JP2010280533A - Method for producing exciton-emitting ZnO scintillator by liquid phase epitaxial growth method - Google Patents

Abstract

When a scintillator crystal is used for radiation detection, an exciton-emitting scintillator with low emission of 450 to 600 nm that can solve the problem that the stability of the discrimination function of radiation detection is impaired by long-wavelength emission with a long fluorescence lifetime. A method for producing a ZnO single crystal is provided.
A non-doped ZnO single crystal is obtained by a liquid phase epitaxial growth method in which ZnO as a solute and a solvent are mixed and melted, and then a ZnO single crystal is grown by directly contacting a substrate with the obtained melt. And ZnO single crystals doped with group III elements and lanthanoid elements.
[Selection] Figure 5

Description

  The present invention relates to a ZnO single crystal used as a scintillator in a scintillation detector.

  There is a scintillation detector as a device for measuring radiation. A typical scintillation detector configuration is shown in FIG. In FIG. 1, when radiation enters the scintillation detector 100, fluorescence corresponding to the radiation incident on the scintillator crystal 110 is generated, and the radiation is detected by detecting this light with a photomultiplier tube or a semiconductor detector 120. Can do.

A TOF (Time of Flight) method has been proposed as a candidate for the next generation scintillator device, and studies are proceeding mainly on fluorides. In order to perform time resolution, it is possible to improve the resolution as the fluorescence lifetime is shorter, and BaF ₂ has been considered to be the most ideal. However, since BaF ₂ has a small light emission amount and a short emission wavelength, an expensive photomultiplier tube (PMT) having a quartz window is required, and a general-purpose PMT cannot be used.

Therefore, an exciton light emitting scintillator of a wide band gap compound semiconductor has been proposed as a scintillator having a short fluorescence lifetime instead of BaF ₂ . If a compound semiconductor such as ZnO or CdS is used as a scintillator, the fluorescence lifetime is short and the emission wavelength is 370 nm, so that a general-purpose PMT or semiconductor detector can be used.

  On the other hand, as a scintillation application, there is an application for confirming the radioactive distribution of contaminants. Conventionally, an autogeography (ARG) or an imaging plate has been used for this confirmation. Since these have been used for several decades, there is an advantage that the operation is stable, but the former has a problem that a developing film is necessary and the radioactivity intensity is not known from the film. In the latter case, the plate itself is easy to use, but it must be analyzed using a large measuring device. In both cases, it is necessary to prepare a measurement sample from the measurement object (contaminant), set it in the apparatus, and determine the exposure time in advance (estimate the appropriate exposure time). However, there is a problem that it takes time until results are obtained.

  If these passive measuring devices emit light when radiation is detected in real time and can be converted into an electrical signal, it can be used for early detection of radioactive contamination and quick response.

  As means for solving the above problems, an exciton light emitting scintillator of a wide band gap compound semiconductor has been proposed by Derenzo et al. (Non-Patent Document 1; Nuclear Instruments and Methods in Physics Research A505 (2003) 111-117, Patent Document) 1; U.S. Patent Application Publication No. 2006/219928).

  If a wide gap type compound semiconductor such as ZnO or CdS is used as a scintillator, the fluorescence lifetime is short, the emission wavelength is 370 to 380 nm, and a general-purpose PMT or semiconductor detector can be used. According to this document, the fluorescence lifetime due to X-ray excitation is as short as 0.11 to 0.82 nsec. However, the scintillator materials described in these documents are polycrystalline. In the case of a polycrystal, there are cases where the emission intensity varies depending on the orientation of the microcrystals and the spatial resolution depends on the particle size. For high spatial resolution and efficient scintillator light emission, the scintillator is preferably a single crystal.

  In addition, an exciton emission scintillator application of ZnO single crystal doped with about 1 ppm to 10 mol% of Group III and lanthanoid has been proposed (Patent Document 2; International Publication No. 2007/094785, Non-Patent Document 2; Nuclear Instruments). and Methods in Physics Research A505 (2003) 82-84).

  According to this document, the fluorescence lifetime of an In-doped ZnO single crystal is described as 0.6 nsec. However, the scintillator materials described in these documents are produced by the “High pressure direct melting technique” method. In this method, high pressure and high temperature are required, the crystal growth cost is high, and a single crystal portion and a polycrystalline portion are mixed, so that the same defects as the polycrystalline body exist.

