 420.degree. Then, in this state, a steady operation is performed for 10 to 30 days to grow a crystal, then the heating furnace is stopped and the temperature is lowered to room temperature, and a ZnO single crystal is taken out.

Here, in order to improve the suitability of the obtained ZnO single crystal for use in piezoelectric semiconductors, particularly acoustic effect type probe materials, ultrasonic amplification materials, piezoelectric transducers, etc. The electrical conductivity of ZnO single crystal is adjusted to 10 ^-3 to 10 ^-6 1 / Ω by appropriately adjusting the concentration of ₂ O _2.
・ Can be adjusted to about cm. In this case, H ₂ O ₂
The concentration is 0.02-0.1mo for 1 liter of alkaline solvent
It is preferably less than 1.

Further, the ZnO single crystal was grown with the H ₂ O ₂ concentration higher than the above value, and the single crystal after the growth was mixed with Al or the like.
It is also possible to dope a valent metal and adjust the electrical conductivity to the above value. When the H ₂ O ₂ concentration is 0.1 mol or more with respect to 1 l of the alkaline solvent, the amount of the H ₂ O ₂ in the ZnO single crystal is
By diffusing 120 ppm of Al, the electrical conductivity of the ZnO single crystal can be adjusted to 10 ⁻³ to 10 ⁻⁶ 1 / Ω · cm. As a diffusion method, for example, a ZnO single crystal is dipped in an Al (OH) ₃ or Al ₂ (CO ₃ ) ₃ solution, and then, in the air or in an O ₂ stream at 700 to 1000 ° C. for 25 hours.
Diffusion processing may be performed for up to 300 hours. Although the solution concentration varies depending on the size of the ZnO single crystal and the amount of the solution, the size of the ZnO single crystal is 5 × 5 × 5 mm and the amount of the solution is 5 m.
In the case of 1, it is preferable to use a solution having an Al concentration of 50 to 200 ppm.

Further, even in the case of a ZnO single crystal grown without using H ₂ O ₂ , 15 to 15% of Li is contained in the ZnO single crystal.
By diffusing 120 ppm, the electrical conductivity in the ZnO single crystal can be adjusted to 10 ⁻³ to 10 ⁻⁶ 1 / Ω · cm. As a diffusion method, for example, Zn in a LiOH solution is used.
Immerse O single crystal, then in air or O ₂ stream 80
The diffusion treatment may be performed at 0 to 1000 ° C. for 100 to 300 hours. The solution concentration varies depending on the size of the ZnO single crystal and the amount of the solution, but when the size of the ZnO single crystal is 5 × 5 × 5 mm and the amount of the solution is 5 ml, the Li concentration is 50 to 200.
It is better to use a ppm solution.

Further, as described above, when the ZnO single crystal is applied to the acoustic effect type probe material or the like, 30 cm ² / V ·
It has mobility (mobility of carrier) of sec or more, and variation in electric conductivity in the single crystal is 10 ²
More preferably, it is within 1 / Ω · cm. The mobility can be adjusted by removing the impurity heavy metal in advance during the firing of the ZnO sintered body according to the above-described growth method. In the conventional method, impurities such as Pb are mixed in the ZnO single crystal, which may reduce the mobility.
Although about 5 ppm of Pb is contained in the ZnO powder, for example, Zn can be used to produce high-purity Zn powder after the high-purity Zn is purified by repeating distillation of Zn. Further, when the ZnO single crystal according to the present invention is used for the acoustic effect type probe material, the temperature difference ΔT between the raw material filling portion and the crystal growing portion is made smaller in the latter half of the first half of the growing process. By controlling, the variation of electrical conductivity is 10 ²
It can be within 1 / Ω · cm, and it is preferable to treat in this way. This temperature difference ΔT is specifically 10 to 25 ° C. in the first half of the growing period and 5 to 10 in the latter half.
By setting the temperature to be ° C, variations in electrical conductivity can be suppressed.

[0020]

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. (Examples 1 and 2) (Growth of single crystal) 500 g of ZnO powder and 500 g of distilled water
After mixing and, molded into a spherical shape of 2-3 mm, 100 ℃
And dried for 2 hours. The obtained dried body was sintered in an Al ₂ O ₃ container at 1100 ° C. for 24 hours in an oxygen atmosphere, and a ZnO sintered body having a particle size of 1 to 2 mm was sieved and selected.

200 g of ZnO sintered body 1 was filled in the growth container 10 shown in FIG. This growth container 10 is provided with thermocouple insertion portions 12 and 12 ', and has an inner diameter of 30 mm and a height of 300.
It has a substantially cylindrical shape of mm, the inner volume is 250 ml, and the inside thereof is coated with Pt. Next, the baffle plate 3 having an opening ratio of 5% is installed in the growing container 10 and the inside of the container 10 is filled with the raw material filling portion 14 and the crystal growing portion 16.
It was divided into Then, on the Pt frame 5, an A shape; 5
Fifteen ZnO seed crystals 7 of × 10 × 1 mm or B shape; 5 × 10 × 0.5 mm were suspended, and this frame 5 was placed in the crystal growth part 16. At this time, the seed crystal 7 was fixed to the frame 5 by penetrating the Pt wire 9 which is an example of a noble metal wire and fastening both ends of the Pt wire 9 to the frame 5.

