WO2009130987A1

WO2009130987A1 - Process for production of zinc oxide single crystal substrate, single crystal substrate grown by the process, and semiconductor light-emitting device comprising the substrate and film formed thereon

Info

Publication number: WO2009130987A1
Application number: PCT/JP2009/056980
Authority: WO
Inventors: 森　信行; 丑田　隆史; 清水　信宏; 登坂上
Original assignee: ユーエムケー・テクノロジー株式会社
Priority date: 2008-04-26
Filing date: 2009-04-03
Publication date: 2009-10-29
Also published as: JPWO2009130987A1; JP5519492B2

Abstract

In a hydrothermal synthesis process of growing a zinc oxide single crystal on a zinc oxide seed crystal under supercritical conditions by using an aqueous KOH solution as the solvent, the stoichiometric composition is attained by cutting a zinc oxide seed crystal in the direction perpendicular to the c-axis; using the c-face as single crystal growth face; using, as the mineralizer, only KOH which has an effect of growing a single crystal in the direction of c-axis; adjusting the temperature difference (ΔT) between dissolution zone (into which zinc oxide is fed) and single crystal zone to 3 to 7° to suppress the mineralizing action of KOH; adding hydrogen peroxide as an oxidizer for attaining the stoichiometric composition; cooling the inside of a single crystal growth vessel prior to the addition to inhibit the decomposition of hydrogen peroxide and thus control the oxygen partial pressure; and using, as the starting zinc oxide, zinc oxide particles which are precipitated by hydrothermal synthesis in an aqueous alkaline solution containing an oxidizer.

Description

Zinc oxide single crystal substrate manufacturing method, single crystal substrate grown by the method, and semiconductor light emitting device formed on the substrate

The present invention relates to a method for producing a zinc oxide single crystal substrate, a single crystal substrate grown by the method, and a semiconductor light emitting device formed on the substrate.

In recent years, as a material constituting an electronic device including a semiconductor element, the energy band width of a direct transition type and a forbidden band is large (ZnO: 3.37 eV), and exciton binding energy (ZnO: 60 meV) is used as another compound semiconductor. Because it is very large compared to the materials used (GaN: 26 meV, ZnSe: 17 meV, etc.), it can be used as a highly efficient light-emitting device, and it can be expected to have excellent characteristics such as a relatively high operating temperature. Zinc oxide single crystals are attracting attention.
In order to create a semiconductor element using these zinc oxide single crystals, a substrate for forming the zinc oxide single crystals is required, but conventionally, zinc oxide single crystals having a crystal structure having no defects cannot be grown. Instead, attempts have been made to use a sapphire substrate or the like for crystal layer formation. However, since the crystal structure is originally different, a zinc oxide single crystal layer cannot be formed directly on the sapphire substrate, and it is not possible to form a zinc oxide single crystal layer without crystal defects even through a buffer layer. It was difficult.

For this reason, in order to realize a semiconductor element made of a material called zinc oxide or a zinc oxide type, a single crystal substrate originally made of zinc oxide having the same crystal structure has been desired.
In addition, as a substrate for a semiconductor element, any of the crystal substrates conventionally tried such as a sapphire substrate is insulative, but zinc oxide can be made conductive by adding a dopant, which makes it convenient as a substrate for a semiconductor element. It has special characteristics.

However, it has been difficult to produce a zinc oxide single crystal substrate having no crystal defects, having a very low impurity concentration, and having a size suitable for semiconductor device production.
As a method for growing zinc oxide single crystals, a method using a hydrothermal synthesis method has been proposed as a method with few crystal defects, and a method according to Non-Patent Documents 1 and 2 has been reported by one of the present inventors.
According to this, a sintered body of zinc oxide as a starting material and a seed crystal are respectively arranged at the lower and upper parts of a container such as platinum, filled with an aqueous solution of potassium hydroxide and lithium hydroxide, and used as an oxygen generator. Under supercritical conditions where hydrogen oxide is added and the temperature in the container is 370 to 400 ° C. and the pressure is 70 to 100 mPa (700 to 1000 kg / cm ² ), the region temperature where the starting material is disposed is set to the region where the seed material is disposed. A zinc oxide single crystal is grown on a seed crystal at a high temperature of 10 to 15 ° C.

JP-A-6-279192 JP 2003-146657 A JP 2003-335516 A JP 2005-225740 A JP 2003-221298 A JP 2006-225213 A Japanese Patent Application Laid-Open No. 7-242496

In addition, a zinc oxide single crystal grown by a hydrothermal synthesis method is prone to so-called oxygen vacancies in which oxygen atoms are missing from the crystal lattice, resulting in crystal defects.
Non-Patent Documents 1 and 2 described above, on the other hand, add hydrogen peroxide to the solvent to maintain the oxygen partial pressure, prevent oxygen vacancies, and make the zinc and oxygen atoms stoichiometrically 1: 1. However, it is difficult to keep the chemically unstable hydrogen peroxide in such a relationship under the actual reaction conditions in the container.
Instead of hydrogen peroxide, it is proposed to use barium peroxide, calcium peroxide, sodium percarbonate, or potassium percarbonate as an oxygen generator (Patent Document 6), and the stoichiometric composition is achieved. The substrate has a resistance value of 1 × 10 ⁹ Ωcm or more and a thickness of 1 mm, and the light transmittance is 75 to 80% or more. The degree of crystal defects and the influence of other impurities have not been clarified. The evaluation of is not clear.

Further, as a semiconductor element substrate, it is necessary to extremely reduce impurities in addition to these crystallinities. Contamination from the growth environment must be reduced as much as possible not only in the starting material but also in the single crystal growth process, and the elements in the alkali material used in the crystal growth described above are no exception. In particular, Li is easy to enter the crystal lattice and has a large influence such as diffusion to the zinc oxide single crystal thin film layer formed on the substrate and affecting the operation of a semiconductor element such as a light emitting diode.
Some light-emitting diodes are not affected by a very small amount of Li contamination, but it is necessary to avoid Li contamination as much as possible except for these applications.