  As a technique for solving the problems of the polycrystal, a scintillator using exciton emission of a wide band gap ZnO single crystal has been proposed (Patent Document 3; International Publication No. 2005/114256 pamphlet, non-patent document). 3; New Energy and Industrial Technology Development Organization, 2005 Research Report “Development of ultra-high-speed scintillator single crystal materials for practical TOF detectors”).

In this report, a hydrothermal synthesis method using a Pt internal autoclave is adopted as a ZnO single crystal growth method. When the same growth method is used, a large ZnO single crystal having high crystallinity can be grown. However, the crystal is subject to ionizing radiation such as alpha rays due to the effects of processing distortion on the crystal surface, fluorescence caused by impurity levels such as interstitial zinc and oxygen defects, and Li mixed from mineralizers used during growth. When irradiated, there was a drawback that a light-emitting component was present in the vicinity of 450 to 600 nm, which is a longer wavelength region than the band edge emission that is exciton emission. Further, the amount of light emitted from the scintillator crystal using ZnO was limited to about 15% of Bi ₄ Ge ₃ O ₁₂ (BGO) which is a typical scintillator crystal (Non-patent Document 4; High Energy News Vol. 21 No. .2 41-50 2002).

  In general, the fluorescence lifetime of exciton emission is short, but the fluorescence lifetime other than exciton emission is long. Therefore, when the scintillator crystal is used for radiation detection, if there is light emission other than exciton light emission, there is a disadvantage that the stability of the discrimination function of radiation detection is impaired, and only exciton light emission becomes dominant. A scintillator crystal has been sought. In this report, improvement of polishing method to reduce processing distortion on the crystal surface and improvement of emission amount due to long wavelength shift of exciton emission wavelength by In doping were seen, but emission of longer wavelength component than exciton emission The problem of high strength could not be solved.

  On the other hand, high-quality ZnO single crystals can be grown by using vapor phase growth methods such as laser pulse deposition (PLD), MBE, and MOCVD. However, the vapor deposition method has a low film formation rate and is 1 μm or more. In order to grow a ZnO single crystal having a film thickness, there is a drawback that it takes a lot of time. In the case of a ZnO single crystal, there is a penetration depth of about 5 to 50 μm with an α-ray or electron beam, and a sufficient thickness to block the α-ray or electron beam with a ZnO single crystal alone can be achieved in a short time by vapor phase growth. It had problems that could not grow.

US Patent Application Publication No. 2006/219928 International Publication No. 2007/095475 Pamphlet International Publication No. 2005/114256 Pamphlet International Publication No. 2007/100146 Pamphlet

Nuclear Instruments and Methods in Physics Research A505 (2003) 111-117 Nuclear Instruments and Methods in Physics Research A505 (2003) 82-84 New Energy and Industrial Technology Development Organization 2005 Research Results Report “Development of Ultra-High Speed Scintillator Single Crystal Materials for Practical TOF Detectors” High Energy News Vol. 21 No. 2 41-50 2002 Appl. Phys. Lett. 78 1237 (2001)

  Under the circumstances described above, a method for manufacturing a ZnO-based scintillator single crystal in which visible light emission other than exciton light emission is suppressed by a liquid phase epitaxial growth method (LPE method) has been demanded.

The present inventors have applied for a method for growing a ZnO single crystal by a solution growth method (Patent Document 4; International Publication No. 2007/100146 pamphlet). Using the method of the application, after it melted by mixing the PbO and Bi ₂ O ₃ or PbF ₂ and PbO is ZnO and the solvent is solute, by contacting the substrate directly resulting melt A non-doped ZnO single crystal can be grown on the substrate. In addition, when the method of the same application is used, a ZnO single crystal containing a group III element such as Al, Ga and In and a lanthanoid element in an amount of 1 mol% or less can also be produced.

  On the other hand, as a result of diligent research, the present inventors have found that a ZnO single crystal grown by a method such as hydrothermal synthesis has n-type crystal defects such as interstitial zinc and oxygen vacancies and a high concentration of impurity Li, It has been found that such crystal defects and impurities Li cause light emission having a long decay lifetime in the wavelength region of 450 to 600 nm when irradiated with α rays or electron beams. Therefore, when using a non-doped ZnO single crystal or a group III element or lanthanoid element-doped ZnO single crystal manufactured by the method of Patent Document 4, an exciton-emitting ZnO single crystal with a small emission of 450 to 600 nm is obtained. Found that can be obtained.

