JP2009234825A - METHOD FOR MANUFACTURING ZnO SINGLE CRYSTAL AND SELF-SUPPORTING WAFER OF ZnO SINGLE CRYSTAL OBTAINED BY IT - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a self-supporting ZnO single crystal having a low Li concentration. <P>SOLUTION: ZnO as a solute is mixed with a solvent and melted. A ZnO single crystal is grown on a seed-crystal substrate 7 by liquid phase epitaxy by bringing the seed-crystal substrate 7 into direct contact with the resulting melt 8 and pulling up the seed-crystal substrate 7 continuously or intermittently. After the ZnO single crystal is grown, by removing the substrate 7 by abrasion or etching and abrading or etching the -c plane side grown by liquid phase epitaxy of the single crystal, the self-supporting ZnO single crystal is obtained. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a ZnO-based semiconductor material, and more particularly to a method for producing a ZnO single crystal useful in the fields of optics and electric / electronic industry and a free-standing ZnO single crystal wafer obtained thereby.

  Conventionally, Si, GaAs, GaN, and the like have been used for optical and electronic devices having various functions. Recently, light-emitting devices and electronic devices using GaN have been actively developed. On the other hand, focusing on oxides, ZnO has been used for varistors, gas sensors, sunscreens, etc. Recently, optical elements, electronic elements, piezoelectric elements, transparent electrodes, etc., due to its optical characteristics, electronic element characteristics and piezoelectric characteristics. Has been attracting attention. In particular, it is known that ZnO has a direct transition type band gap of 3.3 to 3.4 eV like GaN and emits 380 nm ultraviolet light, and emits light of a short wavelength from blue to ultraviolet region. Research and development for applications and applications for semiconductors for light-emitting elements that emit light has become active.

  In addition, unlike a field effect transistor in a ZnO-based semiconductor single crystal stacked body, there is a modulation doping method as one method for causing charge separation by bringing semiconductors into contact with each other without applying an electric field. For example, Japanese Patent Laying-Open No. 2005-72067 (Patent Document 1) discloses that a semiconductor having a wide band gap and a high electron concentration and a semiconductor having a narrow band gap and a high electron mobility are stacked to form an electron from a semiconductor having a high electron concentration. A semiconductor material that satisfies both a high electron concentration and a high electron mobility is disclosed by inducing charge transfer to a semiconductor with high mobility and moving electrons through a layer with high mobility.

Furthermore, in order to manufacture an ultraviolet light emitting device using ZnO, a Zn _1-x Mg _x O of a II-VI group semiconductor mixed crystal having a wide band gap was obtained by a pulse laser deposition method at a growth temperature of 600 ° C. It has been shown that a wider band gap than ZnO can be obtained by adjusting x (A. Ohtomo et.al, Applied Physics Letters, Vol.72, No.19, 11 May 1998, 2466-2468) Patent Document 1). In consideration of application to a light emitting element, it is necessary to adopt a double hetero structure in order to increase the light emission efficiency. By employing the above structure, carrier confinement and light extraction efficiency are improved, and light emission efficiency is improved. In order to form the above structure, it is necessary to sandwich the light emitting layer between an n layer and a p layer having a high band gap, and for this purpose, a ZnO mixed crystal single crystal having a band gap higher than that of ZnO is required, and at the same time, an n-type In addition, a p-type ZnO single crystal is required.

High crystallinity is required to exhibit the electronic element characteristics and optical element characteristics of the ZnO-based semiconductor single crystal. Conventionally, a ZnO-based semiconductor single crystal has been grown by a vapor phase growth method using an insulating substrate such as ZnO, ScAlMgO ₄ and sapphire. In order to achieve high crystallinity, it is necessary to use a substrate with less lattice mismatch. For that purpose, it is desirable to use a ZnO single crystal as a substrate. However, a commercially available ZnO single crystal substrate is grown by a hydrothermal synthesis method, and it is inevitable that Li based on LiOH used as a mineralizer is mixed into the ZnO single crystal. Li in ZnO is easily diffused, and there is a problem that Li moves when the device is operated to make the device operation unstable. The smaller the Li in the ZnO substrate, the better. Therefore, a technique for reducing the Li concentration in the hydrothermal synthesis substrate by post-growth annealing is disclosed in Japanese Patent Application Laid-Open No. 2007-204324 (Patent Document 2). According to the publication, annealing at 1100 ° C. for 1 to 2 hours is about 8.4 × 10 ¹⁶ pieces / cm ³ to 2.0 × 10 ¹⁷ pieces / cm ³ , and annealing at 1300 ° C. for about 2 hours is 9 ×. It is described that the Li concentration in ZnO can be reduced to about 10 ¹⁴ atoms / cm ³ . According to the publication, it is possible to reduce the Li concentration by annealing. However, this method requires an annealing process, and thus the process becomes complicated, and there is a problem that about 1 × 10 ¹⁵ pieces / cm ^{3 of} Li remains even after annealing.

  On the other hand, if the ZnO-based semiconductor single crystal can be made self-supporting by growing a thick ZnO-based semiconductor single crystal and then removing the hydrothermal synthetic substrate by polishing or etching, Li Impurity contamination can be significantly reduced. Furthermore, if an impurity capable of imparting conductivity can be simultaneously doped and self-supported, it is possible to provide a self-supporting conductive ZnO single crystal wafer having a low Li concentration and conductivity.

  The present inventors have previously filed a “method for producing a ZnO single crystal by a liquid phase growth method” (International Publication No. 2007/100146 pamphlet (Patent Document 3). If the invention described in this publication is used, However, when this invention is used, it has been found that the yield is low when a free-standing ZnO single crystal is produced without cracks. As a result of research, it has been found that flux precipitation in the vicinity of the Pt jig holding the substrate is a factor that reduces the yield, and thus, if the method applied by the present inventors is used, the film thickness is large. It is possible to produce a self-supporting ZnO single crystal by growing the ZnO single crystal and removing the substrate used for the growth, but based on the flux component deposited on the Pt jig tool used for the growth. Rack growth, there is a problem that occurs in the cooling and grinding / polishing process.