The problem to be solved is
Improves crystallinity not only in the a-axis and m-axis directions but also in the c-axis direction. Furthermore, the stoichiometric composition is suitable for semiconductor elements such as LEDs and LDs that do not contain defects and impurities that start with Li. A method and a substrate for producing a zinc oxide single crystal (substrate) and a semiconductor element thereof.

In the present invention, an alkaline aqueous solution containing KOH as a mineralizing agent to which an oxidizing agent is added is used to produce a seed at a temperature difference ΔT between a zinc oxide dissolution zone and a zinc oxide single crystal growth zone under supercritical conditions. In the method for growing a single crystal on a crystal, a zinc oxide seed crystal cut out in a plane perpendicular to the c-axis direction is used as the seed crystal, and ΔT is maintained in the range of 3 to 7 ° C. A method for growing a zinc oxide single crystal substrate, comprising growing a single crystal while suppressing growth in the c-axis direction of
As a mineral containing no mineralizer other than KOH as the mineralizer, a single crystal that does not contain harmful Li in a light emitting device,
The above oxidizing agent is H ₂ O _2, and the stoichiometric composition is achieved by suppressing the decomposition of H ₂ O ₂ by cooling the starting materials and the solvent during the addition,
Furthermore, the stoichiometric composition is achieved by using crystal grains in which zinc oxide as the starting material is precipitated by hydrothermal synthesis using a zinc oxide powder sintered body as a raw material.

Further, the present invention is a zinc oxide for a starting material for growing a zinc oxide single crystal by a hydrothermal synthesis method, characterized by comprising crystal grains precipitated by hydrothermal synthesis in an alkaline aqueous solution to which an oxidizing agent is added, It is a starting material that achieves stoichiometric composition in the growth of zinc oxide single crystals by hydrothermal synthesis.
In addition, in an alkaline aqueous solution containing KOH as a mineralizer to which an oxidizing agent is added, on the seed crystal at a temperature difference ΔT between a zinc oxide dissolution zone and a zinc oxide single crystal growth zone under supercritical conditions. In a method for growing a single crystal,
While adding LiOH to the alkaline aqueous solution and suppressing crystal growth in the c-axis direction by KOH, the crystal growth in the a-axis and m-axis directions is promoted,
Next, the single crystal is grown in the c-axis direction of the zinc oxide crystal by KOH by maintaining ΔT in the range of 3 to 7 ° C. in an alkaline aqueous solution not containing LiOH and containing only KOH as a mineralizer. A method of growing a zinc oxide single crystal substrate characterized by growing a single crystal while suppressing the crystallinity improvement and efficient single crystal growth in a zinc oxide single crystal growth that does not strictly require Li-free conditions. Achieve crystal growth.

Further, in the present invention, in an alkaline aqueous solution containing KOH as a mineralizer to which an oxidizing agent is added, the temperature difference ΔT between the zinc oxide dissolution zone and the zinc oxide single crystal growth zone under supercritical conditions A single crystal grown on a seed crystal,
The seed crystal is a zinc oxide seed crystal cut in a plane perpendicular to the c-axis direction,
By maintaining ΔT in the range of 3 to 7 ° C., it consists of a zinc oxide single crystal formed while suppressing the growth in the c-axis direction of the zinc oxide crystal by KOH,
With respect to the emission output of the intrinsic emission wavelength having the peak emission at the wavelength of 3.37 eV in the photoluminescence light by the irradiation of the light of the wavelength of 325.0 nm at the temperature of 11 ° K, the green band emission output of the wavelength of 1.4 to 3.2 eV is A zinc oxide single crystal substrate characterized by having a characteristic that the emission intensity is 0.1% or less,
The single crystal substrate is formed under a condition that does not contain LiOH as a mineralizer, substantially does not contain Li, the oxidizing agent is H ₂ O ₂ , and further, This is a zinc oxide single crystal substrate that achieves a stoichiometric composition and improves crystallinity using crystal grains precipitated by hydrothermal synthesis using zinc oxide powder sintered body as a raw material in zinc oxide.

Furthermore, the present invention provides a temperature difference between a zinc oxide dissolution zone and a zinc oxide single crystal growth zone under supercritical conditions in an alkaline aqueous solution containing KOH as a mineralizer to which an oxidizing agent is added. A single crystal grown on a seed crystal at ΔT,
The seed crystal is a zinc oxide seed crystal cut in a plane perpendicular to the c-axis direction,
By maintaining ΔT in the range of 3 to 7 ° C., it consists of a zinc oxide single crystal formed while suppressing the growth in the c-axis direction of the zinc oxide crystal by KOH,
With respect to the emission output of the intrinsic emission wavelength having the peak emission at the wavelength of 3.37 eV in the photoluminescence light by the irradiation of the light of the wavelength of 325.0 nm at the temperature of 11 ° K, the green band emission output of the wavelength of 1.4 to 3.2 eV is A semiconductor light emitting device comprising a zinc oxide single crystal having a light emission intensity of 0.1% or less as a substrate and a functional single crystal layer formed thereon, wherein the oxidant is a H ₂ O _2, further using the crystal grains was allowed deposited by hydrothermal synthesis of zinc oxide powder sintered zinc oxide as the starting material as a raw material, to achieve a stoichiometric composition, the crystalline It is a semiconductor light emitting device that improves and achieves high functionality.