  According to a preferred embodiment of the present invention, an exciton-emitting ZnO single crystal that emits less light at 450 to 600 nm can be produced. Therefore, when a scintillator crystal is used for radiation detection, long-wavelength emission with a long fluorescence lifetime is possible. Therefore, the problem that the stability of the discrimination function of radiation detection is impaired can be solved.

FIG. 1 shows a typical scintillation detector configuration. FIG. 2 shows a typical LPE (Liquid phase epitaxy: LPE) growth furnace used in the embodiment of the present invention. FIG. 3 is a configuration diagram of the LPE growth furnace used in the examples. FIG. 4 is an α-ray excitation transmission spectrum of the hydrothermal synthesis substrate. FIG. 5 is a transmission spectrum of the α-ray excited scintillator light according to the first embodiment. FIG. 6 is a transmission spectrum of the α-ray excited scintillator light according to the second embodiment. FIG. 7 is a transmission spectrum of the α-ray excited scintillator light according to the third embodiment. FIG. 8 is a transmission spectrum of the α-ray excited scintillator light according to the fourth embodiment. FIG. 9 is a transmission spectrum of the α-ray excited scintillator light according to the fifth embodiment. FIG. 10 is a transmission spectrum of the α-ray excited scintillator light according to Example 6. FIG. 11 is a transmission spectrum of the α-ray excited scintillator light according to Comparative Example 1. 12 is a comparison diagram of the light emission amounts of α-ray excited scintillator light according to Examples 1 to 6 and Comparative Example 1. FIG.

<First Embodiment>
The first embodiment of the present invention is produced by a liquid phase epitaxial growth method in which a ZnO single crystal is grown by bringing a substrate into direct contact with a mixture / melt of a solute ZnO and a solvent. This is a method for manufacturing an exciton-emitting ZnO scintillator.

  That is, after melting and mixing ZnO as a solute and a solvent, the substrate is brought into direct contact with the obtained melt, and further the temperature is lowered, so that the ZnO single crystal precipitated in the supersaturated melt Is a method of manufacturing an exciton-emitting ZnO scintillator by an LPE method in which a substrate is grown on a substrate.

  According to the exciton emission type ZnO scintillator obtained by the manufacturing method according to the present embodiment, only exciton emission having a short fluorescence lifetime at the time of excitation by ionizing radiation such as α rays and electron beams is dominant, and the fluorescence lifetime is reduced. Long light-emitting components are reduced, and the radiation detection discrimination function can be stabilized. Furthermore, the amount of light emitted when irradiated with ionizing radiation such as α rays can be improved to a BGO ratio of 15% or more. Therefore, the ZnO single crystal scintillator obtained in this embodiment can be applied to high-speed detectors such as α rays and electron beams. Examples of the ionizing radiation applied to the exciton-emitting ZnO scintillator include α rays, γ rays, electron beams, X rays, neutron rays, and the like.

<Second Embodiment>
The second embodiment of the present invention is a liquid phase epitaxial growth method in which a ZnO single crystal is grown by bringing a substrate into direct contact with a mixture / melt of ZnO as a solute and a solvent. , In and a lanthanoid element are doped with 585 ppm or less, and a method for producing an exciton-emitting ZnO scintillator.

  According to the exciton emission type ZnO scintillator obtained by the manufacturing method according to the present embodiment, only exciton emission having a short fluorescence lifetime at the time of excitation by ionizing radiation such as α rays and electron beams is dominant, and the fluorescence lifetime is reduced. Long light-emitting components are reduced, and the radiation detection discrimination function can be stabilized. Furthermore, the amount of light emitted when irradiated with ionizing radiation such as α rays can be improved to a BGO ratio of 15% or more. Therefore, the ZnO single crystal scintillator obtained in this embodiment can be applied to high-speed detectors such as α rays and electron beams.

  In the second embodiment, the doping amount of the group III and lanthanoid elements is preferably 585 ppm or less with respect to ZnO. More preferably, it is 146 ppm or less with respect to ZnO, More preferably, it is 59 ppm or less with respect to ZnO, Most preferably, it is non-doped. If it exceeds 585 ppm, the amount of luminescence becomes 15% or less of the BGO ratio, which is not preferable.

<Third Embodiment>
In the third embodiment of the present invention, the solvent is PbF ₂ and PbO, the mixing ratio of the solute and the solvent is solute: solvent = 2 to 20 mol%: 98 to 80 mol%, and the solvent is PbF ₂ mixing ratio of PbO is PbF _2: PbO = 20~80mol%: a 80~20mol%, is a manufacturing method of exciton emission type ZnO scintillator above.