The present inventors diligently studied the above problems and found that there are the following problems. This will be described with reference to FIG. In the conventional method, the crystal layer is grown by liquid phase epitaxial (LPE) using zinc oxide A-1 as a substrate (hereinafter, the grown crystal layer may be referred to as “LPE layer”). After that, the B-3 part in which the Li concentration in the hydrothermal synthesis substrate is reduced and the B-1 part having the original Li concentration are obtained. Further, when the LPE layer B-2 is grown, Li in the hydrothermal synthesis substrate diffuses in the initially grown portion, and a B-4 layer having an increased Li concentration is formed in the LPE growth film. For this reason, only by removing B-1 and B-3, the obtained self-supporting substrate is composed of the LPE layer B-2 and the Li diffusion layer B-4 that should be originally obtained. When an optical element or an electronic element was formed using this as a growth substrate, the B-4 layer having a slightly higher Li concentration than B-2 caused the device characteristics to become unstable.
In addition, there is a considerable lattice mismatch between the ZnO substrate and the grown impurity-doped ZnO film, but thick film growth is also necessary in terms of strain relaxation. However, in vapor phase growth, the growth rate is low, and thick film growth is extremely difficult. On the other hand, in the liquid phase growth method, the growth rate can be controlled by controlling the degree of supersaturation, and a relatively high growth rate is possible.

On the other hand, as described above, an insulating material is often used for a substrate in vapor phase growth and liquid phase growth. For this reason, in order to form an electronic element or an optical element using a ZnO-based mixed crystal single crystal, it is necessary to devise such as forming electrodes in the same direction. This method complicates the manufacturing process of the electronic element and the optical element, which causes high costs, and also has a problem that the electric field is concentrated on a part of the n-type contact layer (n ⁺ ) layer and the element life is shortened. There was a point. However, if an electrically conductive ZnO film is grown thick using an insulating substrate and unnecessary insulating substrates can be removed by polishing or the like, electrodes can be formed on the front and back surfaces, and device characteristics can be obtained even in the device fabrication process. In addition, it can be expected to improve performance in terms of life (see Fig. 2).

  Further, the ZnO-based semiconductor single crystal and the laminated body thereof as a study of the above materials are grown by a vapor phase growth method which is non-thermal equilibrium growth (for example, Japanese Patent Application Laid-Open No. 2003-046081 (Patent Document 4)). Mixing of growth defects is unavoidable, and the crystal quality is not sufficient. In a field effect transistor, a pn junction light emitting element, or the like, which is an example of a conventional semiconductor element, the crystallinity is greatly involved in optical characteristics and semiconductor characteristics. As described above, since the crystal quality of the crystal by the vapor phase growth method was not sufficient, there was a problem that the original performance could not be sufficiently exhibited. Therefore, in order to apply and develop the above-mentioned uses, it is important to establish a method for producing a ZnO single crystal with high crystal quality.

Conventionally, sputtering, CVD, PLD, and the like have been used as methods for growing a ZnO-based semiconductor single crystal. In these methods, the growth orientation of the ZnO-based semiconductor layer was the -c plane orientation. -C-plane growth has a problem that it is difficult to incorporate acceptors (Maki et al. Jpn. J. Appl. Phys. 42 (2003) 75-77) (Non-patent Document 2). In the case of a ZnO-based semiconductor layer, n-type growth is relatively easy, but considering that p-type growth is difficult, it is difficult to grow a p-type layer by -c plane growth. There was a point.
JP-A-2005-72067 JP2007-204324A International Publication No. 2007/100146 Pamphlet JP 2003-046081 A A.Ohtomo et.al, Applied Physics Letters, Vol.72, No.19, 11 May 1998, 2466-2468 Maki et al. Jpn. J. Appl. Phys. 42 (2003) 75-77

The ZnO hydrothermal synthesis substrate is mixed with Li based on the mineralizer, and when this is used as a substrate for the LPE growth of ZnO single crystal, there is a problem that Li diffuses to the LPE growth film side. Furthermore, after growing a thick ZnO single crystal using the LPE growth method, Li diffuses to the growth film side even if the substrate portion is removed by polishing or etching, so that the LPE growth film becomes self-supporting. However, there was a problem of destabilization of device characteristics based on Li. Furthermore, in order to produce a self-supporting ZnO single crystal, when LPE growth of a thick ZnO single crystal is performed, flux precipitation occurs based on the Pt jig tool, and during growth and cooling based on the flux precipitation portion. In addition, there is a problem that cracks are easily generated during grinding / polishing.
Under such circumstances, it is desired to provide a self-supporting ZnO single crystal having a low Li concentration.

The above problems can be solved by the following present invention.
In the first embodiment of the present invention, ZnO as a solute and a solvent are mixed and melted, and then the seed crystal substrate is brought into direct contact with the obtained melt, and the seed crystal substrate is continuously formed. And a step of growing the ZnO single crystal on the seed crystal substrate by pulling up periodically or intermittently. A method for producing a ZnO single crystal by a liquid phase epitaxial growth method. The solvent is preferably a combination of PbO and Bi ₂ O _{3 or} a combination of PbF ₂ and PbO.
The second embodiment of the present invention is a self-supporting ZnO single crystal wafer obtained by the method for producing a ZnO single crystal described in the first embodiment of the present invention, and has a film thickness of 100 μm or more. It is a free-standing ZnO single crystal wafer characterized.
In the present specification, the term “self-supporting” means that the seed crystal substrate used for growth is removed by polishing and / or etching and consists of only a growth layer. The term “continuous” means that the pulling rate is constant in the step of pulling up the seed crystal substrate, and the term “intermittent” means that the pulling rate changes in the step of pulling up the seed crystal substrate. It means to do. The term “solute” refers to a substance that dissolves in a solvent when a solution is made, and the term “solvent” refers to a substance that becomes a medium of a substance that is dissolved when a solution is made.

According to a preferred aspect of the present invention, a thick ZnO single crystal can be obtained. After growing the ZnO single crystal, the seed crystal substrate is removed by polishing or etching, and the -c plane side of the single crystal subjected to liquid phase epitaxial growth is polished or etched to obtain a freestanding ZnO single crystal having a low Li concentration. . The obtained free-standing ZnO single crystal is suitably used as a wafer as it is or as a seed crystal substrate of a ZnO-based mixed crystal single crystal.
The self-standing ZnO single crystal wafer which is a preferred embodiment of the present invention has high crystallinity and can control the carrier concentration while maintaining high carrier mobility. Moreover, Li diffusion from the substrate can be reduced, and unstable operation during device operation can be suppressed. Furthermore, since it is self-supporting, both the + c plane and the −c plane can be used for the surface of the device fabrication substrate, and a growth plane suitable for the target device can be selected.

Hereinafter, the present invention will be described in detail.
In the first embodiment of the present invention, ZnO as a solute and a solvent are mixed and melted, and then the seed crystal substrate is brought into direct contact with the obtained melt, and the seed crystal substrate is continuously formed. And a step of growing the ZnO single crystal on the seed crystal substrate by pulling up periodically or intermittently. A method for producing a ZnO single crystal by a liquid phase epitaxial growth method.