The present invention suppresses crystal growth in the c-axis direction under strict temperature control in which ΔT is in the range of 3 to 7 ° C., regardless of the crystallinity improvement accompanied by the crystal growth suppressing effect in the c-axis direction by LiOH. The most important feature is to improve sex.
In general, the growth of zinc oxide crystals by these hydrothermal synthesis methods uses an alkaline aqueous solution containing KOH and LiOH as a solvent, a dissolved region of zinc oxide as a starting material, and a region in which single crystal growth is performed on a zinc oxide seed crystal. A certain temperature difference ΔT is provided between them to crystallize the zinc oxide transported to the single crystal growth region, and ΔT is generally performed in the range of 10 to 25 ° C.
It is said that KOH and LiOH as alkalis generally act as mineralizers for crystal growth in the c-axis direction of crystals and LiOH in the a- and m-axis directions. It is said that the effect of promoting crystal growth in the c-axis direction is large.
For this reason, according to the above-mentioned Patent Document 7, in the zinc oxide single crystal growth method by the hydrothermal synthesis method, crystal growth in the c-axis direction is promoted by an aqueous potassium hydroxide solution, and then potassium hydroxide and lithium hydroxide are added. A single crystal is efficiently grown by mainly performing crystal growth in the a- and m-axis directions in the aqueous solution.
However, although KOH has a large crystal growth effect as a mineralizer, it tends to cause crystal defects, and LiOH as a mineralizer in the a-axis and m-axis directions suppresses the action of KOH and improves crystallinity. It is thought that.
Accordingly, the crystallinity in the c-axis direction cannot be improved by such a method.

According to the experiments by the present inventors, KOH not only acts as a solvent but also has a remarkably large effect as a mineralizer, affecting the crystallinity. LiOH is a mineral in the a and m-axis directions. In addition to the agent, it is thought that the growth of these crystal axes is balanced by suppressing the action of KOH as a mineralizer.
On the other hand, Li easily enters the zinc oxide single crystal, and further penetrates into the functional layer formed on the substrate as a mobile ion, and causes a harmful effect on the characteristics of the LED, LD, and the like. Therefore, if only KOH is used, these contaminations can be avoided, but if KOH alone is used, the crystal in the c-axis direction proceeds and the crystallinity is not good as described above.
By the way, although these crystal formation reactions are known as hydrothermal deposits in nature, these reactions proceed in a very long time. Since the scale of the deposit is huge, the temperature difference in the deposit is small, and the reaction proceeds in geological time, so to speak, crystals are grown while maintaining the equilibrium condition for a long period of time. Since the hydrothermal process of single crystal growth aimed at by the present invention is essentially the same as these, mineralization by KOH from the viewpoint that the crystallinity can be improved by approaching the equilibrium condition for crystal formation. Attempts were made to control the action of the agent by bringing it closer to these equilibrium conditions.
That is, since the solubility in the solvent depends on the temperature, it is considered that ΔT has the largest influence on the influence of these equilibrium conditions.
According to the confirmation of the present inventors, the optimum ΔT was much smaller than that usually employed and was in the range of 3 to 7 ° C. Even if outside the upper limit of the temperature range, a single crystal can be produced as conventionally performed, but the generation of crystal defects has not been sufficiently suppressed, and the crystallinity has not been satisfactory. Further, if the temperature falls outside the lower limit of the temperature range, single crystal growth does not occur. That is, Patent Document 7 mentions a range of 5 to 25 ° C. as ΔT, which is much smaller than that and the allowable temperature range is only 4 ° C.

With respect to these ΔTs, in Patent Document 6, in order to obtain a stoichiometric composition, in a reaction in which crystal growth is performed in an alkaline aqueous solution containing LiOH and KOH, when ΔT exceeds 10 ° C., zinc oxide As the supersaturation degree of C may collapse at a stretch and nucleation may occur and the efficient growth of seed crystals may be inhibited, it is 10 ° C. or less, preferably 8 ° C. or less, more preferably 6 ° C. or less and 1 ° C. or more. In addition, when the temperature is lower than 1 ° C., the seed crystal is melted or the growth of the seed crystal is rapidly slowed down, and the productivity is inferior.
That is, ΔT is set in the range of 6 ° C. to 1 ° C. to achieve both the crystallinity and the crystal growth rate. These conditions are conditions for coexistence of LiOH and KOH, and are used as a mineralizing agent for LiOH. It is said that the stoichiometric composition has been achieved with respect to these crystal growth effects, but there is no detail about crystallinity.
On the other hand, according to the study by the present inventors, a significant improvement in crystallinity was obtained by maintaining the value of ΔT within the range of 3 to 7 ° C. under the condition that KOH alone does not contain LiOH. .
That is, although these temperature ranges themselves overlap with the above-described conventional attempts, the range is extremely small, and the effect is exhibited as an effect of suppressing crystal growth in the c-axis direction.
As a result, in the present invention, a Li-free zinc oxide single crystal can be grown by not including LiOH.