According to this embodiment, PbO and PbF ₂ form a eutectic system, and LPE growth at a temperature lower than the melting point of PbO or PbF ₂ alone becomes possible, so that evaporation of the solvent can be suppressed. Accordingly, stable crystal growth can be performed with little composition variation, furnace material consumption can be suppressed, and the growth furnace does not have to be a closed system, so that low-cost production is possible.

  Moreover, since the LPE method close to thermal equilibrium growth is used as the crystal growth method, a high-quality ZnO single crystal with few transitions and defects can be manufactured. The doped ZnO single crystal scintillator obtained in this embodiment can be applied to high-speed detectors such as α rays and electron beams.

As the solvent composition, preferably PbF _2: PbO = 20~80mol%: a 80～20Mol%, more preferably PbF _2: PbO = 30~70mol%: a 70～30Mol%, more preferably PbF _2: PbO = 40 to 60 mol%: 60 to 40 mol%. Within this range, the amount of evaporation of the solvents PbF ₂ and PbO can be suppressed. As a result, the fluctuation of the solute concentration is reduced, and the non-doped or group III and lanthanoid-doped ZnO single crystals are stabilized by the LPE method. Can grow into. Furthermore, since the furnace material consumption can be suppressed and the growth furnace does not have to be a closed system, a ZnO single crystal can be grown at a low cost.

The mixing ratio of ZnO as a solute and PbF ₂ and PbO as a solvent is preferably solute: solvent = 2 to 20 mol%: 98 to 80 mol%. More preferably, the solute concentration is 5 mol% to 10 mol%. When the solute concentration is 5 mol% or more, the effective growth rate can be further increased, and when the solute concentration is 10 mol% or less, the temperature at which the solute component is dissolved can be kept lower, and the amount of solvent evaporation can be reduced.

<Fourth Embodiment>
In a fourth embodiment of the present invention, the solvent is PbO and Bi ₂ O ₃ , and the mixing ratio of the solute and the solvent is solute: solvent = 5-30 mol%: 95-70 mol%, and PbO as the solvent And the Bi ₂ O ₃ mixing ratio of PbO: Bi ₂ O ₃ = 0.1 to 95 mol%: 99.9 to 5 mol%.

According to this embodiment, the LPE method close to thermal equilibrium growth is used as the crystal growth method, and PbO and Bi ₂ O ₃ which are fluxes composed of elements having a large ionic radius that are difficult to be incorporated into the ZnO crystal are used. As a result, a high-quality ZnO single crystal with few impurities mixed into the crystal can be produced. In particular, a high-quality ZnO single crystal with reduced contamination of fluorine impurities can be produced. The ZnO single crystal scintillator obtained in this embodiment can be applied to high-speed detectors such as α rays and electron beams.

As the solvent _{composition, PbO: Bi 2 O 3 =} 0.1~95mol%: 99.9~5mol% is preferred. More _{preferably, PbO: Bi 2 O 3 =} 30~90mol%: a 70~10mol%, particularly _{preferably, PbO: Bi 2 O 3 =} 60~80mol%: a 40~20mol%. In the case of a PbO or Bi ₂ O ₃ single solvent, the liquid phase growth temperature becomes high, and therefore a mixed solvent of PbO and Bi ₂ O ₃ having the above mixing ratio is suitable.

The mixing ratio of ZnO as the solute and PbO and Bi ₂ O ₃ as the solvent is preferably solute: solvent = 5-30 mol%: 95-70 mol%. More preferably, the solute concentration is 5 mol% to 10 mol%. When the solute concentration is 5 mol% or more, the growth rate can be further increased, and when the solute concentration is 10 mol% or less, the growth temperature can be suppressed to be lower.

<Crystal growth method suitable for the present invention>
As a method for growing a non-doped ZnO single crystal or a ZnO single crystal doped with a group III or lanthanoid, a vapor phase growth method and a liquid phase growth method have been used roughly. As the vapor phase growth method, chemical vapor transport method (see Japanese Patent Application Laid-Open No. 2004-131301), molecular beam epitaxy or metal organic vapor phase growth method (see Japanese Patent Application Laid-Open No. 2004-84001), sublimation method (Japanese Patent Application Laid-Open No. Hei 5). -70286, etc.) have been used, but there were many dislocations and defects, and the crystal quality was insufficient.