  In order to manufacture a free-standing ZnO single crystal, a thick film growth of the ZnO single crystal is required. In order to use the produced free-standing ZnO single crystal for subsequent device production, it is necessary to have a thickness that does not cause cracks or the like during handling, and the thickness is at least about 50 μm. In order for the free-standing ZnO single crystal obtained after polishing the seed crystal substrate to have a thickness of 50 μm, a growth film thickness of about 100 μm or more is required in consideration of the polishing margin on the front and back surfaces. By using the method of “Preparation method of ZnO single crystal by liquid phase growth method” (International Publication No. 2007/100146 pamphlet) previously filed by the present inventors, a ZnO single crystal having a film thickness of 100 μm or more is grown. Can be made. In this method, a Pt jig is used to hold a hydrothermal synthetic substrate, and the substrate surface is in contact with the melt surface for epitaxial growth. However, since the Pt jig / tool has high thermal conductivity, it has been found that the temperature around the Pt jig / tool decreases, and as a result, flux precipitation is likely to occur near the Pt / tool. It was found that when the flux is deposited, the flux component is taken into the growth film, and cracks based on this are generated during the growth, cooling and polishing / etching, and the yield is lowered.

  In order to solve this problem, the present inventors have conducted intensive research. As a result, the Pt jig is brought into contact with the melt by continuously or intermittently pulling up the Pt jig holding the seed crystal substrate during growth. When the time was reduced, it was found that flux mixing into the film could be reduced, and as a result, the generation of cracks during growth and during polishing / etching could be suppressed. As the shaft pulling method, either continuous or intermittent can be employed, but continuous shaft pulling is superior in terms of stable growth.

  The speed V for continuously pulling up the seed crystal substrate is preferably 2 μm / hr or more and 50 μm / hr or less. More preferably, it is 4 μm / hr or more and 20 μm / hr or less, and further preferably 6 μm / hr or more and 10 μm / hr or less. If it is less than 2 μm / hr, the effect of reducing the entrainment of flux by pulling up the shaft is small, and if it exceeds 50 μm / hr, the substrate may be separated from the melt surface. When the seed crystal substrate is pulled up intermittently, the average speed is preferably within the above range. That is, the average speed v for intermittently pulling up the seed crystal substrate is preferably 2 μm / hr or more and 50 μm / hr or less, more preferably 4 μm / hr or more and 20 μm / hr or less, and further preferably 6 μm / hr or more. 10 μm / hr or less.

The solvent that can be used is not particularly limited as long as it can melt ZnO as a solute, but is preferably a combination of PbO and Bi ₂ O _{3 or} a combination of PbF ₂ and PbO. The mixing ratio of the solute and the solvent is preferably solute: solvent = 5-30 mol%: 95-70 mol% converted to ZnO alone, and more preferably the solute concentration is 5 mol% or more and 10 mol% or less. If the solute concentration is less than 5 mol%, the growth rate is slow, and if it exceeds 10 mol%, the growth temperature increases and the amount of solvent evaporation may increase.

In a preferred embodiment of the present invention, after mixing and melting ZnO as a solute and PbO and Bi ₂ O _{3 as} solvents, a step of bringing a seed crystal substrate into direct contact with the obtained melt, And a step of growing the ZnO single crystal on the seed crystal substrate by pulling the seed crystal substrate continuously or intermittently, and a method for producing a ZnO single crystal by a liquid phase epitaxial growth method. In a preferred embodiment of the present invention, the mixing ratio of the solute and the solvents PbO and Bi ₂ O ₃ is solute: solvent = 5-30 mol%: 95-70 mol%, and the solvent PbO and Bi ₂ O. The mixing ratio with ₃ is PbO: Bi ₂ O ₃ = 0.1 to 95 mol%: 99.9 to 5 mol%. The solvent composition is more preferably PbO: Bi ₂ O ₃ = 30 to 90 mol%: 70 to 10 mol%, and particularly preferably PbO: Bi ₂ O ₃ = 60 to 80 mol%: 40 to 20 mol%. . Since the liquid phase growth temperature becomes high in the solvent of PbO or Bi ₂ O ₃ alone, the PbO and Bi ₂ O ₃ mixed solvent having the above mixing ratio is preferable. The mixing ratio of ZnO as a solute and PbO and Bi ₂ O ₃ as solvents is more preferably a solute concentration of 5 mol% or more and 10 mol% or less. If the solute concentration is less than 5 mol%, the growth rate is slow, and if it exceeds 10 mol%, the growth temperature may increase.

In a preferred embodiment of the present invention, after mixing and melting ZnO as a solute and PbF ₂ and PbO as solvents, a seed crystal substrate is brought into direct contact with the obtained melt, and the seed crystal And a step of growing the ZnO single crystal on the seed crystal substrate by pulling the substrate continuously or intermittently. A method for producing a ZnO single crystal by a liquid phase epitaxial growth method. In a preferred embodiment of the present invention, the mixing ratio of the solute ZnO and the solvents PbF ₂ and PbO is solute: solvent = 2 to 20 mol%: 98 to 80 mol%, and the solvent PbF ₂ and PbO The mixing ratio is PbF ₂ : PbO = 80 to 20 mol%: 20 to 80 mol%. When the mixing ratio of the solvent is PbF ₂ : PbO = 80 to 20 mol%: 20 to 80 mol%, the amount of evaporation of the solvents PbF ₂ and PbO can be suppressed, and as a result, fluctuations in the solute concentration are reduced. A ZnO single crystal can be stably grown. The mixing ratio of the solvent PbF ₂ and PbO is more preferably PbF ₂ : PbO = 60 to 40 mol%: 40 to 60 mol%. The mixing ratio of ZnO as a solute and PbF ₂ and PbO as solvents is more preferable when the solute is 5 to 10 mol%. If the solute concentration is less than 5 mol%, the growth rate is slow, and if it exceeds 10 mol%, the temperature at which the solute component is dissolved increases and the amount of solvent evaporation may increase. In the present invention, a liquid phase growth method is used. Unlike the vapor phase growth method, this method does not require a vacuum system, so that it is possible to produce a ZnO single crystal at low cost and to grow a ZnO single crystal having high crystallinity because of thermal equilibrium growth. Can be made. Further, by controlling the degree of supersaturation, the growth rate can be controlled, and a relatively high growth rate can be realized.