The second feature of the present invention is a hydrothermal synthesis in which zinc oxide, which is a starting material, is used instead of a sintered body of zinc oxide powder that has been widely used in order to achieve a stoichiometric composition and reduce crystal defects. This is because the granular zinc oxide precipitated by the above method is used.
Sintered bodies made from zinc oxide powder are used, including Non-Patent Documents 1 and 2, and these sintered bodies release powdered zinc oxide as the dissolution progresses, and the single crystal growth region It has been regarded as a problem that it has an influence such as intruding into the crystal and adhering to crystals during the growth process. Furthermore, Patent Document 6 describes that oxygen vacancies are generated in zinc oxide as a raw material due to these sintering, and it is described that the sintered body is previously hydrothermally treated in an aqueous solution containing hydrogen peroxide. However, as long as the sintered body is used, these problems have not been solved.
Due to the nature of zinc oxide that is inherently prone to oxygen vacancies, the present inventors have produced strong oxygen vacancies in the sintered zinc oxide, and it is difficult to compensate for the vacancies even when an oxidizing agent is added during the crystal growth process. As a starting material, the idea was to start with zinc oxide that satisfies the stoichiometric composition without oxygen deficiency.
That is, a zinc oxide powder sintered body is used as a raw material, dissolved in an aqueous solution containing an oxidant, and precipitated as zinc oxide by hydrothermal synthesis, and grown until it has a preferred particle size (diameter 2 to 5 mm) in terms of dissolution characteristics. By using the zinc oxide grains as a starting material for growing a single crystal, oxygen deficiency derived from the starting material in the grown single crystal can be eliminated, and scattering of powdered zinc oxide can also be eliminated. It goes without saying that these zinc oxides may be formed as a bulk and then crushed and used as particles of an appropriate size.
Furthermore, although it is thought that strict control of oxygen partial pressure is necessary to achieve the stoichiometric composition, although hydrogen peroxide solution is excellent in maintaining purity, it is chemically active and easily decomposed. Therefore, the decomposition reaction proceeds simultaneously with the addition to the aqueous solution. For this reason, it escapes rapidly as gaseous oxygen from the added aqueous solution and is easily lost from the growth vessel, and it is difficult to accurately control the added amount so as to achieve the calculated oxygen partial pressure.
Therefore, the present inventors cooled the growth vessel after filling the raw material zinc oxide and the alkaline aqueous solution, and suppressed the decomposition reaction of hydrogen peroxide. Specifically, by immersing the growth vessel in liquid nitrogen, freezing the aqueous solution, and adding and injecting hydrogen peroxide water thereon, the vessel is sealed without the decomposition of hydrogen peroxide progressing. It can be performed.
By the above method, the oxygen partial pressure in the single crystal growth vessel is properly maintained, the generation of oxygen vacancies in the grown single crystal is effectively suppressed, and the oxygen vacancies are increased in the growth of the single crystal in the c-axis direction. Therefore, it is possible to grow a complete single crystal while suppressing the occurrence of crystal defects due to the occurrence of the defect.

The zinc oxide single crystal substrate of the present invention has very few crystal defects in the crystal structure grown in the c-axis direction, has improved stoichiometric composition and crystallinity, and is suitable as a substrate for various semiconductor elements, In addition, since it is Li-free, it can exhibit characteristics suitable as a substrate for LEDs and LDs.

Hereinafter, based on description of drawing, the manufacturing method of the zinc oxide single crystal substrate of this invention by a hydrothermal synthesis method is demonstrated.
[Example 1]

FIG. 1 (A) shows a zinc oxide single crystal growing apparatus 11 and FIG. 1 (B) shows a zinc oxide single crystal growing vessel 31 used in the apparatus.
In FIG. 1 (A), the single crystal growing apparatus 11 comprises a high pressure autoclave 12 that can be in a supercritical condition, and

heaters

21 and 22 for controlling temperature conditions along the growing process of the present invention. The entire apparatus is shut off from the outside by a heat insulating material 23 in order to keep the temperature condition constant.
The autoclave 12 is a pressure-resistant container, and a single crystal growing container 31 is set in the accommodating portion 13, and the opening is closed by the lid 16 to seal the inside. The lid includes a packing member 15, and the autoclave main body 14 and the lid 16 are firmly fastened with screws 17 to maintain a sealed state under high pressure.
The upper and lower portions of the outer periphery of the autoclave 12 are kept under a constant temperature condition by the

heaters

21 and 22 arranged above and below.

As shown in FIG. 1B, the single crystal growing vessel 31 is formed of a material that does not elute impurities under the internal reaction conditions, such as platinum (Pt) or gold (Au), and the inside is a single crystal in which a seed crystal is set. The two regions are divided by an internal baffle plate 34, and the flow of the solvent is controlled by a large number of holes provided in the baffle plate 34.
Similarly, seed crystals 41a to 41d are suspended and set inside the container by a suspension made up of a frame 32 made of platinum or the like and a support 33 so as not to elute impurities. At the upper end of the container, a lid 37 for sealing the container after setting starting materials and the like is provided. The lid is provided with a minute opening 38 for injecting a solvent, an oxidizing agent or the like.
Further, a baffle ring 35 having a circulation hole is similarly provided on the outer periphery of the container, and the container is supported in the autoclave accommodating portion 13.

Next, the method for growing a zinc oxide single crystal substrate of the present invention will be described.
First, a cleaned single crystal growth container main body 36 and a lid 37 are prepared, and a raw material region is filled with zinc oxide 51 as a starting material from the container main body opening in a clean room, and then an internal baffle plate 34 is set. The suspension with the seed crystals 41a to 41d suspended is set in the single crystal growth region.
The single crystal growth vessel 36 is covered with a lid 37 and welded. Then, an alkaline aqueous solution is injected from the solution injection micro-opening 38 on the upper surface of the lid, and the entire single crystal growth vessel is immersed in liquid nitrogen and cooled. After injecting the oxygen generator with the aqueous solution in a frozen state, the minute openings are sealed by welding.

The container 31 that has been prepared as described above is set in a state of being suspended by the external buffer ring 35 in the accommodating portion 13 in the autoclave 12. A space 18 of the accommodating portion 13 in the autoclave 12 is filled with pure water having a filling rate (70 to 75%) corresponding to the pressurizing condition, sealed with a packing member 15, and the lid 16 is firmly screwed. The
Thereafter, when the pressure in the autoclave is heated by the