  On the other hand, the liquid phase growth method has an advantage that it is easier to manufacture high quality crystals than the vapor phase growth method because crystal growth proceeds in principle in thermal equilibrium. However, since ZnO has a high melting point of about 1975 ° C. and easily evaporates, it has been difficult to grow a ZnO single crystal using the Czochralski method employed in silicon single crystals and the like. Therefore, as a method for growing the ZnO single crystal, the target substance is dissolved in a suitable solvent, the mixed solution is cooled to a saturated state, and the target substance is grown from the melt. A flux method, a floating zone method, a top seed solution growth (TSSG) method, a solution pulling method, an LPE growth method, and the like have been used.

  As the ZnO single crystal growth method in the present invention, a liquid phase epitaxial method (LPE method), a flux method, a TSSG method, a solution pulling method, and the like can be used. Among these various methods, as a method of growing a ZnO-based single crystal, growth at a relatively low cost and a large area is possible. In particular, considering application to a scintillator, a layer structure according to function is formed. The easy LPE method is suitable. In particular, a non-doped ZnO single crystal or a group III or lanthanoid-doped ZnO single crystal is used as an LPE by using a ZnO single crystal produced by a hydrothermal synthesis method that has a high crystallinity and can easily obtain a large-area substrate. A method of growing by a method (liquid phase homoepitaxial growth method) is suitable.

<LPE growth furnace suitable for the present invention>
A typical LPE growth furnace used in an embodiment of the present invention is shown in FIG. In the LPE growth furnace, a platinum crucible 4 for melting a raw material and storing it as a melt is placed on a crucible base 9 made of mullite (alumina + silica). 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 mullite-made core tube 11 is provided between the heater and the inner wall of the manufacturing furnace, and a mullite-made furnace lid 12 is provided above the core tube 11. A pulling mechanism is provided above the platinum crucible 4. A pulling shaft 5 made of alumina 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 shaft is provided on the upper portion of the pull-up shaft 5.

  Conventionally, alumina and mullite have been exclusively used for the crucible base 9, the furnace core tube 11, the pull-up shaft 5 and the furnace lid 12 in the members constituting the LPE furnace. Therefore, in the temperature range of 700 to 1100 ° C., which is the LPE growth temperature and the raw material melting temperature, the Al component is volatilized from the alumina and mullite furnace materials and dissolved in the solvent, and this is mixed in the ZnO single crystal thin film. it is conceivable that.

In the present invention, control of the group III doping amount is important. It is desirable to control the doping amount of the group III by the charged composition. Therefore, mixing the Al impurities into the LPE-grown ZnO single crystal thin film can be reduced by making the furnace material constituting the LPE furnace a non-Al material. 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 and 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, in a preferred embodiment of the present invention, a non-doped ZnO single crystal or a group III and lanthanoid doped ZnO single crystal is obtained using an LPE growth furnace composed of MgO and / or quartz as a non-Al-based furnace material. To manufacture. 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 furnace, and A mode in which a pulling shaft for moving the substrate up and down is provided, and these members are independently made of MgO or quartz is also preferable.

<Other requirements suitable for the present invention>
In the embodiment of the present invention, in order to control the LPE growth temperature and adjust the solvent viscosity, the ZnO solubility, the evaporation amount of PbF ₂ and PbO, or the evaporation amount of PbO and Bi ₂ O ₃ are not significantly changed. One or two or more third components can be added. For example, examples of the third component include B ₂ O ₃ , P ₂ O ₅ , V ₂ O ₅ , MoO ₃ , WO ₃ , SiO ₂ , MgO, and BaO. Further, Bi ₂ O ₃ may be added as a third component to the solvent of the third embodiment of the present invention.

The substrate used in the embodiment of the present invention is not particularly limited as long as it has a crystal structure similar to ZnO and the grown thin film does not react with the substrate, and a substrate having a close lattice constant is preferably used. For example, _{sapphire, LiGaO 2, LiAlO 2, LiNbO} 3, LiTaO 3, ZnO or the like can be mentioned. Considering that the target single crystal in the present invention is ZnO, ZnO having a high degree of lattice matching between the substrate and the grown crystal is optimal.

  Hereinafter, as a method for growing non-doped ZnO single crystals, group III and lanthanoid doped ZnO single crystals according to an embodiment of the present invention, a method of forming these ZnO single crystal films on a ZnO single crystal substrate by the LPE method will be described. To do. The present invention is not limited to the following examples.