  In a preferred embodiment of the present invention, the ZnO single crystal contains a small amount of a different element. ZnO can exhibit and change its characteristics by doping with different elements. In a preferred embodiment of the present invention, Li, Na, K, Cs, Rb, Be, Ca, Sr, Ba, Cu, Ag, N, P, As, Sb, Bi, B, Tl, Cl, Br, I, Mn One or more selected from the group consisting of Fe, Co, Ni, Cd, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and a lanthanoid element are added. The addition amount is 20 mol% or less, preferably 10 mol% or less, more preferably 1 mol% or less, with respect to ZnO used as a solute. By adding different kinds of elements, there are p-type semiconductors, n-type semiconductors, magnetic semiconductors, conductivity control, varistor applications, piezoelectric body applications, electroluminescent elements, and transparent TFTs.

In a preferred embodiment of the present invention, a ZnO single crystal is used as a seed crystal substrate that is a growth substrate. As a substrate for ZnO single crystal growth, any substrate can be used as long as it has a crystal structure similar to ZnO and the grown thin film does not react with the substrate. Examples thereof include sapphire, LiGaO ₂ , LiAlO ₂ , LiNbO ₃ , LiTaO ₃ , ScAlMgO ₄ , GaN, and ZnO. However, considering that the target single crystal in the present invention is a ZnO single crystal, homoepitaxial growth using a ZnO substrate having a high degree of lattice matching between the substrate and the grown crystal reduces crystallinity, distortion, and warpage of the grown film. And it is preferable in terms of reducing the amount of impurity diffusion from the substrate.

In a preferred embodiment of the present invention, the growth orientation of the ZnO single crystal is the + c plane.
In a preferred embodiment of the present invention, after growing a ZnO single crystal on a seed crystal substrate, the seed crystal substrate used for the growth can be made independent by polishing or etching. At that time, the Li diffusion layer from the substrate side can be removed by removing at least 10 μm, preferably 20 μm or more, on the −c plane side close to the growth substrate. As a substrate removal method, any method of polishing or etching can be adopted, but grinding + polishing which allows easy film thickness control is preferable. A ZnO single crystal is grown on a hydrothermal synthesis substrate. In order to obtain a film thickness of about 50 μm after polishing or etching, a film thickness of about 100 μm is required at the growth stage. After the growth, the substrate side is fixed to the ceramic plate by WAX, and the liquid phase epitaxial growth surface can be flattened by a grinder. The LPE surface side (+ c surface) may be replaced with a ceramic plate, and the substrate thickness may be ground and removed with a grinder to obtain only a liquid phase epitaxial growth film, and polishing may be performed by lapping and polishing the front and back surfaces. At this time, it is possible to remove the Li diffusion layer from the hydrothermal synthesis substrate by polishing the hydrothermal synthesis substrate side of the LPE growth film preferably at 10 μm, more preferably 20 μm or more.

  In a preferred embodiment of the present invention, the ZnO single crystal contains one or more selected from the group consisting of Al, Ga, In, H and F. By containing one or more selected from the group consisting of Al, Ga, In, H, and F, it becomes possible to develop electrical conductivity. If the substrate used for growth is removed by polishing, etching, or the like, a self-supporting conductive substrate is formed, and electrodes can be formed on the front and back of the electronic element and the optical element.

  The second embodiment of the present invention is a self-supporting ZnO single crystal wafer obtained by the method for producing a ZnO single crystal described in the first embodiment of the present invention, and has a film thickness of 100 μm or more. It is a free-standing ZnO single crystal wafer characterized. If the thickness is less than 100 μm, 50 μm cannot be ensured as the thickness after polishing, and it is difficult to use in subsequent device processes. The upper limit of the thickness is not specified, but if it exceeds 500 μm, the growth time becomes longer.

In a preferred aspect of the present invention, the Li concentration of the free-standing ZnO single crystal wafer obtained by the method for producing a ZnO single crystal described in the first embodiment is uniform in the in-plane direction and the thickness direction of the wafer. And preferably 1 × 10 ¹⁵ pieces / cm ³ or less, more preferably 1 × 10 ¹⁴ pieces / cm ³ or less. Here, when the Li concentration is uniform at 1 × 10 ¹⁵ pieces / cm ³ or less, the Li concentration that destabilizes the device operation is 1 × 10 ¹⁵ pieces / cm ³ or less in the entire self-supporting film. means. The uniformity of the Li concentration can be obtained as follows. By measuring the Li concentration at several points on the front and back surfaces after the self-supporting treatment with dynamic SIMS, the Li concentration uniformity in the front and back surfaces was further measured. The Li concentration uniformity in the film thickness direction can be determined by measuring the Li concentration by dynamics SIMS.

  In a preferred embodiment of the present invention, the flatness of at least one surface of the free-standing ZnO single crystal wafer may be such that it can be epitaxially grown. For example, the surface roughness Ra of 50 μm square at an arbitrary position of the ZnO single crystal wafer is preferably 0.5 nm or less, more preferably 0.3 nm or less. If it exceeds 0.5 nm, the LED growth film tends to grow three-dimensionally or pit defects tend to increase, such being undesirable. On the other hand, the flatness of the front and back surfaces is preferably as close as possible. If the flatness is different between the front surface and the back surface, it causes warping. The surface roughness Ra can be measured by using an atomic force microscope (AFM) with respect to a 50 μm square of the free-standing film central part.

In a preferred embodiment of the present invention, the free-standing ZnO single crystal wafer preferably contains Ga, has a carrier concentration of 2.0 × 10 ¹⁷ pieces / cm ^{3 to} 1.0 × 10 ¹⁹ pieces / cm ³ , and The Li concentration is preferably 1 × 10 ¹⁵ pieces / cm ³ or less, more preferably 1 × 10 ¹⁴ pieces / cm ³ or less.
In another preferred embodiment of the present invention, the self-supporting ZnO single crystal wafer contains Al and has a carrier concentration of 2.0 × 10 ¹⁷ pieces / cm ^{3 to} 1.0 × 10 ¹⁹ pieces / cm ³ , In addition, the Li concentration is preferably 1 × 10 ¹⁵ pieces / cm ³ or less, more preferably 1 × 10 ¹⁴ pieces / cm ³ or less.
Furthermore, in another preferred embodiment of the present invention, the self-supporting ZnO single crystal wafer contains In and has a carrier concentration of 2.0 × 10 ¹⁷ pieces / cm ^{3 to} 3.5 × 10 ¹⁷ pieces / cm ³ , In addition, the Li concentration is preferably 1 × 10 ¹⁵ pieces / cm ³ or less, more preferably 1 × 10 ¹⁴ pieces / cm ³ or less.
In the present specification, “carrier concentration” and “carrier mobility” can be measured at room temperature by the Van Der Pauw method using a Hall effect / specific resistance measuring device manufactured by Toyo Technica.