heaters

21 and 22 and reaches a temperature suitable for single crystal growth of about 380 ° C., the pressure of the fluid (pure water) in the housing space 18 and the growth container Both the pressures rise and the equilibrium is maintained inside and outside the growth vessel, and the reaction material filled in the vessel is brought to a supercritical pressure of 100 MPa (1000 kg / cm ² ).
The conditions for this single crystal growth reaction were as follows.
Temperature conditions Autoclave top temperature T1: 415 ° C
Autoclave lower temperature T2: 430 ° C
Single crystal growth region temperature T3: 380 ° C.
Single crystal raw material region temperature T4: 384 to 385 ° C.
ΔT = T4-T3 = 4-5 ° C
Pressure condition: pressure in the growth vessel: 100 MPa (1000 kg / cm ² )
Seed crystal: Size of seed crystal: flaky seed crystal cut in a direction perpendicular to the c-axis having a diameter of 1 to 2 inches and a thickness of 0.5 to 1.0 mm.
Zinc oxide starting material: Granular oxidation deposited by hydrothermal synthesis method under supercritical conditions in alkaline aqueous solution using zinc oxide powder sintered body as raw material of zinc oxide starting material and adding hydrogen peroxide as oxidizing agent Zinc (diameter 2-5mm).
Hydrothermal synthesis in which crystalline zinc oxide, which is the starting material for zinc oxide, is precipitated, is performed under the same conditions as those for the growth of zinc oxide single crystal described below, but a starting material having a stoichiometric composition without oxygen deficiency is obtained. For the purpose, there is no strict restriction on the form of the seed crystal and ΔT, a platinum target is used instead of the seed crystal, and ΔT may be in the range conventionally used.
Solvent: Alkaline aqueous solution with 4 mol / L of KOH dissolved in pure water Filling ratio: Volume ratio of solvent to the space excluding raw materials, baffle plates, seed crystals, suspensions, etc. in the growth vessel: 83%
This filling rate is adjusted so that the internal pressure is 100 MPa (1000 kg / cm ² ) when the environment in the growth vessel heated by the

heaters

21 and 22 is crystal growth temperature: 380 ° C.
An oxygen generator is added to achieve a stoichiometric composition, but hydrogen peroxide (30%), which is a typical oxygen generator, was used. Hydrogen peroxide is most preferred because it does not introduce impurities into the growth vessel.
The amount of hydrogen peroxide solution added was weighed so as to have a stoichiometric composition strictly, and 0.03-0.04 mol / L was added to the alkaline aqueous solution.
The growth of the single crystal took about 3 hours to reach the above reaction conditions, and thereafter, the crystal was grown for 45 days stably maintaining the above conditions to obtain a zinc oxide single crystal.

The form of the zinc oxide single crystal (substrate) thus obtained is a hexagonal crystal structure reflecting the crystal structure as shown in FIG. 2 and FIG. As shown in A), a single crystal of zinc oxide is grown on each of the + c plane (c-axis direction zinc plane) and the −c plane (c-axis direction oxygen plane) of the seed crystal 41.
As is clear from FIG. 2, the zinc oxide single crystal grows in the c-axis direction with respect to the seed crystal 41, whereas large crystal growth occurs in the a-axis direction and the m-axis direction perpendicular to this. I can't see. As for c-axis direction crystal growth, it can be seen that the crystal growth in the + c plane is larger than the crystal growth in the -c plane.
The appearance of the obtained zinc oxide single crystal shows that the seed crystal obtained by hydrothermal synthesis in the presence of lithium hydroxide becomes a yellow colored crystal due to Li contained as an impurity in the crystal. It can be seen that the grown zinc oxide single crystal is colorless and highly transparent.

As will be described later, the zinc oxide

single crystals

52 and 53 are compared with each other in terms of the crystallinity of a single crystal substrate thinly sliced by a high-power microscope, and the zinc oxide single crystal 52 formed on the + c plane of the seed crystal 41 has a c-axis direction. However, the presence of defects could not be recognized in the zinc oxide single crystal 53 formed on the −c plane of the seed crystal 41. This is because when the single crystal is grown on the seed crystal 41 using the single crystal growth apparatus 11 shown in FIG. 1A, the crystal growth rate of the + c plane of the seed crystal 41 is the crystal growth rate of the −c plane. It is thought that it was larger than that. Therefore, in order to eliminate a slight defect in the zinc oxide single crystal 52 formed on the + c plane of the seed crystal, for example, ΔT (T ₄ -T ₃ ) is brought close to the minimum value of 3 ° C. as much as possible. The entire crystal growth rate of the zinc oxide

single crystals

52 and 53 with respect to the crystal 41 is slowed, or a baffle plate that selectively slows the crystal growth is arranged on the + c plane side of the seed crystal 41, etc. It is conceivable to slow the crystal growth rate of the zinc oxide single crystal 52.
As can be seen from these, a slight difference in crystal growth affects the crystallinity. However, if the above ΔT is smaller than 3 ° C., the crystal growth rate is remarkably lowered due to a decrease in transport efficiency or the like. On the other hand, when ΔT exceeds 7 ° C., the crystallinity is affected and it becomes difficult to grow a good single crystal. Therefore, from these conditions, the value of ΔT is limited to a narrow range of 3 to 7 ° C.
Within this range, the temperature range can be adjusted / changed according to other crystal growth conditions, or the optimum value can be selected empirically.

The zinc oxide single crystal 53 on the −c plane of the zinc oxide single crystal obtained under the above growth conditions is 0.5 to 1 mm in a direction perpendicular to the c axis (c plane and horizontal direction) by a diamond cutter or a wire cutter. By slicing to a thickness, polishing and CMP, and cleaning with ultrapure water, a zinc oxide single crystal substrate 54 shown in FIG. 2C is formed. The zinc oxide single crystal substrate 54 has a step terrace structure with a surface roughness on the c-plane of 1 nm or less, and its diameter (a-axis or m-axis direction) is substantially the same as that of the seed substrate 41.

It was confirmed that the lithium content of the formed zinc oxide single crystal substrate 54 was about three orders of magnitude lower than the lithium content of about 1 ppm contained as the seed crystal 41. It can be said that the zinc oxide single crystal substrate 54 containing only this level of lithium essentially does not contain lithium. Further, the zinc oxide single crystal 54 obtained as shown in FIG. 2 (c) is used as the seed crystal 41, and the zinc oxide single crystal growing step by hydrothermal synthesis shown in FIG. It is also possible to grow zinc oxide single crystals with a smaller lithium content. Therefore, the zinc oxide single crystal substrate obtained by the present invention is essentially free of lithium.