<Outline of operating conditions of LPE growth furnace>
The block diagram of the LPE growth furnace used in the Example 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 shaft 5 'made of quartz 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 upper portion of the pull-up shaft 5' made of quartz. Below the platinum crucible 4, a thermocouple 10 for melting the raw material in the crucible is provided.

  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. The temperature is preferably raised to 650 to 1000 ° C, more preferably 700 to 900 ° C, and the mixture is allowed to stand for 2 to 3 hours to stabilize the raw material melt. 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 900 ° 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 adjusted to the melt, a temperature drop is started at a constant temperature or 0.025 to 5.0 ° C./hr to grow a desired non-doped or doped ZnO single crystal on the surface of the substrate. Also during the growth, the substrate is rotated at 5 to 300 rpm by the rotation of the pulling shaft, and is reversely rotated every predetermined time.

  After crystal growth for about 30 minutes to 24 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 thin film. Further, depending on the growth film thickness, it is possible to grow while pulling up the quartz axis with time or continuously.

<Examples 1 to 6, Comparative Example 1>
In this example, a non-doped or Ga-doped ZnO single crystal was produced by a liquid phase epitaxial growth method using PbO and Bi ₂ O ₃ as solvents.

Raw materials were charged in a platinum crucible having an inner diameter of 75 mmφ, a height of 75 mmh, and a thickness of 1 mm according to the formulation shown in Table 1 below. The solvent composition is PbO: Bi ₂ O ₃ = 66.67 mol%: 33.33 mol%, and the solute concentration of ZnO is about 7.00 mol%. The Ga composition is non-doped with respect to ZnO (Example 1), or 29 ppm, 59 ppm, 146 ppm, 293 ppm, 585 ppm, and 2920 ppm (Examples 2 to 6 and Comparative Example 1 respectively).

  The crucible charged with the raw material was placed in the furnace shown in FIG. 3 and melted at a crucible bottom temperature of about 900 ° C. After that, after holding at the same temperature for 3 hours, the temperature was lowered until the crucible bottom temperature reached about 800 ° C., and then a ZnO single crystal substrate having a size of 10 mm × 10 mm × 0.5 mmt in the + C plane orientation grown by the hydrothermal synthesis method And was grown at the same temperature for 12 hours while rotating the pulling shaft made of quartz at 60 rpm. At this time, the rotation direction of the shaft was reversed every 5 minutes. Thereafter, the pulling shaft made of quartz was raised to separate it from the melt, and the shaft was rotated at 200 rpm to shake off the melt component, thereby obtaining a colorless and transparent non-doped or Ga-doped ZnO single crystal thin film.

  Table 2 shows the results of growth time, LPE film thickness, and growth rate of the obtained non-doped or Ga-doped ZnO single crystal thin film. The thickness of these obtained ZnO films was 50 to 70 μm. The LPE growth method had a relatively high growth rate, and the growth rate was 4.2 to 5.8 μm / hr. Therefore, it is possible to easily grow a thickness of 50 μm or more corresponding to the penetration depth of α rays and electron beams.

  Table 2 shows the (002) plane rocking curve half-width (evaluation of crystallinity) after polishing the obtained ZnO film, the presence or absence of long-wavelength light emission of 450 to 600 nm upon α-ray excitation, and the scintillator. The characteristic results are also shown. In the polishing, the obtained ZnO film was lapped and polished to flatten the surface and make the LPE film thickness 40 μm.

As the scintillator characteristics, α-ray excitation transmission spectrum measurement and α-ray excitation scintillator emission amount measurement were performed. ²⁴¹ Am was used as the α-ray source. PMT (R7600: manufactured by Hamamatsu Photonics) was used as a light receiving element, and five surfaces of the polished LPE-grown ZnO single crystal were covered with Teflon (registered trademark) tape so that scintillation light could be obtained from only one surface. At that time, a 1 mm square window was left for passing α-rays. A PMT was installed on the surface where the scintillation light was emitted, and a high voltage was applied to the PMT to amplify the signal and read out. The crest value was amplified by pre-amp, the waveform was adjusted by Pulse shape amp, the signal was obtained as a signal through a multi channel analyzer and used as a signal, and compared with BGO.