  Hereinafter, as a method for growing a ZnO single crystal according to an embodiment of the present invention, a method for growing a ZnO single crystal doped with a group III element on a ZnO substrate by a liquid phase epitaxial (LPE) growth method will be described. The present invention is not limited to the following examples.

A configuration diagram of the furnace used here is shown in FIG.
In the single crystal manufacturing furnace, a platinum crucible 4 for melting a raw material and storing it as a melt is provided on a crucible base 9. Outside the platinum crucible 4 and on the side thereof, three-stage side heaters (upper heater 1, central heater 2, lower heater 3) for heating and melting the raw material in the platinum crucible 4 are provided. . The outputs of the heaters are controlled independently, and the heating amount for the melt is adjusted independently. A core tube 11 is provided between the heater and the inner wall of the manufacturing furnace, and a furnace lid 12 for opening and closing the inside of the furnace is provided above the core tube 11. A pulling mechanism is provided above the platinum crucible 4. A pulling-up shaft 5 is fixed to the pulling mechanism, and a substrate holder 6 and a substrate 7 fixed by the holder are provided at the tip thereof. A mechanism for rotating the pull-up shaft 5 is provided on the pull-up shaft 5. Below the platinum crucible 4, a thermocouple 10 for managing the temperature of the crucible is provided. The non-Al system is suitable for the members constituting the growth furnace. As a non-Al-based furnace material, a ZnO furnace material is optimal, but considering that it is not commercially available, MgO is suitable as a material that does not function as a carrier even when mixed in a ZnO thin film. In view of SIMS analysis results in which the Si impurity concentration in the LPE film does not increase even when a mullite furnace material composed of alumina + silica is used, a quartz furnace material is also preferable. In addition, calcium, silica, ZrO ₂ and zircon (ZrO ₂ + SiO ₂ ), SiC, Si ₃ N _{4 and the} like can be used.

  From the above, it is preferable to grow a ZnO single crystal doped with a group III element using a growth furnace composed of MgO and / or quartz as a non-Al furnace material. And a crucible base for placing the crucible on the crucible, a core tube provided so as to surround the outer periphery of the crucible base, a furnace lid provided on the top of the core tube, for opening and closing the inside of the furnace, and Preferably, the pulling shaft for moving the seed crystal or the substrate up and down is independently made of MgO or quartz.

  In order to melt the raw material in the platinum crucible, the temperature of the production furnace is increased until the raw material is melted. Preferably, the temperature is raised to 800 to 1100 ° C. and left for 2 to 3 hours to stabilize the raw material melt. The standing time may be shortened by stirring with a Pt stirring blade. At this time, an offset is applied to the three-stage heater, and the bottom of the crucible is adjusted to be several degrees higher than the melt surface. Preferably, −100 ° C. ≦ H1 offset ≦ 0 ° C., 0 ° C. ≦ H3 offset ≦ 100 ° C., more preferably −50 ° C. ≦ H1 offset ≦ 0 ° C., 0 ° C. ≦ H3 offset ≦ 50 ° C. After adjusting the bottom temperature of the crucible to a seeding temperature of 700 to 950 ° C. and stabilizing the temperature of the melt, the substrate is brought to the melt surface by lowering the pulling shaft while rotating the substrate at 5 to 120 rpm. Wetted in contact with. After the substrate is made to conform to the melt, the temperature starts to drop at a constant temperature or from 0.01 to 3.0 ° C./hr to grow the target ZnO single crystal on the substrate surface. During the growth, the substrate is rotated at 5 to 300 rpm by the rotation of the pulling shaft, and is reversely rotated at regular time intervals. After crystal growth for about 30 minutes to 100 hours, the substrate is separated from the melt, and the melt component is separated by rotating the pulling shaft at a high speed of about 200 to 300 rpm. Then, it cools to room temperature over 1 to 24 hours, and obtains the target ZnO single crystal.

Example 1
A ZnO single crystal was produced by a liquid phase epitaxial growth method through the following steps. A platinum crucible having an inner diameter of 75 mmΦ, a height of 75 mmh, and a thickness of 1 mm was charged with 33.20 g, 829.53 g, and 794.75 g of ZnO, PbO, and Bi ₂ O ₃ as raw materials, respectively. At this time, the concentration of ZnO as a solute is 7 mol%, and the concentrations of PbO and Bi ₂ O ₃ as solvents are PbO: Bi ₂ O ₃ = 68.5 mol%: 31.5 mol%. The crucible charged with the raw material was placed in the furnace shown in FIG. 3, held at a crucible bottom temperature of about 840 ° C. for 1 hour, and stirred and dissolved by a Pt stirring jig. Thereafter, the temperature is lowered until the bottom temperature of the crucible reaches about 790 ° C., and then contacted with a ZnO single crystal substrate having a size of 10 mm × 10 mm × 0.5 mmt grown in a hydrothermal synthesis method as a seed crystal in a + c plane orientation, Growth was performed at the same temperature for 80 hours while rotating the pulling shaft at 30 rpm. The set temperatures of H1, H2, and H3 were lowered at a rate of -0.1 ° C / hr while maintaining the offset difference. Further, during the LPE growth, that is, over 80 hours, the pulling axis was continuously pulled up by about 500 μm. The shaft pulling speed at this time is about 6.3 μm / hr. At this time, the shaft rotation direction was reversed every two minutes. Thereafter, the pulling-up shaft was raised to separate it from the melt, and the shaft was rotated at 100 rpm to shake off the melt components and then gradually cooled to room temperature to obtain a colorless and transparent ZnO single crystal. After cooling to room temperature, the grown film was taken out from the LPE furnace and observed around the Pt jig, but no flux deposition was observed. The LPE growth film thickness was 391 μm, and the growth rate at this time was about 4.9 μm / hr. Subsequently, the following independence treatment was performed. The back surface (the −c surface side of the hydrothermal synthesis substrate) was fixed to the ceramic plate with WAX. About 50 μm was ground so that the + c plane LPE surface was flattened by a horizontal surface grinder. Then, the self-supporting ZnO single crystal was obtained by reversing the front and back and grinding the amount corresponding to the thickness of the hydrothermal synthetic substrate. The thickness was about 341 μm. The self-supporting film was fixed to the ceramic plate again by WAX, lapped with diamond slurry, and polished with colloidal silica. The polishing amount of the front and back surfaces of the free-standing film was 33 μm on the + c plane and 28 μm on the −c plane, and as a result, a free-standing ZnO single crystal having a thickness of about 280 μm was obtained. There were no cracks in the grinding and polishing process. After the self-supporting treatment, the ± c plane was analyzed by dynamic SIMS to determine the Li concentration. The Li concentration in both the ± c planes was 5 × 10 ¹³ pieces / cm ³ or less, which is the lower limit of detection. In this specification, the dynamic SIMS analysis is performed using a Cameca apparatus under the conditions of primary ion species; O ²⁺ , primary ion acceleration voltage; 8 KeV, measurement temperature; room temperature. Further, the film was cut in a direction perpendicular to the c-plane, and the cut surface was analyzed by SIMS. As a result, the Li concentration was 5 × 10 ¹³ pieces / cm ³ below the detection lower limit. The rocking curve half-value width of the (002) plane showing the crystallinity of the free-standing film is about 33 arcsec, indicating that the crystallinity is high. The carrier concentration exhibiting conductivity was 2.0 × 10 ¹⁷ atoms / cm ³ and the carrier mobility was 165 cm ² / V · sec. It shows that the quality is high also from the high carrier mobility. When flatness was evaluated with an atomic force microscope (AFM) for the 50 μm square of the center of the free-standing film, Ra = 0.3 nm.