In order to form the zinc oxide single crystal substrate 54 of the present invention shown in FIG. 2C, first, a seed crystal 41 having substantially the same diameter as the single crystal substrate 54 in the a-axis and m-axis directions is formed. It is necessary to prepare. A method for producing the seed crystal 41 having such a diameter will be additionally described. Such a seed crystal 41 may be produced by using the equipment described in FIGS. 1A to 1C in the method for producing the zinc oxide single crystal substrate 54 of the present invention described above and performing the same steps.

A method for producing the seed crystal 41 will be described with reference to FIGS.
In the production of the seed crystal 41, LiOH was further added to the alkaline aqueous solution for obtaining the zinc oxide single crystal substrate 54 as the composition of the alkaline aqueous solution injected into the single crystal growth vessel 31. An alkaline aqueous solution of the composition is used. The zinc oxide raw material may be zinc oxide crystal grains having a purity of 7N or higher grown by hydrothermal synthesis, but a zinc oxide sintered body having a purity of 7N or higher may be used. Furthermore, the

heaters

21 and 22 may be driven so that ΔT (T ₄ −T ₃ ) is about 10 ° C. Other conditions are not essentially different from the conditions for producing the zinc oxide single crystal substrate 54 of the present invention.
As a result, as shown in FIGS. 3A and 3B, the seed crystal 61 for forming the seed crystal 41 has a c-axis direction and a direction perpendicular to the c-axis (a Zinc oxide

single crystals

42 and 43 crystal-grown in the axial and m-axis directions) can be obtained. If this is sliced with a diamond cutter or a wire cutter in the same manner as described above and washed, the diameter in the direction perpendicular to the c-axis direction required for the zinc oxide single crystal substrate 54 of the present invention as shown in FIG. For example, the seed crystal 41 having the same size as 1 inch or 2 inches can be formed.
However, in order to obtain the seed crystal 41, it is not always necessary to perform polishing and CMP treatment like the zinc oxide single crystal substrate 54 of the present invention. The reason is that the surface of the seed crystal 41 is dissolved at the initial stage of the single crystal growth step by hydrothermal synthesis shown in FIG. 1 for producing the zinc oxide single crystal substrate 54 of the present invention, and the surface treatment is performed. Because it is done.

In the seed crystal 41 formation step, lithium hydroxide is used for the alkaline aqueous solution, and a temperature difference of about 10 ° C. is used as ΔT, so that the needle is good in the a-axis and m-axis directions perpendicular to the c-axis (the needle is The crystal growth (which does not occur) is performed, the diameter in this direction can be increased, and a zinc oxide single crystal with few crystal defects is grown.
However, the obtained seed crystal 41 contains about 1 ppm of lithium as an impurity, and becomes a transparent but colored yellow single crystal. The lithium contained as an impurity in the seed crystal 41 cannot be precipitated if it exists between the sites of the zinc oxide crystal. However, if it exists between the lattices of the zinc oxide crystal, the zinc oxide single crystal 52 shown in FIG. And 53 in the hydrothermal synthesis process for annealing or annealing.
Therefore, lithium contained in a very small amount in the zinc oxide single crystal substrate 54 of the present invention dissolves the seed crystal 41 with an alkaline aqueous solution in a supercritical state by hydrothermal synthesis for forming the substrate 54 (at the initial growth stage). It is considered that the surface of the seed crystal and dissolution of the seed crystal during the growth period) were slightly dissolved in the alkaline aqueous solution and mixed into the grown zinc oxide single crystal. Therefore, when the zinc oxide single crystal growth by the hydrothermal synthesis described above is performed using the zinc oxide single crystal substrate 54 essentially free of lithium as a seed crystal as described above, the zinc oxide of the present invention obtained thereby is obtained. Even if the single crystal substrate does not contain lithium or contains lithium, an extremely small amount of zinc oxide single crystal substrate can be formed.
In addition, after the growth of the above zinc oxide seed crystal, the c-axis direction zinc oxide single crystal is maintained in a range of 3 to 7 ° C. with an aqueous solution containing only the above-mentioned Li and containing only KOH as a mineralizer. By performing the growth, a zinc oxide single crystal having improved crystallinity in the c-axis direction can be efficiently grown.

As described in detail above, the zinc oxide single crystal substrate 54 formed according to the present invention is essentially free of lithium, is a colorless transparent crystal, and is essentially free of defects and impurities in the crystal surface and crystal. The zinc oxide single crystal having a stoichiometric composition without oxygen vacancies is formed on the formed zinc oxide single crystal substrate 54. This can be confirmed by performing a luminescence inspection.

This will be described with reference to FIG.
FIG. 4 is a diagram showing the results of photoluminescence inspection of the zinc oxide single crystal substrate 54 (shown as A) and the currently available zinc oxide single crystal substrates (shown as B and C) of the present invention.
When each of the substrates (A, B and C) is irradiated with light of 325.0 nm by a He—Cd laser at a temperature of 11 ° K, photoluminescence light is obtained from the zinc oxide single crystal substrate. .
In FIG. 4, emission intensity (vertical axis is logarithmically expressed) generated from each substrate (A, B, and C) is emitted at a band edge emission wavelength (370 nm: 3.37 eV) showing steep emission characteristics. Normalized by intensity, the difference in emission intensity other than the intrinsic emission wavelength is shown.
Currently available zinc oxide single crystal substrates (B and C) emit light with a band edge emission wavelength (370 nm: 3.37 eV) exhibiting steep emission characteristics, and a wavelength range from about 470 nm to about 700 nm (2.9 to 1). .4 eV), and emission having a center at a green wavelength (500 nm to 600 nm) is detected with a broad characteristic. The light emission mechanism of this green light emission (green band light emission) is not always clear, but a combination of multiple point defects caused by interstitial Zn, oxygen deficiency, interstitial zinc, interstitial oxygen, various impurities, etc. It is said that it is caused by. It is considered that the extremely broad light emission characteristics in green light emission (green band light emission) are based on these combined causes.
In contrast, in the zinc oxide single crystal substrate 54 (A) of the present invention, the emission intensity of the green band emission is significantly smaller than the emission intensity of the band edge emission wavelength (370 nm: 3.37 eV) ( The emission intensity ratio is 0.1% or less), and the values measured in these regions are almost noise levels and there is almost no green band emission. According to this, it can be said that the zinc oxide single crystal substrate 54 of the present invention is a single crystal with very few defects and impurities.