In a growth container 10, 3 mol of KOH and 1.
380 ° C. and 700 kg / cm ^{2 in} 5 mol of LiOH
The alkaline solution obtained by dissolving ZnO under the conditions of
Injected. Here, ZnO is sufficiently dissolved and is in a saturated or supersaturated state. The injection amount of the alkaline solution was 80% of the inner volume of the growth container 10. And
Furthermore, 0.06 mol of H _{2 is} added to 1 liter of the alkaline solvent.
O ₂ was injected.

Next, as shown in FIG. 2, the growth container 10
Is installed in the autoclave 20 and the thermocouples 18, 18
Was placed, and then distilled water 22 was injected into the autoclave 20. The injection amount is 70% of the internal volume of the autoclave 20.
%. Next, the autoclave 20 was sealed with the cap 24, and the autoclave 20 was installed in the electric furnace 30. The electric furnace 30 has a two-stage upper and lower structure to enable fine adjustment of the growth temperature, and is equipped with thermocouples 32 and 34.

Next, the temperature of the crystal growing portion 16 was raised so that it was always lower than the temperature of the raw material filling portion 14, and the temperature of the crystal growing portion was raised to 380 ° C. and the raw material filling portion was raised to 395 ° C. In this state, steady operation was performed for 20 days, after which the electric furnace was cooled to room temperature, and then the ZnO single crystal was taken out. (Comparative Examples 1 and 2) The above 3 mol of KOH and 1.5 mo
The same operation as in Examples 1 and 2 was performed except that ZnO was not dissolved in an alkaline solvent composed of 1 liter of LiOH.

(Performance Evaluation) The thickness of the ZnO single crystal obtained in each of the above examples was measured, and the average value (15 pieces) is shown in Table 1. The number of seed crystals dissolved during the growth is also shown.

[0026]

[Table 1]

As shown in Table 1, it can be seen that no seed crystal is dissolved in this example. Moreover, the thickness of the seed crystal is not reduced. From this, it becomes possible to reduce the thickness of the seed crystal by about 1/3 to 1/4 as compared with the conventional method.

[0028]

As described above, according to the present invention,
Since ZnO is dissolved in an alkaline solvent, it is possible to provide a method for growing a ZnO single crystal in which dissolution of a seed crystal is suppressed and growth efficiency is improved when growing a single crystal by a hydrothermal synthesis method. That is, during the growth, the dissolution of the seed crystal hardly progresses, so that the size of the seed crystal can be minimized, which is economically advantageous.

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing an example of a growth container according to the present invention.

FIG. 2 is a schematic sectional view showing an example of a crystal growing apparatus according to the present invention.

[Explanation of symbols]

 1 ZnO Sintered Body 3 Baffle Plate 5 Frame 7 ZnO Seed Crystal 9 Pt Wire 10 Growth Container 12, 12 'Thermocouple Insertion Section 14 Raw Material Filling Section 16 Crystal Growth Section 18, 18' Thermocouple 20 Autoclave 22 Distilled Water 24 Cap 30 Electric furnace 32, 34 thermocouple

Claims (11)

[Claims] 1. When growing a ZnO single crystal using a hydrothermal synthesis method, (1) ZnO and an alkaline solvent are filled in a growth container, and (2) this alkaline solvent is heated to produce Z.
After elution of n ions, a ZnO seed crystal is placed in the above-mentioned growth container, (3) this growth container is then sealed, and a ZnO single crystal is grown at high temperature and high pressure. Single crystal growth method. 2. ZnO is filled in the lower part of the growth container to
2. The growing method according to claim 1, wherein the O seed crystal is filled in the upper part of the growing container, and the lower part of the growing container is adjusted to have a higher temperature than the upper part thereof. 3. The growth container has an upper portion and a lower portion of 360 to 40.
The growing method according to claim 1 or 2, wherein heating is performed at 0 ° C and 380 to 420 ° C. 4. ZnO at 360 to 400 ° C. and 500 to 7
The Zn ion is eluted by being dissolved in the alkaline solvent under a high temperature and high pressure of 00 kg / cm ² to elute Zn ions. 5. The growing method according to claim 1, wherein the ZnO filled in the growing container is a sintered body. 6. When growing a ZnO single crystal using a hydrothermal synthesis method, (1) an alkaline solution containing ZnO and Zn ions and a ZnO seed crystal are placed in a growth container,
(2) Then, the growth container is sealed, and a ZnO single crystal is grown under high temperature and high pressure. 7. ZnO is filled in the lower part of the growth container to obtain Zn
7. The growing method according to claim 6, wherein the O seed crystal is filled in the upper part of the growing container, and the lower part of the growing container is adjusted to have a higher temperature than the upper part thereof. 8. The upper and lower parts of the growing container are set to 360 to 40.
The growing method according to claim 6 or 7, wherein heating is performed at 0 ° C and 380 to 420 ° C. 9. ZnO at 360 to 400 ° C. and 500 to 7
9. The growing method according to claim 6, wherein the alkaline solution containing ZnO and Zn ions is prepared by dissolving under high temperature and high pressure of 00 kg / cm ² . 10. The growing method according to claim 6, wherein the ZnO filled in the growing container is a sintered body. 11. The growing method according to claim 1, wherein a ZnO single crystal obtained by a hydrothermal synthesis method is used as the ZnO seed crystal.