(Example 2-10 and Comparative Example 1)
A self-supporting ZnO single crystal was obtained in the same manner as in Example 1 except that Ga ₂ O ₃ was charged into a Pt crucible. In Example 10, continuous shaft pulling was changed to intermittent shaft pulling. That is, the process of stopping the shaft and raising 100 μm after 16 hours was repeated 5 times, and the total was raised by 500 μm in 80 hours. The charge composition is shown in Table 1. Table 2 shows the physical properties of the obtained free-standing film.

As the amount of Ga ₂ O ₃ charged increased, the carrier concentration increased and the carrier concentration of the self-supporting film of Example 9 charged with 2100 ppm with respect to ZnO was 1.0 × 10 ¹⁹ particles / cm ³ . From these results, it is shown that the carrier concentration can be controlled from 2.0 × 10 ¹⁷ atoms / cm ³ to 1.0 × 10 ¹⁹ atoms / cm ³ by charging Ga ₂ O ₃ . On the other hand, the carrier mobility was 154 cm ² / V · sec in Example 1 in which Ga ₂ O ₃ was not charged and 64 cm ² / V · sec in Example 9 in which 2100 ppm was charged. This seems to be a result of doping Ga, which is a foreign material. The rocking curve half-width of the (002) plane showing crystallinity is 23 to 71 arcsec between 0 to 2100 ppm of Ga ₂ O ₃ , indicating that the crystallinity is extremely high. Further, in Example 1-10, the −c surface portion that is the growth surface on the hydrothermal synthesis substrate side is polished and removed by 12 to 29 μm, and as a result, the Li concentration is 5 × 10 5 that is the SIMS detection lower limit for both the ± c surfaces. It became ¹³ pieces / cm ³ or less.

  Further, in Comparative Example 1 in which the shaft was not pulled up, flux precipitation was observed in the vicinity of the Pt jig / tool. When the growth film was removed from the Pt jig / tool, a crack with the jig / tool as a base point occurred. When the self-supporting treatment was performed using the largest 5 mm square of the cracked fragments, there was no significant difference between Example 9 and Comparative Example 1 in terms of physical properties. Comparing the ninth and tenth embodiments, which differ only in the shaft pulling mechanism, the crystallinity is lowered and the carrier mobility is decreased in the tenth embodiment in which the shaft pulling is intermittent. The reason is that stable LPE growth has not been achieved because the shaft pulling was intermittent.

From the above results, when LPE growth is performed while pulling the shaft continuously or intermittently, flux precipitation based on the Pt jig tool is suppressed, and as a result, the generation of cracks in the process of growth and independence is reduced. It shows what you can do. Further, after the LPE growth, the substrate used for the growth is removed by polishing, and if the −c surface on the hydrothermal synthesis substrate side of the LPE growth film is polished at least 10 μm or more, the Li concentration in the LPE film is 1 × 10 ¹⁵ pieces. / Cm ³ or less. On the other hand, since the carrier concentration of the free-standing ZnO single crystal wafer can be controlled from about 2.7 × 10 ¹⁷ pieces / cm ³ to about 1 × 10 ¹⁹ pieces / cm ³ by controlling the amount of Ga ₂ O ₃ charged. When an optical element or an electronic element is configured using a self-supporting film as a substrate, it becomes a self-supporting conductive substrate, which is more advantageous than an insulating substrate in terms of device manufacturing cost and life.

(Examples 11-18 and Comparative Example 2)
A self-supporting ZnO single crystal was obtained in the same manner as in Example 1 except that Al ₂ O ₃ was charged into a Pt crucible and the charging composition shown in Table 3 was used. In Example 18, continuous shaft pulling was changed to intermittent shaft pulling. That is, the process of stopping the shaft and raising 100 μm after 16 hours was repeated 5 times, and the total was raised by 500 μm in 80 hours. Table 4 shows the physical properties of the obtained self-supporting film. As the Al ₂ O ₃ charge increased, the carrier concentration increased and the carrier concentration of the self-supporting film of Example 17 charged with 2200 ppm with respect to ZnO was 1.0 × 10 ¹⁹ particles / cm ³ . From these results, it is shown that the carrier concentration can be controlled from 2.0 × 10 ¹⁷ pieces / cm ³ to 1.2 × 10 ¹⁹ pieces / cm ³ by charging Al ₂ O ₃ . On the other hand, the carrier mobility was the Al ₂ O ₃ in Example 11 without charged to 163cm ² / V · sec, the Al ₂ O ₃ Example 15 was charged 2200ppm of 95cm ² / V · sec. This is considered to be a result of doping Al as a foreign material. The rocking curve half width of the (002) plane showing crystallinity is 19 to 52 arcsec between 0 to 2179 ppm of Al ₂ O ₃ , indicating that the crystallinity is extremely high. Further, in Example 12-17, the −c surface portion which is the growth surface on the hydrothermal synthesis substrate side is polished and removed by 13-24 μm, and as a result, the Li concentration is 5 × 10 5 which is the SIMS detection lower limit for both ± c surfaces. It became ¹³ pieces / cm ³ or less.

  In Comparative Example 2 in which the shaft was not pulled up, flux precipitation was observed in the vicinity of the Pt jig / tool. When the growth film was removed from the Pt jig / tool, a crack with the jig / tool as a base point occurred. When the self-supporting treatment was performed using the largest 5 mm square of the cracked fragments, there was no significant difference between Example 17 and Comparative Example 2 in terms of physical properties. Comparing Examples 17 and 18 that differ only in the shaft pulling mechanism, the crystallinity is lowered and carrier mobility is decreased in Example 18 in which the shaft pulling is intermittent. The reason is that stable LPE growth has not been achieved because the shaft pulling was intermittent.