The zinc oxide single crystal substrate can be used as a substrate on which a functional thin film constituting a semiconductor element such as a light emitting diode is formed.
As described above, in recent years, a zinc oxide single crystal substrate is required to form a semiconductor element using a zinc oxide single crystal thin film. However, in order to form a zinc oxide single crystal thin film that operates as a semiconductor element, a crystal structure of In addition to being the same zinc oxide, it is essential that the substrate itself is free of harmful impurities and crystal defects.
Of course, the zinc oxide single crystal obtained by the method of the present invention uses high-purity zinc as a starting material. However, in the crystal growth process, the crystal growth rate is controlled by selecting the crystal axis and the mineralizer, oxygen The above characteristics were achieved by a method of adding H ₂ O ₂ for suppressing defects and a growth method using zinc oxide grains from which oxygen defects were eliminated as a starting material.
That is, since the above-mentioned green band emission intensity is extremely low and has been reduced to almost the noise level, this zinc oxide single crystal has almost no crystal defects and impurities to hinder the operation of the semiconductor element. It can be said that it has been resolved.
In addition, a light-emitting element having high emission wavelength purity that is not affected by green band light emission can be formed using this substrate.
Further, as described above, the absence of contamination by Li eliminates an important obstacle in the characteristics of the zinc oxide functional thin film operating as a light emitting diode.

According to the zinc oxide single crystal substrate, as a functional semiconductor single crystal thin film to be formed on the substrate, in addition to zinc oxide, so-called zinc oxide based compounds such as ZnMgO, ZnCdO, ZnSeO, or ZnSO have been proposed so far. These can be applied, and both of them have the same crystal structure and the same lattice constant or the difference between them is extremely small, so that they are excellent single crystal growth substrates.
The inventors of the present invention confirmed the emission characteristics by laminating a zinc oxide single crystal thin film, which is the basic structure of a light emitting diode, using the zinc oxide single crystal substrate obtained as described above.
Here, the present inventors realized a high-quality laminated thin film with few crystal defects by performing homoepitaxial growth on the above-described zinc oxide single crystal substrate using the RS-MBE method.
In short, the characteristics of these functional single crystal thin films basically depend on the crystal structure and quality of the substrate, and the substrate is the basic of the functional single crystal thin film that becomes the semiconductor element formed thereon. The quality is defined, and the device is integrated.

FIG. 6 shows the basic configuration of a light emitting diode whose emission was confirmed using the zinc oxide single crystal substrate 54 formed as described above.
This is because, on a zinc oxide single crystal substrate 54, an n-type cladding layer 301 made of ZnO: Ga doped with gallium as an n-type impurity, an active layer 302 made of ZnO: Cd doped with cadmium, and a p-type impurity. A p-type cladding layer 303 made of ZnO: N doped with nitrogen is stacked. Since the

clad layers

301 and 303 need to confine carriers in the active layer 302, the

clad layers

301 and 303 are selected so as to have a forbidden band width larger than that of the active layer 302.
In the figure, 304 and 305 are electrode layers.

Further, by using the zinc oxide single crystal of the present invention as a substrate, not only the light emitting element structure described above but also a layer structure in which a light emitting element made of a zinc oxide based semiconductor is formed, a semiconductor element exhibiting excellent characteristics is formed. can do.
Thin films formed on a zinc oxide single crystal substrate as exemplified above include zinc oxide semiconductors such as ZnO, ZnMgO compounds, ZnCdO compounds, ZnSeO compounds, and ZnSO compounds. Furthermore, there is a ZnO-based thin film in which a semiconductor element is formed by stacking a plurality of the above thin films by homojunction on a zinc oxide single crystal substrate. Of course, the lattice constant between the zinc oxide single crystal substrate and the ZnO thin film or the ZnO compound thin film, between the ZnO thin film and the ZnO compound thin film, and between the different types of ZnO compound thin films is the same or almost the same, and Since there is no difference in thermal expansion coefficient between them, the crystallinity of the ZnO-based thin film formed by stacking can be improved, and the optical and electrical characteristics and reliability of the semiconductor element can be remarkably improved.

The method for producing a zinc oxide single crystal substrate according to the present invention is excellent in crystallinity, free from crystal defects, has a stoichiometric composition and does not contain harmful Li, such as a light emitting element. It is possible to provide a substrate on which various semiconductor elements are formed, and contribute greatly to the industry together with the semiconductor elements formed on the substrate.

Autoclave (A) and zinc oxide single crystal growth vessel (B) used in the present invention. The figure which shows the external appearance of the zinc oxide single crystal obtained by the hydrothermal synthesis method of this invention. The figure which shows the external appearance of the seed crystal used for the hydrothermal synthesis method of this invention, and the crystal | crystallization. The characteristic view of the zinc oxide single crystal obtained by the hydrothermal synthesis method of this invention, and the zinc oxide single crystal of a prior art example. The zinc oxide single crystal appearance of the present invention. The basic structure of the light emitting element of this invention.