  As can be seen from the results of Examples 1 to 6, when a non-doped or Ga-doped ZnO film was grown using an LPE method close to thermal equilibrium growth, the (002) plane rocking curve half width was 19 to 60 arcsec. Since the half width of the ZnO films according to Examples 1 to 6 is not significantly different from the half width (20 arcsec) of the hydrothermal synthesis substrate used as the substrate, the ZnO films according to Examples 1 to 6 have high crystallinity. It became clear.

  Moreover, as shown in FIGS. 4-11, long wavelength light emission component can be reduced in alpha ray excitation by non-doping or doping Ga with 29 ppm-2920 ppm with respect to Zn. Furthermore, as shown in Table 2 and FIG. 12, the light emission amount of the non-doped or Ga-doped ZnO single crystal is 16% or more in terms of the BGO ratio with the Ga-doped amount in the range of 29 ppm to 585 ppm. Exceeds 15%, which is the amount of light emitted by the system scintillator.

  In the non-doped or Ga-doped ZnO single crystals according to Examples 1 to 6, only exciton luminescence with a short fluorescence lifetime upon excitation by α-rays and electron beams is dominant, and a luminescence component with a long fluorescence lifetime is reduced, thereby detecting radiation. It is possible to stabilize the discrimination function. Therefore, the ZnO single crystal scintillator obtained in this example can be applied to high-speed α-ray and electron beam detectors. On the other hand, the hydrothermal synthetic substrate has a light emission amount of 94%, which is the same as that of BGO.

<Examples 7 to 10>
LPE growth of each element-doped ZnO single crystal was performed by the same method as in Example 1 with the formulation shown in Table 3. Table 4 shows the physical properties of the obtained various element ZnO single crystal films.

  As can be seen from the results of Examples 7 to 10, when a ZnO film doped with each element was grown using the LPE method close to thermal equilibrium growth, the (002) plane rocking curve half width was 22 to 70 arcsec. Since the half width of the ZnO films according to Examples 7 to 10 is not significantly different from the half width (20 arcsec) of the hydrothermal synthesis substrate used as the substrate, the ZnO films according to Examples 7 to 10 have high crystallinity. It became clear. The LPE growth method had a relatively high growth rate, and the growth rate was 2.9 to 5.3 μm / hr. Therefore, it is possible to easily grow a thickness of 50 μm or more corresponding to the penetration depth of α rays and electron beams.

  Moreover, like the result of Examples 1-6, there was no long wavelength light emission component, and all emitted light quantity of alpha ray excitation scintillator light became 38-48% of 15% or more. In the ZnO single crystal doped with Al, In, and lanthanoid shown in this example, only exciton emission with a short fluorescence lifetime upon excitation by α-rays and electron beams is dominant, and a light emitting component with a long fluorescence lifetime is reduced. It becomes possible to stabilize the discrimination function of radiation detection. Therefore, the ZnO single crystal scintillator obtained in this example can be applied to high-speed α-ray and electron beam detectors.

DESCRIPTION OF SYMBOLS 1 ... Upper stage heater 2 ... Center part heater 3 ... Lower stage heater 4 ... Platinum crucible 5 ... Pulling up shaft 6 ... Substrate holder 7 ... Substrate 9 ... Crucible base 11 ... core tube 12 ... furnace lid

Claims (4)

  A method for producing an exciton-emitting ZnO scintillator, characterized by being produced by a liquid phase epitaxial growth method in which a ZnO single crystal is grown by directly contacting a substrate with a mixture / melt of a solute ZnO and a solvent. .   In a liquid phase epitaxial growth method in which a ZnO single crystal is grown by bringing a substrate into direct contact with a mixture / melt of a solute ZnO and a solvent, ZnO is doped with 585 ppm or less of Al, Ga, In, and a lanthanoid element. A method for producing an exciton-emitting ZnO scintillator characterized by the above. The solvent is PbF ₂ and PbO, 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 the solvent PbF ₂ and PbO is PbF ₂ : PbO = 20 to 80 mol%: The method for producing an exciton-emitting ZnO scintillator according to claim 1 or 2, wherein 80 to 20 mol%. The solvent is PbO and Bi ₂ O ₃ , and the mixing ratio of the solute and the solvent is solute: solvent = 5-30 mol%: 95-70 mol%, and the mixing ratio of PbO and Bi ₂ O ₃ as the solvent is _{PbO: Bi 2 O 3 = 0.1~95mol} %: a 99.9~5mol%, the production method of the exciton emission type ZnO scintillator according to claim 1 or 2.