From the above results, when LPE growth is performed while pulling the shaft continuously or intermittently, flux precipitation based on the Pt jig tool is suppressed, and as a result, the generation of cracks in the process of growth and independence is reduced. It shows what you can do. Further, after the LPE growth, the substrate used for the growth is removed by polishing, and if the −c surface on the hydrothermal synthesis substrate side of the LPE growth film is polished at least 10 μm or more, the Li concentration in the LPE film is 1 × 10 ¹⁵ pieces. / Cm ³ or less. On the other hand, the carrier concentration of the free-standing ZnO single crystal wafer can be controlled from 2.0 × 10 ¹⁷ pieces / cm ³ to about 1 × 10 ¹⁹ pieces / cm ³ by controlling the amount of Al ₂ O ₃ charged. When an optical element or an electronic element is configured using a self-supporting film as a substrate, it becomes a self-supporting conductive substrate, which is more advantageous than an insulating substrate in terms of device manufacturing cost and life.

(Examples 19-22)
A self-supporting ZnO single crystal was obtained in the same manner as in Example 1 except that In ₂ O ₃ was charged into a Pt crucible and the charging composition shown in Table 5 was used. Table 6 shows the physical properties of the obtained self-supporting film. As the amount of In ₂ O ₃ charged increased, the carrier concentration increased and the carrier concentration of the self-supporting film of Example 22 charged with 140 ppm with respect to ZnO was 3.5 × 10 ¹⁷ particles / cm ³ . From these results, it is shown that the carrier concentration can be controlled from 2.0 × 10 ¹⁷ atoms / cm ³ to 3.5 × 10 ¹⁷ atoms / cm ³ by charging In ₂ O ₃ . On the other hand, the carrier mobility was 168 cm ² / V · sec in Example 19 in which In ₂ O ₃ was not charged, and 134 cm ² / V · sec in Example 22 in which 140 ppm was charged. This is considered to be a result of doping In which is a foreign substance. The rocking curve half width of the (002) plane showing crystallinity is 19 to 58 arcsec between 0 to 140 ppm of In ₂ O ₃ , indicating that the crystallinity is extremely high. Further, in Example 19-22, the −c surface portion which is the growth surface on the hydrothermal synthesis substrate side is polished and removed by 19-24 μm, and as a result, the Li concentration is 5 × 10 5 which is the SIMS detection lower limit for both ± c surfaces. It became ¹³ pieces / cm ³ or less.

From the above results, it is shown that when LPE growth is performed while pulling the shaft continuously, flux precipitation based on the Pt jig tool is suppressed, and as a result, generation of cracks in the process of growth and independence can be reduced. ing. Further, after the LPE growth, the substrate used for the growth is removed by polishing, and if the −c surface on the hydrothermal synthesis substrate side of the LPE growth film is polished at least 10 μm or more, the Li concentration in the LPE film is 1 × 10 ¹⁵ or less. It becomes possible to. On the other hand, since the carrier concentration of the free-standing ZnO single crystal wafer can be controlled from 2.0 × 10 ¹⁷ pieces / cm ³ to about 3.5 × 10 ¹⁷ pieces / cm ³ by controlling the amount of In ₂ O ₃ charged. When an optical element or an electronic element is configured using the self-supporting film as a substrate, it becomes a self-supporting conductive substrate, which is more advantageous than an insulating substrate in terms of device manufacturing cost and life.

(Example 23)
A ZnO single crystal was produced by a liquid phase epitaxial method by the following steps. Into a platinum crucible having an inner diameter of 75 mmΦ, a height of 75 mmh, and a thickness of 1 mm, 32.24 g, 922.58 g, and 839.88 g of ZnO, PbF ₂ , and PbO were charged as raw materials, respectively. At this time, the concentration of ZnO as a solute is 5 mol%, and the concentrations of PbF ₂ and PbO as solvents are PbF ₂ : PbO = 50 mol%: 50.0 mol%. The crucible charged with the raw material was placed in the furnace shown in FIG. 3, the bottom temperature of the crucible was maintained at about 940 ° C. for 1 hour, and the mixture was stirred and dissolved with a Pt stirring jig. After that, the temperature is lowered until the bottom temperature of the crucible reaches about 835 ° C., and a ZnO single crystal substrate having a size of 10 mm × 10 mm × 529 μmt grown in a hydrothermal synthesis method and having a size of 10 mm × 10 mm × 529 μmt is contacted as a seed crystal. The shaft was grown at the same temperature for 80 hours while rotating the shaft at 30 rpm. The set temperatures of H1, H2, and H3 were lowered at a rate of -0.1 ° C / hr while maintaining the offset difference. Further, during the LPE growth, that is, over 80 hours, the pulling axis was continuously pulled up by about 500 μm. The shaft pulling speed at this time is about 6.3 μm / hr. At this time, the shaft rotation direction was reversed every two minutes. Thereafter, the pulling-up shaft was raised to separate it from the melt, and the shaft was rotated at 100 rpm to shake off the melt components and then gradually cooled to room temperature to obtain a colorless and transparent ZnO single crystal. After cooling to room temperature, the grown film was taken out of the LPE furnace and observed around the Pt jig, but no flux deposition was observed. The LPE growth thickness was 343 μm, and the growth rate at this time was about 4.3 μm / hr. Subsequently, the following independence treatment was performed. The back surface (the −c surface side of the hydrothermal synthesis substrate) was fixed to the ceramic plate with WAX. About 50 μm was ground so that the + c plane LPE surface was flattened by a horizontal surface grinder. Then, the self-supporting ZnO single crystal was obtained by reversing the front and back and grinding the amount corresponding to the thickness of the hydrothermal synthetic substrate. The thickness was about 251 μm. The self-supporting film was fixed to the ceramic plate again by WAX, lapped with diamond slurry, and polished with colloidal silica. The polishing amount of the front and back surfaces of the free-standing film was 24 μm on the + c plane and 18 μm on the −c plane, and as a result, a free-standing ZnO single crystal having a thickness of about 251 μm was obtained. There were no cracks in the grinding and polishing process. After the self-supporting treatment, the ± c plane was analyzed by SIMS to determine the Li concentration. The Li concentration in both the ± c planes was 5 × 10 ¹³ pieces / cm ³ or less, which is the lower limit of detection. The rocking curve half-value width of the (002) plane showing the crystallinity of the free-standing film is about 31 arcsec, indicating that the crystallinity is high. The carrier concentration exhibiting conductivity was 1.0 × 10 ¹⁹ atoms / cm ³ and the carrier mobility was 60 cm ² / V · sec. It shows that the quality is high also from the high carrier mobility. When flatness was evaluated by AFM for the 50 μm square of the free-standing film central portion, Ra = 0.3 nm.