DESCRIPTION OF SYMBOLS 11 Single crystal growth apparatus 12 Autoclave 13 Single crystal growth container accommodating part 14 Autoclave main body 15 Packing member 16 Cover body 17 Screw 18

Internal space

21, 22 Heater 23 Heat insulating material 31 Single crystal growth container 32 Frame 33 Seed crystal support 34 Internal baffle Plate 35 External buffering 36 Container body 37 Growth vessel lid 38 Small opening for injection 41a to 41d Seed crystal 42 Zinc oxide single crystal 43 Zinc oxide single crystal 51 Zinc oxide single crystal 52 Zinc oxide single crystal 53 Zinc oxide single crystal 54 Oxide Zinc single crystal substrate 61 Zinc acid seed crystal 301 n-type cladding layer made of ZnO: Ga 302 Active layer made of ZnO: Cd 303 p-type cladding layer 303 made of ZnO: N
304 electrode 305 electrode

Claims

In an alkaline aqueous solution containing KOH as a mineralizer to which an oxidizing agent is added, a single crystal is formed on the seed crystal at a temperature difference ΔT between the zinc oxide dissolution zone and the zinc oxide single crystal growth zone under supercritical conditions. In the method for growing crystals,
A zinc oxide seed crystal cut out in a plane perpendicular to the c-axis direction as the seed crystal,
A method for growing a zinc oxide single crystal substrate, wherein a single crystal is grown while suppressing growth in the c-axis direction of a zinc oxide crystal by KOH by maintaining ΔT in a range of 3 to 7 ° C.
The method for growing a zinc oxide single crystal substrate according to claim 1, wherein the mineralizer contains no mineralizer other than KOH.
The zinc oxide single crystal substrate growth according to claim 1 or 2, wherein the oxidizing agent is H 2 O 2 and the decomposition of H 2 O 2 is suppressed by cooling the starting material and the solvent at the time of addition. Method.
The method for growing a zinc oxide single crystal substrate according to any one of claims 1 to 3, wherein the zinc oxide as the starting material is a crystal grain precipitated by hydrothermal synthesis using a zinc oxide powder sintered body as a raw material.
A zinc oxide for starting material for growing a zinc oxide single crystal by a hydrothermal synthesis method, characterized by comprising crystal grains precipitated by hydrothermal synthesis in an alkaline aqueous solution to which an oxidizing agent is added.
In an alkaline aqueous solution containing KOH as a mineralizer to which an oxidizing agent is added, a single crystal is formed on the seed crystal at a temperature difference ΔT between the dissolution zone of the starting zinc oxide and the zinc oxide single crystal growth zone under supercritical conditions. In the method for growing crystals,
While adding LiOH to the alkaline aqueous solution and suppressing crystal growth in the c-axis direction by KOH, the crystal growth in the a-axis and m-axis directions is promoted,
Next, the single crystal is grown in the c-axis direction of the zinc oxide crystal by KOH by maintaining ΔT in the range of 3 to 7 ° C. in an alkaline aqueous solution not containing LiOH and containing only KOH as a mineralizer. A method for growing a zinc oxide single crystal substrate, wherein the single crystal is grown while being suppressed.
Grown on seed crystal in temperature difference ΔT between zinc oxide dissolution zone and zinc oxide single crystal growth zone under supercritical conditions in alkaline aqueous solution containing KOH as mineralizer with oxidizing agent added A single crystal,
The seed crystal is a zinc oxide seed crystal cut in a plane perpendicular to the c-axis direction,
By maintaining ΔT in the range of 3 to 7 ° C., it consists of a zinc oxide single crystal formed while suppressing the growth in the c-axis direction of the zinc oxide crystal by KOH,
With respect to the emission output of the intrinsic emission wavelength having the peak emission at the wavelength of 3.37 eV in the photoluminescence light by the irradiation of the light of the wavelength of 325.0 nm at the temperature of 11 ° K, the green band emission output of the wavelength of 1.4 to 3.2 eV is A zinc oxide single crystal substrate characterized by having a characteristic that the emission intensity is 0.1% or less.
It is characterized by being formed under conditions that do not contain LiOH as a mineralizer,
The zinc oxide single crystal substrate according to claim 7.
The zinc oxide single crystal substrate according to claim 7 or 8, wherein the oxidizing agent is H 2 O 2 .
10. The zinc oxide single crystal substrate according to claim 7, wherein the zinc oxide as the starting material is a crystal grain precipitated by hydrothermal synthesis using a zinc oxide powder sintered body as a raw material.
In an alkaline aqueous solution containing an oxidizing agent, not containing LiOH as a mineralizer and containing KOH,
A single crystal grown on a seed crystal at a temperature difference ΔT between a dissolution region of zinc oxide as a starting material and a zinc oxide single crystal growth region under supercritical conditions,
The seed crystal is a zinc oxide seed crystal cut in a plane perpendicular to the c-axis direction,
By maintaining ΔT in the range of 3 to 7 ° C., it consists of a zinc oxide single crystal formed while suppressing the growth in the c-axis direction of the zinc oxide crystal by KOH,
With respect to the emission output of the intrinsic emission wavelength having the peak emission at the wavelength of 3.37 eV in the photoluminescence light by the irradiation of the light of the wavelength of 325.0 nm at the temperature of 11 ° K, the green band emission output of the wavelength of 1.4 to 3.2 eV is A semiconductor light emitting device comprising a zinc oxide single crystal having a light emission intensity of 0.1% or less as a substrate and a functional single crystal layer formed thereon.
The semiconductor light emitting device according to claim 11, wherein the oxidizing agent is H 2 O 2 .
13. The semiconductor light emitting device according to claim 11, wherein the zinc oxide as the starting material is a crystal grain precipitated by hydrothermal synthesis using a zinc oxide powder sintered body as a raw material.