In this example, a dopant imparting conductivity was not added to the grown ZnO, but F doping from PbF ₂ used as a solvent seems to have become a cause of conductivity. From the above results, it is shown that a self-supporting conductive ZnO single crystal can be produced at a Li concentration of 1 × 10 ¹⁵ pieces / cm ³ or less even when PbF ₂ and PbO solvents are used. On the other hand, in the same solvent, F is a conductive expression element, and it becomes difficult to control the conductivity by the amount of dopant charged.

(Examples 24-25)

  In Example 9, Example 24 was performed in the same manner as in Example 9 except that the continuous shaft pulling rate was 2.0 μm / hr. In Example 9, Example 25 was performed in the same manner as Example 9 except that the continuous shaft pulling rate was 50.0 μm / hr. In Examples 24-25, a self-supporting film could be produced without cracking. In the physical properties of the self-supporting films obtained in Examples 24 and 25, the lattice constant and the band gap were almost the same as those in Example 9, but in Example 24 where the growth rate was low, the rocking curve half of the (002) plane was used. The value range narrowed and the crystallinity improved. On the other hand, in Example 25 where the growth rate was high, the full width at half maximum of the rocking curve on the (002) plane was widened, and a decrease in crystallinity was observed.

FIG. 1 is a schematic cross-sectional view for explaining problems in producing a self-supporting Mg-containing ZnO mixed single crystal. FIG. 2 is a configuration diagram showing a conventional element structure and an element structure using an Mg-containing ZnO mixed crystal single crystal obtained by the present invention. FIG. 3 is a block diagram of the furnace used in the examples and comparative examples of the present invention.

Explanation of symbols

1: Upper heater 2: Interruption heater 3: Lower heater 4: Pt crucible 5: Pulling shaft 6: Substrate holder 7: Substrate 8: Melt 9: Crucible base 10: Thermocouple 11: Core tube 12: Furnace lid

Claims (16)

  After the solute ZnO and the solvent are mixed and melted, the seed crystal substrate is brought into direct contact with the obtained melt, and the seed crystal substrate is pulled up continuously or intermittently to form a ZnO single crystal. And a step of growing a crystal on the seed crystal substrate. A method for producing a ZnO single crystal by a liquid phase epitaxial growth method. After mixing and melting ZnO as a solute and solvents PbO and Bi ₂ O ₃ , the seed crystal substrate is brought into direct contact with the obtained melt, and the seed crystal substrate is continuously or The method for producing a ZnO single crystal by a liquid phase epitaxial growth method according to claim 1, further comprising a step of growing the ZnO single crystal on the seed crystal substrate by pulling intermittently. The mixing ratio of the solute and the solvent is solute: solvent = 5 to 30 mol%: 95 to 70 mol%, and the mixing ratio of the solvent PbO and Bi ₂ O ₃ is PbO: Bi ₂ O ₃ = 0.1 to 95 mol. %: 99.9-5 mol% The method for producing a ZnO single crystal according to claim 2. After mixing and melting ZnO as a solute and PbF ₂ and PbO as solvents, the seed crystal substrate is brought into direct contact with the obtained melt, and the seed crystal substrate is continuously or intermittently contacted. The method for producing a ZnO single crystal by a liquid phase epitaxial growth method according to claim 1, further comprising a step of growing the ZnO single crystal on the seed crystal substrate by pulling it up. The mixing ratio of the solute and the solvent is solute: solvent = 2 to 20 mol%: 98 to 80 mol%, and the mixing ratio of PbF ₂ and PbO as the solvent is PbF ₂ : PbO = 20 to 80 mol%: 80 to 20 mol%. The method for producing a ZnO single crystal according to claim 4.   The method for producing a ZnO-based single crystal according to any one of claims 1 to 5, wherein a speed V for continuously pulling up the seed crystal substrate is 2 µm / hr ≤ V ≤ 50 µm / hr.   6. The method for producing a ZnO-based single crystal according to claim 1, wherein an average speed v for intermittently pulling the seed crystal substrate is 2 μm / hr ≦ v ≦ 50 μm / hr.   The method for producing a ZnO-based single crystal according to any one of claims 1 to 7, wherein a film thickness of the ZnO single crystal is 100 µm or more.   The method for producing a ZnO single crystal according to any one of claims 1 to 8, wherein the ZnO single crystal contains one or more selected from the group consisting of Al, Ga, In, H, and F.   The method for producing a ZnO-based single crystal according to any one of claims 1 to 9, wherein the growth orientation of the ZnO single crystal is a + c plane.   The method includes the steps of growing the ZnO single crystal, removing the seed crystal substrate by polishing or etching, and polishing or etching at least 10 μm or more on the −c plane side of the single crystal grown by liquid phase epitaxial growth. 10. A method for producing a ZnO single crystal according to any one of 10 above.   A self-supporting ZnO single crystal wafer obtained by the method for producing a ZnO single crystal according to claim 11, wherein the film thickness is 100 µm or more. The self-standing ZnO single crystal wafer according to claim 12, wherein the Li concentration is uniform with respect to the in-plane direction and the thickness direction of the wafer, and is 1 x 10 ¹⁵ pieces / cm ³ or less. 13. Ga is contained, the carrier concentration is 2.0 × 10 ¹⁷ atoms / cm ^{3 to} 1.0 × 10 ¹⁹ atoms / cm ³ , and the Li concentration is 1e ¹⁵ atoms / cm ³ or less. The self-supporting ZnO single crystal wafer according to 13. Al is contained, the carrier concentration is 2.0 × 10 ¹⁷ pieces / cm ^{3 to} 1.0 × 10 ¹⁹ pieces / cm ³ , and the Li concentration is 1 × 10 ¹⁵ pieces / cm ³ or less. The self-standing ZnO single crystal wafer according to 12 or 13. ¹³ or ¹³ containing In, a carrier concentration of 2.0e ¹⁷ atoms / cm ^{3 to} 3.5 × 10 ¹⁷ atoms / cm ³ , and a Li concentration of 1 × 10 ¹⁵ atoms / cm ³ or less. The self-supporting ZnO single crystal wafer according to 13.

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